Influence of Fischer-Tropsch synthesis (FTS) and hydrocracking (HC) conditions on the product distribution of an integrated FTS-HC process

Abstract

The integration of Fischer-Tropsch synthesis (FTS) with direct hydrocracking (HC) was investigated in a two-stage reactor system, where the influence of FTS performance (concentration of residual H2 and CO as well as amount of heavy hydrocarbons) on HC was studied and the overall performance of the integrated process was analyzed regarding the liquid fuel (C5-C20) selectivity. Sufficient cracking of the C21+ fraction was achieved under FTS conditions and primary cracking was proven as dominant. Higher temperature has a positive effect on HC, but too high temperature would turn out to increase the C1-4 selectivity and a maximum of 70wt.% C5-C20 was obtained. The cracking of FTS hydrocarbons could be divided into two categories, cracking of heavy molecules via liquid phase and cracking of light molecules depending on gas phase. The cracking of lights was slightly suppressed when raising the H2 partial pressure, while the cracking of heavy molecules showed no response. Differently, an increase of the CO partial pressure obviously inhibited the cracking performance of both fractions. The wax flow rate study revealed a quite selective cracking of FTS wax, since the cracking of light hydrocarbons is partial pressure-limited. As long as enough wax covers the catalyst along the bed, a further increase in the catalyst amount has no effect on the final product distribution.