(669a) Measurement and Modeling of Water Isotherms on Zeolite Crystals

A systematic study of water adsorption isotherms of zeolite crystals over a pressure range of 10^-5 to 10^-1 bar and temperatures from 25 to 250 °C was carried out. This work focuses on water isotherms of 3A and 4A zeolite crystals, not pelletized zeolites containing binders. Most water adsorption data reported in the literature result from the use of zeolite pellets or beads forms. To exclude the effects of binders on water adsorption, one must measure adsorption on zeolite crystals without any clays. This is especially important for the design of nontraditional beds with structured contactors, in which different formulations can be used compared to commercial pellets or beads used in packed beds.

Multi-temperature isotherms of 3A crystals are analyzed to determine the isotherm model that offers the best representation of data. Both Toth and dual-site Langmuir model provide reasonable description, but fail in the region of high water loadings over 11 mol/kg. A simple triple-site Langmuir model has been proposed and is proven to satisfactorily model multi-temperature isotherms on 3A and 4A crystals as well as literature data on 3A and 4A pellets. The accuracy of the three sites model (triple-site Langmuir) to describe water adsorption is consistent with and related to the three cation sites (inside α-cage. β-cage, and sodalite cage) in the zeolite LTA structure. Thus, the water molecules have different bonding energy to the cations depending on the locations within the zeolite. These results provide fundamental water isotherms and a simple model to evaluate 3A and 4A in deep dehydration with temperature-swing, or partial pressure-swing adsorption processes, or their combination using non-traditional monolith contactors.