Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
2000-05-18
2001-12-04
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C060S728000, C062S912000
Reexamination Certificate
active
06324867
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to a process and system for liquefying natural gas and in one aspect relates to a process and system for liquefying natural gas wherein the air to the power turbines used in the system is cooled by excess refrigeration from within the system to thereby improve the operating efficiency of the turbines and hence, the overall efficiency of the system.
2. Background
Most Liquid Natural Gas (“LNG”) plants constructed in the last 20 years or so have used industrial gas turbines to drive the refrigeration compressors required to liquefy the natural gas. Typically, these gas turbines have inlet air filters but do not include any means for cooling the inlet air to the turbines. It well known that the amount of power available from a gas turbine is, in part, a function of the inlet air temperature; see “The Refrigerated Gas and Vapor Turbine Cycle”, J. Hilbert Anderson and F. M. Laucks, 87-GT-15, ASME, 1987; “The Anderson Quin Cycle”, J. Hilbert Anderson and W. M. Bilbow, U.S. Department of Energy, Grant #DE-FG01-91CE15535, Final Report, Mar. 19, 1993; U.S. Pat. No. 4,418,527, issued Dec. 6, 1983.
Since the temperature and density of the inlet air changes with the ambient temperature, the amount of power available from a particular turbine varies from day to night and from summer to winter. This change in available power can be quite large; e.g. at times the power available during the hottest summer day can sometimes be less than about 70% of the power available during the coolest winter night. Also, the horsepower from the turbine needed to provide the required refrigeration in an LNG process increases as the heat sink temperature increases (i.e. seawater or air). Due to these varying factors, the gas turbines used in a typical LNG plant usually include gas turbines large enough to supply the required horsepower when operating at the warmest ambient temperatures even though they may only operate at these temperature for short periods of time. This means that most LNG plants have to be significantly overdesigned in order to insure that the required horsepower is always available regardless of the then current ambient temperature.
The effect that the temperature of the inlet air has on the power output of a gas turbine in an LNG process has been recognized. For example, the gas liquefaction system disclosed in U.S. Pat. No. 4,566,885, issued Jan. 28, 1986, is designed with gas turbines large enough to provide the necessary horsepower to liquefy LNG even when operating at the maximum (i.e. warmest) expected ambient temperature. If and when the ambient temperature cools off from this maximum temperature, the turbines can generate additional power which, in turn, can then be used to drive a generator to produce additional electrical power. While this system recovers some of the excess power from the turbines, it still requires unnecessarily large turbines which significantly add to the capital and maintenance costs involved.
Another LNG process in which the temperature of the inlet air to the turbines is used to improve the operation thereof is disclosed in U.S. Pat. No. 5,139,548, issued Aug. 18, 1992 wherein the ambient temperature of the inlet air is periodically predicted. Each predicted temperature is then used to optimize the operating conditions for the system, e.g. minimize the fuel consumption by the turbines at a given LNG production rate. Again, the turbines used in this system must be large enough to produce the horsepower needed at the warmest ambient temperature even though some of the costs can be recouped by reducing the fuel consumption during cooler periods.
While cooling the inlet air for turbines is used in a variety of known commercial operations, e.g. electrical power generation, air conditioning, ice making, etc.; see U.S. Pat. Nos. 5,203,161; 5,321,944; 5,444,971; 5,457,951; 5,622,044; 5,626,019; 5,666,800; 5,758,502; and 5,806,298, it has not found use in gas liquefaction processes such as LNG processes wherein the costs of the turbines required to furnish the power in such operations is a significant factor in the capital and operating costs of the system. With LNG becoming a more important energy source each year, there exists a real need for improving the efficiency of the LNG processes and reducing their costs in order to deliver LNG to market at a competitive price.
SUMMARY OF THE INVENTION
The present invention provides a natural gas liquefaction (LNG) system and process wherein excess refrigeration available in a typical LNG system is used to cool the inlet air to the gas turbines in the system thereby improving the overall efficiency of the system. By maintaining the inlet air for the gas turbines at a constant low temperature, the amount of power generated by the turbines remains at a high level regardless of the ambient air temperature. This allows a LNG plant to be designed for more capacity and allows the plant to operate at a constant production rate throughout the year. Further, since the present invention utilizes the propane circuit which is already present in LNG systems of this type, no additional cooling source is required to carry out the invention.
More specifically, the present invention is especially useful in LNG systems which use first and second closed circuits of first and second refrigerants to cool a feed gas to the low temperatures needed for liquefaction. The first closed circuit carries a first refrigerant (e.g. propane) which is used to initially cool the feed gas (e.g. natural gas). This first circuit, in addition to the necessary heat exchanger needed for cooling the feed gas, also includes a first gas turbine which drives a first compressor which, in turn, compresses and circulates the propane through the first closed circuit. The second closed refrigerant circuit carries a mixed refrigerant “MR” (e.g. nitrogen, methane, ethane, and propane) for further cooling the feed gas to the final low temperature required to produce LNG. The mixed refrigerant is compressed and circulated through the second closed refrigerant circuit by a second compressor which is driven by a second gas turbine.
In accordance with the present invention, the above described LNG system further includes a means for cooling the inlet air to the respective gas turbines. This means is comprised of (1) a cooler positioned in front of the air inlet of each of the respective gas turbines and (2) a closed coolant circuit which is in fluid communication with each of the coolers. Coolant (e.g. water) flows through each of the coolers to cool the ambient air as air flows therethrough and into the turbines.
The closed coolant circuit includes a heat exchanger which is fluidly connected to the first refrigerant circuit whereby at least a portion of the propane in the first refrigerant circuit will flow through the heat exchanger to cool the water in said closed coolant circuit. Preferably, the ambient air is cooled to a temperature no lower than about 5° C. (41° F.) in order to prevent icing in the system. An anti-freeze agent (e.g. ethylene glycol) with corrosion inhibitors can be added to the water as needed.
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“The Refrigerate
Fanning Robert A.
Phan Luan D.
Ryberg Brett L.
Smith Bruce K.
Capossela Ronald
ExxonMobil Oil Corporation
Faulconer Drude
Hoefling Marcy
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