Separation of Heavy Hydrocarbons and NGLs from Natural Gas in Integration with Liquefaction of Natural Gas

Abstract

Described herein is a method of and system for fractionating and liquefying a natural gas feed stream. The natural gas is first fractionated in a scrub column. The overhead vapor from the scrub column is cooled, condensed and divided to form a first, a second and at least one further stream of liquefied first overhead. The first stream of liquefied first overhead is returned to the scrub column as a reflux stream. The second stream of liquefied first overhead forms an LNG product. The further stream of liquefied first overhead is used to provide or generate reflux for a de-methanizer column used to fractionate the bottoms liquid from the scrub column.

1 . A method of fractionating and liquefying a natural gas feed stream, the method comprising:
 (a) introducing the natural gas feed stream into a scrub column in which the natural gas feed stream is separated into a methane-rich vapor fraction collected as a first overhead vapor at the top of the scrub column, and a liquid fraction, enriched in hydrocarbons heavier than methane, collected as a first bottoms liquid at the bottom of the scrub column;   (b) withdrawing a stream of first overhead vapor from the top of the scrub column, and cooling, condensing and dividing said stream to form a first stream of liquefied first overhead, second stream of liquefied first overhead, and at least one further stream of liquefied first overhead;   (c) returning the first stream of liquefied first overhead to the scrub column as a reflux stream introduced into the top the scrub column, thereby providing reflux for the scrub column;   (d) forming a liquefied natural gas (LNG) product stream from the second stream of liquefied first overhead;   (e) withdrawing a stream of first bottoms liquid from the bottom of the scrub column, and introducing said stream into a de-methanizer column in which the stream of first bottoms liquid is separated into a methane-rich vapor fraction collected as a second overhead vapor at the top of the de-methanizer column, and a liquid fraction, enriched in hydrocarbons heavier than methane, collected as a second bottoms liquid at the bottom of the de-methanizer column; and   (f) providing reflux to the de-methanizer column by:
 (1) introducing one of said further streams of liquefied first overhead as a reflux stream into the top of the de-methanizer column; and/or 
 (2) condensing, by indirect heat exchange with one of said further streams of liquefied first overhead, a portion of the second overhead vapor to form a stream of liquefied second overhead that is reintroduced as a reflux stream into the top the de-methanizer column. 
   
     
     
         2 . The method of  claim 1 , wherein step (b) comprises:
 withdrawing the stream of first overhead vapor from the top of the scrub column, cooling and partially condensing said stream, separating the resulting liquid and vapor phases, dividing the separated liquid phase to form the first stream of liquefied first overhead and at least one further stream of liquefied first overhead, and further cooling and condensing at least a portion of the separated vapor phase to form the second stream of liquefied first overhead; and/or   withdrawing the stream of first overhead vapor from the top of the scrub column, cooling and partially condensing said stream, separating the resulting liquid and vapor phases, forming the first stream of liquefied first overhead from at least a portion of the separated liquid phase, further cooling and condensing at least a portion of the separated vapor phase, and dividing the resulting further cooled and condensed stream to form the second stream of liquefied first overhead and at least one further stream of liquefied first overhead.   
     
     
         3 . The method of  claim 2 , wherein:
 the second stream of liquefied first overhead is sub-cooled prior to forming the LNG product stream; or   the cooled and condensed stream is sub-cooled prior to being divided to form the second stream of liquefied first overhead and at least one further stream of liquefied first overhead.   
     
     
         4 . The method of  claim 2 , wherein the stream of first overhead vapor from the top of the scrub column is cooled and partially condensed in a warm bundle of a coil-wound main heat exchanger, and the at least a portion of the separated vapor phase is further cooled and condensed in a middle and/or cold bundle of the coil-wound main heat exchanger. 
     
     
         5 . The method of  claim 2 , wherein the stream of first overhead vapor from the top of the scrub column is cooled and partially condensed in an overhead condenser heat exchanger, and the at least a portion of the separated vapor phase is further cooled and condensed in a coil-wound main heat exchanger. 
     
     
         6 . The method of  claim 1 , wherein step (b) comprises:
 withdrawing the stream of first overhead vapor from the top of the scrub column, cooling and condensing said stream, and dividing the cooled and condensed stream to form the first stream of liquefied first overhead, the second stream of liquefied first overhead, and at least one further stream of liquefied first overhead.   
     
     
         7 . The method of  claim 6 , wherein:
 the cooled and condensed stream is sub-cooled prior to being divided to form the first stream of liquefied first overhead, the second stream of liquefied first overhead, and at least one further stream of liquefied first overhead; or   the cooled and condensed stream is divided to form the first stream of liquefied first overhead and another liquid stream that is then sub-cooled prior to being divided to form the second stream of liquefied first overhead and at least one further stream of liquefied first overhead; or   the second stream of liquefied first overhead is sub-cooled prior to forming the LNG product stream.   
     
     
         8 . The method of  claim 1 , wherein the first stream of liquefied first overhead, used as a reflux stream in step (c), and the further stream(s) of liquefied first overhead, used in step (f), have a flow ratio of at least about 9:1. 
     
     
         9 . The method of  claim 1 , wherein the first stream of liquefied first overhead, used as a reflux stream in step (c), and the further stream(s) of liquefied first overhead, used in step (f), each have a temperature of or below about −40° C. 
     
     
         10 . The method of  claim 1 , wherein the first overhead vapor and second overhead vapor each comprise at least about 95 mole % methane, and the second bottoms liquid comprises less than about 5 mole % methane. 
     
     
         11 . The method of  claim 1 , wherein the method further comprises: withdrawing a stream of second overhead vapor from the top of the de-methanizer column; cooling and condensing all or a portion of said stream to form additional LNG product; and/or using all or a portion of said stream as a fuel stream; and/or exporting all or a portion of said stream as a gaseous natural gas product stream. 
     
     
         12 . The method of  claim 1 , wherein the method further comprises withdrawing a stream of second bottoms liquid from the bottom of the de-methanizer column and fractionating the stream of second bottoms liquid to provide one or more natural gas liquids (NGL) streams. 
     
     
         13 . The method of  claim 12 , wherein the step of fractionating the stream of second bottoms liquid comprises: introducing said stream into a de-ethanizer column in which the stream of second bottoms liquid is separated into ethane-enriched fraction, collected as a third overhead vapor at the top of the de-ethanizer column, and a fraction enriched in hydrocarbons heavier than ethane, collected as a third bottoms liquid at the bottom of the de-ethanizer column. 
     
     
         14 . The method of  claim 13 , wherein the method further comprises:
 withdrawing a stream of third overhead vapor from the top of the de-ethanizer column, and cooling and condensing said stream to form an NGL stream; and/or   withdrawing a stream of third bottoms liquid from the bottom of the de-ethanizer column, and forming one or more NGL streams therefrom.   
     
     
         15 . The method of  claim 13 , wherein the method further comprises providing reflux to the de-ethanizer column by condensing, by indirect heat exchange with a further stream of liquefied first overhead, a portion of the third overhead vapor to form a stream of liquefied third overhead that is reintroduced as a reflux stream into the top the de-ethanizer column. 
     
     
         16 . The method of  claim 13 , wherein the third overhead vapor comprises at least about 95% ethane, and the third bottoms liquid comprises less than about 5 mole ethane. 
     
     
         17 . A system for fractionating and liquefying a natural gas feed stream, the system comprising:
 a scrub column arranged and operable to receive the natural gas feed stream and separate said stream into a methane-rich vapor fraction collected as a first overhead vapor at the top of the scrub column, and a liquid fraction, enriched in hydrocarbons heavier than methane, collected as a first bottoms liquid at the bottom of the scrub column;   a set of conduits, one or more heat exchangers, and optionally one or more separators, arranged and operable to withdraw a stream of first overhead vapor from the top of the scrub column and to cool, condense and divide said stream to form a first stream of liquefied first overhead, second stream of liquefied first overhead, and at least one further stream of liquefied first overhead;   a conduit arranged and operable to return the first stream of liquefied first overhead to the scrub column as a reflux stream introduced into the top the scrub column, thereby providing reflux for the scrub column;   a conduit arranged and operable to withdraw from the system a liquefied natural gas (LNG) product stream formed from the second stream of liquefied first overhead;   a de-methanizer column arranged and operable to receive a stream of first bottoms liquid from the bottom of the scrub column and to separate said stream into a methane-rich vapor fraction collected as a second overhead vapor at the top of the de-methanizer column, and a liquid fraction, enriched in hydrocarbons heavier than methane, collected as a second bottoms liquid at the bottom of the de-methanizer column;   a conduit arranged and operable to withdraw the stream of first bottoms liquid from the bottom of the scrub column and introduce said stream into the de-methanizer column; and   one or both of:
 (1) a conduit arranged and operable to introduce one of said further streams of liquefied first overhead as a reflux stream into the top of the de-methanizer column, thereby providing reflux for the de-methanizer column; 
 (2) a heat exchanger arranged and operable to condense, by indirect heat exchange with one of said further streams of liquefied first overhead, a portion of the second overhead vapor to form a stream of liquefied second overhead that is reintroduced as a reflux stream into the top the de-methanizer column, thereby providing reflux for the de-methanizer column, and a conduit arranged and operable to introduce said further stream of liquefied first overhead into said heat exchanger. 
   
     
     
         18 . A system according to  claim 17 , wherein the set of conduits, one or more heat exchangers, and one or more separators arranged and operable to withdraw, cool, condense and divide the stream of first overhead vapor comprise:
 a conduit arranged and operable to withdraw the stream of first overhead vapor from the top of the scrub column;   a heat exchanger or section of a heat exchanger arranged and operable to cool and partially condense said stream;   a separator arranged and operable to separate the resulting liquid and vapor phases;   a set of conduits arranged and operable to divide the separated liquid phase to form the first stream of liquefied first overhead and a further stream of liquefied first overhead; and   a heat exchanger or section of a heat exchanger arranged and operable to receive, further cool and condense at least a portion of the separated vapor phase to form the second stream of liquefied first overhead.   
     
     
         19 . A system according to  claim 18 , wherein the system further comprises a heat exchanger or section of a heat exchanger arranged and operable to receive and sub-cool the second stream of liquefied first overhead. 
     
     
         20 . A system according to  claim 18 , wherein the section of a heat exchanger arranged and operable to cool and partially condense the stream of first overhead vapor comprises a warm bundle of a coil-wound main heat exchanger, and the section of a heat exchanger arranged and operable to further cool and condense at least a portion of the separated vapor phase comprises a middle and/or cold bundle of the coil-wound main heat exchanger. 
     
     
         21 . A system according to  claim 18 , wherein the heat exchanger arranged and operable to cool and partially condense the stream of first overhead vapor comprises an overhead condenser heat exchanger, and the heat exchanger arranged and operable to further cool and condense at least a portion of the separated vapor phase comprises a coil-wound main heat exchanger. 
     
     
         22 . A system according to  claim 17 , wherein the set of conduits and one or more heat exchangers arranged and operable to withdraw, cool, condense and divide the stream of first overhead vapor comprise:
 a conduit arranged and operable to withdraw the stream of first overhead vapor from the top of the scrub column;   a heat exchanger or section of a heat exchanger arranged and operable to cool and condense said stream; and   a set of conduits arranged and operable to divide the cooled and condensed stream to form the first stream of liquefied first overhead, the second stream of liquefied first overhead, and at least one further stream of liquefied first overhead.   
     
     
         23 . A system according to  claim 22 , wherein the system further comprises a heat exchanger or section of a heat exchanger arranged and operable to sub-cool the cooled and condensed stream prior to said stream being divided to form the first stream of liquefied first overhead, the second stream of liquefied first overhead, and at least one further stream of liquefied first overhead. 
     
     
         24 . A system according to  claim 22 , wherein the system further comprises a heat exchanger or section of a heat exchanger arranged and operable to sub-cool the second stream of liquefied first overhead. 
     
     
         25 . A system according to  claim 17 , wherein the system further comprises:
 a de-ethanizer column arranged and operable to receive a stream of second bottoms liquid from the bottom of the de-methanizer column and to separate said stream into an ethane-enriched fraction, collected as a third overhead vapor at the top of the de-ethanizer column, and a fraction enriched in hydrocarbons heavier than ethane, collected as a third bottoms liquid at the bottom of the de-ethanizer column; and   a conduit arranged and operable to withdraw the stream of second bottoms liquid from the bottom of the de-methanizer column and introduce said stream into the de-ethanizer column.   
     
     
         26 . A system according to  claim 25 , wherein the system further comprises:
 a conduit arranged and operable to withdraw a stream of third overhead vapor from the top of the de-ethanizer column, and one or more heat exchangers arranged and operable to receive, cool and condense said stream to form an NGL stream; and/or   a conduit arranged and operable to withdraw a stream of third bottoms liquid from the bottom of the de-ethanizer column, for forming one or more NGL streams therefrom.   
     
     
         27 . A system according to  claim 25 , wherein the system further comprises a heat exchanger arranged and operable to condense, by indirect heat exchange with a further stream of liquefied first overhead, a portion of the third overhead vapor to form a stream of liquefied third overhead that is reintroduced as a reflux stream into the top the de-ethanizer column, thereby providing reflux for the de-ethanizer column, and a conduit arranged and operable to introduce said further stream of liquefied first overhead into said heat exchanger.