Date of Award

1972

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering

Department

Chemical Engineering

First Advisor

G. David Shilling

Abstract

In order that the automatic control of a process system be optimal, the dynamics of the equipment involved must be throughly understood. Distillation is a complex non-linear process and as such, many of the dynamic system parameters are difficult to predict accurately and must be determined experimentally. The purpose of this study was to develop a series of dynamic models which characterize the response of the liquid temperature on the plates of a pilot Eckey Horizontal Fractionator, to variations in liquid feed rate, liquid feed composition, and vapor feed rate. This was done by experimentally determining the frequency response of the column indirectly, using the pulse technique.

The eight inch, 32 plate column was run as a stripper using the binary methanol-water system. Liquid feeds were introduced at the top with raw steam being fed into the bottom of the column. The column was run at pressures of one atmosphere and about 200 mm Hg. Pulse-like variations were introduced into the liquid feed rate and composition and in the vapor feed rate and the time histories of these pulses and the column responses on several plates were recorded. From these data, values for the system frequency response were calculated numerically using the computerized TAFT routine. This data was then plotted on Bode diagrams. The Bode plots were analyzed to determine the form of the dynamic models of the column and their parameters.

It was found that liquid rate and composition disturbances showed first order responses plus delays which increased with distance from the feed plate. Vapor rate responses also showed first order dynamics but without delays. The major first order time constants were relatively constant on all plates for the liquid and vapor rate responses but increased with distance from the feed plate for liquid composition responses. Of considerable interest was the presence of resonance peaks in the frequency response curves. These are thought to be the result of oscillating composition transients traveling down the column in the liquid and up in the vapor flow. A term to account for resonance was included in the models. It was found that the time delays and first order time constants could be related to the liquid residence time and scale-up equations are developed for applying the results of this study to other Eckey horizontal columns.

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