Energy & Fuels 2002, 16, 485-489
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Tertiary Recycling of Polyethylene to Hydrocarbon Fuel by Catalytic Cracking over Aluminum Pillared Clays George Manos,*,† Isman Y. Yusof,‡ Nicolas H. Gangas,§ and Nikos Papayannakos§ Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K., Chemical Engineering Research Centre, School of Applied Science, South Bank University, London, U.K., and 3Department of Chemical Engineering, National Technical University of Athens, Athens, Greece Received September 24, 2001. Revised Manuscript Received December 8, 2001
The catalytic cracking of polyethylene over an Al pillared saponite, an Al pillared montmorillonite, and their regenerated samples was studied in a semi-batch reactor. Pillared clays were able to convert completely polyethylene in gaseous and liquid hydrocarbons, showing low coking levels. The selectivity and yield to liquid hydrocarbons were high, as the mild acidity of pillared clays avoided excessive cracking to small molecules. Regenerated catalyst samples showed practically identical levels of conversion and selectivity with fresh pillared clay samples. Furthermore, they produced hydrocarbons with practically the same distribution as the fresh samples, confirming that pillared clays can be completely regenerated. Both facts of high yield to liquid products and regenerability make pillared clays potential catalysts for an industrial process of catalytic cracking of plastic waste. The effect of the heating program on the liquid product quality and distribution is also investigated, using two different temperature programs with the same levels of temperature steps, but different duration. The boiling point distribution of the liquid products formed during the second interval of the shorter program was intermediate between this of the lighter liquid produced in the first 10 min of the longer program and the heavier liquid produced between 10 and 20 min. This result clearly shows the importance of the polymer state during each temperature stage.
Introduction The huge amount of waste plastics that resulted from the dramatic increase in polymer production gives rise to serious environmental concerns, as plastic does not degrade and remains in municipal refuse tips for decades. As a solution to this problem, polymer recycling has been suggested via various methods. Between them, thermal and/or catalytic degradation of plastic waste to fuel show the highest potential for a successful future commercial polymer recycling process, especially as we can consider plastic waste to be a cheap source of raw materials in times of accelerated depletion of natural resources. Since pure thermal degradation demands relatively high temperatures and its products require further processing for their quality to be upgraded, catalytic degradation of plastic waste offers considerable advantages.1 It occurs at considerably lower temperatures1 and forms hydrocarbons in the gasoline range,1 eliminating the necessity of further processing. For such a catalytic cracking process, the previously used mainly zeolite-based1-10 catalysts suffer two seri* Corresponding Author: Tel: +44-20-7679 3810. Fax: +44-20-7383 2348. E-mail:
[email protected]. † University College London. ‡ South Bank University. § National Technical University of Athens. (1) Manos, G.; Garforth, A.; Dwyer, J. Ind. Eng. Chem. Res. 2000, 39, 1203-1208.
ous setbacks. They have a very narrow pore size distribution7 in the micropore range and possess strong acidity. It is reasonable to assume that the polymer macromolecules break down first on the external catalytic surface and only small fragments can enter the catalyst pore structure to undergo further reactions. With zeolites this fragmentation progresses to quite small molecules, increasing the yield to gaseous hydrocarbons, which are considered inferior to liquid fuel because of their transportation cost. In the search for suitable catalysts for plastic catalytic cracking we introduced pillared clays,11 that have much (2) Manos, G.; Garforth, A.; Dwyer, J. Ind. Eng. Chem. Res. 2000, 39, 1198-1202. (3) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Appl. Catal. B: Environ. 1993, 2, 153-164. (4) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Energy Fuels 1994, 8, 131-135. (5) Audisio, G.; Bertini, F.; Beltrame, P. L.; Carniti, P. Makromol. Chem., Macromol. Symp. 1992, 57, 191-209. (6) Ng, S. H.; Seoud, H.; Stanciulescu, M.; Sugimoto, Y. Energy Fuels 1995, 9, 735-742. (7) Arandes, J. W.; Abajo, I.; Lopez-Valerio, D.; Fernandez, I.; Azkoiti, M. J.; Olazar, M.; Bilbao, J. Ind. Eng. Chem. Res. 1997, 36, 4523-4529. (8) Arguado, J.; Sotelo, J. L.; Serrano, D. P.; Calles, J. A.; Escola, J. M. Energy Fuels 1997, 11, 1225-1231. (9) Garforth, A. A.; Lin, Y.-H.; Sharratt, P. N.; Dwyer, J. Appl. Catal. A: Gen. 1998, 169, 331-342. (10) Cardona, S. C.; Corma, A. Appl. Catal. B: Environ. 2000, 25, 151-162. (11) Manos, G.; Yusof, I. Y.; Papayannakos, N.; Gangas, N. H. Ind. Eng. Chem. Res. 2001, 40, 2220-2225.
10.1021/ef0102364 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/12/2002
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milder acidity than zeolites12 and have a bimodal pore structure including micropores (
