System and method for producing liquefied natural gas (LNG)

Abstract

The present invention relates to a system and method for producing liquefied natural gas (LNG), comprising the following means: - means for a pre-cooling step (100) of the natural gas stream (1), - means for a liquefying step (200) of the natural gas stream (1), and - means for a sub-cooling step (300) of the natural gas stream (1), wherein the means for the pre-cooling step (100) comprise: • means for pre-cooling the natural gas stream (1) for circulating a mixture of NH3 ammonia refrigerant and water (2), and • means for producing NH3 ammonia refrigerant fed by a line for circulating the low pressure steam generated by the heat of the exhaust gases from at least one gas turbine (401, 402) coupled to the liquefying (200) and / or sub-cooling (300) step.

Description

[0001] Liquefied natural gas (LNG) production system and method

[0002] Technical sector

[0003] The present invention belongs to the technical sector relating to liquefied natural gas (LNG) production systems.

[0004] Background of the invention

[0005] Liquefied natural gas (LNG) is natural gas that has been processed for transportation in a liquid state. Natural gas is transported in this liquid state at atmospheric pressure and in a temperature range of approximately -150°C to -170°C.

[0006] To convert natural gas into liquid, the treated gas is cooled to approximately -162°C, which is the temperature at which methane, its main component, changes phase to a liquid state at atmospheric pressure.

[0007] In the liquefaction process, refrigerant gases are compressed, producing cold liquids such as propane, ethane / ethylene, methane, nitrogen, or mixtures of these, which then evaporate as they exchange heat with the natural gas stream. In this way, the natural gas is cooled to the point where it becomes liquid. Once the gas has been liquefied, it undergoes expansion so it can be stored at atmospheric pressure. The liquefied natural gas (LNG) produced is stored in special tanks and then transferred to special transport tankers.

[0008] Known liquefaction systems are mostly based on industrial or aeroderivative turbines from manufacturers such as General Electric (GE), operating at nominal power levels in the tens of megawatts. Known liquefaction and subcooling nodes or stages are based on large-scale technologies such as the ConocoPhillips Optimized Cascade Process (OCP) (ethylene and methane open cycle) and Air Products AP-X (mixed refrigerant and nitrogen expansion cycle).

[0009] The use of a refrigeration system with an ammonia absorption cycle is also known in the food industry. Explanation of the invention

[0010] The subject of the invention is a system and method for large-scale liquefied natural gas (LNG) production using refrigerants in several stages: pre-cooling, liquefaction, and sub-cooling. The system and method of the invention employ various refrigeration cycles to lower the temperature of a natural gas stream.

[0011] The invention consists of a system and a method for producing liquefied natural gas (LNG), comprising a stream adapted for the circulation of natural gas, the system being adapted for cooling and / or expanding the natural gas stream at low temperatures of -140 e C to -170°C in cooling and / or expansion stages to produce liquefied natural gas (LNG) at atmospheric pressure. The system object of the invention comprises the following means:

[0012] • means for a pre-cooling stage of the natural gas stream,

[0013] • means for a liquefaction stage of the natural gas stream located after the pre-cooling stage, and

[0014] • means for a subcooling stage of the natural gas stream located after the liquefaction stage, where the means for the precooling stage comprise:

[0015] • means for pre-cooling the natural gas stream adapted for the circulation of a mixture of refrigerant ammonia NH3 and water, and

[0016] • means for the production of ammonia NH3 refrigerant fed by a line adapted for the circulation of low-pressure steam, between 1 and 6 bar, generated by the exhaust heat of at least one gas turbine coupled to the liquefaction and / or subcooling stage.

[0017] The expression “below” means that said means or equipment are located downstream of the previous ones.

[0018] It is known that the precooling stage is carried out using propane or a mixed refrigerant as a refrigerant. It is also known that the refrigerant in the subcooling stage, usually a mixed refrigerant, requires cooling in the precooling stage.

[0019] The system and method object of the invention presents several advantages depending on the arrangement of the pre-cooling, liquefaction and sub-cooling stages or nodes.

[0020] In one embodiment, the invention also presents advantages in the system for coupling compressors to industrial gas turbines. The liquefaction and subcooling stages comprise at least two compressors, which may be identical, coupled alternately to two gas turbines, such that the alternating arrangement of the cooling compressors with each gas turbine allows for equipment redundancy to maximize availability. The gas turbines are powered by fuel derived from the vaporized fraction of the natural gas stream after its expansion.

[0021] The plant's thermal demand consists of the fuel required by the turbines to generate electrical power, and the steam demand for the pre-cooling stage. The coupling scheme for the compressors to the drive systems could be the one proposed or another.

[0022] The system and method object of the invention allows obtaining productions of liquefied natural gas (LNG) potentially greater than 12 million tons per year and specific CO2 emissions less than 0.15 ton CO2 / ton LNG for plants with heat rejection at 20 e C by using only two large industrial turbines, on the order of hundreds of megawatts, with waste heat integration to provide thermal power through low-pressure steam to the pre-cooling cycle without requiring additional combustion of natural gas.

[0023] This precooling technology, i.e., using ammonia and water, also allows for the elimination of a compressor and a gas turbine associated with the precooling stage, as it does not require electrical power as the driving force for cooling, but rather heat in the form of low-pressure steam from the heat recovery exchangers of the gas turbine exhaust gases. As indicated, propane is conventionally used for the precooling cycle, and a compressor and a gas turbine are employed. That is, in the system of the invention, the precooling cycle allows for the integration of residual heat from the exhaust gases of the gas turbines of the other nodes, allowing for better fuel utilization and reducing the total number of gas turbines and compressors required compared to other technologies.

[0024] Although the invention allows productions exceeding 12 million tons per year, the plant could be designed for lower productions using lower power turbines. The object of the invention is a method for producing liquefied natural gas (LNG), which comprises a stream adapted for the circulation of natural gas, the method being adapted for cooling and / or expanding the natural gas stream to low temperatures of -140 e C to -170°C in cooling and / or expansion stages to produce liquefied natural gas at atmospheric pressure. The method comprises the following steps:

[0025] - a pre-cooling stage of the natural gas stream,

[0026] - a liquefaction stage of the natural gas stream located after the pre-cooling stage, and

[0027] - a subcooling stage of the natural gas stream located after the liquefaction stage, where the precooling stage comprises:

[0028] • pre-cooling of the natural gas stream by circulating a refrigerant mixture of ammonia NH3 and water, and

[0029] • The pre-cooling for the production of ammonia NH3 is fed by a line for the circulation of low-pressure steam generated by the exhaust heat of at least one gas turbine coupled to the liquefaction and / or subcooling stage.

[0030] Accordingly, the invention relates to a method and a system whereby a treated natural gas stream containing light hydrocarbons, for example, methane, ethane, propane, butane and small amounts of nitrogen N2 under pressure conditions of 30 bar to 80 bar is cooled to low temperatures, for example, from -150 °C to -165 °C, in a series of cooling and / or expansion steps to produce a liquefied product at atmospheric pressure.

[0031] In one embodiment, initial precooling to approximately -30°C is first carried out using an ammonia NH3-water refrigerant absorption system that includes several heat exchangers. In one embodiment, the ammonia NH3-water refrigerant mixture cycle operates at pressures above atmospheric pressure to prevent air from entering the cycle under vacuum conditions.

[0032] As discussed above, the heat for production, for example, in a pure ammonia NH3 refrigerant column, is provided by low-pressure steam generated from the heat of the exhaust gases from the gas turbines, which are used to drive the refrigerant compressors in the subsequent liquefaction and subcooling stages. Since a large part of the waste heat from the exhaust gases is supplied to the pre-cooling stage in the form of low-pressure steam, a specific heat rejection per ton of LNG production to the environment in the liquefaction unit is required, which is, according to the exemplary embodiment, between 40% and 80% higher than a scheme using propane as the working fluid in the pre-cooling, for a process temperature after heat rejection of 20 e C.

[0033] In the method and system object of the invention, the cooling and condensation of the working fluid of the liquefaction stage is also carried out in the pre-cooling stage.

[0034] The proposed invention offers advantages in terms of economies of scale, as it features very low specific power consumption, potentially allowing production rates exceeding 12 Mtpa per liquefaction train with gas turbines with nominal power outputs in the hundreds of megawatts. Therefore, the system and method of the invention would be competitive with large-scale technologies that use propane or mixed refrigerant for precooling and mixed refrigerants / pure components for liquefaction and subcooling, which require a greater number of rotating equipment.

[0035] Brief description of the drawings

[0036] To complement the description being made and in order to help better understand the characteristics of the invention, a drawing is attached as an integral part of said description in which, for illustrative and non-limiting purposes, the following has been represented:

[0037] Figure 1. Shows a diagram of a first embodiment of the liquefied natural gas production system object of the invention.

[0038] Figure 2. Shows a diagram of a second embodiment of the liquefied natural gas production system object of the invention.

[0039] Figure 3. Shows a diagram of a third embodiment of the liquefied natural gas production system object of the invention.

[0040] Figure 4. Shows a diagram of an example of the coupling of refrigeration compressors to gas turbines and heat recovery schemes to generate steam supplied to the pre-cooling stage.

[0041] Examples of embodiments of the invention

[0042] The invention proposes three exemplary embodiments for carrying out the liquefaction and subcooling steps of the precooled stream using different technological elements and working fluids or coolants. Therefore, three liquefaction systems are proposed with a common precooling technology and distinct liquefaction and subcooling cycles, each with different advantages.

[0043] In the embodiments corresponding to Figures 1 and 2, the means for pre-cooling the natural gas stream (1) are thermally decoupled from the subcooling stage (300).

[0044] The technical problem solved by these embodiments is to reduce the thermal load in the pre-cooling stage or node (100), while reducing its dimensions. With the system and method of the object of the invention, a change in the cooling driving force occurs in the pre-cooling stage: through water vapor, thermal power, instead of the compression of a refrigerant, electrical power.

[0045] That the means for cooling the natural gas stream (1) of the pre-cooling stage (100) are thermally decoupled from the sub-cooling stage (300) means that the pre-cooling stage (100) is independent of the sub-cooling stage (300), or what is the same, that the pre-cooling stage (100) is a thermally autonomous stage with respect to the sub-cooling stage (300).

[0046] In this way, the cooling requirements and therefore the thermal demand in said pre-cooling stage (100) and its size are minimized, allowing the thermal demand of the system to be satisfied by residual heat present in the liquefaction plant in an amount greater than 90% of the same.

[0047] Referring to a first embodiment according to Figure 1, a natural gas stream (1) containing, in one example, light hydrocarbons and nitrogen, is subjected to a pre-cooling (100), liquefaction (200) and sub-cooling (300) step.

[0048] The pre-cooling stage (100) comprises an absorption refrigeration system of a mixture of ammonia NH3 refrigerant and water (2), as indicated above.

[0049] A solution of ammonia NH3 refrigerant and water (2) is pumped at a pressure at which pure ammonia NH3 (3) can be condensed at room temperature after a first condenser (109). The concentration of ammonia NH3 refrigerant in the ammonia NH3 refrigerant mixture and water (2) depends on the temperature reached after a second condenser (108) coinciding with that reached in the first condenser (109).

[0050] More specifically, the concentration depends only on the temperature reached in the second condenser (108). The pressure after a pump (101) depends on the temperature reached by the pure NH3 refrigerant (3) in the first condenser (109), which is the same as that reached by the NH3 / water refrigerant mixture (2) in the second condenser (108).

[0051] The means of the pre-cooling stage (100) comprise:

[0052] - a pump (101) for pumping a mixture of ammonia NH3 refrigerant and water (2),

[0053] - a heat exchanger (102) fed by the pump (101),

[0054] - a separation column (103) fed by the heat exchanger (102),

[0055] - a condenser (109) for the condensation of pure ammonia NH3 refrigerant (3) at room temperature,

[0056] - a kettle (104) powered by low-pressure steam,

[0057] - a heat exchanger (105) for subcooling the ammonia refrigerant NH3,

[0058] - a two-phase ammonia expander (106) that feeds a heat exchanger (107) for pre-cooling the natural gas stream (1) and another heat exchanger (110) for cooling and condensing the refrigerant fluid of the liquefaction stage (200),

[0059] - a condenser (108) for the condensation of the refrigerant mixture ammonia NH3-water (2).

[0060] Additionally, a liquid turbine may be used to expand the depleted solution of ammonia NH3 refrigerant and water after the heat exchanger (102) to take advantage of the pressure jump (not shown in the figures). In an exemplary embodiment, the outlet of the two-phase ammonia expander (106) may be partially vaporized.

[0061] Low process temperatures after heat rejection are desirable to obtain high cooling efficiencies. In the pre-cooling stage (100) the thermal service is provided in the form of low pressure steam, at approximately 3 bar, in the reboiler (104) of the column. Heat is efficiently recovered in the heat exchanger (102) to minimize service requirements, while subcooling of the pure liquid ammonia NH3 refrigerant (3) is carried out in the heat exchanger (105) to minimize vapor formation during expansion in the liquid ammonia two-phase expander (106). The liquid ammonia two-phase expander (106) allows to minimize the vaporized fraction by increasing the liquid fraction after expansion, compared to a valve, and therefore to improve the cooling efficiency.In the pre-cooling stage (100) the fraction of ammonia that vaporizes after the expansion is wasted since the pre-cooling is carried out with the latent heat of phase change from liquid to vapor, and not with the sensible heat of the vapor.

[0062] As previously discussed, the designs shown in the embodiments of Figures 1 and 2 allow the subcooling stage (300) to be decoupled from the precooling stage (100), thereby reducing the thermal requirements of the column reboiler (104), such that the steam demand of the reboiler (104) can be practically satisfied with the residual heat of the exhaust gases from the available gas turbines (401, 402), almost completely avoiding the combustion of fuel in boilers for its production, which would lead to a reduction in the thermal efficiency of the plant and an increase in CO2 emissions.

[0063] As mentioned, this pre-cooling technology also allows for the elimination of a compressor and gas turbine required in conventional propane pre-cooling systems, since the driving force for cooling is supplied in the form of low pressure vapor, making the compressor and turbine unnecessary for this pre-cooling stage (100).

[0064] In an exemplary embodiment, the natural gas stream (1) is pre-cooled to approximately -30°C in the heat exchanger (107).

[0065] As indicated in Figure 4, the means for producing ammonia NH3 are fed by a line adapted for the circulation of low pressure steam (17), between 1 and 6 bar, generated by the heat of the exhaust gases (16) of at least one gas turbine (401, 402) coupled to the liquefaction (200) and / or subcooling (300) stage, in the heat exchangers (405, 406).

[0066] In the embodiment shown in Figure 1, in the liquefaction stage (200) the refrigerant fluid is pure ethylene (4) and at least two pressure levels or stages are used for liquefying the natural gas stream (1). The working fluid in the liquefaction stage (200) could alternatively be pure ethane.

[0067] The means of the pre-cooling stage (100) comprise means for cooling and condensing the refrigerant fluid of the liquefaction stage (200), whether this is the refrigerant of any of the embodiments shown in Figures 1 to 3.

[0068] In the embodiments shown, the refrigerant stream from the second liquefaction stage (200) is condensed in the heat exchanger (1 10).

[0069] As indicated, in the first embodiment, shown in Figure 1 , a pure refrigerant, for example ethylene, is used at different pressure levels, at least two pressure levels, to liquefy the natural gas stream (1 ) to reach a temperature of approximately -100 °C. Two or three stages or pressure levels are possible, using a recuperator (203) to subcool the incoming condensed ethylene flows before their expansion.

[0070] The liquefaction stage (200) then comprises two heat exchangers (201) with the natural gas stream (1), two separation vessels (202) at different pressures and the recovery unit (203).

[0071] The natural gas stream (1 ) is liquefied in the heat exchangers (201 ) against the boiling ethylene refrigerant stream (4).

[0072] Liquefaction is carried out at least at two pressure levels or stages, although it could be three. This is because, as it is a pure component, it boils at a constant temperature and, in order to reduce losses, the cooling temperature of the natural gas stream (1) and the heating temperature of the ethylene refrigerant stream (4) must be as close as possible, so it is carried out sequentially at various pressure levels.

[0073] The ethylene vapors after cold recovery are compressed in a compressor (210, 211) with interstage cooling that can be duplicated for redundancy. If necessary, a separation unit for the recovery of natural gas liquids (NGL) and heavy hydrocarbons can be installed between the precooling and liquefaction sections.

[0074] As discussed above, in the embodiment shown in Figure 1, the ethylene refrigerant stream (4) is condensed in the heat exchanger (110) of the pre-cooling stage (100). The ethylene is further subcooled in a gas recuperator (203) to minimize the vaporized fraction when expanding in the valves (204).

[0075] The subcooling stage (300) of the liquefied natural gas stream (1) to, for example, approximately -155°C is carried out with a gas expansion cycle with pure nitrogen (4) as the fluid. Alternatively, the refrigerant may be a noble gas such as pure argon.

[0076] The embodiment example of Figure 1 comprises a recuperator (301) of the refrigerant fluid that is placed prior to an expansion of the refrigerant (4) in an expander (307) for subcooling the natural gas stream (1) in a heat exchanger (304).

[0077] As discussed, said subcooling stage (300) is independent of the precooling stage (100). In addition, the subcooling stage (300) is also independent, i.e. thermally decoupled, from the liquefaction stage (200). This avoids an additional thermal load on the precooling stage (100) and minimizes the steam requirements, so that the demand can be practically met with the exhaust heat of the drive turbines (401, 402) of the compressors (210, 211, 310, 311) and, if necessary, additionally with the exhaust gases from the turbines of a power plant auxiliary to the liquefaction plant.

[0078] As discussed, the subcooling stage (300) corresponding to the embodiment of Figure 1 comprises a nitrogen expander (307). Said expander (307) can be mechanically coupled to a first pre-compression stage (302) to the nitrogen compressor (310, 311) consisting of two stages with intermediate cooling.

[0079] The subcooling stage (300) may comprise a liquefied natural gas vessel (303). In case the plant is slightly deficient in steam, small quantities of fuel gas may be taken from the liquefied natural gas vessel (303) for additional steam generation, conveniently increasing the temperature of the natural gas before expansion in a heat exchanger (304) or two-phase expander, with a minimum increase in specific emissions. Said additional steam may be generated in the heat exchangers (405, 406) by combusting the fuel with the exhaust gases (16) from the turbines by means of the burners (403, 404).

[0080] The natural gas stream (1 ) after the second heat exchanger (201 ) is at approximately -100°C, after which the subcooling stage (300) is carried out, by means of the gas expansion cycle using nitrogen (5) as the working fluid. In particular, the operating temperature range of the expansion cycle is extended to a wider temperature range because ethylene is used as the pure component for liquefaction, with respect to schemes using mixed refrigerant, which allow temperatures below -100 to be reached. eC. However, given the sensible heat exchange at relatively constant heat capacities, the nitrogen flow and temperature in the heat exchanger (304) can be conveniently adjusted to achieve very low temperature approximations between the hot and cold curves. The natural gas stream (1 ) is subcooled to a temperature such that upon expansion in the expander (305) to ambient pressure, a sufficient quantity of fuel (6) is generated in the vapor phase to satisfy the thermal demand of the plant. The cold in the fuel stream (6) is recovered in the exchanger (304) and potentially also in the ethylene recuperator (203). A fuel gas compressor (306) is required to achieve the operating pressure ratio of the gas turbine (401 , 402).

[0081] The liquefied natural gas product (7) from the liquefied natural gas container (303) is directed to storage.

[0082] In an exemplary embodiment, a scrubbing unit may be integrated after the pre-cooling stage (100) to remove heavy hydrocarbons from the natural gas stream (1). These fractions must be removed before cooling to lower temperatures. A two-phase ammonia expander (106) is employed in the liquid NH3 ammonia expansion stage and a heat exchanger (105), for example a gas-liquid subcooler, to maximize efficiency, leading to thermal cooling efficiencies of greater than 50% for pre-cooling.

[0083] The advantages of this first embodiment are the simplicity, ease of operation, operability and controllability of the plant, in addition to the reduction in storage, since the mixing of refrigerant hydrocarbons is avoided and their composition is optimized. Furthermore, printed circuit exchangers and / or plate and fin heat exchangers, typical of pure refrigerants, can potentially be used instead of coil and winding exchangers, which are heavier and less readily available to manufacture. The nitrogen compressor (310, 311) requires approximately two-thirds of the total electrical consumption, thereby reducing costs compared to heavy hydrocarbon compressors and improving safety, as it is an inert fluid.Furthermore, as previously discussed, the use of absorption precooling avoids the need for a refrigerant compressor and the associated drive system, a gas turbine, typical of a propane precooling stage. Furthermore, low heat rejection temperatures, preferably water-cooled, are required to achieve low process temperatures, enabling competitive cooling efficiencies in the precooling loop and low thermal steam demands, which can be met with the available waste heat.

[0084] Additionally, using a redundant scheme for the cooling compressors (210, 21 1 , 310, 31 1 ) driven by industrial turbines (401 , 402) allows the operating conditions in the cycles to be made more flexible, freely selecting the power distribution between the liquefaction (200) and subcooling (300) stages, based on the cut-off temperatures of each stage. This advantage is also inherent to the rest of the embodiment examples shown in Figures 1 to 3.

[0085] In this way, the compressors (210, 21 1 , 310, 31 1 ) of each stage are duplicated, therefore, the following elements are available as shown in figures 1 to 3:

[0086] - two ethylene compressors (210, 211), which are identical.

[0087] - two nitrogen compressors (310, 311), which are identical.

[0088] As can be seen in Figure 4, each gas turbine (401, 402) drives one ethylene compressor (210, 211) and one nitrogen compressor (310, 311). In this way, the plant can operate with only one turbine (401, 402) operating at half capacity.

[0089] In plants designed with conventional technologies (Conoco Phillips Optimized Cascade), operating conditions cannot be freely selected, but are restricted by the fact that each compressor must have a fixed power given by the size of the coupled gas turbine.

[0090] With the dual-turbine system driving two alternating pairs of duplicate compressors, this isn't the case. Only the sum of the power outputs of the two compressors must equal the turbine's output, making the distribution between the compressors more flexible, allowing for the selection of operating conditions.

[0091] As discussed, the subcooling stage (300) in both embodiments corresponding to figures 1 and 2 operate independently of the precooling stage (100), so that the required precooling power is reduced, allowing the steam balance of the column reboiler (104) to be practically satisfied with the heat available from the output gases of the turbines (401, 402).

[0092] If the pre-cooling stage (100) were to pre-cool more of the other loops (e.g., more mixed refrigerant, etc.), there might not be enough steam generated by the turbines (401, 402) to meet demand.

[0093] In the ethylene and nitrogen implementation scheme, it is suggested that if the steam demand is not fully satisfied by the residual heat from the turbines, it can be done by producing it through a greater generation of combustible steam in the liquefied natural gas container (303), but this would involve a relatively small amount.

[0094] In the second embodiment, included in Figure 2, a mixed refrigerant (8) comprising methane, ethylene, propane and butane is used to carry out the liquefaction of natural gas.This embodiment example allows selecting the cut-off temperature of the natural gas stream (9) and methane recycle of the subcooling stage (300) from which the subcooling is carried out by means of the progressive expansion of the natural gas operating with an open methane cycle so as to obtain a first vaporized fraction (10) rich in nitrogen already at high pressure in the first expansion for use as fuel, corresponding to the thermal demand of the liquefaction plant, avoiding a high concentration of nitrogen in the loop by avoiding the recirculation of this stream, and potentially eliminating the need for nitrogen rejection systems in the separation stage of the low-pressure liquefied natural gas LNG and improving the thermodynamic efficiency of the liquefaction, by decreasing the recirculated quantity of this volatile component and consequently the power of the methane compressors.Avoiding the nitrogen rejection unit represents a significant cost reduction and overcomes an inherent problem in liquefaction technologies that employ an open-cycle methane loop. Purging nitrogen during the initial expansion results in less accumulation in the loop.

[0095] According to the above, this possibility is due to the fact that the mixed refrigerant can be used to select the temperature at which expansion begins. This is not possible using a refrigerant such as ethylene, as it is a pure component, since said temperature is limited by the saturation temperature at a pressure close to atmospheric, so as not to operate in a vacuum. Referring to the second embodiment of Figure 2, the subcooling stage (300) comprises a vapor return stream from the expansion of the liquefied stream configured for subcooling the natural gas stream in the subcooling stage (300) and for feeding into a heat exchanger (220) of the liquefaction stage (200), where the outlet temperature of the natural gas stream (9) and the vapor return stream from the liquefaction stage (200) is between -110 e C and -130 eC and the pressure after expansion in an expander (320) of the stream (9) in the subcooling stage (300) is between 10 and 20 bar and because the system also comprises the extraction of a steam stream (10) in the expander (320) as fuel for the system.

[0096] That is, a mixed refrigerant (8) condensed and subcooled in the condenser (110) composed of methane, ethylene, propane and butane is used to carry out the liquefaction at an outlet temperature of the natural gas stream (9) that is carefully selected between, for example, -110 e C and -130 eC, such that after expansion in an expander (320) of the subcooling stage (300) of the natural gas stream (1 ) and of the vapor return stream, an adequate amount of combustible vapor (10) is produced in a two-phase vessel (321 ) to satisfy the thermal demand of the plant. The outlet pressure of the expander (320) is selected, between 10 and 20 bar, coinciding with the pressure ratio of the gas turbines (401 , 402) in operation. If necessary, a small booster compressor can be used to reach higher operating pressures. The liquid stream from the two-phase vessel (321 ) is expanded sequentially in two stages, with recovery of cold gas in heat exchangers (322, 323) to produce a liquefied natural gas product LNG (11 ) at near ambient pressure.The vapor return streams are used to subcool the natural gas before it is fed to expanders (320, 324) in heat exchangers (322, 323), e.g., recuperators, respectively. In addition, the remaining cold is used to pre-cool the discharge of a methane compressor (310, 311, 312). This stream is also directed to the heat exchanger (220) for subsequent cooling and liquefaction.

[0097] Therefore, with the present arrangement, the elimination of a large part of the nitrogen contained in the feed natural gas stream (1) is achieved, avoiding the need for expensive nitrogen rejection units (NRU), obtaining an LNG product according to specifications, for comparatively higher concentrations of N2 in the natural gas stream (1 ). In addition to the advantage of potentially avoiding nitrogen rejection units (NRU), this liquefaction process is thermodynamically more efficient, achieving higher production rates.

[0098] As in the first embodiment, the methane circuit is independent of the pre-cooling stage (100), which allows the steam demand to be met with the residual heat available from the gas turbines (401, 402), without the need for additional boilers and without increasing emissions. However, the sub-cooling stage (300) is not completely decoupled from the liquefaction stage (200), since there is a recirculation of methane from the recuperator (322) to the heat exchanger (220). This implies a greater flow rate of mixed refrigerant (8) which has an impact on a greater thermal demand in (110).Due to the high thermodynamic efficiency of the loop using mixed refrigerant (8) and the minimization of the recirculated flow of methane due to the low temperatures reached in the stream (9) by means of said mixed refrigerant (8), reaching a high liquid fraction after the expansion in the (320), this does not lead to the low pressure steam required in the boiler (104) exceeding that generated with the residual heat of the exhaust gases from the gas turbines (401, 402).

[0099] By means of the nitrogen or methane recuperator (301, 322, 323), according to the embodiment example, prior to expansion, it is possible to decouple the pre-cooling stage (100) from the sub-cooling (300) of natural gas. This does not involve steam consumption because no external heat input is required to generate the refrigerant. This leads to a reduction in the thermal load in the pre-cooling stage (100) due to the configuration of the sub-cooling stage (300), which is not possible in other natural gas liquefaction schemes, for example, in those in which a single mixed refrigerant is used to carry out the liquefaction and sub-cooling of the natural gas stream, or several mixed refrigerants in cascade.

[0100] The advantages of the open-loop mixed refrigerant and methane scheme are slightly higher thermodynamic efficiency and comparatively lower steam demand, resulting in comparatively higher liquefied natural gas (LNG) production and lower specific CO2 emissions. The recovered power from the open-loop expansion stages can be added to the refrigerant compressors by auxiliary engines. Approximately two-thirds of the total work is required by the mixed refrigerant compressor. Mixed refrigerant / methane compressor trains can be duplicated to increase redundancy and availability.

[0101] In the embodiment corresponding to Figure 3, the means for pre-cooling the natural gas stream (1) are thermally coupled with the subcooling stage (300).

[0102] More specifically in the example shown, in the liquefaction stage (200) the refrigerant fluid is pure ethylene (4) and at least two pressure stages are used for the liquefaction of the natural gas stream (1). The working fluid in the liquefaction stage (200) could alternatively be pure ethane.

[0103] The working fluid in the subcooling stage (300) is a mixed refrigerant (12) comprising ethylene, methane and nitrogen, or nitrogen, methane and ethane. Additionally, the mixed refrigerant (12) is condensed in the liquefaction stage (200) of the natural gas stream (1).

[0104] In the pre-cooling stage (100), the refrigerant used in the subsequent liquefaction stage (200), for example pure ethylene (4), is condensed, in which two or three pressure levels are used. The ethylene condenses the mixed refrigerant (12) and the natural gas (1 ) in heat exchangers (231 ) of the liquefaction stage (200). The mixed refrigerant (12) subcools the natural gas (1 ) from approximately -100 e C to a temperature between -140 e C and -160 e C in a cryogenic exchanger (331 ). To do this, the mixed refrigerant is subcooled in the same cryogenic exchanger (331 ) and subsequently expanded in an expander (332), the outlet of which is recirculated to the cryogenic exchanger (331 ), the mixed refrigerant (12) vaporizing as it absorbs heat from the incoming streams. The outlet of the mixed refrigerant at approximately -100 eC in slightly superheated steam conditions is redirected to the recuperator (233) allowing further subcooling of the pure ethylene refrigerant streams (4), minimizing the vaporized fraction thereof after its expansion in the valves (204). The superheated mixed refrigerant exiting the recuperator (233) is redirected to the compressor (310, 311) which consists of two stages with intermediate cooling. By carrying out the subcooling of the natural gas stream by means of latent heat exchange of the mixed refrigerant, as opposed to sensible heat, a reduction in the recirculated flow rate is achieved in the subcooling stage (300).

[0105] On the other hand, the natural gas at the exit of the exchanger (331) is expanded in a two-phase expander (334) to atmospheric pressure and is fed to a separator (333) producing liquefied natural gas LNG (15) and a vaporized fraction that is used as fuel, after recovering cold in the exchangers (331, 233). This fuel stream is divided into a fraction (13) used for the generation of low pressure steam supplied to the pre-cooling cycle (100) and another fraction (14) that is compressed in a compressor (335) and is supplied to the gas turbines (401, 402) that drive the compressors (21 1, 210, 311, 310) of the refrigerants of the subcooling (300) and liquefaction (200) stages. The natural gas outlet temperature after the exchanger (331) is selected to generate the vaporized fraction after the expansion that satisfies the thermal demand of both the gas turbines and the pre-cooling cycle (100).

[0106] In this embodiment, the subcooling stage (300) is not independent, i.e., it is not thermally decoupled from the precooling stage (100), requiring precooling of the mixed refrigerant in a heat exchanger (111) prior to its condensation in the liquefaction stage (200). This also entails an increase in the required ethylene flow (4), which in turn implies a greater thermal demand in the condenser (110) of the precooling stage (100). Therefore, the residual heat of the exhaust gases (16) of the gas turbines (401, 402) alone is not sufficient to generate sufficient steam to satisfy the thermal demand of the reboiler (104) of the precooling stage (100), which requires additional steam generation. In the present configuration, it is estimated that said residual heat from the gas turbines (401, 402) covers approximately 50% of the thermal demand of the boiler (104).The generation of additional steam (17) can be carried out by means of burners (403, 404) in the steam generating boiler (405, 406) coupled to the outlet of the gas turbines (401, 402). That is, the exhaust gas (16) itself is used as an oxidizing agent for the fuel (13). This arrangement for additional steam generation can also be integrated into the previous embodiments, although it is not shown in the figures.

[0107] This results in increased CO2 emissions. However, the subcooling cycle (300) that uses a mixed refrigerant exhibits high thermodynamic efficiency. Since heat exchange occurs through latent heat, vaporization of the mixed refrigerant, as opposed to sensible heat, e.g., the nitrogen expansion cycle in the first embodiment of Figure 1 , the refrigerant / natural gas ratio is much lower, resulting in less compression work. Additionally, for an equivalent volumetric flow rate and pressure ratio, the mixed refrigerant requires less compression power than a component such as nitrogen. Therefore, the specific power consumption in the embodiment of Figure 3 for heat rejection temperatures of 20 eC is only approximately 145 kWh / ton of LNG, while the specific steam demand of the liquefaction plant is approximately 430 kWh / ton of LNG. For the first embodiment in Figure 1, the specific power and steam consumption are approximately 200 kWh / ton and 315 kWh / ton, respectively. The second embodiment in Figure 2, in turn, has a somewhat lower specific power consumption, of approximately 190 kWh / ton, and a steam consumption of 245 kWh / ton. An additional aspect that contributes to greater efficiency in terms of power consumption in the LNG production process is that, since a greater quantity of fuel is required for steam generation, the subcooling temperature of the natural gas required at the exchanger outlet (331 ) is higher, which reduces the cooling capacity in this loop.Finally, this embodiment can also adopt the compressor drive system described in Figure 4 or another one.

[0108] The embodiment shown in Figure 3 shows a relatively greater efficiency improvement when three pressure levels are used in the liquefaction stage (200) with pure ethylene (4), since condensation of the mixed refrigerant (12) also takes place in this stage, with greater heat exchange taking place at low temperatures. By means of this configuration with three pressure levels, not shown in the figures, a specific electrical consumption of approximately 130 kWh / ton LNG is achieved, while the steam demand remains at approximately 430 kWh / ton LNG.

[0109] In all the implementation examples, specific CO2 emissions (due to liquefaction) of less than 0.15 ton / ton are achieved, using industrial turbines with thermal efficiencies between 33-38%.

[0110] The third embodiment therefore achieves a reduction in electricity consumption at the expense of increasing the thermal demand for steam, which leads to an increase in the size of the pre-cooling stage equipment (100). For practical purposes, for an available power determined by the size of the gas turbines, the embodiment shown in Figure 3 allows a greater quantity of natural gas to be processed, producing more LNG, such that the specific emissions (in tonnes of CO2 / tonne of LNG) do not increase drastically.

[0111] The advantages of the third embodiment are primarily a higher production of liquefied natural gas (LNG) compared to the previous examples and other established technologies, at equivalent power supplied by gas turbines. Despite using a mixed refrigerant, the plant's operation is simpler compared to technologies that employ mixed refrigerant vapors (DMR - Dual Mixed Refrigerant, MFC - Mixed Fluid Cascade, and even C3MR), as it consists of only three components: ethylene or ethane (the refrigerant used in the liquefaction stage, the latter also being available in natural gas), methane, and nitrogen (also available in natural gas), avoiding heavier hydrocarbons such as propane and butane. Its optimization is therefore less complex.This embodiment presents a compromise between efficiency and simplicity by using the most efficient system (mixed refrigerant) for the subcooling stage (300) at the lowest temperatures, and simpler schemes for precooling (100) and liquefaction (200) with cooling loops with pure components.

[0112] The embodiment example included in Figure 4 provides for the coupling of the cooling compressors (210, 211, 310, 311, 312) to the gas turbines (401, 402) and the heat recovery exchangers (405, 406) to generate steam supplied to the pre-cooling stage (100).

[0113] Specifically, in the turbines (401, 402) low pressure steam (17) is generated by the residual heat of the exhaust gases (16) from the turbines (401, 402) in the heat exchangers (405, 406). This steam is supplied to the reboiler (104) to purify the mixture of ammonia NH3 and water from the pre-quenching stage (100). The purified ammonia is used as a cooling fluid for said pre-quenching stage (100), as already mentioned.

[0114] If applicable, the excess power of an auxiliary power plant can be coupled to the liquefied gas turbines (401, 402) by means of auxiliary engines.

[0115] Compressor redundancy for each stage is desirable primarily because of the high power outputs achieved with a single unit. It also allows the design operating conditions to be selected to fully utilize the refrigerant cooling capacity, since the relative compressor load is not limited by the fixed sizes of a single turbine. Furthermore, this feature increases availability, as the plant can maintain output above 50% while one of the gas turbines (401, 402) is running.

[0116] In one embodiment, two industrial gas turbines are used as compressor drivers, with power outputs in the range of up to hundreds of megawatts, thermal efficiencies of between 30-40% with pressure ratios of 10-20, and heat rejection up to 20 eC, allows single train capacities greater than 12 mtpa, with specific emissions (due to liquefaction and auxiliary consumption) of less than 0.15 tonnes of CO2 / tonne of liquefied natural gas (LNG). The thermal demand in the form of low pressure steam from the pre-cooling system (100) allows a simplified heat recovery network from the gas turbine exhaust gases in the form of a low pressure steam boiler (405, 406). The auxiliary burners (403, 404) allow additional steam to be generated if required by using the turbine exhaust gases (16) as an oxidising stream. This arrangement allows CO2 emissions to be concentrated in the exhaust gases, which may be beneficial if CO2 capture and sequestration systems are implemented downstream.

[0117] The above configuration could be implemented with smaller turbines, and the configuration of the compressors and turbines could also be different from the above in another embodiment.

Claims

CLAIMS 1 Liquefied natural gas (LNG) production system, comprising a stream adapted for the circulation of natural gas (1 ), the system being adapted for cooling and / or expanding the natural gas stream (1 ) to low temperatures of -140 e C to -170°C in cooling and / or expansion stages to produce liquefied natural gas (LNG) at atmospheric pressure, where the system comprises the following means: - means for a pre-cooling stage (100) of the natural gas stream (1), - means for a liquefaction stage (200) of the natural gas stream (1) located downstream of the pre-cooling stage (100), and - means for a subcooling stage (300) of the natural gas stream (1) located after the liquefaction stage (200), where the means of the precooling stage (100) comprise: • means for pre-cooling the natural gas stream (1 ) adapted for the circulation of a mixture of ammonia NH refrigerant 3 and water (2), and • means for the production of ammonia NH refrigerant 3 fed by a line adapted for the circulation of low pressure steam generated by the heat of the exhaust gases of at least one gas turbine (401, 402) coupled to the liquefaction (200) and / or subcooling (300) stage. 2.- Liquefied natural gas (LNG) production system, according to claim 1, characterized in that the means of the liquefaction stage (200) and the means of the subcooling stage (300) comprise at least two compressors (210, 211, 310, 311, 312) coupled alternately to two gas turbines (401, 402). 3.- Liquefied natural gas (LNG) production system, according to any one of the preceding claims, characterized in that the means of the pre-cooling stage (100) comprise means for cooling and condensing a refrigerant fluid of the liquefaction stage (200). 4.- Liquefied natural gas (LNG) production system, according to claim 3, characterized in that the means of the pre-cooling stage (100) comprise: - a pump (101) for pumping a mixture of ammonia NH refrigerant 3 - water (2), - a heat exchanger (102) fed by the pump (101), - a separation column (103) fed by the heat exchanger (102), - a condenser (109) for the condensation of ammonia NH refrigerant 3 pure at room temperature, - a kettle (104) powered by low-pressure steam, - a heat exchanger (105) for subcooling the ammonia NH refrigerant 3 (3), - a two-phase ammonia expander (106) feeding a heat exchanger (107) for pre-cooling the natural gas stream (1) and another heat exchanger (110) for cooling and condensing the refrigerant fluid from the liquefaction stage (200), - a condenser (108) for the condensation of the ammonia NH refrigerant mixture 3 -water (2). 5.- Liquefied natural gas (LNG) production system, according to any one of the preceding claims, characterized in that the means for pre-cooling the natural gas stream (1) are thermally decoupled from the subcooling stage (300). 6.- Liquefied natural gas (LNG) production system, according to claim 5, characterized in that the means of the subcooling stage (300) comprise a recuperator (301, 322, 323) of a refrigerant fluid. 7.- Liquefied natural gas (LNG) production system, according to any one of claims 5 to 6, characterized in that in the liquefaction stage (200) the refrigerant fluid is pure ethylene or ethane and at least two pressure stages are used for the liquefaction of the natural gas stream (1). 8.- Liquefied natural gas (LNG) production system, according to claim 7, characterized in that the means of the subcooling stage (300) comprise a gas expansion cycle with pure nitrogen or argon refrigerant. 9.- Liquefied natural gas (LNG) production system, according to any one of claims 5 to 6, characterized in that the working fluid in the liquefaction stage (200) is a mixed refrigerant comprising methane, ethylene, propane and butane. 10.- Liquefied natural gas (LNG) production system according to claim 9, characterized in that the subcooling stage (300) comprises a vapor return stream from the expansion of the liquefied stream configured for subcooling the natural gas stream in the subcooling stage (300) and for feeding into a heat exchanger (220) of the liquefaction stage (200), where the outlet temperature of the liquefaction stage (200) of the natural gas stream (9) and of the vapor return stream is between -1 10 e C and -130 eC and the pressure after expansion in an expander (320) of the stream (9) in the subcooling stage (300) is between 10 and 20 bar and because the system also comprises the extraction of a steam stream (10) in the expander (320) as fuel for the system. 11.- Liquefied natural gas (LNG) production system, according to any one of claims 1 to 4, characterized in that the means for pre-cooling the natural gas stream (1) are thermally coupled with the subcooling stage (300). 12.- Liquefied natural gas (LNG) production system, according to claim 11, characterized in that in the liquefaction stage (200) the refrigerant fluid is pure ethylene or ethane and at least two pressure stages are used for the liquefaction of the natural gas stream (1). 13.- Liquefied natural gas (LNG) production system, according to claim 12, characterized in that the working fluid in the subcooling stage (300) is a mixed refrigerant comprising ethylene, methane and nitrogen or ethane, methane and nitrogen. 14.- Liquefied natural gas (LNG) production system, according to claim 13, characterized in that the subcooling stage (300) comprises an outlet of the mixed refrigerant to a recuperator (233) of the liquefaction stage (200) to subcool the pure ethylene or ethane refrigerant. 15.- Method of producing liquefied natural gas LNG, comprising a stream adapted for the circulation of natural gas (1 ), the method being adapted for cooling and / or expanding the natural gas stream (1 ) at low temperatures of -140 eC to -170°C in cooling and / or expansion stages to produce liquefied natural gas at atmospheric pressure, where the method comprises the following stages: - a pre-cooling stage (100) of the natural gas stream (1), - a liquefaction stage (200) of the natural gas stream (1) located after the pre-cooling stage (100), and - a subcooling stage (300) of the natural gas stream (1) located after the liquefaction stage (200), where the precooling stage (100) comprises: • pre-cooling of the natural gas stream (1 ) by circulating a mixture of ammonia NH refrigerant 3 and water (2), and • pre-cooling for the production of ammonia NH refrigerant 3(3) is fed by a line for the circulation of low-pressure steam generated by the exhaust heat of at least one gas turbine (401, 402) coupled to the liquefaction (200) and / or subcooling (300) stage.

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