Rigorous Model For Simulation And Optimization Of Mass Transfer And Reaction Processes In Bubble Column

Numerical simulation has been effectively used to study the gas liquid flow,chemical reaction and mass transfer in bubble column reactors and can provide guidance in optimizing the reactor performance or designing a new reactor.However,the prediction of the species concentration distribution in a bubble column reactor remains challenging.The turbulent mass diffusivity,which is essentially needed for estimating the species concentration distribution,has been usually determined either by using an experimental dispersion coefficient or guessing a turbulent Schmidt number applied as a constant to the whole process.Both methods are empirical and unreliable.In order to overcome the difficulty mentioned above,we have adopted the recently developed computational mass transfer （?）-ε_c model to simulate the species concentration distribution in the bubble column.By using this model the turbulent mass diffusivity and turbulent viscosity can be obtained theoretically,so the analogy between the turbulent mass and momentum transfer can be investigated in the bubble column reactor.At first,the application of the （?）-ε_c model to the chemisorption process of CO₂into Na OH solution in a bubble column reactor is addressed.The population balance model is used to predict the bubble size distribution in the reactor.The gas holdup,bubble size,liquid velocity and the species concentration distributions are obtained by using the present model.The simulated results are shown to be in satisfactory agreement with the experimental data in literature,and the reliability of the （?）-ε_cmodel for simulating the chemisorption process in a bubble column is validated.Next,the （?）-ε_c model is applied to the biodegradation process of toluene in a bubble column and an airlift reactor.On the basis of the simulations,the analogy between the mass and momentum transfer for the bubble column reactor is investigated.And then,the principle of entropy generation extremum and its application to the mass transfer enhancement in the biodegradation process of toluene in a bubble column are presented.The liquid phase dissolved oxygen mass transfer induced entropy generation is defined as the criterion.By maximizing the criterion under the constraint of fixed viscous dissipation,the body force field and the velocity filed are obtained.By using this optimization method the mass transfer of the dissolved oxygen in the liquid phase is enhanced,and the remove efficiency of the toluene in the gas phase increases from 79.6%to 85.5%.Finally,the main research results of this dissertation is summarized,and several aspects about further improvements of the （?）-ε_c model and the optimization method are suggested.The innovation points of this dissertation are as follows:（1）.The computational mass transfer （?）-ε_c model is used to simulate the chemisorption and biodegradation processes in bubble column and airlift reactor.（2）.On the basis of the simulations,the analogy between the mass transfer and momentum transfer for the chemisorption process and biodegradation process in the bubble column is investigated.The results show that this analogy may depend on the rate of chemical or bio-chemical reaction in the bubble column.（3）.The principle of entropy generation extremum is used to enhance the mass transfer in the biodegradation process of toluene in a bubble column.By using this optimization method,the mass transfer of the dissolved oxygen in the liquid phase is enhanced and the remove efficiency of the toluene in the gas phase is increased significantly.