TY - THES ID - 147678941 TI - Development of a non-isothermal kinetic model for hygroscopic materials AU - Verbruggen, Vincent AU - Rongé, Jan AU - Martens, Johan AU - KU Leuven. Faculteit Ingenieurswetenschappen. Opleiding Master of Chemical Engineering (Leuven) PY - 2019 PB - Leuven KU Leuven. Faculteit Ingenieurswetenschappen DB - UniCat UR - https://www.unicat.be/uniCat?func=search&query=sysid:147678941 AB - The relationship between the production of energy and water causes some major issues. Because a shift is expected towards a more energy intensive water production and a water dependent energy production, there is a need for sustainable solutions. In this study, capturing water vapor from air with hygroscopic materials is examined, either to be used to produce water or to convert water into hydrogen. Hygroscopic materials are able to adsorb or absorb water vapor, depending on temperature, relative humidity and structural properties. Two main parts are examined: i) the water uptake capacity and ii) the water uptake rate with a non-isothermal kinetic model to study which parameters are important. An extensive literature study denoted that both materials based on water adsorption and absorption emerge as possible candidates for these applications. Therefore, mesoporous silica gels with pore sizes between 6 and 10 nm were examined and the water uptake curves were determined over a wide range of relative humidities and temperatures. This confirmed that silica gels are able to adsorb water up to one time their own weight, as was observed in literature. The adsorption isotherms of mesoporous silica gels show an increase in amount adsorbed at high relative humidities because of capillary condensation. A temperature rise results in a monotonically decreasing amount adsorbed at constant RH due to the principle of Le Châtelier. The hygroscopic salts on the other hand, absorb several times their own weight because of deliquescence and dissolve themselves in the absorbed water. They are thus able to capture much more water than silica gels but are more difficult to handle. A linkage between the solubility in water and the deliquescence relative humidity was observed. It followed that colligative effects are important because the vapor pressure over the solution is decreased. Furthermore, the adsorption isotherms of PVA sponges and desiccant clay were determined. These materials only adsorb up to 0.3 g/g . Polymers on the other hand, capture almost two times their own weight in water due to absorption. Especially silica gels and hygroscopic salts are considered as interesting options. Subsequently, the kinetics of water vapor adsorption and desorption on silica gels have been examined. A non-isothermal linear driving force model allowed to evaluate the adsorption rate and to determine the mass transfer coefficient k. Temperature effects were taken into account by developing a BET equation that describes the isotherms between 20°C and 50°C. The parameters of this BET equation were observed to vary linearly with temperature. Heat transfer was found to be important in the region of capillary condensation for large particles with a relatively small specific surface area. It was observed that k depends on the amount adsorbed. In the region of low water uptake, the mass transfer coefficient increases because the energy of the bond between the surface and the water molecules decreases towards a complete surface coverage. k then declines, probably because the bridging between the water clusters and the pore walls occurs, impeding diffusion in the gas phase. Eventually, k increases again at high RH, which may be because of a contribution of viscous flow. The diffusivity coefficients are influenced by bed effects. It was observed that the depth of the sample bed is crucial for the adsorption rate and probably causes a larger resistance than heat transfer. ER -