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

Abstract

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 deeply 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. 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.