Fischer-Tropsch Reactor Modelling Considering Vapour-Liquid Equilibrium

The case for investing in alternative fuel-production technology to reduce dependence on crude oil has become increasingly compelling. The Fischer-Tropsch Synthesis (FTS) is an area that is receiving revived interest worldwide as a technology alternative to produce transportation fuels as well as chemicals from syngas. The FTS is the process that converts synthesis gas, i.e. a mixture of carbon monoxide and hydrogen, into a wide range of long chain hydrocarbons. As such, the FTS constitutes a practical way for the chemical liquefaction of solid (coal) or gaseous (natural gas) carbon resources.

It has been generally agreed that a simple polymerization mechanism can be used to describe the distribution of the FTS product. This produces products which follows an Anderson-Schulz-Flory distribution. Thus, plotting the logarithm of the mole fraction versus carbon number will produce a straight line whose slope is related to alpha which is determined by the chain termination and propagation probabilities. In contrast, the products from laboratory and large commercial plants exhibit a two-alpha plot. Several models have been proposed to account for the observed two-alpha values.

Most models do not consider the existence of vapour-liquid equilibrium (VLE) of hydrocarbon products in a typical Fischer-Tropsch reactor. The few that considered VLE do not provide satisfactory explanations for the observed deviation. This work uses VLE to develop a mathematical model to describe the product distribution. The model results show that the effect of vapour-liquid equilibrium is the most probable reason for the observation of a two alpha product distribution. The results also show that liquid removal has no effect on the overall product distribution.

Keywords: Fischer-Tropsch Reactor Modelling, Vapour-Liquid Equilibrium, Product Distribution Model, Chain propagation probability (α), ASF.