Conversion of Polymers Wastes & Energetics

U. Hansen and A. Rinschede

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Waste Disposal Logistics a Prerequisite for Effective Recycling

U. Hansen and A. Rinschede Fraunhofer-Institut für Materialfluß und Logistik, Department of Waste Disposal Logistics, Joseph-von-Fraunhofer Straße 2-4, D-44227 Dortmund, Germany

Legislation has reacted to increasing environmental problems by changing basic regulations especially regarding waste disposal. The recycling of waste will become a standard practice. It is therefore necessary to develop new concepts and technologies which allow for recycling at a high level. Consequently, the use of logistics is an important part of the solution to present problems. The following publication discusses a new software for optimization of internal waste disposal. Also, examples of recycling of plastics and cars are given and approaches towards the development of logistics for suitable redistribution and dismantling are analyzed.

Today, recycling is of growing importance and of great interest to the public. The recycling effort is endless in itself and a basic necessity because our civilization is threatened by growing amounts of waste. The Waste Law stipulates a higher degree of waste avoidance and mandatory recycling. This regulation requires large-scale recycling of industrial products. It also offers the possibility of maintaining responsible care of product content within a production-consumption-disposal cycle. The large product range and high number of component materials call for complex but inexpensive recycling processes. Figure 1 shows present recycling options. It must be understood that not every level of recycling is attainable or feasible for all aggregates, components, or materials; for example, the thermal transformation of duroplastic and elastomers is not possible.

8

Waste disposal logistics

The aim of all efforts should be recycling at a high level. Here lies great development potential for innovative technologies such as recycling of components. The basic aim is to find a scope of use similar to the original product. Fiber reinforced plastics, characterized by high stiffness and low weight, can be used as an example. Instead of milling for use as filler, areas of application should be found where the characteristics of these components can be used directly.

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This procedure is similar to the present material recycling principles where the so-called “cascade of re-utilization” aims for an optimal utilization of existing characteristics in a secondary use. In an economy reorganized into a cyclical system, components and materials would be re-entered into the economic cycle as secondary raw materials, secondary semi-finished products or secondary components. In our highly industrialized society, based on the division of labor, the great variety of material flows cannot be directed to their destinations without thorough planning and coordination. Without planned development, large material quantities would grow to intolerable dimensions because of under-utilization of residues. Conurbational traffic has already reached its limit. Therefore, material and information flows have to be optimized to realize recycling and transport effectively. This is a logistical task. The great importance of logistics in enterprises and economy has been realized only in the last few years. Consequently, its acceptance, and above all, the efficiency of its technical components has grown. This can be seen from an example of manufacturing industry. Here, besides complex production cells, modern logistic components such as fully automated multilevel warehouses, automatically guided transporting systems, automated loading, and transfer points with order-picking and warehouse robots are used. Logistics is divided into four main fields: • procurement logistics • production logistics • distribution logistics • waste disposal logistics. In waste disposal logistics, collecting and recycling are added to the traditional logistic functions such as transport, trans-shipment, and storage. Production and services now use logistics to reduce costs and rationalize operations. In waste disposal, however, the quantity of product-specific residues and waste shows that the logistics is still unknown to many enterprises. This is a disadvantage; only if the quality and the quantity of waste and residues are known, suitable measures can be undertaken to avoid waste and decisions made as to which material can be re-utilized. In order to intensify internal and external recycling, the use of logistics is absolutely necessary. The problems mentioned earlier explain the need for waste disposal logistics.

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Waste disposal logistics

Another important task of waste disposal logistics is to expand internal and external organization systems, which, at present, are mostly concerned with procurement, production, distribution, and waste disposal (Figure 2). This can be achieved, for example, by extending existing material management systems to include waste disposal systems, which either already exists or which

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must be developed. This leads to change in material and information flow because recycling has to be performed to a greater extent in an increasing number of fields. The establishment of a modified business structure is a frequent result (Figure 3). For the effective use of waste disposal logistics, the separation of micro and macro logistics has proven successful (Figure 4): • •

In macro-logistics, elements and systems which connect plants widely separated by geographical distribution (collection, redistribution) are studied. Micro-logistics, on the other hand, combines these elements and systems within a relatively close area, for example within a production or dismantling plant.

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Waste disposal logistics

Below, we give a short description of the software tool for optimization of internal waste disposal and recycling (ELOS) which has been developed by the Fraunhofer - Institut für Materialfluß und Logistik (IML): The basis of the optimization is the collection of data on all internal waste flow systems, which are

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stored in the form of current-state-database. The necessary data can be collected by using existing EDP material or from interviews using machine-readable questionnaires. The actual situation can be graphically presented in the form of a company layout with qualitative and quantitative information concerning existing mate-

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Waste disposal logistics

rial flows and can be statistically evaluated with the help of a related analyzing system. Thus, the cost assessment and analysis of weak points can be performed (Figure 5). A second important part of the system is a database which contains technical information on containers, transportation, warehousing, collecting strategies, methods of waste disposal, recycling potential, and cost assessment. After entering planned procedures, such as the potential for utilizing recycled materials, minimization of waste disposal costs or high disposal security, in a second database further design variants can be generated by interactive planning steps with the help of databases, which are evaluated in respect to their targets. Finally, a choice of evaluated, partially optimized alternatives is available. In the following discussion, two examples of micro and macro-logistics are given followed by some suggestions. The examples were chosen for the following reasons: • Part of the Packing Ordinance has already come into force. Comparable decrees for other products (electronic scrap, used cars) are under discussion in draft form. • Plastics are often used in complex products. If the products are not returned and recycled this has a negative effect on the possible future use of plastics (catchword “recycling quota”). EXAMPLE 1: RETURN LOGISTICS - REDISTRIBUTION (MACRO-LOGISTICS)

Before the planned regulations obliging a producer to take back and recycle certain products come into force, suitable general return strategies have to be developed. The direct technical design of the respective logistics depends on the products and the recycling plans. From economical and ecological points of view, the first requirement should be to use the supply vehicles not only as distribution vehicles but also as waste disposal redistribution vehicles. If this should prove unrealistic, a new concept should be developed. In both cases the concept has to comprise several plans: • a long term (strategic) • a medium term (tactical) • a short term (operational) level

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Strategic planning involves investments of working funds as well as development of concepts for a suitable network of collection points. Here, the collection areas must be defined and the question answered “should centralized or decentralized sites be established (catchword ”combined planning of sites and area")?"

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Waste disposal logistics

FhG has developed a software tool called “Displan”, which can be used for strategic planning in order to find suitable places to concentrate the supply. It should also be possible, with the help of the same tool, to plan the collection points (Figures 6 and 7).

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Tactical planning concerns the combining of orders with operation days as well as tour plans for the individual collection trucks. The operational level includes planning of daily collection tours. By densifying planning data on the operational level input, data for tactical planning are generated. Individual planning should be carried out with the help of a PC where a great range of optimization modules are available. Tour information is taken from street database which are available with high resolution. To implement the plans, data collection trucks should be equipped with a motor fleet information system. Besides the development of such a concept, technical aspects have to be given first consideration. Suitable vehicles are needed for collection of used articles. Requirements depend on size and type of products. An example for this is the recycling of used cars: Besides its topicality (draft of an objective by the Federal Government), this subject is especially interesting because of the great quantity of plastics. On average, 10.4% (about 120 kg) of the weight of a car is made up of different plastics (H. Steinert: Recycling must be given early consideration in the design of a car, FAZ 13/11/1989). At present, the following collection techniques for used cars are possible: 1. Collection via a mobile scrap baling press 2. Collection via retrieving trucks and mounted grapple crane 3. Collection via haulaway trucks 4. Other systems. Given the composition of used cars, variants 1 and 2 would not work. Even cars involved in collisions should not be further damaged during collection. Therefore, collection trucks should be designed in such a way that the outer contour of used cars can be maintained so that parts of the car body can still be re-utilized. Generally, haulaway trucks should be equipped with collecting basins to contain leaking fuels and lubricants. Suitable loading aids must to be developed to allow for continuous loading. If the outer contour of a used car is even partly maintained it can be loaded by a crane and be stacked in the collection truck. Here, loading techniques must be developed carefully so that the car bodies are not dented during loading. Otherwise, window breakage would lead to the materials in the interior being unusable also.

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Waste disposal logistics

As for plastics it should be pointed out that they have a great volume at low weight (EPS is an example). A suitable collection technique must be developed for these materials. Here, the combination of mobile swing-hammer crashers or shrinking devices is possible to improve the loading capacity of the trucks. The collection method must meet quality and purity requirements. EXAMPLE 2: DISMANTLING (MICRO-LOGISTICS)

Before aggregates, components and materials can be recycled, the collected products must be dismantled. At first sight, it seems to be favorable to use production strategies and techniques. A detailed analysis shows, however, that the change from one complex product to a variety of ordered components, raw material, and waste results in definitely changed parameters and problems (Figure 8). Examples for this are: • • • •

increased cycle times due to a variety of types and condition of products high set-up time due to necessary variety of tools changed order conditions possible use of destruction techniques

This field also offers a significant potential for development. The internal information flow connects the necessary work steps. Among others, it should allow for time control. For this purpose, all data on the products which are to be recycled, should be collected before delivery. Important data can be entered into the system with minimum effort by using a scanner-readable questionnaire. Changes or damage to delivered used products should also be considered. At the end of these procedures, all relevant data on delivered products and the whole stock are stored in a central database. In the next step, the necessary recycling processes or the method of retrieval of components or elements must be determined. Depending on production structure and throughput, different informational concepts have to be studied to determine their suitability. For a centrally organized system, all necessary information is stored in a database. Information on each work step is available before the next starts. It is well-understood that such a system requires a widely spread and well functioning communication network. Apart from a high data-processing rate and storage capacity, transmission times have to be considered as well. The advantage of

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this concept is that after determination of the current data, products need only be identified during each work step. The assignment of information to the processed subject is an alternative to central information management. Here, the item is accompanied by all necessary data in machine-readable form (e.g., in the form of programmable data carriers). This allows for a direct control of an automated transport technique. The central production control system is informed only about the part movement. With the help of adequate loading aids, it is relatively easy to automate transport techniques. Requirements for dismantling are, however, more demanding. Prerequisites for automation are either a far-reaching standardization of processes or the use of highly flexible techniques. Both cases differ widely in the size of investment needed, therefore it is important to evaluate the feasible alternatives in detail.

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Waste disposal logistics

This report has shown the role of waste disposal logistics in effective recycling. In the same way as procurement, production, and distribution must be optimized, the various processes in material and information flows must also be optimized and coordinated during recycling and disposal. Here, it has to be considered that in a market economy components and raw materials will only be recycled when the process is as smooth as possible, i.e., at minimum cost. It is the task of waste disposal logistics to make the necessary concepts and technologies available.

F. Pilati et al.

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Powder Coatings from Recycled PET

F. Pilati, M. Toselli, and S. Torricelli Dipartimento di Chimica Applicata e Scienza dei Materiali, Università di Bologna, Italy

C. Stramigioli Dipartimento di Ingegneria Chimica e di Processo, Università di Bologna, Italy

M. Dinelli Inver Spa, Bologna, Italy

INTRODUCTION

The human activities in their different forms are responsible for the pollution problems that have become more and more critical in the last few years. Some of more common actions that have been undertaken to reduce pollution are the recycling of post-consumer materials and elimination of toxic solvents from products such as coatings and adhesives, or from chemical processes in general. The study reported in this paper proposes a contribution to the solution of the pollution problems by suggesting that it is possible to recycle poly(ethylene terephthalate), PET, derived from post-consumer beverage bottles, for the preparation of solvent-free coatings. Research work in the field of polymer recycling is more frequently conducted in the last few years, and different types of recycling have been explored. In several cases, recycled polymers can be considered as a source of molecular fragments which can be used to build again new polymeric materials. A process like this, commonly named as tertiary or chemical recycling, can be used to pro-

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Powder coatings from recycled PET

duce monomers or mixtures of oligomers which can be employed, alone or with other monomers, to produce virgin polymers. The main requirement for the success of this type of recycling is the chemical feasibility and a convenient economic balance. The chemical feasibility of the process depends mainly on the nature of polymer to be recycled. Also, the polymer must be collected in a relatively pure form and must undergo a very extensive demolition of the polymer chains by ‘clean’ chemical reactions, that is by reactions leading to a limited number of chemical compounds at a high yield. On the other hand, the real possibility of recycling is based on the reliability of the process and, particularly, on the economic convenience; the polymeric materials produced by this process should have lower prices than those obtained from monomers. In general, the main prerequisites to meet this last goal are the possibility of a large-scale collection of a high quality waste and an economic chemical process. All the above requirements are commonly met for PET that can easily be transformed (by exchange reactions such as methanolysis or glycolysis) into a mixture of products from which monomers or oligomers can be recovered by a limited number of simple unit operations. Work done recently has shown that, recycled PET, mainly recovered from beverage bottles or films, can be used to produce dimethyl terephthalate, DMT, 1-4 by methanolysis, or bis(hydroxyethyl) terephthalate, BHET, by glycolysis with ethylene glycol,5 or polyols by glycolysis with various glycols.1-4,6-9 DMT and BHET can be re-used to prepare colorless PET of grade suitable for food-contact applications, PET oligomers, mainly for the preparation of PET for non-food products, while polyols can be used as intermediates for new polymeric materi1,4 1,6,7,10,11 However the als such as polyurethanes and unsaturated polyesters. chemical recycling of PET is not limited to the above products, but, in principle, it can be extended to the production of every type of resin which contains terephthalate and/or ethylene glycol units. In other words, PET may be seen as a source of molecular fragments which can be used, alone or together with other monomers, as ‘bricks’ in processes for the rebuilding of new resins. Polymers that contain terephthalate units are employed for instance in coatings and adhesives other than in unsaturated polyesters and in a large number of thermoplastic copolyesters.

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This paper investigates the fundamental aspects (feasibility, properties and economics) of a process which uses PET wastes for the preparation of powder coatings (see Figure 1). The recycling of PET by this process will help in solving of some problems with solid plastic waste and some pollution problems arising from the use of solvents in coatings. A number of resins can be used to prepare powder coatings, among these, for demonstrating the feasibility of the process, we choose as a reference, a polyester resin based on terephthalate (T), neopentyl glycol (NPG) and trimellitic anhydride (TMA) units, similar to some commercial resins. The polyester resins (either obtained from PET or from monomers) were then mixed with an epoxy resin, TiO2 and a catalyst, applied to standard metal sheets and cured in a oven. The properties of the coatings obtained by the resins prepared from PET were then compared with those of the coatings obtained from the reference resin and, finally, an economic balance of the process was established.

Figure 1. Scheme of the use of recycled PET for the production of powder coatings.

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Powder coatings from recycled PET

THE CHEMISTRY OF THE PROCESS

The overall chemical process, leading to the formation of a coating, consists of three principal steps, in the first step polyester resin with hydroxyl terminal groups is prepared, then it is reacted with TMA in order to transform hydroxyl terminal groups into carboxyl groups, and finally this latter resin is mixed with an epoxy resin with which it reacts during the curing stage. The quality of the final coating is expected to depend on the characteristics of the polyester resin, and mainly on its chemical composition, determined by molecular weight distribution, acid value (usually expressed as mg of KOH/g of resin), and functionality. In particular, it is important that the polyester resin has a suitable acid value and carboxylic functionality after the reaction with the TMA because the curing reaction proceeds mainly via reaction of carboxyl with epoxy groups. The purpose of this work was to demonstrate that it is possible to transform recycled PET into a polyester resin with chemical structure, molecular weight distribution, acid value and functionality similar to that of resins (R1 and R2) prepared from monomers and taken as references. The composition of the polyester reference resins was chosen from those typically used for powder coatings. Resin R1 was obtained by reacting first DMT with an excess of NPG, in the presence of Ti(OBu)4 as catalyst, and then with TMA (reactions 1 and 2) (similar polyester resin is marketed for powder coatings by Eastman Chemicals). Resin R2 was obtained in a similar way using a mixture of glycols, NPG and EG, instead of NPG alone (see reactions 1 and 2). Table 1: Characteristic of recycled PET from Tecoplast Size

5 mm

Moisture

0.2 wt%

Elemental analysis Fe

12 ppm

Cl

0.02 wt%

Al

10 ppm

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Recycled PET, in the form of flakes, was purchased from Tecoplast (Italy), and was obtained from the collection of PET beverage bottles clear and light-blue colored; its main characteristics are reported in Table 1. The reactions for preparation of the resins both from monomers and PET, described below, were carried out in a stainless steel reactor (1.8 L of capacity) mounting a paddle agitator, usually driven at 30 rpm, and equipped with facilities for distillation of volatile by-products, for charging reactants and with a valve for the recovering of the products from the bottom of the reactor. As far as

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Powder coatings from recycled PET

possible the reactions were carried out under similar conditions, but we did not try to optimize them with respect to neither molecular weight nor acid value. The amount of reactants used for the preparation of the resins are reported in Table 2. Table 2: Amount of reactants* DMT

EG

PET

NPG

TMA

NPG/PET

g

g

g

g

g

mol/mol

R1

582

-

-

374

109

-

R2

582

38

-

312

125

-

S1

-

-

576

312

109

1

S2

-

-

422

458

92

2

S3

-

-

400

434

87

2

S4

-

-

300

489

66

3

Sample

*all reactions were carried out using Ti(OBu)4 as catalyst (0.1 wt% with respect to DMT or PET)

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REACTION OF PET WITH NPG

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The first goal of the process was to prepare polyester resins from recycled PET with characteristics similar to reference resins obtained from monomers. The more convenient process for this purpose is to cleave back the PET chains to oligomers by glycolysis with NPG (reaction 3), followed by selective removal of ethylene glycol (EG) to decrease the EG/NPG ratio in the reaction mixture. The glycolysis of PET did not present any particular problem, but the presence of a catalyst is needed to reduce the reaction time; in our experiments the rate of glycolysis was increased by adding Ti(OBu)4 as the catalyst (0.1 wt% with respect to the initial amount of PET). Unfortunately the difference in volatility between EG and NPG is not very large and part of NPG distilled off the reactor along with EG, making the complete elimination of EG moieties from the final resin very difficult. Of course, the residual amount of EG moieties in the chains of the final resin can be decreased by increasing the relative amount of NPG added for glycolysis. For this reason we performed several reactions with various NPG /PET ratio; data are reported in Table 2. The first stage, of glycolysis (reaction 3), was carried out at 220-230oC in the closed reactor (there is a slight increase of the pressure up to about 1.5 bar) o for 60 minutes. Then the reaction was continued at 230-240 C with the reactor opened allowing the distillation of a mixture of EG and NPG which was collected in a condenser. When the rate of distillation became very low the pressure in the reactor was decreased (down to about 2 m<W1%-2>bar during 20 minutes) and maintained for 5 minutes. The reactor was then filled with nitrogen and the o temperature was allowed to decrease to 160 C (before the addition of TMA). The chemical composition of the products at the end of this step was deduced from 1H-NMR spectroscopy. As it appears, there is a strong decrease of the EG/NPG molar ratio in the product when the NPG/PET ratio increases; a polyester resin with a residual content of EG moieties as low as 3 mol% was obtained when the initial NPG/PET molar ratio was 4 (based on the monomeric unit of PET). The mol% of EG in the resin, after the first stage of glycolysis, is also reported in Figure 2 against the starting NPG/PET molar ratio.

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Powder coatings from recycled PET

REACTION OF THE HYDROXYL-TERMINATED POLYESTER WITH TRIMELLITIC ANHYDRIDE

The mixtures of oligomers, obtained from monomers, in the case of the reference resin, and from recycled PET, after the glycolysis step, were reacted with TMA under the same reaction conditions. TMA was added when the temperature in o o the reactor was decreased to 160 C, and then allowed to react at 180 C for 110 min (reactions 2 and 4 for resins from monomers and recycled PET, respectively). The final resins were then collected, ground, and analyzed for their

F. Pilati et al.

Figure 2. Resin composition vs the initial (NPG/repeating units of PET) molar ratio in the reactor.

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Powder coatings from recycled PET 1

chemical composition (by FTIR and H-NMR), acid value (by titration), molecular weight (by GPC), and glass transition temperature (by DSC). While the reference resins were white powders, the products obtained from recycled PET were discolored appearing as green-gray powders. However the difference in the chemical structures was limited to the presence of residual EG in the resins derived from PET as measured by spectroscopic techniques. It will be shown below that the color of the polyester resins will not affect significantly the final color of the coating. From 1H-NMR, it was possible to calculate the ratios of the EG/NPG units in the resins. The results obtained, for the reference resin and for samples derived from PET, are reported in Table 3. As expected the EG/NPG ratio is the same at the end of the first step of glycolysis, decreasing from samples S1 to S4. The NPG to terephthalate units (T) ranges from 1.18 to 0.82. Table 3: Composition and characteristic of polyester resins a

a

b

c

f

e

Acid value

mol/mol

mol/mol

mg KOH/mg

R1

-

1.18

62

1520

1.7

58

R2

0.17

1.00

98

1510

2.6

63

S1

0.43

0.82

66

1850

2.2

67

S2

0.20

1.00

58

1800

1.9

63

S3

0.18

0.95

70

1810

2.3

65

S4

0.09

1.10

84

1830

2.7

69

Sample

Mn

d

NPG/T

EG/NPG

Tg o

C

a - from 1H-NMR, b - from titration, c - from GPC using PS standards for calibration, d - average carboxylic functionality, i.e., number avarage of carboxyl groups per molecule, e - from DSC

The acid value is a parameter that is critical to control because it depends primarily on the amount of TMA relative to the molecular weight of the hydroxyl-terminated resins, which in turn can change significantly in a relatively short time of reaction. This leads to a relatively high dispersion of the acid values found for both the reference and the PET-derived resins. Except for resin R2, they are smaller than those of the commercial resins, however, if the acid

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value would be a very critical parameter for the coating properties, it would not be a problem to obtain higher values (and the same values for resins from recycled PET or monomers) by optimizing reaction conditions. Similar comments and conclusions can be extended to the molecular-weight results derived from GPC curves (some of which are reported in Figure 3). The shape of the curves are similar for the resins R1 and R2, obtained from monomers, and those obtained from recycled PET (S1-S4); these latter show very similar molecular weights, independently from the starting ratio NPG/PET, that are slightly higher than those found for the reference resins. As said before for the acid value, it would not be a serious problem to reduce or increase the molecular weight of the resins if necessary.

Figure 3. Typical GPC curves for reference and PET-derived resins; molecular weight calibration is based on polystyrene standards.

From the acid values and the number-average molecular weight, it is possible to calculate the average carboxylic functionality of the polyester resins expressed as an average number of carboxyl groups per chain; the relative data are reported in Table 3. They are relatively low and spread, but this short-comings depend on the reaction conditions, which were not optimized, rather than on the use of recycled PET. o The values of Tg, obtained from DSC curves recorded at 10 C/min, are similar (see Table 3) for both reference and PET-derived resins.

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Powder coatings from recycled PET

CURING AND PROPERTIES OF THE POWDER COATINGS CURING

The polyester resins, prepared as described above, were compounded with the other ingredients of the coating in a twin-screw extruder; all powder coatings were formulated with the same composition as reported in Table 4. The blends were then micronized in a dry mill micronizer under the same conditions. Then the powder coatings were applied on standard UNI panels using an electrostatic o spray gun and cured in an oven at 180 C for 20 minutes. Dry film thicknesses of 170-180 µm were measured. Table 4: Coating composition Components

wt%

Polyester resin

35.2

Epoxy resina

35.2

Flow control agent Degassing agent TiO2d

c

b

1.6 1.0 27.0

a - Araldite GT 7004 from Ciba-Geigy, b - Modaflow from Monsanto, c- Benzoin from BASF, d - Tioxide TR/92 from Tioxide Europe

COATING PROPERTIES

After curing, the coatings obtained from both the reference resins and from PET did not show any readily visible difference. To check the differences in the coatings obtained from monomers and from recycled PET, the most important properties of the coatings (color, gloss, adhesion, and mechanical properties) were evaluated according to ASTM methods for panels after conditioning for 24 h at room temperature. The method employed and the results are listed in Tables 5 and 6 respectively. Based on the properties reported in Table 6 the coatings obtained from monomers and from recycled PET can be considered equivalent. In particular, the color of coating derived from the recycled PET does not show significant difference with that of coatings obtained from monomers.

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Table 5: Tests performed for coatings Test Specular gloss at 60

Method o

ASTM D523 o

Color δE (CIELAB, 10 , D65, specularity included)

ASTM D2244

Buchholtz hardness

DIN 53153

Direct impact

ASTM D2794

Conical mandrel

ASTM D522

Flow

visual

Cross-cut adhesion test

ASTM D3395

Table 6: Results of tests listed in Table 5 Test

R1

R2

S1

S2

S3

S4

Gloss (%)

88

87

80

80

82

90

Color

0

-

0.39

0.37

-

-

Hardness (Buchholz units)

100

100

100

100

110

110

Impact (kg cm)

10

20

20

10

10

10

severe cracking

few cracks

few cracks

severe cracking

no cracks

few cracks

Flow

good

good

good

good

good

good

Adhesion (%)

100

100

100

100

100

100

Mandrel

The differences in molecular weight, molecular weight distribution, acid value, and carboxylic functionality of various resins may be responsible for a slight difference in gloss, impact resistance, and flexibility, however it does not seem that these characteristics of the resins have a strong influence on the properties.

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Powder coatings from recycled PET

ECONOMICS CAPITAL COST

As it is well-known, the economical feasibility of production strongly depends on the size of plant. In the following analysis, a plant able to treat about 1100 t/year of recycled PET corresponding to the production of coating powders of a medium factory is taken into account. The plant consists of a batch reactor with a jacket for heating and cooling, a cooling conveyer belt with a flaker at the end, a tank to collect glycol mixture distilled during the reaction, and a pump. The plant is considered to operate 230 d/year with a daily production of two batches and to be an addition to the existing plant for the production of powder coatings. The installed cost, as sum of the FOB cost, transportation, cost of foundation, erection, and connection to service facilities, was estimated as 0.882 M$ (late 1993). All prices and costs are pertinent to the Italian market. A rate of change 1$=1700 It£ was used. MATERIAL BALANCE

A NPG/EG molar ratio equal to 3 was considered in this situation; as previously stated, 91% of EG units are substituted by NPG units. A summary of the results of the material balance is shown in Table 7. Table 7: Summary of material balance Reactants (g)

Products (g)

PET

1000

-

NPG

1625

1132

TMA

220

-

EG

-

294

Resin

-

1419

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Table 8: Raw material unitary cost Material

Unitary cost ($/kg)

PET

0.53

NPG

1.09

EG

0.39

TMA

2.85

It is easy to note that a glycol mixture is obtained consisting of the replaced EG and excess of NPG. In the following we assume that this mixture can be used, without further purification, in other production in the factory, we consider only the NPG effectively consumed, and we take advantage of a negative cost for the EG obtained. OPERATING COSTS

The unit costs of raw materials used for the economic analysis are reported in Table 8. The incidence of raw materials on the resin cost is: (0.53x1 + 1.09x0.493 + 2.85x0.220 - 0.39x0.294)/1.419 = 1.11 $/kg Other operating costs are quoted in the following: Methane Electrical energy Water Nitrogen Total Utilities Labor (3 units) R&D and quality control Maintenance

Thousand $/year 22.6 26.3 1.0 8.9 58.8 97.1

(5% of the installed cost)

44.1

(11% of the installed cost)

97.0

Depreciation

144.1

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Powder coatings from recycled PET

Table 9: Total production cost 1000$/ year Raw materials Utilities Other operating costs

$/kg

1732.6

1.110

58.8

0.038

382.3

0.245

The results are summarized in Table 9. The total production cost compares favorably with the purchase price of a low-priced polyester resin equal to 1.56 $/kg. Obviously, the results obtained are dependent on the assumed values for all pertinent costs. In particular, the cost of the recycled PET varies considerably depending on the market situation. From a sensitivity analysis performed in correspondence of variations of PET cost, we obtained a total production cost for the resin of 1.319 $/kg and 1.473 $/kg for PET-cost variations of + \20%, respectively. CONCLUSIONS

As an overall conclusion, we can state that it is possible to prepare powder coatings using recycled PET by a chemical process which is feasible and, pro<W1%-2>bably, economically convenient. More detailed conclusions about feasibility and economics of the proposed process can be summarized in the following comments. As regard to feasibility, our results demonstrate that it is possible to use recycled PET for the preparation of polyester resins which are suitable for curing with epoxy resins and which give coatings with properties similar to those obtained from the reference resins prepared from monomers. The major difference between resins prepared from monomers and recycled PET is a residual content of EG moieties in the resins derived from recycled PET which depend by the NPG/PET starting ratio. None of the properties examined seem to be changed significantly by the presence of residual EG up to 30 wt%. Some other minor differences between the coatings prepared from recycled PET and those obtained from monomers, such as acid value and molecular weight, seem to be irrelevant for the properties of the coatings, and also these

F. Pilati et al.

83

parameters can be quite easily modified by a better control of the reaction conditions during the preparation of polyester resin. Thus, the differences in the acid value, molecular weight, and functionality of the resins prepared in our laboratories (either from recycled PET or from monomers) with respect to those of equivalent commercial resins are acceptable considering that we did not try to optimize the reaction conditions (temperature, time, and catalyst of the reaction). Regarding the economics of the process, we have to emphasize that it depends mainly on the price paid for recycled PET which is largely dependent on the collection system developed (which can vary in different countries), and on the market demands. Therefore, the economic convenience of such a process can change from country to country and fluctuate with the cost of the raw materials and of recycled PET. In any case, the process can be carried out in chemical plants which are very similar to those commonly used for the preparation of polyester resins from monomers, and therefore it should not create a problem to design a plant suitable for an easy change of the feed from recycled PET to monomers following the fluctuation of the prices of the raw materials. 1.

REFERENCES

J. Milgrom in Plastics Recycling, Ed. R. J. Ehring, Hanser Publ., Munich, 1989, p. 59. 2. W. De Winter in Review on PET-film Recycling in Recycling of Plastic Materials, Ed. F. P. La Mantia, ChemTec Publishing, Toronto, 1993. 3. Chem. Eng. News, 67, 7 (1989). 4. W. De Winter, A. Marien, W. Heirbaut, and J. Verheijen, Makromol. Chem., Makromol. Symp., 57, 253 (1992). 5. Unpublished results. 6. U. R. Vaida and V. M. Nadkami, Ind. Eng. Chem. Res., 26, 194 (1987). 7. U. R. Vaida and V. M. Nadkami, J. Appl. Polym. Sci., 34, 235 (1987). 8. D. Gintis, Makromol. Chem., Makromol. Symp., 57, 185 (1992). 9. S. Baliga and W. T. Wong, J. Polym. Sci. Part-A, Polym. Chem., 27, 2071 (1989). 10. S. N. Tong, D. S. Chen, C. C. Chen, and L. Z. Chung, Polymer, 24, 469 (1983). 11. K. S. Rebeiz, D. W. Flower, and D. R. Paul, Polym. -Plast. Technol. Eng., 30, 809 (1991).

M. Teller

31

Possible Applications of Pyrolysis Technology in Treatment of Hazardous Wastes and Valuable Materials Recovery

Matthias Teller BC Berlin-Consult GmbH, Am Karlsbad 11, D-10785 Berlin, Germany

Pyrolysis technology has reached by now a state of development that opens more favorable possibilities for waste disposal than some of the traditional techniques. Special complex waste materials can now be recycled or disposed of properly by pyrolytic treatment. Composite materials such as circuit-board waste and complete circuit-boards, mixed plastics, flameproof plastics and shredder residues from car scrapping can be converted into useful materials and environmentally neutral residues by a combination of pyrolysis, gas scrubbing, processing of residues, and incineration. In this process, metals are separated without being oxidized, oils recovered in the gas purification stage, and they can be recycled into the raw material pool with standard methods of petrochemistry. The remaining clean pyrolysis gas can be used directly in the process, and under certain conditions, depending on the type of the input material, permits autothermal operation. A suitable design of the process stages permits the concentration of pollutants and leads to very small mass flow resulting in low disposal costs.

STATE OF DEVELOPMENT IN PYROLYSIS TECHNOLOGY

Since the beginning of the seventies, engineers in industry, and universities are working on the technical development of pyrolysis processes. So far, and with a few exceptions, this technology is not used on a large-scale for treatment of waste and residues. However, this approach undergoes fundamental changes at present. The deciding factors are two development tendencies: On one hand, the number of wastes and residues containing valuable materials which are not

32

Pyrolysis technology in treatment of hazardous wastes

suitable for the traditional thermal disposal process - the incineration - is steadily increasing, and dumping of such materials is more and more given up. On the other hand, basic legal requirements to be met by the emission standards for thermal disposal or utilization are increasingly tightened up, a fact which affects the profitability of incineration plants to a considerable extent. Solutions to this problem are offered by a combination of pyrolysis and incineration, as described in the following. In the past, pyrolysis (i. e., the thermal decomposition of a material under the exclusion of air oxygen) was mainly tested in shaft furnaces, autoclaves, chamber pyrolyses, rotary kilns, and fluidized-bed reactors. Meanwhile, the indirectly heated rotary reactor is considered to be a mature solution for a large-scale technical level. In the field of waste management, pyrolysis was used so far for treating household refuse, hazardous waste, special residues such as tires, sewage sludge, residual oils, oil muds, shredder residues, plastics, and circuit-board 1,2,3 scrap, and for a pyrolytic treatment of contaminated soils. Figure 1 shows a selection of pyrolysis plants for waste treatment. As can be seen, plants with a capacity of 5 to 6 tons per hour are state-of-art.

M. Teller

33

From a technology view point, the processes employed can be divided into three groups of a coupling of pyrolysis and final incineration which is called “graduated incineration” (Figure 2). The first process type directly feeds the pyrolysis gas to the incineration and burns it (plants by Babcock and BC). In the second process type, the pyrolysis gas is cracked in a gas converter, afterwards, cleaned and is then available as lean gas (plant by KWU). In the third process type, the cracked gas is cleaned in a multi-step gas scrubbing before being used for thermal purposes (plants by BC, Kaminsky, Noell, MVU, Veba Oel). The clean pyrolysis gases obtained from the third process type can be used totally or partly for running the pyrolysis. The graduated incineration offers three essential process advantages as compared with the sole incineration. These are: • • •

Resources can be separated and isolated out of the process gently. Noxious materials can be retained and concentrated with a comparably low expenditure for process engineering. The inhomogeneous waste material flows are homogenized to a large extent and can then be burn as pyrolysis gas or pyrolysis coke under very favorable process conditions. The result is a reduction of the flue gases to 30 %.

34

Pyrolysis technology in treatment of hazardous wastes

PROCESS BALANCE

The process advantages of the graduated incineration become clear when a more detailed balance of the process is evaluated. The pyrolysis provides two flows of products (Figure 3): • •

First, the volatile phase, a dust-laden pyrolysis gas, containing constituents of acid gas, ammonia, water, volatile heavy metals, nitrogen, sulphur, and carbon compounds as well as H2, CO, CO2, and a multitude of organics Second, a solid pyrolysis residue is obtained which contains the remaining carbon generated in the thermal cracking and all non-volatile components.

The composition of these two product flows is substantially influenced by the way in which the process is conducted. In general, a dehydrohalogenation and, linked to it, a decomposition of the halogen-organic compounds take place (Figure 4). It can be seen that most of the remaining halogen-organics such as dioxines and furanes enter into the volatile phase and that the remaining solid pyrolysis residue is burdened with noxious materials to such an extent that it is harmless

M. Teller

35

for further handling and utilization. Halogen-organic compounds, still remaining in the volatile phase, do not cause any difficulty. Such compounds are not detectable in the sewage of pyrolysis gas scrubbings because of their good oil solubility and their hydrophobic behavior. They are retained quantitatively in 6 the condensate oils and can be separated from them by well-known processes. Depending on the kind of input material, the residue is more or less contaminated with heavy metals. Because of the activated coke-like structure of the carbon proportions in the residue, however, these heavy metals are bound in such a good manner that elution tests according to DEVS4 do not lead to significant leachings as compared with the limits suggested in the “Technical Directions Waste” (TA Abfall).4 Depending on the input material, metals and inert materials can be separated from the solid residue and be re-used. If there is a sufficient poorness of noxious materials, the remaining pyrolysis coke can be used as a source of substitution energy, for instance, in cement production or also, in combination with a traditional combustible, in a coal-fired power stations.

36

Pyrolysis technology in treatment of hazardous wastes

Pyrolysis tests with plastics have shown1,5 that the volatile phase, i.e., the pyrolysis gases, can reach up to 75 mass percents of the input material. Another 45 mass percents can be condensed as liquid phase by a pyrolysis gas purification in the form of condensation-washing steps. This phase is first of all a mixture of light petrol and coal tar. If the process is conducted properly, aromatic compounds, with the main components such as benzene, toluol, styrene, and naphthalene, constitute up to 95 mass percents. Higher condensing aromatic compounds are mainly anthracene, phenanthrene, pyrene, and chrysene. Thus, the purification of low-temperature carbonization gas provides a raw material much sought after by the petrochemical industry. 5 The quantities of water built by the reaction are between 0.1 and 5 %. Part of the water, obtained from washing the condensate, must be taken from the process to subtract the salts produced. This sewage is of course also loaded with organic materials such as toluol, dimethylacetamide, and diethylacetamide. Tests carried out at Berlin Technical University by order of BC Berlin-Consult GmbH have proven that a biological-adsorptive purification method can decompose the COD of such pyrolysis sewage up to a residual concentration of 300 mg/l of COD. With the same process, nitrogen compounds can be reduced to residual ammonia concentrations of < 10 mg/l.

M. Teller

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After the condensation washing, a clean gas is obtained, free of noxious materials and precursors of noxious materials such as chlorine, sulphur, ammonia, volatile heavy metals, dioxines, and furanes. This clean gas, built up from usual components of burnable gas, which can reach up to 60 mass percents of the input 3 material and thermal values of up to 45 MJ/m , can be directly used for running the pyrolysis. Thus, depending on the type of input material, an autothermal pyrolysis operation becomes possible, and, if occasion arises, part of the clean gas flow can be used for other purposes. These balance values make clear that the pyrolysis technology, including a washing of the pyrolysis gases, permits the splitting up of very heterogeneous input materials, as for instance compounded materials, electronic scrap or shredder residues from car recycling, into the product flows of metals, inert materials, pyrolysis coke, oil, and clean gas. As the following examples will show, the technological expenditures remain far behind that of a traditional thermal disposal.

38

Pyrolysis technology in treatment of hazardous wastes

EXAMPLE: PYROLYSIS OF CIRCUIT-BOARDS

From 1985 to 1987 BC Berlin-Consult GmbH erected a recovery and disposal plant for circuit-boards in a manufacturing plant for a basic circuit-board material in Bernau near Berlin. In this process combination of pyrolysis and incineration (Figure 5), the pyrolysis gas obtained is directly fed to the incineration. It results from the mass balance (Figure 6) that up to 13 mass percents of the input material are recovered in the form of copper. 50 kg per ton of input material are obtained as residue from the flue-gas purification and have to be dumped. Taking the toxicity equivalents of the analogous chlorinated compounds as a basis, the results of measurements, carried out in the clean gas, showed 0.146 ng/m3 for the sum of the brominated dioxines and furanes. This value was reached when a basic circuit-board material was used, the resins of which contained up to 6 mass percents of bromine in the form of flame-retarding additions made of pentabromine-diphenylether. The corresponding coke loads amount to 0.011 ng/g.

M. Teller

39

If the gas is washed between the pyrolysis and the final incineration processes, there is of course no emission of halogenized hydrocarbon on the side of the clean gas. EXAMPLE: PYROLYSIS OF ELECTRONIC SCRAP

A corresponding combination of processes was used in tests to carbonize electronic scrap at low temperatures (Figure 7). In the first test phase, the pyrolysis was followed by a gas quench and washing stage by means of a packed column. Four different charges of electronic scrap, obtained from Siemens, IBM, Edelhoff, and two collectors of electronic scrap in Dresden and Hamburg, were carbonized at low temperatures. Pyrolysis took place at temperatures between o 575 and 645 C, with an average residence time of the materials in this temperature range of 30 minutes. As expected, the tests showed that the pyrolytic treatment of circuit-boards can be applied without any problem to the carbonization of electronic scrap. The following results were obtained as far as the balance of the overall process and the contents of noxious materials are concerned.

40

Pyrolysis technology in treatment of hazardous wastes

A rough balance of the pyrolysis products of the different charges shows that the portion of the remaining solid residue is a subject to strong fluctuations, depending on the input material, and that it can amount up to 55 to 84 mass percents of the input. The solid residue can be mechanically separated into the fractions of metals, glass fibre, inert material, and a fine portion. By the kind of input, the quantities of the individual fractions (Figure 8) can also vary considerably. Processing of metals has to be done in metal-separating plants. In the first step, iron parts and granules of solidified melt (alloys from copper, aluminum, lead, and zinc) can easily be separated. As far as the fraction of glass fibre is concerned, it is checked at present in what quantities and under which conditions they can be integrated into the melting process for recycled colored glass. The fractions of inert materials which, on an average, amount to approximately 8 mass percents only (Figure 9), have to be disposed of in a household-refuse dump. It remains a fine portion with approximately 15 mass percents of the input. Noxious materials which were not converted into the volatile phase, i.e., into the pyrolysis gas, during the pyrolysis process, are bound to the fine portion. An analysis of these fractions suggests to use them separately for thermal purposes. In the sense of a well-rounded disposal concept, it is rec-

M. Teller

41

ommended to design this thermal utilization in such a way that the incineration residues are obtained in the form of vitrified slag (Figure 10). The remainders are the constituents of the volatile phase, the condensate oils from washing the pyrolysis gases, and the clean gas. Present results of condensate oil analyses give no concern regarding their subsequent use. The clean gas is free of any noxious material and can be used for running the pyrolysis. EXAMPLE: PYROLYSIS OF SHREDDER RESIDUES

The process depicted here is also suited for the disposal of shredder residues. So far, this remaining material, of which approximately 400,000 tons are obtained in the Federal Republic of Germany every year from car-crushing plants, is mainly dumped. Due to the content of different noxious materials such as PCB or mineral oils, the acceptance of this disposal option is steadily decreasing, and Federal laws are being prepared which will extremely aggravate a dumping of shredder residues in future.

42

Pyrolysis technology in treatment of hazardous wastes

M. Teller

43

Experience made so far with the carbonization of shredder residues shows that, as long as the suitable process components are chosen, this material can be processed resulting in fractions of inert materials, metals, and vitrified slag (Figure 11). Relative to the waste input, the mass of the remainders obtained from the pyrolysis is between 40 and 55 mass percents. Analysis of the washout of solid residues provides values which meet even the most restrictive effluent 7 conditions in the Federal Republic of Germany. The solid residues of the pyrolysis can be separated into the fractions of metals, inert materials, and carbonated fine portion (Figure 12). Via the process already depicted, the volatile low-temperature carbonization products are decomposed into condensation products and clean gas. The clean gas is used to heat the pyrolysis process, whereas the oils, after a possible decontamination, are used to vitrify the fine portion rich in carbon. 1. 2.

3. 4. 5. 6. 7. 8.

REFERENCES

G. P. Bracker, G. Collin, and E. Michel, Pyrolytische Rohstoff-Rückgewinnung aus unterschiedlichen Sonderabfällen in einem Drehtrommelreaktor. Versuchsergebnisse im halbtechnischen Maßstab, Chem.-Ing.-Tech., 53, 10 (1981). W. Bischofsberger and R. Born in Verfahrens- und umwelttechnische Analyse neuer thermischer Prozesse in der Abfallwirtschaft, Phase 1: Pyrolyse, Berichte aus Wassergütewirtschaft und Gesundheitsingenieurwesen, TU München, 1989. H. Piechura in Erprobung und Optimierung der SalzgitterPyrolyse-Anlage zur thermischen Zersetzung von Sonderabfall mit Energie- und Rohstoffrück gewinnung, Schlußbericht BMFT, Förderkennzeichen 01 VQ 8519, 1989. C. G. B. Frischkorn in Bericht zu Deponieverhalten des Pyrolysereststoffes aus der PKA- Müllpyrolyse und seine Nutzung für die Entsorgung organisch belasteter wässriger Emissionen, Sonderdruck KFA, Julich, 1991. W. Kaminsky, Wertstoffrückgewinnung aus Altgummi durch Pyrolyse, in Elastomere und Umwelt, VDI- Verlag, Düsseldorf, 1991. E. Bilger, Enthalogenierung halogenkohlenwasserstoffhaltiger organischer Flüssigkeiten, Chem.-Ing.-Tech., 62, 4 (1990). E. Pruckner in Verschwelung von Shredderabfällen, KWU Umwelttechnik. R. Martin in Thermische Behandlung der Shredderleichtfraktion, Bayer AG, Leverkusen, 1991.

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117

Continuous Determination of Volatile Organic Breakdown Products of Propellants in Water

Günther Hambitzer and Martin Joos Fraunhofer Institut für Chemische Technologie (ICT), 76327 Pfinztal 1, Joseph von Fraunhofer Straße 7, Germany

Volatile products such as aniline, toluene, and alkylamines are often formed by the decomposition of propellants. These substances can be continuously detected in aqueous solutions by a new mass spectrometric setup. The essential part of this setup is a flow cell coupled with a mass spectrometric unit. A porous, hydrophobic membrane fixes the phase boundary of liquid/gas and serves as a gas inlet for the vacuum. A model explains the pathway of volatile substances dissolved in water. It describes the diffusion in the flow cell, the transfer of volatiles through the phase boundary, and the membrane into the vacuum system. The sensitivity determination of volatile, organic compounds is generally within the range of 1 µg/L to 1 mg/L. Due to the short reaction times, this method enables continuous control of the process and waste water.

INTRODUCTION

The environment of former East Germany has been subjected to considerable environmental pollution as a result of emissions from propellants over many years. On account of their high toxicity, there is a need to determine these substances and their breakdown products in contaminated waters. The disposal of propellants produces process waters in which the breakdown products have to be monitored.1

118

Continuous determination of volatile breakdown products

Figure 1. Schematic presentation of the mass spectrometric apparatus and membrane inlet system.

Many propellants, especially nitroglycerin, contain Centralite 1, which, apart from stabilizing properties, has also gelatinizing (plasticizing) properties. Centralite 1 is a symmetric diethylphenylurea (C17H2O2N2). Its breakdown mostly produces volatile alkylamines and aniline derivatives. In the breakdown of TNT (trinitrotoluene), volatile toluene and partially volatile nitrotoluenes are produced. The rapid analysis and the continuous determination of these volatile breakdown products is needed and can be accomplished by a recently developed mass spectrometric apparatus. EXPERIMENTAL SETUP MASS SPECTROMETRIC APPARATUS WITH MEMBRANE INLET SYSTEM

The core of the setup consists of a flow cell with a porous membrane which allows only gases but no liquids to pass through. A mass spectrometric apparatus is coupled with this flow cell. The porous membrane plays the role of the liquid/gas

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119

phase boundary and also the inlet system to the mass spectrometer. The mass spectrometric apparatus ( Figure 1) can be broken down into two parts, i.e., the mass spectrometer itself as a recording unit and the vacuum apparatus. The vacuum apparatus with its connection to the membrane inlet system (1) possesses two differentially pumped volumes. In the first step, the main part of the gas flowing through the membrane is pumped by a membrane pump (4). A small part of the gas flows via a pressure converter (2), equipped with an adjustable -5 shutter, into the high vacuum (approx. 10 bar) necessary for operating the mass spectrometer. The high vacuum is generated by a turbomolecular pump (5) operated by a preconnected membrane pump (6). The measurement head of the mass spectrometer is arranged so that the molecular stream from the shutter opening is aimed directly at the ion source. This has an effect on reduction of desorption and adsorption processes on the inner wall and thus increase of sensitivity of measurement. FLOW CELL WITH CONTINUOUS DETECTION OF VOLATILE SUBSTANCES IN LIQUIDS

For continuous recording, a flow cell (Figure 2) is flanged onto the mass spectrometric apparatus. A liquid pump conveys a solution through the cell. Gaseous substances are transferred from the liquid into the vacuum through the membrane responsible for the liquid/gas phase boundary. A layer of steel frits (φ = 8 mm) (2) supports the membrane (3), preventing its rupture due to the prevailing difference in pressure. The liquid flows at pressure approaching atmospheric pressure through the cell, whereas on the vacuum side the pressure is 2.2 mbar.

Figure 2. Schematic diagram of flow cell with liquid pump.

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Continuous determination of volatile breakdown products

A teflon-coated Viton ring (4) tightly seals the system preventing any gas from interfering. The membrane is made of Tefzel, a copolymer of Polyethylene and Teflon. The type used has a mean pore radius of 0.05 µm, the thickness of the membrane of 60 µm, and porosity 60 %. EVAPORATION PROCESSES AT THE POROUS MEMBRANE AND MEASUREMENT OF SENSITIVITY

In the following, a model concept is developed which describes the transport of volatile substances, dissolved in water, from the flow cell through the porous membrane, into the vacuum system. CHANNEL FLOW

In the thin layer flow cell (height 0.2 mm), the hydrodynamics are similar to a 2 channel flow. The channel flow in electrochemical cells has been subject to intensive investigation over the last years. The conversion of electrochemical active substances dissolved in water on a channel electrode is comparable with the passage of volatile substances dissolved in water through the phase liquid/gas boundary. The volatile substances are transferred via the membrane into the vacuum system, whereby the concentration drops at the phase boundary. Subsequent transport then takes place from the interior of the solution. In the case of channel flow, a defined diffusion layer is formed. Apart from the substance-specific diffusion constant, the particle flow density is provided through the difference of the concentrations in the liquid interior (bulk) and at the phase boundary. The density of the diffusion layer depends on the third root of the flow rate:2 jLi = Kz D2/3 v1/3 (cBi - cPhi) jLi Kz D2/3 v1/3 cBi cPhi

particle flow density of the volatile species [mol/(min cm2)] cell constant (dependent on cell geometry) diffusion coefficient of the volatile species i [cm2s-1] flow rate of the moving solution [ml/min] bulk concentration of the species i in the solution [mol/L] concentration of the species i at the phase boundary [mol/L]

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FLOW RESISTANCE OF THE MEMBRANE

The evaporating water flows through the membrane into a fine vacuum of approximately two millibars. If the water vapor produced at the phase boundary is completely pumped off, approximately eight milliliters of water per hour and square centimeter of the membrane surface ought to evaporate, according to cal3 culation of the surface-related evaporation rate. In actual fact, however, only 2 th 0.25 ml/(h cm ) evaporates, i.e., less than 1/30 of the calculated quantity. This results in the following model: Directly at the liquid/gas phase boundary, a thin vapor layer is present of saturation pressure of water vapor (approx. 23 mbar at room temperature). Accordingly, a pressure gradient in a direction of a fine vacuum is present in the pores of the membrane. The membrane thus acts as a flow resistance for the water vapor flowing through its pores. On account of the small pore size, there exists a molecular flow of transported gas in the membrane, so that the gas particles are more frequently in contact with walls than other particles of gas. The flow is laminar in the fine vacuum of approx. 2 mbar behind the membrane. PARTIAL PRESSURES OF COMPONENTS IN THE VAPOR LAYER

The concentration of the volatile species at the phase boundary determines their partial pressure in the evaporation area. Raoult’s Law applies to the vapor pressure found above liquid mixture at high concentrations of the components: pA = xA PA pA xA PA

partial pressure of the component A [mbar] mol fraction of the component A in the solution saturation evaporation pressure of component A [mbar]

Accordingly, the partial pressure of the component A is proportional to its molar fraction in the solution and its vapor saturation pressure. Raoult’s Law only applies to ideal solution, i.e., the components of mixture must have chemical similarity and be present at sufficiently high concentration. The usual concentration of components of propellant breakdown are in the region of milligrams to micrograms per liter (relatively low concentration). In this case, Henry’s Law applies:

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Continuous determination of volatile breakdown products

pB = xB KH,B pB partial pressure of the component B [mbar] mol fraction of component B in the solution xB KH,B Henry’s constant of the component B [mbar].

Although the vapor pressure of the dissolved substance, at low concentration, is also proportional to the mol fraction but the proportionality constant is not. However, the vapor pressure of the pure substance is proportional to a substance-specific constant KH (with a dimension of the pressure). Henry’s Law describes a thermodynamic state of equilibrium and is only valid for closed systems in which a vapor medium (area) saturated with the components establishes itself. As described, there exists a practically saturated vapor layer of water (in vapor form) directly at the liquid/gas phase boundary. The flow resistance for the molecular flow in the membrane is, for the evaporated substances, such as gaseous aniline, comparable with that of water vapor. It increases with the root of the molar mass. In practice, the saturated vapor layer, almost equal to partial pressures, must then be formed for the volatile components as corresponds to Henry’s Law. The particle flow density of the volatile substances, from the solution in the vapor layer, occurs because of the low difference of the partial pressure, pH, in accordance with Henry’s Law, and the partial pressure pd, in the vapor layer. jdi = Kdi (pHi - pdi) jdi Kdi pHi pdi

particle flow density of the volatile species i from the phase boundary in the vapor phase [mol/(min cm2)] proportionality constant partial pressure of species i according to Henry Law [mbar] real partial pressure of species i [mbar]. FLOW IN THE MEMBRANE AND IN THE FINE VACUUM

The flow resistance of the membrane produces the difference between the partial pressures of the vapor layer, almost saturated with the components, and the partial pressures in the fine vacuum of the vacuum apparatus. The pump suction output of the vacuum pump (membrane pump) determines the total pressure in the fine vacuum. Thus, the vacuum pump also determines the degree of saturation of the vapor layer and in accordance with the partial pressures of the

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123

components in the vapor layer via the particle stream of the vapor layer through the membrane into a fine vacuum. The pressure gradient in the membrane determines the particle stream density of the volatile species: jvi = KPi(pdi - pvi) jvi KPi pdi pvi

particle flow density of the volatile species in the fine vacuum [mol/(min cm2)l guideline value of the membrane for the species i partial pressure of the volatile species in the almost saturated vapor layer [mbar] partial pressure of the volatile species in the fine vacuum [mbar].

Via a pressure converter, with an adjustable shutter/opening, a small part of the gas flows into the vacuum, necessary for the mass spectrometer, of ap-5 proximately 10 mbar. The mass spectrometer measures the partial pressure in high vacuum, proportional to the partial pressures in fine vacuum. Figure 3 shows the evaporation process of volatile substances dissolved in water through an idealized pore in the membrane.

Figure 3. Schematic diagram of the evaporation of volatile substances dissolved in water through a membrane pore.

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Continuous determination of volatile breakdown products

The liquid/gas phase boundary is formed in the membrane pores. According to Henry’s Law, a specific partial pressure of this substance is reached in the practically saturated vapor layer at the phase boundary corresponding to the concentration of the volatile species. STATIONARY CONDITIONS

In the stationary conditions, all particle flow densities are equal. The following equation applies: jLi = jdi = jvi Consequently, in the stationary conditions, the mass signal is also constant. In external conditions, such as flow rate and temperature of the solution, constantly maintained, the mass signal is directly proportional to the concentration of the volatile species in the solution. The height of the mass signal and/or 4 the limit of evidence is then provided by Henry’s Constant. APPLICATIONS

With the apparatus, aqueous solutions of standard hydrocarbons are measured. Table 1 shows the measured mass signals (always 10 µl/L). With the example of chloroform, trichloroethylene, and methylene chloride, it was possible to demonstrate that the apparatus gives linear readings.5 The limits for other substances were estimated under assumption of linearity and at the region of a few µg/L. With a detection limit of 1 mg/L, aniline shows a considerably lower measurable sensitivity. The diffusion coefficients of the substances listed only differ to a slight extent. With the exception of aniline, all carbohydrates listed possess similar Henry’s constants and thus similar limits of detection. Aniline is markedly adsorbed on the stainless steel wall in the high vacuum system. To obtain short response times and constant mass signals, the high vacuum recipient must be heated.

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Table 1: Mass signals of aqueous solutions of hydrocarbons (10 µl/l) and estimated detection limits

Concentration

Signal

Detection limit

mg/l

10-12 Å

µg/l

83

14.82

9.1

1

Trichloroethylene

95

14.70

14.0

1

Methylene chloride

49

13.36

18.0

1

Methyleneisobutylketone (2-methyl-1-pentanone-4)

43

8.01

7.3

1

Diethylether

31

7.14

2.1

4

Ethyl acetate

43

9.01

8.5

1

Toluene

91

8.72

13.0

<1

n-Hexane

43

6.64

1.8

4

Cyclohexanone

56

7.78

21.7

<0.5

2,2,4-Trimethylpentane

57

6.92

5.3

1

n-Heptane

43

6.83

2.5

3

Aniline

93

10.13

0.04

1000

Name of compound

m/c

Chloroform

CONCLUSIONS

Using the apparatus presented and described above, it is possible to determine traces of volatile breakdown products of propellants. The evidence limit is at 1 µg/L for substances with large Henry’s constants. As a general rule, in gas analysis from the vapor field of a liquid, for example Headspace-GC, a duration of thirty minutes and more must be expected before the vapor phase is in balance with the liquid. In the flow cell with porous, hydrophobic membrane, a stationary condition for the particle stream is reached by itself, thus providing a constant measurement signal, within a few seconds. The mass spectrometric apparatus is suitable for a rapid analysis of samples and for the continuous monitoring of process and waste water.6

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1. 2. 3. 4. 5. 6.

Continuous determination of volatile breakdown products

REFERENCES

G. Bunte, N. Eisenreich, F. Hirth, and H. Krause, Disposal of Propellants and Explosives in Hypercritical Water, Poster Presentation, 23rd International Annual Conference of ICT, Karlsruhe, June 30 - July 3. G. Hambitzer, Doctoral Thesis, Witten, Germany, 1988. M. Joos, Diploma Study, Pfinztal, Germany, 1991. M. Joos and G. Hambitzer, in preparation. G. Hambitzer and M. Joos, Dechema Monographien, 125, Electrochemical Substance Recovery - Basic Requirements and Processing Technique, Publ. Verlag Chemie, Frankfurt a.M., (1992). G. Hambitzer and M. Joos, Chemische Industrie, Maiausgabe, May (1992).

N. Eisenreich, H. Kull, and E. Thinnes

21

Fast Identification of Plastic Materials by Near-Infrared Spectroscopy

Norbert Eisenreich, Harald Kull, and Eric Thinnes Fraunhofer-Institut für Chemische Technologie, ICT, Joseph-von-Fraunhoferstrasse 7, D-76327 Pfinztal 1, Germany

The recycling or the thermal disposal of plastics is only possible after a rigorous process of separation into classes of compatible materials is conducted. An automatic sorting process leads to recyclates which can be re-used in an economic way. Unknown plastic materials such as basic polymers, fillers and some critical additives, e.g., fire retardants, must be identified. Near-infrared spectroscopy (NIRS) allows the identification of basic polymer, because absorption or reflectance spectra deliver “finger prints” of polymers and also provide an additional information on some additives. A NIR-spectrometer based on an acousto-optic tunable filter scans the spectral range required for identification, in milliseconds. A transputer system controls the spectrometer, acquires and analyzes the spectra on-line. The integration into a separation process has been done.

INTRODUCTION

The recycling of plastics follows a step of separation process of mixed materials. Re-use of these materials, as well as thermal disposal, requires a process of selection complicated by the fact that unknown mixtures must be processed. The plastics in industrial wastes, household wastes, or mass consumer products -typically disposed off - consist of a mixture of polymers, modifications, additives, and fillers. Physical methods, such as those based on density, are not sufficiently specific to allow separation. Effective identification processes must use methods which can monitor structural or molecular properties of plastic materials. The method must also be able to withstand an industrial environment and it demands a robust and fast-scanning instrument.

22

Fast identification of plastic materials by NIR

Various standard methods of chemical analysis are under evaluation. First results have been obtained, using X-ray fluorescence and mass spectrometry, but time is still required before they can lead to industrial applications. Possibly, a combination of methods must be used. Several promising methods are being developed at the moment in ICT: • • •

laser pyrolysis and spectroscopy X-ray diffraction near-infrared spectroscopy (NIRS). This paper describes results regarding the application of NIRS. NEAR-INFRARED SPECTROSCOPY

The near-infrared spectral range (NIR) comprises the wavelength of 700 to 2500 nm. Molecules absorb light in the wavelength range by overtone or combination vibrations. The absorbance is reduced by an order of magnitude when compared with fundamental vibrational transitions in the infrared (IR). Condensed materials strongly absorb IR radiation so that only thin films can be studied. The reduced absorbance in the NIR allows the registration of spectra of bulky samples such as are of practical interest in recycling processes. The spectra in the NIR are characteristic for the polymers, similar to the “finger prints” in the IR which enable identification of the most commonly-used materials. The CH, OH, NH, and CO bands, which are mainly observed in the NIR, are of main importance for polymer identification. Consequently, NIR spectra play a significant role in the investigation of polymers.1-3 The NIR spectral range has further advantages: • •

The photo-detectors, such as Ge, InAs, or InGaAs photodiodes, have shorter response time and higher sensitivity than detectors used in the IR Quartz fiber optics, which have low attenuation and are commercially available and inexpensive, can be used

These advantages led to a strong increase in applications of NIRS in industrial process control.4

N. Eisenreich, H. Kull, and E. Thinnes

FAST NIR SPECTROMETER WITH AN ACOUSTO-OPTIC TUNABLE FILTER

23

For fast scanning of the wavelength region of 1000 to 2500 nm, spectrometers based on acousto-optic tunable filters (AOTF) offer essential enhancements. 6,7 They allow millisecond time and wavelength resolution. AOTF use the anisotropic Bragg effect at large angle. This effect is observed in studies of birefringent crystals such as TeO2. Ultrasonic waves generated by piezoelectric transducers form a grating which diffracts incident light. The diffraction results in a beam of zero-order flanked by the diffracted ordinary and extraordinary beam at an angle of 4 degrees, each, and a small wavelength difference. If the frequency of the acoustic wave is changed, the wavelength is also changed due to the constant sound velocity of the crystal (0.65 m/ms). The half wavelength corresponds to the lattice distance, which determines the Bragg diffraction. With some over-simplification, a spectrum is scanned by AOTF by varying the distances of the “grooves” and measuring the different wavelengths at a fixed angle without need to rotate the grating as typically done in conventional grating spectroscopy (for a detailed theoretical analysis, see ear5 lier publication). The spectrometer system7 consists of the light source, an AOTF, fiber optics for absorption and reflection measurements, and the control and data acquisition unit. In the absorption experiments the light source was a halogen lamp with a power of 150 W. A parallel light beam enters the TeO2 crystal where it is diffracted. The extraordinary and/or ordinary beam can be focused onto the sample and the reflected light or transmitted light collected and fed to an InAs photo detector cooled by a thermo-electrical device or by liquid nitrogen. Figure 1 shows a schematic diagram of the spectrometer system. The control and acquisition unit consists of a transputer system, a voltage controlled oscillator (VCO), and a 5 W power amplifier. The transputer system records the signals of the photo detector via a 12 bit analog-to-digital converter at a maximum sampling rate of 500 kHz. It also controls the AOTF operation by the output voltage via a digital-to-analog converter to be fed to the VCO. The VCO supplies the corresponding frequency to the piezo-electric transducer of the AOTF. Parallel to the AOTF control and the data acquisition, the transputer is able to perform other operations such as the correction of the dispersion relation of the AOTF to obtain a spectrum linear with respect to wavelength or wavenumber.

24

Fast identification of plastic materials by NIR

Figure 1. Scheme of the NIR spectrometer equipped with fiber optics and reflecting detector.

It can also evaluate data by producing transmission or absorption spectra or performing a multicomponent analysis. Important for plastic material identification is effective, high speed identification software. The efficiency of the transputer can be increased, if needed, by upgrading to additional mathematical modules in a straight-forward procedure by the transputer links. In the case of low light levels, the system allows a lock-in operation mode without a mechanical light chopper. This is possible by switching the power amplifier periodically and using a lock-in amplifier. Additional features of the AOTF, which are not taken into account at the moment, include: •

The crystal splits up the light in two directions of polarization with only a small difference of the wavelength observed. In the cases when polarization is important, the sample has to be placed between the light source and the AOTF. In the cases when polarization is not important, a dual beam spectrometer or difference spectroscopy can be used.

N. Eisenreich, H. Kull, and E. Thinnes

• • •

25

The spectrometer can operate in emission, absorption, and reflection. Fiber optics can be used. Figure 1 shows a scheme of the spectrometer representing the fiber optic version for reflectance spectroscopy. The AOTF diffracts 60 % of the incident light at a power of 3 W of the amplifier. The wavelength’s resolution depends on the wavelength and it is about 2 to 3 nm in the region of 1000 to 2500 nm. At scan speeds higher than 100 nm/ms the wavelength resolution decreases due to the sound speed of the crystal. A wavelength’s shift has to be corrected in relation to the applied scan speed. At a scan speed of 1500 nm/ms, the wavelength resolution is reduced to 15 nm. NIRS OF PLASTIC MATERIALS 4

Despite increasing interest in NIRS, only a few books with standard spectra are 12 available. These books contain only sparse information on plastic materials. The application of the AOTF spectrometer aims at the registration of spectra of various plastic materials, their modification, and identification of the techniques which can operate under conditions of practical interest. The spectra were recorded in reflection, at high scan speeds, from a moving sample of plastics containing fillers, additives, and pigments. Coatings and covering materials are not considered in this study. To simulate fast moving plastics, a rotating wheel with plastic samples was constructed. It allows recording of absorption or reflection spectra, depending on the optics of the AOTF spectrometer. Spectra were obtained with scan speed of 150 nm/ms corresponding to 100 full range spectra per second. Quartz fiber optics as well as lenses were used. Some spectra of various materials are shown in Figures 2 to 8. The samples differ in the structural composition of aromatic or aliphatic groups. CH bands absorb in the wavelength region of 1100 to 1250 nm (second overtone of CH stretching vibration), 1600 to 1800 nm (first overtone of CH stretching vibrations) and 2150 to 2500 nm (combination band). In CHC13, the CH band is a single-headed band.7,9,12 CH groups in a complex neighborhood show significantly more complex structures which allow an identification. In polyethylene, a nearly pure CH2 leads to a characteristic band of the first overtone different from the bands of CH, CH2, and CH3 groups typical of polypropylene or the aromatic CH band of polystyrene. Figures 2 to 7 illustrate these bands.

26

Fast identification of plastic materials by NIR

Figure 2. Transmission spectrum of polyethylene (additive: oleyl palmitamide).

Figure 3. Transmission spectrum of polyethylene (additive: erucamide).

N. Eisenreich, H. Kull, and E. Thinnes

Figure 4. Reflection spectrum of polystyrene.

Figure 5. Transmission spectrum of polypropylene.

27

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Fast identification of plastic materials by NIR

Figure 6. Resolved CH first overtone of polyethylene.

Figure 7. Resolved CH first overtone of polypropylene.

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29

Figure 8. Reflection spectrum of a silicone rubber.

Different additives, e.g., slipping agents, can, in some cases, be detected by means of small groups (see Figures 2 and 3). The comparison of Figures 6 and 7 (the first CH overtone better resolved) indicates that the wavelength’s resolution of 5 nm is needed to enable a clear identification of most polymers. NH bands appear at 1500 and 2050 nm (e.g., polyurethane spectra of Miller 1,2 and Eichinger ). CO bands can be observed at 1900 to 2100 nm and can be detected in polyurethane as well.1,2,12 In a silicone rubber the first CH overtone is strongly split off and NH band found (Figure 8). • • •

CONCLUSIONS

The spectra in Figures 2 through 8 and the data from the literature1-4,8-12 demonstrate that the polymers of plastic materials can be clearly identified by NIRS in short acquisition times of the spectra (10 ms). Information on additives can be obtained in many cases. Spectra measured in reflection qualitatively show the same band characteristics as absorption spectra.

30

•

1. 2.

Fast identification of plastic materials by NIR

These results encourage the development of sorting systems for plastic materials but strong efforts in development are still needed to adapt this system for practical use in the reflection mode. REFERENCES

D. E. Miller and B. E. Eichinger, Appl. Spectrosc., 44, 887 (1990). H. W. Siesler and K. Holland-Moritz, Infrared and Raman Spectroscopy of Polymers, Marcell Dekker, New York, 1980. 3. Ch. E. Miller, Non-Destructive Analysis of Bulk Polymer Systems, 4th International Conference on Near-Infrared Spectroscopy, Aberdeen, 19-23 August, 1991. 4. NIRS 3. Deutsche Informationstagung über Nahinfrarotspektroskopie, Universität-Gesamthochschule-Duisburg, 14-15 March, 1990. 5. T. Yano and A. Watanabe, Appl. Phys. Lett., 24, 256 (1974). 6. A. Blanc, N. Eisenreich, H. Kull, and W. Liehmann, 19th Intl. ICT-Conference, Proceedings, 74-1 (1988). 7. N. Eisenreich, H. Kull, and T. Messer, French-German Workshop on Optical Measurement Techniques and Fiber Optics, Freiburg (1991). 8. N. Eisenreich, H. Kull, and T. Messer, 22th Intl. ICT-Conference, 99-1 (1991). 9. C. E. Miller, P. G. Edelman, and B. D. Ratner, Appl. Spectrosc., 44, 576 (1990). 10. C. E. Miller, B. E. Eichinger, Appl. Spectrosc., 44, 496 (1990). 11. C. E. Miller, Appl. Spectrosc., 43, 1435 (1989). 12. Sadtler, The Atlas of Near-Infrared Spectra, Sadtler Research Laboratories, Philadelphia, 1981.

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The Disposal of Propellant Components Containing Heavy Metals

W. Böke and G. Hambitzer Fraunhofer-Institut für Chemische Technologie - ICT, Joseph-von-Fraunhofer Straße, D-76327 Pfinztal 1, Germany

In a preliminary study, a concept has been developed for the environmentally safe disposal of igniters containing mercury fulminate used in the ammunition of the former East German Forces (referred to as NVA, Nationale Volksarmee). Apart from technological conventional processing steps, this multi-step disposal process also comprises an electroflotation process for the elimination of heavy metals below the legal limit values and/or for the electrochemical conversion of pollutant substances into materials which are reusable or can be disposed of normally in landfills. Electroflotation processes for the elimination of toxic materials and for cleaning waste water have only come into use in recent times and were mostly tried out on an empirical basis. To establish and optimize process parameters, therefore, further experimental investigations are required.

INTRODUCTION AND OBJECTIVES

In the case of the ammunition of the former NVA, igniters were used which, contrary to the western standard, used mercury fulminate as initial detonating substance. This presents a considerable problem for the proper disposal of these materials. A concept is presented here for a multi-step process to dispose previously dismantled igniters containing mercury fulminate; this concept is based on initial1 experimental studies. The principal objective is the adherence to all regulations laid down by German and international law as regards detriment to the environment, especially observation of the Framework Waste Water Management R eg u la tion ac cording to t h e W a t e r P re se rv a t i o n L a w (Rahmen-Abwasserverwaltungsvorschrift nach dem Wasserhaushaltsgesetz).

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The disposal of propellant components

The principle demanded in these regulations is here drawn into account, i.e., the recycling or reuse of all substances involved in the process as far as possible. The electroflotation principle, for which a place has been reserved as an intermediate step in the disposal concept, provides an important contribution towards achieving this aim. This new process is used in the cleaning of polluted 2 waste water. From a number of samples and references which were available to us,3 we have made the following suppositions as to the composition and properties of the igniters concerned: • •

•

The pan, holding the ignition cap, made of brass with different alloys, has a layer of lacquer inside and has a mass of approx. 300 mg. The igniter mass weighs approx. 30 mg per cap and has the average composition: Mercury fulminate Antimony trisulfide Potassium chlorate

[Hg(CNO)2] [Sb2S3] [KClO3]

25.0 % = 7.50 mg 37.5 % = 11.25 mg 37.5 % = 11.25 mg

The igniter mass is covered with lacquer-coated paper.

These data can deviate considerably according to the country of manufacture and application. Over and beyond this, the igniter may hold additives such as shellac, silicon dioxide, lead oxide as well as different binding agents, up to 10%. TREATMENT OF IGNITERS

Starting with the assumption that the igniters have already come into contact with water during their dismantling, 10 to 30 ml water was added in each case to samples of 1 to 3 ignition caps. Immediately after the addition of water, the water-soluble components of the igniter mass (potassium chlorate and mercury fulminate) start to dissolve, as is recognized by the fact that mercury slowly precipitates in the brass pans of the igniter caps. However, the compactness of the igniter mass and the hydrophobic paper cover retard the dissolution process so that it is incomplete even after 2 days. Obviously the residue remaining in the brass pan after this period still contains undissolved fulminate and chlorate, in addition to the antimony trisulfide which remains insoluble under these conditions.

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The dissolution processes could be considerably accelerated by applying ultrasound. Only after the addition of tensides and ultrasound treatment for approx. 15 minutes in a solution heated up to 55oC, it was possible to separate quantitatively the brass pans from the insoluble and soluble components of the igniter mass. After removal of the amalgamated brass pans and recovering of the insoluble black antimony trisulfide by filtration, which was suffused with remains of the covering paper, it was possible to obtain a clear solution. Determination of Hg, Cu, and Zn by means of AAS showed that this solution still contained Hg ions whereas, for the Hg precipitated onto brass, an equivalent quantity of Zn and Cu, approximately at a ratio of 1:2, had entered into solution. CONCEPT FOR THE DISPOSAL OF IGNITERS

A concept for the disposal of igniters containing mercury fulminate based on the results of previous initial investigations and our objectives of the highest possible environmental compatibility is presented schematically in Figure 1. The numbers cited in the following explanation relate to the numbering of the individual processing steps in the illustration. 1.The dismantled ignition caps were phlegmatized in water and stored on an interim basis. In this process, the dissolution of the soluble parts and/or the separation of the insoluble components and the amalgamation of the brass is set off. The addition of tensides increases the wettability and can accelerate these processes. 2. The solution processes are carried out in an ultrasonic bath up to complete separation of the igniter mass and the brass. According to this, the mercury is to be precipitated out onto the brass to a considerable extent; an equivalent quantity of zinc and copper ions enters into solution. The amalgamated igniter caps are taken out of the process and are transferred for recycling to a special company. This process is possible in existing installations and has already been conducted successfully on a trial (pilot) scale.4 3. The practically insoluble antimony trisulfide, under the conditions of the previous process, is filtered off and - possibly after purification if necessary - reused (e.g., in the rubber industry). 4. The copper and (in smaller concentration) mercury ions contained in the residual solution are removed as far as possible in an electrochemical process step. Both a galvanic separation in a suitable reactor as well as a cementation

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The disposal of propellant components

Figure 1. Preliminary concept for the disposal of igniter caps.

process is conceivable. The separated metals are reused directly or passed on as amalgam to a recycling process (see Step 2). 5. The solution still contains fulminic acid, potassium chlorate, and zinc ions as well as traces of mercury, copper, and possibly antimony ions. In an electroflotation reactor, under suitable conditions, the fulminic acid is reductively decomposed to ammonia and carbon dioxide, and the potassium

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chlorate reduced to potassium chloride. Zinc and the remaining metals precipitate together with the aluminum hydroxide and are contained in the flotate. Ammonia is collected, the ammonium salt is reused. The flotate is disposed in tips, whereby storage on a household waste tip is possible after a positive eluation test. 6. In the final process step, the potassium chloride is precipitated by evaporation. The water is recycled back to the beginning of the entire process; the potassium chloride is deposited on a tip or, under certain circumstances, reused. In the process steps 1 through 4 and 6, we are dealing with conventional, introduced techniques for which the installations and apparatus are commercially available. Through the initial trials, first carried out their operability and suitability for the disposal of igniters, may be considered as being sufficient. ELECTROFLOTATION

It is the electroflotation process here proposed as a 5th processing step which is of decisive importance, particularly for the environmental compatibility of the disposal concept under consideration. It has - as described - the most important task of eliminating all metal ions still contained in the solution down to below their legally allowed limit as well as that of converting the electrochemically/chemically formed decomposition products of the potassium chlorate and the residual fulminic acid into reusable or conventionally disposable substances. In electroflotation, we are concerned with a combination of simultaneously occurring processes - flotation, flocculation, electrochemical reaction - which, individually or partly combined, have already been in use for some time, e.g., in recycling processes. In this combination, however, they have not been applied successfully until quite recently for the cleaning of industrial waste water with a complex composition. Two partial processes are characteristic of electroflotation: 1. Anodic metal decomposition occurs on aluminum (or also iron) electrodes. The conditions (e.g., pH value, temperature) are selected so that the hydroxide formed is precipitated via flocculation with a large surface. On this freshly formed, very reactive hydroxide surface, both dispersed particles as well as emulsified or dissolved substances, including metal ions, can be absorbed.

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The disposal of propellant components

Figure 2. Cobalt concentration versus time.

2. The hydrogen and oxygen bubbles, formed as a result of electrolytic water decomposition, not only cause the flotation, but also make possible homogenous reactions of the dissolved substances, i.e., oxidation via the oxygen produced. The process parameters of electroflotation reactors have, up to now, only been determined empirically; an understanding of the individual partial processes and their interaction makes improvements or optimization of the reactor and the processes possible. DEPLETION OF COBALT

Within the framework of an initial study on electroflotation, different cobalt ni3 trate compounds were electroflotated between two aluminum electrodes 3 cm in size in 100 ml 2 molar KNO3 at currents between 0.5 and 1 Ampere and voltages from 3 to 5 V. Starting with a cobalt concentration of 10 mg/L, the solutions were enriched up to values around 0.1 mg cobalt/L. According to the test conditions, the legal limit value of 1 mg cobalt/L was already reached after 6 to 10 minutes. Figure 2 shows, in exemplary manner, the progression through time of the cobalt content of a solution which experienced electroflotation for 16 minutes at

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a current density of 200 mA/cm2. The cobalt concentrations were determined polarographically. The experiments are continued with solutions which can also contain organic impurities in addition to heavy metals. For this purpose, a thin-layer flow-cell is used in combination with an apparatus developed at the ICT allowing the continuous determination of volatile, organic substances in solutions 5-7 on-line. This experimental arrangement makes possible the identification of individual reaction products and their assignment to the electrochemical parameters of the thin-layer flow-cell. The investigation of individual partial steps of the electroflotation process and the dependence of process parameters and active mechanism is possible in this way. CONCLUSION

Starting with the assumption that every year approximately one thousand million igniters from infantry munitions of the former East German Forces (NVA, Nationale Volksarmee) are to be disposed of, approximately 5.5 tons of mercury are available per year. This alone explains the necessity of granting the highest priority to the adherence of all environmental regulations. The proposed electroflotation process as part of the disposal process is in this context of decisive importance. The preliminary character of the present study explains itself from the necessity of conducting experimental investigations going further in this direction with the aim of determining the process parameters and making the separate process steps more precise, particularly for electroflotation. 1. 2. 3. 4. 5. 6. 7.

REFERENCES

W. Böke and G. Hambitzer in Environment Protection with Munitions and Explosive Materials, Conference Booklet, BAkWVT (Federal Academy for Weapons and Defense Techniques), Mannheim, Germany, 16-17 October, 1991. G. Klose in Experiences with the Clearox Process, Seminar entitled -Modern Processes of Industrial Waste Water Cleaning, Technische Akademie Mannheim, Germany, 24-25 April, 1991. BWB Coblence and Systems Maintenance and Recycling Company, Neubrandenburg, Germany. North German Mercury Recovery Company, Lubeck, Germany. G. Hambitzer, M. Joos, and U. Schriever, Simplified DEMS Apparatus Continuous Determination of Volatile Substances in Process and Waste Water, DECHEMA-Monographien, Vol. 125. M. Joos and G. Hambitzer, Chem. Indus., in print. G. Hambitzer and M. Joos, Conference Booklet, ICT Annual Conference 1992.

A. Pfeil

85

Screening of the Degradation of Plastic Materials in a Composting Medium

Achim Pfeil Fraunhofer-Institut für Chemische Technologie (ITC), Joseph-von-Fraunhofer-Straße 7, D-76327 Pfinztal, Germany

As an alternative in polymer waste utilization, the biodegradation of plastic waste is increasingly under discussion. This concept depends on the polymer degradation by the action of micro-organisms. Focus of attention is the ability of the waste materials to be sufficiently degradable when exposed to the biological attack. A major subject of discussion is thus is a behavior of the polymeric materials in an aerobic and unaerobic medium and how to assess the biodegradation. The following article describes a screening test for the degradability of polymers and plastic materials and recording the degradation history.

SCREENING

Effective screening requires short and reproducible test periods. Short test periods were realized by applying an intense rotting climate. The climate was obtained by a sensitive balancing of the key parameters such as humidity, composition (C to N ratio, pH), aerobic control, and temperature profile. By keeping the parameters constant for each composting cycle comparable temperature profiles were obtained. Figure 1 provides examples of the temperature profiles attained. The temperature curves act as a sensor of the decomposition climate. The resemblance demonstrates that the climate could reproduced within limits which is important for the recording of the degradation history. Working in cycles (1 cycle = 1 week), the history of test samples could be constructed by noting the sample changes occurring per cycle.

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Screening of the degradation of plastic materials

Figure 1. Temperature profiles obtained with standard composting.

COMPOSTING

For effective balancing of the composting parameter, a drum reactor was employed. The important features were: open access and rotability of the drum for homogenization, aerobic control, and moisture adjustment as well as a heat damping capacity. The drum was made from thick wooden planks and had vol3 ume of 1 m . The drum was filled to 3/4 with the composting material made up of chopped green plant materials (grass, cuttings from hedges and trees), wood chips, molasses, horse dung, and ripe compost as inoculator. The C to N ratio of the mixture was adjusted to 25:1, the moisture was set in between 50 and 60 wt% and the pH value as neutral. As a result, the composting material is turned to fresh compost within a period of 7 days. Acting as a reference and as a further sensor of the decomposition climate, the loss of mass and relative tensile strength of jute threads were measured. Figure 2 shows examples of the breakdown of jute threads after exposure to the composting cycles.

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Figure 2. Loss in tensile strength and loss in mass of jute thread.

Table 1: Test methods Macroscopic

Microscopic

Molecular

Mechanical

Microscopy

GPC

Mass, density

Microchemistry

HPLC

TA

Microtomy

GC/IC

Photography

REM

UV/VIS/NIR

Structure

Structure

FTIR, FTIR/ATR

SCREENING OF MATERIAL DECOMPOSITION

The complex nature of the polymeric materials makes it difficult to classify the decomposition process. Consequently the spectrum of the analytical methods is widely spread. Table 1 provides a survey of the analytical methods.

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Screening of the degradation of plastic materials

Figure 3. Example of a linear degradation curve, recorded on massive starch bodies.

Figure 4. Mechanical stability loss of an alloy film.

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Figure 5. Polyethylene film with incorporated starch in cross-section before (left) and after screening (right).

We investigated samples as a cross-section of the waste plastic scene, both single-cell components as well as blends, alloys, and composites. The polymers were both natural (cellulose, starch, PHB/PHV) and synthetic (cellulose derivatives, PE, PP, polyesters, and polyamides). The examples given graphically demonstrate the typical changes in specific properties, i.e., mass, stability, molecular weight, and surface structure. The first example in Figure 3 shows a linear mass dependence exhibited by massive bodies made out of starch. The starch bodies are used in the packaging of ampoules and fast food. This polymer is readily decomposed and breaks down without residues. Alloy type films possess the easy decomposability of starch and at the same time exhibit a high tensile strength. The films are composed of highly destructurized starch and polar synthetics, with a high proportion of starch. The sample in Figure 4 describes the loss in tensile strength for such film. This film consisted of 60 % starch and 40 % synthetic component, the film thickness was 65 µm. LDPE film with a thickness of 36 µm served as a reference.

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Screening of the degradation of plastic materials

Figure 6. Degradation curve of a LDPE film with breakdown initiating additives.

Films made out of polyethylene, in contrast, showed no degradation on screening and behave in a neutral way. For example, a screening up to 7 cycles showed no decomposition, with the exception of oxidative traces on the film surface. Even the inclusion of granular starch did not change the resistance of the polyolefin matrix to the biological attack in the composting medium. It merely produced an enlarged surface due to the preferential breakdown of starch grains. The example in Figure 5 visualizes this effect recorded on a film made out of polyethylene with 8% of incorporated granular starch before and after screening. However, a breakdown of the carbon chain of the polyolefin matrix may be induced by means of special additives. Through heat and oxygen contact, these additives initiate an oxidative chain breakdown process to a level permitting further biological decomposition. Figure 6 shows the degradation history of such an LDPE film containing starch and the special additives. The film thickness was 35 µm, the starch content 8%. Here, the decrease in molecular weight was taken as a parameter characterizing breakdown. By the action of the additives,

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Figure 7. Surface hydrophylization of a LDPE film with breakdown-initiating additives relative to an additive-free film.

the curve progression is significantly changed relative to the additive-free sample. As a reference guideline, the change in tensile strength of jute thread has also been entered. The interaction is most pronounced at the sample surface where the contact with heat and oxygen is the most intimate. Figure 7 demonstrates the enhanced activity as viewed by a FTIR/ATR spectrometer. The surface hydrophylisation of the LDPE film with breakdown-initiating additives relative to a similar film containing no additives is markedly raised. The -1 hydrophylisation was measured by monitoring the oxidation band at 1600 cm in the IR spectrum. CONCLUSIONS

The screening test surveys the degradability in a composting medium. Intermediate products, however, remain undetected. The screening indicates property changes, typically - mass, volume, material strength, macromorphological, and molecular structure. The degradation curves assess: • Significance of volume/weight reduction • Decomposition/non-decomposition of individual components

92

•

Screening of the degradation of plastic materials

The useful combination with subsequent disposal measures such as a specific chemical breakdown or as a sewage treatment. The screening shows that only a few polymeric materials decompose rapidly enough and in a manner suitable for the composting. These are primarily the native materials such as starch, cellulose, and polyhydroxy fatty acids. Typical polymer waste made out of these are found in niches such as fast-food, agriculture, gardening, and cemeteries.

Index

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Index A

cascade of re-utilization 9 CH band 25 chemical oxygen demand 109 circuit-board scrap 32, 38 cobalt 132 combustion 47 composite foil 49, 88 composting 85, 86 concentration profiles 101 condensate oils 35 contaminated soil 32 continuous process 54 crude oil 51 curing 78

acousto-optic filter 23 aerobic control 85 air amount 94 air number 94, 96 ammunition 105, 127 autoclave 32, 50

B BASF 1 biological-adsorptive purification 36 biopolymers 2 bottles 68 Bragg effect 23 brass 129 bromine 38 burning temperature 94 burnout level 98, 99

C capital cost 80 car components 37

D data acquisition 23 decomposition 87 degree of burning 100 diethylacetamide 36 diethylphenylurea 118

diffusion 120 diffusion coefficient 124 dimethylacetamide 36 dioxines 34, 37 drum reactor 86

E electroflotation 128, 130, 131 electronic scrap 39 emission standards 32 erucamide 26 exhaust gas 93 explosives 105

F fiber optics 22 fillers 49, 50 film 68 flame temperature 100 flame-retardant 38 flow cell 119

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Index

channel 120 rate 120 resistance 121 fluidized-bed reactor 32 furanes 34, 37

J

P jute 86, 87

PCB 41 PET 67 phase boundary 121 phenolic resin 49, 62 photo-detector 22 pilot-plant 1 plastics use 57 polarization 24 polyamides 49, 89 polyesters 49, 62, 89 polyethylene 26, 28, 61, 63, 89, 90, 91 polypropylene 27, 28, 61, 89 polystyrene 27 polyvinylchloride 62, 63 powder coatings 67 pressure hydrolysis 105, 108 process balance 34 propellants 117, 127 composition107 properties 78 PVC 2 pyrolysis 31, 59, 61 residue 34

L G galvanic separation 130 gas converter 33 gas scrubbing 35 gasification 47 GC 109 glass fibre 40 glycolysis 68 graduated incineration 33

laser pyrolysis 22 loading aids 19 logistics 7

M macro-logistics 11, 14 mass loss 87 material balance 80 membranes 120, 123 mercury fulminate 127 micro-logistics 11, 18 motor oil 62 MS 118

H halogenic compounds 34 heavy metals 37, 127 Henry Law 121, 122 HPLC 64 hydrocarbon yield 49 hydrogen chloride 52, 54 hydrogenation 46 - 48, 50, 55, 57, 59, 62 hydrolysis conditions 108

N NH band 29 NIR 21, 22, 24 nitrocellulose 93, 95, 100, 106, 109 nitrogen compounds 36 nitroglycerin 109, 118 nitrous oxides 94, 101, 113 NMR 76

I IC 109, 110 igniter 127, 128 ignition 101 incineration 106 apparatus 95 inoculator 86 inorganic fillers 2 input mixture 61 IR 76, 91, 96, 109, 111 isocyanic acid 102

O oleyl palmitamide 26 operating cost 81 operative level 14 organic acids 110

R radiation heat loss 100 Raoult Law 121 reactants 72 reaction products 109 reactive solvents 64 recycling quota 14 residual oil 32 rocket propellants 105 rotary kiln 32 rotting climate 85

Index

temperature progression 96 tensile strength 87 test methods 79, 87 Tg 77 thermodynamic loss 5 tires 32 toluene 36 toxic substances 97 trinitrotoluene 93, 95, 101, 118

S screening 85 separation 21 sequential use 3 shaft furnace 32 shredder residue 32, 41 silicone 29 slipping agents 29 slurry phase 57 software 16 solid residue 40, 109 standard spectra 25 starch 88, 89 stationary conditions 124 strategic planning 14

tactical planning 14

phase 36 products 117

W waste cable 62 disposal logistics 7 hazardous 32 household 32, 61 mixed plastic 49 plastic 58 use 46 wastes carbon-containing 45 household 21 industrial 21

U ultrasound 129 urea 101

V

T

137

visbreaking 52 volatile

X X-ray diffraction 22

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Investigation of Exhaust Gas Products in the Thermal Disposal of Waste Munitions Using Nitrocellulose and TNT as Examples

V. Gröbel, H. H. Krause, and V. Weiser Fraunhofer-Institut für Chemische Technologie, Joseph-von-Fraunhofer-Straße 7, D-76327 Pfinztal 1, Germany

1 INTRODUCTION

The problems involved in the environmentally safe disposal of household waste, industrial waste, and materials, which have passed their storage expiry date, are subject to an increasing public interest. As a universally applicable method, thermal disposal offers the following advantages: • • •

Extensive reduction in waste volume Higher level of efficiency in the decomposition of organic substances A regaining of the energy used in production.

As a result of the increased public awareness about the environment, modern disposal methods must demonstrate the ecological compatibility of emission products. Up to now, recognized processes, capable of performing such a demonstration, have been absent, as the proposed apparatuses1,2 only make use of sample quantities which are too small (in the region of milligrams). In order to obtain valid statements, a combustion apparatus was therefore developed which permits us to investigate solid and liquid waste materials in 3 the context of their compatibility with the environment in thermal disposal.

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Investigation of exhaust gas products

The apparatus makes possible the simulation of the generally used industrial thermal disposal methods (rotary cylinder oven, waste incineration systems) because it works in a closed system with controlled air supply and auxiliary firing. It hereby permits a quantitative recording of the exhaust gas products. By variation of technological process parameters (air amount, burning temperature, reductive atmosphere), it is possible to examine whether the emission limit valth ues in accordance with the 17 Federal (German) Emission Protection Regula4 tion (17. BImSchV) can be adhered to using a favorable incinerator control. In addition, data relevant for evaluation, in the context of fume (i.e., smoke gas) purification, if and when necessary, is also made available. In the following article, the apparatus is described and initial results presented in the field of thermal disposal using waste from military storage depots as an example. Following German reunification, large quantities of propellants and explosives were inherited in the installations of the former East German forces (Nationale Volksarmee, NVA), which now have to be disposed of in an environmentally safe and compatible manner. These materials contain high quantities of nitro groups which emit-large amounts of NOx (up to 2% by volume) into the atmosphere if they are not incinerated properly. Very rapid burning rates result in an insufficient air input, thus producing bad incineration residue rates 5,6 (CO, soot). 2 EXPERIMENTAL BACKGROUND 2.1 INCINERATION APPARATUS

The incineration apparatus shown in Figure 1 was developed and constructed to investigate the burning behavior of solid and liquid substances (e.g., polymers, solvents, propellants, and explosives). A flat flame burner operated by an air/propane gas mixture is mounted. It ignites and heats the sample located on a crucible (diameter: 50 mm). The distance between the crucible and the burning surface can be varied by means of a threaded rod. The incineration air is supplied and controlled via a flow rectifier. This means that the air number, λ, available for the flame, can be adjusted. Gas streams are measured using a suspended particle flowmeter. The glass reactor, positioned on the metal casing, receives the flame in its cylindrical section. A cone-shaped narrowing section channels the smoke gas flow making it possible to take a representative sample. The samples are introduced through a large flange at crucible level. Smaller flanges permit the intro-

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Figure 1. Incineration apparatus for investigating the burning behavior of solid and liquid substances.

duction of thermocouples for measuring temperature or suction withdrawal probes for flame gas sampling. Consequently, axial and vertical flame temperature and flame gas concentration profiles can be recorded. On the reactor, a glass tube is positioned as a smoke gas channel. Samples of these gases are taken for analysis using an incorporated probe unit. The temperature of the smoke gas is measured using thermocouples. 2.2 SAMPLES

Nitrocellulose and TNT are the principal components of propellants and explosives. This is why NC with 13.1 Ma % N and TNT were used as model sub-

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Investigation of exhaust gas products

stances. As fibrous NC burns explosively, it was pressed into cube-shaped standardized blocks (2 g). This reduction in surface decreases the burning rate quite considerably. The oxygen content of NC is 65 mol% of the oxygen required for complete incineration. Consequently, NC also ignites and burns in the absence of surrounding oxygen. In order to achieve an NOx reduction, in a number of NC trials, solid urea was mixed in at a molar nitrogen ratio (NH2 to ONO2) of 1.5. The excess of urea was used to balance a partial thermal decomposition of urea. As it incinerates very slowly, trinitrotoluene (TNT) could be introduced into the apparatus in powder form. It melts first of all and does not ignite until it has reached the evaporation phase. Thus, it burns under normal pressure and in air as a pool fire. 2.3 MEASUREMENT METHODS

Microthermocouples were used to measure the flame gas temperatures (diameter: 100 µm). The thermocouples consisted of PtRh30/PtRh6 alloys, which can measure temperatures up to 2100 K. The small diameter produced only slight irradiation errors (approx. 150 K). The smoke gases are sucked via a sampling tube into evacuated gas sample containers and are investigated for H2, N2, O2, CO, CO2, CH4, C2H6, C2H4, and C2H2 with a gas chromatographic unit specially designed for smoke gas analysis. NOx was measured on-line with an IR analyzer. The water was calculated de3 ductively from the analyses via elementary balances. 3 RESULTS 3.1 TEMPERATURE PROGRESSION

Figure 2a-c shows the temperature progression of the flames of NC, NC-urea mixtures, and TNT in dependence on the air number, λ. The air number describes the quantity of air supplied for incineration standardized (as norm) to the air quantity required via calculation for a complete oxidation of the initial substances. In the case of pure NC, a pronounced maximum temperature could be established under approximately stoichiometric conditions (λ = 1) of 1600 K and with the NC-urea mixture of approx. λ = 1.6 at 1300 K. On the other hand, the TNT flames only show a slight dependence on the air number. In the air

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Figure 2. NOx and temperature progressions dending on the air number, λ, in the burning out of (a) NC, (b) NC-urea, and (c) TNT.

number range investigated, a slight rise without maximum can be observed. The temperatures reached 950 K.

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Investigation of exhaust gas products 3.2 EMISSIONS OF TOXIC SUBSTANCES

NOx is the most important toxic component in substances containing nitro 3 groups. The decisive parameter for controlling the incineration process is the 7 air number, λ. This is why, in Figure 2a-c, the NOx concentrations in the exhaust gas of NC, NC-urea, and TNT flames have been entered over the air number, λ. As concentrations in the smoke gas depend on its dilution with air, the NOx values found were standardized to the nitrogen contained in the fuel. On the burning of NC (Figure 2a), the NOx concentrations pass through a minimum in the range from λ = 0.8 to λ = 1.5. In this range, the temperature passes through a maximum at 1600 K. Urea mixtures show a more prominent minimum at λ = 1.6 (Figure 2b) and a temperature maximum of 1300 K. The relative NOx concentrations are 2 to 3 times lower than with pure NC. The concentration profiles run countercurrent to the temperature in the case of TNT burnouts as well. 3.3 BURNOUT LEVEL

An environmentally relevant discussion of emission products in thermal disposal must, apart from NOx, also consider incompletely converted incineration products such as CO and soot. These components can be assessed globally using 7 the burnout level, α. It is defined as the mol ratio between oxygen already chemically bound, in the smoke gas in relation to the overall value required for complete oxidation. Smoke gas components included in the calculation of the burnout level, α, were: O2, CO2, CO, H2, H2O, CH4, C2H6, C2H4, and C2H2. The calculation of the burnout level does not include any NOx, as nitrogen is not counted as a fuel. Figure 3a-c shows the progression of the burnout level, α, over the air number, λ, for the fuels used. In the case of the NC flames, a plateau with an absolute value of α = 0.65 can be seen for λ < 0.5 in Figure 3a. This corresponds exactly to the quantity of chemically bound oxygen in the fuel. With an insufficient air supply, the NC only reacts with its own oxygen. With an increase in incineration air supply, burning rises sharply, although it does not reach 100% even at high air numbers. Although sufficient air is present inside the reactor chamber, it is not able to mix itself into the hot reaction zone with a sufficient degree of fineness in

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order to burn out rapidly. The NC-urea mixtures do not burn at air numbers below 0.9. As can be seen in Figure 3b, the burnout level, α, reaches 100 % at approximately double the air overdose. In the maximum temperature range, the

Figure 3. Burnout level, α, depending on the air number, λ, in the burning out of (a) NC, (b) NC-urea, and (c) TNT.

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Investigation of exhaust gas products

Figure 4. Adiabatically calculated and experimental temperatures of NC and TNT flames independently of the air number, λ.

air is mixed in completely, so that a maximum burnout is possible. In the case of TNT (Figure 3), burnout increases with the air number and thus with increasing temperature. No complete burnout is obtained. 4 EVALUATION OF MEASUREMENT RESULTS 4.1 COMPARISON OF MEASURED AND CALCULATED TEMPERATURES

In order to calculate the temperature values, a calculation program developed at 8 the ICT was used. It is based on the assumption of a thermodynamic equilibrium in the flame. Figure 4 gives a comparison between calculated and experimentally determined temperatures for NC and TNT. The measured flame temperatures are clearly below those calculated adiabatically. Radiation heat loss and the reaction kinetics not performing completely are the reason for this. The theoretical flame temperatures show in each case a maximum at substoichiometric conditions. An increase in air supply causes the degree of burning to rise and thus the quantity of energy released. Through the injection principle, more atmospheric nitrogen is entrained into the reaction zone at the same time and heated to the reaction temperature. This has an effect causing a heat reduction in the flame. As the supply of air increases, this effect becomes predominant and the flame temperature drops.

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The experimental reaction temperatures of the NC flames show a maximum shifted towards higher air numbers. This is an index for the influence of genuine flow and mixing effects. The atmospheric oxygen is not sufficiently mixed with its reaction partners (micromixture). Apart from a slight rise in temperature, at substoichiometric burning conditions, TNT shows no temperature dependency on the quantity of air supplied. The reason for this lies in the evaporation-controlled TNT burnout. Contrary to NC, TNT does not start burning until it has been subjected to longer heating in a melted condition. After its ignition, only as much TNT evaporates in each case 9 as corresponds to the recycled flame heat. The natural entrainment effect is proportional to the evaporation rate, i.e., surrounding air sucked into the incineration zone per reacting mass is constant. The quantity of incineration air supplied, therefore, has no effect on the flame temperature from a specific threshold value upwards (local stoichiometric mixture). 4.2 COMPARISON OF MEASURED AND CALCULATED CONCENTRATION PROFILES

With the calculation program mentioned in Section 4.1, the smoke gas concentrations depending to the air number, λ, can also be calculated. Calculation was carried out for NC and TNT incineration. The curves shown in Figure 2 show countercurrent tendencies between theoretical and measured NOx concentrations. The equilibrium calculations are always below the measured NOx concentrations. As the thermodynamic balance only describes the upper limit of a reaction under investigation, this means that NOx must already be contained in the initial material. This can only be explained through NO or NO2 being split off through heat in an initial reaction step. Not until a second reaction step is reached, the primarily formed NOx can be reduced. This reduction principally occurs at high temperatures (Figure 2a). We find here the slightest deviation between the measured values and the equilibrium concentrations. At high temperatures, the NOx molecules dissociate to atomic nitrogen and oxygen. These can either combine to form elementary nitrogen and oxygen, or recombine to form nitrogen oxides. According to this mechanism, the equilibrium concentrations form the minimum limit values of the NOx reduction. In the case of TNT burnouts as well, the NOx concentrations run contrary to temperature. Here, the mechanism described above can also be drawn into account by way of explanation.

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Investigation of exhaust gas products 4.3 NOx REDUCTION BY THE MIXING IN OF UREA

The high NOx emissions, in substances with nitro groups, make further reduction measures for NOx necessary. For this reason, the attempt was made to 10,11 achieve an NOx reduction by adding urea as proposed in literature. Urea decomposes into ammonia and isocyanic acid. The ammonia reduces NOx accord10 ing to the known NH2 mechanism. Isocyanic acid reacts with OH radicals to 11 form NCO, which is also in a position to reduce NO. These mechanisms only take place at temperatures between 1250 and 1300 K, as the reaction velocities are too slow below these values and as oxidation to NOx predominates above them. This is why the NOx minimum is shown in Figure 2b as being precisely within this temperature window. The NOx output is reduced by half through the addition of urea. 5 SUMMARY

The apparatus presented is suitable for investigating the burning behavior and toxic substance emission of solid and liquid waste materials on a laboratory scale. In particular, the following possibilities are provided: • • • •

The burning of sufficient quantities of substance under controlled conditions with adjustable air supply and ignition support facility The auxiliary ignition burner makes possible an investigation of substances difficult to ignite as well as low-calorie substances It is possible to make temperature measurements and gas sampling out of the flame zone and the smoke gases Simulation of thermal disposal units and toxic fires

The measurements carried out in this burning apparatus on NC and TNT as model substances for materials containing NO2 groups, produced the following results: •

The supply of air for incineration exerts a great influence on the reaction products: Incompletely oxidized burning products are obtained even at a high excess of air There is a minimum of NOx emissions at an air supply producing maximum temperatures

V. Gröbel, H. H. Krause, and V. Weiser

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1.

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The splitting off of NO and NO2 in an initial reaction stage results in very high NOx emissions which are clearly above the thermodynamic equilibrium The thermodynamic equilibrium concentrations of NOx cannot be reduced via technical incineration processes The admixture of urea shows an additional NOx reduction within the suitable temperature range of the urea mechanism (approx. 1300 K) 6 REFERENCES

W. Merz, H.-J. Neu, M. Kuck, K. Winkler, S. Gorbach, and H. Muffler, Fresenius Z. Anal. Chemie, 325, 449-460 (1986). 2. R. Denig, Fresenius Z. Anal. Chemie, 330, 116-119 (1988). 3. V. Gröbel, N. Eisenreich, H. Krause, and V. Weiser, ICT-Report 11/91 (1991). 4. Seventeenth Regulation on the Implementation of the Federal (German) Emmission Protection Law (Regulation on Incineration Installations for Waste and Similar Fuels, 17. BImSchV, Federal Legal Gazette, 64 30, pages 2545 to 2553, November 1990. 5. T. Härdle, H. Krause, and V. Weiser, 22nd International Annual Conference of ICT, p. 15-1 to 15-13 (1991). 6. V. Gröbel, V. Weiser, N. Eisenreich, and H. Krause, 23rd International Annual Conference of ICT, p. 15-1 to 15-15 (1992). 7. R. Günther, Publ. Springer-Verlag Berlin, Heidelberg, New York, Tokyo 1984. 8. F. Volk and H. Barthelt, ICT-Report 3/82 (1982). 9. V. Weiser, N. Eisenreich, and H. Krause, 22nd International Annual Conference of ICT, p. 101-1 to 101-14, 1991. 10. M. Jødal, C. Nielsen, T. Hulgard, and K. Dam-Johansen, Twenty-Third Symposium (International) on Combustion, The Combustion Institute, 1990. 11. J. A. Caton and D. L. Siebers, Combustion Sci. and Technol., p. 7-16 to 7-17 (1988).

H. Hammer and G. Rauser

45

Reduction of Pollution Through Hydrogenation of Carbon-containing Wastes

Hartmut Hammer and Gerd Rauser RWE - Gesellschaft für Forschung und Entwicklung mbH, Ludwigshafener Straße, D-50389 Wesseling, Germany

The hydrogenating liquefaction is a process which allows the recycling of mixed waste plastics into feedstocks and consequently means the reuse of matter. The process is developed in a 10 kg/h bench scale plant by the RWE \ Gesellschaft für Forschung und Entwicklung mbH. In a thermal pretreatment the feed is converted into a pumpable fluid. At the same time, the major amount of chlorine is split-off. The subsequent hydrogenation leads to a product similar to crude oil and is as such suitable as a feed for refineries.

1 INTRODUCTION

The increasing flood of wastes in connection with steadily decreasing space for deposition represents a growing problem for disposal. Material reuse of unavoidable wastes makes a contribution to the solution of the problem and leads equally to a welcome preservation of resources. Direct material recycling offers solutions of restricted applicability only to unmixed and clean plastic waste. Liquid and solid, dirty and mixed organic synthetic wastes can be transformed nearly unrestrictedly into hydrocarbons of high quality through reaction with hydrogen, the so-called hydrogenation. Development work in Wesseling was supported in a first phase - the laboratory phase - under the title “Reduction of Pollution through Hydrogenation of Carbon-containing Wastes” by the German Minister for Research and Development (BMFT) and the German state North-Rhine-Westfalia (Table 1).

46

Reduction of pollution through hydrogenation

Table 1: Project status Title:

Reduction of pollution through hydrogenation of carbon-containing wastes

Target:

Material use of organic chemical wastes

Support:

Federal Minister of Research and Technology Nordrhein Westfalen State

Table 2: Plastic waste use Procedure

Hierarchy of value

Product value (DM/t)

Regranulation

Use of the macromolecular structure

ca. 400-700

Hydrogenation

Preservation of the basic organic structure

ca. 300

Pyrolysis

Preservation of the basic organic structure

ca. 170

Gasification

Conversion to synthetic gas

<100

Combustion

Use of the heating value

<100

Deposition

Renunciation of heating or material value

ca. -100

The tests in the laboratory were promising. The above mentioned ministries were in support for a second phase of the development, the bench scale plant phase. Since September 1990, a continuously working bench-scale plant with a capacity of 10 kg/h is run to approach the introduction of the process in a technical scale. The costs are about 11 Million DM. 2 HYDROGENATION IN THE HIERARCHY OF VALUE OF THE PROCEDURES FOR UTILIZATION OF PLASTIC WASTE

In Table 2, the possibilities for utilization and disposing of plastic wastes are put in order of a hierarchy of value according to the laws relating to waste and the possibilities of material reuse.

H. Hammer and G. Rauser

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The quoted product values show the potentials of different procedures. For a complete estimation of the economy, the fixed and variable costs of respective production plants have to be considered in addition. Without doubt the most efficient possibility for the use of plastic waste is the direct reuse of plastic products. The macromolecular structure of the compounds is preserved almost entirely. The regranulation, however, is reserved to almost only unmixed and clean plastics. In practice, plastic wastes are typically very dirty and have variable composition. This requires additional washing and separation steps. For some of these mixtures a simple re-melting is possible, but only thick products of a low quality can be produced, therefore this procedure can only make a limited contribution to the disposal of plastic wastes. Plastics are thermally stressed during recycling by re-melting or regranulation. After a limited number of recycling steps their physical properties are changed by degradation so much, that products of fixed specification can no longer be produced. They must be disposed by another procedure. As a solution for the material recycling, the disintegration to gaseous and, especially desired, liquid hydrocarbons offers its service. For these products problems with commercialization are not envisioned because they can be returned into the material cycle via refineries (chemical recycling). Large-scale processes are not yet available. In principle, there are two options in manufacture of hydrocarbons from plastic wastes: hydrogenation and pyrolysis. For both methods, the development work is not finished yet. Gasification of plastic waste leads to a synthesis gas, out of which methanol or other chemicals can be synthesized. Commercial plants are not yet existent. For most plastic wastes, combustion is the only already available utilization possibility. Combustion destroys the organic structure, and only the heating value is used. More than 50% of plastic wastes are still land-fill disposed and as such removed from any utilization at the present time. 3 PRINCIPLE OF HYDROGENATION

Table 3 shows the principle of hydrogenation of plastics. Under conditions of hydrogenating liquefaction - short “hydrogenation” - the macromolecules of the plastics are decomposed thermally in the presence of hydrogen into liquid and gaseous fragments. Instable fragments react with hydrogen. They become saturated and build stable products (oil and gas). Thus the product quality is im-

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Reduction of pollution through hydrogenation

proved distinctively and an essential difference against the pyrolysis procedures is noted. The hydrogenation is regarded as an ecologically beneficial process: heteroatoms such as chlorine, oxygen, nitrogen, and sulphur are split-off and transformed into their hydrogen compounds. These can be handled and disposed with well-known means. The produced oil is largely free of these elements. This is an important quality criterion for subsequent petrochemical treatment. 4 INPUT MATERIALS FOR THE HYDROGENATION PROCESS

Using the high pressure technique, the input materials should be chosen according to economical considerations. In Table 4 possible input materials are summarized. As a matter of principle any organic synthetic waste can be fed into the hydrogenation process. Mixing with other materials, like rests of food in non-re-

Table 4: Input materials for hydrogenation process Solid mixtures of plastic waste - mixed plastic wastes (e.g., fraction from plastic recycling plant, cable casings, wastes from packing materials, commercial plastic waste - compound material (e.g., from automobile scrap, carpet wastes, etc.) Potential quantity > 2Mt/year (Germany) Liquid chemical wastes - waste dyes and varnishes - production wastes Restriction: portion of non-participating substances small

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turnable dishes, rests of soil on farming-foils, or liquid rests in detergent bottles is insignificant for the course of chemical reaction. As input materials from the group of solid wastes are of interest especially the mixed plastic wastes and composite plastic materials, which until now had only a restricted suitability for recycling. Among these are plastic waste fractions from the Dual System Deutschland (DSD, Yellow Ton, Green Point), plants for the recovery of raw materials (plants for sorting of wastes), and from separate pick-up by collecting in sacks. In the field of industrial activities, there are mixed or dirty production wastes such as edge-pieces and imperfect batches generated by the plastics processing industry or the packing branches, which are usable in this process. These wastes are not suitable for regranulation. Composite foils from paper and plastic can also be used for reaction with hydrogen. Furthermore, wastes from cable production and dismantling, rests of carpets and plastic fractions from automobile scrap are prospective input materials for hydrogenation. Liquid chemical wastes such as paints and varnishes and wastes from chemical production plants are suitable for hydrogenation too. Restriction for all input materials is that the portion of non-participating substance should be small. This portion consists of external adhering inert materials; fillers such as chalk, kaolin, quartz, and soot; pigments such as titanium dioxide and ferric oxide; and non-participating substances in composite materials such as metals. The hydrogenating process should be fed with such wastes, which mainly lead to valuable products, namely to hydrocarbons. 5 DEPENDENCE OF HYDROCARBON YIELD ON INPUT MATERIALS

Two essential quality criteria of the input materials determine the yield of oil: •

the percentage of chlorine, oxygen, and nitrogen in plastics. Pure PVC, for example, is mainly converted by hydrogenation to hydrogen chloride, a product which is not desired, but at least can be recovered. This effect is not so profound with polyesters, polyamides or duroplastics such as phenolic resins.

50

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Reduction of pollution through hydrogenation

inorganic additives such as pigments, fillers, and active agents but also non-plastics in composite materials and adhering dirt influence the potential oil yield.

Practical consequences of these influences can be found from Table 5. Yields of liquid hydrocarbons and pumpable residues are given for some typical solid wastes. Table 5: Yields from autoclave hydrogenation tests (wt%) Liquid hydrocarbons

Pumpable residue

PE/PP

90

3

Nylon

70

22

Insulating foam

70

22

Needle felt

64

12

Carpet

36

28

Rubber

20

60

-

83

Plastic from garbage (cleaned)

65

18

Plastic from garbage (not cleaned)

50

27

Car shredder waste (light fraction)

The remaining mass of material (difference to 100 %) consists of gaseous hydrocarbons and hydrogen-containing compounds such as HCl, H2O, etc. Some solid wastes represent suitable input materials which requires only a limited pretreatment or none at all. For other materials, e.g., the light fraction of car shredder material, sufficient enrichment of the plastics component is needed. Similar results were achieved for a multitude of other materials. Included are such materials as plastics from the utilization of waste paper and vulcanized plastics. 6 QUALITY OF PRODUCED HYDROCARBONS

For subsequent treatment and use of the hydrogenation products, it is of importance to compare their quality with other, typically used, raw materials.

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Table 6: Comparison of hydrogenation product with crude oil Crude oil (Kirkouk)

Hydrogenation products from an authentic plastic waste

(wt%

(wt%)

-

2.8

1.4

10.6

Non-aromatic hydrocarbons (<480 C)

62.0

58.5

Benzene, toluene, xylenes

1.6

3.2

12.0

4.8

23.0

20.0

Methane Gaseous hydrocarbons (C2-C4) o

o

Higher aromatics (<480 C) o

Residue (>480 C)

In the hydrogenation reaction of thermoplastics, e.g. polyethylene, the aliphatic structure of the hydrocarbon is preserved, and it is obvious that these materials are especially suitable for subsequent petrochemical treatment. The hydrogenation products can be stored without any difficulties because as saturated hydrocarbons they are almost inert. Plastics hydrogenation products are compared to crude oil in Table 6. The hydrogenation product, obtained from a plastics mixture from a garbage separation, has a smaller amount of higher aromatics, and a larger of gaseous hydrocarbons as compared to crude oil. For a subsequent petrochemical treatment the high amount of aliphatic compounds is of special importance. The hydrogenation products are already refined to some extent which means that the heteroatoms, which especially complicate a subsequent treatment, are largely eliminated. Therefore, hydrogenation products can easily be processed by the common refining and chemical procedures in a crude oil refinery. In addition, the choice of suitable reaction conditions for the hydrogenation allows to reach well-aimed high amounts of gasoline and middle oil in the product mixture.

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Reduction of pollution through hydrogenation

Table 7: Solving of chemical engineering problems by visbreaking Chemical engineering problems: • pumpability of input materials • corrosion by hydrogen chloride • deposition of ammonium chloride Solving problems by pressureless thermal pretreatment: • breaks long chains • lowers viscosity (viscosity proportional to M3.4) • extensive decomposition of Cl-containing compounds to HCl • after pretreatment not more corrosive than bitumen • after pretreatment no deposition of NH4Cl • easily filterable before entering the high pressure zone • limitation to small amount of plastics in the reaction mixture is eliminated • up to 100% plastics becomes liquid and pumpable

7 SOLVING OF CHEMICAL ENGINEERING PROBLEMS BY VISBREAKING

The special problems for the chemical engineering such as corrosion by hydrogen chloride, deposition of ammonium chloride, and pumpability of input materials can all be solved by a visbreaking step. Visbreaking in the oil refining is a well-known, thermal process at low pressure by which distillates are obtained from distillation residues. Use of visbreaking as the first step of hydrogenation allows one to capitalize from the following advantages (Table 7): • •

Long chains of macromolecules are split into smaller fragments The viscosity is proportional to the molecular mass, M,3,4 meaning that decrease in molecular weight by half leads to a lowering of viscosity by about one tenth of its original value

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Figure 1. Flow chart of plastic hydrogenation.

• •

• • •

The organic chlorine, in the input materials, is largely eliminated in gaseous form and can be neutralized in a scrubber The input material for hydrogenation, discharged from chlorine by visbreaking, is comparable in corrosiveness to distillation residues (bitumen), which are normally used as premixing oils. The requirements to the materials for the high pressure part of a plant are essentially diminished, and costs of investment reduced The deposition of ammonium chloride and resulting problems are avoided by separating the chlorine before the hydrogenation step The product from visbreaking can be filtered before entering the high pressure part of plant. Coarse, inert particles can be separated and disturbances of the inlet and outlet system avoided The former limitation to 20-25 wt% of plastics in the input material is no longer relevant. The portion of plastics in the input material can be increased to 100 wt% without any difficulties in pumping.

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Reduction of pollution through hydrogenation

8 PROCEDURE OF THE CONTINUOUS PROCESS

The flow chart of plastic hydrogenation is shown in Figure 1. Freed from non-participating materials such as sand, glass, and metal, plastic wastes are preshredded and fed to the intermediate stock. In the thermal pretreatment process, the plastic waste is suspended in as little oil as possible and subjected to the effect of heat. As a result, the viscosity of the suspension is decreased up to an extent that the reaction product becomes liquid and pumpable by high pressure pumps at elevated temperature. At the same time, hydrogen chloride (HCl), from PVC component, is largely split off. The produced hydrogen chloride is sent directly to the scrubber for neutralization and does not affect the high pressure section of the plant. The hydrogenation preferably occurs in the temperature range between 300 and 500oC at a pressure of 20-40 MPa. The reaction time may vary from 15 minutes to several hours. The reaction parameters depend on quality of the input material and the desired product. The usual target is the production of a maximal quantity of oil, poor in heteroatoms, and a minimization of hydrogenation residue. With this target in mind, the hydrogenating conversion of plastic wastes may be done in a multistage reaction system and with or without a catalyst. The separation of liquid and gaseous phase occurs in a system of a hot and cold separator. Hydrogen chloride, from the chlorine residues which remain in the mixture after pretreatment, ammonia, and other gases in mixture with gaseous hydrocarbons enter a scrubber and are neutralized. These gases can be separated by conventional refining processes into organic and inorganic ingredients. C1-C4 gases can be used as raw material for hydrogen production or as fuel gas. The inorganic ingredients are converted to neutral salts. The liquid hydrocarbon fraction can be processed to chemical raw materials by processes conventionally available in petrochemistry such as refining and distillation. It generally means that it is possible to obtain feedstock for olefin plants and produce specified fuels and fuel oils. Inorganic non-participating substances cannot be converted by hydrogenation. They leave the process, in concentrated form, with the hydrogenation residue. Even, if there was no possibility for subsequent treatment of these residues, the space needed for their deposition decreases by 90% as compared to the original volume of the waste.

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The technical feasibility of the process concept was tested in a bench scale plant which allowed a continuous processing of about 10 kg/h. 9 CHANCES FOR IMPLEMENTATION OF PLASTIC WASTE HYDROGENATION

For further development of the hydrogenation technique for treatment of plastic wastes, the following points are essential: • • • •

Suitable plastic fractions must have long-term availability Necessary disposal credits for hydrogenation of plastic wastes must not compete with “cheaper” regulations in neighboring countries (refuse tourism) A central, large hydrogenation plant is preferably needed which should be attached to a refinery The visbreaking technique should be used

The first mentioned point requires political stability for the legislation in the field of wastes. The validity of item 2 and the realization of the items 3 and 4 allow for a positive prognosis of the realization chances of a large scale plastic waste hydrogenation plant. The commissioned study for the valuation of the hydrogenation technique on a large scale, based on the hydrogenation results in the Wesseling bench scale plant, will soon be completed. After presentation of the results of this study, it will be possible to give a more detailed estimation of chances for implementation of the hydrogenation of plastic wastes. ACKNOWLEDGMENT

This project was supported by the German Minister of Research and Technology and by the German State North-Rhine-Westfalia, support number: 01 ZH 89125 respectively 332-25-37.

CONTENT Polymer Waste From Nuisance to Resource H. H. Krause and J. M. L. Penninger Waste Disposal Logistics - a Prerequisite for Effective Recycling U. Hansen and A. Rinschede Fast Identification of Plastic Materials by Near-Infrared Spectroscopy N. Eisenreich, H. Kull, and E. Thinnes Possible Applications of Pyrolysis Technology in the Treatment of Hazardous Wastes and Recovery of Valuable Materials M. Teller Reduction of Pollution Through Hydrogenation of Carbon-containing Wastes H. Hammer and G. Rauser Recycling of Plastics by Hydrogenation in Slurry Phase M. Gutmann, M. König and M. Marks Powder Coatings from Recycled PET F. Pilati, C. Stramigioli, M. Toselli, S. Torricelli, and M. Dinelli Screening of the Degradability of Plastic Materials in a Composting Medium A. Pfeil Investigation of Exhaust Gas Products in the Thermal Disposal of Waste Munition Using Nitrocellulose and TNT as Examples V. Gröbel, H. H. Krause, and V. Weiser Alkaline Pressure Hydrolysis of Energetic Materials G. Bunte, T. Hirth, H. H. Krause, and N. Eisenreich Continuous Determination of Volatile Organic Breakdown Products of Propellants in Water G. Hambitzer and M. Joos The Disposal of Propellant Components Containing Heavy Metals W. Böke and G. Hambitzer Index

1 7 21 31 45 57 67 85 93 105 117 127 135

H. H. Krause and J. M. L. Penninger

1

Polymer Waste from Nuisance to Resource

H. H. Krause Fraunhofer-Institut für Chemische Technologie - ICT, Postfach 1240, D-76318 Pfinztal (Berghausen), Germany

J. M. L. Penninger Sparqle International, B.V., Boerhaavelaan 18, 7555 BC Hengelo, The Netherlands

Recently, BASF announced the start-up of a pilot-plant, costing 40,000,000 DM, which will convert 15,000 tones per annum of mixed polymer waste.1 A polymer feed is thermally treated in a polymer melt at 300°C. This mild thermal hydrocracking lowers the viscosity of the melt and PVC components in the waste split-off chlorine in the form of hydrochloric acid. The melt is steam-cracked at 400 to 450°C and converted into a mixture of usable hydrocarbons, such as fuel gases, olefins, and aromatics. A high quality naphtha is recovered after distillation in a 45 % yield; a residue of 5 % is retained which contains inorganic fillers and pigments originally prevailing in the polymer products. BASF expects to have a commercial plant ready for start-up by 1996 with a capacity of 300,000 tones per annum. This amounts to approximately 10% of Germany’s mixed polymer waste. The company will save 125 DM worth of raw materials for every tone of converted polymer waste. This “back-to-feed-stock” scheme is only one example in many approaches currently under investigation to mitigate the burden on the environment of polymeric products which are discarded after use. This chemical recycling suits

2

Polymer waste from nuisance to resource

particularly well the polyolefinic wastes, such as polyethylene, polypropylene, polystyrene. These polyolefins command the largest share of the polymer market, but also many other polymers of different constitution find market appreciation and will eventually be wasted. It is questionable to assume that only one technology can give the right solution for all sorts of polymer waste. Let us consider also the problem of waste PVC or waste polymer with a high PVC content. Simple dehydrochlorination, as follows from the thermal cracking in the first stage of the BASF process will not suffice since the product of this thermal treatment will be a coke-like solid, with a relatively low H-to-C ratio and a non-acceptable concentration of chlorine. For these mixed polymer wastes with a high PVC content other technologies will be needed which are better tailored to handle large quantities of HCl and which value HCl as an essential by-product. Thus, the chlorine cycle can be closed by returning HCl into the manufacturing cycle, rather than discarding it into the environment as salts. An example is the fluidized-bed steam gasification of PVC waste, with recovery of 2 inorganic fillers. Another example is the catalytic conversion of polymer waste in a fluidized 3 bed of ZSM-type catalyst, which leads to fuel gases and light aromatics. Both routes intend to circumvent the formation of coke, which is typical for earlier fluid-bed thermal cracking schemes. The perception on how to deal with the massive quantities of polymer wastes which are forced on society once the polymer products are no longer fit for use, is gradually changing. Initially, these products-without-use were only treated as nuisance, because as a waste component in conventional land-fills they were difficult to dispose of. Unlike municipal waste, which consists for the larger part of biodegradable matter, the synthetic polymers have no, or at best a poor biodegradable characteristics. The natural composting processes are thus not powerful enough to convert these materials within an acceptable time span. Furthermore, the composting proceeds through stages where intermediate products are formed. These components can be water-soluble, are leached from the land-fill, and may pollute aquafers. Synthetic polymers with enhanced biodegradable characteristics have been formulated as composite materials with biopolymers. Indeed these composites visually disappear readily when exposed to natural composting environments but this is, at least partly, optical illusion only. The biopolymer degrades

H. H. Krause and J. M. L. Penninger

3

and releases the synthetic polymer, which due to its high level of dispersion prevails as microscale particles which are not visible with the naked eye but are still left in the environment. Nowadays, polymers at the end of its useful live are less and less regarded as the beginning of a waste problem but more and more as an unconventional raw material for new products and services. The new vision on the utilization of fossil resources, called “sequential use of resources” attempts to stretch the reserves of the fossil resources by applying the resource in a time-sequential series of products or services. This chain of sequential use starts with the production of the resource such as natural gas, oil or coal as examples. These resources are committed first to supplying of the raw materials for the manufacturing of chemicals, such as the monomers ethylene, propylene, styrene, adipic acid, diols, vinylchloride, etc. These monomers are subsequently processed into polymers, which, in turn, are transformed into a large variety of products we are familiar with. Once these products do not function anymore as required these products are not discarded but instead collected and prepared for use in subsequent sectors of our economy. The scope of applications for products derived from “prior-used” materials faces several limitations as a result of material requirements, purity specifications, handling characteristics in manufacturing processes, separation problems, logistic problems, etc. The conversion of prior-used material into new material and products requires energy, investment costs, operational costs. The direct material recycling of polymers, which would involve only the collection, size reduction and extrusion of mixed prior-used polymers is probably the most attractive first option in sequential use. But this applies only to a minor fraction of the total amount of polymer waste. Most applications require separation of a polymer mixture, cleaning, etc., and consequently, require consumption of energy and costs. Repeated material recycling suffers furthermore from a steady deterioration of material properties; eventually properties do not match even the most relaxed specifications, and, consequently, the polymer material must be removed from the materials cycle. At this point, a more dramatic change in the sequence is needed. One option could be the conversion of these materials back into polymer feed stocks. Depending on the nature of polymer and purity of the available waste this conversion may aim directly at depolymerization, with recovery of monomer mate-

4

Polymer waste from nuisance to resource

rial. This is a most desired situation, after material recycling, because virgin polymers with top specification can be readily produced in a subsequent polymerization. It is recognized however that this condition is met only incidentally, e.g., the depolymerization of cleaned PET bottles. The bulk of used polymers however prevails as mixture and this would require conversion technology which is more robust and can deal with impurities. One example is the thermal pyrolysis of mixed polymer waste, mostly consisting of poly-olefins, as is highlighted in the introduction to this chapter. The products are excellent feed stocks for the formation of virgin olefins, hence polymeric material, not suited anymore for direct material recycle, is thus chemically recycled and repositioned upstream in the chain of sequential use. Chemical recycling, as a transformation of chemical nature, is subjected to the principles of thermodynamics. This means in practice that each chemical recycle carries a penalty in free energy. This is compensated by energy input, e.g., by converting part of the products or by converting virgin fossil fuels. The question arises now to what extent chemical recycling is useful, considering the repeated thermodynamic losses. This problem is schematically illustrated in Figure 1. Given the enormous market for energy, e.g., for power and transportation of fuels, on the one hand, and the supply of this demand through combustion of virgin fossil resources on the other, the demand on fossil resources for polymer production is only marginal at best. Polymers should therefore be regarded as fossil fuels-in-disguise. Its ultimate destination as an energy source is only temporally restrained, while it provides mankind with a special service as a useful material. Chemical recycling thus makes sense only if the waste polymer in itself is a more attractive raw material for the production of virgin monomers than the primary fossil resource. “More attractive” can be regarded as conversions which consume less energy and/or require less capital resources. A chemical recycling scheme which consumes more energy for the production of virgin monomers or requires a larger capital cost than production from primary fossil resources is thus not a desired option. In this case use of the mixed polymer waste for the production of energy with a high thermodynamic efficiency should be the sensible option; the fractional supply of power which results from polymer-to-energy conversion would otherwise be generated by combustion of an equivalent quantity of primary fossil resources.

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Figure 1. Thermodynamic losses in plastic recycling.

And those resources could have been converted with ease into virgin polymers! Chemical recycling therefore should be evaluated in the total context of supply and demand for energy, such that the consumption of primary fossil resources in total is minimized. This is the challenge to nowadays management of resources. The management of polymer waste utilization is part of a larger and timely effort of how to use in an integrated approach our world’s limited non-renewable resources. This should be done so that fulfilling mankind’s request for materials and energy are met with lowest depletion of resources and lowest burden on environment. With this intention in mind this book was composed as a collection of approaches to technical solutions for the chemical recycling, ultimate mineralization, and the re-use of specific polymer wastes. REFERENCES

1. Chemisch Weekblad, March 12, 1994. 2. Akzo patent application (1994). 3. Arco process, Chem. Engn. News, Febr 14 (1994), p. 27.

Conversion of Polymer Wastes & Energetics Editors H. H. Krause and J. M. L. Penninger

ChemTec Publishing

Copyright © 1994 by ChemTec Publishing ISBN 1-895198-06-2 All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means without written permission of copyright owner. No responsibility is assumed by the Author and the Publisher for any injury or/and damage to persons or properties as a matter of products liability, negligence, use, or operation of any methods, product ideas, or instructions published or suggested in this book.

Printed in Canada ChemTec Publishing 38 Earswick Drive Toronto-Scarborough Ontario MlE lC6 Canada Canadian Cataloguing in Publication Data Main entry under title: Conversion of polymer wastes & energetics Includes bibliographical references and index ISBN 1-895198-06-2 1. Plastics - Recycling. I. Krause, H. H. II. Penninger, Johannes M. L., 1942TP1122.C6 1994

668.4’192

C94-932195-8

M. Gutmann, M. König, and M. Marks

57

Recycling of Plastics by Hydrogenation in Slurry Phase

Manfred Gutmann, Michael König, and Manfred Marks uve - Institute of Technical Chemistry and Environmental Protection Ltd., Rudower Chaussee 5, D-12489 Berlin, Germany

Hydroliquefaction is the most useful process for the recovery of organic compounds from different plastic waste materials. In relation to temperature, pressure, and residence time, the yield of liquid organic material from used plastics was investigated. With special types of catalysts adopted to the type of plastic, it was possible to achieve up to 80% of liquid organic material from plastics. Several examples of plastics conversion in the presence of catalysts and different solvents are presented.

1 INTRODUCTION

After the USA and Japan, the Federal Republic of Germany is the third largest plastics producer in the world. The yearly production is, at present, more than 9.1 million tons of which approximately 5 million tons is subjected to further processing to plastic products within the Federal Republic of Germany. Due to a wide variety of material properties and shaping characteristics, plastics are found in nearly all product lines. In Western Europe, approximately 25% of plastics are used in construction, approximately 21% in packaging, and approximately 15% in the electrical industries. This is followed by the pigment, adhesive, and paint industries with approximately 10%, the furniture and vehicles industry with approximately 7% and the household goods industry with approximately 3%.1 The consumption of plastic products in the Federal Republic of Germany results in approximately 2.5 million tons of plastic wastes produced a year of which approximately 1.3 million tons is local/household waste (Table 1). Only 20% of this quantity is processed at present and the predominant amount (50%)

58

Recycling of plastics by hydrogenation in slurry phase

is either deposited in landfills or incinerated (30%). Table 2 shows the composition of this plastic waste. No other material has, at any time, influenced each field of our live so much as the approximately 50 types of plastics which we deal with every day. For the most part (90%), however, it is the standard plastics polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC), and polymerisates of styrol which can be found almost everywhere. 2 RECYCLING SITUATION

2

The present legal regulations, determining the treatment of wastes, give preference to recycling over incineration or land-filling as the possibly least damaging to the environment due to the fact that the usable materials are brought back into the circle of production. Table 1: Plastic waste produced in Federal Republic of Germany (Federal Statistics Office, 1987) Household waste

1.4 mil. t

1.1 mil. t

tipped

Industrial waste

0.4 mil. t

0.5 mil. t

incinerated

Shredder waste

0.3 mil. t

0.3 mil. t

internal recycling

Total

2.1 mil t

0.1 mil. t

regranulated

0.05 mil. t

products from old plastics

Table 2: Composition of plastic waste1 Polyolefins (PE, PP)

60-65 wt%

Polystyrene (PS)

15-20 wt%

Polyvinylchloride (PVC)

12-17 wt%

Other plastics (polyurethane, polycarbonate, etc.)

approx. 10 wt%

Impurities

3-10 wt%

M. Gutmann, M. König, and M. Marks

59

Table 3: Comparison of product yields (PE/PP) in wt%6 Pyrolysis

Hydrogenation (HC)

Methane

23.1

1.1

Ether(s)

19.1

-

HC gas C2 - C4

20.5

7.0

HC liquid (aliphatic)

4.9

87.7

Benzene

16.6

0.2

Toluene, xylene

7.8

0.2

Higher valent aromatics

8.1

-

-

3.8

HC products with higher b.p.

The conservation of resources is an important function of this recycling process. In practice, however, plastic waste3 is heavily soiled and in various states of composition. Due to the immiscibility of various types of plastic in the borderline range of polymeric phases, the reprocessing methods, practiced at present for mixed plastic materials from local/household waste collections, produce products which are of a low or poor quality. Even if sophisticated reprocessing is applied and quality-improving additives used, it is still not possible to make high-quality products from mixed plastic waste. High quality materials can only be produced after separation of plastic components allowing for a reuse of a material of the same type by similar methods and range of applications. Plastics must not constitute a disposal problem. In the reprocessing methods generally applied nowadays, the molecular structure is, for the most part, retained, whereas it is deliberately destroyed in thermal decomposition processes. It is the aim of material recycling to design a process whereby the secondary raw material obtained can either be fed back into current reprocessing (regranulation), or the synthetic raw material itself can be regained via chemical conversion without restriction of quality.

60

Recycling of plastics by hydrogenation in slurry phase

Figure 1. Principles of reactions - comparison.

3 CHEMICAL REPROCESSING

Via chemical/physical processes, chemical reprocessing regenerates the chemical prestages and building blocks from which plastics are made. We are here for the most part concerned with gaseous or liquid carbon/hydrogen compounds. The possibilities of chemical reprocessing (hydrogenation, hydrolysis, alcoholysis, glycolysis, and pyrolysis) are still in the development phase, so that they cannot yet be economically applied on a large scale. The chemical breakdown of plastics to low-molecular products offers the possibility of decomposing plastics to liquid and gaseous hydrocarbons. In this context we have basically three possibilities: • • •

gasification pyrolysis hydrogenation.

Whereas, in the hydrogenation of mixed plastic wastes (PE/PP), chain-formed liquid hydrocarbons are for the most part produced, gaseous hydrocarbons and aromatics6 are predominant in pyrolysis (Figure 1 and Table 3). The product composition is controlled via the reaction conditions. By contrast to

M. Gutmann, M. König, and M. Marks

61

hydrogenation, which is carried out under increased hydrogen pressure, pyrolysis merely requires exclusion of oxygen to avoid a burning of the products, although considerably higher temperatures are needed than in the case of hydrogenation. Beside the product composition, the product quality is decisively influenced by varying reaction conditions. Under the conditions of hydrogenation, the macromolecular chains are broken down and the fragments saturated by hydrogen. During the course of this process, heteroatoms such as chlorine, nitrogen, oxygen, and sulfur are transferred into the corresponding hydrogen compounds. Contrary to pyrolysis, according to the present state of investigations, the formation of new compounds problematic to the environment such as polycyclic aromatic hydrocarbons can be excluded as regards the hydrogenation pro3,4,5 cess. Table 4: Hydration to 95% liquid and gas production and 5% residue at 70% VR basic load7 Limit parametrs of input product mixture Solid products

5%

Particle size

1 mm

H2O content

10%

easily boiling HC

15%

Alkali salts

2%

Original chlorinated compounds

2%

Original Si compounds

50 ppm

PCB

20 ppm

For the use of comparable polyethylene/polypropylene mixtures, an approx. 90% oil yield is obtained in pressure hydrogenation, this yield value still being at approx. 65% in the case of plastic fractions from household waste separating installations. Table 4 shows the preferred composition for a real (i.e.,

62

Recycling of plastics by hydrogenation in slurry phase

Figure 2. Conversion dependency of used cable materials from temperature (p = 10MPa, t = 30min, plastic/tetralene = 0.1).

practical) initial plastic mixture for slurry phase hydrogenation, as based on laboratory and technical trial results. 4 PRESSURE HYDROGENATION OF PLASTICS

Making use of the principal advantages of reaction technology in the hydrogenation of plastics, we have, as a specific objective, aimed in our work at a reduction of the reaction parameters such as pressure, temperature, hydrogen consumption, and catalyst application with simultaneous quantitative breakdown of hydrocarbons, containing heteroatoms, to saturated paraffins and plastic 8 monomers as a desired result. The following plastic wastes, i.e., polyvinylchloride (PVC) cable waste, polyethylene (PE) cable waste, used “Trabant” car bodies (polyester- and phenolic resins) were mixed with old motor oil, industrial waste oil, and chlorobenzene from waste oil refining processes, and subsequently subjected, in autoclaves, to

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63

Figure 3. (a) Products from hydrogenation of used PE-cable material, (b) Products from hydrogenation of used PVC-cable material.

temperatures up to 500oC and pressures of 20 MPa using disposable catalyst system of slurry phase hydrogenation. First of all, comprehensive characterization of the waste oils, as well as elementary analysis and extractive characterization of the plastic materials involved, was carried out. The PVC cable waste was prominent due to filler contents of up to 40%. By comparison, therefore, investigations were also carried out with pure PVC granules. The temperature dependencies of the conversion processes of PE and PVC in the presence of tetrals are shown in Figure 2. No significant change in the o plastics was observed below 450 C. In the case of polyethylene, temperatures of o 500 C were necessary to obtain a complete conversion.

64

Recycling of plastics by hydrogenation in slurry phase

Figure 4. Structure type analysis of oils from hydrogenation of used PE and PVC.

Apart from the tetraline, other solvents were also tested for their efficiency in hydrogenation (Figure 3). It was found that reactive solvents such as methanol or tetrahydrofuran offer better hydrogenation convertion than the tetraline. The preliminary results need further confirmation. Using semi-preparative HPLC, the oils from plastics hydrogenation were investigated regarding their composition. Apart from the expected aliphatic, aromatics and hetero-compounds are also produced in the hydrogenation of PE and PVC waste cable material (Figure 4). The individual analysis of the fractions initially separated by HPLC allowed to identify decaline and methyl-indene in PE hydrogenation product apart from the homologue series of alkanes. In the case of the single-ring aromatics, benzene homologues could be identified in the first place. The content of hetero-compounds in the oil from the PVC hydrogenation should probably results, to a considerable degree, from plasticizers and organic additives of the plastic which were not hydrogenated. The present results, regarding the breakdown of plastic waste materials via hydrogenation, using reactive solvents (donor effect), show the possibility of reclaiming chemical raw materials which in turn permits their resynthesis to new plastic without a loss in quality (Figure 5).

M. Gutmann, M. König, and M. Marks

65

Figure 5. Products of polymeric conversion.

Through selection of the composition of plastics mixture and reaction conditions of the slurry phase hydrogenation, the yield and distribution of desired products and the paraffin/aromatics composition is controlled. 5 SUMMARY

The process of pressure hydrogenation offers maximum flexibility as a chemical recycling method for plastic wastes. A knowledge of the waste mixture composition allows optimization of the hydrogenation products to be tailored for use in polymer synthesis or as petrochemicals. Presorting of the plastic wastes by type allows for use of the recycled product from this process for production of new plastics or other basic organic chemicals with quality identical to that of newly synthesized materials. ACKNOWLEDGMENT

The authors thank the State Minister for Research and Technology for financial support of this work within the scope of project 30FJ1003B5. 1. 2. 3. 4. 5. 6. 7.

REFERENCES

K. O. Tiltmann in Recycling betrieblicher Abfälle, WEKA-Fachverlage, 1990. Abfallwirtschaftsgesetz, Paragr. 3, Abs. 2. (1986). W. Löffler, Energiewirtschaftliche Tagesfragen, 4, 219-221 (1989). D. Fischer in Handbuch der Altlastensanierung, 3(6), Kap. 5.4.3.5.2. (1989). R. Schaaf, Müll und Abfall, 18, Heft 10, 387-390 (1986). J. Korff and K.-H. Keim, Erdöl, Erdgas, Kohle, 105/5, 223-226 (1989). Versuchsbericht der GfK, CLG, und ICT (1980).

66 8.

Recycling of plastics by hydrogenation in slurry phase Sachstandsbericht zum Vorhaben, Grundlagenuntersuchungen zum thermischen und hydrierenden Abbau von hochsiedenden heteroatomhaltingen Kohlenwasserstoffgemischen und Abprodukten, 2 (1991) an das BMFT (BEO-30FJ1003B5).

G. Bunte, T. Hirth, H. Krause, and N. Eisenreich

105

Alkaline Pressure Hydrolysis of Energetic Materials

G. Bunte, T. Hirth, H. Krause, and N. Eisenreich Fraunhofer-Institut für Chemische Technologie (ICT), Joseph-von-Fraunhofer-Straße 7, D-76327 Pfinztal 1, Germany

Due to the reduction of armament and especially to the German reunification we are met by the objective of the disposal of energetic materials. In the next few years there will be approximately 40,000 t of energetic materials to dispose off. Environmentally friendly disposal methods available for different propellants, explosives, and pyrotechnics are urgently needed. The energetic polymer nitrocellulose is the main component of gun and rocket propellants. One method to dispose off nitrocellulose-containing propellants is the combination of a rapid chemical destruction by pressure hydrolysis and the biological degradation of the reaction mixture. The study describes the results of pressure hydrolysis of different gun and rocket propellants. Under alkaline conditions (propellant to NaOH ratio 2.3:1; reaction temperature 150oC; pressure below 30 bar) biological degradable reaction products were formed. The main products in the liquid phase were simple mono and dicarboxylic acids. Dependent on the reaction conditions 30-50 % of the nitrogen content of the propellants was transformed to nitrite and nitrate. The gaseous nitrogen containing products were N2 (16-46%), N2O (2-23%), NOx (0-5%). Overall 40-60% of the propellant nitrogen was transformed to gaseous products. In the solid residues a nitrogen content between 2 and 9% was found. The residues were mostly due to additives used in propellant manufacturing. In the case of nitrocellulose, the pressure hydrolysis below 30 bar and reaction temperature of about 150oC is sufficient.

1 INTRODUCTION

Old ammunition found in the stocks of the former East German army (NVA = Nationale Volksarmee) amount to approximately 300,000 t. The amount of energetic material contained here is estimated at approximately 40,000 t. The energetic materials consist of gun and rocket propellants, high explosives, and pyrotechnic compositions. The main component of gun and rocket propellants is the energetic polymer nitrocellulose. The high explosives consist

106

Alkaline pressure hydrolysis of energetic materials

mainly of TNT and the nitramines RDX and HMX. The composition of the pyrotechnic ammunition is very complex. For example, NVA-ammunition, to form artificial smoke, was manufactured from Lindan residues, containing a mixture of hexachlorocyclohexanes with alumina. The disposal of these HCH compositions is a serious hazardous waste problem. The objective is to develop specific, environmentally friendly disposal methods for different groups of energetic materials and pyrotechnic compositions. The disposal processes must be: • • • •

safe technically feasible environmentally safe (in respect to soil, water, air) and economic in operation.

The disposal processes are dependent on the composition and thus on the chemical properties of different materials involved. From a view of quantity the disposal of nitrocellulose-based materials (NC) (25,000 t of gun and rocket propellants) is of highest priority. The most common method to dispose off energetic materials is incineration. Different incineration systems either rotary kilns or fluidized-bed incinerators are in use. On incineration of energetic materials high amounts of air pollutants are evolved. Especially, the emission of NOx is a serious problem which requires additional dinox stages to meet the German legal emission limits. Therefore, incineration of energetic materials needs the cost-extensive investment of fluid gas purification. Furthermore, incineration plants do not have an acceptance of general public regardless of technical questions. Hence, alternative disposal methods for energetic materials are highly requested. One way to dispose off nitrocellulose-based propellants is a combination of chemical and biological disposal. This concept means elimination of an explosive character of the energetic material by a simple and cheap chemical reaction producing a biodegradable reaction mixture to study the feasibility of this technique. 1,2 It is known that nitrocellulose can be destroyed by alkaline hydrolysis. The application of this method to complex gun and rocket propellants has to be tested. Preliminary experiments have shown that alkaline hydrolysis of gun propellants under normal pressure conditions needs several hours to decompose the propellant. The alkaline hydrolysis (under pressure below 20 bar) shows

G. Bunte, T. Hirth, H. Krause, and N. Eisenreich

107

comparable results with a reaction temperature of about 150oC in times shorter by a factor of 8-10. To study the feasibility of pressure hydrolysis, as a chemical step in the chemical/biological disposal of NC-based energetic materials, we in3 vestigated the pressure hydrolysis of 5 realistic gun and rocket propellants. Hereby we have to characterize the reaction products to assess the biodegradability of the reaction mixture and also the overall pollutant emission, especially in comparison to incineration techniques. 2 EXPERIMENTAL 2.1 MATERIALS

Five gun and rocket propellants were studied. The compositions are given in Table 1. The propellants had different sizes in the form of sticks and cylinders. To get comparable results, we granted the propellant down to a grain size of 1-5 mm. P1 is a typical single-base propellant the others are so-called double-base propellants. In the case of P5, we found changing amounts of TNT as substitute of DNT. Table 1: Composition of gun and rocket propellants Component

P1

P2

P3

P4

P5

NC

97

80

57.5

57

56

NG

-

14

26.7

28

26

DNT

-

2

8.5

11

9

Centralite

-2

2

2.9

3

2.9

DPA

1

-

-

-

-

Graphite

-

1

-

-

-

Potassium salt

-

1

-

-

-

Lead

-

-

1.4

-

-

Dibutylphthalate

-

-

-

-

4.5

Vaseline

-

-

1

1

1

NC - nitrocellulose, NG - nitroglycerin, DNT - 2,4-dinitrotoluene, DPA - diphenylamine

108

Alkaline pressure hydrolysis of energetic materials

Figure 1. Pressure hydrolysis equipment.

2.2 PRESSURE HYDROLYSIS EQUIPMENT 3

The pressure hydrolysis equipment (Figure 1) consists mainly of an 180 cm autoclave (od = 70 mm, id = 48 mm) in combination with manometer, pressure release valve and sampling valve. Pressure and temperature (thermocouple) are simultaneously registered during the reaction period. The reaction vessel is heated by an electrical resistance heating which allows heating rates of o 10 C/min. 2.3 HYDROLYSIS CONDITIONS

The starting conditions of the reaction were P:H2O = 1:10, P:NaOH = 2,3:1. 120 ml of the reaction mixture were put in the autoclave. The autoclave was closed and evacuated and afterwards filled with helium. This was necessary to determine exactly the molecular nitrogen which was formed during hydrolysis. The o reaction vessel was heated up to 150 C (15 min). This temperature was held over a period of 1 hour. The exothermic hydrolysis leads to a small temperature rise o in the magnitude of 10 C which was compensated by a reduced heating. During o the experiments the reaction temperature could be kept constant within + \3 C. o After the reaction period of 1 hour, the reaction mixture was cooled down to 90 C and separated into gas phase, liquid, and solid residues.

G. Bunte, T. Hirth, H. Krause, and N. Eisenreich

109

2.4 CHARACTERIZATION OF REACTION PRODUCTS

2.4.1 Aqueous Solution

To characterize the composition of the aqueous solutions following analytical methods were used: • • • • • •

determination of chemical oxygen demand (COD) DIN 38409 Teil 41 (DEVH 41) determination of nitrite and nitrate by ion exchange chromatography (IC in combination with conductivity detector, Fa. GAT) determination of short chain carboxylic acids by ion exclusion chromatography (IC with conductivity detector, Fa. GAT) pH value by glass electrodes infrared spectra of the liquid phase using an ATR-Circle Cell in combination with FTIR spectroscopy (Circle Cell with ZnSe-crystal, Fa. Spectra Tech., Inc.) IR-spectra of the residue of the evaporated liquid phase (KBr pellets). 2.4.2 Gas Phase

The gaseous phases were analyzed by: • •

gas chromatography with thermal conductivity detector (TCD) with molecular sieve 13X and Porapak Q columns (FISONS-GC6000 VEGA) NOx measurement by chemiluminescence technique (Model 59A Fa. Rosemount) 2.4.3 Solid Residues

The solid residues were separated and washed with destilled water to the neuo tral point and dried at 60 C. CHN analysis was performed with a CHNO rapid-analyzer (Leybold Heraeus). IR-spectra of the residues were measured as KBr-pellets (NICOLET 60 SX). 3 RESULTS

Nitrocellulose, as the main component of the propellants as well as nitroglycerin, are well decomposed during pressure hydrolysis. The main products are nitrite and nitrate in the aqueous solution as well as short chain organic acids. The chromatogram (Figure 2) by exclusion chromatography is very complex. Malonic acid, succinic acid, glutaric acid, formic acid, acetic acid, and/or

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propionic acid can be identified. These small chain carboxylic acids, as main products of the nitrocellulose hydrolysis, indicate a very good biodegradability. Figure 3 shows an ATR-IR-spectra of the liquid phase. The three intensive bands are carbonate, nitrate, and nitrite. In respect to the nitrogen content of the propellant, a conversion efficiency between 40-60% in nitrate and nitrite is found. The ratio of nitrite to nitrate is between 1.5 and 2. The other part of the nitrogen content of the propellant is transformed to gaseous products. In gaseous products of pressure hydrolysis, N2, N2O, NOx, CO, and CO2 are found. Depending on the reaction conditions the NOx formation is of the order of 0-5%. The nitrogen conversion ratios of the double-base propellant P2 are summarized in Figure 4. Nearly 40% of nitrogen is converted to the harmless molecular nitrogen. Laughing gas is in the amount of 10%.

Figure 2. Ion-exclusion chromatogram of the liquid phase after alkaline pressure hydrolysis of NC.

G. Bunte, T. Hirth, H. Krause, and N. Eisenreich

Figure 3. ATR-IR spectra of two typical liquid phases after alkaline pressure hydrolysis of NC.

Figure 4. N conversion ratio of P2 after alkaline pressure hydrolysis (P2:NaOH = 2.3:1).

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Laughing gas and NOx, which are air pollutants, can be directed to the subsequent biological stages where the denitrification takes place. The solid residues depend on the composition of the propellant: P1: P2: P3: P4: P5:

2-3% residue consisting of DPA and graphite 3-12% centralite; DNT was not found in the residue about 10% were lead oxide Pb2O3 and centralite 5-16% residue under enrichment of centralite 10% residue with enrichment of centralite and dibutylphthalate.

Under the conditions of pressure hydrolysis, diphenylamine as well as centralite - additives in propellants - remain in the solid residue. The enrichment of these compounds in the residues is an advantage of the process because both compounds should not be transferred to the sewage after the biological stages. Both compounds can be separated and disposed off separately. The same applies to the metal additives such as lead oxide which can be removed after hydrolysis. In the case of lead containing propellants the lead content in the aqueous phase has to be checked and controlled. The carbon content of the propellant is transformed to CO2 (about 8%) in the gas phase. Formation of CO (under 1%) and methane (below 0.1%) can be neglected. Depending on the kind of additives between 6 and 10% of carbon are

Figure 5. C conversion ratio of P2 after alkaline pressure hydrolysis (P2:NaOH = 2.3:1).

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Figure 6. N conversion ratio of P5 after pressure hydrolysis under different alkaline conditions. a P5:NaOH = 1:1; b - P5:NaOH = 2.3:1.

found in the solid residue. Main products in the aqueous solution are carboxylic acids, carbonate and hydrogen carbonate. The oxygen demand of organic species in the liquid phase is represented by the COD value. The chemical oxygen demand reaches values of 50-60% carbon. The mean values of several P2 measurements are given in Figure 5. The formation of the reaction products, especially of the gaseous reaction products, is strongly influenced by the reaction conditions. The amount of gaseous products is drastically reduced in stronger alkaline solution (propellant to NaOH ratio1:1). Figure 6 shows the nitrogen conversion ratios on hydrolysis of P5 with lower (1:2.3) and stronger (1:1) alkaline content. The formation of gaseous products is drastically reduced from 20.3% to 2.6%. Particularly important is that NOx formation (below 0.1%) is rather low. For comparison, incineration ratios of propellants conversion from nitrogen to NOx are between 10 and 20%. From the view point of pollutant emission, the advantage of alkaline hydrolysis is evident. Figure 7 shows the carbon conversion ratios with lower and stronger alkaline content. Under more alkaline conditions, the C-content in the residues is re-

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Figure 7. C conversion ratio of P5 after pressure hydrolysis under different alkaline conditions. a - P5:NaOH = 1:1; b - P5:NaOH = 2.3:1.

duced (from 21.9% to 9.4%). The COD value is significantly higher (87% instead of 71%) whereas the gas production (mainly CO2) is drastically reduced (from 4.5% to 0.01%). 4 CONCLUSIONS AND OUTLOOK

Under the conditions of alkaline pressure hydrolysis, nitrocellulose is readily transformed to nitrate and nitrite and short chain carboxylic acids in the liquid phase. In the gaseous phase N2, N2O, and NOx are formed. Dependent on the alkaline content, the NOx amount can be reduced below 0.1% nitrogen content in nitrocellulose. The results of the single-base propellant, P1, show that the additive DPA and graphite remain as residue. Diphenylamine itself does not react under these conditions. It is possible that some DPA may disappear as emulsion during the hydrolysis process. The same occurs with centralite in the double-base propellants such as P2, P3, P4, and P5. The critical component in the double-base propellants is DNT. Here is not clear if all DNT is converted to harmless products. Additional investigations with pure 2,4-DNT show that only 7% of the nitrogen was found as nitrite in the liquid phase. DNT, on the other hand, was not found in the solid residues. Therefore, we assume that DNT is not

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completely converted and may be still present as an emulsion in the aqueous phase. The species: DNT, DPA, and centralite have to be removed from the reaction mixture before biological degradation will be performed. With an exception of these species, the alkaline pressure hydrolysis of propellants delivers a reaction mixture which is completely biodegradable. The two biological steps are anoxic denitrification and normal aerobic treatment. The sludge after the bio4 logical steps is in the range of 5-8% of raw material. For nitrocellulose based propellants, the alkaline pressure hydrolysis in combination with biological treatment seems to be a promising process of disposal and a feasible alternative to the common incineration. For the critical components such as centralite, DPA, DNT, and TNT more 5 drastic conditions must be employed. First experiments of a supercritical hyo drolysis and oxidation (above 374 C and 221 bar) gave good results. It seems possible that under the supercritical water conditions a complete destruction of critical components could be achieved. In the case of hexachlorocyclohexane (HCH) containing pyrotechnic compositions, the hydrolysis, in presence of NaOH, leads to conversions (Cl-) higher 90% after 30 minutes. 1. 2. 3. 4. 5.

5 REFERENCES

K. Fabel, Nitrocellulose, 2. Bd., F. Enke Verlag, Stuttgart, S. 118 f. W. O. Kenyon and H. L. Gray, J. Am. Chem. Soc., 58, 1422 (1936). G. Bunte, T. Hirth, and H. Krause, Druckhydrolyse von Treibmitteln, 23rd ICT Conference, Karlsruhe, June 30-July 3, 1992. A. Dahn and M. Teller, Verfahren zur umweltgerechten Entsorgung von Explosivstoffen, 23rd ICT Conference, Karlsrue, June 30-July 3, 1992. G. Bunte, N. Eisenreich, T. Hirth, and H. Krause, Entsorgung von Treib- und Explosivstoffen durch Prozesse in überkritischem Wasser, 23rd ICT Conference, Karlsrue, June 30-July 3, 1992.