The natural gas liquefaction method is intended for the production of liquefied natural gas (hereinafter - LNG) ensuring the regulation of produced LNG capacity under the conditions of fluctuations in ambient climatic parameters and can be used at gas processing plants located in various regions of the country or the world regardless of the climatic zone.

The technology of processing natural gas to liquefied gas depends significantly on the properties of feed gas, the presence of unwanted impurities (H2O, CO2, H2S, Hg, N2, He, OCS, mercaptans, etc.) and heavy hydrocarbons in it. The treatment of feed gas at gas processing plants includes purification with further compression and cryogenic processing which is a highly energy-consuming process. It is estimated that gas liquefaction module accounts for 45% of the capital expenses for the entire LNG plant representing 25-35% of the total project costs and up to 50% of subsequent operating expenses (Development of technologies for the production of liquefied natural gas [Electronic source] URL: https://chemtech.ru/razvitie-tehnologij-proizvodstva-szhizhe nnogo-prirodnogo- gaza/, access date 28.10.2022).

Liquefaction technologies are based on the use of refrigerating circuits in which the refrigerant cools the counterflow of natural gas by means of gradual expansion and compression. The majority of modem technologies for gas liquefaction assumes the use of three refrigerating circuits as this improves the process of natural gas condensation. In large capacity processes, natural gas is liquefied using two methods: cascade (“propane - ethylene - methane”) or closed refrigerating circuits with the use of mixed refrigerants.

The selection of method for natural gas liquefaction is determined by the original composition of natural gas; at that, the influence of other factors is often not considered. All modem technologies of liquefaction use the potential of natural refrigerants such as water or air depending on the territorial location of plants. However, significant climate warming is observed in the majority of parts of the world in the recent decade and, as a consequence, the mean annual and mean seasonal temperatures of water and air have become considerably higher and the local fluctuations in these temperatures are 10-15 °C. As a result, the assumed cooling potential of natural refrigerants may go down by 30-50% and have sizeable fluctuations depending on the season which is not considered by modem technologies.

PRIOR ART

A combined multi-circuit refrigeration method for gas liquefaction is known which includes sequential cooling of the supplied gas flow, at least in two areas of heat exchange to provide liquefied product where cooling of the supplied gas flow is ensured by means of evaporating refrigerants. The refrigerant is only partially evaporated in the coldest area of heat exchange in the range of coldest temperatures to obtain a partially evaporated refrigerant. The partially evaporated refrigerant is recirculated in the process of recirculating cooling which includes the stages of additional evaporation of the partially evaporated refrigerant in the area of additional heat exchange at the temperatures above the highest temperature in the coldest area of heat exchange, compression of the additionally evaporated refrigerant and cooling of the compressed refrigerant flow in order to obtain the coldest refrigerant. The entire flow of the compressed refrigerant is cooled by means of the cooling stages for the entire flow of the compressed refrigerant in the area of additional heat exchange through indirect heat exchange with an additionally evaporating partially evaporated refrigerant or cooling of the entire flow of the compressed refrigerant in the heat exchange area preceding the coldest area of heat exchange through indirect heat exchange with respective evaporating refrigerant, aftercooling of the compressed refrigerant in the area of additional heat exchange through indirect heat exchange with partially evaporated refrigerant (invention patent RU 2307990, IPC F25J 1/02, filed on 16.03.2004, published on 10.10.2007). The disadvantages of the invention are as follows:

• the application of multi-circuit configuration of liquefaction is accompanied by the big amount of expensive dynamic equipment used; • the use of a gas-liquid flow for cooling in equipment located in series requires observation of certain hydrodynamic conditions of the medium flow which, in its turn, may significantly limit the range of unit operation;

• gas liquefaction method given here is conditioned by the operation of additional areas of heat exchange under harsh temperature conditions linked with the big difference in the temperatures of input flows.

There is a method and device known for cooling of hydrocarbons flow comprising the flow ( 10) of mixed refrigerant including the first mixed refrigerant which passes through one or more heat exchangers (12) obtaining the cooled flow (20) of mixed refrigerant. At least a part of the cooling flow (30) including the second mixed refrigerant is expanded (14) obtaining one or more expanded cooling flows (40a), at least one of which can be sent through one or more heat exchangers (12) to cool the flow (10) of mixed refrigerant thereby obtaining the flow (20) of mixed refrigerant which is used to cool (22) the flow (70) of hydrocarbons. Temperature (Tl) and flow rate (Fl) of at least a part of the cooled flow (20) of mixed refrigerant as well as the flow rate (F2) of flow (30) are continuously monitored using the data on flow rate Fl and temperature Tl (invention patent RU 2469249, IPC F25J 1/02, filed on 10.07.2008, published on 10.12.2012). The disadvantages of the invention are as follows:

• the application of mixed refrigerant in double-circuit liquefaction configurations is accompanied by the use of auxiliary equipment as compared to single-circuit liquefaction configurations;

• under the conditions of fluctuations in ambient climatic parameters, the used coolers (32) will not ensure maintaining the temperatures of refrigerant flows at the levels necessary to achieve the required LNG capacity, therefore LNG production will depend on ambient conditions: the temperature of air or water used for the cooling.

There is also a complex for natural gas processing and liquefaction known including gas processing unit, treated gas liquefaction unit, feed gas main pipeline, commercial gas main pipeline and commercial product transportation unit connected by direct and reverse connections, in particular by pipelines. In this complex, gas processing unit comprises at least feed natural gas treatment section, ethane fraction and natural gas liquids (NGL) extraction section, NGL fractionating section, booster compressor station (BCS), section for commercial gas treatment for liquefaction, ethane fraction treatment section and utilities section containing at least product storage buffer park subsection, boil-off gas compressor subsection and liquefaction refrigerant components treatment subsection and ensures the production of commercial gas as well as commercial gas treated for liquefaction, ethane fraction, propane and/or butane fraction and/or their mixture and pentane-hexane fraction to be supplied to the main pipeline of commercial gas. Treated gas liquefaction unit comprises at least sequentially located precooling, liquefaction and subcooling sections and section of compressors of one or more refrigerants. Commercial product transportation unit consists of at least commercial product cooling section, commercial product main storage park section and shipment section characterized by the fact that downstream BCS, commercial gas is cooled by means of injecting the flow of cold gas coming from treated gas liquefaction unit sections, feed natural gas treatment section of gas processing unit and/or section for commercial gas treatment for liquefaction (gas processing unit) and/or ethane fraction treatment section of gas processing unit are supplemented by the units of extensive gas purification (patent RU 2699160, ICP F25J 3/00, filed on 28.12.2018, published on 03.09.2019). The disadvantage of the invention is the fact that when water and air temperatures increase significantly as compared to the design parameters, the heat exchanging equipment ceases to keep the predetermined process conditions for mass transfer and cryogenic equipment which leads to the decrease in the amount of heat transferred by this equipment to the process flows which has to be compensated by the loss in capacity of the plant in terms of liquefied natural gas.

There is a method of liquefied natural gas flow production known where the process of natural gas flow liquefaction is preceded by the removal of heavy hydrocarbon components having higher molecular weight than the one of butane, including the following stages where natural gas vapor flow (1) with the pressure and temperature of the supplied gas flow is mainly created; the supplied gas flow (1) is routed to a distillation column (10) having two or more stages (11) of separation; lower flow (17) from the bottom of the distillation column (10) and upper flow (16) from the overhead of the distillation column (10) are selected; at that, the upper flow (16) contains a relatively lower amount of heavy hydrocarbon components than the lower flow (17); at least a part of the upper flow (16) is liquefied leading to the production of liquefied natural gas flow; characterized by the fact that the gas flow (1) supply to the distillation column (10) is preceded by the division of the supplied gas flow (1) into the first and second subflows (3 a, 3b) having the selected ratio of the division; the first subflow (3a) is supplied to the distillation column (10) through the first point (7a) of supply at the bottom of the distillation column (10) under the pressure which is at least equal to the pressure of the supplied flow (1) excluding the pressure drop caused by the specified division of the supplied flow (1); at that, the first subflow (3a) is not exposed to heat starting from the point of division of the supplied flow (1) to the first point (7a) of the first subflow (3 a) supply to the distillation column (10); the second subflow (3b) is cooled in the heat exchanger (6) down to the temperature which is lower than the one of the supplied gas flow; and the cooled second subflow (7) is supplied to the distillation column (10) at the second point (7b) of supply located higher than the first point (7a) of supply; at that, the pressure in the pressure reduction device is deliberately not reduced neither in the first nor in the second subflows (3a, 3b) (patent RU 2402592, IPC C10G 5/06, C10L 3/00, F25J 1/02, F25J 3/02, filed on 07.12.2005, published on 27.10.2010). The disadvantages of the invention are as follows:

• lack of feed natural gas preliminary purification from impurities (water, hydrogen sulfide, etc.) which in the course of natural gas condensation will pass into the commercial product degrading its quality;

• low quality of fractionation in the distillation column 10 due to the insufficient number of stages;

• lack of characteristics of the refrigerant (24) supplied to the cooler (26) which shall have the temperature lower than the one of the reflux flow (18); • under the conditions of fluctuations in ambient climatic parameters, the coolers used will not ensure the maintenance of refrigerant flow temperatures at the levels necessary to achieve the required LNG capacity, therefore LNG production will depend on ambient conditions: the temperature of air or water used for cooling.

The most similar to the filed invention is the gas liquefaction method including (a) cooling of the feed gas (1) in the first heat exchange area (21; 705) by indirect heat exchange with one or more flows (23) of the refrigerant, provided in the first refrigerating system; and removal of, essentially, the liquefied feed flow (i.e. after its adiabatic expansion via throttling to the atmospheric pressure the flow contains liquid fraction from 0,25 to 1,0) (25) from the first heat exchange area; (b) further cooling of, essentially, the liquefied feed flow in the second heat exchange area (27; 401; 511; 817) by indirect heat exchange with one or more flows of the refrigerant expanded with work done (29; 205; 405; 509; 515; 619; 712; 719; 815; 823), ensured by the second closed refrigerating system; and removal of additionally cooled, essentially, liquefied feed flow (33) from the second heat exchange area; and (c) expansion with work done (31; 75; 203; 403; 513; 617; 711; 717; 813; 821) of two or more flows (65, 73; 65, 201; 501, 509; 65, 616; 709, 716; 811, 819) of the gaseous cooled and compressed refrigerant in the second refrigerating system; where the operation of the second refrigerating system includes the following stages: (1) compression (83; 305; 507) of one or more vapors (81; 82) of the refrigerant to ensure the flow (59; 517) of compressed refrigerant; (2) cooling of all or part of (59; 61; 306) the compressed refrigerant flow in the third heat exchange area (63; 303; 503; 601; 701; 809) by indirect heat exchange with one or more flows (79; 67 and 301; 407; 505 and 519; 710; 825 and 827) of the refrigerant expanded with work done, to ensure the flow (65; 501; 709; 812) of gaseous cooled down compressed refrigerant, besides, there is no cooling of the feed gas or cooled feed gas flow in this heat exchange area; (3) expansion (31; 31 and 403; 31 and 513; 711 ; 821) with work done of the gaseous cooled and compressed refrigerant flow to ensure the flow of cold refrigerant expanded with work done, ensuring one flow (29; 405; 515; 712; 823) from one or more flows of the refrigerant expanded with work done in the second heat exchange area; and (4) expansion (75 ; 203 ; 31 ; 617; 717 ; 813 ) with work done (73 ; 201 ; 501 ; 616; 716; 811 ) of the gaseous cooled and compressed refrigerant flowto ensure the intermediate temperature flow (77; 205; 301; 505; 619; 719; 815), which is introduced or complements the operating cooling cycle, ensured by the heated cold flow expanded with work done in the second heat exchange area or downstream; besides, the flow rate of the flow (29; 405; 515; 712; 823) of the refrigerant expanded with work done in the second heat exchange area is less than the aggregate flow rate of one or more flows (79; 67+301; 407; 505+519; 710; 825+827) of the refrigerant expanded with work done in the third heat exchange area. Usage of the invention will allow to increase the efficiency and operational flexibility of the gas liquefaction processes (invention patent RU 2331826, IPC F25 J 1/062, filed on 14.09.2004, published on 20.08.2008). The disadvantages of the invention are as follows:

• usage of heat exchanging sections 3, 51 allows to maintain the temperature of the gas routed for liquefaction, and the refrigerant used in the first heat exchange area after the compression, at the required levels under the conditions of fluctuating ambient climatic parameters, but the temperature of the refrigerant used in the second heat exchange area is climate-dependent, as part of the refrigerant flow downstream section 83, routed to the natural gas liquefaction, is not aftercooled in section 71.

• there are methods of liquefying natural gas by using mixed refrigerant, which require ensuring a certain phase equilibrium at the compression stage and air and/or water cooling of the mixed refrigerant flows, and therefore, required temperatures of the mixed refrigerant flows under the conditions of fluctuating ambient climatic parameters, which is not considered in this invention;

• as a cold source for heat exchanging sections 3, 51 , 71 the use of an external circulating refrigerating circuit with evaporating refrigerant is specified, but alternative cold sources allowing to use internal flows of the LNG production are not considered in the invention. A characteristic feature of all methods of LNG production implementation is a search for efficient use of cold introduced in the process both from natural sources - water and air, and from special refrigerants, in particular, light hydrocarbons. In these circumstances if special refrigerants ensure stationary operation of cryogenic equipment then the use of air and water at the initial stage of cooling of the incoming natural gas and cooling compressed hydrocarbon refrigerants makes this stage non- stationary, as the efficiency of the corresponding cooling equipment is determined by the current water and air temperature, which is determined by climatic conditions and is subject to major changes both during the day and in certain months and seasons. Substantial unsteadiness of the initial period of the LNG production leads to changing of the productive capacity of cooling equipment and the facility in general, as increase of the water and air temperature leads to lowering of the heat transfer coefficient and cooling capacity of the heat exchangers and lowering of the LNG production efficiency, while lowering of the air and water temperature leads to an inverse effect. And while comparatively minor daily variations of the temperature mode may be compensated by the air and water flowrate adjustment, their seasonal fluctuations lead to major antibate changes of the LNG production capacity in general. Moreover, the region of the LNG production facility location is also of great importance, as climate determines the selection of the natural refrigerant.

INVENTION DISCLOSURE

During the development of this invention the terms “climate-dependent LNG production”, “climate-independent LNG production”, “level of climate dependency of the LNG production” are used.

Climate-independent LNG production means such production which is isolated from the ambient environment and is an adiabatic object. The technological process of such production is ensured by special refrigerants, the operating mode of adiabatic processes is stationary, which provides for the consistency of the LNG production by commercial product, but at the same time the LNG costs are high due to high operating expenses for provision of the cryogenic LNG production with cold. Climate-dependent LNG production means such production which to a great extent uses the cooling potential of the environment and is a polytropic object. The operating mode of such production is non-stationary due to both daily and seasonal change of the ambient temperature of water and air used as natural refrigerants. Water and air temperature fluctuations may only partly be compensated by changing the flow rate of natural refrigerants into the heat exchanging apparatuses, which leads to fluctuations of the LNG production by commercial product and decrease of its annual yield. Due to the usage of the cooling potential the cost of LNG is somewhat less than with the climate-independent production.

The term “level of climate dependency of the LNG production” allows to estimate the proximity of real production to the climate-independent production and may be characterized numerically with a second midpoint value, i.e. dispersion or ² of the distribution function of the annual capacity of the facility for natural gas processing into LNG by months: x = (H:l ² X|)/i2, where Xj - specific monthly capacity, million tons of gas;

X — average monthly capacity per annum, million tons of gas.

The level of climate dependency with the technological advancements of actual production may vary from a really big number down to zero upon reaching full climate independency, when Xi=const= X.

The objective of the filed invention is to develop a method of efficient natural gas liquefaction for different climatic zones, ensuring as a technical result the increase of the LNG production yield and lowering of the climate dependency level taking into account climatic changes of the ambient temperature during the LNG production facility operation.

The set objective may be resolved by way of having a new method of natural gas liquefaction developed, which foresees performing the natural gas liquefaction process in different climatic zones, namely Arctic (Antarctic), temperate and tropical, including sequential cooling, liquefaction and subcooling of the treated natural gas, including at least one stage compression of the natural gas, with its further routing to at least one multisectional or several one-section heat exchangers located in series for cooling, liquefaction and subcooling of treated natural gas via cold transfer from the circulating flows of one or several refrigerants, containing individual components or their mixture, while each of the used refrigerants undergoes at least one-stage compression, compressed treated natural gas flows and used refrigerants are cooled in coolers using ambient cold, the used refrigerants flows are further cooled with refrigerant flows returned for compression, cooled refrigerants after the expansion are used as cold sources for cooling and/or liquefaction and/or subcooling of treated natural gas, concurrently the liquefaction process includes the aftercooling down to the minimum average monthly climatic zone temperature of the treated natural gas flows and used refrigerants cooled by ambient environment after compression by supplying cold flows to the aftercoolers from external and/or internal source, whereas as cold flows from the external source the flows of cooled natural gas and/or inert gas and/or individual hydrocarbons, circulating in a separate refrigerating circuit, are used, whereas as cold flows from the internal source a part of the treated natural gas flow routed for liquefaction is used (hereinafter - flow A), subject to additional compression and cooling using the ambient cold, further cooling and routing of a part of the compressed and cooled natural gas flow (first part of flow A) to be mixed with natural gas, routed for liquefaction, and part of the compressed and cooled natural gas flow (second part of flow A) after expansion for cooling of used refrigerants and natural gas, routed for mixing, provided however

(a) in single-circuit liquefaction configurations using mixed refrigerant, which is a mixture of at least two or more components, comprising butane, propane, ethane, ethylene, methane, nitrogen and other components, and a single multi-section heat exchanger for cooling, liquefaction and subcooling of the treated natural gas, aftercooling of treated natural gas is performed with aftercoolers, installed on the natural gas flow line downstream its compression between air coolers and/or water coolers and multisectional heat exchanger, aftercooling of the mixed refrigerant is performed with mixed refrigerant aftercoolers, installed downstream each compression stage on the lines between air coolers and/or water coolers and mixed refrigerant separators,

(b) in double-circuit liquefaction configurations using several single-section heat exchangers located in series, where the processes of cooling, liquefaction and subcooling of the treated natural gas take place with two mixed refrigerants, the first of which participates in the precooling of natural gas and consists of butane, propane and ethane and/or ethylene (hereinafter - the first refrigerant), and the second of which participates in liquefaction and subcooling of natural gas and consists of ethane and/or ethylene, methane and nitrogen (hereinafter - the second refrigerant), aftercooling of the treated natural gas and the second refrigerant is provided by aftercoolers of the treated natural gas and the second refrigerant installed on the corresponding natural gas supply lines and the second refrigerant after they are compressed between air and/or water coolers and a single-section heat exchanger providing precooling of the treated natural gas, cooling of the fi t refrigerant is provided by coolers installed on the first refrigerant supply line after each stage of the first refrigerant compressing after the corresponding air and/or water coolers,

(c) in double-circuit liquefaction configurations using a multi-section heat exchanger for cooling and liquefying natural gas treated for liquefaction with a mixed refrigerant, which is a multicomponent mixture, and a subcooler for subcooling natural gas with a refrigerant containing an individual component, the aftercooling of the treated natural gas is provided by aftercoolers of the treated natural gas installed on the natural gas line after its compression between air and/or water coolers and a multi-section heat exchanger, aftercooling of the mixed refrigerant is provided by the aftercoolers of the mixed refrigerant installed after each stage of the mixed refrigerant compressing on the supply lines of the mixed refrigerant between the air and/or water coolers and the separators of the mixed refrigerant, the aftercooling of the individual refrigerant is provided by the aftercoolers of the individual refrigerant installed on the individual refrigerant supply line after compressing refrigerant after air and/or water coolers.

Proposed solution makes it possible to use various options of single- and double-circuit liquefaction configurations as a basis for climate-dependent LNG plants with a reduced level of climate dependence due to the rational placement of additional cold sources - aftercoolers using the cooling potential of external natural refrigerants and internal sources, which provides aftercooling of processed natural gas to a constant temperature corresponding to the minimum average monthly temperature of a given month of the climate zone, which in turn allows the entire LNG production to work consistently for a month with a continuing production program.

It is reasonable to carry out the cooling process of compressed flows of treated natural gas and applied refrigerants in coolers using cold from the environment, namely in the Arctic (Antarctic) and moderate climate by means of air and/or aquatic environment and in tropical climate exclusively by means of the aquatic environment. So, in the Arctic zone on Yamal, unstable air temperature is used in the range from minus 63 to plus 37 ° C, and in the equatorial zone in Nigeria at a fairly stable air temperature (+24...+35oC) more expensive and scarce water (+25...+28oC) is used.

To increase LNG yield in conditions when atmospheric air or river or sea water temperature exceeds rated temperature, defined by maximum designed LNG yield, it is useful to supplement the liquefaction process with a precooling stage of the treated natural gas flows cooled by the external environment and the refrigerants used after compression by means of supplying cold flows to precoolers from the external and/or internal source to reach the rated temperature.

It is reasonable to use gas turbine as compressor drives for external cold source circuits in Arctic (Antarctic) and moderate climates, since the ambient temperature allows to operate at the maximum possible efficiency of a gas turbine. Since in a tropical climate, with an increase of ambient air temperature, the density decreases, the amount of air supplied to the combustion chamber drops and, as a result, the efficiency of gas turbines reduces, therefore it is advisable to use an electric motor.

It is rational to use methane or ethane or ethylene or propane or propylene or butane or isobutane or ammonia or nitrogen or freon or mixtures of individual components or cooled down water as an individual component of an external cold source circulating in a separate refrigerating circuit. The choice of specific refrigerants or their mixtures is defined by the technology used in the LNG production by means of economic calculation and logistic justification.

It is advisable to use boil-off gas flow as an internal source of cold formed during throttling after the subcooling procedure in a heat exchanger or coming from an LNG storage tank, which will enable more efficient distribution of internal process flows and use of own resources to increase energy efficiency of the proposed method of natural gas liquefaction.

LIST OF DRAWINGS

Industrial feasibility of the claimed invention is illustrated by several options of Process Flow Diagrams (Fig. 1-4) and a graphical interpretation of the calculations performed (Fig. 5). The following designations are used on the Process Flow Diagrams (Fig. 1-4):

1-49 - pipelines;

101 - treated natural gas compressor;

102 - mixed refrigerant compressor;

111 - natural gas air cooler;

112, 113 - mixed refrigerant air coolers;

121-124 - mixed refrigerant separators;

131 - natural gas cooler;

132 - natural gas liquefier;

133 - LNG subcooler;

141-145 - valves;

151 - treated natural gas aftercooler;

152, 153 - mixed refrigerant aftercoolers; 201 - treated natural gas compressor;

202 - heavy mixed refrigerant compressor;

203 - high pressure mixed refrigerant compressor;

204 - high pressure mixed refrigerant booster compressor;

211 - natural gas air cooler;

212 - heavy mixed refrigerant air cooler;

213-215 - high pressure mixed refrigerant air coolers;

231, 232 - natural gas coolers;

233 - natural gas liquefier;

234 - high pressure mixed refrigerant cooler;

241-245 - valves;

251 - treated natural gas aftercooler;

252 - heavy mixed refrigerant aftercooler;

253-255 - high pressure mixed refrigerant aftercoolers;

301 - treated natural gas compressor;

302 - mixed refrigerant compressor;

303 - refrigerant compressor with individual component;

304 - refrigerant expander with individual component;

311 - natural gas air cooler;

312, 313 - mixed refrigerant air coolers;

314, 315 - refrigerant air coolers with individual component;

321-323 - mixed refrigerant separators;

33 1 - natural gas cooler;

332 - natural gas liquefier;

333 - LNG subcooler;

334 - recuperative plate heat exchanger;

341-344 - valves;

351 - treated natural gas aftercooler;

352, 353 - mixed refrigerant aftercoolers;

354, 355 - refrigerant with individual component aftercoolers; 401 - treated natural gas compressor;

402 - mixed refrigerant compressor;

403 - internal refrigerant compressor;

404 - internal refrigerant expander;

411 - natural gas air cooler;

412, 413 - mixed refrigerant air coolers;

414, 415 - internal refrigerant air cooler;

421-424 - mixed refrigerant separators;

431 - natural gas cooler;

432 - natural gas liquefier;

433 - LNG subcooler;

441-447 - valves;

451 - internal refrigerant cooler;

452, 453 - mixed refrigerant aftercoolers.

SHORT DESCRIPTION OF DRAWINGS

Figure 1 shows single line diagram of single-circuit liquefaction of natural gas using aftercoolers according to method (a) of the given invention. It is proposed to use two-stage mixed refrigerant compressor 102 driven by a gas turbine. The flow of the mixed refrigerant via pipeline 10 from the mixed refrigerant separator 122 is fed to the first stage of the mixed refrigerant compressor 102, from which the compressed refrigerant flow is directed through the pipeline 11 for atmospheric air cooling to the mixed refrigerant air cooler (hereinafter - air cooler) 112. The gasliquid mixture from air cooler 112 enters mixed refrigerant aftercooler 152 via pipeline 12 and then the cooled flow of the mixed refrigerant is fed to the mixed refrigerant separator 123 via pipeline 13, from which the gas phase via the pipeline 14 enters the second stage of the mixed refrigerant compressor 102. Compressed gas enters the mixed refrigerant air cooler 113 via the pipeline 15 for cooling, from which the gas-liquid mixture is directed to the mixed refrigerant aftercooler 153 via the pipeline 16 and then the cooled refrigerant flow is fed to the mixed refrigerant separator 124 via pipeline 17. The liquid phase from the mixed refrigerant separator 124 via pipeline 24 is throttled at the valve 145 and then the flow via the pipeline 25 is delivered to the mixed refrigerant separator 123.

The treated natural gas is liquefied in the coil-wound heat exchanger consisting of cooler 131, liquefier 132 and subcooler 133.

The treated natural gas is sent through pipeline 1 to treated natural gas compressor 101 for compression, the flow of compressed natural gas through pipeline 2 is sent to natural gas air cooler 111 for cooling, and then the flow of cooled natural gas through pipeline 3 passes through treated natural gas aftercooler 151. Treated natural gas through pipeline 4 from treated natural gas aftercooler 151 (hereinafter - NG), the liquid phase through pipeline 26 from mixed refrigerant separator 123, representing heavy mixed refrigerant (hereinafter - HMR), and the gas phase through pipeline 18 from mixed refrigerant separator 124, representing HP mixed refrigerant (hereinafter - HP MR), are sent to natural gas cooler 131, where they are cooled by the flow of mixed refrigerant in the shell side. Next, the flow of cooled HMR through pipeline 27 is throttled at valve 144 and flowed through pipeline 28 into the shell side of natural gas cooler 131. The flow of cooled HP MR through pipeline 19 is sent to mixed refrigerant separator 121, where it is split into a liquid phase, discharged through pipeline 29 and representing medium mixed refrigerant (hereinafter - MMR), and a gas phase discharged through pipeline 20 representing light mixed refrigerant (hereinafter - LMR). Cooled NG through pipeline 5, flows of MMR through pipeline 29 and flow of LMR through pipeline 20 are sent to natural gas liquefier 132. The flow of cooled MMR through pipeline 30 is throttled at valve 143 and flowed through pipeline 31 to the shell side of natural gas liquefier 132. The flow of cooled LMR through pipeline 21 is supplied to liquefied natural gas subcooler 133. The aftercooled flow of LMR, discharged through pipeline 22, is throttled at valve 142 and is supplied to the shell side of liquefied natural gas subcooler 133 through pipeline 23. The flow of liquefied NG from natural gas liquefier 132 is throttled at valve 141 through pipeline 6 and is sent to liquefied natural gas subcooler 133 through pipeline 7. The flows of mixed refrigerant supplied to the shell side pass through subcooler 133, liquefier 132 and cooler 131 as a downward flow and enter mixed refrigerant separator 122 on the receiving line of the mixed refrigerant compressor 102 as gas flow through pipeline 9 from the coil-wound heat exchanger.

Figure 2 shows a key diagram of the double-circuit natural gas liquefaction with two mixed refrigerants using aftercoolers according to method (b) hereof. The treated natural gas through pipeline 1 is sent for compression to treated natural gas compressor 201, the flow of compressed natural gas is sent for cooling through pipeline 2 to natural gas air cooler 211, and then through pipeline 3 the flow of cooled natural gas passes through treated natural gas aftercooler 251. Treated natural gas through pipeline 4 from natural gas aftercooler 251 (NG), heavy mixed refrigerant through pipeline 30 (HMR) from heavy mixed refrigerant aftercooler 252, high pressure mixed refrigerant through pipeline 19 (HP MR) from HP mixed refrigerant aftercooler 255 are supplied to natural gas cooler 231, where they are cooled by the HMR flow entering the shell side through the pipeline 34. A part of the cooled HMR is throttled at valve 244 through pipeline 33 and is supplied to the shell side of natural gas cooler 231 through pipeline 34. The flows of cooled HP MR through pipeline 20, NG through pipeline 5 and a part of the HMR through pipeline 37 are supplied to the natural gas cooler 232. Cooled HMR through pipeline 38 is throttled at valve 243 and is supplied to the shell side of natural gas cooler 232 through pipeline 39. Cooled NG through pipeline 6 and HP MR through pipeline 21 are supplied to natural gas liquefier 233. The flow of cooled HP MR through pipeline 22 is throttled at valve 242 and flows through pipeline 23 to the shell side of natural gas liquefier 233. The flow of liquefied NG through pipeline 7 from natural gas liquefier 233 is throttled at valve 241. Flows of mixed refrigerants removed in gaseous form from the shell side of natural gas liquefier 233 and natural gas coolers 231, 232 are sent for compression to gas turbine engine compressors 203, 202. The flow of HP MR through pipeline 9, discharged from the shell side of natural gas liquefier 233, is supplied to HP mixed refrigerant compressor 203. The compressed flow of HP MR is sent for cooling through pipeline 10 to HP mixed refrigerant air cooler 213 , the cooled flow of mixed refrigerant through pipeline 11 is further cooled in HP mixed refrigerant aftercooler 253 and is then supplied for compression through pipeline 12 to two-stage HP mixed refrigerant booster compressor 204. The flow of compressed mixed refrigerant through pipeline 13 from the first stage of compressor 204 is sent for cooling to HP mixed refrigerant air cooler 214, then the HP MR flow through pipeline 14 is further cooled in HP mixed refrigerant aftercooler 254 and through pipeline 15 enters the HP mixed refrigerant cooler 234 for cooling by the HMR flow discharged through pipeline 36 from the natural gas cooler 231. The flow of cooled HP MR through pipeline 16 from the HP mixed refrigerant cooler 234 is supplied to the second stage of HP mixed refrigerant booster compressor 204. The compressed refrigerant flow through pipeline 17 is sent for cooling to HP mixed refrigerant air cooler 215, then the HP MR flow through pipeline 18 is further cooled in HP mixed refrigerant aftercooler 255 and is supplied through pipeline 19 to the natural gas cooler 231. HMR flows through pipelines 24- 26, removed from the shell side of natural gas coolers 231, 232 and from the HP mixed refrigerant cooler 234, are mixed and supplied through pipeline 27 to heavy mixed refrigerant compressor 202. The compressed HMR flow through pipeline 28 is sent for cooling to heavy mixed refrigerant air cooler 212, then the cooled HMR flow is sent through pipeline 29 to heavy mixed refrigerant aftercooler 252 for aftercooling and is sent to natural gas cooler 231 through pipeline 30.

Figure 3 shows a key diagram of the double-circuit natural gas liquefaction with one mixed refrigerant and a second refrigerant containing an individual component using aftercoolers according to method (c) hereof. It is assumed that two- stage mixed refrigerant gas turbine engine compressor 302 will be used.

The mixed refrigerant flow through pipeline 10 from mixed refrigerant separator 321 is supplied to the first stage of mixed refrigerant compressor 302, from which the compressed refrigerant flow through pipeline 11 is supplied to mixed refrigerant air cooler 312 for cooling with atmospheric air. The liquid-gas mixture through pipeline 12 from mixed refrigerant air cooler 312 is supplied to mixed refrigerant aftercooler 352 and then the cooled mixed refrigerant flow through pipeline 13 is supplied to mixed refrigerant separator 322, from which the gas phase through pipeline 14 enters the second stage of mixed refrigerant compressor 302. The compressed gas through pipeline 15 is supplied for cooling to the mixed refrigerant cooler 313, from which the liquid-gas mixture through pipeline 16 enters mixed refrigerant aftercooler 353 and then the cooled flow of refrigerant through pipeline 17 is supplied to mixed refrigerant separator 323. The liquid phase through pipeline 22 from mixed refrigerant separator 323 is throttled at valve 344 and then flows through pipeline 23 to mixed refrigerant separator 322.

The treated natural gas is sent through pipeline 1 for liquefaction into a coilwound heat exchanger consisting of natural gas cooler 331 and natural gas liquefier 332. The liquefied gas is sent for subcooling through pipeline 6 to liquefied natural gas subcooler 333, where it is subcooled by the refrigerant flow entering through pipeline 36 and containing an individual component. The subcooled liquefied gas through pipeline 7 is throttled at valve 341.

The treated natural gas through pipeline 1 is sent for compression to treated natural gas compressor 301, the flow of compressed natural gas through pipeline 2 is sent for cooling to natural gas cooler 311, and then the flow of cooled natural gas through pipeline 3 passes through treated natural gas aftercooler 351. Treated natural gas through pipeline 4 from treated natural gas aftercooler 351 (hereinafter - NG), the liquid phase through pipeline 24 from mixed refrigerant separator 322, representing heavy mixed refrigerant (hereinafter - HMR), and the gas phase through pipeline 18 from mixed refrigerant separator 323, representing the high pressure mixed refrigerant (hereinafter - HP MR), are supplied to natural gas cooler

331, where they are cooled by the flow of mixed refrigerant in the shell side. Next, the flow of cooled HMR through pipeline 25 is throttled at valve 343 and flowed through pipeline 26 into the shell side of natural gas cooler 331. The cooled HP MR flow through pipeline 19 and the cooled NG flow through pipeline 5 are supplied to natural gas liquefier 332. The cooled HP MR flow through pipeline 20 is throttled at valve 342 and flowed through pipeline 21 into the shell side of natural gas liquefier

332. Mixed refrigerant flows supplied to the shell side pass through natural gas liquefier 332 and natural gas cooler 331 as a downward flow and enter in mixed refrigerant separator 321 on the receiving line of the mixed refrigerant compressor 302 as gas flow from the coil-wound heat exchanger through pipeline 9.

The refrigerant flow containing an individual component through pipeline 27 from liquefied natural gas subcooler 333 enters recuperative plate heat exchanger 334 and then the heated refrigerant flow through pipeline 28 is supplied to the suction line of individual-component refrigerant compressor 303. The compressed refrigerant flow through pipeline 29 is supplied to individual-component refrigerant air cooler 314 for cooling with atmospheric air. The cooled refrigerant flow through pipeline 30 is supplied to individual-component refrigerant aftercooler 354 for aftercooling and then flows through pipeline 31 to the compressor part of expander 304. The compressed refrigerant flow through pipeline 32 is supplied to air cooler 315 for cooling with atmospheric air. The cooled refrigerant flow through pipeline 33 is supplied to individual-component refrigerant aftercooler 355 for aftercooling then flows through pipeline 34 to the recuperative plate heat exchanger 334 for cooling. The cooled refrigerant flow through pipeline 35 is supplied for expansion to individual-component refrigerant expander 304, and then the flow through pipeline 36 is used as a source of cold to subcool the LNG.

Figure 4 shows a key diagram of the natural gas liquefaction using aftercoolers according to method (a) with treated natural gas used as an internal source of cold.

It is assumed that gas turbine driven two stage mixed refrigerant compressor 402 will be used. Mixed refrigerant flow, over pipeline 12 and downstream from mixed refrigerant separator 422, is directed to the first stage of mixed refrigerant compressor 402, from which the compressed refrigerant flow, over pipeline 13, is subjected to air cooling by means of mixed refrigerant air cooler 412. The resulting gas-liquid mixture is then directed from the air cooler 412, over pipeline 14, to the mixed refrigerant air cooler 452, and then the cooled mixed refrigerant flow, by pipeline 15, enters mixed refrigerant separator 423, from which the gas phase is directed to the second stage of mixed refrigerant compressor 402 through pipeline 16. The compressed gas is then sent through pipeline 17 for cooling to the mixed refrigerant air cooler 413, from which the gas-liquid mixture, over pipeline 18, is delivered to the mixed refrigerant aftercooler 453, and then the cooled refrigerant flow is supplied to the mixed refrigerant separator 424 over pipeline 19. Liquid phase is then directed over pipeline 29, downstream from mixed refrigerant separator 424, and throttled at valve 447, and then flows through pipeline 40 into the mixed refrigerant separator 423.

The treated natural gas is sent through pipeline 1 for liquefaction to coilwound heat exchanger consisting of cooler 431, liquefier 432 and subcooler 432.

The treated natural gas is then sent through pipeline 1 for compression to the treated natural gas compressor 401, then the flow of compressed natural gas, over pipeline 2, is sent for cooling to the natural gas air cooler 411, and then the cooled natural gas, through pipeline 35, is directed in sufficient quantities to the inlet of internal refrigerant compressor 403. Compressed natural gas, over pipeline 36, is supplied for air cooling to the internal refrigerant cooler 414. The cooled gas flow, through pipeline 37, is further cooled in the internal refrigerant cooler 451, from which the cooled natural gas is sent over pipeline 38 to be divided into two parts. The first part, moving through pipeline 39, passes valve 466, then flows through pipeline 40 to undergo mixing, via pipeline 5, with natural gas, which is supplied for liquefaction. The second part of the cooled natural gas, over pipeline 41, is supplied for expansion to the internal refrigerant expander 404, where a cold natural gas flow is formed, exiting trough pipeline 42, part of which, over pipeline 44, is supplied to the internal refrigerant cooler 451, whereas the rest, over pipeline 43, is supplied to the mixed refrigerant aftercoolers 452 and 453 to cool the mixed refrigerant. The natural gas flows, moving through pipelines 45 and 46, downstream from apparatuses 451, 452 and 453, are supplied as a general flow through pipelines 47 to the compressor section of the internal refrigerant expander 404. The compressed natural gas flow, over pipeline 48, is supplied for air cooling to internal refrigerant cooler 415, from which, as a flow and over pipeline 49, it is supplied to the inlet of the internal refrigerant compressor 403. Cooled and treated natural gas (hereinafter - NG), over pipeline 6, with liquid phase moving through pipeline 31 from mixed refrigerant separator 423, constituting heavy mixed refrigerant (hereinafter - HMR), and gaseous phase moving through pipeline 20 from the mixed refrigerant separator 424, constituting high pressure mixed refrigerant (hereinafter - HP MR), are delivered to the natural gas cooler 431 , where they are cooled by a flow of mixed refrigerant in the annular space. Then, the flow of cooled HMR, moving through pipeline 32, is throttled at valve 445 and flowed through pipeline 33 to the annular space of the natural gas cooler 431. Flow of cooled HP MR, over pipeline 21 , is supplied to the mixed refrigerant separator 421, where it is separated into a liquid phase, directed to pipeline 26, which constitutes medium mixed refrigerant (hereinafter - MMR), and gaseous phase, directed to pipeline 22, which constitutes light mixed refrigerant (hereinafter - LMR). Cooled NG, moving through pipeline 7, MMR flows, moving through pipeline 26, and LMR, moving through pipeline 22, are supplied to the natural gas liquefier 432. The flow of cooled MMR, moving through pipeline 27, is throttled at valve 444 and flowed through pipeline 28 into the annular space of the natural gas liquefier 432. The flow of cooled LMR, over pipeline 23, is supplied to the LNG subcooler 433. The aftercooled LMR flow is directed to pipeline 24 and throttled at valve 443, then it is flowed through pipeline 25 into the annular space of the LNG subcooler 433. The flow of liquefied NG, moving over pipeline 8 downstream from natural gas liquefier, is throttled at valve 442 and flowed via pipeline 9 into the LNG subcooler 433. The mixed refrigerant flows, supplied to the annular space, pass through the subcooler 433, liquefier 432 and cooler 431 moving downward and as a gas flow through pipeline 11 downstream from the coil-wound heat exchanger, are eventually delivered into the mixed refrigerant separator 422 on the receiving line of mixed refrigerant compressor 402.

Utilization of treated natural gas flows 43 and 40 to fulfill the function of internal refrigerant for mixed refrigerant flows’ 14 and 18 cooling, as well as treated natural gas 5 flow, enables one to adjust and control the temperature of treated natural gas 6, which is supplied for liquefaction, as well as mixed refrigerant flows 15 and 19 within the range supported by the process equipment stability margins when the latter is being operated against the backdrop of minor natural refrigerants’ temperature fluctuations, and thereby maintain production facility’s LNG output values at a constant level. At the same time, when natural refrigerants’ temperature changes become more significant (daily or seasonal fluctuations), this approach permits one to alter LNG output values relative to the design values or, in other words, reduce facility’s dependance on ambient climatic conditions.

Example 1. Using the operational data acquired from the medium-scale LNG plant, a calculation and comparison of economic efficiency indicators was carried out for a scenario where an aftercooling stage is used for treated natural gas and mixed refrigerant in line with the natural gas liquefaction configuration shown in figure 4.

During the course of LNG production process mathematical modelling, a comparison was made between two possible solutions for the problem: option 1 - natural gas liquefaction configuration using method (a), without implementing aftercooling, as a prototype option (hence, single-circuit NG liquefaction configuration, shown in figure 1, does not feature MR aftercoolers 152, 153 and treated gas aftercooler 151); option 2 - NG liquefaction configuration using method (a), with implementation of aftercooling stage in line with the claimed invention, which is shown in figure 4. For both variants being compared, which are located in the temperate climactic zone, average hourly output dependences were calculated relevant to the average monthly temperature, thereby representing the distribution of production facility’s average monthly LNG output over a time period of several months in year 2020 (figure 5). Data on monthly LNG output versus average monthly temperature for option 1 is provided in table 1 , while data for option 2 is provided in table 2. Annual LNG output data was calculated based on the output values for every month and amounted to: for option 1 - 1.59 million tons, for option 2 - 1.98 million tons (tables 1, 2).

As indicated by the calculations, average production output for the facility during summer months is reduced versus the output during winter months as a result of processes’ dependence on climatic conditions, however activation of the aftercooling circuit, which is described in the invention claim, enables one to significantly reduce this interdependence. When compared to the prototype, minimal assessed LNG output stands at 220 tons per hour, which is 57 tons per hour or 35% more than the prototype output assessment of 163 tons per hour, which further proves that the suggested flow aftercooling method is indeed effective. As a result, annual LNG output for the facility where the suggested methods have been implemented stands at 1.98 million tons, which exceeds prototype results standing at 1.59 million tons. Hence, the initially stated goal has been successfully achieved. Aside from that, the level of dependence on climatic conditions, when the suggested measures are in place, is reduced down to 0.00021 versus 1.084 for the prototype, or, in other words, the factor of climate dependence is virtually eliminated making the suggested LNG production method climate independent.

Detailed techno-economic analysis of the construction projects’ designs for two variants of LNG production facilities and economic efficiency summary indicators associated with the suggested invention and illustrated by the construction of a medium-scale LNG plant, with forecasted LNG sale price at 1412 USD per ton and assessed period of 25 years, prove that, through the increase of capital investments by 1.4% and integration of aftercooling stage, which is enabled by the internal cooling source, one can expect to increase the annual output of the aforementioned plant by 24%, or increase production figures from 1.59 million to 1.98 million tons, and, consequently, achieve 24% revenue growth. At the same time, the increase in operational expenses will be limited to just 18%. Hence, the efficiency of the medium-scale LNG plant project is significantly increased: net present value - by 28 %, return on investment - by 5.8 %. Analysis of the aftercooling stage integration confirms the economic efficiency of the new technical solution.

Calculations associated with LNG plant deployment in various regions of the country that differ from one another in terms of climatic conditions indicate that the impact of the aforementioned conditions on a plant, which design-wise conforms to the option 2 (the suggested invention), is significantly lower than for the option 1 (the prototype), and that in all cases the annual output of the option 2 exceeded that of the prototype (table 3) by 16-24%, which leads to a noticeable reduction in the level of climatic dependence as compared to the prototype. Therefore, the claimed invention may be offering universal improvements and advantages.

The claimed invention offers a solution for the posed problem - development of an efficient natural gas liquefaction method viable for a variety of climatic zones and offering, from a technical standpoint, increased annual LNG yield and reduction of dependence on climatic conditions, with due consideration of ambient temperature fluctuations during the operation of the LNG plant in question. This is achieved by means of natural gas flows’ and refrigerants’ aftercooling following their compression in the aftercooling units using both the internal and external cold sources.