EXPERIMENTAL INVESTIGATION OF POLYTHERMAL SOLUBILITY AND PHASE BEHAVIOR IN THE CaCl₂–METHYLDIETHANOLAMINE–WATER TERNARY SYSTEM

ABSTRACT

The polythermal solubility and phase equilibria of the CaCl₂–CH₃N(C₂H₅OH)₂–H₂O ternary system were systematically investigated over the temperature range from −73.0 to 45.0 °C using the visual–polythermal method. The system was analyzed through its constituent binary subsystems and seven internal sections, enabling construction of a comprehensive polythermal solubility diagram. The crystallization fields of ice, CaCl₂·2H₂O, CaCl₂·4H₂O, CaCl₂·6H₂O, and CH₃N(C₂H₅OH)₂ were identified and their phase boundaries were determined. The results demonstrate that the system belongs to a simple eutectic type, characterized by independent crystallization of components without formation of new chemical compounds. A single invariant ternary eutectic point was established, where ice, CaCl₂·6H₂O, and CH₃N(C₂H₅OH)₂ coexist in equilibrium. The obtained phase diagram provides a thermodynamically consistent description of the system and offers a reliable basis for predicting crystallization limits and evaluating potential technological applications of aqueous calcium chloride–amine systems.

АННОТАЦИЯ

Политермальная растворимость и фазовые равновесия тройной системы CaCl₂–CH₃N(C₂H₅OH)₂–H₂O были систематически исследованы в температурном интервале от −73,0 до 45,0 °C с использованием визуально-политермального метода. Система была изучена на основе анализа бинарных подсистем и семи внутренних сечений, что позволило построить детальную политермальную диаграмму растворимости. Были установлены области кристаллизации льда, гидратов CaCl₂ (CaCl₂·2H₂O, CaCl₂·4H₂O, CaCl₂·6H₂O) и CH₃N(C₂H₅OH)₂, а также определены границы фазовых областей. Показано, что система относится к простому эвтектическому типу, при котором компоненты кристаллизуются независимо, без образования новых химических соединений. Установлена единственная инвариантная тройная эвтектическая точка, в которой лёд, CaCl₂·6H₂O и CH₃N(C₂H₅OH)₂ находятся в равновесии. Полученная фазовая диаграмма обеспечивает термодинамически согласованное описание системы и может служить основой для прогнозирования пределов кристаллизации и оценки технологического применения водных растворов хлорида кальция и метилдиэтаноламина.

Keywords: Polythermal solubility; Phase diagram; Ternary aqueous system; Calcium chloride; Methyldiethanolamine; Crystallization; Phase equilibria; Eutectic system.

Ключевые слова: политермальная растворимость; фазовая диаграмма; тройная водная система; хлорид кальция; метилдиэтаноламин; кристаллизация; фазовые равновесия; эвтектическая система.

INTRODUCTION. Calcium chloride–based aqueous systems are widely applied in chemical, petrochemical, gas treatment, refrigeration, and deicing technologies due to their high solubility and significant hygroscopic properties. At the same time, methyldiethanolamine (MDEA), CH₃N(C₂H₅OH)₂, is extensively used in gas purification processes for selective absorption of acidic gases such as CO₂ and H₂S. In many industrial operations, these components may coexist in aqueous environments, where their phase behavior, crystallization characteristics, and thermodynamic interactions significantly affect process stability and efficiency.

Understanding multicomponent aqueous systems containing inorganic salts and organic amines is essential for the design and optimization of technological processes operating under variable temperature conditions. In particular, phase equilibria and solubility relationships determine crystallization limits, freezing behavior, and the formation of hydrate phases. Accurate phase diagrams allow prediction of stable operating regions and prevention of undesired precipitation during storage or transport.

Binary systems such as CaCl₂–H₂O have been extensively investigated, and their hydrate formation (CaCl₂·2H₂O, CaCl₂·4H₂O, CaCl₂·6H₂O) is well documented in the literature [1]. Similarly, the physicochemical properties of aqueous MDEA solutions, including density, viscosity, and solubility behavior, have been reported in previous studies [2–4]. However, despite the availability of data for individual binary systems, comprehensive investigations of the ternary CaCl₂–MDEA–H₂O system remain limited. In particular, systematic studies covering wide temperature ranges and detailed polythermal solubility diagrams for this system are scarce.

From both theoretical and practical perspectives, investigation of the CaCl₂–MDEA–H₂O ternary system is important for several reasons. First, the coexistence of inorganic electrolytes and organic amines can significantly alter phase stability and crystallization boundaries. Second, temperature-dependent solubility behavior affects freezing points and hydrate formation, which are critical in low-temperature technological processes. Third, knowledge of eutectic compositions and invariant points enables better control of multicomponent solution systems.

In this study, the physicochemical behavior of the CaCl₂–CH₃N(C₂H₅OH)₂–H₂O ternary system was systematically investigated. The system was analyzed through its constituent binary subsystems and seven internal sections to construct a comprehensive polythermal solubility diagram over a wide temperature range. The crystallization fields of ice, CaCl₂ hydrates, and methyldiethanolamine were identified, and phase boundaries were determined. The work aims to provide a thermodynamically consistent description of phase equilibria in this ternary aqueous system and to establish a reliable basis for its potential technological applications.

MATERIALS AND METHODS.

Materials. Industrial-grade calcium chloride (CaCl₂) and chemically pure methyldiethanolamine (MDEA, CH₃N(C₂H₅OH)₂) with a main substance content of 98.0% were used. MDEA is a yellow, amorphous liquid and is readily miscible with water. The solubility of MDEA in water was determined over a range of temperatures and used to construct the binary CH₃N(C₂H₅OH)₂–H₂O phase (solubility) diagram (Figure 1).

Experimental approach

 Polythermal solubility was investigated by the visual–polythermal method. This technique is based on visually observing (i) the temperature at which the first crystals appear during uniform cooling of a solution, or (ii) the temperature at which the last crystals completely disappear during slow heating under continuous stirring.

Figure 1. Polythermal solubility diagram of the CH₃N(C₂H₅OH)₂–H₂O binary system

Apparatus and temperature control. Measurements were performed in a stoppered test tube equipped with a platinum or glass stirrer and a thermometer with 0.1 °C graduation. To ensure uniform cooling/heating, the test tube was placed inside an external sleeve (jacket) containing a cooling mixture; heating was also performed through the same jacket arrangement. Low-temperature conditions were achieved using Dewar vessels charged with liquid nitrogen or dry ice, depending on the target temperature.

Temperature measurement

Two glass thermometers were used to cover the full experimental range: a TN-6 mercury thermometer (−30 to +60 °C) and a TL-15 alcohol thermometer (−100 to +20 °C). This configuration enabled solubility measurements across subzero and near-ambient temperatures relevant to the investigated system.

Preparation of solutions. Binary and ternary mixtures were prepared by weighing the required amounts of CaCl₂, MDEA, and water to achieve targeted compositions. Each mixture was thoroughly homogenized under stirring before thermal cycling. For each composition, crystallization onset (upon cooling) and complete dissolution (upon heating) were determined visually under continuous mixing, and the corresponding equilibrium temperatures were recorded following stabilization.

Construction of phase/solubility diagrams

The ternary CaCl₂–CH₃N(C₂H₅OH)₂–H₂O system was examined by decomposing it into its binary subsystems and by applying seven internal composition sections to reliably map the phase boundaries over the investigated temperature interval (−73.0 to 45.0 °C).  Specifically, three internal sections were drawn from the CaCl₂ vertex toward the CH₃N(C₂H₅OH)₂–H₂O side, and four internal sections were drawn from the CH₃N(C₂H₅OH)₂ vertex toward the CaCl₂–H₂O side.  The collected equilibrium points along these sections were used to delineate crystallization fields and phase boundaries in the ternary diagram.

Isotherms for interpretation. To characterize temperature-dependent changes in phase relations, solubility isotherms were additionally plotted at selected temperatures spaced by 10 °C: −60, −50, −40, −30, −20, −10, 0, 20, 30, and 40 °C. These isotherms were used to track the displacement of phase boundaries with temperature and to provide a clearer interpretation of polythermal behavior.  Figure 1 (to be inserted by the author). Figure 1 presents the binary CH₃N(C₂H₅OH)₂–H₂O solubility diagram constructed from the visual–polythermal measurements. In the context of this work, this binary diagram serves two methodological roles: (i) it provides the reference solubility behavior of MDEA in water needed for designing ternary compositions near the MDEA–H₂O edge, and (ii) it supports consistent interpretation of phase transitions in the ternary system when approaching the CH₃N(C₂H₅OH)₂–H₂O side from the CaCl₂ vertex.

RESULTS AND DISCUSSION.

The polythermal solubility of the CaCl₂–CH₃N(C₂H₅OH)₂–H₂O ternary system was systematically investigated through binary subsystems and seven internal sections over a temperature range from −73.0 to 45.0 °C. Based on the collected experimental data, a comprehensive polythermal phase diagram was constructed, allowing identification of crystallization fields and phase boundaries within the system.

Phase Equilibria and Crystallization Fields

The constructed phase diagram reveals distinct crystallization regions corresponding to ice, CaCl₂ hydrates (CaCl₂·2H₂O, CaCl₂·4H₂O, CaCl₂·6H₂O), and CH₃N(C₂H₅OH)₂. Each component preserves its individuality during crystallization, and no new chemical compounds between CaCl₂ and MDEA were observed. This behavior indicates that the system belongs to a simple eutectic type, where components crystallize independently without forming complex double salts. All crystallization fields converge toward a single invariant ternary eutectic point. At this composition and temperature, ice, CaCl₂·6H₂O, and CH₃N(C₂H₅OH)₂ coexist in thermodynamic equilibrium and crystallize simultaneously. The presence of this invariant point confirms the thermodynamic stability of the system and the absence of additional solid phases.

Analysis of Invariant and Boundary Points

The experimentally determined compositions of liquid phases and corresponding crystallization temperatures are summarized in Table 1.

Table 1.

 Compositions of liquid phases and crystallization temperatures of invariant and boundary points in the CaCl₂–CH₃N(C₂H₅OH)₂–H₂O system.

Table 1 presents the concentrations of CaCl₂, CH₃N(C₂H₅OH)₂, and H₂O in the liquid phase (in wt.%) together with the corresponding crystallization temperatures and the solid phases formed at equilibrium. The table includes both binary and ternary invariant points as well as boundary points between crystallization regions.

From Table 1, it can be observed that the lowest crystallization temperatures correspond to compositions near the ternary eutectic region. At these compositions, three solid phases coexist, confirming the invariant character of the eutectic point. As the composition shifts toward higher CaCl₂ content, crystallization of CaCl₂ hydrates dominates, while increasing MDEA concentration leads to expansion of the CH₃N(C₂H₅OH)₂ crystallization field. The data also show that CaCl₂·6H₂O plays a central role in low-temperature phase equilibria. The crystallization boundaries involving this hydrate extend over a relatively wide composition range, indicating its thermodynamic stability under the investigated conditions. In contrast, the crystallization field of MDEA is comparatively narrower and shifts significantly with temperature decrease.

Temperature Influence and Isothermal Sections

To further analyze temperature-dependent phase behavior, solubility isotherms were constructed at selected temperature intervals. These isothermal sections demonstrate systematic displacement of crystallization boundaries with decreasing temperature. As temperature decreases, the ice crystallization field expands toward higher water concentrations, confirming the dominant influence of water in low-temperature equilibria. The CaCl₂-rich region remains comparatively broad over the investigated temperature range, which is consistent with the high solubility and strong hydration tendency of calcium chloride. The interaction between CaCl₂ hydrates and MDEA does not result in the formation of new solid compounds, reinforcing the classification of the system as a simple eutectic type.

General Discussion

Overall, the obtained polythermal solubility diagram provides a thermodynamically consistent representation of phase equilibria in the CaCl₂–CH₃N(C₂H₅OH)₂–H₂O system. The experimental results demonstrate that phase behavior is primarily governed by hydration phenomena of CaCl₂ and temperature-dependent crystallization of water and MDEA. The absence of new compound formation simplifies thermodynamic interpretation and supports the practical applicability of the system in technological processes operating under varying thermal conditions.

CONCLUSION

A comprehensive polythermal study of the CaCl₂–CH₃N(C₂H₅OH)₂–H₂O ternary system was carried out within a wide temperature range. Based on systematic experimental measurements, a detailed phase diagram was constructed, allowing identification of crystallization regions, phase boundaries, and invariant equilibrium points. The results confirm that the system exhibits simple eutectic behavior, with each component crystallizing independently without formation of additional solid compounds. The invariant ternary eutectic point was determined, and the roles of CaCl₂ hydrates, ice formation, and methyldiethanolamine crystallization in controlling phase equilibria were clarified. The obtained experimental data and constructed polythermal diagram provide a reliable thermodynamic framework for understanding phase behavior in multicomponent aqueous salt–amine systems. These findings can be applied in the design and optimization of technological processes involving concentrated calcium chloride and methyldiethanolamine solutions, particularly under low-temperature operating conditions.

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