Modelling of water vapor adsorption/desorption on hopcalite in an industrial medical air dry
Résumé
The air treatment system is an essential part of medical air installations: it ensures the drying and purification of the compressed air produced, in order to provide healthcare facilities with medical air that complies with the European Pharmacopeia n°1238-medical air in NF EN ISO 7396-1 (ISO, 2016). A medical compressed air dryer is produced by pressure swing adsorption (PSA) using water-selective adsorbents. The production of dry medical air is ensured by air compressors and a column operating in PSA mode with a counter-current regeneration mode. The column is a packed bed with various moisture adsorbents such as alumina, hopcalite and activated carbon in series in the form of granules. Qualitatively, the adsorbents placed at the bottom of the column are the least hydrophilic and adsorb the most moisture, while the adsorbents placed at the top of the column are the most hydrophilic and allow moisture adsorption at low concentrations. In the literature some models to describe the dynamic adsorption of water vapor on alumina are found, contrary to hopcalite where there is limited. Therefore, this study proposes to develop a fickian multiscale model to predict the adsorption and desorption breakthrough fronts on hopcalite as a function of gas flow rate, concentration and temperature. This model is described by mass balance, thermodynamic, hydrodynamics and adsorption/desorption kinetics equations. Several parameters are needed to implement the model, thus the hopcalite is characterized, and water vapor adsorption isotherm measurements are carried out. Experimental adsorption and desorption breakthrough curves at different water vapor concentrations and gas flow rates are measured and used to estimate the axial dispersion, external film mass transfer, mixture diffusion and internal mass transport coefficients involved in the model equations, which are implemented, and solved within COMSOL Multiphysics®. The identified model is validated using different water vapor adsorption and desorption breakthrough curves from those used for parameter identification (Cardenas et al., 2022). The model predictions and the measurements showed a good agreement, quantified using performance indices and confirmed by a Kolmogorov-Smirnov statistic test. The validated model can be used as a predictive tool for the design and optimization of an industrial air dryer.
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