Method for producing acid and alkali, and bipolar membrane electrodialysis device
Patent Information
- Application Number
- AU2025261483
- Authority / Receiving Office
- AU · AU
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-04-24
- Filing Date
- 2025-04-23
- Publication Date
- 2026-10-29
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Figure 00000000_0000_ABST
Abstract
Description
TITLE OF INVENTION: METHOD FOR PRODUCING ACID AND ALKALI, AND BIPOLAR MEMBRANE ELECTRODIALYSIS DEVICE TECHNICAL FIELD
[0001] The present invention relates to a method for producing an acid and an alkali using a bipolar membrane electrodialysis device, a bipolar membrane electrodialysis device, an anion exchange membrane, and a method for operating a bipolar membrane electrodialysis device. BACKGROUND
[0002] As measures against global warming, reduction of CO2 emissions and adsorption and immobilization of CO2 have become major social challenges. Under these circumstances, as shown in FIG. 1, calcium is efficiently extracted from calcium-containing waste with an acid solution obtained by acid / alkali regeneration using bipolar membrane electrodialysis, and CO2 in exhaust gas or the like is directly absorbed by an alkali solution. By causing the two to react with each other to achieve CO2 mineralization, construction of a carbon recycling process for energy saving and resource saving has been proposed.
[0003] In the carbon recycling process shown in FIG. 1, the bipolar membrane electrodialysis device (BMED) uses a bipolar membrane, an anion exchange membrane (AEM), and a cation exchange membrane (CEM); the three types of membranes are arranged between an anode and a cathode to form three compartments, namely, an acid compartment, a raw-material compartment (salt-solution compartment), and an alkali compartment; and an aqueous salt solution (salt solution) is supplied to the raw-material compartment, and electrodialysis of the supplied salt is performed by applying an electric current, whereby an acid and an alkali are produced in the acid compartment and the alkali compartment, respectively. The present inventors have reported that, in the electrodialysis, hydrogen ions (H+) leak from the acid compartment to the raw material compartment through the AEM and that this leakage causes decreases in acid current efficiency and alkali current efficiency. The present inventors have further reported in Non Patent Literature No. 1 that H+ leakage can be prevented and acid current efficiency and alkali current efficiency can be increased by using an AEM having low H+ permeability and, as the salt in the salt solution, NaCl that generates Cl- ions, which more readily permeate the AEM (have higher ion mobility) than SO42- ions. However, further improvement in production efficiency is required, and there is a need to further reduce a specific energy consumption for alkali production. There is also a need to further reduce a specific energy consumption for acid production. Citation List Non Patent Literature
[0004] Non Patent Literature No. 1: “FY2020-FY2021 Results Report: Development of Carbon Recycling and Next-Generation Thermal Power Generation Technologies, etc. / Project for Promoting Next-Generation Thermal Power Generation Technologies / Development of Common Fundamental Technologies for Carbon Recycling Technologies / Research and Development of Ca Extraction from Calcium-Containing Waste and CO2 Mineralization Technology” (published on June 24, 2022 in the Results Report Database of the New Energy and Industrial Technology Development Organization (NEDO)). SUMMARY OF INVENTION Technical Problem
[0005] An object of the present invention is to provide a method for producing an acid and an alkali, a bipolar membrane electrodialysis device, and a method for operating a bipolar membrane electrodialysis device, each of which can reduce a specific energy consumption for alkali production or a specific energy consumption for acid production in production of an acid and an alkali using a bipolar membrane electrodialysis device. Solution to Problem
[0006] In the course of conducting extensive studies on the method for producing an acid and an alkali from a salt solution using a bipolar membrane electrodialysis device with the aim of improving production efficiency, the present inventors have focused on the specific energy consumption for alkali production. In the bipolar membrane electrodialysis device for producing an acid and an alkali, H+ leakage through an AEM (the amount of H+ permeated) reduces current efficiency, so that AEMs having less H+ leakage have been developed and used in the related field. However, such an AEM not only has a small amount of H+ permeated but also has reduced permeability to other ions, thereby causing an increase in cell voltage and, consequently, an increase in specific energy consumption, which has made it difficult to reduce the specific energy consumption. Departing from the approach in the related field, the present inventors have investigated the use of AEMs having a large amount of H+ permeated, which have not been used in the related field, and consequently have found that the specific energy consumption for alkali production can be reduced. The present inventors have also found that the specific energy consumption for alkali production can be optimized by expressing the amount of H+ permeated in terms of a hydrogen ion (H+) permeation flux and using an anion exchange membrane having a specific hydrogen ion permeation flux according to the type of salt in the salt solution to be supplied and an alkali concentration of an alkali aqueous solution to be produced. The present inventors have further found that, by using an anion exchange membrane having a specific hydrogen ion permeation flux according to the type of salt in the salt solution to be supplied and an acid concentration of an acid aqueous solution to be produced, not only the specific energy consumption for alkali production but also the specific energy consumption for acid production can be optimized. The present invention is thus completed.
[0007] That is, the present invention is specified by the following aspects. (1) A method for producing an acid and an alkali, the method including supplying a salt solution to a bipolar membrane electrodialysis device including, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, and then performing electrodialysis to produce an acid and an alkali, in which the anion exchange membrane has a hydrogen ion permeation flux of 7 x 10-8 mol-m’2-s-1 or more for hydrochloric acid or 150 x 10-8 mol-m—2-s-1 or more for sulfuric acid. (2) A bipolar membrane electrodialysis device including, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, in which the anion exchange membrane, an acid compartment in which an acid is produced, the bipolar membrane, an alkali compartment in which an alkali is produced, the cation exchange membrane, and a salt-solution compartment to which a salt solution is supplied constitute one unit, and a plurality of the units are arranged to form an ion exchange section, and the anion exchange membrane is an anion exchange membrane having a hydrogen ion permeation flux of 7 x 10-8 mol-m-2-s-1 or more for hydrochloric acid or 150 x 10-8 mol-m-2-s-1 or more for sulfuric acid. (3) An anion exchange membrane for use in a bipolar membrane electrodialysis device including, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, in which the anion exchange membrane, an acid compartment in which an acid is produced, the bipolar membrane, an alkali compartment in which an alkali is produced, the cation exchange membrane, and a salt-solution compartment to which a salt solution is supplied constitute one unit, and a plurality of the units are arranged to form an ion exchange section, in which the anion exchange membrane has a hydrogen ion permeation flux of 7 x 10-8 mol-m-2-s-1 or more for hydrochloric acid or 150 x 10-8 mol-m-2-s-1 or more for sulfuric acid. (4) A method for operating a bipolar membrane electrodialysis device including, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, by supplying a salt solution to the bipolar membrane electrodialysis device to produce an acid and an alkali, the method including selecting the anion exchange membrane to be used, according to a type of a salt in the salt solution to be supplied and / or an alkali concentration during operation, based on a hydrogen ion permeation flux of the anion exchange membrane for hydrochloric acid or a hydrogen ion permeation flux of the anion exchange membrane for sulfuric acid. (5) The method for operating a bipolar membrane electrodialysis device according to (4), in which, when the anion exchange membrane to be used is selected based on the hydrogen ion permeation flux for hydrochloric acid, either an anion exchange membrane having a hydrogen ion permeation flux of 7 * 10-8 mohm-^s-1 or more or an anion exchange membrane having a hydrogen ion permeation flux of less than 7 * 10-8 mofm-^s-1 is selected as the anion exchange membrane to be used, and when the anion exchange membrane to be used is selected based on the hydrogen ion permeation flux for sulfuric acid, either an anion exchange membrane having a hydrogen ion permeation flux of 150 * 10-8 mobm-2^s-1 or more or an anion exchange membrane having a hydrogen ion permeation flux of less than 150 * 10-8 inohiF's-' is selected as the anion exchange membrane to be used. (6) The method for operating a bipolar membrane electrodialysis device according to (4) or (5), in which a salt solution generated by a carbon recycling process using the bipolar membrane electrodialysis device operated by the operation method according to (4) or (5) is used as the salt solution supplied to the bipolar membrane electrodialysis device. Advantageous Effects of Invention
[0008] The method for producing an acid and an alkali, the bipolar membrane electrodialysis device, and the method for operating a bipolar membrane electrodialysis device according to the present invention can reduce the specific energy consumption for alkali production or the specific energy consumption for acid production, or both, in the production of an acid and an alkali using a bipolar membrane electrodialysis device. BRIEF DESCRIPTION OF DRAWINGS
[0009] [FIG. 1] A diagram for describing a carbon recycling process to which the present invention is applied. [FIG. 2] A schematic diagram of a bipolar membrane electrodialysis device. [FIG. 3] A diagram of an electrodialysis device for measuring an H+ permeation flux. [FIG. 4A] A diagram representing an apparatus for measuring JW and JS and calculation equations therefor. [FIG. 4B] A diagram representing an apparatus for measuring membrane resistance and measurement conditions. [FIG. 4C] A diagram representing an apparatus for measuring a cation transport number. [FIG. 5] A diagram representing apparatus conditions (a) and experimental conditions (b) used in Examples and Reference Examples. [FIG. 6] A diagram representing a relationship between alkali production and cell voltage in Examples and Reference Examples. [FIG. 7] A diagram representing a relationship between alkali production and cell voltage in Examples and Reference Examples. [FIG. 8] A diagram representing a relationship between alkali production and current efficiency in Examples and Reference Examples. [FIG. 9] A diagram representing a relationship between alkali production and current efficiency in Examples and Reference Examples. [FIG. 10] A diagram representing a relationship between alkali production and specific energy consumption for alkali production in Examples and Reference Examples. [FIG. 11] A diagram representing a relationship between alkali production and specific energy consumption for alkali production in Examples and Reference Examples. [FIG. 12] A diagram representing a relationship between acid production and specific energy consumption for acid production in Examples and Reference Examples. DESCRIPTION OF EMBODIMENTS
[0010] The method for producing an acid and an alkali according to the present invention is a method for producing an acid and an alkali, the method including supplying a salt solution to a bipolar membrane electrodialysis device including, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, and then performing electrodialysis to produce an acid and an alkali, in which the anion exchange membrane has a hydrogen ion permeation flux of 7 x 10-8 mol-m’2^’1 or more for hydrochloric acid or 150 x 10-8 mofm-^s-1 or more for sulfuric acid. The bipolar membrane is an ion exchange membrane having a structure in which an anion exchange layer and a cation exchange layer are bonded together, and the bipolar membrane in the present invention is not particularly limited as long as it is an ion exchange membrane having such a structure. Examples of commercially available membranes include Neosepta (registered trademark) BM-1 (manufactured by ASTOM Corporation), Neosepta BPU (manufactured by ASTOM Corporation), and Fumasep FBM (manufactured by FuMA-Tech GmbH). In the present invention, the hydrogen ion (H+) permeation flux of the anion exchange membrane for hydrochloric acid refers to a hydrogen ion permeation flux in electrodialysis measured at 25°C by applying a constant current of 0.1 A in an electrodialysis device in which the anion exchange membrane, with an effective membrane area of 8 cm2, is sandwiched between a 0.5 M HCl aqueous solution and a 1 M NaCl aqueous solution, and the hydrogen ion permeation flux of the anion exchange membrane for hydrochloric acid being 7 x 10-8 mobm'2-s'1 or more means that the hydrogen ion permeation flux measured as described above is 7 x 10-8 mol-mS’1 or more. Here, the effective membrane area is a membrane area through which ions actually permeate, and the same applies below. In the above-described measurement, a NaCl aqueous solution is placed on a negative-electrode side of the anion exchange membrane to be measured as the salt solution, an HCl aqueous solution is placed on a positive-electrode side as the acid solution, and a change in pH over time on the salt-solution side is observed to measure the hydrogen ion permeation flux. A specific method for measuring the hydrogen ion permeation flux is as described in Examples. Hereinafter, unless otherwise specified, the "hydrogen ion permeation flux of the anion exchange membrane for hydrochloric acid" means the hydrogen ion permeation flux under the measurement conditions described above. Furthermore, in the present invention, the hydrogen ion (H+) permeation flux of the anion exchange membrane for sulfuric acid refers to a hydrogen ion permeation flux in electrodialysis measured at 25°C by applying a constant current of 0.1 A in an electrodialysis device in which the anion exchange membrane, with an effective membrane area of 8 cm2, is sandwiched between a 0.5 M H2SO4 aqueous solution and a 1 M Na2SO4 aqueous solution, and the hydrogen ion permeation flux of the anion exchange membrane for sulfuric acid being 150 x 10-8 mobm-2-s-1 or more means that the hydrogen ion permeation flux measured as described above is 150 x 10-8 mol-mS’1 or more. In the above measurement, a Na2SO4 aqueous solution is placed on a negative-electrode side of the anion exchange membrane to be measured as the salt solution, an H2SO4 aqueous solution is placed on a positive-electrode side as the acid solution, and the change in pH over time on the salt-solution side is observed to measure the hydrogen ion permeation flux. A specific method for measuring the hydrogen ion permeation flux in the present invention is as described in Examples. Hereinafter, unless otherwise specified, the "hydrogen ion permeation flux of the anion exchange membrane for sulfuric acid" means the hydrogen ion permeation flux under the measurement conditions described above. In the present specification, mol / L may be expressed as M. In the present invention, the hydrogen ion permeation flux of the anion exchange membrane need only meet either the value specified for hydrochloric acid or the value specified for sulfuric acid, and may meet both. The anion exchange membrane according to the present invention is not particularly limited as long as it satisfies the above-described requirement for hydrogen ion permeation flux.
[0011] The cation exchange membrane in the present invention is not particularly limited as long as it is selectively permeable to cations over anions, and examples thereof include a membrane having a membrane resistance of 0.1 to 10 Q^cm2 (measured at a water temperature of 25°C with 0.5 M NaCl) and a cation transport number of 0.90 to 0.99 (measured at a water temperature of 25°C with 0.5 M NaCl). Examples of the cation exchange membrane include commercially available membranes such as Neosepta CSE (manufactured by ASTOM Corporation) and SELEMION (registered trademark) CMVN (manufactured by AGC Engineering Co., Ltd.). The salt solution in the present invention refers to an aqueous solution of a salt, and examples of the salt include chlorides, nitrates, sulfates, carbonates, and phosphates of an alkali metal. Specific examples thereof include NaCl, KCl, LiCl, NaNO3, KNO3, LiNO3, Na2SO4, K2SO4, Li2SO4, Na2CO3, K2CO3, Na3PO4, and K3PO4. In the present invention, regardless of whether a salt of a monovalent acid such as NaCl or a salt of a divalent or higher-valent acid such as Na2SO4 is used as the salt in the salt solution, it is sufficient that either the specified hydrogen ion permeation flux value for hydrochloric acid or the specified hydrogen ion permeation flux value for sulfuric acid be met. However, when the salt of the monovalent acid is used, the hydrogen ion permeation flux for hydrochloric acid may be used as a criterion for selecting the anion exchange membrane, and when the salt of the divalent or higher-valent acid is used, the hydrogen ion permeation flux for sulfuric acid may be used as a criterion for selecting the anion exchange membrane. The hydrogen ion permeation flux of the anion exchange membrane in the present invention is more preferably 10 x 10-8 mobm-2^s-1 or more for hydrochloric acid, or 200 x 10-8 mohm-2^s-1 or more for sulfuric acid. The upper limit of the hydrogen ion permeation flux is not particularly limited, and examples of the upper limit include 70 x 10-8 mol-m-^s’1 for hydrochloric acid or 600 x 10-8 mol-m-2-s-1 for sulfuric acid. In addition, the anion exchange membrane in the present invention preferably has a membrane resistance measured using 0.5 M NaCl of 0.5 Q^cm2 or less, and preferably has NaCl permeability (salt flux) in a diffusion dialysis system measured using 3 M NaCl of1.0 x 10’4 moPm-2^s-1 or more. Specific methods for measuring the membrane resistance and the salt flux are as described in Examples.
[0012] In the bipolar membrane electrodialysis device according to the present invention, the anion exchange membrane, the acid compartment in which the acid is produced, the bipolar membrane, the alkali compartment in which the alkali is produced, the cation exchange membrane, and the salt-solution compartment to which the salt solution is supplied constitute one unit, and a plurality of the units are arranged to form an ion exchange section. FIG. 2 is a schematic diagram illustrating the configuration of the unit. In FIG. 2, electrodes are disposed at both ends of the ion exchange section, and an electrode solution is supplied to electrode compartments in which the respective electrodes are disposed. In FIG. 2, AEM represents the anion exchange membrane, BP represents the bipolar membrane, and CEM represents the cation exchange membrane. MA represents a salt that dissociates into M+ and A- or into M+ and A2-, and an aqueous solution of the salt (MA) is supplied to the salt-solution compartment. In FIG. 2, water is supplied to the acid compartment and the alkali compartment; however, each compartment may be supplied with water or an aqueous solution containing a salt, and the salt may be the same as or different from the salt supplied to the salt-solution compartment. When aqueous solutions containing a salt are supplied to the acid compartment and the alkali compartment, the aqueous solutions preferably have lower salt concentrations than the aqueous salt solution supplied to the salt-solution compartment. A salt concentration of the salt solution supplied to the salt-solution compartment is preferably 0.5 to 2.0 mol / L, and a salt concentration of the aqueous solution supplied to the acid compartment and the alkali compartment is preferably 0.0001 to 1.0 mol / L. In the acid compartment, the concentration of A- or A2- that has permeated the AEM from the salt-solution compartment and moved into the acid compartment and the concentration of H+ generated in the BP increase, thereby producing an acid. In the alkali compartment, the concentration of M+ that has permeated the CEM from the salt-solution compartment and moved into the alkali compartment and the concentration of OH- generated in the BP increase, thereby producing an alkali. At this time, some of H+ in the acid compartment permeate the AEM and move into the salt-solution compartment, and some of H+ permeate the CEM and move into the alkali compartment. For example, the AEM, the acid compartment, the BP, and the next unit are arranged on the right side of the one unit shown in FIG. 2, that is, immediately to the right of the salt-solution compartment on the right side. Accordingly, some of H+ in the acid compartment of the next unit permeate the AEM and move into the salt-solution compartment of the first unit, and some of H+ permeate the CEM and move into the alkali compartment. An aqueous solution containing the acid is discharged from the acid compartment, and an aqueous solution containing the alkali is discharged from the alkali compartment. The bipolar membrane electrodialysis device according to the present invention may include a supply section and various solution tanks for supplying various solutions to the salt-solution compartment, the acid compartment, the alkali compartment, and the electrode compartment, and a discharge section for discharging the various solutions from the respective compartments; the various solutions discharged from the respective compartments may be supplied again to the respective compartments and circulated.
[0013] In the present invention, the specific energy consumption for alkali production means a specific energy consumption calculated by the following formula (1). In the following formula, the specific energy consumption for NaOH production is calculated. In order to calculate the specific energy consumption for another hydroxide (alkali), the "amount of NaOH per mole (kg)" in the formula (1) may be replaced with the amount (kg) of the other hydroxide per mole.
[0014] [Math. 1] Specific energy consumption = Electric energy at current I (kWh) Amount of alkali generated at current I (kg) = (Cell voltage x I) X 1 hour (h) X 0.001 I x 3600 x Current efficiency x Amount of NaOH per mole (kg)-r Faraday constant = Cell voltage X 0.001 X 96500 Cell voltage -------2------------------- = ■----x0.670 3600 x 0.040 x Current efficiency Current efficiency
[0015] In addition, the current efficiency in the present invention is calculated by the following formula (2). In the formula (2), the change in the number of moles is the change in the number of moles of the produced alkali, the current is the current (A) passed during alkali production, and the time is the time (s) required for alkali production.
[0016] [Math. 2] Change in number of moles X Faraday constant Current efficiency Current x Time x Number of units
[0017] The method for operating a bipolar membrane electrodialysis device according to the present invention is a method for operating a bipolar membrane electrodialysis device including, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, by supplying a salt solution to the bipolar membrane electrodialysis device to produce an acid and an alkali, the method including selecting the anion exchange membrane to be used, according to a type of a salt in the salt solution to be supplied and / or an alkali concentration during operation, based on a hydrogen ion permeation flux of the anion exchange membrane for hydrochloric acid or a hydrogen ion permeation flux of the anion exchange membrane for sulfuric acid. In addition, in the operation method according to the present invention, when the anion exchange membrane to be used is selected based on the hydrogen ion permeation flux for hydrochloric acid, it is preferable to select, as the anion exchange membrane to be used, either an anion exchange membrane having a hydrogen ion permeation flux of 7 x 108 mobm-2^s-1 or more or an anion exchange membrane having a hydrogen ion permeation flux of less than 7 x 10-8 mol-m’2-s’1, and when the anion exchange membrane to be used is selected based on the hydrogen ion permeation flux for sulfuric acid, it is preferable to select, as the anion exchange membrane to be used, either an anion exchange membrane having a hydrogen ion permeation flux of 150 x 10-8 mohif's-' or more or an anion exchange membrane having a hydrogen ion permeation flux of less than 150 x 10-8 mol-m’2-s’1. Furthermore, anion exchange membranes having a hydrogen ion permeation flux of less than 7 x 10-8 mohm-2^s-1 for hydrochloric acid can be further divided into two categories: those having a hydrogen ion permeation flux of 1.0 x 10-8 mobm-2^s-1 or more and less than 7 x 10-8 mol-m’2-s’1, and those having a hydrogen ion permeation flux of less than 1.0 x 10-8 mol-m’2-s’1. Therefore, the anion exchange membrane to be used can be selected from any one of the three categories in total. In addition, anion exchange membranes having a hydrogen ion permeation flux of less than 150 x 10-8 mobm-2^s-1 for sulfuric acid can be further divided into two categories: those having a hydrogen ion permeation flux of 40 x 10-8 mofm-2-s-1 or more and less than 150 x 10-8 mohm-2^s-1, and those having a hydrogen ion permeation flux of less than 40 x 10-8 mol-m’2-s’1. Therefore, the anion exchange membrane to be used can be selected from any one of the three categories in total. When selecting an anion exchange membrane having a hydrogen ion permeation flux of 7 x 10-8 mol-m-2^'1 or more for hydrochloric acid, it is preferable to select an anion exchange membrane having a hydrogen ion permeation flux of 10 x 10-8 mohirV or more for hydrochloric acid; and when selecting an anion exchange membrane having a hydrogen ion permeation flux of 150 x 10-8 mobm-2^s-1 or more for sulfuric acid, it is preferable to select an anion exchange membrane having a hydrogen ion permeation flux of 200 x 10-8 mohm-2^s-1 or more for sulfuric acid.
[0018] Here, the anion exchange membranes classified by the hydrogen ion permeation flux as described above are referred to, in descending order of hydrogen ion permeation flux, as a high H+ permeable membrane, a medium H+ permeable membrane, and a low H+ permeable membrane, respectively, for both hydrochloric acid and sulfuric acid. According to the operation method according to the present invention, for example, when a salt that generates an anion having low mobility in an anion exchange membrane, such as a sulfate (for example, a divalent or higher-valent anion such as SO42-), is used as the salt in the salt solution, a high H+ permeable membrane is selected regardless of the alkali concentration in the produced alkali aqueous solution. When a salt that generates an anion having high mobility in an anion exchange membrane, such as a chloride (for example, a monovalent anion such as Cl-), is used as the salt in the salt solution, a high H+ permeable membrane is selected when the device is operated in a region where the alkali concentration in the produced alkali aqueous solution is relatively low. When the device is operated in a higher alkali concentration region, an anion exchange membrane can be selected, according to the alkali concentration, from a medium H+ permeable membrane or a low H+ permeable membrane. As described above, with the operation method according to the present invention, the anion exchange membrane to be used can be optimized according to the type of salt in the salt solution to be supplied and the alkali concentration during operation, and the specific energy consumption for alkali production can be reduced. In the related field, the anion exchange membrane has not been selected based on the hydrogen ion permeation flux as in the present invention. By the present inventors, the discovery of the method for producing an acid and an alkali using a bipolar membrane electrodialysis device employing an anion exchange membrane having a large amount of H+ permeated, such as a high H+ permeable membrane that has not been used in the related field, made it possible, for the first time, to select an anion exchange membrane based on the hydrogen ion permeation flux, in order to reduce the specific energy consumption for alkali production. In particular, selecting the anion exchange membrane based on the hydrogen ion permeation flux of a high H+ permeable membrane makes it possible to optimize the specific energy consumption for alkali production to a low level.
[0019] In addition, when the bipolar membrane electrodialysis device according to the present invention in which the hydrogen ion permeation flux of the anion exchange membrane is 7 x 10-8 mol-m-^s’1 or more for hydrochloric acid or 150 x 10-8 mol-m-2^1 or more for sulfuric acid is used, it is also possible to operate the device by adjusting, according to the type of salt in the salt solution to be supplied, the alkali concentration during operation in the alkali aqueous solution to be produced. According to the operation method, for example, in a case where the specific energy consumption for alkali production can be reduced regardless of the alkali concentration in the produced alkali aqueous solution, as when sodium sulfate is used in the salt solution, or in a case where the effect of reducing the specific energy consumption for alkali production is exhibited in a relatively low alkali concentration region, as when sodium chloride is used in the salt solution, the alkali concentration can be controlled according to the type of salt in the salt solution to be supplied and adjusted to a region in which the specific energy consumption for alkali production can be reduced, thereby optimizing the specific energy consumption for alkali production.
[0020] The method for producing an acid and an alkali, the bipolar membrane electrodialysis device, and the method for operating a bipolar membrane electrodialysis device according to the present invention can be used in a carbon recycling process as shown in FIG. 1. As shown in FIG. 1, calcium is efficiently extracted from calcium-containing waste with an acid solution obtained by acid and alkali regeneration using bipolar membrane electrodialysis, and CO2 in exhaust gas or the like is directly absorbed by an alkali solution. By causing the two to react with each other, CO2 mineralization is achieved. At this time, the produced solution of an alkali salt such as NaCl is used as the salt solution supplied to the salt-solution compartment and as the solution supplied to the acid compartment and the alkali compartment of the bipolar membrane electrodialysis device, and an acid and an alkali are produced again by electrodialysis in the bipolar membrane electrodialysis device. By repeating the process, carbon recycling can be performed. With the method for operating the bipolar membrane electrodialysis device according to the present invention, the device can be operated by selecting the anion exchange membrane to be used based on the hydrogen ion permeation flux for hydrochloric acid or the hydrogen ion permeation flux for sulfuric acid, according to the type of alkali salt solution generated in the carbon recycling process and the alkali concentration in the alkali solution to be produced, thereby optimizing the specific energy consumption for alkali production. Recently, with the widespread use of lithium batteries and the like, demand for lithium hydroxide as a lithium source has increased, and particularly high-purity lithium hydroxide has been required. Therefore, a method has been proposed for producing lithium hydroxide from these lithium salts by using lithium chloride or lithium sulfate as a raw material and employing electrodialysis with a bipolar membrane; however, it has been difficult to improve production efficiency by reducing the specific energy consumption for lithium hydroxide production, that is, the specific energy consumption for alkali production. With the method for producing an acid and an alkali, the bipolar membrane electrodialysis device, and the method for operating a bipolar membrane electrodialysis device according to the present invention, the specific energy consumption for producing lithium hydroxide from a solution of a lithium salt such as lithium chloride and lithium sulfate by electrodialysis can be optimized to a low level, thereby enabling lithium hydroxide to be produced with high production efficiency. Therefore, the method for producing an acid and an alkali, the bipolar membrane electrodialysis device, and the method for operating a bipolar membrane electrodialysis device according to the present invention can be suitably used not only for alkali production for CO2 immobilization in the carbon recycling process but also for production of lithium hydroxide. Examples
[0021] The present invention will be described below with reference to Examples, but the present invention is not limited to these specific embodiments.
[0022] Characteristics of the membranes used in Examples and Reference Examples are shown in Table 1. In Table 1, CSE represents Neosepta CSE (manufactured by ASTOM Corporation), ASE represents Neosepta ASE (manufactured by ASTOM Corporation), FAS-20 represents FAS-20 (manufactured by Fumatech GmbH), AID represents Neosepta AID (manufactured by ASTOM Corporation), and ACM represents Neosepta ACM (manufactured by ASTOM Corporation). CSE and ASE are membranes commonly used as a cation exchange membrane and an anion exchange membrane, respectively, and ASE corresponds to a medium H+ permeable membrane. AID and ACM are generally referred to as anion exchange membranes having poor (low) H+ permeability and correspond to low H+ permeable membranes. In addition, FAS-20 corresponds to a high H+ permeable membrane. JW in Table 1 represents the water flux in a diffusion dialysis system, and JS represents the salt flux in a diffusion dialysis system.
[0023] [Table 1] Membrane resistance (Q^cm2) Moisture content (-) Ion exchange capacity (meq / g) Membrane thickness (W Jw (10-7 m^s-1) Js (10-4 moPm-2^-1) CEM CSE 2.03 0.27 1.98 148 2.72 2.56 AEM ASE 2.77 0.16 1.96 150 1.37 0.359 FAS-20 0.46 0.11 1.70 23 5.19 2.36 AID 4.2 0.13 1.10 160 0.194 0.0627 ACM 3.23 0.15 1.45 110 0.550 0.00416
[0024] (Measurement of H+ permeation flux) In order to evaluate the H+ permeation flux through a single AEM in electrodialysis, electrodialysis was performed using a small cell shown in FIG. 3. In the evaluation, because it was necessary to keep the pH of the solutions constant in order to calculate the H+ permeation flux more accurately, all the solutions were used for the dialysis experiment promptly after preparation. The laboratory was also ventilated to prevent an increase in carbon dioxide concentration. Sample membranes and end membranes used were each immersed twice in a 0.5 mol / L NaCl aqueous solution for 5 minutes, and then used to start the experiment. Each of the four AEMs used in Examples and Reference Examples was separately sandwiched in the sample-membrane section as a sample membrane, and CSE, which was used in Examples and Reference Examples, was sandwiched in the end-membrane section as the end membranes (CEMs). Electrodialysis was then performed at 25°C for 30 minutes at a constant current of 0.1 A. An effective membrane area was 8 cm2. A 1 mol / L NaCl was used as the electrode solution in the electrode compartments. The volumes of the electrode compartment, the salt-solution compartment, and the acidsolution compartment were each 200 mL. The amount of working solution placed in each of these compartments was 100 mL, and the solutions were stirred at 2500 rpm. After the measurement, in order to regenerate the electrodes, a current was passed at 0.5 A for 15 minutes in the direction opposite to that during the measurement. In the measurement, a NaCl aqueous solution or a Na2SO4 aqueous solution was charged as the salt solution into the salt-solution compartment on the negative-electrode side of the sample membrane, and an HCl aqueous solution or an H2SO4 aqueous solution was charged as the acid solution into the acid-solution compartment on the positive-electrode side of the sample membrane. The pH of the saltsolution compartment was measured continuously and converted into an H+ concentration, and a H+ permeation flux (Ji: mofm-^s-1) was calculated using the following equation (3) from the slope of the change in H+ concentration over dialysis time. In the following equation, V is the volume (m3) of the solution charged into the salt-solution compartment, S is the effective membrane area (m2), and ACi / At is the slope of the change in H+ concentration (concentration gradient) (mol-m-3-s’1). Table 2 shows the conditions of the solutions used in the experiment. Table 3 shows the average H+ permeation flux of each membrane obtained from two measurements performed under each condition.
[0025] [Math. 3] VAC, J , = ------------ ( 3 ) S A t
[0026] [Table 2] Salt-solution compartment Acid-solution compartment Solution Concentration (mol / L) Solution Concentration (mol / L) Condition 1 NaCl aqueous solution 1 HCl aqueous solution 0.5 Condition 2 Na2SO4 aqueous solution 1 H2SO4 aqueous solution 0.5
[0027] [Table 3] Condition 1 Condition 2 ASE 4.4 79 FAS-20 14 291 AID 0.34 21 ACM 0.038 25 (unit in numerical value is x10-8 mol-m-2-s-1)
[0028] (Measurement of JW and JS) Methods for measuring JW and JS in Table 1 are shown below. In order to measure water permeability and co-ion permeability, the water flux JW and the salt flux JS in a diffusion dialysis system were measured using the apparatus shown in FIG. 4A. Here, because the current flowing in the diffusion dialysis system is zero, fluxes of the counter-ion and the co-ion are equal to the salt flux, so that the permeability of the co-ion can be evaluated by measuring JS. In the apparatus, the sample membrane was sandwiched between two containers, Cell I and Cell II. A 3 M NaCl aqueous solution was charged into Cell I, which had a graduated capillary tube, and deionized water and a sensor for measuring conductivity were charged into Cell II. At predetermined measurement intervals, a change in the water level Ah in the capillary tube of Cell I was observed to determine the volume change AV = Ah x a (where a is the cross-sectional area of the capillary tube), and JW was calculated using the equation in FIG. 4A. In addition, at predetermined time intervals, the change in conductivity was measured using the conductivity meter placed in Cell II, the change AC in NaCl concentration was calculated from the conductivity change, and JS was calculated by substituting the calculated value into the equation in FIG. 4A. The temperature during the measurement was 25°C.
[0029] (Measurement of membrane resistance) A method for measuring a membrane resistance in Table 1 is shown below. FIG. 4B shows an apparatus for measuring membrane resistance and the measurement conditions. First, a 0.5 mol / L NaCl aqueous solution was charged as the measurement solution into an acrylic cell having platinum electrodes and a current-carrying area of 0.949 cm2, and a solution resistance (R0) at 25°C was measured at a frequency of 10 kHz using an LCR meter. Thereafter, the sample membrane was sandwiched between the two cells, and a resistance (R1) was measured in the same manner. The membrane resistance (Rm) was calculated from the equation Rm = R1 -Ro. In the equation, Ri [Q^cm2] is the measured resistance with the sample membrane sandwiched between the two cells, Ro [Q^cm2] is the solution resistance measured without the sample membrane, and Rm [Q^cm2] is the membrane resistance.
[0030] (Measurement of cation transport number) A cation transport number was measured by the following method. The membrane to be measured was sandwiched between two cells of the electrodialysis apparatus shown in FIG. 4C, which had an effective membrane area of 8.0 cm2 (2.0 cm x 4.0 cm), and 100 mL of a 0.5 M NaCl solution was charged into each cell. A silver chloride electrode was used as the negative electrode and a silver electrode was used as the positive electrode, and electrodialysis was performed at 25°C by applying a direct current at a current density of 10 mA / cm2 for 75 minutes. Thereafter, the measurement solutions were removed from the two cells and diluted to 300 mL in volumetric flasks, and the change in the number of moles Am during electrodialysis was determined from the conductivity measured using a conductivity meter. The Am was substituted into the equation t+ = Am / Ea to calculate a dynamic transport number, which was taken as the cation transport number. In the equation, t+ represents the dynamic transport number, Am represents the transferred equivalent (change in the number of moles during electrodialysis), Ea represents the theoretical equivalent (= I x t / F), I represents the current [A], t represents the electrodialysis time [s], and F represents the Faraday constant [C / mol]. As a result of the measurement, the cation transport number of CSE was 0.98.
[0031] [Examples 1 to 5] An acid and alkali production experiment was performed using a benchtop bipolar membrane electrodialysis device equipped with the ion exchange membrane unit shown in FIG. 2. The device has 10 cell pairs, with the AEM, acid compartment, BP, alkali compartment, CEM, and salt-solution compartment in the schematic diagram of the bipolar membrane electrodialysis device shown in FIG. 2, constituting one cell pair (unit). The effective membrane area per membrane is 55 cm2. It also has nickel electrodes and electrode compartments at both ends thereof. FIG. 5A shows the apparatus conditions for the device used. FIG. 5B shows the experimental conditions. In the experiment, the predetermined salt solution (salt water), acid solution (solution supplied to the acid compartment), alkali solution (solution supplied to the alkali compartment), and electrode solution shown in FIG. 5B were supplied by pumps to the respective compartments and circulated through the salt-solution tank, acid-solution tank, alkali-solution tank, and electrode-solution tank, respectively. Along with the movement of the ions in the electrodialysis experiment, water (hydrated water) associated with the ions also moves. Therefore, in the experiment, each tank was placed on an electronic balance, and the change in volume over time was calculated from the corresponding change in weight. In the experiment, CSE was used as the cation exchange membrane (CEM), and Neosepta BPU (manufactured by ASTOM Corporation) was used as the bipolar membrane. In addition, FAS-20 was used as the anion exchange membrane (AEM). As the salt in the salt solution, Example 1 used KCl, Example 2 used NaCl, Example 3 used LiCl, Example 4 used Na2SO4, and Example 5 used Li2SO4. In each of Examples, an aqueous solution of the same salt as that in the salt solution supplied to the salt compartment was supplied to the alkali compartment and the acid compartment. However, the initial salt concentrations of the aqueous solutions supplied to the respective compartments were 1.0 M in the salt compartment, 0.001 M in the alkali compartment, and 0.001 M in the acid compartment. As the electrode solution, 1.0 M NaOH was used. The initial volume of each of these solutions was 500 mL. The initial solution volume means the sum of the volumes of solution in each compartment, each solution tank, and the piping connecting each compartment and each solution tank. Electrodialysis was performed at a constant current of 3.0 A in the experiments using LiCl and Li2SO4 aqueous solutions, and at a constant current of 4.4 A in the experiments using aqueous solutions of the other salts. The change over time in the concentration of the solution in each tank was measured using a pH meter or a conductivity meter and recorded on a logging PC. During the electrodialysis, the ion concentration and solution volume in each solution tank, the voltage between the electrodes, and the voltage between the platinum electrodes were measured over time.
[0032] [Reference Examples 1 to 3] Reference Examples 1 to 3 were performed in the same manner as Example 1, except that the anion exchange membrane used was changed. The anion exchange membranes used were ASE, AID, and ACM in the order of Reference Examples 1 to 3.
[0033] [Reference Examples 4 to 6] Reference Examples 4 to 6 were performed in the same manner as Example 2, except that the anion exchange membrane used was changed. The anion exchange membranes used were ASE, AID, and ACM in the order of Reference Examples 4 to 6.
[0034] [Reference Examples 7 to 9] Reference Examples 7 to 9 were performed in the same manner as Example 3, except that the anion exchange membrane used was changed. The anion exchange membranes used were ASE, AID, and ACM in the order of Reference Examples 7 to 9.
[0035] [Reference Examples 10 to 12] Reference Examples 10 to 12 were performed in the same manner as Example 4, except that the anion exchange membrane used was changed. The anion exchange membranes used were ASE, AID, and ACM in the order of Reference Examples 10 to 12.
[0036] [Reference Examples 13 to 15] Reference Examples 13 to 15 were performed in the same manner as Example 5, except that the anion exchange membrane used was changed. The anion exchange membranes used were ASE, AID, and ACM in the order of Reference Examples 13 to 15.
[0037] (Cell voltage) A cell voltage is a voltage across one cell (one unit) including the AEM, the acid compartment, the bipolar membrane (BP), the alkali compartment, the CEM, and the salt solution compartment shown in FIG. 2. In the experiment, platinum strip electrodes were placed, respectively, in contact with the membrane on the anode-compartment side of the membrane adjacent to the anode electrode compartment (anode compartment) and with the membrane on the cathode-compartment side of the membrane adjacent to the cathode electrode compartment (cathode compartment), and a value obtained by dividing the voltage between the two platinum strip electrodes during the experiment by the number of pairs (10 pairs in the experiment) was taken as the cell voltage. FIGS. 6 and 7 show a relationship between the cell voltage and alkali concentration during production. The vertical axis of the figure represents the cell voltage, and the horizontal axis represents the alkali concentration of the alkali aqueous solution produced in the alkali compartment. FIG. 6A shows the results of Example 1 and Reference Examples 1 to 3 in which KCl was used as the salt in the salt solution; FIG. 6B shows the results of Example 2 and Reference Examples 4 to 6 in which NaCl was used as the salt in the salt solution; and FIG. 6C shows the results of Example 3 and Reference Examples 7 to 9 in which LiCl was used as the salt in the salt solution. FIG. 7A shows the results of Example 4 and Reference Examples 10 to 12 in which Na2SO4 was used as the salt in the salt solution; and FIG. 7B shows the results of Example 5 and Reference Examples 13 to 15 in which Li2SO4 was used as the salt in the salt solution. When KCl, NaCl, or LiCl was used as the salt, the cell voltage initially decreased in Examples and Reference Examples and then increased after reaching a certain value. This is because, as time elapses, an alkali and an acid are produced in the 0.001 M aqueous salt solutions initially present in the alkali compartment and the acid compartment, respectively, but it takes time for the produced alkali and acid to circulate through the device and reach uniform concentrations. In addition, the subsequent increase in cell voltage occurs because the salt solution in the salt compartment, which has an initial concentration of 1 M, is desalted over time, thereby decreasing its conductivity. When Na2SO4 or Li2SO4 was used as the salt, the cell voltage initially decreased and then remained at a substantially constant value. Here, when comparing the cell voltages obtained when KCl, NaCl, or LiCl was used as the salt, in the cases of KCl and NaCl, the relationship FAS-20 < ASE = ACM < AID was observed over almost all alkali concentration ranges. In the case of LiCl, the relationship FAS-20 < ASE < AID < ACM was observed over almost all alkali concentration ranges. When Na2SO4 or Li2SO4 was used as the salt, the relationship FAS-20 < ASE < ACM < AID was observed over all alkali concentration ranges. When the cell voltage was compared between the case where a chloride was used as the salt and the case where a sulfate was used as the salt, the cell voltage was higher in the sulfate case than in the chloride case, except for FAS-20, and was particularly high when AID was used with a sulfate. This is because Cl- has higher mobility in the membrane than SO42-. Here, in the descriptions concerning the cell voltage above and the alkali current efficiency and specific energy consumption for alkali production described below, "<" means that the value obtained when the anion exchange membrane indicated on the right is used is higher than the value obtained when the anion exchange membrane indicated on the left is used, and "=" means that the value obtained when the anion exchange membrane indicated on the right is used is substantially the same as the value obtained when the anion exchange membrane indicated on the left is used.
[0038] (Current efficiency) FIGS. 8 and 9 show a relationship between current efficiency for alkali production (alkali current efficiency) and alkali concentration during production. The vertical axis of the figure represents the current efficiency, and the horizontal axis represents the alkali concentration of the alkali aqueous solution produced in the alkali compartment. FIG. 8A shows the results of Example 1 and Reference Examples 1 to 3 in which KCl was used as the salt in the salt solution; FIG. 8B shows the results of Example 2 and Reference Examples 4 to 6 in which NaCl was used as the salt in the salt solution; and FIG. 8C shows the results of Example 3 and Reference Examples 7 to 9 in which LiCl was used as the salt in the salt solution. FIG. 9A shows the results of Example 4 and Reference Examples 10 to 12 in which Na2SO4 was used as the salt in the salt solution; and FIG. 9B shows the results of Example 5 and Reference Examples 13 to 15 in which Li2SO4 was used as the salt in the salt solution. In Examples and Reference Examples, the alkali current efficiency increases in the low-alkali-concentration region because an acid and an alkali are produced when current is passed through the device, but it takes time for the produced acid and alkali to spread uniformly through the device, resulting in low alkali current efficiency at the beginning of operation. Except for the region, the current efficiency when NaCl was used was close to 100%, but decreased as the alkali concentration increased. Similarly, when KCl or LiCl was used, the current efficiency decreased as the alkali concentration increased. For KCl, NaCl, and LiCl, the decrease in current efficiency was greatest for FAS-20, followed by ASE. In almost all alkali concentration regions, the value of the current efficiency was FAS-20 < ASE < AID < ACM when KCl was used, FAS-20 < ASE < AID = ACM when NaCl was used, and FAS-20 < ASE = ACM < AID when LiCl was used. In addition, when Na2SO4 or Li2SO4 was used as the salt, the alkali current efficiency was also low at the beginning of operation, then increased in the low-concentration region, and decreased as the alkali concentration increased. However, FAS-20, ASE, ACM, and AID exhibited approximately the same current efficiency over almost the entire alkali concentration range, and the rates of decrease in current efficiency with increasing alkali concentration were also approximately the same.
[0039] (Specific energy consumption for alkali production) FIGS. 10 and 11 show a relationship between specific energy consumption for alkali production and alkali concentration during production. The vertical axis of the figure represents the specific energy consumption, and the horizontal axis represents the alkali concentration of the alkali aqueous solution produced in the alkali compartment. FIG. 10A shows the results of Example 1 and Reference Examples 1 to 3 in which KCl was used as the salt in the salt solution; FIG. 10B shows the results of Example 2 and Reference Examples 4 to 6 in which NaCl was used as the salt in the salt solution; and FIG. 10C shows the results of Example 3 and Reference Examples 7 to 9 in which LiCl was used as the salt in the salt solution. FIG. 11A shows the results of Example 4 and Reference Examples 10 to 12 in which Na2SO4 was used as the salt in the salt solution; and FIG. 11B shows the results of Example 5 and Reference Examples 13 to 15 in which Li2SO4 was used as the salt in the salt solution. The rapid decrease in specific energy consumption in the low-concentration region accompanies the increase in current efficiency shown in FIGS. 8 and 9 and is considered to occur because, at the beginning of operation, the produced alkali has not yet spread uniformly throughout the alkali-side tank. When KCl, NaCl, or LiCl was used as the salt, FAS-20 had the lowest specific energy consumption in the low-alkali-concentration region. At alkali concentrations higher than those in the region, when KCl or NaCl was used, the specific energy consumption when ACM was used was the lowest, and the values when ASE and AID were used were substantially the same. When LiCl was used, the values when ASE and AID were used were substantially the same and were the lowest. When Na2SO4 or Li2SO4 was used as the salt, in either case, FAS-20 showed the lowest specific energy consumption over the entire alkali concentration range, and the order was FAS-20 < ASE < ACM < AID. From the results shown in FIGS. 10 and 11, based on differences in hydrogen ion permeation flux, an anion exchange membrane that can minimize the specific energy consumption for alkali production can be selected according to the type of salt and the alkali concentration of the alkali aqueous solution to be obtained by operating the bipolar membrane electrodialysis device. For example, when a salt of a monovalent acid such as NaCl was used as the salt in the salt solution, it was possible to select, as the anion exchange membrane, a high H+ permeable membrane having a high hydrogen ion permeation flux, such as FAS-20, when operating in a low alkali concentration region, a low H+ permeable membrane having a low hydrogen ion permeation flux, such as ACM and AID, when operating in a higher alkali concentration region, and, depending on the case, a medium H+ permeable membrane having an intermediate hydrogen ion permeation flux, such as ASE. In addition, when a salt of a divalent or higher-valent acid such as Na2SO4 was used as the salt in the salt solution, it was possible to select a high H+ permeable membrane having a high hydrogen ion permeation flux, such as FAS-20, for use regardless of the alkali concentration during operation. As the hydrogen ion permeation flux of the anion exchange membrane increased, the cell voltage decreased, which led to a reduction in the specific energy consumption for alkali production. However, as the hydrogen ion permeation flux increased, the alkali current efficiency decreased, and the extent of the decrease was particularly large in a high alkali concentration region. Therefore, depending on the type of salt in the salt solution, the relative values of the specific energy consumption for alkali production may be reversed in a high alkali concentration region, and a permeable membrane other than the high H+ permeable membrane may have a lower specific energy consumption for alkali production. When operating in such an alkali concentration region, the specific energy consumption for alkali production could be kept low by using a medium H+ permeable membrane or a low H+ permeable membrane. In addition, as the hydrogen ion permeation flux of the anion exchange membrane increased, the specific energy consumption for alkali production at a low alkali concentration decreased; however, when the hydrogen ion permeation flux became too high, the alkali concentration range in which the specific energy consumption for alkali production could be reduced became narrower. Therefore, in a practical alkali concentration range, an upper limit of the hydrogen ion permeation flux of the high H+ permeable membrane could be determined so that the specific energy consumption for alkali production could be made lower than that of other membranes.
[0040] (Specific energy consumption for acid production) FIG. 12 shows a relationship between specific electrical energy consumption for acid production (specific energy consumption for acid production) and acid concentration. The vertical axis of the figure represents the specific electrical energy consumption, and the horizontal axis represents the acid concentration (H+ concentration) of the acid aqueous solution produced in the acid compartment. FIG. 12A shows the results of Example 1 and Reference Examples 1 and 2 in which KCl was used as the salt in the salt solution; FIG. 12B shows the results of Example 2 and Reference Examples 4 and 5 in which NaCl was used as the salt in the salt solution; and FIG. 12C shows the results of Example 4 and Reference Examples 10 and 11 in which Na2SO4 was used as the salt in the salt solution. In addition, Table 4 shows the numerical values of the respective measurement results.
[0041] [Table 4] Acid concentration (mol / L) Specific energy consumption (kWh / kg-HCl) (kWh / kg-HCl) (kWh / kg-H2SO4) Example 1 Reference Example 1 Reference Example 2 Example 2 Reference Example 4 Reference Example 5 Example 4 Reference Example 10 Reference Example 11 0.2 1.65 1.88 1.91 2.05 2.32 2.47 0.82 1.00 1.32 0.5 1.48 1.54 1.57 1.69 1.78 1.88 0.69 0.93 1.38 0.7 1.57 1.48 1.51 1.75 1.71 1.76 0.67 0.90 1.35
[0042] When KCl, NaCl, or Na2SO4 was used as the salt, the specific energy consumption when FAS-20 was used was the lowest in the low acid concentration region. At acid concentrations higher than those in the region, when KCl or NaCl was used, the values obtained using ASE and AID were substantially the same. When Na2SO4 was used as the salt, in all cases, FAS-20 showed the lowest specific energy consumption over the entire acid concentration range, and the order was FAS-20 < ASE < AID. From the results shown in FIG. 12, based on differences in hydrogen ion permeation flux, an anion exchange membrane that can minimize the specific energy consumption for acid production can be selected according to the type of salt and the acid concentration of the acid aqueous solution to be obtained by operating the bipolar membrane electrodialysis device. For example, when a salt of a monovalent acid such as NaCl was used as the salt in the salt solution, it was possible to select, as the anion exchange membrane, a high H+ permeable membrane having a high hydrogen ion permeation flux, such as FAS-20, when operating in a low acid concentration region, a medium H+ permeable membrane having an intermediate hydrogen ion permeation flux, such as ASE, when operating in a higher acid concentration region, and, depending on the case, a low H+ permeable membrane having a low hydrogen ion permeation flux, such as AID. In addition, when a salt of a divalent or higher-valent acid such as Na2SO4 was used as the salt in the salt solution, it was possible to select a high H+ permeable membrane having a high hydrogen ion permeation flux, such as FAS-20, for use regardless of the acid concentration during operation. As the hydrogen ion permeation flux of the anion exchange membrane increased, the cell voltage decreased, which led to a reduction in the specific energy consumption for acid production. However, as the hydrogen ion permeation flux increased, the acid current efficiency decreased, and the extent of the decrease was particularly large in a high acid concentration region. Therefore, depending on the type of salt in the salt solution, in a high acid concentration region, the specific energy consumption for acid production when a high H+ permeable membrane was used and that when a medium H+ permeable membrane or a low H+ permeable membrane was used may become substantially the same or may be reversed. When operating in such an acid concentration region, the specific energy consumption for acid production could be kept low by using a medium H+ permeable membrane or a low H+ permeable membrane. In addition, as the hydrogen ion permeation flux of the anion exchange membrane increased, the specific energy consumption for acid production at a low acid concentration decreased; however, when the hydrogen ion permeation flux became too high, the acid concentration range in which the specific energy consumption for acid production could be reduced became narrower. Therefore, in a practical acid concentration range, an upper limit of the hydrogen ion permeation flux of the high H+ permeable membrane could be determined so that the specific energy consumption for acid production could be made lower than that of other membranes. INDUSTRIAL APPLICABILITY
[0043] With the method for producing an acid and an alkali, the bipolar membrane electrodialysis device, the anion exchange membrane, and the method for operating a bipolar membrane electrodialysis device according to the present invention, the specific energy consumption for alkali production or the specific energy consumption for acid production, or both, can be reduced, and these inventions can be suitably used in a carbon recycling process and the like. These inventions can also be suitably used for production of lithium hydroxide.
Claims
1. A method for producing an acid and an alkali, the method comprising supplying a salt solution to a bipolar membrane electrodialysis device including, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, and then performing electrodialysis to produce an acid and an alkali,wherein the anion exchange membrane has a hydrogen ion permeation flux of 7 x 10-8 mol-m-^s’1 or more for hydrochloric acid or 150 x 10-8 mol-m-2^1 or more for sulfuric acid.
2. A bipolar membrane electrodialysis device comprising, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane,wherein the anion exchange membrane, an acid compartment in which an acid is produced, the bipolar membrane, an alkali compartment in which an alkali is produced, the cation exchange membrane, and a salt-solution compartment to which a salt solution is supplied constitute one unit, and a plurality of the units are arranged to form an ion exchange section, and the anion exchange membrane is an anion exchange membrane having a hydrogen ion permeation flux of 7 x 10’8 mol-m^s'1 or more for hydrochloric acid or 150 x 10’8 mol-m'2-s-1 or more for sulfuric acid.
3. An anion exchange membrane for use in a bipolar membrane electrodialysis device including, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, in which the anion exchange membrane, an acid compartment in which an acid is produced, the bipolar membrane, an alkali compartment in which an alkali is produced, the cation exchange membrane, and a salt-solution compartment to which a salt solution is supplied constitute one unit, and a plurality of the units are arranged to form an ion exchange section,wherein the anion exchange membrane has a hydrogen ion permeation flux of 7 x 10-8 mofm-2-s-1 or more for hydrochloric acid or 150 x 10’8 mofm-2-s-1 or more for sulfuric acid.
4. A method for operating a bipolar membrane electrodialysis device including, as ion exchange membranes, a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, by supplying a salt solution to the bipolar membrane electrodialysis device to produce an acid and an alkali, the method comprising selecting the anion exchange membrane to be used, according to a type of a salt in the salt solution to be supplied and / or an alkali concentration during operation, based on a hydrogen ion permeation flux of the anion exchange membrane for hydrochloric acid or a hydrogen ion permeation flux of the anion exchange membrane for sulfuric acid.
5. The method for operating a bipolar membrane electrodialysis device according to Claim 4,wherein, when the anion exchange membrane to be used is selected based on the hydrogen ion permeation flux for hydrochloric acid, either an anion exchange membrane having a hydrogen ion permeation flux of 7 x 10-8 mobm-2^s-1 or more or an anion exchange membrane having a hydrogen ion permeation flux of less than 7 x 10-8 mofm-^s-1 is selected as the anion exchange membrane to be used, andwhen the anion exchange membrane to be used is selected based on the hydrogen ion permeation flux for sulfuric acid, either an anion exchange membrane having a hydrogen ion permeation flux of 150 x 10-8 mol-m'2-s-1 or more or an anion exchange membrane having a hydrogen ion permeation flux of less than 150 x 10-8 mohm-^s-1 is selected as the anion exchange membrane to be used.
6. The method for operating a bipolar membrane electrodialysis device according to Claim 4 or 5, wherein a salt solution generated by a carbon recycling process using the bipolar membrane electrodialysis device operated by the operation method according to Claim 4 or 5 is used as the salt solution supplied to the bipolar membrane electrodialysis device.