Equilibria and kinetics of natural gas adsorption on multi-walled carbon nanotube material

Abstract

In this study the equilibrium uptake of methane as the main constituent of natural gas (NG) by the multi-walled carbon nanotubes (MWCNTs) as well as the kinetic of adsorption was investigated at the temperature range 283–318 K and pressure up to 50 bar. The experimental apparatus consisting of a dual adsorption vessel was set up for the measurement of equilibrium adsorption of methane on the adsorbent using a volumetric technique (pressure decay). The average pore diameter, specific surface area and total pore volume of the MWCNT adsorbent were 4.6 nm, 294 m² g⁻¹ and 0.62 cm³ g⁻¹, respectively. The results indicated that an optimal methane storage capacity up to 34 wt% was achieved at a temperature of 283 K and pressure of 50 bar. Several model isotherms such as Langmuir, Freundlich and Sips were examined to fit the equilibrium uptake data. The best fit was obtained with the Sips model based on the values of the regression correlation coefficient. The kinetics of methane adsorption on MWCNT was also investigated and the results revealed fast sorption kinetics for methane adsorption on MWCNTs. Gas adsorption kinetic data were analyzed in terms of the pseudo-first-order kinetic model and intra-particle diffusion model and the model parameters were recovered through a nonlinear fit to the experimental data. Isosteric heat of adsorption was evaluated based on the Clausius–Clapeyron equation at different temperatures. Small values of isosteric heat of adsorption confirmed the physical nature of the adsorption mechanism. The high capacity for natural gas storage also emphasized that MWCNT can be a good candidate as a porous media for natural gas storage compared to a conventional adsorbent such as activated carbon. However, lower temperatures and higher pressures are required to improve the NG storage capacity on the MWCNT adsorbent.