Water adsorption in fresh and thermally aged zeolites: equilibrium and kinetics

Zeolites are widely employed in the industrial drying of gases by Temperature Swing Adsorption (TSA). In a typical TSA process, the adsorbent is packed in a fixed bed, which is sequentially subjected to a “cold” feed (adsorption) and a hot flush (desorption). Due to the hydrothermal stress, adsorbents may suffer from reduced drying capacity in long-term service. The aim of this work is to assess the impact of thermal aging of two zeolite materials (LTA and CHA) having similar pore openings but different Si/Al ratios. We examined how simulated thermal aging affected porous texture, coke deposition and water vapor adsorption equilibrium and kinetics. Both zeolites showed reduced uptake of probe molecules (N 2 at 77 K and CO 2 at 273 K) and water vapor (303 K) when subjected to simulated thermal aging. In Fourier-Transform Infrared Spectroscopy (FTIR) analyses, only the aged LTA zeolite showed bands related to the presence of aromatic compounds. Water vapor adsorption uptake decreased 28.6% and 18.4% for LTA and CHA at 30 mbar, respectively. Kinetic studies indicate a reduction in water diffusion coefficient after the aging cycles. Although LTA has a significantly higher affinity for water as compared to CHA, the latter is much more resistant to hydrothermal aging with comparatively faster water diffusion.