Vacuum Crystallizers produce supersaturated conditions by solvent removal and cooling. A hot concentrated solution is fed to an agitated crystallization chamber maintained at low pressure. The solution boils and cools adiabatically to the boiling point corresponding to the operating pressure. The crystallization follows concentration and the product is removed from the bottom of the vessel. The heat input to produce evaporation comes entirely from the sensible heat of the feed liquor and from the heat of crystallization of the product. It produces supersaturation by the combination of cooling and concentration effects.