2.3.3. Boiling, crystallization and centrifuging
Different types of sugar can be produced : export (VHP or VVHP) and white sugar, for the domestic market; the main difference between them is the chemical treatment, which includes sulphitation in the white sugar production. Prior to sucrose crystallization, water from the syrup is removed using vacuum pans, similarly to evaporators; inside the pans a mixture of sugar crystals and syrup, or mother liquor (the liquid fraction), is formed, named massecuite. After a certain residence time the massecuite is transferred to the crystallizers, which normally consist of mixing tanks with cooling system where sucrose is allowed to crystallize before centrifuging. The massecuite in the crystallizers is sent to centrifuges to separate crystal from run-off syrup (mother liquor), in which hot water and steam are used to wash the crystals. Most sugar factories in Brazil use two-boiling systems, since it is not necessary to completely exhaust molasses, considering that molasses will be used as feedstock for fermentation (Chandel etal.,2014). The syrup obtained previously is conveyed to the subsequent crystallization step, which is carried out in vacuum evaporator crystallizers (batch or continuous) in two or three massecuites. In the two-boiling system, two types of sugars are produced. In the first centrifuge A-massecuite (produced in the first vacuumpan- crystallizer sequence) is fed, sugar crystals are washed with water (and/or steam) and the run-off syrup (mother liquor) is drained, producing grade A sugar and A-molasses. These A-molasses are sent to pan B, where A-molasses are concentrated by evaporation and crystallization is initiated by seeding, consisting of the introduction of fine sucrose crystals. After an appropriate residence time, which depends on the type of equipment and agitation used, the B-massecuite is sent to the crystallizer, which includes a cooling system where further crystallization takes place and the mixture is stored prior to being centrifuged. Afterwards, B-massecuite is centrifuged in continuous basket centrifuges generating grade B sugar (magma) and molasses, which are sent to alcoholic fermentation.This grade B sugar is an intermediate sugar stream that is recycled and used asseed for the crystallization of A-massecuite in pan A. The A-massecuite is sent to the crystallizer, promoting further crystallization, and then it is feed in the basket centrifuge, producing A-sugar and A-molasses. The grade A sugar produced is either white sugar or raw sugar, depending on prior process characteristics.