Liquefied methane production method and liquefied methane production apparatus

Abstract

Provided is a method for producing liquefied methane from a biogas containing methane gas, carbon dioxide gas, nitrogen gas, oxygen gas, argon gas, and water. Said method includes: a first step for separating carbon dioxide gas and water from the biogas to obtain an intermediate gas; and a second step for distilling and separating an intermediate gas by using a distillation column, and obtaining liquefied methane. In the second step, liquefied natural gas is introduced as a refrigerant into a condenser connected to a top portion of the distillation column.

Description

Method for producing liquefied methane and apparatus for producing liquefied methane

[0001] This disclosure relates to a method for producing liquefied methane and an apparatus for producing liquefied methane.

[0002] Natural gas and liquefied natural gas have attracted attention as clean energy sources that emit less carbon dioxide compared to fossil fuels such as coal and oil. However, carbon dioxide is still emitted when natural gas and liquefied natural gas are used as energy sources, and these emissions are steadily increasing. Therefore, alternative energy sources to replace natural gas and liquefied natural gas are needed.

[0003] As an alternative energy source, biogas derived from livestock manure such as dairy cows and beef cattle, food waste, and sewage sludge is attracting attention. Biogas consists of methane, carbon dioxide, nitrogen, and other gases, and is expected to be a clean energy source that can replace natural gas and liquefied natural gas.

[0004] Furthermore, overseas, biogas-derived liquefied methane is used as fuel for ships and trucks. In addition, high-purity liquefied methane has recently been used as rocket fuel, and there is a need to establish a supply system for high-purity liquefied methane (for example, Patent Document 1).

[0005] Japanese Patent Publication No. 2024-136765

[0006] The purpose of this disclosure is to provide a new method and apparatus for producing liquefied methane from biogas.

[0007] [1] A method for producing liquefied methane from biogas containing methane gas, carbon dioxide gas, nitrogen gas, oxygen gas, argon gas, and water, or from concentrated gas obtained by concentrating methane gas in the biogas, comprising: a first step of separating carbon dioxide gas and water from the biogas or concentrated gas to obtain an intermediate gas; and a second step of distilling and separating the intermediate gas using a distillation column to obtain liquefied methane, wherein in the second step, liquefied natural gas is introduced as a refrigerant into a condenser connected to the top of the distillation column.

[0008] [2] The method for producing liquefied methane according to [1], wherein the ratio of the liquefied natural gas introduced into the condenser to the flow rate of the intermediate gas introduced into the distillation column is 1.0 to 2.0, and the ratio of the liquefied natural gas discharged from the condenser to the flow rate of the liquefied natural gas introduced into the distillation column is 0.1 to 0.5.

[0009] [3] A method for producing liquefied methane according to [1] or [2], wherein the capacitor is evacuated.

[0010] [4] A method for producing liquefied methane according to any one of [1] to [3], wherein in the first step, separation by membrane separation or pressure swing adsorption and temperature swing adsorption or pressure swing adsorption are performed in this order.

[0011] [5] The method for producing liquefied methane according to any one of [1] to [4], wherein the biogas contains 50% to 60% by volume of methane gas, 30% to 40% by volume of carbon dioxide gas, and 1% to 15% by volume of nitrogen gas.

[0012] [6] Apparatus for producing liquefied methane from biogas containing methane gas, carbon dioxide gas, nitrogen gas, oxygen gas, argon gas, and water, or concentrated gas obtained by concentrating methane gas in the biogas, comprising: a separation apparatus for separating carbon dioxide gas and water from the biogas or concentrated gas to obtain an intermediate gas; and a distillation apparatus for distilling and separating the intermediate gas to obtain liquefied methane, wherein the distillation apparatus has a distillation column and a condenser connected to the top of the distillation column, and liquefied natural gas is introduced into the condenser as a refrigerant.

[0013] [7] The apparatus for producing liquefied methane according to [6], further comprising a liquefied natural gas outlet pipe for discharging the liquefied natural gas from the condenser.

[0014] [8] The apparatus for producing liquefied methane according to [6] or [7], comprising a vacuum pump for evacuating the capacitor.

[0015] 〔9〕The separation device includes a membrane separation device or a pressure swing adsorption separation device, and a temperature swing adsorption separation device or a pressure swing adsorption separation device, and is the liquefied methane production device according to any one of 〔6〕to 〔8〕.

[0016] 〔10〕The biogas contains 50% to 60% by volume of methane gas, 30% to 40% by volume of carbon dioxide gas, and 1% to 15% by volume of nitrogen gas, and is the method for producing liquefied methane according to any one of 〔6〕to 〔9〕.

[0017] According to the present disclosure, a method and a production device for producing liquefied methane from new biogas can be provided. ​​​​​​​​​​​​​​​​This refers to gases containing nitrogen gas, oxygen gas, argon gas, and water. Biogas may be a gas derived from at least one selected from the group consisting of, for example, livestock manure, food residue, and sewage sludge. The concentration of each gas in the biogas is, for example, 50% to 60% by volume for methane gas and CO2. ２ The gas content is 30% to 40% by volume, nitrogen gas is 1% to 15% by volume, oxygen gas is 0.1% to 5% by volume, argon gas is 0.01% to 1% by volume, and water is 0.01% to 5% by volume. It is preferable that hydrogen sulfide is removed from the biogas beforehand.

[0022] Concentrated Gas In this embodiment, "concentrated gas" refers to gas obtained by concentrating methane gas in biogas. The concentration of methane gas in the concentrated gas is, for example, 75% by volume or more and 97% by volume or less.

[0023] Intermediate Gas In this embodiment, "intermediate gas" refers to CO2 converted from biogas or concentrated gas. ２ This indicates a gas in which water has been separated and the concentration of methane gas is higher than that of biogas or concentrated gas. The concentration of methane gas in the intermediate gas is, for example, between 80% and 99% by volume.

[0024] In this embodiment, "liquefied natural gas" refers to a gas containing chain hydrocarbon gas and nitrogen gas. The liquefied natural gas mainly contains methane gas. The concentrations of each gas in the liquefied natural gas are, for example, 85% to 99% by volume for methane gas (boiling point: -161°C), 0% to 10% by volume for ethane gas (boiling point: -89°C), 0% to 5% by volume for propane gas (boiling point: -42°C), 0% to 1% by volume for isobutane (i-butane) gas (boiling point: -0.5°C), 0% to 1% by volume for normal butane (n-butane) gas (boiling point: -0.5°C), 0% to 0.1% by volume for pentane gas (boiling point: 36°C), and 0% to 0.3% by volume for nitrogen gas (boiling point: -195.8°C).

[0025] 《Step 1》 This step involves converting biogas or concentrated gas into CO2. ２and a step of separating moisture to obtain an intermediate gas. By this step, the CO in the intermediate gas ２ concentration is reduced to 0.0001% by volume or less. By this step, the moisture in the intermediate gas is reduced to 0.0001% by volume or less. Examples of the separation method in this step include temperature swing adsorption method, pressure swing adsorption method, etc. Also, mainly for separating CO ２ methods include membrane separation method, high-pressure water absorption method, chemical absorption method, etc.

[0026] (Temperature Swing Adsorption Method) In this method, biogas or concentrated gas is introduced into an adsorption tower filled with an adsorbent that adsorbs CO ２ and moisture, and CO ２ and water are separated. This method sequentially repeats, for example, an adsorption cycle of (1) adsorption step, (2) heating regeneration step, (3) purge step, and (4) recompression step.

[0027] (1) Adsorption Step The adsorption step is a step of supplying biogas or concentrated gas to an adsorption tower and adsorbing CO ２ and moisture onto the adsorbent to separate CO ２ and moisture from the biogas or concentrated gas. In this step, the temperature of the supplied biogas or concentrated gas is adjusted to, for example, 40°C or less.

[0028] The adsorbent is an adsorbent that can adsorb CO ２ and moisture, and by being heated, the adsorbed CO ２ and moisture are released and the adsorption performance is restored. Examples of such adsorbents include activated alumina, silica gel, hydrophobic zeolite, etc.

[0029] (2) Heating Regeneration Step The heating regeneration step is a step of supplying a gas inert to the adsorbent (hereinafter simply referred to as "inert gas") heated to the adsorption tower after the adsorption step, or directly heating the adsorbent, to desorb CO ２ and moisture from the adsorbent. In other words, the heating regeneration step is a step of making the adsorbent filled in the adsorption tower reusable.

[0030] As an inert gas, for example, CO2 can be extracted from biogas or concentrated gas through an adsorption process. ２ Examples include gases from which moisture has been removed, and nitrogen gas. When such gases are heated and brought into contact with the adsorbent packed inside the adsorption tower, the temperature of the adsorbent surface is increased, and the CO adsorbed on the adsorbent is removed. ２ The adsorbent is then desorbed. This process regenerates the adsorbent. The heating temperature is, for example, 170°C or higher.

[0031] Nitrogen gas is preferably used as the inert gas.

[0032] (3) Purge process The purging process is a process to remove inert gas from the adsorption tower after the heating and regeneration process. In this process, inert gas is removed by introducing gas into the adsorption tower. When the inert gas is nitrogen gas, the gas introduced is, for example, CO2 from biogas or concentrated gas by the adsorption process. ２ and the gas from which moisture has been removed. This process is preferably carried out multiple times until the inert gas is completely removed. Note that the inert gas is adsorbed from the biogas or concentrated gas to remove CO2. ２ Furthermore, if the gas has had moisture removed, this step is unnecessary.

[0033] (4) Restoration process The restoration process involves restoring the adsorption tower after the desorption process to a pressure suitable for the adsorption process by introducing, for example, a high-pressure gas. The high-pressure gas may be, for example, CO2 from biogas after the adsorption process. ２ And use gas from which moisture has been removed.

[0034] In this method, it is preferable to use multiple adsorption towers. For example, when using two adsorption towers, while the adsorption process is being carried out in one tower, the heating and regeneration process, purging process, and repressurization process are carried out in the other tower. By operating the two adsorption towers while switching between them in this manner, CO2 can be continuously and efficiently removed from biogas or concentrated gas. ２ And it enables the separation of water.

[0035] (Pressure swing adsorption method) In this method, CO ２And biogas or concentrated gas is introduced into an adsorption tower filled with an adsorbent that adsorbs moisture, CO ２ The adsorption process also separates water. This method involves sequentially repeating an adsorption cycle consisting of (1) an adsorption step, (2) a purging step, (3) a desorption step, and (4) a repressurization step. Note that steps (2) (purging step) and (4) (repressurization step) are the same as steps (3) (purging step) and (4) (repressurization step) in the temperature swing adsorption method described above, so their explanation is omitted.

[0036] The adsorption process involves supplying biogas or concentrated gas to an adsorption tower and removing CO2. ２ By adsorbing moisture onto an adsorbent, CO2 can be removed from biogas or concentrated gas. ２ This process involves separating the water and other components. This process is carried out, for example, under a pressure of 0.5 MPaG or higher. The adsorbent is the same as the adsorbent that can be used in the temperature swing adsorption method described above, so its explanation is omitted.

[0037] The desorption process involves reducing the pressure in the adsorption tower after the adsorption process to atmospheric pressure (0 MPaG) and removing the CO2 adsorbed onto the adsorbent. ２ The process involves desorption of moisture. In this process, for example, the pressure in the adsorption tower may be reduced to -0.1 MPaG using a vacuum pump.

[0038] (Membrane separation method) In this method, CO ２ Biogas or concentrated gas is introduced into a separation membrane that selectively permeates CO2. ２ This method separates CO2. In this method, biogas or concentrated gas is introduced under compression, and separation proceeds due to the partial pressure difference between membranes. Examples of separation membranes include hollow fiber polymer membranes and those composed of inorganic materials such as zeolites. In this method, it is preferable to compress the gas to 0.8 MPaG or higher. The CO2 separated by this method is... ２ Because methane gas may be present, the separated CO ２ CO again using this method ２ The substance can be separated into methane gas, and the methane gas can be recovered and recycled.

[0039] (High-pressure water absorption method) In this method, under high-pressure conditions, CO ２ By absorbing CO into water２ This method separates the two substances. In this method, biogas or concentrated gas is compressed and introduced, and separation proceeds due to the difference in solubility in water. In this method, it is preferable to compress the gas to 0.9 MPaG or higher.

[0040] (Chemical absorption method) In this method, biogas or concentrated gas is introduced into a chemical absorption solution and a chemical reaction is carried out to remove CO2. ２ This method separates alkaline compounds in the chemical absorption solution from CO2 in the biogas or concentrated gas. ２ Separation proceeds by reacting with [the other compound]. Examples of alkaline compounds include amine compounds.

[0041] The first step preferably includes at least one method selected from the group consisting of temperature swing adsorption and pressure swing adsorption. More preferably, the first step includes multiple separation methods. For example, if the first step includes two separation methods, it is preferable that the combination is one method selected from the group consisting of temperature swing adsorption and pressure swing adsorption, and one method selected from the group consisting of membrane separation, high-pressure water absorption, chemical absorption, and pressure swing adsorption. Among these, it is more preferable that the separation by membrane separation or pressure swing adsorption and then by temperature swing adsorption or pressure swing adsorption is performed in this order. When obtaining an intermediate gas from a concentrated gas, the first step only needs to include at least one method selected from the group consisting of temperature swing adsorption and pressure swing adsorption.

[0042] 《Second Step》 This step involves distilling and separating the intermediate gas using a distillation column to obtain liquefied methane. This method includes (1) a cooling step, (2) a distillation step, and (3) a condensation step.

[0043] (1) Cooling process The cooling process is a process of cooling the intermediate gas obtained in the first process. The temperature of the intermediate gas introduced into the cooling process is preferably 40°C or lower. The pressure of the intermediate gas during the cooling process is, for example, 0.1 to 0.8 MPaG. A portion of the methane gas contained in the intermediate gas may be liquefied by the cooling process. The cooling process may be performed only once or multiple times.

[0044] The intermediate gas after this process may have its pressure adjusted before being introduced into the distillation process described later. Since the pressure during this process is usually higher than the pressure during the distillation process, it is preferable to reduce the pressure.

[0045] (2) Distillation process The distillation process is the process of introducing the intermediate gas into a distillation column and performing distillation. The pressure inside the distillation column during the distillation process is, for example, 0.1 to 0.8 MPaG.

[0046] In this process, nitrogen gas (boiling point at atmospheric pressure: -195.8°C), oxygen gas (boiling point at atmospheric pressure: -183°C), and argon gas (boiling point at atmospheric pressure: -185.8°C) contained in the intermediate gas are separated from methane gas (boiling point at atmospheric pressure: -161.6°C) due to their boiling point differences. The separated methane gas is then converted into liquefied methane by heat exchange with a reboiler, which is usually installed at the bottom of the distillation column.

[0047] (3) Condensation process The condensation process is a process in which methane gas that was not liquefied in the distillation process is condensed in a condenser to obtain liquefied methane.

[0048] The condenser is connected to the top of the distillation column, and methane gas is introduced into the condenser from the top of the column. The methane gas introduced into the condenser condenses, and at least a portion of it becomes liquefied methane. The liquefied methane is returned to the distillation column and recovered. The temperature of the condenser is, for example, below -130°C.

[0049] The methane gas introduced into the condenser is condensed through heat exchange with the liquefied natural gas, which acts as a refrigerant. The initial pressure of the liquefied natural gas introduced into the condenser is, for example, 0.2 to 0.4 MPaG.

[0050] Through heat exchange with methane gas, some of the liquefied natural gas (LNG) is vaporized and converted into natural gas. Normally, to use LNG as natural gas, it is heated to vaporize it. Heating requires a heat source (power). However, this process reduces the power required to heat the LNG. As a result, cost reductions are expected.

[0051] In this process, nitrogen, oxygen, and argon gases in the intermediate gas separated by the distillation process are also introduced into the condenser. Since these nitrogen, oxygen, and argon gases are not liquefied, they can be exhausted or reused as refrigerants to cool the intermediate gas in the cooling process.

[0052] As described above, the liquefied natural gas in this embodiment is a gas that mainly consists of methane, followed by ethane and propane in large quantities. Furthermore, chain hydrocarbon gases with fewer carbon atoms tend to have lower boiling points. In other words, it is thought that the chain hydrocarbon gases with fewer carbon atoms in the liquefied natural gas will preferentially vaporize due to heat exchange between methane and liquefied natural gas. Therefore, it is thought that the saturation temperature of the liquefied natural gas introduced into the condenser will rise over time, making it difficult to maintain the cold temperature required to liquefy methane in the distillation column.

[0053] Therefore, it is preferable to continuously discharge a predetermined amount of liquefied natural gas introduced into the capacitor. This makes it possible to appropriately maintain the temperature inside the capacitor. The pressure of the liquefied natural gas discharged from the capacitor is, for example, 0.05 MPaG or less.

[0054] Furthermore, in order to maintain the temperature inside the condenser more appropriately, it is preferable to adjust the flow rate of liquefied natural gas introduced into the distillation column and the amount of liquefied natural gas discharged from the condenser. The ratio of liquefied natural gas discharged from the condenser to the flow rate of liquefied natural gas introduced into the distillation column (hereinafter also referred to as the "first ratio") may be, for example, 0.1 to 0.5. The first ratio may also be 0.2 to 0.5, 0.2 to 0.4, or 0.2 to 0.3.

[0055] Furthermore, it is preferable to adjust the flow rate of the intermediate gas introduced into the distillation column and the amount of liquefied natural gas introduced into the condenser. This makes it possible to appropriately maintain the composition of the liquefied natural gas in the condenser. The ratio of the liquefied natural gas introduced into the condenser to the flow rate of the intermediate gas introduced into the distillation column (hereinafter also referred to as the "second ratio") may be, for example, 1.0 to 2.0. The second ratio may also be 1.2 to 2.0, 1.3 to 1.8, or 1.4 to 1.6.

[0056] In this embodiment, the capacitor may be evacuated. By evacuating the capacitor, the pressure inside the capacitor decreases, and the temperature inside the capacitor can be lowered. As a result, an improvement in the recovery rate of liquefied methane can be expected. When evacuating, it is preferable that the gas recovered by the vacuum pump after heating is combined with the liquefied natural gas discharged from the capacitor and used as natural gas.

[0057] In this embodiment, a portion of the recovered liquefied methane may become boil-off gas (first boil-off gas), which is the vaporized gas of liquefied methane, due to the heat input from the ambient temperature in the liquefied methane storage tank. Therefore, by reintroducing the first boil-off gas into at least one of the first and second steps, a decrease in the recovery rate of liquefied methane can be suppressed. It is also considered that the first boil-off gas can be used as methane gas.

[0058] Furthermore, when liquefied natural gas (LNG) is stored in a storage tank, there is a risk that boil-off gas (secondary boil-off gas), which is the vaporized gas of LNG, may be generated in the LNG storage tank. For example, by recovering the secondary boil-off gas, it can be used as natural gas.

[0059] <Liquefied Methane Production Apparatus> The liquefied methane production apparatus in this embodiment uses methane gas and CO2. ２ CO2 is obtained from biogas containing nitrogen gas, oxygen gas, argon gas, and water, or from concentrated gas obtained by concentrating methane gas in biogas. ２The apparatus includes a separation device for separating water to obtain an intermediate gas, and a distillation device for distilling and separating the intermediate gas to obtain liquefied methane. The distillation device has a distillation column and a condenser connected to the top of the distillation column. Liquefied natural gas is introduced into the condenser as a refrigerant.

[0060] Figure 1 is a schematic diagram showing an example of the configuration of the liquefied methane production apparatus in this embodiment. The liquefied methane production apparatus 50 will be described below. Note that explanations that overlap with those described in the above-mentioned <Method for Producing Liquefied Methane> will be omitted.

[0061] 《Separation Device》 This device separates CO2 from biogas or concentrated gas. ２ It is a device for separating water and obtaining an intermediate gas. Examples of separation devices include temperature swing adsorption separation devices and pressure swing adsorption separation devices. ２ Examples of devices for separating the substances include membrane separators, high-pressure water absorbers, chemical absorbers, and pressure swing adsorption separators. Figure 1 shows a membrane separator 1 and a temperature swing adsorption separator 10 as examples of separation devices.

[0062] (Temperature Swing Adsorption Separation Device) This device removes CO2 contained in biogas or concentrated gas. ２ And an adsorption tower 11 for adsorbing moisture is installed. The adsorption tower 11 contains CO ２ The device is filled with an adsorbent for adsorbing moisture. Using this device, each step of the temperature swing adsorption method described above is performed, and CO2 is removed from the biogas or concentrated gas. ２ And the moisture is separated.

[0063] The temperature swing adsorption separation apparatus 10 preferably has multiple adsorption towers 11. In Figure 1, the temperature swing adsorption separation apparatus 10 is composed of two adsorption towers (adsorption tower 11a, adsorption tower 11b), and biogas or concentrated gas is alternately discharged to adsorption tower 11a and adsorption tower 11b, thereby continuously and efficiently removing CO from biogas or concentrated gas. ２ This also allows for the separation of moisture.

[0064] (Pressure Swing Adsorption Separator) This apparatus (not shown) is used to remove CO2 contained in biogas or concentrated gas. ２ And an adsorption tower is installed to adsorb moisture. ２ The device is filled with an adsorbent for adsorbing moisture. Using this device, each step of the pressure swing adsorption method described above is performed, and CO2 is removed from the biogas or concentrated gas. ２ And the moisture is separated.

[0065] (Membrane Separation Device) This device contains CO2 contained in biogas or concentrated gas. ２ A separation membrane module is installed to selectively permeate the biogas or concentrated gas. The biogas or concentrated gas is led to the separation membrane module by a compressor (not shown), and the CO2 contained in the biogas or concentrated gas is removed. ２ The CO2 is separated by a separation membrane. ２ Because methane gas may be present, the separated CO ２ CO2 is then separated again using a membrane separation device. ２ The substance can be separated into methane gas, and the methane gas can be recovered and recycled.

[0066] (High-pressure water absorption device) This device (not shown) absorbs CO2 contained in biogas or concentrated gas. ２ A high-pressure absorption tower is installed to absorb CO. ２ It contains water to absorb CO2. The biogas or concentrated gas is led to a high-pressure absorption tower by a compressor, and the CO2 contained in the biogas or concentrated gas is absorbed. ２ They are separated.

[0067] (Chemical absorption device) This device (not shown) absorbs CO2 contained in biogas or concentrated gas. ２ An absorption tower is installed to absorb CO. ２ It contains a processing liquid for absorbing CO2. The biogas or concentrated gas is led to the absorption tower, and the CO2 contained in the biogas or concentrated gas is absorbed. ２ They are separated.

[0068] The separation apparatus preferably includes at least one device selected from the group consisting of a temperature swing adsorption separator and a pressure swing adsorption separator. More preferably, the separation apparatus includes multiple separation devices. For example, if the separation apparatus includes two separation devices, it is preferable that one device selected from the group consisting of a temperature swing adsorption separator and a pressure swing adsorption separator is combined with one device selected from the group consisting of a membrane separator, a high-pressure water absorber, a chemical absorber, and a pressure swing adsorption separator. Among these, it is more preferable that the separation by the membrane separator and the temperature swing adsorption separator is performed in that order. When obtaining an intermediate gas from a concentrated gas, the separation apparatus only needs to include at least one separation device selected from the group consisting of a temperature swing adsorption separator and a pressure swing adsorption separator.

[0069] Distillation Apparatus: In this production apparatus, liquefied methane is obtained by distilling and separating the intermediate gas using a distillation apparatus. Figure 2 is a schematic diagram showing an example of the configuration of the distillation apparatus. The following explanation will refer to Figure 2.

[0070] The intermediate gas obtained by the separation device is cooled, for example, by the main heat exchanger 21. In the main heat exchanger 21, methane gas contained in the natural gas after heat exchange, as described later, may be reused as a refrigerant. Also, as described later, nitrogen gas, oxygen gas, and argon gas separated in the distillation column 23 and not condensed in the condenser 24 may also be reused as refrigerants. The intermediate gas cooled by the main heat exchanger 21 may have its pressure adjusted, for example, by a pressure reducing valve (not shown), before being introduced into the distillation column 23.

[0071] The intermediate gas cooled by the main heat exchanger 21 is introduced into the distillation column 23 and distilled. From the viewpoint of improving the efficiency of distillation, it is preferable to introduce the intermediate gas into the distillation column 23 from the middle of the column by providing an inlet in the middle of the column. The liquefied methane separated by distillation in the distillation column 23 is taken out from the bottom of the distillation column 23 and sent to the liquefied methane storage tank 25.

[0072] The intermediate gas cooled by the main heat exchanger 21 is introduced into the distillation column 23 via a reboiler 22, which is usually installed at the bottom of the column, before being introduced into the distillation column 23. As the intermediate gas is cooled in the reboiler 22, some of the methane gas in the intermediate gas is liquefied, and the intermediate gas and liquefied methane are introduced into the distillation column 23 in a mixed state.

[0073] The methane gas that was not liquefied in the distillation column 23 is condensed by the condenser 24 connected to the top of the distillation column 23, and at least a portion of it becomes liquefied methane. The liquefied methane is returned to the distillation column 23 and sent to the liquefied methane storage tank 25.

[0074] The methane gas introduced into the condenser 24 is condensed by heat exchange with the refrigerant, liquefied natural gas. Through heat exchange with the methane gas, a portion of the liquefied natural gas is vaporized and becomes natural gas. The natural gas may be recovered via the vaporizer 28.

[0075] The manufacturing apparatus may also be equipped with a liquefied natural gas storage tank 26 for storing liquefied natural gas. This allows liquefied natural gas to be stably introduced into the condenser 24 under an appropriate pressure (for example, 0.2 to 0.4 MPaG).

[0076] The nitrogen, oxygen, and argon gases in the intermediate gas separated by the distillation column 23 are also introduced into the condenser 24. These non-liquefiable gases, such as nitrogen, oxygen, and argon, may be exhausted or reused as refrigerants to cool the intermediate gas in the main heat exchanger 21.

[0077] The manufacturing apparatus preferably includes a liquefied natural gas outlet pipe 27 for discharging liquefied natural gas from the condenser 24. This makes it possible to appropriately maintain the temperature inside the condenser 24. The discharged liquefied natural gas may be recovered as natural gas via a vaporizer 28.

[0078] Referring to Figure 3, the manufacturing apparatus may also be equipped with a vacuum pump 31 for evacuating the condenser 24. By evacuating the condenser 24, the pressure inside the condenser 24 is reduced, and the temperature inside the condenser can be lowered. The gas recovered by evacuating is heated by a heating mechanism 30, then joined to a liquefied natural gas outlet pipe 27, and can be used as natural gas via a vaporizer 28. The heating mechanism 30 may be, for example, a heat exchanger, a heater, etc. If the heating mechanism 30 is a heat exchanger, power consumption can be reduced.

[0079] Referring to Figure 4, the vaporizer 28 may be a vaporization cooler 32, and the heating mechanism 30 may be a waste heat recovery heat exchanger 33.

[0080] This manufacturing apparatus may also be equipped with a boil-off gas outlet pipe 29. This allows the first boil-off gas generated in the liquefied methane storage tank 25 to be reintroduced into the distillation column 23.

[0081] Although not shown in the diagram, the system may also be equipped with piping to introduce the second boil-off gas generated in the liquefied natural gas storage tank 26 into the vaporizer 28.

[0082] Examples are described below. However, these examples are not intended to limit the scope of the claims.

[0083] <Example 1> A liquefied methane production apparatus having the configuration shown in Figure 1 was prepared. Biogas derived from livestock manure was prepared. The concentrations of each gas in the biogas were 50% by volume for methane and CO2. ２ The composition of the liquefied natural gas was 40% by volume or less, nitrogen gas was 9% by volume or less, oxygen gas was 1% by volume, argon gas was 0.01% by volume, and water was 0.01% by volume. Liquefied natural gas was prepared. The composition of the liquefied natural gas is as shown in point 3 of Table 1. In this embodiment, in the above-described manufacturing method, the first step was carried out in the order of membrane separation and temperature swing adsorption, followed by the second step. In the membrane separation apparatus, a hollow fiber membrane was used as the separation membrane, and in the temperature swing adsorption separation apparatus, zeolite was used as the adsorbent. The biogas was assumed to have been compressed by a compressor.

[0084] Table 1 shows the flow rate, pressure, temperature, and composition at points 1 to 8 in Figure 1. The composition at each point was analyzed using the fluid simulation software "AVEVA PRO II Simulation".

[0085]

[0086] <Example 2> Liquefied methane was produced in the same manner as in Example 1, except that a liquefied methane production apparatus having the configuration shown in Figure 3 was prepared. A heater was used as the heating mechanism 30.

[0087] Table 2 shows the flow rate, pressure, temperature, and composition at points 1 to 9 in Figure 3. The composition at each point was calculated using the same method as in Example 1.

[0088]

[0089] <Example 3> Liquefied methane was produced in the same manner as in Example 1, except that a liquefied methane production apparatus having the configuration shown in Figure 4 was prepared.

[0090] Table 3 shows the flow rate, pressure, temperature, and composition at points 1 to 13 in Figure 4. The composition at each point was calculated using the same method as in Example 1.

[0091]

[0092] As shown in Table 1, in Example 1, liquefied methane with a purity of 99.99% or higher was obtained from biogas containing 50% by volume of methane gas. Furthermore, the recovery rate of liquefied methane in the distillation process was 91%.

[0093] As shown in Table 2, in Example 2, liquefied methane with a purity of 99.99% or higher was obtained from biogas containing 50 volume% methane gas. Furthermore, the recovery rate of liquefied methane in the distillation process was 98%.

[0094] As shown in Table 3, in Example 2, liquefied methane with a purity of 99.99% or higher was obtained from biogas containing 50% by volume of methane gas. Furthermore, the recovery rate of liquefied methane in the distillation process was 98%.

[0095] Thus, by using the liquefied methane production method and production apparatus described in this disclosure, high-purity liquefied methane can be produced from biogas. Furthermore, since this disclosure effectively utilizes livestock manure that would otherwise be discarded, it can contribute to some of the activities of the Sustainable Development Goals (SDGs).

[0096] The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope of the claims are intended to be included.

[0097] 1. Membrane separation apparatus, 10. Temperature swing adsorption separation apparatus, 11, 11a, 11b. Adsorption columns, 20. Distillation apparatus, 21. Main heat exchanger, 22. Reboiler, 23. Distillation column, 24. Condenser, 25. Liquefied methane storage tank, 26. Liquefied natural gas storage tank, 27. Liquefied natural gas outlet piping, 28. Vaporizer, 29. Boil-off gas outlet piping, 30. Heating mechanism, 31. Vacuum pump, 32. Vaporization cooler, 33. Waste heat recovery heat exchanger, 50. Liquefied methane production apparatus.

Claims

1. A method for producing liquefied methane from biogas containing methane gas, carbon dioxide gas, nitrogen gas, oxygen gas, argon gas, and water, or from concentrated gas obtained by concentrating methane gas in the biogas, comprising: a first step of separating carbon dioxide gas and water from the biogas or concentrated gas to obtain an intermediate gas; and a second step of distilling and separating the intermediate gas using a distillation column to obtain liquefied methane, wherein in the second step, liquefied natural gas is introduced as a refrigerant into a condenser connected to the top of the distillation column.

2. The method for producing liquefied methane according to claim 1, wherein the ratio of the liquefied natural gas introduced into the condenser to the flow rate of the intermediate gas introduced into the distillation column is 1.0 to 2.0, and the ratio of the liquefied natural gas derived from the condenser to the flow rate of the liquefied natural gas introduced into the distillation column is 0.1 to 0.

5.

3. The method for producing liquefied methane according to claim 1 or 2, wherein the capacitor is evacuated.

4. A method for producing liquefied methane according to any one of claims 1 to 3, wherein in the first step, separation by membrane separation or pressure swing adsorption and temperature swing adsorption or pressure swing adsorption are performed in this order.

5. The method for producing liquefied methane according to any one of claims 1 to 4, wherein the biogas contains 50% to 60% by volume of methane gas, 30% to 40% by volume of carbon dioxide gas, and 1% to 15% by volume of nitrogen gas.

6. Apparatus for producing liquefied methane from biogas containing methane gas, carbon dioxide gas, nitrogen gas, oxygen gas, argon gas, and water, or from concentrated gas obtained by concentrating methane gas in the biogas, comprising: a separation apparatus for separating carbon dioxide gas and water from the biogas or concentrated gas to obtain an intermediate gas; and a distillation apparatus for distilling and separating the intermediate gas to obtain liquefied methane, wherein the distillation apparatus has a distillation column and a condenser connected to the top of the distillation column, and liquefied natural gas is introduced into the condenser as a refrigerant.

7. The apparatus for producing liquefied methane according to claim 6, further comprising a liquefied natural gas outlet pipe for discharging the liquefied natural gas from the condenser.

8. The apparatus for producing liquefied methane according to claim 6 or 7, further comprising a vacuum pump for evacuating the capacitor.

9. The apparatus for producing liquefied methane according to any one of claims 6 to 8, wherein the separation apparatus includes a membrane separation apparatus or a pressure swing adsorption separation apparatus and a temperature swing adsorption separation apparatus or a pressure swing adsorption separation apparatus.

10. The apparatus for producing liquefied methane according to any one of claims 6 to 9, wherein the biogas contains 50% to 60% by volume of methane gas, 30% to 40% by volume of carbon dioxide gas, and 1% to 15% by volume of nitrogen gas.

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