Industrial Crystallizers


Whiting Equipment Canada builds a wide range of crystallizers for Chemical Process Industries
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About Our Industrial Crystallizers

Crystallization is the process by which a chemical is separated from solution as a high-purity, definitively shaped solid.

General & Special Purpose Crystallizers

While crystallization is a unit operation embracing well-known concepts of heat and mass transfer, it is nevertheless strongly influenced by the individual characteristics of each material handled. Therefore, each crystallization plant requires many unique features based upon well established general principles. Each application must be evaluated on an individual basis to achieve optimum results. Whiting Equipment Canada builds a wide range of general and special purpose crystallizers for the chemical process industries. Crystallization can be achieved by multiple methods including lowering the solubility of the solution by cooling the liquor, concentrating the solution to saturation via evaporation, and crystallization through chemical reaction.

Batch Vacuum Crystallizer

For special cases requiring very low operating temperatures achieved only by very high vacuum, and for those applications involving relatively small amounts of material – or when the material being processed must be handled on a less than a continuous basis – it is often both convenient and economical to use a Swenson batch vacuum crystallizer. Cycles on batch equipment range from two to eight hours, not including loading and unloading time. At the conclusion of the cycle, the material is deposited in an agitated tank from which it is removed on either a batch or continuous basis for separation and drying. The entire cycle for such equipment may be automated.


Where the material is cooled through a very wide range and/or to a final temperature which requires a very high vacuum, a large ejector or booster is utilized to compress the vapour to a pressure high enough for condensation with available cooling water. In such cases, the batch vacuum crystallizer’s economies of steam usage are approached only by multiple stage continuous equipment of five or more stages.

An added advantage of the batch vacuum type of crystallizer is its capacity for self-cleaning, which is particularly helpful when dealing with materials prone to grow on the walls of continuous crystallization equipment.

Direct Contact Refrigeration Crystallization

When crystallization occurs at such a low temperature that it is impractical to use surface cooling or when the rapid crystallization of solids on the tube walls would foul a conventional surface cooled crystallizer, a draft tube baffle crystallizer (or a forced circulation unit) utilizing the direct contact refrigeration technique can be employed. In this operation, a refrigerant is mixed with the circulating magma within the crystallizer body where it absorbs heat and is vaporized. Refrigerant vapor leaves the surface of the crystallizer similar to water vapor in a conventional evaporative crystallizer. It then must be compressed, condensed and circulated to the crystallizer to maintain continuous operating conditions.

Refrigerants must be relatively insoluble in the solutions processed and have the thermodynamic characteristics to minimize compressor horsepower. Examples are the crystallization of caustic dihydrate with Freon or Propane, and that of Paraxylene with liquid Propane refrigerant.

Draft Tube Baffle Crystallizer

For superior control over crystal size and characteristicsFor superior control over particle size when excessive fine crystals are present, the Swenson draft tube baffle (DTB) crystallizer has been proven highly effective. This type of crystallizer is primarily used in the production of a variety of large-size crystalline materials such as ammonium sulfate, potassium chloride and diammonium phosphate for the fertilizer industry.

The DTB crystallizer is built in both the adiabatic cooling and evaporative (see image) types and consists of a body in which growing crystals are circulated from the lower portion to the boiling surface by means of a large, slow-moving propeller circulator. Surrounding the suspended magma is an annular settling zone from which a stream of mother liquor bearing fine crystals can be removed. These fines are separated from the growing suspension of crystals by gravitational settling in the annular baffle zone.

Fines leaving the baffle zone are sent to a following stage settler or heat exchanger in the case of an evaporative DTB crystallizer. The mother liquor is returned to the suction of the propeller circulator after the fines have been destroyed by heating or mixing with dilute feed or water, depending on the flowsheet.

Low temperature rise at low power input

In the case of adiabatic cooling or evaporative crystallizers, the temperature rise in the circulated magma caused by the mixing of the incoming feed or heated mother liquor at the eye of the propeller is approximately 1°F and thereby limits the supersaturation rate to very low values.  The boiling action is concentrated in the center of the vessel and is well distributed across the surface by means of the vertical draft tube inlet.

Crystallizers of this type typically operate with a suspension of solids ranging from 25-50% apparent settled volume.  The low temperature drop at the boiling surface and the uniform distribution of boiling created by the circulation pattern minimizes crystallization buildup on the walls of the unit and extends the operating cycle.  There are no close clearances where crystal buildup can produce a large reduction in the rate of circulation as in other crystallizer designs.

Baffling allows slurry density control

Swenson DTB equipment is especially useful in multiple stage cooling crystallizer applications where cooling of the feed solution in each stage limits the natural slurry density to a few percent.  By means of the baffled zone, the operating slurry density within the crystallizer can be raised to any convenient value by regulating the slurry underflow rate and removing the remaining mother liquor from the baffle section.

Organic and inorganic chemicals produced by the Swenson draft tube baffle crystallizer include: Ammonium Sulphate, Hypochlorite, Epsom Salt, Potassium Sulfate, Monosodium Glutamate, Borax, Sodium Carbonate Decahydrate, Trisodium Phosphate, Sodium Chlorate, Boric Acid, MAP, Urea, YP soda, etc.

Basic Principles of the DTB Crystallizer:

  • Growing crystals are brought to the boiling surface where supersaturation is most intense and growth is most rapid.
  • The baffle permits separation of unwanted fine crystals from the suspension of growing crystals, thereby affecting control of the product size.
  • Sufficient seed surface is maintained at the boiling surface to minimize harmful salt deposits on the equipment surfaces.
  • Low head loss in the internal circulation paths make large flow’s at low power requirements feasible.

Advantages of the DTB Crystallizer:

  • Capable of producing large singular crystals.
  • Longer operating cycles.
  • Lower operating costs.
  • Minimum space requirements, single support elevation.
  • Adaptable to most corrosion resistant materials of construction.
  • Can be easily instrument-controlled.
  • Simplicity of operation, startup and shutdown.
  • Produces a narrow crystal size distribution for easier drying and less caking.
  • The product size varies only slightly with large changes in production rate.


Fluid Bed Crystallizer

The fluid bed crystallizer is specifically designed for surface cooled crystallization applications. In applications where crystals can be formed simply by cooling the feed solution, (no evaporation required) the fluid bed crystallizer should be considered.

Mechanical Design

  • Small physical size. All the separate items in a conventional shell and tube surface cooled crystallizer (crystallizer body, shell & tube heat exchanger, circulating pump, interconnecting ductwork) are combined into a single piece of equipment, eliminating the need for circulating piping; the heat exchanger and the crystallizer are combined into a single compact unit. The slurry pump is eliminated and replaced by a conventional fan.
  • Low elevation requirements. Typically 9 to 12 feet compared to 30 feet for conventional shell and tube crystallizers, little or no structural steel required.
  • Low maintenance. The only moving part is an air blower and an air damper, no pump seals which might leak.

Process Design

  • Larger crystals are produced because of the reduction in mechanical attrition and formation of nucleation of new seed crystals by eliminating the circulating pump and using air for agitation.
  • Higher liquid heat transfer coefficients are achieved with air agitation, because the liquor flow at the tube surface is more turbulent.
  • Operates at larger temperature differences between cooling medium and liquor temperature without crystal formation on the heat transfer surface. Higher temperature differences can be used because of the high turbulence at the tube surface caused by the bubbles rising through the slurry.
  • Less heat transfer surface required. Depending on the application, the effects of higher heat transfer coefficients combined with operating at larger temperature differences can reduce the required heat transfer by as much as 60%.


Forced Circulation Crystallizer

For feeds where high rates of evaporation are required, where there are scaling compounds, where crystallization is achieved in inverted solubility solutions, or where the solution is of relatively high viscosity, the forced-circulation crystallizer (see figure 11) is the best choice.

This type of unit – also known as the circulating magma crystallizer or the mixed suspension-mixed product removal (MSMPR) crystallizer – consists of a body sized for vapor release with a liquid level high enough to enclose the growing crystals. Suction from the lower portion of the body passes through a circulation pump and a heat exchanger and returns to the body through a tangent or vertical inlet. The heat exchanger is omitted when adiabatic cooling is sufficient to produce a yield of crystals.

The most common use of this crystallizer is as an evaporative crystallizer with materials having relatively flat or inverted solubility. It is also useful with compounds crystallized from solutions with scaling components.

Forced-circulation crystallizers can be operated on a batch basis, but the most frequent use is in the continuous processing of such materials as sodium chloride, sodium sulfate, sodium carbonate monohydrate, citric acid, monosodium glutamate, urea and other similar crystalline materials.


Surface Cooled Crystallizer

For operation at temperatures below which it is not economically feasible to use vacuum equipment, or for solutions with very high boiling point elevations, the Swenson surface cooled crystallizer is generally specified.

The surface cooled unit is a type of forced circulation crystallizer consisting of a shell and tube heat exchanger through which is pumped the slurry of growing crystals, a crystallizer body to provide retention time, and a recirculation pump and piping. Within the crystallizer body is a baffle designed to keep excessively fine crystals separated from the growing magma for size and slurry density control purposes.


Reactive Crystallization

Reactive crystallization is when a solid phase crystalline material results from the reaction of two components. This type of reaction can often be performed more profitably in a crystallizer than in a separate reactor. An example of reactive crystallization is the production of ammonium sulfate from liquid or gaseous ammonia and concentrated sulfuric acid.

The draft tube baffle crystallizer is particularly suited for reactive crystallization. The reactants are mixed in the draft tube of the DTB unit where a large volume of slurry is mixed continuously with the materials to minimize the driving force (supersaturation) created by the reaction.  Removal of the heat produced by the reaction is accomplished by vaporizing water or other solvents as in a conventional evaporative type crystallizer.

Reactive crystallization can also be performed in a forced circulation type crystallizer where the reactants are mixed in the circulation piping.

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