Boil-off gas re-liquefying system

Abstract

A system for reliquefying a boil off gas generated in a storage tank includes a first compressor compressing a partial amount (hereinafter, referred to as ‘fluid a’) of boil off gas discharged from the storage tank, a second compressor compressing another partial amount (hereinafter, referred to as ‘fluid b’) of boil off gas discharged from the storage tank, a second expanding unit expanding a partial amount (hereinafter, referred to as ‘fluid c’) of a flow formed as the fluid a and the fluid b join, a heat-exchanger cooling another partial amount (hereinafter, referred to as ‘fluid d’) of the flow formed as the fluid a and the fluid b join, and a first expanding unit expanding the fluid d cooled by the heat-exchanger, wherein the heat-exchanger heat-exchanges the fluid d with the fluid c as a coolant expanded by the second expanding unit to cool the fluid d.

1 . A system for reliquefying a boil off gas generated in a storage tank comprising:
 a first compressor compressing a partial amount (hereinafter, referred to as ‘fluid a’) of boil off gas discharged from the storage tank;   a second compressor compressing another partial amount (hereinafter, referred to as ‘fluid b’) of boil off gas discharged from the storage tank;   a second expanding unit expanding a partial amount (hereinafter, referred to as ‘fluid c’) of a flow formed as the fluid a and the fluid b join;   a heat-exchanger cooling another partial amount (hereinafter, referred to as ‘fluid d’) of the flow formed as the fluid a and the fluid b join; and   a first expanding unit expanding the fluid d cooled by the heat-exchanger,   wherein the heat-exchanger heat-exchanges the fluid d with the fluid c as a coolant expanded by the second expanding unit to cool the fluid d.   
     
     
         2 . The system of  claim 1 , further comprising a cold heat collecting unit heat-exchanging the fluid d with the boil off gas discharged from the storage tank, as a coolant, to cool the fluid d. 
     
     
         3 . The system of  claim 2 , wherein the fluid d is primarily cooled in the cold heat collecting unit, secondarily cooled by the heat-exchanger, and subsequently expanded by the first expanding unit so as to be reliquefied. 
     
     
         4 . The system of  claim 1 , wherein the fluid c is supplied, to the second expanding unit by way of the heat-exchanger, and the heat-exchanger heat-exchanges the fluid c before passing through the second expanding unit, the fluid c after passing through the second expanding unit, and the fluid d. 
     
     
         5 . The system of  claim 1 , wherein the fluid c which has passed through the second expanding unit and the heat-exchanger joins the boil off gas discharged from the storage tank. 
     
     
         6 . The system of  claim 4 , wherein the fluid c which has passed through the second expanding unit and the heat-exchanger joins the boil off gas discharged from the storage tank. 
     
     
         7 . The system of  claim 6 , further comprising a vapor-liquid separator separating a reliquefied liquefied natural gas (LNG) and a boil off gas remaining in a gaseous state in the fluid d which has passed through the first expanding unit,
 wherein the LNG separated by the vapor-liquid separator is sent to the storage tank, and the boil off gas separated by the vapor-liquid separator joins the boil off gas discharged from the storage tank.   
     
     
         8 . The system of  claim 6 , wherein, in the flow formed as the fluid a and the fluid b join, a remaining flow excluding the fluid c and the fluid d is sent to a fuel consumer. 
     
     
         9 . The system of  claim 8 , wherein the first compressor and the second compressor compress the boil off gas to 10 to 100 bar. 
     
     
         10 . The system of  claim 2 , wherein the fluid c is supplied to the second expanding unit by way of the heat-exchanger, and, the heat-exchanger heat-exchanges the fluid c before passing through the second expanding unit, the fluid c after passing through the second expanding unit, and the fluid d. 
     
     
         11 . The system of  claim 2 , wherein the fluid c which has passed through the second expanding unit and the heat-exchanger joins the boil off gas discharged from the storage tank. 
     
     
         12 . The system of  claim 10 , wherein the fluid c which has passed through the second expanding unit and the heat-exchanger joins the boil off gas discharged from the storage tank. 
     
     
         13 . The system of  claim 12 , further comprising a vapor-liquid separator separating a reliquefied liquefied natural gas (LNG) and a boil off gas remaining in a gaseous state in the fluid d which has passed through the first expanding unit,
 wherein the LNG separated by the vapor-liquid separator is sent to the storage tank, and the boil off gas separated by the vapor-liquid separator joins the boil off gas discharged from the storage tank.   
     
     
         14 . The system of  claim 12 , wherein, in the flow formed as the fluid a and the fluid b join, a remaining flow excluding the fluid c and the fluid d is sent to a fuel consumer. 
     
     
         15 . The system of  claim 14 , wherein the first compressor and the second compressor compress the boil off gas to 10 to 100 bar. 
     
     
         16 . The system of  claim 3 , wherein the fluid c is supplied to the second expanding unit by way of the heat-exchanger, and the heat-exchanger heat-exchanges the fluid c before passing through the second expanding unit, the fluid c after passing through the second expanding unit, and the fluid d. 
     
     
         17 . The system of  claim 3 , wherein the fluid c which has passed through the second expanding unit and the heat-exchanger joins the boil off gas discharged from the storage tank. 
     
     
         18 . The system of  claim 16 , wherein the fluid c which has passed through the second expanding unit and the heat-exchanger joins the boil off gas discharged from the storage tank. 
     
     
         19 . The system of  claim 18 , further comprising a vapor-liquid separator separating a reliquefied liquefied natural gas (LNG) and a boil off gas remaining in a gaseous state in the fluid d which has passed through the first expanding unit,
 wherein the LNG separated by the vapor-liquid separator is sent to the storage tank, and the boil off gas separated by the vapor-liquid separator joins the boil off gas discharged from the storage tank.   
     
     
         20 . The system of  claim 18 , wherein, in the flow formed as the fluid a and the fluid b join, a remaining flow excluding the fluid c and the fluid d is sent to a fuel consumer.