CFD studies on particle-to-fluid mass and heat transfer in packed beds: Free convection effects in supercritical fluids

Abstract

Free convection effects of heat and mass transfer are of prime importance when modeling the behavior of supercritical fluids in packed bed reaction equipment and in solid extractors. Density gradients, caused by temperature or composition variations, tend to control the overall flow pattern of the fluid, making the buoyancy terms of the Navier–Stokes equations an important term to model when doing a computational fluid dynamics (CFD) simulation under the described conditions. The purpose of this work is to visualize by means of CFD techniques the influence of the buoyancy forces over flow patterns and the convective flow in a packed bed filled with a supercritical fluid, because the experimental option is very expensive and time demanding. Computations were made using a three-dimensional CFD modeling strategy. Carbon dioxide in the supercritical conditions was selected as a flowing fluid. Its transport properties at high pressure were incorporated within a CFD commercial code in order to estimate them online within the simulation process. Particle-to-fluid mass transfer in supercritical conditions was analyzed, and transport coefficients were obtained and validated. Recently, Guardo et al. [2006. CFD study on particle-to-fluid heat transfer in fixed bed reactors: convective heat transfer at low and high pressure. Chemical Engineering Science 61, 4341–4353] have presented a correlation for particle-to-fluid heat transfer in supercritical conditions based on an analogy with the correlation proposed by Stüber et al. [1996. Supercritical fluid extraction of packed beds: external mass transfer in upflow and downflow operation. Industrial & Engineering Chemistry Research 35, 3618–3628] for mass transfer. The obtained numerical results presented in this work validate the idea that the modified correlation presented by Guardo et al. [2006. CFD study on particle-to-fluid heat transfer in fixed bed reactors: convective heat transfer at low and high pressure. Chemical Engineering Science 61, 4341–4353] can be used to describe heat transfer phenomena in a packed bed under mixed convection regime at high pressures.