LNG boiloff gas recondensation configurations and methods

Patent metadata

Field	Value
Publication number	US-9927068-B2
Application number	US-201213685201-A
Country	US
Kind code	B2
Filing date	Nov 26, 2012
Priority date	Dec 2, 2011
Publication date	Mar 27, 2018
Grant date	Mar 27, 2018

Abstract

Systems and methods for optimizing the recondensation of boiloff gas in liquid natural gas storage tanks are presented. In especially preferred aspects of the inventive subject matter, BOG from a storage tank is condensed using refrigeration content of a portion of LNG sendout in a direct or indirect manner, and the BOG condensate and LNG sendout portion are combined to form a subcooled stream that is then combined with the balance of the LNG sendout, to be fed to a high pressure pump. Contemplated recondensation operations advantageously occur without using otherwise needed large volume recondensers. Moreover, the condensing and subcooling operations are decoupled from the LNG sendout rate.

First claim

What is claimed is:

1. A method of producing a combined subcooled sendout stream of liquid natural gas (LNG) and boiloff gas (BOG) condensate from a storage tank configured to provide a BOG stream and a sendout LNG stream, comprising:
compressing the BOG stream to thereby produce compressed BOG;
condensing and subcooling a first portion of the compressed BOG using a first portion of the LNG sendout stream to thereby produce a subcooled BOG condensate stream and a subcooled LNG stream;
separately combining the subcooled BOG condensate stream and the subcooled LNG stream in a surge tank to produce a mixed subcooled BOG/LNG stream at a first pressure;
pumping the mixed subcooled BOG/LNG stream to provide a mixed subcooled BOG/LNG stream at a second pressure, wherein the second pressure is higher than the first pressure;
combining the mixed subcooled BOG/LNG stream at the second pressure with a second portion of the LNG sendout stream to thereby produce the combined subcooled sendout stream;
feeding the combined subcooled sendout stream to a high pressure pump;
compressing the combined subcooled sendout stream with the high pressure pump to form a high pressure LNG sendout stream;
recycling a portion of the high pressure LNG sendout stream to the surge tank; and
recycling a vent gas stream from the surge tank into the compressed BOG downstream of the compressing of the BOG stream and upstream of:
1) the condensing and subcooling of the first portion of the compressed BOG and 2) the surge tank; recycling a high pressure pump vent gas stream from the high pressure pump into the compressed BOG downstream of the compressor and upstream of the condensing and subcooling.

2. The method of claim 1 , wherein the condensing and subcooling step comprises using a heat exchanger to condense and subcool the first portion of the compressed BOG.

3. A method of producing a combined subcooled sendout stream of liquid natural gas (LNG) and boiloff gas (BOG) condensate from a storage tank configured to provide a BOG stream and a sendout LNG stream, comprising:
compressing the BOG stream to thereby produce compressed BOG;
condensing the compressed BOG with a first portion of the LNG sendout stream to thereby produce a subcooled BOG condensate stream and a subcooled LNG stream;
separately combining the subcooled BOG condensate stream and the subcooled LNG stream in a surge tank to produce an intermediate BOG/LNG product at a first pressure;
passing a vent gas stream from the surge tank into the compressed BOG the compressing of the BOG stream and upstream of: 1) the condensing of the compressed BOG and 2) the surge tank;
pumping the intermediate BOG/LNG product from the surge tank to a second pressure to thereby provide a mixed subcooled BOG/LNG stream, wherein the second pressure is higher than the first pressure;
combining the mixed subcooled BOG/LNG stream with a second portion of the LNG sendout stream to thereby produce the combined subcooled sendout stream;
and feeding the combined subcooled sendout stream to a high pressure pump;
recycling a portion of the high pressure LNG sendout stream to the surge tank;
recycling a high pressure pump vent gas stream from the high pressure pump into the compressed BOG downstream of the compressor and upstream of the condensing and subcooling.

4. The method of claim 3 , further comprising maintaining a portion of the intermediate BOG/LNG product in the surge tank that is fluidly coupled to the high pressure pump.

5. The method of claim 3 , wherein the condensing step is performed using a heat exchanger in which the first portion of the LNG sendout stream provides refrigeration content.

6. A liquid natural gas (LNG) processing plant, comprising:
a compressor configured to compress a boiloff gas (BOG) stream of an LNG storage tank to thereby produce compressed BOG;
a condensing and subcooling system configured to receive the compressed BOG and a first portion of an LNG sendout stream to thereby produce a subcooled BOG condensate stream and a subcooled LNG stream;
a surge tank configured to separately receive the subcooled BOG condensate stream and the subcooled LNG stream to be mixed as an intermediate BOG/LNG product at a first pressure;
a pump configured to receive the intermediate BOG/LNG product to produce a mixed subcooled BOG/LNG stream at a second pressure, wherein the second pressure is higher than the first pressure;
a flow control element fluidly coupled to the condensing and subcooling system and configured to receive a second portion of the LNG sendout stream and the mixed subcooled BOG/LNG stream to thereby produce a combined subcooled sendout stream;
a high pressure pump fluidly coupled to the flow control element, the high pressure pump configured to receive the combined subcooled sendout stream; and
a recycle line fluidly coupled to the high pressure pump, wherein the recycle line is configured to pass a high pressure pump vent gas stream from the high pressure pump into the compressed BOG downstream of the compressor and upstream of the condensing and subcooling system; a high pressure recycle line configured to recycle a portion of the high pressure LNG sendout stream to the surge tank;
a vent line extending from the surge tank configured to pass a vent gas stream from the surge tank into the compressed BOG downstream of the compressor and upstream of 1) the condensing and subcooling system and 2) the surge tank.

7. The LNG plant of claim 6 , wherein the condensing and subcooling system comprises a heat exchanger configured to use refrigeration content of the first portion of the LNG sendout stream to thereby condense and subcool the compressed BOG.

Assignees

• Fluor Tech Corp

Inventors

• Mak John

Classifications

• F17C2265/037

  with pressurising · CPC title

• F17C2221/033

  Methane, e.g. natural gas, CNG, LNG, GNL, GNC, PLNG · CPC title

• F17C2260/035

  of fluid · CPC title

• F17C2201/0104

  cylindrical · CPC title

• F17C2265/035

  with subcooling the liquid phase · CPC title