Method for producing liquefied methane and apparatus for producing liquefied methane

Abstract

To provide a new method and apparatus for producing liquefied methane from biogas. [Solution] A method for producing liquefied methane from biogas containing methane gas, carbon dioxide gas, nitrogen gas, oxygen gas, argon gas, and water, comprising: a first step of separating carbon dioxide gas and water from the biogas to obtain an intermediate gas; and a second step of distilling 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.

Description

Technical Field

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

Background Art

[0002] Natural gas and liquefied natural gas have attracted attention as clean energy with a low carbon dioxide emission compared to fossil fuels such as coal and oil. However, even when natural gas or liquefied natural gas is used as an energy source, carbon dioxide is emitted, and the emission amount has been on the rise. Therefore, alternative energy to replace natural gas and liquefied natural gas is in demand.

[0003] As such alternative energy, biogas derived from livestock manure such as dairy cows and beef cattle, food residues, sewage sludge, etc. has attracted attention. Biogas is composed of methane gas, carbon dioxide gas, nitrogen gas, etc., and is expected as clean energy to replace natural gas and liquefied natural gas.

[0004] Also, overseas, liquefied methane derived from biogas is used as ship fuel and truck fuel. Furthermore, in recent years, high-purity liquefied methane has been used as rocket fuel, and it is necessary to establish a supply system for high-purity liquefied methane (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] An object of the present disclosure is to provide a method for producing liquefied methane from new biogas and an apparatus for producing the same.

Means for Solving the Problems

[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, A first step of separating carbon dioxide and water from the biogas or concentrated gas to obtain an intermediate gas, The process includes a second step of distilling and separating the aforementioned intermediate gas using a distillation column to obtain liquefied methane, A method for producing 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 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. The method for producing liquefied methane according to [1], wherein 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.

[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 from concentrated gas obtained by concentrating methane gas in the biogas, A separation apparatus for separating carbon dioxide and water from the biogas or concentrated gas to obtain an intermediate gas, The apparatus includes a distillation apparatus for obtaining liquefied methane by distilling and separating the intermediate gas, The distillation apparatus includes a distillation column and a capacitor connected to the top of the distillation column. A liquefied methane production apparatus in which liquefied natural gas is introduced as a refrigerant into the aforementioned condenser.

[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 apparatus for producing liquefied methane according to any one of [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.

[0016]

[10] The method for producing liquefied methane according to any one of [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. [Effects of the Invention]

[0017] This disclosure provides a new method and apparatus for producing liquefied methane from biogas. [Brief explanation of the drawing]

[0018] [Figure 1]FIG. 1 is a schematic diagram showing an example of the configuration of a liquefied methane production apparatus according to the present embodiment. [Figure 2] FIG. 2 is a schematic diagram showing an example of a part of the configuration of a liquefied methane production apparatus according to the present embodiment. [Figure 3] FIG. 3 is a schematic diagram showing another example of the configuration of a liquefied methane production apparatus according to the present embodiment. [Figure 4] FIG. 4 is a schematic diagram showing another example of the configuration of a liquefied methane production apparatus according to the present embodiment.

MODE FOR CARRYING OUT THE INVENTION

[0019] Hereinafter, embodiments of the present disclosure will be described. However, the following description does not limit the scope of the claims.

[0020] <Method for Producing Liquefied Methane> The method for producing liquefied methane according to the present embodiment includes a first step of separating CO2 and moisture from biogas containing methane gas, carbon dioxide gas (CO2), nitrogen gas, oxygen gas, argon gas and moisture or a concentrated gas obtained by concentrating methane gas in biogas to obtain an intermediate gas, and a second step of subjecting the intermediate gas to distillation separation by a distillation column to obtain liquefied methane. In the second step, liquefied natural gas is introduced as a refrigerant into a condenser connected to the top of the distillation column. Hereinafter, each step of the method for producing liquefied methane according to the present embodiment will be described.

[0021] 《Biogas》 In this embodiment, "biogas" refers to a gas containing methane, CO2, nitrogen, oxygen, argon, and water. The biogas may be derived from at least one selected from the group consisting of, for example, livestock manure, food residue, and sewage sludge. The concentrations of each gas in the biogas are, for example, 50% to 60% by volume for methane, 30% to 40% by volume for CO2, 1% to 15% by volume for nitrogen, 0.1% to 5% by volume for oxygen, 0.01% to 1% by volume for argon, and 0.01% to 5% by volume for water. It is preferable that hydrogen sulfide has been 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 gas from which CO2 and water have been separated from biogas or concentrated gas, resulting in a higher concentration of methane gas than that of the biogas or concentrated gas. The concentration of methane gas in the intermediate gas is, for example, 80% by volume or more and 99% by volume or less.

[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 95% by volume for methane gas (boiling point: -161°C), 5% to 10% by volume for ethane gas (boiling point: -89°C), 1% to 5% by volume for propane gas (boiling point: -42°C), 0.1% to 1% by volume for isobutane (i-butane) gas (boiling point: -0.5°C), 0.1% to 1% by volume for normal butane (n-butane) gas (boiling point: -0.5°C), 0.01% to 0.1% by volume for pentane gas (boiling point: 36°C), and 0.01% to 0.3% by volume for nitrogen gas (boiling point: -195.8°C).

[0025] 《1st process》 This process involves separating CO2 and water from biogas or concentrated gas to obtain an intermediate gas. This process reduces the CO2 concentration in the intermediate gas to 0.0001% by volume or less. This process also reduces the water content in the intermediate gas to 0.0001% by volume or less. Examples of separation methods in this process include temperature swing adsorption and pressure swing adsorption. Methods primarily for separating CO2 include membrane separation, high-pressure water absorption, and chemical absorption.

[0026] (Temperature swing adsorption method) In this method, biogas or concentrated gas is introduced into an adsorption tower filled with an adsorbent that adsorbs CO2 and water, and CO2 and water are separated. This method sequentially repeats an adsorption cycle consisting of, for example, (1) an adsorption step, (2) a heating and regeneration step, (3) a purging step, and (4) a repressurization step.

[0027] (1) Adsorption process The adsorption process involves supplying biogas or concentrated gas to an adsorption tower and separating CO2 and moisture from the biogas or concentrated gas by adsorbing them onto an adsorbent. In this process, the temperature of the supplied biogas or concentrated gas is adjusted to, for example, 40°C or lower.

[0028] The adsorbent is capable of adsorbing CO2 and moisture, and is a regenerative adsorbent whose adsorption performance is restored when heated, releasing the adsorbed CO2 and moisture. Examples of such adsorbents include activated alumina, silica gel, and hydrophobic zeolite.

[0029] (2)Heating regeneration process The heating and regeneration process involves supplying a heated inert gas (hereinafter simply referred to as "inert gas") to the adsorption tower after the adsorption process, or directly heating the adsorbent to desorb CO2 and moisture from it. In other words, the heating and regeneration process is the process of making the adsorbent packed in the adsorption tower reusable.

[0030] Examples of inert gases include gases from which CO2 and moisture have been removed by an adsorption process from biogas or concentrated gas, and nitrogen gas. By heating such gases and bringing them into contact with an adsorbent packed in the adsorption tower, the temperature of the adsorbent surface is increased, causing the CO2 and moisture adsorbed on the adsorbent to be desorbed. Through this process, the adsorbent is regenerated. 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, the inert gas is removed by introducing gas into the adsorption tower. If the inert gas is nitrogen gas, the gas introduced is, for example, the gas from which CO2 and water have been removed by the adsorption process from biogas or concentrated gas. It is preferable to perform this process multiple times until the inert gas is completely removed. However, if the inert gas is the gas from which CO2 and water have been removed by the adsorption process from biogas or concentrated gas, this process is unnecessary.

[0033] (4) Repressurization process The repressurization process involves restoring the adsorption tower after the desorption process to a pressure suitable for the adsorption process, for example, by introducing a high-pressure gas. The high-pressure gas used is, for example, the gas from which CO2 and water have been removed from the biogas during the adsorption process.

[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, it is possible to continuously and efficiently separate CO2 and water from biogas or concentrated gas.

[0035] (Pressure swing adsorption method) In this method, biogas or concentrated gas is introduced into an adsorption tower filled with an adsorbent that adsorbs CO2 and water, and CO2 and water are separated. This method sequentially repeats the 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 and (4) repressurization are the same as steps (3) purging and (4) repressurization 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 separating CO2 and moisture from the biogas or concentrated gas by adsorbing them onto an adsorbent. This process is carried out, for example, under a pressure of 0.5 MPaG or higher. The adsorbent is the same as that 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) to desorb the CO2 and moisture adsorbed on the adsorbent. 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, biogas or concentrated gas is introduced into a separation membrane that selectively permeates CO2, and CO2 is separated. In this method, the biogas or concentrated gas is introduced under compression, and the separation proceeds due to the partial pressure difference between the 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. Since the CO2 separated by this method may contain methane gas, the separated CO2 may be separated again into CO2 and methane gas using this method, and the methane gas may be recovered and recycled.

[0039] (High-pressure water absorption method) This method separates CO2 by absorbing it into water under high-pressure conditions. In this method, biogas or concentrated gas is introduced in a compressed state, 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, CO2 is separated by introducing biogas or concentrated gas into a chemical absorption solution and allowing a chemical reaction to occur. In this method, the separation proceeds by reacting an alkaline compound in the chemical absorption solution with the CO2 in the biogas or concentrated gas. 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. Note that 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 process》 This process 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 step is a process of cooling the intermediate gas obtained in the first step. The temperature of the intermediate gas introduced into the cooling step is preferably 40°C or lower. The pressure of the intermediate gas during the cooling step is, for example, 0.1 to 0.8 MPaG. During the cooling step, some of the methane gas contained in the intermediate gas may be liquefied. The cooling step 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 involves 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 step 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 is condensed, 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 condenses through heat exchange with the liquefied natural gas (LNG), which acts as the refrigerant. The initial pressure of the LNG 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, the 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] <Equipment for producing liquefied methane> The liquefied methane production apparatus in this embodiment includes a separation apparatus for separating CO2 and water from biogas containing methane gas, CO2, nitrogen gas, oxygen gas, argon gas, and water, or concentrated gas obtained by concentrating methane gas in biogas, to obtain an intermediate gas, and a distillation apparatus for distilling and separating the intermediate gas to obtain liquefied methane. The distillation apparatus 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 apparatus is for separating CO2 and water from biogas or concentrated gas to obtain an intermediate gas. Examples of separation apparatuses include temperature swing adsorption separators and pressure swing adsorption separators. Applicable apparatuses for separating CO2 include membrane separators, high-pressure water absorbers, chemical absorbers, and pressure swing adsorption separators. Figure 1 shows membrane separator 1 and temperature swing adsorption separator 10 as separation apparatuses.

[0062] (Temperature swing adsorption separation device) This apparatus is equipped with an adsorption tower 11 that adsorbs CO2 and moisture contained in biogas or concentrated gas. The adsorption tower 11 is filled with an adsorbent for adsorbing CO2 and moisture. Using this apparatus, each step of the temperature swing adsorption method described above is performed to separate CO2 and moisture from biogas or concentrated gas.

[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 and adsorption tower 11b), and biogas or concentrated gas is alternately supplied to adsorption tower 11a and adsorption tower 11b, enabling continuous and efficient separation of CO2 and water from biogas or concentrated gas.

[0064] (Pressure swing suction separation device) This apparatus (not shown) is equipped with an adsorption tower for adsorbing CO2 and moisture contained in biogas or concentrated gas. The adsorption tower is filled with an adsorbent for adsorbing CO2 and moisture. Using this apparatus, the steps of the pressure swing adsorption method described above are carried out to separate CO2 and moisture from biogas or concentrated gas.

[0065] (membrane separation equipment) This device is equipped with a separation membrane module (not shown) for selectively permeating CO2 contained in biogas or concentrated gas. Biogas or concentrated gas is supplied to the separation membrane module by a compressor (not shown), and the CO2 contained in the biogas or concentrated gas is separated by the separation membrane. Since methane gas may be present in the CO2 separated by the membrane separator, the separated CO2 may be separated again into CO2 and methane gas by the membrane separator, and the methane gas may be recovered and recycled.

[0066] (High-pressure water absorption device) This apparatus (not shown) is equipped with a high-pressure absorption tower that absorbs CO2 contained in biogas or concentrated gas. The high-pressure absorption tower contains water for CO2 absorption. The biogas or concentrated gas is led to the high-pressure absorption tower by a compressor, where the CO2 contained in the biogas or concentrated gas is separated.

[0067] (Chemical absorption device) This apparatus (not shown) is equipped with an absorption tower that absorbs CO2 contained in biogas or concentrated gas. The absorption tower contains a treatment liquid for absorbing CO2. Biogas or concentrated gas is led to the absorption tower, where the CO2 contained in the biogas or concentrated gas is 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 appropriate pressure (e.g., 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 maintain the temperature inside the condenser 24 appropriately. 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 include 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 include piping for introducing the second boil-off gas generated in the liquefied natural gas storage tank 26 into the vaporizer 28. [Examples]

[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 as follows: methane gas 50% by volume, CO2 40% by volume or less, nitrogen gas 9% by volume or less, oxygen gas 1% by volume, argon gas 0.01% by volume, and water 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 production method described above, the first step was performed 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 1-8 in Figure 1. The composition at each point was calculated using the fluid simulation software "AVEVA PROII Simulation".

[0085] [Table 1]

[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] [Table 2]

[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] [Table 3]

[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% by volume of 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 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 present invention is indicated by the claims rather than the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended. [Explanation of symbols]

[0097] 1 Membrane separation unit, 10 Temperature swing adsorption separation unit, 11, 11a, 11b Adsorption tower, 20 Distillation unit, 21 Main heat exchanger, 22 Reboiler, 23 Distillation tower, 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 equipment.

Claims

1. A method for producing liquefied methane from biogas containing methane gas, carbon dioxide, nitrogen gas, oxygen gas, argon gas, and water, or from concentrated gas obtained by concentrating methane gas in the biogas, A first step of separating carbon dioxide and water from the biogas or concentrated gas to obtain an intermediate gas, The process includes a second step of distilling and separating the aforementioned intermediate gas using a distillation column to obtain liquefied methane, A method for producing 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 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. The ratio of the liquefied natural gas flow rate to the flow rate of the liquefied natural gas introduced into the distillation column to the liquefied natural gas flow rate of the condenser is 0.1 to 0.

5. A method for producing liquefied methane according to claim 1.

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

4. The method for producing liquefied methane according to claim 1, wherein in the first step, separation by membrane separation or pressure swing adsorption and separation by temperature swing adsorption or pressure swing adsorption are performed in this order.

5. The method for producing liquefied methane according to claim 1, 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. An apparatus for producing liquefied methane from biogas containing methane gas, carbon dioxide, nitrogen gas, oxygen gas, argon gas, and water, or from concentrated gas obtained by concentrating methane gas in the biogas, A separation apparatus for separating carbon dioxide and water from the biogas or concentrated gas to obtain an intermediate gas, The apparatus includes a distillation apparatus for obtaining liquefied methane by distilling and separating the intermediate gas, The distillation apparatus includes a distillation column and a capacitor connected to the top of the distillation column. A liquefied methane production apparatus in which liquefied natural gas is introduced as a refrigerant into the aforementioned condenser.

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, further comprising a vacuum pump for evacuating the capacitor.

9. The apparatus for producing liquefied methane according to claim 6, 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 claim 6, 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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