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Abstract

A cryogenic air separation system which is subject to periods of significant changes in product demand is controlled during such periods to minimize the impact of transient operation on product purity. A double-column distillation system is utilized in which a nitrogen-rich liquid is withdrawn from the higher-pressure column and introduced into the lower-pressure column as reflux. An inventory of this liquid is maintained in a holdup tank for storage or withdrawal during periods of transient operation. In addition, nitrogen vapor product from the lower-pressure column is recycled to the higher-pressure column, and the nitrogen vapor recycle rate is controlled as a function of the liquid level in the holdup tank. The flow rate of nitrogen-rich liquid withdrawn from the higher-pressure column is controlled as a function of its composition. The flow ratio of the nitrogen vapor recycle to the nitrogen-rich liquid reflux is controlled as a function of the composition of the nitrogen vapor withdrawn from the lower-pressure column. A feedforward control system increases the flow rate of the nitrogen-rich liquid withdrawn from the higher-pressure column during periods of increasing product demand and decreases the flow rate during periods of decreasing product demand.

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