Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1999-12-16
2001-05-29
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
Reexamination Certificate
active
06237364
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for producing pressurized methane-rich liquid from a methane-rich gas and, more particularly, to a process for producing pressurized liquid natural gas (PLNG) from 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.
One of the distinguishing features of a LNG plant is the large capital investment required for the plant. The equipment used to liquefy natural gas is generally quite expensive. The liquefaction plant is made up of several basic systems, including gas treatment to remove impurities, liquefaction, refrigeration, power facilities, and storage and ship loading facilities.
LNG refrigeration systems are expensive because so much refrigeration is needed to liquefy natural gas. A typical natural gas stream enters a LNG plant at pressures from about 4,830 kPa (700 psia) to about 7,600 kPa (1,100 psia) and temperatures from about 20° C. (68° F.) to about 40° C. (104° F.). Natural gas compositions at atmospheric pressure will typically liquefy in the temperature range between about −165° C. (−265° F.) and −155° C. (−247° F.). This significant reduction in temperature requires substantial refrigeration duty.
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. (−170° F.) 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.). The production of PLNG requires significantly less refrigeration than that required for the production of LNG since PLNG can be more than 50° C. warmer than conventional LNG at atmospheric pressure. Examples of processes for manufacturing PLNG are disclosed in U.S. patent applications Ser. No. 09/099262, 09/099590, and 09/099589 and in U.S. provisional application No. 60/079642. In view of the substantial economic benefits associated with making and transporting PLNG, a continuing need exists for improved processes for producing PLNG.
SUMMARY
An improved process is disclosed for producing from a pressurized methane-rich gas stream a pressurized methane-rich liquid stream having a temperature above −112° C. (−170° F.) and having a pressure sufficient for the liquid to be at or below its bubble point. In this process, a methane-rich liquid stream having a temperature below about −155° C. (−247° F.) is supplied and its pressure is increased. A pressurized methane-rich gas to be liquefied is supplied and introduced to the pressurized methane-rich liquid stream at a rate that produces a methane-rich liquid stream having a temperature above −112° C. (−170° F.) and a pressure sufficient for the liquid to be at or below its bubble point.
In a preferred embodiment, a pressurized liquid natural gas (PLNG) is produced by supplying LNG having a pressure near atmospheric pressure and pumping the LNG to the desired pressure of PLNG to be produced by the process. Natural gas is supplied to the process and the pressure is adjusted either up or down, if needed, to be at essentially the same pressure as the pressurized LNG. Depending on the available pressure of the natural gas, its pressure can be increased by a compression means or decreased by an expansion device such as a Joule-Thomson valve or turboexpander. The pressurized natural gas is then mixed with the pressurized LNG at a rate that produces PLNG having a temperature above −112° C. (−170° F.) and a pressure sufficient for the resulting liquid to be at or below its bubble point. The natural gas may optionally be cooled before it is mixed with the pressurized PLNG by any suitable cooling means. For example, the natural gas may be cooled by indirect heat exchange with an external cooling medium, by an expansion device that reduces the pressure of the natural gas, or by heat exchange with the pressurized LNG. The mixture produced by the mixing of the pressurized LNG and the pressurized natural gas may optionally be passed through a phase separator to remove any gas that remains unliquefied after the mixing. The liquid withdrawn from the separator is then passed to a suitable storage means for storage at a temperature above −112° C. (−170° F.) and a pressure sufficient for it to be at or below its bubble point.
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Doerrler William
Drake Malik N.
ExxonMobil Upstream Research Company
Lawson Gary D.
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