High purity membrane nitrogen

Claim 1

An improved membrane process for the production at high or very high purity, of a less selectively permeable component of a feed gas containing said component and a more selectively permeable component, comprising: (a) introducing the feed gas at a feed pressure to a first stage membrane module of a membrane system containing a total of at least three membrane module stages, said system being capable of selectively permeating the more selectively permeable component thereof; (b) separately removing the less selectively permeable component from the first stage essentially at the feed pressure as retentate, and the more selectively permeable component therefrom at a pressure lower than the feed pressure as permeate; (c) passing the retentate from the first stage to succeeding stages in series, essentially at the feed pressure, as feed gas thereto; (d) separately removing the retentate from each said stage in series essentially at the feed pressure, and the permeate therefrom at a pressure lower than the feed pressure, the retentate from the last stage in series being recovered as high or very high purity gas, without non-membrane treatment for the removal of residual amounts of permeate therein; (e) compressing the permeate from the last stage in series and passing said permeate to an additional membrane module stage as feed gas therefor; and (f) recycling retentate from said additional membrane module stage for blending with the retentate from the next-to-last-stage in series and for passage to the last stage in series, the stage cut of the additional membrane module stage being such that the retentate therefrom has essentially the same composition as the retentate from the next-to-last stage, whereby the less selectively permeable component is advantageously recovered as a high or very high purity product at advantageously low membrane surface area and power requirements.