Hydrothermal stability of zeolite molecular sieves in natural gas drying by temperature swing adsorption

Resumen

Zeolites are well known crystalline aluminosilicates, which may be used in processes that take advantage of their molecular sieving effect, such as natural gas drying. They are often used in cyclic processes that swing pressure and/or temperature to perform adsorption and desorption steps. It is recognized that thermal stress may decrease process performance of the adsorbent upon prolonged use. In this thesis, Chabazite (CHA) and Linde Type A (LTA) cationic zeolites with three different Si/Al ratios and two compensating cations were investigated by thermally aging the samples using a laboratory-scale protocol. Two Premature Aging Protocols – PAPs were proposed that considered the conditions which the adsorbent is exposed to in Temperature Swing Adsorption (TSA) process for natural gas drying. The sample was previously saturated with water and n-heptane vapors (as a reference hydrocarbon) followed by pressurization (30 bar) and heating (573 K) with a mixture of CO2 and CH4 (1:4, v/v). The Si/Al ratios of the CHA and LTA samples under study were 1, 2 and 5 and the compensating cations were Na and K. Pristine and aged samples were characterized and compared, focusing on the interplay between adsorbent features and the hydrothermal stability. X-Ray Diffraction (XRD) analyses showed that all materials remain with a similar crystallinity despite undergoing the aging protocol (except for the LTA with K). X-Ray Photoelectron Spectroscopy associated with 29Al and 27Si Nuclear Magnetic Resonance showed that the bulk Si/Al ratio does not change significantly upon aging, even though there may be Al migration from the outer to the inner framework, leading to an increase in the Si/Al ratio on the external surface of the materials. Chemical analyses reveal that there is an unbalance between Al and the compensating alkali cations in the case of Si/Al = 5, for both CHA and LTA. This excess Al is expected to increase the total acidity, either as Extra Framework Al (EFAl) or as Acid Sites. The enhanced total acidity in the CHA samples was confirmed by NH3-TPD and a massive carbon deposition under the aging protocol was observed in CHA and LTA with Si/Al=5. The K cation provides different features to CHA and LTA materials submitted to the aging protocol. Once Na cations were replaced by K in zeolites, thermal resistance of CHA materials is enhanced. Nonetheless, in LTA zeolites, the presence of K leads to partial material amorphization upon aging, which was verified by XRD and Fourier Transform Infrared Spectroscopy. Adsorption experiments reveal that aged materials presented lower N2 and CO2 adsorption capacities at low temperatures and an increased content of carbon as compared to their pristine counterparts. The water vapor adsorption isotherms at 313 K also showed decreased uptakes for all aged materials as compared to the respective pristine samples. For LTAs, the pristine sample with the lowest Si/Al ratio (=1) in Na form reaches a higher water adsorption capacity at 70 mbar, and the sample has an intermediate deactivation upon aging.