The formation of solids in evaporators is not always undesirable and, indeed, this is
precisely what is required in the evaporator-crystalliser discussed in Chapter 15. The
evaporator–crystalliser is a unit in which crystallisation takes place largely as a result
of the removal of solvent by evaporation. Cooling of the liquor may, in some cases,
produce further crystallisation thus establishing conditions similar to those in vacuum
crystallisation. The true evaporator–crystalliser is distinguished, however, by its use of
an external heat source. Crystallisation by evaporation is practised on salt solutions having
EVAPORATION 819
such as sodium chloride and ammonium
sulphate, which cannot be dealt with economically by other means, as well as those
with inverted solubility curves. It is also widely used in the production of many other
crystalline materials, as outlined by BAMFORTH(46)
. The problem of design for crystallising
equipment is extremely complicated and consequently design data are extremely meagre
and unreliable. This topic is discussed further in Chapter 15.