Computational study for mass transfer on commercial multi-pass trays

A two-dimensional pseudo single phase model is proposed to describe the flow and mass transfer phenomenon on a four-pass distillation tray with a diameter of 12.6 m by combination of computational fluid dynamics(CFD)and computational mass transfer(CMT)equations.The resistance and bubbling effect of the uprising gas phase are considered.The velocity profile and concentration distribution are calculated for the tray designed by traditional equal bubbling area method and for the modified tray under different operational conditions.For the traditional four-pass distillation tray,the liquid loading ratio(Q1/Q2)of 1.0 is first considered.Simulated results show that on its left-side tray there exists a recirculation region,which is in accord with the observations from industrial practice,and a large velocity difference between the arc-shaped region and the main flow region.These two factors account for the lower separation efficiency on the left-side tray compared with that on the right-side tray.Then the influence of Q1/Q2 on tray efficiency is investigated.Simulated results give an optimal Q1/Q2 around 0.8 in terms of tray efficiency.In order to optimize the tray configuration and improve the tray separation efficiency,a modified tray configuration is constructed by introducing several deflectors.The simulation is conducted under the same operational conditions as those for the traditional tray.Calculated results present much more uniform flow regime with the disappearance of recirculation phenomenon and the Murphree tray efficiency is increased by 4.53%—9.22%(Q1/Q2=[0.5,1.5]) compared with the traditional tray.With these deflectors, the optimal Q1/Q2 in terms of tray efficiency changes from 0.8 to 0.7,probably due to the much more uniform flow pattern at low Q1/Q2 as a result of deflectors.