Reliquefaction of compressed vapor

Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction

Reexamination Certificate

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C062S614000

Reexamination Certificate

active

06449983

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for the reliquefaction of a compressed vapour, particularly a method and apparatus which are operable on board ship to reliquefy natural gas vapour.
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, and warming the expanded working fluid in indirect heat exchange with the compressed working fluid, and returning the warmed working fluid to one of the compressors. The natural gas vapour, downstream of a compression stage, is at least partially condensed by indirect heat exchange with the working fluid being warmed. One example of an apparatus for performing such a refrigerant method is disclosed in U.S. Pat. No. 3,857,245.
According to U.S. Pat. No. 3,857,245 the working fluid is derived from the natural gas itself and therefore an open refrigeration cycle is operated. The expansion of the working fluid is performed by a valve. Partially condensed natural gas is obtained.
The partially condensed natural gas is separated into a liquid phase which is returned to storage and a vapour phase which is mixed with natural gas being sent to a burner for combustion. The working fluid is both warmed and cooled in the same heat exchanger so that only one heat exchanger is required. The heat exchanger is located on a first skid-mounted platform and the working fluid compressors on a second skid-mounted platform.
Nowadays, it is preferred to employ a non-combustible gas as the working fluid. Further, in order to reduce the work of compression that needs to supplied externally, it is preferred to employ an expansion turbine rather than a valve in order to expand the working fluid.
An example of an apparatus which embodies both these improvements is given in WO-A-98/43029. Now two heat exchangers are used, one to warm the working fluid in heat exchange with the compressed natural gas vapour to be partially condensed, and the other to cool the compressed working fluid. Further, the working fluid is compressed in two separate compressors, one being coupled to the expansion turbine.
WO-A-98/43029 points out that incomplete condensation of the natural gas vapour reduces the power consumed in the refrigeration cycle (in comparison with complete condensation) and suggests that the residual vapour—which is relatively rich in nitrogen—should be vented to the atmosphere. Indeed, the partial condensation disclosed in WO-A-98/43029 follows well known thermodynamic principles which dictate that the condensate yield is purely a function of the pressure and temperature at which the condensation occurs.
Typically, the liquefied natural gas may be stored at a pressure a little above atmospheric pressure and the boil-off vapour may be partially condensed at a pressure of 4 bar. The resulting partially condensed mixture is typically flashed through an expansion valve into a phase separator to enable the vapour to be vented at atmospheric pressure. Even if the liquid phase entering the expansion valve contains as much as 10 mole per cent of nitrogen at 4 bar, the resulting vapour phase at 1 bar still contains in the order of 50% by volume of methane. In consequence, in a typical operation, some 3000 to 5000 kg of methane may need to be vented daily from the phase separator. Since methane is recognised as a greenhouse gas such a practice would be environmentally unacceptable.
It is therefore desirable to return any flash gas and any uncondensed vapour to the LNG storage tanks of the ship with the condensate. The return of vapour to the storage tanks would in turn tend to enhance the mole fraction of nitrogen in the ullage space of the storage tanks and thereby give rise to two disadvantages. First, as the concentration of nitrogen in the boil-off gas rises, so more work needs to be performed to condense a given proportion of the boil-off gas. Second, variations in the composition of the boil-off gas make the refrigeration cycle more difficult to control.
The method and invention according to the invention are aimed at mitigating the problems that are caused when vapour is returned with condensed natural gas to a liquefied natural gas (LNG) storage tank.
SUMMARY OF THE INVENTION
According to the present invention a method of reliquefying vapour boiled off from liquefied natural gas held in a storage tank comprising compressing the vapour, at least partially condensing the compressed vapour, and returning the condensate to the storage tank, wherein the boiled off vapour is mixed upstream of the compression with liquefied natural gas.
The invention also provides apparatus for reliquefying vapour boiled-off from liquefied natural gas held in a storage tank comprising, the apparatus comprising a flow circuit comprising a vapour path extending from the tank through a compressor to a condenser for at least partially condensing compressed boiled-off vapour and a condensate path extending from the condenser back to the storage tank, wherein the apparatus additionally comprises a conduit for the flow of liquefied natural gas into at least one mixer forming part of the flow circuit upstream of (i.e. on the suction side of) the compressor.
Preferably, the flow of liquefied natural gas is taken from storage, or from the condensate itself en route to storage.
There are various advantages given by the method and apparatus according to the invention. In particular since the nitrogen mole fraction in the liquefied natural gas is less than the nitrogen mole fraction in the boiled-off vapour and even less than that in flash gas formed by the expansion through the valve of the condensed boil-off vapour, dilution of the boiled-off vapour with the liquefied natural gas tends to dampen swings in the composition of the vapour phase in the storage tank that would otherwise occur were the characterising feature of the method and apparatus according to the invention to be omitted. Dilution of the vapour upstream of the compressor makes it possible to reduce fluctuations in the work of compression arising from fluctuations in the temperature of the vapour. These fluctuations arise mainly from changes in the loading of the storage tanks. Preferably, the inlet temperature of the boiled-off vapour to the compressor is maintained substantially constant. If desired, there is an absorber of liquid droplets at a position upstream of the inlet to the compressor so as to remove any residual droplets of liquid hydrocarbon arising from the mixing of the vapour with the liquefied natural gas at the second location though generally this measure will not be necessary. Mixing upstream of the compression is particularly important when the storage tank is only lightly laden with LNG, for example after the main part of the LNG has been off-loaded. During normal operation however, it is preferred to perform the mixing with a stream of LNG that is diverted from the condensation path. It then becomes unnecessary to employ any mechanical pump to withdraw LNG from storage for the purposes of temperature control.
There are a number of different preferred additional locations for effecting the mixing of the boiled-off vapour or its condensate with the liquefied natural gas. A first preferred additional location is downstream of the boiled-off vapour compressor but upstream of the inlet to the condenser for the vapour. Preferably, the mixing at this loca

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