Sorption and condensation of higher hydrocarbons in a Fischer–Tropsch catalyst
Liquid higher hydrocarbons accumulate inside of heterogeneous catalyst pores, resulting in negative influences on reaction rates as well as product distribution, especially for industrial mm-particles. Besides the kinetics of the rate of HC formation, the second parameter determining the pore filling is the effective vapor pressure of each HC with C-number i relative to the saturation value, i.e. Φ = pvap,i,eff/psat,i. Ad- and desorption experiments (Co/Al2O3 FT catalyst, paraffins with C-numbers from 4 to 28) were conducted using a magnetic suspension balance (MSB), and reveal that the sorption equilibrium is determined by multi-layer sorption combined with strong capillary condensation. The experimental sorption data, presented here for the first time for long chain HCs under FT conditions, were also modelled successfully to predict Φ-values in a porous FT catalyst depending on the C-number and pore filling degree. The simulation of the filling process based on the kinetics of the FTS and the new sorption model is in agreement with the experimental data of the process also measured using a MSB.