Backmixing And Periodic Gas-Pulsing Operation In A Multi-stage Segmented Bubble Column

Bubble columns are common multi-phase reactors,due to their simple structure,long liquid residence time,high heat and mass transfer efficiency,low operating and maintenance costs and other outstanding advantages,they are especially suitable for slow reactions and reactions with obvious thermal effect.These reactors have been widely used in petrochemical industry,biological and environmental engineering.The large-scale recirculating flow of the liquid phase in bubble columns without internal components contribute to their serious liquid phase backmixing phenomenon,which have bad effect on the performance of bubble column and made it difficult to meet the needs of production processes.In this thesis,based on the mechanism that the tank-in-series structure can effectively decrease the backmixing in the reactor,we developed a new type of partition plates,which were introduced into the traditional hollow bubble column and form the multi-stage segmented bubble column for the purpose of dividing the structure of the large-scale recirculating flow of the liquid phase to reducing the backmixing effects of the liquid phase.At present,the understanding of the flow and mixing mechanism of the multi-stage segmented bubble column is far from satisfactory,so it is necessary to research into its back-mixing characteristics to provide guidance for the operation and design of the reactors.The experiment was carried out in a bubble column cold mold device with a diameter of 50 cm and a height of 530 cm.The difference between the axial velocity distribution and the residence time distribution of the liquid phase in the bubble column before and after the addition of the partition plats was mainly investigated.The experiment of the axial liquid velocity distribution shows that the addition of the internal partition plates would make the liquid axial velocity distribution in the bubble column become gentle,and the central liquid velocity are decreased,and the reverse liquid velocity near the side wall also decrease.The turning point toward the liquid velocity is closer to the side wall,and the large-scale recirculating flow of the liquid phase is restricted,thereby reducing the liquid phase back-mixing.The pulse tracer method was used to measure the residence time distribution,and the tank-in-series model was used to compare the liquid backmixing before and after adding the partition plates.The results show that the liquid-phase backmixing in the multi-stage segmented bubble column with partition plates inside is significantly reduced comparing to the hollow bubble column,and the number of equivalent tanks N is increased by up to 86.7%compared with the hollow bubble column;the superficial liquid velocity is the most important factor affecting the backmixing effect.Through dimensional analysis and fitting,empirical formulas for predicting the degree of liquid phase backmixing in the bubble column is obtained.Based on the fact that periodic fluid pulses can improve the backmixing characteristics of the reactor,periodic gas-pulsing are introduced in the multi-stage segmented bubble column.Through the measurement of the residence time distribution,it was verified that the periodic gas-pulsing played a role in reducing the back-mixing effect in the multi-stage segmented bubble column.Within the range of experimental conditions,the gas-pulsing operation can increase the number N at most 15.4%.In the control experiment of hollow column,the effect of periodic gas-pulsing pulse operation on the degree of backmixing is quite small.