Improving Agility of Cryogenic Air Separation Plants

Cryogenic air separation plants that produce argon in addition to oxygen and nitrogen use two heat-integrated distillation columns and a side rectifier. The dynamics of the side rectifier are very slow because of three factors:  the large number of separation stages, the small amount of argon in the air feed, and the high product purity. Currently, when the plant shuts down, the liquid inventory in the argon column drains into the upper (low-pressure) column. A normal start-up takes about 10 h to achieve argon product purity. The start-up time can be reduced significantly by using storage vessels to collect the liquid that drains from the argon column upon shutdown and reintroducing the liquid during the subsequent start-up. Compared to a 10 h start-up time with no collection points, the time required to achieve argon product purity is reduced to 3.35 h by using 2 collection points and is reduced to 2.23 h by using 6 collection points.