Cryogenic process for producing ultra high purity nitrogen

In a process for the cryogenic separation of air which comprises nitrogen, oxygen and volatile impurities in an integrated multi-column distillation system wherein an air stream is compressed, freed of condensible impurities, and cooled generating a feed for the integrated multi-column distillation system, the improvement for producing an ultra high purity nitrogen product in a multi-column distillation system comprising a first column and an ultra high purity nitrogen column which comprises:

• a) generating a nitrogen rich vapor containing volatile impurities near the top of the first column and a crude liquid oxygen fraction in the bottom of said first column;
• b) removing and partially condensing at least a portion of said nitrogen rich vapor fraction containing volatile impurities thereby forming a first condensed fraction and an uncondensed fraction;
• c) returning at least a portion of said first condensed fraction as reflux to a column in the distillation system;
• d) removing at least a portion of the uncondensed nitrogen rich vapor fraction rich in volatile impurities generated in step b) as a purge stream
• e) removing a liquid nitrogen fraction from the first column at a point below the removal point for the nitrogen rich vapor containing volatile impurities from the first column;
• f) introducing the liquid nitrogen fraction to an upper part of the ultra high purity nitrogen column as feed;
• g) generating a nitrogen rich vapor fraction containing residual volatile impurities at the top of the ultra high purity nitrogen column and removing that fraction as an overhead;
• and
• h) removing an ultra high purity nitrogen fraction from the ultra high purity nitrogen column.