Method for intensifying mass transfer by electrolyzed water driven double-side air-blowing membrane distillation

Abstract

The invention discloses a method for intensifying mass transfer by electrolyzed water driven bilateral air-blowing membrane distillation, which comprises the following steps: a three-chamber device is sequentially provided with a positive chamber, a negative chamber and a concentration chamber, the positive chamber and the negative chamber are separated by a nylon diaphragm, and the negative chamber and the concentration chamber are separated by a composite membrane; the method comprises the following steps: preparing materials, wherein an alkaline solution is used as electrolyte of an anode chamber and a cathode chamber; water molecule mass transfer: liquid water molecules in the concentration chamber are driven by vapor pressure difference, and the liquid water molecules absorb heat, are vaporized, penetrate through the composite membrane, enter the cathode chamber and are condensed into a liquid state; uniformizing: uniformizing the concentration of the electrolyte in the cathode chamber by using hydrogen generated in the cathode chamber in the water electrolysis process; and air blowing operation: oxygen generated by the anode is blown into the concentration chamber after being subjected to gas-liquid separation. The problem of absorbent regeneration is solved through the electrolyte of alkaline electrolyzed water, low-value oxygen generated by the electrolyzed water is reasonably utilized, and continuous concentration of high-salinity wastewater at room temperature is realized by coupling absorption membrane distillation and bubbling membrane distillation.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane distillation, and particularly to a method for enhancing mass transfer in electrolyzed water-driven double-sided gas-blowing membrane distillation. Background Art

[0003] Traditional industrial treatment of high-salt wastewater mainly includes technologies such as steam compression method, multi-stage flash distillation method, and multi-effect distillation method, but all face problems such as high energy consumption, high investment cost, and easy fouling and corrosion of equipment.

[0004] Using membrane distillation technology to treat high-concentration brine with steam pressure difference as the driving force, compared with pressure-driven membrane processes, it is basically not limited by osmotic pressure, and has advantages such as low energy consumption, high rejection rate, and low operating pressure, and has broad application prospects in the fields of high-salt wastewater treatment, fruit juice concentration, etc.

[0005] However, the phenomena of temperature polarization and concentration polarization in the membrane distillation process will increase the operating energy consumption, and problems such as membrane fouling in the concentration of high-concentration feed liquid and absorbent regeneration will also reduce the stability of the system and limit its industrialized promotion and application.

[0006] Therefore, there is an urgent need for a method for enhancing mass transfer in electrolyzed water-driven double-sided gas-blowing membrane distillation that can solve one or more of the above problems. Summary of the Invention

[0007] To solve one or more problems existing in the prior art, the present invention provides a method for enhancing mass transfer in electrolyzed water-driven double-sided gas-blowing membrane distillation. The technical solution adopted by the present invention to solve the above problems is: a method for enhancing mass transfer in electrolyzed water-driven double-sided gas-blowing membrane distillation, a three-chamber device, the three-chamber device is sequentially provided with an anode chamber, a cathode chamber, and a concentration chamber, the anode chamber is provided with an anode, the cathode chamber is provided with a cathode, the anode chamber and the cathode chamber are separated by a nylon diaphragm, the cathode chamber and the concentration chamber are separated by a composite membrane, the composite membrane includes a porous hydrophobic and breathable polytetrafluoroethylene membrane and a support layer, the support layer is polypropylene, wherein, the concentration chamber is the feed side, and the cathode chamber is the permeation side;

[0008] The method includes: Materials, using an alkaline solution as the electrolyte in the anode chamber and the cathode chamber, the material of the anode is NiCo alloy, and the material of the cathode is NiMo alloy;

[0009] Mass transfer of water molecules, using the steam pressure difference to drive the liquid water molecules on the feed side, the liquid water molecules absorb heat and vaporize and pass through the composite membrane into the permeation side to condense into liquid;

[0010] Homogenization, using the hydrogen gas generated in the cathode chamber during the electrolysis of water to cause homogenization of the electrolyte concentration in the cathode chamber;

[0011] Bubbling operation, the oxygen gas generated by the anode is bubbled into the concentration chamber after gas-liquid separation.

[0012] In some embodiments, the electrolyte is a KOH solution.

[0013] Further, the electrolyte is a 30 wt% KOH solution.

[0014] In some embodiments, the porous hydrophobic and breathable polytetrafluoroethylene membrane faces the electrolyte in the cathode chamber, and the support layer faces the salt solution in the concentration chamber.

[0015] In some embodiments, the material of the anode is NiCo alloy, and the material of the cathode is NiMo alloy.

[0016] In some embodiments, in the bubbling operation, the oxygen gas after gas-liquid separation is bubbled into the concentration chamber at a bubbling rate of 20 mL min -1 .

[0017] In some embodiments, the method further includes: the current density during electrolysis is 400 mA cm -2 .

[0018] The technical effect achieved by the present invention is: based on the above three-chamber device, the porous hydrophobic and breathable polytetrafluoroethylene membrane is used to separate the feed side and the permeation side. The liquid water molecules on the membrane surface of the feed side (high-salt wastewater) complete the mass transfer process of water molecules under the drive of the vapor pressure difference, and then these water molecules are consumed in the process of electrolysis of water. Finally, the concentration of the absorbent (electrolyte) changes little;

[0019] In the case of using the hydrogen gas generated by the cathode to cause homogenization of the electrolyte concentration in the cathode chamber, the thickness of the membrane boundary on the permeation side will be weakened. Therefore, a large vapor pressure difference can always be maintained on both sides of the porous hydrophobic and breathable polytetrafluoroethylene membrane, thereby solving the technical problem of the continuously decreasing mass transfer rate;

[0020] Using the low-value product (oxygen gas) generated by the anode during the electrolysis of water to perform a bubbling operation on the feed side, thereby strengthening the fluid disturbance on the feed side, generating a shear force on the membrane surface, and then reducing the thickness of the liquid boundary layer on the feed side, weakening the influence brought by concentration polarization, and solving the problems of membrane hydrophilic wetting and membrane fouling caused by the concentration polarization phenomenon in the absorption membrane distillation process. The method of the present application does not heat the feed liquid, so temperature polarization is not considered;

[0021] Realize the continuous concentration of high-salt wastewater at room temperature and the continuous driving of alkaline electrolyzed water for membrane distillation-crystallization. In this process, not only can fresh water be provided for alkaline electrolyzed water, but also the purpose of concentrating and crystallizing high-salt wastewater can be achieved;

[0022] Furthermore, adopting the above method of combining three-chamber devices to treat the waste liquid has relatively lower energy consumption than traditional methods, and the equipment cost is not high. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the three-chamber device of the present invention;

[0024] Figure 2 Table of concentration differences between the anode and cathode electrolytes during the electrolysis process;

[0025] Figure 3 Graph of vapor pressure changes corresponding to different concentrations of KCl and KOH aqueous solutions;

[0026] Figure 4 Table of the relationship between the vapor pressure difference across the composite membrane and the concentration rate;

[0027] Figure 5 Table of comparison of the bulging effects on the permeate side, feed side, and both sides simultaneously;

[0028] Figure 6 Table of the influence of different feed liquid concentrations on the flux under the condition of bilateral gas injection;

[0029] Figure 7 Table of comparison of the enhanced mass transfer effect of membrane distillation with different gas injection flow rates;

[0030] Figure 8 Table of the first to third stages of alkaline electrolyzed water-driven gas injection-permeation membrane distillation for segmental concentration of high-salt solution to crystallization;

[0031] Figure 9 Table of the crystallization stage of alkaline electrolyzed water-driven gas injection-permeation membrane distillation for segmental concentration of high-salt solution to crystallization;

[0032] Figure 10 Table of practical comparison of enhanced mass transfer of alkaline electrolyzed water-driven gas injection-permeation membrane distillation (low-concentration KCl solution);

[0033] Figure 11 Table of practical comparison of enhanced mass transfer of alkaline electrolyzed water-driven gas injection-permeation membrane distillation (saturated KCl solution);

[0034] Figure 12 Comparison diagram of scanning electron microscope photos of the surface of the porous hydrophobic breathable polytetrafluoroethylene membrane before and after the experiment.

[0035] Figure 1 Reference numerals:

[0036] 1. Anode chamber, 2. Cathode chamber, 3. Concentration chamber, 4. Nylon diaphragm, 5. Composite membrane, 6. Anode, 7. Cathode. Specific embodiments

[0037] To make the above objects, features, and advantages of the present invention more comprehensible, the following will describe in detail the specific embodiments of the present invention with reference to the accompanying drawings. Many specific details are set forth in the following description to fully understand the present invention. However, the present invention can be implemented in many other ways different from this description, and those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0038] As Figure 1 shown, the present invention discloses a method for enhancing mass transfer in electrolyzed water-driven double-sided gas-blowing membrane distillation, which includes: a three-chamber device, the three-chamber device is sequentially provided with an anode chamber 1, a cathode chamber 2, and a concentration chamber 3, the anode chamber 1 is provided with an anode 6, the cathode chamber 2 is provided with a cathode 7, the anode chamber 1 is separated from the cathode chamber 2 by a nylon diaphragm 4, the cathode chamber 2 is separated from the concentration chamber 3 by a composite membrane 5, the composite membrane 5 includes a porous hydrophobic and breathable polytetrafluoroethylene membrane (PTFE) and a support layer, the support layer is polypropylene (PP), wherein, the concentration chamber 3 is the feed side, and the cathode chamber 2 is the permeation side;

[0039] The method includes: Materials, using an alkaline solution as the electrolyte in the anode chamber and the cathode chamber, the material of the anode is NiCo alloy, and the material of the cathode is NiMo alloy;

[0040] Mass transfer of water molecules, using the vapor pressure difference to drive the liquid water molecules on the feed side, the liquid water molecules absorb heat and vaporize and pass through the composite membrane into the permeation side and condense into liquid;

[0041] Homogenization, using the hydrogen generated in the cathode chamber during the electrolysis of water to cause the homogenization of the electrolyte concentration in the cathode chamber;

[0042] Gas-blowing operation, the oxygen generated by the anode is blown into the concentration chamber after gas-liquid separation.

[0043] Preferably, in the gas-blowing operation, the oxygen after gas-liquid separation is blown into the concentration chamber at a gas-blowing rate of 20 mL min -1 .

[0044] Preferably, the current density during electrolysis is 400 mA cm -2 .

[0045] It should be noted that when implementing, a 30 wt% KOH solution is preferably used as the electrolyte in the anode chamber and the cathode chamber. Furthermore, the material of the anode is NiCo alloy, and the material of the cathode is NiMo alloy. For the composite membrane, the porous hydrophobic and breathable polytetrafluoroethylene membrane faces the electrolyte in the cathode chamber, and the support layer faces the salt solution in the concentration chamber.

[0046] It should be explained that the purpose of selecting the nylon diaphragm is to prevent the mixing of hydrogen and oxygen, and the three-chamber device operates in a constant current mode during electrolysis.

[0047] 1. Using the three-chamber device and the method to concentrate the KCl solution, as shown in Figure 2 It can be seen that during the alkaline electrolysis of water, a hydrogen evolution reaction occurs at the cathode, consuming H2O to produce OH - and H2. Without replenishing the electrolyte solution, the concentration of KOH at the cathode increases. An oxygen evolution reaction occurs at the anode, and the concentration of KOH at the anode also increases but to a lesser extent than at the cathode because the anode consumes OH - to produce H2O and O2. Therefore, the KOH solution can be used as an excellent absorbent in the cathode chamber to couple with membrane distillation to achieve the concentration of high-salt wastewater.

[0048] 2. Using the three-chamber device and the method to concentrate the KCl solution, where a 30 wt% KOH solution is used as the electrolyte in the anode chamber and the cathode chamber, as shown in Figure 3 、 Figure 4 It can be seen that in Figure 3 , the vapour pressure is in KPa and the mass fraction is the mass fraction. The greater the vapour pressure difference between the KCl solution on the feed side and the KOH electrolyte on the permeate side, the greater the membrane flux and the faster the concentration rate. Theoretically, the commonly used 30 wt% KOH aqueous solution in alkaline electrolysis of water can fully serve as an absorbent to concentrate the low-concentration KCl solution to a saturated state or even crystallize it. On the other hand, the coupling of the membrane distillation water migration process and the electrolysis water consumption process can achieve a dynamic balance and the purpose of separating the salt solution by concentration and crystallization.

[0049] 3. Comparative experiment on the effect of gas bubbling. Using the three-chamber device and the method to concentrate the KCl solution, where a 30 wt% KOH solution is used as the electrolyte in the anode chamber and the cathode chamber, oxygen is bubbled into the feed side and hydrogen is bubbled into the permeate side to simulate the process of alkaline electrolysis coupling with gas bubbling to enhance membrane distillation. The concentration of the feed liquid KCl solution is 18 wt%, and the bubbling rates of oxygen and hydrogen are both 20 mL min -1 , as shown in Figure 5As shown, it can be seen that the mass transfer enhancement effect is as follows: both sides > feed side > permeate side. When air is bubbled on the feed side, the feed liquid is disturbed, the thickness of the membrane boundary layer becomes smaller, the vapor pressure at the membrane interface on the feed side increases rapidly, and the mass transfer driving force of the osmotic membrane distillation process increases; while when air is bubbled on the permeate side, in addition to disturbing the permeate liquid, the air bubbling also increases the membrane interface pressure and hinders the transmission of some steam water molecules, ultimately reducing the driving force of the permeant on the water flux, and its strengthening effect is weaker than that of the feed side.

[0050] 4. Double-sided air bubbling test, using the three-chamber device and the method to concentrate KCl solution. Among them, 30wt% KOH solution is used as the electrolyte in the anode chamber and the cathode chamber. Oxygen is bubbled into the feed side, and hydrogen is bubbled into the permeate side to simulate the air-bubbling enhanced membrane distillation process coupled with alkaline water electrolysis. The air-bubbling rates of oxygen and hydrogen are both 20 mL min -1 , and the concentrations of the feed liquid KCl solution are 3wt%, 9wt%, and 18wt% respectively. Combining Figure 6 As shown, it can be seen that under the three feed liquid concentration conditions, air bubbling has an obvious effect on enhancing the mass transfer of membrane distillation. As the concentration of the feed liquid increases, the water flux improvement ratio under the same air intake flow rate increases rapidly. This is because the increase in salt concentration leads to a rapid increase in the viscosity of the solution, the membrane surface boundary layer becomes thicker, and the concentration difference phenomenon is more obvious. The bubbles have a better strengthening effect on the osmotic membrane distillation process with a higher degree of concentration polarization and a thicker membrane surface boundary layer.

[0051] 5. Comparative experiment on the effect of different air-bubbling rates on enhancing the mass transfer of membrane distillation. Using the three-chamber device and the method to concentrate KCl solution. Among them, 30wt% KOH solution is used as the electrolyte in the anode chamber and the cathode chamber. Oxygen is bubbled into the feed side, and hydrogen is bubbled into the permeate side to simulate the air-bubbling enhanced membrane distillation process coupled with alkaline water electrolysis. The air-bubbling rates of oxygen and hydrogen are 0, 10 mL min -1 , 20 mL min -1 , 40 mL min -1 , and the concentration of the feed liquid KCl solution is 18wt%. Combining Figure 7 As shown, it can be seen that when the gas flow rate increases from 0 to mL min -1 , the increase in gas flow rate will increase the membrane flux. This is because the disturbance of the gas to the boundary layer increases, and the effect of weakening concentration polarization gradually becomes stronger. However, when the gas flow rate increases from 20 mL min -1 to 40 mL min -1 , the membrane flux instead begins to decline. This is because the further increase in gas flow rate will block the contact between the feed liquid and the membrane surface, reduce the effective mass transfer area of the membrane, and reduce the driving force of the permeant on the water flux, ultimately resulting in a decrease in the membrane flux.

[0052] 6. Experiment on the staged concentration of high-salt solution to crystallization by alkaline water electrolysis-driven gas-blowing osmotic membrane distillation. The KCl solution was concentrated using the three-chamber device and the method. Among them, 30 wt% KOH solution was used as the electrolyte in the anode chamber and the cathode chamber, and oxygen was blown into the feed side (rate 10 mL min -1 ), and the current density was 200 mA cm -2 . The stage change of the KCl mass fraction is as shown in Figure 8 . Figure 9 . It can be seen that using the three-chamber device and method of the present application can realize the continuous concentration of high-salt solution, from 2.35 - 6.04 wt%, 8.32 - 13.37 wt%, 18.58 - 24.75 wt%. And under the in-situ gas-blowing effect, the mass transfer is enhanced, so that the concentration rate of the high-salt solution is increased, verifying that the overall application of the technical solution is reliable.

[0053] 7. Comparison experiment on the enhanced mass transfer of alkaline water electrolysis-driven gas-blowing osmotic membrane distillation (low-concentration KCl solution). Under the same current density, the influence of gas-blowing and non-gas-blowing in the concentration chamber on the concentration rate was compared. The KCl solution was concentrated using the three-chamber device and the method. Among them, 30 wt% KOH solution was used as the electrolyte in the anode chamber and the cathode chamber, and oxygen was blown into the feed side (rate 20 mL min -1 ), and the current density was 400 mA cm -2 . The change of the KCl mass fraction is as shown in Figure 10 . It can be seen that the hydrogen generated at the cathode and the oxygen blown into the concentration chamber can bring about the homogenization of the KOH and KCl concentrations, thereby effectively increasing the vapor pressure difference on both sides of the composite membrane and realizing enhanced mass transfer, verifying that the overall application of the technical solution is reliable.

[0054] 8. Comparison experiment on the enhanced mass transfer of alkaline water electrolysis-driven gas-blowing osmotic membrane distillation (saturated KCl solution). Under the same current density, the influence of gas-blowing and non-gas-blowing in the concentration chamber on the crystallization rate of saturated KCl solution was compared. The KCl solution was concentrated using the three-chamber device and the method. Among them, 30 wt% KOH solution was used as the electrolyte in the anode chamber and the cathode chamber, and oxygen was blown into the feed side (rate 20 mL min -1 ), and the current density was 400 mA cm -2 . The change of the crystal precipitation mass is as shown in Figure 11 . It can be seen that using the three-chamber device and method of the present application can not only realize the continuous concentration of high-salt solution, but also enhance the rate of crystal precipitation of saturated salt solution; combined with Figure 12 as shown, Figure 12 the interlaced and vertical ones are the membrane fiber filaments of the porous hydrophobic and breathable polytetrafluoroethylene membrane, Figure 12The left figure in the middle shows the electron micrograph after concentrating for 80 h with the feed liquid being saturated KCl solution and oxygen being bubbled in the concentration chamber (rate: 20 mL min -1 ). The right figure shows the electron micrograph after concentrating for 80 h with the feed liquid being saturated KCl solution and no oxygen being bubbled in the concentration chamber. Among them, it can be seen from the left figure that there are basically no obvious large crystals on the membrane surface. From the right figure, it can be seen that there are obviously more large crystals distributed on the membrane surface. By comparing the left figure and the right figure, it can be seen that in-situ gas bubbling can also slow down membrane fouling.

[0055] In summary, on the basis of the above three-chamber device, a porous hydrophobic and breathable polytetrafluoroethylene membrane is used to separate the feed side and the permeation side. The liquid water molecules on the membrane surface of the feed side (high-salt wastewater) complete the mass transfer process of water molecules under the drive of the vapor pressure difference, and then these water molecules are consumed in the process of electrolyzing water. Eventually, the concentration of the absorbent (electrolyte) changes little;

[0056] When the hydrogen gas generated at the cathode is used to equalize the concentration of the electrolyte in the cathode chamber, the thickness of the membrane boundary on the permeation side will be weakened. Therefore, a large vapor pressure difference can always be maintained on both sides of the porous hydrophobic and breathable polytetrafluoroethylene membrane, thereby solving the technical problem of the continuously decreasing mass transfer rate;

[0057] The low-value product (oxygen) generated by the anode during the electrolysis of water is used to perform a bubbling operation on the feed side, thereby strengthening the fluid disturbance on the feed side, generating a shear force on the membrane surface, reducing the thickness of the liquid boundary layer on the feed side, and weakening the influence of concentration polarization, so as to solve the problems of membrane hydrophilic wetting and membrane fouling caused by concentration polarization in the absorption membrane distillation process. The method of this application does not heat the feed liquid, so temperature polarization is not considered;

[0058] Realize the continuous concentration of high-salt wastewater and the continuous driving of membrane distillation-crystallization by alkaline electrolyzed water at room temperature. In this process, not only can fresh water be provided for alkaline electrolyzed water, but also the purpose of concentrating and crystallizing high-salt wastewater can be achieved;

[0059] Furthermore, adopting the above method of combining three-chamber devices to treat the waste liquid has relatively lower energy consumption and equipment cost compared with traditional methods.

[0060] The above-described embodiments only represent one or more implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation to the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the appended claims.

Claims

1. A method for enhancing mass transfer in electrolyzed water-driven double-sided air-blowing membrane distillation, characterized in that A three-chamber device, wherein the three-chamber device is sequentially provided with an anode chamber, a cathode chamber, and a concentration chamber. The anode chamber is provided with an anode, the cathode chamber is provided with a cathode. The anode chamber and the cathode chamber are separated by a nylon diaphragm. The cathode chamber and the concentration chamber are separated by a composite membrane. The composite membrane includes a porous hydrophobic and breathable polytetrafluoroethylene membrane and a support layer. The support layer is made of polypropylene. Among them, the concentration chamber is the feed side, and the cathode chamber is the permeation side; The method includes: materials, using an alkaline solution as the electrolyte in the anode chamber and the cathode chamber; Water molecule mass transfer, driving the liquid water molecules on the feed side by using the vapor pressure difference. The liquid water molecules absorb heat and vaporize and then pass through the composite membrane and enter the permeation side to condense into liquid; Homogenization, using the hydrogen gas generated in the cathode chamber during the electrolysis of water to cause the homogenization of the electrolyte concentration in the cathode chamber; Bubbling operation, the oxygen gas generated by the anode is bubbled into the concentration chamber after gas-liquid separation.

2. The method for enhancing mass transfer by electrolyzed water-driven double-sided air-blowing membrane distillation according to claim 1, characterized in that The electrolyte is a KOH solution.

3. The method for enhancing mass transfer by electrolyzed water-driven bilateral air-blowing membrane distillation according to claim 2, characterized in that The electrolyte is a 30wt% KOH solution.

4. The method for enhancing mass transfer by electrolyzed water-driven double-sided air-blowing membrane distillation according to claim 1, wherein The porous hydrophobic and breathable polytetrafluoroethylene membrane faces the electrolyte in the cathode chamber, and the support layer faces the salt solution in the concentration chamber.

5. The method for enhancing mass transfer by electrolyzed water-driven double-sided air-blowing membrane distillation according to claim 1, wherein The material of the anode is NiCo alloy, and the material of the cathode is NiMo alloy.

6. The method for enhancing mass transfer by electrolyzed water-driven double-sided air-blowing membrane distillation according to claim 1, wherein During the air injection operation, oxygen that has undergone gas-liquid separation is injected into the concentration chamber at an air injection rate of 20 mL / min -1 .

7. The method for enhancing mass transfer by electrolyzed water-driven double-sided air-blowing membrane distillation according to claim 1, wherein The method further includes: the current density during electrolysis is 400 mA cm -2 .

Images