Today, Fischer-Tropsch synthesis is carried out in diverse reactor designs, such as fixed beds, bubble columns or circulating fluid beds. If a fixed bed mode of operation is favoured, the Fischer-Tropsch catalyst will generally, by reason of pressure drop and heat transfer, consist of particles of a few millimetres in size. For particle diameters of more than about one millimetre the effective reaction rate will decrease significantly by pore diffusion due to pore filling of the catalyst with liquid higher hydrocarbons during the synthesis. Furthermore, carbon dioxide and steam form, which induce strong inhibiting effects (above all steam) on the reaction rate. Taking these effects into account, internal pore diffusion was modelled by means of a wax-filled cylindrical single catalyst pore. The modelling was accomplished by a three- dimensional finite element method as well as by a respective differential-algebraic system. Since pore diffusion effects are frequently issue of discussion (1-5) the numerically obtained solution was compared to data calculated analytically.
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