METHOD OF DESTILLING AIR

In a distillation column used in the separation of air, a liquid stream of composition intermediate those occuring at the top and bottom is taken from the column 2 through passage 24 and is reboiled in an external heat exchanger 28 by heat exchange with heat exchange fluid 32. The resultant vapour is returned to the column through the distributor 3O. Its composition matches that of the vapour in the vapour space to which it is returned move closely than that of the vapour in the vapour space above the tray from which the liquid is taken for reboil. The heat exchange fluid is other than one which is taken from a liquid-vapour contact column passed directly to the heat exchanger 26, and then returned directly to the column from which it was taken. The distillation column is preferably the lower pressure column of a double column system, and the heat exchange fluid may be air which is introduced into the column at a level above that from which the liquid stream of intermediate composition is taken. The invention makes possible operation of the column at greater efficiency while still obtaining pure products at the top and bottom of the column.