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Crystallization - White Sugar

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By Bento, Luis San Miguel
Posted on 1970-01-01    Last edited on 2010-12-09

In sugar refining, crystallization is done under vacuum atmosphere in order to have moderate temperatures (65ºC to 75ºC) with a convenient crystal grow velocity. Crystallization is done in single effect evaporators (vacuum pans) where sugar solutions are concentrated from about 78º brix till a supersaturation between 1.10 e 1.20. This concentration corresponds to the meta-stable zone (Chen, 1993) In this zone there is no spontaneous formation of crystals and the existing ones does not dissolves and grow. In order to form crystals in concentrated liquor and to start crystallization, a suspension of powder sugar in ethanol or iso-propanol is added to the liquor.  This operation is named "seeding". These added crystals will grow, during  crystallization, until the desired dimension.

After seeding, boiling proceeds by feeding liquor or syrup to the vacuum pan. The initial conditions of supersaturation must be maintained in this period. This operation is normally done automatically (Azevedo et al.). in order to evaluate water content in massecuites, microwave sensors are used with success. By maintaining these conditions till the pan capacity is reached, sugar with an uniform and desired dimensions is obtained.

In the final of crystallization, massecuite is concentrated in the vacuum pans, before its discharge into storage receivers. This masse is centrifuged in discontinuous high speed centrifuges. In this operation crystals remain in the centrifugal screen and syrup is separated from the masse.  After syrup separation, sugar in the screen is washed with hot water or hot water and steam. Resulting syrups, refined syrups, are returned for next crystallizations. When these syrups attain a certain colour and or purity, they are discharged to the Recovery section.

There are various techniques to make syrups management in crystallization (Boiling Schemes). The most applied boiling scheme is the "Three Boiling Scheme". In this scheme three boilings are made in series. The first boiling fine liquor is fed to the pan. By centrifugation first sugar is separated from the first jet syrup. This syrup is fed to the second boiling and second sugar and second jet syrup is obtained. This syrup is fed to the third boiling and third sugar and third jet syrup is obtained. This syrup is sent to the Recovery section. The three sugars can be commercialized separatly or mixed as a single product.

Other boling scheme is the "In Boiling Scheme". In this scheme the syrup from massecuite centrifugation is recirculated to the vacuum pan and mixed with fine liquor. This recirculation proceeds till the sugar quality is not acceptable for commercialization. The the syrup is discharged to Recovery and a next set of in boilings is iniciated.

Bibliography
 Azevedo S.F., Chorao J., Goncalves M.J., Bento L.S.M., 1993, On-line
          monitoring of white sugar crystallization through software sensors, Proc.
          S.I.T. Conf.,
217-250
Chen J.P.C., 1993, The crystallization of sugar, in Cane Sugar Handbook, Ed.
          J.P.C. Chen, C.C. Chou,  Pub. John Wiley & Sons Inc., 12th Ed., 226-312
Morel du Boil P.G., 1985, Sucrose crystal habit in a refinery, Proc. S. Afr. Sugar
          Technol. Ass.
, 59: 33-38
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
Thompson P., P. Fry, N. hasrajani, 2005, Thoughts on refinery boiling schemes,
          Proc. S.I.T. Conf., 89-100

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