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

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 C-4 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 (E-A,E-FT = 124.8 kJ mol(-1), E-A,E-cr,E-C5-9 = 151.4 kJ mol(-1), E-A,E-iso,E-C5-9 = 155.7 kJ mol(-1)).