Continuous Age Distribution Method for Catalytic Cracking. 1. Proof of Principle

Abstract

A novel steam-deactivation method is described and demonstrated that is able to produce in the laboratory a mixture of cracking catalyst having the same distribution of zeolite surface area as found in the refinery catalytic cracking unit. Both physical and catalytic properties can be closely matched to the refinery catalyst. Based on fundamental kinetic and reactor models, the method has the potential to be quantitatively predictive of refinery results. Deviations from ideal first order tetrahedral Si decay however prevent a priori prediction of refinery performance at this time. We observed slight deviations within the age distribution, but more significant deviations in overall zeolite crystallinity retention were found as a function of refinery fresh catalyst makeup rate, rendering the method semiquantitative at present. The results constitute proof of principle, however, for the method, and further progress on the found nonidealities is anticipated.

Continuous Age Distribution Method for Catalytic Cracking 2. Understanding Nonidealities

“…In an earlier paper, 7 we took the hypothesis that refinery zeolite collapse kinetics were separable into tetrahedral framework aluminum (AlO 4 − , Al T ) and silicon (SiO 4 , Si T ) contributions. The separability hypothesis is common in the study of differential equations 44 and made reasonable in the present case because, as noted above, E-cat dealumination is much faster than Si T collapse.…”

“…This new procedure involved continuously feeding fresh catalyst to a laboratory steaming reactor, while slowly reducing the reactor temperature according to a logarithmic temperature profile. 7 That profile had been derived for the first-order decay of the tetrahedral silica (Si T ) portion of the zeolite, the zeolite being the most active and therefore the most important ingredient in the catalyst. Although fundamental kinetic and reactor models had been used and first-order Si T decay was confirmed by Pine, 8 as well as by fitting 7 published equilibrium catalyst density separation data, 9−14 we nevertheless found substantial deviations from ideal first-order Si T decay.…”

“…7 That profile had been derived for the first-order decay of the tetrahedral silica (Si T ) portion of the zeolite, the zeolite being the most active and therefore the most important ingredient in the catalyst. Although fundamental kinetic and reactor models had been used and first-order Si T decay was confirmed by Pine, 8 as well as by fitting 7 published equilibrium catalyst density separation data, 9−14 we nevertheless found substantial deviations from ideal first-order Si T decay. 7 The deviations were evident both by the classical Arrhenius method and by implementing the new continuous age distribution method (CADM) experimentally.…”

“…7 The deviations were evident both by the classical Arrhenius method and by implementing the new continuous age distribution method (CADM) experimentally. 7 The reasons for the discrepancy between the refinery and laboratory kinetics were not understood. These deviations were significant enough, however, to render the method only semiquantitative, leaving more rigorous reproductions of refinery catalyst properties out of reach.…”

“…7,9,10 More specifically, about 90% or more of the refinery equilibrium catalyst mixture contains zeolite Y whose framework Al T is equilibrated to a low value, typically less than about 6% Al T . 7 Since the framework percentage of Al T is nearly constant, any substantial change in zeolite crystallinity must be due to Si T collapse. To make the evaluation of Si T decay more quantitative, one can easily determine the framework aluminum content, before and after steam deactivation, by correlation 33 with the unit cell size (UCS).…”