Pressure-alternating adsorption type gas production equipment

Abstract

Provided is a pressure-swing-adsorption gas producing apparatus that can satisfactorily perform an adsorption tower desorption step. An off-gas tank T that stores an off-gas and a pressure-boosting unit 10 that boosts the pressure of the off-gas and feeds the off-gas to a membrane separation unit 9, which has a separation membrane 9A through which a miscellaneous gas passes quickly relative to a gas being purified, are provided to the upstream side of the membrane separation unit 9 in an off-gas exhaust path 4 connected to multiple adsorption towers 1. A recycle gas return path 11 that returns a recycle gas that does not pass through the separation membrane 9A to a feedstock gas supply path 3 is provided. An operation control unit is configured such that an off-gas modulator MR is modulated such that the off-gas exhaust flow-rate is a flow-rate at which the off-gas amount exhausted from an adsorption tower 1 during each desorption step is equal to the off-gas amount exhausted from the off-gas tank T between the start of a desorption step at an adsorption tower 1 and the start of a desorption step at a separate adsorption tower 1.

Description

Pressure Swing Adsorption Gas Production Apparatus The present invention provides, for each of a plurality of adsorption towers provided in a form in which an adsorbent for adsorbing the impurity gas is filled from a raw material gas containing the gas to be purified and other impurity gases, and a raw material gas supply passage is connected to one end side and a gas discharge passage for the gas to be purified is connected to the other end side, and an off-gas discharge passage is connected, an operation control unit is provided that sequentially performs, with different phases, an adsorption step of adsorbing the impurity gas from the raw material gas supplied through the raw material gas supply passage and discharging the gas to be purified through the gas discharge passage for the gas to be purified, and a desorption step of discharging the impurity gas through the off-gas discharge passage, a membrane separation unit including a separation membrane that allows the impurity gas to permeate faster than the gas to be purified is provided in the off-gas discharge passage, relates to a pressure swing adsorption gas production apparatus provided with a recycle gas return passage for returning the recycle gas that does not permeate through the separation membrane to the raw material gas supply passage. In such a pressure swing adsorption gas production apparatus, since the off-gas discharged from the adsorption tower contains the gas to be purified, the off-gas after separating the impurity gas from the off-gas in the membrane separation unit is returned to the raw material gas supply passage as recycle gas, thereby reducing the load on the adsorption tower and improving the production efficiency of the gas to be purified. As a conventional example of such a pressure swing adsorption gas production apparatus, there is one in which the off-gas discharged from the adsorption tower through the off-gas discharge passage is guided to a pressurizing unit (compressor), and the off-gas pressurized in the pressurizing unit is guided to the membrane separation unit (see, for example, Patent Document 1). In Patent Document 1, a hydrogen-containing gas containing hydrogen as the gas to be purified and containing oxygen dioxide and methane as impurity gases is described as the raw material gas. Incidentally, in Patent Document 1, the hydrogen-containing gas after passing through the membrane separation unit is supplied to a carbon dioxide separation membrane, and the hydrogen-containing gas from which carbon dioxide has been separated is configured to be returned to the raw material gas supply passage as recycle gas. Although Patent Document 1 does not provide a detailed explanation, it can be considered that in order to avoid adverse effects on the booster section due to operation in an empty state where no off-gas is present, and to lower the internal pressure of the off-gas discharge passage during the desorption process, the amount discharged by the booster section is determined so that the amount of off-gas discharged from the adsorption tower each time a desorption process is performed is equal to the amount discharged by the booster section from the time one adsorption tower starts its desorption process until another adsorption tower starts its desorption process. Patent No. 5039407 In conventional pressure-fluctuation adsorption gas production systems, the off-gas discharged from the adsorption tower through the off-gas discharge channel is pressurized in the pressurization section while filling the off-gas discharge channel, and then guided to the membrane separation section. Therefore, it is difficult to sufficiently reduce the internal pressure of the adsorption tower during the desorption process, which may make it difficult to perform the desorption process properly. To elaborate, although the off-gas discharged from the adsorption tower is sequentially sent to the membrane separation section by the pressurization section, the off-gas discharge passage tends to remain filled with high-pressure off-gas. As a result, it is difficult to sufficiently reduce the internal pressure of the off-gas discharge passage, and consequently, it is difficult to sufficiently reduce the internal pressure of the adsorption tower during the desorption process, which could make it difficult to perform the desorption process successfully. The present invention has been made in view of the above circumstances, and its purpose is to provide a pressure fluctuation adsorption type gas production apparatus that can perform the desorption process of the adsorption tower well. The pressure fluctuation adsorption gas production apparatus of the present invention is configured such that an adsorbent material is filled into a raw material gas containing the gas to be purified and other miscellaneous gases, and each of the multiple adsorption towers is configured such that a raw material gas supply line is connected to one end, a gas to be purified discharge line is connected to the other end, and an off-gas discharge line is also connected. An operation control unit is provided that sequentially performs, with different phases, an operation cycle including an adsorption step, in which the miscellaneous gas is adsorbed from the raw material gas supplied through the raw material gas supply path and the gas to be purified is discharged through the gas to be purified discharge path, and a desorption step, in which the miscellaneous gas is discharged through the off-gas discharge path. The off-gas discharge passage is provided with a membrane separation section equipped with a separation membrane that allows the miscellaneous gas to permeate faster than the gas to be purified. A recycled gas return path is provided for returning the recycled gas that does not permeate the separation membrane back to the raw material gas supply path, and its characteristic configuration is as follows: Upstream of the membrane separation section in the off-gas discharge passage, there is provided an off-gas tank for storing the off-gas discharged through the off-gas discharge passage during the desorption process, and a pressurizing section for supplying the off-gas stored in the off-gas tank to the membrane separation section for membrane separation. An off-gas adjustment unit is provided to adjust the discharge flow rate of the off-gas discharged from the off-gas tank. The operation control unit adjusts the off-gas adjustment unit so that the discharge flow rate of the off-gas is equal to the amount of off-gas discharged from the adsorption tower each time the desorption process is performed, and the amount of off-gas discharged from the off-gas tank between the time the adsorption tower starts the desorption process and the time the other adsorption tower starts the desorption process. In other words, the off-gas discharged from the adsorption tower during the desorption process is temporarily stored in an off-gas tank. The stored off-gas is then pressurized in a booster section and supplied to the membrane separation section. The off-gas, from which impurities have been separated in the membrane separation section, is then returned to the raw material gas supply line as recycled gas through a recycled gas return line. Furthermore, by adjusting the off-gas adjustment unit, the operation control unit maintains the discharge flow rate of the off-gas from the off-gas tank at a rate equal to the amount of off-gas discharged from the adsorption tower each time a desorption process is performed, and the amount of off-gas discharged from the off-gas tank between the start of a desorption process for one adsorption tower and the start of a desorption process for another adsorption tower. This prevents the internal pressure of the off-gas tank from becoming excessively high or low. Therefore, since the off-gas discharged from the adsorption tower during the desorption process is temporarily stored in an off-gas tank, the desorption process can be carried out smoothly by setting the internal pressure of the off-gas tank to a low pressure suitable for the desorption process. In other words, since the off-gas is temporarily stored in an off-gas tank, and the stored off-gas is pressurized in a pressurization unit and supplied to the membrane separation unit, the off-gas can be stored inside the off-gas tank at a sufficiently low pressure, thus enabling a smooth desorption process. Furthermore, by adjusting the off-gas discharge flow rate, it is possible to prevent the internal pressure of the off-gas tank from becoming too high or too low. This allows the off-gas tank to perform the de-attachment process properly, while avoiding situations where excessive internal pressure prevents the de-attachment process from being carried out correctly, or where insufficient internal pressure negatively affects the pressure boosting unit. In short, the characteristic configuration of the pressure fluctuation adsorption type gas production apparatus of the present invention allows for efficient desorption of the adsorption tower. A further characteristic configuration of the pressure fluctuation adsorption type gas production apparatus of the present invention is that a pressure sensor is provided to detect the internal pressure of the off-gas tank. The operation control unit adjusts the off-gas adjustment unit based on the detection information from the pressure sensor to set the off-gas discharge flow rate to a flow rate that maintains the minimum value of the internal pressure of the off-gas tank, which repeatedly increases and decreases, at a set pressure. In other words, based on the detection information from the pressure sensor that detects the internal pressure of the off-gas tank, the operation control unit adjusts the off-gas discharge flow rate to a flow rate that maintains the minimum value of the internal pressure of the off-gas tank, which repeatedly increases and decreases, at a set pressure. As a result, the off-gas discharge flow rate is adjusted to a flow rate that equals the amount of off-gas discharged from the adsorption tower each time a desorption process is performed, and the amount of off-gas discharged from the off-gas tank between the start of a desorption process for one adsorption tower and the start of a desorption process for another adsorption tower. Thus, since the off-gas discharge flow rate is adjusted based on the detection information from a pressure sensor that detects the internal pressure of the off-gas tank, the configuration for adjusting the off-gas discharge flow rate can be simplified and the cost reduced. In short, according to the further characteristic configuration of the pressure fluctuation adsorption type gas production apparatus of the present invention, the configuration for adjusting the off-gas discharge flow rate can be simplified and reduced in cost. A further characteristic configuration of the pressure fluctuation adsorption type gas production apparatus of the present invention is that the operation control unit changes and adjusts the adsorption time for the adsorption process based on at least one of the concentration of the gas to be purified in the raw material gas and the concentration of the gas to be purified discharged from the adsorption tower, and when the adsorption time is changed, the off-gas adjustment unit adjusts the discharge flow rate of the off-gas tank to correct the flow rate so that the amount of off-gas discharged from the adsorption tower before the adsorption time was changed is equal to the amount of off-gas discharged from the off-gas tank between the time the adsorption tower starts the desorption process and the time the other adsorption tower starts the desorption process after the adsorption time has been changed. In other words, since the operation control unit changes the adsorption time for the adsorption process based on at least one of the concentration of the gas to be purified in the raw gas and the concentration of the gas to be purified discharged from the adsorption tower, even if the concentration of the gas to be purified in the raw gas changes, the concentration of the gas to be purified discharged through the gas to be purified discharge channel can be maintained at an appropriate concentration. In other words, for example, by changing the adsorption time to be shortened as the concentration of the gas to be purified in the raw gas decreases, and similarly by changing the adsorption time to be shortened as the concentration of the gas to be purified discharged from the adsorption tower decreases, the concentration of the gas to be purified discharged from the adsorption tower can be appropriately maintained. Incidentally, the adsorption time may be set feedforward in a manner that shortens the adsorption time as the concentration of the gas to be purified in the raw gas decreases, based on the concentration of the gas to be purified in the raw gas and the concentration of the gas to be purified discharged from the adsorption tower, and the adsorption time may also be corrected in a feedback manner in a manner that shortens the adsorption time as the concentration of the gas to be purified discharged from the adsorption tower decreases. Then, when the operation control unit changes the adsorption time, it adjusts the off-gas adjustment unit to correct the discharge flow rate of the off-gas tank to a flow rate equal to the amount of off-gas discharged from the adsorption tower before the adsorption time was changed and the amount of off-gas discharged from the off-gas tank between the start of the desorption process of one adsorption tower and the start of the desorption process of another adsorption tower after the adsorption time has been changed. Therefore, even when the adsorption time is changed, it is possible to prevent the internal pressure of the off-gas tank from becoming excessive or insufficient, thereby avoiding adverse effects on the adsorption process or the pressurization unit. To elaborate, changing the adsorption time immediately alters the time it takes for off-gas discharged from one of the multiple adsorption towers to be discharged from the off-gas tank. In contrast, immediately after changing or adjusting the adsorption time, off-gas is discharged from the adsorption tower where the adsorption process was performed before the adsorption time was changed or adjusted. Therefore, the amount of off-gas discharged from the adsorption tower does not change immediately after the adsorption time is changed or adjusted. As a result, when the adsorption time for the adsorption process is changed to decrease, the off-gas discharge flow rate becomes insufficient, and when the adsorption time for the adsorption process is changed to increase, the off-gas discharge flow rate becomes excessive, causing a large increase or decrease in the internal pressure of the off-gas tank. According to this feature configuration, when the adsorption time is changed, the discharge flow rate of the off-gas tank is corrected to equal the amount of off-gas discharged from the adsorption tower before the adsorption time was changed and the amount of off-gas discharged from the off-gas tank between the start of the desorption process of one adsorption tower and the start of the desorption process of another adsorption tower after the adsorption time has been changed. This makes it possible to suppress a large increase or decrease in the internal pressure of the off-gas tank. In short, according to the further characteristic configuration of the pressure fluctuation adsorption type gas production apparatus of the present invention, the concentration of the gas to be purified discharged from the adsorption tower can be maintained at an appropriate concentration by changing and adjusting the adsorption time, and moreover, when the adsorption time is changed, adverse effects on the adsorption process and the pressurization section can be avoided. A further characteristic configuration of the pressure fluctuation adsorption type gas production apparatus of the present invention is that a raw material gas adjustment unit is provided to adjust the amount of raw material gas supplied to the adsorption tower through the raw material gas supply path. The operation control unit adjusts the raw material gas adjustment unit so that when it corrects the discharge flow rate of the off-gas tank to increase, it corrects the raw material gas supply amount to decrease, and when it corrects the discharge flow rate to decrease, it corrects the raw material gas supply amount to increase. In other words, if the operation control unit corrects the off-gas discharge flow rate to the upward side, the raw material gas supply amount will be corrected to the downward side. Therefore, even if the off-gas discharge flow rate is corrected to the upward side, the concentration of the gas to be purified discharged from the adsorption tower can be maintained at an appropriate concentration. Also, if the off-gas discharge flow rate is corrected to the downward side, the raw material gas supply amount will be corrected to the upward side, thus suppressing a decrease in the production efficiency of the gas to be purified. To elaborate, increasing the off-gas discharge flow rate increases the amount of recycled gas, which in turn increases the amount of gas supplied to the adsorption tower. This tends to increase the amount of miscellaneous gas that is not adsorbed by the adsorption tower, thus lowering the concentration of the target gas discharged from the adsorption tower. However, by correcting the off-gas discharge flow rate to the upward side, and correcting the raw gas supply amount to the downward side, a sudden increase in the amount of gas supplied to the adsorption tower can be avoided, thereby maintaining an appropriate concentration of the target gas discharged from the adsorption tower. Furthermore, when the off-gas discharge flow rate is corrected to decrease, the amount of raw material gas supplied is corrected to increase, thereby suppressing a decrease in the amount of purified gas. Incidentally, when changing the adsorption time for the adsorption process based on the concentration of the gas to be purified discharged from the adsorption tower, if the concentration of the gas to be purified discharged from the adsorption tower falls below the appropriate concentration, the adsorption time will be corrected to decrease again, which may lead to an abnormal condition in which the off-gas discharge flow rate is repeatedly increased. However, if the off-gas discharge flow rate is corrected to increase, the abnormal condition can be suppressed by correcting the raw material gas supply amount to decrease. In short, according to the further characteristic configuration of the pressure fluctuation adsorption type gas production apparatus of the present invention, even when the off-gas discharge flow rate is corrected to increase, the concentration of the gas to be purified discharged from the adsorption tower can be maintained at an appropriate concentration. A further characteristic feature of the pressure fluctuation adsorption type gas production apparatus of the present invention is that the raw material gas is a methane-containing gas containing 40% or more methane as the gas to be purified, and the gas to be purified discharged through the gas to be purified discharge channel is a product gas containing 80% or more methane. In other words, by equipping the system with an adsorption tower and a membrane separation unit, a methane-containing gas containing 40% or more methane can be efficiently purified into a product gas containing 80% or more methane. Incidentally, biogas can be cited as an example of methane-containing gas. Biogas typically consists of 40-60% methane and 40-60% carbon dioxide. Even such biogas can be efficiently purified into a product gas containing 80% or more methane. In short, according to the further characteristic configuration of the present invention, a methane-containing gas containing 40% or more methane as the gas to be purified can be efficiently purified into a product gas containing 80% or more methane. This is a schematic diagram showing a pressure fluctuation adsorption type gas production apparatus. This is a block diagram showing the control configuration. This is a table showing the operating cycle. This is a table showing the relationship between the methane concentration in the raw material gas and the process correction value. This is a table showing the behavior of the adsorption time correction value. This is a diagram showing the change in off-gas tank pressure. This is a diagram showing the change in off-gas tank pressure when the off-gas flow rate is too low. This is a diagram showing the change in off-gas tank pressure when the off-gas flow rate is too high. This is a diagram showing the relationship between off-gas tank pressure and recycled gas flow rate. This is a diagram showing the relationship between differential pressure and correction flow rate. This is a diagram showing the relationship between adsorption time, recycled gas flow rate and off-gas tank pressure. This is a diagram showing the relationship between adsorption time, recycled gas flow rate and off-gas tank pressure. This is a table showing the relationship between the methane concentration in the product gas and the process correction value in the second embodiment. This is a table showing the behavior of the adsorption time correction value in the second embodiment. This is a table showing the relationship between the methane concentration in the raw material gas and the process correction value in the third embodiment. This is a table showing the relationship between the methane concentration in the product gas and the process correction value in the third embodiment. This is a table showing the behavior of the adsorption time correction value in the third embodiment. [First Embodiment] Embodiments of the present invention will be described below with reference to the drawings. (Overall configuration of a pressure fluctuation adsorption type gas production system) In this embodiment, the raw material gas G is a biogas containing 40% or more methane as the gas to be purified and carbon dioxide and other impurities as impurities. The impurities are adsorbed by the adsorbent of the adsorption tower 1, and methane gas containing 80% or more methane is discharged from the adsorption tower 1 as the product gas H. In other words, the biogas used as the raw material gas G is mainly composed of methane and carbon dioxide, with a methane content of approximately 60%. The pressure fluctuation adsorption gas production apparatus of this embodiment is configured to perform gas purification to obtain product gas H containing 80% or more methane. Incidentally, the adsorbent used to pack the adsorption tower 1 can be one in which at least one material selected from activated carbon, molecular sieve carbon, zeolite, and porous metal complex is the main component. In this embodiment, for example, at pore diameters of 0.38 nm or more measured by the MP method, the pore volume at that pore diameter is 0.01 cm³. 3 The pore volume at a pore size of 0.34 nm is 0.20 cm³, and does not exceed 0.34 nm. 3 Molecular sieve carbon with a weight of 1 / g or more is used. As shown in Figure 1, four adsorption towers 1 are provided, namely tower A, tower B, tower C, and tower D. A raw material gas supply passage 3, which supplies raw material gas G pressurized by a compressor 2 acting as a raw material pressurization unit, and an off-gas discharge passage 4, which discharges off-gas, are connected to the lower ends of the four adsorption towers 1. The compressor 2 is equipped with a back pressure valve 2A for maintaining the supply pressure at a set value (for example, 750 kPaG). Upstream of the compressor 2 in the raw material gas supply path 3, a raw material gas adjustment unit MG is provided to adjust the raw material gas supply amount to a target supply amount. The raw material gas adjustment unit MG is configured using a mass flow controller and is configured to detect the flow rate of the raw material gas G flowing through the raw material gas supply path 3, and to control the flow rate of the raw material gas G so that the detected flow rate becomes the target flow rate commanded by the operation control unit F (see Figure 2), which will be described later. Since the details of the mass flow controller are well known, a detailed explanation will be omitted in this embodiment. Furthermore, corresponding to each of the four adsorption towers 1, raw material gas supply valves A1, B1, C1, and D1 are provided to open and close the raw material gas supply passage 3, and off-gas discharge valves A5, B5, C5, and D5 are provided to open and close the off-gas discharge passage 4. A raw material-side concentration sensor SG is provided downstream of the compressor 2 in the raw material gas supply line 3 to detect the methane gas concentration (methane concentration in the raw material gas), which is the concentration of the gas to be purified in the raw material gas G. At the upper end of each of the four adsorption towers 1, a product gas discharge passage 5 is provided, which serves as a purification target gas discharge passage for delivering the product gas H. The product gas H is stored in the product gas tank 6 through this product gas discharge passage 5. In addition, a tower connecting passage 7 is connected to the upper part of each of the four adsorption towers 1 to connect them to one another. Furthermore, corresponding to each of the four adsorption towers 1, there are product gas delivery valves A2, B2, C2, and D2 that open and close the product gas delivery passage 5, and communication / disconnection valves A4, B4, C4, and D4 that open and close the tower connecting passage 7. Incidentally, the product gas delivery passage 5 is equipped with a product-side concentration sensor SH that detects the methane gas concentration (methane concentration in the product gas), which is the concentration of the gas to be purified in the product gas H. An off-gas tank T for storing off-gas is connected to the off-gas discharge passage 4, and an off-gas delivery passage 8 for sending out the stored off-gas is connected to the off-gas tank T. The off-gas delivery passage 8 is equipped with a membrane separation unit 9 with a separation membrane 9A, and a booster pump 10 as a booster unit for boosting the off-gas and supplying it to the membrane separation unit 9. The boost pump 10 is equipped with a pump back pressure valve 10A for maintaining the supply pressure at a set target value (for example, 780 kPaG). The separation membrane 9A of the membrane separation unit 9 is made of a material that allows impurities, mainly composed of carbon dioxide, to permeate faster than methane, the gas to be purified. In other words, by using a material that allows impurities to permeate faster than methane, methane, the gas to be purified, hardly permeates or is virtually impurized, and the impurities, mainly composed of carbon dioxide, among the off-gas stored in the off-gas tank T are separated as membrane permeate gas J. Such a separation membrane 9A may be one in which at least one material selected from cellulose acetate, polyamide, polyimide, polysulfone, polytetrafluoroethylene, polyethersulfone, carbon membrane, microporous glass composite membrane, DDR-type zeolite, highly branched polyimide silica, and polydimethylsiloxane is used as the main component. A recycled gas return channel 11 is provided to supply the off-gas (mainly composed of methane) that did not permeate the separation membrane 9A of the membrane separation section 9 as recycled gas to the raw material gas supply channel 3. Furthermore, an off-gas adjustment unit MR, which adjusts the discharge flow rate of off-gas discharged from the off-gas tank T, is provided in the recycled gas return path 11 in a manner that controls the flow rate of recycled gas to a target flow rate. The off-gas adjustment unit MR, like the raw material gas adjustment unit MG, is configured using a mass flow controller to detect the flow rate of recycled gas flowing through the recycled gas return path 11 and to control the flow rate of recycled gas so that the detected flow rate becomes the target flow rate commanded by the operation control unit F (see Figure 2), which will be described later. In other words, the operation control unit F is configured to command a target flow rate based on the detection information from the pressure sensor 14 that detects the internal pressure of the off-gas tank T (hereinafter referred to as the off-gas tank pressure), and the details thereof will be described later. Incidentally, to explain why the discharge flow rate of off-gas discharged from the off-gas tank T can be adjusted by adjusting the flow rate of recycled gas, in the membrane separation unit 9, the amount of membrane permeate gas increases in direct proportion to the increase in the off-gas discharge flow rate, so the off-gas discharge flow rate and the recycled gas flow rate are directly proportional. (Operational control of a pressure fluctuation adsorption type gas production system) As shown in Figure 2, an operation control unit F is provided to control the operation of the pressure fluctuation adsorption type gas production apparatus. The operation control unit F controls the raw material gas supply valves A1 to D1, the product gas delivery valves A2 to D2, the communication and disconnection valves A4 to D4, and the off-gas discharge valves A5 to D5, so that each of the four adsorption towers 1 is configured to perform the operation cycle shown in the table in Figure 3. That is, each of the four adsorption towers 1 is configured to sequentially execute an operation process defined in a form in which the operation cycle is divided into 16 steps, with different phases. Among the four adsorption towers 1, the operation cycle will be described by taking Tower A as a representative. In Steps 1 to 3, the raw material gas supply valve A1 and the product gas discharge valve A2 corresponding to Tower A are opened, and raw material pressure increase and adsorption corresponding to the adsorption process are performed. That is, while increasing the internal pressure of Tower A, the impurity gas is adsorbed by the adsorbent, and the product gas H is discharged through the product gas discharge path 5. Incidentally, in this adsorption process, the communication intermittent valve A4 and the off-gas discharge valve A5 are closed. In Step 4, the communication intermittent valves A4 and B4 of Tower A and Tower B are opened, and adsorption equalization AB corresponding to the first-stage pressure reduction equalization process for supplying the internal gas of Tower A to Tower B is performed. Incidentally, in this first-stage pressure reduction equalization process, the raw material gas supply valve A1, the product gas discharge valve A2, and the off-gas discharge valve A5 are closed. Incidentally, the meaning of "AB" in the adsorption equalization AB means supplying the internal gas of Tower A on the high-pressure side described above to Tower B on the low-pressure side described later, and the same applies hereinafter. In Step 5, all the valves related to Tower A are closed and standby is performed. In Step 6, the communication intermittent valves A4 and C4 of Tower A and Tower C are opened, and equalization AC corresponding to the middle-stage pressure reduction equalization process for supplying the internal gas of Tower A to Tower C is performed. Incidentally, in this middle-stage pressure reduction equalization process, the raw material gas supply valve A1, the product gas discharge valve A2, and the off-gas discharge valve A5 are closed. In Step 7, the communication intermittent valves A4 and D4 of Tower A and Tower D are opened, and equalization AD corresponding to the final-stage pressure reduction equalization process for supplying the internal gas of Tower A to Tower D is performed. Incidentally, in this final-stage pressure reduction equalization process, the raw material gas supply valve A1, the product gas discharge valve A2, and the off-gas discharge valve A5 are closed. In Steps 8 to 10, the off-gas discharge valve A5 of Tower A is opened, and pressure reduction corresponding to the desorption process is performed. Incidentally, in this desorption process, the raw material gas supply valve A1, the product gas discharge valve A2, and the communication intermittent valve A4 are closed. In step 11, the on-off intermittent valves A4 and B4 of tower A and tower B are opened to perform BA equalization pressure corresponding to the initial stage pressure boosting equalization process for supplying the internal gas of tower B to tower A. Incidentally, in this initial stage pressure boosting equalization process, the raw material gas supply valve A1, the product gas delivery valve A2, and the off-gas discharge valve A5 are closed. In steps 12 and 13, all the valves related to tower A are closed and standby is awaited. In step 14, the on-off intermittent valves A4 and C4 of tower A and tower C are opened to perform CA equalization pressure corresponding to the middle stage pressure boosting equalization process for supplying the internal gas of tower C to tower A. In this middle stage pressure boosting equalization process, the raw material gas supply valve A1, the product gas delivery valve A2, and the off-gas discharge valve A5 are closed. In step 15, all the valves related to tower A are closed and standby is awaited. In step 16, the on-off intermittent valves A4 and D4 of tower A and tower D are opened to perform DA adsorption equalization pressure corresponding to the final stage pressure boosting equalization process for supplying the internal gas of tower D to tower A. Incidentally, in this final stage pressure boosting equalization process, the raw material gas supply valve A1, the product gas delivery valve A2, and the off-gas discharge valve A5 are closed. That is, the operation control unit F is configured to sequentially execute an operation cycle consisting of an adsorption process, an initial stage pressure reduction equalization process, a middle stage pressure reduction equalization process, a final stage pressure reduction equalization process, a desorption process, an initial stage pressure boosting equalization process, a middle stage pressure boosting equalization process, and a final stage pressure boosting equalization process for each of the four adsorption towers 1 in a state where the phases are different. In other words, the operation control unit F is configured to sequentially execute an operation cycle consisting of an adsorption process, a pressure reduction equalization process, a desorption process, and a pressure boosting equalization process for each of the four adsorption towers 1 in a state where the phases are different. And as shown in the lower part of FIG. 3, for each of steps 1 to 16, X, t1, t2, and t3 are defined as the step times (seconds) for executing each step, and each process is configured to be executed according to the defined step times. Incidentally, the step times for Step 1, Step 5, Step 9, and Step 13 correspond to the adsorption time correction value X used to change and adjust the adsorption time for executing the adsorption process. As described later, the system is configured so that the adsorption time for executing the adsorption process is changed and adjusted by correcting the adsorption time correction value X. As described above, a raw material gas adjustment unit MG is provided upstream of the compressor 2 in the raw material gas supply passage 3. This raw material gas adjustment unit MG is configured to detect the flow rate of the raw material gas G flowing through the raw material gas supply passage 3 and to control the raw material gas supply amount so that the detected flow rate becomes the target flow rate set by the operation control unit F. In other words, the operation command unit 16 instructs the operation control unit F to provide a target supply amount of raw material gas G, and the operation control unit F instructs the raw material gas adjustment unit MG to provide a target flow rate based on the instructed target supply amount. Furthermore, the target supply amount of raw material gas G is determined experimentally according to the type of raw material gas G, such as its methane concentration. Incidentally, during the startup operation of the pressure fluctuation adsorption type gas production system, the operator commands the operation control unit F from the operation command unit 16 to set a target flow rate for the recycled gas flowing through the recycled gas return path 11. The operation control unit F then commands the off-gas adjustment unit MR to set the target flow rate. Subsequently, when an automatic operation command is issued from the operation command unit 16, the operation control unit F commands the off-gas adjustment unit MR to set the target flow rate. The details of this process will be described later. (Adsorption time adjustment control) The operation control unit F is configured to change and adjust the adsorption time for the adsorption process based on the detection information from the raw material concentration sensor SG. In this embodiment, the adsorption time correction value X corresponding to the time in step 1, the time in step 5, the time in step 9, and the time in step 13 is configured to be increased or decreased all at once. Incidentally, the time in step 1 corresponds to the adsorption time in tower A, the time in step 5 corresponds to the adsorption time in tower B, the time in step 9 corresponds to the adsorption time in tower C, and the time in step 13 corresponds to the adsorption time in tower D. For example, the adsorption time of tower A is the sum of the adsorption time correction value X from step 1, the step time from step 2, and the step time from step 3. By correcting the adsorption time correction value X from step 1, the adsorption time of tower A is changed and adjusted, and the same applies to the adsorption times of towers B, C, and D. In this embodiment, the relationship between the methane gas concentration in the raw material gas detected by the raw material concentration sensor SG and the process correction value is predetermined as shown in the table in Figure 4. When starting operation of the pressure fluctuation adsorption gas production apparatus, the adsorption time correction value X is set to a preset initial value (for example, 100 seconds) to begin operation. Subsequently, after each adsorption process in each of the adsorption towers 1, the average methane concentration in the raw material gas, which is the average value of the methane concentration in the raw material gas during that cycle, is calculated. The average methane concentration in the raw material gas is calculated by sampling the detection information from the raw material-side concentration sensor SG at set time intervals (for example, 500 ms) and calculating the average of the sampled detection values. Next, as shown in Figure 5, a process correction value is determined based on the relationship between the average methane concentration in the raw material gas and Figure 4. The value obtained by adding this process correction value to the current adsorption time correction value (initial value) X is set as the new adsorption time correction value X for the next adsorption process. For example, if an adsorption process with y adsorption steps is performed with an adsorption time correction value X set to 100 seconds, and the average methane concentration in the raw material gas during the cycle is 57%, then in the next adsorption process with y+1 steps, the adsorption time correction value X will be set to 94 seconds, which is the sum of 100 seconds and the process correction value -6 seconds. The same applies to subsequent adsorption processes with y+2 to y+6 steps. (Control of recycled gas flow rate) The operation control unit F is configured to adjust the off-gas adjustment unit MR so that the flow rate of the recycled gas is equal to the amount of off-gas discharged from the adsorption tower 1 each time a desorption process is performed, and the amount of off-gas discharged from the off-gas tank T from the time one adsorption tower 1 starts a desorption process until another adsorption tower 1 starts a desorption process. Incidentally, the reason why the discharge flow rate of off-gas discharged from the off-gas tank T can be adjusted by adjusting the flow rate of recycled gas is that, as described above, in the membrane separation unit 9, the amount of membrane permeate gas increases in direct proportion to the increase in the discharge flow rate of off-gas, and therefore the discharge flow rate of off-gas and the flow rate of recycled gas are directly proportional. In other words, the off-gas discharged by one adsorption tower 1 performing the desorption process is stored in the off-gas tank T, and this stored off-gas needs to be discharged from the off-gas tank T between the start of the desorption process of one adsorption tower 1 and the start of the next desorption process of another adsorption tower 1. In this embodiment, the operation control unit F is configured to adjust the off-gas adjustment unit MR in order to maintain the relationship between the amount of off-gas discharged from the adsorption tower 1 each time a desorption process is performed, the amount of membrane permeate gas separated by the membrane separation unit 9 between the start of the desorption process of adsorption tower 1 and the start of the desorption process of another adsorption tower 1, and the amount of recycled gas returned to the raw material gas supply line 3 through the recycled gas return line 11 between the start of the desorption process of adsorption tower 1 and the start of the desorption process of another adsorption tower 1, in the relationship shown in equation (1) below. Recycled gas amount = Off-gas amount - Membrane permeation gas amount -------- (1) To elaborate, as shown in Figures 6-8, the off-gas tank pressure, which is the internal pressure of the off-gas tank T, increases as each of the four adsorption towers 1 performs the desorption process, but decreases as the off-gas is discharged through the off-gas delivery passage 8. Furthermore, if the relationship in equation (1) above is maintained, the off-gas tank pressure will repeatedly increase and decrease while maintaining an appropriate minimum pressure (local minimum) and an appropriate maximum pressure, as shown in Figure 6. In contrast, if the amount of recycled gas is less than "off-gas amount - membrane permeate gas amount," the off-gas tank pressure gradually increases, as shown in Figure 7, making it impossible to properly perform the depressurization process. Furthermore, if the amount of recycled gas is greater than "off-gas amount - membrane permeate gas amount," as shown in Figure 8, the off-gas tank pressure will gradually decrease, and the off-gas tank pressure in the off-gas tank T will become too low (negative pressure), which may cause problems such as damage to the booster pump 10. In this embodiment, the operation control unit F is configured to maintain the relationship in equation (1) above by determining a target flow rate to set the minimum off-gas tank pressure (minimum value) to a target pressure (for example, 2 kPaG) based on the pressure detection information from the pressure sensor 14 that detects the off-gas tank pressure, and by commanding the off-gas adjustment unit MR to the determined target flow rate. Specifically, as shown in Figure 9, the operation control unit F calculates the differential pressure Pt between the minimum pressure (minimum value) and the target pressure (for example, 2 kPaG) each time the minimum pressure (minimum value) of the off-gas tank T occurs repeatedly. Based on the calculated differential pressure Pt, it then calculates the corrected flow rate of the target flow rate to be commanded to the off-gas adjustment unit MR, and the currently commanded target flow rate is corrected by the corrected flow rate and the resulting flow rate is commanded to the off-gas adjustment unit MR as the new target flow rate. As shown in Figure 10, the corrected flow rate is calculated as a value of 1 NL / min when the differential pressure Pt is 1 kPaG, and the larger the differential pressure Pt, the larger the calculated flow rate. As a result, as shown in Figure 9, the system is configured so that the minimum pressure (minimum value) of the off-gas tank pressure approaches the target pressure (for example, 2 kPaG) by correcting the target flow rate of the recycled gas. In Figure 10, an example is shown where the relationship between differential pressure Pt and corrected flow rate is set as a linear equation. However, the relationship between differential pressure Pt and corrected flow rate can be changed in various ways, such as by setting it as a quadratic equation. Incidentally, in this embodiment, as described above, during the startup operation of the pressure fluctuation adsorption type gas production apparatus, the operator commands the operation control unit F from the operation command unit 16 to set a target flow rate for the recycled gas flowing through the recycled gas return path 11. Specifically, while monitoring the off-gas tank pressure detected by the pressure sensor 14, the operator commands a target flow rate such that the lowest (minimum) off-gas tank pressure becomes the target pressure (for example, 2 kPaG). Then, when the minimum pressure (minimum value) of the off-gas tank pressure reaches the target pressure (for example, 2 kPaG), an automatic operation command is issued from the operation command unit 16. In this case, the operation control unit F sets the target flow rate while calculating the corrected flow rate based on the differential pressure Pt as described above, and then commands the off-gas adjustment unit MR to set the target flow rate. Furthermore, even during the startup operation of the pressure fluctuation adsorption type gas production apparatus, the operation control unit F may set a target flow rate while determining the corrected flow rate based on the differential pressure Pt, as described above, and then command the off-gas adjustment unit MR to set the target flow rate. (Correction control of recycled gas flow rate) As described above, the operation control unit F is configured to change the target flow rate commanded to the off-gas adjustment unit MR in accordance with any changes or adjustments made to the adsorption time during the adsorption process, thereby feedforward-adjusting the recycle gas flow rate. In other words, when the operation control unit F changes the adsorption time, it is configured to adjust the off-gas adjustment unit MR in order to correct the flow rate of the recycled gas to a flow rate that is equal to the amount of off-gas discharged from the adsorption tower 1 before the adsorption time was changed and the amount of off-gas discharged from the off-gas tank T between the time the adsorption tower 1 starts the desorption process and the time the other adsorption tower 1 starts the desorption process after the adsorption time has been changed. In other words, changing the adsorption time immediately alters the time it takes for a portion of the off-gas discharged from one of the four adsorption towers 1 to be returned to the raw material gas supply line 3 as recycled gas. In contrast, immediately after changing or adjusting the adsorption time, off-gas is discharged from the adsorption tower 1, which performed the adsorption process before changing or adjusting the adsorption time. Therefore, the amount of off-gas discharged from the adsorption tower 1 does not change immediately after changing or adjusting the adsorption time. As a result, when the adsorption time for the adsorption process is changed to decrease, the recycled gas flow rate becomes insufficient, and when the adsorption time for the adsorption process is changed to increase, the recycled gas flow rate becomes excessive, causing the off-gas tank pressure in off-gas tank T to increase or decrease significantly. For example, as shown in Figure 11, if the adsorption time is changed to a decreasing state, the off-gas tank pressure will gradually increase due to insufficient recycled gas flow rate. In this embodiment, when the adsorption time is changed or adjusted, the system is configured to maintain the appropriate recycled gas flow rate by feedforward adjustment in accordance with that change or adjustment. Specifically, the operation control unit F is configured to change the target flow rate commanded to the off-gas adjustment unit MR in accordance with any changes made to the adsorption time, in order to maintain a constant relationship between the product of the reference time and the recycled gas flow rate. The reference time is the period during which off-gas is discharged from each of the adsorption towers 1, or in other words, the time between the start of off-gas discharge from one adsorption tower 1 and the start of off-gas discharge from the next adsorption tower 1. Specifically, these correspond to the time from when the off-gas discharge from Tower D begins until the off-gas discharge from Tower A begins, the time from when the off-gas discharge from Tower A begins until the off-gas discharge from Tower B begins, the time from when the off-gas discharge from Tower B begins until the off-gas discharge from Tower C begins, and the time from when the off-gas discharge from Tower C begins until the off-gas discharge from Tower A begins, corresponding to the time from when the off-gas discharge from Tower C begins until the off-gas discharge from Tower D begins. Each of these times includes an adsorption time correction value X. Therefore, if the adsorption time is changed or adjusted, the system is configured to determine a new target recyclable gas flow rate by dividing the product of the reference time before the adsorption time change / adjustment and the recyclable gas flow rate by the reference time after the adsorption time change / adjustment, and then command the off-gas adjustment unit MR with the determined recyclable gas flow rate as the target flow rate. For example, as shown in Figure 12, changing the adsorption time to a decreasing state will increase the recycled gas flow rate, thereby suppressing the increase in off-gas tank pressure. Although not shown in the diagram, increasing the adsorption time reduces the recycled gas flow rate, which in turn suppresses the decrease in off-gas tank pressure. (Correction control of raw material gas supply) The operation control unit F is configured to correct the amount of raw material gas G supplied to the adsorption tower 1 by commanding the raw material gas adjustment unit MG to a target amount of raw material gas supply that is increased or decreased by the amount corresponding to the increase or decrease in the recycled gas flow rate when the recycled gas flow rate is increased or decreased by the "recycled gas flow rate correction control" described above. In other words, increasing the recycled gas flow rate causes a rapid increase in the amount of gas supplied to the adsorption tower 1, which may increase the amount of miscellaneous gas supplied to the adsorption tower 1 and potentially decrease the methane concentration of the product gas H. In this embodiment, when the adsorption time is changed to a decreasing state by "adsorption time adjustment control," and the recycled gas flow rate is increased by "recycled gas flow rate correction control," the amount of raw material gas G supplied is reduced by an amount corresponding to the increase in the recycled gas flow rate, thereby suppressing a decrease in the methane concentration of the product gas H. [Second Embodiment] Next, a second embodiment of the pressure fluctuation adsorption type gas production apparatus will be described. This second embodiment shows another embodiment of the "adsorption time adjustment control" in the first embodiment, and the basic configuration is the same as that of the first embodiment. Therefore, in the following description, the differences from the first embodiment will be described in detail. In the first embodiment described above, the operation control unit F was shown as an example in which it changes and adjusts the adsorption time for the adsorption process based on the detection information of the raw material side concentration sensor SG. However, in this second embodiment, the adsorption time is changed and adjusted based on the detection information of the product side concentration sensor SH. In other words, as shown in the table in Figure 13, the relationship between the methane gas concentration in the product gas detected by the product-side concentration sensor SH and the process correction value is predetermined. When starting the operation of the pressure fluctuation adsorption gas production apparatus, the adsorption time correction value X is set to a preset initial value (for example, 100 seconds) and operation is started. Thereafter, for each cycle in which the adsorption process is performed in each of the adsorption towers 1, the average methane concentration in the raw material gas, which is the average value of the methane concentration in the product gas during that cycle, is calculated. The average methane concentration in the product gas is calculated by sampling the detection information of the product-side concentration sensor SH at set time intervals (for example, 500 ms) and calculating the average of the sampled detection values. Next, as shown in Figure 14, when the adsorption process of the first cycle is completed, a process correction value is determined based on the relationship between the average methane concentration in the product gas and Figure 13. The value obtained by adding the determined process correction value to the current adsorption time correction value (initial value) X is set as the new adsorption time correction value X for the next adsorption process. Subsequently, when the adsorption process of the second and subsequent cycles is completed, a process correction value is calculated based on the relationship between the average methane concentration in the product gas and Figure 13. The calculated process correction value and all the process correction values ​​obtained when the adsorption process of the previous cycle was completed are added together to obtain an integrated value. This integrated value is then added to the initial value (for example, 100 seconds) to obtain a new adsorption time correction value X for the next adsorption process. For example, if the average methane concentration in the product gas is 82% when the first adsorption process with the adsorption time correction value X set to 100 seconds is performed, then in the next adsorption process with y+1 steps, the adsorption time correction value X will be set to 97 seconds, which is the sum of the initial value of 100 seconds and the process correction value of -3 seconds. Next, if the average methane concentration in the product gas is 84% ​​when the second adsorption process is completed, then in the next adsorption process (number of subsequent adsorption processes y+2), the adsorption time correction value X will be 96 seconds, which is the sum of the initial value of 100 seconds, the process correction value of the first cycle of -3 seconds, and the process correction value of the second cycle of -1 second. The same applies to subsequent adsorption steps y+3 to y+6. In this second embodiment as well, by performing "corrective control of the recycled gas flow rate," rapid fluctuations in the off-gas tank pressure are suppressed. In particular, when correcting the adsorption time based on the methane gas concentration in the product gas, if the adsorption time for the adsorption process is changed to a reduced level, the recycled gas flow rate may become insufficient and the off-gas tank pressure may increase. This can worsen the desorption conditions in the desorption process (reduced pressure) of the adsorption tower 1, making it impossible to properly desorb the unwanted gases. As a result, the adsorption time may be repeatedly changed to a reduced level, potentially leading to an uncontrollable abnormal state where the adsorption time is drastically reduced and the recycled gas flow rate drastically increases. However, by performing "correction control of the recycled gas flow rate," the occurrence of such an abnormal state can be suppressed. In this second embodiment as well, the decrease in methane gas concentration in the product gas can be suppressed by performing "supply correction control of raw material gas." In particular, when correcting the adsorption time based on the methane gas concentration in the product gas, increasing the recycle gas flow rate increases the amount of miscellaneous gas supplied to the adsorption tower 1, which lowers the methane concentration in the product gas H. Consequently, the adsorption time is repeatedly reduced and the recycle flow rate is increased, which may lead to an abnormal situation where the methane concentration in the product gas H remains low. However, by implementing "raw material gas supply correction control," the occurrence of such an abnormal situation can be suppressed. [Third Embodiment] Next, a third embodiment of the pressure fluctuation adsorption type gas production apparatus will be described. This third embodiment shows another embodiment of the "adsorption time adjustment control" in the first embodiment, and the basic configuration is the same as that of the first embodiment. Therefore, in the following description, the differences from the first embodiment will be described in detail. In this third embodiment, the operation control unit F changes and adjusts the adsorption time for the adsorption process based on the detection information from the raw material side concentration sensor SG and the detection information from the product side concentration sensor SH. Specifically, as shown in the table in Figure 15, the relationship between the methane gas concentration in the raw material gas detected by the raw material-side concentration sensor SG and the process correction value is predetermined, and as shown in the table in Figure 16, the relationship between the methane gas concentration in the product gas detected by the product-side concentration sensor SH and the process correction value is predetermined. Then, when starting operation of the pressure fluctuation adsorption type gas production apparatus, the adsorption time correction value X is set to a preset initial value (for example, 100 seconds) and operation begins. Subsequently, for each cycle in which the adsorption process is performed in each adsorption tower 1, the average methane concentration in the raw material gas, which is the average value of the methane concentration in the raw material gas during that cycle, and the average methane concentration in the product gas, which is the average value of the methane concentration in the product gas during that cycle, are calculated. Furthermore, the average methane concentration in the raw material gas is calculated by sampling the detection information from the raw material concentration sensor SG at set time intervals (e.g., 500 ms) and determining the average of the sampled detection values. Similarly, the average methane concentration in the product gas is calculated by sampling the detection information from the product concentration sensor SH at set time intervals (e.g., 500 ms) and determining the average of the sampled detection values. Next, a process correction value for raw material gas G is determined based on the relationship between the average methane concentration in the raw material gas and Figure 15. Similarly, a process correction value for product gas H is determined based on the relationship between the average methane concentration in the product gas and Figure 16. As shown in Figure 17, the current adsorption time correction value (initial value) X, the process correction value for the raw material gas G corresponding to the adsorption process of the previous cycle, and the process correction value for the product gas H corresponding to the adsorption process of the previous cycle are added together to obtain the new adsorption time correction value X for the next adsorption process. For example, when the first adsorption process with y elapsed adsorption steps is performed with the adsorption time correction value X set to 100 seconds, if the methane concentration in the raw material gas is 57% and the average methane concentration in the product gas is 82%, then in the next adsorption process with y+1 elapsed adsorption steps, the adsorption time correction value X will be set to 91 seconds, which is the sum of the initial value of 100 seconds, the process correction value for the raw material gas G of -6 seconds, and the process correction value for the product gas H of -3 seconds. Next, when the second adsorption process is completed, if the average methane concentration in the raw material gas is 59% and the average methane concentration in the product gas is 84%, then calculate the process correction value for raw material gas G "-2 seconds" and the process correction value for product gas H "-1 second". Furthermore, add the process correction value for product gas H in the previous adsorption process of y steps "-3 seconds" and the process correction value for product gas H in the current adsorption process of y+1 steps "-1 second" to obtain the cumulative value of the process correction value for product gas H "-4 seconds". Then, the adsorption time correction value X is calculated by adding the initial value of "100 (seconds)", the process correction value for raw material gas G "-2 (seconds)", and the integrated value of the process correction value for product gas H "-4 (seconds)", resulting in "94 (seconds)". The same applies to the subsequent cycles y+3 to y+6. In this third embodiment as well, by performing "corrective control of the recycled gas flow rate," rapid fluctuations in the off-gas tank pressure are suppressed. In particular, when correcting the adsorption time based on the methane gas concentration in the product gas, if the adsorption time for the adsorption process is changed to a reduced level, the recycled gas flow rate may become insufficient and the off-gas tank pressure may increase. This can worsen the desorption conditions in the desorption process (reduced pressure) of the adsorption tower 1, making it impossible to properly desorb the unwanted gases. As a result, the adsorption time may be repeatedly changed to a reduced level, potentially leading to an uncontrollable abnormal state where the adsorption time is drastically reduced and the recycled gas flow rate drastically increases. However, by performing "correction control of the recycled gas flow rate," the occurrence of such an abnormal state can be suppressed. In this third embodiment as well, the decrease in methane gas concentration in the product gas can be suppressed by performing "supply correction control of raw material gas." In particular, when correcting the adsorption time based on the methane gas concentration in the product gas, increasing the recycle gas flow rate increases the amount of miscellaneous gas supplied to the adsorption tower 1, which lowers the methane concentration in the product gas H. Consequently, the adsorption time is repeatedly reduced and the recycle flow rate is increased, which may lead to an abnormal situation where the methane concentration in the product gas H remains low. However, by implementing "raw material gas supply correction control," the occurrence of such an abnormal situation can be suppressed. [Other embodiments] Next, other embodiments are listed. (1) In the first to third embodiments described above, the off-gas adjustment unit MR, which adjusts the discharge flow rate of the off-gas discharged from the off-gas tank T, is provided downstream of the membrane separation unit 9. However, the off-gas adjustment unit MR may also be installed at a location corresponding to the space between the booster pump 10 and the membrane separation unit 9. (2) In the first to third embodiments described above, the example given was that in the "raw material gas supply correction control," the raw material supply amount is reduced in accordance with the increase in recycled gas. However, the concentration of miscellaneous gases in the recycled gas may be detected, and the raw material supply amount may be reduced by an amount equivalent to the increase in miscellaneous gases in the recycled gas. (3) In the first to third embodiments described above, an example was given in which the off-gas adjustment unit MR is configured using a mass flow controller. However, the off-gas adjustment unit MR may also be configured in a manner in which the discharge amount of the boost pump 10, which constitutes the boost unit, is adjusted while detecting the gas flow rate with a flow sensor. (4) In the first to third embodiments described above, the example given was that the raw material gas G is a biogas containing methane as the gas to be purified and carbon dioxide, etc. as miscellaneous gas. However, various gases can be used as the raw material gas G, such as gas obtained by reforming city gas, that is, gas containing hydrogen as the gas to be purified and containing carbon dioxide, carbon monoxide, and nitrogen as miscellaneous gas. (5) In the first to third embodiments described above, a gas production apparatus equipped with four adsorption towers 1 was described as a pressure fluctuation adsorption type apparatus. However, the apparatus may be equipped with three or five or more adsorption towers 1, and the adsorption process, the pressure reduction equalization process, the desorption process, and the pressure boost equalization process may be executed sequentially in each of the adsorption towers 1 with different phases. (6) In the first to third embodiments described above, the raw material gas supply passage 3 and the off-gas discharge passage 4 are connected to the lower end of the adsorption tower 1, and the gas to be purified discharge passage 5 is connected to the upper end of the other end of the adsorption tower 1. However, the system may also be implemented in which the raw material gas supply passage 3 and the off-gas discharge passage 4 are connected to the upper end of the adsorption tower 1, and the gas to be purified discharge passage 5 is connected to the lower end of the other end of the adsorption tower 1. Furthermore, the connection point of the off-gas discharge channel 4 to the adsorption tower 1 may be either the end of the adsorption tower 1 to which the gas to be purified discharge channel 5 is connected, or the middle of the adsorption tower 1 in the longitudinal direction. In short, the off-gas discharge channel 5 just needs to be connected to the adsorption tower 1. Furthermore, the configurations disclosed in the above embodiments (including other embodiments, the same applies hereinafter) can be applied in combination with configurations disclosed in other embodiments, provided that no inconsistencies arise. Moreover, the embodiments disclosed herein are illustrative, and the embodiments of the present invention are not limited thereto, and can be modified as appropriate without departing from the object of the present invention. 1 Adsorption tower 3. Raw material gas supply route 4. Off-gas discharge channel 5. Gas discharge path to be purified (product gas delivery path) 9A Separation membrane 9 Membrane separation section 11. Recycled gas return route 12 Off-gas adjustment section 14. Pressure Sensor 15. Raw Material Gas Adjustment Section F Operation Control Unit G. Raw material gas (biogas) H. Gas to be purified (product gas) T Off-gas tank

Claims

Revised 05 / 11 / 2018, Page 1 of 2, Claims:

1. A pressure-alternating adsorption gas production device comprising: multiple adsorption towers, each filled with adsorbent material, in which the gas mixture is adsorbed from the source gas, including the target gas for purification and the gas mixture; a source gas delivery path connected to one end of each adsorption tower; a target gas for purification discharge path connected to the other end of each adsorption tower; and a waste gas discharge path connected to each adsorption tower. An operation control unit that operates the adsorption towers sequentially, with different phases, and operating cycles that include the adsorption process in which the gas mixture is adsorbed from the source gas delivered through the source gas delivery path, with the target gas for purification discharged through the target gas for purification discharge path.The adsorption process involves the gas mixture being vented through a discharge path. A membrane separation unit is positioned along the discharge path, incorporating a separation membrane that allows the gas mixture to pass through faster than the target gas for purification. A recycling return path is also in place, where recycled gas that does not pass through the separation membrane is returned to the supply path from the source. A pressure booster unit and waste gas tank are located upstream of the membrane separation unit in the discharge path. The waste gas tank stores the waste gas vented through the discharge path during the discharge process.And a pressure acceleration unit that increases the pressure of the flue gas stored in the flue gas tank and delivers the flue gas to the membrane separation unit for membrane separation. In the membrane separation unit, the pressure-switched adsorption type gas production device is integrated with a flue gas adjustment unit that adjusts the flue gas discharge flow rate from the flue gas tank and a flue gas adjustment unit operation control unit, where the flue gas discharge flow rate is the flow rate of the amount of flue gas discharged from one of the adsorption towers each time the discharge process is performed, equal to the amount of flue gas discharged from the flue gas tank during the time from when one adsorption tower starts the discharge process until when another adsorption tower starts the discharge process.----------------------------------------------------------------------------------------Page 1 of 2 pages Claims 1. Pressure-switched adsorption type gas production device integrated with: Multiple adsorption towers, each filled with an adsorbent through which a gas mixture is adsorbed from source gases including the target gas for purification and the gas mixture,The gas delivery path from the source is connected to one end of each adsorption tower, the target gas vent path for purification is connected to the other end of each adsorption tower, and the waste gas vent path is connected to each adsorption tower. The control unit operates the adsorption towers sequentially, using different phases. The operating cycle includes the adsorption process where the gas mixture is adsorbed from the source gas delivered through the source delivery path, with the target gas for purification vented through the target gas vent path, and the adsorption process where the gas mixture is vented through the waste gas vent path. A membrane separation unit is provided in the waste gas vent path and includes a separation membrane that allows the gas mixture to pass through faster than the target gas for purification. A recycling gas return path is also provided, where recycled gas that does not pass through the separation membrane is returned to the source gas delivery path. A pressure boosting unit and waste gas tank are provided on the opposite side of the membrane separation unit in the waste gas vent path.The waste gas tank stores the waste gas that is vented through the waste gas vent path during the discharge process, and the pressure acceleration unit increases the pressure of the waste gas stored in the waste gas tank and delivers the waste gas to the membrane separation unit for membrane separation. In the membrane separation unit, the pressure-switched adsorption gasification device is integrated with a waste gas adjustment unit that adjusts the vent flow rate of the waste gas vented from the waste gas tank and a control unit for adjusting the waste gas adjustment unit. The waste gas vent flow rate is the flow rate equal to the amount of waste gas vented from one of the adsorption towers each time the discharge process is performed, equal to the amount of waste gas vented from the waste gas tank during the time from when one adsorption tower begins the discharge process until when another adsorption tower begins the discharge process.

2. The pressure-switched adsorption gasification device according to claim 1 is integrated with a pressure sensor that detects the pressure in the waste gas tank (page 2 of 2), which the control unit adjusts, based on data from the pressure sensor.The waste gas adjustment unit, where the waste gas discharge flow rate is the minimum flow rate in the area of ​​the internal pressure of the waste gas tank, which is repeatedly increased and decreased, is maintained at the set pressure 3. The alternating pressure adsorption type gas production device according to claim 2, where the control unit changes the adsorption time during which the adsorption process is carried out, based on at least one of the target gas concentrations for purification in the source gas and the target gas concentration for purification discharged from the discharge tower, and in the case of changes in the discharge time,The control unit adjusts the flue gas adjustment unit to correct the flue gas discharge flow rate from the flue gas cylinder to a flow rate equal to the volume of flue gas discharged from one of the absorption towers before the discharge time change, equal to the volume of flue gas discharged from the flue gas cylinder during the time from when one absorption tower begins the discharge process after the discharge time change until when another absorption tower begins the discharge process.

4. A pressure-switched absorption gas production device under claim 3, which is further incorporated with a source gas adjustment unit that adjusts the source gas delivery rate to the absorption tower via the source gas delivery path, where the control unit adjusts the source gas adjustment unit to correct the source gas delivery rate to a value in the event that the flue gas discharge flow rate from the flue gas cylinder is corrected to a higher value.And to correct the gas delivery rate from the source to a higher value in the event that the discharge flow rate is corrected to a lower value.

5. Pressure-switched adsorption gas production equipment under any one of claims 1 to 4, where the source gas is a methane-containing gas containing 40% or more methane as the target gas for purification, and the target gas for purification discharged through the target gas for purification discharge path is a purified gas containing 80% or more methane;