Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1999-10-21
2001-03-13
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S619000, C062S639000
Reexamination Certificate
active
06199403
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a process for separating a multi-component feed stream using fractionation and producing a pressurized, refrigerated liquid product. More specifically, the invention relates to a process for separating a multi-component stream containing methane and at least one high volatility component having a relative volatility greater than that of methane and producing pressurized liquefied natural gas.
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.
Natural gas often contains diluent gases such as nitrogen and helium. The presence of these gases reduces the heating value of the natural gas. Also, certain of these gases may have independent commercial uses if they can be separated from the natural gas. Consequently, the separation of diluent gases from natural gas may have twofold economic benefit, namely, enhancement of the natural gas heating value and production of a marketable gas such as helium. LNG plants also remove the nitrogen from the natural gas because the nitrogen will not remain in the liquid phase during transport of conventional LNG, which is at or near atmospheric pressure.
In general, most known natural gas separation processes comprise at least three distinct operative steps or stages. These include (1) a preliminary gas treatment step for the removal of water and acidic gases such as carbon dioxide and hydrogen sulfide, (2) a natural gas liquids product separation step using low but non-cryogenic temperatures for the separation and recovery of the ethane and heavier hydrocarbon components, and (3) a nitrogen separation or rejection step, often referred to as Nitrogen Rejection Units (NRUs). The nitrogen rejection is generally effected by the cooling of the nitrogen-containing natural gas and fractionating it in a distillation column.
It has recently been proposed to produce methane-rich liquid having a temperature above about −112° C. (−170° F.) And a pressure sufficient for the liquid to be at or below its bubble point. This pressurized liquid natural gas is sometimes referred to as PANG to distinguish it from LNG which is at or near atmospheric pressure. The pressure of PANG will typically be above about 1,380 pa (200 Pisa). One of the advantages of a process for producing PANG is that pressurized liquefied natural gas can contain up to about 10 mole percent nitrogen. However, the nitrogen lowers the heating value of the PANG and increases the bubble point of the PANG product. There is therefore a need for an improved process for removing nitrogen from a natural gas stream and simultaneously producing PANG.
SUMMARY
The invention relates generally to a liquefaction process in which a feed stream containing methane and at least one high-volatility component, such as helium and nitrogen, that has a relative volatility greater than that of methane produces a pressurized liquefied product rich in methane that is substantially free of the higher volatility component. For illustrative purposes, it will be assumed that the more volatile component is nitrogen.
In this inventive process a liquefied, multi-component feed stream is fed to a hydraulic expander means such as one or more hydraulic turbines. The multi-component feed stream is rich in methane and has at least one high volatility component that has a relative volatility greater than that of methane. The feed stream is at or below the feed stream's bubble point and has a temperature above about −112°C. (−170° F.). The expander means reduces the pressure of the feed stream and cools the feed stream, producing gas and liquid phases during pressure reduction. From the expander means the liquid and vapor phases are fed to a separation system to separate the liquid and vapor phases. An overhead vapor stream, enriched in the volatile component, is withdrawn from the separation system. A portion of the overhead vapor stream is preferably withdrawn as a vapor product stream for use as fuel gas or for further processing. The remaining portion of the vapor stream is preferably condensed using either an internal or external refrigeration system. After being condensed, the liquid stream is preferably fed to an upper region of the separation system. A liquid stream rich in methane is recovered from the separation system and pumped to a higher pressure and heated, preferably by indirect heat exchange with the feed stream, to produce a pressurized liquefied product stream having a pressure sufficient for the product stream to be at or below its bubble point and having a temperature above about −112° C. (−170° F.). In the preferred embodiment, the heat exchange between the high-pressure methane-rich stream and the feed stream reduces the refrigeration requirements for the liquefaction process.
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Cole Eric T.
Stone Brandon T.
Capossela Ronald
ExxonMobil Upstream Research Company
Lawson Gary D.
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