Chitosan-coated zeolite with liquid-blocking and air-permeable functions as well as preparation method and application of chitosan-coated zeolite

Abstract

The invention provides chitosan-coated zeolite with liquid blocking and ventilating functions as well as a preparation method and application thereof, and relates to the technical field of enhanced mass transfer of equipment for adsorption heat exchange. According to the invention, a film coating technology and an adsorption technology are combined, the surface of 13X zeolite is coated with a layer of liquid-resistant breathable film-chitosan film, and vinyl trimethoxy silane is adopted to carry out chemical crosslinking on the chitosan film, so that the pore size and distribution of the chitosan film are regulated and controlled, and liquid free water in a system is regulated and controlled. The coated zeolite prepared by the invention has excellent liquid resistance, reduces the content of free water in a bed layer so as to enhance the overall performance and efficiency of a system, and is more suitable for being applied to a direct contact type adsorption heat pump system.

Description

Technical Field

[0001] The present invention relates to the technical field of enhanced mass transfer of equipment for adsorption heat exchange, and particularly relates to a chitosan-coated zeolite, a preparation method thereof, and a high-temperature adsorption heat pump based on the chitosan-coated zeolite for generating steam. Background Art

[0002] Adsorption heat pumps are driven by low-grade heat sources and rely on the adsorption / desorption cycle of gas-solid working fluids to produce refrigeration or heating effects. The direct contact method is used for adsorption heat pumps with a 13X zeolite-water working fluid pair, which can directly transform hot water into high-temperature steam. However, the heat and mass transfer efficiency of direct contact heat pumps is limited by the performance of the adsorbent.

[0003] 13X zeolite has attracted much attention in the field of heat storage due to its excellent thermal stability, high heat storage density, economy, and environmental friendliness. However, the hydrophilicity of 13X results in excessive free water inside the particles, and its use in adsorption heat pump systems will lead to large resistance to the migration of liquid water in the adsorption-desorption cycle, limiting the heat and mass transfer efficiency of the adsorption heat pump system, increasing the regeneration energy consumption, and reducing the system performance. Therefore, how to control the migration of liquid water in the adsorption bed is of great significance for enhancing heat and mass transfer in the adsorption heat pump system. Summary of the Invention

[0004] In view of the above problems, the present invention selects a chitosan membrane as a liquid-blocking and air-permeable membrane, coats the chitosan membrane on the surface of the zeolite, prevents the infiltration of external liquid free water into the particles, further enhances the generation process and desorption process of high-temperature steam, and improves the overall performance of the entire adsorption heat pump system.

[0005] One object of the present invention is to provide a preparation method of chitosan-coated zeolite.

[0006] Another object of the present invention is to provide a chitosan-coated zeolite prepared by the above preparation method.

[0007] A third object of the present invention is to provide an application of the chitosan-coated zeolite in an adsorption heat pump system.

[0008] In order to achieve the above objects of the present invention, the following technical solutions are specifically adopted:

[0009] In the first aspect, the present invention provides a preparation method of chitosan-coated zeolite, including the following steps:

[0010] (1) Dissolve chitosan in an aqueous acetic acid solution to form a chitosan membrane solution, and then add vinyltrimethoxysilane (VTMS) to the chitosan membrane solution for cross-linking reaction to obtain a chitosan-coated solution;

[0011] (2) Pretreat the 13X zeolite, and then coat the pretreated 13X zeolite with a chitosan coating solution by means of a coating machine and an air spray gun. The coated zeolite is subjected to drying and ethanol immersion treatment to obtain chitosan-coated zeolite.

[0012] Step (1):

[0013] Preferably, the weight-average molecular weight of chitosan is 200,000 - 1,000,000, preferably 1,000,000;

[0014] Preferably, the volume fraction of acetic acid in the acetic acid aqueous solution is 1% - 3%, preferably 2%;

[0015] Preferably, the mass fraction of chitosan in the chitosan membrane solution is 1% - 3%, preferably 2%;

[0016] Preferably, the added volume of vinyltrimethoxysilane accounts for 0.1% - 2% of the volume of the chitosan membrane solution, preferably 1.5%;

[0017] Preferably, the crosslinking reaction temperature is 50 - 80°C, preferably 65°C, and the reaction time is 1 - 3 h, preferably 2 h.

[0018] Step (2):

[0019] Preferably, the pretreatment of the 13X zeolite includes the following steps: calcine the 13X zeolite to a completely dry state and then adsorb it to saturation.

[0020] Preferably, the coating includes the following steps: add the pretreated 13X zeolite into a coating machine and roll it, and then spray the chitosan coating solution on the surface of the 13X zeolite with an air spray gun, and dry and cure to form a chitosan film.

[0021] Preferably, the temperature of the coating machine is 60 - 80°C, preferably 65°C; the rotation speed of the coating machine is 10 - 50 r / min, preferably 20 r / min; the inclination angle of the coating machine is 30 - 60°, preferably 45°;

[0022] Preferably, the spraying parameters include: spraying pressure is 300 - 500 kPa, preferably 400 kPa; spraying flow rate is 5 - 6 ml / min, preferably 5.56 ml / min; nozzle diameter is 1 - 1.5 mm, preferably 1.3 mm; spraying distance is 10 - 30 cm, preferably 20 cm; spraying width is 8 - 12 cm, preferably 10 cm.

[0023] Preferably, the chitosan coating ratio (the mass ratio of chitosan and zeolite in the chitosan coating solution is the coating ratio) is 6% - 10%, preferably 8%;

[0024] Preferably, the drying temperature is 100 - 130°C, preferably 100°C;

[0025] Preferably, the ethanol soaking time is 24 - 48 h, preferably 48 h.

[0026] In a second aspect, the present invention provides a chitosan-coated zeolite prepared by the preparation method of the above chitosan-coated zeolite.

[0027] In a third aspect, the present invention further provides an application of the above chitosan-coated zeolite in a direct contact adsorption heat pump system.

[0028] Beneficial effects

[0029] The present invention combines the film coating technology and the adsorption technology, and coats a liquid-blocking and air-permeable membrane - chitosan membrane on the surface of 13X zeolite to regulate the liquid free water in the system. By the liquid-blocking effect of the membrane, the content of free water in the bed layer is reduced, thereby enhancing the overall performance and efficiency of the system.

[0030] The chitosan-coated zeolite (13X zeolite@chitosan membrane) prepared by the method of the present invention has excellent liquid-blocking ability, strengthens the high-temperature steam generation time and the zeolite regeneration process, improves the system performance, and is more suitable for the application of the direct contact adsorption heat pump system.

[0031] The addition of VTMS crosslinking agent introduces hydrophobic ethylene groups into the membrane, enhancing the anti-swelling effect of the membrane and improving the stability of the membrane in water; the silane crosslinking agent used for pervaporation membrane separation generally adopts a silane with a hydrophilic group after hydrolysis (the difference is that the pervaporation separation hydrophilic silane aims to provide hydrophilicity to improve the separation efficiency, while the hydrophobic silane aims to balance the hydrophilicity and hydrophobicity of the membrane and strengthen the stability of the membrane).

[0032] Combining the chitosan membrane and the zeolite adsorbent together, the coating of the chitosan membrane not only plays a role in liquid-blocking and air-permeable, but also solves the problem that the zeolite will be pulverized when used in water, improving the recyclability of the zeolite. At the same time, the chitosan coating is used for the adsorption heat pump, and the membrane can be used under a large temperature difference. The use conditions of the chitosan membrane in the pervaporation field are all temperature conditions below 100 °C (the membrane has the ability to withstand a large temperature difference, and the membrane layer can play a role in buffering thermal stress).

[0033] The present invention has been described in detail above, but the above embodiments are essentially illustrative only and are not intended to limit the present invention. In addition, the present invention is not limited by any theory described in the foregoing prior art or the invention content or the following examples. Description of the drawings

[0034] Figure 1 It is a schematic diagram of an adsorption heat pump system;

[0035] Figure 2 It is a process flow chart of the preparation of the coated zeolite.

[0036] Figure 3 It is the schematic diagram of the synthesis of coated zeolite;

[0037] Figure 4 It is the static adsorption capacity of different coated zeolites;

[0038] Figure 5 It is the diagram of the adsorption heat test device;

[0039] Figure 6 It is the adsorption heat test of coated zeolite;

[0040] Figure 7 It is the influence of chitosan molecular weight on the liquid blocking rate of coated zeolite;

[0041] Figure 8 It is the influence of coating ratio on the liquid blocking rate of coated zeolite;

[0042] Figure 9 It is the influence of VTMS content on the liquid blocking rate of coated zeolite;

[0043] Figure 10 It is the diagram showing the liquid blocking effect of coated zeolite, where (a) 13X; (b-c) front and side liquid blocking effect diagrams of VCZ-1.5;

[0044] Figure 11 It is the Fourier infrared spectrum diagram of chitosan membrane and coated zeolite;

[0045] Figure 12 It is the TG-DTG curves of different coated zeolites, where (left) VCZ-0; (right) VCZ-1.5;

[0046] Figure 13 It is the pore size distribution diagram of chitosan membranes with different molecular weights;

[0047] Figure 14 It is the surface electron microscope images of coated zeolite at different magnifications, where (a1-a3) 13X; (b1-b3) VCZ-0; (c1-c3) VCZ-1.5;

[0048] Figure 15 It is the cross-section electron microscope images of coated zeolite at different magnifications, where (a1-a4) 13X; (b1-b4) VCZ-0; (c1-c4) VCZ-1.5;

[0049] Figure 16 It is the cross-section electron microscope images of chitosan membrane at magnifications of 20,000 and 40,000, where (a1-a2) VCZ-0; (b1-b2) VCZ-1.5;

[0050] Figure 17 It is the diagram of the distribution positions of temperature measurement points in the adsorption heat pump bed layer;

[0051] Figure 18 Thermal response curves of the steam generation process for different zeolites, where (left) 13X; (right) VCZ-1.5;

[0052] Figure 19 Comparison of the steam generation mass and steam generation rate for different zeolites;

[0053] Figure 20 Thermal response curves of the zeolite regeneration process for different zeolites, where (a) 13X; (b) VCZ-1.5;

[0054] Figure 21 Free water content and its removal energy consumption of different zeolites;

[0055] Figure 22 Comparison of the COP performance of different zeolites;

[0056] Figure 23 Comparison of the SHP performance of different zeolites. Detailed implementation manners

[0057] The present invention will be further described below in conjunction with embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of the present invention claimed.

[0058] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0059] Drugs and reagents:

[0060] In this study, 13X zeolite (average particle size 3.8 mm) was used as a commercially formed zeolite, provided by the Zhengzhou National Engineering Research Center for Comprehensive Utilization of Non-Metallic Mineral Resources. This zeolite was prepared from 20% binder and 80% zeolite powder. The relevant information of the experimental reagents required for this experimental study is given in Table 1. Preparation conditions of the coated zeolite: the molecular weight of chitosan was 200,000 - 1,000,000; the content of VTMS cross-linking agent was 0 - 2% (v / v); the cross-linking reaction temperature of the coating solution was 65 °C, and the reaction time was 2 h; the coating ratio of the chitosan coating solution was 6 - 10%.

[0061] Table 1 Experimental reagents

[0062]

[0063] System experiments and equipment:

[0064] The system experimental equipment is shown in Table 2.

[0065] Table 2 Experimental equipment

[0066]

[0067]

[0068] Example 1

[0069] Preparation of 13X zeolite@chitosan membrane:

[0070] (1) Weigh 8 g of chitosan with a molecular weight M W = 100W, dissolve the chitosan in 400 ml of 2v% acetic acid solution to form a chitosan membrane solution, then add 1.5v% vinyltrimethoxysilane (VTMS) crosslinking agent dropwise to the membrane solution, stir and react with a magnetic stirrer, the reaction temperature is 50 °C, the reaction time is 2 h, and the reacted membrane solution is the chitosan coating solution.

[0071] (2) First, calcine 100 g of zeolite 13X at 550 °C for 2 h until it reaches a completely dry state, then place a saturated sodium chloride solution in a closed device, and the zeolite adsorbs until it reaches a saturated state in the closed environment. Set the temperature of the coating machine to 65 °C and the rotation speed of the coating machine to 20 r / min. Put the saturated-adsorbed zeolite into the coating pan and rotate it for preheating for 15 min. After the temperature stabilizes, use an air spray gun to spray the chitosan coating solution onto the surface of the zeolite (the conditions and parameters of spraying and coating are shown in Table 3), dry and cure to form a chitosan membrane, and the chitosan coating ratio is selected to be 8%. After spraying, put the coated zeolite into an oven and dry it at 100 °C for 2 h, then soak it in absolute ethanol for 48 h. Finally, the dried zeolite is the chitosan-coated zeolite. The coated zeolite is denoted as VCZ-x, where x represents the volume content of VTMS.

[0072] Table 3 Spraying conditions and parameters

[0073]

[0074] Example 2

[0075] The content of VTMS crosslinking agent in the coating solution is 0.5v%, and the preparation of the remaining coated zeolite is the same as that in Example 1.

[0076] Example 3

[0077] The content of VTMS crosslinking agent in the coating solution is 1v%, and the preparation of the remaining coated zeolite is the same as that in Example 1.

[0078] Example 4

[0079] The content of VTMS crosslinking agent in the coating solution is 2v%, and the preparation of the remaining coated zeolite is the same as that in Example 1.

[0080] Example 5

[0081] During the spraying process, the coating ratio of the chitosan film solution was 9%, and the preparation of the remaining coated zeolite was the same as in Example 1.

[0082] Example 6

[0083] During the spraying process, the coating ratio of the chitosan film solution was 10%, and the preparation of the remaining coated zeolite was the same as in Example 1.

[0084] Example 7

[0085] The molecular weight M of chitosan W = 20W, and the preparation of the remaining coated zeolite was the same as in Example 1.

[0086] Example 8

[0087] The molecular weight M of chitosan W = 50W, and the preparation of the remaining coated zeolite was the same as in Example 1.

[0088] Comparative Example 1

[0089] Commercially available 13X spherical zeolite particles with a diameter of 3.8 mm. The zeolite particles do not have the ability to block liquid, and the liquid blocking rate is 0.

[0090] Comparative Example 2

[0091] Zeolite chemically grafted with silane VTMS on zeolite 13X (Li G, Xue B, He X, et al. Superhydrophobic Surface-modified zeolite to regulate the migration of nonadsorbed liquid water in an Open-loop adsorption heat pump[J]. Applied Thermal Engineering, 2022, 215:118929.). Under the same test conditions, its liquid blocking rate is 30%, which is lower than that of the coated zeolite prepared in the present invention.

[0092] Comparative Example 3

[0093] The zeolite with a core-shell structure coated with polyurethane (He X, Xue B, Wei R, et al. A novel hydrophobic shell of bio-based polyurethane coated on zeolite 13X: For designing a high-efficiency adsorption heat pump[J]. Energy Conversion and Management, 2024, 313: 118612.), under the same test conditions, its liquid-blocking rate is about 70%, which is lower than that of the coated zeolite prepared in the present invention.

[0094] Comparative Example 4

[0095] The content of VTMS cross-linking agent is 0, and the preparation of the remaining coated zeolites is the same as that in Example 1. Under the same test conditions, its liquid-blocking rate is about 50%, which is lower than that of the chitosan-coated zeolite of the present invention.

[0096] Comparative Example 5

[0097] The difference between this comparative example and Example 1 is that VTMS is replaced by hydrophilic tetraethoxysilane (TEOS). Under the same test conditions, its liquid-blocking rate is 40%. This is because the hydrophilicity of the chitosan film after cross-linking with TEOS is improved, making the film prone to swelling in water, resulting in the destruction of the film structure. While the vinyl hydrophobic group of VTMS can improve the anti-swelling effect of the film, so the liquid-blocking rate is lower than that of the chitosan-coated zeolite of the present invention.

[0098] Performance Testing and Characterization of Chitosan-Coated Zeolite

[0099] 1. Static water adsorption capacity test

[0100] The static adsorption capacity is one of the key indicators to measure the performance of adsorbent materials. It refers to the mass of water contained in each unit mass of zeolite when the zeolite is adsorbed to equilibrium for a long time at a certain temperature and humidity, which directly affects the energy density of the adsorption material. In an adsorption heat pump system, the higher the static adsorption capacity of the zeolite, the better its adsorption performance. When coating the zeolite, it must be noted that during the zeolite coating process, chitosan may block the pores inside the zeolite, thus occupying the adsorption sites and reducing the adsorption performance of the zeolite. Therefore, first saturate the zeolite with adsorption, and the pores of the zeolite are occupied by water molecules, which can effectively prevent the coating material from entering the pores and protect its structural integrity. By testing the static adsorption capacity of the zeolite sample, the adsorption performance before and after zeolite coating is evaluated. According to GB / T 6287-2021, the static adsorption capacity of the zeolite sample is tested, and the test conditions are: at room temperature, with a relative humidity of about 85%.

[0101] Figure 4Test results of static adsorption capacity of 13X zeolite and different coated zeolites. As shown in the figure, the static adsorption capacity of 13X zeolite is the largest, and the adsorption capacity at saturation can be 0.280 g / g. After being coated with chitosan, the adsorption capacity of the zeolite decreases slightly, with the lowest being 0.270 g / g. Compared with 13X zeolite, for the coated zeolite prepared by pretreatment followed by coating, its static adsorption capacity changes little. The lowest VCZ-0 only decreases by 3.6%, which indicates that during the preparation of the pretreated coated zeolite, only a small part of chitosan penetrates into the zeolite pores and occupies the active sites of the zeolite, resulting in a decrease in the static adsorption capacity. The test results of static water adsorption capacity show that the numerical values of the static adsorption capacity of the coated zeolite samples change little basically.

[0102] 2. Adsorption heat test

[0103] Adsorption heat refers to the heat released by the interaction between zeolite and water adsorption, and it is also an index to measure the adsorption capacity of zeolite. For a direct-contact adsorption heat pump, the magnitude of its adsorption heat determines the quality of steam generation and plays a decisive role in the performance of the system. It has been determined above that the coating process does not affect the static adsorption capacity of the zeolite sample. To further verify that the adsorption performance of the coated zeolite is not affected during the preparation process, the adsorption heats of zeolite 13X and the coated zeolite are tested. The contact between zeolite and water belongs to physical adsorption. According to the method proposed by the inventor for measuring the integral adsorption heat of zeolite, the adsorption heat of 13X zeolite is tested. The adsorption heat test device is as Figure 5 shown, and its main structure includes a heat insulation layer, a heat insulation cover, a thermocouple, a magnetic stirrer and a data acquisition instrument.

[0104] The following are the specific test steps for adsorption heat:

[0105] (1) Take about 10 g of the zeolite sample regenerated at 130 °C (the regeneration temperature of the system experiment is 130 °C), ensure that its initial water content is the same, seal it in a bag for use.

[0106] (2) Pour 50 ml of deionized water into the test device, put in a magnetic stir bar, and set the rotation speed to 30 r / min.

[0107] (3) After turning on the data acquisition instrument, quickly pour the zeolite into the test device, and then immediately cover it tightly with the heat insulation cover to avoid heat loss.

[0108] (4) Observe the temperature changes of each measurement point on the data acquisition instrument. After the temperatures of each measurement point are stable and remain unchanged, the test is completed. Turn off the test device, remove the heat insulation cover, and record the mass of the test device.

[0109] (5) During the test, the steam mass is equal to the mass difference before and after the adsorption of the test device. To ensure the reliability of the experimental results, each material is tested 3 times, and the average value is taken as the adsorption heat value, which is calculated according to Equation (1).

[0110]

[0111] Where: Q loss — The heat loss of the test device (kJ), which is calculated by the following Equation (2).

[0112]

[0113] Where: T inside — The temperature below the heat insulation cover of the test device, K;

[0114] T room — The indoor temperature, K;

[0115] h cr — The natural convection coefficient, W·m -2 ·K -1 ;

[0116] σ cover — The thickness of the heat insulation cover on the top of the test device, m;

[0117] λ cover — The thermal conductivity of the heat insulation cover material of the test device, W·m -1 ·K -1 .

[0118] Figure 6 are the test results of the adsorption heat of the zeolite samples. The adsorption heat of different zeolite samples changes slightly, indicating that chitosan only forms a layer of film on the surface of the zeolite, does not significantly affect the structure of the zeolite, and its adsorption performance is not affected either. Among them, the adsorption heat values of 13X are 902.55 kJ / kg respectively, and the lowest adsorption heat value of the coated zeolite is 889.07 kJ / kg. Compared with 13X, it only drops by 0.98% at the lowest, which is consistent with the change law of the static adsorption capacity. Based on the comprehensive test results of the static adsorption capacity and adsorption heat, it can be concluded that the adsorption and heat storage performance of the coated zeolite samples will not be affected.

[0119] 3. Liquid retention rate test

[0120] 13X zeolite is suitable for adsorption heat pumps due to its good adsorption performance. However, when it adsorbs water, a large amount of free water will enter the interior of the particles, resulting in high regeneration energy consumption and restricting the performance of the system. To test the liquid-blocking effect of coated zeolite on free water, an experimental device was self-made in this study to test the liquid-blocking effect of zeolite in direct contact with liquid water. A certain mass of zeolite samples was taken, saturated with adsorption, immersed in hot water, and then taken out and the free water on the surface of the zeolite was dried. The liquid-blocking effect of the coated zeolite was evaluated by measuring the mass difference before and after it was put into water. Taking 13X zeolite as a reference, the liquid-blocking rate was defined as the ratio of the reduced content of free water in the coated zeolite particles to the theoretical content of free water inside the 13X zeolite particles. The parameters affecting the liquid-blocking effect of chitosan-coated zeolite are mainly: the molecular weight of chitosan, the coating ratio of zeolite, and the content of VTMS cross-linking agent. The following will deeply explore its influence on the liquid-blocking rate. To ensure the reliability of the experimental data, each group of tests was carried out more than three times.

[0121] The purpose of this experiment was to explore the influence of different molecular weights on the liquid-blocking effect of chitosan-coated zeolite. Chitosans with molecular weights of 200,000, 500,000, and 1,000,000 were selected as membrane materials. 1.5% (v / v) of VTMS cross-linking agent was added to the membrane solution respectively, and the coating ratio was selected as 8%. Then, according to the above-mentioned test method, it was immersed in hot water at 100 °C for 30 min. The test results are as Figure 7 shown. As the molecular weight increased, the liquid-blocking effect of the chitosan membrane improved. This is because chitosan with a high molecular weight has longer molecular chains and higher entanglement during film formation, forming a more complex network structure, reducing the voids between molecules, and thus making the film structure more compact. On the contrary, the film structure formed by low-molecular-weight chitosan is relatively loose and the liquid-blocking performance is weak. By exploring the influence of different molecular weights of chitosan on the liquid-blocking rate, chitosan with a molecular weight of 1,000,000 was determined to be used as the coating material.

[0122] When coating zeolite, it is crucial to ensure the uniformity of the zeolite particle coating. Only by determining the appropriate chitosan coating ratio can a uniform and continuous coating layer be formed on the surface of the zeolite. By testing the liquid-blocking rates of zeolites with different coating ratios, the coating ratio of chitosan-coated zeolite was optimized. Chitosan membrane with a molecular weight of 1,000,000 was selected as the coating material, and other test conditions remained unchanged. The test results of the liquid-blocking rates of chitosan-coated zeolites with different coating ratios are as Figure 8As shown, for the coated zeolites prepared with 4% and 6% coating ratios, their liquid-blocking rates are only 0.183 and 0.652 respectively. At this time, the chitosan film fails to uniformly coat the zeolite particles. When the coating ratio reaches 8%, the liquid-blocking rate reaches 0.952, which is basically the same as that of the 10% coated zeolite, indicating that an 8% coating ratio can uniformly coat a layer of chitosan film on the surface of the zeolite. With the increase of the coating ratio, it is difficult to further improve the liquid-blocking rate of the film. On the contrary, it may cause the film layer to be too thick or locally piled up, affecting the air permeability of the coated zeolite. In summary, considering the cost of the membrane material and the coating uniformity, an 8% coating ratio is selected as the optimal coating ratio.

[0123] VTMS as a cross-linking agent can regulate the pore structure and permeability of the chitosan film, improve the thermal stability, mechanical properties and denseness of the film, and is an important factor affecting the liquid-blocking rate of the chitosan film. Under the condition of determining the optimal molecular weight and the optimal coating ratio, the content of the VTMS cross-linking agent is adjusted to test the influence of the chitosan-coated zeolite on the liquid-blocking rate, and its liquid-blocking rate at different temperatures and different times is tested. Figure 9 The influence of different VTMS contents on the liquid-blocking rate of the chitosan-coated zeolite is shown. With the increase of the content of the VTMS cross-linking agent, the liquid-blocking rate of the chitosan-coated zeolite is significantly improved. When the content of VTMS is 1.5% (v / v), the liquid-blocking rate of the coated zeolite is close to the highest value and can reach 0.957. On the one hand, with the increase of the cross-linking agent content, the cross-linking structure of the chitosan film becomes more compact, the pore size and volume of the film both decrease, and the regulation of the film pore size makes it difficult for liquid water to enter the zeolite pore channels through the film. On the other hand, the chitosan film itself is a hydrophilic film and is prone to swelling in water, resulting in the influence on the structural stability of the film and the increase of the liquid permeability of the film. While the cross-linking of VTMS introduces a small amount of vinyl structure into the film, reducing the hydrophilicity of the film and enhancing its anti-swelling property. In addition, through the comparison at different times and different temperatures, it can also be concluded that the increase of the VTMS cross-linking agent content is also beneficial to improving the thermal stability of the film in water and is more suitable for the adsorption heat pump system. After putting 13X zeolite and the coated zeolite VCZ-1.5 into water for 30 minutes, the liquid-blocking effect is as Figure 10 shown, and it can also be seen that VCZ-1.5 basically achieves the liquid-blocking effect.

[0124] 4. Fourier Transform Infrared Spectroscopy

[0125] Infrared spectroscopy is an analytical technique based on the transition of molecular vibration and rotation energy levels, used to study the structure and chemical composition of substances. According to the different absorption of infrared light with different frequencies by the sample, the abscissa is the infrared radiation wavelength (cm -1 ), and the ordinate is the transmittance (%). The resolution of the spectrum measurement is 4 cm -1 , and the wavelength range is 4000 - 400 cm -1 .

[0126] Figure 11 In the middle (left) is the Fourier transform infrared spectrum of chitosan membrane (CS membrane) and cross-linked chitosan membrane (CS-VTMS), which explains the chemical changes during the cross-linking process of chitosan and VTMS. The absorption peak at 3200-3600 cm -1 is the characteristic peak of hydroxyl (-OH) in chitosan, and the absorption peak at 2800-3000 cm -1 is the stretching vibration of C-H. The characteristic peaks at 1657 cm -1 , 1600 cm -1 are the amide I band (stretching vibration of C=O) and amide II band (stretching vibrations of N-H and C-N) respectively. The introduction of VTMS disrupts the hydrogen bonds within and between chitosan molecules, and the vibrations of these two peaks are enhanced. The characteristic peak at 1408 cm -1 is caused by the deformation of CH2 vibration in the vinyl group, which is consistent with the weak band of the vinyl characteristic peak reported by Gellermann between 1400-1415 cm -1 . The bending vibration of =C-H at 650-820 cm -1 proves that the vinyl group in VTMS is successfully introduced into the chitosan membrane. The stretching vibration peaks of C-O and Si-O at 900-1100 cm -1 . Compared with the CS membrane, the peak value of this peak becomes wider due to the cross-linking reaction between VTMS and chitosan, generating Si-O-C bonds, which overlap with the characteristic peak of C-O. The peak at 540 cm -1 is the symmetric stretching vibration peak of Si-O-Si, because VTMS first hydrolyzes to form silanol, and a part of the Si-O bonds self-condense to form Si-O-Si bonds. The infrared spectrum analysis shows that the cross-linking reaction between VTMS and chitosan occurs successfully.

[0127] Figure 11 In the middle (right) is the Fourier transform infrared spectrum of zeolite 13X and zeolite 13X@CS-VTMS. It can be seen from the figure that the FTIR spectra of 13X zeolite and VCZ-x are basically the same, which proves that the structure of zeolite 13X is basically not damaged before and after coating. The characteristic peaks of hydroxyl appear near 1650 cm -1 and 3400 cm -1 . The characteristic peaks in the range of 400-1200 cm -1 are the T-O4 structure in zeolite 13X. Compared with 13X zeolite, the characteristic peak of C-H appears at 1420 cm -1 in VCZ-x, which proves that the chitosan membrane is successfully coated on the surface of zeolite 13X.

[0128] 5. Thermal stability

[0129] In the adsorption heat pump system, the highest temperature in the reactor can reach 220 °C, so zeolite is required to have good thermal stability. 13X zeolite has good thermal stability and can be applied to this heat pump system. However, the film layer in zeolite 13X@chitosan belongs to polymer materials, and its applicable range in the adsorption heat pump needs to be tested. A thermogravimetric analyzer (TG) was used to test its heating curve at a heating rate of 10 °C / min under a nitrogen atmosphere.

[0130] Figure 12 Figures 4 and 5 are the TG-DTG curves of coated zeolites VCZ-0 and VCZ-1.5. By comparing with 13X zeolite analysis, it can be concluded that the thermal weight loss trends of VCZ-0 and VCZ-1.5 are the same, and their weight loss mainly has three trends. The first stage is the desorption of adsorbed water in the coated zeolite. The second stage is the thermal degradation of the chitosan film. First, the glycosidic bond decomposes to form a series of low fatty acids such as acetic acid and butyric acid. The third stage is the complete degradation of chitosan molecules. The weight loss in the first and second stages of VCZ-0 and VCZ-1.5 almost coincides, including both the desorption of zeolite water and the thermal degradation of the chitosan film, reaching the maximum at 287.84 °C. The third-stage decomposition occurs near 517.37 °C, which is the complete degradation of the chitosan film. The maximum decomposition temperature of VCZ-1.5 is 33.39 °C higher than that of 13X. This is because VTMS crosslinking strengthens the crosslinking degree of chitosan molecular chains, and the film has stronger mechanical properties. At the same time, VTMS crosslinks with the chitosan film and introduces Si-O bonds into the film. The bond energy of Si-O bonds is much larger than that of C-O bonds, so it has better thermal stability. Generally speaking, the chitosan-coated zeolite prepared by VTMS crosslinking does not show obvious decomposition within 220 °C, which proves its feasibility in the high-temperature heat pump system.

[0131] 6. BET Test

[0132] BET is a method for testing the specific surface area and pore size distribution of porous materials. It evaluates the specific surface area and pore structure of materials by testing the adsorption of gases on the material surface. The model used in this test is Micromeritics ASAP 2460, and the test samples are chitosan films (containing VTMS crosslinking) prepared with chitosans of different molecular weights. The test conditions are: N2 adsorption, degassing temperature 120 °C, and degassing time 9 h.

[0133] Figure 13Figure 0 shows the pore size distribution diagrams of chitosan films with different molecular weights. It can be seen from the figure that the pore sizes of chitosan films are concentrated in the range of 2 - 10 nm. The specific surface area, pore volume, and mesopore diameter data measured are shown in Table 4. The chitosan film prepared with a molecular weight of 200,000 has the largest pore volume and pore diameter, while the chitosan film prepared with a molecular weight of 1,000,000 has the smallest pore volume and pore diameter. This is because the larger the molecular weight, the longer the molecular chains and the higher the entanglement degree during the film-forming process, forming a more complex network structure, reducing the intermolecular voids, and thus making the film structure more compact. The smaller the pore volume and pore diameter of the film, the stronger the ability to block liquid water.

[0134] Table 4 Specific surface area, pore volume, and pore size of chitosan films with different molecular weights

[0135]

[0136] 7. Scanning Electron Microscope

[0137] As previously mentioned, the main challenge in the preparation of coated zeolites is how to uniformly and completely coat the film on the surface of zeolite particles. The microstructure of zeolite 13X@chitosan was observed and analyzed by scanning electron microscope (SEM). Figure 14 Figure 14 shows the surface morphologies of different zeolite particles. a, b, and c represent zeolite 13X, VCZ-0, and VCZ-1.5 respectively, and a1 - a3 are the morphological characteristic diagrams of the zeolite particle surfaces at different magnifications (500, 2000, and 5000 times). By comparing the surface morphologies of 13X, VCZ-0, and VCZ-1.5, it can be seen that the surface of the coated zeolite is relatively uniform and smooth, while the surface of the uncoated zeolite 13X is uneven. Through the magnification, it can be found that the chitosan film uniformly and completely covers the zeolite surface, and the film layer is dense with relatively small pore diameters. Basically, no zeolite particles can be seen on the coated zeolite. The chitosan film layer on the zeolite surface blocks the direct contact between liquid water and zeolite. Water can only pass through the pores of the film in the form of gaseous molecules when contacting the zeolite, while there are many pores and voids on the surface of the uncoated zeolite particles, and liquid water can easily pass through and adhere to the interior of the zeolite pores, which makes the regeneration energy consumption of the adsorption heat pump extremely large. The coated zeolite can perfectly solve this problem.

[0138] Figure 15Cross-sectional electron micrographs of the interface between the zeolite core layer and the membrane layer in zeolite samples at different magnifications (500, 1000, 2000, and 5000 times). Microstructural characterization shows that a dense heterogeneous interface is formed between 13X zeolite and the chitosan membrane, with good interfacial compatibility between the two phases and no obvious defect structures in the interfacial transition region. The membrane layer has a uniform thickness distribution, with an average thickness of 8 - 10 μm. Further observation reveals that the chitosan membrane layer exhibits specific orientation arrangement characteristics on the zeolite surface. The formation of this anisotropic structure can be attributed to the directional rolling of particles during the coating process, resulting in an ordered orientation structure of the chitosan membrane layer on the particle surface. Through the magnification, it can also be seen that the zeolite particles at the interface of the coated zeolite maintain their due pores like 13X, indicating that during the spraying and coating process, the chitosan coating material does not block the pores of the zeolite particles. Therefore, the adsorption performance of the coated zeolite remains basically unchanged.

[0139] Figure 16 SEM images of the cross-sections of VCZ-0 and VCZ-1.5 membrane layers magnified 20,000 times and 40,000 times. It can be seen from the figures that VCZ-0 has larger membrane pore sizes and higher porosity, while VCZ-1.5 has smaller membrane pore sizes and is denser. This is because VTMS crosslinks the chitosan molecular chains tightly through siloxane, forming a more complex network structure, reducing the intermolecular voids, and changing the microstructure of the chitosan membrane, thus better preventing the penetration of liquid water. The SEM images of the cross-sections of VCZ-0 and VCZ-1.5 membrane layers prove that the VTMS crosslinking agent can well regulate the pore size distribution of the chitosan membrane.

[0140] Application Example Adsorption Heat Pump Steam Generation System

[0141] The adsorption heat pump system test bench in this study uses the method of direct contact heat exchange. The adsorption working pair is zeolite-water. The zeolite and water are in direct contact to release a large amount of adsorption heat and convert part of the liquid water into high-temperature steam. This system belongs to the second type of heating heat pump and is driven by medium and low-temperature waste heat. As Figure 1 is a schematic diagram of this test bench. Table 5 introduces the experimental equipment and related model parameters of this system.

[0142] Table 5 Experimental Equipment Information

[0143]

[0144] Experimental Operation Steps and Parameters:

[0145] When carrying out the preparatory work for the system cycle experiment, the regular equipment maintenance procedures should be given priority. To strengthen the insulation device to reduce heat loss, it is necessary to first update the heat insulation layer outside the conveying pipeline and the reaction device, and then clean the scaling substances deposited inside the heat exchanger to eliminate the attachments on the pipe wall and reduce the thermal resistance, so as to improve the heat transfer efficiency of the test device. The water in the low-temperature circulating water pump also needs to be replaced, and the coil pipes should be cleaned to remove impurities such as rust and scale. After all the preparatory work is completed, fill the adsorption bed with zeolite, prepare to adjust the pipeline valves, and start the cycle experiment.

[0146] Steam generation process:

[0147] A complete system cycle test includes a steam generation process and a zeolite regeneration process. By adjusting 6 valves, this experimental operation process can be completed. The specific experimental operation steps are as follows:

[0148] (1) First, turn on the constant temperature water bath in advance and heat the water to 80°C, then turn on the low-temperature water circulation pump for refrigeration, and finally turn on the heating tapes of the reactor and the inlet pipeline for heat preservation to make each heating equipment reach the target temperature.

[0149] (2) Conduct the pre-inlet water operation, insulate the inlet pipeline, adjust valves V1 and V2, turn on the metering pump to let the low-temperature inlet water pass through the inlet pipeline and discharge from the bottom of the reactor, and end the pre-inlet water operation when the inlet water temperature at the reactor outlet reaches the target value.

[0150] (3) Open valves V2, V3, and V5, close valves V1, V4, and V6, and control the inlet water flow by adjusting the frequency of the frequency converter to make it enter the reactor smoothly.

[0151] (4) Monitor the temperature changes of each temperature measurement point on the data acquisition instrument in real time, and record the steam mass through an electronic balance.

[0152] (5) When the water level rises to the top of the reactor, the temperature measurement point at that place will suddenly drop below 100°C, and the steam generation process ends. Immediately turn off the metering pump and the frequency converter.

[0153] (6) Open valve V1 to drain the liquid water in the reactor, and then close all valves. The steam generation process ends, and prepare for the next regeneration process.

[0154] Zeolite regeneration process:

[0155] (1) First, load the dried silica gel into the drying tube.

[0156] (2) Open valve V6, and air passes through the air compressor, dryer, and heater in turn and discharges from the pipeline. When the air reaches the set temperature and humidity and remains stable, the preheating process is completed.

[0157] (3) Turn on the data acquisition instrument, close valve V6, open valves V1, V3, and V4. The hot dry gas enters the reactor and starts to regenerate the zeolite. The temperature at each temperature measurement point gradually rises to the set temperature. When all temperature measurement points are stable and remain unchanged, it indicates that the desorption process of the zeolite is completed.

[0158] (4) Adjust the valves and turn off all equipment to make the system in a closed state, preventing the zeolite in the reactor from contacting the outside world and affecting the initial state of the zeolite. The end of the zeolite regeneration process also means the completion of one round of system experiments. To reduce the accidental errors brought by the experiments, three parallel cyclic experiments will be carried out on each group of zeolite samples under the same operating condition parameters. The operating condition parameters and uncertainties are shown in Table 6:

[0159] Table 6 Experimental operating condition parameters

[0160]

[0161]

[0162] System evaluation method

[0163] An adsorption heat pump system is an energy system that uses solid adsorbents to achieve heat transfer through the adsorption and desorption processes. Its working principle is based on the mass balance and energy balance in thermodynamics. To effectively evaluate the overall performance of the adsorption heat pump system, a comprehensive analysis of aspects such as mass balance and energy balance must be carried out, and the coefficient of performance (COP h ) efficiency COP ex and the heating efficiency (SHP) of the system must be calculated. Through comprehensive evaluation, it can provide strong support for the optimal design and operation strategy formulation of the adsorption heat pump system, and promote its wide application in the field of energy conservation and environmental protection.

[0164] In the adsorption heat pump system, the whole process follows the law of conservation of energy. Heat input and output are involved in the energy transfer process. Through energy accounting, the utilization efficiency of the adsorption heat pump using external heat sources and their energy in the whole cycle process can be understood. According to the energy balance, the energy input into the reactor by the system includes the heat of the inlet water and the heat released by the contact between the zeolite and water. The heat output from the reactor includes the heat contained in the generated steam and the sum of the sensible heat accumulated by the system. The sensible heat accumulated by the system is the sensible heat of the reactor, zeolite, and free water. The energy balance equation is shown in Equation (3):

[0165] Q w,in +Q ads =Q s,out +Q acc (3)

[0166] Where: Q w,in— Heat input from the system inlet water, kJ;

[0167] Q ads — Total heat of adsorption released by zeolite adsorption, kJ;

[0168] Q s,out — Heat for generating steam, kJ;

[0169] Q acc — Sensible heat accumulated in the system, kJ.

[0170] The heat terms in the energy balance equation (3) are given by (4) to (7):

[0171] Q w,in =c p,w m w,in (T w,in -T ref ) (4)

[0172] Where: c p,w — Specific heat capacity at constant pressure of water, kJ·kg -1 ·K -1 ;

[0173] m w,in — Mass of the inlet water, kg;

[0174] T w,in — Temperature of the water input to the system, K;

[0175] T ref — Reference temperature (enthalpy value is 0), K.

[0176] Q ads =m z,dry (x eq -x ini )ΔH ads,zw (5)

[0177] Where: ΔH ads,zw — Heat of adsorption released by zeolite adsorbing water, kJ / kg.

[0178] Since the high-temperature steam generated in the system is superheated steam, this part of the heat should include the following three parts: The first part is the sensible heat when the inlet water temperature reaches the boiling point, the second part is the latent heat of vaporization of water, and the third part is the heat contained in the high-temperature steam continuing to heat up to become superheated, as shown in Equation (6).

[0179] Q s,out =m s,out [c p,w (T bp -T ref )+γ+c p,s (T s -Tbp )] (6)

[0180] Where: T bp —The boiling point temperature of water, K

[0181] γ—The latent heat of vaporization of water, kJ / kg;

[0182] c p,s —The specific heat capacity of steam, kJ·kg -1 ·K -1 ;

[0183] T s —The temperature of superheated steam, K.

[0184] The sensible heat accumulated in the system includes: the sensible heat accumulated in the reactor, the sensible heat accumulated in the zeolite, and the heat of the drained water, which is represented by Equation (7).

[0185] Q acc = m z,dry c p,z (T z,fin - T z,ini ) + [m drain + m z,dry (x eq - x ini )]c p,w (T w,fin - T w,ini )

[0186] + m r c p,r (T r,fin - T r,ini )(7)

[0187] Where: C p —The specific heat capacity of zeolite, kJ·kg -1 ·K -1 ;

[0188] T z,fin —The final temperature of zeolite at the end of steam generation, K;

[0189] T z,ini —The initial temperature of zeolite before steam generation starts, K;

[0190] m r —The mass of the reactor, kg;

[0191] C p,r —The specific heat capacity of the reactor, kJ·kg -1 ·K -1 ;

[0192] T r,fin —The temperature of the reactor at the end of steam generation, K;

[0193] T r,ini — Temperature of the reactor before the start of steam generation, K.

[0194] The energy consumption during the regeneration process mainly includes: the energy consumption required for the removal of free water in the bed, the energy consumption required for the desorption of adsorbed water from the zeolite, and the increase in the sensible heat of the zeolite and the reactor. The formula is shown in (8).

[0195] Q reg = Q reg,free + Q reg,ads + Q reg,z+r (8)

[0196] Where: Q reg — Total energy consumption required for zeolite regeneration, kJ;

[0197] Q reg,free — Energy consumption required for the removal of free water from the zeolite, kJ;

[0198] Q reg,ads — Energy consumption required for the desorption of adsorbed water from the zeolite, kJ;

[0199] Q reg,z+r — Increase in the sensible heat of the zeolite and the reactor, kJ.

[0200] The calculation methods of the energies in formula (8) are given in (9) - (11).

[0201] Q reg,free = m z,dry γ(x fin - x ini ) (9)

[0202] Q reg,ads = m z,dry ΔH ads,zs (x eq - x ini ) (10)

[0203] Q reg,z+r = (m z,dry c p,z + m r c p,r )(T g - T z,fin ) (11)

[0204] Where: T g — Temperature of the hot dry air inlet during the regeneration process, K.

[0205] Coefficient of performance (COP h) is a key indicator for measuring the heating performance of an adsorption heat pump, which is defined as the energy that the system can provide under the condition of consuming unit energy. The higher the coefficient of performance (COP), the better the heating performance of the system. The experimental system belongs to the second type of heat pump - temperature rise heat pump, and its purpose is to improve the quality of waste heat. Therefore, its COP is generally less than 1. h The calculation formula of COP is shown in (12):

[0206]

[0207] Efficiency COP ex refers to the maximum ability of the system to convert into useful work under specific environmental conditions, and is defined as the ratio of the useful output to the total input. In the adsorption heat pump system, the input is the value of the driving heat source (industrial waste heat), and the output is the exergy of the generated high-temperature steam relative to the environmental state. Its calculation formula is shown in (13):

[0208]

[0209] where: T0 - environmental temperature, °C;

[0210] T s - steam temperature, °C;

[0211] T L - waste heat temperature, °C.

[0212] The heating efficiency (SHP) of the system represents the efficiency of each unit mass of zeolite in the system to generate high-temperature steam, and is also a key indicator for evaluating the performance of the adsorption heat pump. Its calculation formula is given in (14):

[0213]

[0214] where: t gen - the time required for the steam generation process, s;

[0215] t reg - the time required for the zeolite regeneration process, s.

[0216] Experimental results of the adsorption heat pump system

[0217] This experiment studied the adsorption performance and chemical properties of zeolite 13X@chitosan, and compared these results with commercially available 13X zeolite samples. The results showed that zeolite 13X@chitosan could meet the requirements of the adsorption heat pump system to a certain extent. Therefore, in this experiment, the coated zeolite VCZ-1.5 with the best liquid blocking rate was prepared in batches and used together with zeolite 13X in the adsorption heat pump system experiment to evaluate its specific performance in the adsorption heat pump system. At the same time, the study also explored the dynamic thermal response of the bed layer and the high-temperature steam generation capacity of 13X zeolite and VCZ-1.5 materials during the system cycle. In addition, this study further evaluated the influence of zeolite 13X@chitosan membrane on the performance of the adsorption heat pump system by analyzing indicators such as the free water content in the bed layer, desorption energy consumption, coefficient of performance (COP), and heating effectiveness (SHP).

[0218] 1. Steam generation stage

[0219] 1.1 Analysis of dynamic thermal response of the bed layer

[0220] During the experiment, in order to study the temperature changes at different positions in the adsorption bed, four thermocouples were arranged in the adsorption bed reactor in this experiment to monitor the temperature fluctuations at each position in real time. Figure 17 The schematic diagram of the positions of these temperature measurement points is shown.

[0221] Before starting to study the dynamic thermal response of the steam generation process, it is necessary to point out that since the temperature measurement point (T bot ) at the bottom of the adsorption bed first contacts the low-temperature influent water, this study did not use its data as valid data for subsequent analysis during the steam generation stage. Figure 18 The figure on the left shows the thermal response curves of each temperature measurement point in the 13X adsorption bed over time during the steam generation stage. The temperature changes of the three temperature measurement points show the same trend: rising rapidly first, and then dropping rapidly after reaching the highest value.

[0222] At the initial stage of the steam generation stage, the temperatures in the lower temperature measurement areas (T 1 / 3 and T 2 / 3 ) were maintained at about 125°C, mainly due to the following two factors: First, the steam generation stage starts after the regeneration of the zeolite is completed, and at this time, the temperature of the zeolite is close to the temperature of the hot dry air during the regeneration process; second, because the regeneration process of the zeolite uses 130°C hot dry gas as waste heat, and at the same time, part of the heat is lost due to the pre-influent water section. The temperature measurement point at the top (T top ) is relatively low at about 115°C because it is close to the top outlet of the adsorption bed and is connected to the flange at the top of the reactor, and it is affected by heat radiation. The temperature change curve at the middle 1 / 3 of the adsorption bed is like T 1 / 3As shown, at the 137s mark, the temperature at this position rises sharply because the influent water comes into contact with the regenerated zeolite at the bottom of the adsorption bed, and a large amount of heat is released during the adsorption of zeolite and water. During this process, the concentrated release of the adsorption heat causes partial phase change of the liquid water, and the water molecules rapidly undergo phase change to generate steam. The steam continues to migrate upward. During this process, part of the steam is adsorbed by the zeolite and continuously releases the adsorption heat, resulting in T 1 / 3 The temperature at the position reaches the peak temperature of 202 °C at 208s, indicating that the zeolite reaches the adsorption equilibrium state. Subsequently, as the water level continues to rise, the temperature at this point begins to drop rapidly and drops to 100 °C at 256s, indicating that the water level has reached this point, and the temperature at this point finally stabilizes and approaches the level of the influent water temperature. For the temperature change at 2 / 3 (T 2 / 3 ), the starting time of the temperature rise is delayed by 109s compared to T 1 / 3 and starts to rise at 244s. This time delay phenomenon is because there is an obvious time lag effect during the upward migration of steam in the bed from bottom to top. When the high-temperature steam reaches this position, the adsorption heat released by the zeolite rapidly increases the temperature at this point to 226 °C and reaches the adsorption saturation state. The peak temperature of T 2 / 3 is about 24 °C higher than that of T 1 / 3 , mainly because the liquid water has not reached this area in time. This temperature distribution characteristic fully reflects the time difference in the heat and mass transfer process inside the adsorption bed.

[0223] The bed thermal response curve of the temperature measurement point (T top ) at the top of the adsorption bed is given by the blue curve. This temperature measurement point is a key position for monitoring the high-temperature steam transfer process. The temperature change of T top can be divided into three typical stages: the initial stable period, the rapid rise period, and the gradual decline period. The initial temperature of T top is about 11 °C lower than that of T 1 / 3 and T 2 / 3 . This temperature difference is mainly because the pipeline at the top of the adsorption bed is connected to the flange. Although effective heat insulation measures have been taken for the experimental device, heat loss still cannot be completely avoided during the pre-influent stage. The temperature response of T top also shows an obvious lag phenomenon. The temperature rise time at T top is 358s, which lags behind T 1 / 3 and T 2 / 3 by 229s and 124s respectively. This time difference is mainly because: only when the zeolite bed at the bottom reaches the adsorption equilibrium and thermodynamic equilibrium, can the high-temperature steam gradually migrate upward and finally reach the top. T topThe highest temperature reached 224 °C at 514 s. After maintaining this temperature for 131 s, the temperature at this point dropped suddenly. This is because the zeolite was gradually approaching the adsorption saturation state, resulting in a reduction in the release of adsorption heat. At the same time, the rise of the liquid water level promoted the transfer of part of the heat to the liquid phase. When the temperature dropped below 100 °C at 716 s, it meant that the liquid water had completely covered the reactor bed, marking the end of the steam generation stage. The whole process lasted for 717 s. The thermal response curve of this temperature measurement point provided a reliable basis for in-depth study of the heat and mass transfer laws in the adsorption heat pump system.

[0224] Figure 18 The dynamic thermal response curve of the coated zeolite VCZ-1.5 during the steam generation stage is shown in the middle (right). The spatial distribution of the specific temperature measurement points in the figure is also as Figure 17 shown. The dynamic thermal response curve of VCZ-1.5 indicates that its temperature change trend is basically the same as that of zeolite 13X, but there are significant differences in the specific parameters of the temperature change, indicating that the chitosan-coated zeolite has a certain influence on the heat and mass transfer laws in the bed.

[0225] T 1 / 3 The temperature at the position started to rise rapidly at 137 s and reached a peak of 168 °C at 168 s and then dropped rapidly below 100 °C, which was 88 s earlier than that of 13X zeolite. This early phenomenon can be attributed to the effect of the chitosan coating. The chitosan membrane played a role in blocking the liquid and allowing air permeability, preventing the liquid water from entering the interior of the zeolite particles, thereby accelerating the rise of the water level and the mass transfer process. Due to the insufficient thermodynamic equilibrium time, the highest temperature at this position was about 34 °C lower than that of 13X. At the T 2 / 3 position, the thermal response curve also showed an early characteristic. The temperature started to rise at 166 s and reached a peak of 219.6 °C at 258 s and maintained for 56 s. Compared with 13X, the peak appeared about 102 s earlier. This change is related to the migration of water on the surface of the zeolite particles after coating. The residence time of the liquid water in the bed was shortened, resulting in a corresponding reduction in the adsorption equilibrium time. The top temperature measurement point (T top) The temperature started to rise at 273 s and reached a peak of 212.55 °C at 379 s, maintaining for 164 s. Compared with 13X, the maximum temperature at this point decreased by 11.78 °C, but the duration extended by 25.2%. This phenomenon can be explained as follows: due to the shortening of the adsorption equilibrium time in the bottom area of the adsorption bed, the un-released adsorption heat gradually accumulates and transfers upward, finally achieving double equilibrium of thermodynamics and adsorption at the top. Both the thermodynamic equilibrium and adsorption equilibrium of the whole bed layer show a hysteresis phenomenon. This indicates that the coated zeolite can maintain the thermodynamic equilibrium state for a long time and conduct heat and mass transfer, which is beneficial to the generation of steam. Due to the liquid-blocking ability of the membrane, the rising rate of the water level accelerates, resulting in an accelerated mass transfer rate, and the time of the whole steam generation stage is shortened, about 610 s. The thermal response curve of VCZ-1.5 also proves its excellent performance in the adsorption heat pump system.

[0226] 1.2 Steam generation quality and rate

[0227] The steam generation quality and rate of zeolite 13X and VCZ-1.5 are as Figure 19 shown. The data in the figure are the average values obtained from three parallel experiments to reduce the influence of accidental errors. It can be seen from the figure that the steam generation quality and rate of 13X and VCZ-1.5 are not very different. The steam generation quality of 13X zeolite in the adsorption heat pump system is 28.33 g, and the steam quality of VCZ-1.5 is 29.52 g, which is 4.2% more than that of 13X. This is because when the coated 13X zeolite generates high-temperature steam, the liquid-blocking effect of the chitosan film layer makes the liquid water rise to the top of the adsorption bed faster, and less heat released by the zeolite during the whole process is used for the sensible heat increase of the liquid free water, which means more adsorption heat is used for steam generation, resulting in an increase in steam quality. The steam generation rates of zeolite 13X and VCZ-1.5 are 0.11 g / s and 0.10 g / s respectively. This is because during the steam generation stage of VCZ-1.5, although the chitosan film has a good liquid-blocking effect, its air permeability effect is also hindered, so the adsorption rate of the zeolite is affected, thus delaying the time for the zeolite to reach the adsorption equilibrium, resulting in a slower heat release rate of the zeolite, and the time for steam generation is relatively extended, but the adsorption heat used for steam generation remains basically unchanged, and the generated mass is not very different. In summary, the steam generation rate of VCZ-1.5 is slightly lower than that of 13X. Generally speaking, the adsorption performance of zeolite is not affected during the coating process. Using VCZ-1.5 for the adsorption heat pump to generate high-temperature steam also has a good effect, further indicating that the coated zeolite has higher potential application value compared with 13X zeolite.

[0228] 2 Thermal response curve during the zeolite regeneration stage

[0229] 2.1 Bed layer thermal response curve

[0230] When analyzing the bed thermal response during the regeneration stage of wet zeolite, the thermocouple at the top (T top ) of the adsorption bed did not come into contact with the zeolite particles, making it impossible to effectively monitor the temperature change at the temperature measurement point at the top of the adsorption bed. Therefore, this study only carried out a detailed analysis of the temperature dynamic changes at three temperature measurement points in the lower part of the adsorption bed. In the adsorption heat pump system, the study of temperature changes during the zeolite regeneration stage is crucial. The temperature responses of zeolite 13X and zeolite VCZ-1.5 samples during regeneration were compared. The selected temperature measurement points were located at the bottom, 1 / 3, and 2 / 3 of the adsorption bed respectively to observe the temperature dynamic changes at different positions.

[0231] The bed thermal response curves of 13X zeolite and coated zeolite VCZ-1.5 during the zeolite regeneration stage are as Figure 20 shown. The temperature changes of zeolite 13X and VCZ-1.5 samples during regeneration both showed the characteristics of first decreasing and then slowly increasing. The zeolite regeneration stage occurs after the steam generation stage. Therefore, at the beginning of the experiment, the initial temperatures of the zeolites were roughly the same. As the hot dry air entered the reactor, the free water remaining in the adsorption bed that had not been adsorbed absorbed heat and evaporated rapidly during the heating process, resulting in a rapid drop in the temperatures at the three temperature measurement points. Subsequently, the temperature entered a plateau period. When the temperature at the T 2 / 3 position rose, it indicated that the free water in the bed had been completely removed. Then, some of the free water and adsorbed water inside the particles also began to desorb gradually. With the continuous introduction of hot dry air, the temperature at the T 2 / 3 position continued to rise and finally approached 130 °C, indicating that the zeolite at this position had completed desorption. Since T 1 / 3 was at a lower position and the temperature rose slowly, when all the temperature measurement points stabilized at 130 °C, the zeolite regeneration stage was completed.

[0232] Although the temperature change trends of zeolite 13X and VCZ-1.5 were generally the same, there were significant differences in the time of temperature increase. From the data shown in the figure, the time when the temperature of the 13X sample started to rise at the T 2 / 3 position was 629 s, while the corresponding time for the VCZ-1.5 sample was 388 s, which was 241 s earlier than that of 13X. This phenomenon was mainly because the chitosan film after coating VCZ-1.5 had the effect of blocking liquid and allowing air permeability, resulting in a decrease in the free water content in the bed. The energy consumption for desorbing the free water between particles decreased significantly. Therefore, less hot dry air was required, resulting in a reduction in the time plateau at the T 2 / 3 position. This was also closely related to the migration law of the free water inside the zeolite particles during the steam generation stage. At the same time, the time when the temperature of the VCZ-1.5 sample rose at the T 1 / 3 and T bot positions was also advanced. The temperature of 13X zeolite at the T 2 / 3 and T 1 / 3and T bot The times from the start of temperature rise to stabilization at around 130 °C at [T], [T], and [T] are 3668 s, 3089 s, and 2561 s respectively, while VCZ-1.5 only takes 3048 s, 2534 s, and 2049 s, indicating that 2 / 3 at [T], 1 / 3 at [T], bot and at [T], the desorption times of VCZ-1.5 are reduced by 20.3%, 18.0%, and 20.0% respectively compared to 13X. This is because the liquid-blocking effect of VCZ-1.5 makes it almost impossible for there to be liquid free water inside the zeolite particles, and only adsorbed water needs to be desorbed during the entire desorption process, while 13X needs to desorb both free water and adsorbed water simultaneously. Therefore, the desorption time of VCZ-1.5 is significantly reduced compared to 13X. During the entire zeolite regeneration stage, 13X takes 4329 s, while VCZ-1.5 only takes 3426 s, with the overall time reduced by 20.6% and the regeneration energy consumption significantly decreased. The existence of the coating structure effectively blocks the penetration of liquid water into the interior of the particles, reduces the water content inside the particles, and makes the temperature jump time of the coated zeolite earlier than that of the 13X zeolite sample in the reactor. This finding provides an important basis for studying the heat and mass transfer laws of zeolite particles in the adsorption bed.

[0233] 2.2 Free water content and regeneration energy consumption

[0234] After the end of the steam generation stage, the liquid water in the adsorption bed is discharged from the bottom of the reactor, and then the adsorption bed needs to be regenerated by hot dry air. Although most of the unadsorbed liquid water has been removed, a certain amount of free water will still remain in the particles and in the inter-particle gaps, and this free water will affect the temperature change in the adsorption bed layer during the regeneration stage of the wet zeolite. The core objective of this study is to improve the overall performance of the system by reducing the free water content and regeneration energy consumption in the bed layer. According to the calculation formulas described in Chapter 2, relevant calculations were carried out on the 13X and VCZ-1.5 packed beds of zeolite, and the calculation results are as Figure 21 shown.

[0235] From Figure 21It can be seen that the free water content in the 13X zeolite system is 0.307 g / g, while the free water content of the VCZ-1.5 sample after coating is only 0.118 g / g, which is a 61.6% decrease compared to 13X. The energy consumption for removal during the regeneration stage of 13X zeolite is 693.79 kJ, and that of VCZ-1.5 is 265.97 kJ. After the 13X zeolite is coated with chitosan, its energy consumption for removal decreases by 61.2%. From the perspective of the structural changes of the material, for the zeolite coated with chitosan, a liquid-blocking and air-permeable membrane is added to its surface, which can prevent liquid free water from entering the interior of the zeolite particles during the steam generation stage. Therefore, after the steam generation stage ends, the free water in the adsorption bed layer only remains the water adsorbed by the zeolite and the free water between the particles. Due to the significant decrease in the free water in the adsorption bed, the energy consumption for removal also decreases accordingly, indicating that the zeolite after chitosan coating can effectively regulate the migration of free water in the adsorption heat pump system. The decrease in the free water content and its regeneration energy consumption indicates that the 13X zeolite after chitosan coating can effectively optimize the performance of the adsorption heat pump system.

[0236] 3. System Performance Evaluation

[0237] 3.1. System Heating System and Efficiency

[0238] The coefficient of performance (COP h ) is an important indicator for evaluating the heating performance of a heating-type heat pump, and its value reflects the heating efficiency of the heat pump system. The value of COP h is mainly determined by the heating capacity of the heat pump system and the energy consumed during the regeneration process. As Figure 22 shown, the COP h of 13X zeolite used in the adsorption heat pump system is 0.095. Although this value is low, for an adsorption heat pump system, its main purpose is to improve the quality of waste heat. Therefore, even if the COP h is low, the system can still play a good role. The COP h of VCZ-1.5 is 0.214, which is a 125.3% increase compared to 13X, indicating that the heating performance of the 13X zeolite after coating with chitosan has been significantly improved. Specifically, there are two reasons for the increase in the COP h of the chitosan-coated zeolite sample: Firstly, the chitosan membrane has a liquid-blocking and air-permeable effect, and the free water in the zeolite particles is significantly reduced, resulting in a significant reduction in the energy consumed for removing free water during the regeneration process of the heat pump system, effectively reducing the regeneration energy consumption. Secondly, during the steam generation stage, the liquid-blocking effect of the coated zeolite accelerates the rising rate of liquid water, accelerating the heat and mass transfer rate of the adsorption reactor, concentrating more adsorption heat at the top of the adsorption bed, and generating more high-temperature steam. Considering the above factors, the COP h performance of the system has been significantly improved.

[0239] Coefficient of Performance (COP) ex It reflects the energy availability. According to the calculation formula of COP ex it can be seen that the ambient temperature and the regeneration temperature remain the same in the system experiments of zeolite 13X and VCZ-1.5. Therefore, the temperature of the high-temperature steam and the COP h variation law will directly affect the COP ex variation. As Figure 22 shown, the COP of zeolite 13X ex is 0.179, and the COP of VCZ-1.5 ex has increased to 0.391 compared to 13X. This is attributed to the significant improvement of the COP ex while the temperature of the high-temperature steam has only decreased slightly. Therefore, the COP of VCZ-1.5 ex is consistent with the variation trend of the COP h . The results of the COP ex and the COP h show that the energy utilization efficiency has also been significantly improved when the chitosan-coated zeolite is used in the adsorption heat pump system.

[0240] 3.2 Heating Power

[0241] The heating power (SHP) of the system is used to describe the ability of the adsorbent to absorb or release heat during the temperature change process, reflecting the rate of heat output of the system. Figure 23 The SHP results of the zeolite 13X and VCZ-1.5 samples in the system experiments are shown. The SHP of zeolite 13X and VCZ-1.5 are 0.026 W / kg and 0.032 W / kg respectively. Compared with 13X, the SHP of VCZ-1.5 has increased by 23.1%. The chitosan-coated zeolite plays a role in blocking liquid and allowing air permeability during the steam generation stage, accelerating the rising speed of the liquid level, promoting heat and mass transfer in the bed layer, increasing the mass of the high-temperature steam, and reducing the time of the entire steam generation stage, thereby improving the SHP of the system. In summary, the chitosan-coated zeolite shows better performance in the rate of heat output than 13X.

[0242] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: within the scope of the spirit and essence defined by the claims of the present invention, the technical solutions described in the foregoing embodiments can be modified, or some or all of the technical features can be equivalently replaced; and these modifications or replacements are still within the scope defined by the claims of the present invention.

Claims

1. A preparation method of chitosan-coated zeolite, characterized in that, It includes the following steps: (1) Dissolve chitosan in an acetic acid aqueous solution to form a chitosan membrane solution, and then add vinyltrimethoxysilane to the chitosan membrane solution for cross-linking reaction to obtain a chitosan coating solution; (2) Pretreat 13X zeolite, and then use the chitosan coating solution to coat the pretreated 13X zeolite with the help of a coating machine and an air spray gun. The coated zeolite is subjected to drying and ethanol immersion treatment to obtain chitosan-coated zeolite.

2. The preparation method according to claim 1, characterized in that, In step (1), the weight-average molecular weight of chitosan is 200,000 - 1,000,000, preferably 1,000,000.

3. The preparation method according to claim 1, characterized in that, In step (1), the mass fraction of chitosan in the chitosan membrane solution is 1% - 3%, preferably 2%.

4. The preparation method according to claim 1, characterized in that, In step (1), the added volume of vinyltrimethoxysilane accounts for 0.1% - 2% of the volume of the chitosan membrane solution, preferably 1.5%.

5. The preparation method according to claim 1, characterized in that, In step (1), the cross-linking reaction temperature is 50 - 80°C, and the reaction time is 1 - 3 h.

6. The preparation method according to claim 1, characterized in that, In step (2), the pretreatment of 13X zeolite includes the following steps: calcine 13X zeolite to a bone-dry state, and then adsorb it to saturation.

7. The preparation method according to claim 1, wherein In step (2), the coating includes the following steps: add the pretreated 13X zeolite into a coating machine and roll it, and then use an air spray gun to spray the chitosan coating solution on the surface of 13X zeolite, and dry and cure to form a chitosan membrane; Preferably, the temperature of the coating machine is 60 - 80°C, the rotation speed of the coating machine is 10 - 50 r / min, and the inclination angle of the coating machine is 30 - 60°; Preferably, the spraying parameters include: spraying pressure is 300 - 500 kPa, spraying flow rate is 5 - 6 ml / min, nozzle diameter is 1 - 1.5 mm, spraying distance is 10 - 30 cm, and spraying width is 8 - 12 cm.

8. The preparation method according to claim 1, wherein, In step (2), the mass ratio of chitosan to zeolite in the chitosan coating solution is 6% - 10%, preferably 8%.

9. A chitosan-coated zeolite, characterized in that, Prepared by the preparation method according to any one of claims 1 - 8.

10. An application of the chitosan-coated zeolite according to claim 9 in a direct contact adsorption heat pump system.

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