Doctoral Dissertations

Date of Award

12-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

George C. Frazier

Committee Members

Bruns D. Bogue, C. Moore, D. Clark

Abstract

Reactive distillation is becoming a popular technique for chemical separation because it has many advantages over conventional distillation processes. The use of reactive distillation allows one to utilize the heat of reaction to enhance mass transfer, to influence the chemical equilibrium by removing the reaction products, and in some cases to overcome the restrictions of azeotropic mixtures. Although the results of many studies have been published dealing with the simulation of reactive distillation in plate columns, few publications have dealt with the simulation of reactive distillation in packed columns. In this study, a mathematical model was developed to simulate reactive distillation processes in packed columns. In addition, the esterification reaction of methanol and acetic acid was studied experimentally to verify the model. Because of the corrosive nature of the system used, a packed column is more advantageous than a plate column since corrosion resistance packings are available and the columns are potentially less expensive. To accurately represent the mass and heat transfer in packed columns, the model considered both the mass and heat balance equations and the vapor-liquid interfacial flux calculations for multicomponent systems. The concentration and temperature profiles were calculated along the column using a Runge-Kutta method to solve the set of differential equations representing the system. An 84-inch-long laboratory packed column with a 1.5-inch i.d. was built and filled with 1/4-inch intallox saddle packing in order to perform the esterification reaction to compare with the predictions of the model. Selected reflux ratios, methanol-to-acid ratios, and feed rates were used to study the effects of these variables on distillate purity and reactant conversion. The concentration and temperature profiles predicted by the model showed good agreement with the experimental results. The maximum error was 20% in one run, and the minimum error was 0.56% in another run. The average error of all of the runs was about 10%, which is considered satisfactory since many variables and parameters affected both the experimental results and model predictions. The simulation showed that the results are relatively sensitive to the mass transfer coefficient for the vapor-liquid interface. It is therefore important that an accurate mass transfer correlation be available for the packing of interest for use in precise column design. Reflux ratio affects both distillate purity and fi^actional conversion. At a constant boil-up rate, distillate purity increased with increase in reflux ratio, while the fi-actional conversion decreased. It therefore appears that an optimal value of the reflux ratio may exist which may be different fi-om that obtained for conventional distillation. At constant reflux ratio and boil-up rate, the increase in methanol to acid feed ratio increased the conversion but reduced the distillate purity. The conversion also increased when a low feed rate was used with small changes in the distillate purity. Liquid hold-up is a particularly important factor in the reactive distillation of the methanol-acetic acid system. This study showed that if a packing were available capable of providing double the hold-up of the intallox packing used in this study then the reactant conversion potentially could be increased from 86% to 95% in the laboratory column. This study showed, further, that should the mass transfer model be simplified, the computational time can be reduced by as much as 20% in some cases, but with a little loss in accuracy. The simplified model can be used for preliminary, scoping calculations required for column optimization, but the complete model is recommended for finalizing the design.

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