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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