Multiscale modeling of fixed-bed reactors with porous (open-cell foam) non-spherical particles: Hydrodynamics

Saurish Das,N.G Deen,J.A.M Kuipers

doi:10.1016/j.cej.2017.10.047

Abstract

An accurate numerical model is proposed to simulate flow through cylindrical fixed-bed reactors with randomly packed porous non-spherical particles. The length scale for flow outside the porous particles (made of open-cell foam) is O(102) higher than the size of the internal micro-pores of the particles. To capture the flow at these two different length scales, a multiscale modeling approach, derived using volume averaging theory (VAT), is developed. The flow through and around the porous particles is computed as a single hydrodynamic field in a Cartesian computational domain. The flow within the inter-particle space is fully resolved, whereas, flow at the scale of the intra-particle micro-pores is not resolved and instead represented by closure terms. Random packings of cubic and cuboid particles in cylindrical columns of different diameter are generated using a glued-sphere Discrete Element Method (DEM) approach. The packing structures for different particle-column combinations are analysed. The effects of particle size/shape, column diameter and internal porosity of the particles on the overall pressure drop and flow distribution are investigated. The macroscopic Reynolds number (based on the particle equivalent diameter and the superficial velocity of the bed) is varied from 0.1 to 400. The effect of Reynolds number on pressure drop is analyzed, as well as the reduction in pressure drop due to the presence of the intra-particle pores. In addition, our numerical simulations have helped to elucidate the detailed fluid-solid interaction in complex bi-disperse, dual porosity porous media.

Highlights

Materials of interest such as catalysts are usually noble or transition metals
To overcome problems related to hot spot formation, multi-tubular packed bed reactors are preferably used, where a large number of parallel slender columns are placed inside a cooling jacket
Random packings by using each of the particle types are generated in four different columns of diameter D = 30 mm, 55 mm, 83 mm and 120 mm

Summary

Introduction

To synthesize the final catalyst, these catalytically active materials are usually deposited on so called catalyst carriers. The final catalyst can be in the shape of pellets, extrudates or structured, and in fixed-bed reactors these catalyst particles are usually packed randomly. The reactions (often heterogeneous) take place at the surface of the catalyst carriers and are usually highly exothermic; and proper cooling is required to avoid thermal runway. The diameter of an individual reactor tube is small and is in the same order as that of the catalyst particle diameter. As a result, it provides efficient cooling of each individual tube, thereby suppressing severe radial temperature gradients

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Conclusion