Material-cylinder-free continuous multistage countercurrent electrodialysis system and electrodialysis method

Abstract

The invention belongs to the technical field of electrodialysis, and particularly relates to a material-cylinder-free continuous multistage countercurrent electrodialysis system and an electrodialysis method. The material-cylinder-free continuous multistage countercurrent electrodialysis system comprises more than two stages of membrane stacks, each compartment of each stage of membrane stack is not provided with a material cylinder, and the compartments of each stage of membrane stack are directly connected through pipelines. The feed liquid in each compartment of each stage of membrane stack in the electrodialysis system is organized in a countercurrent manner, and in-stack self-circulation of the feed liquid in each compartment is realized through a circulating pump. The system is simple, convenient to connect, quick to start and quick in dynamic response. The interstage flow is stable, and the whole system is convenient to control.

Description

Technical Field

[0001] The present invention belongs to the field of electrodialysis technology, and specifically, relates to a non-tank continuous multi-stage countercurrent electrodialysis system and an electrodialysis method. Mainly, when electrodialysis (ordinary electrodialysis, bipolar membrane electrodialysis, etc.) is used to treat a salt-containing liquid to achieve desalination, salt concentration or acid-base regeneration, an electrodialysis system and an electrodialysis method are provided to reduce the energy consumption of electrodialysis treatment of salt-containing liquid, improve current efficiency and desalination rate. Background Art

[0002] Electrodialysis is a membrane separation technology that utilizes the selective permeability of ion exchange membranes to migrate charged ions across the membrane under the action of an electric field to achieve solution desalination and salt concentration. In conventional electrodialysis (hereinafter referred to as ordinary electrodialysis), there are alternating anion and cation exchange membranes in the membrane stack. The membrane stack structure (taking sodium chloride as an example) and the schematic diagram of ion migration are shown in the figure below. Figure 1 As shown, AM is an anion exchange membrane and CM is a cation exchange membrane. An ion exchange membrane is a thin film with ion selective permeability, which allows ions with opposite electrical properties to the fixed groups on the membrane to pass through, while blocking ions with the same electrical properties as the fixed groups on the membrane. That is, a cation exchange membrane (hereinafter referred to as a cation membrane) allows positively charged ions to pass through, while blocking negatively charged ions; an anion exchange membrane (hereinafter referred to as an anion membrane) allows negatively charged ions to pass through, while blocking positively charged ions. The raw material solution (salt-containing solution) enters the desalination chamber (that is, circulates between the desalination chamber and the material cylinder), and under the action of the electric field, anions and cations migrate across the membrane to the concentration chambers on both sides, and finally a desalted feed solution is obtained in the desalination chamber, and a concentrated salt solution is obtained in the concentration chamber.

[0003] Based on conventional electrodialysis, new electrodialysis technologies such as bipolar membrane electrodialysis, selective electrodialysis, and displacement electrodialysis have been developed. Among them, the bipolar membrane is an ion exchange membrane with special functions. It can dissociate water molecules in the membrane into H under the action of an electric field. + and OH - and released outside the membrane. The bipolar membrane releases H + The positive side releases OH - The negative side is called the negative side. When the bipolar membrane is used in combination with anion and cation exchange membranes, bipolar membrane electrodialysis can regenerate salt into corresponding acids and bases, while desalting the feed liquid (Industrial Water Treatment, 2021, 41(05): 36-41.).

[0004] Figure 2The diagram shows the membrane stack structure (taking sodium sulfate as an example) and ion migration diagram of conventional "acid-salt-base" three-chamber bipolar membrane electrodialysis. AM is an anion exchange membrane (hereinafter referred to as anion membrane), CM is a cation exchange membrane (hereinafter referred to as cation membrane), and BM is a bipolar membrane; the compartment between AM and CM is called the salt chamber, and a salt-containing feed solution (aqueous solution of sodium sulfate) is initially added to the salt chamber; the compartment between AM and BM is called the acid chamber, and the compartment between CM and BM is called the base chamber; under the action of the electric field, the water molecules in the bipolar membrane dissociate into H + and OH - , move toward the cathode and anode into the acid chamber and alkaline chamber respectively, H + With the anions (SO4 2- ) combine in the acid chamber to obtain acid (H2SO4); OH - The cations (Na + ) to obtain a base (NaOH) in the base chamber.

[0005] Will Figure 2 The anionic membrane in the "acid-salt-alkali" three-chamber bipolar membrane electrodialysis stack shown in the figure is removed, that is, the salt chamber and the acid chamber are combined to obtain a "salt-alkali" two-chamber structure membrane stack. The conventional "salt-alkali" two-chamber bipolar membrane electrodialysis stack structure (taking sodium gluconate as an example) and the schematic diagram of ion migration are shown in Figure 3(a). Among them, CM is a cation exchange membrane, BM is a bipolar membrane; the compartment between BM and CM is the salt chamber, and a salt-containing feed solution (aqueous solution of sodium gluconate) is initially added to the salt chamber; the compartment between CM and BM is the alkali chamber; under the action of the electric field, the water molecules in the bipolar membrane dissociate into OH - and H + , respectively migrate into the base chamber and the salt chamber, and the cations in the salt chamber (Na + ) migrates into the alkali chamber, obtains alkali (NaOH) in the alkali chamber, and obtains a mixture of acid and salt (a mixture of gluconic acid and remaining sodium gluconate) in the salt chamber.

[0006] Likewise, Figure 2 The cation membrane in the "acid-salt-base" three-chamber bipolar membrane electrodialysis stack shown in the figure is removed, that is, the salt chamber and the alkali chamber are combined to obtain an "acid-salt" two-chamber structure membrane stack. The conventional "acid-salt" two-chamber bipolar membrane electrodialysis stack structure (taking pentamethylenediamine hydrochloride as an example) and the schematic diagram of ion migration are shown in Figure 3(b). AM is an anion exchange membrane, BM is a bipolar membrane; the compartment between BM and AM is the acid chamber; the compartment between AM and BM is the salt chamber, and a salt-containing feed solution (aqueous solution of pentamethylenediamine hydrochloride) is initially added to the salt chamber; under the action of the electric field, the water molecules in the bipolar membrane dissociate into H + and OH - , respectively migrate into the acid chamber and the salt chamber, and the anions in the salt chamber (Cl -) migrates into the acid chamber, obtains acid (hydrochloric acid) in the acid chamber, and obtains a mixture of alkali and salt (a mixture of pentamethylenediamine and remaining pentamethylenediamine hydrochloride) in the salt chamber.

[0007] In addition, there are other types of electrodialysis membrane stack structures, such as double decomposition electrodialysis and ion exchange electrodialysis (figure omitted).

[0008] Traditionally, electrodialysis is performed in batch mode, e.g. Figure 4 As shown in the figure: add the initial feed liquid of each chamber into the feed tank, turn on the power supply and the circulation pump of each chamber, and the feed liquid circulates between the electrodialysis membrane stack and the feed tank until the predetermined desalination rate or concentration or concentration multiple is reached, and then the feed liquid of each chamber is discharged. The traditional batch operation mode requires many auxiliary equipment and frequent loading and unloading of feed liquid, especially in the later stage of the electrodialysis process, the ions migrate against the concentration, which increases the energy consumption. The situation of bipolar membrane electrodialysis is similar. The problem of reverse concentration migration is more serious in the batch operation mode because H + And Yu OH - The migration activity of anion exchange membranes in aqueous solution is significantly higher than that of other anions. + The blocking effect of cation exchange membrane on OH is limited. - The blocking effect is also limited. With the increase of the concentration of regenerated acid and alkali in the acid chamber and alkali chamber, the H + It is very easy to "leak" into the salt chamber through the anion exchange membrane, and OH in the alkali chamber - It is very easy to "leak" into the salt chamber through the cation exchange membrane. In the batch operation mode, the "leakage" in the later stage of bipolar membrane electrodialysis is more serious, making it difficult to regenerate bipolar membrane electrodialysis to obtain high acid and alkali concentrations. In addition, the "leakage" reduces the current efficiency of bipolar membrane electrodialysis, resulting in increased energy consumption.

[0009] To this end, people have proposed a multi-stage countercurrent electrodialysis method (CN113477090A; Journal of Process Engineering, 2010, 10(02): 276-81.; CN201010034136.6, CN201010150056.7), in which the feed liquids at each stage are organized in a countercurrent manner, which reduces energy consumption and increases acid concentration compared to the same operating conditions without classification. However, no specific electrodialysis system solution is given.

[0010] The conventional multi-stage countercurrent electrodialysis system is a batch multi-stage countercurrent electrodialysis system, such as Figure 5As shown, the feed liquids of each compartment (more than two compartments) of each level of membrane stack are organized in a countercurrent manner, but each level still adopts batch operation, that is, the feed liquids of the concentration chamber and desalination chamber of ordinary electrodialysis, or the feed liquids of the acid chamber, salt chamber, and alkali chamber of bipolar membrane electrodialysis are organized in a countercurrent manner in the whole process, but each level is like batch single-stage electrodialysis or bipolar membrane electrodialysis. When the treatment reaches a predetermined level, the finished liquid of the current level cylinder is transferred to the next level cylinder (as the initial liquid of the next level), and the finished liquid of the previous level cylinder is transferred to the current level cylinder (as the initial liquid of the current level), and a new batch is started. The batch multi-stage countercurrent electrodialysis system reduces the concentration difference on both sides of the membrane to a certain extent, and alleviates the problem of high energy consumption when ions migrate against concentration, but the operation process is cumbersome and difficult to manage. Each level (set) of equipment (electrodialyzer) must undergo a time-varying process, regardless of whether the constant pressure or constant current mode is adopted.

[0011] To this end, people have proposed a continuous multi-stage countercurrent electrodialysis system, such as Figure 6 As shown (shown are two compartments, such as ordinary electrodialysis or two-compartment bipolar membrane electrodialysis). Since the electrodialysis membrane stack needs to maintain a sufficient linear velocity (usually the liquid flow linear velocity is about 3 cm / s) to ensure mixing and mass transfer in the membrane stack, the feed liquid in each compartment of each level of the membrane stack needs to maintain a high-speed self-circulating flow (circulating through the membrane stack outlet, the material cylinder, and the membrane stack inlet), and at the same time, a small portion of the feed liquid is diverted from the membrane stack outlet to the material cylinder of the next level of the membrane stack (Desalination, 2003, 153 (1-3): 371-6.).

[0012] Figure 6 The system shown still retains the material tanks of the liquid in each compartment of each level of membrane stack. The outlet of each level of membrane stack is connected to the material tank of the next level of membrane stack, and the flow between each level is regulated by valves ( Figure 6 The disadvantage is that it is difficult to ensure the consistency of flow between each level unless a complex flow control system based on the feedback of the liquid level position of each cylinder is added.

[0013] and Figure 6 Another continuous multi-stage countercurrent electrodialysis system similar to the system shown is: the cylinders of each membrane stack are connected to the cylinders of the next membrane stack, and the liquid in each cylinder flows into the next cylinder in an overflow manner depending on the liquid level difference (CN113477089A) ( Figure 7 ), which can basically ensure that the liquid level of each cylinder is maintained within a certain range, but the flow rate between each level still fluctuates and has poor stability.

[0014] The above-mentioned continuous countercurrent multi-stage electrodialysis system ( Figure 6 , Figure 7) The advantage of retaining the cylinders of each level of membrane stack is that the cylinders of each level of membrane stack are connected to the atmosphere, and the fluid mechanics of each level of membrane stack during operation is similar to that of the single-stage batch operation membrane stack, and the pressure distribution is also similar. However, the disadvantages are: the system connection is complex and there are many devices; the existence of the cylinder increases the liquid storage volume of the entire system, resulting in slow startup of the system at startup / after cleaning, slow dynamic response, unstable inter-stage flow, and difficulty in implementing automatic control. Summary of the invention

[0015] In order to solve the problems of complex connection, multiple equipment, slow startup, slow dynamic response, unstable inter-stage flow, and difficulty in automatic control of the above-mentioned continuous countercurrent multi-stage electrodialysis system with various levels of cylinders, the present invention provides a cylinder-free continuous multi-stage countercurrent electrodialysis system and an electrodialysis method.

[0016] The technical principle of the present invention is as follows: since a sufficient linear velocity needs to be maintained inside the electrodialysis membrane stack to ensure mixing and mass transfer within the membrane stack, if the compartments of each level of the electrodialysis membrane stack are simply directly connected in series, the residence time of the slurry flowing through the membrane stack will be short and the one-way mobility will be very low. Therefore, the slurry in each compartment needs to maintain high-speed self-circulating flow inside each level of the membrane stack (circulating through the membrane stack outlet, circulating pump, and membrane stack inlet). Therefore, each compartment of each level of the membrane stack of the present invention maintains a high-speed self-circulating flow of "membrane stack outlet, circulating pump, and membrane stack inlet". loop (flowmeters, valves, heat exchangers, etc. can also be installed on the high-speed self-circulating loop as needed), but no material cylinder is set on the high-speed self-circulating loop (that is, the high-speed self-circulating loop is isolated from the outside atmosphere); on the other hand, a small part of the liquid is diverted from any position of the high-speed self-circulating loop of the compartment of the upper-level membrane stack and directly enters any position of the high-speed self-circulating loop of the corresponding compartment of the current-level membrane stack, but it does not overlap with the position where a small part of the liquid is diverted from the high-speed self-circulating loop of the compartment of the current-level membrane stack to the corresponding compartment of the next-level membrane stack. As a result, not only the number of connections and equipment in the system is reduced, but also the liquid storage volume of the entire system is greatly reduced, and conditions for implementing automatic control are provided.

[0017] The cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention comprises membrane stacks of two or more levels, wherein each compartment of each level of the membrane stack is not provided with a cylinder, and each compartment of each level of the membrane stack is directly connected by pipelines.

[0018] According to the cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention, the feed liquid in each compartment of each stage of the membrane stack in the electrodialysis system is organized in a countercurrent manner, and the feed liquid in each compartment is self-circulated in the stack through a circulation pump.

[0019] According to the cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention, preferably, the electrodialyzer of the electrodialysis system includes a common electrodialyzer, a three-chamber bipolar membrane electrodialyzer or a two-chamber bipolar membrane electrodialyzer.

[0020] According to the cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention, as an option, when the electrodialyzer of the electrodialysis system is a common electrodialyzer, the membrane stack of the common electrodialyzer includes two compartments, namely a desalination compartment and a concentration compartment;

[0021] The countercurrent arrangement includes: the initial liquid of the desalting chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage membrane stack; the initial liquid of the concentrating chamber enters from the N-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the first-stage membrane stack; wherein N≥2.

[0022] According to the cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention, as another option, when the electrodialyzer of the electrodialysis system is a three-chamber bipolar membrane electrodialyzer, the membrane stack of the three-chamber bipolar membrane electrodialyzer includes three compartments, namely, an acid chamber, a salt chamber and an alkali chamber;

[0023] The countercurrent arrangement includes: the initial liquid of the salt chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage membrane stack; the initial liquid of the acid chamber and the initial liquid of the alkali chamber enter from the N-stage membrane stack, pass through each stage of the membrane stack in sequence, and flow to the first-stage membrane stack; wherein, N≥2.

[0024] According to the cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention, as another option, when the electrodialyzer of the electrodialysis system is a two-chamber bipolar membrane electrodialyzer, the membrane stack of the two-chamber bipolar membrane electrodialyzer includes two compartments, namely a salt chamber and an alkali chamber;

[0025] The countercurrent arrangement includes: the initial liquid of the salt chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage membrane stack; the initial liquid of the alkali chamber enters from the N-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the first-stage membrane stack; wherein N≥2;

[0026] Alternatively, the membrane stack of the two-chamber bipolar membrane electrodialyzer includes two compartments, namely a salt chamber and an acid chamber; the countercurrent organization includes: the initial liquid in the salt chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage membrane stack; the initial liquid in the acid chamber enters from the N-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the first-stage membrane stack; wherein N≥2.

[0027] According to the cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention, preferably, the manner in which the compartments of each stage of the membrane stack are directly connected by pipelines includes:

[0028] The outlet of each compartment of each membrane stack is connected to the inlet of the corresponding compartment of the next membrane stack; or,

[0029] The outlet of each compartment of each membrane stack is connected to the outlet of the corresponding compartment of the next membrane stack; or,

[0030] The inlet of each compartment of each membrane stack is connected to the outlet of the corresponding compartment of the next membrane stack; or,

[0031] The inlet of each compartment of each level of membrane stack is connected to the inlet of the corresponding compartment of the next level of membrane stack.

[0032] The present invention also provides an electrodialysis method based on any of the above-mentioned cylinder-free continuous multi-stage countercurrent electrodialysis systems, when the electrodialyzer in the electrodialysis system is a common electrodialyzer, comprising the following steps:

[0033] The outlet salt concentration of the final desalination chamber of a common electrodialyzer is used to feedback-regulate the operating voltage of the final stage, or the flow rate of the feed liquid of the first desalination chamber is feedback-regulated.

[0034] According to the electrodialysis method of the present invention, it is further preferred that the outlet salt concentration is lower than the set value, and the final stage operating voltage is reduced; the outlet salt concentration is higher than the set value, and the final stage current density is lower than the maximum current density that the ion exchange membrane can withstand, and the final stage operating voltage is increased; wherein, the final stage operating voltage adjustment increment = -(outlet salt concentration - set value) * K1; wherein K1 is a negative value, which is the correlation coefficient between the outlet salt concentration and the final stage operating voltage, obtained by the fitting relationship between the two;

[0035] or,

[0036] The outlet salt concentration is higher than the set value, and the current density of the final stage is equal to or higher than the maximum current density that the ion exchange membrane can withstand, thereby reducing the flow rate of the feed liquid of the first-stage desalination chamber, wherein the flow rate adjustment increment of the feed liquid of the first-stage desalination chamber=-(outlet salt concentration-set value)*K2; wherein K2 is a positive value, which is the correlation coefficient between the outlet salt concentration and the flow rate of the feed liquid of the first-stage desalination chamber, and is obtained by a fitting relationship between the two.

[0037] According to the electrodialysis method of the present invention, it is further preferred that the outlet salt concentration is represented by a conductivity value detected by a conductivity meter, wherein:

[0038] The conductivity value of the outlet salt concentration is lower than the set value, and the final stage operating voltage is reduced; the conductivity value of the outlet salt concentration is higher than the set value, and the current density of the final stage is lower than the maximum current density that the ion exchange membrane can withstand, and the final stage operating voltage is increased; wherein, the increment of the final stage operating voltage adjustment = -(conductivity value of the outlet salt concentration - set value)*K3; wherein K3 is a negative value, which is the correlation coefficient between the conductivity value of the outlet salt concentration and the final stage operating voltage, and is obtained by the fitting relationship between the two;

[0039] or,

[0040] The conductivity value of the outlet salt concentration is higher than the set value, and the current density of the final stage is equal to or higher than the maximum current density that the ion exchange membrane can withstand, thereby reducing the flow rate of the feed liquid of the first-stage desalination chamber, wherein the flow rate adjustment increment of the feed liquid of the first-stage desalination chamber=-(conductivity value of the outlet salt concentration-set value)*K4; wherein K4 is a positive value, which is the correlation coefficient between the conductivity value of the outlet salt concentration and the flow rate of the feed liquid of the first-stage desalination chamber, and is obtained by a fitting relationship between the two.

[0041] The electrodialysis method based on any of the above-mentioned cylinder-free continuous multi-stage countercurrent electrodialysis systems of the present invention, when the electrodialyzer in the electrodialysis system is a bipolar membrane electrodialyzer, comprises the following steps:

[0042] The outlet salt concentration of the final salt chamber of the bipolar membrane electrodialyzer is used to feedback-regulate the operating voltage of the final stage, or the flow rate of the feed liquid of the first salt chamber is feedback-regulated.

[0043] According to the electrodialysis method of the present invention, it is further preferred that the outlet salt concentration is lower than the set value, and the final stage operating voltage is reduced; the outlet salt concentration is higher than the set value, and the final stage current density is lower than the maximum current density that the ion exchange membrane can withstand, and the final stage operating voltage is increased; wherein, the final stage operating voltage adjustment increment = -(outlet salt concentration - set value) * K1; wherein K1 is a negative value, which is the correlation coefficient between the outlet salt concentration and the final stage operating voltage, obtained by the fitting relationship between the two;

[0044] or,

[0045] The outlet salt concentration is higher than the set value, and the current density of the final stage is equal to or higher than the maximum current density that the ion exchange membrane can withstand, thereby reducing the flow rate of the feed liquid of the first-stage salt chamber, wherein the flow rate adjustment increment of the feed liquid of the first-stage salt chamber = -(outlet salt concentration - set value)*K2; wherein K2 is a positive value, which is the correlation coefficient between the outlet salt concentration and the flow rate of the feed liquid of the first-stage salt chamber, and is obtained by the fitting relationship between the two.

[0046] According to the electrodialysis method of the present invention, it is further preferred that the outlet salt concentration is represented by a conductivity value detected by a conductivity meter, wherein:

[0047] The conductivity value of the outlet salt concentration is lower than the set value, and the final stage operating voltage is reduced; the conductivity value of the outlet salt concentration is higher than the set value, and the current density of the final stage is lower than the maximum current density that the ion exchange membrane can withstand, and the final stage operating voltage is increased; wherein, the increment of the final stage operating voltage adjustment = -(conductivity value of the outlet salt concentration - set value)*K3; wherein K3 is a negative value, which is the correlation coefficient between the conductivity value of the outlet salt concentration and the final stage operating voltage, and is obtained by the fitting relationship between the two;

[0048] or,

[0049] The conductivity value of the outlet salt concentration is higher than the set value, and the current density of the final stage is equal to or higher than the maximum current density that the ion exchange membrane can withstand, thereby reducing the flow rate of the feed liquid of the first-stage salt chamber, wherein the flow rate adjustment increment of the feed liquid of the first-stage salt chamber = -(conductivity value of the outlet salt concentration - set value)*K4; wherein K4 is a positive value, which is the correlation coefficient between the conductivity value of the outlet salt concentration and the flow rate of the feed liquid of the first-stage salt chamber, and is obtained by the fitting relationship between the two.

[0050] The common electrodialysis and bipolar membrane electrodialyzer in the present invention are conventional electrodialyzer equipment, and can be operated in conventional ways, including constant current, constant pressure or variable pressure, variable current, preferably constant pressure.

[0051] The number of stages of the non-tank continuous multi-stage countercurrent electrodialysis system of the present invention is preferably two or three stages.

[0052] The temperature of the feed solution in each compartment of the present invention adopts the temperature of conventional electrodialysis operation, usually in the range of 5 to 50°C; the flow rate in each compartment adopts the conventional flow rate, usually in the range of 1 to 10 cm / s; the current density adopts the conventional current density, usually in the range of 10 to 100 mA / cm 2 range.

[0053] The ion exchange membrane used in the conventional electrodialysis and bipolar membrane electrodialyzer of the present invention is not particularly limited, and any ion exchange membrane known to those skilled in the art can be used, which can be purchased from the market. For example, conventional electrodialysis anion exchange membrane, cation exchange membrane and bipolar membrane, and monovalent anion and cation selective membranes can also be used.

[0054] The advantages of the present invention are:

[0055] 1. Simple system and convenient connection. The present invention solves the problems of complex connection and multiple equipment of the continuous countercurrent multi-stage electrodialysis system with various levels of material tanks. Even compared with the overflow mode of "the material liquid of each level of material tank flows into the next level in an overflow manner by relying on the liquid level difference", it also greatly simplifies the whole system device and connection.

[0056] 2. Fast startup and dynamic response. The present invention solves the problems of slow startup and slow dynamic response of the continuous countercurrent multi-stage electrodialysis system with various levels of material cylinders, and achieves a short startup time. The reason is that the dead volume of the system of the present invention is small;

[0057] 3. Stable inter-stage flow. The present invention solves the problem of unstable inter-stage flow in a continuous countercurrent multi-stage electrodialysis system with various stages of feed cylinders. The high-speed self-circulating loop of each stage of the membrane stack of the present invention is isolated from the outside atmosphere, achieving stable transportation of inter-stage feed liquid, and the flow between each stage is consistent (the inter-stage flow depends on the flow of the feed pump. As long as the flow of the feed pump is stable, the flow between each stage can be stable).

[0058] 4. The whole system is easy to control. The present invention solves the problem that it is not easy to implement automatic control for a continuous countercurrent multi-stage electrodialysis system with various levels of material cylinders. The continuous countercurrent multi-stage electrodialysis system with various levels of material cylinders needs to go through the flow change of the feed pump - the change of the liquid level position of the material cylinder - the change of the overflow flow - the change of the liquid level position of the next level of material cylinder, and the feedback adjustment is slow and the process is long. The electrodialysis system of the present invention is fully closed, and the change of the flow rate of the feed pump can be immediately reflected in the outlet flow and the treatment effect, and the control means responds quickly; it only needs to measure / monitor the conductivity of the outlet liquid at each level to determine the operating status of the membrane stack at this level; it only needs to adjust the operating voltage of the final membrane stack to quickly affect the outlet salt concentration. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 The diagram shows the membrane stack structure of ordinary electrodialysis (taking sodium chloride as an example) and ion migration; AM is an anion exchange membrane and CM is a cation exchange membrane.

[0060] Figure 2 The diagram shows the membrane stack structure (taking sodium sulfate as an example) and ion migration of a conventional "acid-salt-base" three-chamber bipolar membrane electrodialysis; AM is an anion exchange membrane, CM is a cation exchange membrane, and BM is a bipolar membrane.

[0061] Figure 3(a) is a schematic diagram of a conventional "salt-alkali" two-chamber bipolar membrane electrodialysis membrane stack structure (taking sodium gluconate as an example) and ion migration; CM is a cation exchange membrane and BM is a bipolar membrane.

[0062] FIG3( b ) is a schematic diagram of a conventional “acid-salt” two-chamber bipolar membrane electrodialysis membrane stack structure (taking pentamethylenediamine hydrochloride as an example) and ion migration; AM is an anion exchange membrane and BM is a bipolar membrane.

[0063] Figure 4 This is the batch operation mode of ordinary electrodialysis (three membrane stacks are shown to operate independently).

[0064] Figure 5 It is a multi-stage countercurrent electrodialysis system operated in batch mode (a three-stage two-compartment system is shown).

[0065] Figure 6 The invention is a continuous multi-stage countercurrent electrodialysis system (a three-stage two-compartment system is shown) in which the outlet of each membrane stack is connected to the material tank of the next membrane stack.

[0066] Figure 7 A continuous multi-stage countercurrent electrodialysis system (a three-stage two-compartment system is shown) in which the cylinders of each stage of membrane stack are connected to the cylinders of the next stage of membrane stack.

[0067] Figure 8This is a tank-free continuous multi-stage countercurrent electrodialysis system of the present invention (a three-stage two-compartment system is shown).

[0068] Fig. 9 This is a cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention (a three-stage system is shown, with only one compartment connection drawn, including a flow meter, valve, and heat exchanger) (export-import mode).

[0069] Fig.10 This is a cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention (a three-stage system is shown, with only one compartment connection drawn, including a flow meter, valve, and heat exchanger) (outlet-outlet mode).

[0070] Fig.11 This is a cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention (a three-stage system is shown, with only one compartment connection drawn, including a flow meter, valve, and heat exchanger) (inlet-outlet mode).

[0071] Fig.12 This is a cylinder-free continuous multi-stage countercurrent electrodialysis system of the present invention (a three-stage system is shown, with only one compartment connection drawn, including a flow meter, valve, and heat exchanger) (inlet-inlet mode). DETAILED DESCRIPTION

[0072] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods; the materials, reagents, etc. used in the following examples, unless otherwise specified, can be obtained from commercial channels.

[0073] The non-tank continuous multi-stage countercurrent electrodialysis system of the present invention is as follows Figure 8 As shown (the three-stage two-compartment system is shown, which is for illustration only. The electrodialysis system includes an electrodialysis system with two or more compartments), the feed liquid in each compartment (more than two compartments) of each level of membrane stack is organized in a countercurrent manner, and each compartment of each level of membrane stack is not provided with a material cylinder, and the feed liquid in each compartment is realized by a circulating pump to achieve high-speed self-circulation in the stack; the compartments of each level of membrane stack are directly connected (by pipes, valves, etc.), so that the feed liquid (interstage feed liquid) is transported from each compartment of each level of membrane stack to the corresponding compartment of the next level of membrane stack (i.e., the next membrane stack along the feed liquid transport direction), thereby realizing continuous feeding and continuous discharging of the multi-stage countercurrent electrodialysis system.

[0074] Specifically, for ordinary electrodialysis, the two compartments refer to the desalting compartment and the concentrating compartment (such as Figure 1As shown in the figure, the "liquid is organized in a countercurrent manner" means that the initial liquid of the desalting chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in turn, and flows to the Nth stage (N≥2) membrane stack; the initial liquid of the concentrating chamber enters from the Nth stage (N≥2) membrane stack, passes through each stage of the membrane stack in turn, and flows to the first stage of the membrane stack. In the process of the initial liquid of the desalting chamber flowing from the first stage of the membrane stack to the Nth stage of the membrane stack step by step, ions gradually migrate out, and the ion concentration in the desalting chamber decreases step by step along the flow direction; while in the process of the initial liquid of the concentrating chamber flowing from the Nth stage of the membrane stack to the first stage of the membrane stack step by step, ions gradually migrate into the desalting chamber, and the ion concentration in the concentrating chamber increases step by step along the flow direction. "The liquid is organized in a countercurrent manner" makes the desalting liquid with high ion concentration and the concentrated liquid with high ion concentration at the same level, and the desalting liquid with low ion concentration and the concentrated liquid with low ion concentration at the same level, thereby reducing the concentration difference on both sides of the ion exchange membrane, thereby reducing the energy consumption when the ions migrate against the concentration and improving the current efficiency.

[0075] Specifically, for three-compartment bipolar membrane electrodialysis, the three compartments are an acid compartment, a salt compartment, and a base compartment (e.g. Figure 2 As shown), the "liquid is organized in a countercurrent manner" means that the initial liquid of the salt chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage (N≥2) membrane stack; the initial liquid of the acid chamber and the initial liquid of the alkali chamber enter from the N-stage (N≥2) membrane stack, pass through each stage of the membrane stack in sequence, and flow to the first-stage membrane stack.

[0076] Specifically, for two-chamber bipolar membrane electrodialysis, the two compartments refer to the salt chamber and the alkali chamber (as shown in Figure 3(a)), or, the acid chamber and the salt chamber (as shown in Figure 3(b)), and the "liquid is organized in a countercurrent manner" means: the initial liquid of the salt chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage (N≥2) membrane stack; the initial liquid of the alkali chamber enters from the N-stage (N≥2) membrane stack, passes through each stage of the membrane stack in sequence, and flows to the first-stage membrane stack; or, the initial liquid of the salt chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage (N≥2) membrane stack; the initial liquid of the acid chamber enters from the N-stage (N≥2) membrane stack, passes through each stage of the membrane stack in sequence, and flows to the first-stage membrane stack.

[0077] Specifically, the method of "directly connecting the compartments of each level of membrane stack" includes: the outlet of each compartment of each level of membrane stack is connected (by pipes, valves, etc.) to the inlet of the corresponding compartment of the next level of membrane stack (hereinafter referred to as the "outlet-inlet" method, such as Fig. 9 The outlet of each compartment of each membrane stack is connected to the outlet of the corresponding compartment of the next membrane stack (hereinafter referred to as the "outlet-outlet" method, such as Fig.10 The inlet of each compartment of each membrane stack is connected (by pipes, valves, etc.) to the outlet of the corresponding compartment of the next membrane stack (hereinafter referred to as the "inlet-outlet" method, such as Fig.11As shown); and, the inlet of each compartment of each membrane stack is connected (by pipes, valves, etc.) to the inlet of the corresponding compartment of the next membrane stack (hereinafter referred to as the "inlet-inlet" method, such as Fig.12 shown).

[0078] It should be noted that the present invention Figure 9-12 The "feed tank" and "discharge tank" in Figure 4-Figure 7 The "material tank" in the. Figure 9-12 The "feed tank" and "discharge tank" do not participate in the self-circulation loop of the membrane stack, but only indicate the source and destination of the feed liquid. In the industrial site, they are equivalent to "feed liquid temporary storage tanks" and their volumes can be large or small.

[0079] The material-free continuous multi-stage countercurrent electrodialysis system of the present invention may be disturbed during actual industrial operation (i.e., continuous feeding and continuous discharging). For example, the concentration and flow rate of the feed solution at the first stage (level 1) inlet (i.e., the feed solution at the first stage (level 1) desalination chamber inlet of ordinary electrodialysis, or the feed solution at the first stage (level 1) salt chamber inlet of bipolar membrane electrodialysis) may fluctuate (due to the reasons of the preceding process), affecting the salt concentration at the final stage (level N) outlet. To this end, the present invention also provides an electrodialysis method based on the material-free continuous multi-stage countercurrent electrodialysis system.

[0080] The electrodialysis method based on the material-free continuous multi-stage countercurrent electrodialysis system of the present invention uses the salt concentration at the outlet of the final stage (N stage) desalination chamber of ordinary electrodialysis, or the salt concentration at the outlet of the final stage (N stage) salt chamber of bipolar membrane electrodialysis, to feedback adjust the operating voltage of the final stage (N stage), or feedback adjust the flow rate of the feed liquid of the first stage (1st stage) desalination chamber or salt chamber. That is, the outlet salt concentration is lower than the set value (i.e., the target value), and the final stage (N stage) operating voltage is reduced; the outlet salt concentration is higher than the set value (i.e., the target value), and the current density of the final stage (N stage) is lower than the maximum current density that the ion exchange membrane can withstand, and the final stage (N stage) operating voltage is increased; wherein, the final stage (N stage) operating voltage adjustment increment = -(outlet salt concentration - set value) * K1, K1 is the correlation coefficient between the outlet salt concentration and the final stage (N stage) operating voltage (Note: K1 itself is a negative value), which is related to the membrane stack and the material system being treated, and is obtained from the fitting relationship between the two. The final stage (N stage) operating voltage can be manually adjusted near the set value, the corresponding outlet salt concentration can be detected, and 1 data point can be obtained. This can be repeated to obtain at least 2 data points to establish the fitting relationship. Of course, more data points will make the fitting relationship more accurate; the outlet When the salt concentration is higher than the set value (i.e., the target value), and the current density of the final stage (N stage) is equal to or higher than the maximum current density that the ion exchange membrane can withstand, the flow rate of the feed liquid of the first stage (1 stage) desalination chamber or salt chamber is reduced, wherein the flow rate adjustment increment of the feed liquid of the first stage (1 stage) desalination chamber or salt chamber = -(outlet salt concentration - set value) * K2, K2 is the correlation coefficient between the outlet salt concentration and the flow rate of the feed liquid of the first stage (1 stage) desalination chamber or salt chamber (note: K2 itself is a positive value), which is related to the membrane stack and the treated material system, and is obtained from the fitting relationship between the two. The flow rate of the feed liquid of the first stage (1 stage) desalination chamber or salt chamber can be manually adjusted near the set value, the corresponding outlet salt concentration can be detected, and 1 data point can be obtained. This can be repeated to obtain at least 2 data points, and the fitting relationship can be established. Of course, more data points will make the fitting relationship more accurate.

[0081] Specifically, the outlet salt concentration is represented by a conductivity value detected by a conductivity meter, and the conductivity meter detection is preferably online monitoring.

[0082] Specifically, if the conductivity value of the outlet salt concentration is lower than the set value (i.e., the target value), the final stage (N stage) operating voltage is reduced; if the conductivity value of the outlet salt concentration is higher than the set value (i.e., the target value), and the current density of the final stage (N stage) is lower than the maximum current density that the ion exchange membrane can withstand, the final stage (N stage) operating voltage is increased; wherein, the increment of the final stage (N stage) operating voltage adjustment = -(conductivity value of the outlet salt concentration - set value) * K3, K3 is the correlation coefficient between the conductivity value of the outlet salt concentration and the final stage (N stage) operating voltage (Note: K3 itself is a negative value), which is related to the membrane stack and the material system being treated, and is obtained from the fitting relationship between the two. The final stage (N stage) operating voltage can be manually adjusted near the set value, the corresponding conductivity value of the outlet salt concentration is detected, and 1 data point is obtained. This is repeated to obtain at least 2 data points, and the fitting relationship can be established. Of course, more data points will make the fitting relationship more accurate. The conductivity value of the outlet salt concentration is higher than the set value (i.e., the target value), and the current density of the final stage (N stage) is equal to or higher than the maximum current density that the ion exchange membrane can withstand, and the flow rate of the feed liquid of the first stage (1 stage) desalination chamber or salt chamber is reduced, wherein the flow rate adjustment increment of the feed liquid of the first stage (1 stage) desalination chamber or salt chamber=-(conductivity value of the outlet salt concentration-set value)*K4, K4 is the correlation coefficient between the conductivity value of the outlet salt concentration and the flow rate of the feed liquid of the first stage (1 stage) desalination chamber or salt chamber (note: K4 itself is a positive value), which is related to the membrane stack and the treated material system, and is obtained from the fitting relationship between the two. The flow rate of the feed liquid of the first stage (1 stage) desalination chamber or salt chamber can be manually adjusted near the set value, the conductivity value of the corresponding outlet salt concentration is detected, and 1 data point is obtained. This is repeated to obtain at least 2 data points, and the fitting relationship can be established. Of course, more data points will make the fitting relationship more accurate.

[0083] Example 1

[0084] The present invention uses a cylinder-free continuous three-stage countercurrent common electrodialysis system to treat sodium chloride aqueous solution. The common electrodialyzer is a plate-frame type, and the membrane stack is composed of 4 RXAM anion membranes (Hebei Yadeshi Environmental Protection Equipment Co., Ltd.) and 5 SHCM cation membranes (Shanghai Xiangfeng Chemical Co., Ltd.). The membrane stack structure is referenced Figure 1 As shown, the effective area of ​​each membrane in the membrane stack is 0.0088m 2 The anode plate is a titanium-coated ruthenium electrode, and the cathode plate is a stainless steel electrode. The separators and screens are made of polypropylene, the separators are non-loop separators, and the screens are woven mesh. Fig. 9 shown.

[0085] Fill each level of membrane stack with water in advance. Start the circulation pumps of the desalination chamber and concentration chamber of each level of ordinary electrodialysis membrane stack, and the liquid flow linear velocity in each compartment is 3cm / s. Turn on the DC power supply of each level of membrane stack, and adopt the constant voltage mode. The operating voltages of the first, second and third level membrane stacks are 2.8V, 3V and 3.2V respectively. A 30g / L sodium chloride aqueous solution is introduced into the inlet of the desalination chamber of the first level membrane stack through the feed pump, with a flow rate of 800mL / h; deionized water is introduced into the inlet of the concentration chamber of the third level membrane stack through the feed pump, with a flow rate of 120mL / h; 0.3L of 30g / L sodium sulfate solution is introduced into the electrode chamber of each level of membrane stack. The conductivity of the outlet feed liquid of the desalination chamber of the third level membrane stack is monitored online.

[0086] After 80 minutes, the conductivity of the outlet liquid of the desalination chamber of the third-stage membrane stack stabilized at 3mS / cm. At this time, the timing was started and the volume of the feed liquid and the outlet liquid of each compartment was measured. After 120 minutes, the ordinary electrodialysis operation was stopped, and 1.59L of 3g / L NaCl aqueous solution was obtained at the outlet of the desalination chamber of the third-stage membrane stack, and 0.25L of sodium chloride aqueous solution with a concentration of 174g / L was obtained at the outlet of the concentration chamber of the first-stage membrane stack. According to calculations, the energy consumption of ordinary electrodialysis concentration and recovery of sodium chloride is 0.24kWh / kgNaCl, and the membrane flux is 10.5mol NaCl / m 2 / h, average current efficiency 70.4%. During operation, the fluctuation range of sodium chloride concentration of the outlet liquid of the third-stage membrane stack desalination chamber was 3g / L±0.04g / L.

[0087] Comparative Example 1

[0088] refer to Figure 6 The continuous three-stage countercurrent conventional electrodialysis system shown is used to treat sodium chloride aqueous solution. The other configurations of the electrodialysis device are the same as those in Example 1, but the connection relationship of the three-stage membrane stack is referenced to Figure 6 As shown. The volume of the material cylinder of each level of membrane stack desalination chamber and concentration chamber is 50mL, which is connected between the membrane stack outlet and the circulation pump inlet of the corresponding compartment to form a circulation loop of "membrane stack outlet, material cylinder, circulation pump, membrane stack inlet"; at the same time, a small part of the liquid is diverted from the membrane stack outlet to the material cylinder of the next level of membrane stack.

[0089] The other operations during the operation were the same as those in Example 1, but the membrane stacks and the feed tanks at each level were filled with water in advance; a 30 g / L sodium chloride aqueous solution was introduced into the feed tank inlet of the desalination chamber of the first-stage membrane stack through a feed pump; and deionized water was introduced into the feed tank inlet of the concentration chamber of the third-stage membrane stack through a feed pump. The conductivity of the feed solution at the outlet of the desalination chamber of the third-stage membrane stack was monitored online.

[0090] After 160 minutes, the conductivity of the outlet liquid of the desalination chamber of the third-stage membrane stack stabilized at 3-4mS / cm. At this time, the timing was started and the volume of the feed liquid and the outlet liquid of each compartment was measured. After 120 minutes, the ordinary electrodialysis operation was stopped. 1.594L of 3.7g / L NaCl aqueous solution was obtained at the outlet of the desalination chamber of the third-stage membrane stack, and 0.245L of sodium chloride aqueous solution with a concentration of 172 / L was obtained at the outlet of the concentration chamber of the first-stage membrane stack. According to calculations, the energy consumption of ordinary electrodialysis concentration and recovery of sodium chloride is 0.244kWh / kgNaCl, and the membrane flux is 10.17NaCl / m 2 / h, current efficiency 69.3%. During operation, the sodium chloride concentration of the outlet liquid of the third-stage membrane stack desalination chamber fluctuated within the range of 3.7g / L±1.1g / L, and the liquid level in the tank was difficult to stabilize, rising or falling continuously, and the interstage flow had to be adjusted manually.

[0091] Example 2

[0092] The present invention uses a cylinder-free continuous three-stage countercurrent "acid-salt-alkali" three-chamber bipolar membrane electrodialysis system to treat lithium sulfate aqueous solution and regenerate sulfuric acid and lithium hydroxide. The three-chamber bipolar membrane electrodialyzer is a plate-frame type, and the membrane stack is composed of 4 AMVN negative membranes (Asahi Glass Co., Ltd.), 4 SHCM positive membranes (Shanghai Xiangfeng Chemical Co., Ltd.) and 5 TRJBM bipolar membranes (Beijing Tingrun Membrane Technology Development Co., Ltd.). The membrane stack structure is referenced Figure 2 As shown, the effective area of ​​each membrane in the membrane stack is 0.0088m 2 The anode plate is a titanium-coated ruthenium electrode, and the cathode plate is a stainless steel electrode. The separators and screens are made of polypropylene, the separators are non-loop separators, and the screens are woven mesh. Fig.10 shown.

[0093] Fill each level of membrane stack with water in advance. Start the circulation pumps of the acid chamber, salt chamber and alkali chamber of each level of the "acid-salt-alkali" three-chamber bipolar membrane electrodialysis membrane stack, and the liquid flow linear velocity in each compartment is 3cm / s. Turn on the DC power supply of each level of membrane stack, and adopt the constant voltage mode. The operating voltages of the first, second and third level membrane stacks are 12V, 12V and 12V respectively. At the outlet of the salt chamber of the first level membrane stack, a 150g / L lithium sulfate aqueous solution is introduced through the feed pump at a flow rate of 600mL / h; at the outlet of the acid chamber of the third level membrane stack, deionized water is introduced through the feed pump at a flow rate of 600mL / h; at the outlet of the alkali chamber of the third level membrane stack, deionized water is introduced through the feed pump at a flow rate of 600mL / h; 0.3L of 0.35mol / L sodium sulfate solution is introduced into the polar chamber of each level of membrane stack. The conductivity of the feed liquid at the outlet of the salt chamber of the third level membrane stack is monitored online.

[0094] After 18 minutes, the conductivity value of the feed liquid at the outlet of the salt chamber of the third-stage membrane stack stabilized at 2.6mS / cm. At this time, the timing was started, and the volume of the feed liquid and the discharge liquid in each compartment was measured. After 120 minutes, the "acid-salt-alkali" three-chamber bipolar membrane electrodialysis operation was stopped, and 0.61L of 2.42g / L lithium sulfate aqueous solution was obtained at the outlet of the salt chamber of the third-stage membrane stack, 1.51L of 1.08mol / L sulfuric acid aqueous solution was obtained at the outlet of the acid chamber of the first-stage membrane stack, and 1.48L of 2.20mol / L lithium hydroxide aqueous solution was obtained at the outlet of the alkali chamber of the first-stage membrane stack. According to calculations, the energy consumption of treating lithium sulfate with the "acid-salt-alkali" three-chamber bipolar membrane electrodialysis is 3.20kWh / kg LiOH, Li + The membrane flux is 14.09 mol / m 2 / h, SO4 2- The membrane flux is 6.77 mol / m 2 / h, average current efficiency 78.5%. During operation, the fluctuation range of lithium sulfate concentration in the salt chamber outlet of the third-stage membrane stack was 2.42g / L±0.1g / L.

[0095] Comparative Example 2

[0096] use Figure 6 The continuous three-stage countercurrent "acid-salt-alkali" three-chamber bipolar membrane electrodialysis system shown in the figure treats lithium sulfate aqueous solution and regenerates sulfuric acid and lithium hydroxide. The other settings of the three-chamber bipolar membrane electrodialysis device are the same as those in Example 2, but the connection relationship of the three-stage membrane stack is referenced to Figure 6 As shown. The volume of the material cylinder of the acid chamber, salt chamber and alkali chamber of each level of membrane stack is 50mL, which is connected between the membrane stack outlet and the circulation pump inlet of the corresponding compartment to form a circulation loop of "membrane stack outlet, material cylinder, circulation pump, membrane stack inlet"; at the same time, a small part of the material liquid is diverted from the membrane stack outlet to the material cylinder of the next level of membrane stack.

[0097] The other operations during the operation were the same as those in Example 2, but the membrane stacks and the material cylinders at each level were filled with water in advance; a 150 g / L lithium sulfate aqueous solution was introduced into the inlet of the salt chamber material cylinder of the first-stage membrane stack through a feed pump at a flow rate of 600 mL / h; deionized water was introduced into the inlet of the acid chamber material cylinder of the third-stage membrane stack through a feed pump at a flow rate of 600 mL / h; deionized water was introduced into the inlet of the alkali chamber material cylinder of the third-stage membrane stack through a feed pump at a flow rate of 600 mL / h. The conductivity of the feed solution at the outlet of the salt chamber of the third-stage membrane stack was monitored online.

[0098] After 40 minutes, the conductivity value of the feed liquid at the outlet of the salt chamber of the third-stage membrane stack stabilized at 6.6mS / cm. At this time, the timing was started, and the volume of the feed liquid and the discharge liquid in each compartment was measured. After 120 minutes, the "acid-salt-alkali" three-chamber bipolar membrane electrodialysis operation was stopped, and 0.65L of 6.1g / L lithium sulfate aqueous solution was obtained at the outlet of the salt chamber of the third-stage membrane stack, 1.49L of 1.08mol / L sulfuric acid aqueous solution was obtained at the outlet of the acid chamber of the first-stage membrane stack, and 1.46L of 2.20mol / L lithium hydroxide aqueous solution was obtained at the outlet of the alkali chamber of the first-stage membrane stack. According to calculations, the energy consumption of treating lithium sulfate with the "acid-salt-alkali" three-chamber bipolar membrane electrodialysis is 3.35kWh / kg LiOH, Li + The membrane flux is 13.89 mol / m 2 / h, SO4 2- The membrane flux is 6.67 mol / m 2 / h, with an average current efficiency of 77.3%. During operation, the lithium sulfate concentration of the salt chamber outlet of the third-stage membrane stack fluctuated within the range of 6.1g / L±0.2g / L, and the liquid level in the tank was difficult to stabilize, rising or falling continuously, and the interstage flow had to be adjusted manually.

[0099] Example 3

[0100] The present invention uses a continuous three-stage countercurrent "salt-alkali" two-chamber bipolar membrane electrodialysis system without a material cylinder to treat a sodium gluconate aqueous solution and regenerate it into gluconic acid and sodium hydroxide. The two-chamber bipolar membrane electrodialyzer is a plate-frame type, and the membrane stack is composed of 4 SHCM cation membranes (Shanghai Xiangfeng Chemical Co., Ltd.) and 5 TRJBM bipolar membranes (Beijing Tingrun Membrane Technology Development Co., Ltd.). The membrane stack structure is shown in Figure 3 (a). The effective area of ​​each membrane in the membrane stack is 0.0088m 2 The anode plate is a titanium-coated ruthenium electrode, and the cathode plate is a stainless steel electrode. The separators and screens are made of polypropylene, the separators are non-loop separators, and the screens are woven mesh. Fig.11 shown.

[0101] Fill each level of membrane stack with water in advance. Start the circulation pumps of the salt chamber and alkali chamber of each level of the "salt-alkali" two-chamber bipolar membrane electrodialysis membrane stack, and the liquid flow linear velocity in each compartment is 3cm / s. Turn on the DC power supply of each level of membrane stack, and use the constant voltage mode. The operating voltages of the first, second, and third level membrane stacks are 12.5V, 12.5V, and 12V, respectively. At the outlet of the salt chamber of the first-level membrane stack, a 300g / L sodium gluconate aqueous solution is introduced through a feed pump with a flow rate of 830mL / h; at the outlet of the alkali chamber of the third-level membrane stack, deionized water is introduced through a feed pump with a flow rate of 420mL / h; 0.3L of 0.5mol / L sodium sulfate solution is introduced into the polar chambers of each level of membrane stack. Monitor the conductivity of the inlet liquid of the salt chamber of the third-level membrane stack online.

[0102] After 23 minutes, the conductivity of the feed solution at the salt chamber inlet of the third-stage membrane stack stabilized at 6.6 mS / cm. At this time, the timing was started and the volume of the feed solution and the discharge solution of each compartment was measured. After 120 minutes, the "salt-alkali" two-chamber bipolar membrane electrodialysis operation was stopped, and Na + 1.54L of 5g / L gluconic acid aqueous solution was used to obtain 0.96L of 1.96mol / L sodium hydroxide aqueous solution at the inlet of the alkali chamber of the first-stage membrane stack. The energy consumption of the "salt-alkali" two-chamber bipolar membrane electrodialysis treatment of sodium gluconate was calculated to be 2.24kWh / kgNaOH, Na + The membrane flux is 9.08 mol / m 2 / h, the average current efficiency is 67.01%. During the operation, the Na content of the inlet liquid of the salt chamber of the third-stage membrane stack is + The concentration fluctuation range is 5g / L±0.2g / L.

[0103] Comparative Example 3

[0104] refer to Figure 7 The continuous three-stage countercurrent "salt-alkali" two-chamber bipolar membrane electrodialysis system shown is used to treat sodium gluconate aqueous solution and regenerate gluconic acid and sodium hydroxide. The other settings of the two-chamber bipolar membrane electrodialysis device are the same as those in Example 3, but the connection relationship of the three-stage membrane stack is referenced to Figure 7 As shown. The volume of the material cylinder of the salt chamber and alkali chamber of each membrane stack is 50mL, which is connected between the membrane stack outlet and the circulation pump inlet of the corresponding compartment, forming a circulation loop of "membrane stack outlet, material cylinder, circulation pump, membrane stack inlet"; the material cylinder of each membrane stack is connected to the material cylinder of the next membrane stack, and the material liquid of each material cylinder flows into the next level in an overflow manner depending on the liquid level difference.

[0105] The other operations during the operation were the same as those in Example 3, but the membrane stacks and the material tanks at each level were filled with water in advance. A 300 g / L sodium gluconate aqueous solution was introduced into the inlet of the salt chamber material tank of the first-stage membrane stack through a feed pump at a flow rate of 830 mL / h; deionized water was introduced into the inlet of the alkali chamber material tank of the third-stage membrane stack through a feed pump at a flow rate of 420 mL / h. The conductivity of the feed solution at the outlet of the salt chamber material tank of the third-stage membrane stack was monitored online.

[0106] After 50 minutes, the conductivity of the feed liquid at the outlet of the salt chamber of the third-stage membrane stack stabilized at 9.7 mS / cm. At this time, the timing was started and the volume of the feed liquid and the outlet liquid of each compartment was measured. After 120 minutes, the "salt-alkali" two-chamber bipolar membrane electrodialysis operation was stopped, and Na +1.58L of gluconic acid aqueous solution with a concentration of 7.2g / L was used to obtain 0.92L of sodium hydroxide aqueous solution with a concentration of 1.91mol / L at the outlet of the alkali chamber of the first-stage membrane stack. According to calculations, the energy consumption of the "salt-alkali" two-chamber bipolar membrane electrodialysis treatment of sodium gluconate is 2.31kWh / kgNaOH, Na + The membrane flux is 8.49 mol / m 2 / h, the average current efficiency is 65.03%. During the operation, the Na + The concentration fluctuation range is 7.2g / L±0.3g / L.

[0107] Example 4

[0108] The cylinder-free continuous two-stage countercurrent "acid-salt" two-chamber bipolar membrane electrodialysis system of the present invention is used to treat the aqueous solution of pentamethylenediamine hydrochloride and regenerate it into hydrochloric acid and pentamethylenediamine. The two-chamber bipolar membrane electrodialyzer is a plate-frame type, and the membrane stack is composed of 4 AMVN anionic membranes (Asahi Glass Co., Ltd.) and 5 TRJBM bipolar membranes (Beijing Tingrun Membrane Technology Development Co., Ltd.). The membrane stack structure is shown in Figure 3 (b), and the effective area of ​​each membrane in the membrane stack is 0.0088m 2 ; The anode plate is a titanium-coated ruthenium electrode, and the cathode plate is a stainless steel electrode; the separator and mesh are both made of polypropylene, the separator is a non-loop separator, and the mesh is a woven mesh type. Fig.12 shown.

[0109] Fill each level of membrane stack with water in advance. Start the circulation pumps of the acid chamber and salt chamber of each level of "acid-salt" two-chamber bipolar membrane electrodialysis membrane stack, and the liquid flow linear velocity in each compartment is 3cm / s. Turn on the DC power supply of each level of membrane stack, adopt the constant voltage mode, and the operating voltages of the first and second level membrane stacks are 12V and 12V respectively. At the inlet of the salt chamber of the first level membrane stack, a 0.52mol / L aqueous solution of pentamethylenediamine hydrochloride is introduced through the feed pump at a flow rate of 800mL / h; at the inlet of the acid chamber of the second level membrane stack, deionized water is introduced through the feed pump at a flow rate of 800mL / h; 0.3L of 0.25mol / L sodium sulfate solution is introduced into the polar chamber of each level of membrane stack. Monitor the conductivity of the inlet liquid of the salt chamber of the second level membrane stack online.

[0110] After 15 minutes, the conductivity value of the inlet liquid of the salt chamber of the second-stage membrane stack stabilized at 6.9mS / cm. At this time, the timing was started, and the volume of the feed liquid and the outlet liquid of each compartment was measured. After 120 minutes, the "acid-salt" two-chamber bipolar membrane electrodialysis operation was stopped, and 1.45L of a pentamethylenediamine aqueous solution with a Cl- concentration of 3.6g / L was obtained at the inlet of the salt chamber of the second-stage membrane stack, and 1.75L of a hydrochloric acid aqueous solution with a concentration of 0.87mol / L was obtained at the inlet of the acid chamber of the first-stage membrane stack. According to calculations, the energy consumption of the "acid-salt" two-chamber bipolar membrane electrodialysis to treat pentamethylenediamine hydrochloride is 0.109kWh / mol HCl, and the Cl- membrane flux is 6.16mol / m 2 / h, average current efficiency 54.42%. During operation, the fluctuation range of Cl- concentration of the inlet liquid of the salt chamber of the second-stage membrane stack was 3.6g / L±0.2g / L.

[0111] Comparative Example 4

[0112] refer to Figure 7 The continuous two-stage countercurrent "acid-salt" two-chamber bipolar membrane electrodialysis system shown is used to treat the aqueous solution of pentamethylenediamine hydrochloride to regenerate hydrochloric acid and pentamethylenediamine. The other settings of the two-chamber bipolar membrane electrodialysis device are the same as those in Example 4, but the connection relationship of the two-stage membrane stack is referenced to Figure 7 As shown. The volume of the cylinders of the acid chamber and salt chamber of each membrane stack is 50mL, which are connected between the membrane stack outlet and the circulation pump inlet of the corresponding compartment, forming a circulation loop of "membrane stack outlet, cylinder, circulation pump, membrane stack inlet"; the cylinders of each membrane stack are connected to the cylinders of the next membrane stack, and the liquid in the cylinders of each level flows into the next level in an overflow manner depending on the liquid level difference.

[0113] The other operations during the operation were the same as those in Example 4, but the membrane stacks and the material tanks at each level were filled with water in advance. A 0.52 mol / L aqueous solution of pentamethylenediamine hydrochloride was introduced into the inlet of the salt chamber material tank of the first-stage membrane stack through a feed pump at a flow rate of 800 mL / h; deionized water was introduced into the inlet of the acid chamber material tank of the second-stage membrane stack through a feed pump at a flow rate of 800 mL / h. The conductivity of the material solution at the outlet of the salt chamber material tank of the second-stage membrane stack was monitored online.

[0114] After 30 minutes, the conductivity value of the outlet liquid of the salt chamber of the second-stage membrane stack stabilized at 8.9mS / cm. At this time, the timing was started, and the volume of the feed liquid and the outlet liquid of each compartment was measured. After 120 minutes, the "acid-salt" two-chamber bipolar membrane electrodialysis operation was stopped, and 1.50L of a pentamethylenediamine aqueous solution with a Cl- concentration of 4.7g / L was obtained at the outlet of the salt chamber of the second-stage membrane stack, and 1.70L of a hydrochloric acid aqueous solution with a concentration of 0.86mol / L was obtained at the outlet of the acid chamber of the first-stage membrane stack. According to calculations, the energy consumption of the "acid-salt" two-chamber bipolar membrane electrodialysis to treat pentamethylenediamine hydrochloride is 0.118kWh / mol HCl, and the Cl- membrane flux is 5.71mol / m2 / h, average current efficiency 50.3%. During operation, the fluctuation range of Cl- concentration of the outlet liquid of the salt chamber of the second-stage membrane stack was 4.7g / L±0.3g / L.

[0115] Example 5

[0116] The present invention uses a cylinder-free continuous three-stage countercurrent "acid-salt-alkali" three-chamber bipolar membrane electrodialysis system to treat sodium sulfate aqueous solution and regenerate sulfuric acid and sodium hydroxide. The three-chamber bipolar membrane electrodialyzer is a plate-frame type, and the membrane stack is composed of 4 AMVN negative membranes (Asahi Glass Co., Ltd.), 4 SHCM positive membranes (Shanghai Xiangfeng Chemical Co., Ltd.) and 5 TRJBM bipolar membranes (Beijing Tingrun Membrane Technology Development Co., Ltd.). The membrane stack structure is referenced Figure 2 As shown, the effective area of ​​each membrane in the membrane stack is 0.0088m 2 The anode plate is a titanium-coated ruthenium electrode, and the cathode plate is a stainless steel electrode. The separators and screens are made of polypropylene, the separators are non-loop separators, and the screens are woven mesh. Fig.10 shown.

[0117] Fill each level of membrane stack with water in advance. Start the circulation pumps of the acid chamber, salt chamber and alkali chamber of each level of the "acid-salt-alkali" three-chamber bipolar membrane electrodialysis membrane stack, and the liquid flow linear velocity in each compartment is 3cm / s. Turn on the DC power supply of each level of membrane stack, and adopt the constant voltage mode. The operating voltages of the first, second and third level membrane stacks are 12V, 12V and 12V respectively. At the outlet of the salt chamber of the first level membrane stack, a 200g / L sodium sulfate aqueous solution is introduced through the feed pump at a flow rate of 600mL / h; at the outlet of the acid chamber of the third level membrane stack, deionized water is introduced through the feed pump at a flow rate of 600mL / h; at the outlet of the alkali chamber of the third level membrane stack, deionized water is introduced through the feed pump at a flow rate of 600mL / h; 0.3L of 0.35mol / L sodium sulfate solution is introduced into the polar chamber of each level of membrane stack. The conductivity of the feed liquid at the outlet of the salt chamber of the third level membrane stack is monitored online.

[0118] After 17 minutes, the conductivity of the feed liquid at the outlet of the salt chamber of the third-stage membrane stack stabilized at 6.9 mS / cm, a 4.82 g / L sodium sulfate aqueous solution was obtained at the outlet of the salt chamber of the third-stage membrane stack, a 1.24 mol / L sulfuric acid aqueous solution was obtained at the outlet of the acid chamber of the first-stage membrane stack, and a 2.52 mol / L sodium hydroxide aqueous solution was obtained at the outlet of the alkali chamber of the first-stage membrane stack.

[0119] After 30 minutes, the flow rate of the sodium sulfate aqueous solution entering the salt chamber outlet of the first-stage membrane stack was artificially increased by 5% to 630mL / h (simulating the fluctuation of the raw material liquid flow rate in production). After about 15 minutes, the conductivity of the feed liquid at the salt chamber outlet of the third-stage membrane stack increased and stabilized at 10.8mS / cm; through the preliminary experiment (fitting relationship between operating voltage and conductivity of outlet feed liquid), K3=-0.122 / (mS / cm) was obtained, multiplied by -(10.8mS / cm-6.9mS / cm), and the increment of the operating voltage of the third-stage membrane stack was obtained to be 0.47V. After implementing this increment, about 5 minutes later, the conductivity of the feed liquid at the salt chamber outlet of the third-stage membrane stack was restored to 7.1mS / cm; after 30 minutes, the sodium sulfate aqueous solution entering the salt chamber outlet of the first-stage membrane stack was artificially increased to 630mL / h (simulating the fluctuation of the raw material liquid flow rate in production). After about 15 minutes, the conductivity of the feed liquid at the salt chamber outlet of the third-stage membrane stack increased and stabilized at 10.8mS / cm; through the preliminary experiment (fitting relationship between operating voltage and conductivity of outlet feed liquid), K3=-0.122 / (mS / cm) was obtained, multiplied by -(10.8mS / cm-6.9mS / cm), and the increment of the operating voltage of the third-stage membrane stack was obtained to be 0.47V. After implementing this increment, about 5 minutes later, the conductivity of the feed liquid at the salt chamber outlet of the third-stage membrane stack was restored to 7.1mS / cm; The sodium sulfate aqueous solution was switched to a sodium sulfate aqueous solution with a concentration of 210 g / L (the concentration was increased by 5%, and the flow rate was maintained at 630 mL / h to simulate the fluctuation of the raw material solution concentration in production). After about 15 minutes, the conductivity of the feed solution at the outlet of the salt chamber of the third-stage membrane stack increased and stabilized at 10.9 mS / cm; through preliminary experiments (the fitting relationship between the flow rate of the feed solution and the conductivity of the outlet feed solution), K4=8.66 (mL / h) / (mS / cm) was obtained, and multiplied by -(10.9 mS / cm-6.9 mS / cm), the increase in the flow rate of the sodium sulfate aqueous solution entering the salt chamber outlet of the first-stage membrane stack was obtained to be -35 mL / h. After implementing this increase, about 5 minutes later, the conductivity of the feed solution at the outlet of the salt chamber of the third-stage membrane stack recovered to about 6.9 mS / cm.

[0120] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A non-tank continuous multi-stage countercurrent electrodialysis system, comprising two or more membrane stacks, characterized in that: No material cylinder is provided in each compartment of each level of the membrane stack, and each compartment of each level of the membrane stack is directly connected by pipelines.

2. The non-tank continuous multi-stage countercurrent electrodialysis system according to claim 1, characterized in that: The feed liquid in each compartment of each level of membrane stack in the electrodialysis system is organized in a countercurrent manner, and the feed liquid in each compartment is self-circulated in the stack through a circulation pump.

3. The non-tank continuous multi-stage countercurrent electrodialysis system according to claim 2, characterized in that: The electrodialyzer of the electrodialysis system includes a common electrodialyzer, a three-chamber bipolar membrane electrodialyzer or a two-chamber bipolar membrane electrodialyzer.

4. The non-tank continuous multi-stage countercurrent electrodialysis system according to claim 3, characterized in that: The electrodialyzer of the electrodialysis system is a common electrodialyzer, and the membrane stack of the common electrodialyzer includes two compartments, namely a desalination compartment and a concentration compartment; The countercurrent arrangement includes: the initial liquid of the desalting chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage membrane stack; the initial liquid of the concentrating chamber enters from the N-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the first-stage membrane stack; wherein N≥2.

5. The non-tank continuous multi-stage countercurrent electrodialysis system according to claim 3, characterized in that: The electrodialyzer of the electrodialysis system is a three-chamber bipolar membrane electrodialyzer, wherein the membrane stack of the three-chamber bipolar membrane electrodialyzer includes three compartments, namely, an acid chamber, a salt chamber and an alkali chamber; The countercurrent arrangement includes: the initial liquid of the salt chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage membrane stack; the initial liquid of the acid chamber and the initial liquid of the alkali chamber enter from the N-stage membrane stack, pass through each stage of the membrane stack in sequence, and flow to the first-stage membrane stack; wherein, N≥2.

6. The non-tank continuous multi-stage countercurrent electrodialysis system according to claim 3, characterized in that: The electrodialyzer of the electrodialysis system is a two-chamber bipolar membrane electrodialyzer, wherein the membrane stack of the two-chamber bipolar membrane electrodialyzer includes two compartments, namely a salt chamber and an alkali chamber; The countercurrent arrangement includes: the initial liquid of the salt chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage membrane stack; the initial liquid of the alkali chamber enters from the N-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the first-stage membrane stack; wherein N≥2; Alternatively, the membrane stack of the two-chamber bipolar membrane electrodialyser includes two compartments, namely a salt compartment and an acid compartment; The countercurrent arrangement includes: the initial liquid of the salt chamber enters from the first-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the N-stage membrane stack; the initial liquid of the acid chamber enters from the N-stage membrane stack, passes through each stage of the membrane stack in sequence, and flows to the first-stage membrane stack; wherein N≥2.

7. The non-tank continuous multi-stage countercurrent electrodialysis system according to any one of claims 1 to 6, characterized in that: The ways of directly connecting the compartments of each level of membrane stack through pipelines include: The outlet of each compartment of each membrane stack is connected to the inlet of the corresponding compartment of the next membrane stack; or, The outlet of each compartment of each membrane stack is connected to the outlet of the corresponding compartment of the next membrane stack; or, The inlet of each compartment of each membrane stack is connected to the outlet of the corresponding compartment of the next membrane stack; or, The inlet of each compartment of each level of membrane stack is connected to the inlet of the corresponding compartment of the next level of membrane stack.

8. An electrodialysis method based on the cylinder-free continuous multi-stage countercurrent electrodialysis system according to any one of claims 1 to 6, wherein the electrodialyzer in the electrodialysis system is a common electrodialyzer, comprising the following steps: The outlet salt concentration of the final desalination chamber of a common electrodialyzer is used to feedback-regulate the operating voltage of the final stage, or the flow rate of the feed liquid of the first desalination chamber is feedback-regulated.

9. The electrodialysis method according to claim 8, characterized in that When the outlet salt concentration is lower than the set value, the final stage operating voltage is reduced; when the outlet salt concentration is higher than the set value and the final stage current density is lower than the maximum current density that the ion exchange membrane can withstand, the final stage operating voltage is increased; wherein, the final stage operating voltage adjustment increment = -(outlet salt concentration - set value) * K1; wherein K1 is a negative value, which is the correlation coefficient between the outlet salt concentration and the final stage operating voltage, obtained by the fitting relationship between the two; or, The outlet salt concentration is higher than the set value, and the current density of the final stage is equal to or higher than the maximum current density that the ion exchange membrane can withstand, thereby reducing the flow rate of the feed liquid of the first-stage desalination chamber, wherein the flow rate adjustment increment of the feed liquid of the first-stage desalination chamber=-(outlet salt concentration-set value)*K2; wherein K2 is a positive value, which is the correlation coefficient between the outlet salt concentration and the flow rate of the feed liquid of the first-stage desalination chamber, and is obtained by a fitting relationship between the two.

10. The electrodialysis method according to claim 9, characterized in that The outlet salt concentration is represented by the conductivity value detected by a conductivity meter, wherein: The conductivity value of the outlet salt concentration is lower than the set value, and the final stage operating voltage is reduced; the conductivity value of the outlet salt concentration is higher than the set value, and the current density of the final stage is lower than the maximum current density that the ion exchange membrane can withstand, and the final stage operating voltage is increased; wherein, the increment of the final stage operating voltage adjustment = -(conductivity value of the outlet salt concentration - set value)*K3; wherein K3 is a negative value, which is the correlation coefficient between the conductivity value of the outlet salt concentration and the final stage operating voltage, and is obtained by the fitting relationship between the two; or, The conductivity value of the outlet salt concentration is higher than the set value, and the current density of the final stage is equal to or higher than the maximum current density that the ion exchange membrane can withstand, thereby reducing the flow rate of the feed liquid of the first-stage desalination chamber, wherein the flow rate adjustment increment of the feed liquid of the first-stage desalination chamber=-(conductivity value of the outlet salt concentration-set value)*K4; wherein K4 is a positive value, which is the correlation coefficient between the conductivity value of the outlet salt concentration and the flow rate of the feed liquid of the first-stage desalination chamber, and is obtained by a fitting relationship between the two.

11. An electrodialysis method based on the cylinder-free continuous multi-stage countercurrent electrodialysis system according to any one of claims 1 to 6, wherein the electrodialyzer in the electrodialysis system is a bipolar membrane electrodialyzer, comprising the following steps: The outlet salt concentration of the final salt chamber of the bipolar membrane electrodialyzer is used to feedback-regulate the operating voltage of the final stage, or the flow rate of the feed liquid of the first salt chamber is feedback-regulated.

12. The electrodialysis method according to claim 11, characterized in that When the outlet salt concentration is lower than the set value, the final stage operating voltage is reduced; when the outlet salt concentration is higher than the set value and the final stage current density is lower than the maximum current density that the ion exchange membrane can withstand, the final stage operating voltage is increased; wherein, the final stage operating voltage adjustment increment = -(outlet salt concentration - set value) * K1; wherein K1 is a negative value, which is the correlation coefficient between the outlet salt concentration and the final stage operating voltage, obtained by the fitting relationship between the two; or, The outlet salt concentration is higher than the set value, and the current density of the final stage is equal to or higher than the maximum current density that the ion exchange membrane can withstand, thereby reducing the flow rate of the feed liquid of the first-stage salt chamber, wherein the flow rate adjustment increment of the feed liquid of the first-stage salt chamber = -(outlet salt concentration - set value)*K2; wherein K2 is a positive value, which is the correlation coefficient between the outlet salt concentration and the flow rate of the feed liquid of the first-stage salt chamber, and is obtained by the fitting relationship between the two.

13. The electrodialysis method according to claim 12, characterized in that: The outlet salt concentration is represented by the conductivity value detected by a conductivity meter, wherein: The conductivity value of the outlet salt concentration is lower than the set value, and the final stage operating voltage is reduced; the conductivity value of the outlet salt concentration is higher than the set value, and the current density of the final stage is lower than the maximum current density that the ion exchange membrane can withstand, and the final stage operating voltage is increased; wherein, the increment of the final stage operating voltage adjustment = -(conductivity value of the outlet salt concentration - set value)*K3; wherein K3 is a negative value, which is the correlation coefficient between the conductivity value of the outlet salt concentration and the final stage operating voltage, and is obtained by the fitting relationship between the two; or, The conductivity value of the outlet salt concentration is higher than the set value, and the current density of the final stage is equal to or higher than the maximum current density that the ion exchange membrane can withstand, thereby reducing the flow rate of the feed liquid of the first-stage salt chamber, wherein the flow rate adjustment increment of the feed liquid of the first-stage salt chamber = -(conductivity value of the outlet salt concentration - set value)*K4; wherein K4 is a positive value, which is the correlation coefficient between the conductivity value of the outlet salt concentration and the flow rate of the feed liquid of the first-stage salt chamber, and is obtained by the fitting relationship between the two.

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