Industrial Crystallizers

Several types of crystallizers are commercially available. The choice of crystallizer depends on the material to be crystallized and the solvent, the method of crystallization, the product specifications, in particular the crystal size distribution, and the flexibility of the design in cases where products of various coarseness (L50) must be crystallized on demand in the same equipment.

Here only three large scale evaporative and cooling crystallizers will be discussed. These examples mainly serve to illustrate that in practice and in particular for a large scale crystallizer several compartments can be distinguished, where different processes may dominate.

Forced Circulation Crystallizer

The forced circulation (FC) crystallizer is the most widely used crystallizer. It is most common in multistage flash evaporative crystallization of salts with a flat solubility curve. It is the least expensive vacuum crystallizer, especially for evaporation of substantial amounts of water. The crystallizer as depicted in Figure 9 comprises two separate bodies that can be designed separately for evaporation and crystallization and for heat input.

The boiling zone with often a tangential inlet of the incoming flow should be large enough for vapour

Industrial Images Crystallizers
Figure 9 Forced circulation crystallizer with a tangential inlet (Swenson type). Reproduced with permission from Bennett (1993).

release, while the vessel bulk zone should maintain a sufficiently large volume to retain the growing crystals until the supersaturation is consumed. A slurry pump circulates the crystal slurry through the tubes of the heat exchanger, that can act as an internal fines dissolver, back into the boiling zone. This pump also creates most of the attrition fragments that may grow out as secondary nuclei, although usually an (axial-type) centrifugal pump is applied to minimize the attrition in order to get a sufficiently large mean crystal size. Because the forced circulation causes good mixing, the FC crystallizer is often modelled as a one compartment or MSMPR crystallizer in spite of its various zones.

The supersaturation and the turbulence may however differ locally.

Draft Tube Baffled Crystallizer

In this draft tube baffled (DTB) crystallizer with an external heat exchanger, as depicted in Figure 10, the heat duty is also separated from the crystallizer body. Fines removal has been realized by installing a skirt baffle that creates a settling or annular zone. The flow in the draft tube thus has to be upwards: this is effected by the impeller that also creates most of the attrition fragments. The fines flow can be diluted or heated to partly or totally dissolve

Msmpr Baffles

Figure 10 Stirred draft tube baffle (DTB) crystallizer with an external heat exchanger and fines destruction (Swenson type). Reproduced with permission from Bennett (1993).

the fines. An increase in fines flow increases the number of fines that are removed from the crystallizer, but also the cut size of the fines. The fines loop in this way serves as an actuator that can be applied for control of the mean crystal size, although the variation in mean crystal size that can be achieved is limited.

The addition of an elutriation leg at the bottom of the crystallizer or the addition of another type of classifier allows classification of the product flow, and thus also serves as an actuator to influence the CSD of the product.

Obviously the various zones of the DTB crystallizer have different functions, and different supersaturation and turbulence values can be expected, in particular for large scale crystallizers.

The DTB crystallizer can also be applied as cooling crystallizer by using the heat exchangers as a cooling system.

Fluidized Bed Crystallizer

A fluidized bed crystallizer (Figure 11) is especially designed to produce large and uniformly sized crystals. The heat duty and the crystallizer body are again separated. At the top of the bed the crystals are settled, and only the fines leave the crystallizer with the exhausted mother liquor to be circulated through the heat exchanger after mixing with the feed stream. The hot circulated flow enters the vaporizer head, where the solvent is flashed off. The supersaturated solution leaves the vaporizer through the downcomer, and enters the densely packed fluidized bed at the bottom of the crystallizer. The supersaturation is consumed on its way up, and a coarse product leaves the crystallizer at the bottom.

Secondary nucleation here results from crystal-crystal collisions. Also for this crystallizer several functions can be identified that are restricted to various zones in the crystallizer. The supersaturation and the turbulence are also not evenly distributed.

Figure 10 Stirred draft tube baffle (DTB) crystallizer with an external heat exchanger and fines destruction (Swenson type). Reproduced with permission from Bennett (1993).

Draft Tube Seals
Figure 11 Fluidized bed crystallizer (Swenson type). Reproduced with permission from Bennett (1993).

Since the residence time of the crystals can be considerably increased, this type of crystallizer is commonly used to produce large crystals.

This crystallizer can also be used for cooling crystallization. No vapour chamber is then needed. Other well-known cooling crystallizers are: first, a Swenson type where the suspension is circulated through a heat exchanger; second, a direct cooling crystallizer, where a refrigerant is introduced directly into a draft tube crystallizer; and third, a cooling disc crystallizer, that can be regarded as a compact cascade of cooling crystallizers with various cooling elements that are scraped by rotating wipers.

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