Adsorption of light gases and gas mixtures on zeolites and nanoporous carbons
Mitchell, Lucas Alexander
:
2014-04-01
Abstract
The ability to accurately predict mixture adsorption equilibrium is vital for
future gas separation technologies. This research concentrates on the investigation
and prediction of adsorption equilibrium of pure components and gas mixtures. The
systems studied are of importance to the field of personal medical oxygen generation.
Two commercial adsorbents are investigated: a LiLSX zeolite and a carbon
molecular sieve. Adsorption equilibria are measured on the LiLSX zeolite for nitrogen
and oxygen, both as pure gases and binary mixtures. The binary measurements
consist of Henry’s law behavior with one component in excess as well as binary
isotherms for a range of compositions. Binary Henry’s law behavior is modeled by
adding virial excess mixture coefficients to the ideal adsorbed solution theory. The
mixture coefficients are evaluated solely from the binary Henry’s law behavior, and
the binary isotherms for a range of compositions are predicted accurately. Adsorption
isotherms are measured on the carbon molecular sieve for oxygen, nitrogen, and argon
at pressures up to 100 bar. The capacities of the gases are higher than expected, with
the carbon molecular sieve having capacities higher than BPL activated carbon and
capacities similar to a superactivated carbon.
An approach to model adsorption of pure gases using classical density functional
theory is developed. Fundamental measure theory is used as an accurate
method to describe the hard sphere interaction potential. To address the intermolecular
and intramolecular attractive potentials, a version of the statistical associating
fluid theory is used. The new approach models adsorption of molecules of increasing
complexity on flat walls, with predictions comparing well with simulation data in the
literature. The new approach is extended by incorporating the 10-4-3 Steele potential
to describe slit-shaped carbon pores. Nitrogen is first modeled inside the pores, where
the density profiles are combined to produce a pore size distribution and describe the
nitrogen isotherm. Pore density profiles are modeled for n-pentane and, using the
pore size distribution obtained from nitrogen, a pentane isotherm is shown to agree
well with experimental data.