Section 191, Section 192, Section 193, Section 194 and Section 195 discuss the behavior of pure single-component fluids during evaporation. However, in the chemical and petroleum industries in particular, many processes involve the evaporation (and condensation) of binary (n = 2) and multicomponent (n > 2) mixtures, where n is the number of components. There are thermodynamic advantages, also, in using mixtures of refrigerants which evaporate and condense over a range of temperatures. In the boiling of binary and multicomponent mixtures, the heat transfer and the mass transfer processes are closely linked, with the evaporation rate often being limited by the mass transfer processes. This is significantly different from single-component systems, where interfacial mass transfer rates are normally very high.

In this section the differences to be expected in the basic physical processes will be considered. This will lead to a presentation of the available information on pool boiling of mixtures (Section 197). Finally, the problems of predicting heat transfer rates in forced convective evaporation of mixtures will be examined (Section 198). Starting with the early work of Bonilla and Perry (1941) and Cichelli and Bonilla (1944), a considerable body of published information on pool boiling of biliary mixtures has been built up. There is less information on forced convective evaporation though recent work has revealed some surprising features.

A. Elementary phase equilibrium

(a) Binary systems

If we consider a mixture of vapours consisting of components A and B which has a total pressure  p then we can regard this pressure as being made up of partial pressures  p_A and  p_B such that  p =  p_A +  p_B. Strictly speaking, the concept of partial pressure applies only to ideal vapour mixtures where the partial pressure p_i, related to the mole fraction ỹ_i, of the ith component in the mixture. Thus: