Scraped heat exchanger

Rice, starch, milk and water are added into the mixing tank and are heated at a preset temperature to about 60°C for pre-soaking. If required, flavourings and sugar are added at that point. The mix is then transferred to a scrape heat exchanger and the mixture is heated to approx. 115-121°C with a holding time of 15 to 20 minutes. 

The heat exchanging surface of the extruder is relatively large compared to various other reactors. Also, due to its functioning as a scraped heat exchanger, the thermal homogenization of the product is reasonable. 

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. 

Skelland, A.H.P., Oliver, D.R. and Tooke, S. Brit. Chem. Eng. 7 (1962) 346. Heat transfer in a water-cooled scraped-surface heat exchanger. 

Bott, T.R. Brit. Chem. Eng. II (1966) 339. Design of scraped-surface heat exchangers. 

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

In one continuous method (17), the waxy oil charge—for example, in the case of a topped crude—is diluted with an equal volume of propane and the mixture is chilled to about —25° F. by means of heat exchange with cold filtrate in double-pipe scraped-surface equipment. This chilled mixture is then mixed with propane which has been chilled by autorefrigeration to —45° F. to bring the ratio of propane to oil to about 5 to 1. Ninety per cent of the refrigeration required is accomplished in this manner the remaining 10% is obtained by evaporating propane from the diluted mixture. 

Figure 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger, (b) Scraped inner surface of a double-pipe exchanger, (c) Shell-and-tube exchanger with fixed tube sheets, (d) Kettle-type reboiler, (e) Horizontal shell side thermosiphon reboiler, (f) Vertical tube side thermosiphon reboiler, (g) Internal reboiler in a tower, (h) Air cooler with induced draft fan above the tube bank, (i) Air cooler with forced draft fan below the tube bank.

Z. Tadmor, Corotating Disk Scraped Surface Heat Exchangers, Food Techn., Dec 1985 pp. 66-74. 

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