Abstract K2CO3 is a promising salt for thermochemical heat storage. For a high performance, the thermochemical reaction must take place as close as possible to the equilibrium, while ensuring sufficient reaction rates. In this work, we studied the near-equilibrium hydration kinetics of K2CO3 and other salts (CuCl2, MgCl2 and LiCl). We proposed a generic two-step mechanism for the hydration of salts, consisting of (1): adsorption of water vapour and dissolution of ions from the initial phase (a wetting film) and (2): formation of the hydrate crystal (crystallisation from the wetting film). The two steps are assumed to be in momentarious balance during the hydration process. As a result, nucleation is rate limiting at low supersaturations of water vapour (inside the metastable zone), and water diffusion to the wetting film is rate limiting at high supersaturations (outside the metastable zone). We have seen that the vapour pressure of the wetting film stabilises at the metastable zone boundary p*. The driving force for hydration outside the metastable zone (MZ) is therefore the pressure difference between the atmospheric vapour pressure and the vapour pressure of the wetting film, p − p*. Non-Parametric Kinetic analysis of the hydration of K2CO3 indicates that nucleation plays a central role inside the metastable zone (at low supersaturations) as expected. Outside the MZ, the analysis suggests a steady conversion rate, in agreement with a water vapour diffusion limitation. The diffusion limited process at high supersaturations hardly depends on the temperature, but mainly on the pressure difference, as expected. It is further shown that the diffusion limited process can be characterised with an apparent activation energy. However, this apparent activation energy is in fact the hydration enthalpy and does not refer to a real energy barrier.

中文翻译：

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作为两步过程的盐水合：水吸附和水合物形成

摘要 K2CO3 是一种很有前途的热化学蓄热盐。为了获得高性能，热化学反应必须尽可能接近平衡，同时确保足够的反应速率。在这项工作中，我们研究了 K2CO3 和其他盐类（CuCl2、MgCl2 和 LiCl）的近平衡水合动力学。我们提出了盐类水合的通用两步机制，包括（1）：从初始相（润湿膜）中吸附水蒸气和溶解离子；（2）：形成水合物晶体（从润湿膜）。假设这两个步骤在水合过程中处于瞬时平衡。因此，在水蒸气的低过饱和度（亚稳态区域内）下，成核是限速的，并且水扩散到润湿膜的速度在高过饱和度（亚稳态区域外）受到限制。我们已经看到润湿膜的蒸气压在亚稳态边界 p* 处稳定。因此，亚稳区 (MZ) 外水合的驱动力是大气蒸气压与润湿膜的蒸气压之间的压差 p - p*。K2CO3 水合的非参数动力学分析表明，正如预期的那样，成核在亚稳态区域内（在低过饱和度下）起着核心作用。在 MZ 之外，分析表明转化率稳定，与水蒸气扩散限制一致。正如预期的那样，高过饱和度下的扩散限制过程几乎不取决于温度，而主要取决于压差。进一步表明，扩散受限过程可以用明显的活化能来表征。然而，这个表观活化能实际上是水合焓，并不是真正的能量屏障。