As mention previously, impurities can hinder crystallization process. In the past, before clarification process is matured, a lot of loss generated in this process due to crystal growth failure. The quality of sugar was also not good due to impurities attached to the crystal. The next processes are as following.
Evaporation is a process to increase concentration by water removal. The sugar solution is heated above its boiling point to vaporize water. Water vapor which is lighter will go to the top of evaporator vessel and transferred to a condenser to cool it. Cooled water obtained in the condenser is called as condensate. The condensate is pumped out from condenser vessel thus water can stay vaporized. To help the process, the evaporator often equipped with a vacuum pump or barometric condenser which help to reduce the boiling point of the solution. By reducing boiling point, the heat required to evaporate will be less, and sugar will not be exposed to a high temperature which may destroy sucrose content. In sugar factory, it is common to have 2 step multi-effect evaporator. First is low vacuum evaporator and next is high vacuum evaporator. Both may have a 5-effect type. Low vacuum is used in the first stage to remove most of the water and avoid too much entrainment of sugar solution to the condensate. In the next step, higher vacuum is used when the solution has been concentrated. Thus, less entrainment will likely occur. At this step, the concentration shall reach beyond its solubility limit to permit crystallization to occur.
Crystallization is another important step in cane sugar processing. Concentrated sugar solution from evaporation step is transferred to crystallization vessel which in some design also in vacuum condition. Often, a seed is added during the process. Crystal seed is required to initiate crystallization process and accelerate crystal growth. Seed acts as a nucleus and the sucrose will attach to it layer by layer and the crystal will grow. Recycled liquor from separation process also can be used for seeding. However, impurities in the liquor may have an adverse effect. Since it is necessary to grow crystal to an acceptable size, residence time becomes an important factor in this process.
- Separation and Drying:
Crystal slurry will be processed by a centrifugal separator to separate liquid from the crystal. As implied by its name, it uses centrifugal force to throw water from the crystal. Often, clean water is sprayed to the crystal during the process. It will improve the cleanliness of the crystal. However, it may dissolve the crystal which may increase process loss. The excess water is called as mother liquor.
After separated, the crystal shall be dried through a fluidized dryer to get dry sugar crystal.