Forced circulation (FC) is the most widely supplied method of crystallization. The Swenson forced circulation crystallizer is the best choice for applications that require high rates of evaporation and for solutions with relatively flat or inverted solubility, that contain scaling components, or exhibit high viscosity.
A forced circulation crystallizer—also known as a circulating magma crystallizer or mixed suspension mixed product removal (MSMPR) crystallizer—consists of a body sized for vapor release with a liquid level high enough to enclose the growing crystals. Suction from the lower portion of the body passes through a circulation pump and a heat exchanger and returns to the body through a draft tube (DT) or tangential inlet. The heat exchanger is omitted when adiabatic cooling is sufficient to produce a yield of crystals.
When the heat exchanger is used, it is normally one- or two-pass and is designed for relatively low temperature rises in the solution. This limits the supersaturation of scaling components when heating materials of inverted solubility. In most applications, the steam to liquid delta-T is also limited to prevent mass boiling within the tubes or vaporization at the tube wall.
Forced circulation crystallizers are found in sizes ranging from two-foot diameter laboratory models to over 40-foot diameter units for continuous processing. Per pound of product, it is ordinarily the most inexpensive type of equipment available, particularly when substantial amounts of evaporation are required.
Forced circulation crystallizers can be operated on a batch basis, but their most frequent use is for continuous processing of such materials as sodium chloride, sodium sulfate, sodium carbonate monohydrate, lithium hydroxide monohydrate, citric acid, and similar crystalline materials.