Evaporation in Sugar Factories

Evaporation is the process of concentrating a solution by boiling to convert some of the liquid to vapor.
There are two classes of evaporation: Heating evaporation (conversion of liquid to vapor by heating under normal pressure) and Flashing evaporation (conversion of liquid to vapor by heating under vacuum).
In the sugar factory, water is evaporated from juice in two stations: Evaporation station and Crystallization station.
In the evaporation station, thin juice with about 15% dry substance (DS) is concentrated to produce a thickened juice with about 60% DS, called thick juice.
Thin juice consists of nonvolatile solutes (sucrose and nonsucrose substances) and a volatile solvent (water).

Evaporation is carried out in multiple-effect evaporators to achieve maximum steam economy.
Each effect is arranged in series and operated so that each succeeding one operates under a higher vacuum (lower pressure).
This arrangement allows the juice to be drawn from one vessel to the next and permits it to boil at progressively lower temperatures.
To obtain a low boiling temperature in the evaporators, a condenser keeps the juice under vacuum, usually with the help of a vacuum pump placed between the evaporator and the condenser.

Sugar juice can contaminate the water system during the sugar milling process.
In an MEE (Multiple Effect Evaporator), the vapor from the first effect is used to heat the juice in the second effect, and so forth.
The first effect of the MEE is usually driven by steam, and the condensate from this effect is normally returned to the boiler feed water stream.
The concentrated juice (final evaporator syrup) is removed from the last effect by a pump.

A severe problem in cane sugar processing is the deposition of scale on the evaporator tubes or plates.
Scale deposits consist of calcium and silicate salts, enmeshed in a matrix of organic material that can accumulate very rapidly, causing a severe drop-off in heat transfer and efficiency of evaporation.
Cane mills are forced to clean their evaporators in the order of every 10-15 days, using mixtures of acid and alkali and mechanical cleaning.
Evaporator scaling is a problem in cane sugar refining and beet sugar production as well, but not to the extent that it is in raw sugar production.

Crystallization is a natural or artificial process of formation of solid crystals precipitating from a solution or a melt.
Crystallization is an aspect of precipitation, obtained through a variation of the solubility conditions of the solute in the solvent.
It consists of 2 major steps: Nucleation and Crystal Growth.
Nucleation is a step where the solute molecules dispersed in the solvent start to gather into clusters.
Crystal growth is the subsequent growth of the nuclei that succeeds in achieving the critical cluster size.
Crystal growth depends upon the conditions of super saturation.

Initially, a sub-saturated standard liquor is pumped into the vacuum pan.
The vacuum pan is equipped with a Calandria, where steam flows at a pressure greater than atmospheric pressure and releases heat to the massecuite (mixture of raw sugar and syrup) as it condenses.
A Barometric Condenser helps in maintaining the partial vacuum (0.2-0.3 bar) and 65-70°C temperature, as it absorbs the steam and condenses it by means of cooling.

Crystallization in a factory takes place under vacuum and the simultaneous processes of mass transfer and vacuum are necessary to keep the temperature at a low enough level to minimize color formation and the inversion or degradation of sucrose in the process.
The process has been carried out in batch vacuum pans, but more recently continuous systems have been introduced.
Crystallization needs to be carried out in a number of stages to reduce the sucrose content in final molasses.

The final evaporator syrup is pumped to a vacuum pan, where it is evaporated to super saturation to cause sugar crystallization.
The vacuum pan is designed to handle viscous materials and is a vertical cylinder with its bottom designed to allow easy removal of the crystallized mass.
The heating elements used in vacuum pans are short, large-diameter vertical tubes (calandria pans) or coils around the inner surface of the truncated cone of the pan (coil pans).

LOADING - Standard liquor is made to enter the vacuum pan, followed by supplying steam, which increases the temperature.
CONCENTRATION - Evaporation of water leads to an increase in the concentration of the juice into super saturation level.
SEEDING - Seeds of sucrose crystals dissolved in an alcoholic solution are added to the vacuum pan.
STABILIZATION - Super saturation conditions are maintained till the crystals attain their final size 0.5μm and a brix level of 90.4.

Massecuite from the vacuum pans is sent to centrifugal machines in which the crystals are separated from the mother liquor.
A centrifugal consists of a cylindrical perforated basket lined with a screen of perforated sheet metal.
The basket, enclosed in a metal casing, is mounted on a vertical shaft that rotates the basket, imparting centrifugal force to the massecuite.