Natural gas liquefaction apparatus

Abstract

A natural gas liquefaction device is provided that can prevent the performance of the device from being degraded by foreign substances mixed in during the process. 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, and is characterized in that the pour point of the lubricating oil applied to the natural gas pressurization part of the gas compressor is lower than the temperature of the cooled natural gas at the downstream end of the heat exchanger.

Description

Technology Field

[0001] The present invention relates to a natural gas liquefaction device, and more specifically, to a natural gas liquefaction device capable of preventing the performance of the device from being degraded by foreign substances mixed in during the process. 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] Since such natural gas liquefaction facilities must control natural gas to the desired pressure and temperature, they use multiple compressors and coolers (heat exchangers). As a result, process-related foreign substances (e.g., lubricating oil from compressors) may be mixed in during liquefaction, which can degrade the performance of the equipment.

[0005] For example, liquid substances such as lubricating oil can be mixed in during the compression process and cause problems at unspecified locations within the device as they flow. Furthermore, as they cool, their fluidity decreases, making them prone to sticking; this can negatively affect devices that pass the fluid (e.g., heat exchangers). Since this issue degrades the performance of the entire liquefaction system, it must be addressed. Prior art literature

[0006] Korean Patent Publication No. 10-1341798, (December 17, 2013) The problem to be solved

[0007] The technical objective of the present invention is to solve these problems by providing a natural gas liquefaction device capable of preventing the performance of the device from deteriorating due to foreign substances (e.g., compressor lubricating oil) mixed in during the process, and simultaneously to provide a natural gas liquefaction device to which an improved cooling process is applied, which improves the cooling process of the liquefaction device.

[0008] 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

[0009] 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, wherein the pour point of the lubricating oil applied to the natural gas pressurization part of the gas compressor is lower than the temperature of the cooled natural gas at the downstream end of the heat exchanger.

[0010] The above lubricating oil can be mixed with the natural gas that has passed through the gas compressor and passed to the downstream end of the heat exchanger without losing fluidity.

[0011] After passing the above lubricating oil through the expansion valve, it can be filtered by a filter that filters out solid substances.

[0012] The filter may include a metal fiber filter disposed downstream of the expansion valve to filter the lubricating oil.

[0013] The above lubricating oil may include a cylinder oil that is applied to the pressurizing cylinder of the gas compressor and has a pour point lower than the temperature of the cooled natural gas at the downstream end of the heat exchanger.

[0014] The above cylinder oil may be a substance with a viscosity index of 130 or higher and a pour point that is at least 1°C lower than the temperature of the cooled natural gas downstream of the heat exchanger.

[0015] The heat exchanger may include two or more heat exchangers that gradually lower the temperature on a pipeline through which natural gas passes, thereby controlling the temperature of the cooled natural gas at the downstream end of the heat exchanger to be higher than the pour point of the lubricating oil.

[0016] The heat exchanger may include a low-temperature heat exchanger that receives the recovery gas discharged from the gas-liquid separation drum and exchanges heat with natural gas, and a high-temperature heat exchanger disposed upstream of the low-temperature heat exchanger and exchanges heat with natural gas before the recovery gas that has passed through the low-temperature heat exchanger enters the low-temperature heat exchanger.

[0017] The above heat exchanger may further include an independent heat exchanger that cools natural gas with an independent refrigerant cycle between the high-temperature heat exchanger and the low-temperature heat exchanger.

[0018] The above refrigerant cycle may include a refrigerant supply pipe that supplies a refrigerant other than the recovery gas to the independent heat exchanger, a refrigerant recovery pipe that recovers the 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 refrigerant through a refrigerant compressor, a refrigerant condenser, and a first expansion section, and cools the refrigerant to a first temperature at the downstream end of the first expansion section.

[0019] 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 refrigerant introduced into the above refrigerant supply pipe to be cooled to a second temperature.

[0020] The above refrigerant cycle may further include a second expansion section disposed in the refrigerant supply pipe, which expands the refrigerant cooled to the second temperature to cool it down to a third temperature lower than the first temperature. Effects of the invention

[0021] According to the present invention, it is possible to prevent the degradation of a device's performance caused by foreign substances introduced during the natural gas liquefaction process, particularly in the process. For example, it can effectively resolve issues such as the degradation of cooler performance caused by foreign substances, such as lubricating oil, which are difficult to separate during the compression process, entering the cooler during the liquefaction process. Furthermore, since the present invention resolves these problems while minimizing structural changes to the system, it can be applied to various systems (natural gas liquefaction devices and systems), and by also providing an improved cooling process, the performance of the device can be further enhanced. Brief explanation of the drawing

[0022] FIG. 1 is a diagram showing the configuration of a natural gas liquefaction device according to one embodiment of the present invention. Figure 2 is a configuration diagram showing the independent heat exchanger and refrigerant cycle of the natural gas liquefaction device of Figure 1 in more detail. Figure 3 is a table illustrating the properties of a material applicable as a lubricant of the present invention. Specific details for implementing the invention

[0023] 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.

[0024] 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)].

[0025] Hereinafter, a natural gas liquefaction device according to the present invention will be described in detail with reference to FIGS. 1 to 3.

[0026] FIG. 1 is a diagram showing the configuration of a natural gas liquefaction device according to one embodiment of the present invention, and FIG. 3 is a table illustrating the properties of a material applicable as a lubricant of the present invention.

[0027] Referring to FIG. 1, the natural gas liquefaction device (1) according to the present invention includes a heat exchanger (20) and a gas compressor (15, 17) upstream of the heat exchanger (20). The present invention is formed so that the lubricating oil mixed into the natural gas from the gas compressor (15, 17) does not adhere to the heat exchanger (20) due to cooling. The present invention achieves this objective by adjusting the pour point of the lubricating oil applied to the gas compressor.

[0028] In other words, in the present invention, even if the lubricating oil mixed with natural gas in the compression process reaches the cooling section, the pour point of the lubricating oil is adjusted to be lower than the temperature of the cooled natural gas at the rear end of the heat exchanger (20), so the lubricating oil passes through the heat exchanger (20) without losing its fluidity. Through this, the problem of the lubricating oil blocking the flow path in conventional heat exchangers can be resolved.

[0029] The natural gas liquefaction device (1) of the present invention is configured as follows. The 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, and is characterized in that the pour point of the lubricating oil applied to the natural gas pressurization part of the gas compressor (15, 17) is lower than the temperature of the cooled natural gas at the rear end of the heat exchanger (20).

[0030] In one embodiment of the present invention, the lubricating oil (i.e., the lubricating oil applied to the gas compressor) can be mixed with the natural gas that has passed through the gas compressor (15, 17) and passed to the downstream end of the heat exchanger (20) without losing fluidity, and after passing through the expansion valve (25), can be filtered by a filter (30) that filters solid matter. Such a filter (30) may include, for example, a metal fiber filter placed downstream of the expansion valve (25) to filter the lubricating oil.

[0031] In addition, in one embodiment of the present invention, the heat exchanger (20) may include two or more heat exchangers (e.g., a high-temperature heat exchanger, a low-temperature heat exchanger, and an independent heat exchanger) that lower the temperature in stages on a pipeline through which natural gas passes, thereby adjusting the temperature of the cooled natural gas at the downstream end of the heat exchanger (20) to be higher than the pour point of the lubricating oil mixed in the natural gas. Through this, the pour point of the lubricating oil and the temperature resulting from pre-cooling in the heat exchanger (20) can be mutually adjusted so that the lubricating oil does not lose its fluidity while passing through the heat exchanger.

[0032] In addition, the heat exchanger (20) is divided into a section that uses cryogenic recovery gas generated during the supercooling process of natural gas as a refrigerant and a section that uses a refrigerant other than the recovery gas, so that thermal efficiency can be increased during multi-stage cooling. That is, the present invention provides an improved cooling process through the configuration of the heat exchanger (20), thereby exhibiting the effect of improving the liquefaction performance of the device.

[0033] In addition, non-flammable refrigerants other than the recovery gas can be used, which prevents the risk of fire caused by the excessive use of conventional flammable refrigerants and improves the safety of the device.

[0034] Hereinafter, the structure and effects of the present invention will be explained in more detail through an embodiment of the present invention. The process will be explained in order with reference to FIG. 1, and the lubricating oil will be explained in more detail during the compression process.

[0035] Referring to FIG. 1, the natural gas feed stream (10) refers to a flow of natural gas before liquefaction. The natural gas feed stream (10) has methane as its main component 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. The substances removed through pretreatment may be, for example, hydrogen sulfide, mercury, carbon dioxide, and water.

[0036] The natural gas feed stream (10) may be 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) may be partially crossed / joined with the gas recovery lines (40, 41, 42, 43, 11).

[0037] 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).

[0038] The gas compressor (15, 17) is configured to receive and compress natural gas from this natural gas feed stream (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 compressor (15, 17) may change to approximately 250 bar and 55°C.

[0039] The gas compressor (15, 17) may be formed, for example, as a reciprocating compressor equipped with a piston. However, since the gas compressor (15, 17) can be implemented in various forms capable of fluid compression, it is not necessary to limit it thereto. In this embodiment, the gas compressor (15, 17) may be a two-stage compressor including a first gas compression stage (15) and a second gas compression stage (17), and may be a reciprocating compressor.

[0040] The lubricating oil may be applied to the piston or cylinder of the gas compressor (15, 17). That is, since the lubricating oil is applied to the pressurizing part that pressurizes the natural gas, the mixing of the lubricating oil into the natural gas during the natural gas compression process is inevitable. In the present invention, the pour point of the lubricant applied to the gas compressor (15, 17) is adjusted to be lower than the temperature at which the natural gas has completed pre-cooling, i.e., the temperature of the cooled natural gas at the rear of the heat exchanger (20).

[0041] Therefore, even if the lubricating oil of the gas compressor is mixed with natural gas during the compression process and flows through a subsequent path, it passes through the heat exchanger without reaching the pour point, so the lubricating oil does not adhere to the heat exchanger. Since the lubricating oil passes through the heat exchanger (20) without losing fluidity at a temperature higher than the pour point, the problem of the heat exchanger (high-temperature heat exchanger, low-temperature heat exchanger, independent heat exchanger, etc. constituting the heat exchanger) becoming clogged due to adhesion is eliminated. In other words, the lubricating oil can be mixed with the natural gas that has passed through the gas compressor (15, 17) and pass through the downstream end of the heat exchanger (20) without losing fluidity.

[0042] To this end, the lubricating oil of the gas compressor may be configured as follows. For example, the lubricating oil may include cylinder oil that is applied to the pressurizing cylinder of the gas compressor (15, 17) and has a pour point lower than the temperature of the cooled natural gas at the downstream end of the heat exchanger (20). For example, in this embodiment, the cooled natural gas at the downstream end of the heat exchanger (20) may be -65°C, and the pour point of the cylinder oil used as lubricating oil may be a temperature lower than that.

[0043] Figure 3 shows examples of materials (such as R100) that can be used as such lubricants. Since the lubricating performance of the oil is affected by viscosity, it is desirable to have an appropriate viscosity and a low pour point temperature. For example, a cylinder oil that can be used as a lubricant may be a material with a viscosity index of 130 or higher and a pour point that is at least 1°C lower than the temperature of the cooled natural gas (in this embodiment -65°C) at the downstream end of the heat exchanger (20).

[0044] The lubricating oil is not particularly limited as long as it has such performance. The lubricating oil may be a synthetic or natural material and does not need to be limited by its composition. The term "lubricating oil" includes cylinder oil applied to the cylinders of the gas compressor (15, 17) and may also include oil applied to the other side of the gas compressor if the performance is equivalent. Since such foreign substances, such as lubricating oil, are mixed into the natural gas during the process (particularly the compression process), they can be distinguished from the impurities originally mixed in the natural gas feed stream. The present invention is characterized by preventing the performance of the device from being degraded by such foreign substances (particularly lubricating oil) mixed into the natural gas during the process.

[0045] In this way, the lubricating oil mixed in the gas compressor (15, 17) passes through the heat exchanger (20) and can be filtered into a solid state at the filter (30) located downstream of the expansion valve (25). That is, by adjusting the pour point of the lubricating oil, the point at which the temperature drops below the pour point of the lubricating oil due to cooling can be adjusted to the downstream of the expansion valve (25). Therefore, the present invention has the characteristic of being able to carry out the liquefaction process without placing a separate filter upstream of the expansion valve (25), and since the filter (30) only needs to be applied downstream of the expansion valve (25), the device configuration is simple. However, this configuration does not mean excluding the placement of additional filters to treat other impurities or to filter a portion of the lubricating oil between the gas compressor (15, 17) and the expansion valve (25) as needed.

[0046] As described above, the filter (30) can be formed into a metal fiber filter or the like, and can be modified into various forms within the limits of being able to filter lubricating oil whose temperature has dropped below the pour point. Multiple filters may be placed downstream of the expansion valve. In this way, the pour point of the lubricating oil applied to the gas compressor (15, 17) can be controlled to resolve the problem of the heat exchanger, etc. being clogged by the lubricating oil, and the performance of the device can be maintained without problems.

[0047] However, this configuration is not possible by adjusting only the pour point of the lubricating oil; it is also possible to adjust the cooling process as needed to maintain the temperature at the downstream end of the heat exchanger (20) higher than the pour point temperature. For example, the heat exchanger (20) may include two or more heat exchangers that gradually lower the temperature on the piping line through which natural gas passes, thereby controlling the temperature of the cooled natural gas at the downstream end of the heat exchanger (20) to be higher than the pour point of the lubricating oil.

[0048] That is, the multi-stage cooling structure of the heat exchanger (20) described below is not only capable of improving the efficiency of the cooling process, but is also advantageous for maintaining the temperature at the rear end of the heat exchanger (20) higher than the pour point temperature of the lubricating oil.

[0049] That is, by appropriately adjusting the cooling performance of the heat exchanger (20), the temperature of the cooled natural gas at the rear end of the heat exchanger (20) is controlled, so the pour point temperature of the lubricating oil and the temperature of the cooled natural gas at the rear end of the heat exchanger (20) can be organically controlled with each other. Therefore, there is no need to limit the embodiment of the present invention to this embodiment, and the invention can be modified in various ways, such as using a lubricating oil having a different pour point that exhibits an equivalent effect depending on the temperature at the rear end of the heat exchanger. Therefore, the present invention can also have versatility that allows it to be applied in various ways.

[0050] In addition, as described below, the present invention can also improve the performance of the cooling process through a multi-stage cooling structure of the heat exchanger (20). Below, after describing the remaining configuration of the present invention, the heat exchanger (20) will be described in more detail.

[0051] Referring to FIG. 1, the gas compressors (15, 17) may include a gas compressor intercooler (16) on a piping line connecting the discharge portion of the first gas compression section (15) and the inlet portion of the second gas compression section (17). The gas compressor intercooler (16) can cool the gas stream, whose temperature has risen while being compressed by the first gas compression section (15), to the inlet temperature of the second gas compression section (17). Additionally, a gas compressor aftercooler (18) may be installed on a piping line (19) connecting the discharge portion of the second gas compression section (17) and the heat exchanger (20) to cool the gas stream, whose temperature has risen while being compressed by the second gas compression section (17).

[0052] 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).

[0053] 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.

[0054] 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.

[0055] In this process, the aforementioned lubricating oil is mixed with natural gas and passes through the heat exchanger (20) without losing fluidity and without reaching the pour point. Therefore, the problem of performance degradation caused by the influx of foreign substances during the compression and pre-cooling processes (especially the problem of the lubricating oil adhering to the heat exchanger and blocking it) is resolved, and the device operates normally.

[0056] If necessary, a filter of a suitable type (not shown) may be placed between the gas compressor (15, 17) and the heat exchanger (20) to remove impurities. For example, a filter may be placed to properly treat 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 added to the appropriate location in the liquefaction process as needed.

[0057] 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.

[0058] For example, the cooled natural gas stream (24) discharged from the heat exchanger (20) can be isenthalpicly expanded to 4 to 8 barg or 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.

[0059] 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.

[0060] 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.

[0061] The aforementioned filter (30) is positioned between the expansion valve (25) and the gas-liquid separation drum (27). The filter (30) may be installed to remove impurities that have been converted into a solid state by cooling (e.g., heavy hydrocarbons in addition to lubricating oil). The filter (30) may be structured to filter out solid substances through filtration. Within such limits, the filter (30) is not particularly limited, so various structures can be applied for effective filtering. Preferably, the aforementioned metal fiber filter may be applied, and other various filters capable of processing lubricating oil cooled below its pour point may also be applied. In this way, by adjusting the pour point of the lubricating oil, it is possible to pass the lubricating oil mixed with natural gas through a heat exchanger (heat exchangers forming the heat exchange section) and then process it downstream of the expansion valve (25).

[0062] 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). 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 stream of recovered gas 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.

[0063] 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.

[0064] 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) 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).

[0065] The gas purging valve (49) may be connected to a power generation module (52) that generates power using the recovered gas as fuel. The power generation module may provide at least a portion of the generated power to the use 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 lines (40, 41, 42, 43, 11) but is supplied as fuel and used for the purpose of producing power.

[0066] 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.

[0067] 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). By recycling available resources within the liquefaction device or facility in this way, the efficiency of the device can be further improved.

[0068] The power produced by the recovery gas can be recovered and used to drive the device. 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 independent 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.

[0069] 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.

[0070] 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. In addition, as described above, by controlling the pour point of the gas compressor lubricating oil, the lubricating oil mixed into the natural gas also passes through the heat exchanger without reaching the pour point, thereby preventing the deterioration of the heat exchanger's performance.

[0071] Below, the cooling process of a heat exchanger composed of multiple heat exchangers is described in more detail.

[0072] 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 integrate a section utilizing the recovered gas as a refrigerant on the gas recovery line (40, 41, 42, 43, 11) and a section circulating an independent cooling cycle to cool the compressed natural gas in multiple stages. Since the cooling section is configured in stages in this way, it is advantageous for adjusting the end temperature, and thus, as described above, it is possible to adjust the temperature of the cooled natural gas at the end of the heat exchanger (20) to be higher than the pour point temperature of the lubricating oil. The heat exchanger (20) may be formed in a form in which two or more heat exchangers are arranged in multiple stages.

[0073] The heat exchanger constituting the heat exchange section (20) 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 that has passed through 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.

[0074] 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.

[0075] In this way, the heat exchanger (20) 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 and 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. The same applies to the independent heat exchangers described later, and each heat exchanger can be appropriately insulated to minimize heat inflow from the surroundings.

[0076] Additionally, the heat exchanger may further include an independent heat exchanger (221) that cools natural gas in an independent refrigerant cycle between a high-temperature heat exchanger (211) and a low-temperature heat exchanger (212). The refrigerant cycle of the independent heat exchanger (221) may include a refrigerant supply pipe (222) that supplies a refrigerant other than the recovered gas to the independent heat exchanger (221), a refrigerant recovery pipe (223) that recovers the refrigerant from the independent heat exchanger (221), and a refrigerant circulation pipe (224) that is connected between the refrigerant supply pipe (222) and the refrigerant recovery pipe (223) and passes the 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 cools the refrigerant to a first temperature at the downstream end of the first expansion section (305). Since these refrigerant cycles are independent, they are advantageous for temperature control, and the type of refrigerant can be changed as needed.

[0077] Hereinafter, the independent heat exchanger and the refrigerant cycle will be explained in more detail with reference to Fig. 2, which illustrates the refrigerant cycle of Fig. 1 in detail.

[0078] Figure 2 is a configuration diagram showing the independent heat exchanger and refrigerant cycle of the natural gas liquefaction device of Figure 1 in more detail.

[0079] Referring to FIG. 2, the refrigerant cycle is composed of an independent cooling cycle that circulates the refrigerant to an independent heat exchanger (221). The refrigerant cycle may include 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.

[0080] In the refrigerant circulation pipe (224), a refrigerant compressor (301), a refrigerant condenser (303), and a first expansion section (305) that cool the refrigerant through compression, condensation, and expansion may be arranged in sequence. Each is described as follows.

[0081] 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.

[0082] 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.

[0083] 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 (inflow from the refrigerant compressor) mixed into the refrigerant during the compression process into a liquid state. An oil circulation pipe (307) for recirculating the separated lubricating oil to the refrigerant compressor (301) may be formed between the oil separator (302) and the refrigerant compressor (301).

[0084] 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).

[0085] 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).

[0086] 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.

[0087] 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).

[0088] 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.

[0089] 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).

[0090] The refrigerant supply pipe (222) branches off at an appropriate point of the refrigerant circulation pipe to supply refrigerant to an independent heat exchanger (221). 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 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.

[0091] 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.

[0092] 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.

[0093] A second expansion section (310) that cools the refrigerant cooled to a second temperature to a third temperature may be installed in the refrigerant supply pipe (222). That is, the refrigerant that has been properly pre-cooled can be further cooled to reach the target temperature before being supplied to the independent heat exchanger (221). In other words, a second expansion section (310) that expands the refrigerant cooled to a second temperature and cools it to a third temperature (approximately -49.52°C), which is lower than the first temperature (lowest temperature of the refrigerant circulation pipe), may be installed in the refrigerant supply pipe (222). The second expansion section (310) may also be formed as an expansion valve 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.

[0094] Therefore, the independent heat exchanger (221) is supplied with a 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.

[0095] 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. Such an independent cooling cycle may be applied.

[0096] Non-flammable refrigerants can be used in such independent cooling cycles. Generally, propane-based refrigerants are used, which pose a high risk of ignition; however, in the case of the refrigerant cycle described above, non-flammable refrigerants composed of azeotropic or non-azeotropic mixtures (e.g., HFC-based azeotropic mixed non-flammable refrigerants, HFO-based non-azeotropic mixed non-flammable refrigerants, etc.) can be applied. Therefore, the safety of the device is also enhanced. By applying a heat exchanger (20) that includes such an independent refrigerant cycle, the efficiency and safety of the cooling process can also be improved.

[0097] 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

[0098] 1: Natural gas liquefaction unit 10: Natural gas feed stream 13: Gas compressor suction drum 15, 17: Gas compressor 20: Heat exchange section 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 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 unit 211: High-temperature 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: 1st heat exchange module 307: Oil circulation pipe 308: Branch pipe 309: Second heat exchange module 310: Second expansion section 311: Refrigerant compressor suction drum

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 discharges it as liquid liquefied natural gas and cryogenic recovery gas, wherein the pour point of the lubricating oil applied to the natural gas pressurization part of the gas compressor is lower than the temperature of the cooled natural gas at the downstream end of the heat exchanger, and the heat exchanger includes two or more heat exchangers that gradually lower the temperature on the piping line through which the natural gas passes to control the temperature of the cooled natural gas at the downstream end of the heat exchanger to be higher than the pour point of the lubricating oil, and the heat exchangers include a low-temperature heat exchanger that receives the recovery gas discharged from the gas-liquid separation drum and exchanges heat with the natural gas, a high-temperature heat exchanger disposed upstream of the low-temperature heat exchanger and exchanges heat between the recovery gas that has passed through the low-temperature heat exchanger and the natural gas before it enters the low-temperature heat exchanger, and an independent heat exchanger that cools the natural gas with an independent refrigerant cycle between the high-temperature heat exchanger and the low-temperature heat exchanger. A natural gas liquefaction device comprising, wherein the refrigerant cycle comprises a refrigerant supply pipe that supplies a refrigerant other than the recovery gas to the independent heat exchanger, a refrigerant recovery pipe that recovers the 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 refrigerant through a refrigerant compressor, a refrigerant condenser, and a first expansion section, and cools the refrigerant to a first temperature at the rear end of the first expansion section, and 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 refrigerant introduced into the refrigerant supply pipe to a second temperature. Claim 2 A natural gas liquefaction device according to claim 1, characterized in that the lubricating oil is mixed with the natural gas passing through the gas compressor and passes to the downstream end of the heat exchanger without losing fluidity. Claim 3 A natural gas liquefaction device according to paragraph 2, characterized in that the lubricating oil is filtered by a filter that filters solid substances after passing through the expansion valve. Claim 4 A natural gas liquefaction device according to paragraph 3, characterized in that the filter comprises a metal fiber filter disposed downstream of the expansion valve to filter the lubricating oil. Claim 5 A natural gas liquefaction device according to claim 1, characterized in that the lubricating oil comprises a cylinder oil applied to the pressurizing cylinder of the gas compressor and having a pour point lower than the temperature of the cooled natural gas at the downstream end of the heat exchanger. Claim 6 A natural gas liquefaction device according to claim 5, characterized in that the cylinder oil is a substance having a viscosity index of 130 or higher and a pour point that is at least 1°C lower than the temperature of the cooled natural gas at the downstream end of the heat exchanger. Claim 7 delete Claim 8 delete Claim 9 delete Claim 10 delete Claim 11 delete Claim 12 A natural gas liquefaction device according to claim 1, wherein the refrigerant cycle further comprises a second expansion section disposed in the refrigerant supply pipe, which expands the refrigerant cooled to the second temperature to cool it down to a third temperature lower than the first temperature.

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