Adsorption, Kinetics, and Diffusion of Methane on Nanoporous Activated Carbon for Adsorbed Natural Gas Storage Systems

The adsorption–desorption kinetic behaviors of
methane in
nanoporous carbon adsorbent KS-HA-1 were studied over a pressure range
of 1 Pa to 5.28 MPa and at eight different temperatures between 143
and 333 K. The equilibrium methane adsorption onto KS-HA-1 was measured
up to 21 MPa. The pseudo-first-order (PFO), pseudo-second-order (PSO),
Elovich, and intraparticle diffusion (IPD) models were tested to describe
the experimental kinetic curves. The PSO model provided the best description,
indicating that the kinetics of methane adsorption onto the adsorbent
was determined by the availability of adsorption sites that are narrow
micropores with a size <1.2 nm. At 5.28 MPa, the effective diffusivity
of methane increased from 0.303 × 10–6 to 5.256
× 10–6 cm2/s with a temperature
rise from 143 to 333 K. The experimental data were used to develop
an approach for predicting adsorption kinetic parameters under arbitrary
thermodynamic conditions. The molecular dynamics method was employed
to evaluate both the equilibrium methane adsorption and the self-diffusion
coefficient in model slit-like pores with widths that matched the
pore sizes in KS-HA-1 as estimated by the NLDFT analysis. It was established
that at constant pressure, the self-diffusivity increased with increasing
pore sizes and temperature and dropped with increasing pressure.