Crystallizer Configurations

For continuous crystallization processes several types of crystallizer have been developed, which are used for different applications. The most common types of crystallizers are listed here.

• Forced circulation crystallizer (see Figure 1A). The most widely used crystallizer. It is often treated as a well mixed crystallizer, although several studies have shown that large variations in supersaturation exist within the crystallizer volume. Therefore crystal growth is limited to a small part of the crystallizer (in the vicinity of the boiling zone), and depending on the temperature rise in the external heat exchanger, even dissolution of the fine crystals will take place in that part of the crystallizer. Actuators to control the crystal size distribution are limited although configurations exist with an elut-riation leg in which selective removal takes place. The level of supersaturation can be affected by control of the evaporation rate, while the profile can be influenced by adapting the circulation flow rate through the external heat exchanger. The main operational problems encountered with this type of crystallizer are scaling in the boiling zone or in the heat exchanger.

Figure 1 (A) Forced circulation and (B) stirred draft tube baffle (DTB) crystallizer with an external heat exchanger and fines destruction. Both crystallizers are produced by USFilter's HPD.

Draft tube baffled (DTB) crystallizer (see Figure 1B). This crystallizer enables the generation of larger crystals using fines removal, which is done by installing a skirt baffle to create a settling or annular zone. The flow in the draft tube thus has to be upwards, which 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 the fines. An increase in the fines removal flow increases the number of fines that are removed from the crystallizer, but also increases 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 elutriation leg, when present, serves more as a washing device to remove the impurities from the crystal than as a product classifier. The DTB cry-stallizer is among the best-studied crystallizers, because of the low order oscillations which are often observed. Other operational problems are scaling in the boiling and baffle zones. Fluidized-bed crystallizer (see Figure 2A). This cry-stallizer is especially designed to produce large and uniformly sized crystals. 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 crystal-lizer. The supersaturation is consumed on its way up, and a coarse product leaves the crystallizer at the bottom. The main control problems are stabilizing the fluidized bed and keeping the supersaturation in the circulation loop, and specifically in the boiling zone and the downcomer, within certain limits to prevent spontaneous (primary) nu-cleation. Also low-order cycling occurs in this type of crystallizer, which is however much less well studied than the DTB crystallizer. Although fines dissolution already takes place in the circulation loop, a separate fines removal loop can be installed to control the CSD in the crystallizer. In addition a clear liquor overflow stream is sometimes used to control the slurry density in the fluidizer (double draw off).

• Cooling crystallizer (see Figure 2B). In this crystal-lizer the slurry is circulated through a heat exchanger. For crystallization from solution the slurry is pumped through a tube and shell heat exchanger, with a AT range between the tube and the wall of 5-10°C. The temperature decrease in the heat exchanger must be controlled precisely.

As can be seen from these descriptions, the number of available process inputs to manipulate the CSD in the crystallizer is rather limited. The actuator most used

Figure 2 (A) Fluidized bed crystallizer from USFilter's HPD and (B) cooling crystallizers from Swenson.

in control studies is the fines dissolution flow rate, although in industrial crystallizers this flow cannot be manipulated freely. It is constrained on the lower side by the heat input of the system and by the maximum temperature increase of the fines flow. Selective product removal using an external product classifier such as a wet screen or a hydrocyclone seems to be an attractive additional process actuator method, which can be implemented irrespective of the crystallizer used.

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