Reliquefaction of boil-off from liquefied natural gas
First Claim
1. In a process for liquifying boil-off gas resulting from the evaporation of liquified natural gas contained in a storage vessel, the boil-off gas being cooled and liquified in a closed-loop refrigeration system and then returned to said storage vessel wherein, said closed-loop refrigeration system comprises the steps:
- compressing nitrogen as a working fluid in a compressor system to form a compressed working fluid;
splitting said compressed working fluid into a first and second stream;
isenthalpically expanding said first stream to produce a cooled first stream, then warming against boil-off gas and compressed working fluid; and
isentropically expanding the second stream to form a cooled expanded stream which is then warmed against boil-off gas to form at least partially condensed boil-off prior to warming against the working fluid and prior to return to the compressor system;
the improvement for reliquifying a boil-off gas having from about 0 to 5% nitrogen by volume, which comprises;
(a) effecting isenthalpic expansion of said first stream when the nitrogen content is from about 0-5%, under conditions such that at least a liquid fraction is generated at a pressure higher than the isentropically expanded stream;
(b) warming the liquid fraction against the partially condensed boil-off gas and against the compressed working fluid prior to returning said fractions to the compressor system.
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Accused Products
Abstract
The present invention relates to an improved process for the reliquefaction of boil-off gas containing up to 10% nitrogen resulting from the evaporation of liquefied natural gas (LNG) contained in a storage vessel. In the process, a closed-loop refrigeration cycle is utilized wherein an isenthalpically expanded stream is warmed against an initially cooled boil-off stream. The boil-off LNG stream is initially cooled by indirect heat exchange with an isentropically expanded refrigerant stream.
74 Citations
13 Claims
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1. In a process for liquifying boil-off gas resulting from the evaporation of liquified natural gas contained in a storage vessel, the boil-off gas being cooled and liquified in a closed-loop refrigeration system and then returned to said storage vessel wherein, said closed-loop refrigeration system comprises the steps:
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compressing nitrogen as a working fluid in a compressor system to form a compressed working fluid; splitting said compressed working fluid into a first and second stream; isenthalpically expanding said first stream to produce a cooled first stream, then warming against boil-off gas and compressed working fluid; and isentropically expanding the second stream to form a cooled expanded stream which is then warmed against boil-off gas to form at least partially condensed boil-off prior to warming against the working fluid and prior to return to the compressor system; the improvement for reliquifying a boil-off gas having from about 0 to 5% nitrogen by volume, which comprises; (a) effecting isenthalpic expansion of said first stream when the nitrogen content is from about 0-5%, under conditions such that at least a liquid fraction is generated at a pressure higher than the isentropically expanded stream; (b) warming the liquid fraction against the partially condensed boil-off gas and against the compressed working fluid prior to returning said fractions to the compressor system. - View Dependent Claims (2, 3, 4, 5, 6)
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7. In a process for liquifying boil-off gas resulting from the evaporation of liquified natural gas contained in a storage vessel, the boil-off gas being cooled and liquified in a closed-loop refrigeration system and then returned to said storage vessel wherein said closed-loop refrigeration system comprises the steps:
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compressing nitrogen as a working fluid in a compressor system to form a compressed working fluid; splitting said compressed working fluid into a first and second stream; isenthalpically expanding said first stream to produce a cooled first stream, then warming against recycle compressed working fluid and boil-off gas; isentropically expanding the second stream to form a cooled expanded stream which is then warmed against boil-off gas and working fluid prior to return to the compressor system; the improvement for reliquefying a boil-off gas containing from about 5 to 10% nitrogen by volume which comprises; (a) effecting isenthalpic expansion of said first stream under conditions such that at least a liquid fraction is generated at a pressure higher than the isentropically expanded stream; (b) separating any vapor fraction, if generated, from the liquid fraction; (c) warming the vapor fraction, if generated, against boil-off gas and recycle compressed working fluid; (d) splitting the liquid fraction formed in step (a) into a first major portion and a second minor portion; (e) warming the first major portion of the liquid fraction against boil-off gas in parallel with the warming of said isentropically expanded second stream; and (f) isenthalpically expanding the second minor portion to produce a second cooled liquid fraction and a second vapor fraction and then warming the second cooled liquid fraction and said second vapor fraction against the partially condensed boil-off gas for effecting final condensation. - View Dependent Claims (8, 9, 10, 11, 12, 13)
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Specification