STEADY-STATE FISCHER-TROPSCH SYNTHESIS IN SUPERCRITICAL PROPANE

Fischer-Tropsch synthesis (FTS) on a precipitated iron catalyst was conducted in a fixed bed reactor at baseline conditions of 1.48 MPa, 250 degrees C, 2L(NTP)/(h.g of cat), H-2/CO = 0.67, and in supercritical propane (total pressure of 7 MPa) while keeping the synthesis partial pressure and gas flow rate constant. The catalyst activity and lumped hydrocarbon product distribution under the supercritical conditions were similar to those obtained during reaction at the baseline conditions; however, higher selectivities of 1-olefins were obtained during the supercritical phase FTS. This suggests that the rate of FTS is not diffusion limited under the reaction conditions used in this study, whereas the secondary olefin hydrogenation and isomerization reactions are diffusion limited. An independent test in which nitrogen was used instead of propane showed that operation at a total pressure of 7 MPa does not have any effect on olefin selectivity. The observed increase in olefin selectivity during the supercritical FTS is due to higher diffusivities and desorption rates of high molecular weight olefins relative to normal FTS where the catalyst pores are filled with liquid hydrocarbons (wax).