Abstract: In order to boost production of a product (A) of an existing separation plant (X, 1), an additional plant (Y) is integrated with the original plant so as to enable the original plant (X) to produce more of that product (A+B), whilst the additional plant may or may not necessarily itself produce the same product directly. For example, air is separated in a first unit, which is an existing double column distillation plant, to produce an oxygen rich fluid. So as to increase the production of the oxygen rich fluid, a second unit, which is a wash column (15), is integrated with the first unit. Air (41) is separated in the single nitrogen wash column (15) to remove oxygen and gaseous nirogen (42) is produced at the top of the column. The wash column is fed with liquid nitrogen (39) from the high pressure column (25) of an existing air separation unit.