Mjw. Frank et al., MODELING OF SIMULTANEOUS MASS AND HEAT-TRANSFER WITH CHEMICAL-REACTION USING THE MAXWELL-STEFAN THEORY .2. NONISOTHERMAL STUDY, Chemical Engineering Science, 50(10), 1995, pp. 1661-1671
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.