Mechanism Of Microbubble Generation Enhanced By Swirling Flow And Its Application In Slurry Reactor

The bubble column reactor with the characteristics of strong mixing ability,high efficiency of heat and mass transfer,uniform temperature distribution,etc.,has been widely applied in gas-liquid and gas-liquid-solid reaction processes,especially for exothermic reactions,such as Fischer-Tropsch synthesis and residue hydrogenation.The bubble size distribution determines the gas-liquid flow regimes and mass transfer rate in the reactor.The smaller the bubble size is,the larger the gas-liquid interface is,which is beneficial for gas-liquid mass transfer.Therefore,the microbubble is an effective method to improve the mass transfer rate of the gas-liquid reaction and gasliquid-solid reaction controlled by mass transfer.Compared with the microporous and mechanical shearing microbubble generators,the venturi microbubble generator breaks the gas into multiple microbubbles by the liquid shear force.It has wide applications with the advantages of simple structure and low energy consumption.However,the traditional venturi microbubble generator can only generate microbubbles well with a high liquid velocity and low gas-liquid ratio,which is hard to meet the needs of process intensification of the petrochemical reaction processes operating under high gas-liquid ratio and large flux conditions.Therefore,the development of microbubble generation methods and equipment suitable for a large throughput and a high gas-liquid ratio not only has great meaning for reducing reactor volume,improving equipment efficiency and realizing the strengthening of the chemical reaction process,but also reducing the operating cost and resource consumption.Moreover,it can be promoted to mineral flotation,aquaculture and other industries,which have broad application prospects.In this thesis,a new swirl-venturi microbubble generator is proposed,which performs better microbubble generation performance at the high gas-liquid ratio,generates smaller bubbles,and strengthens the gas-liquid mass transfer rate.The strengthening mechanism of the swirling flow field on bubble formation and breakup process in the venturi microbubble generator is revealed,and the optimal operating region was obtained.However,there is a potential problem using the microbubbles in the slurry reactor of the residue hydrogenation reaction,that is,the turbulence caused by microbubbles is weak,which is unfavorable to the gas-liquid-solid mixing in the slurry reactor.In response to this problem,a process intensification method of using microbubbles to enhance mass transfer and macro bubbles to promote mixing with a"bimodal" distribution of bubble sizes is proposed.Correspondingly,a combined gas distributor consisting of a swirl-venturi microbubble generator and a ring-shaped gas distributor was designed,and analyzing the influence of the bubble size distribution on the hydrodynamics,liquid phase mixing and liquid volumetric mass transfer coefficient in the slurry reactor.Finally,the design principle of the combined gas distributor is proposed,and it is used in the design of the internals of the 10,000-ton slurry reactor of residue hydrogenation.The main work and achievements of this thesis are as follows:1.It is found that the poor coincidence between the bubble trajectories and the area with a high turbulent dissipation rate is the main reason that limits bubble breakup probability and frequency.This thesis took the lead in proposing the design idea that creates a high turbulent dissipation rate area and match the bubble trajectories by introducing the swirling liquid flow into the traditional venturi microbubble generator,and a new swirl-venturi microbubble generator is fabricated.The experimental results show that under the same operating conditions,compared with the traditional venturi microbubble generator,the swirl-venturi microbubble generator generates more bubbles with narrower bubble size distribution and higher microbubble fraction in the air-water system.In summary,the swirl-venturi microbubble performs a stronger microbubble generation ability.To be specific,the bubble Sauter mean diameter is reduced by 20%-50%,the maximum stable bubble size is reduced by 30%-50%,and the liquid volumetric mass transfer coefficient is increased by 5%-30%.Empirical correlations of the Sauter mean bubble size of the venturi microbubble generator and the swirl-venturi microbubble generator were fitted,with an error of ± 30%.2.In order to reveal the strengthening mechanism of the swirling flow on the microbubble generation,the formation process of a single bubble and the detachment process of multiple bubbles in the swirl-venturi microbubble generator were studied.The experimental results show that with the increase of Re_L,the effect of the swirling field is enhanced,four single bubble formation patterns(nucleation-expansiondetachment,nucleation-detachment,expansion-detachment,and detachment)and four multi-bubble growth and detachment patterns(uniform,chaotic,alternating and pulsation)appear in sequence.And the bubble detachment diameter and growth time both decrease first and then remain flat with the increase of Re_L.When Re_L is large enough,affected by the strong negative liquid pressure and the precessing vortex core of the swirl-venturi microbubble generator,the bubbles always fall off in the form of bubble clusters composed of several bubble sub-clusters,and the pulsation interval between two bubble clusters increases with the increase of Re_L,decreases with the increase of Fr_G.The map of the bubble formation patterns,growth and detachment patterns changing with Re_L and Fr_G was drawn according to the experimental results.Finally,the optimal operating region was determined to be the overlapping area of the detachment formation pattern,and the pulsation growth and detachment pattern:Re_L>5859,Fr_G>0.0837.3.Comparative analysis of the single bubble breakup process in the swirl-venturi microbubble generator and the traditional venturi microbubble generator.And the strengthening mechanism of the swirl flow field on the bubble breakup is revealed by analyzing the bubble breakup pattern,the location distribution of the bubble initial breakup,the bubble swarm transportation,and the daughter bubble size distribution.Different from the venturi microbubble generator where only tensile breakup pattern exists,there are two more bubble breakup patterns(i.e.,dynamic erosive breakup and static erosive breakup)in the swirl-venturi microbubble generator.As the liquid velocity increases,the probability of tensile breakup and dynamic erosive breakup decreases,but static erosive breakup starts to dominate the breakup of bubbles.Affected by the radial pressure in the swirl-venturi microbubble generator,the gas exists in the form of a gas core in the converging section and the throat.In contrast to this,the gas core disintegrates and breaks into multiple microbubbles when it enters the diverging section.Besides,the location distribution of the bubble initial breakup of the static erosive breakup pattern is more concentrated near the wall.Under the same experimental conditions,the bubble break s more severely and generates more daughter bubbles with narrower size distribution in the swirl-venturi microbubble generator compared with that in the venturi microbubble generator.Considering the bubble growth and breakup comprehensively,the design principle of the swirl-venturi microbubble generator is revised:the formation of single bubbles should conform to the detachment pattern,the growth and detachment of multiple bubbles should conform to the pulsation pattern,and the bubble breakup should conform to the static erosive breakup pattern.Therefore,the optimal operating region is the overlapping area of these three modes:Re_L>17078,Fr_G>0.0837.4.The swirl-venturi microbubble generator is used in the slurry reactor of residue hydrogenation.To meet the needs of industrial installations to enhance mixing and mass transfer,solving the problem that the liquid turbulence caused by the microbubbles is weak and unfavorable for gas-liquid-solid phases mixing,the idea of using bubbles with a "bimodal" size distribution is proposed.To be specific,the microbubbles enhance mass transfer and macro bubbles promote mixing.Based on this design idea,a combined gas distributor consisting of a swirl-venturi microbubble generator and a ring-shaped gas distributor is designed and fabricated.The effect of bubble size distribution on gas holdup,solid holdup,liquid phase mixing and liquid volumetric mass transfer coefficient in the slurry reactor was studied by experiments.Based on the experimental results,three principles for the design of the bubble size "bimodal"distribution are proposed:first,the superficial gas velocity of the macro bubble should be larger than 0.077 m/s;second,the ratio of the air intake of the swirl-venturi microbubble generator to the total intake air αSVMG should not be larger than 30%;third,the diameter difference between the macro bubble and the microbubble should be greater than 2 mm.