(48g) Optimization of Multi-Pass Distillation Tray Column by CFD Method

Authors: 
Yuan, X. G. - Presenter, Tianjin University
Gao, X. - Presenter, Tianjin University
Liu, C. - Presenter, State key laboratory of chemical engineering, Tianjin University
Yu, G. - Presenter, Tianjin University


Multi-pass distillation tray column has been widely used in chemical and petrochemcial industries. Traditional design method of multi-pass distillation tray column was mainly based on empirical models. There was very few published work on how to optimize multi-pass distillation tray by CFD method in order to increase the separation efficiency. In this paper, CFD method are employed to simulate the flow and mass transfer performance in a 12.6m in diameter four-pass distillation tray column and optimized configuration of the tray was proposed.

Based on our previous work (AIChE meeting, Distillation 2003: 479-496), a CFD model to simulate the flow and mass transfer performance of the four-pass distillation tray was proposed. Using this model, flow pattern and concentration distribution of the liquid on different trays of the column can be simulated. Using the concentration profile of the vapor and liquid phase on different trays, column efficiency can be calculated. By changing the configuration of the tray and downcomer, which include tray length, weir length, weir height, shape of the downcomer and shape of the inlet weir, flow patterns of the liquid phase on the tray can be modified. By analyzing the column efficiency at different flow patterns, optimized configuration of the tray and downcomer can be obtained.

At first only single tray was simulated. During the simulation, two important assumptions are made, one is the length of flow path between side-tray and mid-tray are equal, the other is initial velocity and concentration of vapor and liquid at the inlet are uniform. The results show the existence of liquid circulation area is apparent on side-tray rather than mid-tray, which is in agreement with literature. From the velocity and concentration profile it can be found that the maldistribution on side-tray is clearer than mid-tray due to existence of liquid circulation and velocity distribution has much influence on concentration distribution. Tray efficiency at different liquid load ratio can be calculated. It was found that tray efficiency is the highest at the liquid load ratio of 0.5 between side-tray and mid-tray.

Then the structure of downcomer was modified into a specified configuration, which can lead the liquid to flow along a certain path. Simulation result shows liquid circulation area becomes smaller and velocity and concentration distribution on side-tray become relatively uniform. In this case, mass transfer is intensified and tray efficiency was increased by 6.4%. It is concluded that the modified structure of tray was much better.

Keywords four-pass distillation tray CFD mass transfer tray efficiency tray optimization