Method for extracting ethane from an initial natural gas stream and corresponding plant

Abstract

A method includes: recovering and compressing a head stream coming from a separation column, to form a stream of compressed purified natural gas; liquefying the stream of compressed purified natural gas in a liquefaction unit to form a stream of a pressurized liquefied natural gas; flash expanding of the stream of pressurized liquefied natural gas and recovering in a storage; recovering and compressing of a flow of flash gas coming from the expanding; separating the flow of compressed flash gas ( 132 ) into a fuel stream and a recycle stream; cooling and expanding the recycle stream, then introducing the cooled and expanded recycle stream at a head stage of the separation column.

1 . A method to extract ethane from a stream of initial natural gas, comprising:
 cooling a stream of initial natural gas in at least one first upstream heat exchanger, to form a stream of cooled natural gas;   separating the stream of cooled natural gas into a liquid flow and a gas flow;   expanding the liquid flow and introducing at least one stream coming from the liquid flow at a first level into a methane and C2+ hydrocarbons separation column;   forming a turbine feed stream from the gas flow;   expanding the turbine feed stream in a dynamic expansion turbine to form an expanded stream and introducing the expanded stream coming from the dynamic expansion turbine into the separation column at a second level,   introducing a bottom stream rich in C2+ hydrocarbons recovered from the separation column into a fractionation column, and recovering a flow of ethane from the fractionation column;   recovering and compressing at least a part of a head stream coming from the separation column, to form a stream of compressed purified natural gas;   liquefying the stream of compressed purified natural gas in a liquefier, to form a stream of a pressurized liquefied natural gas;   flash expanding the stream of pressurized liquefied natural gas and recovering expanded liquefied natural gas in a storage;   recovering at least one flow of flash gas coming from the flash expanding of the stream of pressurized liquefied natural gas;   compressing the at least one flow of flash gas to form a compressed gas;   separating the flow of compressed flash gas into a fuel stream and a recycle stream;   at least partial cooling and expanding of the recycle stream to form a cooled and expanded recycle stream, followed by introducing the cooled and expanded recycle stream at a head stage of the separation column.   
     
     
         2 . The method according to  claim 1 , wherein a concentration of methane in the recycle stream is greater than 90 mol %. 
     
     
         3 . The method according to  claim 1 , wherein introducing the cooled and expanded recycle stream at a head stage of the separation column is carried out at a first level starting from the top of the separation column. 
     
     
         4 . The method according to  claim 1 , comprising introducing the recycle stream in the first heat exchanger and cooling the recycle stream by heat exchange in the first heat exchanger with the head stream coming from the separation column. 
     
     
         5 . The method according to  claim 1 , comprising separating the gas flow into the turbine feed stream introduced into the dynamic expansion turbine, and into a reflux stream introduced into the separation column, at a level lower than a level of introduction of the recycle stream, after cooling the reflux stream in a second upstream heat exchanger and static expansion of the reflux stream. 
     
     
         6 . The method according to  claim 5 , wherein cooling the recycle stream includes flowing the recycle stream through the second heat exchanger. 
     
     
         7 . The method according to  claim 1 , wherein the expanding the recycle stream includes flowing the recycle stream through a static expansion valve. 
     
     
         8 . The method according to  claim 1 , comprising placing at least a part of the compressed purified head natural gas in heat exchange with the flow of flash gas in a downstream heat exchanger. 
     
     
         9 . The method according to  claim 1 , comprising removing a recirculation stream from the stream of compressed purified natural gas, upstream of the liquefaction unit, the recirculation stream being cooled, expanded, and introduced into the separation column. 
     
     
         10 . The method according  claim 1 , comprising expanding the stream of pressurized liquefied natural gas a dynamic or static expander, followed by introducing an expanded expanding stream of pressurized liquefied natural gas into a flash drum to be separated into the expanded liquefied natural gas introduced into the storage, and the at least one flow of flash gas. 
     
     
         11 . The method according to  claim 1 , comprising forming the at least one flow of flash gas in the storage when the expanded liquefied natural gas is introduced into the storage. 
     
     
         12 . The method according to  claim 11 , comprising directly introducing the stream of pressurized liquefied natural gas into the storage, without flowing through a flash drum. 
     
     
         13 . The method according to  claim 1 , wherein compressing the head stream coming from the separation column is carried out in at least one first compressor coupled to the dynamic expansion turbine and then in a compression machine comprising successively a second compressor, a cooler for the gas compressed in the second compressor, and a third compressor, to form the stream of compressed purified natural gas. 
     
     
         14 . The method according to  claim 1 , comprising cooling and partially condensing a head flow coming from the fractionation column to form a cooled and at least partially condensed head flow and then introducing the cooled and at least partially condensed head flow into a head drum, recovering the flow of ethane at the head of the head drum, the bottom of the head drum being reintroduced as reflux into the fractionation column. 
     
     
         15 . The method according to  claim 1 , comprising forming the turbine feed stream which is sent to the dynamic expansion turbine without separation from an entire gas flow coming from the separation of the stream of cooled natural gas. 
     
     
         16 . An ethane extraction installation to extract ethane from a stream of initial natural gas, comprising:
 at least one first upstream heat exchanger configured to cool a stream of initial natural gas, to form a stream of cooled natural gas;   a separator to separate the stream of cooled natural gas into a liquid flow and a gas flow;   a liquid flow expander;   a methane and C2+ hydrocarbons separation column and an inlet to introduce at least one stream coming from the expanded liquid flow into the separation column, at a first level;   a dynamic expansion turbine configured to expand a turbine feed stream formed from the gas flow, configured to form an expanded stream and an inlet to introduce the expanded stream from the dynamic expansion turbine into the separation column at a second level,   a fractionation column, an inlet to introduce a bottom stream rich in C2+ hydrocarbons coming from the separation column into the fractionation column, and an outlet to recover a flow of ethane from the fractionation column;   an outlet to recover at least a part of a head stream coming from the separation column and a compressor to compress the at least a part of a head stream coming from the separation column, to form a stream of compressed purified natural gas;   a liquefier configured to liquefy the stream of compressed purified natural gas to form a stream of pressurized liquefied natural gas;   a flash expander of the stream of pressurized liquefied natural gas to form a flash expanded liquefied natural gas and a storage to recover the flash expanded liquefied natural gas;   an outlet to recover at least one flow of flash gas from the flash expander;   a compressor of the at least one flow of flash gas;   a separator of the flow of compressed flash gas into a fuel stream and a recycle stream;   a cooler/expander to cool and at least partially expand the recycle stream and form a cooled and expanded recycle stream, and to introduce the cooled and expanded recycle stream at a head stage of the separation column.