System And Method For Cooling A Bog Stream

Abstract

A system for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquefaction plant, comprising a line ( 10 ) for feeding BOG into a compressor ( 11; 11 ″) prior to heat exchange with a closed-loop refrigeration system. The refrigeration system comprising compressors ( 2, 3, 4 ) and expanders( 8, 9 ) and a number of heat ex-changers for heat exchange with the BOG stream. The expanders ( 8, 9 ) are arranged in series. A precooler in the feed line ( 10 ) is fluidly connected ( 32, 33 ) to the closed-loop refrigeration system, whereby said BOG is pre-cooled in heat exchange with a portion of the coolant in the closed-loop refrigeration system prior to said compression.

1 . A method for cooling a liquid natural gas (LNG) boil-off gas (BOG) stream ( 10 ) prior to compression by at least one compressor ( 11 ;  11 ′,  11 ″) in a boil-off reliquefaction plant where at least a portion of the BOG stream following compression is reliquefied in heat exchange with a coolant in a closed-loop refrigeration system comprising a heat exchanger unit ( 5 ,  6 ,  6 B,  7 ) having at least two stages, the method comprising the steps of:
 feeding ( 33 ;  33 ′,  33 ″;  37 ) a fraction of said coolant from a take-off location ( 48 ) in the closed loop refrigeration system, to at least one heat exchanging location ( 30 ;  30 ′,  30 ″) upstream of the at least one compressor ( 11 ;  11 ′,  11 ″);   bringing the coolant and the BOG stream into a heat exchanging relationship at the at least one heat exchanging location ( 30 ;  30 ′,  30 ″); and   feeding ( 32 ;  32 ′,  32 ″;  38 ) the coolant from the at least one heat exchanging location ( 30 ;  30 ′,  30 ″), to a re-entry location ( 49 ) in the closed loop refrigeration system, downstream of said take-off location ( 48 ).   
     
     
         2 . The method of  claim 1 , wherein the BOG stream compression comprises a first BOG compression stage ( 11 ″), upstream of a second BOG compression stage ( 11 ′). 
     
     
         3 . The method of  claim 2 , wherein the coolant is brought into a heat exchanging relationship at a first heat exchanging location ( 30 ′) upstream of the first BOG compression stage ( 11 ″), and at a second heat exchanging location ( 30 ″) upstream of the second BOG compression stage ( 11 ′). 
     
     
         4 . The method of  claim 2 , further comprising the steps of:
 feeding ( 37 ) a fraction of said coolant from the take-off location ( 48 ) in the closed loop refrigeration system, to a first heat exchanging location ( 30 ′) upstream of the first BOG compression stage ( 11 ″);   bringing the coolant and the BOG stream into a heat exchanging relationship at the first heat exchanging location ( 30 ′);   feeding ( 36 ) the coolant from the first heat exchanging location ( 30 ′), to a second heat exchanging location ( 30 ″) upstream of the second BOG compression stage ( 11 ′);   bringing the coolant and the BOG stream into a heat exchanging relationship at the second heat exchanging location ( 30 ″); and   feeding ( 38 ) the coolant from the second heat exchanging location ( 30 ″), to the re-entry location ( 49 ) in the closed loop refrigeration system.   
     
     
         5 . The method of  claim 1 , wherein the fraction of the coolant is fed from a take-off location ( 48 ) between a third heat exchanger stage ( 6 B) and a second heat exchanger stage ( 6 ) in the heat exchanger unit, and wherein coolant is fed to a re-entry location ( 49 ) between the third heat exchanger stage ( 6 B) and the second heat exchanger stage ( 6 ) in the heat exchanger unit. 
     
     
         6 . The method of  claim 5 , wherein said take-off location ( 48 ) and said re-entry location ( 49 ) are located downstream of the third heat exchanger stage ( 6 B) in the closed loop refrigeration system heat exchanger unit. 
     
     
         7 . The method of  claim 1 , wherein the fraction of the coolant is fed from a take-off location ( 48 ), and returned to the re-entry location ( 49 ), both locations in a first region (A) in the closed loop refrigeration system, downstream of a first heat exchanger stage ( 5 ) and upstream of a second heat exchanger stage ( 6 ) in the heat exchanger unit. 
     
     
         8 . The method of  claim 1 , wherein the fraction of the coolant is fed from a take-off location ( 48 ), and returned to the re-entry location ( 49 ), both locations in a second region (B) in the closed loop refrigeration system, downstream of a second heat exchanger stage ( 6 ) and upstream of a third heat exchanger stage ( 6 B) in the heat exchanger unit. 
     
     
         9 . The method of  claim 1 , wherein the fraction of the coolant is fed from a take-off location ( 48 ), and returned to the re-entry location ( 49 ), both locations in a third region (C) in the closed loop refrigeration system, downstream of a third heat exchanger stage ( 6 B) and upstream of a fourth heat exchanger stage ( 7 ) in the heat exchanger unit. 
     
     
         10 . An apparatus for cooling a liquid natural gas (LNG) boil-off gas (BOG) flowing in a BOG conduit ( 10 ), prior to compression by at least one compressor ( 11 ;  11 ′,  11 ″) in a boil-off reliquefaction plant where at least a portion of the BOG stream following compression is reliquefied in heat exchange with a coolant in a closed-loop refrigeration system comprising a heat exchanger unit ( 5 ,  6 ,  6 B,  7 ) having at least two stages, said apparatus comprising:
 at least one heat exchanger ( 30 ;  30 ′,  30 ″) fluidly connected to the BOG conduit ( 10 ), upstream of the at least one compressor ( 11 ;  11 ′,  11 ″);   a first line ( 33 ;  33 ′,  33 ″;  37 ) for feeding a fraction of said coolant from a take-off location ( 48 ) in the closed loop refrigeration system, to the at least one heat exchanger ( 30 ;  30 ′,  30 ″); and   a second line ( 32 ;  32 ′,  32 ″;  38 ) for feeding coolant from the at least one heat exchanger ( 30 ;  30 ′,  30 ″), to a re-entry location ( 49 ) in the closed loop refrigeration system, downstream of said take-off location ( 48 ).   
     
     
         11 . The apparatus of  claim 10 , wherein the at least one compressor comprises a first BOG compressor ( 11 ″) arranged in the BOG conduit ( 10 ) upstream of a second BOG compressor ( 11 ′). 
     
     
         12 . The apparatus of  claim 11 , further comprising a first heat exchanger ( 30 ′) upstream of the first BOG compressor ( 11 ″), and at a second heat exchanger ( 30 ″) upstream of the second BOG compressor ( 11 ′). 
     
     
         13 . The apparatus of  claim 12 , wherein
 the first line ( 37 ) fluidly connects the coolant take-off location ( 48 ) in the closed loop refrigeration system with a coolant inlet of the first heat exchanger ( 30 ′);   an intermediate line ( 36 ) fluidly connects a coolant outlet of the first heat exchanger ( 30 ′) with a coolant inlet of the second heat exchanger ( 30 ″); and   the second line ( 38 ) fluidly connects a coolant outlet of the second heat exchanger ( 30 ″) with the re-entry location ( 49 ) in the closed loop refrigeration system.   
     
     
         14 . The apparatus of  claim 10 , wherein the take-off location ( 48 ) is located in a line ( 20 ′) between a third heat exchanger stage ( 6 B) and a second heat exchanger stage ( 6 ) in the heat exchanger unit, and wherein the re-entry location ( 49 ) is located in the line ( 20 ′) between the third heat exchanger stage ( 6 B) and the second heat exchanger stage ( 6 ) in the heat exchanger unit, downstream of the take-off location ( 48 ). 
     
     
         15 . The apparatus of  claim 10 , wherein the take-off location ( 48 ) and the re-entry location ( 49 ) are located in a first region (A) in a section ( 17 ) of the closed loop refrigeration system, downstream of a first heat exchanger stage ( 5 ) and upstream of a second heat exchanger stage ( 6 ) in the heat exchanger unit. 
     
     
         16 . The apparatus of  claim 10 , wherein the take-off location ( 48 ) and the re-entry location ( 49 ) are located in a second region (B) in a section ( 17 B) of the closed loop refrigeration system, downstream of a second heat exchanger stage ( 6 ) and upstream of a third heat exchanger stage ( 6 B) in the heat exchanger unit. 
     
     
         17 . The apparatus of  claim 10 , wherein the take-off location ( 48 ) and the re-entry location ( 49 ) are located in a third region (C) in a section ( 18 ) of the closed loop refrigeration system, downstream of a third heat exchanger stage ( 6 B) and upstream of a fourth heat exchanger stage ( 7 ) in the heat exchanger unit. 
     
     
         18 . An apparatus for substantially reliquefying liquid natural gas (LNG) boil-off gas (BOG) in a boil-off reliquefaction plant, comprising a BOG feed line ( 10 ) and a compressor ( 11 ) fluidly connected to heat exchangers ( 5 ,  6 ,  7 ) and a return line ( 13 ) for re-liquefied BOG connected to ultimate outlet of the heat exchangers, wherein said heat exchangers the BOG is heat exchanged with a closed-loop refrigeration system comprising a coolant, said apparatus comprising:
 a compression unit ( 2 ,  3 ,  4 ) for compressing said coolant; a coolant line ( 15 ) fluidly connecting the compression unit with an inlet of a first coolant passage ( 51 ) of a first heat exchanger ( 5 ), said first heat exchanger also having a second coolant passage ( 52 ) and a BOG passage ( 53 );   a coolant line ( 16 ) fluidly connecting an outlet of the first heat exchanger's first coolant passage ( 51 ) with a first expander ( 8 );   a coolant line ( 17 ) fluidly connecting the outlet of the first expander ( 8 ) with an inlet of a first coolant passage ( 61 ) of a second heat exchanger ( 6 ), said second heat exchanger also having a second coolant passage ( 62 ) and a BOG passage ( 63 );   a coolant line ( 18 ) fluidly connecting an outlet of the second heat exchanger's first coolant passage ( 61 ) with a second expander ( 9 );   a coolant line ( 19 ) fluidly connecting the outlet of the second expander ( 9 ) with an inlet of a second coolant passage ( 72 ) of a fourth heat exchanger ( 7 ), said fourth heat exchanger also comprising a BOG passage ( 73 );   an outlet of said second fluid passage ( 72 ) of the fourth heat exchanger ( 7 ) being fluidly connected ( 20 ) to an inlet of said second fluid passage ( 62 ) of the second heat exchanger ( 6 );   an outlet of said second fluid passage ( 62 ) of the second heat exchanger ( 6 ) being fluidly connected to an inlet of said second fluid passage ( 52 ) of the first heat ex-changer ( 5 ); and   an outlet of said second fluid passage ( 52 ) of the first heat exchanger ( 5 ) being fluidly connected to an inlet of said compression unit ( 2 ,  3 ,  4 ).   
     
     
         19 . An apparatus for substantially reliquefying liquid natural gas (LNG) boil-off gas (BOG) in a boil-off reliquefaction plant, comprising a BOG feed line ( 10 ) and a compressor ( 11 ) fluidly connected to heat exchangers ( 5 ,  6 ,  7 ) and a return line ( 13 ) for re-liquefied BOG connected to an ultimate outlet of the heat exchangers, wherein the BOG is heat exchanged with a closed-loop refrigeration system comprising a coolant, said apparatus comprising:
 a compression unit ( 2 ,  3 ,  4 ) for compressing said coolant;   a coolant line ( 15 ) fluidly connecting the compression unit with an inlet of a first coolant passage ( 51 ) of a first heat exchanger ( 5 ), said first heat exchanger also having a second coolant passage ( 52 ) and a BOG passage ( 53 );   a coolant line ( 16 ) fluidly connecting an outlet of the first heat exchanger's first coolant passage ( 51 ) with a first expander ( 8 );   a coolant line ( 17 ) fluidly connecting the outlet of the first expander ( 8 ) with an inlet of a first coolant passage ( 61 ) of a second heat exchanger ( 6 ), said second heat exchanger also having a second coolant passage ( 62 ) and a BOG passage ( 63 ); a coolant line ( 17 B) fluidly connecting an outlet of said first coolant passage ( 61 ) of the second heat exchanger ( 6 ) to an inlet of a first coolant passage ( 61 B) of a third heat exchanger ( 6 B);   a coolant line ( 18 ) fluidly connecting an outlet of the third heat exchanger's first coolant passage ( 61 B) with a second expander ( 9 );   a coolant line ( 19 ) fluidly connecting the outlet of the second expander ( 9 ) with an inlet of a second coolant passage ( 72 ) of a fourth heat exchanger ( 7 ), said fourth heat exchanger also comprising a BOG passage ( 73 );   an outlet of said second fluid passage ( 72 ) of the fourth heat exchanger ( 7 ) being fluidly connected ( 20 ) to an inlet of a second fluid passage ( 62 B) of the third heat ex-changer ( 6 B);   an outlet of said second fluid passage ( 62 B) of the third heat exchanger ( 6 B) being fluidly connected ( 20 ′) to an inlet of the second coolant passage ( 62 ) of said second heat exchanger ( 6 );   an outlet of said second fluid passage ( 62 ) of the second heat exchanger ( 6 ) being fluidly connected to an inlet of said second fluid passage ( 52 ) of the first heat ex-changer ( 5 ); and   an outlet of said second fluid passage ( 52 ) of the first heat exchanger ( 5 ) being fluidly connected to an inlet of said compression unit ( 2 ,  3 ,  4 ).   
     
     
         20 . The apparatus of  claim 10 , wherein the coolant comprises nitrogen. 
     
     
         21 . The apparatus of  claim 10 , for use on a floating vessel. 
     
     
         22 . The method of  claim 1 , wherein the coolant comprises nitrogen. 
     
     
         23 . The method of  claim 1 , for use on a floating vessel.