Abstract--A new explicit relation is proposed for the prediction of the enhancement factor for reversible reactions of finite rate in chemically loaded solutions which also allows for unequal diffusivities. The relation for the enhancement factor is not based on an approximation of the absorption process, but is derived from a similarity which can be observed between the results of the approximation for an irreversible (1,1) order reaction given by, for example, DeCoursey (surface renewal model), and the exact numerical results. (1967) were replaced by the roots of these ratios in order to adapt the enhancement factors to the penetration theory. In general, this adaptation of the solution of Secor and Beutler gave reasonably good results, however, for some situations with unequal diffusivities deviations up to 20% were found. The results of the present approximation were for various reactions compared to the numerical enhancement factors obtained for the model based on the Higbie penetration theory. Generally, the agreement was reasonably good. Only 26 of 2187 preselected simulations (1.18%) had a deviation which was larger than 20%, while the average deviation of all simulations was 3.3%. The deviations increased for solutions with a substantial chemical loading in combination with unequal diffusivities of the components. For reactions with a kinetic order unequal to unity, the Ha number had to be multiplied by a factor, w,/,~t~ so that E, = ~fHaA in the regime 2

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A fundamental description of gas-liquid mass transfer with reversible consecutive reaction has been given. The Higbie penetration theory has been used and numerical simulations were carried out for isothermal absorption. Although the model can be adapted to reactions of general stoichiometric and kinetic orders, results have been provided for unit orders only. In Part I, results have been presented for the case of equal diffusivities of all the chemical species involved. In Part II, the effect of unequal diffusivity on the absorption rate and, hence, the overall enhancement factor in consecutive reaction systems has been presented in detail. Results presented here are dedicated to the case where both reaction steps are considered to be reversible. Finally, the model presented in this paper has been used to determine the selectivity towards the intermediate species using data previously reported in literature. The model system of chlorination of p-cresol in 1,2,4-dichlorobenzene has been used for this purpose. The strong influence of the mass transfer parameters on the selectivity of the intermediate has been shown.

A fundamental description of non-isothermal mass transfer accompanied by a single reversible chemical reaction has been presented. The description is based on the Higbie penetration theory. Arrhenius type dependence of solubility, reaction rates and diffusivities on temperature has been assumed. Special emphasis has been paid to bimolecular irreversible reactions where depletion of the liquid phase reactant occurs. In addition, the mass transfer behavior in the infinite enhancement regime has also been presented. It has been shown that the Shah criterion fails under conditions where depletion of the liquid phase reactant occurs. In the infinite enhancement regime, the non-isothermal enhancement factor is dependent on the ratio of the diffusivities of the reactants, the ratio of the initial stoichiometric reactant concentrations and the activation energies of solubility and reactant diffusivity. These characteristics of the infinite non-isothermal enhancement factor have been reported earlier by Asai et al. {1985, A.I.Ch.E.J. 31, 1304-1312). Additionally, it has been shown that, for bimolecular irreversible reactions, the use of correlations for interracial temperature rise that assume all heat to be released at the interface is not valid for systems with low Lewis numbers but also not for systems where depletion of the liquid phase reactant occurs. Further. the model has been used to study the effect of reversibility on bimolecular reactions. The effect of temperature dependence of the solubility of the gaseous component and diffusivities of the various species on the overall enhancement has been presented. Since the non-isothermal enhancement factor of bimolecular reversible reactions is dependent on various parameters, it is not possible to determine its value by analytical or via approximate techniques. One is forced to use numerical methods for this purpose instead. !2 1997