CFD Modeling Of Hydrodynamic Characteristics In Gas-liquid Two-phase Reactors

The gas-liquid stirred tank and bubble reactor are widely used in the petroleum, chemical industry and other fields. Now using CFD numerical simulation method to research reactors can provide valid references to its structure design, optimization and operation. The Euler-Euler two-phase model and standard k-ε turbulence model were adopted to simulate the flow field inside the two types of reactor. The effects of stirring speeds, gas flow rates and gas distribution modes on the macroscopic and microscopic characteristics of flow field are investigated.Based on the hypothesis of a single bubble size, flow field of the three layers combined impellers gas-liquid stirred tank was simulated by Fluent. The influences of stirring speeds and gas flow rates on gas holdup, gassed mixing power demand and mixing time were studied. The results show that:The axial distribution of local gas holdup presents a multi peak form. The overall gas holdup and the local gas holdup increase with the increasing of the stirring speed or gas flow rate. The relative power consumption (RPD) decreases with the increasing of the gas flow rate. The results of the simulation are in good agreement with the experimental trend in literature. Improving the stirring speed can effectively reduce the mixing time, but the mixing efficiency is reduced. The impact of gas flow rate on mixing time is complex.The whole structure of gas-liquid stirred tank is reformed by replacing the impellers with three layer ring pipe gas distributors. The effect of gas distribution ratio and methods on the flow field were investigated. The results show that:The overall gas holdup increases with the increasing of superficial gas velocity while only the bottom distributor introduces gas. When all the three layers distributors add gas, the larger gas flow rate ratio of the bottom distributor, the greater the overall gas holdup. The radial distribution of local gas holdup presents multi peak form because of superficial gas velocity and gas distribution method. Velocity field distribution in axial direction inside the reactor is very non-uniform, there are many local cyclic flows. Liquid velocity distribution in axial section presents a central peak form, the zero speed zone range and location change with axial height. The axial liquid velocity distribution is affected by axial height, superficial gas velocity and gas distribution methods. Increasing the gas flow rate of bottom distributor and keeping the middle and upper distributor introducing gas can get a larger liquid circulation velocity and smaller mixing time.Numerical simulation results are reasonable. The results can provide effective guidance for optimizing the operation and further design of bubble reactor.