The limiting mutual diffusion coefficients of Fischer-Tropsch synthesis products in near-critical hydrocarbons

Abstract

The Fischer-Tropsch synthesis (FTS) is used to convert synthesis gas into petroleum products such as gasoline and diesel fuel. It was developed in Germany during WW 11 as an alternative fuel source during the fuel embargo and is still used as a major source of fuel in South Africa. The objective of our work will be to run the reaction in the supercritical region with a hydrocarbon solvent in order to utilize the advantages of supercritical fluids (SCFs). SCFs have the advantages of high diffusivities, low viscosities, and increased mass transfer. Effective pore diffusivities for the reactants and reaction products become important when modeling the reaction. One of the most important factors in determining effective diffusivities is the molecular diffusivity of the organic in the supercritical fluid. However, data for diffusivities in supercritical fluids are scarce. Because diffusion coefficients cannot be determined a priofi, it is necessary to measure them. We have utilized the Taylor dispersion technique to measure the limiting mutual diffusion coefficients of some FTS products, namely 1-octene and 1-tetradecene, in subcritical and supercritical ethane and propane in the temperature range 293.2-338.25 K and the pressure range 55.2-110.3 bar. It has been found that diffusion coefficients are a strong function of the solvent density and the molecular weight and diameter of the solute and solvent. We have correlated the data to predict the diffusion coefficients using the rough-hard-spheres (RHS) theory; however, it has been found that this theory underpredicts the data. Therefore, we have used modifications to the RHS theory of diffusion by Sung and Stell (1984) to better predict the data. These modifications to the RHS theory predict the data we obtained within the experimental limits.