Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1999-10-21
2001-05-01
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S619000, C062S048200
Reexamination Certificate
active
06223557
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a process for liquefying a multi-component feed stream using cryogenic fractionation. More specifically, the invention relates to a process to liquefy a natural gas stream containing a component more volatile than methane to produce pressurized liquefied natural gas (PLNG) that is lean in the more volatile component.
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 PLNG to distinguish it from LNG which is at or near atmospheric pressure. The pressure of PLNG will typically be above about 1,380 kPa (200 psia). One of the advantages of a process for producing PLNG is that pressurized liquefied natural gas can contain up to about 10 mole percent nitrogen. However, the nitrogen lowers the heating value of the PLNG and increases the bubble point of the PLNG product. There is therefore a need for an improved process for removing nitrogen from a pressurized natural gas stream and simultaneously producing PLNG.
SUMMARY
The invention relates generally to a separation process in which a pressurized 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. For illustrative purposes, it will be assumed that the primary separation is between N
2
and CH
4
.
In the preferred embodiment of this invention, a process is disclosed for separating nitrogen from a nitrogen-containing, pressurized natural gas to produce a pressurized liquid natural gas that is lean in nitrogen and having a temperature above about −112° C. (−170° F.). The pressurized natural gas feed stream is passed to a fractionation column at a pressure above about 1,380 kPa (250 psia). The pressure of the feed natural gas is preferably above about 4,137 kPa (600 psia) and it is expanded by a suitable expansion means to a lower pressure prior to being passed to the fractionation column. The fractionation column produces a first liquid stream that is lean in nitrogen and a first vapor stream that has enhanced nitrogen content. The vapor stream is then cooled to produce a vapor phase and a liquid phase. The vapor and liquid phases are then phase separated to produce a second vapor stream and a second liquid stream. The second liquid stream is returned to the fractionation column as reflux. The second vapor stream is preferably used to cool the incoming feed stream. The first liquid is removed from the fractionation system as a product stream lean in nitrogen and having a temperature above about −112° C. (−170° F.) and a pressure sufficient for the liquid product to be at or below its bubble point.
Optionally, the feed stream is separated into a first feed stream and a second feed stream. The first feed stream is cooled by indirect heat exchange with a process-derived stream from a fractionation column. The second feed stream is cooled by indirect heat exchange with a process-derived liquid from the fractionation column. The first and the second feed streams are then combined and passed to the fractionation column.
One advantage of the present invention is that pressurized liquid product can be produced that is lean in nitrogen with only one fractionation column without having to reduce to fractionation column to need atmospheric pressure which is the conventional practice for removing nitrogen from liquefied natural gas.
BRIEF DESCRIPTION OF THE DRAWING
The present invention and its advantages will be better understood by referring to the following detailed description and the attached drawing which is a schematic flow diagram of one embodiment of this invention. The drawing is not intended to exclude from the scope of the invention other embodiments which are the result of normal and expected modifications of the embodiment disclosed in the drawing. Various required subsystems such as valves, flow stream mixers, control systems, and sensors have been deleted from the drawing for the purposes of simplicity and clarity of presentation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
It has been discovered that a pressurized natural gas stream containing methane and a relatively volatile component such as nitrogen can be cryogenically separated with only minimal need for auxiliary cryogenic refrigeration to produce a pressurized liquefied natural gas that is substantially free of nitrogen without reducing the pressure to near atmospheric pressure.
In accordance with this discovery, the present invention provides a process for separation of pressurized liquefied natural gas containing methane and at least one high volatility component, such as helium and nitrogen. The separation process produces a pressurized liquid natural gas that is substantially free of the high volatility component and that has a temperature above about −112° C. (−170° F.) and a pressure sufficient for the liquid product to be at or below its bubble point. This methane-rich product is sometimes referred to in this description as pressurized liquid natural gas (“PLNG”).
The term “bubble point” as used in this description is the temperature and pressure at which a liquid begins to convert to gas. For example, if a certain volume of PLNG is held at constant pressure, but its temperature is increased, the temperature at which bubbles of gas begin to form in the PLNG is the bubble point. Similarly, if a certain volume of PLNG is held at constant temperature but the pressure is reduced, the pressure at which gas begins to form defines the bubble point. At the bubble point, the liquefied gas is saturated liquid.
The first consideration in cryogenic processing of natural gas is contamination. The raw natural gas feed stock suitable for the process of this invention may comprise natural gas obtained from a crude oil well (associated gas) or from a gas well (non-associated gas). The composition of natural gas can vary significantly. As used herein, a natural gas stream contains methane (C
1
) as a major component. The natural gas will typically also contain ethane (
Capossela Ronald
ExxonMobil Upstream Research Company
Lawson Gary D.
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