CFD modelling of multiphase flow distribution in trickle beds

Abstract

Abstract Multiphase flow in trickle-bed reactors (TBR) is known to be extremely complex and depends on a multitude of effects including the physico-chemical properties of both gas, liquid and solid phases, the ratio of column diameter to particle diameter and most importantly the gas and liquid superficial velocities. Despite several works devoted to the experimental investigation of liquid distribution, there is yet no universal agreement on the influence of interstitial phenomena on overall TBR hydrodynamics. Consequently, a Eulerian multiphase model was developed to predict the liquid holdup and pressure drop in the trickling flow regime with a 3D computational grid. The multiphase model was optimized in terms of mesh density and time step for the successful hydrodynamic validation activities. The model predictions correctly handled the effect of different numerical solution parameters. Afterwards, particular attention is paid to the consequences on flow development and hydrodynamic parameters of imposing liquid maldistribution at the bed top with three types of liquid distributors. Several computational runs were carried out querying the effect of gas and liquid flow rate on overall hydrodynamics. Computational fluid dynamics (CFD) predictions demonstrated that liquid flow rate had a prominent effect on radial pressure drop profiles at the higher values whereas the gas flow rates had it major outcome at lower regimes. Regarding the liquid holdup predictions, several time averaged for radial and axial profiles illustrated that a five times increase on liquid flow rate cannot be matched by an equivalent change on gas flow rate. The increase in both flow rates was found to smooth the oscillatory behaviour of local phenomena, but the gas flow rate had an outstanding consequence on both hydrodynamic parameters. Finally, CFD simulations at atmospheric conditions were compared with the pressurized ones. Liquid holdup fluctuations of about 25% between the liquid-rich and the gas-rich zone can be smoothened as long as the operating pressure is increased until 30 bar.