Bulk phase and intrinsic interfacial resistances to evaporation under vacuum

Abstract

To assess the importance of bulk phase heat transfer resistances in vacuum evaporation processes, measurements are made of the evaporation rate of triethanolamine at subatmospheric pressures. Spontaneous interfacial convection at the evaporating vapor:liquid surface is suppressed by the addition of sufficient amounts of a surface-active solute. At pressures of 100 and 21 Pa the contribution of kinetic mobility of molecules at the interface is negligible and the experimental results are predicted by accounting for the heat transfer resistance in the bulk liquid phase only. Furthermore, these results confirm the validity of published natural convection heat transfer correlations for this system. Next, experimental measurements of evaporative flux are made at low pressures (≤0.1 Pa), for which the limited kinetic mobility of molecules adds a significant resistance. Results are seen to, be consistent only with an evaporation coefficient E = 0.3, suggesting that even trace quantities of a soluble surface-active agent can dramatically alter the interfacial character of triethanolamine.