Patent Description:
A cold refrigerant produced at a different location, such as liquefied nitrogen gas ("LIN"), can be used to liquefy natural gas. For example, <CIT> describes shipping liquid nitrogen or liquid air from a market place to a field site where it is used to liquefy natural gas. The LNG is shipped back to the market site in the tanks of the same cryogenic carrier used to transport the liquefied nitrogen or air to the field site. Regasification of the LNG is carried out at the market site, where the excess cold from the re-gasification process is used to liquefy nitrogen or air for shipping to the field site.

However, since the natural gas from the regasification of LNG must be at a higher pressures (e.g., greater than <NUM> bar (<NUM> psi)) for introduction into the gas sales pipeline, the total energy needed for both the production of LIN and the re-pressurization of natural gas can be significantly greater than the energy needed to produce LNG using conventional processes. Therefore, there is a need to develop more energy efficient methods to produce LIN and high pressure natural gas from the regasification of LNG.

Furthermore, the process of <CIT> requires the integration of the complete LNG value chain. That is, there must be integration of the production of LNG using LIN as the cold refrigerant, the shipping of LIN to the natural gas resource location, the shipping of LNG to regasification locations, and the production of LIN using the available exergy from the regasification of LNG. This value chain is further described in <CIT> and<CIT>.

The production of LNG at the gas resource site using LIN as the sole refrigerant may require a LIN to LNG ratio of greater than <NUM>:<NUM>. For this reason, the production of LIN at the regasification site favors a greater than <NUM>:<NUM> LIN to LNG ratio in order to ensure that only the LNG produced using the LIN is then required to liquefy the needed amount of nitrogen. The matching of the LIN to LNG ratio at both the LNG plant and the regasification plant allows for an easier integration of the LNG value chain since LNG from additional production sources is not needed.

<CIT> describes a process where the vaporization of LNG is used to produce LIN, where the LIN to LNG ratio that is used is greater than <NUM>:<NUM>. In <CIT> the LNG is vaporized close to atmospheric pressure. Therefore, since the standardized pressure at which LNG must be when entering the gas sales pipeline is greater than <NUM> bar (<NUM> psi), a significant amount of energy is required to compress the natural gas to pipeline pressure. As such, there is a need for a method which allows pumping the LNG to higher pressures prior to vaporization in order to minimize the required amount of natural gas compression.

<CIT> and <CIT> and <CIT> describe methods where LNG is first pressurized to the pipeline transport pressure prior to vaporization of the LNG. In these disclosures, the vaporizing LNG is used to condense the nitrogen gas and is used as the interstage coolant for the multistage compression of the nitrogen gas to a pressure of at least <NUM> bar (<NUM> psi). The interstage cooling of the nitrogen gas using the vaporizing and warming of the natural gas allows for cold compression of the nitrogen gas which significantly reduces its energy of compression. However, in these disclosures a LIN to LNG ratio of less than <NUM>:<NUM> is used to produce the LIN and high pressure natural gas. This low LIN to LNG ratio does not allow for point-to-point integration of the regasification plant with the LNG plant, since a LIN to LNG ratio of at least <NUM>:<NUM> is typically required to produce LNG using LIN as the sole refrigerant.

<CIT> describes a method where LNG from multiple production sources are used to produce the LIN needed for LNG production at one production site. However, this multi-source LNG value chain arrangement significantly complicates the LNG value chain.

Therefore, there remains a need to develop an energy efficient method for producing LIN and high pressure natural gas from the regasification of LNG. There is further a need for an integrated method that is able to utilize a LIN to LNG ratio that is greater than <NUM>:<NUM>, or more preferably greater than <NUM>:<NUM>.

Other background references include <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, and <CIT>.

The present invention is concerned with a method for producing a liquefied nitrogen gas (LIN) stream at a liquid natural gas (LNG) regasification facility as defined in claim <NUM>.

Various specific embodiments and versions of the present invention will now be described, including preferred embodiments and definitions that are adopted herein. While the following detailed description gives specific preferred embodiments, those skilled in the art will appreciate that these embodiments are exemplary only, and that the present invention can be practiced in other ways. Any reference to the "invention" may refer to one or more, but not necessarily all, of the embodiments defined by the claims. The use of headings is for purposes of convenience only and does not limit the scope of the present invention.

All numerical values within the detailed description and the claims herein are modified by "about" or "approximately" the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

As used herein, "auto-refrigeration" refers to a process whereby a fluid is cooled via a reduction in pressure. In the case of liquids, auto-refrigeration refers to the cooling of the liquid by evaporation, which corresponds to a reduction in pressure. More specifically, a portion of the liquid is flashed into vapor as it undergoes a reduction in pressure while passing through a throttling device. As a result, both the vapor and the residual liquid are cooled to the saturation temperature of the liquid at the reduced pressure. For example, auto-refrigeration of a natural gas may be performed by maintaining the natural gas at its boiling point so that the natural gas is cooled as heat is lost during boil off. This process may also be referred to as a "flash evaporation.

As used herein, the term "compressor" means a machine that increases the pressure of a gas by the application of work. A "compressor" or "refrigerant compressor" includes any unit, device, or apparatus able to increase the pressure of a gas stream. This includes compressors having a single compression process or step, or compressors having multi-stage compressions or steps, or more particularly multi-stage compressors within a single casing or shell. Evaporated streams to be compressed can be provided to a compressor at different pressures. Some stages or steps of a cooling process may involve two or more compressors in parallel, series, or both. The present invention is not limited by the type or arrangement or layout of the compressor or compressors, particularly in any refrigerant circuit.

As used herein, "cooling" broadly refers to lowering and/or dropping a temperature and/or internal energy of a substance, such as by any suitable amount. Cooling may include a temperature drop of at least about <NUM>, at least about <NUM>, at least about <NUM>, at least about <NUM>, at least about <NUM>, at least about <NUM>, or least about <NUM>, or at least about <NUM>, or at least about <NUM>, or at least about <NUM>, or at least about <NUM>. The cooling may use any suitable heat sink, such as steam generation, hot water heating, cooling water, air, refrigerant, other process streams (integration), and combinations thereof. One or more sources of cooling may be combined and/or cascaded to reach a desired outlet temperature. The cooling step may use a cooling unit with any suitable device and/or equipment. According to some embodiments, cooling may include indirect heat exchange, such as with one or more heat exchangers. In the alternative, the cooling may use evaporative (heat of vaporization) cooling and/or direct heat exchange, such as a liquid sprayed directly into a process stream.

As used herein, the term "expansion device" refers to one or more devices suitable for reducing the pressure of a fluid in a line (for example, a liquid stream, a vapor stream, or a multiphase stream containing both liquid and vapor). Unless a particular type of expansion device is specifically stated, the expansion device may be (<NUM>) at least partially by isenthalpic means, or (<NUM>) may be at least partially by isentropic means, or (<NUM>) may be a combination of both isentropic means and isenthalpic means. Suitable devices for isenthalpic expansion of natural gas are known in the art and generally include, but are not limited to, manually or automatically, actuated throttling devices such as, for example, valves, control valves, Joule-Thomson (J-T) valves, or venturi devices. Suitable devices for isentropic expansion of natural gas are known in the art and generally include equipment such as expanders or turbo expanders that extract or derive work from such expansion. Suitable devices for isentropic expansion of liquid streams are known in the art and generally include equipment such as expanders, hydraulic expanders, liquid turbines, or turbo expanders that extract or derive work from such expansion. An example of a combination of both isentropic means and isenthalpic means may be a Joule-Thomson valve and a turbo expander in parallel, which provides the capability of using either alone or using both the J-T valve and the turbo expander simultaneously. Isenthalpic or isentropic expansion can be conducted in the all-liquid phase, all-vapor phase, or mixed phases, and can be conducted to facilitate a phase change from a vapor stream or liquid stream to a multiphase stream (a stream having both vapor and liquid phases) or to a single-phase stream different from its initial phase. In the description of the drawings herein, the reference to more than one expansion device in any drawing does not necessarily mean that each expansion device is the same type or size.

The term "gas" is used interchangeably with "vapor," and is defined as a substance or mixture of substances in the gaseous state as distinguished from the liquid or solid state. Likewise, the term "liquid" means a substance or mixture of substances in the liquid state as distinguished from the gas or solid state.

A "heat exchanger" broadly means any device capable of transferring heat energy or cold energy from one media to another media, such as between at least two distinct fluids. Heat exchangers include "direct heat exchangers" and "indirect heat exchangers. " Thus, a heat exchanger may be of any suitable design, such as a co-current or counter-current heat exchanger, an indirect heat exchanger (e.g. a spiral wound heat exchanger or a plate-fin heat exchanger such as a brazed aluminum plate fin type), direct contact heat exchanger, shell-and-tube heat exchanger, spiral, hairpin, core, core-and-kettle, double-pipe or any other type of known heat exchanger. "Heat exchanger" may also refer to any column, tower, unit or other arrangement adapted to allow the passage of one or more streams there through, and to affect direct or indirect heat exchange between one or more lines of refrigerant, and one or more feed streams.

As used herein, the term "indirect heat exchange" means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other. Core-in-kettle heat exchangers and brazed aluminum plate-fin heat exchangers are examples of equipment that facilitate indirect heat exchange.

As used herein, the term "natural gas" refers to a multi-component gas obtained from a crude oil well (associated gas) or from a subterranean gas-bearing formation (nonassociated gas). The composition and pressure of natural gas can vary significantly. A typical natural gas stream contains methane (Ci) as a significant component. The natural gas stream may also contain ethane (C<NUM>), higher molecular weight hydrocarbons, and one or more acid gases. The natural gas may also contain minor amounts of contaminants such as water, nitrogen, iron sulfide, wax, and crude oil.

Described herein are systems and processes where LIN and natural gas that is at sufficiently high pressure such that it is suitable for pipeline transport (e.g., <NUM> bar (<NUM> psia) or greater) are produced by indirect heat exchange of at least one nitrogen gas stream with at least two LNG streams within at least one heat exchanger where the LNG streams are at different pressures. In some embodiments, the LIN and high pressure natural gas are produced by the indirect heat exchange of at least one nitrogen gas stream with at least three, or at least four, LNG streams in a multi-stream heat exchanger where each of the LNG streams are at a different pressure from the other LNG streams.

For example, a single LNG stream may be pressurized, for example by using one or more pumps, to an intermediate pressure. The intermediate pressure LNG stream is then split into at least two LNG streams. At least one of the LNG streams is let down in pressure, for example using one or more expansion devices, such as valves, hydraulic turbines, or other devices as known in the art. The reduced pressure LNG stream(s) are then conveyed to at least one heat exchanger. At least one of the LNG streams that is at the intermediate pressure is additionally pressurized using one or more pumps to a pressure higher than the intermediate pressure, such as a pressure equal to or higher than the natural gas sales pipeline pressure. The additionally pressurized LNG stream(s) are then piped to the at least one heat exchanger. The at least two LNG streams undergo indirect heat exchange with at least one nitrogen gas stream within the at least one heat exchanger, whereby the nitrogen gas stream is liquefied forming LIN. According to the present invention, there are at least two LNG streams where the pressures of each LNG stream are independent and different from each other and at least one of the at least two LNG streams is a reduced pressure LNG stream, which is provided at a pressure that is between <NUM> to <NUM> bar (<NUM> to <NUM> psi) and reduced in pressure to form the reduced pressure LNG stream.

In a preferred embodiment, a single LNG stream is introduced to the system. In some embodiments, the LNG stream that enters the system is at a pressure of greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia). The LNG stream that enters the system may be at a pressure of less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia). For example, in some embodiments, the LNG stream that enters the system may be at a pressure of from about <NUM> bar (<NUM> psia) to about <NUM> bar (<NUM> psia), or from about <NUM> bar (<NUM> psia) to <NUM> bar (<NUM> psia), or at a pressure typical for the transport of LNG, such as about <NUM> bar (17psia).

The LNG stream is then pressurized using one or more pumps to an intermediate pressure. The intermediate pressure is a pressure greater than <NUM> bar (<NUM> psia), or may be greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia). The intermediate pressure is less than <NUM> bar (<NUM> psia), or may be less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia). According to the invention, the intermediate pressurized LNG stream is a pressure from <NUM> to <NUM> bar (<NUM> to <NUM> psia), or may be from <NUM> to <NUM> bar (<NUM> to <NUM> psia), or from <NUM> to <NUM> bar (<NUM> to <NUM> psia).

The pressurized LNG stream is then split into two or more streams. For example, the pressurized LNG stream may be split into three or four LNG streams. All but one of the pressurized LNG streams are then reduced in pressure using one or more expansion devices, such as valves, hydraulic turbines, or a combination of devices, where each of the reduced pressures is different from the other reduced pressures. Thus, in an embodiment where the pressurized LNG stream was split into three LNG streams, two of the LNG streams are reduced to different pressures using one or more valves and one LNG stream is not reduced in pressure. Likewise, in an embodiment where the pressurized LNG stream was split into four LNG streams, three of the LNG streams would be reduced in pressure to different pressures using one or more valves and one LNG stream is not reduced in pressure. The LNG stream that is not reduced in pressure is pressurized using one or more pumps to a pressure that may be equal to or higher than the natural gas sales pipeline pressure, such as greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia).

In an embodiment, where the pressurized LNG stream was split into at least four streams, the pressures of each stream are different from one another. For example, the pressure of the first LNG stream may be reduced to a value from <NUM> bar (<NUM> psia) to <NUM> bar (<NUM> psia), or from <NUM> bar (<NUM> psia) to <NUM> bar (<NUM> psia), or from <NUM> bar (<NUM> pisa) to <NUM> bar (<NUM> psia). The pressure of the second LNG stream may be between <NUM> to <NUM> bar (<NUM> to <NUM> psia), or from <NUM> to <NUM> bar (<NUM> to <NUM> psia), or from <NUM> to <NUM> bar (<NUM> to <NUM> psia). The pressure of the third LNG stream may be between <NUM> bar (<NUM> psia) and the intermediate pressure, or from <NUM> to <NUM> bar (<NUM> to <NUM> psia), or from <NUM> to <NUM> bar (<NUM> to <NUM> psia), or from <NUM> to <NUM> bar (<NUM> to <NUM> psia). The fourth LNG stream is pressurized using one or more pumps to a pressure that may be equal to or higher than the natural gas sales pipeline pressure, such as greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia).

The reduced pressure LNG streams and the additionally pressurized LNG stream are all piped to at least one heat exchanger, and in preferred embodiments, are piped to a single multi-stream cryogenic heat exchanger. The LNG streams undergo indirect heat exchange with a nitrogen gas stream that is also piped to the heat exchanger. Suitable heat exchangers include, but are not limited to, cryogenic heat exchangers, which may include brazed aluminum type heat exchangers, spiral wound type heat exchanger, and printed circuit type heat exchangers. As it is known in the art, a suitable heat exchanger will allow for indirect heat exchange between the LNG streams and the nitrogen gas stream while preventing or minimizing indirect heat exchange between the LNG streams. The nitrogen gas stream is at least partially liquefied within the heat exchange such that less than <NUM> mol%, or less than <NUM> mol%, or less than <NUM> mol%, or less than <NUM> mol%, or less than <NUM> mol%, or less than <NUM> mol%, or less than <NUM> mol%, or less than <NUM> mol% of the stream remains in the vapor phase.

The pressure of the nitrogen gas stream that is piped to the heat exchanger may be greater than <NUM> bar (<NUM> psia), or greater than the critical point pressure of the nitrogen gas stream, or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia).

The composition of the nitrogen gas stream may be at least <NUM>% nitrogen, or at least <NUM>% nitrogen, or at least <NUM>% nitrogen, or at least <NUM>% nitrogen, or at least <NUM>% nitrogen, or at least <NUM>% nitrogen. The nitrogen gas stream may comprise other gaseous impurities, such as other components found in air, such as oxygen, argon and carbon dioxide.

The pressures, flow rates and heat exchanger outlet temperatures of the LNG streams entering the multi-stream heat exchanger may be chosen to allow for close matching of the nitrogen gas stream's cooling curve with the warming curves or the composite warming curve of the LNG streams. In some embodiments, it is preferred that the heat exchanger outlet temperatures of the additionally pressurized LNG stream be greater than -<NUM>, or greater than -<NUM>, or greater than -<NUM>, or greater than -<NUM>, or greater than -<NUM>, or greater than -<NUM>, or greater than -<NUM>, or greater than -<NUM>, or greater than -<NUM>, or greater than -<NUM>, or greater than <NUM>, or greater than <NUM>. In some embodiments, the heat exchanger outlet temperature of the additionally pressurized LNG stream may be from - <NUM> to <NUM>, or from -<NUM> to <NUM>, or from -<NUM> to -<NUM>, or from -<NUM> to -<NUM>. The additionally pressurized LNG streams once vaporized may be at a sufficient pressure to enter the gas sale pipeline or be utilized within the regasification plant without requiring additional compression. It is preferred that heat exchanger outlet temperatures of the reduced pressure LNG streams be less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>. In some embodiments, the heat exchanger outlet temperature of the reduced pressure LNG streams is from -<NUM> to -<NUM>, or from -<NUM> to -<NUM>, or from -<NUM> to -<NUM>. The reduced pressure LNG streams may be fully or partially vaporized within the at least one heat exchanger.

After exiting the at least one heat exchanger, the reduced pressure LNG streams may be separated into their liquid and gas components. The liquid component of the reduced pressure LNG streams may be pumped to pressure greater than or equal to the pressure of the additionally pressurized LNG streams and then recycled back to the at least one heat exchanger. The gas component of the reduced pressure LNG streams may be pressurized in compressors to pressures suitable to introduce the compressed gases to the sale gas pipeline or to pressures suitable for use of the compressed gases within the regasification plant. It is often preferred that compressed gases be mixed with some or all the of the vaporized additionally pressurized LNG streams prior to distributing the gases. In a preferred embodiment, the heat exchanger outlet temperature of the reduced pressure LNG streams are sufficiently low to allow for cold compression of the gases to pressures suitable for use without requiring any intercooling of the gases during compression.

In some embodiments, all or a portion of the additionally pressurized LNG streams, after flowing through the at least one heat exchanger, may be piped to at least one second heat exchanger. Alternatively, all or a portion of the additionally pressurized LNG streams may bypass the at least one heat exchanger and may be piped directly to the at least one second heat exchanger. The at least one second heat exchanger can be used for indirect heat exchange of the additionally pressurized LNG streams with the at least one nitrogen gas stream prior to compression of the nitrogen gas stream. The cooling of the at least one nitrogen gas stream with the additionally pressurized LNG streams may occur before one or more of the compression stages of the at least one nitrogen gas stream. The cooling of the at least one nitrogen gas stream with the additionally pressurized LNG streams may occur after intercooling and/or aftercooling of the nitrogen gas stream. As it is known in the art, intercooling and aftercooling of gases may involve the removal of heat from gases after compression by indirect heat exchange with the environment. It is common for the heat to be removed using air or water from the environment. The cooling of the at least one nitrogen gas stream with all or a portion of the additionally pressurized LNG streams prior to compression of the at least one nitrogen gas stream may allow for compression of the at least one nitrogen gas at suction temperatures less than <NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>. The cold compression of the at least one nitrogen gas stream significantly reduces the energy of compression of said gas.

The process described herein has the advantage of liquefying an at least one nitrogen gas stream into an at least one LIN stream by utilizing at least two LNG streams where the required compression of the vaporized LNG streams may be significantly less than prior art. For example, <CIT> discloses a process where the vaporization of LNG is used to produce LIN. The method of <CIT> has the advantage that a LIN to LNG ratio of greater than <NUM>:<NUM> is used to produce the LIN. However, <CIT> has the disadvantage that the single LNG stream is vaporized close to atmospheric pressure. Since natural gas must be admitted to the gas sales pipeline at a high pressure (greater than <NUM> bar (<NUM> psi)), a significant amount of compression is required to pressurize the natural gas to the pipeline pressure. The compression of the close to atmospheric pressure natural gas stream would mostly likely involve the use of multiple compression stages with a significant amount of intercooling and aftercooling of the natural gas stream occurring after each compression stage. The compression of this natural gas stream would require a significant amount of capital investment in compressors and coolers within the regasification plant. It would also be an energy intensive process that would most likely eliminate any thermodynamic advantage in utilizing the available exergy in regasifying the LNG to produce the LIN. In contrast to <CIT>, the system and method described herein only requires compression of a fraction of the total LNG flow. In some embodiments of this invention, the reduced pressure LNG streams account for no more than <NUM>% of the total LNG flow, or less than <NUM>% of the total LNG flow, or less than <NUM>% of the total LNG flow. Another advantage of the present system and method is that the compression of the reduced pressure LNG stream gases may occur at temperatures less than - <NUM>. The cold compression of the reduced pressure LNG stream gases significantly reduces the amount of energy needed for compressing the gases.

For example, in embodiments where the LNG stream is split into four streams, the three reduced pressure streams may account for less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, of the total LNG flow. In some embodiments, the lowest pressure LNG stream may account for less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>% of the total LNG flow. In some embodiments, the second lowest pressure LNG stream may account for less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, of the total LNG flow. In some embodiments, the third lowest pressure LNG stream may account for less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, or less than <NUM>%, of the total LNG flow. In some embodiments, the highest pressure LNG stream may account for greater than <NUM>%, or greater than <NUM>%, or greater than <NUM>%, or greater than <NUM>%, or greater than <NUM>%, or greater than <NUM>%, of the total LNG flow.

This present system and method also has the additional advantage of liquefying an at least one nitrogen gas stream to form at least one LIN stream by utilizing an at least two LNG streams where the total LIN to LNG ratio is greater than <NUM>:<NUM>. For example, <CIT> and <CIT> and <CIT> disclose methods where the LNG is first pressurized to the pipeline transport pressure prior to vaporization of the LNG. In these references, the vaporizing LNG is used to condense the nitrogen gas and used as the coolant within the intercoolers between the multistage compression of the of the nitrogen gas to a pressure at least greater than <NUM> bar (<NUM> psi). The intercooling of the nitrogen gas using the vaporizing and warming natural gas allows for cold compression of the nitrogen gas which significantly reduces its energy of compression. The methods and processes described in all three of these references have the disadvantage that a LIN to LNG ratio of less <NUM>:<NUM> is used to produce the LIN and high pressure natural gas. This low LIN to LNG ratio does not allow for point-to-point integration of the regasification plant with a LNG plant since a LIN to LNG ratio of <NUM>:<NUM> or greater is typically required to produce LNG using LIN as the sole refrigerant. In the regasification plants described in <CIT> and <CIT> and <CIT>, LNG sourced from conventional LNG plants would need to be used in addition to the LNG produced from the LIN. In contrast, the system and method described herein, has the advantage that it allows for the energy efficient production of LIN using a LIN to LNG ratio of greater than <NUM>:<NUM>. The matching of the LIN to LNG ratio at both the LNG plant and the regasification plant allows for an easier integration of the LNG value chain since LNG from conventional production sources is not needed. Additionally, certain embodiments of this system and method allow for one or more of the vaporizing LNG streams to be used to cool the nitrogen gas stream prior to compression of the nitrogen gas stream in order to improve process efficiency.

Having described various aspects of the systems and methods herein, further specific embodiments of the invention include those set forth in the following paragraphs as described with reference to the Figures. While some features are described with particular reference to only one Figure (such as <FIG>, <FIG>, or <FIG>), they may be equally applicable to the other Figures and may be used in combination with the other Figures or the foregoing discussion.

<FIG> illustrates a system where LIN and pressurized natural gas for pipeline transport are produced by indirect heat exchange of at least one nitrogen gas stream with two or more LNG streams in at least one heat exchanger where each of the LNG streams is at a different pressure. A nitrogen gas stream <NUM> is provided to the system. The nitrogen gas stream <NUM> comprises nitrogen gas and may contain less than <NUM> ppm impurities, such as oxygen, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm, or less than <NUM> ppm impurities. The nitrogen gas stream <NUM> may be provided from any available source, for example, it may be provided from commonly known industrial processes for separating nitrogen gas from air such as membrane separation, pressure swing adsorption separation, or cryogenic air separation. In some preferred embodiments, the nitrogen gas stream <NUM> is provided from a cryogenic air separation system. Such systems may be preferred as they can provide high purity nitrogen gas streams (e.g., less than <NUM> ppm impurities, such as O<NUM>) at high quantities (e.g., greater than <NUM> MSCFD). The nitrogen gas stream <NUM> may be provided to the system at a pressure that is greater than atmospheric pressure, or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia).

The nitrogen gas stream <NUM> may be conveyed or transported, for example be piped, to a compressor <NUM>. The compressor <NUM> increases the pressure of the nitrogen gas streams to a pressures greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psi). In some embodiments, the compressor <NUM> increases the pressure of the nitrogen gas stream to a pressure greater than the critical point pressure of the nitrogen gas stream. The compression of the nitrogen gas stream may occur in a single stage or in multiple stages of compression. In some embodiments, more than one compressor may be used, where the compressors are parallel, in series, or both. The high pressure nitrogen gas stream <NUM> may then be split into two streams 112a and 112b which are then piped to heat exchangers <NUM> and <NUM> where they are liquefied by heat exchange with vaporizing LNG streams to form high pressure LIN stream <NUM>.

With reference to <FIG>, a LNG stream <NUM> is introduced to the system and is pressurized to an intermediate pressure to form intermediate pressure LNG stream <NUM>. The LNG stream <NUM> may be pressurized utilizing means known in the art, for example a pump <NUM>. The intermediate pressure LNG stream <NUM> is split into at least two LNG streams, a first LNG stream <NUM> and a second set LNG stream <NUM>. The first LNG stream <NUM> may be reduced in pressure by flowing through one or more valves <NUM> to form a reduced pressure LNG stream <NUM>. The pressure of the reduced pressure LNG stream <NUM> may be less than less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia), or less than <NUM> bar (<NUM> psia). The pressure of the reduced LNG stream <NUM> may be greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia). In some embodiments, the pressure of the reduced LNG stream <NUM> may be from about <NUM> bar (<NUM> psia) to about <NUM> bar (<NUM> psia), or from about <NUM> bar (<NUM> psia) to <NUM> bar (<NUM> psia). According to the present invention, there is at least one reduced pressure LNG stream, which is provided at a pressure that is between <NUM> to <NUM> bar (<NUM> to <NUM> psi) and reduced in pressure to form the reduced pressure LNG stream. The reduced pressure LNG stream <NUM> is then conveyed to a first heat exchanger <NUM> where the reduced pressure LNG stream <NUM> is vaporized by heat exchange with the nitrogen gas stream 112a. The outlet temperature of the vaporized, reduced pressure LNG stream <NUM> as it leaves the heat exchanger <NUM> may be less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than - <NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>. The vaporized, reduced pressure LNG stream <NUM> may then be cold compressed in compressor <NUM> to a pressure greater than <NUM> bar (<NUM> psia) to form compressed natural gas stream <NUM>. The compression of the vaporized, reduced pressure LNG stream <NUM> may occur in a single stage or multiple stages of compression. The second LNG stream <NUM> is pumped in pump <NUM> to produce an increased pressured LNG stream <NUM>. The pressure of the increased pressured LNG stream <NUM> may be a greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia). The increased pressure LNG stream <NUM> is then piped to a second heat exchanger <NUM> where the LNG stream is vaporized by heat exchange with nitrogen gas stream 112b. The vaporized, increased pressure LNG stream <NUM> may have outlet temperatures of greater than -<NUM>, or greater than <NUM>, or greater than <NUM>, or greater than <NUM>, or greater than <NUM>. The vaporized, increased pressurized LNG stream <NUM> may be combined with the compressed natural gas stream <NUM> to form high pressure natural gas stream <NUM> that is suitable for transport in the gas sales pipeline.

The high pressure LIN streams 113a and 113b exiting the heat exchangers <NUM> and <NUM> may be combined into one stream <NUM> and may then be further cooled in a heat exchanger <NUM>. In some embodiments, the high pressure LIN streams 113a and 113b are each introduced individually into the heat exchanger <NUM>, while in other embodiments, the high pressure LIN streams are combined as shown in <FIG> before being introduced into the heat exchanger. In some embodiments, the high pressure LIN stream <NUM> is sub-cooled in a flash gas heat exchanger <NUM> to form a sub-cooled high pressure LIN streams <NUM>. The sub-cooled high pressure LIN stream <NUM> may then be let down in pressure using two-phase hydraulic turbines, single-phase hydraulic turbines, valves, or other common devices known in the art. In a preferred embodiment, the sub-cooled high pressure LIN stream <NUM> is let down in pressure using two-phase hydraulic turbines <NUM> for the last stage of pressure reduction. The reduced pressure LIN stream <NUM> can then be separated into a vapor component as nitrogen flash gas streams <NUM> and a liquid component as product LIN streams <NUM>. The nitrogen flash gas stream <NUM> can then be sent back to the flash gas exchanger <NUM> where it can be utilized to cool the high pressure LIN stream <NUM> through indirect heat exchange. The warmed nitrogen flash gas streams <NUM> can then be cold compressed into a recycled nitrogen gas streams <NUM>. The compression of the warmed nitrogen flash gas streams may occur in a single stage or multiple stages of compression <NUM>. The recycled nitrogen gas stream <NUM> can then be mixed with the nitrogen gas streams <NUM> before one of the nitrogen gas streams stages of compression <NUM>.

<FIG> illustrates an embodiment where a single multi-stream heat exchanger <NUM> is utilized. This embodiment has the advantage that less piping is required for transporting the LNG streams and the LIN streams. Similar to the system of <FIG>, in <FIG> a LNG stream <NUM> is introduced to the system and is pressurized <NUM> to an intermediate pressure. The intermediate pressure LNG stream <NUM> is split into a first LNG stream <NUM> and a second LNG stream <NUM>. The first LNG stream <NUM> may be reduced in pressure by flowing through one or more valves <NUM> to form a reduced pressure LNG stream <NUM> which is then introduced to the multi-stream heat exchanger <NUM>. The vaporized, reduced pressure LNG stream <NUM> that exits the multi-stream heat exchanger <NUM> may then be cold compressed in compressor <NUM> to a pressure greater than <NUM> bar (<NUM> psia) to form compressed natural gas stream <NUM>. The second LNG stream <NUM> is pumped in pump <NUM> to produce an increased pressured LNG stream <NUM> which is introduced to the multi-stream heat exchanger <NUM> where the LNG stream is vaporized by heat exchange with nitrogen gas stream <NUM>. The vaporized, increased pressure LNG stream <NUM> exiting the multi-stream heat exchanger <NUM> may be combined with the compressed natural gas stream <NUM> to form high pressure natural gas stream <NUM> that is suitable for transport in the gas sales pipeline.

Like in <FIG>, <FIG> also shows a nitrogen gas stream <NUM> entering the system and being piped to compressor <NUM>. The compressed high pressure nitrogen gas <NUM> enters the multi-stream heat exchanger <NUM> where it is liquefied by heat exchange with the vaporizing LNG streams to form a high pressure LIN stream <NUM>. The high pressure LIN stream <NUM> can then be sub-cooled in a flash gas exchanger <NUM> to form a sub-cooled high pressure LIN stream <NUM>. The pressure of the sub-cooled high pressure LIN stream <NUM> can then be let-down <NUM>, such as in a two-phase hydraulic turbine, to form a reduced pressure LIN stream <NUM>. The reduced pressure LIN stream <NUM> can then be separated into a nitrogen flash gas stream <NUM> and a product LIN stream <NUM>. The nitrogen flash gas stream <NUM> can then be sent back to the flash gas exchanger <NUM> where it can be utilized to cool the high pressure LIN stream <NUM> through indirect heat exchange. The warmed nitrogen flash gas streams <NUM> can then be cold compressed <NUM> into a recycled nitrogen gas streams <NUM> which can then be mixed with the nitrogen gas streams <NUM> before one of the nitrogen gas streams stages of compression <NUM>.

<FIG>illustrates a system where LIN and pressurized natural gas for pipeline transport are produced by indirect heat exchange of a nitrogen gas stream and four LNG streams at different pressures. A main LNG stream <NUM> is pressurized <NUM> to an intermediate pressure to form an intermediate pressure LNG stream <NUM>. The intermediate pressure LNG stream <NUM> is at a pressure of from <NUM> to <NUM> bar (<NUM> to <NUM> psia), or may be from <NUM> to <NUM> bar (<NUM> to <NUM> psia), or from <NUM> to <NUM> bar (<NUM> to <NUM> psia). The intermediate pressure LNG stream is split into four LNG streams, a first LNG stream <NUM>, a second LNG stream <NUM>, a third LNG stream <NUM>, and a fourth LNG stream <NUM>. The first, second and third LNG streams may be reduced in pressure using one or more valves <NUM>, <NUM>, and <NUM> to produce a first reduced pressure LNG stream <NUM>, a second reduced pressure LNG stream <NUM>, and a third reduced pressure LNG stream <NUM>, respectively. The pressure of the first reduced pressure LNG stream <NUM> may be between <NUM> to <NUM> bar (<NUM> to <NUM> psia). The pressure of the second reduced pressure LNG stream <NUM> may be between <NUM> to <NUM> bar (<NUM> to <NUM> psia). The pressure of the third reduced pressure LNG stream <NUM> may be between <NUM> bar (<NUM> psia) and the intermediate pressure. The pressures of the first, second and third reduced pressure LNG streams are independent and different from each other. According to the present invention, there is at least one reduced pressure LNG stream, which is provided at a pressure that is between <NUM> to <NUM> bar (<NUM> to <NUM> psi) and reduced in pressure to form the reduced pressure LNG stream. The fourth LNG stream <NUM> is pressurized using one or more pumps <NUM> to a pressure that may be greater than <NUM> bar (<NUM> psia), or more likely, to a pressure that may be greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), to form an additionally pressurized LNG stream (<NUM>). The three reduced pressure LNG streams <NUM>, <NUM>, and <NUM> and the additionally pressurized LNG stream <NUM> are all piped to a single, multi-stream cryogenic heat exchanger <NUM>. Suitable cryogenic heat exchangers include, but are not limited to, brazed aluminum type heat exchangers, spiral wound type heat exchanger, and printed circuit type heat exchangers. As it is known in the art, a suitable type heat exchanger will allow for indirect heat exchange between the four LNG streams <NUM>, <NUM>, <NUM>, and <NUM> and the nitrogen gas stream <NUM> while preventing or minimizing indirect heat exchange between the LNG streams. The first <NUM>, second <NUM>, and third <NUM> reduced pressure LNG streams exit the multi-stream cryogenic heat exchanger <NUM> as a first vaporized, reduced pressure LNG stream <NUM>, a second vaporized, reduced pressure LNG stream <NUM>, and a third vaporized, reduced pressure LNG stream <NUM>, respectively. The pressure, flow rates and heat exchanger outlet temperatures of the reduced pressure LNG streams may be chosen to allow for close matching of the temperature versus heat transfer curves within the heat exchanger. It is preferred that temperatures of the vaporized, reduced pressure LNG streams be less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, less than -<NUM>. The vaporized, reduced pressure LNG streams may be fully or partially vaporized within the cryogenic heat exchanger. After exiting the heat exchanger <NUM>, the vaporized, reduced pressure LNG streams may be separated into their liquid and gas components. The liquid component of the vaporized, reduced pressure LNG streams may be pumped to pressure equal to or greater than the pressures of the additionally pressurized LNG stream and then recycled back to the cryogenic heat exchanger (not shown in <FIG>for simplicity). The gas component of the vaporized, reduced pressure LNG streams may be pressurized in compressors <NUM> to a pressure suitable to introduce the compressed natural gas stream <NUM> to the sale gas pipeline <NUM> or to pressures suitable for use of the compressed natural gas stream within the regasification plant. Suitable pressures for the compressed natural gas stream may be greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or greater than <NUM> bar (<NUM> psia), or may be greater than <NUM> bar (<NUM> psia). In a preferred embodiment of this invention, the temperatures of the vaporized, reduced pressure LNG streams are sufficiently low so as to allow for cold compression of the gases to pressures suitable for use without requiring any intercooling of the gases during compression. It is often preferred that compressed natural gas stream be mixed with some or all the of the vaporized, additionally pressurized LNG stream <NUM> to form a high pressure natural gas stream <NUM> prior to distributing the gases to the gas sales pipeline or other users.

The additionally pressurized LNG stream <NUM> exits the multi-stream cryogenic heat exchanger <NUM> as stream <NUM> which may then be piped to at least one or two more heat exchangers <NUM> and <NUM> to further cool the nitrogen gas stream at the warmer end of the nitrogen gas stream cooling curve. The pressures, flow rates and heat exchanger outlet temperatures of the additionally pressurized LNG stream may be chosen to allow for close matching of the temperature versus heat transfer curves within the heat exchangers. It is preferred that the temperature of the vaporized, additionally pressurized LNG stream <NUM> be greater than <NUM>, or greater than <NUM>, or greater than <NUM>, or greater than <NUM>.

<FIG>shows a nitrogen gas stream <NUM> entering the system. The nitrogen gas stream may be mixed with a recycled nitrogen gas stream <NUM>. The gas mixture, here still referred to as the nitrogen gas stream, may then be piped to at least one heat exchanger <NUM> where it is cooled by indirect heat exchange with all or a portion of the of the additionally pressurized LNG stream <NUM> to form an intercooled nitrogen gas stream <NUM>. The additionally pressurized LNG stream may be piped to the at least one heat exchanger after flowing through the multi-stream cryogenic heat exchanger or, in some embodiments not shown, may bypass the multi-stream cryogenic heat exchanger and proceed directly to the heat exchanger. In some embodiments, the cooling of the nitrogen gas stream with the additionally pressurized LNG stream may occur before one or more of the compression stages of the nitrogen gas stream. In some embodiments, the cooling of the nitrogen gas stream with the additionally pressurized LNG streams may occur after cooling of the nitrogen gas stream with the environment. The intercooled nitrogen gas stream may have a temperature of less than <NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>. The cold compression of the intercooled nitrogen gas stream significantly reduces the energy of compression of said gas. <FIG>shows that the intercooled nitrogen gas stream <NUM> is then piped to a booster compressor <NUM> to form a high pressure nitrogen gas stream <NUM>. The pressure of the high pressure nitrogen gas stream <NUM> is a pressure greater than <NUM> bar (<NUM> psia), or greater than the critical point pressure of the nitrogen gas stream, or greater than <NUM> bar (<NUM> psia). The compression of the intercooled nitrogen gas stream may occur in a single stage or in multiple stages of compression. The high pressure nitrogen gas stream <NUM> may then be piped to at least one heat exchanger <NUM> where it is cooled by indirect heat exchange with all or a portion of the of the additionally pressurized LNG stream <NUM> to form an aftercooled nitrogen gas stream <NUM>. In some embodiments, the cooling of the high pressure nitrogen gas stream with the additionally pressurized LNG stream may occur after cooling of the nitrogen gas stream with the environment. The aftercooled nitrogen gas stream <NUM> may have a temperature of less than <NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>, or less than -<NUM>. The aftercooled nitrogen gas stream <NUM> is then piped to the multi-stream cryogenic heat exchanger <NUM> where it is liquefied into a high pressure LIN stream <NUM> by heat exchange with the vaporizing LNG streams <NUM>, <NUM>, <NUM>, and <NUM>.

The LIN stream <NUM> shown in <FIG>may be further sub-cooled in a flash gas exchanger <NUM>. The sub-cooled high pressure LIN stream <NUM> is let down in pressure using one or more or combinations of two-phase hydraulic turbines, single-phase hydraulic turbines, valves, or other common devices known in the art <NUM>. In a preferred embodiment of this invention, the sub-cooled high pressure LIN stream is let down in pressure using a two-phase hydraulic turbine for its last stage of pressure reduction. The reduced pressure LIN stream <NUM> is then separated into its vapor component as nitrogen flash gas stream <NUM> and its liquid component as product LIN stream <NUM>. The nitrogen flash gas stream is sent to the flash gas exchanger <NUM> where it acts to cool the high pressure LIN stream <NUM> through indirect heat exchange. The warmed nitrogen flash gas stream <NUM> is then cold compressed <NUM> into a recycled nitrogen gas stream <NUM>. The compression of the warmed nitrogen flash gas stream may occur in a single stage or multiple stages of compression. The recycled nitrogen gas stream <NUM> is then mixed with the nitrogen gas stream <NUM> before one of the nitrogen gas stream stages of compression.

A simulation was conducted to model the cooling curves exhibited by the nitrogen gas stream and LNG streams of a system configured as in <FIG>. <FIG> shows the cooling curve for a nitrogen gas stream <NUM> along with a composite warming curve of four LNG streams <NUM> that utilize the system in <FIG>. In the simulation, the nitrogen gas stream <NUM> enters the multi-stream heat exchanger <NUM> at a pressure of <NUM> bar (<NUM> psia). The first reduced pressure LNG stream <NUM> enters the heat exchanger at a pressure of <NUM> bar (<NUM> psia) and exits <NUM> the heat exchanger at a temperature of -<NUM>. The second reduced pressure LNG stream <NUM> enters the heat exchanger at a pressure of <NUM> bar (<NUM> psia) and exits <NUM> the heat exchanger at a temperature of -<NUM>. The third reduced pressure LNG stream <NUM> enters the heat exchanger at a pressure of <NUM> bar (<NUM> psia) and exits <NUM> the heat exchanger at a temperature of -<NUM>. The additionally pressurized LNG stream <NUM> enters the heat exchanger at a pressure <NUM> bar (<NUM> psi) and exits <NUM> the heat exchanger at a temperature of -<NUM>. The first, second and third reduced pressure LNG streams accounts for <NUM>%, <NUM>% and <NUM>% of the total LNG flow, respectively. The additionally pressurized LNG stream accounts for the remaining balance (<NUM>%) of the LNG flow. For this example, the heat exchanger was designed for a minimum approach temperature of <NUM>. It had a log mean temperature difference of <NUM> for a heat duty of <NUM> MW. As seen in <FIG>, by varying the pressure and amount of LNG in each stream, the composite warming curve of the four LNG streams are able to approximate the cooling curve of the nitrogen gas stream. This allows for efficient use of the exergy of the system when forming the LIN and the regasification of the LNG.

Certain embodiments and features have been described using a set of numerical upper limits and a set of numerical lower limits. It should be appreciated that ranges from any lower limit to any upper limit are contemplated unless otherwise indicated. All numerical values are "about" or "approximately" the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

Claim 1:
A method for producing a liquefied nitrogen gas (LIN) stream (<NUM>; <NUM>; <NUM>) at a liquid natural gas (LNG) regasification facility comprising:
(a) providing a nitrogen gas stream (<NUM>; <NUM>; <NUM>);
(b) providing at least two LNG streams (<NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM>, <NUM>, <NUM>) where the pressures of each LNG stream are independent and different from each other,
wherein at least one of the at least two LNG streams is an additionally pressurized LNG stream, which is pressurized using one or more pumps to form the additionally pressurized LNG stream (<NUM>; <NUM>; <NUM>);
(c) liquefying the nitrogen gas stream by indirect heat exchange of the nitrogen gas stream with the LNG streams (<NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM>, <NUM>, <NUM>) in at least one heat exchanger (<NUM>, <NUM>; <NUM>; <NUM>);
(d) vaporizing at least a portion of the two LNG streams to produce at least two natural gas streams (<NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM>, <NUM>, <NUM>);
(e) compressing at least one of the two natural gas streams (<NUM>; <NUM>; <NUM>, <NUM>, <NUM>) to form compressed natural gas (<NUM>; <NUM>; <NUM>);
wherein the method is characterized in that at least one of the at least two LNG streams is a reduced pressure LNG stream (<NUM>; <NUM>; <NUM>, <NUM>, <NUM>), which is provided at a pressure that is between <NUM> to <NUM> bar (<NUM> to <NUM> psi) and reduced in pressure to form the reduced pressure LNG stream (<NUM>; <NUM>; <NUM>, <NUM>, <NUM>).