Scraped-Surface Exchangers

Scraped-surface exchangers are particularly suitable for heat transfer with crystalhzation, heat transfer with severe folding of surfaces, heat transfer with solvent extraction, and heat transfer of high-viscosity fluids. They are extensively used in paraffin-wax plants and in petrochemical plants for ciystallization. 

Harriott, P. Chem. Eng. Prog. Symp. Ser. No. 29, 55 (1959) 137-9. Heat transfer in scraped surface exchangers. 

FIG. 11 -34 Scraper blade of scraped-surface exchanger. Henry Vogt Machine Co., Inc.)... 

Scraped-surface exchangers have a rotating element with spring-loaded scraper blades to scrape the inside surface (Fig. 11-34). Generally a... 

Operational Characteristics Feed enters ai higher saturation temperature then is maintained in the crystallizer and is cooled in sheil-and-tube or scraped-surface exchangers to remove sensible heat and heat of crystallization. Solids encrustation problems are generally confined to the cooling surfaces... 

Cooling crystallization can be accomplished reliably by using scraped-surface exchangers however, vertical shell end tube exchengers can be used if periodic thawing of solute from (he inside of the tubes is not overly disruptive to the process. Heat transfer coefficieatu of 500-100 fcJ/h-m2 - °C (25-50 Bui/h-ft2- °F) are typical for scraped-surface units. 

Anchor-agitated vessels are nearly always operated batchwise in continuous processes, heat transfer to or from viscous liquids is often accomplished in scraped-surface exchangers. Typically these are double-pipe exchangers with a fairly large central tube 100 to 300 mm (4 to 12 in.) in diameter jacketed with steam or cooling liquid. The inside surface of the central tube is wiped by one or more longitudinal blades mounted on a rotating shaft. 

HEAT-TRANSFER COEFFICIENTS IN SCRAPED-SURFACE EXCHANGERS. Assume that the bulk temperature of the liquid at some location along the exchanger is T, and the temperature of the heat-transfer surface is T . Assume for the present that T, > T. Consider a small element of area of the heat-transfer surface over which the blade has just passed. Any liquid that was previously on this surface element has be ti removed by the blade and replaced by other liquid at temperature T. Heat flows from the surface to the liquid during the time interval fr, which is the time until the next scraper blade passes the surface element, removes the liquid, and redeposits new liquid on the surface. 

Liquid-solid suspensions, viscous aqueous and organic solutions, and numerous food products, such as margarine and orange juice concentrate, are often cooled or heated in a scraped-surface exchanger. This consists of a double-pipe heat exchanger with a jacketed... 

In another method of classification of crystallizers, the equipment is classified according to the method of suspending the growing product crystals. Examples are crystallizers where the suspension is agitated in a tank, is circulated by a heat exchanger, or is circulated in a scraped surface exchanger. 

Incremental ketone dewaxing plants (see Figure 20) use a combination of Scraped Surface Exchangers that use cold filtrate for cooling the slurry followed by Scraped Surface Chillers, which use a refrigerant (propane, propylene, ammonia) to cool the slurry to the filtration temperature. Solvent may be added at the beginning or along the train in increments . 

Scraped Surface Chiller. Usually a double pipe design of 6, 8,12 inch diameter with internal scrapers to remove wax from the cold walls. T5 ically a refrigerant such as propane, propylene or ammonia is used on shell side. Used in conventional dewaxing Scraped Surface Exchanger. Usually a double pipe design of 6, 8,12 inch diameter with internal scrapers to remove wax from the cold walls. Typically cold filtrate is used on shell side. Used in conventional dewaxing... 

Double-pipe scraped surface exchangers for both solvent dewaxing and wax recrystallizing process sections. 

---