An experimental and theoretical study of desorption with chemical reaction in a packed column : the carbon dioxide-monoethanolamine water system

Abstract

In contrast to gas absorption, the most expensive part of the gas purification process, the stripping of dissolved gases from spent chemically reactive absorbing solutions has received remarkably little attention from either a fundamental or a practical view point. The effect of operating variables on the plate efficiency or packed height of regenerators is largely unknown and no design procedures are available for their estimation. In this thesis an extensive set of measurements of mass transfer coefficients obtained in a pilot scale regenerator using the Carbon dioxide-Monoethenolamine system have been obtained. The desorption column was of 0.152 m (6 in.) in diameter and packed with 0.0127 m (1/2 in.) Raschig rings to a height of 1.64 m. The effect of five operating variables: liquid flow rate; vapour flow rate; pressure; (or temperature) carbonation ratio and MEA concentration on mass transfer coefficients has been investigated.Equilibrium solubility data for Carbon dioxide - Monoethanolamine - Water system has been correlated. It has been found that the equilibrium partial pressure of carbon dioxide is independent of monoethanolamine concentration below a carbonation ratio of about 0.45 and depends only on carbonation ratio and temperature. For carbonation ratios of greater than about 0.45, the equilibrium partial pressure of carbon dioxide also depends on monoethanolamine concentration. This is also in conformance with the reaction mechanism of carbon dioxide with monoethanolamine. The results reported here represent the first comprehensive investigation of the influence of plant operating variables on the performance of a solvent regeneration column of semi-commercial scale. The key finding is that the 173 experimentally determined measured mass transfer coefficients are predictable from first principles to within + 25% on average. Stripping process appear to be controlled entirely neither by the gas nor the liquid film resistances; rather, each is important with the dominance of one over the other varying with the vertical position in the column.The overall mass transfer coefficient based on the liquid phase was found to increase with increasing liquid flow rate for most of the operating conditions. The effect of liquid flow rateis carbonation ratio dependent. The effect is stronger at high carbonation ratios and becomes weaker at low carbonation ratios. It is particularly interesting to note that for very low carbonation ratios, increasing liquid flow rate can actually decrease the overall mass transfer coefficient.Carbonation ratio has a very strong effect on the overall mass transfer coefficient. The effect of carbonation ratio is gas rate dependent; at lower gas rate, the effect is strongerand at igh rates the effect is moderate. Over the range of operating variables investigated, an increase in vapour flow rate increases the overall mass transfer coefficient.The dependence of vapour flow rate is a function of carbonation ratio; being stronger at lower carbonation ratios and weaker at high carbonation ratios.The influence of pressure is peculiar: the gas film resistence decreases and the liquid film resistance increases as the pressure is increased. Thus, exhibiting a minimum value of the overall mass transfer coefficient. An increase in monoethanolamine concentration results in a decrease of overall mass transfer coefficient. The effect of reboiler efficiency on the overall mass transfer coefficient has also been evaluated. An increase in the reboiler efficiency decreases the performance of the desorption column but the overall performance increases as the efficiency of the reboiler increases. However, the effect of the reboiler efficiency is very mild.Two models, referred to as the physical and the chemical model, based on the two film theory were proposed to explain the effect of various operating variables on the mass transfer coefficient. The chemical model is found capable of correctly predicting the magnitude and trend of KOLa a with changing operating conditions within the accuracy of the experimental data. The effect predicted by the physical model on the other hand is higher by a factor of about 2 as compared with the experimental findings.Finally, a detailed procedure for the design of desorption columns and the evaluation of the performance of existing desorption columns for CO2-MEA-H2O system based on the chemical model is presented. Only fundamental physico-chemical data are required for the design and this permits optimum conditions for packed regenerators to be chosen. Computer programs for rigorous design and evaluation of existing desorption columns are also presented.