Crystallizers

Our evaporators and crystallizers are implemented for zero liquid discharge applications among other applications to convert dilute or concentrated salt containing water streams into solid salt crystals and clean water. Crystallization is a solid-liquid separation technique in which solid crystals are formed from a liquid solution. Crystallizers can eliminate liquid wastes to create zero liquid discharge (ZLD). Crystallization is divided into two stages: primary nucleation and secondary nucleation. Primary nucleation involves the growth of new crystals. Secondary nucleation perpetuates growth and is the main stage that causes the mass production of crystals. There are three types of crystallization processes: concentration, cooling (under vacuum or with a heat exchanger), or by reaction or equilibrium displacement.

Selecting crystallizers requires an analysis of application requirements. For example, a brine crystallizer processes wastewater and produces both solid salt crystals and clean water. A resin crystallizer allows for the crystallization of flake or amorphous resin pellets. A vertical continuous cooling crystallizer (VCCC) is used to crystallize highly viscous and extremely slowly crystallizing fill masses. Other application-specific crystallizers are also available.

Crystallizers can either stand alone or be combined with other technologies, such as a brine concentrator or evaporator. Steam-driven evaporators remove water from a solution or slurry, but the discharge is still in liquid and not crystal form. During evaporation, a product is concentrated by boiling off the solvent, generally water. A brine concentrator is a specific type of evaporator used to turn waste-saturated industrial wastewater into distilled water for reuse. A typical brine concentrator can recover 95 to 99% of wastewater for reuse. Evaporators and crystallizers often replace steam hosts when a plant’s original host is lost.

Brine crystallizers are suitable for recovering salts from waste water that can then be used or sold. In this way, a crystallizer maximizes waste stream usage and helps plants meet zero liquid discharge (ZLD) requirements. Crystallizers are used in manufacturing, chemical processing, mining, petrochemical refining, and electrical component manufacturing applications.

In the design and manufacturing of crystallizers, our work focuses on the crystallization of solutions and not on the melt crystallization. Our expertise covers the three types of crystallization process:

We possess the know-how and expertise for all of the following types of crystallization equipment: with total or partial classification, involving the recirculation of the magma, with or without settling zones.

• Forced Circulation Crystallizer

• Oslo Type Crystallizer (classified-suspension crystallizer)

• DTB crystallizer (draft - tube - baffle crystallizer)

• Induced Circulation Crystallizer

• Cooling Crystallizers

• Evaporative crystallizers

• Freeze crystallizers

• Melt crystallizers

• Vacuum Crystallizers

Forced Circulation Crystallizer

• An "active volume", designed case by case, to get both required residence time for crystal growth and mother liquor desuper saturation

• A given agitation (recirculation rate) rated to control the extent of super saturation arising from the evaporation, and to keep the temperature difference in the heat exchanger within reasonable limits

• A special design of the liquid-vapor separation area to minimize the carry over losses and avoid the formation of an excessive amount of fines, which is highly detrimental to crystal growth.

Continuous Forced Circulation Crystallizer: depending upon specific process requirements, additional devices for our Crystallizers can be provided. They include:

• Internal baffles, used mainly for excess mother liquor overflow and /or withdrawal of fines when crystal growth is slow or disturbed by impurities build-up (see Figure 2)

• Elutriation leg, to improve product purity and to deliver a narrow crystal size distribution

• An internal scrubbing section to reduce to very low value the carry over losses, or even to provide stripping or absorption devices when a volatile compound must be recovered

Continuous Forced Circulation Crystallizer:

Forced circulation crystallizers are of the (Mixed Suspension Mixed Product Removal) MSMPR type and operate either on controlled or "natural" slurry density depending upon process requirements and/or unit material balance.

These crystallization systems can be either single or multiple effects and the vapor recompression concept (either thermal or mechanical) is often applied. Usually, these crystallizers operate from low vacuum to atmospheric pressure.

As a rule, these crystallization units are used for high evaporation rates and when crystal size is not of the utmost importance or if crystals grow at a fair rate.

Almost any material of construction can be considered for the fabrication of these crystallizers.

It is worth bearing in mind that the heating element is omitted for vacuum cooling crystallizers.

Typical products produced with these crystatllizers are:

• NaCl (food or technical grade sodium chloride)

• KNO₃

• Na₂, SO₄, K₂ SO₄

• NH₄Cl

• Na₂CO₃H₂O

• Citric acid

When the problem of scaling impedes the process of concentration, a crystallizer design similar to the one described above is proposed. This applies for CaSO4 saturated solutions, like fertilizer grade phosphoric acid, demineralization effluents, vinasses.

Oslo Type Crystallizer (Classified-Suspension Crystallizer)

The Oslo type crystallizer also called classified-suspension crystallizer is the oldest crystallizer design developed for the production of large, coarse crystals.

The basic crystallizer design criteria are two fold:

• Desuper saturation of the mother liquor by contact with the largest crystals present in the crystallization chamber

• Keeping most of the crystals in suspension without contact by a stirring device, thus enabling the production of large crystals of narrow size distribution

The classifying crystallization chamber is the lower part of the crystallization unit. The upper part is the liquor-vapor separation area where super saturation is developed by the removal of the solvent (water for most applications). The slightly supersaturated liquor flows down through a central pipe and the super saturation is relieved by contact with the fluidized bed of crystals. The desuper saturation occurs progressively as the circulating mother liquor moves upwards through the classifying bed before being collected in the top part of the chamber. Then it leaves via the circulating pipe and after addition of the fresh feed, it passes through the heat exchanger where heat make-up is provided. It is then recycled to the upper part.

Additional devices, such as described for the forced circulation crystallizer, are of course available.

It is worth bearing in mind that the operating costs of the Oslo type crystallizer unit are much lower than with any other type when both large and coarse crystals are required. Since crystals are not in contact with any agitation device, the amount of fines to be destroyed is lower and so is the corresponding energy requirement.

This Oslo type crystallizer (classified - suspension crystallizer) allows long cycles of production between washing periods.

In addition to usual process operations, the Oslo type crystallizer has also found a number of interesting applications, e.g. for reaction-crystallization and for separation-crystallization when several chemical species are involved.

Most of the Oslo type crystallization units are of the "close type." However, the "open" type (refer to Figure 2) is worth to be considered when very large settling areas are required or when the vessel must be fabricated out of high cost alloys or metals.

Typical crystallizer products are:

• (NH₄)₂ SO₄

• Na₂SO₄

• AgNO₃

• Hydrated mono sodium glutamate

• Mono ammonium phosphate (MAP)

DTB Crystallizer (Draft - Tube - Baffle Crystallizer)

The Draft Tube Baffle Crystallizer is an elaborated Mixed Suspension Mixed Product Removal (MSMPR) design, which has proven to be well suited for vacuum cooling and for processes exhibiting a moderate evaporation rate. The concept is such that if no (or little) heat make-up is required, it results in a rather compact arrangement; therefore the initial investment is minimized.

As a rule, these units operate with a rather low super saturation, which is sometimes a limitation to crystal growth, so that very large crystals can be produced only by providing extensive and costly dissolving of fines.

The Draft Tube Baffle unit (Figure 1) includes a baffled area (settling zone), peripheral to the active volume, from where excess of mother liquor and/or fines are removed for further processing. The necessary agitation of the suspension mixed with the incoming feed solution is provided by a bottom entry agitation at moderate energy consumption.

Draft Tube Baffle crystallizers are often equipped with an elutriation leg below the body to classify the crystals.

When destruction of fines not needed or wanted, baffles are omitted and the internal circulation rate is set to have the minimum nucleating influence on the suspension (Draft Tube design, draft-tube crystallizer).

When large evaporation rates are required, an external heating loop must be provided, making the arrangement less competitive from an initial cost standpoint.

The Draft Tube Baffle Crystallizer, which can be considered when crystallization can be achieved with natural suspension, has proven to be well suited to many applications such as:

• boric acid

• Na₂SO₄. 10H₂0 (Glauber salt)

• melamine

• citric acid

• NaCIO₃

Induced Circulation Crystallizer

The induced circulation crystallizer design has been recently developed to provide additional agitation of the active volume of forced circulation crystallizers with the use of only one pump, it operates similarly to a Draft Tube Baffle crystallizer but without the internal agitation device. The main applications are for evaporative crystallization cases. The unit also operates according to the Mixed Suspension Mixed Product Removal (MSMPR) principle and all options described for the other designs are of course available for this concept. The equipment is able to produce a narrow crystal size distribution. Like other designs, it can be fabricated in almost any material of construction. Performances and product quality are equivalent to those of a Draft Tube Baffle unit designed to the same specification, but appear to be limited to non-viscous solutions as the induced flow would be quite limited when the mother liquor exhibits a high viscosity.

We have completed over 40 crystallizer installations for evaporating and crystallizing organic and inorganic compounds including: ammonium sulfate, ammonium chloride, ammonium hydroxide, ammonium nitrate, sodium sulfate, sodium carbonate, urea, calcium acetate, calcium chloride, lithium chloride, chromium trioxide, cupric sulfate (blue vitriol), ferric chloride, barium chloride, sodium chloride, sugar, MSG, boric acid, adipic acid, MgSO4, KCl, silver nitrate, zinc sulfate, caprolactam, acetic acid, lactose, sodium cyanide, etc. Some of the larger units furnished: Dupont: Crystallizers for caprolactam sulfate and adipic acid.
 

Crystallizer Experience Includes:

• Allied Chemical: (NH4)2SO4, adipic acid and caprolactam sulfate.

• Ames Goldsmith: 100 T/d silver nitrate crystallizer.

• J. Huber: MVR evaporator for Na2SO4 salt cake.

• Kalium Corp: KCl evaporator/crystallizer

• Pacific Coast Borax: KCl evaporator/crystallizer.

• Monsanto: Boric acid evaporator crystallizer.

• Sherrit Gordon Mines: 2x 300 T/d (NH4)2SO4 classifying growth crystallizer.

• American Potash: Evaporators and crystallizers for – soda ash Na2CO3, 1,000 T/d potash KCl, 1,000 T/d K2SO4, boric acid and Na2SO4 salt cake.

• National Lead: Sodium fluoride concentration systems.

• American Cyanamid: Sodium cyanide concentration and salt cake crystallization.

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