Combination of Fischer–Tropsch Synthesis and Hydroprocessing in a Single-Stage Reactor. Part I. Mathematical Modeling of the Reaction Kinetics

Abstract

A model for the hydroprocessing taking place on Ni/ZSM-5 in a single stage with Fischer–Tropsch synthesis was developed on the basis of various studies in the literature for the hydroprocessing of n-alkanes and FT waxes. Several assumptions were necessary: (i) lumping together of hydrocarbons, partly distinguishing between n-alkanes and isomers, (ii) chain reaction from n-alkanes to isomers to cracked products, (iii) ideal hydrocracking (primary cracking with equal cracking probability for C4 to CNC-4). Additionally, considering calculations from literature data for isomerization equilibrium constants as well as relative cracking and isomerization rates of hydrocarbons, it was possible to decrease the number of parameters. Using a model with predetermined kinetic parameters for the Fischer–Tropsch synthesis and its product distribution, the remaining parameters of the hydroprocessing model could be fitted to the experimental results of Co and Ni/ZSM-5 in a dual layer configuration. Considering phase equilibrium of the sampling conditions in both the Fischer–Tropsch and the hydroprocessing model increased the accuracy. The physical mixture configuration could be described with sufficient accuracy as well. Fischer–Tropsch synthesis and hydro-cracking and -isomerization show a similar temperature dependence of the reaction rate constants (EA,FT = 124.8 kJ mol–1, EA,cr,C5–9 = 151.4 kJ mol–1, EA,iso,C5–9 = 155.7 kJ mol–1).