Liquid maldistribution and mass transfer efficiency in randomly packed distillation columns

The design and scale-up of packed columns have been traditionally based on one-dimensional models due to the lack of understanding of flow distributions in the packing. This often leads to unreliable design and limits the application of packed columns, particularly on large scales.; The objective of this thesis is to develop theoretical models to predict flow distribution and separation efficiency of randomly packed columns, thus providing more rigorous design tools for such columns.; To study the liquid distribution in a randomly packed column, a 0.6 m diameter air-water column has been constructed with a special liquid collector equipped at the bottom of the column. This liquid collector was installed to serve three purposes, namely, (1) to measure the liquid radial distribution, (2) to support the packing, and (3) to distribute the inlet gas flow.; The effects of liquid distributor design, operating condition, packed bed height, and liquid physical properties on liquid distribution have been experimentally determined. Two different designs of liquid distributor were used: one was a standard commercial ladder-type distributor, another was a modification of the first one which only allowed the liquid to enter the central region of the column (covered 43% of the column cross sectional area). The liquid distributions were measured over a relatively wide range of operating conditions: the gas flow rate was varied from 0 to 3.0 kg/m2s with three different liquid flow rates: 2.91, 4.78 and 6.66 kg/m2s. The packed bed height was varied from 0.9 m to 3.5 m to examine the flow distribution development along the bed height. Three different systems (water/air, aqueous detergent solution/air, and Isopar/air) were employed to study the effect of liquid properties such as surface tension and viscosity on liquid distribution. These systems were chosen since they had relatively large differences in surface tension and viscosity. The measured liquid distribution data were used to validate the models developed in this study.; The predictive models for hydrodynamics and mass transfer in randomly packed columns have been established. These models were based on theoretical volume-averaged Navier-Stokes equations and mass transfer equations. The complicated two-phase flow behavior and mass transfer characteristics in randomly packed columns have been modeled in the following aspects: (a) flow resistance offered by the solid packing, (b) pressure drop of two-phase flow across the packed column, (c) liquid and gas spreading (dispersion for volume fraction), (d) inter-phase mass transfer and effective mass transfer area, (e) mass dispersion in both radial and axial directions, (f) turbulent flow, and (g) void fraction variation in radial direction.; The models were solved with the aid of the modern Computational Fluid Dynamics (CFD) package CFX4.2 developed by AEA technology plc. The predicted liquid flow distribution, pressure drop, concentration profile and separation efficiency (HETP) were compared with experimental data obtained in this study and FRI (Fractionation Research, Inc.) data at various conditions. In general the predictions agree well with the experimental data, indicating the suitability of the proposed models for the simulation of hydrodynamics and mass transfer in randomly packed columns.