Coupled Transport Processes in Zero-Gravity Distillation Columns

Abstract

Distillation is an essential process in the chemical industry. To cope with small production quantities of specialized chemicals, modular production plants have gained increasing attention in the past years. Zero-gravity distillation is a small-scale distillation process, which offers high separation efficiencies and can be used as a part of modular production plants. Instead of gravity, capillary forces are utilized to sustain the fluid circulation. In order to apply the process in industry, the influence of the operating parameters and the capillary structure on the process variables has to be understood. In this thesis, the results of experiments and simulations of zero-gravity distillation are presented and discussed. The experimental setup, which is similar to a flat heat pipe, allows measuring the wall and vapor temperatures, the fluid composition at the condenser, the pressure, and the liquid-vapor interface shape. Two different capillary structures were investigated at infinite reflux with mixtures of water and ethanol. For the extensively studied triangular groove structure, the overall average ethanol fraction, the heat flux, and the inclination of the system were varied. Negative inclination angles indicate that the condenser lay higher than the evaporator. The separation efficiency defined as the difference between the ethanol mole fraction at the condenser and the system average ethanol mole fraction increased with decreasing overall average ethanol fraction, decreasing heat flux, and decreasing inclination angle. For the advanced rectangular groove structure, three fluid compositions were tested. By comparing the two groove structures, no clear conclusion could be drawn on which capillary structure promotes separation. In the theoretical model, heat transfer, mass transfer, and hydrodynamics are coupled for zero-gravity distillation in channel-shaped capillary structures. In contrast to existing models for porous media as capillary structures, the influence of the variation of the area occupied by the liquid along the flow direction is accounted for. Additionally, heat conduction in the channel wall is included. The theoretical model of zero-gravity distillation was applied to a re-entrant channel structure at different values of the heat flux, the inclination, and the radius of the circular part of the cross section. As in the experiments, the separation improved with decreasing heat flux, which means the difference between the ethanol mole fractions between evaporator and condenser increased. Changing the inclination of the system did not lead to changes in the distribution of the component mole fractions in the system. With decreasing the radius of the circular part of the capillary structure cross section, an improved separation was observed. Thus, an influence of the capillary structure design on the separation was shown.