Liquefaction of Natural Gas

Abstract

A method and apparatus for liquefying natural gas vapour is provided. Firstly, liquid natural gas is sub-cooled at a first heat exchanger using a liquid coolant such as liquid nitrogen. The sub-cooled liquid natural gas is then used to condense the natural gas vapour at a second heat exchanger.

Description

FIELD OF THE INVENTION[0001]The present invention relates to liquefaction of natural gas vapour, particularly during the transport and storage of liquefied natural gas (LNG). In particular, but not exclusively, the present invention relates to the handling of boil off gas in a bunker vessel used to refuel LNG-powered vessels.BACKGROUND TO THE INVENTION[0002]Liquefied natural gas (LNG) is natural gas (typically methane—CH4) that has been liquefied, typically to make it more manageable during storage and / or transport. At atmospheric pressure, this means the temperature of the LNG is reduced to around −163 degrees centigrade or below.[0003]LNG carriers are vessels used to carry LNG across large distances, particularly to carry LNG from gas producing nations to gas consuming nations. The journey times of such vessels are significant, measured in days, weeks and even months. During such time, the LNG is stored at low temperature in order that it does not vaporise.[0004]Notwithstanding th...

AI Technical Summary

Benefits of technology

The invention provides an efficient way to make liquid natural gas (LNG) by first cooling down existing LNG to a sub-cooled state using a liquid coolant, which then directly cools the natural gas vapor. This process is much more energy efficient than creating LNG directly from the natural gas vapor. The resulting LNG can then be stored and reused in future cycles. The invention is compact and efficient, as there is no need for separate tanks for the coolant and the generated LNG.

Problems solved by technology

However, steam turbine propulsion is a relatively inefficient means to drive a large ship.

The diesel engine in such a ship is unable to handle BOG and instead an independent LNG re-liquefaction plant is provided on the vessel.

Furthermore, as well as high power requirements, the re-liquefaction plant is typically unable to handle high boil-off rates.

Excess BOG that the LNG re-liquefaction plant is unable to handle is burnt in a gas combustion unit (GCU) and thereby wasted.

As mentioned above, while this offers a potential method for handling BOG needed to provide propulsion and / or electricity, there remains the issue of handling such BOG when the required load does not match the BOG present.

Simply burning the BOG in a GCU is both wasteful and polluting.

Indeed, environmental regulations in many territories prohibit the handling of BOG in this way within close proximity to shore.

Furthermore, using LNG as a primary fuel source creates its own challenges.

However, as both vessels are stationary during this process, the BOG created cannot be used for propulsion and must be handled in some other manner.

A re-liquefaction plant is impractical for a relatively small vessel such as a bunker vessel due to its large energy requirements and, as mentioned above, burning off excess BOG is both wasteful and environmentally unsound.

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