A method for temperature difference enhanced bipolar membrane electrodialysis process

Abstract

This invention relates to a method for temperature-differential enhanced bipolar membrane electrodialysis. A three-compartment bipolar membrane electrodialysis device is employed, wherein the membrane stack of the device is arranged sequentially in the order of cation exchange membrane – bipolar membrane – anion exchange membrane. Adjacent membranes are separated by partitions to form acid, salt, and alkali compartments. The electrode plates and the bipolar membranes on both sides of the membrane stack form two polar liquid compartments. Each compartment is connected to a corresponding container via a pump. Heating the alkali or acid solution establishes a temperature difference across the bipolar membrane, accelerating ion migration, increasing the water dissociation rate within the bipolar membrane, increasing the concentration of acid and alkali produced, improving current efficiency, and reducing energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of bipolar membrane electrodialysis technology, specifically involving establishing a temperature difference across a bipolar membrane to accelerate ion migration, increase the water dissociation rate within the bipolar membrane, thereby increasing the concentration of acid and alkali produced, increasing the current density, and reducing energy consumption. Background Technology

[0002] Concentrated seawater treatment often employs evaporation crystallization, but this method suffers from high equipment investment and energy consumption, as well as low efficiency. Membrane distillation, also based on heat-driven evaporation, typically requires draw solution from thermally regenerated feed liquid, which, limited by thermodynamic constraints, has not yet achieved large-scale industrial application. Bipolar membrane electrodialysis (BMED), building upon electrodialysis (ED), electrolyzes water molecules through a bipolar membrane interlayer, simultaneously achieving desalination of high-salinity wastewater and the preparation of acids and alkalis, without the need for added chemical reagents and with virtually no byproducts.

[0003] Many domestic and international scholars have conducted research on the treatment of concentrated brine from seawater desalination and other types of high-salinity wastewater using bipolar membrane electrodialysis (BMED). Research results indicate that BMED has significant potential for near-zero emissions in real-world production processes. In bipolar membrane electrodialysis, temperature has the most significant impact. This invention explores the mass transfer mechanism under the coupling effect of temperature and electric fields in the bipolar membrane process, aiming to better enhance the bipolar membrane electrodialysis process, thereby reducing energy consumption, increasing current density, and improving the quality of the produced acids and alkalis. Summary of the Invention

[0004] This invention aims to explore the impact of temperature difference on the bipolar membrane electrodialysis process. By controlling the acid and alkali solutions to remain stable at different temperatures, the temperature difference is applied to the bipolar membrane to investigate the changes in the bipolar membrane electrodialysis process under temperature difference conditions. The process is comprehensively evaluated by combining key indicators such as acid and alkali concentration, salt chamber conductivity changes, current efficiency, and energy consumption. This will better enhance bipolar membrane electrodialysis technology, reducing energy consumption, increasing current density, improving the quality of produced acids and alkalis, and increasing its efficiency and economy, providing new practical ideas for research and application in related fields.

[0005] The technical solution of this invention is as follows:

[0006] 1. A method for temperature-differential enhanced bipolar membrane electrodialysis, characterized in that a three-compartment bipolar membrane electrodialysis device is adopted, wherein the membrane stack of the bipolar membrane electrodialysis device is arranged sequentially in the order of cation exchange membrane - bipolar membrane - anion exchange membrane, and adjacent membranes are separated by partitions to form acid chamber, salt chamber and alkali chamber, and the electrode plates and the bipolar membranes on both sides of the membrane stack form two polar liquid chambers; each compartment is connected to a corresponding container through a pump; a temperature difference is established on both sides of the bipolar membrane to accelerate ion migration, increase the water dissociation rate in the bipolar membrane, increase the concentration of acid and alkali produced, improve current efficiency and reduce energy consumption;

[0007] The steps of the temperature difference-enhanced bipolar membrane electrodialysis process are as follows:

[0008] (1) Four liquid solutions, namely acid solution, alkali solution, salt solution and polar solution, are introduced into the four chambers of the bipolar membrane electrodialysis device, and the alkali solution is heated to establish a temperature difference on both sides of the bipolar membrane;

[0009] (2) Turn on the peristaltic pump and set the flow rate value, turn on the DC power supply, set the target voltage, and run the bipolar membrane electrodialysis device;

[0010] (3) During the process, record the membrane stack voltage, power supply voltage and current, and the temperatures of acids, bases and salts, and take samples on time to perform acid-base titration or conductivity and pH measurement, and calculate current efficiency and energy consumption.

[0011] (4) Stop the experiment and turn off the DC power supply when the current drops below 0.6A under constant voltage operating conditions.

[0012] The method for a temperature difference-enhanced bipolar membrane electrodialysis process is characterized in that: the NaCl solution in the salt chamber is 50 g / L to 70 g / L, and the initial acid and alkali concentrations are 0.05 mol / L.

[0013] The method for a temperature difference-enhanced bipolar membrane electrodialysis process is characterized in that the membrane surface flow rate is 4-8 cm / s.

[0014] The method for a temperature difference-enhanced bipolar membrane electrodialysis process is characterized in that the temperature difference is 0, 5, 10, and 15°C.

[0015] The method for temperature difference enhanced bipolar membrane electrodialysis is characterized in that the temperature difference across the membrane can also be obtained by heating the acid chamber solution.

[0016] The beneficial effects of this invention are:

[0017] This invention controls the acid and alkali solutions to remain stable at different temperatures, allowing the temperature difference to act on the bipolar membrane, thereby accelerating ion migration, increasing the water dissociation rate within the bipolar membrane, increasing the concentration of produced acid and alkali, improving current efficiency, and reducing energy consumption. It has very promising application prospects. Attached Figure Description

[0018] Figure 1 Schematic diagram of temperature difference-enhanced bipolar membrane electrodialysis;

[0019] Figure 2 The change of electrical conductivity over time under temperature difference conditions at a salinity of 50 g / L;

[0020] Figure 3 The change of acid concentration in an acid chamber with a salinity of 50 g / L over time under temperature difference conditions;

[0021] Figure 4 The change of alkali concentration in an alkali chamber with a salinity of 50 g / L over time under temperature difference conditions;

[0022] Figure 5 The change of current efficiency over time under temperature difference conditions at a salinity of 50 g / L;

[0023] Figure 6 Energy consumption at a salinity of 50 g / LL varies over time under temperature difference conditions;

[0024] Figure 7 The change of electrical conductivity over time under temperature difference conditions at a salinity of 70 g / L;

[0025] Figure 8 The change of acid concentration in an acid chamber with a salinity of 70 g / L over time under temperature difference conditions;

[0026] Figure 9 The change of alkali concentration in an alkali chamber with a salinity of 70 g / LL over time under temperature difference conditions;

[0027] Figure 10 The change of current efficiency over time under temperature difference conditions at a salinity of 70 g / LL;

[0028] Figure 11 Energy consumption at a salinity of 70 g / L varies over time under temperature difference conditions;

[0029] Figure 12 Comparison of conductivity changes in the alkali and acid chambers upon heating;

[0030] Figure 13 Comparison of changes in acid and alkali concentrations during heating in the alkali and acid chambers;

[0031] Figure 14 Comparison of heating current efficiency changes in alkali and acid chambers;

[0032] Figure 15 Comparison of energy consumption changes for heating in the alkali and acid chambers; Detailed Implementation

[0033] The invention will be further described below with reference to the accompanying drawings:

[0034] Example 1

[0035] Four initial solutions were prepared with the following concentrations: 50 g / L NaCl solution, 0.05 mol / L HCl solution (acid chamber), 0.05 mol / L NaOH solution (base chamber), and 0.2 mol / L Na₂SO₄ solution (electrode chamber). A BMED experiment was conducted under constant current conditions, with a current density set at 35 mA / cm². 2 ,like Figure 1 As shown, the salt chamber temperature and acid chamber temperature were both 20℃. The alkali chamber solution temperature was increased to 20, 25, 30, and 35℃. The temperature difference was applied to the bipolar membrane, and the change in salt chamber conductivity over time was measured. Figure 2 As shown, the conductivity decreased from 62 ms / cm at the initial moment to 8 ms / cm. The lowest conductivity was 4.4 ms / cm at a temperature difference of 5℃, followed by 7.3 ms / cm at a temperature difference of 10℃. The conductivity at temperature differences of 5, 10, and 15℃ was lower than that under isothermal conditions at the same time point. The decrease in conductivity was most significant at a temperature difference of 10℃, 6.84 ms / cm lower than under isothermal conditions, with an average reduction rate of approximately 18.84%.

[0036] like Figure 3 and Figure 4 As shown, the initial acid and alkali concentrations were 0.05 mol / L. Under temperature differences of 0, 5, 10, and 15℃, the resulting acid concentrations were 0.64, 0.66, 0.65, and 0.69 mol / L, and the resulting alkali concentrations were 0.56, 0.57, 0.58, and 0.61 mol / L, respectively. The temperature difference slightly increased the final acid and alkali concentrations. The average acid concentration was 14% higher than that under isothermal operation, and the alkali concentration was about 4% higher than that under isothermal operation. Increasing the alkali chamber temperature was more conducive to increasing the acid concentration. As the feed salinity increased, the acid and alkali concentrations also increased accordingly. Due to the increased alkali chamber temperature, the leakage of OH- from the alkali chamber to the salt chamber was accelerated; therefore, the final alkali concentration was lower than the acid concentration.

[0037] like Figure 5As shown, when the temperature difference is 5℃, the average current efficiency can be increased by 3.7% compared to the isothermal condition, and by about 1.3% when the temperature difference is 15℃. Appropriate temperature differences can improve current efficiency. By default, the current efficiency is 0 at time 0. Under the influence of an applied electric field, ions in the solution undergo directional migration and transmembrane transport. Simultaneously, a concentration gradient forms on the membrane surface under the influence of the electric field. The velocity of ions approaching the membrane layer is significantly faster than the velocity of ions moving away from the membrane layer, thus increasing the current efficiency. As ions migrate to both sides of the salt chamber, the ion concentration in the salt chamber gradually decreases, and the current efficiency gradually decreases after reaching its maximum. On the one hand, the increased salt chamber concentration leads to a corresponding increase in conductivity. Within a certain range, the current density is proportional to the bandwidth (potential difference between the two ends of the bipolar membrane). When the initial salt chamber concentration increases, the solution resistivity decreases; therefore, increasing the current density is beneficial to improving the current efficiency.

[0038] Energy consumption is calculated per kg of acid. Figure 6 At a salinity of 50 g / L, energy consumption was reduced by 9.5% and 4.8% under temperature differences of 10 and 15 °C, respectively, compared to isothermal conditions. Similar to current efficiency, increasing the alkali chamber temperature decreases solution viscosity, reduces ion transport resistance, and enhances ion thermal mobility, thus promoting transmembrane migration and reducing process energy consumption. Energy consumption remained relatively stable between 40 and 120 min, as the membrane stack voltage also remained stable during this period, resulting in relatively stable energy consumption under constant current conditions. As the ion concentration in the salt chamber decreased, energy consumption gradually increased after 120 min, and the salt chamber conductivity dropped to 10 mS / cm at 200 min. Ignoring energy consumption in this range, the integral area under the energy consumption curve at a temperature difference of 15 °C between 0 and 160 min was 628.8, while the corresponding integral area under isothermal operation was 729.9, representing a reduction in energy consumption of approximately 13.85%.

[0039] Example 2

[0040] Four initial solutions were prepared with the following concentrations: 70 g / L NaCl solution, 0.05 mol / L HCl solution (acid chamber), 0.05 mol / L NaOH solution (base chamber), and 0.2 mol / L Na₂SO₄ solution (electrode chamber). A BMED experiment was conducted under constant current conditions, with a current density set at 35 mA / cm². 2 The salt chamber and acid chamber temperatures were set to 20°C, while the alkali chamber solution temperatures were increased to 20, 25, 30, and 35°C. This temperature difference was applied to the bipolar membrane, and the change in salt chamber conductivity over time was measured. Figure 7As shown, under isothermal conditions at a salinity of 70 g / L, it takes 340 minutes to reduce the conductivity of the salt chamber to below 10 mS / cm, while it takes 300 minutes for temperature differences of 5℃ and 10℃, and 320 minutes for a temperature difference of 15℃. Under temperature difference conditions, the conductivity change is almost the same for the first 160 minutes. After 160 minutes, the conductivity at a temperature difference of 15℃ is slightly higher than that at 5℃ and 10℃, but still lower than that under isothermal operation. Temperature difference is beneficial to the desalination effect of bipolar membrane electrodialysis; increased temperature can increase the diffusion rate of solutes in water, reduce water viscosity, and improve desalination efficiency. However, excessively high temperatures inhibit heat transfer, especially when treating higher concentrations of brine.

[0041] Figure 8 and Figure 9 It was found that the acid concentration increased most significantly under a 15℃ temperature difference at a salt concentration of 70 g / L, with the highest concentration being approximately 13.1% higher than that under isothermal conditions, and the base concentration was approximately 3.8% higher. The initial acid and base concentrations were 0.05 mol / L. Under temperature differences of 0, 5, 10, and 15℃, the acid concentrations were 0.84, 0.85, 0.86, and 0.95 mol / L, respectively, and the base concentrations were 0.79, 0.8, 0.81, and 0.82 mol / L, respectively. The temperature difference increased both the final acid and base concentrations. The 15℃ temperature difference resulted in the most significant increase in acid and base concentrations, with the highest acid concentration exceeding that under isothermal conditions by 13.1% and the highest base concentration by 3.8%. The effect of temperature difference on increasing acid concentration was greater than that on increasing base concentration.

[0042] The current efficiency curve measured at a salinity of 70 g / L is similar to that at a salinity of 50 g / L, such as... Figure 10 As shown, a temperature difference of 15℃ has the most significant effect on improving current efficiency, with the maximum improvement reaching 83.2% compared to isothermal conditions. This is followed by a temperature difference of 10℃, with the maximum improvement reaching 48.3% compared to isothermal conditions. The average current efficiency under a temperature difference of 5℃ is also slightly higher than that under isothermal conditions. During the experiment, the current efficiency improved under the influence of temperature difference. At a temperature difference of 15℃, the average efficiency was about 17% higher than under isothermal conditions, followed by a temperature difference of 10℃, with an average efficiency improvement of 5.4%.

[0043] Energy consumption measured at 70 g / L salinity as a function of time is as follows: Figure 11 As shown, within 0 to 350 minutes, compared with isothermal conditions, the average energy consumption decreased by approximately 6.1%, 14.6%, and 26.8% for temperature differences of 5℃, 10℃, and 15℃, respectively. The effect of temperature difference on energy consumption is related to the effect of current efficiency; as current efficiency increases, energy consumption decreases accordingly.

[0044] The effects of temperature increases in the acid and alkali chambers on the bipolar membrane electrodialysis process were investigated. Figure 12 , Figure 13 , Figure 14and Figure 15 As shown, when the temperature of the acid chamber or the alkali chamber is increased by 5℃, the changes in conductivity, acid and alkali concentration, current efficiency and energy consumption of the salt chamber are almost the same. Therefore, there is no significant difference in the effect of maintaining the same temperature difference on the left or right side of the bipolar membrane.

[0045] The examples of the invention have been described above, but the invention is not limited to the examples described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the invention. All such equivalent modifications or substitutions are included within the scope defined by the claims of this application.

[0046] Matters not covered in this invention are common knowledge.

Claims

1. A method for temperature difference-enhanced bipolar membrane electrodialysis, characterized in that, A bipolar membrane electrodialysis device with a three-compartment structure is adopted. The membrane stack of the bipolar membrane electrodialysis device is arranged in the order of cation exchange membrane - bipolar membrane - anion exchange membrane. Adjacent membranes are separated by partitions to form acid chamber, salt chamber and alkali chamber. The electrode plates and the bipolar membranes on both sides of the membrane stack form two polar liquid chambers. Each compartment is connected to the corresponding container through a pump. The temperature difference established on both sides of the bipolar membrane accelerates the ion migration rate, increases the water dissociation rate in the bipolar membrane, increases the concentration of acid and alkali produced, improves the current efficiency and reduces energy consumption. The steps of the temperature difference-enhanced bipolar membrane electrodialysis process are as follows: (1) Pass the four liquid solutions of acid, alkali, salt and polar liquid into the four chambers of the bipolar membrane electrodialysis device, and heat the alkali chamber solution or the acid chamber solution to establish a temperature difference on both sides of the bipolar membrane. The temperature difference is 5℃, 10℃ or 15℃. (2) Turn on the peristaltic pump and set the flow rate value, turn on the DC power supply, set the target voltage, and run the bipolar membrane electrodialysis device; (3) During the process, record the membrane stack voltage, power supply voltage and current, and the temperatures of acids, bases and salts, and take samples on time to perform acid-base titration or conductivity and pH measurement, and calculate current efficiency and energy consumption. (4) Stop the experiment and turn off the DC power supply when the current drops below 0.6A under constant voltage operating conditions.

2. The method for a temperature difference-enhanced bipolar membrane electrodialysis process according to claim 1, characterized in that: The NaCl solution in the salt chamber is 50 g / L to 70 g / L, and the initial acid and base concentrations are 0.05 mol / L.

3. The method for a temperature difference-enhanced bipolar membrane electrodialysis process according to claim 1, characterized in that: The flow rate at the membrane surface is 4–8 cm / s.

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