Apart from the fundamental arrangements shown in Fig. (4), several other
hybrid types of crystallizer designs are used in industry. Fig. 7 shows a
collection of the simplest category
Figure 7 FC-type crystallizers
The stirred-tank crystallizer (1) is selected for vacuum-cooling
crystallization. The horizon-tal crystallizer (3) is also a vacuum-cooled
crystallizer, and is characterized by several stages arranged in series within
a single outer shell. Compared to the single-stage, vertical, agitated-tank
crystallizer, a horizontal multi-stage crystallizer can result in lower
investment costs. The draft-tube crystallizer (2) can operate with low-speed
circulators, and therefore better control of the process supersaturation, and
lower primary nucleation are possible. This type is used for products which
have low growth rates and narrow metastable ranges. The forced-circulation
crystallizer (4) is comparable in function to the draft-tube crystallizer. The
con¬trolled recirculation of the suspension is carried out by means of an
axial-flow pump through an external heat exchanger. This type of crystallizer
can also be used for vacuum-cooling crystallization (5).
In FC
crystallizers, the suspension density is determined by the mass flux. Higher
densities (e. g., for achieving a faster desupersaturation rate) can be
achieved in Fluidized Bed or DTB crystallizers if clarified solution is removed
from the crystallizer separately (see fig. 4, centre and right). The
low-intensity recirculation of the suspension, the removal of the fines, and
the separate removal of clear liquor, as well as the classifying effect,
contribute strongly to the production of coarser crystals. Thus, products such
as ammonium sulphate, potassium chloride or urea are produced in this type of
crystallizer, with average particle sizes of about 1.5 mm.
Still
coarser particles can be produced in fluidized-bed type crystallizers. Best
known is the "Oslo"-type (fig. 4, right). At present, two variations of this
crystallizer type exist. The original, also known by the name "Krystal” has
certain operating problems due to the for-mation of incrustations that quickly
hinder circulation. The more recent "MESSO" variant was developed especially
for crystallization of substances prone to form incrustations, and does not
have these problems: By reversing the flow in the evaporation section, the
superheated solution from the heat exchanger moves from the walls to the
center, from where it is removed. This means that the solution supersaturation
increases (as evapora-tion occurs) in a zone away from wall surfaces, and the
formation of incrustations in the evaporation section is thus avoided.
Operating times of several weeks can be achieved by such Fluidized Bed
crystallizers.
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