42 - Evaporation

Since evaporators dealing with boiling solutions, and in particular with solutions with non-volatile solutes, any calculations must account for the effect of boiling point elevation.

The vapor pressure of an aqueous solution is less than that of pure water at the same temperature; so the boiling point of the solution will be higher than that of the water. This is called Boiling Point Elevation or vapor pressure lowering.

Note that the equilibrium vapor rising from a solution exhibiting boiling point elevation will exist at a temperature and pressure such that it is superheated with respect to pure vapor. The vapor rises at the solution boiling point, elevated with respect to the pure component boiling point. The vapor, however, is solute free, so it won’t condense until the extra heat corresponding to the elevation is removed, thus it is superheated.