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Crystallization

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By Bento, Luis San Miguel
Posted on 2006-09-18    Last edited on 2013-09-15

 

Sucrose crystallize in monoclinic crystals, with dimensions between some decimal of millimeters up to some centimeters, in commercial sugars. In sugar refining, this operation is done under vacuum 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 1,15 e 1,25. In this zone spontaneous formation of crystals does not occurs and the existing ones does not dissolve and will 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 (Seeding).  These added crystals will grow, during the crystallization, until the desired dimension. Another process consist in fedding the pan with a seed magma. This magma can be prepared throgh a crystallization at low temperature. This process will produce crystals of high quality.

After crystals formation, 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. By maintaining these conditions,  sugar with a uniform and the desired dimensions will be obtained, when the pan capacity is reached.  

During crystallization process, sucrose diffuses through the syrup layer involving crystals, and will be fixed into the crystalline mesh, at the position more thermodynamically stable. When this operation is done at a velocity slow enough to allow  non sucrose molecules be removed to the syrup, a high quality sugar crystal is obtained. However, in practice, there is not a perfect separation between sucrose and non sucrose molecules, and some impurities remain in the crystals. From these impurities the ones that most affect sugar quality are those with colour and those with high molecular weight that in some sugar applications, as in soft drinks, produces turbidity to the final product. 

In the final of crystallization operation, massecuite is concentrated in the vacuum pans, before be discharged into the storage receivers (mixers or crystallizers).

Bibliography

Chen J.P.C. (1993) “The crystallization of sugar”, in Cane Sugar Handbook, Ed.
            J.P.C. Chen, C.C.  Chou, Pub. John Whiley & Sons, 12ªEd, 226-312
Mantovani G., Vaccari G., "Crystallization" in Sugar Technology, Ed, P.W. van der
            Poel, H. Schiweck, T. Schwartz, Verlag Dr. Albert Bartens, Berlin, 1998
Van Hook Andrew, (1961) "Crystallization - Theory and Practice", Pub. Reinhold
          Pub. Corp., Chapman & Hall Ltd. London

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