CONICET | Buscador de Institutos y Recursos Humanos

Isomerization-cracking of long paraffins to produce octane enhancers for the gasoline pool is a complex refinery operation that needs simultaneous adjustment of the molecular size of the feed (cracking) and skeletal branching (isomerization). As both reactions are acid-catalyzed it is very difficult to adjust the acidity distribution of a single catalyst to have a proper balance. In this work the idea is tried of concentrating isomerization on one catalyst type and cracking on another one. The catalyst used is a mechanical mixture of a isomerization catalyst (Pt/WO3-ZrO2) (PtWZ) and an cracking one (Pt/SO4-2-ZrO2) (PtSZ) intimately mixed and working at the same reaction conditions. The tests were performed using n-hexadecane as a model feedstock. The screening of pressure and temperature conditions indicates that the best match for both catalysts is 20 atm and 225°C. Stable activity levels for both catalysts can only be got at a H 2/n-C16 ratio of 10. An adequate space velocity for increasing the yield to iC5-9 and preventing consecutive cracking is 18.4 h-1. At this process conditions, several catalyst mixture compositions were tried. The results indicate that for most composite formulations the distribution of products is similar to that obtained on single PtWZ. Mixtures (PtWZ+PtSZ) that contain 25 % or 50 % PtSZ have a better product distribution, with the maximum displaced to higher carbon numbers, with a reduced yield to isobutane and light gases. Despite the differences in product selectivity depending on mixture composition, from a global process point of view most operation points lie on a linear plot of octane number gain (∆RON) versus yield of light gases (C1-4) with a singular inflexion point for mixtures (PtWZ+PtSZ) containing more than 50% PtSZ. This indicates that RON gains can only be obtained at the expense of the liquid yield in spite of the composite composition. However fixing the weigth percent of PtSZ below 50% the RON gain is more important than the decrease in liquid yield. The relative absence of volcano plots of optimality for the composition domain seems to indicate that most isomerization and cracking reactions occur in sequential way, i.e. most products react only once on the catalyst surface and then are desorbed. In this sense they obey the pattern of ideal hydrocracking. For this reason all branching occurs first and cracking occurs the last. The already cracked products undergo much fewer branching reactions afterwards. For this reason the most appropriate catalyst for isomerization-cracking seems to be PtWZ because it leads to more highly branched carbenium ions in the adsorbed state before they suffer scission into smaller fragments.

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