Research On Three-phase Hydrodynamics And Bubble Behavior Of Two-stage Loop Airlift Reactor

As a significant type of multiphase reactor,loop airlift reactors are designed based on the bubble columns,and have many advantages,like simple structure,low energy consumption,low shear rate as without mechanical mixing facilities,and efficient in mass and heat transfer.Due to these advantages,loop airlift reactors have been widely used in bioengineering,chemical engineering and environmental engineering.By structural improvement,single stage loop airlift reactors can be constructed into multistage loop airlift reactors,which have been proved to have higher gas holdup,less liquid backmixing,and better mass transfer performance.In multi-stage loop airlift reactors,although the flowing channel is fixed and fluid shows directional flow,the flow and mass transfer are extremely complicated,especially in gas-liquid-solid three phase systems.Therefore,in this work,local hydrodynamic characteristics and bubble behaviors of three-phase system were studied in a two-stage loop airlift reactor.In this work,a two-stage loop airlift reactor and its detecting system were designed.By experimental methods,effects of operating parameters(superficial gas velocity and solid loading)on hydrodynamic parameters were researched,including bubble circulation regimes,gas holdup,liquid velocity,and axial profiles of the slurry concentration.The results showed that in this two-stage loop airlift reactor,under low superficial gas velocity,different features of bubble circulation regime were displayed in the downcomer of first(lower)and second(upper)stages.And the increase in both superficial gas velocity and solid loading can promote regime transition.Gas holdup of two stages' downcomer increased with the superficial gas velocity,while declined with the increasing solid loading.Also,local gas holdup in second stage is higher than that in first stage.In the complete bubble circulation regimes,the changing trend of liquid velocity as superficial gas velocity and solid loading is similar with that of gas holdup.For gas-liquid-solid three phase systems,empirical correlations used to predict gas holdup and liquid velocity under complete bubble circulation regime were proposed.And the axial profiles of the slurry concentration can be considered to be uniform in complete bubble circulation regime.Using dual-tip electrical conductivity probe and data acquisition system,the bubble behaviors were measured in single stage and two-stage loop airlift reactors,and the comparison between these two reactors was conducted.Results showed that in loop airlift reactors,bubble diameters are small at low axial positions.In two-stage loop airlift reactor,the bubble Sauter diameter decreases with increasing solid loading.And with low solid holdup,the bubble size in two-stage loop airlift reactor is much larger than that in single stage loop airlift reactor,while the relationship is opposite with higher solid holdup.