CFD Simulation Of The Performance Of Carbon Dioxide Absorption Taylor Flow Microreactor

Absorption is a typical unit operation in chemical engineering.In the process of gas-liquid mass transfer,how to improve the efficiency of gas-liquid mass transfer is an important issue in process intensification.As a new type of process intensification equipment,the short transfer time,large specific surface area of mass transfer,and intrinsic safety can be achieved in micro-reactor,which is an attractive form of mass transfer equipment to enhance gas-liquid mass transfer process.Based on the enhancement of the gas-liquid mass transfer and reaction process of CO₂in[Bmim]/BF₄/MEA aqueous solution via Taylor flow microchannel reactor,this paper uses computational fluid dynamics（CFD）method to study the hydrodynamics performance,mass transfer performance and reaction performance of Taylor flow microchannel reactor at the scale of single channel and Taylor unit cell,providing a certain basis for the design and scale-up of Taylor flow microreactor.Firstly,on the single-channel scale,the flow in the T-type single-channel micro-reactor was simulated via the VOF method.The distribution of gas-liquid two-phase flow pattern was obtained,and the mechanisms of bubble formation in the microchannel and its transition conditions were analyzed.The pressure drops,gas holdup,bubble length and liquid slug length of the microchannel reactor at different gas-liquid superficial velocities were studied and compared with the literature correlation.Secondly,on the scale of Taylor unit cell,the flow of Taylor unit cell was simulated by CFD method,and the liquid phase mixing performance of Taylor cell was investigated.The residence time distribution（RTD）curve of Taylor unit cell was obtained by particle tracking method and tracer injection method,and the axial dispersion number VDN used to characterize the axial diffusion performance was calculated.The simulation results show that there are two symmetrical vortices in the liquid slug of Taylor unit cell,which constitute the recirculation zone.The liquid film between the bubble surface and the pipe wall flows in the opposite direction.The different regimes of axial diffusion properties were identified by Taylor unit cell RTD curves at different Pe numbers.Under the pure convection regime（Pe=∞）,keeping Taylor flow with short cell length,long and thin liquid film and small bubble velocity is beneficial to reduce axial back mixing,and the empirical correlation of VDN is regressed.Under diffusion regime（Pe<10²）,the axial diffusion is independent of the bubble velocity,and it is proved that the CSTR-PFR model cannot accurately predict the Taylor unit cell RTD curve at the diffusion regime.Finally,CFD simulation of gas-liquid mass transfer and reaction process was carried out in Taylor unit cell.The gas-liquid mass transfer performance and reaction performance of Taylor cell were investigated in local and global ways respectively.The simulation results of gas-liquid mass transfer performance show that the special distribution of local mass transfer performance on the surface of Taylor bubble reflects the essential characteristics of the enhancement of gas-liquid mass transfer in Taylor flow.The dynamic mass transfer process of Taylor unit cell is divided into convection regime and diffusion regime,and the mass transfer film saturationφ>0 is a sign of the shift from convection control to diffusion control.In addition,the influence of Taylor cell parameters on the overall mass transfer performance was also investigated.Mass transfer film saturationφ_filmis a function of Fo number,available modifies the empirical correlation of mass transfer.The simulation results of the reaction performance show that the local enhancement factor of the Taylor bubble surface has a peak value in the left and right cap of bubble respectively,showing the main location of the reaction on the bubble surface.The larger the specific surface area,the higher the enhancement factor,which is consistent with the characteristic that the reaction of CO₂in[Bmim]BF₄/MEA aqueous solution is a fast reaction.In addition,the reactivity of Taylor cells at other Da numbers were also investigated.