The generated supersaturation (4) can be adjusted by
varying the recirculation through the crystallizer. As more solution is
recirculated, points (2) and (4) approach nearer to point (3). The coordinates
of point (1), i.e. the feed mass flow and the crystallizer cooling (operating)
temperature, define the production capacity of the system. The recirculation in
the crystallizer is adapted to this production capacity, and the
supersaturation is kept within the metastable range. The circulation flow,
therefore, is a major design criterion. The circulation flow can be calculated
by equation
1
.
This is a limiting relationship for the production capacity of
a crystallizer, and if exceeded, spontaneous nucleation can occur. Relatively
high circulation flows are usually necessary even for minor production
capacities, because the typical metastable field has a range of a only few g/l:
as an example, for a production of 1 t/h and a Delta C of 1 g/l, the
circulation flow would be 1000 m
3/h.
Fig. 6 illustrates the relationship
between desupersaturation rate and total crystal surface area. The generation
of supersaturation drives crystal growth on the crystal surface area A1 while
the suspension is circulated through the crystallizer. Looking at the
recirculation as a series of differential elements, a typical saw-tooth
function is evident. Because residual supersaturation is added to the fresh
supersaturation generated in the element that follows, all supersaturation
should preferably be consumed within a single loop. If there is a large amount
of crystal surface area available (A2), the desupersaturation (measured in kg
mass/m2 of crystal surface area) of the liquid is faster, resulting in lower
residual su-persaturation at the completion of the loop. Suspension densities
(mass of crystals/mass of suspension) between 15 and 25 wt.-%, usually satisfy
this requirement.
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