Heat sensitive

Separation of high-molecular-weight heat-sensitive materials. High-molecular-weight materials are often heat sensitive and as such are usually distilled under vacuum to reduce their boiling temperature. 

In summary, distillation is not well suited for separating either low-molecular-weight materials or high-molecular-weight heat-sensitive materials. However, distillation might still be the best method for these cases, since the basic advantages of distillation... 

Reactive and heat-sensitive components must be removed early. 

M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

Liquid chromatography, having a resolving power generally less than that of gas phase chromatography, is often employed when the latter cannot be used, as in the case of samples containing heat-sensitive or low vapor-pressure compounds. 

A group of aminoxanthenes, ie, pyra2oloxanthenes, is used as color formers ia pressure or heat-sensitive imaging papers (43). These compounds are colorless, but, upon contact with acidic electron-accepting material, are converted to resonance forms that are lightly colored. An example is stmcture [58294-05-6] (35), which forms upon the condensation of A[,A/-diethyl-y -aminophenol with phthalic anhydride, followed by addition of 6-hydroxyinda2ole ia 80% sulfuric acid (44). 

Temperature and pressure are not considered as primary operating variables temperature is set sufficiendy high to achieve rapid mass-transfer rates, and pressure is sufficiendy high to avoid vaporization. In Hquid-phase operation, as contrasted to vapor-phase operation, the required bed temperature bears no relation to the boiling range of the feed, an advantage when heat-sensitive stocks are being treated. 

Pharmaceutical Processes. The pharmaceutical industry is a principal user of extraction because many pharmaceutical intermediates and products ate heat-sensitive and cannot be processed by methods such as distillation. A usehil broad review can be found in the Hterature (241). 

Vitamins. The preparation of heat-sensitive natural and synthetic vitamins (qv) involves solvent extraction. Natural vitamins A and D are extracted from fish Hver oils and vitamin E from vegetable oils (qv) Hquid propane [74-98-6] is the solvent. In the synthetic processes for vitamins A, B, C, and E, solvent extraction is generally used either in the separation steps for intermediates or in the final purification. 

MiscelUneous. Mahc acid is used in pharmaceuticals (qv), cosmetics (qv), dentifrices (qv), metal cleaning, electroless plating (46), wash-and-wear textile finishing (47—49), for stabilization of heat-sensitive copying paper (50), as an inhibitor of gelation, livering, and agglomeration in cellulose nitrate Hqueurs, and in many other appHcations. 

The common types of dryers are rotary, hearth, flash (spray), and fluidized beds (10). Hot gases are used invariably to remove moisture. The gas flow can be either cocurrent or countercurrent to the flow of soHds, the former tends to be more efficient. In the hearths, the gas flow is countercurrent as the soHds are raked down from one hearth to the next below. Flash dryers are very rapid because the soHds are exposed only briefly to the hot gases. Fluidized-bed dryers, which use hot gases to suspend the soHds, are rapid and efficient, but require elaborate dust coHection systems. These are preferred when fine soHds are involved, and are used commonly for drying fine coal. Indirect-fired dryers are used when the soHds are heat sensitive or combustible. 

When the dryer is seen as a heat exchanger, the obvious perspective is to cut down on the enthalpy of the air purged with the evaporated water. Minimum enthalpy is achieved by using the minimum amount of air and cooling as low as possible. A simple heat balance shows that for a given heat input, minimum air means a high inlet temperature. However, this often presents problems with heat-sensitive material and sometimes with materials of constmction, heat source, or other process needs. AH can be countered somewhat by exhaust-air recirculation. 

Heat-SensitiZingProcess. Another process used to make latex mbber articles of thicker section involves sensitizing the compound so that it coagulates when heated to a given temperature, then using heated molds to build the article to the desired thickness. Ammonia-preserved latex is used in this process, and polyether, polythioether, or poly(vinyl methyl ether) (PVME) can be used as heat-sensitizing agents. 

Selection of the most suitable machine for a given requirement is an extremely complex process. Added to variations in the properties of the different materials, many of the machines involved have been specifically developed or adapted to perform only particular tasks. The principal factors which must be addressed are toughness/britdeness, hardness, abrasiveness, feed size, cohesity, particle shape and stmcture, heat sensitivity, toxicity, explodability, and specific surface. 

Heat Sensitivity. Only 1—2% of appHed energy is effectively used for size reduction. The remainder is mainly converted to heat, which is absorbed by the grinding air, product, and equipment. 

Sorbate combined with mild heat has a synergistic effect with regard to microbial destmction thus, in the presence of 0.025—0.06 wt % sorbate, products such as apple juice, peach and banana sHces, fmit salads, and strawberries can be treated with less severe heat treatments to extend shelf life (119,120). Sorbates increase the heat sensitivity of various spoilage fungi under varying conditions of pH and water activity (121—124). A similar synergistic effect has been reported for the combination of sorbate with irradiation (125). 

Sundower Seed. Compared to the FAO/WHO/UNU recommendations for essential amino acids, sunflower proteins are low in lysine, leucine, and threonine for 2 to 5-year-olds but meet all the requirements for adults (see Table 3). There are no principal antinutritional factors known to exist in raw sunflower seed (35). However, moist heat treatment increases the growth rate of rats, thereby suggesting the presence of heat-sensitive material responsible for growth inhibitions in raw meal (72). Oxidation of chlorogenic acid may involve reaction with the S-amino group of lysine, thus further reducing the amount of available lysine. 

Chemical indicators for dry-heat steriliza tion are available either in the form of pellets enclosed in glass ampuls, or in the form of paper strips containing a heat-sensitive ink. The former displays its end point by melting, the latter by a color change (see Chromogenic materials). 

Considering their heat sensitivity, the separation of fatty acids and rosin with minimal degradation by fractional distillation under vacuum and/or in the presence of steam is surprisingly good (3). Tad od rosin (TOR) contains about 2% fatty acid and smad amounts of neutrals. Tad od fatty acid (TOFA) contains as Htde as 1.2% rosin and 1.7% neutrals. In typical U.S. TOFA, 49% of the fatty acids is oleic, 45% linoleic, and 3% palmitic, stearic, and eicosatrienoic acid. TOR and TOFA are upgraded to resins and chemicals for the manufacture of inks (qv), adhesives (qv), coatings (qv), and lubricants (see Lubrication AND lubricants). 

Thermal degradation of isocyanates occurs on heating above 100—120°C. This reaction is exothermic, and a mnaway reaction can occur at temperatures >175° C. In view of the heat sensitivity of isocyanates, it is necessary to melt MDl with caution and to foUow suppHers recommendation. Disposal of empty containers, isocyanate waste materials, and decontamination of spilled isocyanates are best conducted using water or alcohols containing small amounts of ammonia or detergent. Eor example, a mixture of 50% ethanol, 2-propanol, or butanol 45% water, and 5% ammonia can be used to neutrali2e isocyanate waste and spills. Spills and leaks of isocyanates should be contained immediately, ie, by dyking with an absorbent material, such as saw dust. 

Low viscosity cellulose acetate is used in lacquers and protective coatings for paper, metal, glass, and other substrates and as an adhesive for cellulose photographic film because of its quick bonding rate and excellent bond peel strength (135) (see Coatings). Heat-sensitive adhesives for textiles have also been prepared from cellulose acetate (136). Extmded cellulose acetate film makes an excellent base for transparent pressure-sensitive tape (137) (see Adhesives). 

Supercritical and Freeze Drying. To eliminate surface tension related drying stresses in fine pore materials such as gels, ware can be heated in an autoclave until the Hquid becomes a supercritical fluid, after which drying can be accompHshed by isothermal depressurization to remove the fluid (45,69,72) (see Supercritical fluid). In materials that are heat sensitive, the ware can be frozen and the frozen Hquid can be removed by sublimation (45,69). 

The suitabiHty and economics of a distillation separation depend on such factors as favorable vapor—Hquid equiHbria, feed composition, number of components to be separated, product purity requirements, the absolute pressure of the distillation, heat sensitivity, corrosivity, and continuous vs batch requirements. Distillation is somewhat energy-inefficient because in the usual case heat added at the base of the column is largely rejected overhead to an ambient sink. However, the source of energy for distillations is often low pressure steam which characteristically is in long supply and thus relatively inexpensive. Also, schemes have been devised for lowering the energy requirements of distillation and are described in many pubHcations (87). 

The absolute pressure may have a significant effect on the vapor—Hquid equiHbrium. Generally, the lower the absolute pressure the more favorable the equiHbrium. This effect has been discussed for the styrene—ethylbenzene system (30). In a given column, increasing the pressure can increase the column capacity by increasing the capacity parameter (see eqs. 42 and 43). Selection of the economic pressure can be faciHtated by guidelines (89) that take into consideration the pressure effects on capacity and relative volatiHty. Low pressures are required for distillation involving heat-sensitive material. 

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