CFD study and experimental validation of trickle bed hydrodynamics under gas, liquid and gas/liquid alternating cyclic operations

Abstract

Computational fluid dynamics (CFD) simulations of a trickle bed reactor under gas, liquid, and gas/liquid alternating cyclic operations have been performed by using an unsteady multiple-Euler framework by means of the commercial software FLUENT. Electrical capacitance tomography (ECT) imaging was applied to track the liquid holdup waves along the bed and to validate the CFD simulation results. Comprehensive simulations were conducted to examine the effects of reactor geometry meshing (i.e., one, two, or three-dimensional), domain initialization, numerical schemes (i.e., 1st and 2nd order upwind or 3rd order MUSCL), drag forces, constant coefficients of drag force terms (i.e., Ergun constants), gas and liquid properties and particle size. The important outcome of this work was that a one-dimensional multiple-Euler CFD model with first order numerical scheme was sufficient to accurately capture the non-steady state hydrodynamic behavior of trickle bed reactors under different cyclic operation strategies. This indicates that the alignment of flow and mesh practically eliminates (or minimizes) the numerical diffusion. Moreover, application of uniform bed porosity can predict the hydrodynamics parameters under cyclic operations. The model was able to predict accurately the morphological (i.e., the details of rise and falling or breakthrough and tail as well as plateau) characteristics of a liquid wave inside the bed for all the examined cyclic strategies, including base–peak and ON–OFF modes. In addition, simulation results were in excellent consistency with experimental data in terms of both value and behavior (i.e., fluctuations) of overall bed pressure drop. It was found that the liquid–solid drag force is the main cause of wave attenuation along the bed. The performed case studies with the CFD model revealed the significance of different parameters (e.g., Ergun constants, gas and liquid properties, packing size).