Adsorbent filters often use activated carbons to remove a wide range of toxic chemicals from air at high separation ratios. Many toxic compounds, such as pesticides, contain bulky functional groups or have significant branching. It is of interest to know if filter performance for these compounds could be diffusion limited due to steric hindrances in commercially available amorphous carbons. Most mass transfer studies in porous adsorbents have focused on highly ordered crystalline structures, such as zeolites, or carbon molecular sieves that have a very narrow pore size distribution, with the uniform structures allowing for shape selective separations. Few studies have attempted to find steric effects for amorphous adsorbents.
A concentration swing frequency response (CSFR) technique was used to measure mass transfer rates of various C10 hydrocarbons to explore steric hindrance effects on diffusion in amorphous BPL activated carbon. With the various C10 hydrocarbons, there are enough carbons to create different ring or branched shapes, but not so many that volatility is greatly reduced. By plotting amplitude ratios as a function of frequency and fitting to a mathematical model derived from transfer functions, this technique can easily distinguish among different mass transfer rate mechanisms and permits the accurate calculation of diffusion coefficients. The hydrocarbon adsorbates of interest are decane, α-pinene, limonene, and decalin. The fluorocarbon, perfluorodecalin, is also investigated. The CSFR experiment was performed at multiple gas-phase concentrations for each adsorbate, and a micropore diffusion model fits the data well. Adsorption isotherms were measured for each adsorbate on BPL activated carbon and described using a Toth model. Predicted isotherm slopes derived from the CSFR experiments compare well with the equilibrium isotherm slopes. For each adsorbate, the measured diffusion coefficients are found to be concentration dependent and increase as the adsorbate concentration increases in a manner consistent with Darken thermodynamic correction factors.
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