Fischer-Tropsch Synthesis in a Fixed Bed Reactor

A numerical simulation that models the Fischer-Tropsch (FT) synthesis in a tubular multitube reactor packed with an iron-based catalyst is conducted to assess the effects of process parameters on product distribution. The study adopts the alkyl and alkenyl mechanisms in predicting the formation of paraffins and olefins. The effects of the desorbed hydrocarbons on the gaseous flow and reaction kinetics are accounted for in the computational algorithm. The extent of the variation of the syngas molar feed ratio, reactor inlet pressure, and reactor length on paraffin and olefin selectivities and mass flow rates is documented. Three distinct regions of the FT synthesis in the packed tube are documented. In the first region, the polymerization reactions are characterized with the absence of termination reactions that result in chain propagation reactions reaching higher carbon atom numbers with increasing axial length. The beginning of the second region is marked with the initial formation of desorbed species. The second region is characterized initially with chain termination reactions reaching higher carbon atom numbers with increasing axial length. This results in the decrease of the extent of the chain propagation reactions to lower carbon atom numbers, which itself limits the termination reactions to lower carbon atom numbers. This is the only region where liquid olefin and paraffins are formed, as the end of the second region is marked with the propagation reactions not reaching carbon number atoms beyond n = 19. In the third region, with the chain propagation reactions keep diminishing to lower carbon atom numbers, the termination reactions themselves decrease to lower carbon atom numbers. This region is characterized with constant gas flow rates, as in the absence of desorbed liquids any decrease in syngas results in the formation of low carbon number gaseous olefins and paraffins.