Experimental phase diagram study of the binary KCl-Na₂CO₃ system

Highlights

• A complete phase diagram of the KCl-Na₂CO₃ has experimentally been developed and validated.

• Liquidus temperatures have been identified using a new simple method based on DSC measurement.

• A precise eutectic composition has been experimentally identified based on the Tammann diagram.

• Thermo-physical properties of eutectic and off-eutectic compositions have been measured by DSC.

• The existence of a solid solution in the KCl-Na₂CO₃ system has been confirmed based on the Tammann diagram and SEM-EDS.

Abstract

The phase diagram of the KCl-Na₂CO₃ system was experimentally investigated for the complete range of compositions using differential scanning calorimetry (DSC). The results are in excellent agreement with the phase diagram that is computed by the two-sublattice models. Both DSC experimental curves and the Tammann diagram were used to determine the eutectic point and the results were compared. The latter provides more reliable and accurate information with the eutectic composition found to be 50.41 ± 0.78 wt%Na₂CO₃(41.67 ± 0.68 mol% Na₂CO₃). Physical properties at the eutectic point including solidification/melting temperature and latent heat of fusion/solidification were also studied. The results show a melting point of 581.76 ± 0.57 °C and latent heat of fusion of 260.40 ± 8.69 J/g. The occurrence of subcooling with a temperature difference of 5.58 ± 0.67 °C was also identified. The existence of a solid solution was confirmed in the current investigation using the Tammann diagram and Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM-EDS).

Introduction

High-temperature eutectic molten salt phase change materials (PCMs) are receiving growing attention in thermal energy storage (TES) technologies. This is due to their potential for higher temperature energy storage capacity, an increase in system efficiency and further cost reduction in heat and power generation [[1], [2], [3], [4], [5], [6]]. Of most interest to concentrating solar power (CSP) developers [5,7,8] in this space is the development of storage materials to provide dispatchable energy from the next generation of receivers and power blocks at temperatures between 550−720 °C. To evaluate the potential suitable PCMs knowledge of the phase diagrams, phase equilibrium charasteristics and the thermodynamic properties of the material is required [9]. That is because phase diagram provides important information on the indication of key temperature-composition points, e.g. eutectic, peritectic and congruent meltiong compositins [10]. The techniques used for the devolpmnet of phase diagram, e.g. thermal analysis, microscopic and x-ray diffraction method, help fundamentally undreastand further thermo-physical and equilibrium characteristics of PCMs. These include enthalpy change during the phase transition, melting/freezing temperature, phase separation, super-cooling state and chemical stability, which are a pre-requisite for PCM selection and designing latent heat storage [10]. The overall objective of this paper is to provide this knowledge.

Thermal analysis of different PCMs, such as paraffines and hydrate salts have been previously studied based on dynamic DSC measurement [[11], [12], [13], [14]]. It was found that the melting temperature process is significantly affected by heating and cooling rates using this method. The typical heating/cooling rates used for this method is 0.5−20 K/min. The peak and endpoint temperature of DSC curves were found to be higher for the use of higher rating rate. Therefore, a wider phase transition range was obtained. Rathgeber et al. [12] recommended a slow dynamic heating rate of 0.5 K/min is required to determine the phase transition temperature with higher accuracy. In regards to high-temperature molten salt PCMs, less attention has been paid to the significance of heating/cooling rate on the characterizing of DSC curves for the identification of accurate melting temperature process, phase diagram, liquidus temperature and eutectic point. For example, the earlier work of Nakayoshi et al. [15] and Hames et al. [16] on developing the phase diagram of the LiCl-KCl-UCl₃ system, identified the melting temperature transition based on the DSC method focusing on the initial and final points of deviation from the baseline with the use of only one heating/cooling rate of 5 K/min. For thermophysical characterization of different PCMs, the effect of heating and cooling rates showed different results. In regards to the enthalpy of fusion for paraffine based PCMs, Jin et al. [11] and Rady et al. [13] reported it is independent on the heating rate. However, Sun et al. [14] reported it is affected by heating rate but without any established pattern. For a molten salt system e.g. KCl-LiCl and Na₂CO₃-NaCl system, it was found that the effect of heating and cooling rates in the range 5−20 °C/min on the latent heat of fusion is not significant with the experimental uncertainty of ∼5 % [17,18]. Findings in the literature showed that the effect of heating/cooling rates for the accurate identification of thermal characterization of PCMs need to be carefully given attention where dynamic DSC measurement is carried out. Therefore, this study aims to accurately measure thermal behaviour (e.g, phase diagram, eutectic point and thermal characterization) of high-temperature eutectic molten salt PCM based on the dynamic DSC measurement.

The potassium-sodium chloride and/or carbonate salt binary, ternary or quaternary mixtures (NaCl-KCl-Na₂CO₃-K₂CO₃) could be potentially considered for a high-temperature TES over 550 °C based on their eutectic melting temperature [19,20]. The phase diagram of the NaCl-KCl system has previously been studied and their thermodynamic properties have been measured at the eutectic point, 50 mol% NaCl-50 mol% KCl [21]. The potential of the two binary eutectic systems of 40.55 wt%NaCl- 49.45 Na₂CO₃ [18] and 52.81 wt%K₂CO₃-47.19 wt% Na₂CO₃ [22] as a suitable high-temperature PCM has also been previously confirmed, focusing on the thermophysical properties of melting point, latent heat and heat capacity, together with the thermal stability of the system.

The experimental phase diagram of the KCl-Na₂CO₃ system based on thermal analysis and visual polythermal, which was studied between 1910 and 1958, has previously been reviewed by Lindberg at al. [20] and Yaokawa et al. [19]. Among different experimental studies, only the work of Sato [23] reported both solidus and liquidus temperature. Regarding the liquidus temperature, different studies showed some inconsistencies in the results. However, the reason is not clear due to the lack of information on the detailed method used for developing the phase diagram. Therefore, part of the current investigation aims to develop a complete phase diagram of the KCl-Na₂CO₃ system based on DSC experiments focusing on measuring both liquidus and solidus curves. Volkova and Bergmann evaluated the eutectic composition of 44.52 mol%Na₂CO₃-55.48 mol%KCl at a melting temperature of 588 °C by a stable diagonal cross-section of Na₂CO₃-KCl on the liquidus projection of reciprocal system [24]. Meanwhile, Yaokawa et al. [19] calculated the phase diagram of the Na₂CO₃-KCl system based on two sublattice models and predicted the eutectic composition of ∼ 42.62 mol% Na₂CO₃. This indicates there is a difference of approximately 2 mol% in the reported eutectic composition. Given the discrepancies in the literature and given that there is no experimental data available on the eutectic composition of the Na₂CO₃-KCl system, this paper aims to accurately determine the eutectic point using a DSC method together with a Tammann plot assessment. To assess the suitability of this system as a potential high-temperature PCM, a preliminary study on the thermophysical properties of the salt system at a measured eutectic and off-eutectic compositions has been carried out. In addition, the existence of the solid solution pure component -rich composition of this system is yet to be experimentally studied. Therefore, SEM-EDS and DSC coupled with a Tammann diagram have been used to fill this gap.