Hydrogel for membrane distillation, membrane module and distillation method

Abstract

The invention provides hydrogel for membrane distillation, a membrane module and a distillation method, the hydrogel takes polyurethane as a support material, and the polyurethane is loaded with polyvinyl alcohol, or a reaction product of polyvinyl alcohol and sodium alginate, or a reaction product of polyvinyl alcohol, sodium alginate and carboxylated carbon nanotubes. The water-polymer interaction in the hydrogel is utilized to change the state of a hydrogen bond network and water, so that the water evaporation enthalpy is reduced. Under the condition of not increasing energy consumption, the permeation flux of the system is improved, so that the specific energy consumption of produced water is reduced, meanwhile, the operation conditions are optimized, and a new strategy is provided for solving the problem of low heat utilization rate of membrane distillation.

Description

Technical Field

[0001] The invention belongs to the technical field of membrane distillation, and in particular relates to a hydrogel, a membrane component and a distillation method for membrane distillation. Background Art

[0002] Thermal conductive vacuum membrane distillation (CH-VMD) can effectively reduce the negative impact of temperature polarization, making the permeation flux higher than traditional membrane distillation. However, the thermal efficiency and permeation flux of the system are still the key factors restricting its development. In the prior art, it has been proposed to increase the solar interfacial evaporation rate by using hydrogels, and highly hydratable light-absorbing hydrogels (h-LAH) with polyvinyl alcohol and chitosan as hydratable skeletons and polypyrrole as light absorbers. It can reduce the energy of water evaporation (<50%). It has also been proved that improving the hydratability of h-LAH ​​can change the state of water and partially activate water, thereby promoting water evaporation and increasing the solar steam generation rate to 3.6kg / m 2 ·h. Some people also proposed to prepare hydrogels that can withstand high salt water by introducing polyelectrolyte materials. Anionic polyelectrolyte-based hydrogels (APH) with polyvinyl alcohol as the skeleton and poly(3,4-ethylenedioxythiophene):poly(sodium p-benzenesulfonate) as the light absorber were prepared by freeze-thaw method. As an evaporator, it has both photothermal performance (evaporating seawater) and electrostatic repulsion (avoiding solid salt crystallization). It can be seen that hydrogel materials have been used in the research of improving water evaporation due to their unique three-dimensional network structure and internal water state, but most of the current research only focuses on solar interfacial evaporation. Summary of the invention

[0003] In view of this, the present invention aims to propose a hydrogel, a membrane assembly and a distillation method for membrane distillation, which utilizes the water-polymer interaction in the hydrogel to change the hydrogen bond network and the state of water, thereby reducing the enthalpy of water evaporation. The permeation flux of the system is increased without increasing energy consumption, thereby reducing the specific energy consumption of water production, while optimizing the operating conditions, providing a new strategy for solving the problem of low heat utilization rate of membrane distillation.

[0004] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0005] A hydrogel for membrane distillation uses polyurethane as a supporting material, polyvinyl alcohol is loaded on the polyurethane, or a reaction product of polyvinyl alcohol and sodium alginate, or a reaction product of polyvinyl alcohol, sodium alginate and carboxylated carbon nanotubes.

[0006] Preferably, the thickness of the polyurethane is 2 to 4 mm.

[0007] The present invention also provides a method for preparing the hydrogel for membrane distillation as described above, comprising the following steps:

[0008] 1) washing a polyurethane having a length of 8 to 9 cm, a width of 2 to 3 cm, and a thickness of 2 to 4 mm repeatedly with anhydrous ethanol and deionized water, and then drying at 40 to 60° C. for 1 to 2 hours;

[0009] 2) 10-20 g of a 10.0-15.0 wt% polyvinyl alcohol solution and 120-150 μL of a 40-50 wt% glutaraldehyde solution are mechanically stirred and uniformly mixed, which is referred to as solution a; glutaraldehyde, as a cross-linking agent, reacts with hydroxyl groups in polyvinyl alcohol and sodium alginate to form covalent bonds, thereby enhancing the three-dimensional network structure of the hydrogel and thus improving its mechanical strength and stability.

[0010] 3) Add 500-550 μL of 1-1.2 mol / L HCl solution to solution a, and mix evenly by mechanical stirring;

[0011] 4) Add the dried polyurethane to the mixture in step 3), spread evenly, and let it stand for 2 to 4 hours;

[0012] 5) Soaking the polyurethane treated in step 4) in water for 24 to 48 hours to obtain hydrogel SH-1.

[0013] Preferably, in step 3), 0.1-0.2 g of sodium alginate is further added to solution a to prepare hydrogel SH-2; the remaining steps are the same.

[0014] Preferably, in step 3), 0.1-0.2 g of sodium alginate and 0.1-0.3 g of hydroxylated carbon nanotubes are added to solution a to prepare hydrogel SH-3; the remaining steps are the same.

[0015] The present invention also provides a membrane module comprising the hydrogel for membrane distillation as described above.

[0016] Preferably, it is composed of a feed side and a permeate side, with two layers of sealing rings in the middle to prevent liquid leakage; the external heat source transfers heat to the hydrogel and the feed liquid through the heat carrier, and the hydrogel with a length of 8 to 9 cm, a width of 2 to 3 cm, and a thickness of 2 to 4 mm is placed in the feed channel with a depth of 3 to 6 mm and close to the heat carrier; the effective area is 16 to 20 cm 2 The polytetrafluoroethylene hydrophobic membrane is placed on the other side of the heat carrier.

[0017] Preferably, the polytetrafluoroethylene membrane (PTFE) has a membrane thickness of 20 to 80 μm, a membrane pore size of 0.1 to 0.2 μm, a porosity of 80% to 85%, a contact angle of 130° to 145°, and a liquid osmotic pressure of 130 to 135 kPa.

[0018] The present invention also provides a method for heat-conducting vacuum membrane distillation, using a membrane assembly including the hydrogel for membrane distillation as described above, the heat-conducting vacuum membrane distillation system is composed of a cold water circulation system, a heating system and a vacuum condensation collection system; an external heat source transfers heat to a feed liquid through an aluminum heat carrier; a peristaltic pump is used to circulate the feed liquid; a vacuum pump is used to generate a vacuum on the permeation side of the membrane; water vapor is condensed on the permeation side to form a permeate; the feed flow rate is 5 to 10 cm / s, the heat source temperature is 100 to 200°C, and the vacuum degree is 0.08 to 0.1 MPa.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] The present invention targets the CH-VMD system, and first prepares a polyurethane (PU) / polyvinyl alcohol (PVA) hydrogel with polyurethane as the supporting material, and then introduces polyelectrolyte sodium alginate (SA) to prepare a negatively charged PU / PVA / SA hydrogel. On this basis, a heat conductive material carboxylated carbon nanotube (MWCNT-COOH) is added to prepare a PU / PVA / SA / MWCNT-COOH hydrogel. The above three hydrogels are characterized and analyzed, and their effects on the temperature, permeation flux, thermal efficiency and specific energy consumption inside the CH-VMD system are compared. Then, the optimization effect on the system is verified from the perspective of the change of water state inside the hydrogel. Further promote the development of membrane distillation technology.

[0021] The theoretical value of the evaporation enthalpy of pure water is 2357.6 kJ / kg, while the measured and calculated evaporation enthalpy of SH-1 is 1195 kJ / kg, the evaporation enthalpy of SH-2 is 1136 kJ / kg, and the evaporation enthalpy of SH-3 is 1082 kJ / kg. Compared with pure water, the evaporation enthalpy of all hydrogels is reduced.

[0022] The flux of the conventional CH-VMD system is 11.16 L / m 2 ·h, and after adding SH-1 hydrogel as evaporation material, the system flux can reach 17.94L / m 2 h, which is 60.75% higher than the conventional CH-VMD system (SH-0). This is because the PVA in SH-1 is a hydrophilic polymer, and its hydrophilic polymer chains are cross-linked in a large amount of water to form a unique three-dimensional network structure. The hydrogen bonding between water molecules and polymer chains weakens the hydrogen bonding between water molecules, thereby reducing the energy demand for evaporation, so the flux is improved to a certain extent. The flux of SH-2 is 20.38L / m 2h, compared with the pure PVA gel of SH-1, the addition of sodium alginate increases the number of hydrophilic groups in the hydrogel, which increases the proportion of intermediate water produced in the hydrogel, thereby further increasing the evaporation rate of water by 13.60%. The flux of SH-3 is as high as 22.35L / m 2 ·h. Compared with SH-2, SH-3 improves the heat transfer efficiency by introducing the high thermal conductivity of carboxylated carbon nanotubes, reduces the heat loss to the circulating feed liquid, and increases the evaporation rate by 9.67%.

[0023] The effects of hydrogels SH-1, SH-2, and SH-3 on the CH-VMD desalination flux. SH-1 can increase the system's permeation flux by more than 48% at different salt concentrations. SH-2 and SH-3, due to the presence of sodium alginate polyelectrolyte, enable the hydrogels to produce negatively charged groups, thereby repelling ions in seawater, further increasing the permeation flux to more than 52%, and the salt retention rate is more than 99%.

[0024] The TE values ​​of the CH-VMD systems with hydrogels are all higher than those of the CH-VMD systems without hydrogels, among which the TE (thermal efficiency) value of SH-1 can be as high as 82.8%; the TE (thermal efficiency) value of SH-2 is 77.1%; and the TE (thermal efficiency) value of SH-3 is 75.0%. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic diagram of the membrane assembly;

[0026] Figure 2 Schematic diagram of the experimental setup of the CH-VMD system;

[0027] Figure 3 For the preparation of hydrogel;

[0028] Figure 4 Cross-sectional electron micrographs of different hydrogels (a) SH-1; (b) SH-2; (c) SH-3

[0029] Figure 5 Fourier transform infrared spectra of polyurethane and hydrogels SH-1, SH-2, and SH-3;

[0030] Figure 6 (a) DSC test graph of hydrogel SH-1, SH-2, and SH-3; (b) integral graph of evaporation enthalpy calculation;

[0031] Figure 7 The evaporation performance test diagram of hydrogel in CH-VMD system: (a) temperature change at the same position on the surface of SH-0, SH-1, SH-2, and SH-3; (b) system flux comparison diagram;

[0032] Figure 8 The desalination performance test diagram of the hydrogel in the CH-VMD system: (a) the flux of the system when the feed liquid is 0.8wt% NaCl solution; (b) the flux of the system when the feed liquid is 3.5wt% NaCl solution; (c) the flux of the system when the feed liquid is 10wt% NaCl solution; (d) the comparison of the flux and salt retention rate of different hydrogels under different feed water concentrations;

[0033] Fig. 9 This is a comparison chart of the thermal efficiency and specific energy consumption of the system when using different hydrogels. DETAILED DESCRIPTION

[0034] Unless otherwise defined, the technical terms used in the following examples have the same meanings as those generally understood by those skilled in the art to which the present invention belongs. The test reagents used in the following examples, unless otherwise specified, are all conventional biochemical reagents; the experimental methods, unless otherwise specified, are all conventional methods.

[0035] The present invention will be described in detail below with reference to the embodiments.

[0036] The experimental materials and reagents used in the present invention are shown in Table 1.

[0037] Table 1

[0038]

[0039] The experimental instruments used in the present invention are shown in Table 2.

[0040] Table 2

[0041]

[0042] The present invention adopts a commercial hydrophobic flat polytetrafluoroethylene membrane (PTFE) with a membrane thickness of 50 μm, a membrane pore diameter of 0.1 μm, a porosity of 81%, a contact angle of 143°, and a liquid osmotic pressure of 132 kPa.

[0043] The size of the membrane module is 80mm×20mm×4mm, which can accommodate a hydrophobic membrane with an effective area of ​​16cm2. Its structure is as follows Figure 1 As shown. The membrane assembly is composed of two parts, the feed side and the permeate side, with two layers of sealing rings in the middle to prevent liquid leakage. The external heat source transfers heat to the hydrogel and the feed liquid through the heat carrier. The hydrogel with a length of 8cm, a width of 2cm and a thickness of 3mm is placed in the feed channel with a depth of 4mm close to the heat carrier. The effective area is 16cm 2 The PTFE hydrophobic membrane is placed on the other side of the heat carrier.

[0044] The present invention adopts the CH-VMD system of laboratory scale, such as Figure 2As shown. The experimental device consists of a cold water circulation system, a heating system and a vacuum condensation collection system. The external heat source transfers heat to the feed liquid through an aluminum heat carrier. A peristaltic pump is used to circulate the feed liquid. The vacuum pump is used to generate a vacuum on the permeate side of the membrane, and water vapor is condensed on the permeate side to form a permeate.

[0045] The effective area is 16cm 2 A PTFE membrane (2 cm × 8 cm) was placed in a feed channel with a depth of 4 mm, and the feed channel was connected to a water inlet and a water outlet for the circulation of the feed liquid.

[0046] The heat carrier uses an aluminum sheet with a high thermal conductivity, and the aluminum sheet is perforated to avoid blocking the transmembrane process of water vapor.

[0047] A peristaltic pump is used to circulate the feed solution, and water vapor is condensed on the permeate side. A vacuum pump is used to create a vacuum (0-100 kPa) on the permeate side of the membrane.

[0048] The external heat source was set to a temperature of 200°C, a vacuum of 0.1 MPa, and a feed flow rate of 6.08 cm / s.

[0049] All temperature data in the experiment were measured by a multi-channel temperature recorder. The temperature recorder was used to measure the internal temperature change of the heat carrier from the beginning of heating to 2 hours after operation, and the measurement point was set on the upper surface of the hydrogel. The conductivity of the feed liquid and the produced water was measured using a Lei magnetic conductivity meter. The experiment was repeated three times and the results were reported as the average value.

[0050] Example 1

[0051] The hydrogel is loaded on polyurethane, such as Figure 3 As shown, the specific implementation steps are as follows:

[0052] ① Wash the polyurethane with a length of 8 cm, a width of 2 cm, and a thickness of 3 mm repeatedly with anhydrous ethanol and deionized water, and then dry it at 45°C for 1 hour.

[0053] ② Mechanically stir 10 g of 10.0 wt% PVA solution and 120 μL of 50 wt% GA solution until uniform, which is called solution a.

[0054] ③ Add 500 μL of 1.2 mol / L HCl solution to solution a and mix evenly by mechanical stirring.

[0055] ④ The above mixture was evenly applied on the dried polyurethane and allowed to stand for 2 hours to obtain the prepared polyurethane-based hydrogel sample.

[0056] ⑤ Soak the prepared sample in 500 mL of deionized water for 24 hours to obtain the final hydrogel sample and name it SH-1.

[0057] ⑥ Using the same method, 0.1 g of sodium alginate was added to solution a to prepare hydrogel SH-2.

[0058] ⑦ In the same way, 0.1 g sodium alginate and 0.2 g hydroxylated carbon nanotubes were added to solution a to prepare hydrogel SH-3.

[0059] The obtained hydrogel was analyzed:

[0060] ① The morphology and structure of SH-1, SH-2 and SH-3 were characterized by scanning electron microscopy (SEM). Figure 4 As shown; it can be seen that the interior of the hydrogel is a porous network structure with an average pore size of about 10μm. The pores of the polyurethane itself and the porous structure inside the hydrogel provide physical conditions for the rapid evaporation of water and the diffusion of steam. After the addition of SA, the porous structure of the hydrogel still exists, and the hydrogel pore wall contains 1-2μm holes, showing the typical structure of a uniform hydrogel. SH-3 is composed of a porous network structure with a pore size of 50-200μm, and there are 1-2μm holes distributed on the pore wall, indicating that the addition of MWCNT-COOH has no obvious effect on the internal morphology of the hydrogel.

[0061] ② The chemical composition of the hydrogel was analyzed by Fourier transform infrared spectroscopy (FTIR). Figure 5 As shown, SH-1 has three main peaks located at 3298 cm -1 、2916.4cm -1 and 1090.3cm -1 The 1090.3 cm -1 It is the characteristic peak of PVA. The OH stretching absorption peak in the infrared spectrum curve of SH-2 moves to a lower wave number, which can indicate that the presence of SA weakens the hydrogen bonding interaction between PVA molecules. Its carboxyl and hydroxyl groups will form hydrogen bonds with the hydroxyl groups in PVA. The formation of such hydrogen bonds will cause the stretching vibration of the OH bond of the hydroxyl group in PVA to change, which in turn causes the OH characteristic peak to red shift. Polyurethane is at 1537.3cm -1 The characteristic peaks at 1091cm are NH in-plane bending vibration and CN stretching vibration, while the peaks at 1091cm -1The characteristic peak at is the result of CC stretching vibration. The spectrum of SH-3 shows the characteristic peaks of SH-2 and polyurethane at the same time, indicating the feasibility of in-situ synthesis of composite hydrogels in polyurethane. Compared with SH-1, the -OH peak of SH-3 moves to the high wavenumber direction, that is, blue shift occurs. This blue shift phenomenon may be caused by the opening of the π bond in MWCNT-COOH, and the formation of hydrogen bonds between the abundant hydroxyl and carboxyl groups in MWCNT-COOH and the hydroxyl groups of PVA macromolecules.

[0062] ③ Use differential scanning calorimetry (DSC) to detect the heat change of the material under the temperature / time program to verify that the evaporation enthalpy of water in the hydrogel can be reduced. Figure 6 As shown in (a), the heat flow signal first rises with the increase of temperature to reach the highest signal, and then suddenly drops until it becomes smooth. This process represents the evaporation of water. It can be seen from the figure that the composite hydrogels SH-2 and SH-3 have wider peaks and gradually decay. Therefore, compared with SH-1, SH-2 and SH-3 show different evaporation behaviors. The integral of the heat flow curve over time represents the evaporation enthalpy of water, such as Figure 6 As shown in (b), the area of ​​the peak in the figure represents the size of the enthalpy change, that is, the total amount of heat absorbed in the process. The theoretical value of the evaporation enthalpy of pure water is 2357.6 kJ / kg, while the evaporation enthalpies of SH-1, SH-2 and SH-3 calculated by measurement are 1195 kJ / kg, 1136 kJ / kg and 1082 kJ / kg respectively. Compared with pure water, the evaporation enthalpy of all hydrogels is lower. Therefore, less energy is required when using hydrogels for evaporation, and when the total heat input into the system remains unchanged, the permeation flux of the system can be improved.

[0063] Effects of hydrogels SH-1, SH-2, and SH-3 on evaporation of CH-VMD system:

[0064] Experimental process:

[0065] CH-VMD experiments were carried out using different prepared hydrogels.

[0066] The feed liquid was pure water, the feed flow rate was 6.08 cm / s, the heat carrier temperature was 60°C, and the vacuum degree was 100 kPa. The flux of the CH-VMD system was compared when hydrogels SH-1, SH-2, SH-3 and no hydrogel (SH-0) were used. At the same time, the temperature of the upper surface of the hydrogel was measured from the beginning of heating to 2 hours after operation using a temperature recorder.

[0067] Experimental results:

[0068] like Figure 7As shown in (a), when no hydrogel is used, the temperature at the same point in the feed channel rises slowly with time, and reaches a peak after 100 minutes, with the highest temperature being only 35.6°C. On the contrary, after adding different hydrogels to the system, their surface temperatures are much higher than SH-0, and can reach above 40°C. Among them, the surface temperature of SH-3 rises particularly quickly, and the highest temperature can reach 44°C. This is because there are carboxylated carbon nanotubes in SH-3, and carbon nanotubes themselves have excellent thermal conductivity. The presence of carboxyl groups and hydroxyl groups will increase the interaction between carbon nanotubes and PVA, water, etc., and produce hydrogen bonds. It can increase the heat conduction path and heat transfer efficiency, thereby improving the thermal conductivity of carboxylated carbon nanotubes. In addition, the presence of carboxyl groups enhances the hydrophilicity of the carbon nanotube surface, making it easier to form a closer contact with water or other liquid media, accelerating heat transfer. At the same time, the enhancement of hydrophilicity can reduce thermal resistance and increase heat transfer rate. Therefore, when SH-3 is used, its upper surface can obtain more heat.

[0069] Depend on Figure 7 (b) It can be seen that the flux of the conventional CH-VMD system is 11.16 L / m 2 ·h, after adding SH-1, the flux can reach 17.94L / m 2 h, an increase of 60.75%. This is because the PVA in SH-1 is a hydrophilic polymer, and its hydrophilic polymer chains are cross-linked in a large amount of water to form a unique three-dimensional network structure. The hydrogen bonding between water molecules and polymer chains weakens the hydrogen bonding between water molecules, thereby reducing the energy demand for evaporation, so the flux is improved to a certain extent. When SH-2 is used, the system flux is 20.38L / m 2 h, this is because the addition of sodium alginate increases the number of hydrophilic groups in the hydrogel, which increases the proportion of intermediate water produced in the hydrogel, thereby increasing the evaporation rate of water by 13.6%. After using SH-3, the flux is as high as 22.35L / m 2 ·h. Compared with SH-2, SH-3 improves the heat transfer efficiency and reduces the heat loss of the circulating feed liquid by introducing the high thermal conductivity of carboxylated carbon nanotubes, which increases the evaporation rate by 9.67%.

[0070] In addition, using the same energy input U in , compared with the known theoretical value of liquid water evaporation enthalpy 2357.6 kJ / kg, the equivalent evaporation enthalpy of hydrogel was estimated:

[0071] U in =E equ ·m g =E0·m0

[0072] Among them, U inis the energy used for water evaporation; E0 is the known enthalpy of evaporation of water, 2357.6 kJ / kg; m0 is the mass change of pure water when only pure water is used for testing; m g is the mass change of pure water after adding hydrogel; E equ is the equivalent evaporation enthalpy of water in the hydrogel.

[0073] The calculated results are shown in Table 3. Compared with the known evaporation enthalpy of liquid water, the equivalent evaporation enthalpy of water in other hydrogels is reduced to varying degrees. This result has the same trend as the result obtained by DSC test, which verifies that hydrogel can reduce the evaporation enthalpy of water, thereby increasing the permeation flux of the system.

[0074] Table 3

[0075]

[0076] Effects of hydrogels SH-1, SH-2, and SH-3 on CH-VMD desalination flux

[0077] Experimental process:

[0078] CH-VMD experiments were carried out using different prepared hydrogels.

[0079] The feed flow rate is 6.08 cm / s, the heat carrier temperature is 60°C, and the vacuum degree is 100 kPa.

[0080] The experiment investigated the effects of different hydrogels on the system flux when the feed solution was set to solutions with different salinities.

[0081] Experimental results:

[0082] like Figure 8 As shown in Figures ac, at different concentrations, the permeation flux of the system is still significantly improved after the hydrogel is added. Among them, SH-1 can increase the permeation flux of the system by more than 48% at different salt concentrations, while SH-2 and SH-3, due to the presence of sodium alginate polyelectrolyte, can generate negatively charged groups in the hydrogel to repel ions in seawater, further increasing the permeation flux to more than 52%. Figure 8 (d) It can be concluded that the hydrogel is less affected by concentration. Regardless of the type of hydrogel, the permeation flux of the system in salt water of different concentrations fluctuates slightly, but is generally stable within a certain range, and the salt retention rate is above 99%.

[0083] The energy consumption of the MD system is a key factor restricting its development, so it is necessary to analyze the thermal efficiency and specific energy consumption of the system.

[0084] The CH-VMD experiment was carried out using the three hydrogels prepared by the method in Example 1. When the CH-VMD system uses hydrogels, it has a great advantage in flux compared to not using hydrogels. The thermal efficiency and specific energy consumption of the three prepared hydrogels were analyzed and compared.

[0085] Experimental results:

[0086] like Fig. 9 It represents the TE and SEC of the CH-VMD system when using different types of hydrogels. The results show that the TE value of the CH-VMD system with hydrogel is higher than that of the CH-VMD system without hydrogel, among which the TE value of SH-1 can be as high as 82.8%, which is 19.6% higher than that of SH-0. This is because the presence of hydrogel reduces the heat required for water evaporation, making the system have a higher permeation flux, thereby generating more heat for water evaporation at the same heat carrier temperature, that is, Q v value, thereby obtaining a higher TE. Although the permeation flux of SH-2 and SH-3 is larger than that of SH-1, their TE values ​​are lower than that of SH-1. This is because the water evaporation enthalpy of SH-1, SH-2 and SH-3 is reduced by 710.9 kJ / kg, 907.7 kJ / kg and 1036 kJ / kg respectively compared with the evaporation enthalpy of pure water, among which the degree of reduction of SH-2 and SH-3 is 27.7% and 45.7% greater than that of SH-1 respectively. The permeation flux of the three hydrogels increased by 7.23 L / m 2 h, 8.23L / m 2 h, 9.47L / m 2 ·h, where the increase of SH-2 and SH-3 is 13.7% and 30.9% greater than that of SH-1, respectively. It can be seen that the increase of flux of SH-2 and SH-3 is not as great as the decrease of evaporation enthalpy, so the calculated TE value is smaller than SH-1 but greater than SH-0. This shows that using hydrogel to increase the water evaporation rate is an effective way to increase the TE of the system. SEC represents the energy consumption per unit of water production, which is positively correlated with heat input and negatively correlated with TE. The lower its value, the less heat is required for the system to produce unit mass of pure water. As can be seen from the figure, when hydrogel is used, the SEC of the system is significantly reduced, among which SH-3 can be as low as 0.70kWh / L, which is 48.2% lower than SH-0 of 1.36kWh / L; the SEC value of SH-1 is 0.80kWh / L, and the SEC value of SH-2 is 0.75kWh / L, which is also significantly reduced compared with SH-0.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A hydrogel for membrane distillation, characterized in that: Polyurethane is used as a supporting material, and polyvinyl alcohol, or a reaction product of polyvinyl alcohol and sodium alginate, or a reaction product of polyvinyl alcohol, sodium alginate and carboxylated carbon nanotubes are loaded on the polyurethane.

2. The hydrogel for membrane distillation according to claim 1, characterized in that: The thickness of polyurethane is 2 to 4 mm.

3. A method for preparing the hydrogel for membrane distillation as claimed in claim 1 or 2, characterized in that: The following steps are included: 1) washing a polyurethane having a length of 8 to 9 cm, a width of 2 to 3 cm, and a thickness of 2 to 4 mm repeatedly with anhydrous ethanol and deionized water, and then drying at 40 to 60° C. for 1 to 2 hours; 2) Mechanically stir 10-20 g of 10.0-15.0 wt% polyvinyl alcohol solution and 120-150 μL of 40-50 wt% glutaraldehyde solution, which is called solution a; 3) Add 500-550 μL of 1-1.2 mol / L HCl solution to solution a, and mix evenly by mechanical stirring; 4) Add the dried polyurethane to the mixture in step 3), spread evenly, and let it stand for 2 to 4 hours; 5) Soaking the polyurethane treated in step 4) in water for 24 to 48 hours to obtain hydrogel SH-1.

4. The hydrogel for membrane distillation according to claim 3, characterized in that: In step 3), 0.1-0.2 g of sodium alginate is added to solution a to prepare hydrogel SH-2; the remaining steps are the same.

5. The hydrogel for membrane distillation according to claim 3, characterized in that: In step 3), 0.1-0.2 g of sodium alginate and 0.1-0.3 g of hydroxylated carbon nanotubes are added to solution a to prepare hydrogel SH-3; The rest of the steps are the same.

6. A membrane module, characterized in that: The invention comprises the hydrogel for membrane distillation as claimed in any one of claims 1 to 5.

7. The membrane module according to claim 6, characterized in that: It is composed of a feed side and a permeation side, with two layers of sealing rings in the middle to prevent liquid leakage; the external heat source transfers heat to the hydrogel and the feed liquid through the heat carrier; the hydrogel with a length of 8 to 9 cm, a width of 2 to 3 cm, and a thickness of 2 to 4 mm is placed in a feed channel with a depth of 3 to 6 mm and close to the heat carrier; the effective area is 16 to 20 cm 2 The polytetrafluoroethylene hydrophobic membrane is placed on the other side of the heat carrier.

8. The membrane module according to claim 7, characterized in that: The polytetrafluoroethylene membrane has a membrane thickness of 20 to 80 μm, a membrane pore size of 0.1 to 0.2 μm, a porosity of 80% to 85%, a contact angle of 130° to 145°, and a liquid osmotic pressure of 130 to 135 kPa.

9. A method for thermally conductive vacuum membrane distillation, characterized in that: A membrane assembly comprising a hydrogel for membrane distillation as described in any one of claims 1 to 5 is used, and the heat-conducting vacuum membrane distillation system consists of a cold water circulation system, a heating system and a vacuum condensation collection system; an external heat source transfers heat to a feed liquid through an aluminum heat carrier; a peristaltic pump is used to circulate the feed liquid; a vacuum pump is used to generate a vacuum on the permeation side of the membrane; water vapor is condensed on the permeation side to form a permeate; and the process is carried out under the conditions of a feed flow rate of 5 to 10 cm / s, a heat source temperature of 100 to 200°C, and a vacuum degree of 0.08 to 0.1 MPa.

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