Experimental Study And Numerical Simulations Of Bubble Column With A CFD-PBM Coupled Model

As an important type of multiphase contactors, bubble column reactors are widelyused in chemical, petrochemical and biochemical industries. However, industrialreactors are often operated at high pressure or with high viscosity liquid. Studies on theeffect of these factors will not only help to optimize the operation of the reactor, but alsomeet the needs of the industrial reactor design and scale-up. In this work, the effects ofviscosity and pressure on the hydrodynamics and mass transfer behavior in a bubblecolumn were studied based on a CFD-PBM coupled model, and validated byexperimental measurements. The main contents include:The effect of liquid viscosity on the total gas holdup, gas holdup of large bubbles,gas holdup of small bubbles, radial profiles of local gas holdup and liquid velocity, andbubble size distribution were studied in the homogeneous and heterogeneous flowregimes by both experiments and numerical simulations with the CFD-PBM coupledmodel. The simulation results agreed well with the experimental data of bubble columnhydrodynamics in a wide range of gas velocities and liquid viscosities.The interfacial area were calculated from the local gas holdup and bubble sizedistribution, and the liquid-side mass transfer coeffcient was calculated by the slippenetration model. The effect of liquid viscosity on the gas-liquid mass transfercoefficient was then studied. The predicted interfacial area and volumetric mass transfercoefficients were consistent with experimental data for different liquid viscositiesreported in the literature. The simulation results confirmed that the volumetric masstransfer coefficients and gas holdup follow a similar trend with the superficial gasvelocity, which was observed in experiments.Based on the Young-Laplace equation and Bernoulli equation, the dynamic processof bubble/droplet breakup was analyzed in detail. The time scale of the internal flowinside a bubble/droplet and the time scale of neck shrinking were introduced, whichwere validated with experimental data. Using these two time scales, a unifiedmechanism for bubble/droplet breakup due to turbulent eddy collision was proposed.The mechanism also included the effect of pressure on bubble breakup. Compared withexperimental data, the new model could quantitatively predict not only the non-linear effects of pressure and bubble diameter on bubble breakup rate, but also the differentdaughter size distribution of bubble and droplet breakup.The combined effects of the gas sparger and liquid viscosity on average gas holdup,large bubble and small bubble gas holdup in a bubble column were studied byexperiments. The effect of sparger on the distribution of the total gas holdup wasprimarily due to its effect on small bubble gas holdup. Based on the Drift-flux model,the flow regime transition with two different spargers and with different liquidviscosities was analyzed. A correlation in term of dimensionless superficial gas velocitywas obtained for the prediction of the fraction of small bubbles in the gas phase atdifferent liquid viscosities, which was suitable for both sinter porous plate andperforated plate sparger.