Method for natural gas liquefaction

Abstract

FIELD: gas industry.SUBSTANCE: invention relates namely to technologies for liquefying natural gas, storing it and shipping it. The method for liquefying natural gas includes purifying and drying natural gas to obtain a prepared gas stream, after which pre-cooling, liquefaction and sub-cooling of the prepared natural gas are carried out in a natural gas liquefaction unit containing air cooling devices and complex air cleaning devices, using a refrigerant containing nitrogen, methane, ethylene, propane and isobutane. The refrigerant is compressed in a multi-stage compressor, followed by cooling and partial condensation of the compressed refrigerant flow in air-cooling units and its separation into a gas phase and a liquid phase – a heavy mixed refrigerant. The resulting gas phase is compressed to the final pressure. The refrigerant, compressed to the final pressure, is partially condensed in air-cooling devices to obtain a vapour-liquid mixture, which is separated into a gas phase, which is a light mixed refrigerant, and a liquid phase, which is a medium mixed refrigerant. The liquid phase, which is a heavy mixed refrigerant, is cooled, followed by throttling and evaporation in a cryogenic heat exchanger. The gas phase, which is a light mixed refrigerant, is liquefied and subcooled, and the liquid phase, which is a medium mixed refrigerant, is subcooled, followed by expansion of the light and medium fractions of the refrigerant at different temperature levels and their use as cold sources for liquefaction and subcooling of natural gas in the main heat exchanger. The air supplied to the inlet of air cooling devices, as well as the air supplied to the inlet of complex air cleaning devices of gas turbine drives of refrigerant compressors, is pre-cooled by means of adiabatic air cooling devices installed in front of the air intake devices of the said devices and units, by spraying the cooling medium into the air flows created by the fans.FIELD: increased energy efficiency and productivity.7 cl

Description

[0001] Field of technology to which the invention relates

[0002] The invention relates to the gas industry, specifically to technologies for the liquefaction, storage, and shipment of natural gas. The proposed technical solution can be used in the construction and operation of industrial complexes for the liquefaction of natural gas, including process units for preparation, liquefaction, storage, and shipment, as well as auxiliary systems that ensure the functioning of technological processes. The invention can be applied at large- and medium-tonnage LNG production facilities, including fixed and offshore sites, as well as at complexes connected to the gas transmission system, to ensure efficient and sustainable LNG production in various climatic conditions and expand the logistical capabilities of its supply.

[0003] Technology Level

[0004] As an analog, the applicant considers a method for operating a liquefaction plant for liquefying a hydrocarbon-rich gas stream. This method includes controlling the flow rate of gas supplied to a pre-cooling stage, a liquefaction stage, and a sub-cooling stage using a control unit. According to this method, a hydrocarbon-containing stream, such as natural gas, is subjected to stepwise cooling in a liquefaction apparatus by heat exchange with fractions of a circulating refrigerant mixture in a generally closed mixed refrigerant cycle. The natural gas stream sequentially undergoes pre-cooling, liquefaction, and deep cooling (sub-cooling) stages. In this process, the refrigerant mixture, completely evaporated during these stages, is compressed in a compressor unit, cooled, and separated into a liquid high-boiling fraction and a gaseous second fraction.The high-boiling fraction is fed to the pre-cooling stage, where it is cooled, throttled, and generates refrigeration capacity by cooling itself and the main stream. The second fraction is also fed to the pre-cooling stage, cooled there, and then, after cooling, is separated in an intermediate separator located after the pre-cooling stage into a liquid medium-boiling fraction and a gaseous low-boiling fraction. The medium-boiling fraction is fed to the liquefaction stage, where it is cooled, throttled, and self-cooled, providing refrigeration for liquefying the natural gas. The low-boiling fraction is also fed to the liquefaction stage, cooled there, and then fed to the deep cooling stage, where, after throttling, it provides refrigeration for subcooling the natural gas stream. [DE102011113548A, published 21.03.2013].

[0005] A disadvantage of this analog is its focus primarily on controlling flow rates in the overall refrigerant circuit without in-depth optimization of the thermodynamic liquefaction cycle itself. The described method does not disclose specific refrigerant compositions or offer solutions for their selection to minimize energy losses. This technical solution does not include operations for the recovery of secondary energy resources, such as heat from exhaust gases of power equipment or cold from throttled process streams. Furthermore, comprehensive integration of the resulting boil-off gas into the process cycle for its full recovery is not considered. Consequently, energy losses remain significant, and the potential for improving the overall energy efficiency of the process is underutilized.

[0006] The applicant considers as a prototype a METHOD FOR LIQUEFACTION of a hydrocarbon-rich fraction, previously prepared for liquefaction, in particular natural gas, comprising: a) pre-cooling, liquefaction and sub-cooling of the prepared natural gas in one multi-section heat exchange apparatus due to the transfer of cold from a refrigerant, which is a multi-component mixture; b) compression of the refrigerant used in at least two stages; c) partial condensation of the compressed stream of refrigerant supplied for cooling, at least after the penultimate stage of the compressor; d) compression of the resulting gas phase, which is a stream of mixed refrigerant after separation of the heavy mixed refrigerant from it, to the final pressure; d) partial condensation of the refrigerant compressed to the final pressure, producing a gas phase representing a mixed high-pressure refrigerant;e) cooling the resulting liquid phase, which is a heavy mixed refrigerant, followed by throttling and evaporation of the hydrocarbon-rich fraction and the gas phase, which is a high-pressure mixed refrigerant, which, after partial condensation, is separated into a gas phase, which is a light mixed refrigerant, and a liquid phase, which is a medium mixed refrigerant; g) liquefaction and supercooling of the gas phase, which is a light mixed refrigerant, supercooling of the liquid phase, which is a medium mixed refrigerant;both fractions are expanded at different temperature levels and used as sources of cold and for liquefying and supercooling the hydrocarbon-rich fraction, wherein the composition of the refrigerant used is selected such that the end boiling point at the operating pressure of the gas phase, which is a light mixed refrigerant, is lower than the start boiling point at the operating pressure of the liquid phase, which is a heavy mixed refrigerant, characterized in that the excess amount of the liquid phase, which is a medium mixed refrigerant, is directed from the gas phase separation separator, which is a high-pressure mixed refrigerant, to the compressed refrigerant separation separator after the penultimate compression stage. [RU2811216, published 11.01.2024].;

[0007] A drawback of the closest technical solution is the lack of operations to ensure climate stability and maximum energy efficiency under high ambient temperatures. The prototype does not provide for adiabatic cooling of the air supplied to the air-cooling units and gas turbine drives, which leads to a significant drop in their efficiency during hot periods, increased energy costs for refrigerant compression, and a reduction in overall process performance. The prototype also does not implement additional measures for the recovery of secondary energy resources (heat from turbine exhaust gases, cold from throttled flows) and the full inclusion of boil-off gas in the process cycle, limiting the potential for improving the overall energy efficiency of the plant.

[0008] Disclosure of the essence of the invention

[0009] The technical problem solved by the claimed invention is to eliminate the shortcomings of the prototype and create a method for liquefying natural gas with increased energy efficiency and productivity, which ensures the stability of the liquefaction process mode during fluctuations in ambient temperature, optimization of heat flows in the refrigeration cycle and a reduction in the specific energy costs for compressing the refrigerant.

[0010] The technical result of the claimed invention is to increase the energy efficiency and productivity of natural gas liquefaction.

[0011] The said technical result is achieved in that the method for liquefying natural gas includes purifying and drying natural gas to obtain a prepared gas stream, after which the prepared natural gas is pre-cooled, liquefied and sub-cooled in a natural gas liquefaction unit containing air cooling devices and complex air cleaning devices, using a refrigerant containing nitrogen, methane, ethylene, propane and isobutane; compressing the refrigerant in a multi-stage compressor with subsequent cooling and partial condensation of the compressed refrigerant stream in air cooling devices and separating it into a gas phase and a liquid phase - a heavy mixed refrigerant; compressing the resulting gas phase, which is a stream of mixed refrigerant after separating from it the liquid phase - a heavy mixed refrigerant, to the final pressure;cooling and partially condensing a refrigerant in air-cooling apparatuses, compressed to a final pressure, to obtain a vapor-liquid mixture; separating said vapor-liquid mixture into a gas phase, which is a light mixed refrigerant, and a liquid phase, which is a medium mixed refrigerant; cooling said liquid phase, which is a heavy mixed refrigerant, followed by throttling and evaporation in a cryogenic heat exchanger;subsequent liquefaction and subcooling of the gas phase, which is a light mixed refrigerant, and subcooling of the liquid phase, which is a medium mixed refrigerant, with subsequent expansion of the light and medium fractions of the refrigerant at different temperature levels and using them as cold sources for liquefaction and subcooling of natural gas in the main heat exchanger, wherein the air supplied to the inlet of the air cooling devices used for cooling and condensing the refrigerant streams, as well as the air supplied to the inlet of the integrated air cleaning devices of the gas turbine drives of the refrigerant compressors, are pre-cooled by means of adiabatic air cooling devices installed in front of the air intake devices of the said devices and apparatuses, by spraying the cooling medium into the air streams created by the fans.

[0012] In particular, the heat from the exhaust gases of the gas turbine drives of the refrigerant compressors is utilized in a waste heat exchanger, transferring it to the coolant oil circuit, which is used as a heat source in the heat exchangers of the natural gas preparation unit;

[0013] In particular, adiabatic air cooling devices are equipped with nozzles for supplying cooling medium.

[0014] In particular, evaporated prepared water is used as a cooling medium.

[0015] In particular, the supply and regulation of the supply of the cooling medium to the nozzles is carried out by means of pumps by turning on / off the required number of pumps and nozzles.

[0016] In particular, when removing heavy hydrocarbons at the stage of natural gas preparation, the cooled and throttled gas flow is directed to a plate recuperator mounted after the throttle valve to utilize the cold;

[0017] In particular, the boil-off gas generated during the storage and shipment of liquefied natural gas is compressed and sent to the general recovery flow.

[0018] In particular, propane and isobutane in the composition of the multi-component refrigerant contain impurities of n-butane, hexane and heptane.

[0019] In particular, the air entering the air cooling devices and the input of the complex air cleaning devices of the gas turbine drives of the refrigerant compressors is additionally cooled by means of adiabatic air cooling devices.

[0020] In particular, water is used as a cooling medium.

[0021] Implementation of the invention

[0022] Increased energy efficiency in this application refers to a reduction in the specific energy consumption per unit of final product—one ton of liquefied natural gas. This is achieved through operations that minimize energy losses at all stages of the process cycle and maximize the utilization of the process's internal energy resources.The specific mechanisms, according to the claimed method, are: the use of multi-stage compression of the refrigerant with intermediate cooling, which brings the compression process closer to isothermal, significantly reducing the specific energy consumption due to the decrease in the volume and temperature of the refrigerant before each subsequent stage; the use of a multi-component refrigerant containing nitrogen, methane, ethylene, propane and isobutane during liquefaction, which ensures the formation of a more optimal temperature profile in heat exchangers and reduces thermodynamic losses; the implementation of heat recovery from the exhaust gases of the gas turbines driving the refrigerant compressors, with the transfer of this heat for heating the process flows at the stage of preparation of natural gas, which reduces the need for external energy sources; as well as the return of the boil-off gas formed during storage and shipment into the process cycle after its compression instead of combustion or discharge.An objective and measurable criterion for achieving this result is the specific energy consumption, expressed in kilowatt-hours per ton of LNG produced (kWh / t LNG).

[0023] In this application, productivity improvement is defined as the ability of the process to maintain nominal output with minimal deviations and downtime, which is equivalent to an increase in average annual liquefied natural gas production with the same production capacity. Key factors here include process stability at all liquefaction stages and a reduction in scheduled and emergency shutdowns due to thermal imbalances, equipment contamination, or logistical failures. The objective criterion is the capacity utilization factor, calculated as the ratio of the actual volume of liquefied natural gas produced over a period to the maximum possible volume during 24-hour operation at nominal mode. An additional criterion may be the number of emergency shutdowns per year, which directly impacts operational continuity and overall output.

[0024] Increased climate stability as an additional technical benefit of the claimed method implies the ability to maintain the design performance and energy efficiency of the liquefaction process under significant seasonal and daily fluctuations in ambient air temperature, especially in hot climates. The primary technical solution achieving this result is the adiabatic pre-cooling of air supplied to the inlet of air-cooling units (ACUs) and integrated air purification devices through the evaporation of fine droplets of a cooling medium, such as treated water, in the airflow. If necessary, the intensity of this cooling is adjusted by varying the flow of cooling medium through nozzles, preventing overheating of the incoming air and the subsequent reduction in heat exchange efficiency during the refrigerant compression and condensation stages.A measurable criterion for achieving this result is the percentage reduction in the productivity of the liquefaction process when the ambient temperature increases to an extremely high level (for example, +35°C).

[0025] Expanded logistics capabilities as an additional technical benefit of the proposed method refers to increased flexibility and reliability in finished product sales through diversification of shipping operations and transport types. This method enables the shipment of liquefied natural gas not only through marine terminals but also through land-based loading points into tank trucks and using bunkering vessels. This ensures the prompt redistribution of product depending on demand and the logistics situation, preventing forced downtime in the liquefaction process due to a lack of available transport. An objective criterion for achieving this benefit is the proportion of production downtime caused by logistical constraints.

[0026] According to the claimed invention, a method for liquefying natural gas includes pre-cooling, liquefying, and sub-cooling prepared natural gas in a natural gas liquefaction unit containing air coolers and integrated air purification units, using cold transfer from a refrigerant, which is a multi-component mixture containing nitrogen, methane, ethylene, propane, and isobutane. The combination of refrigerant components is selected based on their thermodynamic properties, which enable the creation of a mixed refrigeration cycle with an optimal temperature profile.Each component of the mixture operates within its own effective temperature range: propane and isobutane provide pre-cooling in the high- and medium-temperature zone (approximately +30°C to -30°C), ethylene and methane are used in the low-temperature stages for deep cooling and liquefaction (approximately -30°C to -90°C and below), and nitrogen, which has the lowest boiling point, is used in the cryogenic zone to achieve the final liquefaction temperature and subcool the LNG. This combination of refrigerants with different boiling points creates a smooth and consistent thermal gradient in the main heat exchanger, which minimizes the average temperature difference between the cooled natural gas and the refrigerant. In particular, the use of isobutane, which has an optimal boiling point of approximately -11.7°C and a high heat of vaporization, ensures efficient heat removal in the initial stages of cooling, preventing localized thermal stresses.As a result, achieving the same final temperature of liquefied natural gas requires less refrigerant compression work in compressors, which directly reduces specific energy consumption, expressed in kWh per ton of LNG. Thus, the use of the proposed multicomponent mixture containing nitrogen, methane, ethylene, propane, and isobutane ensures the technical result—increased energy efficiency of the liquefaction process—by creating an optimal temperature profile in heat exchangers, minimizing thermodynamic losses during heat transfer, and correspondingly reducing energy costs for refrigerant compression.

[0027] According to the invention, multi-stage refrigerant compression is achieved with partial condensation after each compression stage. Multi-stage compression, unlike single-stage compression, significantly reduces the energy required to compress the working fluid due to two key principles: bringing the compression process closer to isothermal conditions and reducing mechanical losses. In a multi-stage compressor, the refrigerant is compressed sequentially in several stages, and after each stage, the compressed and heated flow is sent to an air cooler for intermediate cooling and partial condensation before entering the next stage. This intermediate cooling significantly reduces the refrigerant temperature, which has several positive effects. First, cooling the gas reduces its specific volume, meaning the subsequent compression stage expends significantly less work to further increase the pressure.Secondly, the thermal load on the equipment is reduced, increasing the reliability and service life of compressor units. Thirdly, partial condensation after the compression stages, starting with the penultimate ones, allows for the efficient separation of the refrigerant into fractions (including the separation of heavy components such as propane and isobutane) for their optimal use in the various temperature circuits of the cycle. This process organization minimizes the total compression work required to maintain the required pressure drop in the refrigeration system and increases the efficiency of the entire liquefaction cycle. Reducing compressor energy consumption is the main contributor to the reduction in specific energy consumption per ton of LNG produced (kWh / t LNG).Thus, the implementation of a multi-stage refrigerant compression operation with partial condensation after each stage ensures the achievement of a technical result - increased energy efficiency - due to a significant reduction in the compression work, a decrease in mechanical and thermal losses, and the creation of conditions for the effective fractionation of a multi-component refrigerant.

[0028] According to the invention, the resulting gas phase, which is a mixed refrigerant stream after separating the liquid phase—the heavy mixed refrigerant—to a final pressure, is compressed. After the initial stages of compression and partial condensation of the multicomponent refrigerant, its phase separation occurs: the liquid phase (the heavy mixed refrigerant), rich in components with a higher boiling point (such as propane and isobutane), is separated from the stream and is subsequently used to provide refrigeration during the pre-cooling stages. The remaining gas phase consists predominantly of lighter components (nitrogen, methane, and ethylene), which must be compressed to a higher, "final" pressure for efficient operation in the low-temperature portion of the cycle. Compressing this gas phase, depleted in heavy components, rather than the entire initial mixture to maximum pressure, yields a significant energy gain.Since the work required to compress a gas to a given pressure directly depends on its mass and initial volume, separating and removing a portion of the flow as a liquid phase before the final compression stages reduces the volume and mass of the gas subject to further compression. This reduces both the number of compression stages for the remaining flow and the power consumed by the compressors in these final stages. As a result, the total work expended on compressing the refrigerant throughout the cycle is reduced. Furthermore, this separate compression of fractions with different volatility allows for more precise adjustment of the thermodynamic parameters of each fraction to a specific temperature level in the main heat exchanger, further minimizing irreversible losses during heat exchange between the refrigerant and natural gas.Thus, the operation of additional compression of the light gas phase to the final pressure ensures the achievement of the technical result - increased energy efficiency of the liquefaction process - due to the rational division of the refrigerant flow and optimization of the compressor equipment operation, which leads to a reduction in the specific energy consumption for compression.

[0029] According to the invention, partial condensation of a refrigerant compressed to a final pressure is performed, producing a vapor-liquid mixture. This feature is a critical process step that directly impacts the efficiency of subsequent refrigerant separation and, consequently, the technical result of increasing the energy efficiency of the liquefaction process. After the gas phase, enriched in light components, is compressed to a high final pressure, it is sent to a partial condensation stage. In this context, "partial" means that the flow is not cooled until all components are completely condensed, but rather to a temperature at which only a certain portion of them condenses, primarily the heavier components remaining in the gas phase (e.g., ethylene, some methane), while the lighter components (nitrogen, a significant portion of methane) remain in a gaseous state.This operation is performed, with the primary cooling of the stream in question occurring in the pre-cooling section of the main heat exchanger. There, heat from the compressed refrigerant is transferred to cooler process streams within the cycle, such as refrigerant return streams or partially cooled natural gas. The resulting vapor-liquid mixture is a two-phase system with a distinct compositional separation: the liquid phase is enriched in components with a relatively higher condensation temperature (the middle fraction), while the vapor phase is enriched in components with an extremely low condensation temperature (the light fraction). This preliminary phase separation is a preparatory step for the subsequent effective mechanical separation of the mixture in the separator. Without the partial condensation operation, separation would be impossible or extremely ineffective, since all the refrigerant would remain in a homogeneous gaseous state.Thus, partial condensation creates the necessary conditions for the subsequent cascade use of the refrigerant, where different fractions with different refrigeration potentials are used to cool the natural gas at different temperatures. This maximizes the use of the cold generated in the cycle and minimizes heat recovery due to suboptimal temperature differences in the heat exchangers. Consequently, partial condensation of the refrigerant compressed to the final pressure ensures the achievement of a technical result—improved energy efficiency—by creating conditions for the subsequent effective fractionation of the multi-component refrigerant and its cascade use. This leads to reduced thermodynamic losses in the main heat exchanger and a reduction in the energy required to compress the circulating flows.

[0030] According to the invention, the vapor-liquid mixture is separated into a gas phase, representing a light mixed refrigerant, and a liquid phase, representing a medium mixed refrigerant. This feature is a logical continuation and materialization of the advantages established in the previous stages and serves as the basis for constructing a highly efficient cascade refrigeration cycle, directly influencing the technical result achieved – increased energy efficiency and productivity of the liquefaction process. Separation of the vapor-liquid mixture obtained after partial condensation is carried out in standard separation equipment, such as a gravity or centrifugal separator (throttle separator), where, under the action of gravity or inertia, stratification of the medium occurs: the liquid phase, having a higher density, collects in the lower part of the apparatus, and the gas phase occupies the upper part.The key factor isn't the separation itself, but rather the fact that this operation results in the formation of two target streams with clearly differentiated compositions and, consequently, different thermodynamic properties. The light mixed refrigerant (gas phase), consisting predominantly of nitrogen and methane, is characterized by an extremely low boiling point and is designed to provide deep cooling during the final stages of LNG liquefaction and subcooling. The medium mixed refrigerant (liquid phase), enriched with ethylene, propane, and possibly residual methane, has a higher condensation / boiling point and is used for heat removal at intermediate temperature levels. This targeted separation of the stream into fractions allows for their independent and optimal downstream processing: the light fraction can be further compressed (if required), liquefied, and subcooled, while the medium fraction can be subcooled and throttled.This makes it possible to feed these fractions into the main heat exchanger in different cross-sections, precisely matching their temperature potential. As a result, a multi-stage cold source is created in the heat exchanger, rather than a single source, allowing for a significantly better matching of the natural gas and refrigerant cooling curves, reducing the average temperature difference in the unit. Reducing the temperature difference is the primary way to reduce thermodynamic losses during heat exchange, which directly translate into excess compressor operation. Thus, separating the vapor-liquid mixture into light and medium fractions achieves the technical result of increased energy efficiency by creating conditions for cascaded refrigerant use, which minimizes irreversible heat transfer losses in the main heat exchanger and reduces the specific energy consumption of the liquefaction cycle.

[0031] According to the invention, the liquid phase, which is a heavy mixed refrigerant, is cooled, followed by expansion and evaporation in the main heat exchanger to provide natural gas cooling. This feature is a key element in the utilization of low-grade cold within the process cycle and directly impacts the technical result of increasing the energy efficiency of the liquefaction process. The heavy mixed refrigerant, separated at an earlier stage (after the initial compression and condensation stages), consists predominantly of components with relatively high boiling points, such as propane, isobutane, and possibly heavier hydrocarbons. This liquid phase, already under pressure, undergoes additional cooling before its useful use.Cooling of the heavy refrigerant can be achieved through heat exchange with other, colder flows within the cycle (for example, with the return flow of cold refrigerant or with the natural gas flow after throttling), which corresponds to the principle of cold recovery and minimization of external energy costs. After cooling, the flow of heavy liquid refrigerant is sent to throttling—an irreversible adiabatic process of sharply reducing pressure by passing through a throttling valve or regulator. As a result of throttling, the flow temperature is significantly lowered, and some of the liquid instantly evaporates, converting the flow into a cold vapor-liquid mixture. This cold mixture is then fed to the main heat exchanger (or to its corresponding section designed for high-temperature cooling), where it completely evaporates, thereby extracting heat from the cooled natural gas during its pre-cooling stage.Thus, the cold generated by throttling and evaporating the heavy refrigerant fraction is effectively utilized within the process itself, reducing the load on the main low-temperature stages of the cycle. This reduces the amount of refrigerant required to be compressed, liquefied, and subcooled in the low-temperature section of the unit, and consequently lowers the energy consumption of the compressors serving these stages. Utilizing the cold generated by throttling the heavy fraction is a form of energy recovery within the cycle, directly contributing to a reduction in specific energy consumption (kWh / t LNG). Furthermore, the controlled evaporation of the heavy refrigerant in the main heat exchanger ensures stable and efficient heat removal during the initial stage of natural gas cooling, preventing the condensation of unwanted heavy components and contributing to an overall improvement in process reliability and performance.Thus, the operation of cooling, throttling and evaporation of a heavy mixed refrigerant ensures the achievement of a technical result - increased energy efficiency - due to the useful utilization of the cold generated during throttling, the recovery of heat flows within the cycle and a reduction in the load on the main energy-consuming stages of compression and cooling.

[0032] According to the invention, air supplied to the inlet of the air-cooling devices of the liquefaction unit, as well as air supplied to the inlet of the air-cooling devices of the gas preparation unit (including the air-cooling units of the heavy hydrocarbon removal unit and the air-cooling units installed downstream of the compressor for feeding the treated gas to the liquefaction unit) and the integrated air cleaning devices of the gas-turbine drives of the refrigerant compressors, are pre-cooled by means of adiabatic air cooling devices installed upstream of the air intake devices of the said devices and units, by spraying a cooling medium, such as treated water, into the air flows generated by the fans. This feature is key to ensuring climate stability and energy efficiency of the entire liquefaction process and directly affects the achievement of the claimed technical result.Adiabatic cooling devices utilize a simple yet highly effective physical principle: spraying a finely dispersed cooling medium, such as water, into the discharge air stream causes its intense evaporation. The phase transition from liquid to vapor requires a significant amount of thermal energy (heat of vaporization), which is extracted from the air flow itself, resulting in significant cooling without performing mechanical work—that is, adiabatically. Applying this process to the air supplied to the inlet of air-cooled coolers (ACOs) solves the critical problem of reduced heat transfer efficiency under high ambient temperatures. ACOs, being the primary heat-removal devices for compressed refrigerants, are critically dependent on the incoming air temperature.Adiabatic pre-cooling of this air allows its temperature at the AVO inlet to be maintained at a consistently low level (e.g., no higher than +25°C) even in extreme external heat (+35°C and above). This ensures high temperature head in the AVO heat exchangers, prevents a drop in the refrigerant condensation rate, and, consequently, eliminates a reduction in the overall performance of the refrigeration cycle and LNG production. Simultaneous cooling of the air entering the integrated air cleaning units of the gas turbine drives solves another, but no less important, problem. Gas turbines, which drive refrigerant compressors, are among the main energy consumers in a gas liquefaction complex. Their capacity and specific fuel consumption directly depend on the temperature and density of the air at the compressor inlet.Cooling this air through adiabatic evaporation increases its density, leading to an increase in the mass flow rate through the turbine. As a result, the gas turbine's power output increases, while the specific fuel consumption per unit of generated mechanical energy decreases. This directly reduces the energy costs of refrigerant compression, which is one of the largest expense items in the liquefaction cycle. Thus, the adiabatic pre-cooling feature for two key systems provides a synergistic effect: it stabilizes and improves the efficiency of both the heat-removal circuit (AVO) and the energy-supply circuit (gas turbines). This allows the system to maintain nominal performance and low specific energy consumption over a wide range of external temperatures, which is the essence of climate stability.Consequently, this feature ensures the achievement of a technical result - increased energy efficiency, productivity and climate stability of the liquefaction process - due to the stabilization of the thermodynamic operating conditions of the main energy-consuming units (AVO and GTU) in a hot climate, which prevents a drop in LNG production and an increase in specific energy costs (kW h / t LNG).

[0033] According to the invention, the exhaust heat from gas turbine drives of refrigerant compressors can be recovered in a heat recovery heat exchanger, transferring it to a heat transfer oil circuit, which, if necessary, is used as a heat source in heat exchangers of the natural gas treatment unit. This feature further contributes to the technical result of increased energy efficiency. Without recovery, this heat is dissipated into the atmosphere and is a wasted energy resource. Transferring heat to the heat transfer oil circuit allows it to be accumulated and used for process needs, for example, to preheat feed gas during the removal of acid gases and methanol or to regenerate adsorbents in the drying module. This reduces or completely eliminates the need for external heat sources (such as steam boilers or direct gas heaters) for these operations.Consequently, the overall fuel consumption of the complex is reduced when implementing the claimed method, which directly reduces the specific energy consumption per unit of LNG production (kWh / t LNG). This feature thus further contributes to the technical result—increased energy efficiency—by utilizing waste heat from gas turbines to cover the internal thermal loads of the process, thereby reducing the consumption of primary fuel or steam from external sources.

[0034] According to the invention, adiabatic air cooling devices can be equipped with nozzles for supplying a cooling medium. This feature further enhances climate stability, energy efficiency, and process stability. The need to regulate the supply of cooling medium typically arises under variable external conditions, such as changes in ambient temperature and humidity throughout the day or season. Using nozzles with a fixed flow rate under conditions such as high humidity can result in incomplete evaporation of water, leading to excessive moisture flow and potential problems in downstream equipment. In hot, dry weather, on the other hand, a fixed flow rate may be insufficient to achieve the required cooling.The use of multiple nozzle groups, each connected to a separate pump, allows the automatic control system, based on sensor signals, to regulate the overall water flow by turning on or off the required number of pumps and their associated nozzle groups. This ensures a stepped yet precise dosing of sprayed water, maintaining optimal air cooling through adiabatic saturation without excessive water consumption or the risk of over-humidification. This solution maintains a stable and specified air temperature at the inlet of air-cooled units (ACUs) and gas turbine units (GTUs) regardless of weather conditions. A stable ACU inlet temperature ensures consistently high heat removal efficiency, preventing fluctuations in refrigerant condensation and disrupting the heat balance of the cycle.A stable air temperature and humidity at the gas turbine inlet ensure consistent operating parameters, preventing power loss and increasing specific fuel consumption. This feature thus further enhances technical performance—improving climate resilience and energy efficiency—by enabling adaptive control of the adiabatic cooling process by regulating the number of operating nozzle groups. This maintains the stability of key equipment across a wide range of external conditions and optimizes water and energy consumption.

[0035] According to the invention, during the removal of heavy hydrocarbons during the natural gas preparation stage, the cooled and throttled gas flow can be directed to a plate recuperator installed downstream of the throttle valve for cold recovery. This feature further contributes to the technical result of increasing the energy efficiency of the entire liquefaction process. In a standard heavy hydrocarbon removal process, cooling of the feed gas for condensation of heavy fractions can be accomplished solely with an external refrigerant, which places an additional load on the gas preparation refrigeration system. However, during the throttling (sharp reduction in pressure) of the prepared gas flow, it undergoes adiabatic expansion and significant cooling. The resulting cold stream is typically then sent for separation or to the next process unit, and its refrigeration potential is often not fully utilized.Installing a plate recuperator immediately after the throttle valve enables heat exchange between this cold, throttled stream and the original, warmer gas flow entering the heavy hydrocarbon removal stage. In the recuperator, the cold from the throttled stream is transferred to pre-cool the incoming feedstock. This allows the required low temperature for condensing heavy hydrocarbons to be achieved at a lower cost, since some of the cooling is now provided by recovering internal cold, rather than solely by external refrigerant. Consequently, the use of this recuperator improves the energy efficiency of the heavy hydrocarbon removal unit, reducing the load on its own refrigeration system and lowering the energy consumption required to prepare the feedstock before feeding it to the liquefaction unit.Thus, this feature additionally influences the achievement of the technical result - increased energy efficiency - due to the useful recovery of cold generated during throttling in the gas preparation process, which reduces the energy costs of the technological preparation line and, as a result, contributes to the reduction of the overall specific energy consumption of the natural gas liquefaction complex.

[0036] According to the invention, boil-off gas generated during the storage and shipment of liquefied natural gas can be compressed and fed into the general recovery stream. This feature further enhances the energy efficiency, productivity, and environmental friendliness of the process. Boil-off gas inevitably forms during LNG storage in isothermal tanks due to external heat inputs, as well as during pumping and loading (shipping) operations. In traditional systems, this gas, if its volume is small, is often flared or used as fuel for on-site use, which is not optimal in terms of fully utilizing its energy and material potential. Compression of boil-off gas allows its pressure to be increased to a level at which it can be effectively returned to the process cycle.Directing the compressed stream into the main recovery stream (for example, at the inlet of the gas treatment unit or directly into the feed gas stream before the liquefaction unit) ensures its complete reintegration into the process. This leads to several positive effects. First, valuable hydrocarbon feedstock (methane) is returned to the production cycle, increasing the overall yield of marketable LNG and reducing direct material losses. Second, fuel gas consumption for flares and other needs is reduced or completely eliminated, which can now be covered by the recycled methane, reducing the specific energy consumption of the complex. Third, greenhouse gas and hydrocarbon emissions into the atmosphere are minimized, improving environmental safety. Fourth, the pressure in the LNG storage tanks is stabilized, preventing uncontrolled pressure increases and improving operational safety.Thus, this feature additionally influences the achievement of technical results - increased energy efficiency and productivity - due to the return and beneficial use of the evaporated product, which increases the final LNG yield, reduces the consumption of external fuel and prevents production losses.

[0037] Thus, the claimed method is implemented as a single technological process, including the stages of preparation, liquefaction, storage, and shipment of natural gas. The sequential execution of these operations ensures efficient gas purification, cooling, and liquefaction using an optimized refrigeration cycle, as well as safe and economical storage and shipment of the finished product. The use of a multi-component refrigerant of a specified composition during liquefaction, adiabatic air pre-cooling, and boil-off gas recovery improves the energy efficiency of the process, reduces operating costs, and minimizes product loss. This interconnected sequence of operations reflects the logical and technological integrity of the method and ensures the stability and flexibility of its implementation.

[0038] Implementation examples

[0039] First implementation example:

[0040] This example implements the basic configuration of the method.

[0041] Natural gas is fed to a treatment unit, including separation, purification, and removal of heavy hydrocarbons. The treated gas is then sent for liquefaction. Liquefaction occurs in the main heat exchanger through heat exchange with a multicomponent refrigerant, a mixture of nitrogen, methane, ethylene, propane, and isobutane. The refrigerant is circulated through multistage compression with intermediate and final condensation. After the initial compression stages, the refrigerant is partially condensed, the separated liquid phase (heavy mixed refrigerant) is cooled, throttled, and sent for evaporation in one section of the main heat exchanger. After separation, the gas phase is pressurized to final pressure, partially condensed, and separated into light and medium fractions. The light fraction is liquefied and subcooled, while the medium fraction is subcooled.Both subcooled fractions are expanded and used as cold sources in the corresponding sections of the main heat exchanger for liquefying and subcooling natural gas. Air supplied to the inlet of the air coolers and integrated air cleaning units of the gas turbine drives is pre-cooled adiabatically by spraying water. The resulting liquefied natural gas is stored. The resulting boil-off gas is compressed.

[0042] Second implementation example:

[0043] The method is implemented as in the first embodiment, additionally performing the operation according to claim 2. The exhaust heat from the gas turbine drives of the refrigerant compressors is recovered in a waste heat exchanger, transferring it to the heat transfer oil circuit. The heated heat transfer oil is then used as a heat source in the heat exchangers of the natural gas preparation unit, for example, to preheat the feed stream in the heavy hydrocarbon removal module.

[0044] Third implementation example:

[0045] The method is carried out as in Example 1 or 2, additionally applying the features of dependent clauses 3, 4, and 5. Adiabatic air cooling is performed using nozzles. When heavy hydrocarbons are removed during the preparation stage, the cooled and throttled gas flow is directed to a plate recuperator, mounted downstream of the throttle valve, for cold recovery (clause 4). The stripping gas generated during liquefied natural gas storage is compressed and sent to the general process flow for recovery (clause 5).

[0046] To confirm the impact of the claimed method on achieving the technical result—increasing the energy efficiency and productivity of the liquefaction process, ensuring its climate stability—a comprehensive experiment was conducted using mathematical modeling. Five process flow diagrams were modeled:

[0047] 1. A method similar to patent DE102011113548A (published 03 / 21 / 2013).

[0048] 2. Prototype method according to patent RU2811216C1 (published 11.01.2024).

[0049] 3. The claimed method in the basic configuration according to Implementation Example 1.

[0050] 4. The claimed method with turbine heat recovery according to Example of implementation 2.

[0051] 5. The claimed method with controlled adiabatic cooling and cold recovery according to Example of implementation 3.

[0052] The simulation was conducted under identical initial conditions: a design LNG capacity of 5 million tons per year, variable ambient temperatures from -35°C to +35°C throughout the year, and an identical incoming natural gas composition (85% methane, 8% ethane, 4% propane, 3% heavy hydrocarbons). The evaluation period was one year of operation with hourly dynamic calculations of all process parameters.

[0053] In terms of energy efficiency, the implementation of the declared method in all its configurations showed specific energy consumption in the range of 260–285 kW h per ton of produced LNG.

[0054] All the specified ranges for the proposed method are significantly lower than those of known solutions. The specific energy consumption of the prototype method was 347 kWh / t of LNG, while that of the comparable method was 380 kWh / t of LNG. Thus, the proposed method provides an 18–32% reduction in energy consumption relative to the prototype and a 25–32% reduction relative to the comparable method. Energy efficiency is confirmed by calculating the refrigeration cycle efficiency, which for the proposed method was 72–74%, compared to 58% for the prototype and 52% for the comparable method.

[0055] In terms of climate stability, the process implemented using the proposed method demonstrated the ability to maintain stable performance under significant fluctuations in ambient temperature. At a critical temperature of +35°C, LNG productivity for the proposed method decreased by only 5–9% of the nominal value, depending on the configuration (the lowest loss was observed in Example 3). Meanwhile, the prototype method lost 23% of its performance, while the comparable method lost 30%. This result was achieved by performing adiabatic pre-cooling of the air supplied to the cooling and power supply systems, which enabled the maintenance of stable operating temperatures for key equipment.

[0056] Process mode stability, assessed by the number of conditional emergency shutdowns per year, was also higher for the proposed method across all its variants. This indicator amounted to 0.5–0.9 shutdowns per year, compared to 2.5 shutdowns per year for the prototype method and 3.1 shutdowns per year for the analogue method. This increased stability is due to the combined effect of the method's operations, including temperature stabilization and, if necessary, recovery of boil-off gas.

[0057] Thus, the results of mathematical modeling have experimentally confirmed that the execution of operations in the sequence and combination provided by the claimed method provides a synergistic effect, expressed in a significant increase in energy efficiency, productivity and climate stability of the natural gas liquefaction process in comparison with the closest known technical solutions.

Claims

1. A method for liquefying natural gas, comprising: purifying and drying natural gas to obtain a prepared gas stream, followed by pre-cooling, liquefaction and sub-cooling of the prepared natural gas in a natural gas liquefaction unit containing air cooling units and integrated air cleaning devices, using a refrigerant containing nitrogen, methane, ethylene, propane and isobutane; compressing the refrigerant in a multi-stage compressor, followed by cooling and partial condensation of the compressed refrigerant stream in air cooling units and its separation into a gas phase and a liquid phase - a heavy mixed refrigerant; compressing the resulting gas phase, which is a stream of mixed refrigerant after separation from it of the liquid phase - a heavy mixed refrigerant, to a final pressure; cooling and partial condensation of the refrigerant in air cooling devices, compressed to the final pressure, to produce a vapor-liquid mixture;separating said vapor-liquid mixture into a gas phase, which is a light mixed refrigerant, and a liquid phase, which is a medium mixed refrigerant; cooling said liquid phase, which is a heavy mixed refrigerant, followed by throttling and evaporation in a cryogenic heat exchanger;subsequent liquefaction and subcooling of the gas phase, which is a light mixed refrigerant, and subcooling of the liquid phase, which is a medium mixed refrigerant, with subsequent expansion of the light and medium fractions of the refrigerant at different temperature levels and using them as cold sources for liquefaction and subcooling of natural gas in the main heat exchanger, wherein the air supplied to the inlet of the air cooling devices used for cooling and condensing the refrigerant streams, as well as the air supplied to the inlet of the integrated air cleaning devices of the gas turbine drives of the refrigerant compressors, are pre-cooled by means of adiabatic air cooling devices installed in front of the air intake devices of the said devices and apparatuses, by spraying the cooling medium into the air streams created by the fans.

2. The method according to paragraph 1, characterized in that the heat from the exhaust gases of the gas turbine drives of the refrigerant compressors is utilized in a heat exchanger-utilizer, transferring it to the coolant oil circuit, which is used as a heat source in the heat exchangers of the natural gas preparation unit.

3. The method according to paragraph 1, characterized in that the adiabatic air cooling devices are equipped with nozzles with an adjustable supply of cooling medium.

4. The method according to paragraph 1, characterized in that when removing heavy hydrocarbons at the stage of preparing natural gas, the flow of cooled and throttled gas is directed into a plate recuperator mounted after the throttle valve for cold recovery.

5. The method according to paragraph 1, characterized in that the stripping gas formed during storage and shipment of liquefied natural gas is compressed and sent to the general recovery flow.

6. The method according to paragraph 1, characterized in that the air entering the air cooling devices and the input of the complex air cleaning devices of the gas turbine drives of the refrigerant compressors is additionally cooled by means of adiabatic air cooling devices.

7. The method according to paragraph 1, characterized in that water is used as the cooling medium.