CFD Simulation and Observation of Liquid Flow in Spinning Band Distillation Column
|Keywords||Spinning band distillation CFD flow regime mass transfer|
Application of spinning band distillation (SBD) can be traced back to last century, which can be used to separate liquid compounds with the property of close boiling-point and thermo-sensitive. The band rotates in column to make the up-vapor and down-condensate close contacted, so that the mass and the heat can be transferred sufficiently and the compounds can be separated perfectly. As the special structure and principle, SBD can operate at very high vacuum condition and large number of plates. Moreover, it also has the properties of low holdup and low pressure drop in column. So far, the foundational study of SBD is just very scarce. We would like to use the software of CFD to simulate the liquid field of the SBD and then validate it in experiments, which is a new and very popular method in these years.CFD simulations with software Fluent6.2 were mainly accomplished to study flow field of SBD column. With the 2D model of SBD column, we studied the distribution of liquid phase and velocities, turbulence and relative pressure in column. Besides, we proved the results of simulation correct in the experiments.With the three important suppositions, we predigested the condition of the 3D simulation and studied the turbulent model of CFD under the condition of steady temperature and pressure. In condition of moving reference frame, the three style of turbulent models: Standard k-ε,Realizable k-εand RNG k-εwere simulated and compared. In the simulation, the three-step iterative method was used to calculate the flow field, and at last we found that the Standard k-εmodel failed to simulate the violent rotational flow and curve flow on surface, however, the Standard k-εmodel well simulated the field where was slightly influenced by spinning band in the column. In addition, the Realizable k-εmodel a little failed in simulating the axial velocity in the liquid field and was also not as good as RNG k-εmodel in iteration, so we can conclude that the RNG k-εmodel was the best model to simulate the turbulent flow in SBD column.After the experimental study of SBD column, we observed the distribution and regime of liquid field under many different rotating speeds and liquid-in velocities. At last, we found that there were four liquid regimes in the SBD column, which were stripe flow, film flow, wave-film, turbulence-film. Moreover, we built a mathematical model of turbulence about the liquid flow regime. By doing this, we get the critical values of Recomb corresponding to the four different flow regimes. Furthermore, we have studied the influence of gas-velocity in column. With observation in experiments, we got the conclusion that the gas-velocity almost did not disturb the distribution and regime of liquid in column. And under normal operation, the jet-flow was just less than 5% in volume fraction of the whole holdup, so we would neglect the influence of jet on mass transfer in SBD column and just consider the most important part of film on column surface. Finally, by consulting the model of wet wall column, an empirical formula was built to describe the mass transfer in SBD column. Although the empirical formula was not proved, it was just a reference for following study.