Continuing with my previous post Separation techniques: Crystallization, now I would like to explain to you as simply as possible because the correct choice of a crystallizer is of the utmost importance.

It is extremely important to clarify that a crystallizer; it must fundamentally fulfill the condition of creating an oversaturated solution since crystallization cannot occur without supersaturation.

One way to classify crystallization apparatus is based on the method used to create supersaturation:

• Over-saturation produced by cooling without appreciable evaporation, for example, tank crystallizers.
• Over-saturation produced by evaporation, with appreciable cooling, for example, crystallization evaporators, crystallizers-evaporators.
• Evaporation combined with adiabatic cooling: vacuum crystallizers.

Types of crystallizers.

Crystallizers of mixed suspension and withdrawal of combined products.

This type of equipment sometimes called a circulating magma crystallizer, is the most important one used today. In most commercial equipment of this type, the uniformity of the suspension of the product solids in the crystallizer body is sufficient for the theory to be applied. Even when certain characteristics and different varieties are included in this classification, the equipment operating at the highest capacity is of the type in which the vaporization of a solvent, usually water, is usually produced.

Surface cooled crystallizer.

This type of crystallizer is commonly used for some materials, such as potassium chlorate, since it is possible to incorporate a forced circulation tube and shell exchanger, in direct combination with a body of extraction tube crystallizer.

Careful attention must be paid to the temperature difference between the cooling medium and the slurry circulating in the exchanger tubes.

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Furthermore, the trajectory and flow velocity of the slurry inside the crystallizer body must be such that the volume contained in the body is active. This means that there may be crystals suspended inside the body due to turbulence and that they are effective in relieving the supersaturation created by the reduction of the temperature of the slurry as it passes through the exchanger. This type of equipment produces crystals in the mesh range of 30 to 100. The design is based on the permissible heat exchange rates and the retention that is required for the growth of the product crystals.

Forced circulation evaporation crystallizer.

In this crystallization device, the slurry leaving the body is pumped through a circulation pipe and through a shell heat exchanger, where its temperature rises from 2 to 6 ° C. since this heating is done without vaporization, the materials of normal solubility should not produce sedimentation in the tubes. The heated liquor, which returns to the body through a recirculation line, mixes with the slurry and raises its temperature locally, near the point of entry, which causes boiling on the surface of the liquid. During the subsequent cooling and vaporization to reach the equilibrium between the liquid and the vapor, the supersaturation that is created causes sedimentation in the swirling body of the suspended crystals, until they go away again through the circulation pipe. The amount and speed of the recirculation, the size of the body and the type and speed of the circulation pump are critical design concepts, in order to obtain predictable results. If the crystallizer is not of the evaporation type and depends only on adiabatic evaporation cooling to achieve good performance, the heating element will be omitted. The feed is admitted to the circulation line, after removing the slurry, at a point sufficiently below the free surface of the liquid, to avoid instantaneous vaporization during the mixing process.

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Draft tube and baffle crystallizer (DTB).

Since mechanical circulation greatly influences the level of nucleation within the crystallizer, many designs using circulators located within the crystallizer body have been developed, thereby reducing the pumping load exerted on the circulator. This technique reduces the power consumption and the tip speed of the circulator and, therefore, the nucleation speed.

The suspension of the product crystals is maintained by a large and slow-moving propeller, surrounded by an extraction tube inside the body. The propeller directs the grout towards the surface of the liquid, to avoid that the solids short-circuit the area of ​​more intense supersaturation. The cooled slurry returns to the bottom of the container and recirculates through the propeller.

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This design consists of a characteristic of the destruction of fine particles comprising the zone of settlement that surrounds the body of the crystallizer, the circulation pump, and the heating element. The latter provides sufficient heat to satisfy the requirements of evaporation and raise the temperature of the solution removed from the settler, in order to destroy all the small crystalline particles that are removed. The thick crystals are separated from the fine particles in the area by gravitational sedimentation.

Direct contact cooled crystallizer.

For some applications, such as obtaining ice from seawater, it is necessary to reach temperatures so low that cooling through the use of refrigerants is the only economic solution. In these systems, it is sometimes not practical to use surface cooling equipment, because the allowable temperature difference is so low (less than 3 ° C), that the heat exchange surface becomes excessive or because the viscosity is so high that The mechanical energy applied by the circulation system is greater than that which can be obtained with reasonable differences in temperature. In these systems, it is convenient to mix the refrigerant with the slurry which is cooled in the crystallizer, so that the heat of vaporization of the refrigerant is relatively immiscible with the mother liquor and capable of undergoing separation, compression, condensation and subsequent recycling in the crystallization system.

This technique was very suitable to reduce the problems that are associated with the accumulation of solids on a cooling surface. The use of direct contact cooling also reduces the overall energy needs of the process, since it is a refrigeration process that includes two fluids, a higher temperature difference is required, on a general basis, when the refrigerant must first cool some solution intermediate, like the calcium chloride brine, and that solution, in turn, cools the mother liquor in the crystallizer. Equipment of this type has worked properly at temperatures as low as -59 ° C (-75 ° F).

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Extraction tube crystallizer (DT).

This crystallizer can be used in systems where the destruction of fine particles is not desired or necessary. In those cases, the diverter is omitted and the size of the internal circulator is determined so that it has a minimal nucleation influence on the suspension.
In the DT and DBT crystallizers, the circulation speed that is reached is usually much higher than that obtained in a similar crystallizer with forced circulation. Therefore, the equipment is applied when it is necessary to circulate large quantities of slurry, to minimize the levels of supersaturation inside the equipment.

In general, the method is required to have long operational cycles with materials capable of growing on the walls of the crystallizer. The extraction and diverter tube designs are commonly used for the production of granular materials, from 8 to 30 mesh, such as ammonium sulfate, potassium chloride and other inorganic and organic crystals.

In general, what classifies a crystallizing team is the movement of the solution within the team and the way in which the solid is later separated.

How to choose which one to use?

First:

Choose a means of generation of supersaturation based on the characteristics of the solubility-temperature curves of the substance to be crystallized.

Second:

Decide if the crystallization will be batch or continuous. The Batch design is the simplest but requires more control of variables. Continuous design generates large productions (more than one ton per day or flow rates greater than 20 m3 per hour).

The final choice of the team will also depend on other aspects such as:

• Type and size of crystals to be produced.
• Physical characteristics of the feed.
• Resistance to corrosion.

Factors to Consider:

• Solvent power: It must be able to easily dissolve the solute and then allow obtaining the desired crystals.
• Purity: It must not introduce impurities that affect the appearance and properties of the crystal.
• Chemical reactivity: Must be stable.
• Handling and processing: preferably low viscous and with a melting temperature below 5 ° C. Of low flammability and toxicity.

Refences:

• https://www.sciencedirect.com/science/article/pii/0255270188850141
• http://www.thermopedia.com/content/680/
• http://crystallisation.pbworks.com/f/Swenson+Crystallization+Equipment.pdf
• http://www.swensontechnology.com/crystallizers/
• Crystallization and Crystallizers, Jean-Paul Duroudier
• Crystallization (Fourth Edition), J.W. Mullin

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