Scraped-Surface Crystallizer

Scraped-Surface Crystallizer For relatively small-scale apph-cations a number of ciystallizer designs employing direct neat exchange between the shiny and a jacket or double wall containing a cooling medium have been developed. The heat-transfer surface is scraped or agitated in such a way that the deposits cannot build up. 

These results show that it may be more efficient to depectinize fruit juices before their concentration by freezing because this would give minimum losses at lower heat removal rates and thus at conditions of more economical operation. The implication of these results for the design of a scraped surface crystallizer are currently being examined. 

Production rates of melt crystallization of organic materials usually are low enough to warrant the use of scraped surface crystallizers like that of Figure 16.10(a). A major difficulty in the production of crystals is the occlusion of residual liquor on them which cuts the overall purity of the product, especially so because of low temperatures near the eutectic and the consequent high viscosities. Completeness of removal of occluded liquor by centrifugation or filtration often is limited because of the fragility and fineness of the organic crystals. 

Brodie Crystallizer-Purifier [Fig. 1414(c)]. This equipment combines a horizontal scraped surface crystallizer with a vertical purifying section. The capacity and performance of the purifier... 

All feeds were prepared in Armstrong scraped surface crystallizers. 

The scraped-surface crystallizer provides an effective and inexpensive method of producing slurry in equipment which does not require expensive installation or supporting structures. 

Brodie Crystallizer-Purifier [Fig, 16.15(c)], This equipment combines a horizontal scraped surface crystallizer with a vertical purifying section. The capacity and performance of the purifier depends strongly on the sizes of the crystals that enter that zone. In order to ensure adequate crystal size, residence times in the crystallizing zone as long as 24 hr may be needed. No data of... 

Scraped surface crystallizer Distillation columns See pressure vessels, packing, and trays length, m 7.0 280.0 41,000 40,000 0.7 ... 

A scraped-surface crystallizer is similar in principle to the tank type, but the cooling surfaces are continually scraped or agitated to prevent fouling by deposited crystals and to promote heat transfer. These crystallizers are suitable for processing high-viscosity liquors. Scraped-surface crystallizers can be operated batch-wise, with recirculation of the mother liquor, or continuously. A disadvantage of this type is that they tend to produce very small crystals. 

Double-Pipe Scraped-Surface Crystallizer This type of equipment consists of a double-pipe heat exchanger with an internal agitator fitted with spring-loaded scrapers that wipe the wall of the inner pipe. 

In the group of suspension processes, there are the jacket cooled and the directly (e.g., by inert gases) cooled processes. The Amoco process, on the one hand, and the Maruzen or Chevron process, on the other hand, are, according to Ransley (1984), representatives for such processes. Other developments are scrapes crystallizers according to the Humble-Oil, the Krupp-Harpen (see Ritzer 1973), or the Hoechst AG (DE-PS 1969) process which are used for the production of p-nitrochlorobenzene. These scraped surface crystallizers are in fact solid layer processes, but in the handling of the products—crystals in suspension—they have to be treated like suspension processes. A number of processes that feature a combination of a scraped crystallizer and suspension techniques will be discussed. 

Basically the same idea was used by Brodie (1971) to create the Brodie Purifier of Union Carbide Corp. (GB-PS 1968), which features a horizontally scraped surface crystallizer with a vertical purification section. The performance of the apparatus depends strongly on the size of the crystals entering the purification section. Walas (1988) reports that as much as 24h residence time is needed to create the correct crystal size. He also presents data about p-dichlorobenzene purification. Mullin (1988) reports that in addition to the p-dichlorobenzene, large-scale production has also been installed for naphthalene. Figure 7.20 shows a diagram of the Purifier. 

Suspension melt crystallization crystals and melt same temperature, design on degree of supersaturation, separation of crystals from melt depends on density difference in countercurrent operation. Scraped surface crystallizer. Section 4.6. The suspension methods have slower rates of crystal growth compared with the solid layer processes. 

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