Generation And Evolution Of Microbubbles In Fixed Bed Bubble Reactor

Fixed bed bubble reactor is a special reactor in which solid particles are stationary,gas and liquid flow upwardly,liquid phase is the continuous phase,and gas phase is the dispersed phase.Gas phase is dispersed in liquid phase in the form of bubbles,and bubble size is critical to the interphase mass transfer.Gas distributors in conventional fixed bed bubble reactor produce bubbles whose size is more than 1 mm.It is not conducive to the interphase mass transfer,especially at low gas liquid ratio.In this paper,a new fixed bed bubble reactor is proposed,which uses Venturi type gas-liquid distributor to produce microbubbles.Microbubble has characteristics of large specific surface area,slow rising rate and fast dissolving speed,which can enhance the interphase mass transfer.Generation mechanism of microbubble in Venturi bubble generator and Venturi type gas-liquid distributor were studied to illuminate ways of generation and evolution of microbubbles.Mechanism of evolution of microbubble was revealed by studying motion law of bubbles and bubble behaviors in liquid phase bed and structured packing bed.The main work and results are as follows:(1)Microbubble generation performance of Venturi bubble generator and Venturi type gas-liquid distributor were studied by using the high-speed camera and CFD simulation.Design principles of Venturi bubble generator and Venturi type gas-liquid distributor were proposed,and optimal structural parameters and operating conditions were obtained.Firstly,bubble size and its distribution were investigated for Venturi bubble generator under different operating conditions and structural parameters.An empirical correlation was proposed through the regression analysis of experimental data.It is also found that bubble Sauter mean diameter fluctuates with time under condition of low gas-liquid ratio.Secondly,on the basis of verifying the accuracy of CFD simulation,influence of operating conditions on the axial distribution of velocity and pressure in Venturi bubble generator was discussed.It is found that increase of liquid velocity or decrease of gas velocity makes axial distribution of pressure steeper,and shear force of liquid phase on gas phase is enhanced,which is conducive to generate microbubbles.Results also show that bias flow of liquid in expanding section is consistent with the experimental results under low gas liquid ratio,and the bias flow phenomenon disappears with increase of gas velocity.Finally,effects of bed type,spacing between Venturi bubble generator,and tilt angle of Venturi type gas-liquid distributor on bubble size were investigated.It is found that increasing the spacing is beneficial to reduce coalescence between bubbles.The recommended spacing is not less than 78 mm,and Venturi-type gas-liquid distributor has a good performance of anti-plate tilt.(2)Based on analysis of forces acting on bubbles of different sizes in the liquid-phase bed,the high-speed camera and particle trajectory velocimetry(PTV)were used.Motion low of bubbles with different size and distribution and motion low of bubbles in different bed height were revealed.It is found that tracks of bubbles can be divided into three kinds:track of bubbles with a diameter of less than 1 mm is a straight line;track of bubbles with a diameter between 1.0 and 5.5 mm is a uniformly oscillating S-shaped or "zhizi" curve;track of bubbles with diameter of 5.5 mm to 8.5 mm is a curve which is occasionally bent.Bubbles produced by venturi bubble enter into the liquid phase bed in the form of bubble bunch,and the width of bubble bunch increases with height of bed,gas and liquid flow rate.In the region where is near Venturi type gas-liquid distributor,bubble behavior is mainly controlled by liquid phase,and bubbles of different sizes show similar behavior,while bubble behavior is mainly affected by itself in the region where is away from Venturi type gas-liquid distributor.(3)Three main bubble coalescence and four main bubble breakup mechanisms in the structured packing were found by using the high-speed camera.Difference of bubble behavior between regular bubbles and microbubbles was compared.Three main bubble coalescence mechanism are decelerating coalescence,compression coalescence and cleaning coalescence.Four main bubble breakup mechanism are shear breakup,impact breakup,acceleration breakup and retention breakup.Moreover,with increase of the spacing of packing,there are more and more bubbles coalescence and breakup mechanism.In the structured packing with a spacing of 3 mm,regular bubbles whose size matches with the spacing of packing have a variety of bubble behaviors,including coalescence,fragmentation,deformation and free motion,while behaviors of microbubble are mainly coalescence and free motion.Compared with empty bed,bubble Sauter mean diameter increases significantly after bed was filled with catalyst,and the fixed bed had an obvious influence on microbubble coalescence.With increase of size of spherical particle,bubble Sauter mean diameter increases firstly and then decreases.When the size of spherical particle is 3 mm,bubble Sauter mean diameter is the largest.It is suggested choosing large size large size spherical particles helps to maintain the microbubble state in fixed bed column reactor.