Volume Frequency Response Method for Determining Mass Transfer Mechanisms of O2 in Carbon Molecular Sieve 3K172
Date of Award
Open Access Thesis
Carbon molecular sieves (CMS) have grown more desirable over the years as an adsorbent for industrial separation processes as CMS technology has advanced. CMS is commonly used in nitrogen inerting (i.e, oxygen removal from air), carbon dioxide removal from methane and oxygen purification. However, knowledge of the dynamic behavior of these gases on CMS is needed to design and operate efficient and effective separation processes. For this reason, the mass transfer resistances within the micropore at both low and high frequencies were characterized using frequency response, COMSOL Multiphysics modeling, and MATLAB optimization , because frequency response methods have demonstrated the ability to discriminate between limiting mass transfer mechanisms.
This method is performed through a sinusoidal perturbation of volume, pressure, or concentration. Each method has unique advantages and disadvantages that need to be considered when determining the most appropriate for the adsorbate-adsorbent at hand. Due to the robustness and applicability of the volumetric frequency response system (VFRS), the current study focused on a previously constructed VSFR system to utilize the wide range of frequencies it handles, allowing for analysis of both slow and fast diffusing gases. Mass transfer mechanisms were identified utilizing the data obtained from this VFRS system and fitting it to a mathematical model for oxygen adsorbed by Shirasagi CMS 3K 172 from Takeda Chemicals at 750 torr at 20, 30, 40 and 50 °C and 100 and 200 torr at 25 °C. Three distinct zones were identified in which isothermal local equilibrium, micropore diffusion resistance, and mouth resistance dominated at low, intermediate and high frequencies, respectively. The model did not fit the phase lag data well at high frequencies but showed that an increase in temperature resulted in a decrease in the amplitude.
Additionally, a parametric study was performed to demonstrate the effect of the heat transfer coefficient, heat of adsorption, adsorbent heat capacity, micropore diffusion, and mouth resistance on the adsorption kinetics. For the base case of oxygen at 760 torr and 20 °C, curves displayed a delayed drop in intensity as micropore diffusion limitations decreased. With an increase in mouth resistances, the slope of the intensity curves became steeper and the phase lag was shifted right. An increase in the heat capacity of the adsorbent caused a developing hill between 0.001 and 0.015 Hz, while an increase in the heat of adsorption shifted this hill downwards. An increase in the heat transfer coefficient caused an increase in the starting location of the intensity curve until equilibrium was reached and an increase in the heat transfer coefficient no longer had an effect. The heat transfer coefficient had no effect on phase lag amplitude.
Smithson, O.(2020). Volume Frequency Response Method for Determining Mass Transfer Mechanisms of O2 in Carbon Molecular Sieve 3K172. (Master's thesis). Retrieved from https://scholarcommons.sc.edu/etd/6176