Molten salt (MS) mixtures are used for the transport (HTF-heat transfer fluid) and storage of heat (HSM-heat storage material) in Concentration Solar Plants (CSP). In general, alkaline and earth-alkaline nitrate/nitrite mixtures are employed. Along with its upper stability temperature, the melting point (liquidus point) of a MS mixture is one of the main parameters which defines its usefulness as a HTF and HSM medium. As a result, we would like to develop a predictive model which will allow us to forecast freezing points for different MS mixture compositions; thus circumventing the need to determine experimentally the phase diagram for each MS mixture. To model ternary/quaternary phase diagram, parameters for the binary subsystems are to be determined, which is the purpose of the concerned work. In a binary system with components A and B, in phase equilibrium conditions (e.g. liquid and solid) the chemical potentials (partial molar Gibbs energy) for each component in each phase are equal. For an ideal solution it is possible to calculate the mixing (A+B) Gibbs energy:ΔG = ΔH − TΔS = RT(xAlnxA + xBlnxB) In case of non-ideal solid/liquid mixtures, such as the nitrates/nitrites compositions investigated in this work, the actual value will differ from the ideal one by an amount defined as the “mixing” (mix) Gibbs free energy. If the resulting mixtures is assumed, as indicated in the previous literature, to follow a “regular solution” model, where all the non-ideality is considered included in the enthalpy of mixing value and considering, for instance, the A component:
ΔG≡0=(ΔHA−TΔSA)+(ΔH¯mixAL−TΔS¯mixAL)−(ΔH¯mixAS−TΔS¯mixAS)
where the molar partial amounts can be calculated from the total value by the Gibbs Duhem equation:
(ΔH¯mixAL=ΔHmix−XBLdΔHmixdXBL)L;(ΔH¯mixAS=ΔHmix−XBSdΔHmixdXBS)S
and, in general, it is possible to express the mixing enthalpy for solids and liquids as a function of the mol fraction:
ΔHLmix=XALXBL(a1+b1XAL+c1XALXBL),ΔHSmix=XASXBS(a2+b2XAS+c2XASXBS)

From the latter expressions it can be possible to modelize the phase diagram of a binary mixtures by using the a,b and c couples of parameters. To calculate those coefficients a method commonly employed in literature is to measure the mixing enthalpies, or to use one reported of the enthalpy of mixing (for instance for the liquid state) and calculate the other one using the phase diagram points. A direct ΔHmix (in solid or liquid phase) measurement can be difficult to carry out using common DSC equipment generally present in research laboratories. In fact, such determinations can be, in principle, performed, but the obtained data will be affected by large experimental errors. On the other hand, it is possible to obtain values with great precision regarding the algebraic sum of mixing enthalpies and the phase diagram trend. For this reason, only the phase diagrams are proposed to be used to calculate a, b, c parameters, and, subsequently, the total (liquid-solid algebraic sum) enthalpy of mixing will be employed to verify their validity.

At this aim, a C++ code was assessed and used. Three binary mixtures were considered by combining NaNO3, KNO3 and NaNO2.

Topics
Phase transitions, Continuum mechanics, Enthalpy, Thermodynamics, Associated liquids, Chemical properties, Educational assessment
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