JP7721286B2 - Carbon dioxide circulation system and adsorbent for gas drying - Google Patents

Description

The present invention relates to an adsorbent for gas drying, a gas dryer, and a carbon dioxide circulation system.

Gas drying adsorbents that dry gas by adsorbing moisture contained in the gas have been known for some time. For example, Patent Document 1 discloses a methane production apparatus that includes a gas drying adsorbent that dries exhaust gas from methane ( _CH4 ) combustion, and that produces methane using carbon dioxide in the exhaust gas from which water vapor has been removed by the gas drying adsorbent.

Japanese Patent Application Laid-Open No. 2019-142806

In the methane production apparatus of Patent Document 1, a carbon dioxide recovery vessel filled with zeolite 13X is used to recover carbon dioxide from flue gas after water vapor has been removed. Because zeolite 13X adsorbs water vapor, the flue gas must be sufficiently dried. Therefore, in the methane production apparatus of Patent Document 1, a gas dryer filled with NaY-type zeolite, which contains sodium ions as cations, is installed upstream of the carbon dioxide recovery vessel. However, because NaY-type zeolite adsorbs a certain amount of carbon dioxide in the flue gas along with water vapor, drying the flue gas reduces the carbon dioxide content of the flue gas. A reduction in the carbon dioxide content in the flue gas also reduces the efficiency of methane production in the methane production apparatus.

The present invention was made to solve the above-mentioned problems, and aims to provide a technology that prevents a decrease in the carbon dioxide content of gas in a gas drying adsorbent that dries gas containing carbon dioxide.

The present invention has been made to solve at least some of the above-mentioned problems, and can be realized in the following forms.

(1) According to one aspect of the present invention, there is provided an adsorbent for drying gas that dries the gas by adsorbing moisture contained in the gas. This adsorbent for drying gas includes zeolite containing magnesium ions as cations.

According to this configuration, the gas drying adsorbent comprises zeolite containing magnesium ions as cations. Compared to zeolite containing sodium ions as cations, zeolite containing magnesium ions has almost the same moisture absorption capacity, but a lower carbon dioxide absorption capacity. This suppresses carbon dioxide adsorption when drying gas containing carbon dioxide, thereby preventing a decrease in the carbon dioxide content of the gas being dried.

(2) In the gas drying adsorbent of the above embodiment, the molar ratio of magnesium ions to cations contained in the zeolite may be 0.8 or greater. According to this configuration, the molar ratio of magnesium ions to cations contained in the zeolite is 0.8 or greater. By setting the ratio of magnesium ions to cations to 0.8 or greater, the carbon dioxide absorption performance of the gas drying adsorbent can be further reduced. Therefore, when drying a gas containing carbon dioxide, a decrease in the carbon dioxide content of the gas to be dried can be further suppressed.

(3) In the gas drying adsorbent of the above embodiment, the zeolite may contain a composite oxide of aluminum and silicon, and the composition ratio of silicon to aluminum in the composite oxide may be 2 or more and 5 or less. According to this configuration, the zeolite contains a composite oxide of aluminum and silicon, and the composition ratio of silicon to aluminum in this composite oxide is 2 or more and 5 or less. This increases the ion exchange capacity of the zeolite, allowing the zeolite to contain more magnesium ions. Therefore, when drying a gas containing carbon dioxide, adsorption of carbon dioxide is further suppressed, further suppressing a decrease in the carbon dioxide content of the gas being dried.

(4) In the gas drying adsorbent of the above embodiment, the zeolite may be Y-type zeolite, and the silicon to aluminum composition ratio in the composite oxide may be 2.75. According to this configuration, the zeolite is Y-type zeolite with pores large enough to allow magnesium ions to pass through, making the zeolite more likely to contain magnesium ions as cations. This further suppresses carbon dioxide adsorption when drying a gas containing carbon dioxide, thereby further preventing a decrease in the carbon dioxide content of the gas being dried.

(5) In the gas drying adsorbent of the above embodiment, the zeolite may be a Y-type zeolite having a composition of Mg _y Na _(6.4-2y) Al _6.4 Si _17.6 O ₄₈ , where y is greater than 2.23. According to this configuration, the Y-type zeolite, which has a silicon to aluminum composition ratio of 2.75, has pores large enough to allow magnesium ions to pass through and contains more magnesium ions as cations than sodium ions. This allows the carbon dioxide absorption capacity to be further reduced without substantially changing the moisture absorption capacity compared to zeolites containing sodium ions as cations. Therefore, when drying a gas containing carbon dioxide, a decrease in the carbon dioxide content of the gas to be dried can be suppressed.

(6) According to another aspect of the present invention, a gas dryer is provided. This gas dryer includes the above-described gas drying adsorbent and a container that contains the gas drying adsorbent. According to this configuration, the gas dryer includes the above-described gas drying adsorbent. As a result, the gas dried by this gas dryer contains a relatively large amount of carbon dioxide, allowing for efficient use of carbon dioxide in a system that utilizes carbon dioxide.

(7) According to yet another aspect of the present invention, a carbon dioxide circulation system is provided. This carbon dioxide circulation system includes the above-mentioned gas dryer, which dries a mixed gas containing carbon dioxide and moisture supplied from an external gas generation unit by adsorbing moisture in the mixed gas onto the gas drying adsorbent; a carbon dioxide capture unit that captures carbon dioxide from the mixed gas dried in the gas dryer; and a hydrocarbon production unit that generates hydrocarbon compounds using the carbon dioxide captured in the carbon dioxide capture unit and supplies the hydrocarbon compounds to the external gas generation unit. According to this configuration, in the carbon dioxide circulation system, the mixed gas supplied from the gas generation unit is dried in the gas dryer. At this time, the above-mentioned gas drying adsorbent provided in the gas dryer adsorbs moisture but does not adsorb much carbon dioxide, so the mixed gas sent from the gas dryer to the carbon dioxide capture unit contains a relatively large amount of carbon dioxide. The carbon dioxide captured from the mixed gas sent to the carbon dioxide capture unit serves as a raw material for hydrocarbon compounds produced in the hydrocarbon production unit. The produced hydrocarbon compounds are supplied to the external gas generation unit. In the gas generation unit, for example, thermal energy is extracted by combustion of hydrocarbon compounds, and a mixed gas containing carbon dioxide and moisture is generated. This generated mixed gas is then dried again in a gas dryer, and carbon dioxide is then captured in a carbon dioxide capture unit as a raw material for hydrocarbon compounds. In this way, the carbon dioxide circulation system configured as described above can extract energy such as combustion heat while circulating carbon between the gas generation unit and the carbon dioxide circulation system. In this carbon dioxide circulation system, a regeneration process is performed in the gas dryer, in which moisture adsorbed on the gas drying adsorbent is desorbed from the gas drying adsorbent by heating or other means and discharged outside the system. The above-mentioned gas drying adsorbent has a lower ability to adsorb carbon dioxide than, for example, zeolite containing sodium ions as cations. Therefore, less carbon dioxide is discharged outside the system together with moisture during the regeneration process. This allows for improved carbon circulation throughout the system when carbon is circulated between the gas generation unit and the carbon dioxide circulation system.

The present invention can be realized in various forms, such as a control method for a gas dryer and a carbon dioxide circulation system, a computer program for performing moisture adsorption and recovery in a gas dryer and a carbon dioxide circulation system, a server device for distributing the computer program, and a non-transitory storage medium on which the computer program is stored.

1 is a schematic diagram showing a schematic configuration of a carbon dioxide circulation system according to a first embodiment. FIG. 2 is a conceptual diagram illustrating the distribution of adsorbed H ₂ O in a gas dryer. FIG. 1 is a diagram illustrating the _CO2 loss rate in a gas dryer. FIG. 1 is a diagram showing the relationship between the proportion of unused adsorbent and the amount of CO ₂ loss.

First Embodiment
FIG. 1 is an explanatory diagram showing a schematic configuration of a carbon dioxide circulation system 1 according to a first embodiment. The carbon dioxide circulation system 1 includes a gas dryer 10, a carbon dioxide separation unit 20, a mixed gas supply flow path 30, a hydrogen supply source 41, a hydrogen supply flow path 42, a reactor 50, a raw material gas flow path 60, a reactant gas flow path 70, a heat transfer medium flow path 80, and a control unit 90. The carbon dioxide circulation system 1 is used in conjunction with a mixed gas supply source 5 that supplies a mixed gas containing carbon dioxide (CO ₂ ). The mixed gas supply source 5 is, for example, a combustion furnace in a factory, and the mixed gas contains, in addition to CO ₂ , oxygen (O ₂ ), nitrogen (N ₂ ), moisture (H ₂ O), and the like. The moisture contained in the mixed gas includes gaseous moisture (water vapor) and liquid moisture (e.g., liquid droplets). The carbon dioxide circulation system 1 is a system for circulating carbon by using _CO2 contained in a mixed gas supplied from a mixed gas supply source 5 to generate methane ( _CH4 ) which the mixed gas supply source 5 uses for combustion. Note that in this embodiment, the carbon dioxide circulation system 1 generates _CH4 , but it is also possible to generate hydrocarbon compounds other than _CH4 . _The carbon dioxide circulation system 1 is also applicable to the generation of hydrocarbon compounds composed of carbon and hydrogen, such as ethane ( _C2H6 ) and propane ( _C3H8 ), and hydrocarbon compounds composed mainly of _carbon and hydrogen, such as methanol ( _CH3OH ). The mixed gas supply source 5 corresponds to the "gas generating section" in the claims.

The gas dryer 10 is an apparatus for separating _H2O from a mixed gas and includes a container 12 containing a gas drying adsorbent 11. The gas drying adsorbent 11 is a material capable of absorbing _H2O , and in this embodiment, Y-type zeolite containing magnesium ions as cations is used. The gas drying adsorbent 11 can desorb and regenerate the adsorbed H2O by utilizing the heat of a heat medium flowing through a heat medium flow path 80 (described later) or the heat of a heater (not shown). The _H2O desorbed from the gas drying adsorbent ₁₁ is released to the outside of the system via a discharge valve 14 from a discharge flow path 13 by a purge gas supplied by a purge gas supply unit (not shown). Features of the gas drying adsorbent 11 will be described later.

The carbon dioxide separation unit 20 includes a first _CO2 separator 21 and a second _CO2 separator 22. The carbon dioxide separation unit 20 is a device for separating and capturing _CO2 from the mixed gas. The carbon dioxide separation unit 20 corresponds to the "carbon dioxide capture unit" in the claims.

The first _CO2 separator 21 contains a _CO2 adsorbent 21a, such as zeolite, activated carbon, or silica gel, that has _CO2 storage capacity. A hydrogen injection unit 21b is provided inside the first _CO2 separator 21, which injects hydrogen supplied from the hydrogen supply passage 42 into the first _CO2 separator 21. The first _CO2 separator 21 is connected to a mixed gas supply passage 30, a raw material gas passage 60, and a first exhaust passage 21c. The _CO2 contained in the mixed gas supplied from the mixed gas supply passage 30 is absorbed in the _CO2 adsorbent 21a, and the remaining components in the mixed gas are released from the first exhaust passage 21c to the outside of the system via a first exhaust valve 21d. The _CO2 desorbed from the _CO2 adsorbent 21a is purged by _H2 injected from the hydrogen injection unit 21b and is sent to the raw material gas passage 60 together with _H2 .

The second _CO2 separator 22 has the same shape and capacity as the first _CO2 separator 21 and contains a _CO2 adsorbent 22a. Like _{the CO2} adsorbent 21a, the CO2 adsorbent 22a is a material capable of absorbing _CO2 and has absorption performance comparable to that of the _CO2 adsorbent 21a. The second CO2 separator ₂₂ is provided with a hydrogen injection unit 22b that injects hydrogen supplied from the hydrogen supply passage 42 into the second _CO2 separator 22. The second CO2 separator ₂₂ is connected to a mixed gas supply passage 30, a raw material gas passage 60, and a second exhaust passage 22c. The _CO2 contained in the mixed gas supplied from the mixed gas supply passage 30 is absorbed in the _CO2 adsorbent 22a, and the remaining components in the mixed gas are released to the outside of the system from the second exhaust passage 22c via a second exhaust valve 22d. The CO ₂ desorbed from the CO ₂ adsorbent 22 a is purged by H ₂ injected from the hydrogen injection part 22 b and is sent to the raw material gas flow path 60 together with H ₂ .

The mixed gas supply flow path 30 is a gas flow path for supplying the mixed gas supplied from the mixed gas supply source 5 to the carbon dioxide separation unit 20 via the gas dryer 10, and is configured to include a plurality of gas pipes. The mixed gas supply flow path 30 is provided with a first mixed gas supply valve 31 and a second mixed gas supply valve 32. The mixed gas supplied from the mixed gas supply source 5 is dried in the gas dryer 10 and then supplied to the first CO2 separator 21 via the first _mixed gas supply valve 31. The mixed gas supplied from the mixed gas supply source 5 is supplied to the second _CO2 separator 22 via the second mixed gas supply valve 32. The opening and closing of the first mixed gas supply valve 31 and the second mixed gas supply valve 32 are each controlled by a control unit 90. The mixed gas supply flow path 30 is provided with a temperature sensor, a flow _rate sensor, and a _CO2 concentration sensor (not shown) for measuring the temperature, flow rate, and _CO2 concentration of the mixed gas supplied to the first CO2 separator 21 and the second _CO2 separator 22.

The hydrogen supply source 41 is, for example, a water electrolysis device or a hydrogen tank, and will be described as a water electrolysis device here. The hydrogen supply source 41 generates H ₂ to be supplied to the first CO ₂ separator 21 and the second CO ₂ separator 22.

The hydrogen supply flow path 42 is a gas flow path for supplying H ₂ generated in the hydrogen supply source 41 to the first CO ₂ separator 21, the second CO ₂ separator 22, and the raw material gas flow path 60, and is configured to include a plurality of gas pipes. The hydrogen supply flow path 42 is provided with a first hydrogen supply valve 43, a second hydrogen supply valve 44, and a third hydrogen supply valve 45. When the first hydrogen supply valve 43 is opened, the H ₂ generated in the hydrogen supply source 41 is injected into the inside of the first CO ₂ separator 21. When the second hydrogen supply valve 44 is opened, the H ₂ generated in the hydrogen supply source 41 is injected into the inside of the second CO ₂ separator 22. When the third hydrogen supply valve 45 is opened, H ₂ is added from a hydrogen addition unit 46 to the raw material gas flowing through the raw material gas flow path 60. The first hydrogen supply valve 43, the second hydrogen supply valve 44, and the third hydrogen supply valve 45 are each controlled to open and close by the control unit 90. The hydrogen supply flow path 42 is provided with a temperature sensor and a flow rate sensor (not shown) for measuring the temperature and flow rate of the _H gas supplied to the first _CO2 separator 21, the second _CO2 separator 22, and the raw material gas flow path 60.

The reactor 50 is a container for producing _CH4 by a methanation reaction therein, and contains a catalyst 51. The catalyst 51 contains a metal having methanation performance, such as Ru or Ni. A raw material gas passage 60 and a reaction gas passage 70 are connected to the reactor 50. The reactor 50 produces _CH4 by a methanation reaction using a raw material gas containing _CO2 and _H2 supplied through the raw material gas passage 60. The reactor 50 corresponds to the "hydrocarbon production section" in the claims.

The raw material gas flow path 60 is a gas flow path for supplying the raw material gas containing H ₂ and CO ₂ sent out from the first CO ₂ separator 21 and the second CO ₂ separator 22 to the reactor 50, and is configured to include a plurality of gas pipes. The raw material gas flow path 60 is provided with a first raw material gas valve 61 and a second raw material gas valve 62. The first raw material gas valve 61 and the second raw material gas valve 62 are each controlled to open and close by the control unit 90. Specifically, when the raw material gas inside the first CO ₂ separator 21 is to be supplied to the reactor 50, the first raw material gas valve 61 is controlled to be in an open state, and the second raw material gas valve 62 is controlled to be in a closed state. When the raw material gas inside the second CO ₂ separator 22 is to be supplied to the reactor 50, the first raw material gas valve 61 is controlled to be in a closed state, and the second raw material gas valve 62 is controlled to be in an open state. The raw material gas flow passage 60 is provided with a temperature sensor, a flow rate sensor, and a CO ₂ concentration sensor (not shown) for measuring the temperature, flow rate, and CO ₂ concentration of the flowing raw material gas.

The reaction gas flow path 70 is a gas flow path for supplying _CH4 produced in the reactor 50 to the mixed gas supply source 5, and is configured to include a plurality of gas pipes. A heat exchanger 71 is provided in the reaction gas flow path 70. The reaction mixed gas containing _CH4 discharged from the reactor 50 is first supplied to the heat exchanger 71. In the heat exchanger 71, _H2O is separated from the reaction mixed gas. The _H2O separated from the reaction mixed gas is discharged to the outside of the system via an _H2O discharge flow path 72. The reaction gas containing _CH4 from which _H2O has been separated is supplied to the mixed gas supply source 5 via the reaction gas flow path 70.

The heat medium flow path 80 is a flow path through which a heat medium (thermal fluid) such as oil flows, and supplies heat generated in the reactor 50 by the methanation reaction to the carbon dioxide separation unit 20. The heat medium flow path 80 is provided with a first flow path switching valve 81 and a second flow path switching valve 82. The heat medium flow path 80 includes three flow paths (a first heat medium flow path 83, a second heat medium flow path 84, and a third heat medium flow path 85) separated by these two valves. The first heat medium flow path 83 supplies the heat medium to the first _CO2 separator 21. The second heat medium flow path 84 supplies the heat medium to the second _CO2 separator 22. The third heat medium flow path 85 is provided with a pump 86 and a temperature adjustment unit 87. The heat medium in the third heat medium flow path 85 flows through the first heat medium flow path 83 or the second heat medium flow path 84 by driving the pump 86, and then returns to the third heat medium flow path 85 to be circulated. The temperature adjusting unit 87 is a device for adjusting the temperature of the heat medium, and when the temperature of the heat medium heated in the reactor 50 or the heat exchange unit 71 is higher than the set temperature, it adjusts the temperature by adding a heat medium at room temperature. Also, when the temperature of the heat medium is lower than the set temperature, in addition to adjusting the flow rate, it heats the heat medium to the set temperature using a heater or the like. The pump 86 and the temperature adjusting unit 87 are controlled by the control unit 90.

The first flow path switching valve 81 is a three-way valve for switching the destination (first heat medium flow path 83 or second heat medium flow path 84) of the heat medium sent out from the third heat medium flow path 85 by the pump 86. The second flow path switching valve 82 is linked to the switching of the first flow path switching valve 81 and is a three-way valve for switching the source (first heat medium flow path 83 or second heat medium flow path 84) of the heat medium returning to the third heat medium flow path 85. The switching of the first flow path switching valve 81 and the second flow path switching valve 82 is each controlled by the control unit 90.

The first heat medium flow path 83 includes a flow path that passes through the inside of the first _CO2 separator 21, and is configured to be able to supply heat from the heat medium to the _CO2 adsorbent 21a. The first _CO2 separator 21 is configured with a double pipe, and a heat medium flow path is formed between the outer pipe and the inner pipe.

The second heat medium flow path 84 includes a flow path that passes through the inside of the second _CO2 separator 22, and is configured to be able to supply heat from the heat medium to the CO2 adsorbent 22a. The second CO2 separator ₂₂ is configured with a double pipe, and a heat medium flow path is formed between the outer pipe and the inner pipe.

The third heat transfer medium flow path 85 includes a flow path that passes through the interior of the reactor 50 and is configured to transfer heat generated by the methanation reaction inside the reactor 50 to the heat transfer medium. Here, the reactor 50 is configured with a double pipe, with the catalyst 51 disposed in the inner pipe and a heat transfer medium flow path formed between the outer pipe and the inner pipe.

The control unit 90 is a computer comprising ROM, RAM, and a CPU, and performs overall control of the carbon dioxide circulation system 1. The control unit 90 is electrically connected to the valves and sensors (temperature sensor, flow rate sensor, concentration sensor, etc.) provided in each of the above-mentioned flow paths, as well as the pump 86 and temperature adjustment unit 87, and controls the various valves, pump 86, temperature adjustment unit 87, etc. based on the measured values of the sensors, etc.

In the carbon dioxide circulation system 1, the mixed gas supplied from the mixed gas supply source 5 is dried in the gas dryer 10 by adsorption and removal of _H2O . The dried mixed gas is sent to the carbon dioxide separation unit 20. In the carbon dioxide separation unit 20, the dried mixed gas is supplied to either the first _CO2 separator 21 or the second _CO2 separator 22. The _CO2 separator supplied with the dried mixed gas adsorbs and recovers _CO2 in the mixed gas. At this time, in the other of the first _CO2 separator 21 or the second _CO2 separator 22, _H2 is supplied by the hydrogen supply source 41, so that the _CO2 adsorbed in the _CO2 adsorbent is desorbed from the _CO2 adsorbent, and the raw material gas containing _CO2 and _H2 is sent to the reactor 50. In this way, in the carbon dioxide separation unit 20, while either the first _CO2 separator 21 or the _second CO2 separator ₂₂ is adsorbing _CO2 from the mixed gas, the other of the first CO2 separator 21 or the second _CO2 separator 22 supplies the raw material gas to the reactor 50. This allows the raw material gas containing _CO2 and _H2 to be supplied to the reactor 50 without interruption.

In the reactor 50, _CH4 is produced using the raw material gas supplied from the carbon dioxide separation unit 20. The produced _CH4 is discharged from the reactor 50 as a reaction mixture gas together with _H2O , a by-product. _H2O contained in the reaction mixture gas is removed from the reaction mixture gas in the heat exchange unit 71. The reaction gas from which _H2O has been removed is supplied to the mixed gas supply source 5 via the reaction gas flow path 70. In this embodiment, the mixed gas supply source 5 generates thermal energy by burning _CH4 , and the generated thermal energy is used to generate electricity and drive various devices. The mixed gas containing _CO2 generated by the combustion of _CH4 in the mixed gas supply source 5 is supplied to the gas dryer 10. As a result, by using the mixed gas supply source 5 in combination with the carbon dioxide circulation system 1, thermal energy can be obtained in a circulated state without discharging carbon outside the system.

In the carbon dioxide circulation system 1, the gas drying adsorbent 11 that has adsorbed _H2O is heated in the gas dryer 10, thereby desorbing the adsorbed _H2O and regenerating the gas drying adsorbent 11. When the gas drying adsorbent 11 is regenerated, gas components other than _H2O adsorbed in the gas drying adsorbent 11 are also desorbed from the gas drying adsorbent 11. The gas components including H2O desorbed from the gas drying adsorbent 11 are discharged outside the system by a purge gas. Note that the method for regenerating the _gas drying adsorbent 11 that has adsorbed _H2O is not limited to the above-mentioned method, and may be performed using only a purge gas.

Next, the gas drying adsorbent 11 will be described. The gas drying adsorbent 11 is a zeolite containing magnesium ions as cations. Specifically, the gas drying adsorbent 11 contains Y-type zeolite having a composition of _{Mg2.8Na0.8Al6.4Si17.6O48} . That is _, the gas drying adsorbent 11 of this embodiment contains a composite oxide of aluminum and silicon, _and the composition ratio _of silicon to aluminum in this composite oxide is 2.75. Furthermore _, the molar ratio of magnesium ions to cations contained in the Y-type zeolite is 0.78.

The ratio of magnesium ions to cations contained in the gas drying adsorbent 11 is measured by inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, a certain amount of gas drying adsorbent 11 is dissolved in a solution, and the amount of cations and magnesium contained in the gas drying adsorbent 11 are measured. Next, the ratio of the amount of magnesium to the measured amount of cations is calculated, and the molar ratio of magnesium ions to cations contained in the Y-type zeolite is calculated. The ratio of magnesium ions to cations contained in the gas drying adsorbent 11 may also be measured by energy dispersive X-ray spectroscopy (EDX). Specifically, the gas drying adsorbent 11 is polished to form a sample with an exposed cross section. The cross section of this sample is observed using EDX, and the total number of observed cation atoms and the total number of magnesium atoms within a specified range are counted. The ratio of the number of magnesium atoms to the total number of counted cation atoms is taken as the molar ratio of magnesium ions to cations contained in the Y-type zeolite. Additionally, the proportion of magnesium ions among the cations contained in the gas drying adsorbent 11 may be measured using atomic absorption spectrometry (AAS).

The silicon to aluminum composition ratio in the composite oxide contained in the Y-type zeolite is measured using ICP-AES or AAS. The type of zeolite contained in the gas drying adsorbent 11 is identified based on the results of measurements using X-ray diffraction (XRD).

Next, we will explain the manufacturing method of the gas drying adsorbent 11. First, 10 g of zeolite pellets (NaY-type zeolite, Tosoh HSZ-320NAD1C, Si/Al ratio = 2.75, composition Na: 6.4, Al: 6.4, Si: 17.6, O: 48) were immersed in 50 mL of 2 mol/L magnesium nitrate solution and allowed to stand overnight at 60°C with occasional stirring. After standing overnight, the sample was cooled to room temperature, filtered, and washed three times with 500 mL of ion-exchanged water. The washed sample was again immersed in 50 mL of fresh 2 mol/L magnesium nitrate solution and allowed to stand overnight at 60°C, followed by similar washing. The immersion, standing, and washing process was repeated once more, followed by drying, to obtain the zeolite for the gas drying adsorbent 11. This manufacturing method exchanges the zeolite cations from sodium ions to magnesium ions, enabling the cation exchange rate, based on the number of ion exchange sites in the zeolite, to be 80% or more.

Next, the characteristics of the gas drying adsorbent 11 of this embodiment will be explained using the performance of a gas dryer 10 equipped with the gas drying adsorbent 11. Here, the performance of the gas dryer 10 of this embodiment will be explained while comparing it with the performance of a gas dryer equipped with a comparative gas drying adsorbent. Here, the comparative gas drying adsorbent is a Y-type zeolite containing sodium ions as cations, and is zeolite prepared before immersion in the magnesium nitrate solution in the above-mentioned manufacturing method. Hereinafter, the gas drying adsorbent 11 of this embodiment will be referred to as "MgY-type zeolite," and the comparative gas drying adsorbent will be referred to as "NaY-type zeolite."

Prior to evaluating the performance of the gas dryer, adsorption isotherms for _CO2 and _H2O were prepared for each of MgY-type zeolite and NaY-type zeolite. _{The CO2} adsorption isotherm was prepared by vacuum pretreating 1 g of gas drying adsorbent at 150°C and then measuring the amount of _CO2 adsorbed at 30°C using a volumetric fully automatic adsorption measuring device (BELSORP-MAX, manufactured by Microtrack Bell). The _H2O adsorption isotherm was prepared by vacuum pretreating 0.1 g of gas drying adsorbent at 150°C and then measuring the amount of _H2O adsorbed at 25°C using the volumetric fully automatic adsorption measuring device, as in the case of _CO2 .

In the performance evaluation test of the gas dryer, in an experiment to dry a mixed gas containing CO ₂ and H ₂ O, the conditions for the mixed gas and the gas dryer were set as follows.
Mixed gas flow rate: 6 m ³ /h
Dew point: 25°C (H ₂ O concentration: 3%)
_CO2 concentration: 10%
This results in a weight of _H2O flowing into the gas dryer of 0.145 kg _H2O per hour, and a weight of _CO2 flowing into the gas dryer of 1.18 kg _CO2 per hour.
Gas dryer Capacity: 0.26 ^m3
Packing weight of gas drying adsorbent: 1.82 kg _ads (bulk density: 0.7 kg/L)

In the gas dryer, the gas drying adsorbent is brought into contact with the mixed gas for 2.5 hours (adsorption step), and then the gas drying adsorbent is regenerated (regeneration step). In this case, to adsorb all of the _H2O flowing into the gas dryer, 1 kg _ads of gas drying adsorbent must adsorb 0.08 kg of _H2O per hour. Therefore, the amount of _H2O adsorption required for the adsorbent in one adsorption step is 0.2 kg _H2O /kg _ads .

Based on the _H2O adsorption isotherms of MgY-type zeolite and NaY-type zeolite, the saturated _H2O adsorption capacity of both MgY-type zeolite and NaY-type zeolite was 0.3 kg _H2O /kg _ads . Therefore, there is no difference in _H2O storage capacity, i.e., the mixed gas drying capacity, between MgY-type zeolite and NaY-type zeolite. This indicates that the drying performance of the gas dryer does not change even if MgY-type zeolite is used instead of NaY-type zeolite. In the gas dryer performance evaluation test described above, both MgY-type zeolite and NaY-type zeolite have sufficient capacity to adsorb _H2O , so the regeneration cycle (2.5 hours) can be extended (e.g., to 3.5 hours). However, to maintain system redundancy, the regeneration timing of the gas drying adsorbent was maintained at 2.5 hours.

FIG. 2 is a conceptual diagram illustrating the distribution of adsorbed H ₂ O within a gas dryer. Under the conditions described above, approximately 30% of the adsorbent packed in the gas dryer is not used to adsorb H ₂ O. As shown in FIG. 2, when the mixed gas flows from one side of the gas dryer to the other (the direction indicated by the white arrow in FIG. 2), the H ₂ O contained in the mixed gas is adsorbed by the gas drying adsorbent packed upstream of the mixed gas in the gas dryer. Therefore, the gas drying adsorbent downstream of the mixed gas in the gas dryer is not used to adsorb H ₂ O. Since the mixed gas from which H ₂ O has been removed passes through this unused adsorbent (the "H ₂ O unadsorbed portion" in FIG. 2), CO ₂ contained in the mixed gas is adsorbed. That is, in this evaluation test, CO ₂ is adsorbed by 0.61 kg of the gas drying adsorbent in one adsorption process.

FIG. 3 is a diagram illustrating the _CO2 loss rate in the gas dryer. As described above, when the _CO2 concentration in the mixed gas is 10%, the total amount of _CO2 flowing into the gas dryer in one adsorption step is 2.95 kg. From the _CO2 adsorption isotherms of MgY-type zeolite and NaY-type zeolite, the " _CO2 adsorption amount per unit weight" at a _CO2 partial pressure of 10 kPa is 0.031 kg for MgY-type zeolite and 0.059 kg for NaY-type zeolite. Therefore, the " _CO2 adsorption amount in the unused adsorbent" is 0.019 kg for MgY-type zeolite and 0.036 kg for NaY-type zeolite. The _CO2 adsorbed by this "unused adsorbent" is the amount of _CO2 lost and discharged outside the system during the regeneration step. The ratio of _CO2 released outside the system in the regeneration step to the total amount of _CO2 flowing into the gas dryer in one adsorption step (" _CO2 loss rate") is smaller for MgY-type zeolite (0.65%) than for NaY-type zeolite (1.21%), so MgY-type zeolite can reduce the amount of _CO2 released outside the system. In other words, MgY-type zeolite can suppress a decrease in the carbon dioxide content of the mixed gas more effectively than NaY-type zeolite.

In addition, the gas dryer provided in the carbon dioxide circulation system is repeatedly regenerated by operating the carbon dioxide circulation system for a long period of time. Therefore, the _CO2 discharged outside the system during the regeneration process cannot be used to produce hydrocarbon compounds in the carbon dioxide circulation system. As described above, MgY-type zeolite can suppress a decrease in the carbon dioxide content of the mixed gas, and therefore can supply more carbon dioxide to the carbon dioxide separation section in the subsequent stage than NaY-type zeolite. This can improve the carbon circulation rate throughout the carbon dioxide circulation system.

Figure 4 shows the relationship between the proportion of unused adsorbent and the amount of _CO2 loss. As mentioned above, the amount of _CO2 discharged outside the system during the regeneration process of the gas drying adsorbent, i.e., the amount of _CO2 loss, is determined by the proportion of unused adsorbent in the gas dryer (the "unadsorbed _H2O portion" in Figure 2). Therefore, the relationship between the proportion of unused adsorbent in the gas dryer (proportion relative to the total weight of the gas drying adsorbent) and the _CO2 loss rate was compared for MgY-type zeolite and NaY-type zeolite. The results are shown in Figure 4.

The horizontal axis of Figure 4 shows the proportion of unused adsorbent in the gas dryer. Specifically, the proportion of unused adsorbent is expressed as the ratio between the total weight of _H2O flowing into the gas dryer and the weight of _H2O that can be adsorbed by the adsorbent packed in the gas dryer. The total weight of _H2O flowing into the gas dryer can be expressed as α x β (kg), where α (kg/hour) is the weight of _H2O flowing into the gas dryer per unit time and β (hours) is the time for one cycle. Furthermore, the weight of _H2O that can be adsorbed by the gas drying adsorbent packed in the gas dryer can be expressed as γ x Wsat (kg), where γ (kg) is the weight of the gas drying adsorbent packed in the gas dryer and Wsat (kg/kg) is the weight of _H2O that can be adsorbed by the gas drying adsorbent per unit weight. The ratio α×β to γ×Wsat, i.e., (α×β)/(γ×Wsat), is the ratio of the weight of the gas drying adsorbent used when all of the _H2O flowing into the gas dryer is adsorbed by the gas drying adsorbent to the total weight of the gas drying adsorbent packed in the gas dryer. Therefore, {1-(α×β)/(γ×Wsat)} is the proportion of the gas drying adsorbent packed in the gas dryer that is not used to adsorb _H2O . In other words, the larger {1-(α×β)/(γ×Wsat)}, the greater the proportion of unused adsorbent packed in the gas dryer. Note that, for a gas dryer in actual use, α×β/γ<Wsat must be satisfied. If α×β/γ>Wsat, there will be no gas drying adsorbent unused to adsorb _H2O . The vertical axis of Figure 4 represents the " _CO2 loss rate" shown in Figure 3.

As shown in Figure 4, comparing MgY-type zeolite (solid line L1 in Figure 4) and NaY-type zeolite (dashed line L2 in Figure 4), it was found that the difference in _CO2 loss rate increases as the proportion of unused adsorbent (horizontal axis) increases. Here, when the proportion of unused adsorbent is less than 0.3, the difference in _CO2 loss rate between MgY-type zeolite and NaY-type zeolite decreases, but the redundancy of the _H2O storage performance of the gas dryer decreases, and _H2O may leak from the gas dryer due to sudden fluctuations in gas flow rate, etc. Furthermore, when the proportion of unused adsorbent is greater than 0.6, the _CO2 loss rate of MgY-type zeolite is less than half that of NaY-type zeolite, so the _{CO2 loss} rate reduction effect is significant. However, as the proportion of unused adsorbent increases, the utilization rate of the gas drying adsorbent when adsorbing _H2O from the mixed gas decreases. Furthermore, if the proportion of unused adsorbent is greater than 0.6, excess gas drying adsorbent will be present, which will lead to an increase in the energy required to desorb H ₂ O by thermal regeneration (such as sensible heat loss). Therefore, the range of the proportion of unused adsorbent is preferably 0.3≦{1−(α×β)/(γ×Wsat)}≦0.6 (the range indicated by the double-ended arrow A1 in Figure 4).

As described above, the gas drying adsorbent 11 of this embodiment includes zeolite containing magnesium ions as cations. Zeolite containing magnesium ions has almost the same _H2O storage performance as zeolite containing sodium ions as cations, but has a lower _CO2 storage performance. This suppresses _CO2 adsorption when drying a mixed gas containing _CO2 , thereby suppressing a decrease in the carbon dioxide content of the mixed gas discharged from the gas dryer 10.

Furthermore, the gas drying adsorbent 11 of this embodiment contains Y-type zeolite having a composition _{of Mg2.8Na0.8Al6.4Si17.6O48} . That is _, the aluminum-to _- silicon composition ratio in the aluminum-silicon composite oxide is _2.75 . This increases the ion exchange capacity of the zeolite, and the zeolite has pores large enough to allow magnesium ions to pass through, allowing the zeolite to contain a larger amount of magnesium ions. Therefore, compared to zeolites containing sodium ions as cations, the gas drying adsorbent 11 can reduce the _CO2 storage capacity without significantly changing the _H2O storage capacity. This prevents a decrease in the carbon dioxide content of a _CO2 -containing mixed gas when drying the mixed gas.

Furthermore, according to the gas drying adsorbent 11 of this embodiment, the aluminum to silicon composition ratio in the aluminum and silicon composite oxide is 2 to 5, with a relatively high aluminum content. This improves the hydrophilicity of the zeolite, making it easier to adsorb moisture. This therefore improves the dryness of the mixed gas discharged from the gas dryer 10.

Furthermore, according to the carbon dioxide circulation system 1 of this embodiment, the mixed gas discharged from the mixed gas supply source 5 is dried in the gas dryer 10. At this time, the gas drying adsorbent 11 provided in the gas dryer 10 adsorbs _H2O but does not adsorb much _CO2 , so the mixed gas sent from the gas dryer 10 to the carbon dioxide separation unit 20 contains a relatively large amount of _CO2 . The _CO2 recovered from the mixed gas sent to the carbon dioxide separation unit 20 becomes a raw material for _CH4 generated in the reactor 50. This generated _CH4 is supplied to the mixed gas supply source 5. In the mixed gas supply source 5, thermal energy is extracted by combustion of _CH4 , and a mixed gas containing _CO2 and _H2O is generated. This generated mixed gas is dried again in the gas dryer 10, and then _CO2 is recovered as a raw material for _CH4 in the carbon dioxide separation unit 20. In this way, the carbon dioxide circulation system 1 can extract energy such as combustion heat while circulating carbon between the mixed gas supply source 5 and the carbon dioxide circulation system 1. In the carbon dioxide circulation system 1, when the gas drying adsorbent 11 adsorbs _H2O in the gas dryer 10, a regeneration process is carried out in which the _H2O is desorbed from the gas drying adsorbent 11 by heating or the like and discharged outside the system. The gas drying adsorbent 11 adsorbs _CO2 less easily than, for example, zeolite containing sodium ions as cations, and therefore less _CO2 is discharged outside the system together with _H2O in the regeneration process. This makes it possible to improve the carbon circulation rate throughout the system when carbon is circulated between the mixed gas supply source 5 and the carbon dioxide circulation system 1.

<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.

[Modification 1]
In the above-described embodiment, the gas drying adsorbent 11 is used in the gas dryer 10 included in the carbon dioxide circulation system 1. However, the application field of the gas drying adsorbent 11 is not limited to this. For example, since a gas containing _CO2 and _H2O can be separated into _CO2 and _H2O with high accuracy, the separated _CO2 and _H2O can each be used for different purposes.

[Modification 2]
In the above-described embodiment, the gas drying adsorbent 11 contains _Y -type zeolite having a composition of _{Mg2.8Na0.8Al6.4Si17.6O48 ,} in which the molar ratio of magnesium ions to _cations contained in the zeolite is _0.78 . In this Y-type zeolite, the molar ratio of magnesium ions to cations is 0.78, but the molar ratio of magnesium ions to cations is preferably 0.8 or greater. This further reduces the _CO2 absorption capacity of the gas drying adsorbent. Therefore, when drying a gas containing carbon dioxide, a decrease in the carbon dioxide content of the gas can be further suppressed. In the above-described manufacturing method, by further repeating the series of steps of immersion, standing, and washing, a zeolite with a molar ratio of magnesium ions to cations of 0.8 or greater can be obtained.

[Modification 3]
In the above-described embodiment, the zeolite contained in the gas drying adsorbent 11 is Y-type zeolite in which the silicon to aluminum composition ratio in the composite oxide is 2.75. However, the silicon to aluminum composition ratio is not limited to this and may be 2 or more and 5 or less. When the silicon to aluminum composition ratio is 2 or more and 5 or less, the ion exchange capacity of the zeolite can be increased, and therefore, the zeolite can contain more magnesium ions.

[Modification 4]
In the above-described embodiment, the carbon dioxide circulation system 1 includes the gas dryer 10, the carbon dioxide separation unit 20, the hydrogen supply source 41, the reactor 50, and the like. However, the configuration of the carbon dioxide circulation system 1 is not limited to this. For example, the carbon dioxide separation unit 20 does not need to include two _CO2 separators. Even if there is only one, it is possible to supply a raw material gas containing _CO2 and _H2 to the reactor 50 by switching between a _CO2 adsorption process and a _CO2 desorption process.

This aspect has been described above based on embodiments and variations, but 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 its spirit or the scope of the claims, and equivalents are included in this aspect. Furthermore, if a technical feature is not described as essential in this specification, it may be deleted as appropriate.

DESCRIPTION OF SYMBOLS 1... Carbon dioxide circulation system 5... Mixed gas supply source 10... Gas dryer 11... Gas drying adsorbent 12... Storage vessel 20... Carbon dioxide separation section 41... Hydrogen supply source 50... Reactor

Claims (6)

1. A carbon dioxide circulation system comprising:
a gas dryer including a gas drying adsorbent including zeolite containing magnesium ions as cations and a container for accommodating the gas drying adsorbent, the gas dryer drying the mixed gas containing carbon dioxide and moisture by adsorbing the moisture in the mixed gas supplied from an external gas generating unit to the gas drying adsorbent;
a carbon dioxide recovery unit that recovers carbon dioxide from the mixed gas dried in the gas dryer;
a hydrocarbon production unit that produces hydrocarbon compounds using the carbon dioxide recovered in the carbon dioxide recovery unit and supplies the hydrocarbon compounds to the external gas generation unit,
Carbon dioxide circulation system. 2. The carbon dioxide circulation system according to claim 1,
The gas drying adsorbent has a molar ratio of magnesium ions to cations contained in the zeolite of 0.8 or more.
Carbon dioxide circulation system. The carbon dioxide circulation system according to claim 1 or 2,
The zeolite contains a composite oxide of aluminum and silicon,
the composition ratio of silicon to aluminum in the composite oxide is 2 or more and 5 or less;
Carbon dioxide circulation system. 4. The carbon dioxide circulation system according to claim 3,
The zeolite is a Y-type zeolite,
The composition ratio of silicon to aluminum in the composite oxide is 2.75.
Carbon dioxide circulation system. 2. The carbon dioxide circulation system according to claim 1,
The zeolite is a Y-type zeolite having a composition of Mg _y Na _(6.4-2y) Al _6.4 Si _17.6 O ₄₈ ,
y is equal to or greater than 2.23;
Carbon dioxide circulation system. An adsorbent for drying gas that dries the gas by adsorbing moisture contained in the gas,
Zeolite containing magnesium ions as cations,
The zeolite contains Mg _y Na _{(6.4-2 years)} Al _6.4 Si _17.6 O ₄₈ is a Y-type zeolite having a composition
y is equal to or greater than 2.23;
Adsorbent for gas drying.