Method of cooling boil off gas and an apparatus therefor

Abstract

The disclosure relates to a method and apparatus for cooling, preferably liquefying a boil off gas (BOG) stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, said method comprising at least the steps of: -compressing a boil off gas stream ( 01 ) from said liquefied cargo in three or more stages of compression comprising at least a first stage ( 65 ), a second stage ( 70 ) and final stage ( 75 ) to provide a compressed discharge stream ( 06 ), wherein intermediate compressed BOG streams ( 02, 04 ) are provided between consecutive stages of compression; -cooling the compressed discharge stream ( 06 ) to provide a cooled compressed discharge stream ( 07 ); -heat exchanging an expanded, optionally further cooled, portion of the cooled compressed discharge stream ( 07 ), with (i) one or more intermediate compressed BOG streams ( 04 ) from consecutive stages selected from between the second and final stages ( 75 ) of compression to provide one or more cooled intermediate compressed BOG streams ( 05 ) and optionally (ii) one or more portions ( 07 a, 108 a ), optionally after further cooling, of the cooled compressed discharge stream ( 07 ); and -passing the one or more cooled intermediate compressed BOG streams ( 05 ) to the next stage of compression ( 75 ).

1 . A method of cooling a boil off gas stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere and selected from the group consisting of liquefied petroleum gas, liquefied petrochemical gas selected from propylene, and liquefied ammonia, said method comprising at least the steps of:
 compressing a boil off gas stream from said liquefied cargo in three or more stages of compression comprising at least a first stage, a second stage and final stage to provide a compressed discharge stream, wherein intermediate compressed BOG streams are provided between consecutive stages of compression; 
 cooling the compressed discharge stream against a water stream to provide a cooled, fully condensed, compressed discharge stream; 
 heat exchanging an expanded, optionally further cooled, portion of the cooled, fully condensed, compressed discharge stream, with (i) one or more intermediate compressed BOG streams from consecutive stages selected from between the second and final stages of compression to provide one or more cooled intermediate compressed BOG streams and optionally (ii) one or more portions, optionally after further cooling, of the cooled compressed discharge stream; and 
 passing the one or more cooled intermediate compressed BOG streams to the next stage of compression. 
 
     
     
         2 . The method of  claim 1  comprising at least the steps of:
 compressing the boil off gas stream in a first stage of compression to provide a first intermediate compressed BOG stream as an intermediate compressed BOG stream; 
 compressing the first intermediate compressed BOG stream, optionally after heat exchange to provide a cooled first intermediate compressed BOG stream, in a second stage of compression to provide a second intermediate compressed BOG stream as an intermediate compressed BOG stream. 
 
     
     
         3 . The method of  claim 1  further comprising the steps of:
 splitting the cooled, fully condensed, compressed discharge stream into a continuing cooled compressed discharge stream and a cooled compressed discharge side stream; 
 expanding the cooled compressed discharge side stream to provide a expanded cooled discharge stream. 
 
     
     
         4 . The method of  claim 3  further comprising the step of:
 heat exchanging the expanded cooled discharge stream with an intermediate compressed BOG stream from consecutive stages selected from between the second and final stages of compression to provide a cooled intermediate compressed BOG stream. 
 
     
     
         5 . The method of  claim 3  comprising the further step of:
 flashing the expanded cooled discharge stream to provide an overhead expanded cooled discharge stream; 
 heat exchanging the overhead expanded cooled discharge stream with an intermediate compressed BOG stream from consecutive stages selected from between the second and final stages of compression to provide a cooled intermediate compressed BOG stream. 
 
     
     
         6 . The method of  claim 4  wherein the intermediate compressed BOG stream is a second intermediate compressed BOG stream and the cooled intermediate compressed BOG stream is a cooled second intermediate compressed BOG stream. 
     
     
         7 . The method of  claim 3  comprising the further step of:
 heat exchanging the expanded cooled discharge stream with the continuing cooled compressed discharge stream to provide a further cooled compressed discharge stream. 
 
     
     
         8 . The method of  claim 7  comprising the further steps of:
 splitting the further cooled compressed discharge stream into a continuing further cooled compressed discharge stream and a further cooled compressed discharge side stream; 
 expanding the further cooled compressed discharge side stream to provide an expanded further cooled discharge stream. 
 
     
     
         9 . The method of  claim 8  comprising the further step of:
 heat exchanging the expanded further cooled discharge stream with an intermediate compressed BOG stream from consecutive stages selected from between the first and final stages of compression to provide a cooled intermediate compressed stream. 
 
     
     
         10 . The method of  claim 8  comprising the further step of:
 flashing the expanded further cooled discharge stream to provide an overhead expanded further cooled discharge stream; 
 heat exchanging the overhead expanded further cooled discharge stream with an intermediate compressed BOG stream from consecutive stages selected from between the first and final stages of compression to provide a cooled intermediate compressed BOG stream. 
 
     
     
         11 . The method of  claim 9  wherein the intermediate compressed BOG stream is a first intermediate compressed BOG stream and the cooled intermediate compressed BOG stream is a cooled first intermediate compressed BOG stream. 
     
     
         12 . The method of  claim 9  comprising the further step of:
 heat exchanging the expanded further cooled discharge stream with the continuing further cooled compressed discharge stream to provide a cooled return stream. 
 
     
     
         13 . The method of  claim 12  further comprising the step of:
 expanding the cooled return stream to provide an expanded cooled return stream. 
 
     
     
         14 . The method according to  claim 1 , wherein the liquefied cargo is LPG, such as LPG comprising at least 3.5 mol % ethane. 
     
     
         15 . The method of  claim 1  wherein the compressed discharge stream is cooled against a seawater stream to provide the cooled compressed discharge stream. 
     
     
         16 . The method of  claim 1  wherein the first stage, second stage and final stage of compression are stages of a multi-stage compressor. 
     
     
         17 . An apparatus to cool a boil off gas stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere and selected from the group consisting of liquefied petroleum gas, liquefied petrochemical gas selected from propylene, and liquefied ammonia, said apparatus comprising at least:
 a compression system to compress a boil off gas stream from a liquefied cargo, said compression system comprising three or more stages of compression comprising at least a first stage, a second stage and final stage to provide a compressed discharge stream, wherein intermediate compressed BOG streams are provided between consecutive stages of compression, 
 a first heat exchanger to cool the compressed discharge stream against a water stream to provide a cooled, fully condensed, compressed discharge stream; 
 a first discharge stream pressure reduction device to expand an optionally further cooled, portion of the cooled, fully condensed, compressed discharge stream to provide an expanded, optionally further cooled portion of the cooled, fully condensed, compressed discharge stream, 
 one or more further heat exchangers to heat exchange an expanded, optionally further cooled, portion of the cooled, fully condensed, compressed discharge stream, with (i) one or more intermediate compressed BOG streams from consecutive stages selected from between the second and final stages of compression to provide one or more cooled intermediate compressed BOG streams to the next stage of compression and optionally (ii) one or more portions, optionally after further cooling, of the cooled compressed discharge stream. 
 
     
     
         18 . An apparatus to cool a boil off gas stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere and selected from the group consisting of liquefied petroleum gas, liquefied petrochemical gas selected from propylene, and liquefied ammonia, said apparatus comprising at least:
 a compression system to compress a boil off gas stream from a liquefied cargo, said compression system comprising three or more stages of compression comprising at least a first stage, a second stage and final stage to provide a compressed discharge stream, wherein intermediate compressed BOG streams are provided between consecutive stages of compression; 
 a first heat exchanger to cool the compressed discharge stream against a water stream to provide a cooled, fully condensed, compressed discharge stream; 
 a first discharge stream pressure reduction device to expand an optionally further cooled, portion of the cooled, fully condensed, compressed discharge stream to provide an expanded, optionally further cooled portion of the cooled, fully condensed, compressed discharge stream; 
 one or more further heat exchangers to heat exchange an expanded, optionally further cooled, portion of the cooled, fully condensed, compressed discharge stream, with (i) one or more intermediate compressed BOG streams from consecutive stages selected from between the second and final stages of compression to provide one or more cooled intermediate compressed BOG streams to the next stage of compression and optionally (ii) one or more portions, optionally after further cooling, of the cooled compressed discharge stream; and 
 wherein the apparatus is operable using the method of  claim 1 .