To crystallize sucrose, sugar solutions must have a concentration higher than saturation. To achieve this quickly, liquor feeding the vacuum pans must have a solids concentration about 78º brix, in order to attain quickly the desired sobresaturantion. Therefore fine liquor must be concentrated from about 62 brix to 78º brix. This concentration is normally done in double effect evaporators with heating calandrias. Steam at 1kg/cm2 is fed to the first body calandria. Second body is heated with vapors from the first body. Vapors released in the last body are condensate through a barometric condenser.
Simplified scheme of a double effect evaporator
Evaporators must be designed in a way that a good liquor circulation is achieved in the interior of the bodies, avoiding points with high residence time, specially near the calandria surface. This is important in order to avoid colour formation in the liquor.
There are different evaporator types: as the calandria evaporator, falling film evaporato and the plate evporator.
Smejkal Q., R. Schick, L-G Fleischer, 2005, Influence of reduced temperature during evaporation and
crystallization on sugar quality, Proc. of S.I.T. Conf., 61-72
Vidal M.C., F.C.C. Broughton. 1989, The new plate evaporator at Sidul Refinery, Proc. of S.I.T. Conf., 1-13