Chemistry Reference
In-Depth Information
160
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0
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Temperature (°C)
Figure 4.1.
Solubility data for lactose from the literature (McLeod, 2007) (¤ Hudson, 1904,
&
1908;
Saillard, 1919; + Gillis, 1920;
*
Herrington, 1934;
h
Rozanov, 1962; Foremost Foods,
1970; Visser, 1982).
over a period up to 48 h (but times as short as 12 h have been used) to allow
first nucleation and then growth of lactose crystals. In many industrial plants,
the final temperature will be between 15 and 258C. If it is assumed that the
concentrate is cooled to 208C, that sufficient time is allowed for the solution
to come to equilibrium with the crystals and that all crystals are of sufficient
size, that there is 100% recovery through the recovery and washing zone, and
ignoring the losses incurred by recycled lactose from the washing stages, then
the theoretical yield can be calculated as follows: in concentrate, there are
110 g of lactose per 100 g of water; at 208C, 100 g of water can contain 19.1 g
of lactose in solution, at equilibrium, therefore, 90.9 g lactose will crystallise,
giving a theoretical yield of 90.9 g/110 g or 82.6% w/w.
Normal yields will be less than this because of losses due to (i) dissolved
lactose and fine crystals that are not recovered during the recovery/washing
operations and are lost in the mother liquor and (ii) recycled lactose from the
washing operation increasing the water load through the plant.
To increase this yield, operating conditions must be changed; several
things can be done to improve the yield.
1. Increase the concentration exiting the evaporators. The limit to this,
without crystallisation occurring in the evaporators, is the solubility
limit at the final temperature of the evaporators. If the concentration
