Chemical Engineering Science 59 (2004) 2433–2445 www.elsevier.com/locate/ces Thermal pyrolysis of polypropylene: eect of reux-condenser on the molecular weight distribution of products Deepyaman Seth, Amitava Sarkar * Department of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1 Received 26 June 2003; received in revised form 24 February 2004; accepted 2 March 2004 Abstract When polypropylene is heated in an inert atmosphere and the volatile products are condensed, the molecular weight distribution (MWD) of the product obtained is very wide. In addition, this product is very waxy, has poor ow properties, and is unusable as a liquid fuel. To improve the product quality, ZSM-5 and other catalysts are usually used in the recycling process. However, these catalysts are costly. On the other hand, if the higher boiling fractions of the volatiles evolved due to degradation are not allowed to escape, then the MWD of the nal product can be improved. This paper compares the eects of degradation with and without a reux on the MWD of the product. Initial MWD of the polypropylene determined by GPC was used for all the simulations. In solving the governing population balance equation for the polymer degradation process, the rate constant for degradation has been tted to the total weight loss of the polypropylene at a given temperature. The problems arising from the use of moment method with an assumed gamma-distribution are also discussed. A xed-pivot discretization technique has been adopted for solving the governing equations. ? 2004 Elsevier Ltd. All rights reserved. Keywords: Polymer degradation; Thermal pyrolysis; Simulation; Population balance; Numerical analysis; Moment method 1. Introduction Recently, much attention has been paid to the recycling of waste polymers by thermal or catalytic pyrolysis as a method to recover value added products or energy via the produc- tion of high-value petrochemical feedstock or synthetic fuel fractions. Pyrolysis has been investigated at laboratory and pilot plant level, and some semi-commercial installations have been built for the treatment of plastic wastes (Murty et al., 1996; Ding et al., 1997; Horvat and Ng, 1999; Cardona and Corma, 2000; Wong and Broadbelt, 2001; U car et al., 2002). These processes have not yet been commercialized due to several operational diculties on one hand and a lack of reliable theoretical models for designing such pieces of equipment on an industrial scale on the other. Unlike thermolysis in a constant-volume liquid phase, py- rolysis usually refers to a melted polymer thermally or cat- alytically depolymerizing to lower molecular weight (MW) compounds that vaporize and reduce the melt volume. ∗ Corresponding author. Tel.: +1-519-888-4567x2879; fax: +1-519-746-4979. E-mail address: amit@cape.uwaterloo.ca (A. Sarkar). The vaporization process makes pyrolysis kinetics more complex and dicult to characterize quantitatively than the solution phase thermolysis. The escape of higher MW com- ponents from the reaction mixture reduces the commercial value of the nal distillate products by increasing its vis- cosity and no measure has been taken so far to recycle the higher MW components back to the melter. The theory of polymer degradation is basic to the under- standing of polymer recycling. The use of continuous distri- bution function in MW is the basis of the continuous distri- bution kinetics theory (Aris and Gavalas, 1966). A polymer sample usually consists of a large number of similar species diering in chain length (or MW). As the size or MW of a particle in the polymer system is a discrete function, the discrete form of the fragmentation equation is perhaps more appropriate, essentially for long times, even though the con- tinuous fragmentation equation is useful for nding explicit solutions. However, if the MW of the polymer sample is large, the number of points for which the equation would be written is very large. In this case, the discretization points would start from the lowest molecular weight monomer. Successive discretization points would be a multiple of this monomer MW. The possibility of a grid that is ne at lower MW and is coarse at higher MW is not possible for the 0009-2509/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.03.008