CO2 COLLECTION SYSTEM

Abstract

A CO.sub.2 collection system is configured to collect CO.sub.2 by bringing CO.sub.2-containing gas into contact with a reaction liquid containing NaOH and stored in a reaction tank to produce a target product. The target product to be extracted from the reaction liquid is adjusted according to a maximum temperature of the reaction liquid after production of NaHCO.sub.3 in the reaction liquid, an extraction temperature that is a temperature of the reaction liquid when the target product is extracted from the reaction tank, and an initial concentration of the NaOH in the reaction liquid.

Claims

1-68. (canceled)

69. A CO.sub.2 collection system configured to collect CO.sub.2 by bringing CO.sub.2-containing gas into contact with a reaction liquid containing NaOH and stored in a reaction tank to produce a target product, wherein the target product to be extracted from the reaction liquid is adjusted by adjusting a maximum temperature of the reaction liquid after production of NaHCO.sub.3 in the reaction liquid, an extraction temperature that is a temperature of the reaction liquid when the target product is extracted from the reaction tank, and an initial concentration of the NaOH in the reaction liquid.

70. The CO.sub.2 collection system according to claim 69, wherein the target product is an aqueous NaHCO.sub.3 solution, the maximum temperature of the reaction liquid after the production of the NaHCO.sub.3 in the reaction liquid is less than 65 C., the extraction temperature is 0 C. or more and 40 C. or less, and the initial concentration of the NaOH is more than 0% and equal to or less than a NaOH concentration required to produce the NaHCO.sub.3 with saturated solubility at the extraction temperature that is derived based on correspondence between a temperature of the reaction liquid and the NaOH concentration required to produce the NaHCO.sub.3 with saturated solubility.

71. The CO.sub.2 collection system according to claim 70, wherein the extraction temperature is 10 C. or more and less than 20 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 3.86 to 4.57%.

72. The CO.sub.2 collection system according to claim 70, wherein the extraction temperature is 20 C. or more and less than 30 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 4.57 to 5.29%.

73. The CO.sub.2 collection system according to claim 70, wherein the extraction temperature is 30 C. or more and less than 40 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 5.29 to 5.38%.

74. The CO.sub.2 collection system according to claim 69, wherein the target product is an aqueous sodium sesquicarbonate solution, the maximum temperature of the reaction liquid after the production of the NaHCO.sub.3 in the reaction liquid is less than 65 C., the extraction temperature is 0 C. or more and 40 C. or less, and the initial concentration of the NaOH is more than 0% and equal to or less than a NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is derived based on correspondence between a temperature of the reaction liquid and the NaOH concentration required to produce the Na.sub.2CO.sub.3 with saturated solubility.

75. The CO.sub.2 collection system according to claim 74, wherein the extraction temperature is 10 C. or more and less than 20 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 9.43 to 16.22%.

76. The CO.sub.2 collection system according to claim 74, wherein the extraction temperature is 20 C. or more and less than 30 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 16.22 to 29.96%.

77. The CO.sub.2 collection system according to claim 74, wherein the extraction temperature is 30 C. or more and less than 40 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 29.96 to 36.98%.

78. A CO.sub.2 collection system configured to collect CO.sub.2 by bringing CO.sub.2-containing gas into contact with a reaction liquid containing NaOH and stored in a reaction tank to produce a predetermined target product, wherein a maximum temperature of the reaction liquid after production of NaHCO.sub.3 in the reaction liquid is determined according to the target product to be extracted from the reaction liquid, and an initial concentration of the NaOH in the reaction liquid is determined based on the target product to be extracted from the reaction liquid and an extraction temperature that is a temperature of the reaction liquid when the target product is extracted from the reaction tank.

79. The CO.sub.2 collection system according to claim 78, wherein the target product is an aqueous NaHCO.sub.3 solution, the maximum temperature of the reaction liquid after the production of the NaHCO.sub.3 in the reaction liquid is less than 65 C., the extraction temperature is 0 C. or more and 40 C. or less, and the initial concentration of the NaOH is more than 0% and equal to or less than a NaOH concentration required to produce the NaHCO.sub.3 with saturated solubility at the extraction temperature that is derived based on correspondence between a temperature of the reaction liquid and the NaOH concentration required to produce the NaHCO.sub.3 with saturated solubility.

80. The CO.sub.2 collection system according to claim 79, wherein the extraction temperature is 10 C. or more and less than 20 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 3.86 to 4.57%.

81. The CO.sub.2 collection system according to claim 79, wherein the extraction temperature is 20 C. or more and less than 30 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 4.57 to 5.29%.

82. The CO.sub.2 collection system according to claim 79, wherein the extraction temperature is 30 C. or more and less than 40 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 5.29 to 5.38%.

83. The CO.sub.2 collection system according to claim 78, wherein the target product is an aqueous sodium sesquicarbonate solution, the maximum temperature of the reaction liquid after the production of the NaHCO.sub.3 in the reaction liquid is less than 65 C., the extraction temperature is 0 C. or more and 40 C. or less, and the initial concentration of the NaOH is more than 0% and equal to or less than a NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is derived based on correspondence between a temperature of the reaction liquid and the NaOH concentration required to produce the Na.sub.2CO.sub.3 with saturated solubility.

84. The CO.sub.2 collection system according to claim 83, wherein the extraction temperature is 10 C. or more and less than 20 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 9.43 to 16.22%.

85. The CO.sub.2 collection system according to claim 83, wherein the extraction temperature is 20 C. or more and less than 30 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 16.22 to 29.96%.

86. The CO.sub.2 collection system according to claim 83, wherein the extraction temperature is 30 C. or more and less than 40 C., and the initial concentration of the NaOH is more than 0% and an upper limit is 29.96 to 36.98%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is a conceptual diagram showing the configuration of a CO.sub.2 collection system in a first embodiment.

[0016] FIG. 2 is a diagram showing a first-order approximation equation for a temperature (liquid temperature) and a NaOH concentration required to produce NaHCO.sub.3 with saturated solubility.

[0017] FIG. 3 is a conceptual diagram showing the correspondence between an initial NaOH concentration and a target product in the first embodiment.

[0018] FIG. 4 is a diagram showing a first-order approximation equation for a temperature (liquid temperature) and a NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility.

[0019] FIG. 5 is a conceptual diagram showing the correspondence between an initial NaOH concentration and a target product in a second embodiment.

[0020] FIG. 6 is a conceptual diagram showing the correspondence between an initial NaOH concentration and a target product in a third embodiment.

[0021] FIG. 7 is a conceptual diagram showing the correspondence between an initial NaOH concentration and a target product in a fourth embodiment.

[0022] FIG. 8 is a conceptual diagram showing the correspondence between an initial NaOH concentration and a target product in a fifth embodiment.

[0023] FIG. 9 is a conceptual diagram showing the configuration of a CO.sub.2 collection system in a sixth embodiment.

[0024] FIG. 10 is a conceptual diagram showing the correspondence between an initial NaOH concentration and a target product.

MODES FOR CARRYING OUT THE INVENTION

First Embodiment

[0025] A first embodiment of a CO.sub.2 collection system will be described with reference to FIGS. 1 to 3.

1-1. Target Product

[0026] In a CO.sub.2 collection system 1, a target product to be produced by bringing CO.sub.2-containing gas into contact with a reaction liquid containing NaOH can be an aqueous solution or a solid of NaHCO.sub.3, an aqueous solution or a solid of Na.sub.2CO.sub.3, or an aqueous solution or a solid of sodium sesquicarbonate that is a mixture of NaHCO.sub.3 and Na.sub.2CO.sub.3. In the first embodiment, the target product is an aqueous NaHCO.sub.3 solution.

1-2. CO.SUB.2.-Containing GAS

[0027] As shown in FIG. 1, the CO.sub.2 collection system 1 of the present embodiment is configured to collect CO.sub.2 from exhaust gas that is CO.sub.2-containing gas emitted from a CO.sub.2 emission facility 100. In this specification, the CO.sub.2-containing gas refers to a gas containing CO.sub.2 as a component. The CO.sub.2-containing gas may be a gas containing only CO.sub.2 as a component, or may be a gas further containing unavoidable impurities. The CO.sub.2-containing gas may be a mixed gas in which CO.sub.2 and other substances are mixed as components. The proportion of CO.sub.2 in the mixed gas is not limited, and the main component having the largest proportion in the mixed gas may be CO.sub.2 or a substance other than CO.sub.2.

1-3. CO.SUB.2 .Emission Facility 100

[0028] The CO.sub.2 emission facility 100 shown in FIG. 1 is not particularly limited as long as it is a facility that emits CO.sub.2-containing gas, and examples of the facility include a facility having a boiler, a fuel cell, an incinerator, and a heat treatment facility. An exhaust duct 20 is connected to the CO.sub.2 emission facility 100, and exhaust gas G0 that is CO.sub.2-containing gas is emitted via the exhaust duct 20. The temperature of the exhaust gas G0 emitted from the CO.sub.2 emission facility 100 is not particularly limited, and is preferably high. For example, the temperature can be within a range of 100 C. to 300 C. In the present embodiment, the exhaust gas G0 emitted from the CO.sub.2 emission facility 100 has a temperature of 140 C.

1-4. CO.SUB.2 .Collection System 1

[0029] The CO.sub.2 collection system 1 of the present embodiment mainly includes a CO.sub.2 collection device 10, a gas channel 21, a moisture removal filter 30, an air pump 40, a liquid temperature adjustment device 50, and a filter 60. Each component will be described below.

1-5. Gas Channel 21

[0030] The gas channel 21 allows CO.sub.2-containing gas to flow. In the first embodiment, the gas channel 21 is formed by a pipe connected to the exhaust duct 20 that is connected to the CO.sub.2 emission facility 100. Part of the exhaust gas G0 that is CO.sub.2-containing gas flowing through the exhaust duct 20 flows through the gas channel 21.

[0031] In the first embodiment, taking into consideration that, if the amount of CO.sub.2-containing gas bubbling in a reaction tank 11 described later is excessively large, it is difficult to obtain fine bubbles in a reaction liquid, only part of the exhaust gas G0 emitted from the CO.sub.2 emission facility 100 is caused to flow through the gas channel 21, and the remaining exhaust gas that is not caused to flow through the gas channel 21 is released to the outside via the exhaust duct 20.

1-6. Moisture Removal Filter 30

[0032] The moisture removal filter 30 is provided in the gas channel 21, and removes moisture from CO.sub.2-containing gas flowing through the gas channel 21. Although illustration is omitted, the moisture removal filter 30 is composed of a water separator including a moisture separation portion that separates water vapor contained in CO.sub.2-containing gas as a liquid, and a gas passage portion through which the CO.sub.2-containing gas from which the water vapor has been separated and removed passes. The separated moisture is stored in a tank that is not shown and discharged as appropriate. The exhaust gas that has passed through the gas passage portion is discharged into a pipe 31 that communicates with the air pump 40 described later. By removing water using the moisture removal filter 30, water is prevented from accumulating in the downstream air pump 40.

1-7. Air Pump 40

[0033] The air pump 40 is connected to the moisture removal filter 30 via the pipe 31, and is configured to suck CO.sub.2-containing gas. The suction of the air pump 40 causes part of the exhaust gas to flow from the exhaust duct 20 to the gas channel 21. The sucked CO.sub.2-containing gas is supplied to the reaction tank 11 described later via a pipe 41. The configuration of the air pump 40 is not limited, and the air pump 40 is preferably a diaphragm pump in which gas does not come into direct contact with a drive unit of the air pump 40.

[0034] The drive of the air pump 40 is controlled by a pump control unit 45. The pump control unit 45 switches the suction operation of the air pump 40 ON and OFF based on a detection result from a pH sensor 13 that detects the pH of the reaction liquid in the reaction tank 11 described later. When the suction operation of the air pump 40 is turned ON, bubbling of the CO.sub.2-containing gas in the reaction tank 11 described later is started. When the suction operation of the air pump 40 is turned OFF, the bubbling is stopped.

[0035] Regarding the flow rate of the CO.sub.2-containing gas sucked by the air pump 40, the operation of the air pump 40 can be controlled by the pump control unit 45 based on a detection result from a temperature sensor 12 that detects the temperature (liquid temperature) of a reaction liquid P in the reaction tank 11 described later. Therefore, the amount of the CO.sub.2-containing gas supplied to the reaction tank 11 described later can be adjusted to control reaction heat produced in the reaction tank 11. Thus, the temperature of the reaction liquid P can be set to a predetermined temperature. In the first embodiment, the temperature of the reaction liquid P is adjusted by the liquid temperature adjustment device 50 described later, in addition to the control on the flow rate in the air pump 40. In order to minimize energy required to adjust the temperature of the reaction liquid P, the length of the pipe forming the gas channel 21 and the heat dissipation properties of the pipe can be adjusted in advance according to the target temperature of the reaction liquid P.

1-8. CO.SUB.2 .Collection Device 10

[0036] The CO.sub.2 collection device 10 collects CO.sub.2 from CO.sub.2-containing gas by bringing the CO.sub.2-containing gas into contact with the reaction liquid stored in the reaction tank 11. The reaction liquid is an aqueous NaOH solution containing NaOH. The NaOH concentration in the reaction liquid will be described later.

[0037] The CO.sub.2-containing gas is supplied to the reaction tank 11 via the pipe 41. The tip of the pipe 41 is located near the inner bottom of the reaction tank 11, and is configured to discharge the CO.sub.2-containing gas into the reaction liquid to cause bubbling. The reaction tank 11 is provided with the temperature sensor 12 that detects the temperature (liquid temperature) of the reaction liquid P, and the pH sensor 13 that detects the pH of the reaction liquid P.

[0038] The reaction tank 11 is provided with the liquid temperature adjustment device 50 capable of heating or cooling the reaction liquid in the reaction tank 11. A liquid temperature adjustment device control unit 55 that controls the operation of the liquid temperature adjustment device 50 is connected to the liquid temperature adjustment device 50. The liquid temperature adjustment device control unit 55 is configured to control the heating or cooling operation of the liquid temperature adjustment device 50 based on the liquid temperature that is the temperature of the reaction liquid P detected by the temperature sensor 12 provided in the reaction tank 11. Based on the liquid temperature detected by the temperature sensor 12, the pump control 45 controls the operation of the air pump 40 and the liquid temperature adjustment device control unit 55 controls the operation of the liquid temperature adjustment device 50 as described above. Thus, the temperature of the reaction liquid or an aqueous product solution in the reaction tank 11 can be maintained at a predetermined temperature.

1-9. CO.SUB.2 .Immobilization Reaction

[0039] By bringing CO.sub.2-containing gas into contact with a reaction liquid containing NaOH (aqueous NaOH solution) in the reaction tank 11, a reaction of formula 1 below occurs and then a reaction of formula 2 below occurs in the reaction tank 11. In this specification, formulas 1 and 2 below are referred to also as CO.sub.2 immobilization reaction.

##STR00001##

[0040] In the reaction tank 11 shown in FIG. 1, neither NaHCO.sub.3 nor Na.sub.2CO.sub.3 is present before the start of the reactions. Along with the progress of the reactions, however, one of the following states will occur: a state in which Na.sub.2CO.sub.3 is produced and NaHCO.sub.3 is not present, a state in which part of Na.sub.2CO.sub.3 further reacts with CO.sub.2 to produce NaHCO.sub.3 and both are present, or a state in which all Na.sub.2CO.sub.3 is converted to NaHCO.sub.3 and disappears and NaHCO.sub.3 is present. Both NaHCO.sub.3 and Na.sub.2CO.sub.3 produced by the above reactions are dissolved in the water in the reaction tank 11 and are in the form of aqueous solutions. In this specification, NaHCO.sub.3, Na.sub.2CO.sub.3, and a mixture of the two are collectively referred to as product, and these aqueous solutions are collectively referred to as aqueous product solution. In the products, a product to be obtained with high purity is referred to as target product.

[0041] In the first embodiment, the CO.sub.2-containing gas supplied from the pipe 41 is brought into contact with the aqueous NaOH solution in the reaction tank 11 by bubbling. Therefore, the reactions can be started. In order to increase the frequency of contact between the CO.sub.2-containing gas and the aqueous NaOH solution, the CO.sub.2-containing gas is preferably discharged in the form of fine bubbles during bubbling. The fine bubbles can be formed by a fine bubble forming device (not shown) provided at the tip of the pipe 41.

[0042] If the exhaust gas emitted from the CO.sub.2 emission facility 100 contains a substance that inhibits the above reactions in the reaction tank 11, a filter (not shown) that removes the substance that inhibits the above reactions is preferably provided at a position upstream of the reaction tank 11, for example, on the gas channel 21, between the moisture removal filter 30 and the air pump 40, or between the air pump 40 and the reaction tank 11. If the exhaust gas emitted from the CO.sub.2 emission facility 100 does not contain any components other than CO.sub.2, or if it is clear that the exhaust gas does not contain any substance that inhibits the above reactions in the reaction tank 11, there is no need to provide the filter.

[0043] An exhaust unit 14 shown in FIG. 1 discharges, from the reaction tank 11 to the filter 60, CO.sub.2-removed gas from which CO.sub.2 has been removed in the reaction tank 11. The filter 60 captures harmful components in the CO.sub.2-removed gas. The configuration of the filter 60 is not limited. In the first embodiment, the filter 60 is configured to remove water-soluble substances (e.g., NaOH in the reaction liquid that is sprayed by bubbling in the reaction tank 11 and reaches the exhaust unit 14, and nitrogen oxides NOx contained in the exhaust gas) in the CO.sub.2-removed gas by bubbling the CO.sub.2-removed gas in water W stored in the filter 60 and passing it through the water W. The CO.sub.2-removed gas that has passed through the filter 60 is released to the outside of the CO.sub.2 collection system 1 via an external release unit 61.

[0044] The target product can be collected as follows. When the target product produced by the reactions of formulas 1 and 2 above is in the form of an aqueous solution, the aqueous solution of the target product is discharged to the outside via an openable and closable drain cock 70 provided in the reaction tank 11 and collected in a collection container 75. When the target product is in the form of a solid, the solid target product is discharged to the outside together with the aqueous solution in the reaction tank 11 via the drain cock 70 provided in the reaction tank 11 and collected in the collection container 75, and then the solid target product is separated and collected by filtration, centrifugation, etc. The collected target product can be used as a resource, for example, as a cleaning agent, an antiseptic, or a herbicide.

[0045] After the target product is collected from the reaction tank 11, the drain cock 70 is closed, and an aqueous NaOH solution that is the reaction liquid to be used in the next reaction is supplied into the reaction tank 11 from a reaction liquid supply unit that is not shown. The drain cock 70 is closed in a normal state in which the target product is not collected.

1-10. Initial NaOH Concentration, Maximum Temperature of Reaction Liquid, and Extraction Temperature

[0046] The concentration of NaOH in the reaction liquid stored in the reaction tank 11 before the start of the above CO.sub.2 immobilization reaction is referred to as initial NaOH concentration. In the first embodiment, the initial NaOH concentration is determined based on a target product to be extracted from the reaction liquid, a maximum temperature of the reaction liquid after NaHCO.sub.3 is produced in the reaction liquid, and an extraction temperature that is a liquid temperature when the target product is extracted from the reaction tank 11.

[0047] In the first embodiment, the target product to be extracted from the reaction liquid is an aqueous solution of NaHCO.sub.3(sodium bicarbonate) as described above. The initial concentration of NaOH is set to a value more than 0% and equal to or less than a NaOH concentration required to produce NaHCO.sub.3 with saturated solubility at the extraction temperature that is derived based on the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility.

[0048] Regarding the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility, the saturated solubility of NaHCO.sub.3 at 10 C. intervals from 0 C. to 40 C. is first shown in an upper part of Table 1 below. The NaOH concentration required to produce NaHCO.sub.3 with saturated solubility at each temperature is calculated as shown in a lower part of Table 1 below.

TABLE-US-00001 TABLE 1 Temperature ( C.) 0 10 20 30 40 Saturated solubility of 7.00 8.10 9.60 11.10 12.70 NaHCO.sub.3 (%) NaOH concentration required 3.33 3.86 4.57 5.29 5.38 to produce NaHCO.sub.3 with saturated solubility (%)

[0049] A first-order approximation equation for the temperature (liquid temperature) and the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility shown in the lower part of Table 1 can be calculated as shown in formula 3 below, and can be shown as in FIG. 2.

[00001]$\begin{array}{cc}y=0.144x+6.82\left(R^2=0.996\right)& \left(\mathrm{Formula}3\right)\end{array}$

(where y is the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility, and x is the liquid temperature)

[0050] In the first embodiment, the approximation equation shown in formula 3 above is used as the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility. The initial NaOH concentration is set within a first concentration range shown in FIG. 3 that is a range more than 0% and equal to or less than a NaOH concentration A required to produce NaHCO.sub.3 with saturated solubility at the extraction temperature that is calculated based on the approximation equation of formula 3 above. The format of the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility is not limited, and may be a higher-order approximation equation instead of the first-order approximation equation in the first embodiment, or may be determined by a map or a logical formula.

[0051] The maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to less than 65 C. Since the above CO.sub.2 immobilization reaction is an exothermic reaction, the temperature of the reaction liquid increases during CO.sub.2 immobilization. However, the temperature (liquid temperature) of the reaction liquid is adjusted to less than 65 C. by the above air pump 40 and the above liquid temperature adjustment device 50.

[0052] The extraction temperature that is the liquid temperature when the target product is extracted from the reaction tank 11 can be set within a range of general outside air temperature. In the first embodiment, the extraction temperature is set to 0 C. or more and 40 C. or less.

[0053] The upper limit A of the first concentration range shown in FIG. 3 can be set to 3.86 to 4.57% when the extraction temperature is 10 C. or more and less than 20 C. based on the values in the lower part of Table 1 above, and can preferably be set to 3.86%. The upper limit A of the first concentration range can be set to 4.57 to 5.29% when the extraction temperature is 20 C. or more and less than 30 C., and can preferably be set to 4.57%. The upper limit A of the first concentration range can be set to 5.29 to 5.38% when the extraction temperature is 30 C. or more and less than 40 C., and can preferably be set to 5.29%. In FIG. 3, the symbol B indicates a NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is calculated based on an approximation equation of formula 4 described later.

1-11. Start and Stop of Reaction

[0054] In the CO.sub.2 collection system 1 of the first embodiment, the suction operation of the air pump 40 is turned ON to start bubbling CO.sub.2-containing gas in the reaction tank 11, thereby bringing the CO.sub.2-containing gas into contact with the reaction liquid having the initial NaOH concentration. In this way, the CO.sub.2 immobilization reaction is started. When the pH sensor 13 then detects that the pH of the reaction liquid in the reaction tank 11 has reached a value corresponding to the pH at which the total amount of NaOH has been converted to NaHCO.sub.3, the suction operation of the air pump 40 is turned OFF to stop bubbling the CO.sub.2-containing gas, thereby stopping the CO.sub.2 immobilization reaction. Then, the target product is extracted from the reaction tank 11 via the drain cock 70 and collected in the collection container 75.

1-12. Actions and Effects

[0055] In the CO.sub.2 collection system 1 of the first embodiment, the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to less than 65 C., and the initial NaOH concentration is within the above first concentration range. Therefore, NaHCO.sub.3 produced in the reaction tank 11 is not thermally decomposed and is not supersaturated. Thus, NaHCO.sub.3 does not precipitate. As a result, a highly pure aqueous NaHCO.sub.3 solution can be obtained as the target product, and the target product can selectively be produced easily. Since NaHCO.sub.3 does not precipitate, clogging of the fine bubble forming device that is not shown and is provided at the tip of the pipe 41 can be prevented.

[0056] In the first embodiment, the initial NaOH concentration is determined based on the approximation equation of formula 3 above. Instead of this, modification 1-1, 1-2, or 1-3 below can be adopted.

[0057] In modification 1-1, the approximation equation of formula 3 above is not used. Based on the values of the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility shown in the lower part of Table 1 above, the extraction temperature is set to 10 C. or more and less than 20 C., the initial concentration of NaOH is set within the first concentration range shown in FIG. 3 and is more than 0%, and the upper limit A can be set to 3.86 to 4.57%. The initial concentration of NaOH is preferably more than 0% and 3.86% or less.

[0058] In modification 1-2, the approximation equation of formula 3 above is not used. Based on the values of the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility shown in the lower part of Table 1 above, the extraction temperature is set to 20 C. or more and less than 30 C., the initial concentration of NaOH is set within the first concentration range shown in FIG. 3 and is more than 0%, and the upper limit A can be set to 4.57 to 5.29%. The initial concentration of NaOH is preferably more than 0% and 4.57% or less.

[0059] In modification 1-3, the approximation equation of formula 3 above is not used. Based on the values of the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility shown in the lower part of Table 1 above, the extraction temperature is set to 30 C. or more and less than 40 C., the initial concentration of NaOH is set within the first concentration range shown in FIG. 3 and is more than 0%, and the upper limit A can be set to 5.29 to 5.38%. The initial concentration of NaOH is preferably more than 0% and 5.29% or less.

[0060] In any of modifications 1-1, 1-2, and 1-3 above, the precipitation of NaHCO.sub.3 is suppressed at the extraction temperature. Therefore, the same actions and effects as those of the first embodiment are attained.

Second Embodiment

[0061] In the second embodiment, the configurations of the devices are similar to those in the first embodiment shown in FIG. 1.

2-1. Target Product

[0062] In the first embodiment, the target product is the aqueous NaHCO.sub.3 solution. In the second embodiment, the target product is an aqueous sodium sesquicarbonate solution in which NaHCO.sub.3 and Na.sub.2CO.sub.3 are dissolved in water.

2-2. Initial NaOH Concentration, Maximum Temperature of Reaction Liquid, and Extraction Temperature

[0063] In the second embodiment, the initial concentration of NaOH is set to a value more than 0% and equal to or less than a NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is derived based on the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility.

[0064] Regarding the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility, the saturated solubility of Na.sub.2CO.sub.3 at 10 C. intervals from 0 C. to 40 C. is first shown in an upper part of Table 2 below. The NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at each temperature is calculated as shown in a lower part of Table 2 below.

TABLE-US-00002 TABLE 2 Temperature ( C.) 0 10 20 30 40 Saturated solubility of Na.sub.2CO.sub.3 7.00 12.50 21.50 39.70 49.00 (%) NaOH concentration required 5.28 9.43 16.22 29.96 36.98 to produce Na.sub.2CO.sub.3 with saturated solubility (%)

[0065] A first-order approximation equation for the temperature (liquid temperature) and the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility shown in the lower part of Table 2 can be calculated as shown in formula 4 below, and can be shown as in FIG. 4.

[00002]$\begin{array}{cc}y=1.112x+3.7\left(R^2=0.966\right)& \left(\mathrm{Formula}4\right)\end{array}$

(where y is the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility, and x is the liquid temperature)

[0066] In the second embodiment, the approximation equation shown in formula 4 above is used as the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility. The initial NaOH concentration is set within a second concentration range shown in FIG. 5 that is a range more than 0% and equal to or less than a NaOH concentration B required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is calculated based on the approximation equation of formula 4 above. The format of the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility is not limited, and may be a higher-order approximation equation instead of the first-order approximation equation in the first embodiment, or may be determined by a map or a logical formula.

[0067] The maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to less than 65 C. as in the first embodiment. The extraction temperature that is the liquid temperature when the target product is extracted from the reaction tank 11 is also set to 0 C. or more and 40 C. or less as in the first embodiment. The other configurations are similar to those in the first embodiment.

[0068] The upper limit B of the second concentration range shown in FIG. 5 can be set to 9.43 to 16.22% when the extraction temperature is 10 C. or more and less than 20 C. based on the values in the lower part of Table 2 above, and can preferably be set to 9.43%. The upper limit B of the second concentration range can be set to 16.22 to 29.96% when the extraction temperature is 20 C. or more and less than 30 C., and can preferably be set to 16.22%. The upper limit B of the second concentration range can be set to 29.96 to 36.98% when the extraction temperature is 30 C. or more and less than 40 C., and can preferably be set to 29.96%. In FIG. 5, the symbol A indicates the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility at the extraction temperature that is calculated based on the approximation equation of formula 3 described above.

2-3. Start and Stop of Reaction

[0069] In the CO.sub.2 collection system 1 of the second embodiment, the suction operation of the air pump 40 is turned ON to start bubbling CO.sub.2-containing gas in the reaction tank 11, thereby bringing the CO.sub.2-containing gas into contact with the reaction liquid having the initial NaOH concentration. In this way, the CO.sub.2 immobilization reaction is started. When the pH sensor 13 then detects that the pH of the reaction liquid in the reaction tank 11 has reached a value corresponding to the pH at which the total amount of NaOH has been consumed and the ratio of Na.sub.2CO.sub.3 to NaHCO.sub.3 is 50:50 as desired, the suction operation of the air pump 40 is turned OFF to stop bubbling the CO.sub.2-containing gas, thereby stopping the CO.sub.2 immobilization reaction.

2-4. Actions and Effects

[0070] In the CO.sub.2 collection system 1 of the second embodiment, the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to less than 65 C. Therefore, NaHCO.sub.3 produced in the reaction tank 11 is not thermally decomposed. Since the initial NaOH concentration is within the above second concentration range, Na.sub.2CO.sub.3 is not supersaturated and does not precipitate. As a result, a highly pure aqueous sodium sesquicarbonate solution in which NaHCO.sub.3 and Na.sub.2CO.sub.3 are dissolved in water can be obtained as the target product, and the target product can selectively be produced easily. Since Na.sub.2CO.sub.3 does not precipitate, the reactivity of the reaction between Na.sub.2CO.sub.3 and CO.sub.2 for producing NaHCO.sub.3 shown in formula 2 above is prevented from decreasing, and the CO.sub.2 collection rate is improved.

[0071] In the second embodiment, the initial NaOH concentration is determined based on the approximation equation of formula 4 above. Instead of this, modification 2-1, 2-2, or 2-3 below can be adopted.

[0072] In modification 2-1, the approximation equation of formula 4 above is not used. Based on the values of the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility shown in the lower part of Table 2 above, the extraction temperature is set to 10 C. or more and less than 20 C., the initial concentration of NaOH is set within the second concentration range shown in FIG. 5 and is more than 0%, and the upper limit B can be set to 9.43 to 16.22%. The initial concentration of NaOH is preferably more than 0% and 9.43% or less.

[0073] In modification 2-2, the approximation equation of formula 4 above is not used. Based on the values of the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility shown in the lower part of Table 2 above, the extraction temperature is set to 20 C. or more and less than 30 C., the initial concentration of NaOH is set within the second concentration range shown in FIG. 5 and is more than 0%, and the upper limit B can be set to 16.22 to 29.96%. The initial concentration of NaOH is preferably more than 0% and 16.22% or less.

[0074] In modification 2-3, the approximation equation of formula 4 above is not used. Based on the values of the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility shown in the lower part of Table 2 above, the extraction temperature is set to 30 C. or more and less than 40 C., the initial concentration of NaOH is set within the second concentration range shown in FIG. 5 and is more than 0%, and the upper limit B can be set to 29.96 to 36.98%. The initial concentration of NaOH is preferably more than 0% and 29.96% or less.

[0075] In any of modifications 2-1, 2-2, and 2-3 above, the precipitation of Na.sub.2CO.sub.3 can be suppressed at the extraction temperature. Therefore, the same actions and effects as those of the second embodiment are attained.

Third Embodiment

[0076] In the third embodiment, the configurations of the devices are similar to those in the first embodiment shown in FIG. 1.

3-1. Target Product

[0077] In the third embodiment, the target product is solid NaHCO.sub.3.

3-2. Initial NaOH Concentration, Maximum Temperature of Reaction Liquid, and Extraction Temperature

[0078] In the third embodiment, the initial concentration of NaOH is set to be equal to or more than the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility at the extraction temperature that is derived based on the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility, and to be equal to or less than the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is derived based on the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility.

[0079] In the third embodiment, the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility at the extraction temperature that is derived based on the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce NaHCO.sub.3 with saturated solubility is the NaOH concentration A calculated based on the approximation equation of formula 3 above in the first embodiment.

[0080] In the third embodiment, the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is derived based on the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility is the NaOH concentration B calculated based on the approximation equation of formula 4 above in the second embodiment.

[0081] Therefore, in the third embodiment, the initial NaOH concentration is set to a third concentration range shown in FIG. 6 that is a range equal to or more than the NaOH concentration A required to produce NaHCO.sub.3 with saturated solubility at the extraction temperature that is calculated based on the approximation equation of formula 3 above and equal to or less than the NaOH concentration B required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is calculated based on the approximation equation of formula 4 above.

[0082] In the third embodiment, the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to less than 65 C. as in the first embodiment. The extraction temperature that is the liquid temperature when the target product is extracted from the reaction tank 11 is also set to 0 C. or more and 40 C. or less as in the first embodiment. The other configurations are similar to those in the first embodiment.

[0083] The lower limit A of the third concentration range shown in FIG. 6 can be set to 3.86 to 4.57% when the extraction temperature is 10 C. or more and less than 20 C. based on the values in the lower part of Table 1 above, and can preferably be set to 3.86%. The upper limit B can be set to 9.43 to 16.22% based on the values in the lower part of Table 2 above, and can preferably be set to 9.43%.

[0084] The lower limit A of the third concentration range can be set to 4.57 to 5.29% when the extraction temperature is 20 C. or more and less than 30 C. based on the values in the lower part of Table 1 above, and can preferably be set to 4.57%. The upper limit B can be set to 16.22 to 29.96% based on the values in the lower part of Table 2 above, and can preferably be set to 16.22%.

[0085] The lower limit A of the third concentration range can be set to 5.29 to 5.38% when the extraction temperature is 30 C. or more and less than 40 C. based on the values in the lower part of Table 1 above, and can preferably be set to 5.29%. The upper limit B can be set to 29.96 to 36.98% based on the values in the lower part of Table 2 above, and can preferably be set to 29.96%.

3-3. Start and Stop of Reaction

[0086] In the CO.sub.2 collection system 1 of the third embodiment, the suction operation of the air pump 40 is turned ON to start bubbling CO.sub.2-containing gas in the reaction tank 11, thereby bringing the CO.sub.2-containing gas into contact with the reaction liquid having the initial NaOH concentration. In this way, the CO.sub.2 immobilization reaction is started. When the pH sensor 13 then detects that the pH of the reaction liquid in the reaction tank 11 has reached a value corresponding to the pH at which the total amount of NaOH has been consumed and the ratio of Na.sub.2CO.sub.3 to NaHCO.sub.3 is 50:50, the suction operation of the air pump 40 is turned OFF to stop bubbling the CO.sub.2-containing gas, thereby stopping the CO.sub.2 immobilization reaction.

3-4. Actions and Effects

[0087] In the CO.sub.2 collection system 1 of the third embodiment, the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to less than 65 C. Therefore, NaHCO.sub.3 produced in the reaction tank 11 is not thermally decomposed. Since the initial NaOH concentration is within the above third concentration range, Na.sub.2CO.sub.3 is not supersaturated and does not precipitate, but NaHCO.sub.3 is supersaturated and precipitates in the reaction tank 11. As a result, the only product that precipitates and becomes a solid in the reaction tank 11 is NaHCO.sub.3. Therefore, highly pure solid NaHCO.sub.3 can be obtained as the target product, and the target product can selectively be produced easily. Since Na.sub.2CO.sub.3 does not precipitate, the reactivity of the reaction between Na.sub.2CO.sub.3 and CO.sub.2 for producing NaHCO.sub.3 shown in formula 2 above is prevented from decreasing, and the CO.sub.2 collection rate is improved.

[0088] In the third embodiment, the initial NaOH concentration is determined based on the approximation equations of formulas 3 and 4 above. Instead of this, modification 3-1, 3-2, or 3-3 below can be adopted.

[0089] In modification 3-1, the approximation equations of formulas 3 and 4 above are not used. Based on the values of the NaOH concentration A required to produce NaHCO.sub.3 with saturated solubility shown in the lower part of Table 1 above and the values of the NaOH concentration B required to produce Na.sub.2CO.sub.3 with saturated solubility shown in the lower part of Table 2 above, the extraction temperature is set to 10 C. or more and less than 20 C., the initial concentration of NaOH is set within the third concentration range shown in FIG. 6, the lower limit A can be set to 3.86 to 4.57%, and the upper limit B can be set to 9.43 to 16.22%. The initial concentration of NaOH can be set to 3.86% or more and 9.43% or less.

[0090] In modification 3-2, the approximation equations of formulas 3 and 4 above are not used. Based on the values of the NaOH concentration A required to produce NaHCO.sub.3 with saturated solubility shown in the lower part of Table 1 above and the values of the NaOH concentration B required to produce Na.sub.2CO.sub.3 with saturated solubility shown in the lower part of Table 2 above, the extraction temperature is set to 20 C. or more and less than 30 C., the initial concentration of NaOH is set within the third concentration range shown in FIG. 6, the lower limit A can be set to 4.57 to 5.29%, and the upper limit B can be set to 16.22 to 29.96%. The initial concentration of NaOH can be set to 4.57% or more and 16.22% or less. In modification 3-3, the approximation equations of formulas 3 and 4 above

[0091] are not used. Based on the values of the NaOH concentration A required to produce NaHCO.sub.3 with saturated solubility shown in the lower part of Table 1 above and the values of the NaOH concentration B required to produce Na.sub.2CO.sub.3 with saturated solubility shown in the lower part of Table 2 above, the extraction temperature is set to 30 C. or more and less than 40 C., the initial concentration of NaOH is set within the third concentration range shown in FIG. 6, the lower limit A can be set to 5.29 to 5.38%, and the upper limit B can be set to 29.96 to 36.98%. The initial concentration of NaOH can be set to 5.29% or more and 29.96% or less.

[0092] In any of modifications 3-1, 3-2, and 3-3 above, only NaHCO.sub.3 precipitates at the extraction temperature and solid NaHCO.sub.3 can be collected. Therefore, the same actions and effects as those of the third embodiment are attained.

Fourth Embodiment

[0093] In the fourth embodiment, the configurations of the devices are similar to those in the first embodiment shown in FIG. 1.

4-1. Target Product

[0094] In the fourth embodiment, the target product is an aqueous Na.sub.2CO.sub.3 solution or solid Na.sub.2CO.sub.3.

4-2. Initial NaOH Concentration, Maximum Temperature of Reaction Liquid, and Extraction Temperature

[0095] In the fourth embodiment, the initial concentration of NaOH is a value more than 0% and is within a fourth concentration range shown in FIG. 7.

[0096] In the fourth embodiment, the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to 65 C. or more. Since the above CO.sub.2 immobilization reaction is an exothermic reaction, the temperature of the reaction liquid increases during CO.sub.2 immobilization. When the temperature (liquid temperature) of the reaction liquid is less than 65 C., heating is performed to a temperature of 65 C. or more by the above air pump 40 and the above liquid temperature adjustment device 50. The extraction temperature that is the liquid temperature when the target product is extracted from the reaction tank 11 is set to 0 C. or more and 40 C. or less as in the first embodiment. The other configurations are similar to those in the first embodiment.

4-3. Start and Stop of Reaction

[0097] In the CO.sub.2 collection system 1 of the fourth embodiment, the suction operation of the air pump 40 is turned ON to start bubbling CO.sub.2-containing gas in the reaction tank 11, thereby bringing the CO.sub.2-containing gas into contact with the reaction liquid having the initial NaOH concentration. In this way, the CO.sub.2 immobilization reaction is started. When the pH sensor 13 then detects the pH of the reaction liquid in the reaction tank 11 indicating that the total amount of NaOH has been consumed, the suction operation of the air pump 40 is turned OFF to stop bubbling the CO.sub.2-containing gas, thereby stopping the CO.sub.2 immobilization reaction.

4-4. Actions and Effects

[0098] In the CO.sub.2 collection system 1 of the fourth embodiment, the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to 65 C. or more. Therefore, NaHCO.sub.3 produced in the reaction tank 11 is thermally decomposed. Therefore, when the initial NaOH concentration is within the above fourth concentration range, that is, the entire range, an aqueous Na.sub.2CO.sub.3 solution or solid Na.sub.2CO.sub.3 is present as a product but NaHCO.sub.3 is not present in the reaction tank 11. As a result, highly pure Na.sub.2CO.sub.3 can be obtained as the target product, and the target product can selectively be produced easily. Since NaHCO.sub.3 is not present, clogging of the fine bubble forming device that is not shown and is provided at the tip of the pipe 41 due to the precipitation of NaHCO.sub.3 can be prevented.

Fifth Embodiment

[0099] In the fifth embodiment, the configurations of the devices are similar to those in the first embodiment shown in FIG. 1.

5-1. Target Product

[0100] In the fifth embodiment, the target product is solid Na.sub.2CO.sub.3.

5-2. Initial NaOH Concentration, Maximum Temperature of Reaction Liquid, and Extraction Temperature

[0101] In the fifth embodiment, the initial concentration of NaOH is set to be equal to or more than the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is derived based on the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility.

[0102] In the fifth embodiment, the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is derived based on the correspondence between the temperature of the reaction liquid and the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility is the NaOH concentration calculated based on the approximation equation of formula 4 above in the second embodiment.

[0103] In the fifth embodiment, the initial NaOH concentration is set within a fifth concentration range shown in FIG. 8 that is a range equal to or more than the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility at the extraction temperature that is calculated based on the approximation equation of formula 4 above.

[0104] In the fifth embodiment, the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to 65 C. or more as in the fourth embodiment. The extraction temperature that is the liquid temperature when the target product is extracted from the reaction tank 11 is also set to 0 C. or more and 40 C. or less as in the fourth embodiment. The other configurations are similar to those in the first embodiment.

[0105] The lower limit B of the fifth concentration range shown in FIG. 8 can be set to 9.43 to 16.22% when the extraction temperature is 10 C. or more and less than 20 C. based on the values in the lower part of Table 2 above, and can preferably be set to 9.43%. The lower limit B of the fifth concentration range can be set to 16.22 to 29.96% when the extraction temperature is 20 C. or more and less than 30 C. based on the values in the lower part of Table 2 above, and can preferably be set to 16.22%. The lower limit B of the fifth concentration range can be set to 29.96 to 36.98% when the extraction temperature is 30 C. or more and less than 40 C. based on the values in the lower part of Table 2 above, and can preferably be set to 29.96%.

5-3. Start and Stop of Reaction

[0106] In the CO.sub.2 collection system 1 of the fifth embodiment, the suction operation of the air pump 40 is turned ON to start bubbling CO.sub.2-containing gas in the reaction tank 11, thereby bringing the CO.sub.2-containing gas into contact with the reaction liquid having the initial NaOH concentration. In this way, the CO.sub.2 immobilization reaction is started. When the pH sensor 13 then detects the pH of the reaction liquid in the reaction tank 11 indicating that the total amount of NaOH has been consumed, the suction operation of the air pump 40 is turned OFF to stop bubbling the CO.sub.2-containing gas, thereby stopping the CO.sub.2 immobilization reaction.

5-4. Actions and Effects

[0107] In the CO.sub.2 collection system 1 of the fifth embodiment, the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid is set to 65 C. or more. Therefore, NaHCO.sub.3 produced in the reaction tank 11 is thermally decomposed. Since the initial NaOH concentration is set to the above fifth concentration range, Na.sub.2CO.sub.3 is supersaturated and precipitates in the reaction tank 11. As a result, the only product that precipitates and becomes a solid in the reaction tank 11 is Na.sub.2CO.sub.3. Therefore, highly pure solid Na.sub.2CO.sub.3 can be obtained as the target product by filtration or centrifugation of the collected matter in the collection container 75, and the target product can selectively be produced easily.

[0108] In the fifth embodiment, the initial NaOH concentration is determined based on the approximation equation of formula 4 above. Instead of this, modification 5-1, 5-2, or 5-3 below can be adopted.

[0109] In modification 5-1, the approximation equation of formula 4 above is not used. Based on the values of the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility shown in the lower part of Table 2 above, the extraction temperature is set to 10 C. or more and less than 20 C., the initial concentration of NaOH is set within the fifth concentration range shown in FIG. 8, and the lower limit B can be set to 9.43 to 16.22%. The initial concentration of NaOH is preferably 9.43% or more.

[0110] In modification 5-2, the approximation equation of formula 4 above is not used. Based on the values of the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility shown in the lower part of Table 2 above, the extraction temperature is set to 20 C. or more and less than 30 C., the initial concentration of NaOH is set within the fifth concentration range shown in FIG. 8, and the lower limit B can be set to 16.22 to 29.96%. The initial concentration of NaOH is preferably 16.22% or more.

[0111] In modification 5-3, based on the values of the NaOH concentration required to produce Na.sub.2CO.sub.3 with saturated solubility shown in the lower part of Table 2 above, the extraction temperature is set to 30 C. or more and less than 40 C., the initial concentration of NaOH is set within the fifth concentration range shown in FIG. 8, and the lower limit B can be set to 29.96 to 36.98%. The initial concentration of NaOH is preferably 29.96% or more.

[0112] In any of modifications 5-1, 5-2, and 5-3 above, the only product obtained as a solid at the extraction temperature is Na.sub.2CO.sub.3. Therefore, highly pure solid Na.sub.2CO.sub.3 can be obtained as the target product, and the same actions and effects as those of the fifth embodiment are attained.

Sixth Embodiment

[0113] As shown in FIG. 9, the CO.sub.2 collection system 1 of the sixth embodiment includes an adjustment unit 80 and an initial NaOH concentration setting unit 81. The other configurations in the sixth embodiment are the same as those in the first embodiment shown in FIG. 1, and therefore the same reference numerals as in the first embodiment are used and the description thereof is omitted.

6-1. Adjustment Unit 80 and Initial NaOH Concentration Setting Unit 81

[0114] The adjustment unit 80 shown in FIG. 9 adjusts the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid, the extraction temperature, and the initial concentration of NaOH in the reaction liquid to adjust the target product. In the sixth embodiment, the adjustment unit 80 adjusts the temperature (liquid temperature) of the reaction liquid by adjusting the flow rate of the air pump 40 through the pump control 45 and by controlling the drive of the liquid temperature adjustment device 50 through the liquid temperature adjustment device control unit 55. Further, the initial NaOH concentration of the reaction liquid to be introduced into the reaction tank 11 is adjusted via the initial NaOH concentration setting unit 81 described later.

[0115] The initial NaOH concentration setting unit 81 shown in FIG. 9 is configured to introduce the reaction liquid having the initial NaOH concentration adjusted by the adjustment unit 80 into the reaction tank 11 from a reaction liquid introduction unit 15 provided in the reaction tank 11.

[0116] In the sixth embodiment, the adjustment unit 80 includes a selection unit 82 for a user to select a desired target product, and the product selected by the user via the selection unit 82 is set as the target product. Then, the initial NaOH concentration of the reaction liquid and the temperature (liquid temperature) of the reaction liquid are adjusted according to the set target product. The correspondence between the initial NaOH concentration and the target product can be shown as in FIG. 10.

6-2. Adjustment of Target Product

[0117] The adjustment of the target product in the sixth embodiment will be described in detail below.

[0118] In the sixth embodiment, when the user selects an aqueous NaHCO.sub.3 solution as the target product via the selection unit 82, the initial NaOH concentration is adjusted within the first concentration range via the initial NaOH concentration setting unit 81 as in the first embodiment, and the maximum temperature of the reaction liquid and the extraction temperature are also adjusted via the air pump 40 and the liquid temperature adjustment device 50 as in the first embodiment. Thus, the aqueous NaHCO.sub.3 solution can be obtained as the target product as in the first embodiment.

[0119] By adjusting the initial NaOH concentration, the maximum temperature of the reaction liquid, and the extraction temperature as in modifications 1-1, 1-2, and 1-3 above, the aqueous NaHCO.sub.3 solution may be obtained as the target product as in modifications 1-1, 1-2, and 1-3.

[0120] In the sixth embodiment, when the user selects an aqueous sodium sesquicarbonate solution as the target product via the selection unit 82, the initial NaOH concentration, the maximum temperature of the reaction liquid, and the extraction temperature are adjusted via the initial NaOH concentration setting unit 81, the air pump 40, and the liquid temperature adjustment device 50 as in the second embodiment. Thus, the aqueous sodium sesquicarbonate solution can be obtained as the target product as in the second embodiment.

[0121] In the sixth embodiment, when the user selects solid NaHCO.sub.3 as the target product via the selection unit 82, the initial NaOH concentration, the maximum temperature of the reaction liquid, and the extraction temperature are adjusted via the initial NaOH concentration setting unit 81, the air pump 40, and the liquid temperature adjustment device 50 as in the third embodiment. Thus, solid NaHCO.sub.3 can be obtained as the target product as in the third embodiment.

[0122] In the sixth embodiment, when the user selects either an aqueous Na.sub.2CO.sub.3 solution or solid Na.sub.2CO.sub.3 as the target product via the selection unit 82, the initial NaOH concentration, the maximum temperature of the reaction liquid, and the extraction temperature are adjusted via the initial NaOH concentration setting unit 81, the air pump 40, and the liquid temperature adjustment device 50 as in the fourth embodiment. Thus, the aqueous Na.sub.2CO.sub.3 solution or solid Na.sub.2CO.sub.3 can be obtained as the target product as in the fourth embodiment.

[0123] In the sixth embodiment, when the user selects solid Na.sub.2CO.sub.3 as the target product via the selection unit 82, the initial NaOH concentration, the maximum temperature of the reaction liquid, and the extraction temperature are adjusted via the initial NaOH concentration setting unit 81, the air pump 40, and the liquid temperature adjustment device 50 as in the fifth embodiment. Thus, solid Na.sub.2CO.sub.3 can be obtained as the target product as in the third embodiment.

6-3. Actions and Effects

[0124] In the CO.sub.2 collection system of the sixth embodiment, the target product to be extracted from the reaction liquid is adjusted by adjusting the maximum temperature of the reaction liquid after the production of NaHCO.sub.3 in the reaction liquid, the extraction temperature that is the temperature of the reaction liquid when the target product is extracted from the reaction tank, and the initial concentration of NaOH in the reaction liquid. Thus, the product can selectively be produced easily, and the desired target product can be extracted with high purity.

[0125] The present invention is not limited to the above embodiments and modifications, and can be applied to various embodiments without departing from the spirit and scope of the present invention.