1University College London, Torrington Place, London, United Kingdom

Crystallization and Precipitation

Standard Article

John W.Mullin1

DOI: 10.1002/14356007.b02_03
Article Online Posting Date: January 15, 2003 on Ullmans Encyclopaedia of Industrial chemistry

5. Crystallization from Solutions

Solution crystallizers are generally classified according to the method by which supersaturation is achieved, e.g., cooling, evaporation, vacuum (adiabatic cooling), reaction, salting out. The designation controlled denotes supersaturation control; classifying refers to classification of product size. The term mixed-suspension mixed-product removal is abbreviated as MSMPR (see Section Crystallizer Modeling and Design).

5.1. Cooling Crystallizers

5.1.1. Nonagitated Vessels

The simplest type of cooling crystallizer is the unstirred tank: a hot feedstock solution is charged to the open vessel where it is allowed to cool, often for several days, by natural convection. Metallic rods may be suspended in the solution so that large crystals can grow on them and reduce the amount of product that sinks to the bottom of the crystallizer. The product is removed by hand.

Because cooling is slow, large interlocked crystals are usually obtained and retention of mother liquor is unavoidable. As a result, the dried crystals are generally impure. Because of the uncontrolled nature of the process, product crystals range from a fine dust to large agglomerates.

Labor costs are generally high, but the method is economical for small batches because capital, operating, and maintenance costs are low. However, the productivity of this type of equipment is low and space requirements are high.

5.1.2. Agitated Vessels

Installation of an agitator in an open-tank crystallizer generally results in smaller, more uniform crystals and reduced batch time. The final product tends to have a higher purity because less mother liquor is retained by the crystals after filtration and more efficient washing is possible. Water jackets are usually preferred to coils for cooling because the latter often become encrusted with crystals and cease to operate efficiently. Where possible, the internal surfaces of the crystallizer should be smooth and flat to suppress encrustation.

An agitated cooler is more expensive to operate than a simple tank crystallizer, but it has a much higher productivity. Labor costs for product handling may still be rather high. The design of tank crystallizers varies from shallow pans to large cylindrical tanks.

The large agitated cooling crystallizer shown in Figure 22A has an upper conical section which slows down the upward velocity of liquor and prevents the crystalline product from being swept out with the spent liquor. An agitator located in the lower region of a draft tube circulates the crystal slurry (magma) through the growth zone of the crystallizer. If required, cooling surfaces may be provided inside the crystallizer.

/ Figure 22. Cooling crystallizers
A) Internal circulation through a draft tube; B) External circulation through a heat exchanger
a) Calming section; b) Growth zone; c) Draft tube

Use of external circulation allows good mixing inside the crystallizer and high rates of heat transfer between the liquor and coolant (Fig.22B). An internal agitator may be installed in the crystallization tank if needed. The liquor velocity in the tubes is high; therefore, small temperature differences are usually adequate for cooling purposes and encrustation on heat-transfer surfaces can be reduced considerably. The unit shown may be used for batch or continuous operation.

5.1.3. Direct-Contact Cooling

The use of a conventional heat exchanger and the problems caused by crystal encrustation can be avoided by employing direct-contact cooling (DCC) in which supersaturation is achieved by allowing the process liquor to come into contact with a cold heat-transfer medium. Other potential advantages of DCC over conventional indirect-contact methods include better heat transfer and smaller cooling load. However, problems are also associated with DCC crystallization; these include product contamination from the coolant and the cost of extra processing required to recover the coolant for further use.

A solid, liquid, or gaseous coolant can be used; heat exchange may occur via the transfer of sensible or latent heat. The coolant may or may not boil during the operation, and it can be miscible or immiscible with the process liquor. Thus, four basic types of DCC crystallization are possible [7]:

1. Immiscible, boiling, solid or liquid coolant: heat is removed mainly by transfer of latent heat of sublimation or vaporization.

2. Immiscible, nonboiling, solid, liquid, or gaseous coolant: mainly sensible heat transfer.

3. Miscible, boiling, liquid coolant: mainly latent heat transfer.

4. Miscible, nonboiling, liquid coolant: mainly sensible heat transfer.

Crystallization processes employing DCC have been used successfully in the dewaxing of lubricating oils, the desalination of water, and the production of inorganic salts from aqueous solution [7].

5.2. Evaporating Crystallizers

If the solubility of a solute in a solvent is not appreciably decreased by lowering the temperature, the appropriate degree of solution supersaturation can be achieved by evaporating some of the solvent. Evaporation techniques have been used for centuries to crystallize salts; the simplest method—utilization of solar energy—is still employed commercially throughout the world [58]. Common salt is produced widely from brine in steam-heated evaporators, and similar evaporating crystallizers, often in multiple-effect series, are used in sugar refining. Many types of forced-circulation evaporating crystallizers are now in large-scale use [1-8].

Evaporating crystallizers are normally operated under reduced pressure to aid in solvent removal, minimize heat consumption, or decrease the operating temperature of the solution; they are best described as “reduced-pressure evaporating crystallizers”.

5.3. Vacuum (Adiabatic Cooling) Crystallizers

A vacuum crystallizer operates on a slightly different principle from the reduced-pressure evaporating crystallizer described in the previous section.

Supersaturation is achieved in a vacuum crystallizer by simultaneous evaporation and adiabatic cooling of the feedstock. A hot, saturated solution is fed into an insulated vessel maintained under reduced pressure. If the feed liquor temperature is higher than the boiling point of the solution under the low pressure existing in the vessel, the liquor cools adiabatically to this temperature. The sensible heat and any heat of crystallization liberated by the solution evaporate some of the solvent and concentrate the solution.

5.4. Continuous Crystallizers

Many different continuously operated crystallizers are available, but the majority can be divided into three basic types: forced-circulation, fluidized-bed (Oslo), and draft-tube agitated units. A small selection of the large number of commercial types available is described.

Forced-Circulation Crystallizers. A Swenson forced-circulation crystallizer that operates under reduced pressure is shown in Figure 23. A high recirculation rate through the external heat exchanger is used to provide good heat transfer and minimize encrustation. The crystal magma is circulated from the lower conical section of the evaporator body through the vertical tubular heat exchanger and reintroduced tangentially into the evaporator below the liquor level to create a swirling action and prevent flashing (sudden evaporation). Feedstock enters on the pump inlet side of the circulation system. Product crystal magma is removed below the conical section.

/ Figure 23. Forced-circulation Swenson crystallizer
a) Evaporator; b) Heat exchanger; c) Pump

Fluidized-Bed Crystallizers. In an Oslo fluidized-bed crystallizer, a bed of crystals is suspended in the vessel by the upward flow of supersaturated liquor in the annular region surrounding a central downcomer (Fig.24). Although originally designed as classifying crystallizers, fluidized-bed Oslo units are now frequently operated in a mixed-suspension mode to improve productivity, although this reduces product crystal size. A cooling-type Oslo crystallizer operates in the classifying mode as follows. The hot, concentrated feed solution is fed into the vessel at a point directly above the inlet to the circulation pipe. Saturated solution from the upper regions of the crystallizer, together with the small amount of feedstock, is circulated through the tubes of the heat exchanger, which is cooled by forced circulation of water or brine. On cooling, the solution becomes supersaturated, but not enough for spontaneous nucleation to occur; great care, in fact, is taken to prevent this. Product crystal magma is removed from the lower regions of the vessel.

/ Figure 24. Oslo cooling crystallizer
a) Downcomer; b) Pump; c) Heat exchanger

Draft-Tube Agitated Vacuum Crystallizers. A Swenson draft-tube-baffled (DTB) vacuum unit is shown in Figure 25. A relatively slow-speed propeller agitator is located in a draft tube which extends to a few inches below the liquor level in the crystallizer. Hot, concentrated feedstock enters at the base of the draft tube. The steady movement of magma and feedstock up to the surface of the liquor produces a gentle, uniform boiling action over the whole cross-sectional area of the crystallizer. The degree of supercooling thus produced is very low (<1°C), and in the absence of violent vapor flashing, both excessive nucleation and salt buildup on the inner walls are minimized. The internal baffle in the crystallizer forms an annular space in which agitation effects are absent. This provides a settling zone that permits regulation of the magma density and control of the removal of excess nuclei. An integral elutriating leg may be installed underneath the crystallization zone (as depicted in Fig.25) to effect some degree of product classification.

/ Figure 25. Swenson draft-tube-baffled (DTB) crystallizer
a) Boiling surface; b) Draft tube; c) Baffle; d) Settling zone; e) Elutriating leg

The Standard–Messo turbulence crystallizer (Fig.26) is another successful draft-tube vacuum unit. Two liquor flow circuits are created by concentric pipes: an outer ejector tube with a circumferential slot and an inner guide tube. Circulation is effected by a variable-speed agitator in the guide tube. The principle of the Oslo crystallizer is utilized in the growth zone; partial classification occurs in the lower regions, and fine crystals segregate in the upper regions. The primary circuit is created by a fast upward flow of liquor in the guide tube and a downward flow in the annulus; liquor is thus drawn through the slot between the ejector tube and the baffle, and a secondary flow circuit is formed in the lower region of the vessel. Feedstock is introduced into the guide tube and passes into the vaporizer section where flash evaporation takes place. Nucleation, therefore, occurs in this region, and the nuclei are swept into the primary circuit. Mother liquor can be drawn off via a control valve, thus providing a means of controlling crystal slurry density.

/ Figure 26. Standard–Messo turbulence crystallizer
a) Draft tube; b) Downcomer; c) Circumferential slot

The Escher–Wyss Tsukishima double-propeller (DP) crystallizer (Fig.27) is essentially a draft-tube agitated crystallizer with some novel features. The DP unit contains an annular baffled zone and a double-propeller agitator which maintains a steady upward flow inside the draft tube and a downward flow in the annular region. Very stable suspension characteristics are claimed.

/ Figure 27. Escher–Wyss Tsukishima double-propeller (DP) crystallizer
a) Baffle; b) Draft tube; c) Elutriating leg; d) Heat exchanger