Adsorption Equilibria and Mass Transfer in Porous Adsorbents

Abstract

Adsorption is an interfacial phenomenon in which intermolecular forces between a molecule and a surface create an adsorbed phase with different properties than the bulk fluid. Variation in the adsorbed phase among different adsorbate and solid adsorbent pairs is the driving force for many gas-phase separations. To efficiently design these separation processes, detailed characterization of the adsorbed phase over a range of operating conditions is required. The two main characteristics of the adsorbed phase that need to be well understood are adsorption equilibria and mass transfer rates, i.e. how much and how fast molecules are adsorbed.

The purpose of this work is to use fundamental principles of adsorption to measure and interpret adsorption equilibria and mass transfer rates in various systems of interest. A concentration-swing frequency response (CSFR) method was used to measure mass transfer rates of a series of hydrocarbons in BPL activated carbon. Hydrocarbons with different ring and branched structures were used to test for steric effects on diffusion in the amorphous adsorbent. A correlation between rigid-ring structures and lower diffusivity was found. CSFR was also used to measure diffusion rates of CO2 in large single crystals of Cu-BTC, a metal-organic framework (MOF). Many MOFs have been studied as adsorbents for carbon capture and sequestration, but diffusion rates in the literature are scarce. The single crystal morphology of Cu-BTC allowed accurate measurements of micropore diffusion coefficients. Mass transfer rates were also measured on bidisperse pellets of a highly stable MOF, UiO-66, for CO2 and ethane. Macropore diffusion was determined to be the controlling resistance for both adsorbates. Volumetric methods were used to measure high pressure oxygen isotherms on a series of MOFs. MOFs with coordinatively unsaturated Cu metal sites were found to be promising candidates for oxygen storage, with capacities greater than current state of the art adsorbents. Finally, novel adsorbents were synthesized for a CO2 scrubber in a rebreather apparatus. The challenge for these adsorbents was obtaining high CO2 capacities at ambient temperatures, despite low CO2 partial pressures in water saturated conditions, while preventing mass transfer limitations.