Fluid dynamic modelling of bubble column reactors

Numerical simulations of rectangular shape bubble column reactors (BCR) are validated starting from preliminary simulations aimed at identifying proper simulation parameters for a given system and resulting up to the numerical simulation with mass transfer and chemical reactions. The transient, three dimensional simulations are carried out using FLUENT software and the results obtained for a system with low gas flow rate (48 L/h) indicated that we need enough fine mesh grid and appropriate closure of interfacial forces to predict reliably plume oscillation period, liquid axial velocity and gas holdup profiles. In case of high flow rate (260 L/h), we compared the results for the effect of different interfacial closure forces and change in inlet boundary condition for gas volume fraction. There is no change in hydrodynamic results when there is change in gas volume fraction at inlet boundary condition. The effect of virtual mass interfacial force on the simulation results is also negligible. However, the major effects of applying lift force on results of plume oscillation period, liquid axial velocity and gas holdup is predicted. For comparable simulation results to experimental data, it is suggested that requirement of enough fine grids and appropriate correlations for interfacial forces, especially the combination of drag and lift forces is necessary. To study the bubble size distribution in BCR the numerical simulations are carried out with QMOM population balance technique for air-water fluid system. After finalization of the generic moment boundary conditions with simulations with PBM using QMOM without breakage and coalescence phenomena, then we simulated the system with breakage and coalescence and eventually, the simulation results are compared with experimental and simulation data taken from the scientific literature. For better hydrodynamics results of BCR as compared to experimental results, the interfacial lift force with combination of drag force is predicted for QMOM. The discretization scheme for gas volume fraction and moments of first order upwind provided the expected results of bubble size distribution. The simulation result of QMOM with breakage and coalescence models were also in good agreement with hydrodynamics experimental results and simulation results of class methods and DQMOM for bubble size distribution results. The modelling of chemical absorption of pure CO2 gas in caustic solution is carried out in a rectangular BCR with identical simulation parameters settings of previous work. For applicability of available kinetic and physical data we developed concentration differential equations to estimate the species molar concentration with respect to time in MATLAB code. The obtained profiles of evaluation of concentration and pH were in similar fashion as compared to available CFD simulated concentration and pH profiles at a point in the bubble column with respect to time. CFD simulation taking into account the mass transfer and chemical reaction, the E-E approach is used with assumption of uniform bubble size for modelling of chemisorption of the CO2 gas bubbles into NaOH aqueous solution. The adopted models successfully predicted the hydrodynamics results and are in good agreement with experimental and simulation results, however, reaction processes results are not as per expectation and further improvement in adopted simulation methods is required for better results.