Simulation And Backflow Analysis Of Hydraulics Of Combined Directed Valve Tray

The flow patterns and hydraulics of gas-liquid flow on a commercial scale distillation tray is of great importance in determining tray efficiency, and closely related to the tower internal structure. Due to its good performance in hydrodynamics and mass transfer, combined directed valve tray has been widely used in industrial processes. In this work, with the target of optimizing the arrangement of combined directed valve tray, we operated accurate CFD simulations and quantitative analysis of flow fields based on the experimental data. The regional and quantitative characteristics of the complex flow system were investigated in detail. The main research contents and conclusions were as follows:The interphase drag force is mainly considered for building the inter-phase momentum transfer source, and a new correlation of average gas hold-up was fitted and built in the source term for CFD simulations based on experimental data. A three dimensional model of gas-liquid two phase flow of 1.2 m diameter combined directed valve trays was developed with the Euler-Euler framework and standard k-ε turbulence model. The simulation was carried out by software Fluent 6.3 in an air-water system with float valves fully open.The flow pattern characteristics gas-liquid two phase flow was investigated.Clear liquid height was applied to verify the validity of the simulation, and the predicted liquid height from CFD simulation was in good agreement with the results calculated from the volume fraction correlations. The influences of liquid phase velocity, weir height and super gas velocity on clear liquid height were discussed. The results of CFD simulation showed that there were an obvious decline of clear liquid height exist in the valve arrangement area, especially in guiding holes. The phenomenon of liquid phase circulations was founded to be exist in around float valves, arcuate regions and overflow weir outlet, which affect the efficiency of trays largely. The driving effect of the guide hole in the float valve on the upper layer was analyzed, and the results showed and proved that the trapezoidal guide float valve has a better guiding ability than the rectangular guide valve.For an in-depth analysis on the impact of the plate structure to backflow,6 plates were designed by varying numbers and positions of the trapezoidal guide float valve on the basis of the industrial tray. Non-ideal flow of liquid phase on the trays was quantified by backflow volume ratio T% and backflow flux ratio Q%. Time-averaged values of T% and Q%, Tav% and Qav% were calculated respectively to represent the liquid phase back-mixing degree of different tray geometries. The results showed that different numbers and positions of two kinds of valves had remarkable influence on flow field,backflow volume ratio and flux ratio.By reasonable arrangement, Tav% could reduce from 38.2% to 30.4% with backflow volume decreased by 20.4% and Qav% reduce from 31.1% to 25.2% with backflow flow rate decreased by 19.0% compared to the industrial tray. It could largely decrease the liquid phase back-mixing degree and improve efficiency of the tower plate through reasonable tray structures. This study was expected to provide a guidance for tray design and optimization.