Experimental Studies And CFD Simulations Of Fluid Flow And Mass Transfer In A Structured Packed Column At Elevated Pressure

The performance of Mellapak 250Y corrugated structured packing in distillation applications at pressures ranging from 0.3 to 2.0MPa had been analysed by using HTU-NTU method. These data were obtained in a 150mm diameter distillation column operated with n-butane/n-pentane system at total reflux. In considering the axial backmixing effects, the height of an overall gas phase transfer unit, HTUOG, was divided into two parts. One part represented the height of an overall gas phase transfer unit, if no backmixing occurs, designated as HTU*OG, and the other part, designated as the height of a backmixing unit (HBU), represented the backmixing effects. The HTUOG was evaluated from the measured concentration profile of n-butane in liquid phase. The HBU obtained experimentally was correlated in terms of the properties of the separating materials and the equivalent diameter of the structured packing. Our result showed that HTUOG varied from 0.3810 to 0.5454m with pressure increasing from 1.017 to 1.924MPa. It indicated the overall efficiency of the structured packing decreased gradually at high pressure, as a result of the vapor backmixing. An experimental study of the extent of axial backmixing in both gas and liquid phases was conducted in a 150mm I.D. column packed with Mellapak 350Y corrugated structured packing. The column was operated at pressures ranging from 0.3 to 2.0MPa with nitrogen and water flowing countercurrently through the packing under trickle-flow conditions. The gas and liquid flowrates were varied from 2.0 to 5.0 m3h-1 (at experimental conditions) and from 0.3 to 1.2 m3h-1 respectively. The amount of axial backmixing was experimentally evaluated by the pulse response techniques using hydrogen in the gas phase and an aqueous solution of NaCl in the liquid phase as inert tracers. The response of the tracer was monitored by means of thermal conductivity in the gas phase and electrical conductance in the liquid phase. The experimentally determined RTD curves were interpreted in terms of the diffusion-type model. The model parameters (backmixing coefficient and interstitial velocity) were determined by the time domain analysis of the response curves. The results indicated that axial backmixing in the gas increased notably with gas flowrate and slightly with operating pressure and liquid flowrate. And the liquid-phase axial backmixing was an increasing function of both gas and liquid flowrates and insensitive to pressure. Various correlations were developed for reproducing the experimental mixing data obtained under one-and two-phase flow conditions. The agreement between experimental and correlated data appeared to be acceptable and within ±20% of difference. According to the definition of the Representative Elementary Unit (REU), the volume averaging technique was applied to derive the governing flow equations under one-and two-phase flow conditions. Additional terms appeared in the averaged governing equations were porosity, interphase forces and hydraulic dispersivity. Depending on these governing equations, a commercial CFD (Computational Fluid Dynamics) code (PHOENICS3.3) was used to predict the fluid dynamics behavior of gas and liquid phases. The simulated profiles of pressure, velocity and concentration of the tracer were presented. Then, the CFD results had been used to fit the diffusion-type model and the obtained axial backmixing coefficient had an average error of 22% with that evaluated by the experimental data.