Prediction of Axial Mixing for a Structured Packed Column Using a Two‐equation Model

Abstract

A theoretical model based on the recently developed c 2 À e c two-equation method is proposed to predict the axial mixing behavior in a structured packed column. By solving the proposed model, the process of tracer injection experiments for determining the axial back-mixing coefficient of structured packing can be simulated. Consequently, the axial Bodenstein number and dispersion coefficient under liquid single-phase or gas-liquid two-phase flow conditions can be calculated. The validation of the proposed method is tested by simulating the twophase fluid flow behavior in Flexipac 2 structured packing. The simulated results are compared with the experimental data and satisfactory agreement is found between the simulation and the experiments. IntroductionAxial mixing, or axial dispersion, is an important nonideal flow behavior occurring in packed beds [1]. With regard to experimental investigations of axial mixing behavior in packed beds, much effort has been focused on random packing and only a few studies have been devoted to structured packing. Among them, the work of Mak et al. [2] is an early reported study of structured packing columns. The authors measured the axial dispersion coefficient of single-phase flow in a pulsed packed column. Ricardo et al. [1,3] investigated the axial mixing behavior in a large scale packed column for four different packings under single and two-phase flow conditions.The axial mixing behavior in a structured packed column is usually measured by tracer injection experiments [1][2][3][4][5]. Since this kind of experiment is usually known to be expensive and laborious, and can only be performed in laboratories, the use of virtual experiments involving a computational fluid dynamics (CFD) method is a good alternative.During recent decades, many theoretical research works have been undertaken on structured packing using computational fluid dynamics (CFD) techniques [6][7][8][9][10][11][12][13]. However, most of them were devoted to the computation of velocity distribution and pressure drop. In order to simulate the hydrodynamics of liquid-phase flow in structured packing filled with catalyst pellets, Gulijk [10] presented a Toblerone model, which simplified the problem of multiphase flow to a single phase in the channels, and facilitated the calculation of the transversal dispersion in the structured packing using the CFX code. Krishna et al. [11][12][13] carried out a series of experimental and theoretical studies to investigate the flow and mass transfer behavior within a KATAPAK-S structured packing containing a catalyst. In their work, the liquid-phase residence time distribution [11], radial and axial dispersion [12], gas and liquid-phase mass transfer [13] were investigated by the CFD method. A similar numerical study was also reported by Chen [14]. However, the simulated dispersion coefficients in these works were considerably underestimated in comparison to the experimental values. This is mainly due to the lack of proper theoretical models to describe the tracer transport pr...

Dynamic Simulation of Continuous-Contact Separation Processes with the Moment Transformation Method

“…Axial dispersion is also important in bubble columns and extraction columns. There are a number of studies that address the issue of axial dispersion in column packings. − The effect of axial dispersion on separation efficiency in the context of the classic design methods is also discussed in King’s book…”

“…There are a number of studies that address the issue of axial dispersion in column packings. [20][21][22][23][24] The effect of axial dispersion on separation efficiency in the context of the classic design methods is also discussed in King's book. 1 With the present model, axial dispersion can be dealt with very easily and intuitively.…”