Application Of Double-salt Composite Activated Carbon Fibres In A Multi-circulation Adsorption Atmospheric Water Harvesting Systems

Fresh water is one of the important substances on which human beings depend for survival and development.However,freshwater resources are unevenly distributed,and the scarcity of freshwater resources is becoming increasingly serious due to population growth,social development,climate warming and environmental pollution.At the same time,the atmosphere is rich in water,about six times the total amount of global surface water,and is not limited by the geographical environment,the impact on the environment during water production is small,and it is a good source of fresh water,so atmospheric water harvesting technology is an important way to alleviate water tension.Sorption-based atmospheric water harvesting（SAWH）offers an accessible method for fresh water harvesting.Rapid adsorption-desorption kinetics and mass production of adsorbents are core to realizing high water productivity.Although metal organic frameworks（MOFs）,hygroscopic gels and composite adsorbents based on MOFs and gels have a very high water yield of 1.30～2.12g_water/g_sorbent/day per unit of material due to their superior hygroscopic capacity or fast adsorption/desorption kinetics),they suffer from complex preparation processes,high costs and are currently difficult to mass produce.The daily water yield is only 0.046～0.75g_water/day.In contrast,although porous material-hygroscopic salt（Such as Li Cl,Ca Cl₂,etc.）composite adsorbents have a lower daily water yield per unit of adsorbent,they have a higher daily water yield instead due to the simple preparation process,low cost and the ability to achieve batch production.For example,composite adsorbents consisting of activated carbon fibres,silica sol and Li Cl can be used to obtain daily water withdrawals of up to 3.9～7.7 g_water/day.It should be noted that the strong hygroscopicity of Li Cl is a double-edged sword,which on the one hand can greatly increase the hygroscopicity of porous materials,but on the other hand also poses the risk of corrosion of the system by salt solution overflow and the high desorption enthalpy resulting in the need for high desorption temperatures.In recent years,organic weak acid salts have started to be used in solid dehumidification systems due to their low corrosiveness.Compared to Li Cl-modified composite adsorbents,organic weak acid salt-modified composite adsorbents have a lower adsorption capacity but better adsorption/desorption kinetics.In addition,some research in dehumidification and adsorption atmospheric water harvesting technology has shown that the inherent properties of one salt can be modified by the addition of another salt.The selection of a suitably proportioned dual salt composite can improve dehumidification performance.Therefore,the combination of Li Cl and organic weak acid may be a way to increase the daily water harvesting of adsorbent per unit mass in rapid circulation atmospheric water harvesting system.In this paper,a series of double salt composite adsorbents were prepared and their application in adsorbent atmospheric water harvesting system was studied.The main work and conclusion of this paper are as follows:（1）Comparison and analysis of the operating mechanisms and working principles of various types of adsorption atmospheric water harvesting systems,theoretical analysis of the operating strategies of adsorption atmospheric water harvesting systems and comparison of the advantages and disadvantages of single and multiple cycles.（2）Potassium formate and sodium acetate modified activated carbon-silica sol composite adsorbents AS/pf and AS/S were prepared respectively,and their adsorption/desorption performance,theoretical daily water capacity and economy were tested and analyzed,and compared with Li Cl-modified composite adsorbent AS/L.It was found that although the adsorption capacity of AS/pf and AS/S was not as good as that of AS/L,the adsorption/desorption rate was faster,and AS/S was particularly suitable for multi-cycle the adsorption-based atmospheric water harvesting system with a cycle time of 3h.（3）Seven different ratios of double salt composite adsorbents were prepared using lithium chloride and sodium acetate and compared with single salt composite adsorbents AS/L and AS/S.When compared with the AS/L composite adsorbents,the equilibrium adsorption volumes of the double salt composite adsorbents were reduced to varying degrees.However,when the ratio of lithium chloride to sodium acetate is appropriate,the release of water vapour can be significantly increased thanks to faster adsorption and desorption rates.For example,at 25°C&70%RH,the adsorption rate of the double salt sample（AS/LS2～5）was 8%to 31%higher than that of the single salt sample（AS/L）;at 70°C&20%RH,the desorption rate of the double salt sample（AS/LS2～5）was 76%to 140%higher than that of AS/L.（4）Under the same adsorption/desorption time ratio,with the increase of the specific gravity of CH3COONa,the water yield of double-salt samples first increased and then decreased.When the cycle time is 180 min,the optimal adsorption/desorption time ratio is 2:1.The sample AS/LS3 has the largest water yield under the four adsorption/desorption time ratios,which is 69%higher than that of AS/L,and the theoretical daily water yield in multi-cycle mode can reach 3.5 kg_water/kg_adsorbent·day under the working conditions of 25°C&70%RH adsorption and 70°C&10%RH desorption,which is 330%higher than that in single-cycle mode.（5）A small and sustainable adsorbed atmospheric water harvesting device was developed,and a new"1+n"mode was proposed and compared with the continuous cycle mode and batch treatment cycle mode.Daily water harvesting in islands and arid areas of China was predicted by using AS/LS3,a double salt composite adsorbent with the best comprehensive performance,and a new"1+n"cycle model.Indoor atmospheric water harvesting experiments were carried out to test the water harvesting effect of the system under experimental conditions,and to verify the possibility of water harvesting of the material.