JP2010209036A - Methane concentration method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently concentrate methane contained in a biogas largely changing component concentrations by a PSA method to give a product gas. <P>SOLUTION: The methane concentration method includes, by alternately changing an adsorption process for increasing pressure of an adsorption tower packed with an adsorbent to a relatively high pressure and a regeneration process having a relatively low pressure, concentrating methane contained in a biomass by a pressure swing adsorption separation method for carrying out gas separation, deriving concentrated methane from the adsorption tower to store it in a product tank and supplying concentrated methane from the adsorption tower to a user. In the method, pressure of the adsorption tower or the product tank is measured to obtain a maximum pressure and the amount of the biogas to be introduced to the adsorption tower is adjusted on the basis of the obtained maximum pressure. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a methane concentration method, and more particularly, to a method for concentrating methane in digestion gas (biogas) obtained by anaerobic fermentation such as livestock manure, sewage treatment plant, food waste, etc., by a pressure fluctuation adsorption separation method. .

  As a method of concentrating specific components in the mixed gas, the adsorption tower packed with the adsorbent is switched between a relatively high pressure adsorption step and a relatively low pressure regeneration step so that the specific component in the mixed gas is A pressure fluctuation adsorption separation method (PSA method) in which product gas is produced by concentration is known. In this PSA method, the concentration of at least one kind of gas is detected by a gas concentration detector at the outlet of the adsorption tower in order to keep the quality of the product gas constant even if the composition and flow rate of the mixed gas as the raw material gas changes. A method has been proposed in which the gas concentration detector is connected to a PLC and the switching cycle of the adsorption tower is automatically changed by the PLC (see, for example, Patent Document 1).

JP 2003-19415 A

  However, when a mixed gas whose composition varies, such as biogas, is used as the raw material gas, the pressure during the adsorption process (adsorption pressure) may not increase sufficiently due to the composition variation, and the purity of the product gas varies. I had to do it. In addition, the gas concentration detector is generally expensive and has problems such as poor responsiveness and low reliability compared to a pressure gauge and a flow meter, resulting in an increase in equipment cost and maintenance cost. The price was also affected.

  Therefore, the present invention provides a product gas of stable quality by using a pressure gauge that is inexpensive and excellent in responsiveness and reliability in a methane concentration method in which methane contained in biogas is concentrated by the PSA method to obtain a product gas. It aims at providing the methane concentration method which can manufacture (concentrated methane) at low cost.

  In order to achieve the above object, the methane concentration method of the present invention switches alternately between an adsorption step in which the pressure of the adsorption tower packed with the adsorbent is relatively high and a regeneration step in which the pressure is relatively low. Methane contained in biogas is concentrated by a pressure fluctuation adsorption separation method that performs gas separation, the condensed methane is led out from the adsorption tower and stored in a product tank, and the concentrated methane is supplied from the product tank to the user In the concentration method, the pressure in the adsorption tower or the product tank is measured to obtain a maximum pressure, and the amount of biogas introduced into the adsorption tower is adjusted based on the obtained maximum pressure.

  Further, the methane concentration method of the present invention is a pressure at which gas separation is performed by alternately switching between an adsorption step in which the pressure of the adsorption tower packed with the adsorbent is relatively high and a regeneration step in which the pressure is relatively low. In the methane concentration method of concentrating methane contained in biogas by a variable adsorption separation method, deriving the concentrated methane from the adsorption tower and storing it in a product tank, and supplying the concentrated methane from the product tank to a user, The maximum pressure is determined by measuring the pressure in the adsorption tower or the product tank, the calculated maximum pressure is compared with a preset target maximum pressure, and the bio to be introduced into the adsorption tower when the maximum pressure is lower than the target maximum pressure. The gas amount is adjusted to increase, and when the maximum pressure is higher than the target maximum pressure, the amount of biogas introduced into the adsorption tower is adjusted to decrease. That.

  Furthermore, the methane concentration method of the present invention is characterized in that the amount of biogas introduced into the adsorption tower is adjusted by controlling the rotational speed of a compressor that compresses the biogas introduced into the adsorption tower. Further, the pressure in the adsorption tower or the product tank is measured at a preset time interval, and the current measurement pressure (P0), the previous measurement pressure (P1), and the two previous measurement pressures (P2) are measured. ), And when (P0) <(P1) and (P0) <(P2) are satisfied, the previous measured pressure (P1) is set as the maximum pressure. Furthermore, the maximum pressure is obtained by measuring the pressure in the product tank.

  According to the methane concentration method of the present invention, even if the composition of the biogas as a raw material varies, the variation in the methane concentration in the concentrated methane as the product gas can be suppressed. In addition, when the methane concentration in biogas increases, the amount of biogas introduced is reduced, so the power consumption of the compressor can be reduced, and the recovery rate can be reduced by not increasing the methane concentration of concentrated methane more than necessary. Can be prevented.

It is a systematic diagram showing an example of a pressure fluctuation adsorption separation device to which the methane concentration method of the present invention is applied. It is explanatory drawing which shows each process of a pressure fluctuation adsorption separation apparatus. It is a figure which shows the pressure change in an adsorption tower. It is a figure which shows the relationship between a methane density | concentration and a pressure change. It is explanatory drawing of the procedure which judges a maximum pressure. 1 is a system diagram of a pressure fluctuation adsorption separation device used in Example 1. FIG. It is a figure which shows the change of the raw material methane density | concentration in Example 2, an inverter rotation speed, and product purity.

  A pressure fluctuation adsorption separation apparatus (PSA apparatus) shown in FIG. 1 is for concentrating methane in a biogas, which is a raw material gas, and supplying it to a user, and includes two molecules packed with molecular sieve carbon as an adsorbent. Adsorption towers A and B, a compressor 11 that compresses biogas that is a raw material gas, a cooler 12 that cools the biogas that has been heated to high temperature by compression heat, and removes moisture, and concentrated methane that is a product gas Product tank 13 and inlet valves 14a and 14b programmed for opening and closing timing by PLC or the like (hereinafter referred to as “a” for the adsorption tower A side valve and “b” for the adsorption tower B side valve) Exhaust valves 15a and 15b, pressure equalizing valves 16 and 17, purge valves 18a and 18b, outlet valves 19a and 19b, a flow rate adjusting valve 20 for adjusting the flow rate of the purge gas to a predetermined flow rate, and a product gas flow rate are adjusted. Pressure adjustment 21 and a flow rate regulator 22, a pressure gauge 23 for measuring the pressure in the product tank 13, a PLC 24 for obtaining a maximum pressure from a change in pressure measured by the pressure gauge 23, and a maximum pressure obtained by the PLC 24 Is provided with a digital indicating adjuster 25 for PID control of the inverter frequency, and an inverter 26 for controlling the rotational speed of the compressor 11 in accordance with an instruction from the digital indicating adjuster 25. The compressor 11 is provided with a back pressure adjusting valve 27 for adjusting the compressor discharge pressure so as not to exceed a preset pressure.

  This PSA device separates methane and carbon dioxide in biogas by repeating each step shown in FIG. 2, concentrates methane, collects product gas mainly composed of methane, and supplies it to the user. . In FIG. 2, only the path through which the gas flows in each step is shown. Hereinafter, each process is demonstrated in order of a process.

  FIG. 2A shows a state in which the adsorption tower A is in the pressurizing step and the adsorption tower B is in the first half of the regeneration step, the inlet valve 14a of the adsorption tower A is opened, and the compressor 11 is placed in the adsorption tower A. The biogas compressed to a predetermined pressure is introduced and the inside of the tower is pressurized. At this time, the purge valve 18b and the exhaust valve 15b of the adsorption tower B are opened, and the flow rate of part of the product gas in the product tank 13 is adjusted by the flow rate adjusting valve 20 so that the purge valve 18b is moved to the outlet side of the adsorption tower B. The gas in the adsorption tower B is purged and discharged from the inlet side to the outside through the exhaust valve 15b.

  FIG. 2B shows a state in which the adsorption tower A is in the adsorption (product take-out) step and the adsorption tower B is in the latter half of the regeneration step, the outlet valve 19a is opened, and the outlet gas of the adsorption tower A is the outlet valve 19a. In addition to being sent to the product tank 13, a part of the outlet gas is divided into the flow rate adjusting valve 20 side and used as the purge gas for the adsorption tower B. The exit gas of the adsorption tower A is a gas in which carbon dioxide in the biogas is adsorbed by molecular sieve carbon packed in the tower, and methane in the biogas is concentrated to a predetermined concentration.

  FIG. 2C shows a state in which the adsorption tower A is switched from the adsorption process to the decompression and pressure equalization process, and the adsorption tower B is switched from the regeneration process to the pressure and pressure equalization process, and the inlet valve 14a and the outlet valve of the adsorption tower A 19a, the exhaust valve 15b and the purge valve 18b of the adsorption tower B are closed, and the upper and lower pressure equalizing valves 16, 17 are opened. In this pressure equalization process, the gas rich in methane in the adsorption tower A having a relatively high pressure in the tower after completion of the adsorption process is recovered in the adsorption tower B having a relatively low pressure in the tower after the regeneration process is finished. As a result, the adsorption tower A is depressurized and the adsorption tower B is pressurized.

  FIG. 2D shows a state in which the adsorption tower A is switched to the regeneration process and the adsorption tower B is switched to the pressurization process, the exhaust valve 15a of the adsorption tower A is opened, and the gas in the tower is released to the atmosphere. When the pressure in the tower is lowered, the carbon dioxide adsorbed on the molecular sieve carbon is desorbed and discharged out of the tower. Further, the purge valve 18a is opened, and a part of the product gas in the product tank 13 is adjusted in flow rate by the flow rate adjusting valve 20 and introduced into the outlet of the adsorption tower A to purge carbon dioxide remaining in the tower. In the adsorption tower B, the inlet valve 14b is opened, and a biogas having a predetermined pressure is introduced into the tower and pressurized.

  FIG. 2E shows a state in which the outlet valve 19b is opened and the adsorption tower A is switched from the pressurizing process to the adsorption process while the adsorption tower A continues the regeneration process. A part of the outlet gas of the tower B is introduced as a purge gas, and the gas in the tower is continuously discharged from the inlet side of the adsorption tower A through the exhaust valve 15a. Further, most of the outlet gas of the adsorption tower B is sent to the product tank 13 as product gas.

  FIG. 2F shows a state in which the adsorption tower A is switched from the regeneration process to the pressurization and pressure equalization process, and the adsorption tower B is switched from the adsorption process to the decompression and pressure equalization process, and the exhaust valve 15a and purge valve of the adsorption tower A are shown. 18a, the inlet valve 14b and the outlet valve 19b of the adsorption tower B are closed, and the pressure equalizing valves 16 and 17 are opened, and the methane-rich gas in the adsorption tower B after the adsorption process is finished, the regeneration process is finished. It is recovered in the adsorption tower A.

  By repeating such pressurization, adsorption, decompression pressure equalization, regeneration, and pressure equalization in both towers A and B, a product gas obtained by separating carbon dioxide from biogas and concentrating methane is obtained. It is done. A check valve may be used instead of the outlet valves 19a and 19b, or an orifice may be used instead of the flow rate adjustment valve 22. Moreover, it can also be set as the multi-column type PSA apparatus provided with 3 or more adsorption towers, and a 1-column type PSA apparatus can also be used. Furthermore, in the regeneration step, the inside of the tower may be evacuated by a vacuum pump as necessary. As the adsorbent, molecular sieve carbon is optimal, but zeolite, activated carbon and the like can also be used.

  FIG. 3 shows changes in pressure in both towers A and B and the product tank 13 in one cycle. The pressure equalization step is a step of making the pressures of both the towers A and B substantially equal, and one of the towers is in a reduced pressure state and the other tower is in a pressurized state. In this pressure equalization process, since the pressure in the product tank 13 is higher than the pressure in both towers A and B, the product gas does not flow into the product tank 13, and the product gas in the product tank 13 is used at a constant flow rate. Therefore, the pressure in the product tank 13 gradually decreases. At this time, the biogas compressed by the compressor 11 is circulated to the suction side of the compressor 11 through the back pressure regulating valve 27, and the introduction of the biogas to both towers A and B is interrupted. .

  Even if the pressure equalization process is completed and the process proceeds to the adsorption process, the pressure is maintained until the pressure in the adsorption tower becomes higher than the pressure in the product tank 13. The pressure is gradually decreasing. When the pressure of the adsorption tower in the adsorption process becomes higher than the pressure of the product tank 13, the outlet gas of the adsorption tower flows into the product tank 13 as the product gas, so that the pressure of the product tank 13 is the adsorption which is performing the adsorption process. Rise with tower pressure.

  On the other hand, the composition of biogas obtained by processing livestock manure in a fermenter is generally about 60% methane (volume%, the same applies hereinafter), and most of the remainder is carbon dioxide. The composition of the biogas obtained from the fermenter fluctuates due to the input of waste other than the above, troubles in temperature control of the fermenter, the frequency of manure and urine input, and the temperature difference of the waste according to the season. The methane concentration in the inside fluctuates.

  When the concentration of methane in the raw biogas decreases, the amount of carbon dioxide adsorbed by the adsorbent increases relatively, and the amount of concentrated methane (product gas) flowing into the product tank 13 decreases. When the gas adsorption tower introduction amount and the product gas supply amount from the product tank 13 are constant, when the methane concentration of the biogas used as a raw material is lowered, as shown in FIG. Decreases with decreasing methane concentration. Furthermore, since the amount of carbon dioxide adsorbed by the adsorbent increases, the adsorbent breaks through in a short time, and the pressure in the adsorption tower does not rise sufficiently. It becomes impossible to adsorb, and the methane concentration of the product gas flowing into the product tank 13 decreases, so that the product gas with a predetermined purity cannot be supplied.

  The biggest factor related to the performance of the PSA device is the adsorption pressure. Therefore, when the methane concentration in the biogas decreases, the adsorption capacity of the adsorbent is increased by sufficiently increasing the pressure of the adsorption tower to the predetermined adsorption pressure. It is possible to suppress the decrease in the concentration of concentrated methane from the adsorption tower toward the product tank by sufficiently adsorbing the carbon dioxide in the biogas to the adsorbent.

  In order to increase the pressure in the adsorption tower, it is only necessary to increase the amount of raw material gas introduced into the adsorption tower. Therefore, the amount of raw material gas introduced is changed according to the variation in the methane concentration in the biogas that is the raw material gas. However, in order to measure the methane concentration in the biogas, a gas concentration detector must be used. As described above, the gas concentration detector is expensive and has problems with responsiveness and reliability. Therefore, using a pressure gauge that is inexpensive and excellent in responsiveness and reliability, obtain the maximum pressure by measuring the adsorption tower pressure and product tank pressure that change according to the fluctuation of the methane concentration in biogas. By adjusting the amount of biogas introduced into the adsorption tower based on the maximum pressure, fluctuations in the methane concentration of the product gas can be minimized.

  The position where the pressure gauge for obtaining the maximum pressure is installed can be arbitrarily selected as long as the pressure changes according to the fluctuation of the methane concentration in the biogas, but the pressure fluctuation range is larger than that of the adsorption tower. It is recommended to select a narrow product tank. Moreover, although the procedure for obtaining the maximum pressure can be selected as appropriate, as shown in FIG. 5, the pressure in the adsorption tower or the product tank is measured at a preset time interval, and the measured current pressure (P0) is measured. The previous measured pressure (P1) and the previous measured pressure (P2) are compared, and when (P0) <(P1) and (P0) <(P2) are satisfied, By determining the measurement pressure (P1) as the maximum pressure (Pmax), it is possible to avoid the influence of measurement noise and reliably determine the maximum pressure.

  For example, as shown in the present embodiment, the pressure in the product tank 13 is continuously measured by the pressure gauge 23, the measured pressure is read by the PLC 24 at intervals of 1 second, and the pressure (P0) at the time of reading is 1 second. When the previously read pressure (P1) and the pressure read (P2) two seconds ago are always compared, and (P0) <(P1) and (P0) <(P2) are satisfied, one second before The pressure (P1) read in is determined as the maximum pressure (Pmax).

  The obtained maximum pressure (Pmax) is sent from the PLC 24 to the digital indicator / regulator 25, and the inverter frequency is PID controlled based on the maximum pressure (Pmax). 11 is controlled. That is, when the maximum pressure in the product tank 13 decreases, it is determined that the methane concentration in the biogas has decreased, and the number of rotations of the compressor 11 is increased to increase the amount of biogas introduced into the adsorption tower. Further, when the maximum pressure in the product tank 13 increases, it is determined that the methane concentration in the biogas has increased (returned), and the rotation speed of the compressor 11 is decreased to reduce the amount of biogas introduced into the adsorption tower. . Thereby, the amount of biogas introduced into the adsorption tower can be adjusted to the optimum amount according to the methane concentration in the biogas, and the concentrated methane flowing into the product tank 13 from the adsorption tower, that is, the methane concentration in the raw material gas Even if fluctuates, fluctuations in the methane concentration in the product gas can be minimized, and a product gas having a relatively stable methane concentration can be supplied to the user.

  For example, a reference composition of biogas as a raw material is set in advance. For example, when the reference composition is set to a methane concentration of 60%, the target maximum pressure of the product tank 13 according to the performance and configuration of the PSA device, the product gas specification, etc. Ps) is set in advance, for example, 0.6 MPa (gauge pressure, the same applies hereinafter), the measured maximum pressure (Pmax) of the product tank 13 is compared with the target maximum pressure (Ps), and the maximum pressure (Pmax) When the pressure is lower than the target maximum pressure (Ps), the inverter frequency is set high so that the amount of biogas introduced into the adsorption tower increases, and when the maximum pressure is higher than the target maximum pressure, the amount of biogas introduced into the adsorption tower By adjusting the inverter frequency to be low, product gas having a stable purity (methane concentration) can be continuously supplied.

  However, when the methane concentration in the biogas decreases, that is, when the carbon dioxide concentration increases, increasing the amount of biogas introduced increases the pressure and increases the amount of carbon dioxide adsorbed. Since the carbon adsorption capacity is limited, carbon dioxide breaks through, so it is difficult to keep the purity of the product gas constant, but the purity fluctuation range should be within a range of several percent. it can.

  The data sent from the PLC 24 to the digital indicating adjuster 25 may be data corresponding to the obtained maximum pressure (Pmax), and the difference (Ps−Pmax) between the target maximum pressure (Ps) and the maximum pressure (Pmax). Alternatively, data corresponding to (Pmax−Ps) may be used. In addition, the adjustment of the amount of biogas introduced into the adsorption tower is optimal in terms of cost and accuracy by controlling the inverter of the compressor, but it can also be performed by other known methods depending on the configuration of the compressor. is there.

  Experiments were performed using the experimental apparatus shown in FIG. Note that the same components as those of the PSA device shown in FIG. In this experimental apparatus, methane gas and carbon dioxide are introduced into the mixing tank 53 from the respective cylinders 51 and 52 in a predetermined amount, and the mixed gas is used as a mixed gas simulating a biogas having a predetermined composition. Then, the pressure was adjusted to a predetermined pressure by the pressure adjusting valve 54, and the raw material gas was adjusted to a predetermined flow rate by the raw material gas flow controller 55 to obtain the raw material gas. Each adsorption tower A and B was filled with 2 liters of molecular sieve carbon as an adsorbent, and pressure gauges 23a and 23b were provided to monitor the pressure.

The pressure of the raw material gas is set to 0.6 MPa, the pressure during regeneration is set to atmospheric pressure, the pressure equalizing time is set to 2 seconds, and the adsorption time is set to 98 seconds. While adjusting the amount of gas from each cylinder 51, 52 while keeping the amount of product gas discharged from the product tank 13 constant by the controller 56, the methane concentration of the raw material gas is reduced to 60%, 55%, and 50%. The maximum adsorption pressure of the adsorption towers A and B at each methane concentration was measured, and the methane concentration (product purity) of the product gas was calculated from the carbon dioxide concentration measured by the carbon dioxide concentration meter 57. Table 1 shows the relationship between the adsorption pressure and the product purity with respect to the raw material methane concentration.

Next, the introduction amount of the source gas was adjusted by the source gas flow rate regulator 55 so that the adsorption pressure was the same. Table 2 shows the relationship between the raw material gas introduction amount, the adsorption pressure, and the product purity when the raw material gas introduction amount when the methane concentration is 60% is 1, and the methane concentration is 55% and the methane concentration is 50%.

  As shown in Table 1, when the raw material methane concentration is reduced to 50%, when the present invention is not applied, the product purity is decreased to 92.5%. As can be seen from the table, even when the raw material methane concentration is 50%, the product purity is reduced only to 97%.

The biogas obtained from an actual anaerobic fermentor was used as a raw material gas using the PSA apparatus shown in FIG. Each adsorption tower A and B is filled with 33 liters of molecular sieve carbon as an adsorbent, the pressure equalization time is 5 seconds, the adsorption time is 150 seconds, the regeneration pressure is atmospheric pressure, and the product gas supply amount is 5.2 m ^3. / Hr (atmospheric pressure, 0 ° C. converted value), the target maximum pressure (Ps) of the product tank 13 is set to 0.6 MPa, and based on the fluctuation of the maximum pressure (Pmax) of the product tank 13 as described above. The number of revolutions of the compressor 11 is controlled by an inverter so that the amount of biogas introduced can be automatically adjusted so that the maximum pressure (Pmax) is 0.6 MPa, which is the target maximum pressure (Ps). During the experiment, the methane concentration fluctuates in the range of 62 to 53%, and the inverter frequency automatically changes according to the product tank pressure. When the methane concentration is 60% or more, 50 Hz, and when the methane concentration is 53% It became 60 Hz.

  The state of the methane concentration (raw material methane concentration A), the inverter frequency B, and the product purity C in the biogas when continuously operated for 12 days is shown in FIG. As is clear from FIG. 7, even if the biogas composition (methane concentration) fluctuates, the rotational speed of the compressor 11 is inverter-controlled based on the maximum pressure in the product tank 13, and the biogas is introduced into the adsorption tower. It can be seen that the product purity C could be controlled in the range of 97.5 to 98.5% by adjusting the amount.

  DESCRIPTION OF SYMBOLS 11 ... Compressor, 12 ... Cooler, 13 ... Product tank, 14a, 14b ... Inlet valve, 15a, 15b ... Exhaust valve, 16, 17 ... Pressure equalizing valve, 18a, 18b ... Purge valve, 19a, 19b ... Outlet valve, DESCRIPTION OF SYMBOLS 20 ... Flow regulating valve, 21 ... Pressure regulator, 22 ... Flow regulator, 23 ... Pressure gauge, 24 ... PLC, 25 ... Digital indicator regulator, 26 ... Inverter, 27 ... Back pressure regulating valve, 51, 52 ... Cylinder 53 ... Mixing tank, 54 ... Pressure regulating valve, 55 ... Raw material gas flow controller, 56 ... Product gas flow controller, 57 ... Carbon dioxide concentration meter, A, B ... Adsorption tower

Claims (5)

  Included in biogas by the pressure fluctuation adsorption separation method in which gas separation is performed by alternately switching between an adsorption step in which the pressure of the adsorption tower packed with the adsorbent is relatively high and a regeneration step in which the pressure is relatively low. Concentrate methane, extract the concentrated methane from the adsorption tower, store it in the product tank, and measure the pressure in the adsorption tower or the product tank in the methane concentration method for supplying concentrated methane from the product tank to the user Then, the maximum pressure is obtained, and the amount of biogas introduced into the adsorption tower is adjusted based on the obtained maximum pressure.   Included in biogas by the pressure fluctuation adsorption separation method in which gas separation is performed by alternately switching between an adsorption step in which the pressure of the adsorption tower packed with the adsorbent is relatively high and a regeneration step in which the pressure is relatively low. Concentrate methane, extract the concentrated methane from the adsorption tower, store it in a product tank, and measure the pressure in the adsorption tower or the product tank in the methane concentration method for supplying concentrated methane from the product tank to the user Then, the maximum pressure is determined, the determined maximum pressure is compared with a preset target maximum pressure, and when the maximum pressure is lower than the target maximum pressure, the amount of biogas introduced into the adsorption tower is increased, When the maximum pressure is higher than the target maximum pressure, adjustment is made so that the amount of biogas introduced into the adsorption tower is reduced.   The method for concentrating methane according to claim 1 or 2, wherein the amount of biogas introduced into the adsorption tower is adjusted by controlling the number of revolutions of a compressor that compresses the biogas introduced into the adsorption tower.   The pressure of the adsorption tower or the product tank is measured at a preset time interval, and the measured current pressure (P0), the previous measured pressure (P1), and the previous measured pressure (P2) The previous measured pressure (P1) is set as the maximum pressure when (P0) <(P1) and (P0) <(P2) are satisfied. The methane concentration method of any one of Claims.   The methane concentration method according to any one of claims 1 to 4, wherein the maximum pressure is obtained by measuring a pressure in the product tank.

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