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Method of Repressurising the Pretreatment PSA of a Cryogenic Air Separation Unit

IP.com Disclosure Number: IPCOM000019388D
Publication Date: 2003-Sep-12
Document File: 1 page(s) / 24K

Publishing Venue

The IP.com Prior Art Database

Abstract

Cryogenic air separation units (ASUs) generally use a pair of adsorbers to remove contaminants from the feed air, such as water vapour or carbon dioxide, which would freeze out in the downstream cryogenic equipment. The use of pressure swing adsorbers (PSAs) for this duty can be the preferred technology. A PSA does not require regeneration gas heating; and it does not require feed air chilling, owing to its relative insensitivity to feed air temperature. Unfortunately the short cycle time, and hence the necessary rapid repressurisation time of a PSA, can result in adverse switch pressure effects on the downstream cryogenic equipment during the changeover period of the 2 PSA vessels.

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Method of Repressurising the Pretreatment PSA of a Cryogenic Air Separation Unit

Cryogenic air separation units (ASUs) generally use a pair of adsorbers to remove contaminants from the feed air, such as water vapour or carbon dioxide, which would freeze out in the downstream cryogenic equipment.

The use of pressure swing adsorbers (PSAs) for this duty can be the preferred technology. A PSA does not require regeneration gas heating; and it does not require feed air chilling, owing to its relative insensitivity to feed air temperature. Unfortunately the short cycle time, and hence the necessary rapid repressurisation time of a PSA, can result in adverse switch pressure effects on the downstream cryogenic equipment during the changeover period of the 2 PSA vessels.

Typically, to repressurise the offline vessel of a 2 vessel PSA requires over 10% of the design feed air for about 20% of the PSA onstream time. This can result in severe pressure and composition fluctuations in the ASU products. Sometimes it can be necessary to use an expensive 3 vessel PSA system in order to minimise these switch effects . However, even then the switch effects can be unacceptable.

For a conventional cryogenic oxygen cycle, it is known that one can sometimes withdraw a substantial high pressure nitrogen (HPGAN) stream from the ASU HP column. This HPGAN, perhaps in conjunction with simultaneous (or sequential) feed air repressurisation, can be used to repressurise the offline PSA. This would result i...