Masters Theses

Date of Award

5-1999

Degree Type

Thesis

Degree Name

Master of Science

Major

Chemical Engineering

Major Professor

Jack Watson

Abstract

Distillation is the preferred separation technique in the chemical industry because of its operational simplicity and efficiency. However, distillation has thermodynamic and transport limitations which determine the number of stages and the energy required to separate the components of a mixture. In this study, several types of laboratory-scale experiments were designed and conducted to investigate the hypothesis that an applied electric field on the order of a few kilovolts per centimeter enhances component separation, heat transfer, and mass transfer in a distillation processFor boiling polar- polar and polar-nonpolar mixtures, the relative volatility, known as the separation factor in a distillation process, has been shown experimentally to increase by as much as 10% in a single distillation stage. The electric field effects were only observed when there was a polar component present in the system. In these experiments, the more volatile component was the component whose concentration was increased in the vaporThe results of batch distillation experiments showed an increase in distillate concentration and a small increase in distillate flow rate with negligible power input from the applied voltage.

Experiments were performed to investigate the mechanisms involved in separation enhancements with an applied electric field. These included experiments varying the shape and separation of the electrodes and the strength and polarity of the electric field. The experiments showed that greater voltage differences led to higher separation efficiency. However, varying the electrode separation at a given potential difference, and thus the electric field strength, had little effect on the vapor composition. The geometry of the system was important in maximizing the separation improvements due to the applied voltage difference because the increase in separation efficiency was reduced under intense electrohydrodynamic conditions where liquid dynamics such as dripping, splashing, or jetting occurred. A large rise in the slope of the current, as opposed to a small, steady increase, as the applied voltage was increased indicated when the electric field enhancements had been reduced. This elevated current was sometimes accompanied by the formation of microdroplets at the surface of the liquidThese findings, in combination with calculations of the interface charge density, suggested that improvements in the separation efficiency achieved by an applied voltage difference were induced by charge accumulation at the vapor-liquid interface rather than electric field effects in the bulk liquid region of the still.

Mass transfer enhancements have been observed in a distillation stage due to electrohydrodynamic spraying. Applying voltage to a distillation tray increased the interfacial contact area between liquid and vapor by decreasing the size of the bubbles rising through the liquid in the distillation stage. This transport enhancement led to increased separation of the components and increased plate efficiency.

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