Investigation of the Catalytic Pyrolysis of High-Density Polyethylene over a HZSM-5 Catalyst in a Laboratory Fluidized-Bed Reactor P. N. Sharratt* and Y.-H. Lin † Environmental Technology Centre, Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, U.K. A. A. Garforth and J. Dwyer Centre for Microporous Materials, Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, U.K. High-density polyethylene (HDPE) was pyrolyzed over HZSM-5 catalyst using a specially developed laboratory fluidized-bed reactor operating isothermally at ambient pressure. The influence of reaction conditions including temperature, ratios of HDPE to catalyst feed, and flow rates of fluidizing gas was examined. The sodium form of siliceous ZSM-5, silicalite, containing very few or no catalytically active sites, gave very low conversions of polymer to volatile hydrocarbons compared with HZSM-5 (Si/Al ) 17.5) under the same reaction conditions. Experiments carried out with HZSM-5 gave good yields of volatile hydrocarbons with differing selectivities in the final products dependent on reaction conditions. Catalytic pyrolysis of HDPE performed in the fluidized-bed reactor was shown to produce valuable hydrocarbons in the range of C 3 -C 5 carbon number with a high olefinic content. The production of olefins with potential value as a chemical feedstock is potentially attractive and may offer greater profitability than production of saturated hydrocarbons and aromatics. 1. Introduction The disposal of municipal and industrial waste is recognized to be a major environmental problem. Land- fill is becoming much more expensive and of question- able desirability for many localities. The destruction of wastes by incineration is prevalent, but this practice is expensive and often generates problems with unac- ceptable emissions. Another alternative would be true recycling, i.e., to convert the waste material into prod- ucts that can be reused and significantly reduce the net cost of disposal (Lee, 1995). Possible technologies for the conversion of waste to useful products have attracted research in the area of thermal degradation. Workers in Japan have developed a dual fluidized-bed process for obtaining medium- quality gases from municipal solid waste (Kagayama et al., 1980; Igarashi et al., 1984). Thermal cracking of waste polymer using kilns or fluidized beds has been piloted on a significant scale in Europe (Kaminsky et al., 1995; Kaminsky, 1995; Conesa et al., 1994). Other processes using a pilot-plant fluidized-bed reactor or an internally circulating-fluidized bed (ICFB) reactor to pyrolyze plastic waste have also been tried in North America (Scott et al., 1990; Sodero et al., 1996). How- ever, the thermal degradation of polymers to low mo- lecular weight materials has a major drawback in that a very broad product range is obtained. In addition, these processes require high temperatures, typically more than 500 °C and even up to 900 °C. Catalytic pyrolysis provides a means to address these problems. Suitable catalysts can have the ability to control both the product yield and product distribution from polymer degradation as well as to reduce significantly the reaction temperature, potentially leading to a cheaper process with more valuable products. In contrast to thermal degradation research, catalytic pyrolysis has been carried out by considering a variety of catalysts (Aguado et al., 1996; Audisio et al., 1992; Ohkita et al., 1993; Sakata et al., 1996) with little emphasis on the reactor design, with only simple adiabatic batch and fixed-bed reactors being used (Ishi- hara et al., 1993; Songip et al., 1993; Mordi et al., 1994). Also, even though catalysis has been used, this is often done by thermally degrading the polymer and passing the degradation products through the catalyst (Songip et al., 1993, Ohkita et al., 1993). The use of fixed beds where polymer and catalyst are contacted directly leads to problems of blockage and difficulty in obtaining intimate contact over a significant portion of the reactor volume. Without good contact the formation of large amounts of residue is likely, and scale up to industrial scale is not feasible. Much less is known about the performance of catalyst in polymer degradation using a fluidized-bed reactor. Scott et al. (1990) report the use of a fluidized bed containing sand, activated carbon, or an iron-loaded carbon as the solid medium. While the authors claim catalytic effects, the reactions were carried out at temperatures typical for pyrolysis: 500-790 °C. Hard- man et al. (1993) used a fluidized bed containing quartz sand, silica, or other refractory materials. Again, relatively high operating temperatures are suggested, with 450-550 °C being preferred. Although zeolite catalysts were used in some trials, the results for those trials are sketchy and were carried out at temperatures in excess of 430 °C. The objective of this current work was to explore the capabilities of a laboratory catalytic fluidized-bed reac- tion system using a zeolite catalyst (i) for the study of product distributions and (ii) for identification of suit- able reaction conditions for achieving waste polymer recycling. * Corresponding author. Telephone: +44 (161) 200 4367. Fax: +44 (161) 200 4399. E-mail: p.n.sharratt@umist.ac.uk. † Current address: Department of R&D, Kaohsiung Chem- istry, P.O. Box 90583, Chiu-Chu-Tang, 840 Kaohsiung, Tai- wan, Republic of China. 5118 Ind. Eng. Chem. Res. 1997, 36, 5118-5124 S0888-5885(97)00348-5 CCC: $14.00 © 1997 American Chemical Society