Isopiestic Determination Of The Activities And Thermodynamic Simulation On The System Lithium Nitrate-potassium Nitrate-water

Facing energy crisis, more new alternative energy is needed to search for. Thermal energy storage technologies store solar energy, industrial waste heat and other energy firstly, and then reuse them, which protect the environment and save the energy. Large area in northern China, a long time use of boiler burning hot water transported to warm the cold room, not only consumes a lot of coal and natural gas, but also produces large amounts of harmful gases. However, in most area in northern China, there are abundant sunshine during the day and the temperature is much lower in the night. Phase change materials store thermal energy via latent heat, which absorb and release thermal energy when their phases change, can be conveniently used for years. Thus our interest focuses on exploring and designing room-temperature phase change energy storage materials which are cheap and safe and will be used in the buildings for energy-saving. During a variety of phase change materials, the inorganic hydrated salts are safer and most are much cheaper than other types. From a great number of inorganic multi-component aqueous salt solution systems, we newly found the LiNO3-KNO3-M(NO3)n-H2O system whose phase-change temperature or eutectic temperature might between 15℃and 25℃.In order to determine the composition and temperature of the eutectic point of the system, we did a lot of experimental and theoretical research as follows:1. Water activities of KNO3-H2O,LiNO3-H2O and LiNO3-KNO3-H2O were determined by isopiestic method at 298.15 K. The reference was NaCl when the ionic strength below 6.7 mol·kg-1 while H2SO4 was used as reference when the ionic strength was larger. We accurately determined the water activities of the three systems from dilute to concentrated solution in a water bath whose temperature was kept at 298.15±0.01 K. We also determined the solubility and water activity of KNO3 at 298.15 K under the same condition. The reliability of the data and the method was verified toward the comparison to the data from literatures, and the use of second reference CaCl2.2. Water activities of KNO3-H2O,LiNO3-H2O and LiNO3-KNO3-H2O were determined by isopiestic method at 273.15 K. The reference was NaCl when the ionic strength below 5.6 mol·kg-1, while H2SO4 was used as reference when the ionic strength was larger. We accurately determined the water activities of the three systems from dilute to concentrated solution in a bath (the medium is the mixture of industrial alcohol and water by the ratio of 1:4 whose temperature was kept at 273.15±0.01 K. CaCl2 was used as second reference which verified the reliability. 3. Pitzer-Simonson-Clegg(PSC) model was selected to represent the measured water activities of single electrolyte solutions. We determined the optimized pure electrolyte parameters by fitting the PSC model to the measured binary water activities, then recalculated the water activities and compared to the measured ones, finding that a good agreement exist between the measured water activities and the calculated ones by the model.4. Pitzer-Simonson-Clegg model was selected to predict and calculate the thermodynamic properties of the LiNO3-KNO3-H2O system. We determined a first minimal set of mixture parameters by fitting the PSC model to the measured ternary water activities, then a second set of mixture parameters by fitting the PSC model to the measured ternary water activities and the experimental isothermal solubilities. First of all, we only used the pure electrolyte parameters to predict the solubility phase diagram of the LiNO3-KNO3-H2O ternary system, and found out great deviation between the predicted values and the measured water activities, which means that the binary model can not predict the solubility property of the ternary system. Then we did the prediction with both the pure electrolyte parameters and the first set of mixture parameters, still the result was not very good, maybe the error of the measurement of the water activity came out because of the increasing viscosity. Finally, we calculated the solubility phase diagram with the second set of mixture parameters and the pure electrolyte parameters, and the agreement was very good. The parameters we got would be used to predict the solubility phase diagram of the system LiNO3-KNO3-M(NO3)n-H2O.