Crystallizers Swenson-Walker

A solution containing 23 per cent by mass of sodium phosphate is cooled from 313 to 298 K in a Swenson-Walker crystalliser to form crystals of Na3P04.12H20. The solubility of Na3P04 at 298 K is 15.5 kg/100 kg water, and the required product rate of crystals is 0.063 kg/s. The mean heat capacity of the solution is 3.2 kJ/kg deg K and the heat of crystallisation is 146.5 kJ/kg. If cooling water enters and leaves at 288 and 293 K, respectively, and the overall coefficient of heat transfer is 140 W/m2 deg K, what length of crystalliser is required ... 

The circulating-magma design describes equipment in which crystals are intentionally transported with the liquor to the region where supersaturation is being created. Swenson-Walker, Wulff-Bock, and doublepipe crystallizers are examples of this type. Forced-circulation design is common. 

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 crystallisers can be operated batchwise, with recirculation of the mother liquor, or continuously. A disadvantage of this type is that they tend to produce very small crystals. A typical unit is the Swenson-Walker crystalliser shown in Volume 2. 

The Swenson-Walker crystallizer (Figure 837) developed in the early 1930s is a well-known example of a trough crystallizer with internal agitation and... 

Figure 8.37. Swenson-Walker crystallizer. (After Seavoy and Caldwell, 1940)...

Crystallizers similar to the Swenson-Walker type with semi-cylindrical (U-type) or nearly cylindrical (0-type) cross-section have been used for many years in the sugar industry for the crystallization of concentrated molasses. Units fitted with water-cooled jacket are still employed, but these show poor heat transfer characteristics scraper-stirrers are not permitted on account of the damage they do to the crystals. Accordingly, many other types of cooling arrangement have been tried (Seavoy and Caldwell, 1940). 

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