Crystallization is the process of forming solid material from a liquid solution or melt, where the solid material formed has crystalline (as opposed to amorphous) structure. A crystallization process generally has the following characteristics:
· The feed material is either in solution or is a liquid above the melting point of the solid phase. If in solution, there may be more than one solvent present.
· There may be dissolved or solid impurities present. Some impurities may have very similar properties to the solute (especially for side-products from organic reactions). During crystallization, impurities may remain in solution, crystallize separately, or incorporate in some way into the product crystals.
· The product material is solid, and present as particles in a range of sizes.
· The product is generally surrounded by mother liquor.
· The waste stream from the process is liquid, containing both residual dissolved product and impurities.
The general advantages of crystallization as a process are:
· High purification can be obtained in a single step.
· Produces a solid phase which may be suitable for direct packaging and sale.
· Operates at a lower temperature and with lower energy requirements than corresponding distillation separations.
· Plant can be simple and easy to construct and maintain.
· May be more economic than alternative separation processes.
The general disadvantages are:
· Generally only purifies one component.
· Yield is limited by phase equilibria.
· Process kinetics are more complex and less well-understood than some alternatives; obtaining detailed kinetic parameters involves complex experimental procedures.
Crystallization is used industrially in a wide variety of processes. Typical applications include bulk chemicals, such as salt, sugar and fertilizers; high value-added products such as specialty chemicals and pharmaceuticals; and difficult separations such as ortho- and para-xylene.
Crystallization processes relating to a single crystal and to multiple crystals in vessels are discussed below while typical equipment used for crystallization are dealt with in a separate article on Crystallizers.
