Plastics, Rubber, and Paper Recycling - American Chemical Society

Chapter 15

Advances in Plastics Recycling Thermal Depolymerization of Thermoplastic Mixtures 1

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch015

Mark W. Meszaros

Amoco Chemical Company, 200 East Randolph Drive, Chicago, IL 60601-7125

Technologies which convert plastic wastes back to their starting materials are a promising development in plastics recycling. These starting materials can then be reassembled back into new polymers or used as petrochemical feedstocks. An initial challenge for these new recycling technologies is to develop cost effective and versatile process units which can handle the variety of mixed plastic resins and additives typically found in post-consumer waste streams. The Conrad Unit has successfully met this challenge and demonstrated this technology during extensive parametric studies evaluating various resin streams and operating conditions. A new approach to recycling post-use plastics is underway in the United States and elsewhere that may offer a means of significantly increasing the overall quantity of plastics that can be recycled.(2-5) Called "advanced recycling technologies," these new approaches may provide advantages that overcome problems associated with some conventional plastic recycling efforts: namely, costly sorting of the different types of plastics; concerns about quality of end products; and finding reliable markets for products made from the recycled material. Advanced recycling recovers the chemical or feedstock components of plastics. These versatile and marketable end-products are the building blocks from which new plastics and a variety of other products can be manufactured. This is achieved by converting plastics back to raw materials, either directly to monomers or to the petrochemical feedstocks that are used to make monomers and many other petrochemical products. Advanced recycling processes can accept almost all types of plastics and do not require them to be washed or sorted by color or type. Advanced recycling also eliminates some of the grinding, shredding and extruding processes used in conventional plastics recycling. Lastly, the chemical and feedstock products from an advanced recycling process are virtually indistinguishable from products made from virgin materials and will have wide market applicability. Several advanced recycling processes are under development in Europe, Japan, Canada and the United States and will supplement existing conventional recycling processes.( 2-7) Integration of these new recycling options with the existing plastics Current address: Flinn Scientific, Inc., P.O. Box 219, Batavia, IL 60510 0097-6156/95/0609-0170$12.00/0 © 1995 American Chemical Society In Plastics, Rubber, and Paper Recycling; Rader, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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recycling infrastructure will provide a long-term solution for plastics by increasing the amount and types of plastics recycled. Conrad Recycling Project. The Conrad recycling process converts mixtures of plastics into a liquid petrochemical product that is similar to a high quality "sweet" crude oil or a partially refined crude oil fraction such as naphtha or gas oil. The Conrad process, like all advanced recycling processes currently under development, is not economical without tipping fees or subsidies. It is, however, a reliable and robust recycling unit capable of recycling mixed plastic waste streams into easily marketable products at a lower overall cost than conventional recycling processes. Advanced recycling processes are inherently uneconomic because the principle product, a naphtha-gas oil stream, has a value of only six to eight cents per pound at current crude oil prices of eighteen dollars a barrel. With this revenue stream, it is difficult to cover manufacturing and capital costs unless economies of scale are reached with large, refinery-scale facilities. Unfortunately, the cost for collection, transportation, and logistics to acquire and prepare the feedstock is substantial. Nevertheless, advanced recycling processes may provide the most economical route to handle large volumes of mixed plastics. Germany, which has federal mandates to recycle 60 percent of all plastic packaging, has recently announced projects to recycle over a billion pounds per year of plastics using advanced recycling processes.(6,7) The German projects are all large-scale processes integrated with an existing refinery or petrochemical facility. Because of the structure of the petrochemical industry in the United States, smaller process units that convert plastics into easily transportable liquids may be more attractive. The Conrad process meets this requirement, and the American Plastics Council (APC) is working with Conrad Industries, Inc., (Chehalis, Washington, 206-7484924) to demonstrate the viability of smaller advanced recycling process for recycling plastics back into liquid petrochemical feedstocks. The project is part of APC's overall program to increase the recycling rate for plastics and to develop or improve recycling technologies. The Conrad process was chosen because of its simplicity, versatility and safety features. More important, its size and low cost allows for a rapid progression from pilot to commercial scale process units. The objective of the partnership between Conrad and APC is to determine the technical and economic feasibility of recycling plastics back to liquid petrochemical feedstocks. To meet this objective, a series of parametric studies has been conducted to examine how various types of plastics perform under different operating conditions. Studies on various chlorine capture processes are also being conducted. Data acquired from these studies have been used to define process conditions for commercial-scale demonstrations, identify potential design improvements for future units, and develop preliminary process economics. Conrad Recycling Process. The Conrad recycling process is an auger kiln reactor that applies heat to plastics and/or tires in the absence of oxygen to produce liquid petroleum, solid carbonaceous material and noncondensable gases.(#) The unit was originally designed to recycle tires, but with minor modifications is well suited for plastics. Developmental studies on plastics and tires are ongoing at the Chehalis facility where two Conrad process units are operational. Parametric studies on plastics have been performed primarily on a 200 lb/hr pilot unit but a 2000 lb/hr commercial scale unit is also available. Both units are fully automated and process data is downloaded to a computer on a continuous basis. The feed system for the pilot unit is designed for granulated or pelletized plastics, consisting of a simple rotary air lock followed by an auger feeder. Feed systems to handle whole plastic bottles, films, and other plastic components are under investigation and will be incorporated in future Conrad recycling units. The horizontal auger kiln, or retort, is of a proprietary design and is well-suited for the

In Plastics, Rubber, and Paper Recycling; Rader, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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depolymerization of plastics to volatile products. The gases exit at the top of the discharge end of the retort and pass through a venturi scrubber, bubble cap condenser, and water cooled condenser. Gases that are not condensed are used within the system to generate heat for the process. Solid products, such as carbonaceous or inorganic materials exit at the bottom of the discharge end of the retort to the solids collection drum. The operating conditions for the auger kiln are adjusted to maximize liquid product yields while still providing enough gas to sustain fuel requirements for the heating of the retort. High temperatures produce high yields of light hydrocarbons (C i - C5) but unless a facility is located in close proximity to a market for light gases, the product gases will be difficult to market. At current operating conditions, liquid products represents about 70 to 80% of the final output of this system. The carbon produced is of such a small yield (1-3%) and low quality due to contamination from inorganic impurities that it will have limited markets. The liquid product is shipped to refineries and plastics production facilities for conversion into feedstocks for products such as synthetic fibers, new plastics and other petroleum-based derivatives. Results & Discussion Thermal degradation of plastics to liquid hydrocarbons is a simple concept, similar to many refinery cracking processes. Waste plastics, however, are not crude oil and pose some intriguing challenges, including logistics, economics, and the ability to produce a marketable product. Favorable economics are difficult to achieve because the volume of plastics will never approach the scale of most petrochemical operations since only 3-4% of crude oil is converted into plastics. In addition, preprocessing the plastics, e.g. excessive sorting, washing and size reduction, is expensive. The economics of an advanced recycling process will improve if the process can handle and remove the impurities typically found in post-use plastics such as surface contamination (e.g. food, detergents, water, paper), additives and fillers incorporated in plastics, and nonhydrocarbon elements such as chlorine and nitrogen. On the positive side, plastics are aliphatic-rich materials that easily convert into a product stream, rich in petrochemical building blocks or a high octane refinery feedstock. BP Chemicals have demonstrated that the liquid resulting from the thermal cracking of mixed plastics is an excellent feedstock for steam crackers or cat crackers.(9) In the steam cracker, the plastics generated feed produced higher yields of ethylene (34% vs. 28%), propylene (17% vs. 15%) and butylene (12% vs. 7%) compared to the traditional naphtha feed. In the cat cracker, plastics generated feed gave over 86% yield of a naphtha grade product versus 62% yield from vacuum oil. The challenge, therefore, is to develop a simple and robust process unit that can accommodate a variable feedstream and produce a consistent product, free of nonhydrocarbon impurities. The Conrad project has demonstrated that an auger kiln reactor can meet this challenge and may offer some economic and operational advantages over other processes because of the lack of feed preparation required, variable residence time in the reactor, and the simplicity and robustness of the unit. Feed preparation is simplified in the Conrad process because minimal size reduction, washing, and removal of non-plastic contaminants is required. Plastics are introduced into the retort without oxygen through a rotary air lock if the plastics are granulated, or by using a ram feeder (e.g. V-ram, reciprocating, or conical auger) if the plastic products are still in their original form. Inorganic contaminants do not disrupt the process and ultimately exit the unit with any unreacted materials or carbonaceous products. The plastics do not have to be fluidized or molten to enter the process resulting in reduced preprocessing requirements. The Conrad process allows for a variable residence time in the reactor producing a more uniform product distribution. The plastics initially melt upon entering the hot retort and the auger keeps the molten mass moving. The molten plastics continue to



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be heated until carbon-carbon bond cleavage (depolymerization) begins and continue until volatile products are produced. The gaseous products are swept out of the reactor and nonvolatile fragments continue to be heated until becoming volatile. Plastics that are easily depolymerized (e.g. polystyrene or polypropylene) are quickly cracked and removed from the reaction vessel before undergoing further cracking, thus keeping molecular weight as high as possible. Plastics that require more energy to cleave their carbon-carbon backbone (e.g. polyethylene) are either catalyzed by reactive species (radical or carbonium ions), generated by the easily cracked polymers, or remain in the retort until sufficient energy is applied to affect depolymerization. Parametric studies sponsored by the APC on a fluidized bed unit at the Energy and Environmental Research Center (EERC) at the University of North Dakota showed a synergistic effect from the thermal cracking of plastic mixtures.(V0) A 50:50 mixture of polypropylene and polyethylene cracked at a lower temperature than polyethylene does alone and produced a more narrow distribution of products than either polypropylene or polyethylene would separately. Polypropylene normally cracks at a lower temperature than polyethylene, and the consensus is that the reactive species from this depolymerization catalyzes the cracking of polyethylene which in turn hinders further cracking of the polypropylene fragments to lighter products by chain termination steps. The result is that both polymers depolymerize at polypropylene cracking temperature and more attractive products are produced. The Conrad unit has also proven to be both a robust and efficient unit. During the parametric study, the system continuously ran for five day periods. It takes between 8 to 12 hours for the unit to warm-up and line-out, but after start-up minor changes in feed rate, auger rotation, and oven temperatures are easily accomplished. Additional work on a versatile feed system is required but once the plastics entered the reaction vessel, the auger kiln easily handled them. Parametric Study. An eighteen-month parametric study has been underway to assess the Conrad recycling process.(ii) The objectives of this study were to identify process bottlenecks, develop operating parameters, and begin to assess product value and markets. A base feed mixture of 60:20:20 high-density polyethylene, polypropylene and polystyrene (HDPE:PP:PS) was chosen as representative of the major constituents found in the post-consumer plastic stream and was used during the start-up and initial phases of the study. Off-spec resin pellets were used as feed material throughout the first phases of the study to eliminate feed composition variation. It also simplifies the preparation of mixtures and feeding the unit. The last phase of the program focused on using post-consumer plastics to gain an understanding of the effect that contamination and variable feed stream have on the process. Base Feed. Overall, liquid product yields and composition from the 60:20:20 HDPE:PP:PS base feed exceeded expectations. However, a delicate balance does exist between temperature (Table I), feed rate, residence time and product yields. Lower temperatures produce a high quality naphtha grade stream and high yields of paraffin waxes (which result from incomplete depolymerization of the plastics, especially polyethylene). Reducing the feed rate and slowing the auger rotation results in a reduction of wax formation and increased yields of liquid products. Better insulation and more efficient heating of the discharge end of the retort also reduces the yield of waxes in the solids collection drum. Because of these challenges, reported liquid yields from the early phases of the parametric study are the combined liquid product yields and paraffin wax products yield found in the solids collection drum.


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Table I. Product yields from Base Mixture at Various Reaction Temperatures Oven Temperature Solid Yield Liquid Yield Gas Yield (wt%) (wt%) (wt%) (°F) 1450 28 64 8 1300 48 51 1 1200 73 27 1100 79 21 Retort temperatures are about 200°F cooler than oven. Combination of liquids and paraffin waxes from solids collection drum. b

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Part of the difficulty in determining the optimum combination of temperature, feed rate and auger speed is inherent in the design of the pilot unit. The Conrad pilot unit is a single pass retort with a relatively short kiln length (~6 ft. heated zone). Commercial scale Conrad units consist of two larger retorts in series that substantially increases residence time. Nevertheless, valuable engineering and design data has been collected from the pilot unit that will undoubtedly improve the efficiency of the Conrad recycling process. Gas chromatographic analyses of the liquid and gas products from the first phase of the parametric study are shown in Table II. The liquid products from the first phases of the parametric study were combined and 6,000 gallons of product oil were shipped to the Lyondell-Citco Refinery in Houston Texas, and used as a feed in its coker unit. The coker unit was chosen because it minimizes risks to the refinery and the light end gases from the coker go directly to several monomer production facilities, including Mobil's steam cracker. Analysis of the oil shipped to Lyondell is shown in Tables III and IV. 8

Table II. GC Analysis of Products from Depolymerization of Base Mixture 1100 Oven Temp. (°F) 1200 1300 Auger Temp (°F) 1095 980 895 Partial Oil Analysis (wt%) Benzene 7.0 2.3 1.1 Toluene 20.0 11.0 9.1 Ethylbenzene 9.0 5.7 4.8 Styrene 20.0 17.0 14.7 10.0 9.4 6.8 Cio C11-C15 14.5 16.6 13.1 8.6 9.9 14.5 C16-C20 3.0 3.5 6.4 C21-C25 1.7 1.7 4.5 C26-C30 C31-C40 1.1 1.5 4.2 Partial Gas Analysis (vol.%) H 4.0 2.6 2.2 Methane 18.0 13.6 10.5 Ethylene 19.4 18.1 13.8 Propylene 21.4 21.8 21.0 Isobutylene 5.0 6.1 6.8 Paraffin waxes found in the liquid product tanks are included in this analysis. Waxes from the solids handling drum are not part of this analysis. Auger temperature was measured via thermocouple inserted inside the auger shaft at the midpoint of the retort. b

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Table III. Simulated and Actual Distillation of Conrad Product Oil Weight % Simulated Distillation Actual Distillation Temperature (°F) Temperature (°F) 10 146 191 20 244 188 30 288 294 40 304 334 50 339 395 60 403 472 70 474 564 80 563 654 87.5^ 669 636 90 684 — 99 899 — The oil began to crack at 669°F and the distillation was suspended. Table IV. Analysis of Product Oil Shipped to Lyondell-Citco Specific Gravity 0.8860 RVP,psi 4.1 Pour Point, °F 20 Viscosity @ 75 °F, est 3.5 Viscosity @ 122 °F, est L5

LDPE, PS, and PET-rich feeds. To increase the understanding of process capabilities and product yields , studies were performed using the base feed spiked with either LDPE, PET, or additional PS. Again, only off-spec resin pellets were used to simplify the studies and reduce the variables. An LDPE-rich feed mixture containing 20:48:16:16 LDPE:HDPE:PP:PS behaved similarly to the base mixture and resulted in similar product yields and composition (Table V). Slightly higher yields of C12-C15 aliphatics were observed and higher molecular weight aliphatics decreased. No operating difficulties were observed. A PS-rich feed mixture containing 48:16:36 HDPE:PP:PS also behaved similarly to the base mixture and resulted in similar product yields and composition. In the liquid product, styrene yields almost doubled and slightiy lower yields of higher molecular weight (>Ci2) aliphatics were observed, as would be expected. Ethylbenzene yields did not significantly increase with the increased polystyrene level suggesting that ethylbenzene may not result exclusively from polystyrene depolymerization but rather from the cyclization and dehydrogenation of various aliphatic compounds at elevated temperatures. A PET-rich feed mixture containing 20:48:16:16 PET:HDPE:PP:PS did not behave like the base mixture. It was difficult to run the mixture at the lower temperatures (1100-1200°F), due to extensive production of solids. Upon analysis, the solids were identified as >95% terephthalic acid. The remaining 5% of the solids were found to be mono and bis-hydroxyethyl esters. The solids were easily filtered, but due to the design of the pilot unit a substantial portion of the solids remained in the cooling tower, product tanks, and piping. Terephthalic acid yields were estimated at around 15 mol% at 1200°F. At higher temperatures, the PET and/or terephthalate moieties decarboxylated to produce C O 2 and aromatic products. There must be enough water encapsulated in the PET and other resins to supply the water for hydrolysis and at the lower temperatures, the terephthalic acid sublimes out of the retort before it undergoes decarboxylation.


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Table V. G C Analysis of Products From Depolymerization of Base Mixture and LDPE, PS, P E T Feed Base + LDPE Base + PS Base + PET Temp. (*F) Oven 1300 1300 1200 1200 1100 1300 1200 1100 1450 Auger* 1100 1000 980 870 1010 925 1100 910 1180 Yields (wt%) Liquids 41 59 45 50 45 32 55 36 50 Solids 3 8 1 7 21 32 3 8 28 Gas 56 47 47 67 38 20 18 60 22 Aliphatics 8.5 30.5 30.9 16.7 46.3 43.1 50.8 23.5 46.5

Plastics, Rubber, and Paper Recycling - American Chemical Society

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