RONCO and Coeuret [ l]
succeeded in predicting the rate of mass transfer with homogeneous second order irreversible reaction through a unique expression derived after using the reaction factor criterium. A thorough comparison among most of the available, theoretical and experimental, expressions for the reaction factor revealed that the maximum deviation with respect to the results given by the proposed expression was never larger than 10 per cent. On the same line of reasoning they[2] pointed out that a suitable expression for mass transfer coefficient calculations was Kishinevskii's[3] since it was the only one providing an explicit expression for the enhancement or reaction factor (Q). Yet it has the disadvantage of being very difficult to handle without an electronic calculator.
The purpose of this communication is to show that by extending previous approximations [4], to some of the very well known models, a simple explicit expression relating 0 with the kinetic and physical parameters may be found. When compared with exact results obtained by numerical integration of the non linear set of governing differential equations, this simple expression shows itself to be a good approximation for the maximum deviation with respect to exact results does not exceed 8 per cent even for the most unfavourable case herewith analyzed.
Van Krevelen and Hoftijzer[S] presented their rather simple method to extend film theory results for first to second order irreversible reactions. Their approximation was firstly tested by Peaceman[6] claiming a maximum deviation with respect to exact results of the order of 8 per cent, but more recently by Santiago and Farina[4], who solved the problem numerically, showed such agood agreement between approximate and exact results that the resulting Van Krevelen and Hoftijzer's expression can be regarded as almost exact (maximum deviation less than 2 per cent). Thus we shall first try to generalize Van Krevelen and Hoftijzer's method by considering the full governing differential equations, so as to include most of the existing mass transfer models.
Consider a general binary diffusion-reaction problem defined by the equation of continuity for species A and B, ' reference value (see caption to Fig. I) tResearch Fellows of the "Consejo National gaciones Cientificas y T6cnicas of Argentina".