MODELING OF SIMULTANEOUS MASS AND HEAT-TRANSFER WITH CHEMICAL-REACTION USING THE MAXWELL-STEFAN THEORY .2. NONISOTHERMAL STUDY

In Part I a general applicable model has been developed which calculat es mass and heat transfer fluxes through a vapour/gas-liquid interface in case a reversible chemical reaction with associated heat effect ta kes 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 isot hermal absorption of a pure gas A in a solvent containing a reactive c omponent B has been studied. In this paper the influence of thermal ef fects on the mass transfer rates is investigated, with special attenti on to the concentrated systems. The thermal effects arise as a consequ ence of enthalpy changes due to phase transitions and chemical reactio n. Account is taken of the influence of temperature gradients on (i) t he solubility of the gaseous component in the liquid phase, (ii) the c hemical reaction rate and (iii) the mass transfer coefficients in the liquid phase. Numerical simulations show that, when compared to the co rresponding isothermal case, the thermal effects can affect the mass t ransfer rates by as much as a factor of 30. In case of high Lewis numb ers the numerically calculated mass transfer rates can very well be pr edicted from an approximate analytical expression, which has been pres ented 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 ca se of a second-order chemical reaction it was shown that thermal effec ts may change the maximum enhancement factor and consequently shift th e absorption from the instantaneous regime to the pseudo-first-order r egime. Further, it is concluded that there may exist non-isothermal ga s-liquid absorption systems where minor changes in parameters appearin g in the heat balance, e.g. binary mass transfer coefficients, chemica l reaction rate constant, Le' number or heat transfer coefficients, ma y result in drastically altered system behaviour. For situations in wh ich thermal effects are significant, also the vaporization of the liqu id mixture should be taken into account, especially when the calculate d interface temperature is near or exceeds the boiling temperature of the liquid.