(80b) a Study of Paraffin Product Distribution in Fischer-Tropsch Synthesis

Natural resources (coal, natural gas, biomass and wastes) can be converted into liquid hydrocarbon fuel via the Fischerâ??Tropsch Synthesis (FTS) reaction. Although the products of FTS cover an extremely wide range, and include olefins, paraffins and oxygenates with carbon numbers extending from C₁ to more than C₁₀₀, the major FT products are paraffins and olefins. The Anderson-Schulz-Flory (ASF) distribution model has been used to describe FTS product distribution, where the mole fraction of hydrocarbons is governed by a chain growth probability factor α (Note that in this model 0< α<1).  

Some researchers have chosen to study the ASF distribution of olefins and paraffins separately. The published results of the ASF distribution of paraffin products show deviations from the ideal ASF distribution, such as a high yield of methane; a distinct change in the slope of the line between light hydrocarbons and long chain hydrocarbons; and in some cases, the values of α >1 have been observed for light hydrocarbons (C₃-C₅). Unfortunately, none of the kinetic and mechanistic chain growth models can fully explain the paraffin product distribution that have been obtained experimentally. Nowadays, a few researchers try to link the FT products distribution with thermodynamics.  

In the current work, a model based on thermodynamic quasi-equilibrium was introduced to describe the paraffin product distribution in FTS. The model was compared to experimental data obtained from two groups of experiments: one using wide ranges of H₂/CO/CO₂ mixtures over both cobalt- and iron-based catalysts and the other by co-feeding olefin and paraffin products into the FT reactor with or without feeding CO/H₂. In addition, experimental data from other researchers, covering a wide range of catalysts, operating conditions and reactor types, was also used to test the model. It is shown that the measured data follows the equilibrium curves quite closely. Thereafter, Aspen simulations, incorporating the thermodynamic quasi-equilibrium model, have been used to simulate the production of FTS by varying the reaction conditions (temperature, pressure, feeding different kinds of olefin and paraffin products into the reactor). The comparison between the Aspen simulation results and the data obtained experimentally revealed that most of the experimental results were quite close to those arrived at through Aspen thermodynamic simulations, which indicate that the paraffin product distribution can be described by quasi-thermodynamic equilibrium.