Boil-off gas re-liquefying device and method for ship

Abstract

Disclosed is a re-liquefying device using a boil-off gas as a cooling fluid so as to re-liquefy the boil-off gas generated from a liquefied gas storage tank provided in a ship. A boil-off gas re-liquefying device for a ship comprises: a multi-stage compression unit for compressing boil-off gas generated from a liquefied gas storage tank; a heat exchanger in which the boil-off gas generated from the storage tank and the boil-off gas compressed exchange heat; a vaporizer for heat exchanging the boil-off gas cooled by the heat exchanger and a separate liquefied gas supplied to a fuel demand source of a ship, and thus cooling the boil-off gas; an intermediate cooler for cooling the boil-off gas that has been cooled by the heat exchanger; and an expansion means for branching a part of the boil-off gas, which is supplied to the intermediate cooler, and expanding the same.

1 - 15 . (canceled) 
     
     
         16 . A BOG reliquefaction apparatus for ships for transportation of liquefied gas, the BOG reliquefaction apparatus comprising:
 a multistage compressor comprising a plurality of compression stage parts and compressing BOG discharged from a storage tank storing liquefied gas;   a heat exchange unit reliquefying the BOG compressed by the multistage compressor by cooling the BOG compressed by the multistage compressor through heat exchange; and   a third expansion unit decompressing the BOG reliquefied by the heat exchange unit,   wherein the heat exchange unit comprises:
 a heat exchanger cooling the BOG compressed by the multistage compressor through heat exchange between the BOG compressed by the multistage compressor and the BOG supplied from the storage tank to the multistage compressor and not subjected to compression; and; 
 an intermediate cooler expanding some of the compressed BOG while cooling the remaining compressed BOG through heat exchange between the expanded BOG and the remaining compressed BOG. 
   
     
     
         17 . The BOG reliquefaction apparatus for ships according to  claim 16 , wherein the heat exchange unit comprises a vaporizer vaporizing liquefied gas to be supplied as fuel to a fuel demand site in the ship while cooling the compressed BOG through heat exchange between the compressed BOG and the liquefied gas to be supplied as fuel. 
     
     
         18 . The BOG reliquefaction apparatus for ships according to  claim 16 , wherein the heat exchange unit comprises heat exchanger, at least one of one or more intermediate coolers and the vaporizer to cool the compressed BOG through at least two stages. 
     
     
         19 . The BOG reliquefaction apparatus for ships according to  claim 16 , wherein the expanded BOG discharged from the intermediate cooler after heat exchange is returned upstream of one of the plurality of compression stage parts of the multistage compressor. 
     
     
         20 . The BOG reliquefaction apparatus for ships according to  claim 19 , wherein when the BOG reliquefaction apparatus includes one or more intermediate coolers, the one or more intermediate coolers are connected to each other in series and the expanded BOG used as a refrigerant for an intermediate cooler disposed upstream among the intermediate coolers is supplied farther downstream among the compression stage parts of the multistage compressor than the expanded BOG used as a refrigerant for an intermediate cooler disposed downstream among the intermediate coolers. 
     
     
         21 . The BOG reliquefaction apparatus for ships according to  claim 16 , wherein the heat exchange unit comprises:
 a first expansion unit cooling some BOG branched off from the compressed BOG cooled by the heat exchanger by expanding some BOG branched off therefrom;   a first intermediate cooler cooling the compressed BOG through heat exchange between the expanded BOG cooled by the first expansion unit and the remaining compressed BOG not branched off to the first expansion unit;   a second expansion unit cooling some BOG branched off from the BOG cooled by the first intermediate cooler by expanding some BOG branched off therefrom; and   a second intermediate cooler cooling the remaining BOG not branched off to the second expansion unit through heat exchange between the expanded BOG cooled by the second expansion unit and the remaining BOG not branched off to the second expansion unit, followed by supplying the remaining BOG to the third expansion unit.   
     
     
         22 . The BOG reliquefaction apparatus for ships according to  claim 16 , wherein the heat exchange unit comprises:
 a first expansion unit cooling some BOG branched off from the compressed BOG cooled by the heat exchanger by expanding some BOG branched off therefrom;   a first intermediate cooler cooling the compressed BOG through heat exchange between the expanded BOG cooled by the first expansion unit and the remaining compressed BOG not branched off to the first expansion unit;   a vaporizer heating liquefied gas to be supplied as fuel to a fuel demand site in the ship while cooling the BOG cooled by the first intermediate cooler through heat exchange between the BOG cooled by the first intermediate cooler and the liquefied gas to be supplied as fuel to the fuel demand site;   a second expansion unit cooling some BOG branched off from the BOG cooled by the vaporizer by expanding some BOG branched off therefrom; and   a second intermediate cooler cooling the remaining BOG not branched off to the second expansion unit through heat exchange between the expanded BOG cooled by the second expansion unit and the remaining BOG not branched off to the second expansion unit,   wherein the BOG cooled by the second intermediate cooler is supplied to the third expansion unit and the liquefied gas heated by the vaporizer is supplied to the fuel demand site in the ship.   
     
     
         23 . The BOG reliquefaction apparatus for ships according to  claim 16 , wherein the heat exchange unit comprises:
 a multistream heat exchanger in which the heat exchanger is integrated with the intermediate cooler; and   a multistream expansion unit cooling some BOG branched off from the compressed BOG to be supplied to the multistream heat exchanger by expanding some BOG branched off therefrom,   wherein, in the multistream heat exchanger, the compressed BOG is cooled by the BOG not subjected to compression and the expanded BOG through heat exchange between the BOG not subjected to compression, the compressed BOG, and the expanded BOG cooled by the multistream expansion unit.   
     
     
         24 . The BOG reliquefaction apparatus for ships according to  claim 23 , wherein the heat exchange unit further comprises a vaporizer cooling the BOG cooled by the multistream heat exchanger through heat exchange between the BOG cooled by the multistream heat exchanger and liquefied gas to be supplied as fuel to a fuel demand site in the ship,
 wherein the BOG cooled by the vaporizer is supplied to the third expansion unit and the liquefied gas heated by the vaporizer is supplied to the fuel demand site in the ship.   
     
     
         25 . The BOG reliquefaction apparatus for ships according to  claim 16 , wherein the heat exchange unit comprises:
 a vaporizer heating liquefied gas to be supplied as fuel to a fuel demand site in the ship while cooling the BOG cooled by the heat exchanger through heat exchange between the BOG cooled by the heat exchanger and the liquefied gas to be supplied as fuel to the fuel demand site;   a second expansion unit cooling some BOG branched off from the compressed BOG cooled by the vaporizer by expanding some BOG branched off therefrom; and   a second intermediate cooler cooling the compressed BOG through heat exchange between the expanded BOG cooled by the second expansion unit and the remaining compressed BOG not branched off to the second expansion unit,   wherein the BOG cooled by second intermediate cooler is supplied to the third expansion unit and the liquefied gas heated by the vaporizer is supplied to the fuel demand site in the ship.   
     
     
         26 . The BOG reliquefaction apparatus for ships according to  claim 16 , further comprising:
 a gas/liquid separator separating BOG having passed through the third expansion unit into gaseous BOG and reliquefied BOG to supply the reliquefied BOG or the reliquefied BOG and the non-reliquefied gaseous BOG to the storage tank, or to supply the reliquefied BOG to the storage tank while recirculating the gaseous BOG to the multistage compressor.   
     
     
         27 . A BOG reliquefaction method for ships for transportation of liquefied gas, comprising:
 compressing BOG discharged from a storage tank storing liquefied gas;   cooling the compressed BOG through multiple stages; and   decompressing reliquefied BOG generated by cooling the compressed BOG,   wherein cooling the compressed BOG comprises:   a heat exchange step in which the compressed BOG is cooled through heat exchange between the compressed BOG and BOG to be compressed; and   an intermediate heat exchange step in which some BOG is branched off from the compressed BOG cooled in the heat exchange step and is expanded and the remaining BOG not branched off from the compressed BOG is cooled through heat exchange between the expanded BOG and the remaining compressed BOG.   
     
     
         28 . The BOG reliquefaction method for ships according to  claim 27 , wherein cooling the compressed BOG comprises a vaporization step in which the compressed BOG is cooled and liquefied gas to be supplied as fuel to a fuel demand site in the ship is vaporized through heat exchange between the compressed BOG and the liquefied gas to be supplied as fuel. 
     
     
         29 . The BOG reliquefaction method for ships according to  claim 27 , wherein cooling the compressed BOG comprises cooling the compressed BOG through multiple stages that comprise at least one of the heat exchange step, the intermediate heat exchange step and the vaporization step,
 wherein the intermediate heat exchange step is performed at least once.   
     
     
         30 . The BOG reliquefaction method according to  claim 29 , further comprising
 a gas/liquid separation step in which the decompressed BOG is separated into gaseous BOG and reliquefied BOG,   wherein the reliquefied BOG separated by the gas/liquid separation step is returned back to the storage tank, and the gaseous BOG separated by the gas/liquid separation step is returned back to the storage tank or recirculated to the step of compressing BOG.