Abstract: 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 and is at least partial reboiled in an external heat exchanger by heat exchange with a heat exchange fluid. The resultant vapor is returned to the column. Its composition matches that of the vapor in the vapor space to which it is returned more closely than that of the vapor in the vapor 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 vapor contact column, is passed directly to the heat exchanger, and is then returned directly to the column from which it was taken.

Abstract: Air is fractionated in a double distillation column comprising lower pressure and higher pressure columns. A condenser/reboiler provides reboiled oxygen to the bottom of the column. Liquid nitrogen reflux is introduced into the top of the column. Oxygen, nitrogen and argon-enriched streams are withdrawn from the column. The argon-enriched stream is separated in a further distillation column and a product argon stream is withdrawn therefrom. A liquid stream comprising oxygen and nitrogen is withdrawn from an intermediate level of the column, is at least partially reboiled in a heat exchanger, and is returned to the column at a level where the composition of the vapor corresponds approximately to that of the reboiled liquid. Heating for the heat exchanger is provided by a fluid stream withdrawn from the second distillation column. The fluid stream is returned to the second column.