Fouling crystallizers

Evaporator type Very viscous > 1000 Medium viscosity < 1000 max Low viscosity < 100 Foaming Scaling or fouling Crystals produced Solids in suspension ... 

Very viscous (above 2000 mN s/m2) Med. viscosity (100-1000 mN s/m2) Low viscosity to water (max. 100 mN s/m2) Foaming Scaling or fouling Crystal producing Solids in suspension ... 

Agitated horizontal film (votator) usually to condition foodsmffs, crystallize, and react. Especially for viscous feed. OK for foaming, fouling, crystal formation, and suspended solids. Viscosities >2000 mPa s. Relative to agitated film retention time of 1 1 and volume 1 1. Overall heat transfer coefficient 2 kW/m °C, decreasing with increasing viscosity 3 to 12 kW/m. See also evaporation, Section 16.11.4.1. 

Long tube, rising film especially for clear and relatively dilute feed, OK for foanting, but not for fouling, crystal formation, or suspended solids. Viscosities 150 to 1000 mPa s. AT > 14°C. Select AT to create annular or mist-annular flow. Overall heat transfer coefficient 2 to 5 kW/m °C, decreasing with increasing viscosity. HDHl = 10 to 10 Pa s. Area 3 to 300 m . 

Especially for viscous feed. OK for foaming, for fouling, crystal formation and suspended solids. Viscosities > 2000 mPa s. 

Although many commercial crystallizers operate with some form of selective crystal removal, such devices can be difficult to operate because of fouling of heat exchanger surfaces or blinding of screens. In addition, several investigations identify interactions between classified fines and course product removal as causes of cycling of a crystal size distribution (7). Often such behavior can be rninirnized or even eliminated by increasing the fines removal rate (63,64). 

Random and structured packings are susceptible to surface fouling due to process conditions and/or the presence of oxygen as may be related to bacterial growth. Some systems will precipitate solids or crystals from solution usually due to the temperature and concentration effects. Bravo [135] discusses air-water stripping and illus-... 

In the case of crystallization and suspended solid fouling, the process is usually of type A. However, when the fouling is of the crystallization type with a pure compound crystallizing out, the fouling approaches type B and the equipment must be cleaned at frequent intervals. 

Phosphonates exhibit all the properties of polyphosphates, such as threshold effect, crystal distortion, and sequestration, but are superior in their effectiveness. They provide good chelates for calcium, magnesium, iron, and copper and are commonly used where iron fouling is a problem. Their sequestering properties are generally superior to other common chelants, such as EDTA and NTA. 

This chip micro reactor was designed to have transparent reaction sections, to provide a large specific surface area of the flowing liquid, and to minimize fouling by crystallization of insoluble photoproducts [72-74]. 

This type is similar in principle to the tank type, but the cooling surfaces are continually scraped or agitated to prevent the fouling by deposited crystals and to promote heat transfer. They are suitable for processing high-viscosity liquors. Scraped-surface... 

Although chromate is the best aqueous corrosion inhibitor available, its use has been severely curtailed due to toxicity and environmental concerns ( ). One of the more successful non-chromate treatments involves the use of phosphate/phosphonate combinations. This treatment employs high levels of orthophosphate to promote passivation of the metal surfaces. Therefore, it is important to control calcium phosphate crystallization so that high levels of orthophosphate may be maintained in the system without fouling or impeding heat-transfer functions. 

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