Product selectivity effects during cracking of alkanes at very short and longer times on stream

Cracking of C7, C10, C12 and C14 n-alkanes, over USY, Beta and ZSM-5 zeolites was carried out in a reaction system which allows to obtain instantaneous conversions at very short times on stream. With this system the influence of operation variables, chain length of the paraffin fed, and catalyst structure on product selectivity was established. Positive and negative effects of catalyst decay on product selectivity were obtained, and this behaviour could be simulated by using a deactivation model in which the rate of deactivation depends on the product concentration. It was observed that olefin/paraffin, branched/normal paraffin, branched/normal olefin ratios and aromatic selectivities are affected by chain length, conversion level, and time on stream. Their evolution was explained by taking into account bimolecular reactions of hydrogen and hydride transfer, and the effect of deactivation at very short times on stream, similar to those in FCC units, could be determined. Furthermore, the catalyst structure showed an significant effect on space demanding reactions, such as bimolecular ones. The importance of that effect follows the order USY > Beta > ZSM-5.