CFD Modeling Of Hydraulics And Mass Transfer Of Distillation Trays And Experimental Studies Of New Tray

A computational fluid dynamics (CFD) model was developed to predict the hydraulics and mass transfer of distillation trays. The gas and liquid phases were simulated in the Eulerian framework as two interpenetrating phases. A turbulence model based on the k ?εequations for the mixture of the two phases was used. This turbulence model was suitable for computations at all phase fraction values and can be reduced to the equivalent single phase model in the extremes when only one or other of the phase was present. Interaction between the two phases occurred via interphase momentum and mass transfer sources were calculated using the Krishna and Higbie correlations respectively.The CFD model was applied to simulate the flow patterns and hydraulics of a commercial-scale 1.2m diameter sieve tray. To testify the validity of mixture turbulence model, predicted clear liquid heights by using mixture phase and single phase turbulence model were compared with the experimental data from the literatures. The comparison revealed that the mixture turbulence model gave better predictions than single phase turbulence model. To validate of the CFD model for all operating statuses of sieve tray, hydraulics of weeping, normal and entrainment statue of a 0.5×0.5m rectangular tray was simulated. Predicted weeping rate, clear liquid height and entrainment were found to be in good agreement with the experimental data. Valve tray was the other kind of tray applied widely in chemical engineering. In this paper, the CFD model was also used to simulate the hydraulics of an open valve tray. To provide the closure relation for the model, correlation of average gas hold up fraction was built by estimated experimental data of clear liquid heights. Several simulations were carried out for a 0.5×0.5m rectangular open valve tray with varying superficial gas velocity, weir height and liquid weir load. The clear liquid heights determined by these simulations were in good agreement with the experimental data, and the validity of the CFD model for vavle tray was proved.Based on the hydraulics simulation, mass transfer of a 1.2m diameter sieve tray was predicted. The simulated results including the outlet concentration, the Murphree tray efficiency and the overall column efficiency agreed reasonably with the experimental data. The rules of concentration field of two phases, point efficiencies and transferred mass between two phases on sieve tray were obtained. Results showed that foaming layer and gas wimbles were the major interphase mass transfer zones, and the effective interfacial area and liquid flow field were the main factors influencing interphase mass transfer.To improve liquid flow field and interphase mass transfer on distillation tray, a CFD quasi-single phase model was used for a diameter 2.44m sieve tray. Improving -tray structure was used to avoid the uneven fluid flow of liquid. The results of simulations showed that adding trapeziform inlet weir or guide plates could reduce retrograde flow effectively. For simulated sieve tray, the 16.5% trapeziform inlet weir, or ten guide plates with the length of 0.2m were the best way to achieve uniform liquid flow on the tray.Because of the lower of mass transfer, efficiency and processing capacity, sieve tray does not suit the case of large ratio of liquid to gas, such as absorption, high pressure distillation. To overcome these shortcomings of sieve tray, the new gas-liquid co-current flow tray was invented in this paper. Its hydrodynamics was investigated by experiment, including clear liquid height, pressure drop and entrainment. The experimental results showed that clear liquid height, pressure drop and entrainment increased as the increase of liquid or gas flow, and the performance of gas-liquid co-current flow tray was better than sieve tray under the operating condition of large ratio of liquid to gas.