Intensification of adsorption-based oxygen concentrators using layered beds
Creators: Sankruthi, Sripradha
Publisher: name: University of Alberta Library
Year: 2023

<p>Adsorption-based oxygen concentrators are modular devices that take in air and operate using electricity to produce 95% pure oxygen. These devices consume less unit energy and are more economical when producing oxygen on a small-to-medium scale (&lt; 300 tons oxygen per day), compared to cryogenic distillation. State of the art, commercial oxygen concentrators use low-silica lithium exchanged zeolite (LiLSX) adsorbents in vacuum swing processes to achieve this in the most efficient way possible. In recent times, due to a surge in demand for lithium worldwide, many industries, including adsorbent manufacturers, are faced with rising costs and frequent short supply. This has persuaded many manufacturers of oxygen concentrators to revert to adsorbents like sodium-exchanged zeolites (13X), despite their inferior performance. This presents an opportunity to explore adsorption-processes with multiple adsorbents in various proportions arranged in the form of layers. This thesis aims to present a method to study processes involving layered adsorbent beds, and how such systems can be designed and optimized to minimize energy and maximize productivity.<br>
In this work, two common adsorbents have been chosen - LiLSX and 13X to study the behaviour of layered beds. Isotherms of the two adsorbents are measured between 0-12 bar pressure using volumetry. The experimental data is fit to the Single-site Langmuir (SSL) equation. First, the two adsorbents are considered individually. Unary and binary breakthrough experiments are performed on the single adsorbent systems. A mathematical model has been developed to describe various steps of an adsorptive separation process. PSA experiments are conducted on a test rig constructed in-house to validate the model. A set of optimization studies are performed to get the purity-recovery, and energy-productivity limits in various processes using the two adsorbents individually. <br>
 A two-layer adsorbent bed is considered in this study, consisting of LiLSX and 13X. For any given proportion of LiLSX to 13X, two possible configurations of the layered bed are possible, depending on which adsorbent is placed near the feed end. At all stages in this study, these two configurations have been analyzed independently to check for any differences in performance between them. Breakthrough experiments are performed to understand dynamics of gas flow, temperature and composition across the layered bed. The mathematical model is extended to describe adsorption in a layered bed system. This enhanced model is validated by performing unary and binary breakthrough experiments through the layered beds. PSA experiments are performed on the test-rig to further validate the process model. Optimization studies are performed to understand purity-recovery and energy-productivity limits of layered beds with various proportions of LiLSX. Special emphasis is given to understand the differences between the operation of a small lab-scale column with finite heat transfer and an adiabatic column with no heat transfer. It is found that the order in which the layers are arranged does not matter in systems where heat is removed easily. In large scale columns, which tend to behave more adiabatically, placing the adsorbent with lower nitrogen capacity (13X in this case) near the feed end gives higher recovery than the reverse configuration.</p>