Cryogenic system for producing low-purity oxygen

Claim 1

A process for producing low-purity oxygen from feed air by low-temperature distillation comprising the steps of: (a) supplying a high-pressure gas feed stream, comprising at least 35 percent of said feed air, at pressure of at least 65 psia, in a cleaned, cooled state, (b) distilling said high-pressure gas feed stream in a higher-pressure distillation column so as to produce first intermediate oxygen-enriched liquid at the lower end and first nitrogen-rich gas at the upper end of said column; (c) heat exchanging the first nitrogen-rich gas with colder oxygen-enriched liquid so as to condense the first nitrogen-rich gas as reflux for said higher-pressure distillation column and a lower-pressure distillation column while simultaneously vaporizing the oxygen-enriched liquid as vapor for upward flow through said lower-pressure distillation column; (d) supplying a low-pressure gas feed stream, comprising no more than 65 percent of said feed air, at pressure of from 40 to 80 psia, but at least 10 psia less than the pressure of said high pressure gas feed stream, in a cleaned, cooled state; (e) distilling the low-pressure gas feed stream against a colder liquid reflux in an auxiliary distillation column so as to produce second intermediate oxygen-enriched liquid at the lower end and second nitrogen-rich gas at the upper end of said auxiliary distillation column; (f) expanding a portion of the intermediate oxygen-enriched liquid and introducing same to the lower-pressure distillation column; (g) removing a portion of the second nitrogen-rich gas from the upper end of the auxiliary distillation column, in an amount equal to the molar flow rate of between 20 and 70 percent of the molar flow rate of the low-pressure gas feed stream, as product; (h) expanding substantially the remainder of the unexpanded intermediate oxygen-enriched liquid, separately from the liquid of step (f), and indirectly heat exchanging said remainder with unremoved second nitrogen-rich gas outside of the aforementioned distillation columns, condensing said unremoved second nitrogen-rich gas and at least partially vaporizing the remainder of the intermediate oxygen-enriched liquid; (i) introducing at least a part of the condensed second nitrogen-rich vapor to the auxiliary distillation column as said colder liquid reflux therefore; (j) introducing the expanded and at least partially vaporized oxygen-enriched mixture form in step (h) to the lower pressure distillation column; and (k) distilling the streams introduced to the lower pressure distillation column so as to produce a product stream of low-purity oxygen at the bottom thereof and a nitrogen-rich gas-stream at the top thereof.