Scraped Surface

The Phillips process is a two-stage crystallisation process that uses a pulsed column in the second stage to purify the crystals (79,80). In the pulsed column, countercurrent contact of the high purity PX Hquid with cold crystals results in displacement of impurities. In the first stage, a rotary filter is used. In both stages, scraped surface chillers are used. This process was commercialized in 1957, but no plants in operation as of 1996 use this technology. 

Insofar as they are used to purify other products, several processes used in the refinery fall under the classification of dewaxing processes however, such processes must also be classified as wax production processes (2). Most commercial dewaxing processes utilize solvent dilution, chilling to crystallize the wax, and filtration (28). The MEK process (methyl ethyl ketone—toluene solvent) is widely used. Wax crystals are formed by chilling through the walls of scraped surface chillers, and wax is separated from the resultant wax—oil—solvent slurry by using fliUy enclosed rotary vacuum filters. 

One of the early column crystallizers was that iatroduced for the separation of xylene isomers (see Xylene and Ethylbenzene). In this unit, shown schematically ia Eigure 25, -xylene crystals are formed ia a scraped-surface chiller above the column and fed to the column. The crystals move downward counter-currenfly to impure Hquid ia the upper portion of the column and melted -xylene ia the lower part of the column. Impure Hquor is withdrawn from an appropriate poiat near the top of the column of crystals while pure product, xylene, is removed from the bottom of the column. The pulse unit drives melt up the column as reflux and iato a product receiver. 

Co.). In direct contact with the scraped surface is the process fluid which may deposit ciystals upon chilling or be extremely fouhng or of very high viscosity. Motors, chain drives, appropriate guards, and so on are required for the rotating element. For chilling service with a refrigerant in the outer shell, an accumulator drum is mounted on top of the unit. 

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. 

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. 

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

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. 

The primary consideration for all AEM analysis is that the specimen be thin (generally carbon coated electron microscope grid either dry or in a suitable liquid. If a liquid suspension is used in preparing the specimen, it is important that all elements of interest are insoluble in that liquid. Only particles thin enough to meet AEM thin-film criteria (15) should be analyzed quantitatively. Scraping surface particles from a catalyst pellet for specimen preparation may be more useful than grinding the entire pellet. 

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... 

Scraped surface Organic compounds, where fouling is a problem, viscous materials Chlorobenzenes, organic acids, paraffin waxes, naphthalene, urea... 

Arrows indicate that metal show.i is somewhere in range indicated by extreme of dashed line. Theae data for matals are baaed on scraped surfaces. The data for unacraped (presumably oxidised) metals were somewhat lower (io, e negative) in the series. 

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

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. 

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