Methane Purification System and Method

Abstract

A process for producing high-purity methane is disclosed wherein a mixture containing predominantly methane, and at least one higher hydrocarbon, such as ethane or propane, and nitrogen is liquefied and fed to a first fractionation column that removes nitrogen and other light gases from the mixture through the top of the first fractionation column as a vapor. The remainder of the mixture is fed to a second fractionation column that removes the higher hydrocarbons and other heavier components through the bottom of the second fractionation column as a liquid producing high-purity methane out the top of the second fractionation column as a vapor. The high-purity methane vapor is warmed, compressed, and liquefied by heat exchange. The liquid leaving the second fractionation column can be further purified in a stripping column to remove non-hydrocarbon components such as carbon dioxide.

What is claimed is: 
     
         1 . A system for purifying a natural gas stream to produce a methane stream comprising:
 a. A natural gas inlet line configured to receive a liquid natural gas stream;   b. a first heat exchanger having a first heat exchanger warming passage in downstream fluid communication with the natural gas inlet line, said first heat exchanger having a first heat exchanger warming passage outlet;   c. a first expansion device in downstream fluid communication with the natural gas inlet line, said first expansion device having a first expansion device outlet;   d. a nitrogen removal column having a nitrogen removal column inlet in downstream fluid communication with the first heat exchanger warming passage outlet and the first expansion device outlet, said nitrogen removal column having a nitrogen removal column first vapor outlet, a nitrogen removal column liquid outlet and a nitrogen removal column reflux fluid inlet;   e. a second heat exchanger having a nitrogen reflux vapor cooling passage in downstream fluid communication with the nitrogen removal column first vapor outlet and a nitrogen reflux vapor cooling passage outlet in fluid communication with the nitrogen removal column reflux fluid inlet;   f. a first hydrocarbon product control valve in downstream fluid communication with the nitrogen removal column liquid outlet, said first hydrocarbon product control valve having a first hydrocarbon product control valve outlet;   g. a demethanizer having a demethanizer fluid inlet, a demethanizer first vapor outlet through which a methane-enriched vapor stream flows, a demethanizer reflux fluid inlet and a demethanizer liquid outlet;   h. a third heat exchanger having a third heat exchanger warming passage and a third heat exchanger cooling passage, said third heat exchanger warming passage in downstream fluid communication with the demethanizer first vapor outlet;   i. a compressor in downstream fluid communication with the third heat exchanger warming passage, said compressor having a compressor outlet;   j. an aftercooler in downstream fluid communication with the compressor outlet, said aftercooling having an aftercooler outlet;   k. said third heat exchanger cooling passage in downstream fluid communication with the aftercooler outlet so that a methane-enriched fluid passing through the cooling passage is cooled by a methane-enriched fluid passing through the third heat exchanger warming passage;   l. a hydrocarbon product stream warming passage configured to receive a fluid stream from the first hydrocarbon product control valve outlet, further cool a methane-enriched fluid stream downstream of the aftercooler and direct warmed fluid from the hydrocarbon product stream warming passage to the demethanizer fluid inlet; and   m. a supplemental cooling circuit configured to cool the second heat exchanger.   
     
     
         2 . The system of  claim 1  wherein the first heat exchanger warming passage is downstream of first expansion device. 
     
     
         3 . The system of  claim 2  further comprising a supplemental expansion device having a supplemental expansion device outlet and wherein the first expansion device is configured to receive a first portion of a liquid natural gas stream and the supplemental expansion device is configured to receive a second portion of the liquid natural gas stream and wherein the first heat exchanger warming passage outlet and the supplemental expansion device outlet are configured to direct natural gas fluid to the nitrogen removal column. 
     
     
         4 . The system of  claim 3  wherein the nitrogen removal column inlet is a first inlet that is in downstream fluid communication with the first heat exchanger warming passage outlet, said nitrogen removal column further including a second inlet that is in downstream fluid communication with the supplemental expansion device outlet. 
     
     
         5 . The system of  claim 3  wherein the nitrogen removal column inlet is in downstream fluid communication with both the first heat exchanger warming passage outlet and the supplemental expansion device outlet. 
     
     
         6 . The system of  claim 1  wherein the first expansion device is downstream of first heat exchanger warming passage. 
     
     
         7 . The system of  claim 1  further comprising a nitrogen removal column second vapor outlet. 
     
     
         8 . The system of  claim 1  further comprising a nitrogen separator in downstream fluid communication with the second heat exchanger outlet, said nitrogen separator having a rejected nitrogen vapor outlet and a nitrogen reflux liquid outlet, said nitrogen reflux liquid outlet configured to direct nitrogen reflux liquid to the nitrogen removal column reflux fluid inlet. 
     
     
         9 . The system of  claim 1  wherein the nitrogen removal column includes a reboiled feed vapor inlet and further comprising a nitrogen removal column reboiler heat exchanger configured to receive liquid from the nitrogen removal column liquid outlet, direct a nitrogen removal column reboiled feed vapor to the reboiled feed vapor inlet and direct fluid to the second expansion device. 
     
     
         10 . The system of  claim 1  wherein the nitrogen removal column includes a reboiler liquid outlet and a reboiled feed vapor inlet and further comprising a nitrogen removal column reboiler heat exchanger configured to receive liquid from the reboiler liquid outlet and direct a nitrogen removal column reboiled feed vapor to the reboiled feed vapor inlet. 
     
     
         11 . The system of  claim 1  wherein the third heat exchanger includes the hydrocarbon product stream warming passage. 
     
     
         12 . The system of  claim 1  further comprising a fourth heat exchanger and wherein the demethanizer includes a demethanizer reflux vapor outlet, said fourth heat exchanger configured to receive a reflux vapor from the demethanizer reflux vapor outlet, cool the reflux vapor and direct a reflux liquid to the demethanizer fluid inlet and wherein the supplemental cooling circuit is configured to also cool the fourth heat exchanger. 
     
     
         13 . The system of  claim 1  wherein the second heat exchanger includes a demethanizer reflux vapor cooling passage having a demethanizer reflux vapor cooling passage outlet and further comprising a demethanizer reflux separator in downstream fluid communication with the demethanizer reflux vapor cooling passage outlet, said demethanizer reflux separator having a methane-enriched vapor outlet and a demethanizer reflux liquid outlet, said demethanizer reflux liquid outlet configured to direct reflux liquid to the demethanizer reflux fluid inlet and said methane-enriched vapor outlet configured to direct methane-enriched vapor to the warming passage of the third heat exchanger. 
     
     
         14 . The system of  claim 1  further comprising a carbon dioxide stripper having a carbon dioxide stripper vapor inlet and a carbon dioxide stripper liquid outlet, said carbon dioxide stripper inlet in downstream fluid communication with the demethanizer liquid outlet. 
     
     
         15 . The system of  claim 14  wherein the first heat exchanger has a first heat exchanger cooling passage and further comprising a fifth heat exchanger having the hydrocarbon product stream warming passage and a fifth heat exchanger methane cooling passage, said hydrocarbon product stream warming passage in downstream fluid communication with the carbon dioxide stripper liquid outlet, and said fifth heat exchanger methane cooling passage configured to receive fluid from the third heat exchanger cooling passage and to direct cooled fluid to the first heat exchanger cooling passage. 
     
     
         16 . The system of  claim 15  further comprising a sixth heat exchanger having a sixth heat exchanger methane cooling passage configured to receive fluid from the fifth heat exchanger methane cooling passage and wherein the supplemental cooling circuit and the sixth heat exchanger include a sixth heat exchanger warming passage. 
     
     
         17 . The system of  claim 15  further comprising a second hydrocarbon product control valve having a second hydrocarbon control valve inlet in downstream communication with the nitrogen removal column liquid outlet and a second hydrocarbon control valve outlet configured to direct fluid to the demethanizer fluid inlet. 
     
     
         18 . The system of  claim 15  further comprising a seventh heat exchanger having a seventh heat exchanger cooling passage having a seventh heat exchanger cooling passage outlet, said seventh heat exchanger cooling passage configured to receive a liquid stream from the carbon dioxide stripper liquid outlet and wherein the seventh heat exchanger cooling passage outlet is configured to direct fluid to the hydrocarbon product stream warming passage of the fifth heat exchanger, said seventh heat exchanger also having a seventh heat exchanger warming passage having a seventh heat exchanger warming passage outlet, said seventh heat exchanger warming passage in downstream fluid communication with the demethanizer liquid outlet and wherein the seventh heat exchanger warming passage is configured to direct fluid into the carbon dioxide stripper. 
     
     
         19 . The system of  claim 15  wherein the first heat exchanger has a first heat exchanger cooling passage in downstream fluid communication with the fifth heat exchanger methane cooling passage. 
     
     
         20 . The system of  claim 1  wherein the first heat exchanger has a first heat exchanger cooling passage in downstream fluid communication with the third heat exchanger cooling passage. 
     
     
         21 . The system of  claim 1  wherein the supplemental cooling circuit uses nitrogen as a refrigerant. 
     
     
         22 . The system of  claim 1  wherein the supplemental cooling circuit is a closed loop circuit. 
     
     
         23 . The system of  claim 1  wherein the supplemental cooling circuit is an open loop circuit. 
     
     
         24 . The system of  claim 1  further comprising a pump configured to receive a liquid natural gas stream from the natural gas inlet line and direct the liquid natural gas stream to the first heat exchanger warming passage. 
     
     
         25 . A method for purifying a natural gas stream to produce a methane stream comprising the steps of:
 a. warming a liquid natural gas stream containing methane, nitrogen and other hydrocarbons;   b. expanding a liquid natural gas stream containing methane, nitrogen and other hydrocarbons;   c. directing a warm and expanded natural gas stream to a nitrogen removal column;   d. forming a first nitrogen-rich vapor stream and hydrocarbon product stream in the nitrogen removal column;   e. forming a reflux stream using the first nitrogen-rich vapor stream and directing the reflux stream back to the nitrogen removal column;   f. warming the hydrocarbon product stream to form a warmed hydrocarbon product stream.   g. directing the warmed hydrocarbon product stream to a demethanizer;   h. forming a methane-enriched vapor stream and a demethanizer product stream in the demethanizer;   i. warming the methane-enriched vapor stream;   j. compressing the warmed methane-enriched vapor stream to form a compressed methane vapor stream;   k. cooling the compressed methane vapor stream using the warming of the methane-enriched vapor stream during step i. and the warming of the hydrocarbon product stream of step f. to form a methane-enriched fluid stream; and   l. further cooling the methane fluid stream to form a methane product liquid stream.   
     
     
         26 . The method of  claim 25  using a supplemental cooling circuit to form the reflux stream during step e. and to form the methane product liquid stream during step  1 . 
     
     
         27 . The method of  claim 26  wherein the supplemental cooling circuit uses nitrogen as a refrigerant. 
     
     
         28 . The method of  claim 25  wherein step a. is performed before step b. 
     
     
         29 . The method of  claim 25  wherein step b. is performed before step a. 
     
     
         30 . The method of  claim 25  further comprising the step of directing the demethanizer product stream to a carbon dioxide stripper.