Natural gas liquefaction apparatus

Abstract

A natural gas liquefaction device is provided. The natural gas liquefaction device comprises a gas compressor that receives and compresses natural gas from a natural gas feed stream, a heat exchanger that cools the natural gas passing through the gas compressor through heat exchange, an expansion valve that expands and supercools the natural gas cooled through the heat exchanger, and a gas-liquid separation drum that separates the phases of the gas-liquid mixture generated by passing through the expansion valve and separates and discharges the liquid liquefied natural gas and cryogenic recovery gas. The heat exchanger comprises a first cooling unit that cools the natural gas using the recovery gas as a refrigerant, and a second cooling unit that cools the natural gas using an azeotropic or non-azeotropic non-combustible mixed refrigerant circulating in an independent cooling cycle independent of the first cooling unit, thereby improving both the cooling efficiency of the natural gas and the safety of the equipment.

Description

Technology Field

[0001] The present invention relates to a natural gas liquefaction device, and more specifically, to a natural gas liquefaction device having an improved cooling process that increases cooling efficiency and enhances the safety of the device. Background Technology

[0002] Fossil fuels are energy sources used in various fields. Petroleum and natural gas are representative fossil fuels used as fuel for most power generation facilities (thermal power plants, combined cycle power plants, etc.) and transportation means, with the exception of nuclear-based facilities. Petroleum is extracted in liquid form, while natural gas is extracted in gaseous form, along with petroleum.

[0003] Since natural gas is extracted in a gaseous state, it is necessary to convert it into a liquid state that is easy to handle for storage, transportation, etc. Natural gas can be liquefied by cooling it below the liquefaction temperature while considering pressure conditions, and it can also be compressed and used as needed. An example of a liquefaction facility is shown in Korean Patent 10-1341798, etc.

[0004] In the natural gas liquefaction process, it is necessary to apply an appropriate refrigerant to the system. Using an inappropriate refrigerant in the process may lead to reduced efficiency and make it difficult to reach the target temperature. Furthermore, while safety issues may arise, changing the refrigerant or the consequent system modifications is not easily accomplished due to the complexity and numerous variables involved. For example, commonly used hydrocarbon refrigerants (e.g., propane) are dangerous due to their high flammability, yet they cannot be replaced because system improvements are not easy. Prior art literature

[0005] Korean Patent Publication No. 10-1341798, (December 17, 2013) The problem to be solved

[0006] The technical objective of the present invention is to solve these problems by providing a natural gas liquefaction device having an improved cooling process that increases cooling efficiency and enhances the safety of the device. The present invention aims to simultaneously achieve the objectives of ensuring the safety of natural gas liquefaction facilities by limiting the use of highly flammable refrigerants (e.g., propane) and improving the efficiency of the liquefaction process.

[0007] The technical problems of the present invention are not limited to those mentioned above, and other unmentioned technical problems will be clearly understood by those skilled in the art from the description below. means of solving the problem

[0008] The natural gas liquefaction device according to the present invention comprises: a gas compressor that receives and compresses natural gas from a natural gas feed stream; a heat exchanger that cools the natural gas passing through the gas compressor through heat exchange; an expansion valve that expands and supercools the natural gas cooled by passing through the heat exchanger; and a gas-liquid separation drum that separates the phases of the gas-liquid mixture generated by passing through the expansion valve and separates and discharges the liquid liquefied natural gas and cryogenic recovery gas. The heat exchanger comprises a first cooling unit that cools the natural gas using the recovery gas as a refrigerant, and a second cooling unit that cools the natural gas using an azeotropic or non-azeotropic non-combustible mixed refrigerant circulating in an independent cooling cycle independent of the first cooling unit.

[0009] The first cooling unit may include a low-temperature heat exchanger that exchanges heat between the recovery gas discharged from the gas-liquid separation drum and natural gas, and a high-temperature heat exchanger located upstream of the low-temperature heat exchanger that exchanges heat between the recovery gas discharged from the low-temperature heat exchanger and natural gas before it enters the low-temperature heat exchanger.

[0010] The second cooling unit may include an independent heat exchanger that pre-cools natural gas with the azeotropic or non-azeotropic non-combustible mixed refrigerant between the high-temperature heat exchanger and the low-temperature heat exchanger.

[0011] The second cooling unit may further include a refrigerant supply pipe that supplies the azeotropic or non-azeotropic non-combustible mixed refrigerant to the independent heat exchanger, a refrigerant recovery pipe that recovers the azeotropic or non-azeotropic non-combustible mixed refrigerant from the independent heat exchanger, and a refrigerant circulation pipe connected between the refrigerant supply pipe and the refrigerant recovery pipe, which passes the azeotropic or non-azeotropic non-combustible mixed refrigerant through a refrigerant compressor, a refrigerant condenser, and a first expansion unit, and cools it to a first temperature at the downstream end of the first expansion unit.

[0012] The above refrigerant supply pipe is branched from the front end of the first expansion section of the above refrigerant circulation pipe, thermally contacts the rear end of the first expansion section, and is connected to the above independent heat exchanger, thereby enabling the azeotropic or non-azeotropic non-combustible mixed refrigerant introduced into the above refrigerant supply pipe to be cooled to a second temperature.

[0013] It may further include a second expansion section disposed in the refrigerant supply pipe, which expands the azeotropic or non-azeotropic non-combustible mixed refrigerant cooled to the second temperature to cool it down to a third temperature lower than the first temperature.

[0014] It may further include a first heat exchange module that exchanges heat between the downstream end of the first expansion section of the refrigerant circulation pipe and the refrigerant supply pipe.

[0015] It may further include an oil separator disposed between the refrigerant compressor and the refrigerant condenser of the above refrigerant circulation pipe, and an oil circulation pipe for recirculating the separated lubricating oil between the oil separator and the refrigerant compressor.

[0016] It may further include a branch pipe branched from the front end of the first expansion section of the refrigerant circulation pipe and connected to the refrigerant compressor, and a second heat exchange module that exchanges heat between the branch pipe and the oil circulation pipe.

[0017] The above azeotropic or non-azeotropic non-combustible mixed refrigerant may include at least one of an azeotropic mixture of the hydrofluorocarbon series and a non-azeotropic mixture of the hydrofluoroolefin series.

[0018] The heat exchanger may further include an auxiliary expansion valve that lowers the temperature of the natural gas to below the pour point of the lubricating oil applied to the gas compressor between the independent heat exchanger and the low-temperature heat exchanger, and at least one oil filter that removes the lubricating oil from the natural gas that has passed through the auxiliary expansion valve upstream of the low-temperature heat exchanger.

[0019] The above oil filter may include a first filter capable of filtering liquid oil and a second filter capable of filtering vapor oil. Effects of the invention

[0020] According to the present invention, natural gas can be liquefied using a highly effective cooling process. In particular, by using a process that utilizes a non-flammable refrigerant, the safety of the device can be enhanced while simultaneously improving the efficiency of the cooling process. This allows for an increase in the production capacity of liquefied natural gas while simultaneously enhancing the safety of the facility. Furthermore, impurities such as lubricating oil within the natural gas can be effectively removed through an appropriate filtering structure. Such a natural gas liquefaction device can be applied to various facilities, including onshore liquefaction facilities and onboard vessels. Brief explanation of the drawing

[0021] FIG. 1 is an overall configuration diagram of a natural gas liquefaction device according to one embodiment of the present invention. FIG. 2 is a configuration diagram showing the configuration of the second cooling unit of the natural gas liquefaction device of FIG. 1 in more detail. Figure 3 is a performance table comparing an example of the azeotropic or non-azeotropic non-combustible mixed refrigerant of the present invention with a conventional propane refrigerant. FIG. 4 is a diagram showing the configuration of a heat exchanger of a natural gas liquefaction device according to another embodiment of the present invention. Specific details for implementing the invention

[0022] The advantages and features of the present invention and the methods for achieving them will become clear by referring to the embodiments described below in detail together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below but may be implemented in various different forms. These embodiments are provided merely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined only by the claims. Throughout the specification, the same reference numerals refer to the same components.

[0023] In this specification, 'line' refers to a fluid flow path and / or a fluid flow formed along said flow path; each line may refer to a pipe forming the fluid flow path, or it may refer to the fluid (or fluid stream) itself flowing along said pipe. Additionally, the pressure unit bara refers to absolute pressure [bar (absolute)], and barg refers to gauge pressure [bar (gauge)].

[0024] FIG. 1 is an overall configuration diagram of a natural gas liquefaction device according to one embodiment of the present invention.

[0025] Referring to FIG. 1, a natural gas liquefaction apparatus (1) according to the present invention receives a feed gas (natural gas before liquefaction treatment, the main component of which is methane) and performs a liquefaction process including compression, pre-cooling, and isenthalpic expansion. In the pre-cooling process, the natural gas before liquefaction is sufficiently cooled by different cooling sections (a first cooling section and a second cooling section). At least one of the two cooling sections cools the natural gas with an independent cooling cycle composed only of a non-flammable refrigerant.

[0026] Preferably, the cooling cycle of one (the second cooling unit) can be configured with an azeotropic or non-azeotropic non-combustible mixed refrigerant, and the other (the first cooling unit) can be cooled with cryogenic recovery gas, which is a byproduct of the natural gas liquefaction process. This not only enables highly efficient cooling but also prevents the risk of fire caused by the excessive use of conventional flammable refrigerants. Consequently, the safety of the equipment performing the natural gas liquefaction process can be significantly improved.

[0027] The natural gas liquefaction device (1) of the present invention is configured as follows. A natural gas liquefaction device (1) comprises a gas compressor (15, 17) that receives and compresses natural gas from a natural gas feed stream (10), a heat exchanger (20) that cools the natural gas passing through the gas compressor (15, 17) through heat exchange, an expansion valve (25) that expands and supercools the natural gas cooled by passing through the heat exchanger (20), and a gas-liquid separation drum (27) that separates the phases of the gas-liquid mixture generated by passing through the expansion valve (25) and separates and discharges the liquid liquefied natural gas and cryogenic recovery gas. The heat exchanger (20) comprises a first cooling unit (210) that cools the natural gas using the recovery gas as a refrigerant, and a second cooling unit (220) that cools the natural gas using an azeotropic or non-azeotropic non-combustible mixed refrigerant circulating in an independent cooling cycle independent of the first cooling unit (210).

[0028] At this time, the azeotropic nonflammable mixed refrigerant refers to a nonflammable azeotropic mixed refrigerant, and the non-azeotropic nonflammable mixed refrigerant refers to a non-flammable non-azeotropic mixed refrigerant. These are all nonflammable mixed refrigerants and are applied as refrigerants for the second cooling unit (220). Although not limited thereto, for example, the azeotropic or non-azeotropic nonflammable mixed refrigerant may include at least one of a hydrofluorocarbon series azeotropic mixture (e.g., HFC series refrigerant-HFC-507A) and a hydrofluoroolefin series non-azeotropic mixture (e.g., HFO series refrigerant-HFO-448A).

[0029] In one embodiment of the present invention, the first cooling unit (210) and the second cooling unit (220) are combined to enable multi-stage cooling. For example, the first cooling unit (210) may include a low-temperature heat exchanger (212) that exchanges heat between the recovery gas discharged from the gas-liquid separation drum (27) and natural gas, and a high-temperature heat exchanger (211) that exchanges heat between the recovery gas discharged from the low-temperature heat exchanger (212) and natural gas before it enters the low-temperature heat exchanger (212) upstream of the low-temperature heat exchanger (212). Additionally, the second cooling unit (220) may include an independent heat exchanger (221) that pre-cools natural gas with an azeotropic or non-azeotropic non-combustible mixed refrigerant between the high-temperature heat exchanger (211) and the low-temperature heat exchanger (212).

[0030] Additionally, the second cooling section (220) includes a refrigerant supply pipe (222) that supplies an azeotropic or non-azeotropic mixed refrigerant to an independent heat exchanger (221), a refrigerant recovery pipe (223) that recovers an azeotropic or non-azeotropic non-combustible mixed refrigerant from the independent heat exchanger (221), and a refrigerant circulation pipe (224 in FIG. 2) that passes the azeotropic or non-azeotropic non-combustible mixed refrigerant between the refrigerant supply pipe (222) and the refrigerant recovery pipe (223) to a refrigerant compressor (301 in FIG. 2), a refrigerant condenser (303 in FIG. 2), and a first expansion section (305 in FIG. 2), and cools it to a first temperature (approximately -6.28°C) at the downstream end of the first expansion section (305), and the refrigerant supply pipe (222) is branched off from the upstream end of the first expansion section (305) of the refrigerant circulation pipe (224). After thermally contacting the rear end of the first expansion section (305) and connecting it to an independent heat exchanger (221), the azeotropic or non-azeotropic non-combustible mixed refrigerant introduced into the refrigerant supply pipe (222) can be cooled to a second temperature (approximately -1.28℃).

[0031] In addition, by further including a second expansion section (310 in FIG. 2) disposed in the refrigerant supply pipe (222) to expand the azeotropic or non-azeotropic non-combustible mixed refrigerant cooled to a second temperature and cool it down to a third temperature (approximately -40.52°C) lower than the first temperature, an extremely effective cooling cycle using an azeotropic or non-azeotropic non-combustible mixed refrigerant can be realized. Below, the configuration and effects of the present invention will be explained in more detail based on such an embodiment of the present invention.

[0032] First, the overall configuration of the natural gas liquefaction device (1) is described in detail with reference to FIG. 1. After describing the overall configuration, the second cooling unit (220) is described in more detail.

[0033] The natural gas feed stream (10) refers to a flow of natural gas before liquefaction. The natural gas feed stream (10) is primarily composed of methane and may contain impurities (e.g., heavy hydrocarbons, lubricating sulfates, nitrogen, etc.). Impurities in the natural gas feed stream (10) can be partially removed through pretreatment. Any impurities that are not removed can be removed at appropriate locations during the natural gas liquefaction process. Substances removed through pretreatment may include, for example, hydrogen sulfide, mercury, carbon dioxide, and water.

[0034] In this embodiment, the natural gas feed stream (10) is joined with the recovered gas separated in the gas-liquid separation drum (27). Since the recovered gas is part of the natural gas that remains in the gaseous state even after passing through the liquefaction process, the recovered gas can be recirculated to the liquefaction process to prevent the natural gas from being wasted during the process. To this end, the natural gas feed stream (10) can be partially crossed / joined with the gas recovery lines (40, 41, 42, 43, 11).

[0035] The recovered gas can be joined to the natural gas feed stream (10) along the gas recovery lines (40, 41, 42, 43, 11). Since the recovered gas is a gaseous component (containing nitrogen) even after the liquefaction process, the composition of the natural gas feed stream (10) does not change even if the recovered gas is joined. However, since repeated circulation can increase the nitrogen content in the natural gas, the amount of circulation can be controlled by discharging the recovered gas from the gas recovery lines (40, 41, 42, 43, 11). This will be described later.

[0036] Gas compressors (15, 17) receive and compress natural gas from these natural gas feed streams (10). After compression, at least a portion of the natural gas may be in a supercritical state. For example, the pressure and temperature conditions of the compressed natural gas passing through the gas compressors (15, 17) may change to about 250 bar and 55°C.

[0037] The gas compressor (15, 17) may, for example, be equipped with multi-stage rotating blades to compress the fluid. It may also be formed as a reciprocating compressor equipped with a piston. The gas compressor (15, 17) may be implemented in various forms capable of compressing the fluid. In this embodiment, the gas compressor (15, 17) may be a two-stage compressor comprising a first gas compression stage (15) and a second gas compression stage (17), and may be a reciprocating compressor.

[0038] In this case, a gas compressor intercooler (16) may be installed on the piping line connecting the discharge portion of the first gas compression unit (15) and the inlet portion of the second gas compression unit (17) to cool the gas stream, whose temperature has risen while being compressed by the first gas compression unit (15), to the inlet temperature of the second gas compression unit (17). Additionally, a gas compressor aftercooler (18) may be installed on the piping line (19) connecting the discharge portion of the second gas compression unit (17) and the heat exchanger (20) to cool the gas stream, whose temperature has risen while being compressed by the second gas compression unit (17).

[0039] In this configuration, natural gas can be compressed to about 96 barg in the first gas compression section (15) and cooled to about 55°C in the gas compressor intercooler (16), and can be compressed to about 250 barg in the second gas compression section (17) to be converted into a supercritical or dense phase. The natural gas compressed in this way can be cooled to about 55°C in the gas compressor aftercooler (18) and then supplied to the heat exchange section (20).

[0040] A gas compressor suction drum (13) for separating and removing liquid substances may also be installed upstream of the gas compressor (15, 17). The gas compressor suction drum (13) can capture any condensation within the natural gas to protect the head of the gas compressor (15, 17) from liquid droplets. The gas compressor suction drum (13) receives a natural gas feed stream (10), separates the liquid components, and supplies only natural gas in a gaseous state to the gas compressor (15, 17). The gas compressor suction drum (13) may be formed, for example, as a gas-liquid separation device that separates the phases of a gas-liquid mixed fluid using a density difference.

[0041] The heat exchanger (20) cools the high-pressure natural gas that has passed through the gas compressor (15, 17) through heat exchange. This process is a precooling process for expanding the high-pressure natural gas before liquefying it, and is organically integrated with subsequent processes. That is, the cryogenic recovery gas generated in the expansion valve (25) described later and separated in the gas-liquid separation drum (27) can be used as a refrigerant in the precooling process, and the recovery gas that has undergone this process can be added to the natural gas feed stream (10) for repeated use.

[0042] The heat exchanger (20) includes a first cooling unit (210) that cools natural gas using a recovery gas as a refrigerant, and a second cooling unit (220) that cools natural gas using an azeotropic or non-azeotropic non-combustible mixed refrigerant circulating in an independent cooling cycle independent of the first cooling unit (210). Since the recovery gas, which is the refrigerant of the first cooling unit (210), is continuously generated during the natural gas liquefaction process, there is no problem in replenishing it. Therefore, the problem of refrigerant leakage in a conventional closed cycle can also be effectively solved.

[0043] In addition, the second cooling unit (220) cools by circulating a non-flammable refrigerant through an independent cooling cycle [refrigerant supply pipe (222), refrigerant recovery pipe (223), and refrigerant circulation pipe (224)], so it is free from the risk of fire even if some leakage occurs due to an accident. Therefore, the equipment becomes very safe. Furthermore, as described later, since an extremely effective cooling cycle is formed using an azeotropic or non-azeotropic non-flammable mixed refrigerant, effective liquefaction of natural gas is also possible through sufficient pre-cooling. The heat exchange unit (20) will be described in more detail later.

[0044] If necessary, it is also possible to place a filter (not shown) of a suitable type between the gas compressor (15, 17) and the heat exchanger (20). For example, a filter may be placed to remove liquid and / or vapor impurities remaining in the natural gas after passing through the gas compressor (15, 17). The filter may remove the impurities by a gas-liquid separation method or by an adsorption method. The filter may be placed at the appropriate location in the liquefaction process as needed.

[0045] Natural gas pre-cooled in the heat exchanger (20) passes through the expansion valve (25) and is further cooled. The expansion valve (25) can supercool the natural gas through an isenthalpic expansion process. The expansion valve (25) can be formed as a Joule-Thomson valve, and the compressed natural gas can be expanded and cooled by the Joule-Thomson process after pre-cooling. The expansion process is an isenthalpic process in which the volume of the natural gas changes due to the pressure difference and can be cooled to a cryogenic temperature. In this process, a portion of the natural gas may be partially liquefied and a gas-liquid mixture may be created through phase changes, such as the generation of flash gas.

[0046] For example, the cooled natural gas stream (24) discharged from the heat exchanger (20) can be isenthalpicly expanded to 4 to 8 barg, 5 to 7 barg, or about 6 barg as it passes through the expansion valve (25). The expansion valve (25) can expand the high-pressure natural gas pre-cooled through the Joule-Thomson process to cool it to a lower temperature. The temperature of the fluid stream (26) produced as a result of the Joule-Thomson expansion can be lowered to about -130°C to -160°C and can be partially liquefied.

[0047] That is, the compressed and cooled natural gas passes through the expansion valve (25) and is cooled to a cryogenic temperature, which can be converted into a two-phase gas-liquid mixture. The gas-liquid separation drum (27) downstream of the expansion valve (25) separates the gas-liquid mixture generated by passing through the expansion valve (25) into phases and discharges it as a liquid phase and a gas phase. That is, the gas-liquid separation drum (27) separates the gas-liquid mixture of natural gas generated by the supercooling process into liquid liquefied natural gas and gaseous cryogenic recovery gas and discharges them.

[0048] That is, liquefied natural gas (LNG) is produced by passing through the expansion valve (25) and the gas-liquid separation drum (27), which can be obtained by separating the gas-liquid mixture. The liquid component separated from the gas-liquid mixture is separated into liquefied natural gas, and the gas component is separated into cryogenic recovery gas and discharged. The cryogenic recovery gas is supplied to the aforementioned heat exchanger (20), used as a refrigerant for the pre-cooling process, and then joins the natural gas feed stream (10). Meanwhile, the liquefied natural gas is stored in a storage tank (29). The storage tank (29) can be positioned, for example, at the downstream end of the gas-liquid separation drum (27) and connected to the liquefied natural gas discharge line (28). The liquefied natural gas can be accumulated in the storage tank (29) and then supplied to the point of use.

[0049] A filter module (30) may be placed between the expansion valve (25) and the gas-liquid separation drum (27). The filter module (30) may be installed to remove impurities (e.g., heavy hydrocarbons and lubricating oil, etc.) that have been converted into a solid state by cooling. The filter module (30) may be structured to filter out solid substances through filtering. Within such limits, the filter module (30) is not particularly limited, so various structures can be applied for effective filtering. In this way, impurities can also be removed at appropriate points during the liquefaction process.

[0050] Below, the configuration of the heat exchange section and the structure of the independent cooling cycle included in the heat exchange section will be explained in more detail.

[0051] Referring to FIG. 1, the heat exchanger (20) may be structured to cool high-pressure natural gas compressed in gas compressors (15, 17) to different temperatures in multiple stages. The heat exchanger (20) may cool the compressed natural gas in multiple stages by including a section that uses the recovered gas as a refrigerant on the gas recovery line (40, 41, 42, 43, 11) and a section that circulates an independent cooling cycle. The heat exchanger (20) includes a first cooling section (210) that cools the natural gas using the recovered gas as a refrigerant, and a second cooling section (220) that cools the natural gas using an azeotropic or non-azeotropic non-combustible mixed refrigerant circulating in an independent cooling cycle independent of the first cooling section (210).

[0052] The first cooling section (210) may include a low-temperature heat exchanger (212) that exchanges heat between the recovered gas discharged from the gas-liquid separation drum (27) and natural gas, and a high-temperature heat exchanger (211) that exchanges heat between the recovered gas discharged from the low-temperature heat exchanger (212) and natural gas before it enters the low-temperature heat exchanger (212) upstream of the low-temperature heat exchanger (212). Accordingly, the natural gas entering the heat exchange section (20) first exchanges heat with the high-temperature heat exchanger (211) and then finally exchanges heat with the low-temperature heat exchanger (212) to reach the target temperature.

[0053] The high-temperature heat exchanger (211) and the low-temperature heat exchanger (212) may be structured to be connected in series on the gas recovery line (40, 41, 42, 43, 11). For example, the gas recovery line (40, 41, 42, 43, 11) may include a line (40) that supplies recovery gas from the gas-liquid separation drum (27) to the low-temperature heat exchanger (212), a line (41) that supplies the recovery gas heat-exchanged in the low-temperature heat exchanger (212) to the high-temperature heat exchanger (211), and a line (42, 43, 11) that supplies the recovery gas that has passed through the high-temperature heat exchanger (211) to the natural gas feed stream (10) to join them. The high-temperature heat exchanger (211) and the low-temperature heat exchanger (212) are connected in series to pass the recovered gas in the opposite direction to the transfer direction of the high-pressure natural gas, and the temperature of the high-pressure natural gas that has passed through the gas compressor (15, 17) can be lowered step by step.

[0054] In this way, the first cooling section (210) is formed with a structure that exchanges heat between the recovered gas and natural gas in stages. By integrating the structure for recovering cryogenic recovered gas with the cooling structure through heat exchange, a highly energy-efficient system is formed. For example, the high-temperature heat exchanger (211) and the low-temperature heat exchanger (212) can be formed as a printed circuit heat exchanger (PCHE) or an aluminum plate heat exchanger, and the same applies to the independent heat exchangers described later. Each heat exchanger can be appropriately insulated to minimize the inflow of heat from the surroundings.

[0055] Meanwhile, the second cooling unit (220) pre-cools natural gas with an azeotropic or non-azeotropic non-combustible mixed refrigerant between the high-temperature heat exchanger (211) and the low-temperature heat exchanger (212). That is, natural gas that has been primarily cooled in the high-temperature heat exchanger (211) is pre-cooled to an intermediate temperature in the second cooling unit (220) before being cooled to a target temperature in the low-temperature heat exchanger (212). The second cooling unit can cool to a target temperature by circulating the azeotropic or non-azeotropic non-combustible mixed refrigerant in an extremely efficient process. The present invention implements this as follows.

[0056] The second cooling unit (220) may include an independent heat exchanger (221) that pre-cools natural gas with an azeotropic or non-azeotropic non-combustible mixed refrigerant between the high-temperature heat exchanger (211) and the low-temperature heat exchanger (212) of the first cooling unit (210). The second cooling section (220) may include a refrigerant supply pipe (222) that supplies an azeotropic or non-azeotropic non-combustible mixed refrigerant to an independent heat exchanger (221), a refrigerant recovery pipe (223) that recovers the azeotropic or non-azeotropic non-combustible mixed refrigerant from the independent heat exchanger (221), and a refrigerant circulation pipe (224) connected between the refrigerant supply pipe (222) and the refrigerant recovery pipe (223) to pass the azeotropic or non-azeotropic non-combustible mixed refrigerant through a refrigerant compressor (301 in FIG. 2), a refrigerant condenser (303 in FIG. 2), and a first expansion section (305 in FIG. 2) and cool it to a first temperature at the downstream end of the first expansion section (305).

[0057] Since the cooling cycle of the second cooling unit (220) is briefly illustrated in FIG. 1, the second cooling unit (220) will be explained in more detail with reference to FIG. 2, which illustrates it in detail.

[0058] FIG. 2 is a configuration diagram illustrating the configuration of the second cooling unit of the natural gas liquefaction device of FIG. 1 in more detail, and FIG. 3 is a performance table comparing an example of the azeotropic or non-azeotropic non-combustible mixed refrigerant of the present invention with a conventional propane refrigerant. The pipelines shown in FIG. 2 also represent the fluid flow along the pipelines.

[0059] Referring to FIG. 2, the second cooling unit (220) is composed of an independent heat exchanger (221) and an independent cooling cycle that circulates an azeotropic or non-azeotropic non-combustible mixed refrigerant to the independent heat exchanger (221). In order to appropriately control the final temperature of the refrigerant supplied to the independent heat exchanger (221) and improve the efficiency of the cooling cycle, the cooling cycle includes a loop-shaped refrigerant circulation pipe (224), a refrigerant supply pipe (222) that supplies the cooled refrigerant to the independent heat exchanger (221), and a refrigerant recovery pipe (223) that recovers the refrigerant to the refrigerant circulation pipe (224) after heat exchange.

[0060] As described, the refrigerant circulation pipe (224) is configured with a refrigerant compressor (301), a refrigerant condenser (303), and a first expansion section (305) in sequence, which cool an azeotropic or non-azeotropic non-combustible mixed refrigerant, which is the refrigerant of the second cooling section, through compression, condensation, and expansion. In the following description of the second cooling section (220), the azeotropic or non-azeotropic non-combustible mixed refrigerant will be simply referred to as the refrigerant.

[0061] The refrigerant compressor (301) can receive and compress refrigerant through at least two paths and discharge it. The refrigerant compressor (301) is positioned on the refrigerant circulation pipe (224) and connected to the refrigerant recovery pipe (223). Thus, the refrigerant circulating in the refrigerant circulation pipe (224) and the refrigerant recovered in the refrigerant recovery pipe (223) can be mixed and compressed. To the extent that such operation is possible, the refrigerant compressor (301) does not need to be limited to a specific structure. For example, the refrigerant discharged from the refrigerant compressor (301) may be in a temperature range of approximately 90°C.

[0062] The refrigerant discharged from the refrigerant compressor (301) passes through the refrigerant condenser (303) and is condensed. The refrigerant condenser (303) may include an air-cooled condenser, but is not limited thereto, and the refrigerant condenser (303) may also have various structures capable of cooling and condensing the compressed refrigerant. For example, the refrigerant discharged from the refrigerant condenser (303) may be appropriately cooled and be in a temperature range of approximately 50°C.

[0063] An oil separator (302) may also be placed between the refrigerant compressor (301) and the refrigerant condenser (303). The oil separator (302) is also placed on the refrigerant circulation pipe (224). The oil separator (302) may be formed as a gas-liquid separation device, etc., and may be applied to separate the lubricating oil mixed in the refrigerant during the compression process into a liquid state. An oil circulation pipe (307) may be formed between the oil separator (302) and the refrigerant compressor (301) to recirculate the separated lubricating oil to the refrigerant compressor (301).

[0064] The oil circulation pipe (307) may include a structure that exchanges heat with the refrigerant on the refrigerant circulation pipe (224) to prevent overheating of the compressor. For example, it may include a branch pipe (308) that branches off from the front end of the first expansion section (305) of the refrigerant circulation pipe (224) and is connected to the refrigerant compressor (301), and a second heat exchange module (309) that exchanges heat between the branch pipe (308) and the oil circulation pipe (307). Through this, the lubricating oil mixed in the refrigerant during the compression process can be recovered, but overheating caused by repeated recovery can be prevented. The branch pipe (308) may be formed in multiple numbers, and the second heat exchange module (309) may be connected between at least one of the multiple branch pipes (308) and the oil circulation pipe (307).

[0065] The refrigerant that has passed through the refrigerant condenser (303) can be stored in the refrigerant storage tank (304) and then supplied to the first expansion unit (305). The refrigerant storage tank (304) can serve to regulate the flow rate of the refrigerant circulating through the refrigerant circulation pipe (224) and can also serve as a kind of accumulator. The aforementioned branch pipe (308) can be branched between the refrigerant storage tank (304) and the first expansion unit (305) and connected to the refrigerant compressor (301). For example, the refrigerant storage tank (304) can store the refrigerant in a temperature range of approximately 50°C and then supply it to the first expansion unit (305).

[0066] The refrigerant discharged from the refrigerant storage tank (304) is supplied to the first expansion section (305) along the refrigerant circulation pipe (224) and then cooled by depressurization in the first expansion section (305). The first expansion section (305) may be formed, for example, by an expansion valve that expands and cools the fluid through a Joule-Thomson process. However, it is not limited thereto, and other types of expanders may be applied if possible. The refrigerant passing through the first expansion section (305) can be cooled to a temperature of approximately -6.28°C.

[0067] That is, the refrigerant in the refrigerant circulation pipe (224) can be cooled to the lowest temperature of the refrigerant circulation pipe (224), which is a first temperature (approximately -6.28℃), after passing through the refrigerant compressor (301), the refrigerant condenser (303), and the first expansion section (305) at the rear end of the first expansion section (305).

[0068] The first expansion unit (305) is connected to a temperature sensor (401) installed at the rear end and can be operated by taking into account the temperature at the rear end. The temperature sensor can be formed as a thermocouple, and this configuration can also be the same for the second expansion unit (310) described later. In the case of the second expansion unit (310), it is placed in the refrigerant supply pipe (222), and the temperature sensor (401) can be installed in the refrigerant circulation pipe (224). The second expansion unit (310) will be described again later.

[0069] The refrigerant circulation pipe (224) sequentially passes through the refrigerant compressor (301), the refrigerant condenser (303), and the first expansion section (305) to cool the refrigerant to a first temperature. The refrigerant that has passed through the first expansion section (305) is supplied back to the refrigerant compressor (301) (after heat exchange) to repeat the compression process. With this structure, the first temperature of the refrigerant can be generated in the refrigerant circulation pipe (224).

[0070] The refrigerant supply pipe (222) branches off at an appropriate point of the refrigerant circulation pipe to supply refrigerant to an independent heat exchanger. The refrigerant supply pipe (222) branches off from the front end of the first expansion section (305) of the refrigerant circulation pipe (224), thermally contacts the rear end of the first expansion section (305), and then connects to the independent heat exchanger (221). Through this, the azeotropic or non-azeotropic non-combustible mixed refrigerant introduced into the refrigerant supply pipe (222) can be cooled to a second temperature. The second temperature is a temperature higher than the first temperature and may be approximately -1.28°C.

[0071] That is, as illustrated, the refrigerant supply pipe (222) can be branched from the refrigerant circulation pipe (224) at the front end of the first expansion section (305) to separate and discharge some of the refrigerant before expansion from the refrigerant circulation pipe (224). The discharged refrigerant is thermally contacted with the rear end of the second expansion section (310) within the refrigerant supply pipe (222) to exchange heat and is cooled to a second temperature. For example, as illustrated, a first heat exchange module (306) that exchanges heat between the rear end of the first expansion section (305) of the refrigerant circulation pipe (224) and the refrigerant supply pipe (222) can be arranged to cool the refrigerant in the refrigerant supply pipe (222) in the first stage through heat exchange.

[0072] That is, the refrigerant is continuously circulated on the refrigerant circulation pipe (224) to generate a first temperature, and at least a portion of the refrigerant can be branched from the refrigerant circulation pipe (224) to cool to a second temperature through heat exchange and then supplied to the refrigerant supply pipe (222). The refrigerant remaining in the refrigerant circulation pipe (224) passes through the first heat exchange module (306) and is then supplied to the refrigerant compressor (301) to be compressed again.

[0073] A second expansion section (310) is provided in the refrigerant supply pipe (222) to cool the refrigerant cooled to a second temperature to a third temperature. That is, the refrigerant that has been properly pre-cooled is additionally cooled to bring the refrigerant to a target temperature before being supplied to the independent heat exchanger (221). In other words, a second expansion section (310) can be provided in the refrigerant supply pipe (222) to expand an azeotropic or non-azeotropic non-combustible mixed refrigerant cooled to a second temperature and cool it to a third temperature (approximately -49.52°C) which is lower than the first temperature (lowest temperature of the refrigerant circulation pipe). The second expansion section (310) can also be formed as an expansion valve, etc., that expands and cools the fluid through a Joule-Thomson process, and can cool the cooled fluid to a lower target temperature (third temperature) by isenthalpic expansion.

[0074] Accordingly, the second cooling unit (220) of the present invention uses a single refrigerant, but can supply the refrigerant with its temperature lowered to a point that is not generally expected by adjusting the temperature of the refrigerant in stages. In addition, since this cooling cycle is implemented using only azeotropic or non-azeotropic non-combustible mixed refrigerants, both process efficiency and safety are greatly improved.

[0075] Therefore, the independent heat exchanger (221) is provided with an azeotropic or non-azeotropic non-combustible mixed refrigerant cooled to a third temperature (approximately -49.52°C), which is sufficient to cool the natural gas to an intermediate temperature between the two heat exchangers on the connecting line (22) connecting the high-temperature heat exchanger and the low-temperature heat exchanger. Based on the third temperature, the high-temperature heat exchanger can pre-cool the natural gas to a temperature higher than that, and the low-temperature heat exchanger can pre-cool it to a temperature lower than that, so that it can finally be cooled to an operating temperature at which effective natural gas liquefaction is possible at the expansion valve.

[0076] The refrigerant that has passed through the independent heat exchanger (221) is recovered to the refrigerant circulation pipe (224) through the refrigerant recovery pipe (223), and a refrigerant compressor suction drum (311) may be placed in the refrigerant recovery pipe (223) to protect the refrigerant compressor (301) head from liquid droplets, etc. The refrigerant recovered from the independent heat exchanger (221) to the refrigerant recovery pipe (223) may be in a temperature range of approximately -35°C and, after passing through the refrigerant compressor suction drum (311), may be supplied to the refrigerant compressor (301) to be compressed again. In this way, natural gas can be effectively pre-cooled by repeating the cooling cycle.

[0077] The cooling cycle of the second cooling unit (220) described above consists only of an azeotropic or non-azeotropic non-flammable mixed refrigerant. FIG. 3 shows an example of such an azeotropic or non-azeotropic non-flammable mixed refrigerant and a comparison of its performance with that of a conventional propane-based refrigerant (e.g., HC-290). As described above, the azeotropic non-flammable mixed refrigerant of the present invention is a non-flammable azeotropic mixed refrigerant and includes an azeotropic mixture of the hydrofluorocarbon series (e.g., HFC-507A), and the non-azeotropic non-flammable mixed refrigerant may be a non-azeotropic mixed refrigerant and include a non-azeotropic mixture of the hydrofluoroolefin series (e.g., HFO-448A).

[0078] As confirmed in (b) of the table, the exemplified azeotropic or non-azeotropic non-combustible mixed refrigerant is a non-combustible and low-toxicity refrigerant of Class 1 in the Safety group (based on the safety class under ISO 817, 2014 revision), and is extremely safe compared to conventional propane refrigerant. By configuring the aforementioned cooling cycle with such azeotropic or non-azeotropic non-combustible mixed refrigerant, not only the safety of the equipment (especially fire prevention) but also the efficiency of cooling to pre-cool natural gas to the target temperature can be secured.

[0079] Hereinafter, with reference to FIG. 1, the remaining components of the present invention and the recovery gas circulation process will be explained in more detail.

[0080] Referring again to FIG. 1, the recovered gas discharged from the gas-liquid separation drum (27) is recirculated along the gas recovery line (40, 41, 42, 43, 11) connected to the heat exchanger (20) described above. The gas recovery line (40, 41, 42, 43, 11) may refer to a pipe through which the discharged recovered gas travels, as well as a recovered gas stream flowing through it. The gas recovery line (40, 41, 42, 43, 11) serves to heat-exchange the recovered gas discharged from the gas-liquid separation drum (27) at the heat exchanger (20) and then join it to the natural gas feed stream (10). The gas recovery line (40, 41, 42, 43, 11) may refer to a piping line or the entire flow of the recovery gas that circulates the recovery gas from the gas-liquid separation drum (27) to the natural gas feed stream (10), and may be formed by including a plurality of lines located in the recovery gas flow path as described above.

[0081] A gas purging valve (49) may also be installed on the gas recovery line (40, 41, 42, 43, 11) to discharge at least a portion of the recovered gas to the outside of the gas recovery line (40, 41, 42, 43, 11). The gas purging valve (49) has the function of controlling the flow path and can change the flow path at the point where the piping line is connected or joined to change the direction of flow of the recovered gas or distribute it. For example, the amount of recovered gas discharged through the gas purging valve (49) can be changed according to the concentration of nitrogen contained in the liquefied natural gas discharged from the gas-liquid separation drum (27). That is, an allowable nitrogen concentration of the liquefied natural gas, which is the final product of the device, can be set, and the amount of recovered gas discharged can be precisely controlled using the gas purging valve (49) so that the nitrogen concentration is maintained at a level lower than that concentration.

[0082] The arrangement structure of the gas recovery line (40, 41, 42, 43, 11) is described in more detail as follows. A recovery gas compressor (45, 47) may be installed on the gas recovery line (40, 41, 42, 43, 11) to compress the recovery gas that has passed through the heat exchanger (20) described above and provide it as a natural gas feed stream (10). The recovery gas compressor (45, 47) may be a two-stage compressor including a first recovery gas compression section (45) and a second recovery gas compression section (47). On the piping line connecting the discharge section of the first recovery gas compression section (45) and the inlet section of the second recovery gas compression section (47), a recovery gas compressor intercooler (46) may be installed to cool the recovery gas stream, whose temperature has risen while being compressed by the first recovery gas compression section (45), to the inlet temperature of the second recovery gas compression section (47). Additionally, a recovery gas compressor aftercooler (48) may be installed on a piping line (11) [i.e., part of the gas recovery line (40, 41, 42, 43, 11)] that joins the natural gas feed stream (10) from the discharge section of the second recovery gas compression section (47) to cool the recovery gas stream whose temperature has risen while being compressed by the second recovery gas compression section (47).

[0083] In this embodiment, the gas purging valve (49) can be connected to a power generation module (52) that generates power using the recovered gas as fuel. The power generation module can provide at least a portion of the generated power to a use site of the natural gas liquefaction device (1). That is, the recovered gas discharged from the gas purging valve (49) is not discarded even after being discharged from the gas recovery line (40, 41, 42, 43, 11) but is supplied as fuel and used for the purpose of producing power.

[0084] For example, a branch line (44) may be formed that branches off from the gas recovery line (42, 43) between the heat exchanger (20) and the recovery gas compressor (45, 47) among the gas recovery lines (40, 41, 42, 43, 11) to the power supply module (52), and a portion of the recovery gas may be supplied to the power generation module (52) through the branch line (44). The gas purging valve (49) is formed as a three-way valve that controls the flow path at the confluence of the gas recovery lines (40, 41, 42, 43, 11) and the branch line (44), so that it can distribute the recovery gas and simultaneously control the discharge amount and the circulation amount.

[0085] In addition, the evaporated gas generated by the evaporation of liquefied natural gas in the storage tank (29) can also be recovered and used for power generation. To this end, an evaporated gas line (50) capable of supplying evaporated gas may be placed between the branch line (44) and the storage tank (29). In this way, available resources within the liquefaction device or facility can be recycled, thereby further improving the efficiency of the device.

[0086] The power produced by the recovery gas can be recovered and used to drive the device again. The power generation module (52) generates power using the recovery gas discharged from the gas purging valve (49) as fuel, and the generated power can be supplied to the gas compressor (15, 17), the recovery gas compressor (45, 47), and other driving devices (e.g., the refrigerant compressor of the second cooling unit). That is, it is possible to receive part or all of the power required by the natural gas liquefaction device (1) from the power generation module (52) driven by the discharged recovery gas. In this way, the device can be operated energy-efficiently by utilizing the recovery gas in various ways.

[0087] The power generation module (52) may include, for example, a generator and a gas engine, and the gas engine may be driven by gas fuel including recovered gas, and the generator may be connected to the gas engine to operate. The power generation module (52) may be implemented in various forms capable of generating power by receiving gas fuel such as recovered gas. The power generated by the power generation module (52) is not limited to electric power and may be mechanical driving power. For example, power may be produced by a generator connected to the gas engine and supplied to each compressor, etc., or it is possible to directly transmit the mechanical driving power of the gas engine to each compressor, etc. without a generator.

[0088] By configuring the natural gas liquefaction device (1) in this manner, the pre-cooling process of natural gas and the supercooling process accompanied by fluid expansion can be organically integrated so that the entire process operates in an energy-efficient manner. In addition, by using two types of refrigerants—azeotropic or non-azeotropic non-combustible mixed refrigerant—a highly efficient and safe pre-cooling process can be implemented. By configuring the natural gas liquefaction device (1) in this manner, liquefied natural gas can be produced.

[0089] Hereinafter, other embodiments of the present invention will be described in detail.

[0090] Since another embodiment of the present invention is substantially the same as the previously described embodiment except that an auxiliary expansion valve and an oil filter are applied to the heat exchanger, the differences from the previously described embodiment will be explained in detail, and for matters not otherwise mentioned, please refer to the previously described explanation.

[0091] FIG. 4 is a diagram showing the configuration of a heat exchanger of a natural gas liquefaction device according to another embodiment of the present invention.

[0092] Referring to FIG. 4, a natural gas liquefaction device according to another embodiment of the present invention has a structure (auxiliary expansion valve and oil filter) added to the heat exchanger (20) for removing lubricating oil mixed in from a gas compressor (see 15, 17 in FIG. 1). The lubricating oil is applied to the gas compressor (15, 17) (e.g., the cylinder of the compressor), but since it can cause problems in other parts of the process after being mixed with natural gas during the compression process, it is desirable to remove it at an appropriate location.

[0093] In particular, since lubricating oil mixed with natural gas can adhere to the cooling device [especially the low-temperature heat exchanger (212)] when cooled below the pour point and hinder the flow of the fluid (i.e., natural gas), such a problem is resolved as follows.

[0094] For example, the heat exchanger (20) may further include an auxiliary expansion valve (320) that lowers the temperature of natural gas to below the pour point of the lubricating oil applied to the gas compressor (see 15 and 17 in FIG. 1) between the independent heat exchanger (221) and the low-temperature heat exchanger (212) as in FIG. 4, and at least one oil filter (330) that removes the lubricating oil from the natural gas that has passed through the auxiliary expansion valve (320) upstream of the low-temperature heat exchanger (212).

[0095] That is, considering that the heat exchanger (20) has a multi-stage cooling structure, a structure is applied to lower the natural gas temperature outside the heat exchanger to below the pour point of the lubricating oil, and the lubricating oil that has lost its fluidity can be filtered in advance outside the heat exchanger.

[0096] According to this configuration, the lubricating oil is removed as follows. That is, referring to FIG. 4, after passing through the independent heat exchanger (221) and before reaching the low-temperature heat exchanger (212), the natural gas is cooled below the pour point of the lubricating oil by an auxiliary expansion valve (320). Since the lubricating oil loses fluidity and becomes solid or semi-solid when cooled below the pour point, and is prone to solidification, the lubricating oil is cooled below the pour point in advance before reaching the low-temperature heat exchanger (212) and removed outside the heat exchanger. The lubricating oil cooled below the pour point is removed by an oil filter (330).

[0097] The oil filter (330) is positioned between the auxiliary expansion valve (320) and the low-temperature heat exchanger (212). The oil filter (330) may be formed to redundantly filter oil in liquid and vapor phases, taking into account the fluid phase (e.g., supercritical phase) caused by expansion. That is, the oil filter (330) may include a first filter (331) capable of filtering liquid oil and a second filter (332) capable of filtering vapor oil. Each may be one or multiple, and the number may be increased or decreased as needed. For example, the first filter (331) may be formed as a coalescing type filter media that filters liquid oil from gas (natural gas), and the second filter (332) may be formed as an absorber type filter media that filters vapor oil from gas (natural gas). These may be appropriately redundantly positioned as illustrated.

[0098] This arrangement does not exclude the additional application of an oil filter (330') at another location, so as needed, any additional oil filter (330') can be placed at another location where the lubricating oil can be filtered.

[0099] Therefore, even if natural gas is cooled to a temperature (e.g., -65°C) lower than the lubricating oil pour point (e.g., -45°C) in the low-temperature heat exchanger (212), the problem of the heat exchanger's flow path being blocked by the lubricating oil does not occur. For example, natural gas can be cooled to -35°C (251 barg) in the independent heat exchanger (221), then cooled to -45°C (120 barg) at the lubricating oil pour point in the auxiliary expansion valve (320), and then cooled again to -65°C (119.8 barg) in the low-temperature heat exchanger (212). In this way, since the lubricating oil in the natural gas is already cooled below the pour point before reaching the low-temperature heat exchanger (212) and then removed by the oil filter (330), the problem caused by the lubricating oil in the low-temperature heat exchanger (212) practically does not occur.

[0100] At this time, the auxiliary expansion valve (320) can be formed as a Joule Thomson valve, and the oil filter (330) can be formed as a structure capable of draining. For example, the oil filter (330) can repeatedly discharge the lubricating oil to the outside by opening the drain valve (not shown) after filtering. In this way, problems such as the lubricating oil adhering to the heat exchanger can be very effectively resolved by cooling the lubricating oil below its pour point outside the heat exchanger and then removing it.

[0101] Although embodiments of the present invention have been described above with reference to the attached drawings, those skilled in the art will understand that the present invention may be implemented in other specific forms without changing its technical concept or essential features. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. Explanation of the symbols

[0102] 1: Natural gas liquefaction unit 10: Natural gas feed stream 13: Gas compressor suction drum 15, 17: Gas compressor 20: Heat exchanger 22: Connection line 24: Natural gas stream 25: Expansion valve 26: Fluid stream 27: Gas-liquid separation drum 28: Liquefied natural gas discharge line 29: Storage tank 30: Filter module 40, 41, 42, 43, 11: Gas recovery line 44: Branch line 45, 47: Recovery gas compressor 49: Gas purging valve 50: Evaporative Gas Line 52: Power Generation Module 210: First cooling section 211: High-temperature section heat exchanger 212: Low-temperature heat exchanger 220: Second cooling section 221: Independent heat exchanger 222: Refrigerant supply pipe 223: Refrigerant return pipe 224: Refrigerant circulation pipe 301: Refrigerant compressor 302: Oil separator 303: Refrigerant condenser 304: Refrigerant storage tank 305: First expansion section 306: First heat exchange module 307: Oil circulation pipe 308: Branch pipe 309: Second heat exchange module 310: Second expansion section 311: Refrigerant compressor suction drum 320: Auxiliary expansion valve 330: Oil filter 331: First filter 332: Second filter

Claims

Claim 1 A gas compressor that receives and compresses natural gas from a natural gas feed stream; a heat exchanger that cools the natural gas passing through the gas compressor through heat exchange; and an expansion valve that expands and supercools the natural gas cooled after passing through the heat exchanger. The apparatus includes a gas-liquid separation drum that separates the gas-liquid mixture generated by passing through the expansion valve and separates and discharges the liquid liquefied natural gas and cryogenic recovery gas. The heat exchanger comprises a first cooling unit that cools the natural gas using the recovery gas as a refrigerant, and a second cooling unit that cools the natural gas using an azeotropic or non-azeotropic non-combustible mixed refrigerant circulating in an independent cooling cycle independent of the first cooling unit. The first cooling unit includes a low-temperature heat exchanger that exchanges heat between the recovery gas discharged from the gas-liquid separation drum and the natural gas, and a high-temperature heat exchanger that exchanges heat between the recovery gas discharged from the low-temperature heat exchanger and the natural gas before it enters the low-temperature heat exchanger. The second cooling unit includes an independent heat exchanger that pre-cools the natural gas using the azeotropic or non-azeotropic non-combustible mixed refrigerant between the high-temperature heat exchanger and the low-temperature heat exchanger, and the independent A natural gas liquefaction device comprising: a refrigerant supply pipe for supplying the azeotropic or non-azeotropic non-combustible mixed refrigerant to a heat exchanger; a refrigerant recovery pipe for recovering the azeotropic or non-azeotropic non-combustible mixed refrigerant from the independent heat exchanger; and a refrigerant circulation pipe connected between the refrigerant supply pipe and the refrigerant recovery pipe, which passes the azeotropic or non-azeotropic non-combustible mixed refrigerant through a refrigerant compressor, a refrigerant condenser, and a first expansion section, and cools the azeotropic or non-azeotropic non-combustible mixed refrigerant to a first temperature at the rear end of the first expansion section, wherein the refrigerant supply pipe branches off from the front end of the first expansion section of the refrigerant circulation pipe, thermally contacts the rear end of the first expansion section, and is connected to the independent heat exchanger, thereby cooling the azeotropic or non-azeotropic non-combustible mixed refrigerant introduced into the refrigerant supply pipe to a second temperature. Claim 2 delete Claim 3 delete Claim 4 delete Claim 5 delete Claim 6 A natural gas liquefaction device according to claim 1, further comprising a second expansion section disposed in the refrigerant supply pipe to expand the azeotropic or non-azeotropic non-combustible mixed refrigerant cooled to the second temperature and cool it to a third temperature lower than the first temperature. Claim 7 A natural gas liquefaction device according to claim 1, further comprising a first heat exchange module that exchanges heat between the downstream end of the first expansion section of the refrigerant circulation pipe and the refrigerant supply pipe. Claim 8 A gas compressor that receives and compresses natural gas from a natural gas feed stream; a heat exchanger that cools the natural gas passing through the gas compressor through heat exchange; and an expansion valve that expands and supercools the natural gas cooled after passing through the heat exchanger. The apparatus includes a gas-liquid separation drum that separates the gas-liquid mixture generated by passing through the expansion valve and separates and discharges the liquid liquefied natural gas and cryogenic recovery gas. The heat exchanger comprises a first cooling unit that cools the natural gas using the recovery gas as a refrigerant, and a second cooling unit that cools the natural gas using an azeotropic or non-azeotropic non-combustible mixed refrigerant circulating in an independent cooling cycle independent of the first cooling unit. The first cooling unit includes a low-temperature heat exchanger that exchanges heat between the recovery gas discharged from the gas-liquid separation drum and the natural gas, and a high-temperature heat exchanger that exchanges heat between the recovery gas discharged from the low-temperature heat exchanger and the natural gas before it enters the low-temperature heat exchanger. The second cooling unit includes an independent heat exchanger that pre-cools the natural gas using the azeotropic or non-azeotropic non-combustible mixed refrigerant between the high-temperature heat exchanger and the low-temperature heat exchanger, and the independent A natural gas liquefaction apparatus comprising: a refrigerant supply pipe for supplying the azeotropic or non-azeotropic non-combustible mixed refrigerant to a heat exchanger; a refrigerant recovery pipe for recovering the azeotropic or non-azeotropic non-combustible mixed refrigerant from the independent heat exchanger; and a refrigerant circulation pipe connected between the refrigerant supply pipe and the refrigerant recovery pipe, which passes the azeotropic or non-azeotropic non-combustible mixed refrigerant through a refrigerant compressor, a refrigerant condenser, and a first expansion section, and cools the azeotropic or non-azeotropic non-combustible mixed refrigerant to a first temperature at the downstream end of the first expansion section; an oil separator disposed between the refrigerant compressor and the refrigerant condenser of the refrigerant circulation pipe; and an oil circulation pipe for recirculating the separated lubricating oil between the oil separator and the refrigerant compressor. Claim 9 A natural gas liquefaction device according to claim 8, further comprising a branch pipe branched from the front end of the first expansion section of the refrigerant circulation pipe and connected to the refrigerant compressor, and a second heat exchange module that exchanges heat between the branch pipe and the oil circulation pipe. Claim 10 In claim 1, the natural gas liquefaction device comprises at least one of an azeotropic mixture of a hydrofluorocarbon series and a non-azeotropic mixture of a hydrofluoroolefin series. Claim 11 A gas compressor that receives and compresses natural gas from a natural gas feed stream; a heat exchanger that cools the natural gas passing through the gas compressor through heat exchange; and an expansion valve that expands and supercools the natural gas cooled after passing through the heat exchanger. The system includes a gas-liquid separation drum that separates the gas-liquid mixture generated by passing through the expansion valve and separates and discharges the liquid liquefied natural gas and cryogenic recovery gas. The heat exchanger comprises a first cooling unit that cools the natural gas using the recovery gas as a refrigerant, and a second cooling unit that cools the natural gas using an azeotropic or non-azeotropic non-combustible mixed refrigerant circulating in an independent cooling cycle independent of the first cooling unit. The first cooling unit includes a low-temperature heat exchanger that exchanges heat between the recovery gas discharged from the gas-liquid separation drum and the natural gas, and a high-temperature heat exchanger that exchanges heat between the recovery gas discharged from the low-temperature heat exchanger and the natural gas before it enters the low-temperature heat exchanger. The second cooling unit includes an independent heat exchanger that pre-cools the natural gas using the azeotropic or non-azeotropic non-combustible mixed refrigerant between the high-temperature heat exchanger and the low-temperature heat exchanger. A natural gas liquefaction apparatus comprising a heat exchanger, an auxiliary expansion valve that lowers the temperature of the natural gas to below the pour point of the lubricating oil applied to the gas compressor between the independent heat exchanger and the low-temperature heat exchanger, and at least one oil filter that removes the lubricating oil from the natural gas that has passed through the auxiliary expansion valve upstream of the low-temperature heat exchanger. Claim 12 In claim 11, the above oil filter comprises a first filter capable of filtering liquid oil and a second filter capable of filtering vapor oil, in a natural gas liquefaction device.

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