Single- and two-phase flow in fixed-bed reactors: MRI flow visualisation and lattice-Boltzmann simulations

Three-dimensional structural magnetic resonance imaging (MRI) and MRI veloc imetry have been used to fully characterise the structure of the interparti cle pore space and the single-phase Row held in a packed bed of alumina cat alyst particles. Three orthogonal components of the velocity (V-x, V-y and V-g) are acquired such that the fluid velocity vector is determined at a po re-scale resolution of 156 mum The pore space has been analysed by unambigu ously partitioning the pore space into individual poles. Characteristics of the individual pores are combined with the MRI velocity data to determine quantitative statistical information concerning Row through these pores. Th e ability of the lattice-Boltzmann simulation technique to predict the flow field visualised by MRI is also demonstrated by performing the simulation on a lattice derived directly from the MRI experimental three-dimensional i mage of the structure of the packed bed. A direct comparison of the MRI and lattice-Boltzmann results shows there is good agreement between the two me thods. Using the pore analysis in conjunction with the velocity information , the flow field through the pore space is shown to be highly heterogeneous with 40% of the fluid Rowing through only 10% of the pores. We also show t hat the lattice-Boltzmann data may be used to calculate average molecular d isplacement propagators similar to those acquired experimentally for such s ystems. The effect of the wall on the fluid velocity and porosity is calcul ated as a function of distance from the wall. Some difference between the M RI and lattice-Boltzmann results are observed close to the wall because of inertial effects in the high velocity channels which are not simulated by t he lattice-Boltzmann method. Finally, we present initial results from the e xtension of this work to two-phase flow in packed beds. A case study of the visualisation of the extent of wetting of the packing as a function of tim e following start-up is presented. (C) 2001 Elsevier Science Ltd. All right s reserved.