Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1999-10-21
2001-02-27
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S619000, C062S623000, C062S048200
Reexamination Certificate
active
06192705
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to an improved process for reliquefaction of pressurized boil-off gas from 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.
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. to about 40° C. Natural gas, which is predominantly methane, cannot be liquefied by simply increasing the pressure, as is the case with heavier hydrocarbons used for energy purposes. The critical temperature of methane is −82.5° C. This means that methane can only be liquefied below that temperature regardless of the pressure applied. Since natural gas is a mixture of gases, it liquefies over a range of temperatures. The critical temperature of natural gas is typically between about −85° C. and −62° C. Natural gas compositions at atmospheric pressure will typically liquefy in the temperature range between about −165° C. and −155° C. Since refrigeration equipment represents such a significant part of the LNG facility cost, considerable effort has been made to reduce refrigeration costs.
Many systems exist in the prior art for the liquefaction of natural gas by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the gas liquefies. Conventional liquefaction cools the gas to a temperature of about −160° C. at or near atmospheric pressure. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, and methane. Although many refrigeration cycles have been used to liquefy natural gas, the three types most commonly used in LNG plants today are: (1) “cascade cycle” which uses multiple single component refrigerants in heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature, (2) “expander cycle” which expands gas from a high pressure to a low pressure with a corresponding reduction in temperature, and (3) “multi-component refrigeration cycle” which uses a multi-component refrigerant in specially designed exchangers. Most natural gas liquefaction cycles use variations or combinations of these three basic types.
One proposal for reducing refrigeration costs is to produce liquefied natural gas at temperatures above −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 referred to as PLNG to distinguish it from LNG which is at or near atmospheric pressure. PLNG requires significantly less refrigeration since PLNG can be more than 50° C. warmer than conventional LNG. For most natural gas compositions, the pressure of the PLNG ranges between about 1,380 kPa (200 psia) and about 3,450 kPa (500 psia). In the storage, transportation, and handling of PLNG, there can be a considerable amount of “boil-off.” A needs exist for a process for re-liquefying PLNG boil-off gas to again produce PLNG and at the same time having power requirements that are economic.
SUMMARY
This invention relates to a process for reliquefying pressurized boil-off gas produced by pressurized liquid natural gas. In this process, refrigeration duty is provided to a heat exchanger by means of a refrigeration cycle, preferably a closed-cycle refrigeration system have mixed refrigerants as the cooling medium. Pressurized natural gas is fed through the heat exchanger, which at least partially liquefies the natural gas. The natural gas is then expanded to a lower pressure to produce a liquid stream having a temperature above about −112° C. (−170° F.) and having a pressure sufficient for the liquefied stream to be at or below its bubble point. The liquid stream is then passed to a first phase separator to remove from the liquid stream any vapors that may exist after the expansion step. A boil-off vapor to be reliquefied is passed through the heat exchanger, thereby providing refrigeration duty to the heat exchanger for cooling the feed natural gas and warming the incoming boil-off gas. The boil-off gas is then compressed and cooled before being recycled back through the heat exchanger for further cooling of the boil-off gas. The compressed, cooled boil-off gas is then expanded to a lower pressure and passed to a second phase separator. The second phase separator produces a vapor stream and a liquid stream. The vapor stream produced by the second separator is removed from the process for further use preferably as pressurized fuel, and more preferably the removal for use as fuel occurs after the vapor stream has passed through the heat exchanger for warming of fuel. The liquid stream produced by the second phase separator is passed to the first phase separator to produce a pressurized product stream having a temperature above about −112° C. and a pressure sufficient for the liquid to be at or below its bubble point.
An advantage of this process is that vapors produced by loading of ships and other storage containers with PLNG can be liquefied with minimal recompression of the vapors. This process also reduces the total compression required by the recovering for use as fuel a portion of the vapor to be reliquefied. This is advantageous since the vapor portion removed as fuel contains a significantly higher concentration of nitrogen than the nitrogen concentration in the liquefied gas product. Removal of nitrogen from the process in accordance with this invention requires up to seven percent less overall compression for the liquefaction plant than would be required if the nitrogen was not removed and all of the vapor was liquefied.
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pate
Capossela Ronald
ExxonMobil Upstream Research Company
Lawson Gary D.
LandOfFree
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