TRANSPORT PHENOMENA IN LIQUIDS EVAPORATING AT LOW PRESSURES

The hydrodynamic stability of a binary mixture of low surface tension evaporating into a vacuum is examined using linear stability theory. The presence of the less volatile component is shown to lower the stability limit for vapor recoil from that of a corresponding pure component, due to the interaction of the heat and mass transfer mechanisms to product instability. Trends are shown to be sensitive to the difference in volatilities between the two components. The criterion for fluid inertia instability remains unchanged from that of the pure component. The difference in volatility between the two components may either be stabilizing or destabilizing for the moving boundary mechanism, depending on whether interfacial cooling produces hot spots at the troughs or crests of a wave. The torpid patches observed experimentally during evaporation of two component low surface tension mixtures cannot be attributed to an increase in the intrinsic stability of the interface. Rather, the accumulation of the less volatile constituent suppresses evaporation and induces a condition of virtual instability to the interface.; The application of the concept of addition of resistances is shown to accurately predict fluxes for triethanolamine (TEA) evaporating into a vacuum. Using natural convection correlations for heat transfer in the liquid phase and kinetic theory for the vapor phase, it is shown that it is not necessary to measure the surface temperature in order to predict mass fluxes. For pressures greater than 20 Pa, the flux of TEA is determined solely by resistance to heat transfer in the bulk liquid phase, the intrinsic resistance to molecular interchange at the surface being negligible. At lower pressures, vapor phase resistance becomes important. For TEA selectively contaminated with Igepal-CO-970, evaporating at pressures less than 0.1 Pa, experiments reveal that the intrinsic evaporation coefficient is 0.3. Apparent evaporation coefficients calculated as correction factors to the actual evaporation rates are shown to be of little predictive value for evaporation rates because of their highly situation-specific nature.; The approximate solution for evaporation of binary mixtures from a laminar jet shows that the interaction of the surface cooling and depletion effects are important even for correct prediction of qualitative trends. Mass transfer effects are shown to be more significant than surface cooling in determining separation factors. Low initial temperature gives the best separation for the ethyl hexyl phthalate-ethyl hexyl sebacate (EHP-EHS) mixture, whereas a temperature of 135(DEGREES)C gives the best separation for the normal octyl phthalateethyl hexyl sebacate (NOP-EHS) mixture. Even for ideal mixtures, the separation factor depends on the initial composition. Greater contact time decreases the separation for the EHP-EHS system, and has little effect on the NOP-EHS case. The effect of overbearing pressure is to vary the separation from the thermodynamic limit (high overbearing pressures and low evaporation rates) to the kinetic limit (low pressure and high evaporation rates). The presence of heat and mass transfer resistances in the liquid cause the evaporation factor to drop well below the thermodynamic limit for most situations encountered in practice.