Spherical photo-thermal evaporator and preparation method and application thereof

Abstract

The invention discloses a spherical photo-thermal evaporator and a preparation method and application thereof, and belongs to the technical field of solar seawater desalination. The preparation method specifically comprises the following steps: (1) carrying out ultrasonic cleaning, flushing and drying on light plastic balls for later use; (2) respectively dissolving polyvinyl alcohol and tannic acid into deionized water and an organic solvent, stirring and mixing, and adding a light absorption material to obtain a precursor solution; (3) immersing the light plastic balls into the precursor solution, taking out the light plastic balls after the surfaces are uniformly covered with the gel solution, and standing and curing at room temperature; and (4) immersing into an aqueous solution containing Fe < 3 + > ions, standing, and cleaning to obtain the composite material. The invention has the advantages of low cost, simple preparation process and the like, and shows excellent desalting capacity and salt-resistant stability in desalting and long-acting desalting processes of salt water with different concentrations.

Description

Technical Field

[0001] The present invention relates to the technical field of solar seawater desalination, and more particularly to a spherical photothermal evaporator and a preparation method and application thereof. Background Art

[0002] Global water scarcity is becoming increasingly severe, and climate change-induced droughts and high temperatures are further exacerbating this challenge. Although seawater is abundant on Earth, accounting for 97.5% of global water resources, traditional desalination technologies such as reverse osmosis and multi-stage flash evaporation are difficult to widely deploy in energy-scarce regions due to high energy consumption, complex infrastructure, and high costs. Therefore, developing clean, sustainable alternative technologies has become a key priority for the scientific and engineering communities.

[0003] Solar interfacial evaporation, an emerging technology for freshwater harvesting, has attracted considerable attention due to its low energy consumption, environmental friendliness, and cost-effectiveness. This technology utilizes photothermal materials to convert solar energy into localized heat, enabling efficient water evaporation. In recent years, researchers have developed a variety of photothermal materials (such as metal nanoparticles, semiconductors, and carbon-based materials) and matrix materials (such as hydrogels and porous wood) to enhance the performance of solar evaporators.

[0004] While researchers have significantly improved evaporation rates, key challenges hinder the practical application of solar desalination. For example, during long-term desalination, salt crystals precipitated on the surface of the photothermal material block incident light and affect water transport, thereby reducing the evaporation rate and limiting the performance and lifespan of the evaporator. Furthermore, the evaporator's operating capacity is highly dependent on the incident angle of sunlight, which in actual outdoor applications varies dynamically from 0° to 180°.

[0005] Therefore, how to increase the evaporation rate of photothermal materials and improve the performance and life of evaporators is an urgent problem that needs to be solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the object of the present invention is to provide a spherical photothermal evaporator and a preparation method and application thereof to address the deficiencies in the prior art.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for preparing a spherical photothermal evaporator specifically comprises the following steps:

[0009] (1) Ultrasonic cleaning, rinsing, drying and standby of the lightweight plastic balls;

[0010] (2) Dissolving polyvinyl alcohol (PVA) and tannic acid (TA) in deionized water and an organic solvent, respectively, stirring and mixing, and adding a light absorbing material to obtain a precursor solution;

[0011] (3) Immerse the lightweight plastic ball in the precursor solution, take it out after the surface is evenly covered with the gel solution, and let it stand at room temperature to solidify. Repeat the above steps to obtain a lightweight plastic ball coated with a photothermal film;

[0012] (4) Immerse the lightweight plastic ball coated with the photothermal film in a 3+ The spherical photothermal evaporator is obtained by placing the spherical photothermal evaporator in an aqueous solution of ions, letting it stand and washing it.

[0013] Furthermore, in the above step (1), the lightweight plastic ball is made of polystyrene and has a diameter of 1 cm to 1 m, preferably 2.4 cm.

[0014] A further beneficial effect of the above method is that the size of the photothermal evaporation balls is controllable and can be prepared on a large scale, simply by changing the size of the inner core of the lightweight plastic ball.

[0015] Furthermore, in the above step (1), the reagent for ultrasonic cleaning is deionized water, and the time is 10 minutes; the reagent for rinsing is ethanol.

[0016] A further beneficial effect of the above method is that impurities on the surface of the lightweight plastic balls can be removed by ultrasonic cleaning and flushing.

[0017] Furthermore, in the above step (2), the mass ratio of polyvinyl alcohol to tannic acid is 1:(0.5-2), preferably 1:1.

[0018] A further benefit of this approach is that the concentration of the polyvinyl alcohol (PVA) aqueous solution and the ratio of PVA to tannic acid are adjustable, specifically adjusting the ratio of the components in the precursor solution based on the desired properties of the photothermal evaporation layer. The tannic acid addition ratio affects the uniformity of the photothermal film on the sphere surface and the self-cleaning ability of the evaporator. In terms of mass ratio, a 1:1 ratio of PVA to tannic acid exhibits the best rotational self-cleaning properties for salt crystals.

[0019] Furthermore, in the above step (2), the organic solvent is at least one of methanol and ethanol, preferably ethanol; and the mass ratio of deionized water to the organic solvent is 1:(0.2~8).

[0020] A further beneficial effect of the above method is that the cross-linking mechanism of polyvinyl alcohol and tannic acid originates from the hydrogen bond cross-linking behavior occurring in different solutions. In aqueous solution, polyvinyl alcohol and tannic acid are easily cross-linked to form a hydrogel, and can be uniformly dissolved in a mixed solution of ethanol and water.

[0021] Furthermore, in the above step (2), the light absorbing material is a carbon material having excellent photothermal evaporation performance, preferably at least one of modified needle coke, graphene, carbon nanotubes, coal-based carbon and biochar.

[0022] Furthermore, in the above step (3), during the static curing process, as the ethanol evaporates, the polyvinyl alcohol and tannic acid undergo rapid hydrogen bond cross-linking and coat the lightweight plastic balls.

[0023] Furthermore, in the above step (4), Fe 3+ The aqueous solution of ions is Fe2(SO4)3 solution, in which Fe 3+ The concentration of ions is 0.01~5 mol / L.

[0024] A further beneficial effect of adopting the above is that Fe 3+ The ions coordinate and cross-link with the phenolic hydroxyl groups in tannic acid, which can improve the morphology, hydrophilicity, mechanical strength and photothermal evaporation performance of the spherical photothermal evaporator. 3+ The anions in the aqueous solution of ions have the Hofmeister effect. Different anions will affect the structure of the photothermal film and the rotation behavior of the evaporator. Compared with Cl - , NO3 - ions, the evaporator treated with Fe2(SO4)3 solution has better mechanical strength and rotary self-cleaning ability.

[0025] Furthermore, in the above step (4), the standing time is 2 to 24 hours; and the cleaning reagent is deionized water.

[0026] The further beneficial effect of adopting the above method is that, during the standing process, the Fe 3+ The coordination and crosslinking of the ions with the phenolic hydroxyl groups in the tannic acid enhances the mechanical strength and evaporation performance of the spherical photothermal evaporator. The spherical photothermal evaporator is obtained after the residual substances on the surface of the sphere are thoroughly washed with deionized water.

[0027] The present invention also seeks to protect a spherical photothermal evaporator prepared by the above preparation method.

[0028] The present invention also seeks to protect the use of a spherical photothermal evaporator prepared by the above-mentioned preparation method in solar seawater desalination, which exhibits excellent salt resistance and photothermal evaporation performance.

[0029] It can be seen from the above technical solution that compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] 1. This invention uses lightweight polystyrene spheres as the core. Based on the heterogeneous solvent crosslinking mechanism of polyvinyl alcohol and polyphenols, a one-step hydrogen bond crosslinking method induced by rapid ethanol evaporation is used to coat the spheres with a uniform photothermal film. The film is then immersed in an Fe2(SO4)3 solution to form metal-hydroxyl coordination, ultimately resulting in a spherical photothermal evaporator with rotating self-cleaning capabilities. This preparation process is simple, low-cost, and scalable.

[0031] 2. The present invention solves the difficult problem of coating the thin film material on the surface of the sphere. The process of coating the photothermal evaporation layer only involves a one-step rapid hydrogen bond cross-linking reaction. The synthesis steps are simple, the cost is low, and it can be prepared on a large scale. The geometric characteristics of the sphere in the present invention enable the evaporator to have the ability to absorb sunlight at all angles. The lightweight plastic sphere core enables the evaporator to float on the water surface. The photothermal film coated on the plastic sphere has a sparse pore structure after being soaked in Fe2(SO4)3 solution. Compared with the dense porous structure of the gel material, this is conducive to promoting the upward transmission of brine along the surface of the evaporator, so that the evaporator has a low water content and is more sensitive to the gravity deviation of the precipitated salt crystals. Thanks to the water transmission capacity of the hydrophilic PVA-TA cross-linked network and the geometric characteristics of the sphere, the evaporator exhibits excellent evaporation capacity and rotational self-cleaning ability during the evaporation of brine of different concentrations.

[0032] 2. The spherical photothermal evaporator of this invention features rotational self-cleaning capabilities. Its lightweight core makes the evaporator highly sensitive to center-of-gravity shifts caused by precipitated salt crystals, enabling timely removal of these crystals. Its curved surface allows the evaporator to absorb sunlight from all angles, significantly enhancing its light absorption capacity. Furthermore, this invention offers advantages such as low cost and a simple manufacturing process. It demonstrates excellent desalination capacity and salt resistance stability in desalination of saline solutions of varying concentrations, as well as in long-term desalination processes.

[0033] 3. Taking the evaporation of sodium chloride (NaCl) solution as an example, when the mass ratio of PVA and TA is 1:1, the evaporator can achieve 2.32 kg m for 3.5 wt%, 10 wt% and 20 wt% NaCl solutions under one sun illumination. -2 h -1 , 1.92 kg m -2 h -1 and 1.72 kg m -2 h -1The evaporation rate surpasses all the reported spherical photothermal evaporators using carbon materials as photothermal conversion materials. In addition, when evaporating a 20 wt% NaCl solution, the evaporator rotated three times within 50 minutes, indicating that this spherical photothermal evaporator is more sensitive to salt crystals precipitated on the surface. When the Yellow Sea water was subjected to a 10-day continuous cycle test (one sunlight for 8 hours per day), the evaporator rotated 1-2 times per day, and the average evaporation rate remained at 2.04 kg m -2 h -1 The spherical photothermal evaporator exhibited excellent salt stability, and its sensitivity to surface salt crystallization exceeded that of all reported evaporators with rotary self-cleaning capabilities. In an outdoor desalination experiment using the spherical photothermal evaporator on real seawater, the ion content in the collected condensed water met drinking water standards.

[0034] 4. In addition to showing excellent desalination ability for NaCl solution (simulated seawater), the spherical photothermal evaporator of the present invention also shows strong universality. It also shows excellent evaporation ability and salt resistance stability for other inorganic salt solutions such as sodium sulfate, potassium chloride and calcium chloride. It provides a new low-cost route for the commercial development of solar seawater desalination and inorganic industrial wastewater treatment, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The scanning electron microscope images of the photothermal films on the surfaces of the spherical photothermal evaporators in Examples 1 and 3 are shown;

[0036] Figure 2 These are optical photographs of the rotational behavior of the spherical photothermal evaporators of Examples 1 and 3 under one sun intensity;

[0037] Figure 3 The evaporation performance of the spherical photothermal evaporator in Example 1 and Example 3 for NaCl solutions with different concentrations;

[0038] Figure 4 The evaporation performance of the spherical photothermal evaporator in Example 1 for desalination of Yellow Sea water for 10 consecutive days;

[0039] Figure 5 is the ion content of the condensed water collected by the spherical photothermal evaporator in Example 1. DETAILED DESCRIPTION

[0040] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0041] In the following examples, modified needle coke was prepared by high-temperature calcination and acid treatment of petroleum coke purchased from Shanghai Petrochemical.

[0042] Example 1

[0043] The preparation method of the spherical photothermal evaporator specifically comprises the following steps:

[0044] (1) Use a lightweight polystyrene sphere with a diameter of 2.4 cm as the core, immerse it in deionized water for ultrasonic treatment for 10 min, then rinse with ethanol and dry it for later use;

[0045] (2) First, 1.5 g of polyvinyl alcohol particles were dissolved in 20 mL of deionized water and heated in a water bath at 90 °C for 1 h to completely dissolve them. Then, 0.15 g of modified needle coke was added as a light absorbing material. Separately, 1.5 g of tannic acid powder was dissolved in 20 mL of ethanol and magnetically stirred for 30 min. Finally, the polyvinyl alcohol aqueous solution and the tannic acid ethanol solution were thoroughly mixed under stirring to obtain a precursor solution of the photothermal evaporation material.

[0046] (3) Immerse the lightweight polystyrene ball in the precursor solution for 1 minute, take it out and let it stand at room temperature for 2 minutes to allow the ethanol to evaporate. Repeat this immersion-evaporation operation until the surface of the ball is evenly covered with a layer of black photothermal film, thereby obtaining a lightweight plastic ball coated with a photothermal film;

[0047] (4) Immerse the lightweight plastic ball coated with the photothermal film in a 2 wt% Fe2(SO4)3 solution and let it stand for 24 hours to allow the Fe 3+ The spherical photothermal evaporator is obtained by fully reacting the spherical surface with tannic acid and then thoroughly washing the residue with deionized water.

[0048] Taking the evaporation of NaCl solution as an example, the desalination performance of the spherical photothermal evaporator prepared in Example 1 was studied. First, scanning electron microscopy revealed that the prepared photothermal film had a sparse pore structure like an open-cell foam ( Figure 1 (a) The structural characteristics of this photothermal film directly affect the rotation behavior of the spherical photothermal evaporator. When the spherical photothermal evaporator is placed in a 20 wt% NaCl solution under a single solar intensity, the evaporator exhibits three rotation behaviors within 50 minutes, occurring at 21 minutes and 1 second, 33 minutes and 56 seconds, and 46 minutes and 40 seconds ( Figure 2 (a) The evaporator desalinated 3.5 wt%, 10 wt%, and 20 wt% NaCl solutions under one sun, achieving 2.32 kg m -2 h -1 , 1.92 kg m -2 h-1 and 1.72 kgm -2 h -1 The evaporation rate ( Figure 3 (a) surpasses all previously reported spherical photothermal evaporators using carbon materials as photothermal conversion materials. During a 10-day continuous cycle test of Yellow Sea water (8 hours of sunlight per day), the evaporator rotated 1-2 times per day, and the average evaporation rate remained at 2.04 kg m -2 h -1 ( Figure 4 ), showing excellent salt resistance stability, and the sensitivity of the spherical photothermal evaporator to surface salt crystallization exceeds that of all reported evaporators with rotary self-cleaning capabilities. The spherical photothermal evaporator was used to conduct outdoor desalination experiments on real seawater, and the ions (Na + Mg 2+ , K + , Ca 2+ ) content meets the drinking water standards set by the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA) ( Figure 5 ).

[0049] This spherical photothermal evaporator not only has excellent desalination and self-cleaning capabilities for NaCl solutions, but is also universally applicable to other inorganic salt solutions (such as sodium sulfate, potassium chloride, and calcium chloride).

[0050] Example 2

[0051] The steps are roughly the same as those in Example 1, except that the effects of different ratios of PVA and TA on the evaporator's rotary self-cleaning ability due to different uniformities of the photothermal film are compared to obtain the optimal preparation conditions.

[0052] Specifically, in Example 1, 1.5 g of tannic acid powder was added to prepare precursor solution A. For comparison, the amount of tannic acid powder added was adjusted to 0.75 g to prepare precursor solution B; and the amount of tannic acid powder added was adjusted to 3 g to prepare precursor solution C. The other steps were the same, and three spherical photothermal evaporators, including those in Example 1, were obtained.

[0053] The optimal preparation conditions of the evaporator were evaluated by the rotational self-cleaning behavior of the three spherical photothermal evaporators in NaCl solution. Scanning electron microscopy revealed that the three prepared photothermal films all had a porous structure. Among them, the uniformity of film A on the surface of the polystyrene sphere was better, while the uniformity of film B and film C was poor. The three spherical photothermal evaporators were placed in a 20 wt% NaCl solution under one solar intensity, and evaporators B and C did not show any rotation behavior within 50 minutes. The results show that when the mass ratio of PVA and TA is 1:1, the photothermal film prepared in Example 1 can be evenly coated on the surface of the polystyrene sphere, and exhibits excellent rotational self-cleaning ability during the desalination process of the NaCl solution.

[0054] Example 3

[0055] The effect of different photothermal film structures on the evaporator's rotary self-cleaning ability was investigated. The polystyrene balls coated with photothermal film were immersed in different Fe 3+ In the solution, whether the spherical photothermal evaporator with different microstructures will also exhibit rotation behavior in NaCl solution.

[0056] The polystyrene spheres coated with the photothermal film prepared in Example 1 were used as a basis. For comparison, the Fe2(SO4)3 solution in Example 1 was replaced with an aqueous FeCl3 solution with a different mechanism of action. All other steps remained the same, resulting in two spherical photothermal evaporators, including the one in Example 1. The effects of different anions in the coordination solution on the photothermal film structure and the evaporator's rotational behavior were investigated.

[0057] The effects of different photothermal film structures on the evaporator's rotational self-cleaning ability were evaluated by measuring the desalination and salt resistance of two spherical photothermal evaporators in NaCl solution. Scanning electron microscopy revealed that Example 1 was subjected to SO4 2- The solution-treated photothermal film has a sparse pore structure like an open-cell foam, while the Cl - The solution-processed photothermal film has a three-dimensional network porous structure ( Figure 1 b). The two spherical photothermal evaporators were placed in a 20 wt% NaCl solution at a sunlight intensity of one sun. In Example 1, SO4 2- The evaporator treated with the solution showed three rotation behaviors within 50 min, while the evaporator treated with Cl - The solution-treated evaporator showed no rotation behavior ( Figure 2 b). The desalination capacity of the two evaporators was evaluated under one sun. The evaporator with rotational behavior in Example 1 showed 2.32 m -2 h -1 、1.92m -2 h-1 and 1.72 kg m -2 h -1 The evaporation rate of the evaporator without rotation behavior is relatively high, which is 2.58 m -2 h -1 , 2.13 m -2 h -1 and 1.92 kg m -2 h -1 ( Figure 3 b). The results show that the microstructure of the photothermal film affects the rotation behavior of the spherical photothermal evaporator, and a high evaporation rate and a rotational self-cleaning ability cannot be achieved at the same time. In contrast, the salt resistance of the evaporator is more important under actual working conditions. Example 1 SO4 2- The treated spherical photothermal evaporator has a slightly reduced evaporation rate while maintaining its rotational self-cleaning capability, and is also highly competitive in practical desalination applications.

[0058] Example 4

[0059] Investigate the effect of the size of the evaporator on its rotary self-cleaning ability.

[0060] The steps were roughly the same as those in Example 1, except that the diameters of the polystyrene plastic balls were changed to 1 cm, 2.4 cm, and 4 cm, respectively. The other steps were the same, and three spherical photothermal evaporators including Example 1 were obtained.

[0061] The effect of evaporator size on its rotational self-cleaning ability was observed through the rotational behavior of three spherical photothermal evaporators in NaCl solution. The three spherical photothermal evaporators were placed in a 20 wt% NaCl solution under one sunlight intensity. The evaporator with a diameter of 1 cm showed one rotation behavior within 50 minutes, the evaporator with a diameter of 2.4 cm showed three rotation behaviors, and the evaporator with a diameter of 4 cm did not show any rotation behavior. The results show that the size of the evaporator also has an important influence on its rotational self-cleaning behavior. Considering that salt crystallization on the surface of the evaporator will seriously affect the desalination stability in the actual desalination process, the high sensitivity to salt crystallization gives the 2.4 cm diameter evaporator unique application potential.

[0062] The above experiments demonstrate that the present invention solves the problem of coating the sphere surface with a thin film material. The photothermal evaporation coating process involves only a single, rapid hydrogen-bonding cross-linking reaction, resulting in a simple synthesis process, low cost, and scalable production. The lightweight plastic sphere core enables the evaporator to float on the water surface and is sensitive to the gravitational shift of precipitated salt crystals. Thanks to the water-transport capacity of the hydrophilic PVA-TA cross-linked network and the geometric properties of the sphere, the evaporator has the ability to absorb sunlight from all angles and exhibits excellent evaporation and rotational self-cleaning capabilities for salt solutions of varying concentrations, demonstrating broad application prospects.

[0063] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a spherical photothermal evaporator, characterized in that: The specific steps include: (1) Ultrasonic cleaning, rinsing, drying and standby of the lightweight plastic balls; (2) Dissolving polyvinyl alcohol and tannic acid in deionized water and an organic solvent, respectively, stirring and mixing, and adding a light absorbing material to obtain a precursor solution; (3) Immerse the lightweight plastic ball in the precursor solution, take it out after the surface is evenly covered with the gel solution, and let it stand at room temperature to solidify. Repeat the above steps to obtain a lightweight plastic ball coated with a photothermal film; (4) Immerse the lightweight plastic ball coated with the photothermal film in a 3+ The spherical photothermal evaporator is obtained by placing the spherical photothermal evaporator in an aqueous solution of ions, allowing it to stand, and washing it.

2. The method for preparing a spherical photothermal evaporator according to claim 1, characterized in that: In step (1), the lightweight plastic ball is made of polystyrene and has a diameter of 1 cm to 1 m.

3. The method for preparing a spherical photothermal evaporator according to claim 1, characterized in that: In step (1), the ultrasonic cleaning agent is deionized water, and the time is 10 minutes; the rinsing agent is ethanol.

4. The method for preparing a spherical photothermal evaporator according to claim 1, characterized in that: In step (2), the mass ratio of the polyvinyl alcohol to tannic acid is 1:(0.5~2).

5. The method for preparing a spherical photothermal evaporator according to claim 1, characterized in that: In step (2), the organic solvent is at least one of methanol and ethanol; and the mass ratio of the deionized water to the organic solvent is 1:(0.2-8).

6. The method for preparing a spherical photothermal evaporator according to claim 1, characterized in that: In step (2), the light absorbing material is at least one of modified needle coke, graphene, carbon nanotubes, coal-based carbon and biochar.

7. The method for preparing a spherical photothermal evaporator according to claim 1, characterized in that: In step (4), the Fe-containing 3+ The aqueous solution of ions is Fe2(SO4)3 solution, in which Fe 3+ The concentration of ions is 0.01~5 mol / L.

8. The method for preparing a spherical photothermal evaporator according to claim 1, characterized in that: In step (4), the standing time is 2 to 24 hours; the cleaning reagent is deionized water.

9. A spherical photothermal evaporator prepared by the preparation method according to any one of claims 1 to 8.

10. Use of a spherical photothermal evaporator prepared by the preparation method according to any one of claims 1 to 8 in solar seawater desalination.

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