Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1999-12-16
2001-07-10
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
Reexamination Certificate
active
06257017
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the handling of pressurized liquefied natural gas and, more particularly, to a process for producing a displacement gas suitable for unloading containers having pressurized liquefied natural gas contained therein.
BACKGROUND OF THE INVENTION
Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called “LNG”) for transport to market.
It has been recently proposed to transport natural gas at temperatures above −112° C. (−170° F.) and at pressures sufficient for the liquid to be at or below its bubble point temperature. For most natural gas compositions, the pressure of the natural gas at temperatures above −112° C. will be between about 1,380 kPa (200 psia) and about 4,500 kPa (650 psia). This pressurized liquid natural gas is referred to as PLNG to distinguish it from LNG, which is transported at near atmospheric pressure and at a temperature of about −162° C. (−260° F.).
If PLNG is unloaded from a container by pumping the PLNG out and allowing the container pressure to decrease, the decompression of the PLNG can lower the temperature in the container below the permitted design temperature for the container. If the pressure in the container is maintained as the PLNG is removed to avoid such temperature reduction, the vapor remaining in the container will contain a significant mass percentage of the container's original cargo. Depending upon the pressure and temperature of storage and the composition of the PLNG, the vapors may constitute from about 10 to 20 percent of the mass of PLNG in the container before the liquid was removed. It is desirable to remove as much of this gas as is economically possible while keeping the container at approximately the same temperature as the PLNG before unloading.
SUMMARY
This invention relates to a process for producing a displacement gas from a multi-component, pressurized, liquefied gas rich in methane to be used in displacing a pressurized liquefied gas from one or more containers. As a first step, the pressure of the multi-component, pressurized, liquefied gas is increased and it is then separated into a first fraction and a second fraction. The first fraction is expanded by a suitable expansion means to reduce its pressure, and the expanded fraction is passed to a separation means, such as a conventional separation means or a conventional fractionator, which produces a liquid stream and a gas stream. The second fraction is heated to convert it to a vaporous product stream. A portion of the vaporous product stream is withdrawn and the withdrawn portion is expanded to reduce its pressure and temperature. The expanded vaporous stream is then passed to the separation means. The remaining portion of the vaporous product stream is discharged from the process. The liquid product produced by the separation means is recycled by being combined with the multi-component, pressurized, liquefied gas before the multi-component, pressurized, liquefied gas is pressurized in the first step of this process. The gas stream produced by the separation means is expanded to reduce its pressure and temperature. This expanded gas stream is available for use as a displacement gas for displacing liquefied gas from one or more containers.
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Doerrler William
Drake Malik N.
ExxonMobil Upstream Research Company
Lawson Gary
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