Nitrogen rejection from natural gas with CO2 and variable N2 content

Claim 1

In a cryogenic nitrogen rejection process for a natural gas feed stream comprising a double distillation cycle having a high pressure distillation stage and a low pressure distillation stage, the feed stream being introduced into the high pressure distillation stage wherein a nitrogen containing overhead and a hydrocarbon-containing bottoms are maintained while yielding a waste nitrogen overhead stream and a methane product bottoms stream from the low pressure distillation stage, the method for treating a natural gas feed stream containing variable amounts of methane, nitrogen, carbon dioxide and ethane-plus hydrocarbons which comprises (a) removing a vapor methane stream from the bottoms of the low pressure distillation stage and warming it to ambient temperature, (b) compressing the vapor methane stream and condensing at least a portion of the compressed methane stream against the waste nitrogen overhead stream from the low pressure distillation stage, (c) expanding the condensed methane stream through a valve and combining it with the methane bottoms of the low pressure distillation stage to provide additional refrigeration to the methane bottoms, (d) withdrawing a liquid methane stream possessing the additional refrigeration from the bottoms of the low pressure distillation stage, and (e) vaporizing the liquid methane stream against nitrogen overhead of the high pressure distillation stage to condense at least a portion of the nitrogen overhead to provide sufficient nitrogen reflux to the high pressure distillation stage for scrubbing substantially all of the carbon dioxide out of the feed stream in the high pressure distillation stage and keeping it soluble in the hydrocarbon bottoms, and methane reboil to the low pressure distillation stage.

Claim 6

A method for rejecting nitrogen from a natural gas stream containing variable amounts of methane, nitrogen, carbon dioxide and ethane-plus hydrocarbons which comprises (a) passing the natural gas stream through a cooling zone, (b) introducing the cooled natural gas stream into the high pressure distillation stage of a double distillation cycle comprising a high pressure distillation stage and a low pressure distillation stage, (c) fractionally distilling the natural gas stream in the high pressure distillation stage at a pressure sufficient to provide an overhead consisting essentially of nitrogen substantially free of methane and carbon dioxide and a bottoms comprising substantially all of the carbon dioxide dissolved in the ethane-plus hydrocarbons, (d) withdrawing a liquid side stream consisting essentially of nitrogen and methane substantially free of carbon dioxide from an intermediate level of the high pressure distillation stage, expanding the side stream through a valve and introducing it into an intermediate level of the low pressure distillation stage, (e) condensing the nitrogen overhead from the high pressure distillation stage by heat exchange with a methane cycle to provide nitrogen reflux for the high pressure distillation stage and a nitrogen liquid stream, which methane cycle comprises (i) removing a vapor methane stream from the bottoms of the low pressure distillation stage and warming it, (ii) compressing the vapor methane stream and condensing at least a portion of the compressed methane stream against a waste nitrogen overhead stream from the low pressure distillation stage, (iii) expanding the condensed methane stream through a valve and combining it with the methane bottoms of the low pressure distillation stage to provide additional refrigeration to the methane bottoms, (iv) withdrawing a liquid methane stream possessing the additional refrigeration from the bottoms of the low pressure distillation stage, and (v) vaporizing the liquid methane stream against the nitrogen overhead of the high pressure distillation stage to condense at least a portion of the overhead to provide sufficient nitrogen reflux to the high pressure distillation stage for scrubbing the carbon dioxide out of the feed stream and methane reboil to the low pressure distillation stage, (f) expanding the nitrogen liquid stream through a valve into the top of the low pressure distillation stage, and (g) fractionally distilling the nitrogen and methane in the low pressure distillation stage to recover a waste nitrogen vapor overhead stream and a methane product bottoms stream.

Claim 7

A method for rejecting nitrogen from a natural gas stream containing variable amounts of methane, nitrogen, carbon dioxide and ethane-plus hydrocarbons which comprises (a) passing the natural gas stream through a cooling zone, (b) introducing the cooled natural gas stream into the high pressure distillation stage of a double distillation cycle comprising a high pressure distillation stage and a low pressure distillation stage, (c) fractionally distilling the natural gas stream in the high pressure distillation stage at a pressure sufficient to provide an overhead consisting essentially of nitrogen substantially free of methane and carbon dioxide and a bottoms comprising substantially all of the carbon dioxide dissolved in the ethane-plus hydrocarbons, (d) withdrawing a liquid side stream consisting essentially of nitrogen and methane substantially free of carbon dioxide from an intermediate level of the high pressure distillation stage, expanding the side stream through a valve and introducing it into an intermediate level of the low pressure distillation stage, (e) condensing the nitrogen overhead from the high pressure distillation stage by heat exchange with a methane cycle to provide nitrogen reflux for the high pressure distillation stage and a nitrogen liquid stream, which methane cycle comprises (i) removing a vapor methane stream from the bottoms of the low pressure distillation stage and warming it, (ii) compressing the vapor methane stream and condensing at least a portion of the compressed methane stream against a waste nitrogen overhead stream from the low pressure distillation stage, (iii) expanding the condensed methane stream through a valve and combining it with the methane bottoms of the low pressure distillation stage to provide additional refrigeration to the methane bottoms, (iv) withdrawing a liquid methane stream possessing the additional refrigeration from the bottoms of the low pressure distillation stage, and (v) vaporizing the liquid methane stream against the nitrogen overhead of the high pressure distillation stage to condense at least a portion of the overhead to provide sufficient nitrogen reflux to the high pressure distillation stage for scrubbing the carbon dioxide out of the feed stream and methane reboil to the low pressure distillation stage, (f) expanding the nitrogen liquid stream through a valve into the top of the low pressure distillation stage, and (g) fractionally distilling the nitrogen and methane in the low pressure distillation stage to recover a waste nitrogen vapor overhead stream and a methane product bottoms stream.