Patent Application
20240003619
The present disclosure concerns natural gas liquefaction systems and methods.
Natural gas, mainly composed of methane and including minor amounts of heavier hydrocarbons, such as ethane, propane, butanes, pentanes, hexanes, benzene, toluene and others, as well as possibly carbon dioxide, is becoming an increasingly popular source of energy, due to its large availability and reduced environmental impact. However, transportation of natural gas from the site of extraction, where the source of natural gas is available, to the end user, is a main concern. One cost-effective and safe method of transporting natural gas over long distances is to liquefy the natural gas and to transport it in tanker ships, often referred to as LNG carriers. At destination the liquefied natural gas is transformed back in its gaseous state and made available to the end users.
Different systems and technologies have been developed to liquefy natural gas (NG) and produce liquefied natural gas (LNG), using single or combined refrigeration circuits, wherein one or more refrigerant fluids are cyclically compressed and condensed in a condenser, expanded and heated in heat exchange relationship with natural gas and/or with another refrigerant fluid. Thermal energy is removed from the natural gas by the refrigerant fluid and rejected in the environment through the condenser, where compressed refrigerant is chilled and condensed by heat exchange against a cold source, e.g. air or water.
Several different natural gas liquefaction systems and methods have been developed over the decades and are known to those skilled in the art.
Natural gas liquefaction is an energy consuming process and continuing efforts are being made to improve the overall efficiency of LNG production systems.
Specifically, energy is needed to drive the compressors required to compress the refrigerant(s). Additional energy is required by ancillary facilities and units of the natural gas liquefaction system such as, but not limited to, gas pre-treatment facilities. These are required to remove undesirable components from the raw natural gas coming from the gas field, prior to liquefaction. Components to be removed from the raw natural gas include, for instance: heavy hydrocarbons (HHC), such as pentane and heavier hydrocarbons, benzene, toluene, xylene; mercaptans; hydrogen sulfide (H2S); carbon dioxide; and moisture (H2O).
Several natural gas pre-treatment facilities are used to remove one or more of the above-mentioned undesired components. By way of example, such pretreatment facilities include, but are not limited to, sweetening facilities, adapted to remove carbon dioxide and other acid gas components (hydrogen sulfide, mercaptans, and the like), de-hydration facilities, adapted to remove moisture, optionally fractionation systems or the like, adapted to remove and recover heavy hydrocarbons.
There is a continuous need for further improving the energy efficiency of natural gas liquefaction systems and methods.
According to an aspect, disclosed herein is a system for producing liquefied natural gas, comprising a natural gas feed and a natural gas liquefaction facility having a refrigeration circuit. The refrigeration circuit comprises a refrigeration compressor adapted to compress at least one refrigerant fluid, a driver adapted to drive the refrigeration compressor, a cooler to remove heat (thermal energy) from the refrigerant fluid during or after compression, and a heat exchanger configured to receive a flow of natural gas from the natural gas feed and remove heat therefrom by heat exchange against the expanded refrigerant fluid.
The system for producing liquefied natural gas further includes a thermal energy storage system adapted to receive and store therein thermal energy rejected by the natural gas liquefaction system at a low temperature and recovered by means of a heat pump. The heat pump is adapted to collect low-temperature thermal energy rejected from the natural gas liquefaction system and transfer the rejected thermal energy to the thermal energy storage system at a higher temperature, i.e. at a temperature higher than the temperature at which the thermal energy is rejected.
Furthermore, the system may comprise at least one processing facility, powered by thermal energy from the thermal energy storage system. In some embodiments, the thermal energy stored in the thermal energy storage system may be used for power generation, i.e. thermal energy can be delivered to a thermodynamic cycle which converts thermal energy into mechanical energy, subsequently converted into electric energy.
As used herein, the term “refrigerant” or “refrigerant fluid” is any fluid capable of undergoing thermodynamic transformations of compression, cooling and expansion, in order to extract heat from the natural gas to be liquefied and to reject thermal energy removed from the natural gas.
In many LNG systems, one or more refrigerant fluids, different from the natural gas itself, are used in various combinations and are processed in closed loops, i.e. closed cycles. Some LNG systems, however, use a flow of liquefied natural gas as the refrigerant fluid in an open loop, i.e. in an open circuit, without requiring additional refrigerant fluids, different from the natural gas itself. The novel features disclosed herein can be employed in both kinds of LNG systems, as will be illustrated below, with reference to some exemplary embodiments.
The thermal energy, which is recovered through the heat pump, may be part of the thermal energy rejected from the refrigerant fluid during or after compression and prior to expansion thereof.
In addition, or alternatively, in some embodiments, the recovered low-temperature rejected thermal energy may include heat rejected from one or more processing facilities of the LNG system, different from the refrigeration circuit. For instance, rejected heat can be recovered by the batch-wise operating regeneration of a dehydration unit provided for removal of moisture from the raw natural gas prior to liquefaction in a natural gas pre-treatment facility.
The energy efficiency of the LNG system is improved, as at least part of the rejected thermal energy is not released in the environment, but rather collected in the thermal energy storage system for use by an ancillary processing facility requiring thermal energy for the operation thereof, for instance, or for other uses, such as electric power generation.
The heat pump allows storage of the thermal energy at a temperature higher than the temperature of rejection, making the thermal energy more valuable for use in thermodynamic cycles. The power required to drive the heat pump is less than the useful power recovered through the heat pump.
For instance, the natural gas liquefaction system may include one or more pre-treatment facilities, adapted to receive and pre-treat natural gas prior to deliver the natural gas to the natural gas liquefaction facility. Pre-treatment requires thermal energy, which can be entirely or at least partially provided by the thermal energy storage system.
In some embodiments, the driver which drives into rotation the refrigeration compressor or compressor train of the refrigeration circuit may include an electric motor. The electric motor can be powered by electric power generated by an electric generator driven by a thermal energy conversion system adapted to convert thermal energy into mechanical energy and to drive the at least one electric generator therewith.
The thermal energy conversion system can comprise an internal combustion engine, such as in particular a gas turbine engine, fueled with natural gas directly or indirectly delivered by the natural gas feed. In advantageous embodiments, a waste heat recovery unit adapted to recover waste heat from the internal combustion engine and further adapted to transfer waste heat to the thermal energy storage system can be provide, for further improving the energy efficiency of the system.
The LNG system may further include a carbon dioxide capturing facility adapted to receive flue gas from the internal combustion engine and remove carbon dioxide therefrom. Thermal energy required to operate the carbon dioxide capturing facility can be at least partly provided by the thermal energy storage system.
Further embodiments and additional features of the system according to the present disclosure are set forth here below with reference to the enclosed drawings.
According to another aspect, the present disclosure relates to a method for liquefying natural gas with a natural gas liquefaction system.
The method includes flowing natural gas in a heat exchanger of a natural gas liquefaction system and removing thermal energy from the natural gas by heat exchange against a refrigerant fluid. Low-temperature thermal energy is removed from the refrigerant fluid through a refrigerant cooler. Moreover, the method comprises the step of recovering low-temperature thermal energy rejected from the natural gas liquefaction system and transferring the rejected thermal energy to a thermal energy storage system through a heat pump at a temperature higher than the temperature at which the thermal energy has been rejected.
Reference is now made briefly to the accompanying drawings, in which:
In general terms, disclosed herein are systems and methods adapted to improve the overall efficiency of a natural gas liquefaction system, wherein thermal energy (heat) rejected by the natural gas liquefaction system is exploited to reduce the amount of energy required to operate the system. Specifically, part of the low-temperature thermal energy removed from the natural gas during the liquefaction process is recovered and used for various purposes, instead of being released in the environment. The recovered low-temperature thermal energy is transferred to a thermal energy storage system at a temperature higher than the temperature at which it is rejected, using a heat pump. The heat pump can be driven by electric energy also used to drive the refrigeration compressor of the refrigeration cycle. The overall energy efficiency of the system is thus increased, since low-temperature thermal energy is exploited for operating one or more thermal energy requiring facilities of the system. The ecological footprint of the LNG production system can be reduced.
In addition to recovering thermal energy from the refrigerant of one or more refrigeration cycles, low-temperature thermal energy can be recovered from any location of the natural gas liquefaction system, where thermal energy is rejected at a temperature sufficiently above ambient temperature.
When multiple refrigeration circuits are used, rejected thermal energy can be recovered from at least one of the refrigeration circuits. In embodiments, one or more refrigeration circuits may include more than one compressor or compressor stage. Thermal energy can be recovered between two sequentially arranged compressors or compressor stages, through an intercooling heat exchanger, for instance, or downstream the last compressor, or compressor stage, possibly upstream of a refrigerant condenser.
In general, not the entire rejected thermal energy is recovered, but only a fraction thereof, while a residual fraction of thermal energy at near-to-ambient temperature can be rejected in the environment.
While recovering thermal energy from the refrigerant after compression thereof is particularly beneficial, thermal energy can be recovered also from other sources of rejected thermal energy. A suitable source of recoverable low-temperature thermal energy can be the pre-treatment section of the LNG system, at the outlet of a reactor and/or drier regeneration system, for instance.
Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function and use of the systems, devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the systems, devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. Features described or illustrated in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention.
Referring now to the drawings,
After removal of undesired components such as acid gas (carbon dioxide, hydrogen sulfide, mercaptans), heavy hydrocarbons and moisture therefrom, pretreated natural gas (NG) is delivered from the gas pre-treatment facility 5 to a natural gas liquefaction facility schematically shown at 7, including a refrigeration circuit. The natural gas liquefaction facility 7 can be configured as any natural gas liquefaction facility known in the art. By way of example, but without limitation, the natural gas liquefaction facility 7 can be based on using any suitable liquefaction process. By way of example only, and not limitation, any of the following types of liquefaction processes could be used: single refrigeration cycles, such as nitrogen cycles and single mixed refrigerant cycles, double refrigeration cycles, such as propane-mixed refrigerant LNG cycles, double mixed refrigerant cycles, three refrigerant cycles, or others.
The natural gas liquefaction facility 7 may include one or more refrigeration circuits in combination, with one or more heat exchangers adapted to bring the natural gas flow into heat exchange relationship with one or more refrigerant fluids, depending upon the kind of natural gas liquefaction facility 7 used. In general terms, the natural gas liquefaction facility 7 includes at least one compressor arrangement, comprised of one or more compressors, adapted to compress, i.e. pressurize, at least one refrigerant fluid. A driver arrangement drives one or more compressors into rotation. The driver arrangement may in turn include one or more drivers. The compressed refrigerant fluid is cooled and condensed in a condenser arrangement, which may in turn comprise one or more condensers adapted to remove heat from the compressed refrigerant fluid. A refrigerant expansion arrangement is further included in the natural gas liquefaction facility 7 and adapted to expand the cooled and condensed refrigerant(s). A heat exchanger of the natural gas liquefaction facility 7 is configured to receive a flow of natural gas and remove heat therefrom by heat exchange with the expanded refrigerant fluid, wherein the heat exchanger can include one or more heat exchanger units.
The structure, arrangement and operation of the various components of the natural gas liquefaction facility 7 can vary and largely depend upon the gas liquefaction technology used. For the sake of the present disclosure, what matters is that the natural gas liquefaction facility 7 includes a compressor arrangement comprised of one or more compressors and relevant driver(s), condensers, expanders and heat exchangers in combination, to cause one or more refrigerant fluids to undergo cyclic thermodynamic transformations, whereby thermal energy is removed from the natural gas NG flowing through the LNG production system 1 and rejected therefrom.
For the sake of simplicity, the natural gas liquefaction facility 7 is represented in
Thermal energy transferred in heat exchanger 17 from the natural gas NG to the refrigerant fluid, is removed from the refrigerant fluid in condenser 13. As will become apparent from the following description, according to various embodiments the natural gas liquefaction system 1 may include more than one condenser, and/or different refrigerant coolers in combination, wherefrom heat extracted from the natural gas is rejected.
Liquefied natural gas LNG is collected in a liquefied natural gas storage and offloading facility 19 including an LNG storage tank 22 and a cryogenic pump 20.
To improve the energy efficiency of the system 1, a thermal energy storage system 21 is provided, wherein thermal energy rejected by the natural gas liquefaction facility 7 is collected at a temperature higher than the rejection temperature, i.e. the temperature at which the thermal energy, or part thereof, is rejected from the natural gas liquefaction facility 7.
A heat pump 23 driven by an electric motor 25 is provided to recover low-temperature thermal energy rejected from the refrigerant fluid and transfer said thermal energy at a higher temperature in the thermal energy storage system 21. A low-temperature heat transfer fluid circuit 23A is provided between the condenser 13 and the cold side of the heat pump 23, and a high-temperature heat transfer fluid circuit 23B is provided between the hot side of the heat pump 23 and the thermal energy storage system 21. Heat transfer fluids circulate in the respective circuits 23A, 23B by means of pumps (not shown).
In one implementation the heat pump 23 can be a trans-critical heat pump.
Mechanical power generated by the electric motor 25 is used to transfer the thermal energy from a lower temperature at the condenser 13 to the higher temperature at the thermal energy storage system 21.
The electric motor 25 and the compression system 9 can be powered by an electric distribution grid as described in greater detail later on with reference to the following figures.
Instead of rejecting the thermal energy removed from the natural gas in the environment, as in LNG systems of the current art, said thermal energy is at least partly collected and stored in the thermal energy storage system 21 at a temperature suitable for use in other processing facilities of the system 1.
In the exemplary embodiment of
The natural gas liquefaction and heat recovery method performed by the system 1 of
With continuing reference to
In
Heat rejected at the cooler 13 is recovered via heat pump 23, while heat rejected at the condenser 13A is rejected in the environment. In general, therefore, only a fraction of the rejected thermal energy from the refrigerant fluid is recovered from the compressed refrigerant fluid(s) used in the natural gas liquefaction facility 7. The amount of thermal energy recovered through the heat pump 23 and the amount of thermal energy rejected in the environment depend upon the temperature in the low-temperature side of the heat pump 23 and upon the temperature at which the refrigerant fluid condenses.
In
In one implementation, electric energy can be generated by an electric generator 33 driven by a thermal energy conversion system 35, herein also referred to shortly as a thermodynamic system 35, which converts thermal power into mechanical power. The thermodynamic system 35 may include an open thermodynamic cycle, such as a Bryton cycle, using a gas turbine engine. The gas turbine engine may be fueled with natural gas NG from the natural gas feed 3. In other embodiments, the thermodynamic system 35 may include a closed thermodynamic cycle, such as a Rankine cycle using water or an organic fluid as a working fluid.
In embodiments, the thermodynamic system 35 may include a combined top, high-temperature cycle and a bottom, low-temperature cycle, for instance a high-temperature gas turbine cycle, the waste heat whereof is used to power a bottom Rankine cycle, e.g. a steam or organic Rankine cycle.
As schematically depicted in
With continuing reference to
The system 1 of
As noted above, the natural gas liquefaction facility 7 is represented herein in a simplified manner, as including a simple refrigeration circuit. In embodiments, the natural gas liquefaction facility 7 may include one or more closed or open refrigeration circuits in combination, with one or more heat exchangers adapted to bring the natural gas flow into heat exchange relationship with one or more refrigerant fluids, depending upon the kind of natural gas liquefaction facility 7 used.
As in
Thermal energy removed from the natural gas NG in the heat exchanger 17 and absorbed by the refrigerant fluid is removed from the refrigerant fluid in cooler 13 and condenser 13A and at least partly transferred, via heat pump 23 driven by electric motor 25 and relevant circuits 23A, 23B, to the thermal energy storage system 21 at a temperature higher than the temperature at which heat is discharged from cooler 13.
The natural gas NG is cooled until reaching a liquid state and liquefied natural gas LNG is collected in LNG storage and offloading facility 19, including storage tank 22 and cryogenic pump 20.
As in the previously described embodiments, also in the embodiment of
Similarly to the embodiment of
In the embodiment of
In some embodiments, an additional heating unit, for example a heater and/or a superheater, can be provided to add further thermal energy to the process fluid (steam or vapor) of the thermodynamic system 35, if the thermal energy from the thermal energy storage system 21 is not sufficient. In the schematic of
The thermodynamic circuit of steam or vapor turbine 51 further includes a condenser 59, a working fluid storage tank 60 and a pump 61.
The working fluid of the thermodynamic system 35 can be for example water, if a steam Rankine cycle is used, or an organic fluid, such as pentane, cyclopentane, carbon dioxide and the like, if an organic Rankine cycle (ORC) is used.
In order to further improve the overall energy efficiency of the system 1, in some embodiments the system 1 may further include one or more renewable energy collectors, to collect and exploit energy from renewable energy sources, such as solar energy or wind energy. In one implementation a concentrated solar power plant (CSP plant) 71 can be functionally coupled to the thermal energy storage system 21. The CSP plant 71 can include any kind of solar concentrator, for instance using heliostats, parabolic troughs, or the like. Solar energy is collected in form of heat, transferred via a heat transfer circuit 73 to the thermal energy storage system 21, and stored therein.
In further embodiments, renewable energy sources may be used to generate electric power, that in turn can be distributed, through the electric energy distribution grid 29, to one or more users or units connected thereto. In
In addition to, or instead of photovoltaic panels 77, a different kind of energy collector can be used to collect energy from renewable sources, for instance wind turbines 83 of a wind farm, electrically coupled to the electric energy distribution grid 29 via inverters 85.
With continuing reference to
The system 1 of
More specifically, the top thermodynamic cycle 35A can include an internal combustion engine. In one implementation, the top thermodynamic cycle 35A includes a gas turbine engine 91 comprised of an air compressor 93, a power turbine 95 and a combustion chamber 97. The gas turbine engine 91 can be fueled with natural gas from the natural gas feed 3 (duct 99) and/or from the BOG duct 59. The gas turbine engine 91 is drivingly coupled to an electric generator 33A, to convert mechanical power generated by the gas turbine engine 91 into electric power, which is delivered to the electric energy distribution grid 29.
Waste heat can be recovered from the flue gas of the gas turbine engine 91 in a waste heat recovery unit 100. In an implementation, the waste heat recovery unit 100 includes a waste heat recovery heat exchanger 101 (shortly referred to as WHR heat exchanger 101). A working fluid of the closed bottom thermodynamic cycle circulates in the WHR heat exchanger 101 in heat exchange relationship with the flue gas from the gas turbine engine 91. The bottom thermodynamic cycle 35B can be a Rankine cycle, preferably using an organic fluid. Pressurized working fluid is vaporized and possibly superheated in the WHR heat exchanger 101 and expanded in the vapor turbine 51, cooled and condensed in the condenser 59, collected in tank and pumped by pump 61 to the WHR heat exchanger 101 again. As in
In embodiments, further thermal power can be recovered from the flue gas in an auxiliary waste heat recovery heat exchanger 103 (auxiliary WHR heat exchanger 103) of the waste heat recovery unit 100, arranged along the flue gas path, downstream of the WHIR heat exchanger 101. Waste heat recovered in the auxiliary WHR heat exchanger 103 is delivered through a heat transfer circuit 107 to the thermal energy storage system 21.
If the temperature of the waste heat in the auxiliary WHR heat exchanger 101 is too low for direct transfer to the thermal energy storage system 21, said waste heat can be pumped to the thermal energy storage system 21 by heat pump 23 or by a further heat pump (not shown) for the same purpose.
Flue gas from the WHR heat exchanger 101 (and optionally auxiliary WHR heat exchanger 103) can be treated in a carbon dioxide capture facility 41 prior to be discharged in the environment through a stack 105, to remove carbon dioxide therefrom, and thus reduce the environmental impact of the system 1.
Carbon dioxide can be captured in the carbon dioxide capture facility 41 using any suitable post-combustion carbon capturing system or equivalent system aimed at separating and concentrating carbon dioxide generated by hydrocarbon combustion, for further uses. In embodiments of the present disclosure, thermal power from the thermal energy storage system 21 can be used to entirely or partly powering the carbon dioxide capturing facility 41. In
The carbon dioxide capturing facility 41 can comprise carbon dioxide post treatment and export. Carbon dioxide flowing from the carbon dioxide capturing facility 41 and from the gas pre-treatment facility 5 can be gathered and transferred outside the LNG system 1 as schematically shown in
While in the previously described embodiments electric energy generated by a thermodynamic system 35 is used to drive both the heat pump 21 and the compressor arrangement 47 of the natural gas liquefaction facility 7, in other embodiments the compressor arrangement 47 can be driven by an internal combustion engine, such as a gas turbine engine, while an electric generator, driven by the same or an additional internal combustion engine, can be used to power the heat pump 23 and other ancillary equipment and facilities of the system 1.
In the above description reference has been made to a generic natural gas liquefaction facility 7, since features of the present disclosure can be beneficial in combination with any natural gas liquefaction system.
With continuing reference to
By way of example,
While in all the above-mentioned cycles one or more refrigerants are processed in closed circuits, LNG systems using open refrigeration circuits are known, wherein the same liquefied natural gas is used as the refrigerant. Rejected heat recovery via the heat pump 23 can be provided also in such open-circuit LNG systems. An exemplary open circuit using LNG as refrigerant for natural gas liquefaction is shown schematically in
In addition to recovering low-temperature thermal energy rejected from the refrigerant coolers of the refrigeration circuit, in some embodiments, low-temperature thermal energy can be recovered also from other low-temperature heat sources of the system 1, for instance from the pre-treatment section 5 during batchwise regeneration cycles performed therein.
While the invention has been described in terms of various specific embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without departing form the spirit and scope of the claims. In addition, unless specified otherwise herein, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102020000030023 | Dec 2020 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/025467 | 11/26/2021 | WO |
