Method for treating a multi-phase hydrocarbon stream and an apparatus therefor

Abstract

A multi-phase hydrocarbon stream ( 145 ) is treated to provide a treated liquid hydrocarbon stream ( 165 ), such as a liquefied natural gas (LNG) stream. The multi-phase hydrocarbon stream ( 145 ) is passed to a first gas/liquid separator ( 150 ), wherein it is separated at a first pressure to provide a first separator hydrocarbon vapour stream ( 205 ) and a first separator bottoms stream ( 155 ). The first separator bottoms stream ( 155 ) is then separated in a second gas/liquid separator ( 160 ) at a second pressure that is lower than the first pressure, to provide a second separator hydrocarbon vapour stream ( 175 ) and a treated liquid hydrocarbon stream ( 165 ). The second separator hydrocarbon vapour stream ( 175 ) is compressed in an overhead stream compressor ( 180 ) to provide a stripping vapour stream ( 185 ) which is passed to the first gas/liquid separator ( 150 ).

1 . A method of treating a multi-phase hydrocarbon stream to provide a treated liquid hydrocarbon stream, comprising at least the steps of:
 producing a multi-phase hydrocarbon stream from natural gas, said multi-phase hydrocarbon stream comprising a vapour phase and a liquid phase;   passing the multi-phase hydrocarbon stream to a first gas/liquid separator;   separating the multi-phase hydrocarbon stream in the first gas/liquid separator at a first pressure to provide a first separator hydrocarbon vapour stream, comprising hydrocarbons and nitrogen, and a first separator bottoms stream;   separating the first separator bottoms stream in a second gas/liquid separator at a second pressure to provide a second separator hydrocarbon vapour stream and a treated liquid hydrocarbon stream in the form of LNG, wherein the second pressure is lower than the first pressure;   compressing the second separator hydrocarbon vapour stream in an overhead stream compressor to provide a stripping vapour stream; and   passing the stripping vapour stream to the first gas/liquid separator at a level gravitationally lower than the level at which the multi-phase hydrocarbon stream is passed to the first gas/liquid separator.   
     
     
         2 . The method according to  claim 1 , wherein said compressing of the second separator hydrocarbon vapour stream by the overhead stream compressor provides the stripping vapour stream at a third pressure, which is equal to or greater than the first pressure. 
     
     
         3 . The method according to  claim 1 , further comprising:
 deriving a low pressure fuel gas stream from the first separator hydrocarbon vapour stream;   passing the low pressure fuel gas stream to a combustion device at a fuel gas pressure not higher than the pressure of the first separator hydrocarbon vapour stream.   
     
     
         4 . The method according to  claim 3 , wherein the first pressure of the first gas/liquid separator is at or above the fuel gas pressure and the first separator hydrocarbon vapour stream nor the low pressure fuel gas stream is compressed before use in the combustion device. 
     
     
         5 . The method according to  claim 3 , wherein the combustion device is one from the group consisting of a furnace, a boiler, a dual fuel diesel engine. 
     
     
         6 . The method according to  claim 3 , wherein the step of deriving the low pressure fuel gas stream from the first separator hydrocarbon vapour stream comprises the step of:
 warming the first separator hydrocarbon vapour stream against a warming stream in fuel gas heat exchanger to provide the low pressure fuel gas stream and a cooled warming stream.   
     
     
         7 . The method according to  claim 6 , wherein the step of producing a multi-phase hydrocarbon stream from said natural gas comprises:
 cooling a part of the natural gas as said warming stream in the fuel gas heat exchanger against the first separator hydrocarbon vapour stream, to provide said cooled warming stream in the form of a cooled process stream.   
     
     
         8 . The method according to  claim 1 , wherein the first separator hydrocarbon vapour stream comprises between from 30 mol % to 95 mol % of nitrogen. 
     
     
         9 . The method according to  claim 1 , wherein the treated liquid hydrocarbon stream contains less than 1 mol % nitrogen. 
     
     
         10 . The method according to  claim 1 , wherein the step of producing the multi-phase hydrocarbon stream from said natural gas comprises cooling. 
     
     
         11 . The method according to  claim 1 , wherein the step of producing the multi-phase hydrocarbon stream from said natural gas comprises:
 providing a hydrocarbon supply stream from a natural gas stream at elevated pressure;   extracting a continuing hydrocarbon stream from the hydrocarbon supply stream;   passing the continuing stream to a cooling and liquefaction unit wherein it is cooled an at least partially liquefied to provide an at least partially liquefied hydrocarbon stream;   passing the at least partially liquefied hydrocarbon stream to an inlet of at least one hydrocarbon stream expansion device and therein reducing the pressure of the at least partially liquefied hydrocarbon stream to provide the multi-phase hydrocarbon stream.   
     
     
         12 . The method according to  claim 11 , further comprising:
 splitting the hydrocarbon supply stream into a high pressure fuel gas stream and said continuing hydrocarbon stream, which high pressure fuel gas stream has one or both of a nitrogen content of lower than 15 mol % and a pressure of higher than 15 bara.   
     
     
         13 . An apparatus for treating a multi-phase hydrocarbon stream comprising a liquid phase and a vapour phase to provide a treated liquid hydrocarbon stream in the form of LNG, comprising at least:
 means for producing a multi-phase hydrocarbon stream from natural gas, said means comprising at least one of a liquefaction unit and one or more hydrocarbon stream expansion devices;   a first gas/liquid separator arranged to receive the multi-phase hydrocarbon stream and separate it into a first separator hydrocarbon vapour stream, comprising hydrocarbons and nitrogen, and a first separator bottoms stream, said first gas/liquid separator having a first inlet for feeding the multi-phase hydrocarbon stream into the first gas/liquid separator, a first outlet for discharging the first separator hydrocarbon vapour stream from the first gas/liquid separator, a second outlet for discharging the first separator bottoms stream from the first gas/liquid separator and a second inlet, located at a level gravitationally lower than said first inlet, for feeding a stripping vapour stream into the first gas/liquid separator;   a second gas/liquid separator arranged to receive the first separator bottoms stream and separate it into a second separator hydrocarbon vapour stream and a treated liquid hydrocarbon stream in the form of LNG, said second gas/liquid separator having a first inlet in fluid communication with the second outlet of the first gas/liquid separator, for feeding the first separator bottoms stream into the second gas/liquid separator, a first outlet for discharging the second separator hydrocarbon vapour stream from the second gas/liquid separator and a second outlet for discharing the treated liquid hydrocarbon stream from the second gas/liquid separator;   a bottoms stream expansion device arranged between the second outlet of the first gas/liquid separator and the first inlet of the second gas/liquid separator, to reduce the pressure of the first separator bottoms stream; and   an overhead stream compressor to compress the second separator hydrocarbon vapour stream to provide the stripping vapour stream, said overhead stream compressor having an inlet in fluid communication with the first outlet of the second gas/liquid separator to receive the second separator hydrocarbon vapour stream, and an outlet in fluid communication with the second inlet of the first gas/liquid separator for discharging the stripping vapour stream.   
     
     
         14 . The apparatus according to  claim 13 , wherein the one or more hydrocarbon stream expansion devices is connected to the liquefaction unit and downstream thereof, to expand an at least partially liquefied hydrocarbon stream discharged from the liquefaction unit, to provide the multi-phase hydrocarbon stream, said one or more hydrocarbon stream expansion devices having an inlet for receiving the at least partially liquefied hydrocarbon stream and an outlet for discharging the multi-phase hydrocarbon stream, wherein said outlet is connected to the first inlet of the first gas/liquid separator. 
     
     
         15 . The apparatus according to  claim 13 , further comprising:
 a combustion device operating at a fuel gas pressure not higher than the pressure of the first separator hydrocarbon vapour stream, the combustion device arranged to receive low pressure fuel gas derived from the first separator hydrocarbon stream.   
     
     
         16 . The apparatus according to  claim 15 , wherein there is no compressor between the first outlet of the first gas/liquid separator and the combustion device. 
     
     
         17 . The apparatus according to  claim 15 , wherein the combustion device is one from the group consisting of a furnace, a boiler, a dual fuel diesel engine. 
     
     
         18 . The method according to  claim 1 , wherein the first separator hydrocarbon vapour stream has a pressure in the range of from 2 to 15 bara. 
     
     
         19 . The method according to  claim 8 , wherein the first separator hydrocarbon vapour stream has a pressure in the range of from 2 to 15 bara. 
     
     
         20 . The method according to  claim 10 , wherein the step of producing the multi-phase hydrocarbon stream from said natural gas further comprises changing the pressure of the natural gas.