Abstract: An air separation unit including an air intake, a high pressure cryogenic distillation column receiving air from the air intake and producing an oxygen rich liquid reflux, and a low-pressure cryogenic distillation column. An adaptive controller controls the flow rate of the oxygen-rich liquid from the first distillation column into the second distillation column, wherein during plant upsets, the adaptive controller maintains the level of oxygen-rich liquid in the first distillation column at a substantially constant level by adjusting a flow rate of the oxygen rich liquid into the second distillation column, and wherein the argon content of the argon-rich output stream from the second distillation column remains substantially constant.