Two Step Nitrogen and Methane Separation Process

Abstract

A system and method for removing nitrogen and producing a high pressure methane product stream from natural gas feed streams is disclosed. A system for also producing natural gas liquids in conjunction with nitrogen removal from natural gas feed streams is also disclosed. The system and method of the invention are particularly suitable for use with feed streams in excess of 50 MMSCFD and up to 750 MMSCFD and containing up to 75 ppm carbon dioxide. Typical power requirements for compressing the methane product stream to produce a suitably high pressure stream for sale are reduced according to the system and method of the invention.

I claim: 
     
         1 . A method for removing nitrogen and for producing a high pressure methane product stream and a natural gas liquids stream, the method comprising:
 providing a feed stream comprising nitrogen, methane, ethane, and propane;   separating the feed stream into a first overhead stream and a first bottoms stream in a first fractionating column;   separating the first overhead stream into a second overhead stream and a second bottoms stream in a second fractionating column;   separating at least a portion of the first bottoms stream into a third overhead stream and a natural gas liquids product stream in a third fractionating column;   dividing the second bottoms stream into a first split stream, a second split stream, and a third split stream;   cooling the first split stream through heat exchange with the second overhead stream;   reducing the pressure of the cooled first split stream by passing through a first expansion valve to form a low pressure stream;   reducing the pressure of the second split stream by passing through a second expansion valve to form an intermediate pressure stream;   mixing the third split stream and the third overhead stream to form a high pressure stream;   cooling the feed stream prior to entering the first fractionating column through heat exchange with the second overhead stream, low pressure stream, intermediate pressure stream, high pressure stream, and at least a portion of the first bottoms stream prior to entering the third fractionating column;   cooling the first overhead stream prior to entering the second fractionating column through heat exchange with the second overhead stream, low pressure stream, and intermediate pressure stream;   compressing the low pressure stream to a pressure substantially equal to that of the intermediate pressure stream and combining the two streams to form a first compressed stream;   compressing the first compressed stream to a pressure substantially equal to that of the high pressure stream and combining the two streams to form a second compressed stream;   compressing the second compressed stream to a produce a high pressure methane product stream.   
     
     
         2 . The method of  claim 1  wherein the feed stream is cooled to between −130° F. and −175° F. prior to entering the first fractionating column. 
     
     
         3 . The method of  claim 2  wherein the pressure of the feed stream is between 500 psia and 650 psia when it enters the first fractionating column. 
     
     
         4 . The method of  claim 3  wherein the feed stream comprises up to 75 ppm carbon dioxide. 
     
     
         5 . The method of  claim 4  wherein the first bottoms stream comprises substantially all of the carbon dioxide from the feed stream and the first overhead stream is substantially free of carbon dioxide. 
     
     
         6 . The method of  claim 3  wherein the first overhead stream is not condensed prior to cooling and no reflux stream is recycled to the first fractionating column. 
     
     
         7 . The method of  claim 2  further comprising supplying at least a portion of the energy released from cooling the feed stream to the first fractionating column and supplying at least a portion of the energy released from cooling the first overhead stream to the second fractionating column. 
     
     
         8 . The method of  claim 7  further comprising supplying at least a second portion of the energy released from cooling the feed stream to the third fractionating column.