The present invention relates to processes and systems for liquefying natural gas. In one aspect the invention relates to such processes and systems wherein common compression string(s) are used to compress and recycle the refrigerants used in a plurality of individual trains which, in turn, are used for liquefying natural gas.
Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
Large volumes of natural gas (i.e. primarily methane) are located in remote areas of the world. This gas has significant value if it can be economically transported to market. Where the gas reserves are located in reasonable proximity to a market and the terrain between the two locations permits, the gas is typically produced and then transported to market through submerged and/or land-based pipelines. However, when gas is produced in locations where laying a pipeline is infeasible or economically prohibitive, other techniques must be used for getting this gas to market.
A commonly used technique for non-pipeline transport of gas involves liquefying the gas at or near the production site and then transporting the liquefied natural gas to market in specially-designed storage tanks aboard transport vessels. The natural gas is cooled and condensed to a liquid state to produce liquefied natural gas at substantially atmospheric pressure and at temperatures of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.) (xe2x80x9cLNGxe2x80x9d), thereby significantly increasing the amount of gas which can be stored in a particular storage tank. Once an LNG transport vessel reaches its destination, the LNG is typically off-loaded into other storage tanks from which the LNG can then be revaporized as needed and transported as a gas to end users through pipelines or the like.
As will be understood by those skilled in the art, plants used to liquefy natural gas are typically built in stages as the supply of feed gas, i.e. natural gas, and the quantity of gas contracted for sale, increase. Each stage normally consists of a separate, stand-alone unit, commonly called a train, which, in turn, is comprised of all of the individual components necessary to liquefy a stream of feed gas into LNG and send it on to storage. As used hereinafter, the term xe2x80x9cstand-alone trainxe2x80x9d means a unit comprised of all of the individual components necessary to liquefy a stream of feed gas into LNG and send it on to storage. As the supply of feed gas to the plant exceeds the capacity of one stand-alone train, additional stand-alone trains are installed in the plant, as needed, to handle increasing LNG production.
In typical LNG plants, each stand-alone train includes at least a cryogenic heat exchange system for cooling the gas to a cryogenic temperature, a separator (i.e. a xe2x80x9cflash tankxe2x80x9d), a xe2x80x9creject gasxe2x80x9d heat exchanger, and a fuel gas compressor. As used herein, a xe2x80x9ccryogenic temperaturexe2x80x9d includes any temperature of about xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.) and lower. LNG is typically stored at substantially atmospheric pressure and at temperatures of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.). To reduce the pressure of feed gas during liquefaction, it is typically passed from the cryogenic heat exchange system across an expansion valve or hydraulic turbine in a stand-alone train (i.e. xe2x80x9cflashedxe2x80x9d) before it is passed into the separator (i.e. the flash tank). As the pressure of the cooled feed gas is reduced to produce LNG, some of the gas flashes and becomes vapor. LNG is removed from the flash tank and is pumped from its respective stand-alone train on to a storage tank for further handling.
In somewhat greater detail, each stand-alone train is comprised of a cryogenic heat exchange system which, in turn, utilizes two or more refrigerant circuits, acting in series, to cool the feed gas down to the cryogenic temperature needed for liquefaction. Typically, the first circuit carries a first refrigerant (e.g. propane) which is compressed by a first compression string in the stand-alone train and is circulated through a series of primary heat exchangers to heat exchange with and initially cool the feed gas. Typically, the second refrigerant circuit carries a second refrigerant, e.g., a mixed refrigerant xe2x80x9cMRxe2x80x9d (e.g. nitrogen, methane, ethane, and propane) which is compressed by a second compression string in the stand-alone train and is circulated first through a series of propane heat exchangers and then through a main cryogenic heat exchanger to thereby complete the cooling of the feed gas to produce the LNG. In some cases, the cryogenic heat exchange system utilizes a cascade refrigeration system, a dual mixed refrigerant system, or some other refrigeration system, as will be familiar to those skilled in the art.
In some cases, the economics of an LNG plant may be improved by driving the compressors in both the first and second compression strings through one or more common shafts. However, this does not overcome all of the disadvantages associated with each stand-alone train in an LNG plant requiring its own dedicated, compression strings. For example, a complete stand-alone train, including two or more compression strings, must be installed in a plant each time it becomes desirable to expand the LNG plant production capacity, which can add significantly to the capital and operating costs of the plant. Further, if any refrigerant compressor, or its driver (e.g., a gas turbine) fails, in a particular stand-alone train, the affected stand-alone train must be shut down until the failed compressor and/or driver can be repaired. LNG production at the plant is significantly reduced during the down time. Still further, anytime a stand-alone train is shut down due to failure of a compression string, the temperature in the main cryogenic heat exchanger of that stand-alone train will rise substantially thereby requiring xe2x80x9crecoolingxe2x80x9d of the main heat exchanger to the cryogenic temperature before the train can be put back into production.
It is desirable to improve processes and systems for liquefying natural gas to lower the costs of LNG production as much as possible so that LNG can continue to be delivered to market at a competitive price.
The present invention provides natural gas liquefaction systems and processes wherein a first refrigerant and a second refrigerant are treated as a utility, and are supplied from a common source to a plurality of dependent trains in an LNG plant. This allows the dedicated, compression strings, which are normally found in each stand-alone train of a multi-train LNG plant, to be replaced by common compression strings which, in turn, supply the refrigerants to more than one dependent train in the plant. As used hereinafter, the term xe2x80x9cdependent trainxe2x80x9d includes any unit in an LNG plant that lacks its own, dedicated compression string.
More specifically, the present invention relates to an LNG system that is comprised of two or more dependent trains, each of which converts a feed gas into LNG. Each dependent train includes at least a first refrigerant circuit and a second refrigerant circuit, in series, which cool the feed gas to the cryogenic temperature needed for LNG. The first refrigerant (e.g. propane) flows through a series of primary heat exchangers in the first refrigerant circuit to initially cool the feed gas. A second refrigerant (e.g. mixed refrigerant comprised of nitrogen, methane, ethane, and propane) flows through a cryogenic heat exchange system, comprised of one or more individual heat exchangers, in the second refrigerant circuit to further cool the gas and convert it into LNG. This invention is applicable to other types of cryogenic heat exchange systems, including without limitation those with cascade refrigeration systems that use two or more refrigeration systems, those with a dual mixed refrigerant system, or those with some other refrigeration system, as will be familiar to those skilled in the art. For example, without limiting the scope of this invention, this invention is applicable to cascade refrigeration systems with three refrigeration loops in which the refrigeration from one stage is used to condense the compressed refrigerant in the next stage.
In dependent trains of the present invention, dedicated compression strings for circulating desired refrigerants through their respective circuits are not required. Instead, a set of common compression strings are provided in the present system to supply refrigerants from a common source to more than one of the dependent trains in the LNG plant.
If more than one set of common compression strings are required due to the increasing size of an LNG plant (i.e. number of dependent trains to be serviced), a plurality of first compression strings are provided and manifolded together so that compressed first refrigerant from the first compression strings can be directed to various dependent trains as needed. Likewise, a plurality of second compression strings can be manifolded together whereby the second refrigerant from the second compression strings can be directed to various dependent trains as needed.
It will be recognized that by treating all of the refrigerants in an LNG plant as a utility (i.e. a single first refrigerant supply, a single second refrigerant supply, etc.) and by using independent, common compression strings to supply the refrigerants to the respective refrigerant circuits in a plurality of dependent trains, a significant number of benefits will be realized.