(335c) Trace Gas Separation and Concentration By MOF Vacuum Swing Absorption System

Separation represents an important and often highly energy-intensive unit operation in a wide range of medical, chemical, and other industrial processing industries. Some major examples of industrially significant separations include the separation of nitrogen and oxygen, removal of volatile organics from process streams, separation of alkenes from alkanes, and separation of CO₂ from a wide range of process streams and as well as the capture of off-gassing from the processing of nuclear fuels. These separations are traditionally performed using a range of processes such as cryogenic distillation, adsorbent beds, and vacuum- or pressure-swing adsorption processes. Cryogenic distillation is hard to scale down and requires a significant amount of energy, especially for the separation and capture of low-concentration components. Thus, new and improved separations are needed to address increased selectivity and permeability and achieve an industrially significant value, particularly from low-concentration process streams. Metal-organic frameworks (MOFs) have shown great promise as a pathway to introducing selectivity and allowing the capture of trace contaminates as well as the separation of higher-level components. However, they are typically produced as small, nanometer- to submicron-scale particle powders, making them impractical to integrate into many applications, leading to dusting and the disintegration of the particles.

Mainstream Engineering has successfully demonstrated a highly scalable and tunable process to produce high porosity-engineered MOF-polymer beads. This platform approach, applicable to various MOF materials, can control bead size down to 0.1 mm with high MOF loading, no loss of active MOF surface area, high stability, low-pressure drop, and no dusting. The process has enabled the production of tens of kilograms per day of highly reproducible MOF beads and beads from a wide range of MOFs. In this paper, we will discuss the application of these engineered MOF beads to a series of vacuum swing separation unit operations to separate and capture low levels of gasses initially, krypton (140 ppm in the air) and xenon (1000 ppm in the air) from the off-gassing from the reprocessing of used nuclear fuel. We will discuss the application and scale-up of both the engineered MOF beads and the vacuum swing adsorption system and the effects of vacuum cycling conditions, capture temperature, and gas flow rates on the adsorption process for the capture and selective separation of krypton and xenon. The effects of improving the selectivity and capture efficiency of the MOFs on the overall process performance and economics will be discussed, as well as the effect of the process conditions on the cycle life. The development of an automated benchtop MOF-based vacuum swing system will be discussed, as will the application of the system and engineered MOF materials to other separations.