Measurements and Modeling of Water Adsorption Isotherms of Zeolite Linde-Type A Crystals

Abstract

A systematic study of water adsorption isotherms of zeolite 3A and 4A 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. Zeolite 3A crystals show approximately 12% less swing working capacity for partial pressure swing processes at room temperature for deep dehydration, when compared to 4A crystals. However, the 3A crystal can be regenerated at a lower temperature than the 4A crystal. The 4A crystal has a steeper water isotherm and higher heat of adsorption at zero coverage than 3A. Our results on 3A and 4A crystals are compared to literature data for pelletized 3A and 4A zeolites which contain binders. A new triple-site Langmuir model has been proposed and provides a satisfactory description when compared to other models, such as the Toth model and dual-site Langmuir model. The triple-site Langmuir model is consistent with the Linde-Type A crystal having three different sites for water adsorption. The improvement in fitting is significant in the region where water loadings approach a maximum. This new model was also successfully applied to existing literature data sets for the water isotherm at multiple temperatures for 3A and 4A beads. 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 nontraditional monolith contactors.

Steam adsorption on molecular sieve 3A for sorption enhanced reaction processes

“…Whereas the GSTA model can be seen as a multisite Langmuir isotherm with equal site contribution to the adsorption capacity (Ladshaw et al 2015), the activation energies for the corresponding dual-site Langmuir can be determined. Although the experiments in this work are performed at elevated temperatures and pressures compared to previous works, the approximate energies found for the α and β cage, − 70 and − 10 kJ mol −1 respectively, are in agreement with the range reported in literature (Gabruś et al 2015;Llano-Restrepo and Mosquera 2009;Ladshaw et al 2015;Wang 2020;Loughlin 2009;Lin et al 2015;Simo et al 2009). Note that the contribution to the total capacity is considered equal, as for the GSTA model.…”

“…The difference is in the energy attributed to a specific site for the heterogeneous Langmuir model compared to the energy attributed to the event of n molecules adsorbing for the GSTA model. In support of this, Wang (2020) recently showed that the same dataset can be described by a triple-Langmuir model using the same number of fitting parameters (Wang 2020). Nonetheless, the adjustable number of equilibrium parameters makes the GSTA model flexible and capable of describing water adsorption (as well as other components) on molecular sieves (Ladshaw et al 2015).…”

“…In literature, between 2 and 7 (Gabruś et al 2015;Llano-Restrepo and Mosquera 2009;Ladshaw et al 2015;Wang 2020;Loughlin 2009;Lin et al 2015) different energetic sites have been proposed for water adsorption on LTA zeolites. Clearly, the LTA framework consists of two distinct cavities which are accessible to water, the α and β cages (Breck et al 1956;Reed and Breck 1956).…”

“…These sites correspond to the α cage and the β cage in the micropore and a mesopore contribution. Recently, a similar approach was used to describe water adsorption on zeolites 3A and 4A with a triple Langmuir isotherm (Wang 2020). It has been shown that a triple-site Langmuir is able to describe the water adsorption in zeolite 3A (Grace Davison) compared to the seven-site GSTA model (Llano-Restrepo and Mosquera 2009; Wang 2020).…”

“…Nonetheless, the adjustable number of equilibrium parameters makes the GSTA model flexible and capable of describing water adsorption (as well as other components) on molecular sieves (Ladshaw et al 2015). However, the more parameters (m + 2 for the GSTA model) a model contains, typically the better the data description will be, as demonstrated by the recent results from Wang (2020).…”