Modelling of simultaneous mass and heat transfer with chemical reaction using the Maxwell-Stefan theory - II. Non-isothermal study

Abstract

In Part I a general applicable model has been developed which calculates mass and heat transfer fluxes through a vapour/gas-liquid interface in case a reversible chemical reaction with associated heat effect takes place in the liquid phase. In this model the Maxwell-Stefan theory has been used to describe the mass transport. Also in Part I the isothermal absorption of a pure gas A in a solvent containing a reactive component B has been studied. In this paper the influence of thermal effects on the mass transfer rates is investigated, with special attention to the concentrated systems. The thermal effects arise as a consequence of enthalpy changes due to phase transitions and chemical reaction. Account is taken of the influence of temperature gradients on (i) the solubility of the gaseous component in the liquid phase, (ii) the chemical reaction rate and (iii) the mass transfer coefficients in the liquid phase. Numerical simulations show that, when compared to the corresponding isothermal case, the thermal effects can affect the mass transfer rates by as much as a factor of 30. In case of high Lewis numbers the numerically calculated mass transfer rates can very well be predicted from an approximate analytical expression, which has been presented in this paper. In most cases this is also a reasonable estimate of the mass transfer rate in case the Lewis number equals unity. In case of a second-order chemical reaction it was shown that thermal effects may change the maximum enhancement factor and consequently shift the absorption from the instantaneous regime to the pseudo-first-order regime. Further, it is concluded that there may exist non-isothermal gas-li1uid absorption systems where minor changes in parameters appearing in the heat balance, e.g. binary mass transfer coefficients, chemical reaction rate constant, Lé number or heat transfer coefficients, may result in drastically altered system behaviour. For situations in which thermal effects are significant, also the vaporization of the liquid mixture should be taken into account, especially when the calculated interface temperature is near or exceeds the boiling temperature of the liquid.