Study On Bubble Formation Characteristics Of Metal Porous Membrane And Mass Transfer Process In Gas-Liquid Stirred Tanks

The gas-liquid two phase in the gas-liquid stirred tank can be quickly and fully dispersed and mixed under the shear of the impeller,so that the gas-liquid stirred tanks has excellent mixing and mass transfer efficiency,resulting the wide use in biological fermentation,organic synthesis,oxidation,hydrogenation et al.The method of generating microbubbles by porous membrane can be used as a gas distribution device in gas-liquid stirred tanks because of its simple structure and good dispersion characteristics.The coupling of this technology with the gas-liquid stirred tank is one of the development directions of multiphase stirred tank in the future.In this work,firstly,the characteristics of microbubbles generated on the surface of metal porous membrane were studied by non-invasive digital image recognition method in a transparent square cavity flow channel.The effects of liquid flow rate,superficial gas velocity,liquid viscosity,liquid surface tension and average pore size of metal porous membrane surface on the average diameter of bubbles(d_av)were analyzed.The results show that d_avdecreases with the increase of liquid velocity,the increase of superficial gas velocity and the decrease of surface tension.Viscosity has a dual effect on d_av,which is with the increase of viscosity,d_avdecreases first and then increases,and d_avis the lowest when the viscosity is 1.32 m Pa·s.Based on the experimental results,an empirical correlation describing the relationship between the average bubble diameter d_avand Reynolds number Re,capillary number Ca,gas-liquid ratio u_g/u _Lis proposed.Then,the porous membrane was used as a gas sparger and applied to a gas-liquid stirred tank.Two triple-impeller combinations:a half-elliptical disk turbine and two down-pumping wide-blade hydrofoils（HEDT+2WH_D）and three down-pumping wide-blade hydrofoils（3WH_D）were used to study the mass transfer characteristics.The mass transfer performance of porous plate spargers(FS₁₅₀)and ring spargers（RS）with a diameter of 150 mm was studied under the same operating conditions.The results show that the volumetric mass transfer coefficient（k _La）of the HEDT+2WH_D+FS₁₅₀is greater than that of the HEDT+2WH_D+RS at the same input power,while the k _La of the HEDT+2WH_D+FS₁₅₀is close to that of the 3WH_D+FS₁₅₀.The above results show that the application of the metal porous film spargers in the gas-liquid stirred tanks can effectively improve the mass transfer rate in the stirred tanks.Finally,the effect of distance between the impeller and the gas sparger Ls on the k_La was studied.The HEDT+2WH_Dwas used to compare the k_La of the gas spargers with FS₂₀₀and FS₁₅₀under the same operating conditions.The results show that under the same input power and the same impellers,the sequence of k_La values is FS₂₀₀（L_S20）>FS₂₀₀（L_S50）>FS₁₅₀（L_S50）≈FS₂₀₀（L_S100）.When the impellers combination is 3WH_D,by comparing the k _La achieved by the vaulted porous spargers（VS）and FS₁₅₀and by studying the influence of L_Son k_La when using VS,it can be found that under the same input power,the sequence of k _La achieved by different structural parameters is VS（L_S20）>VS（L_S50）≈VS（L_S100）>FS₁₅₀（L_S50）.