Method and apparatus for the reliquefaction of a vapour

Abstract

Boiled off liquefied natural gas flows from a storage tank and is compressed in vapour compression stages. The resulting compressed vapour is condensed in a condenser and the condensate returned to the tank. The condenser is cooled by a working fluid, for example nitrogen, flowing in a Brayton cycle. The Brayton cycle includes a heat exchanger which removes heat of compression from the compressed natural gas vapour upstream of its passage through the condenser. In addition a part of the working fluid is withdrawn from a region of the Brayton cycle intermediate the working fluid outlet from the condenser and the working fluid inlet to the heat exchanger and the withdrawn working fluid flows through another heat exchanger in which it removes heat of compression from the natural gas vapour intermediate the compression stages. The withdrawn working fluid is returned to the Brayton cycle.

Description

[0001]This invention relates to a method and apparatus for the reliquefication of a vapour, particularly a method and apparatus which are operable on board ship to reliquefy natural gas vapour.[0002]Natural gas is conventionally transported over large distances in liquefied state. For example, ocean going tankers are used to convey liquefied natural gas from a first location in which the natural gas is liquefied to a second location in which it is vaporised and sent to a gas distribution system. Since natural gas liquefies at cryogenic temperatures, i.e.. temperatures below −100° C., there will be continuous boil-off of the liquefied natural gas in any practical storage system. Accordingly, apparatus needs to be provided in order to reliquefy the boiled-off vapour. In such an apparatus a refrigeration cycle is performed comprising compressing a working fluid in a plurality of compressors, cooling the compressed working fluid by indirect heat exchange, expanding the working fluid, an...

AI Technical Summary

Benefits of technology

[0017]Preferably when the said storage tank is fully laden with liquefied natural gas, the condenser is operated such that sub-cooled liquefied natural gas exits from it. Sometimes, however, when the said storage tank contains only a relatively small amount of liquefied natural gas return of the condensate to the tank has the effect of enriching the boiled-off vapour in nitrogen. In consequence the vapour presented to the condenser for condensation may contain an excess of nitrogen with the consequence that not only is the condensate not sub-cooled bu

Problems solved by technology

Since methane is recognised as a greenhouse gas such a practice would be environmentally unacceptable.

Another problem associated with the operation of the apparatus according to WO-A-98/43029, is that there are considerable thermodynamic inefficiencies caused by a mismatch between, on the one hand, the temperature and enthalpy of the compressed natural gas and, on the other hand, the temperature and enthalpy of the working fluid.

Since the nitrogen mole fraction in the liquefied natural gas is less than the nitrogen mole fraction in the boiled-off vapour and

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