Dehydrating and drying agents

Drying Agents. See under Dehydrating and Drying Agents in Vol 3 of Encycl, p D43... 

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. 

The crude product was washed successively with concentrated HCI, 62% HjSO, concentrated HjSO and 5% NaOH solution. It was dehydrated over a drying agent and then refractionated in a still. 20 parts of bis(2,2,2-trifluoroethyl) ether were recovered (BP 62.5° to 63.5°C). 

Calcium bromide (CaBr ) is used as a developer for photographic film and paper and as a dehydrating agent (drying agent), food preservative, and Are retardant. 

The possibility of an attack by the drying agent on the compound to be dried must always be considered carefully and if necessary examined by small-scale tests, as has been mentioned before. Most notable for such unwanted aggression amongst the popular drying agents is the molecular sieve. Various grades of molecular sieve have different activities and specialist publications should be consulted for details. The main reactions which must be kept in mind are that they can catalyse the dehydrochlorination of alkyl chlorides and the condensation of ketones, they can dehydrate alcohols and can form exposive compounds from nitroalkanes (see Section 4.6.3.). 

The normal crystalline zeolites contain water molecules which are coordinated to the exchangeable cations. These structures can be dehydrated by heating under vacuum, and in these circumstances, the cations move position at the same time, frequently settling on sites with a lower coordination number. The dehydrated zeolites are extremely good drying agents, absorbing water to get back to the preferred hydrated condition. 

For extraction of nonpolar analytes, drying agents are mixed with the matrix to adsorb moisture before extraction. Hydromatrix (Celite 566) has been used frequently. Sodium sulfate, and calcium sulfate (Drierite) are also used to dehydrate the matrix. Ratios of sample to drying agent of 1 1 up to 1 5 have been... 

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