Mixing baffle crystallizers

A simple method for implementation of classified-fines removal is to remove slurry from a settling zone in the crystallizer. The settling zone can be created by constructing a baffle that separates the zone from the well-mixed portion of the vessel—recall, for example, the draft-tube-baffle crystallizer described in Section V—or, in small-... 

Figures 11.3-6 and 11.3-7 show four types of mixed suspension crystallizers, several of which ate fitted with upflow draft tubes and internal annular baffles. An external circulation lorq> generally emanates from the upper section of the baffled zone and enters near the propeller in the bottom of the draft tube. A stream containing fines and/or a stream of clear liquor ate ul from the upper section of the annular baffled zone.

As mentioned above batch crystallizers are usually simple vessels provided with some means of mechanical agitation or particulate fluidization. These have the effect of reducing temperature and concentration gradients, and maintain crystals in suspension. Baffles may be added to improve mixing and heat exchange or vacuum systems may be added, as appropriate. Various design combinations are available and some are illustrated in Figure 7.1. 

Special devices for classification of crystals may be used in some applications. Figure 14 shows a draft-tube-baffle (DTB) crystallizer that is designed to provide preferential removal of both fines and classified product. As shown, feed is introduced to the fines circulation line so that any nuclei formed upon introduction of the feed can be dissolved as the stream flows through the fines-dissolution heat exchanger. The contents of the crystallizer are mixed by the impeller, which forces the slurry to flow in the indicated direction. A quiescent zone is formed between the... 

Draft tube-baffle type of crystallizer Mixed suspension-mixed product removal... 

When, in the first case, the two liquids, kept at different temperatures, are mixed with a small water-to-ethyl acetate volumetric ratio (1 100), using a suitable stirrer and no bafiles, a W/O emulsion forms first. Without baffles, the emulsion droplets do not break down. Then, as they cool and water/ethyl acetate diflfuse, solubility in the emulsion droplets decreases and crystals precipitate on the droplet surfaces. When crystallization is complete, agglomerates have formed. This mechanism is called emulsion solvent diffusion or ESD. 

A semibatch reaction crystallizer with a profiled bottom and/ or a draft tube was used by Stavek et al. (1988) to study the influence of hydrodynamic conditions on the controlled doublejet precipitation of silver chloride microcrystals. The crystallizer is a cylindrical vessel (14cm in diameter) equipped with four radial baffles and a six-pitched-blade impeller to provide good mixing. Silver nitrate and potassium chloride solutions were added at a constant rate to the crystallizer, which contained aqueous gelatin solution. The temperature and silver ion concentration in the crystallizer were controlled during the run. 

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