Fischer–Tropsch synthesis with in situ H2O removal – Directions of membrane development

Abstract

H2O is the main by-product of the Fischer–Tropsch (FT) reaction. The motivation for in situ H2O removal by means of hydrophilic membranes is threefold: (a) to reduce H2O promoted catalyst deactivation, (b) to boost the reactor productivity, and (c) to displace the water gas shift (WGS) equilibrium to enhance the conversion of CO2 to hydrocarbons. The selective removal of H2O from a complex mixture of H2, CO, CO2 and hydrocarbons under FT conditions is a challenging task for membranes and membrane preparation.The potential of the FT membrane reactor concept is illustrated by mathematical case studies. The results of the experimental demonstration of in situ H2O removal under reactive FT conditions with an amorphous silica membrane and a ceramic supported polymer membrane prove the concept but confirm the high demands on membrane permeance, permselectivity and stability. Based on mathematical case studies, experimental results and a literature study on hydrophilic membranes, the following critical membrane specifications are formulated: (a) permeance QH2O>1×10−7mol/(sm2Pa) and (b) permselectivities SH2O/H2, SH2O/CO, SH2O/CO2>75.With regard to the literature on hydrophilic membranes, microporous zeolite membranes outperform amorphous microporous membranes and polymer membranes. A new type of membrane prepared by direct hydrothermal synthesis on α-Al2O3 supports is a thin defect free hydroxy sodalite (H-SOD) film with a layer thickness of 2μm. This type of membrane offers high permselectivities and high H2O fluxes at temperatures up to 200°C and is therefore, a promising candidate for in situ H2O removal during FT synthesis.