Numerical And Experimental Study Of Transport Phenomena In A Structured Packed Column At High Pressure

To better understand the mass-transfer behaviors in the structured packed columns at high pressure, the performance of structured packing Mellapak 350Y was undertaken in a distillation column with an 8.0m height and 150mm inner diameter at high-pressure and under total reflux. The n-butane and iso-butane were used as the test system. The operating pressure range was 0.70-1.9MPa. The liquid flow rates were varied from 10.45 to 67.14 m³/(m²Â·h). The results showed that the separation efficiency of the packing increased with the increase of the operating pressure. Below 1.70MPa, the values of HETP decreased with the increase of gas F-factor at constant operating pressure. At 1.90MPa, however, the efficiency deteriorated before flooding point.A single-phase three-dimensional CFD model is built to numerically calculate the liquid velocity distribution in the structured packing. To test the calculated results, the velocity profile within the structured packing is experimentally measured using the LDV method. The comparison indicates that the CFD simulation is confirmed very well with the experimental flow pattern. Based on this finding, the backmixing behavior of the gas-phase flow is simulated at high pressure. The pressure range is 0.30¹.80MPa. The simulated result is compared with our previous experimental data. It is found from the comparison that both indicate similar tendency on the effect of the operating pressure and gas flow rate on the gas backmixing coefficient, but, the experimental data are higher than the simulated results.Using the volume of fluid (VOF) method, a two-phase flow CFD model is developed for the prediction of the total liquid holdup and the effective wetted area in gas-liquid countercurrent columns containing structured packings. The simulated system and conditions are exactly the same as in the high-pressure distillation experiments. In the present model, two source terms are added into the momentum transport equation to describe the effects of surface tension and interface stress shear on the flow. The calculated liquid holdup and effective wetted area are compared with the values predicted by Gualito model.Based on the hydrodynamics simulation work, the mass transfer process within the structured packed distillation columns is also simulated at high pressure operation. In the simulations, the (-|c₂)-ε_c model, replacing the common Schmidt number analogy, is introduced to calculate the turbulent diffusion coefficient. By the model calculations, the concentration distribution can be obtained, and subsequently by the Fenske equation, the HETPs are estimated. The modeling results agree reasonably with experimental data, and the mean relative deviation is 25%.