Liquid-vapor contact method and apparatus

A method of distilling air in at least one liquid-vapor contact column in which a boiling liquid phase comprising a mixture of oxygen, nitrogen and argon is contacted intimately and undergoes mass exchange with a condensing vapor phase comprising oxygen, nitrogen and argon, said method including the steps of:

• (i) providing reboil at a bottom region and reflux at a top region of a first liquid-vapor contact column
• (ii) taking from a chosen level in the first column at least one liquid stream out of mass exchange relationship with vapor, said stream having a composition intermediate the extremes of composition that obtain in the column;
• (iii) heat exchanging said at least one liquid stream of intermediate composition with a heat exchange fluid in a region external to the column so as to boil at least part of the stream, said heat exchange fluid flowing along a path other than one in which it passes directly from one liquid-vapor contact column, undergoes phase change in heat exchange relationship with said liquid stream of intermediate composition and returns directly to the same column;
• and
• (iv) returning at least one stream of the boiled liquid to said first column or another liqud-vapor contact column;
• wherein:
• the composition of the said returning stream matches more closely the composition of vapor at or adjacent the column level where said stream is returned than it does the composition of the vapor in mass exchange relationship with the liquid at said chosen level from which said at least one liquid stream is taken.