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Study On Phase Diagram Of Ca(NO3)-Mg(NO32-H2O System And Design Of PCM Using Thermodynamic Model

Posted on:2015-04-21Degree:MasterType:Thesis
Country:ChinaCandidate:X L YuFull Text:PDF
GTID:2181330431450695Subject:Inorganic Chemistry
Abstract/Summary:PDF Full Text Request
Because of the continuous consumption of mineral resources and problem ofenvironmental pollution, people are becoming more and more aware of the importance ofefficient utilization and development of renewable energy. Phase change material (PCM),which can store and release latent heat during its phase transition and maintain at a constanttemperature, has become a hot topic in the research. It can effectively relieve the imbalancebetween supply and demand of energy in time and space. Molten salt hydrates PCM is one ofthe most widely application latent heat storage materials. The key used in commercialapplication is to find out PCMs with cheap price, safety, high performance and suitabletemperature. Traditional method for looking for PCMs is not only time–consuming, but alsodifficult to find real eutectic point. An efficient way to search PCM is to build complete phasediagram of relevant system combined with a thermodynamic model. Based on abundant, lowcost and high enthalpy value of calcium and magnesium salt, the phase diagram of theCa(NO3)2–Mg(NO3)2–H2O ternary system was studied. Also, combined with the experimentalsolubility data of the above system in this work, the phase diagram ofCa(NO3)2–Mg(NO3)2–LiNO3–H2O quaternary system was predicted by using thermodynamicmodel. A eutectic point which can be used as a potential phase change material was obtained.The main content of this paper are as follows:(1) A method for precise determination of calcium and magnesium in theCa(NO3)2Mg(NO3)2H2O ternary system was discussed. The total content of calcium andmagnesium was elaborately determined by EDTA mass titration for mixture solutioncontaining calcium nitrate and magnesium nitrate. The content of calcium can bequantitatively separated from magnesium using calcium oxalate gravimetric method. Then thecontent of magnesium can be obtained by subtraction. The results show that when the massratio of Mg2+:Ca2+is less than10:1, the relative errors of analysis results can be controlledwithin±0.3%. Compared with classic method, it meets requirement of solubility data withhigh accuracy.(2) Based on the inconsistency of solubility data from literature, the solubility isothermsof the Ca(NO3)2Mg(NO3)2H2O ternary system at273.15K、298.15K and323.15K havebeen elaborately determined by isothermal equilibrium method. And the correspondingphase diagram was obtained. The results show that there are two solubility branches forCa(NO3)2·4H2O and Mg(NO3)2·6H2O at273.15and298.15K. which are different from theexperimental data of literatures. Because of the high viscosity of calcium nitrate, only thesolubility branch corresponding to solid phase Mg(NO3)2·6H2O and solubility of Ca(NO3)2H2O system at323.15K were determined.(3) The modified BET model was supplied to calculate isotherms of theCa(NO3)2–Mg(NO3)2–H2O system at different temperatures only with binary parameters. Theresults show that with the increase of temperature, the deviation of calculated value andexperimental data increase. In order to obtain accurate information of the phase diagram ofthe system, the empirical interaction parameter ABwas fitted to the experimental data inthis work. Combined with binary parameters and AB, the isotherms ofCa(NO3)2–Mg(NO3)2–H2O system were recalculated. The complete phase diagram of theternary system from273.15K to373.15K was predicted and was compared with literaturevalue. The results show that the calculated values are in agreement with experimental data inthis work. To some degree, both of the predicted value and experimental data are reliable.(4) Combined with model parameters of the relative systems, the modified BET modelwas supplied to predict phase diagram of Ca(NO3)2–Mg(NO3)2–LiNO3–H2O system withoutany quaternary system experimental data. The ternary eutectic point Ca(NO3)2·4H2O(48.3%) Mg(NO3)2·6H2O (14.7%) LiNO3·3H2O (37%) with melting temperature291K wasobtained. The predicted eutectic material was measured by means of heat storage performance.Results show that in the environment of8~21C, the eutectic material maintain at15C formore than300minutes. Its heat enthalpy is102.8J·g–1. It can be taken as potential PCM withexcellent heat storage property. Meanwhile, the results suggest that thermodynamic model hasimportant theoretical significance for the design of molten salt hydrates as PCM.
Keywords/Search Tags:Calcium nitrate, Magnesium nitrate, Lithium nitrate, BET model, Solubilityphase diagram, Phase change material
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