Experiment And Numerical Simulation Of Local Gas Dispersion In Gas-Liquid Reactors

Gas-liquid and gas-liquid-solid mechanically stirred reactors are widely applied in many process industries, for example, petrochemical, biochemical and sewage treatment process. With the intensification of gas-liquid reaction process and the scale-up of equipment, new requirement raised for optimizing the existing reactors or designing novel reactors with high performance. Investigation on the local gas dispersion in these reactors can provide guidance in optimization and design. However, due to the complexity of the gas-liquid system and the restriction of measurement techniques, the study on local gas dispersion under high superficial gas velocity is still weak in recent research work. In present study, the local gas dispersion in gas-liquid reactors was investigated in detail under medium and high superficial gas velocity by both experimental measurement and numerical simulation. The main contributions of the paper include:The local gas holdup and bubble size distribution were measured with double-tip conductivity probes in a dual-impeller agitated vessel under medium and high superficial gas velocity. The effects of stirring speed, gas inlet rate, impeller combinations on gas dispersion performance were investigated. It was found that the effects of stirring speed and gas inlet rate on gas dispersion decrease under medium and high superficial gas velocity compared with that under lower superficial gas velocity. Gas dispersion became worsen under medium and high superficial gas velocity. For the stirred vessel with impeller combination of pitch up turbine as upper impeller and concave blade turbine as lower impeller (PTU-CBT), the lower impeller discharge capacity of dispersing gas to the bottom region decreases with increasing of the gas inlet rate, local gas holdup increases greatly in lower impeller region while lightly in upper impeller region and upper circulation region. It is reasonable to mount the pipe gas distributors near wall region in stirred vessel under high superficial gas velocity condition.The gas dispersion under relatively high superficial gas velocity was simulated numerically with computation fluid dynamics (CFD). The Euler-Euler multiphase flow model, multi-reference frame method and a transport equation for the bubble number density (BND) function were used in the numerical simulation. The gas and liquid velocity, gas holdup and bubble size were achieved. The simulation results are in good agreement with the experimental value measured with double-tip conductivity probes, which indicated that the numerical method in our work could well predict the gas dispersion in stirred vessel under relatively high superficial gas velocity. The numerical simulation results show that, with the increasing of gas inlet rate, the eddy flow and back mixing of gas and liquid weaken, the maximum velocity of gas and liquid in impeller region decreases. Under high superficial gas velocity, the uniformity of bubble size distribution decrease, bubbles are inclined to coalesce in surface region. With increasing of the stirring speed, the global mean bubble size falls and the global gas holdup increases.The fluid flow in a boiling stirred vessel and gas residence time distribution in an aerated stirred vessel were numerically simulated, respectively. The phase change mode couple CFD method was used to predict the hydrodynamics in boiling stirred vessel. The numerical simulation results show that, gas holdup distribution in boiling stirred vessel is quite different from that in aerated stirred vessel. When heat was introduced from the bottom of the vessel, the boiling zone is mainly in surface region. The gas holdup in surface region is very high while is almost zero in bottom region. DPM method and tracer method were adopted to predict the gas residence time distribution. The numerical simulation results show that, bubble size, stirring speed and gas inlet rate have great effects on gas disperson and back mixing in aerated stirred vessel.Among the investigation of many dual-impeller combinations with experimental and CFD method, it was found that PCBDT-CBDT impeller combination is the best for gas dispersion under the same operating condition. The numerical simulation results also show that impeller combination with holes on the blades is more effective for gas dispersion than the corresponding impeller combination without holes on the blades. Gas distributor with large diameter is better for gas dispersion for agitated vessel mounted lower radial impeller under medium and high superficial gas velocity.The gas dispersion in bubble columns was experimental investigated and numerically simulated. Flow pattern in bubble column can be divided into bubbling region, transient flow region and fully developed flow region from lower side to upper side. Gas holdup and bubble size change sharply with the axial height in transient flow region while almost keeps unchanged in fully developed flow region. To improve the fluid flow, novel technique of mounting double stage gas distributor and flow draft tube in the bubble column was adopted. Bubble column with double stage gas distributors can improve the uniformity of oxygen distribution for oxidation reaction system compared with that with single stage gas distributor. Bubble column with flow draft tube can improve the liquid circulation and make gas liquid reaction evenly in the bubble column.