Font Size: a A A

Mathematical Models And Numerical Simulation Of Multiphase Flow And Crystallization In Gas-liquid/Slurry Reactors

Posted on:2019-03-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q LiFull Text:PDF
GTID:1311330545952098Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
Gas-liquid/slurry reactors are widely used in petrochemical,biochemical and environmental engineering processes.There are many mathematical modeling and numerical simulation investigations on these reactors.Because of the complex interactions among bubbles and that of bubbles with liquid,in addition to the diversity of the combination of interphase force models,bubble induced turbulence models and bubble aggregation and breakage models,there is no universally applicable set of models.Mathematical models and numerical simulation studies on bubble columns and airlift loop reactors are carried out in this work based on the computational fluid dynamics-population balance equation(CFD-PBE)coupled model.Firstly,a CFD-PBE coupled model for the gas-liquid two phase system is developed using an in-house code programmed with the Fortran language.In order to overcome the shortcoming of over-estimated large particle fractions by the fixed pivot method,we has realized for the first time the application of the cell average method,which is developed based on the fixed pivot method,in the CFD-PBE coupled model.The model is applied to simulate the gas-liquid flow in a bubble column,and the PBE is solved by using either the cell average method or the fixed pivot method.The simulated results of the gas holdup and axial liquid velocity by using both methods are in reasonable agreement with the experimental data,validating positively this CFD-PBE model.However,the cell average method performs better in predicting the mean bubble diameter and the bubble size distribution(BSD).The fixed pivot method gives a larger mean bubble diameter and a wider BSD,and it also overestimates the probability density function of large bubbles.Then,the effect of superficial gas velocity is investigated numerically.It is indicated that with the increase of superficial gas velocity,the total gas holdup in the bubble column increases and the mean bubble diameter increases slightly first and then decreases.Secondly,a CFD-PBE solver is developed in the framework of OpenFOAM(an open source CFD package written in C++)and verified on a bubble column.Then,the CFD-PBE-PBE coupled model and solver for the gas-liquid crystallization system are developed by coupling the gas-liquid mass transfer,reaction processes,and the second PBE for the crystallization.This solver is used to realize for the first time the fully coupled simulation of a gas-liquid crystallization system.The formation of calcium carbonate via the reaction of gaseous CO2 with Ca(OH)2 solution in an airlift crystallizer is simulated using this solver.The effects of enhancement factor and crystallization kinetics and operation conditions are systematically investigated.The results indicate that the enhancement factor slightly influences the variation of solution pH and the concentration of Ca2+,while basically it has no effect on particle diameter and crystal size distribution(CSD).Crystallization kinetics significantly influences the simulation results.The model can accurately predict the variation of pH,Ca2+ concentration and the mean crystal size with time when an appropriate crystallization kinetics model is used.The increase in superficial gas velocity results in the increasing consumption rates of OH-and Ca2+,and the particle diameter increases slightly.Higher initial concentrations of reactants lead to a smaller particle diameter and a narrower CSD.At last,mathematical models and numerical simulation studies on bubble columns are generally carried out under atmospheric pressure,while most industrial bubble columns are operated under elevated pressure.Therefore,some modifications are made in the drag force model and the bubble breakage model in the gas-liquid CFD-PBE solver.By introducing modification factors,the effect of pressure on the gas holdup and bubble size is demonstrated.This solver is employed to simulate the bubble columns operated at different pressures,and the predicted gas holdup is in reasonable agreement with the reported experimental results.At higher superficial gas velocities,the predicted bubble diameter decreases with the increase of pressure.Predicted results of BSDs show the probability distribution function of large bubbles decreases and that of small bubbles increases with increasing pressure,indicating that the bubble breakage rate is enhanced.
Keywords/Search Tags:Bubble column, Airlift loop crystallizer, Gas-liquid two-phase flow, Computational fluid dynamics, Population balance equation
PDF Full Text Request
Related items