RELIQUEFACTION OF BOIL OFF GAS
First Claim
1. A method of reliquefying a part of the boil-off gas from a ship'"'"''"'"'s cargo of liquefied natural gas, which boil-off gas has been compressed for feeding as fuel to the ship'"'"''"'"'s motor, said method comprising the steps of separating a portion of the boiloff gas streAm that has been compressed for feeding to the ship'"'"''"'"'s boilers, further compressing said separated portion to a higher pressure, cooling said further compressed portion to condense it, the cooling being effected in part by utilizing at least part of the sensible cold in the boil-off gas prior to compression and in part by cold obtained by expanding the condensate, recovering reliquefied boil-off gas from the liquid fraction of the expanded condensate, and injecting liquefied natural gas into the boil-off gas at a point before the further compressed portion is cooled to condense it.
1 Assignment
0 Petitions
Accused Products
Abstract
A method is provided for reliquefying a part of the gas boiling off from a ship'"'"''"'"'s cargo of liquefied natural gas and normally used to fuel the ship'"'"''"'"'s motor. The method comprises separating a portion of the boil-off gas stream that has been compressed for use as fuel, further compressing the separated portion, condensing the further compressed material and recovering the liquid fraction from the condensate. The extent of reliquefaction is increased by injecting liquefied natural gas from the cargo into the boil-off gas before condensing the latter and/or expanding a portion of the condensed material and recycling it.
105 Citations
12 Claims
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1. A method of reliquefying a part of the boil-off gas from a ship'"'"''"'"'s cargo of liquefied natural gas, which boil-off gas has been compressed for feeding as fuel to the ship'"'"''"'"'s motor, said method comprising the steps of separating a portion of the boiloff gas streAm that has been compressed for feeding to the ship'"'"''"'"'s boilers, further compressing said separated portion to a higher pressure, cooling said further compressed portion to condense it, the cooling being effected in part by utilizing at least part of the sensible cold in the boil-off gas prior to compression and in part by cold obtained by expanding the condensate, recovering reliquefied boil-off gas from the liquid fraction of the expanded condensate, and injecting liquefied natural gas into the boil-off gas at a point before the further compressed portion is cooled to condense it.
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2. A method as claimed in claim 1 in which all the sensible cold in the uncompressed boil-off gas between the temperature at which it boils off from the cargo and ambient temperature is used in the cooling of the further compressed portion of boil-off gas to condense it.
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3. A method as claimed in claim 1 in which the liquefied natural gas is injected into the separated portion of the compressed boil-off gas stream.
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4. The method claimed in claim 1 in which liquefied natural gas is injected into the boil-off gas at a point before the further compressed portion is cooled to condense it and a portion of the condensate is recycled after expansion to a pressure which is below the maximum pressure of the further compressed portion, wherein said portion of condensate is combined with said liquefied natural gas and the combined material is injected into the separated portion of the compressed boil-off gas stream before said separated portion reaches its maximum pressure as a result of said further compression.
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5. A method as claimed in claim 1 in which at least a part of the power required for further compressing the separated portion of boil-off gas is provided by a gas engine which is fuelled by boil-off gas.
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6. A method as claimed in claim 1 further comprising the step of recycling a portion of the condensate after expanding said portion to a pressure which is below the maximum pressure of the further compressed portion.
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7. In a ship having a cargo tank for carrying liquefied natural gas, first gas compressor means and means for feeding boil-off gas from said tank to said first compressor means for compressing for use as fuel to the ship'"'"''"'"'s motor, an improvement further comprising:
- i. second gas compressor means;
ii. heat exchange means;
iii. expansion valve means;
iv. gas/liquid separator means;
v. first conducting means for feeding boil-off from said cargo tank through said heat exchange means as a first coolant stream prior to feeding it to said first compressor means;
vi. second conducting means for passing a portion of the compressed boil-off gas from said first compressor means to said second compressor means for further compression;
vii. third conducting means for passing the further compressed boil-off gas from said second compressor means through said heat exchange means in indirect counter-current heat exchange relationship with said first coolant stream, said further compressed boil-off gas stream being cooled and condensed in said heat exchange means;
viii. fourth conducting means for passing the cooled and condensed gas stream from said heat exchange means through said expansion valve means to expand same and thence to said gas/liquid separator means;
ix. gas recovery means for recovering gas from said gas/liquid separator means and passing it as a second coolant stream through said heat exchange means in indirect countercurrent heat-exchange relationship with said further compressed gas stream;
x. injection means for injecting gas from said cargo tank into boil-off gas at a point before said boil-off gas is passed to said heat exchange means for cooling and condensing; and
xi. liquid recovery means for recovering liquid from said gas/liquid separator means and returning it as reliquefied boil-off gas to said cargo tank.
- i. second gas compressor means;
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8. In a ship having a cargo tank for carrying liquefied natural gas, first gas compressor meaNs and means for feeding boil-off gas from said tank to said first compressor means for compressing for use as fuel to the ship'"'"''"'"'s motor, an improvement further comprising:
- i. second gas compressor means;
ii. heat exchange means;
iii. expansion valve means;
iv. gas/liquid separator means;
v. first conducting means for feeding boil-off gas from said cargo tank through said heat exchange means as a first coolant stream prior to feeding it to said first compressor means;
vi. second conducting means for passing a portion of the compressed boil-off gas from said compressor means to said second compressor means for further compression;
vii. third conducting means for passing the further compressed boil-off gas from said compressor means through said heat exchange means in indirect counter-current heat exchange relationship with said first coolant stream, said further compressed boil-off stream being cooled and condensed in said heat exchange means;
viii. fourth conducting means for passing the cooled and condensed gas stream from said heat exchange means through said expansion valve means to expand same and thence to said gas/liquid separator means;
ix. gas recovery means for recovering gas from said gas/liquid separator means and passing it as a second coolant stream through said heat exchange means in indirect counter-current heat-exchange relationship with said further compressed gas stream;
x. expanding means for expanding and recycling a portion of the further compressed gas stream after it has been cooled and condensed by passage through saud heat exchange means; and
xi. liquid recovery means for recovering liquid from said gas/liquid separator means and returning it as reliquefied boil-off gas to said cargo tank.
- i. second gas compressor means;
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9. The improvement as in claim 8, further comprising:
- xii. injection means for injecting liquefied gas from said cargo tank into said boil-off gas at a point before said boil-off gas is passed to said heat exchange means for cooling and condensing.
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10. A method of reliquifying a part of the boil-off gas from a ship'"'"''"'"'s cargo of liquified natural gas, which boil-off gas has been compressed for feeding as fuel to the ship'"'"''"'"'s motor, said method comprising the steps of separating a portion of the boil-off stream that has been compressed for feeding to the ship'"'"''"'"'s boilers, further compressing said separated portion to a higher pressure, cooling said further compressed portion to condense it, the cooling being effected in part by utilizing at least part of the sensible cold in the boil-off gas prior to compression and in part by cold obtained by expanding the condensate, recovering reliquified boil-off gas from the liquid fraction of the expanded condensate, and recycling a portion of the condensate after expanding said portion to a pressure which is below the maximum pressure of the further compressed portion.
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11. A method as claimed in claim 10 in which all the sensible cold in the uncompressed boil-off gas between the temperature at which it boils off from the cargo and ambient temperature is used in cooling of the further compressed portion of the boil-off gas to condense it.
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12. A method as claimed in claim 10 in which at least a part of the power required for further compressing the separated portion of boil-off gas is provided by a gas engine which is fuelled by boil-off gas.
Specification