Expander-Based LNG Production Processes Enhanced With Liquid Nitrogen

Abstract

A method for producing liquefied natural gas (LNG). A natural gas stream is directed to a mechanical refrigeration unit to liquefy the natural gas stream and form a pressurized liquefied natural gas (LNG) stream with a pressure greater than 50 psia (345 kPa) and less than 500 psia (3445 kPa). A liquid refrigerant subcooling unit is provided at a first location. Liquid refrigerant is produced at a second location that is geographically separate from the first location. The produced liquid refrigerant is transported to the first location. The pressurized LNG stream is subcooled in the liquid refrigerant subcooling unit by exchanging heat between the pressurized LNG stream and at least one stream of the liquid refrigerant to thereby produce an LNG stream.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application claims the benefit of U.S. Provisional Patent Application 62 / 266,979, filed Dec. 14, 2015 entitled EXPANDER-BASED LNG PRODUCTION PROCESSES ENHANCED WITH LIQUID NITROGEN, the entirety of which is incorporated by reference herein.[0002]This application is related to U.S. Provisional Patent Application No. 62 / 266,976 titled “Method and System for Separating Nitrogen from Liquefied Natural Gas Using Liquefied Nitrogen;” U.S. Provisional Patent Application No. 62 / 266,983 titled “Method of Natural Gas Liquefaction on LNG Carriers Storing Liquid Nitrogen;” and U.S. Provisional Patent Application No. 62 / 622,985 titled “Pre-Cooling of Natural Gas by High Pressure Compression and Expansion,” all having common inventors and assignee and filed on an even date herewith, the disclosure of which is incorporated by reference herein in their entirety.BACKGROUND[0003]Field of Disclosure[0004]The disclosure relates generally to the field of...

AI Technical Summary

Benefits of technology

The present disclosure describes methods and systems for liquefying natural gas to produce liquefied natural gas (LNG). The methods involve using a mechanical refrigeration unit to liquefy the natural gas stream and form a pressurized LNG stream, followed by a liquid refrigerant subcooling unit that further subcools the pressurized LNG stream to produce the final LNG stream. The system also involves using a mechanical refrigeration unit to liquefy the natural gas stream and form a pressurized LNG stream, followed by a liquid nitrogen subcooling unit that further subcools the pressurized LNG stream to produce the final LNG stream and at least one vaporized liquid nitrogen stream. The technical effects of this patent include improved efficiency and flexibility in the production of LNG and increased safety during the liquefaction process.

Problems solved by technology

FLNG is a technology solution for monetizing offshore stranded gas where it is not economically viable to construct a gas pipeline to shore.

Although FLNG has several advantages over conventional onshore LNG, significant technical challenges remain in the application of the technology.

Although expander-based process has its advantages, the application of this technology to an FLNG project with LNG production of greater than 2 million tons per year (MTA) has proven to be less appealing than the use of the mixed refrigerant process.

The size of the expander-based process train is limited since its refrigerant mostly remains in the vapor state throughout the entire process and the refrigerant absorbs energy through its sensible heat.

Furthermore, the limitations in compander horsepower size results in parallel rotating machinery as the capacity of the expander-based process train increases.

However, the equipment count, complexity and cost all increase with multiple expander trains.

The processes disclosed by these two patents have the disadvantage of using a mechanical refrigeration system while still requiring a significant of amount of LIN to produce the LNG.

In FLNG applications, space for storage of LIN either topside or in the hull of the floating structure may be limited.

The technology, however, is still limited to a capacity of less than 1.5 MTA.

However, the technology has the disadvantage of a high equipment count and increased complexity due to its requirement for two independent refrigeration loops and the compression of the feed gas.

Furthermore, the high pressure operation also means that the equipment and piping will be much heavier than that of other expander-based processes.

According to Maunder, including a liquefying expander in the process significantly reduces the recycle gas rate and the overall required refrigeration power.

However, the technology is still limited to capacity of less than 1.5 MTA and it requires the use of a liquefying expander, which is not standard equipment for LNG production.

The technology has also been shown to be less efficient than the technologies described by Foglietta and Minta for the liquefaction of lean natural gas.

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