applied catalysis zyxwvutsr A ELSEVIER Applied Catalysis A: General 110 ( 1994) 109-l 19 Effect of coke deposition on the effective diffusivity of catalyst pellets S. Al-Bayaty, D.R. Acharya, R. Hughes* zyxwvutsrqponmlkjihgfed Department of Chemical Engineering, University zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR of Salford, Salford M 5 4WT, UK (Received 28 September 1993, revised manuscript received 17 December 1993) zyxwvutsrqponmlkjihgfe Abstract A chromatographic pulse injection technique has been used with a modified Wicke-Kallenbach diffusion cell to measure the diffusivity of He-N2 mixtures in silica-alumina, a commercial zeolitic cracking catalyst and chromia-alumina pelleted catalysts containing increasing amounts of coke. While the results confirmed that diffusivity decreases with increasing coke content, the variation in the measured diffusivity with temperature suggested that diffusion occurs in the transition region for all pellets. The pore size distribution analysis indicated that the coke has a considerable blocking effect on the mesopores for silica-alumina and chromia-alumina catalysts while for zeolites the large mesopores and some macropores are affected. Key words: coking; cbromia-alumina; diffusion; silica-alumina; zeolites 1. Introduction The effect of coke deposition on the transport of reactants and products in porous catalyst pellets has been the subject of a number of investigations over the past few decades. Whether deposition of coke restricts the pore openings or blocks these off completely is still a subject of uncertainty and will depend on the extent of coke formation, the morphology of the coke deposits within the pores, the nature of the coke forming reaction and the structure of the catalyst itself. The distribution of coke would also be expected to be influenced by the operating regime of the catalyst pellet i.e. whether it was in the kinetic or diffusional controlled regimes. Early work by Haldeman and Botty [ 11, using a variety of physical techniques to examine tht! coke deposits, suggested that the coke consisted of filmy aggregates of sizes less than 10 nm. At the low coke levels used in their work, no significant change in catalyst area or *Corresponding author. Tel. ( +44-61)745 5081, fax. ( +4461)745 5999 0926-860X/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved .SSDlO926-860X (93)E0261-A