Crystallizers are used in industry to achieve liquid-solid separation. They are an important piece of chemical processing equipment because they are capable of generating high purity products with a relatively low energy input.
Forced-circulation crystallization is the most widely used crystallization method in industry.
Forced-circulation crystallizers, such as the one shown below, are evaporative crystallizers. They create a super-saturated solution by evaporating the solvent of a saturated solution. The solute of this supersaturated solution then cools, forming crystals. These types of crystallizers are classified as mixed-suspension, mixed-product-removal (MSMPR) crystallizers. The key assumption of an MSMPR crystallizer is that the slurry is perfectly mixed and uniform throughout the system.
The feed slurry is first heated in a heat exchanger, then pumped to the main body of the crystallizer. Vaporization occurs at the top surface of the slurry, while nucleation occurs near the bottom of the crystallizer body. The crystals are removed and vaporized solvent is condensed and returned to the crystallizer body.
There are several adaptations that can be added to a forced-circulation crystallizer to help narrow the crystal size distribution. Options include baffling, a conical entrance, and an elutriation step. A conical entrance promotes more thorough mixing, which creates a more uniform slurry mixture. Baffling is used to remove fines from the mixture, so that they can be recirculated for further growth. Elutriation, like baffling, removes smaller particles from the slurry, except elutriation separates particles based on weight rather than size. Elutriation is the process of flowing air over particles. Lighter particles are picked up by the air stream, while heavier ones remain in place.
Forced-circulation crystallizers are commonly used to produce salts and chemicals in industry. The forced-circulation crystallizer shown on the left is used to produce sodium sulfate, while that on the right is used in the agricultural industry.
Forced-circulation crystallizers are often arranged in stages. The two-stage crystallizer shown below is used by an agricultural chemical company to evaporate phosphoric acid from a mixture and recover solids.
Draft tube baffle crystallizers are used to control crystal size and characteristics.
Draft Tube Baffle (DTB) crystallizers, such as the one shown below, are used in applications in which excessive nucleation occurs. Small, unwanted crystals, called fines, are removed from larger particles using gravitational settling and recycled through the crystallizer again, resulting in maximum crystal recovery and larger crystals.
DTB crystallizers have two distinct volumes. In the baffled region gravitational settling separates larger crystals from fines. The larger crystals settle between the baffle and draft tube, as shown below, and are removed in the product slurry, while the fines are recirculated after being redissolved in a heat exchanger. The evaporated solvent is then condensed, cooled, and returned to the region of active crystallization.
DTB crystallizers typically have circulation rates of 1 to 4 times/min and a total residence time of 4 to 6 hours.
The two DTB crystallizers shown below are used to produce ammonium sulfate. DTB crystallizers are also used for the purification of organic compounds as well as the production of salts.
Examples of crystals that are formed using a DTB crystallizer are shown below.
Surface-cooled crystallizers combine a draft tube baffle crystallizer body with a heat exchanger. Slurry is drawn from the crystallizer body and then cooled before being pumped back into the crystallizer body.
Crystallizers such as these are the most useful for operations in which the solution's boiling point is extremely high, or when such low temperatures are required that evaporation by vacuum is not possible.
Surface-cooled crystallizers such as the one shown here are used to produce sodium chloride from caustic solutions, sodium carbonate decahydrate from waste solutions, and sodium chlorate from sodium chloride saturated solutions.
Vacuum crystallizers use a condenser with a booster to maintain a vacuum inside the crystallizer body. This vacuum makes it possible to generate a supersaturated solution when very low operating temperatures are needed. Vacuum crystallizers may be continuous or batch. The batch vacuum crystallizer is particularly useful when processing materials that tend to grow on the walls of continuous crystallization equipment.
The vacuum crystallizer shown below is used to produce Glauber's Salt at a mineral plant. The crystallizer body can be seen on the left, with the condenser and booster slightly above and to the right of it.
Scraped surface crystallizers such as the one shown below are used as low energy, low cost means of crystallization for small scale operations. They are used primarly for the crystallization of melts.
A scraped surface crystallizer consists of a jacketed pipe in which a cooling medium between the pipe wall and the jacket remove heat from the slurry, causing crystallization. Inside the pipe, large scrapers wipe the solid deposits from the wall to prevent build up. Scraped surface crystallizers are often grouped together in assemblies, as shown below.
Genck, Wayne J. A Clearer View of Crystallizers. Chemical Engineering. 7(2011): 28 - 32. Print.
Mersmann, A. Crystallization Technology Handbook. New York, NY: Marcel Dekker, Inc., 1995. Print.
Mullin, J.W. Crystallization. 3rd ed. Stoneham, MA: Butterworth-Heinemann, 1993. Print.
Myerson, Allan S. Handbook of Industrial Crystallization. Stoneham, MA: Butterworth- Heinemann, 1993. Print.
Perry, Robert H., and Don W. Green. Perry's Chemical Engineers' Handbook. 7th ed. New York: McGraw-Hill Inc., 1997. Print.
Walas, Stanley M. Chemical Process Equipment Selection and Design. Stoneham, MA: Butterworth-Heinemann, 1990. Print.