Question

how does a crystalliser work

Answer 1

There is a wide variety of equipment used to carry out the crystallization process, called crystallizers. Such equipment can be classified into four broad types:
Bulk solution crystallizers. Crystals are suspended in solution for a significant time while nucleation and growth occurs.
Precipitation vessels. Feed streams entering the vessel generate high supersaturation levels (by chemical reaction, drowning or salting out), very rapidly forming large numbers of small crystals.
Melt crystallizers forming multiple crystals. The bulk (typically > 90%) of the solution or melt forms crystals either in suspension or on a cooled surface. Impurities remain in the small amount of uncrystallized mother liquor.
Melt crystallizers forming large high-purity single crystals. Crystals form very slowly from high-purity melts, yielding large, pure and defect-free crystals. These are typically used for semiconductor manufacture.
All these type of equipment have aspects in common:
A region where supersaturation is generated to drive the crystallization.
A region where crystals are in contact with supersaturated solution for crystal growth. In some cases, crystals are present throughout the vessel, suspended by some form of agitation; in other cases, the crystals occupy only part of the vessel, typically as a fluidized bed.
Selection/Design
The selection of the appropriate crystallizer for a particular task will depend upon the feed material available, the properties of the system and the product requirements of the customer. A typical design sequence involves:
Basic data collection.
Selection of supersaturation generation method.
Choice of batch or continuous operation.
Choice of specific equipment type.
Bench and pilot scale tests.
Full scale design.
A full procedure is too complex for this article; however some key aspects are outlined below. For further details, refer to any of the books given in the reference list.
Supersaturation generation
Five main methods can be used to generate supersaturation:
Cooling, using the vessel walls, internal coils, or by pumping mother liquor through an external heat exchanger. This is used when solubility changes significantly with temperature and when the feed stream is near saturation at a high temperature.
Evaporation, by heating the mother liquor or reducing the pressure to form a boiling zone at the top of the vessel. This can be used for a wide range of systems, although it is more energy-intensive than cooling.
Reaction, where feed streams enter and mix resulting in a chemical reaction generating the product, usually at high levels of supersaturation.
Drowning out, where a miscible solvent is added resulting in a mixture in which the product is less soluble. This has similar characteristics to reaction crystallization.
Salting out, where a salt with a common ion is added to precipitate the product from solution. Again, this has similar characteristics to reaction crystallization
Other techniques, e.g., pressure change, are occasionally found.
Crystallizer mode
Crystallizers can be designed to operate in either batch or continuous mode (and, rarely, combinations of the two).
Batch crystallization is generally easier to control and is more flexible. It can operate over a wide range of conditions.
Continuous crystallizers produce a consistent product and are generally smaller and more energy efficient than batch equipment for the same production rate. Thus, continuous crystallizers are favored for high-production rate systems. However, they operate over only a narrow range of conditions, so more process knowledge is generally required to make sure they produce the required product specification.
Standard equipment
For solution crystallizers, the simplest equipment is an agitated, cooled vessel. Although simple, it is far from optimal in terms of hydrodynamics, with poor crystal suspension. A draft tube and baffles are often added to improve suspension characteristics, and this leads to designs such as the Swenson Draft Tube Baffled (DTB) and Oslo-Krystal crystallizers.
The Swenson DTB has the main recirculation provided by a propeller inside a draft tube, with a settling zone to allow fines to be removed and dissolved, and a product elutriation leg, where large crystals are extracted against an upward flow carrying small crystals back into the vessel. Both evaporative and cooling versions are found.
The Oslo-Krystal unit has a fluidized bed of crystals, suspension and agitation being provided by an external circulation loop of either crystal magma or relatively crystal-free solution.
For further information on industrial solution crystallizers, refer to Myerson (1992), Mullin (1993) or the SPS Crystallization Manual.
Melt crystallizers fall into three general categories. The first has crystals in suspension within the vessel; crystals form on a cooled wall and are removed by a scraper. A melting zone can be added to increase purification of the crystals.