Investigation into the Hydration Behavior of K₂CO₃ Packed Beds:An NMR Study

K2CO3 is seen as a promising heat storage material, available for applications in the domestic sector. For practical purposes, the material is hereby often employed in a packed bed containing millimeter-sized particles. To gain more insight into the hydration behavior of these packed beds, quantitative NMR measurements, capable of following the in-situ hydration behavior, are presented for the first time. It is found that hydration behavior varies significantly, depending on the specific hydration conditions that are chosen. At low airflows hydration is found to proceed via a hydration front, while higher airflows cause the hydration front to widen. Since an increase in flow rate coincided with an increase in the supplied water vapor, hydration is eventually found to proceed in a uniform manner. A comparison between TGA and NMR measurements shows that the overall packed bed hydration kinetics hereby transition to the reaction kinetics of single K2CO3 particles. Graphical Abstract: [Figure not available: see fulltext.]</p

Hydration fronts in packed particle beds of salt hydrates:Implications for heat storage

Hydration of packed beds of salt hydrate particles underpins the working principle of low temperature thermochemical energy storage (TCES). Typically, the salt hydrate particles are millimeter sized. An isothermal model for packed bed hydration is formulated, and it is shown that for millimeter sized particles hydration can be described as an advection-reaction process. Traveling wave solutions have been obtained that describe a moving hydration front. The speed of the hydration front is about five orders of magnitude slower than the air velocity in the particle bed. The width of the hydration front is under relevant TCES conditions between 10 and 100 cm. Therefore, hydration fronts will only develop in meter-sized packed beds. A constant hydration rate (and power output) is related to the existence of a traveling hydration front. Therefore, constant hydration rates and power output can only be expected for meter sized TCES reactors. Finally, the influence of temperature gradients is analyzed for the case that the front width is smaller than the bed size. The temperature lift and power output are calculated. Future steps should involve a more detailed description of temperature gradients and a quantitative analysis of finite size effects.</p

Accelerating the hydration reaction of potassium carbonate using organic dopants

Potassium carbonate has recently been identified as a promising candidate for thermochemical energy storage. However, as for many salt hydrates, the reaction kinetics is limited, and moreover, the hydration transition is kinetically hindered due to a metastable zone, involving limited mobility. This work aims to improve mobility by using organic potassium dopants, it shows that doping with potassium-formate and -acetate, can accelerate the hydration reaction. It has been shown that these dopants can enhance the hydration rate by two mechanisms i.e. introducing mobility due to adsorption of more water or introducing more surface area, where water adsorption can occur. This work opens up new possibilities for organic dopants to enhance the performance of salt hydrates.</p

Understanding the hydration process of salts:the impact of a nucleation barrier

The solid-state hydration of salts has gained particular interest within the frame of thermochemical energy storage. In this work, the water vapor pressure–temperature (p–T) phase diagram of the following thermochemical salts was constructed by combining equilibrium and nonequilibrium hydration experiments: CuCl2, K2CO3, MgCl2·4H2O, and LiCl. The hydration of CuCl2 and K2CO3 involves a metastable zone of ca. 10 K, and the induction times preceding hydration are well-described by classical homogeneous nucleation theory. It is further shown for K2CO3 (metastable) and MgCl2·4H2O (not metastable) through solubility calculations that the phase transition is not mediated by bulk dissolution. We conclude that the hydration proceeds as a solid–solid phase transition, mobilized by a wetting layer, where the mobility of the wetting layer increases with increasing vapor pressure. In view of heat storage application, the finding of metastability in thermochemical salts reveals the impact of nucleation and growth processes on the thermochemical performance and demonstrates that practical aspects like the output temperature of a thermochemical salt are defined by its metastable zone width (MZW) rather than its equilibrium phase diagram. Manipulation of the MZW by e.g. prenucleation or heterogeneous nucleation is a potential way to raise the output temperature and power on material level in thermochemical applications

Influence of buoyancy on drainage of a fractal porous medium

The influence of stabilizing hydrostatic pressure gradients on the drainage of a fractal porous medium is studied. The invasion process is treated with invasion percolation (IP) in a gradient. Fractality is mimicked by randomly closing bonds of a network. Two length scales govern the problem: the characteristic length of the pore structure xi(s) and a length scale xi(g) above which buoyancy determines the structure of the cluster. When xi(s)xi(g), gravity becomes important and xi(g) scales with the bond number B as xi(g)proportional toB(-0.57), as in ordinary IP, while the fractal dimension becomes equal to the Euclidean one. When xi(g)xi(s) the fractal dimension of the invading cluster equals the Euclidean one and xi(g)proportional toB(-0.69)

Defect-driven water migration in MgCl\u3csub\u3e2\u3c/sub\u3e tetra- and hexahydrates

\u3cp\u3eThis molecular dynamics simulations study elucidates how water diffusion in MgCl\u3csub\u3e2\u3c/sub\u3e·nH\u3csub\u3e2\u3c/sub\u3eO (n=4 and 6) is facilitated by defects and dopants. Both the impact of a single vacancy (one water molecule was removed) and an interstitial (one water molecule was added) on long range water motion has been investigated. Spontaneous vacancy-interstitial pair defect formation was not observed, which is in line with the predicted high energy costs for defect formation: 150 and 200 kJ/mol for the tetrahydrate and hexahydrate, respectively. The vacancy defects did not show long range mobility, which we relate to the strong bonding of water in the coordination shell of the Mg-ion that prevents water molecules from shifting to neighbouring magnesium ions. On the contrary, the addition of an extra water molecule, facilitates long range motion, which was found as a sequence of hopping events. The interstitial motion is anisotropic in both tetrahydrate and hexahydrate crystals, as interstitials preferentially reside at locations of unfavourable Cl-Cl interactions. Strikingly, Mg ↔ Ca substitutional defects neither increase the mobility of vacancies nor facilitates interstitial motion in the MgCl\u3csub\u3e2\u3c/sub\u3e lattice. While Ca dopants slightly facilitate vacancy formation, it also stabilizes interstitial water molecules by incorporating these molecules in its coordination shell. As a consequence, the interstitial becomes trapped and loses its mobility. Therefore, Ca dopants will not increase the hydration/dehydration kinetics of MgCl\u3csub\u3e2\u3c/sub\u3e hydrates and cannot be used to boost the power output of MgCl\u3csub\u3e2\u3c/sub\u3e-based heat storage devices.\u3c/p\u3

Impact of Atmospheric CO2on Thermochemical Heat Storage Capabilities of K2CO3

This work investigates the reactions occurring in K2CO3-H2O-CO2 under ambient CO2 pressures in temperature and vapor pressure ranges applicable for domestic thermochemical heat storage. The investigation shows that depending on reaction conditions, the primary product of a reaction is K2CO3·1.5H2O, K2CO3·2KHCO3·1.5H2O, or a mixture of both. The formation of K2CO3·1.5H2O is preferred far above the equilibrium conditions for the hydration reaction. On the other hand, the formation of double salt is preferred at conditions where hydration reaction is inhibited or impossible, as the thermogravimetric measurements identified a new phase transition line below the hydration equilibrium line. The combined X-ray diffraction, thermogravimetric analysis, and Fourier-transform infrared spectroscopy study indicates that this transition line corresponds to the formation of K2CO3·2KHCO3, which was not observed in any earlier study. In view of thermochemical heat storage, the formation of K2CO3·2KHCO3·(1.5H2O) increases the minimum charging temperature by approximately 40 °C. Nevertheless, the energy density and cyclability of the storage material can be preserved if the double salt is decomposed after each cycle