The role of diffusion in alkyl-benzenes catalytic cracking

The present study reports the catalytic cracking of three hydrocarbon model compounds: cumene, 1,3-di-iso-propyl-benzene (1,3-DIPB) and 1,3,5-tri-iso-propyl-benzene (1,3,5-TIPB). Experiments were developed in a novel CREC Riser Simulator, a catalytic reactor that mimics the operating conditions of large-scale fluid catalytic cracking (FCC) units. Reaction testing involved different reaction times and temperatures, using Y-zeolites of different crystal sizes 0.4 and 0.9 mum). It was shown that cumene cracking and the cumene cracking products are unaffected by the zeolite crystal size. It was proven that 1,3-DIPB cracking; it remains unaltered with changes in crystal size with however, crystal dimensions affecting product selectivities. Regarding the 1,3,5-TIPB cracking, at 350-450degreesC using the 0.4 mum Y-zeolites, higher 1,3,5-TIPB conversions were obtained. Differences on 1,3,5-TIPB conversions were however, less significant at 500-550degreesC. It is postulated that the 1,3,5-TIPB conversion differences are caused by constrained diffusional transport in the Y-zeolite crystals and that the increase of temperature changes the catalytic cracking of 1,3,5-TIPB from the diffusionally controlled regime to the kinetically controlled regime. (C) 2002 Elsevier Science B.V. All rights reserved.