Modelling reaction and diffusion in a wax-filled hollow cylindrical pellet of Fischer Tropsch catalyst

Abstract

Previous modelling of fixed-bed, Fischer-Tropsch (FT) reactors has demonstrated the advantages relative to spherical pellets of using cylindrical and shaped pellets to provide improved transport attributes under conditions relevant to industrial operation. However, mass transport models have focussed on the investigation of transport within pellets with spherical symmetry, whilst detailed investigations of more complex shapes have not been undertaken. Here, a pseudo-isothermal, steady-state, two-dimensional model was investigated for catalyst pellets of cylindrical form, both solid and hollow. A cobalt-based catalyst was considered at conditions where the rate of condensable hydrocarbon generation is large enough to result in the accumulation of liquid hydrocarbons in the pores of a catalyst. It was found that effectiveness factors were bounded by those of sphere and slab above and below Thiele moduli of ~0.75 and ~1.15, respectively, for the conditions examined, with the effectiveness factors exceeding those of both sphere and slab models between these moduli. Here, comparisons were made on the basis of the characteristic diffusion length, the catalyst particle's volume divided by its external surface area. However, values of the FT chain growth parameter, α, between these values of Thiele modulus were lower than both those of sphere and slab geometry, and thus under these conditions hollow cylinders gave the greatest methane selectivity. Highlights A 2-D model was investigated for the FT reaction for solid and hollow cylindrical Co catalyst pellets Conditions considered where liquid hydrocarbons accumulate in the pores Effectiveness factor exceeded that of spherical and slab geometry for Thiele moduli range 0.75-1.15 FT growth parameter, α, in this range was lowest with cylinders and so gave greatest CH 4 selectivity

Mass transfer advantage of hierarchical structured cobalt‐based catalyst pellet for Fischer–Tropsch synthesis

“…47,48 Although the assumption of the pellet with wax fully-filled has been applied for simplicity in almost all numerical modeling studies of FTS catalyst pellet. 5,[7][8][9]14,38,[42][43][44] The studies by Jess et al 43,44,49 indicated there might exist only partly filled state in pores under practical industrial FTS conditions. Furthermore, the introduction of macropores in pore structure is in favor of a reduction in the filling degree of liquid wax.…”

“…Therefore, the syngas must solubilize in the liquid film first and further diffuse into the pores and reach the internal active sites, accompanying with FTS reaction. 5,[7][8][9]14,38,[42][43][44] The external diffusion limitations in the gas and liquid films were assumed to be negligible because of the high gas linear velocity employed in industrial reactors. 45 Since the mesopores formed by ammonia solution etching were mainly dispersed on the internal surface of the macropores and diffusion length in mesopores was relatively short, the internal diffusion limitations in such mesopores were not considered in the pellet modeling.…”

“…The concentrations of species CO, H 2 , H 2 O and C 6 H 14 dissolved in the wax phase assumed to be in equilibrium with those in the gas phase can be described by Henry's law. 5,8,45 Henry's law constants for gases and light hydrocarbons in n-paraffins were calculated from Marano and Holder's correlation. 46 Wax accumulation in FTS catalyst pores is a non-negligible factor leading to strong internal diffusion limitations because of the low diffusivity of reactants and products in liquid wax.…”

“…formation. [5][6][7][8][9] Therefore, a great number of studies have been devoted to developing a viable catalyst that can balance high product yield and reasonable pressure drop simultaneously.…”

“…13 The other approach is to alter the geometry of pellets for reducing the pressure drop of catalyst bed and enhancing the transport process. The simulation studies 4,5,8,14 indicated that in comparison to spherical pellets, more complex shapes, that is, trilobes or hollow cylindrical pellets, exhibit excellent hydrodynamic, transport characteristics under industrial conditions, whereas, the low mechanical strength for hollow cylinder pellet would limit its industrial application. 5 The approaches mentioned above, either the nonuniform distribution of active components or the adjustment on the geometry of pellets, mainly focus on how to shorten the diffusion distance.…”