The Binary Alkali Nitrate and Chloride Phase Diagrams: NaNO 3 -KNO 3 , LiNO 3 -NaNO 3 , LiNO 3 -KNO 3 , and NaCl-KCl
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MOLTEN salts such as alkali and alkaline nitrates/ nitrites/chlorides are used as solar thermal energy storage media, namely heat transfer and storage fluids (HTF) in a thermal energy storage system (TES) in concentrated solar power (CSP) plants. HTF is used to produce steam in a boiler via a heat exchanger to run a steam turbine for electricity generation. Broader operating temperature range (low melting point and high decomposition point) of HTF is desirable for achieving higher power cycle efficiency (solar to electric conversion efficiency) for CSP plants. The melting point of HTF should ideally be close to ambient temperature for reducing the initial energy requirement for melting.[1,2] Understanding of the appropriate phase diagrams of multicomponent molten nitrate and/or chloride salts to MEHEDI BIN MOHAMMAD, PETER CADUSCH, GEOFFREY ALAN BROOKS, and M. AKBAR RHAMDHANI are with the Department of Mechanical and Product Design Engineering, Swinburne University of Technology, Hawthorn, VIC, 3122, Australia. Contact e-mail: [email protected]. Manuscript submitted September 19, 2017. Article published online September 10, 2018. 3580—VOLUME 49B, DECEMBER 2018
obtain low melting point eutectic mixture is important when designing a HTF. Mathematical formulation or software simulation can limit the need for costly and difficult experimentation for obtaining a desirable composition of higher order components.[3,4] The higher order systems can be computed using their simple constituent binary phase systems. Thus, understanding binary phase equilibria is important to formulate the multicomponent systems.[5–9] Generally, phase diagrams can be constructed through equilibration experiments using techniques such as differential scanning calorimetry (DSC)/differential thermal analysis (DTA), dilatometry, X-ray diffraction (XRD), hot-stage microscope, and electron probe microanalysis (EPMA).[10–14] Phase diagrams can also be obtained by calculation using assumed solution model in a computer simulation program, such as MTDATA, HSC, FactSage, ThermoCalc, Lukas, Calphad, and Matlab.[10,15–20] A mathematical modeling of binary ionic system to determine the congruently melting eutectic composition has been based on different numerical approaches involving various nonideal mixing models such as the Margules equation, excess Gibbs energy methods including nonrandom two liquid equation (NRTL),
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modified Van Laar equations, Wilson equation, universal quasi-chemical equation (UNIQUAC), and lattice theory or quasi-chemical equation.[21–33] Binary nitrate solid–liquid phase equilibrium calculations are usually modeled as a neutral, noninteracting, and nonaqueous mixtures.[26] Most recent modelings of various nitrate and chloride systems have been based on the regular solution model.[34–51] The NaNO3-KNO3 system has been studied heavily compared to the other binary alkali systems.[14,40,45–54] Zhang et al.[42] studied the binary NaNO3-KNO3 using two models, the Henrian and the regular solution
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