Energy & Fuels 1990, 4, 407-411
407
Fast Pyrolysis of Plastic Wastes D. S. Scott,* S. R. Czernik, J. Piskorz, and D. St. A. G. Radlein Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 2N7 Received March
7, 1990.
Revised Manuscript Received May 14, 1990
Preliminary results using an atmospheric-pressure fluidized-bed fast pyrolysis process have been obtained for the fast pyrolysis of commerical grades of polystyrene, poly(vinyl chloride), and polyethylene. On the basis of these tests, polyethylene was selected for detailed study because of the difficulty in regenerating the monomer in good yields and because of its abundance as a waste material—about 70% of all plastic waste. A test program was undertaken to attempt to produce This approach not only disposes higher value products from polyethylene, rather than monomer. of the waste but also offers the possibility of cost return. Both thermal and catalytic pyrolysis tests have been done in an atmospheric-pressure fluidized-bed reactor. Initial results suggest that catalytic pyrolysis can give promising yields of a liquid hydrocarbon product that could be refined to yield a transportation fuel. Product composition from the pyrolysis of polyethylene can be controlled over a wide range by a suitable combination of type of catalyst, temperature, and particle size.
Introduction
represent 20-30%. Of the organic waste stream, that is, after removal of glass, metals, etc., plastics are about 9-12% by weight. In addition to the presence of plastics in municipal waste streams, many wastes collected from manufacturing or service industries may contain much higher proportions of plastics. Of the plastics discarded as waste, packaging materials make up 50-70% of the total. Of this amount, 89% are polyolefins (polyethylene, polypropylene, polystyrene and poly (vinyl chloride)), with polyethylene being about 63% of all packaging waste. Previous work by others has shown that some polymeric materials, e.g., acrylics, can be deand composed thermally in high yields to the monomers these can be recycled. The reuse of these materials therefore becomes a problem of sorting or of isolation of a “pure” source. Unfortunately, polyethylene and polypropylene do not appear to give high yields of ethylene or propylene on decomposition, with known technology. Therefore, it would be desirable to be able to convert these waste polyolefins into products of some value other than the monomers, because the material disposal could be accomplished while producing products of enough value to offset the disposal costs to some degree. The present research was undertaken to explore the value of fast pyrolysis, either thermal or catalytic, as a technology for achieving this objective.
The disposal of municipal and industrial waste is now recognized to be a major environmental problem in North America. The conventional solution of landfilling is becoming too expensive and of questionable desirability for many localities. The destruction of wastes by incineration is becoming more prevalent, but this practice is often expensive also and often generates problems with unacceptable emissions. A third alternative would be true recycling, that is, to convert the waste material into products that can be reused. If a reasonably economical process can be used, some cost recovery from the products would also be possible, and the net cost of disposal might be significantly reduced. Of the possible technologies for the conversion of waste to useful products, one that has attracted some study and development effort is thermal pyrolysis. A number of projects at various scales and with varying success have been reported. For example, Japanese workers have developed fluidized-bed thermal pyrolysis processes based on the work of Kunii and others1,2 primarily for the production of medium quality gas at scales to 150 metric tons/day of waste. Other processes, using kilns or fluid beds,3,4 have also been piloted on significant scales in Europe. Some attempts at full-scale pyrolysis of waste have also been made in North America, but this technology seems not to have been adopted by others. Municipal waste consists in large part of paper and woody materials to the extent of 59-63%. Pyrolysis processes for biomass materials, either for gasification or for conversion to high yield of liquids, have been developed in recent years.5,6 In particular, we have developed in recent years a simple and potentially economic atmospheric-pressure fluidized-bed fast pyrolysis process, now known as the Waterloo fast pyrolysis process (WFPP). This process gives high conversions of biomass to liquids and would allow conversion to liquids of the cellulose or lignocellulosics in waste materials to be achieved. Another and more troublesome component of waste streams is plastics, inasmuch as they are not presently biodegradable. While they make up only 7-9% of the weight of the total waste stream, by volume they may *
Experimental Section Three of the most common polymers, poly(vinyl chloride), polystyrene, and polyethylene, were selected for pyrolysis tests. The experiments were carried out in an apparatus originally (1) Kunii, D.; Kunugi, T. J. Inst. Pet. Jpn. 1973, 16, 20. (2) Kagayama, M.; Igarashi, M.; Hasegawa, M.; Fukada, J.; Kunii, D. In Thermal Conversion of Solid Wastes and Biomass; Jones, J. L., Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980; p 527. (3) Kaminsky, W.; Sinn, H. In Thermal Conversion of Solid Wastes and Biomass; Jones, J. L, Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980; p 423. (4) Kaminsky, W. J. Anal. Appl. Pyrolysis 1985, 8, 439. (5) Buekens, A. G.; Schoeters, J. G. In Thermal Conversion of Solid Wastes and Biomass; Jones, J. L, Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980; p 397. (6) Scott, D. S.; Piskorz, J. In Bioenergy 84; Egneus, H., Ellegard, A., Eds.; Elsevier: London, 1955; Vol. Ill, p 15.
To whom correspondence should be addressed.
0887-0624/90/2504-0407102.50/0
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1990 American Chemical Society
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Energy & Fuels, Vol.
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FEEDER
Figure
1
designed for the fast pyrolysis of biomass,7,8 in which a modification of the feeding tube was made. Biomass was normally fed pneumatically into the sand fluidized bed, but in the case of polymers this type of feeding immediately resulted in plugging of the inlet tube by melting polymer. Plastic particles thus were introduced into the top of the reactor and allowed to drop freely into the fluid bed. Gas flow rates and particle size were adjusted so that the settling velocity was at least twice as high as the fluidizing gas velocity. Polymer particles were fed at a rate of 30-60 g/h entrained in a stream of nitrogen. Nitrogen was also normally used as the fluidizing gas. A process flow diagram of the experimental system is shown in Figure 1. Nitrogen entered the base of the reactor to fluidize the bed of Ottawa sand or of activated carbon catalyst particles (250-590 µ ). The process operated under atmospheric pressure. All solid, liquid, and gaseous products were collected, and a material balance was attempted. Volatile products after leaving the reactor were collected in the ice-water, or ice-salt, condensers followed by a cotton-wool filter, an activated carbon adsorber, and a gas bag. Solids (char, coke) were partly blown out to the cyclone and char pot and were partly retained in the fluid bed. The reactor and the various units of the collection line were weighed before and after the runs to determine the amounts of products. Amounts of gas were calculated as the sum of individual components, which were determined quantitatively by gas chromatography using an MS5 column and TC detector for hydrogen, a Carbowax column with flame ionization detection (FID) for to C4 hydrocarbons, and a Petrocol column and FID for higher hydrocarbons. Liquid product was analyzed qualitatively by GC/MS, which permitted the identification of components, and quantitatively by GC using the Petrocol column. The cotton-wool filter and activated carbon adsorber were washed out in a Soxhlet apparatus with hexane, and the extracts were then analyzed in the same way as the liquid
product. The activated carbon used in most of the catalytic tests was a commercial product, Galgón F400, derived from bituminous coal. In one test a coconut shell activated carbon (MCB) was used. Activated carbon with different contents of iron was also employed as a catalyst. In preparing this material, the Galgón F400 carbon was soaked in an amount of aqueous ferric nitrate sufficient to give the desired iron loading on the catalyst and the iron treated carbon was then dried. Prior to a pyrolysis run, the catalyst was reduced for 1 h in a stream of hydrogen at 500 °C. Of the plastics selected for pyrolysis tests, all were commercial-grade molding powders screened to the desired particle size. The one sample of industrial plastic waste tested consisted of scrap (7) Scott, D. S.; Piskorz, J. Can. J. Chem. Eng. 1984, 62, 404. (8) Scott, D. S.; Piskorz, J.; Radlein, D. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 581.
Table I. Fast Pyrolysis of Poly(vinyl chloride) (Goodrich Geon 30; Bench-Scale Unit; Run PI)0 temperature, °C amount fed, g feed particle size, yields, % feed char
0
520 20.0
