Mechanistic modeling of catalytic cracking chemistry

This work develops and refines models for two aspects of the Fluidized Catalytic Cracking (FCC) process. The first section presents the development of a model for droplet vaporization in the riser section of the FCC process, focusing on the role of cracking kinetics in mass transfer processes. The second section continues an ongoing effort in modeling catalytic cracking kinetics in the reactor section of the process.; Specifically, in the first part of this work, the heat-up and vaporization times of vacuum gas oil droplets in a FCC riser are investigated. Previous models of FCC drop vaporization have not included the effect of thermal cracking reactions and the primary goal of this work was to determine the set of conditions under which cracking reactions could significantly shorten vaporization times. At high reaction temperatures (755K < T < 800K), droplet evaporation rates were significantly enhanced by thermal cracking. For extremely high reaction temperatures (T > 800K) vaporization rates were controlled by cracking rates, even for large droplets. An empirical model suitable for inclusion in on-line modeling was developed for the vaporization process. In the empirical model, vaporization times were proportional to the third power of initial droplet diameter when thermal cracking was the dominant process. In situations where thermal cracking was significant but not dominant, (700K < T < 800K), vaporization times were proportional to the 1.15–0.42 power of diameter.; In the second part of this work, a molecularly explicit model of the catalytic cracking of n-heptane on ZSM-5 zeolite catalyst was developed. An algorithm based on Boolean relation matrices was used to describe the structure of hydrocarbons and carbenium ions, and to account for each elementary step on both Lewis and Bronsted sites using well-known concepts in carbenium ion chemistry. Kinetic equations incorporating maximum information about the complex reaction networks for the catalytic cracking of paraffins were derived computationally, accounting for competitive adsorption of reactants and products on the active sites in the zeolite cages. The complete reaction network was used to obtain a limited set of elementary single event rate parameters, that are independent of the feedstock, valid for the catalytic cracking of any mixture of paraffins, olefins, naphthenes and aromatics on bifunctional catalysts. Finally, the effect of catalyst properties and reaction temperature on model behavior is investigated. The calculations are based on data from microactivity test experiments of n-heptane cracking over ZSM-5 catalysts for 15 different cases, with various Si/Al ratios and operating temperatures.