Performance of diffusion-optimised Fischer-Tropsch catalyst layers in microchannel reactors at integral operation

Abstract

Microchannel reactors offer a solution to utilize highly active catalysts for the Fischer-Tropsch process. The use of a wall-coated catalyst improves temperature control and prevents the negative correlation between pressure drop and catalyst efficiency. Diffusion limitations are, however, still a concern as a high reactor productivity demands a large catalyst layer thickness, to increase catalyst holdup while using as few channels as possible. Utilizing transport pores is one way of optimising the catalyst and achieving greater layer thicknesses while maintaining a good product selectivity. In this publication, we describe an isothermal and isobaric microchannel-reactor with a novel product film formation model and an improved selectivity description for accurate calculation of the product distribution. The optimisation of catalyst layers by defining ideal thicknesses and transport pore fraction is tested within realistic integral operation of the catalyst layers. Because the catalysts selectivity is strongly affected by the local syngas ratio the interplay of diffusion effects and convection in the gas phase is of major importance for the accurate prediction of catalyst behaviour. Non-optimised layers with great layer thickness and strong impact of diffusion limitations improve their performance with increasing conversion. Optimised layers with ideal amounts of transport pores and very thin layers, for which diffusion restrictions are less significant, on the other hand, exhibit an opposite behaviour and do not benefit from high conversions. These findings can also improve the interpretation of experimental results, which are often conducted at different conversion levels.