The Boc Group, Inc.

John Douglas Oakey

Paul Higginbotham

Stephen Roger Clare

John Marshall

Robert A Mostello

Graeme John Dunn

David J Kamrath

David C Gladman

Brian Morice Davies

Robert Owen

Nitrogen is rejected from a pressurized feed gas stream comprising methane and nitrogen by cooling the feed gas stream in a main heat exchanger and rectifying the cooled feed gas stream in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure column in heat exchange relationship with the lower pressure column. A pump withdraws a methane product stream from the column. The methane product stream is pressurized by the pump and is vaporized in the main heat exchanger. The pressurized feed gas stream is expanded with the performance of external work in an expansion turbineupstream of the main heat exchanger.

Nitrogen rejection method and apparatus

Nitrogen is rejected from a pressurised feed gas stream comprising methane and nitrogen by cooling the feed gas stream in a main heat exchangerand rectifying the cooled feed gas stream in a double rectification columncomprising a higher pressure rectification columna lower pressure rectification column, and a condenser-reboilerplacing the higher pressure columnin heat exchange relationship with the lower pressure columnA pumpwithdraws a methane product stream from the columnThe methane product stream is pressurised by the pumpand is vaporised in the main heat exchangerThe pressurised feed gas stream is expanded with the performance of external work in an expansion turbineupstream of the main heat exchanger

Nitrogen is rejected from a feed natural gas stream comprising methane and nitrogen so as to form a primary methane product. The mole fraction of nitrogen in the feed natural gas increases over a period of time. The feed natural gas stream is cooled in a main heat exchanger and is rectified in a double rectification column. A primary product methane stream and a secondary nitrogen-enriched product stream are withdrawn from the rectification column. The secondary nitrogen-enriched product stream has a mole fraction of methane at or above a chosen minimum value when the said mole fraction of nitrogen is at a minimum. When the said mole fraction of nitrogen rises to a value at which the mole fraction of methane in the secondary nitrogen-enriched product stream falls below the chosen minimum, a part of the feed gas is introduced through conduit into the secondary nitrogen-enriched product stream so as to restore its mole fraction of methane to the chosen minimum value or a value thereabove.

Nitrogen is rejected from a feed gas stream comprising methane and nitrogen so as to form a methane product. The feed gas is separated in a double rectification column comprising a higher pressure rectification column 14, a lower pressure rectification column 16, and a condenser-reboiler 18. Product methane is withdrawn from the column 16 by a pump 42 and is vaporised. A flow of gas is recycled from the column 16 to the column 14, being warmed in main heat exchanger 4. A first part of the warmed recycle gas being compressed in compressor 48 to a first pressure and introduced into the higher pressure rectification column 14. A second part of the warmed recycle gas is compressed in the compressor 48 and condensed in a second condenser-reboiler associated with an intermediate main exchange region of the lower pressure column 16 and is returned to the higher pressure rectification column 14.

Nitrogen is rejected from a feed gas stream comprising methane and nitrogen so as to form a methane product. The feed gas stream is cooled in a main heat exchanger and is rectified in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column. At least part of the feed gas stream is expanded into the higher pressure rectification column through an expansion valve, the feed gas stream being partially liquefied upstream of the double rectification column. Over a period of time the mole fraction of nitrogen in the feed gas mixture increases. At a suitable increased nitrogen mole fraction, operation of a second double rectification column in parallel with the first double rectification column is commenced.

A method of forming a methane product by rejecting nitrogen from a feed gas stream comprising methane and nitrogen is disclosed. After being cooled in a main heat exchanger, the feed gas stream is rectified in a double rectification column comprising a higher pressure column and a lower pressure column. A gas flow is recycled from the lower pressure column to the higher pressure column. Part of the recycle flow is compressed in a compressor, cooled, liquefied in a condenser-reboiler and introduced into the higher pressure column. Another part of the recycle flow is also compressed in the compressor, cooled, liquefied in the main heat exchanger or by expansion, and introduced into the double rectification column in liquid state. At least part of the cooling of both parts of the recycle gas is performed in the main heat exchanger. A liquid methane product is withdrawn from the lower pressure column.

Introgen rejection method

Nitrogen is rejected from a feed gas stream comprising methane and nitrogen so as to form a methane product. The feed gas stream is cooled in a main heat exchanger and rectified in a double rectification column comprising a higher pressure column, a lower pressure column and a condenser-reboiler placing the higher and lower pressure rectification columns in heat exchange relationship with each other. At least part of the feed gas stream is expanded through a valve into the higher pressure rectification column and the feed gas stream is partially liquefied upstream of the double rectification column. Over a period of time, the mole fraction of nitrogen in the feed gas stream may increase. The operating pressure of the lower pressure rectification column is periodically increased in response to increases in the mole fraction of nitrogen. A back pressure valve may be periodically adjusted to effect the increase in pressure.

Nitrogen is produced by separation of it from air. Nitrogen so separated is condensed. Most or all of the nitrogen is separated by rectification. At least some of the condensed nitrogen is employed as reflux in the rectification. The nitrogen is both separated and condensed at three or more different pressures.

Production of nitrogen

Air is separated in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a first condenser-reboiler, of which the condensing passages communicate with an upper region of the higher pressure rectification column, and the reboiling passage communicate with a lower region of the lower pressure rectification column. A second condenser-reboiler is operated so as to reboil a liquid fraction obtained in the higher pressure rectification column. A stream of the air to be separated is partially or totally condensed by indirect heat exchange with a stream of nitrogen condensed in the first condenser-reboiler. At least part of the stream of condensed nitrogen is taken as product downstream of its heat exchange with the second stream of compressed air.

Air separation

Air is separated in an arrangement of rectification columns comprising a double rectification column having a higher pressure column and a lower pressure rectification column, an intermediate pressure rectification column, and a side rectification column communicating with the lower pressure column. The side rectification column is provided with a condenser and the intermediate pressure rectification column with a reboiler and a further condenser. A stream of oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column through an outlet and is separated in the intermediate pressure rectification column. The reboiler is heated by a stream typically withdrawn from the side rectification column through an outlet thereof. The condensers of the side rectification column and the intermediate column are cooled by streams having different oxygen mole fractions.

Air is separated in an arrangement of rectification columns comprising a double rectification column (which has a higher pressure column and a lower pressure column), an intermediate pressure rectification column and a side rectification column communicating with the lower pressure column. The side rectification column is provided with a condenser and the intermediate pressure rectification column with a reboiler and a condenser. A stream of oxygen-enriched liquid and a stream of liquid air are withdrawn from the higher pressure column through outlets and are separated in the intermediate pressure rectification column. The reboiler is heated by a stream typically withdrawn from the side rectification column through an outlet. A stream of liquid is withdrawn through an outlet at the bottom of the intermediate pressure rectification column is used to cool the condenser and is separated in the lower pressure rectification column. The mole fraction of oxygen in the stream withdrawn through the outlet is intermediate the oxygen mole fractions in the streams fed to the column via the outlets of the high pressure column.

Air is separated in an arrangement of rectification columns comprising a double rectification column (which has a higher pressure column and a lower pressure column), an intermediate pressure rectification column, and a side rectification column communicating with the lower pressure column. The side rectification column is provided with a condenser, and the intermediate pressure rectification column with a reboiler and a further condenser. A stream of oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column through an outlet and is separated in the intermediate pressure rectification column. The reboiler is heated by a stream typically withdrawn from the side rectification column through an outlet. A stream of liquid is withdrawn from the bottom of the intermediate pressure rectification column, is employed to cool the further condenser of the intermediate pressure rectification column, and is partially vaporised therein. A stream of residual liquid flows from such further condenser and is used to cool the condenser of the side rectification column.

A stream of precooled and purified air is introduced into a double rectification column comprising a higher pressure rectification column and a lower pressure rectification column and is separated therein into an oxygen-rich fraction and a nitrogen-rich fraction. A stream of argon-enriched oxygen vapor flows from an outlet of the lower pressure rectification column into a side column in which argon is separated therefrom. An oxygen-enriched liquid air stream is taken from an outlet at the bottom of the higher pressure rectification column. A vaporous oxygen-enriched air stream is introduced into the lower pressure rectification column through an inlet above the outlet. At least part of the oxygen-enriched liquid is separated in a further or intermediate pressure rectification column provided with a reboiler, thereby forming a vapor depleted of oxygen and a liquid air stream further enriched in oxygen. At least one stream of the further-enriched liquid is vaporised to form the oxygen-enriched vapor that is introduced into the lower pressure rectification column. A part of the oxygen-depleted vapor is condensed in a condenser and is taken as product or reintroduced into the lower pressure rectification column. The partial reboiling in the reboiler is effected by indirect heat exchange with a vapor stream withdrawn from an intermediate region of the side column. The condenser is cooled by a stream of liquid withdrawn from the further rectification column.

A stream of precooled and purified air is introduced into a double rectification column comprising a higher pressure rectification column and a lower pressure rectification column and is separated therein into an oxygen-rich fraction and a nitrogen-rich fraction. A stream of argon-enriched oxygen vapour flows from the lower pressure rectification column into a side column in which argon is separated therefrom. An oxygen-enriched liquid air stream is taken from the bottom of the higher pressure rectification column. A vaporous oxygen-enriched air stream is introduced into the lower pressure rectification column above the point at which argon-enriched oxygen vapour is removed. At least part of the oxygen-enriched liquid is partially reboiled in a reboiler and is separated in a further rectification column, thereby to form a vapour depleted of oxygen and a liquid air stream further enriched in oxygen. At least one stream of the further-enriched liquid is vaporised to form the oxygen-enriched vapour that is introduced into the lower pressure rectification column. A part of the oxygen-depleted vapour is condensed and is taken as product or reintroduced into the lower pressure rectification column. The partial reboiling in the reboiler is effected by indirect heat exchange with a stream of argon-enriched oxygen vapour withdrawn from the outlet, or in an alternative process with a stream withdrawn from an intermediate region of the side column.

In order to provide `ultra high purity` nitrogen having diminished concentrations of light and heavy impurities in comparison with nitrogen produced by conventional cryogenic air separation, the nitrogen product from a conventional cryogenic air separation column is introduced into the bottom of a liquid-vapor contract column 2 fitted with a condenser 8 to provide reflux. A liquid nitrogen stream having a reduced concentration of heavy impurities is withdrawn from the column 2 through an outlet 22 situated at a level a few trays below the top tray in the column 2. The liquid nitrogen is then subjected to two stages of flash separation. In the first stage the liquid is passed through valve 24 into a phase separator 26. In the second stage, the resulting liquid from the first stage, having a reduced concentration of light impurities, is passed through valve 32 into a phase separator 34. Liquid nitrogen product is withdrawn from the phase separator 34 through outlet 38.

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Godalming, United Kingdom

John Douglas Oakey