Crystallization in a Sugar Refinery
Theory of sugar crystallization
Growth mechanisms
The growth kinetic is consistent with Burton, Cabrera and Franck’s BCF law (see section 2.3.7).
Figure 3.1. Growth variations according to supersaturation (BCF law)
Non-sugars are adsorbed on the surface, taking the place of the sugar molecules and slowing the integration of molecules on the crystalline faces. Accordingly, BCF growth should be multiplied by a reduction factor K between 0 and 1 and which is an increasing function of the purity (see section 3.1.3) according to Table 3.2, taken from the work of Bartens [BAR 98].
|
Purity |
0.7 |
0.8 |
0.9 |
1 |
|
K |
0 |
0.09 |
0.38 |
1 |
Table 3.1. Reduction factor
The supersaturation that we typically employ depends on the quality of sugar present in the magma. We can draw the table as follows:
|
Refined sugar |
1.05 |
|
Sugar A (white) |
1.10-1.15 |
|
Sugar B (raw) |
1.10-1.20 |
|
Sugar C (after production) |
1.15-1.25 |
Table 3.2. Typical supersaturations S = 1 + а
The BCF growth rate is of the same order of magnitude as that of other crystals (organic or not). It is upon crossing the diffusion layer that the transfer is slowed, due to the very high viscosity of sugar solutions. Consequently, diffusivity is very low (see section 3.2.2).
