Carbon dioxide recovery system

Abstract

To provide a carbon dioxide recovery system for eliminating carbon dioxide through pressure reduction executed by a vacuum pump, which can reduce the fluctuation width of an elimination amount of carbon dioxide per unit time.SOLUTION: This carbon dioxide recovery system comprises: an adsorber storing an adsorbent capable of adsorbing carbon dioxide, in which an exhaust gas flows; a purge gas supply unit that can supply purge gas inside; a feed-out flow path for feeding out a mixed gas containing the carbon dioxide and the purge gas fed out from the absorber during supplying of the purge gas inside; a vacuum pump provided in the feed-out flow path and capable of reducing pressure inside; and a control unit that controls the vacuum pump. The control unit controls the vacuum pump so as to provide a pressure reduction period divided into two which is a period for executing inside pressure reduction by the vacuum pump during an elimination period for supplying the purge gas inside and eliminating the carbon dioxide from the absorbent.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide capture system. [Background technology]

[0002] Conventionally, there has been known a technique for recovering carbon dioxide by adsorbing the carbon dioxide contained in exhaust gas onto an adsorbent. For example, Patent Document 1 discloses a methane production apparatus that desorbs the carbon dioxide adsorbed onto an adsorbent housed in an adsorber by supplying hydrogen as a purge gas, and then produces methane using a mixed gas containing carbon dioxide and hydrogen delivered from the adsorber. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6791177 Summary of the Invention [Problem to be solved by the invention]

[0004] In a desorption process for desorbing carbon dioxide from an adsorbent, in addition to supplying a purge gas, the pressure inside the adsorber may be reduced by a vacuum pump to promote carbon dioxide desorption. During the desorption process, the amount of carbon dioxide desorbed per unit time tends to increase over time while depressurization is being performed, and tends to decrease over time while depressurization is stopped. Thus, the amount of carbon dioxide desorbed per unit time increases or decreases depending on whether depressurization is performed or stopped. If the fluctuation range of this increasing or decreasing amount of carbon dioxide desorbed per unit time is large, this can lead to problems such as a need to widen the control range of the hydrogen flow rate added to adjust the H / CO ratio in the mixed gas to a suitable value and an increase in the amount of power required to operate a compressor that compresses the mixed gas used to produce hydrocarbons. Patent Document 1 does not take into account the fluctuation range of the amount of carbon dioxide desorbed per unit time.

[0005] The present invention has been made to solve at least some of the above-mentioned problems, and aims to provide a technology that can reduce the range of fluctuation in the amount of carbon dioxide desorption per unit time in a carbon dioxide capture system that desorbs carbon dioxide by reducing pressure using a vacuum pump. [Means for solving the problem]

[0006] The present invention has been made to solve at least part of the above-mentioned problems, and can be realized in the following aspects.

[0007] (1) According to one aspect of the present invention, there is provided a carbon dioxide capture system for capturing carbon dioxide from exhaust gas containing carbon dioxide, the carbon dioxide capture system including: an adsorber containing an adsorbent capable of adsorbing carbon dioxide and into which the exhaust gas flows; a purge gas supply unit capable of supplying a purge gas to the interior; a delivery flow path connected to the adsorber for delivering a mixed gas containing carbon dioxide and the purge gas that is delivered from the adsorber when the purge gas is supplied to the interior; a vacuum pump provided in the delivery flow path for reducing the pressure inside the interior; and a control unit for controlling the vacuum pump, wherein the control unit controls the vacuum pump so that a decompression period, during which the vacuum pump reduces the pressure inside the interior, is provided twice or more times during a desorption process in which the purge gas is supplied to the interior to desorb carbon dioxide from the adsorbent.

[0008] According to this configuration, the desorption process includes two or more decompression periods, during which the vacuum pump decompresses the interior of the adsorber. Therefore, the length of each decompression period is shortened compared to a configuration in which only one decompression period of the same length as the combined duration of the two or more divided decompression periods is provided during the desorption process. Furthermore, the length of each non-decompression period, during which the vacuum pump stops decompressing the interior of the adsorber, is also shortened. Therefore, the shortened length of each decompression period prevents an excessive increase in the amount of carbon dioxide desorbed per unit time, and the shortened length of each non-decompression period prevents an excessive decrease in the amount of carbon dioxide desorbed per unit time. As a result, the range of fluctuation in the amount of carbon dioxide desorbed per unit time can be reduced in a carbon dioxide capture system that desorbs carbon dioxide by decompressing the interior of the adsorber using a vacuum pump.

[0009] (2) In the carbon dioxide capture system of the above aspect, the control unit may adjust the length of each of the decompression periods so that the fluctuation range of the ratio of hydrogen to carbon dioxide in the mixed gas falls within a first set range set for the ratio. According to this configuration, the ratio of hydrogen to carbon dioxide in the mixed gas can be kept within the first set range, so that the mixed gas can be delivered with a ratio suitable for producing hydrocarbons.

[0010] (3) In the carbon dioxide capture system of the above form, a surge tank capable of storing the mixed gas is provided downstream of the position where the vacuum pump is provided in the delivery flow path, and the control unit may adjust the length of each of the decompression periods so that the fluctuation range of the pressure in the surge tank falls within a second set range set for the pressure. According to this configuration, the fluctuation range of the pressure inside the surge tank can be kept within the second set range, so that the carbon dioxide capture system can be provided with a surge tank of a size suitable for that fluctuation range of the pressure.

[0011] The present invention can be realized in various aspects, for example, in the form of a carbon dioxide capture method, a carbon dioxide capture system control method, a computer program for controlling a carbon dioxide capture system, a hydrocarbon production system, a control method for these systems, a computer program for controlling these systems, a server device for distributing these computer programs, a non-transitory storage medium on which the computer program is stored, etc. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is an explanatory diagram illustrating the configuration of a carbon dioxide capture system according to an embodiment; [Figure 2] FIG. 2 is an explanatory diagram showing switching between an adsorption process, a desorption process, and a cooling process. [Figure 3] FIG. 10 is an explanatory diagram of control of a vacuum pump in a comparative example. [Figure 4] FIG. 10 is an explanatory diagram showing the fluctuation range of the H2 / CO2 ratio in the mixed gas in a comparative example. [Figure 5] FIG. 10 is an explanatory diagram showing the fluctuation range of pressure in a surge tank in a comparative example. [Figure 6] FIG. 4 is an explanatory diagram of control of a vacuum pump in the present embodiment. [Figure 7] FIG. 10 is an explanatory diagram of adjustment of the length of a depressurization period by a control unit. [Figure 8] FIG. 10 is an explanatory diagram showing a fluctuation range narrowed by adjusting the length of the pressure reduction period. [Figure 9] FIG. 10 is an explanatory diagram showing a fluctuation range narrowed by adjusting the length of the pressure reduction period. [Figure 10] FIG. 10 is a diagram illustrating the effect of changing the number of decompression periods provided during the desorption process. [Figure 11] FIG. 10 is a diagram illustrating the effect of changing the number of decompression periods provided during the desorption process. [Figure 12] FIG. 10 is a diagram illustrating the effect of changing the number of decompression periods provided during the desorption process. [Figure 13] FIG. 10 is a diagram illustrating the effect of changing the number of decompression periods provided during the desorption process. DETAILED DESCRIPTION OF THE INVENTION

[0013] <Embodiment> FIG. 1 is an explanatory diagram illustrating the configuration of a carbon dioxide capture system 1 according to one embodiment of the present invention. The carbon dioxide capture system 1 is a system that captures carbon dioxide from exhaust gas containing carbon dioxide. The carbon dioxide capture system 1 includes a first adsorber 10, a second adsorber 20, a third adsorber 30, a combustion facility EG, an exhaust gas supply flow path 50, a hydrogen supply source HY, a hydrogen supply flow path 70, a mixed gas delivery flow path 80, a hydrocarbon synthesis unit 90, a heat medium flow path 100, and a control unit 110. The first to third adsorbers 10, 20, and 30 may also be collectively referred to simply as adsorbers.

[0014] The first adsorber 10 is a device for recovering carbon dioxide from exhaust gas. The first adsorber 10 houses a first adsorbent 12 capable of adsorbing carbon dioxide therein, and exhaust gas flows into the first adsorber 10. In the carbon dioxide recovery system 1, exhaust gas discharged from the combustion equipment EG flows into the first adsorber 10. Examples of the first adsorbent 12 include zeolite, activated carbon, and silica gel. The second adsorber 20 and the third adsorber 30 are adsorbers similar to the first adsorber 10. The second adsorber 20 and the third adsorber 30 house therein a second adsorbent 22 and a third adsorbent 32 similar to the first adsorber 10. The first to third adsorbents 12, 22, and 32 may also be collectively referred to simply as adsorbents.

[0015] The combustion equipment EG is a combustion furnace in a factory. The exhaust gas discharged from the combustion equipment EG contains oxygen, nitrogen, water, and the like in addition to carbon dioxide adsorbed by the first to third adsorbents 12, 22, and 32. The exhaust gas supply passage 50 connects the combustion equipment EG to each of the first to third adsorbers 10, 20, and 30 and is a gas passage for supplying the exhaust gas discharged from the combustion equipment EG to the first to third adsorbers 10, 20, and 30. The exhaust gas supply passage 50 is provided with a first exhaust gas supply valve 51, a second exhaust gas supply valve 52, and a third exhaust gas supply valve 53 for adjusting the supply of exhaust gas to each of the first to third adsorbers 10, 20, and 30. By controlling the opening and closing of these valves, it is possible to switch between on and off the supply of exhaust gas to each of the first to third adsorbers 10, 20, and 30. The opening and closing of each of the first to third exhaust gas supply valves 51 to 53 is controlled by a control unit 110, which will be described later.

[0016] The exhaust gas discharge flow path 60 connects each of the first to third adsorbers 10, 20, and 30 to the outside of the carbon dioxide capture system 1, and is a gas flow path for discharging the exhaust gas that has been discharged from the combustion equipment EG and then passed through each of the first to third adsorbers 10, 20, and 30 to the outside of the carbon dioxide capture system 1. The exhaust gas discharge flow path 60 is provided with a first exhaust gas discharge valve 61, a second exhaust gas discharge valve 62, and a third exhaust gas discharge valve 63 for adjusting the exhaust gas discharge from each of the first to third adsorbers 10, 20, and 30. By controlling the opening and closing of these valves, it is possible to switch between performing and stopping the exhaust gas discharge from each of the first to third adsorbers 10, 20, and 30. The opening and closing of each of the first to third exhaust gas discharge valves 61 to 63 is controlled by a control unit 110, which will be described later.

[0017] In the exhaust gas supplied from the combustion equipment EG to the first to third adsorbers 10, 20, 30 via the exhaust gas supply passage 50, carbon dioxide contained in the exhaust gas is adsorbed by the first to third adsorbents 12, 22, 32. The remaining exhaust gas that is not adsorbed by the first to third adsorbents 12, 22, 32 is discharged to the outside of the carbon dioxide capture system 1 via the exhaust gas discharge passage 60.

[0018] The hydrogen supply source HY is a water electrolysis device. The hydrogen supply flow path 70 is a gas flow path for supplying hydrogen from the hydrogen supply source HY to the first to third adsorption devices 10, 20, and 30. A first hydrogen supply valve 71, a second hydrogen supply valve 72, and a third hydrogen supply valve 73 are provided in the hydrogen supply flow path 70. By controlling the opening and closing of these valves, it is possible to switch between on and off the supply of hydrogen to each of the first to third adsorption devices 10, 20, and 30. The opening and closing of each of the first to third hydrogen supply valves 71 to 73 is controlled by a control unit 110, which will be described later.

[0019] The hydrogen supplied from the hydrogen supply source HY through the hydrogen supply passage 70 to the first to third adsorbers 10, 20, 30 is used as a purge gas for desorbing CO2 adsorbed in the first to third adsorbents 12, 22, 32. In other words, the hydrogen supply source HY corresponds to a purge gas supply unit capable of supplying purge gas into the first to third adsorbers 10, 20, 30.

[0020] The mixed gas delivery passage 80 is connected to each of the first to third adsorption devices 10, 20, and 30, and is a passage for delivering the mixed gas delivered from the first to third adsorption devices 10, 20, and 30 to a hydrocarbon synthesis apparatus 90 (described later) when a purge gas is supplied to the inside of the first to third adsorption devices 10, 20, and 30. The mixed gas contains carbon dioxide and hydrogen as a purge gas. The mixed gas delivery passage 80 is provided with a first mixed gas delivery valve 81, a second mixed gas delivery valve 82, and a third mixed gas delivery valve 83. By controlling the opening and closing of these valves, it is possible to switch between executing and stopping delivery of the mixed gas from each of the first to third adsorption devices 10, 20, and 30.

[0021] Furthermore, a vacuum pump 86 is provided in the mixed gas delivery passage 80. More specifically, the vacuum pump 86 is provided downstream of the positions of the first to third mixed gas delivery valves 81 to 83 in the mixed gas delivery passage 80. The vacuum pump 86 can reduce the pressure inside any one of the first to third adsorbers 10, 20, and 30 that is spatially connected to the vacuum pump 86 when any one of the first to third mixed gas delivery valves 81 to 83 is opened. The lower the pressure inside the adsorber, the more rapidly carbon dioxide is desorbed from the adsorbent.

[0022] A bypass flow path BY is connected to a portion of the mixed gas delivery flow path 80 between the positions where the first to third mixed gas delivery valves 81 to 83 are provided and the position where the vacuum pump 86 is provided. The bypass flow path BY is a flow path used when sending the mixed gas to a compressor 87 (described later) without passing through the vacuum pump 86. Whether or not the mixed gas is circulated through the bypass flow path BY is determined by the control unit 110 controlling the opening and closing of an on-off valve 84 and an on-off valve 85. The on-off valve 84 is provided in the mixed gas delivery flow path 80. The on-off valve 85 is provided in the bypass flow path BY.

[0023] Furthermore, a compressor 87 and a surge tank 88 are provided in the mixed gas delivery passage 80 downstream of the position where the vacuum pump 86 is provided. The compressor 87 compresses the mixed gas sent from the upstream side of the mixed gas delivery passage 80 and sends the compressed mixed gas downstream. The higher the pressure of the mixed gas, the more efficient the synthesis of hydrocarbons in a hydrocarbon synthesis unit 90, which will be described later.

[0024] Surge tank 88 is a tank that is disposed downstream of compressor 87 and can temporarily store the mixed gas delivered from compressor 87. The delivery port of surge tank 88, which delivers the mixed gas toward mixed gas delivery passage 80 on the side connected to hydrocarbon synthesis unit 90, is adjusted so that the pressure or flow rate of the delivered mixed gas is constant.

[0025] A hydrogen addition flow path 89 is connected to the mixed gas delivery flow path 80 at a portion downstream of the position where the surge tank 88 is provided. The hydrogen addition flow path 89 is a flow path for adding hydrogen to the mixed gas that flows into the hydrocarbon synthesis device 90, which will be described later. In hydrocarbon synthesis, the ratio of hydrogen to carbon dioxide (H2 / CO2 ratio) in the mixed gas used as the raw material affects the efficiency of hydrocarbon synthesis. In other words, the hydrogen addition flow path 89 is a flow path for adjusting the H2 / CO2 ratio in the mixed gas to a value suitable for hydrocarbon synthesis. The flow rate of hydrogen added from the hydrogen addition flow path 89 to the mixed gas delivery flow path 80 is adjustable. The hydrocarbon synthesized in the carbon dioxide capture system 1 is methane, and the suitable H2 / CO2 ratio for methane synthesis is 4.

[0026] The hydrocarbon synthesis unit 90 is a unit that synthesizes methane by a methanation reaction using the mixed gas supplied from the first to third adsorbers 10, 20, and 30 via the mixed gas delivery passage 80.

[0027] The heat medium flow path 100 is a flow path through which a heat medium flows that exchanges heat with the first to third adsorbents 12, 22, and 32. Heat generated in the hydrocarbon synthesis unit 90 by the methanation reaction is supplied to the first to third adsorbers 10, 20, and 30 via the heat medium flowing through the heat medium flow path 100. The heat medium flow path 100 is provided with a first flow path switching valve 104, a second flow path switching valve 105, a third flow path switching valve 106, and a fourth flow path switching valve 107. The first to fourth flow path switching valves 104 to 107 are all three-way valves that switch the flow path through which the heat medium flows from among the first flow path 101, the second flow path 102, and the third flow path 103 included in the heat medium flow path 100. The first to third flow paths 101 to 103 are flow paths that pass between the outer pipe and the inner pipe of the first to third adsorbers 10, 20, and 30, which are configured as double pipes or shell-and-tube structures. For example, when the flow path through which the heat medium flows is switched to the first flow path 101, heat is supplied to the first adsorbent 12 housed in the first adsorption device 10 from the heat medium flowing through the first flow path 101. In Fig. 1, the portion of the first adsorption device 10 through which the heat medium passes is illustrated by dotted hatching.

[0028] The heat medium flow path 100 is also provided with a pump 108 and a temperature adjustment unit 109. The pump 108 circulates the heat medium within the heat medium flow path 100 through the flow paths spatially connected to the pump 108 by switching the first to fourth flow path selector valves 104 to 107 among the first to third flow paths 101 to 103, and can change the flow direction of the heat medium. The temperature adjustment unit 109 is a device that can adjust the temperature of the heat medium, and when the temperature of the heat medium heated in the hydrocarbon synthesis unit 90 is higher than a set temperature, it adjusts the temperature by adding a room-temperature heat medium. When the temperature is lower than the set temperature, in addition to adjusting the flow rate, it may also heat the heat medium to the set temperature using a heater or the like.

[0029] The control unit 110 is a computer including a ROM, a RAM, and a CPU, and performs overall control of the carbon dioxide capture system 1. The control unit 110 controls the vacuum pump 86, as well as various valves and pumps provided in each of the above-mentioned flow paths, the temperature adjustment unit 109, etc. The control unit 110 causes each of the first to third adsorption units 10, 20, 30 to perform an adsorption process, a desorption process, and a cooling process, which will be described later.

[0030] 2 is an explanatory diagram showing switching between the adsorption process, desorption process, and cooling process in the first to third adsorbers 10, 20, and 30. In the carbon dioxide capture system 1, the first to third adsorbers 10, 20, and 30 each perform an adsorption process in which carbon dioxide is adsorbed from the exhaust gas onto the first to third adsorbents 12, 22, and 32, respectively; a desorption process in which H2 as a purge gas is supplied into the first to third adsorbers 10, 20, and 30 after the adsorption process to desorb carbon dioxide from the first to third adsorbents 12, 22, and 32; and a cooling process in which the first to third adsorbents 12, 22, and 32 after the desorption process are cooled. As shown in FIG. 2, the first to third adsorbers 10, 20, and 30 repeat different processes in the order of cycle 1, cycle 2, and cycle 3. That is, the control unit 110 causes the first to third adsorption units 10, 20, and 30 to repeatedly execute the adsorption process, the desorption process, and the cooling process. Note that a room temperature heat medium is supplied to the adsorption units during the adsorption process and the cooling process through a flow path (not shown) formed in each of the first to third adsorption units 10, 20, and 30 (a flow path different from the heat medium flow path 100). On the other hand, a heated heat medium is supplied to the adsorption unit during the desorption process through the heat medium flow path 100. In other words, the control unit 110 executes a heat medium delivery to circulate the heat medium through the adsorption unit during the desorption process.

[0031] When the first adsorber 10 is performing the adsorption process, the first exhaust gas supply valve 51 and the first exhaust gas discharge valve 61 are in an open state, and the first hydrogen supply valve 71 and the first mixed gas delivery valve 81 are in a closed state. In this state, carbon dioxide contained in the exhaust gas supplied from the combustion equipment EG to the first adsorber 10 is adsorbed by the first adsorbent 12. The remaining exhaust gas that is not adsorbed by the first adsorbent 12 is discharged to the outside of the carbon dioxide capture system 1 via the exhaust gas discharge flow path 60. During the adsorption process, a room-temperature heat medium is supplied to the first adsorber 10 via a flow path (not shown in the figure) formed in the first adsorber 10 (a flow path different from the heat medium flow path 100) to cool the first adsorbent 12.

[0032] When the first adsorber 10 is performing the desorption process, the first exhaust gas supply valve 51 and the first exhaust gas discharge valve 61 are closed, and the first hydrogen supply valve 71 and the first mixed gas delivery valve 81 are open. The on-off valve 84 is open, and the on-off valve 85 is closed. In this state, hydrogen supplied to the first adsorber 10 from the hydrogen supply source HY acts as a purge gas to desorb the carbon dioxide adsorbed in the first adsorbent 12. The desorbed carbon dioxide is delivered together with hydrogen to the mixed gas delivery path 80 as a mixed gas. During the desorption process, the vacuum pump 86 is operated to reduce the pressure inside the first adsorber 10, and the first to fourth path switching valves 104 to 107 are switched to switch the path through which the heat transfer medium flows from the second path 102 to the first path 101 (see FIG. 2 ). At this time, heat is supplied to the first adsorbent 12 housed in the first adsorption device 10 from the heat medium circulating through the first flow path 101 by the pump 108.

[0033] When the first adsorption device 10 is performing the cooling step, the first exhaust gas supply valve 51 and the first exhaust gas discharge valve 61 are closed, and the first hydrogen supply valve 71 and the first mixed gas delivery valve 81 are also closed. During the cooling step, the flow path through which the heat medium flows is switched from the first flow path 101 to the third flow path 103 by switching the first to fourth flow path switching valves 104 to 107 (see FIG. 2). During the cooling step, the heat medium at room temperature is supplied to the first adsorption device 10 through a flow path (not shown in the figure) formed in the first adsorption device 10 (a flow path different from the heat medium flow path 100) to cool the first adsorbent 12.

[0034] When the second and third adsorption devices 20, 30 are caused to perform the adsorption process, desorption process, and cooling process, respectively, the various valves of the second and third adsorption devices 20, 30 corresponding to the various valves of the first adsorption device 10 are opened, closed, and switched in the same manner as when the first adsorption device 10 is caused to perform the above-mentioned adsorption process, desorption process, and cooling process.

[0035] In the carbon dioxide capture system 1, the control unit 110 controls the vacuum pump 86 so that the decompression period, during which the vacuum pump 86 reduces the pressure inside the adsorber, is set two or more times during the desorption step. Details will be described using FIG. 6.

[0036] FIG. 3 is an explanatory diagram of the control of the vacuum pump 86 in a carbon dioxide capture system of a comparative example. The carbon dioxide capture system of the comparative example is the same as the carbon dioxide capture system 1 of the present embodiment, except that the vacuum pump 86 is controlled so that only one depressurization period is provided during the desorption process. The horizontal axis of FIG. 3 represents time, with time elapsed toward the right side of the drawing. On the vertical axis of FIG. 3, ON represents a state in which the vacuum pump 86 is depressurizing the interior of the adsorber, and OFF represents a state in which the vacuum pump 86 is not depressurizing the interior of the adsorber. The period Tcyc represents the length of each desorption process. As described above, in the carbon dioxide capture system of the comparative example, only one depressurization period is provided during the desorption process, and as shown in FIG. 3, the period Tcyc includes one depressurization period To and one non-depressurization period Tn. The non-depressurization period Tn is a period in which the vacuum pump 86 is not depressurizing the interior of the adsorber. Note that the period 2Tcyc is twice as long as the period Tcyc. The period from the end of the period Tcyc until the end of the period 2Tcyc corresponds to a period during which an adsorber different from the adsorber that executed the desorption process during the period Tcyc is made to execute the desorption process.

[0037] FIG. 4 is an explanatory diagram showing the fluctuation range of the H2 / CO2 ratio in the mixed gas delivered from the delivery port of the surge tank 88 in a carbon dioxide capture system of a comparative example. As described above, the delivery port of the surge tank 88 refers to the delivery port that delivers the mixed gas toward the mixed gas delivery passage 80 connected to the hydrocarbon synthesis unit 90. The horizontal axis of FIG. 4 represents time, as does the horizontal axis of FIG. 3. The vertical axis of FIG. 4 represents the H2 / CO2 ratio in the mixed gas delivered from the delivery port. The max and min shown on the right side of FIG. 4 represent the maximum and minimum values ​​of the fluctuating H2 / CO2 ratio in the mixed gas. In other words, the range between max and min corresponds to the fluctuation range of the H2 / CO2 ratio in the mixed gas delivered from the surge tank 88.

[0038] During the non-depressurization period Tn after the desorption step begins, carbon dioxide is desorbed from the adsorbent by supplying hydrogen and heat from the heat transfer medium. At the beginning of the non-depressurization period Tn, a large amount of carbon dioxide is adsorbed onto the adsorbent, resulting in a large amount of carbon dioxide desorbed per unit time. However, as the amount of carbon dioxide adsorbed onto the adsorbent decreases, the amount of carbon dioxide desorbed per unit time also decreases. In other words, during the non-depressurization period Tn, the amount of carbon dioxide desorbed per unit time tends to decrease over time. Reflecting this situation, Figure 4 shows that the H2 / CO2 ratio in the mixed gas discharged from the surge tank 88 increases as the non-depressurization period Tn progresses.

[0039] On the other hand, when the depressurization period To begins after the non-depressurization period Tn ends, hydrogen is supplied, heat is supplied from the heat transfer medium, and the pressure inside the adsorber is reduced, thereby promoting desorption of carbon dioxide from the adsorbent. After the depressurization period To begins, the amount of carbon dioxide desorbed per unit time increases as the pressure inside the adsorber progresses. That is, during the depressurization period To, the amount of carbon dioxide desorbed per unit time tends to increase over time. Reflecting this situation, FIG. 4 shows that the H2 / CO2 ratio in the mixed gas discharged from the surge tank 88 decreases as the depressurization period To progresses.

[0040] FIG. 5 is an explanatory diagram showing the fluctuation range of the pressure inside the surge tank 88 in a carbon dioxide capture system of a comparative example. The horizontal axis of FIG. 5 represents time, as do the horizontal axes of FIGS. 3 and 4. The vertical axis of FIG. 5 represents the pressure inside the surge tank 88. The max and min shown on the right side of FIG. 5 represent the maximum and minimum values ​​of the fluctuating pressure inside the surge tank 88. In other words, the range between max and min corresponds to the fluctuation range of the pressure inside the surge tank 88. FIG. 5 shows that after the non-depressurization period Tn starts, the pressure inside the surge tank 88 decreases due to the amount of carbon dioxide desorbed per unit time decreasing over time, and that after the depressurization period To starts, the pressure inside the surge tank 88 increases due to the amount of carbon dioxide desorbed per unit time increasing over time.

[0041] Fluctuations in the H2 / CO2 ratio in the mixed gas discharged from the surge tank 88 result from increases or decreases in the amount of carbon dioxide desorbed per unit time as the pressure is reduced and the non-pressure is switched between. Fluctuations in the pressure within the surge tank 88 result from increases or decreases in the amount of mixed gas discharged, which accompanies increases or decreases in the amount of carbon dioxide desorbed per unit time. These fluctuations are mitigated by the temporary storage of the mixed gas in the surge tank 88. However, a certain degree of fluctuation remains in both cases, as shown by the range between max and min in Figure 4 and the range between max and min in Figure 5. The widening of the fluctuation range of the H2 / CO2 ratio in the mixed gas discharged from the surge tank 88 leads to a wider control range of the hydrogen flow rate added from the hydrogen addition flow path 89 to the mixed gas delivery flow path 80 in order to adjust the H2 / CO2 ratio in the mixed gas to a desired value. The control range of the hydrogen flow rate refers to the control range of the hydrogen flow rate required to adjust the H2 / CO2 ratio in the mixed gas to a desired value. Furthermore, among these fluctuation ranges, an increase in the fluctuation range of the pressure inside the surge tank 88 leads to an increase in the amount of electricity required to operate the compressor 87 that compresses the mixed gas used to produce hydrocarbons. In particular, the higher the upper limit of the fluctuation range due to an increase in the fluctuation range of the pressure inside the surge tank 88, the greater the amount of electricity required to operate the compressor 87 when the pressure inside the surge tank 88 is at the upper limit. In this regard, in the carbon dioxide capture system 1 of this embodiment, the vacuum pump 86 is controlled so that depressurization periods, during which the vacuum pump 86 depressurizes the inside of the adsorber, are provided two or more times during the desorption process, thereby narrowing the fluctuation range of the amount of carbon dioxide desorbed per unit time that increases or decreases depending on the switching between depressurization periods and non-depressurization periods.

[0042] FIG. 6 is an explanatory diagram of the control of the vacuum pump 86 in the carbon dioxide capture system 1 of this embodiment. As described above, in the carbon dioxide capture system 1, the control unit 110 controls the vacuum pump 86 so that two or more depressurization periods are provided during the desorption process. The horizontal and vertical axes in FIG. 6 are the same as the horizontal and vertical axes in FIG. 3. FIG. 6 shows a case where the vacuum pump 86 is controlled so that two depressurization periods are provided during the desorption process. In the carbon dioxide capture system 1, as shown in FIG. 6, two depressurization periods To and two non-depressurization periods Tn are included in the period Tcyc. Note that the total length of the depressurization periods To provided twice during the period Tcyc in FIG. 6 is the same as the length of the depressurization period To provided only once during the period Tcyc in FIG. 3.

[0043] For ease of explanation, FIG. 6 shows the two depressurization periods To as being equal in length. However, in the carbon dioxide capture system 1, the length of each depressurization period To is adjusted. Specifically, the control unit 110 adjusts the length of each depressurization period so that the fluctuation range of the H2 / CO2 ratio in the mixed gas falls within a first set range. The first set range is a range set for the H2 / CO2 ratio in the mixed gas, and a H2 / CO2 ratio in the mixed gas that falls within this range is considered to be a ratio suitable for the methanation reaction. The control unit 110 also adjusts the length of each depressurization period so that the fluctuation range of the pressure in the surge tank 88 falls within a second set range. The second set range is a range set for the pressure in the surge tank 88, and a pressure in the surge tank 88 that falls within this range is considered to be a suitable pressure taking into account the power consumption of the compressor 87. That is, in the carbon dioxide capture system 1 of this embodiment, the control unit 110 adjusts the length of each decompression period while taking into consideration both that the fluctuation range of the H2 / CO2 ratio in the mixed gas falls within a first set range and that the fluctuation range of the pressure in the surge tank 88 falls within a second set range.

[0044] FIG. 7 is an explanatory diagram of adjustment of the length of the decompression period by the control unit 110. Both FIGS. 7(A) and 7(B) show a case in which the vacuum pump 86 is controlled so that three decompression periods are set during the desorption process in the carbon dioxide capture system 1. The horizontal and vertical axes of FIGS. 7(A) and 7(B) are the same as the horizontal and vertical axes of FIG. 6. FIG. 7(A) shows three decompression periods To, each with a different length. FIG. 7(B) shows three decompression periods To, each with an adjusted length from the state shown in FIG. 7(A). In detail, FIG. 7(B) shows a state after adjustment in which the lengths of the first decompression period To and the third decompression period To are extended and the length of the second decompression period To is shortened from the state shown in FIG. 7(A). The total length of the first decompression period To that is extended from the state shown in Figure 7(A) and the third decompression period To that is extended from the state shown in Figure 7(A) may be the same as or different from the length of the second decompression period To that is shortened from the state shown in Figure 7(A).

[0045] FIG. 8 is an explanatory diagram showing the narrowed fluctuation range achieved by adjusting the length of the decompression period. Both FIGS. 8(A) and 8(B) show the fluctuation range of the H2 / CO2 ratio in the mixed gas delivered from the surge tank 88. The horizontal and vertical axes of FIGS. 8(A) and 8(B) are the same as those of FIG. 4. FIG. 8(A) shows the fluctuation range of the H2 / CO2 ratio when the lengths of the three decompression periods provided during the desorption process are the lengths shown in FIG. 7(A). FIG. 8(B) shows the fluctuation range of the H2 / CO2 ratio when the lengths of the three decompression periods provided during the desorption process are the adjusted lengths shown in FIG. 7(B). Each of FIGS. 8(A) and 8(B) shows a first setting range R1 corresponding to the first setting range described above.

[0046] As shown by point Pe in Figure 8(A), when the lengths of the three decompression periods are the lengths shown in Figure 7(A), the fluctuation range of the H2 / CO2 ratio in the mixed gas does not fall within the first set range R1. On the other hand, as shown in Figure 8(B), when the lengths of the three decompression periods are the lengths shown in Figure 7(B), the fluctuation range of the H2 / CO2 ratio in the mixed gas falls within the first set range R1.

[0047] FIG. 9 is an explanatory diagram showing the narrowed fluctuation range achieved by adjusting the length of the decompression period. Both FIGS. 9(A) and 9(B) show the fluctuation range of the pressure in the surge tank 88. The horizontal and vertical axes of FIGS. 9(A) and 9(B) are the same as those of FIG. 5. FIG. 9(A) shows the fluctuation range of the pressure when the lengths of the three decompression periods provided during the desorption process are the lengths shown in FIG. 7(A). FIG. 9(B) shows the fluctuation range of the pressure when the lengths of the three decompression periods provided during the desorption process are the adjusted lengths shown in FIG. 7(B). Each of FIGS. 9(A) and 9(B) shows a second setting range R2 corresponding to the second setting range described above.

[0048] As shown by point Bo in FIG. 9(A), when the lengths of the three depressurization periods are the lengths shown in FIG. 7(A), the fluctuation range of the pressure in the surge tank 88 does not fall within the second set range R2. On the other hand, as shown in FIG. 9(B), when the lengths of the three depressurization periods are the lengths shown in FIG. 7(B), the fluctuation range of the pressure in the surge tank 88 falls within the second set range R2. In this way, in the carbon dioxide capture system 1, the control unit 110 adjusts the length of each of the two or more depressurization periods provided during the desorption process so that the fluctuation range of the H2 / CO2 ratio in the mixed gas and the fluctuation range of the pressure in the surge tank 88 each fall within a suitable range.

[0049] FIG. 10 is a diagram illustrating the effect of changing the number of pressure reduction periods provided during the desorption process. The horizontal axis of FIG. 10 indicates the number of decompression periods provided during the desorption process. That is, "1" on the horizontal axis indicates the results for the carbon dioxide capture system of the comparative example (when the desorption process is provided once), and "2" and "3" on the horizontal axis indicate the results for the carbon dioxide capture system 1 of this embodiment (when the desorption process is provided twice or three times). Here, when the decompression period provided during the desorption process is two or three times, the length of each decompression period To is adjusted to narrow the fluctuation range, as shown in FIG. 7(B). Furthermore, when the decompression period provided during the desorption process is two or three times, the total length of the two (or three) decompression periods is the same as the length of that single decompression period provided once during the desorption process. The horizontal axes of FIGS. 11 to 13 are the same as those of FIG. 10. The vertical axis of Fig. 10 indicates the fluctuation range of the H2 / CO2 ratio in the mixed gas delivered from the surge tank 88. More specifically, the vertical axis of Fig. 10 indicates a relative value when the fluctuation range when the decompression period provided during the desorption process is one is set to 1.0. As shown in Fig. 10, the fluctuation range of the H2 / CO2 ratio in the mixed gas delivered from the surge tank 88 decreases as the number of decompression periods provided during the desorption process increases.

[0050] Figure 11 is an explanatory diagram of the effect of changing the number of decompression periods provided during the desorption process. The vertical axis of Figure 11 represents the fluctuation range of the pressure inside the surge tank 88. More specifically, the vertical axis of Figure 11 represents a relative value when the fluctuation range when the decompression period provided during the desorption process is one is set to 1.0. As shown in Figure 11, the fluctuation range of the pressure inside the surge tank 88 decreases as the number of decompression periods provided during the desorption process increases.

[0051] Figure 12 is an explanatory diagram of the effect of varying the number of pressure reduction periods provided during the desorption process. The vertical axis of Figure 12 shows the control range of the hydrogen flow rate added from the hydrogen addition flow path 89 to the mixed gas delivery flow path 80 to adjust the H2 / CO2 ratio in the mixed gas delivered from the surge tank 88. More specifically, the vertical axis of Figure 12 shows a relative value when the control range when the pressure reduction period provided during the desorption process is one is set to 1.0. As shown in Figure 12, the control range of the hydrogen flow rate becomes smaller as the number of pressure reduction periods provided during the desorption process increases.

[0052] FIG. 13 is an explanatory diagram of the effect of changing the number of decompression periods provided during the desorption process. The vertical axis of FIG. 13 represents the reduction rate of the amount of power required to operate the compressor 87. Here, the amount of power required to operate the compressor 87 corresponds to the total amount of power required to operate the compressor 87 when each adsorber is sequentially executing the desorption process, divided by the number of desorption processes. In other words, the amount of power required to operate the compressor 87 corresponds to the average amount of power required to operate the compressor 87 per desorption process. Specifically, the vertical axis of FIG. 13 represents the reduction rate of the amount of power required to operate the compressor 87 when the desorption process is performed once, compared to the amount of power required to operate the compressor 87 when the desorption process is performed twice (three times). As shown in FIG. 13, the reduction rate of the amount of power required to operate the compressor 87 increases as the number of decompression periods provided during the desorption process increases.

[0053] As described above, according to the carbon dioxide capture system 1 of the above-described embodiment, two or more decompression periods are provided in the desorption process (see, for example, FIGS. 6 and 7). Therefore, compared to a configuration in which only one decompression period of the same length as the combined duration of two or more divided decompression periods is provided during the desorption process (see FIG. 3), the length of one decompression period is shortened (compare the lengths of the decompression periods To in FIGS. 3 and 6). Furthermore, the length of one non-decompression period during the non-decompression period of the desorption process is also shortened (compare the lengths of the non-decompression periods Tn in FIGS. 3 and 6). Therefore, shortening the length of one decompression period can prevent an excessive increase in the amount of carbon dioxide desorbed per unit time, and shortening the length of one non-decompression period can prevent an excessive decrease in the amount of carbon dioxide desorbed per unit time. As a result, in the carbon dioxide capture system 1 that desorbs carbon dioxide by decompression using the vacuum pump 86, the fluctuation range of the amount of carbon dioxide desorbed per unit time can be reduced. As a result, it is possible to narrow the fluctuation range of the H2 / CO2 ratio in the mixed gas discharged from the surge tank 88 (Figure 10), narrow the fluctuation range of the pressure inside the surge tank 88 (Figure 11), narrow the control range of the hydrogen flow rate added to adjust the H2 / CO2 ratio in the mixed gas (Figure 12), and improve the reduction rate of the amount of electricity required to operate the compressor 87 (Figure 13).

[0054] Furthermore, in the carbon dioxide capture system 1 of the above-described embodiment, the control unit 110 adjusts the length of each depressurization period To so that the fluctuation range of the H2 / CO2 ratio in the mixed gas falls within a first set range set for the H2 / CO2 ratio in the mixed gas. Therefore, the H2 / CO2 ratio in the mixed gas can be kept within the first set range (see first set range R1 in Figures 8(A) and 8(B)), and therefore a mixed gas with a ratio suitable for producing hydrocarbons can be sent to the hydrocarbon synthesis unit 90.

[0055] Furthermore, in the carbon dioxide capture system 1 of the embodiment described above, the length of each depressurization period To is adjusted so that the fluctuation range of the pressure inside the surge tank 88 falls within a second set range set for the pressure inside the surge tank 88. Therefore, the fluctuation range of the pressure inside the surge tank 88 can be kept within the second set range (see second set range R2 in FIGS. 9(A) and 9(B)), so that the carbon dioxide capture system 1 can be provided with a surge tank 88 of a size that is appropriate for that fluctuation range of pressure. In other words, the carbon dioxide capture system 1 can be provided with a surge tank 88 of a size that is neither too large nor too small for the pressure inside the surge tank 88 that falls within the second set range R2.

[0056] Furthermore, for example, if the fluctuation range of the pressure inside the surge tank 88 when one depressurization period is provided during the desorption process is 1.0, and the fluctuation range of the pressure inside the surge tank 88 when two or more depressurization periods are provided during the desorption process is 0.5, then if 1.0 is an acceptable fluctuation range, the size of the surge tank 88 provided in the carbon dioxide capture system 1 may be reduced. In this case, even if the fluctuation range in the surge tank 88 becomes 1.0 due to the reduction in size, this can help reduce the size of the entire carbon dioxide capture system 1, as long as 1.0 is an acceptable fluctuation range.

[0057] <Modification of this embodiment> The present invention is not limited to the above-described embodiment, and can be embodied in various forms without departing from the spirit of the invention. For example, the following modifications are also possible.

[0058] In the above-described embodiment, the control unit 110 adjusts the length of each depressurization period while taking into consideration both that the fluctuation range of the H2 / CO2 ratio in the mixed gas falls within the first set range and that the fluctuation range of the pressure in the surge tank 88 falls within the second set range, but this is not limiting. The control unit 110 may adjust the length of each depressurization period while taking into consideration only one of that the fluctuation range of the H2 / CO2 ratio in the mixed gas falls within the first set range and that the fluctuation range of the pressure in the surge tank 88 falls within the second set range.

[0059] In the above-described embodiment, in Fig. 6, which shows an example in which the number of decompression periods provided during the desorption process is two, the non-decompression period Tn arrives first after the desorption process starts, but this is not limited to this. When the number of decompression periods provided during the desorption process is two, the decompression period To may arrive first after the desorption process starts. Of course, the same applies when the number of decompression periods provided during the desorption process is three or more.

[0060] In the above-described embodiment, in FIG. 6 , which illustrates an example in which the number of decompression periods provided during the desorption process is two, the number of non-decompression periods Tn provided during the desorption process and the number of decompression periods To provided during the desorption process are the same, two, but this is not limited to this. When the number of decompression periods provided during the desorption process is two or more, the number of non-decompression periods Tn provided during the desorption process may be different from the number of decompression periods To provided during the desorption process. Of course, the same applies when the number of decompression periods provided during the desorption process is three or more.

[0061] This aspect has been described above based on embodiments and modifications. However, the above-described embodiments are intended to facilitate understanding of this aspect and are not intended to limit this aspect. This aspect may be modified or improved without departing from the spirit and scope of the claims, and equivalents thereof are included in this aspect. Furthermore, if a technical feature is not described as essential in this specification, it may be deleted as appropriate. [Explanation of symbols]

[0062] 1...Carbon dioxide capture system 10...First adsorption device 12...First adsorbent 20…Second adsorption device 22…Second adsorbent 30...Third adsorption device 32…Third adsorbent 50...Exhaust gas supply passage 51...First exhaust gas supply valve 53...Third exhaust gas supply valve 60...Exhaust gas discharge passage 61...First exhaust gas discharge valve 63...Third exhaust gas discharge valve 70...Hydrogen supply channel 71...3 Hydrogen supply valve 73...Third hydrogen supply valve 80...mixed gas delivery passage 81...First mixed gas delivery valve 83...Third mixed gas delivery valve 84...Open / close valve 85...Open / close valve 86...Vacuum pump 87...Compressor 88...Surge tank 89...Hydrogen addition channel 90...Hydrocarbon synthesis unit 100...heat medium flow path 101...First flow path 102...Second flow path 103...Third flow path 104...First flow path switching valve 105...Second flow path switching valve 106...Third flow path switching valve 107...Fourth flow path switching valve 108...Pump 109...Temperature adjustment section 110...Control unit

Claims

1. A carbon dioxide capture system that captures carbon dioxide from exhaust gas containing carbon dioxide, an adsorber containing an adsorbent capable of adsorbing carbon dioxide and into which the exhaust gas flows; a purge gas supply unit capable of supplying a purge gas to the interior; a delivery passage connected to the adsorber for delivering a mixed gas containing carbon dioxide and the purge gas delivered from the adsorber when the purge gas is supplied to the interior of the adsorber; a vacuum pump provided in the delivery flow path and capable of reducing the pressure inside the delivery flow path; a control unit that controls the vacuum pump, the control unit controls the vacuum pump so that a decompression period, a period during which the vacuum pump depressurizes the interior, is set in two or more parts during a desorption process in which the purge gas is supplied to the interior and carbon dioxide is desorbed from the adsorbent.

2. 2. The carbon dioxide capture system of claim 1, The control unit adjusts the length of each of the decompression periods so that a fluctuation range of the ratio of hydrogen to carbon dioxide in the mixed gas falls within a first set range set for the ratio.

3. The carbon dioxide recovery system according to claim 1 or 2, further comprising: a surge tank that is provided in the delivery flow path downstream of a position where the vacuum pump is provided and that is capable of storing the mixed gas; The control unit adjusts the length of each of the depressurization periods so that the fluctuation range of the pressure in the surge tank falls within a second set range set for the pressure.

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