Precooling

To a mixture of 0.3 mol of the diol (see 111, Exp. 60) and 5 ml of pyridine were added with swirling and cooling in a bath of -30°C 85 g of precooled (-30°C) thionyl chloride. The addition was carried out in 10-g portions over a period of 20 min. The cooling bath was then removed and the temperature of the mixture was allowed to rise to 25 or 30°C in 4 h (occasional cooling may be necessary). Some boiling stones were then added and the mixture was warmed at 40 c for 45 min in a water-pump vacuum. Subsequently, the mixture was vigorously shaken with 200 ml of ice water and 20 ml of pyridine. Three extractions with diethyl ether were carried out. The extracts were successively washed with 3 N HCl and water and... 

Potassium hydride (1 eq.) was washed with hexanes and suspended in anhydrous ether at 0°C. 7-Bromoindole was added as a solution in ether. After 15 min, the solution was cooled to — 78°C and t-butyllithium (2 eq.) which had been precooled to — 78°C was added by cannula. A white precipitate formed. After 10 min DMF (2 eq.) was added as a solution in ether. The reaction mixture was allowed to warm slowly to room temperature and when reaction was complete (TLC) the suspension was poured into cold 1 M H3PO4. The product was extracted with EtOAc and the extract washed with sat. NaHCOj and dried (MgS04). The product was obtained by evaporation of the solvent and purified by chromatography on silica gel (61% yield). 

Adsorption (qv) of gases has been reviewed (40,50) (see also Adsorption, gas separation). Adsorption, used alone or in combination with other removal methods, is excellent for removing pollutant gases to extremely low concentrations, eg, 1 ppmv. When used in combination, it is typically the final step. Adsorption, always exothermic, is even more attractive when very large gas volumes must be made almost pollutant free. Because granular adsorbent beds ate difficult to cool because of poor heat transfer, gas precooling is often practiced to minimize adsorption capacity loss toward the end of the bed. Pretreatment to remove or reduce adsorbable molecules, such as water, competing for adsorption sites should also be considered (41). 

The second Hquefaction process is carried out at temperatures from 261 to 296 K, with Hquefaction pressures of about 1600—2400 kPa (16—24 atm). The compressed gas is precooled to 277 to 300 K, water and entrained oil are separated, and the gas is then dehydrated ia an activated alumina, bauxite, or siHca gel drier, and flows to a refrigerant-cooled condenser (see Drying agents). The Hquid is then distilled ia a stripper column to remove noncombustible impurities. Liquid carbon dioxide is stored and transported at ambient temperature ia cylinders containing up to 22.7 kg. Larger quantities are stored ia refrigerated iasulated tanks maintained at 255 K and 2070 kPa (20 atm), and transported ia iasulated tank tmcks and tank rail cars. 

Chloroformates are shipped in nonretumable 208-L (55-gal) polyethylene dmms with carbon steel overpacks or high density polyethylene dmms. Eor bulk shipments, insulated stainless-steel tank containers and tmcks provide secure protection. Tank tmck and rail car quantities are shipped using equipment dedicated for these types of products. Materials such as isopropyl chloroformate, benzyl chloroformate, and j -butyl chloroformate that require refrigeration are precooled when shipped in bulk containers. Bulk shipments that are precooled must proceed to the destination without layover. Dmm shipments of IPCE, BCE, and SBCE must be shipped in refrigerated containers. Many of the chloroformates are only shipped in tmck load shipments. The U.S. Department of Transportation (DOT) Hazardous Materials Regulations control the shipments of chloroformates, as described in Table 3. 

Engine Expansion Cycle. In this process, a refrigerant fluid is compressed, optionally precooled, and expanded to a lower pressure through an expander while producing work to reduce both the enthalpy and temperature of the fluid. The expanded refrigerant fluid is warmed to provide cryogenic refrigeration. 

Fig. 3. Propane precooled mixed refrigerant process for natural gas Hquefaction.

FIG. 11-116 Propane precooled mixed-refrigerant cycle cooling curve for natural gas. 

Installation of heat-exchange equipment to precool and dehumidify the feed gas to an absorber also deserves consideration in order to take advantage of the cooling effects created by vaporization of solvent in the lower sections of the tower. 

For materials with very low melting points it is sometimes convenient to use dilute solutions in acetone, methanol, pentane, diethyl ether or CHCI3-CCI4. The solutions are cooled to -78° in a dry-ice/acetone bath, to give a slurry which is filtered off through a precooled Buchner funnel. Experimental details, as applied to the purification of nitromethane, are given by Parrett and Sun [J Chem Educ 54 448 7977]. 

Triethylammonium hydroiodide [4636-73-1 j M 229.1, m 181 . Purified as for triethylammonium bromide, except the soln for pptn was precooled acetone at -10 and the ppte was twice recrystd from a cooled acetone/hexane mixture at -10°. 

Identify cases where feed or intermediate heat exchangers would be helpful, when only one of the operating lines is too far from the equilibrium curve. A large gap for the bottom section could indicate potential for feed preheat or an interreboiler. A large gap for the top section could portend precooler or intercondenser need. 

The reaction mixture is not cooled during the addition of the last two or three portions of phenyl isocyanate, so that the final temperature is near 25° this procedure prevents separation of the sodium salt of cyanophenylurea, which crystallizes readily at low temperatures. For the same reason, the filtered solution of the salt is not precooled, but rather is cooled during the precipitation of the free cyanourea. 

B. 2,4,6-Tnbromohenzoic acid. A 5-1. three-necked flask, equipped with a mechanical stirrer and thermometer and surrounded by an ice-salt bath, is charged with a precooled mixture of 1.02 1. of concentrated sulfuric acid and 500 ml. of water. Cooling during the reaction period is assisted by the ice-salt bath, but is effected chiefly by periodic additions of large amounts... 

Wet scrubbers rely on a liquid spray to remove dust particles from a gas stream. They are primarily used to remove gaseous emissions, with particulate control a secondary function. The major types are venturi scrubbers, jet (fume) scrubbers, and spray towers or chambers. Venturi scrubbers consume large quantities of scrubbing liquid (such as water) and electric power and incur high pressure drops. Jet or fume scrubbers rely on the kinetic energy of the liquid stream. The typical removal efficiency of a jet or fume scrubber (for particles 10 g. or less) is lower than that of a venturi scrubber. Spray towers can handle larger gas flows with minimal pressure drop and are therefore often used as precoolers. Because wet scrubbers may contribute to corrosion, removal of water from the effluent gas of the scrubbers may be necessary. 

Orifice scrubbers generally do not require precleaning, unless the waste gas contains large pieces of debris. Precooling may be necessary for high temperature waste gas flows which increase the evaporation of the scrubbing liquid. 

For absorption applications, precoolers (e.g., spray chambers, quenchers) may be needed to saturate the gas stream or to reduce the inlet air temperature to acceptable levels to avoid solvent evaporation or reduced absorption rates. 

A solution of 16jS-methyl-l la,17a,21-trihydroxy-5j5-pregnane-3,20-dione 21-acetate (52), 45 g, in dioxane (297 ml) is cooled to 15° and treated over a 5 min period with a solution of bromine (34.2 g) in dioxane (594 ml) precooled to 18°. After 2 min a solution of sodium acetate (60 g) in water (600 ml) is added and the mixture poured into ice water (8 liters). The precipitate is filtered off, washed to neutrality with water, and dried to give the crude dibromide (53), 55.7 g mp 125-126° (dec.) [aJu 58°. A mixture of dibromide (53), 55.5 g, lithium bromide (27.8 g), lithium carbonate (27.8 g) and DMF (1.11 liters) is refluxed under rapid stirring for 6 hr. The mixture is concentrated under vacuum to about 250 ml, poured into ice water (8 liters) containing hydrochloric acid (250 ml), and extracted with methylene dichloride. The extracts are washed to neutrality with water and evaporated to dryness. The residue is dissolved in acetone, evaporated to dryness under reduced pressure, redissolved in acetone and crystallized by the additon of hexane. This gives the dienone (54) 24.4 g, mp 236-239°. 

A detailed procedure for the use of MCPBA recently appeared in Reagents for Organic Synthesis by Fieser and Fieser. The commercially available MCPBA (Aldrich) is 85% pure the contaminant, m-chlorobenzoic acid, can be removed by washing with a phosphate buffer of pH 7.5. The epoxidation is usually performed as follows a solution of 3 -acetoxy-5a-androst-16-ene (2.06 g, 6.53 mmoles) in 25 ml of chloroform (or methylene dichloride) is chilled to 0° in a flask fitted with a condenser and drierite tube and treated with a solution of commercial MCPBA (1.74 g, 20% excess) in 25 ml chloroform precooled to the same temperature. The mixture is stirred and allowed to warm to room temperature. After 23 hr (or until TLC shows reaction is complete) the solution is diluted with 100 ml chloroform and washed in sequence with 100 ml of 10% sodium sulfite or sodium iodide followed by sodium thiosulfate, 200 ml of 1 M sodium bicarbonate and 200 ml water. The chloroform extract is dried (MgS04) and evaporated in vacuo to a volume of ca. 10 ml. Addition of methanol (10 ml) followed by cooling of the mixture to —10° yields 0.8 gof 16a,17a-epoxide mp 109.5-110°. Additional product can be obtained by concentration of the mother liquor (total yield 80-90%). 

Androst-4-ene-3,17,19-trione (0.23g) is added to a precooled solution of 0.23 g of potassium hydroxide in a mixture of 1 ml of water and 5 ml of methanol. The reaction mixture is stirred at 5-10° under nitrogen for 3 hr, diluted with benzene (30 ml) and washed with water (2 x 10 ml). The aqueous layer is reextracted with benzene and the benzene solution dried and evaporated. The crude crystalline product is filtered through 8 g of Merck silicagel and eluted with benzene-ethyl acetate (9 1) to yield 0.19 g of 19-norandrostenedione (88%) mp 169-171° 141° (CHCI3). 

A piece of sodium metal stored under kerosene m a metal container is removed from ajar and blotted with dry napkin or filter paper With a sharp knife, the layer of oxides IS removed until a shiny surface appears The removed layer is then destroyed carefully by adding very small pieces (not larger than 0 5 cm) to precooled 200 mL of methanol or ethanol... 

Vor-kiihler, m. forecooler, primary cooler, -kuhlung, /. forecooling, precooling, -kultur, /. preliminary culture, vorkiipen, t,.t. bottom with a vat dye,... 

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