Enhancing internal mass transport in Fischer–Tropsch catalyst layers utilizing transport pores

Abstract

Internal mass transport limitations inside Fischer-Tropsch catalysts due to the slow diffusion of reactants in the liquid-filled pores may significantly alter the selectivity and achievable productivity. In this work, diffusive restrictions for planar catalyst layers were investigated by mathematical modeling and simulation. A one-dimensional model utilizing empirical kinetics, incorporating transport pores as an additional pathway for mass transport and taking into account heat production, allows for calculation of catalyst efficiency and productivity towards C 5+ products. As diffusional mass transport leads to strong concentration gradients that impair selectivity, an optimum layer thickness with maximum C 5+ productivity can be found. Additional transport pores enhance the mass transport but reduce the amount of active phase, which requires a trade-off by optimizing the fraction of transport pores and layer thickness. For reference conditions, the catalyst layer with an ideal amount of transport pores and ideal thickness exhibits a productivity that is about 47% higher than that for the best layer without transport pores. This improvement requires transport pores with diameters not larger than about 60 μm. While the improvement potential significantly depends on the effective diffusivities, the effect of heat generation was found to be negligible.

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

“…[24][25][26] By using transport pores the internal diffusion is enhanced and can improve the observed selectivities, as shown in the group of Montes 18,27 and also by our previous work. 22 Our existing simulation work 28 predicts the improved selectivity but also predicts an improved total productivity, which does not agree well with the experimental evidence. This can be a result of the simplified model, not taking axial convection into account.…”

“…Initial estimation lead to upper limits for transport pore diameters of 1-2 μm under all circumstances. 28 For transport pore fractions exceeding 10% the limit can increase to 10 μm; later 3D simulations confirmed this. 57 Thus, transport pores can be about three orders of magnitude larger than the typical mesopores of catalysts.…”

“…This expression is as used in our previous works. 28,34 It contains the modification of the effective diffusivity to include diffusion in transport pores. This modification is valid for sufficiently small transport pores to ensure a homogeneous concentration profile.…”

“…Despite an abundance of simulation work on the FT process, to the authors' knowledge, there is no work satisfyingly describing and comparing the behaviour of catalysts under integral operation for different diffusion lengths or for employment of transport pores. Simulation work with differential reactors was either only focusing on pore optimisation in general 29,30 or elaborating specifically on the relation between diffusion, selectivity and pore filling degree 28,[31][32][33][34] or FT kinetics. 35,36 When complete reactors were considered often fluidized catalyst systems have been used, [37][38][39] which are useful for kinetic studies but not directly applicable to fixed bed reactors.…”

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

“…Gardezi and Joseph (2015) modelled the performance of egg-shell cobalt silica FT catalysts and concluded that such distributions afforded more efficient metal usage over conventional configurations of catalyst and might allow for the use of larger particles, thus reducing the pressure drop across a fixed bed reactor. Becker et al (2014Becker et al ( , 2016 examined the introduction of wide, transport pores aimed at improving mass transport, promoting C 5+ selectivity and increasing effectiveness factor. The resulting numerical model was based on a micro-structured reactor of slab geometry with a catalyst coated channel wall, with the kinetics derived from the work of Yates and Satterfield (1991).…”