Mixed-Cation Zeolites with Minimum Lithium and Silver for Air Separation

Abstract

Air separation is a key process in today’s industry and is achieved by various methods including but not limited to the energy intensive cryogenic distillation of liquefied air and pressure swing adsorption process. The cryogenic process is preferable for high-volume production of Oxygen and Nitrogen from atmospheric air while the pressure swing adsorption process is more applicable for low-to-medium volume production. In this dissertation, we focused mainly on air separation by adsorption which is based on the unique adsorption property of zeolites with high N2/O2 selectivities. An introductory chapter is presented here-in that gives an in-depth picture of various air separation processes and technologies. The second chapter evaluates carbon dioxide, water vapor, and methane on Li-LSX (where LSX denotes low-silica X-zeolite with Si/Al = 1.0) as a superior adsorbent for air separation at low pressure. Characteristic adsorption isotherms to very low partial pressures (to a few ppm at 1 atm) were measured for Li-LSX and compared with the conventional synthetic 13X zeolite as well as two ion-exchanged zeolites in K-LSX and Ca-LSX. Though Li-LSX has been the sorbent of choice since its invention for air separation by pressure swing adsorption and vacuum swing adsorption, the demand for lithium has steeply risen due to its application in lithium ion-batteries for energy storage as well as the fact that its reserves are dwindling, thereby driving up its cost, we set out in the third chapter to develop new zeolites in which lithium is substantially reduced and replaced by a low cost alkali-earth metal cation, in this case, Ca2+. To accomplish this ground-breaking task, mixed-cation LiCa-LSX zeolites containing minimum lithium were prepared by exchanging small fractions of Li+ into Ca-LSX, followed by dehydration under mild conditions to avoid equilibration/migration of the lithium cations. Results after comparing the mixed-cation samples against the pure-cation samples based on their oxygen productivity performance by pressure swing adsorption via a model simulation showed that the LiCa-LSX samples yielded significantly higher O2 product productivities at the same product purity and recovery than their pure-cation precursor, Ca-LSX and only 25% less the Li-LSX. Chapter 4 involves the desulfurization of natural gas using nitrogen-doped carbon. Comparisons of adsorption isotherms for hydrogen sulfide and methane showed that the nitrogen-doped carbon sample (7 wt% N2) adsorbs hydrogen sulfide 5 times more and adsorbs methane 1.3 times less than commercial Calgon BPL 12x30 activated carbon respectively. The regeneration energy required for the synthesized nitrogen-doped carbon sample was very low and required approximately 8 minutes at 333 K for complete hydrogen sulfide desorption. In chapter 5, more mixed-cations, LiSr-LSX, AgCa-LSX and AgSr-LSX containing minimum lithium and silver were prepared by exchanging small fractions of Li+ into Sr-LSX and Ag+ into Sr-LSX and Ca-LSX respectively. Strong evidences were provided that significant fractions of the exchanged Li+ and Ag+ remained in SIII and SII* respectively after comparisons of the N2/O2 adsorption isotherms and isosteric heats of adsorption of the mixed-cation and pure-cation samples. Chapter 6 covers the study of Sr-LSX zeolite and its possibility of completely replacing Li-LSX for air separation in adsorption processes. The O2 productivity performance results from this study were quite promising and are discussed in detail along with treatment conditions for adsorption capacity optimization.