Desiccation of Air

It is frequently necessary to dry the air for chemical and metallurgical processes. For example, air required to be ozonised for ventilating or sterilising purposes must first be dried; and the efficiency of a blast furnace in the production of pig-iron is greatly enhanced by the desiccation of the blast. One convenient method consists in cooling the air by passage through chambers fitted with pipes through which cooled brine from a refrigerator is passed. The moisture is deposited as ice on the tubes, the escaping air having been cooled to about -5° C. (23° F.), and its moisture content reduced to approximately 2.6 grams per kilogram of dry air.

In the Daubine-Roy process of Desiccation of Air is made to ascend a tower and during the process to pass through trays of calcium chloride, the temperature of which is kept low (namely, 4° to 5° C.) by means of water coolers. The limits of hydration range from the monohydrate to the octahydrate; thus

CaCl₂.H₂O + 7H₂O ⇔ CaCl₂. 8H₂O,

the monohydrate being periodically regenerated by passing hot gases through the towrer at a temperature not exceeding 200° C. Experience shows that 240 kilograms of the monohydrate spread out in a layer 24 cm. deep and cooled by tubes containing water to 4° or 5° C. will per hour desiccate 300 cubic metres of air percolating it from above downwards; it will continue to do this for 4 hours, reducing the humidity of the air from an average of about 15 grams of moisture per cubic metre to one of about 1.5 grams. To desiccate a gas to this extent by refrigeration would require a temperature of about -15° C.

For Desiccation of Air for laboratory purposes small quantities of air are frequently dried by passage over anhydrous calcium chloride - which is a slightly more powerful desiccator than the monohydrate. A better reagent is sulphuric acid. Gases may be led directly through the concentrated acid or, better, through tubes or towers containing pumice, glass beads, or other suitable substances moistened with the acid. The concentrated acid reduces the moisture content to about 1 milligram in 4000 litres.

The most powerful desiccator available to the chemist is phosphorus pentoxide. Prolonged exposure of a gas to this reagent, conveniently scattered over glass-wool, reduces the moisture content to about 1 milligram in 40,000 litres.