Date of Award
Doctor of Engineering
Washkewicz College of Engineering
Portable or small scale pressure swing adsorption (PSA) systems have gained increasing popularity in both industry and literature due to the commercial success of personal oxygen concentrators (POCs). While these processes have much in common with larger PSA systems, significant differences exist that make understanding process limitations difficult. These include faster cycle times, smaller adsorbent particles, and a reduced column size. Macropore diffusion is traditionally assumed to control the mass transfer rate in columns packed with zeolite particles in an oxygen production process. While numerous studies have confirmed this assumption for the particle size used in industrial size PSA processes, it has not been validated for the much smaller particle size used in small scale PSA. Smaller particles improve the mass transfer rate by increasing interfacial area per volume as well as decreasing diffusion distance. Despite this reduction, small scale PSA simulations often still assume a mass transfer rate solely limited by macropore diffusion. This approach fails to adequately account for the influence of other mass transfer mechanisms whose impact increases due to particle size reduction. This study experimentally demonstrates the dominant mass transfer mechanism is no longer macropore diffusion for the particle size used in small scale PSA for oxygen production. Depending on the gas velocity, axial dispersion effects either become the limiting mechanism or equally as important as macropore diffusion. It also shows that improperly accounting for axial dispersion effects has a significant impact on the mass transfer coefficient estimation, often measured with breakthrough experiments. An important limitation for small scale PSA processes is the limit on adsorbent utilization. Decreasing cycle time for a PSA process typically results in a gain in adsorbent utilization, often represented in industry by the bed size factor (BSF). Increasing adsorbent utilization is represented by a decrease in BSF. A low BSF is desirable because it represents a smaller overall process size, which is highly attractive for portable systems. Currently, there is no consensus in literature if a lower limit for the BSF exists and what may cause it. In this study, a two column small scale PSA process was used to measure the cycle time of a minimum BSF. It represents the first experimental literature example of a minimum BSF for a two column air separation process. The data was then used with a literature model to better understand why the minimum was occurring and what was primarily causing it. It was determined that macropore diffusional resistance is the primary cause of a minimum BSF.
Moran, Aaron A., "Limits of Small Scale Pressure Swing Adsorption" (2018). ETD Archive. 1050.