Operation of a cryogenic air separation unit which intermittently uses air feed as the repressurization gas for a two bed PSA system

In a process for the cryogenic distillation of an air feed to produce a nitrogen-enriched waste stream and product streams comprising a nitrogen rich stream and an oxygen rich stream, said process comprising the steps of:

• (a) compressing the air feed to an elevated pressure;
• (b) pretreating the compressed air feed in a two bed pressure swing adsorption system to remove impurities comprising carbon dioxide and water to produce an impurity-depleted air feed wherein said two bed pressure swing adsorption system has an intermittent repressurization gas requirement which is satisfied by intermittently diverting a portion of the impurity-depleted air feed;
• (c) cooling the remaining portion of the impurity-depleted air feed in a cooling system to a temperature near its dew point;
• (d) introducing the cooled, impurity-depleted air feed into a cryogenic distillation column system comprising a higher pressure column and a lower pressure column wherein at least a portion of the cooled, impurity-depleted air feed is specifically fed to the higher pressure column;
• (e) removing a crude liquid oxygen stream from the bottom of the higher pressure column, reducing the pressure of at least a first portion thereof and feeding the reduced pressure portion to the lower pressure column wherein it is distilled into the nitrogen rich stream which is removed from the top of the lower pressure column and the oxygen rich stream which is removed from the bottom of the lower pressure column;
• (f) removing the nitrogen-enriched waste stream from an upper intermediate location of the lower pressure column;
• the improvement for at least partially eliminating reductions in the purity of the product streams caused by intermittently diverting of a portion of the impurity-deleted air feed in step (b) for the intermittently required repressurization gas in the pressure swing adsorption system, said improvement comprising reducing the flow of both the nitrogen-enriched waste stream removed in step (f) and the crude liquid oxygen stream removed in step (e) during those intermittent periods when repressurization gas is required in the pressure swing adsorption system wherein said reducing of the flow of both the nitrogen-enriched waste stream removed in step (f) and the crude liquid oxygen stream removed in step (g) is implemented by a feedforward control system whereby said reducing is performed prior to, or simultaneously with, the beginning of the intermittent periods when repressurization gas is required in the pressure swing adsorption system.