| The gas-liquid reaction process of synthesis of acetic acid,acetic anhydride and propionic acid by carbonylation is controlled by gas-liquid mass transfer.The stirred tanks are mostly used in carbonylation process.Due to strong corrosiveness of reaction medium,not only the reaction and separation equipment need to be made of corrosion-resistant materials such as zirconium and Hastelloy,resulting in high manufacturing and maintenance costs,but also causing the problems such as large mechanical stirring vibration and easy seal leakage.The industrial devices have experienced production accidents caused by mechanical seal leakage.The liquid nozzle with simple structure(static equipment)is used to replace the mechanical stirring(moving equipment),and high efficiency gas-liquid dispersion and mixing are achieved in the reactors through using vertical downward high-speed liquid jet to break up and disperse the bubbles,which can not only solve the vibration of stirrer and the leakage of mechanical seal but also can strengthen the gas-liquid mass transfer and improve reaction efficiency.The jet bubbling reactor with submerged liquid nozzle and gas distributor has the characteristics of simple structure,safety,reliability,and low maintenance cost.However,the breakup,transport,and dispersion effects of vertical downward submerged liquid jet on the bubbles and its regulation rules are still unclear,which limits the scale-up design and engineering application of jet bubbling reactor.In this thesis,the jet bubbling reactor is taken as the research object.The breakup of single bubble and transport and dispersion behaviors of bubble swarm in the vertical downward submerged liquid jet field(jet field for short)are investigated through high-speed camera,bubble image velocimetry,visual observation,acoustic emission detection and other methods.Two bubble breakup patterns,jet breakup and jet-vortex breakup,are discovered.The periodic vortex-like motion of bubble swarm under high liquid jet Reynolds number is observed.Three kinds of dispersion states,flooding,loading,and complete dispersion,appear as the liquid jet velocity increases.Empirical models of bubble breakup parameters are established,the movement evolution and transport laws of bubble swarm are clarified,and the acoustic emission detection method of gas-liquid dispersion state is invented.Furthermore,the effects of internal structural parameters and operating parameters on the gas-liquid mass transfer and liquid backmixing in a jet bubbling reactor are revealed.Based on the control of liquid backmixing degree and bubble size distribution,the process intensification methods of gas-liquid mass transfer are proposed.Finally,a mathematical model of jet bubbling reactor coupled with reaction kinetic equation of synthesis of acetic acid by carbonylation is established,which can predict the methanol conversion rate and acetic acid yield in an industrial reactor and guide the design and scale-up of the reactor.The main research work and results of this thesis are as follows:1.The breakup behaviors of single bubble in the jet field are investigated through the high-speed camera.Two bubble breakup patterns,i.e.jet breakup and jet-vortex breakup are observed.As the jet velocity and mother bubble size increase,the probability of jet-vortex breakup events increases,and the probability of jet breakup events decreases.The bubble breakup region(0-25 times the nozzle outlet diameter below the nozzle,0-2 times the nozzle outlet diameter on both sides of the jet axis)is determined.The empirical correlations of bubble breakup frequency,average number of daughter bubbles,and daughter bubble size distribution are established,and the relative deviation between the calculated values and experimental values is less than 20%.Among them,the bubble breakup frequency increases with the mother bubble size and turbulent dissipation rate.The average daughter bubble number increases with the breakup Weber number,and the curve of the daughter bubble size distribution is L-shaped.2.The movement behaviors of bubble swarm in the jet field are studied through the bubble image velocimetry.It is found that under high liquid jet Reynolds number(Rej)the periodic vortex-like motion of bubble swarm appears,and the unimodal bubble size distribution caused by bubble vortex transporting effect occurs.When the Rej is less than 14000,the bubble swarm trajectory is linear.The bubble size distribution is bimodal and the number density function(NDF)of large bubbles(>5 mm)in bottom region of the reactor is larger than that in middle and top regions.When the Rej>14000,the vortex-like motion of bubble swarm is observed,and the bubble vortices periodically oscillate.With the Rej increasing,the flapping frequency and moving region of bubble vortices increase,and the bubble size distribution at different axial heights are unimodal and basically the same.Below the nozzle outlet,the bubble rise velocity distribution is downward in the center and upward in both sides and is significantly affected by the axial height and jet velocity.Above the nozzle outlet,the bubbles move upward,and the bubble rise velocity in the reactor middle is greater than that near the wall,whose distribution is hardly affected by axial height and jet velocity.3.The gas-liquid dispersion characteristics in the jet field are studied through visual observation.It is found that with an increase of the liquid jet velocity(Uj),three dispersion states,flooding,loading,and complete dispersion,appear in sequence.And the uj at the transition point from the flooding to the loading is named flooding jet velocity(ujf),and the uj at the transition point between the loading and the complete dispersion is defined as complete dispersion jet velocity(ujcd).The acoustic emission detection method of gas-liquid dispersion state in the jet field is established,and the gas-liquid dispersion state is distinguished by the characteristic parameter of the slope k of the curve of fluctuation distribution index of the acoustic signal standard deviation(F1)versus the uj.When k=0,it is under the flooding.When k>0,the loading occurs.When k<0,the complete dispersion appears.The average relative error between the acoustic emission detection values of the ujf and ujcd and the detection values through visual observation is less than 5%.4.The effects of structural parameters of gas distributor and liquid nozzle on the gas-liquid mass transfer performance in a jet bubbling reactor are studied using the dynamic dissolved oxygen method.It is found that bubble breakup efficiency and gas-liquid mass transfer can be intensified through increasing the velocity and shear stress of radial jet at gas distributor outlet when the input power is constant.Intensification methods of gas-liquid mass transfer in a jet bubbling reactor are proposed,such as increasing the liquid nozzle outlet diameter,reducing the gas distributor diameter,and shortening the distance between the liquid nozzle and the gas distributor.Incrasing the velocity and shear stress of the radial jet at the gas distributor outlet,the bubble size is reduced,the overall gas holdup rises,and the liquid volumetric mass transfers coefficient(kLa)rises.An empirical correlation between the kLa and gas input power,liquid input power,and internal structural parameters is built,and the relative deviation between the calculated values and experimental values is less than 20%.5.A tanks-in-series with backflow model is established to characterize the liquid backmixing in a jet bubbling reactor.Through analyzing the results of cold-model experiments,it is found that the liquid backmixing degree is determined by the circulation flow of upward in the center and downward near the wall caused by gas bubbling,the circulation flow of downward in the center and upward near the wall caused by liquid jet,and the local short-circuit of fresh liquid feed.Increasing the liquid jet velocity or decreasing the superficial gas velocity,the liquid circulation caused by gas bubbling is weakened and the liquid circulation caused by liquid jet is strengthened,so that the liquid backmixing degree first decreases and then increases.When the superficial gas velocity is low,and the jet velocity is high,compared with the liquid feeding from the nozzle,the amount of liquid short-circuiting is higher when the liquid is fed from the bottom,resulting in a larger liquid backmixing degree.While under other operating conditions,the liquid backmixing degree feeding from the bottom is smaller than that feeding from the nozzle.Furthermore,the reaction kinetic equation of methanol carbonylation to acetic acid is coupled with the flow model,and the mathematical model of jet bubbling reactor is established.The relative deviation between the model calculation values of methanol conversion rate and acetic acid yield and the industrial values is less than 1%,which verifies the accuracy of the model.Through simulation analysis,it is found that with an increase of the reaction temperature,methanol feed concentration,and effective reaction volume,the methanol conversion rate increases and the acetic acid yield decreases.Increasing the liquid circulation flow or superficial gas velocity,both methanol conversion rate and acetic acid yield increase. |
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