Doctoral Dissertations

Date of Award

12-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

O. A. Basaran

Committee Members

C. H. Byers, L. J. Gray, J. W. Prados

Abstract

Design and scaleup of distillation equipment requires an understanding of the vapor-liquid equilibria as well as an understanding of the column hydraulics. This work focuses on two problems of fluid mechanics important in understanding the behavior of distillation columns: the shapes and stability of dielectric drops in an electric field and the flow patterns on distillation trays. Axisymmetric equilibrium shapes and stability of dielectric drops subject to an applied electric field are determined by solving simultaneously the Young-Laplace equation for drop shape and the Maxwell equations for field distribution. Both linearly and nonlinearly polarizable drops are studied, as are drops that are pendant/sessile on a supporting plate and those that are floating freely between two plates. The range of parameters for which hysteresis in drop deformation can be observed is given for linearly and nonlinearly polarizable drops. Properly accounting for the effects of nonlinear polarization brings the theoretical results of this work into accord with previously published experimental results. Detailed examination of the electric fields inside nonlinearly polarizable drops reveals that they are very nonuniform, in contrast to the nearly uniform fields usually found in linearly polarizable drops. The concept of depth averaging the equations of motion for a thin film (relative to the length of the distillation tray) is used to reduce the three-dimensional, free surface flow problem to two dimensions. A one-dimensional model is first used to gain understanding into the behavior of the bulk flow, ignoring any edge effects. A rise in the height profile on the tray, or hydraulic jump, is predicted. The effects of Reynolds number, capillary number (surface tension), tray geometry, and film thickness are all examined. The importance of the downcomers is shown. The two-dimensional model allows examination not only of rectangular trays, but also round trays and trays which are annular arcs, so called "race track" trays. A large zone of recirculation is predicted near the walls of round trays. This is confirmed in published experimental results and in experimental observations reported here. Race track trays did not show recirculation even at large arc angles.

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