Patent Grant
10663115
The invention relates to the liquefaction of natural gas to form liquefied natural gas (LNG) using liquid nitrogen (LIN) as a coolant, and more specifically, to the storage and/or transport of liquid nitrogen to an LNG liquefaction location using an LNG storage tank.
LNG production is a rapidly growing means to supply natural gas from locations with an abundant supply of natural gas to distant locations with a strong demand of natural gas. The conventional LNG cycle includes: (a) initial treatments of the natural gas resource to remove contaminants such as water, sulfur compounds and carbon dioxide; (b) the separation of some heavier hydrocarbon gases, such as propane, butane, pentane, etc. by a variety of possible methods including self-refrigeration, external refrigeration, lean oil, etc.; (c) refrigeration of the natural gas substantially by external refrigeration to form LNG at near atmospheric pressure and about −160° C.; (d) transport of the LNG product in ships or tankers designed for this purpose to a market location; and (e) re-pressurization and re-gasification of the LNG to a pressurized natural gas that may distributed to natural gas consumers. Step (c) of the conventional LNG cycle usually requires the use of large refrigeration compressors often powered by large gas turbine drivers that emit substantial carbon and other emissions. Large capital investments—on the order of billions of US dollars—and extensive infrastructure may be required as part of the liquefaction plant. Step (e) of the conventional LNG cycle generally includes re-pressurizing the LNG to the required pressure using cryogenic pumps and then re-gasifying the LNG to form pressurized natural gas by exchanging heat through an intermediate fluid but ultimately with seawater, or by combusting a portion of the natural gas to heat and vaporize the LNG. Generally, the available exergy of the cryogenic LNG is not utilized.
A cold refrigerant produced at a different location, such as liquefied nitrogen gas (“LIN”), can be used to liquefy natural gas. A process known as the LNG-LIN concept relates to a non-conventional LNG cycle in which at least Step (c) above is replaced by a natural gas liquefaction process that substantially uses liquid nitrogen (LIN) as an open loop source of refrigeration and in which Step (e) above is modified to utilize the exergy of the cryogenic LNG to facilitate the liquefaction of nitrogen gas to form LIN that may then be transported to the resource location and used as a source of refrigeration for the production of LNG. U.S. Pat. No. 3,400,547 describes shipping liquid nitrogen or liquid air from a market place to a field site where it is used to liquefy natural gas. U.S. Pat. No. 3,878,689 describes a process to use LIN as the source of refrigeration to produce LNG. U.S. Pat. No. 5,139,547 describes the use of LNG as a refrigerant to produce LIN.
The LNG-LIN concept further includes the transport of LNG in a ship or tanker from the resource location to the market location and the reverse transport of LIN from the market location to the resource location. The use of the same ship or tanker, and perhaps the use of common onshore tankage, are expected to minimize costs and required infrastructure. As a result, some contamination of the LNG with LIN and some contamination of the LIN with LNG may be expected. Contamination of the LNG with LIN is likely not to be a major concern as natural gas specifications (such as those promulgated by the United States Federal Energy Regulatory Commission) for pipelines and similar distribution means allow for some inert gas to be present. However, since the LIN at the resource location will ultimately be vented to the atmosphere, contamination of the LIN with LNG (which, when regasified as natural gas, is a greenhouse gas more than 20 times as impactful as carbon dioxide) must be reduced to levels acceptable for such venting. Techniques to remove the residual contents of tanks are well known but it may not be economically or environmentally acceptable to achieve the needed low level of contamination to avoid treatment of the LIN or vaporized nitrogen at the resource location prior to venting the gaseous nitrogen (GAN). What is needed is a method of using LIN as a coolant to produce LNG, where if the LIN and the LNG use common storage facilities, any natural gas remaining in the storage facilities is effectively purged prior to filling the storage facilities with LIN.
The invention provides a method for loading liquefied nitrogen (LIN) into a cryogenic storage tank initially containing liquid natural gas (LNG) and a vapor space above the LNG. First and second nitrogen gas streams are provided. The first nitrogen stream has a lower temperature than the second nitrogen gas stream. While the LNG is offloaded from the storage tank, the first nitrogen gas stream is injected into the vapor space. The storage tank is then purged by injecting the second nitrogen gas stream into the storage tank to thereby reduce a natural gas content of the vapor space to less than 5 mol %. After purging the storage tank, the storage tank is loaded with LIN.
The invention also provides a method of purging a cryogenic storage tank initially containing liquid natural gas (LNG) and a vapor space above the LNG. A first nitrogen gas stream is provided having a temperature within 20° C. of a normal boiling point of the first nitrogen gas stream. A second nitrogen gas stream is provided having a temperature within 20° C. of a temperature of the LNG. The first nitrogen gas stream and the second nitrogen gas stream are slip streams from a nitrogen liquefaction process. The LNG is offloaded from the storage tank while the first nitrogen gas stream is injected into the vapor space. The second nitrogen gas stream is injected into the storage tank, to thereby reduce a methane content of the vapor space to less than 5 mol %. After injecting the second nitrogen gas stream into the storage to tank, the storage tank is loaded with liquid nitrogen (LIN).
The invention also provides a dual-use cryogenic storage tank for alternately storing liquefied natural gas (LNG) and liquid nitrogen (LIN). A liquid outlet is disposed at a low spot in the tank and permits liquids to be removed from the tank. One or more nitrogen gas inlet ports are disposed at or near a top of the tank. The one or more gas inlet ports introduce nitrogen gas into the tank as LNG is removed from the tank through the liquid outlet. One or more additional nitrogen gas inlet ports are disposed near the bottom of the tank and permit additional nitrogen gas to be introduced into the tank. One or more gas outlet ports permit removal of gas from the tank as the additional nitrogen gas is introduced into the tank. One or more liquid inlet ports permit a cryogenic liquid such as LIN to be introduced into the tank while the additional nitrogen gas is removed from the tank through the one or more gas outlet ports.
Various specific aspects and versions of the present disclosure will now be described, including preferred aspects and definitions that are adopted herein. While the following detailed description gives specific preferred aspects, those skilled in the art will appreciate that these aspects 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 aspects defined by the claims. The use of headings is for purposes of convenience only and does not limit the scope of the present invention. For purposes of clarity and brevity, similar reference numbers in the several Figures represent similar items, steps, or structures and may not be described in detail in every Figure.
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, 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 by any suitable, desired, or required amount. Cooling may include a temperature drop of at least about 1° C., at least about 5° C., at least about 10° C., at least about 15° C., at least about 25° C., at least about 35° C., or least about 50° C., or at least about 75° C., or at least about 85° C., or at least about 95° C., or at least about 100° C. 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 aspects, 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 (1) at least partially by isenthalpic means, or (2) may be at least partially by isentropic means, or (3) 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 medium to another medium, 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, printed-circuit, 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 therethrough, 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 (non-associated gas). The composition and pressure of natural gas can vary significantly. A typical natural gas stream contains methane (C1) as a significant component. The natural gas stream may also contain ethane (C2), 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.
Certain aspects 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.
All patents, test procedures, and other documents cited in this application are fully incorporated by reference to the extent such disclosure is not inconsistent with this application and for all jurisdictions in which such incorporation is permitted.
Described herein are methods and processes to purge an LNG transport tank using nitrogen gas so that the tank subsequently may be used to transport LIN. Specific aspects of the disclosure 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, they may be equally applicable to the other Figures and may be used in combination with the other Figures or the foregoing discussion.
The liquefied compressed nitrogen gas stream 114 is passed through a second heat exchanger 122, where it is further cooled via indirect heat exchange with a flash nitrogen gas stream or boil-off nitrogen gas stream 124, the source of which will be further described herein. The subcooled liquefied nitrogen gas stream 126 is expanded, preferably in a work-producing expander 128, to form a partially liquefied nitrogen gas stream where the pressure of the partially liquefied nitrogen gas stream is a pressure suitable for transport of the formed LIN stream 136 to storage. Alternatively, the work-producing expander 128 may be followed by an expansion valve (not shown) to further reduce the pressure of the subcooled liquefied nitrogen gas stream to form the partially liquefied nitrogen gas stream. The work-producing expander 128 may be operationally connected to a generator 130, which may in turn directly or indirectly provide the power to drive the motors, compressors, and/or pumps in system 100 or other systems. The partially liquefied nitrogen gas stream 132 is directed to a separation vessel 134, where the previously mentioned flash nitrogen gas stream or boil-off nitrogen gas stream 124 is separated from the LIN stream 136. The LIN stream 136 may be sent to a land-based or ship-based storage tank, and in a disclosed aspect, may be stored in a dual purpose storage tank configured to store LNG at one time and LIN at another time, as will be further described. The boil-off nitrogen gas stream 124 enters the second heat exchanger 122 at a temperature near the normal boiling point of nitrogen, or approximately −192° C., and cools the liquefied compressed nitrogen gas stream 114. In an aspect, the temperature of the boil-off nitrogen gas stream 124 is within 20° C., or within 10° C., or within 5° C., or within 2° C., or within 1° C. of −192° C. The warm flash or boil-off nitrogen gas stream 138 exits the second heat exchanger 122 at a temperature close to the temperature of the LNG, which is likely to be close to the boiling point of LNG, i.e., −157° C. In an aspect, the temperature of the warmed boil-off nitrogen gas stream is within 20° C., or within 10° C., or within 5° C., or within 2° C., or within 1° C. of −157° C. The warmed boil-off nitrogen gas stream 138 is compressed in a boil-off nitrogen gas compressor 140, which is driven by a second motor 142 or other motive force, to thereby form a compressed boil-off nitrogen gas stream 144. The compressed boil-off nitrogen gas stream 144 is combined with the nitrogen gas stream 102 to be recycled through system 100.
As previously discussed, to fully take advantage of the benefits of an LNG-LIN process, it is preferable to transport LNG from its production location to its regasification location in the same tank that transports LIN from the LNG regasification location to the LNG production location. Such a dual-use tank is shown in
A process or method of purging tank 200 according to disclosed aspects is shown in
The remaining vapor is then purged from the vapor space 302 of the tank 200 through the one or more gas outlet ports 208 by injecting a cold nitrogen gas stream into the tank through the additional gas inlet ports 212 (
Aspects of the disclosure may be modified in many ways while keeping with the spirit of the invention. For example, throughout this disclosure the proportion of methane in the vapor space of the tank has been described as a mol % by mass. Alternatively, as natural gas may be comprised of more than just methane, it may be advantageous to instead speak of the proportion of non-nitrogen gases present in the vapor space as measured by a mol % by mass. Additionally, the number and positioning of the gas inlet ports 206, gas outlet ports 208, and additional gas inlet ports 212 may be varied as desired or required.
The aspects disclosed herein provide a method of purging a dual-use cryogenic LNG/LIN storage tank. An advantage of the disclosed aspects is that natural gas in stored/transported LIN is at an acceptably low level. Another advantage is that the disclosed method of purging permits the storage tank to be essentially emptied of LNG. No remainder or “heel” is required to remain in the tank. This reinforces the dual-use nature of the tank, and further lowers the natural gas content in the tank when LIN is loaded therein. Still another advantage is that the nitrogen gas used for purging is taken from the LIN production/LNG regasification system. No additional purge gas streams are required to be produced. Yet another advantage is that the gas purged from the storage tank can be recycled back into the LIN production system. This closed system reduces or even eliminates undesired emissions into the atmosphere.
Aspects of the disclosure may include any combinations of the methods and systems shown in the following numbered paragraphs. This is not to be considered a complete listing of all possible aspects, as any number of variations can be envisioned from the description above.
1. A method for loading liquefied nitrogen (LIN) into a cryogenic storage tank initially containing liquid natural gas (LNG) and a vapor space above the LNG, the method comprising:
providing a first nitrogen gas stream and a second nitrogen gas stream, where the first nitrogen stream has a temperature lower than a temperature of the second nitrogen gas stream;
offloading the LNG from the storage tank while injecting the first nitrogen gas stream into the vapor space;
purging the storage tank by injecting the second nitrogen gas stream into the storage tank, to thereby reduce a methane content of the vapor space to less than 5 mol %; and
after purging the storage tank, loading the storage tank with LIN.
2. The method of paragraph 1, wherein the temperature of the first nitrogen gas stream is within 5° C. of a normal boiling point of the first nitrogen gas stream.
3. The method of paragraph 1 or paragraph 2, wherein the temperature of the second nitrogen gas stream is within 5° C. of a temperature of the LNG.
4. The method of any one of paragraphs 1-3, wherein the first nitrogen gas stream and the second nitrogen gas stream are slip streams from a nitrogen liquefaction process.
5. The method of paragraph 4, further comprising using available cold from regasification of the LNG to liquefy the nitrogen in the nitrogen liquefaction process.
6. The method of paragraph 4, further comprising expanding a pressurized liquefied nitrogen gas stream in the nitrogen liquefaction process to produce LIN and a boil-off nitrogen gas stream, wherein a portion of the boil-off nitrogen gas stream is the first nitrogen gas stream.
7. The method of paragraph 6, further comprising, prior to expanding the pressurized liquefied nitrogen gas stream, cooling the pressurized liquefied nitrogen gas stream using the boil-off nitrogen gas stream to produce a warm boil-off nitrogen gas stream, wherein a portion of the warm boil-off nitrogen gas stream is the second nitrogen gas stream.
8. The method of paragraph 4, wherein a gas stream ejected from the storage tank during LIN loading is mixed with a nitrogen gas stream within the nitrogen liquefaction process.
9. The method of paragraph 8, wherein the nitrogen gas stream within the nitrogen liquefaction process comprises the second nitrogen gas stream.
10. The method of any one of paragraphs 1-9, wherein a gas stream ejected from the storage tank during LIN loading is mixed with a boil-off natural gas stream.
11. The method of any one of paragraphs 1-10, wherein a gas stream ejected from the storage tank from the purging of the storage tank is mixed with an LNG boil-off gas stream.
12. The method of any one of paragraphs 1-11, wherein a methane content of a gas in the vapor space prior to injecting the second nitrogen gas stream is less than 20 mol %.
13. The method of any one of paragraphs 1-12, wherein a methane content of a gas in the vapor space prior to loading the LIN into the tank is less than 2 mol %.
14. The method of any one of paragraphs 1-13, wherein a methane content of the LIN after being loaded in the storage tank is less than 100 ppm.
15. The method of any one of paragraphs 1-14, wherein the first nitrogen gas stream and the second nitrogen gas stream have an oxygen concentration of less than 1 mol %.
16. The method of any one of paragraphs 1-15, wherein a gas stream ejected from the storage tank during LIN loading is mixed with a natural gas stream created by regasification of the LNG.
17. A method of purging a cryogenic storage tank initially containing liquid natural gas (LNG) and a vapor space above the LNG, the method comprising:
providing a first nitrogen gas stream with a temperature within 20° C. of a normal boiling point of the first nitrogen gas stream;
providing a second nitrogen gas stream with a temperature within 20° C. of a temperature of the LNG;
wherein the first nitrogen gas stream and the second nitrogen gas stream are slip streams from a nitrogen liquefaction process;
offloading the LNG from the storage tank while injecting the first nitrogen gas stream into the vapor space;
injecting the second nitrogen gas stream into the storage tank, to thereby reduce a methane content of the vapor space to less than 5 mol %; and
after injecting the second nitrogen gas stream into the storage tank, loading the storage tank with liquid nitrogen (LIN).
18. A dual-use cryogenic storage tank for alternately storing liquefied natural gas (LNG) and liquid nitrogen (LIN), comprising:
a liquid outlet disposed at a low spot in the tank and configured to permit liquids to be removed from the tank;
one or more nitrogen gas inlet ports disposed at or near a top of the tank, the one or more gas inlet ports configured to introduce nitrogen gas into the tank as LNG is removed from the tank through the liquid outlet;
one or more additional nitrogen gas inlet ports disposed near the bottom of the tank and configured to permit additional nitrogen gas to be introduced into the tank;
one or more gas outlet ports configured to permit removal of gas from the tank as the additional nitrogen gas is introduced into the tank; and
one or more liquid inlet ports configured to permit a cryogenic liquid such as LIN to be introduced into the tank while the additional nitrogen gas is removed from the tank through the one or more gas outlet ports.
While the foregoing is directed to aspects of the present disclosure, other and further aspects of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
This application claims the priority benefit of U.S. Patent Application No. 62/463,274 filed Feb. 24, 2017 entitled “METHOD OF PURGING A DUAL PURPOSE LNG/LIN STORAGE TANK”, the entirety of which is incorporated by reference herein.
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