THE EFFECT OF CHEMICAL TREATMENT ON ADSORPTION OF NATURAL GAS BY MULTI-WALLED CARBON NANOTUBES: SORPTION EQUILIBRIA AND THERMODYNAMIC STUDIES

In this study, adsorption of methane as the main constituent of natural gas was studied on pristine multi-walled carbon nanotubes (MWCNTs), after which purification and different chemical treatments of MWCNTs were performed to enhance the natural gas adsorption capacity. The results revealed that chemical treatment of the MWCNTs in the presence of H2SO4/HNO3 acidic mixture in 3:1 volume ratio enhanced considerably the adsorption capacity of natural gas (an optimal up to 45 mmol/g) at a temperature of 298.15 K and pressure of 50 bar, compared to the pristine MWCNTs (about 27 mmol/g) at the same operating conditions. This effect can be attributed to the opening of the nanotubes caps with a major alteration in their structural properties due to chemical treatment. The experimental data of adsorption were almost equally well described by Langmuir; Freundlich and Sibs Equations to determine the model isotherms. The best fit was obtained by the Sips model isotherm with the r-squared value near to unity. Furthermore, using the experimental data obtained in different temperatures the isosteric heat of natural gas adsorption onto pristine MWCNTs was also calculated in the interested range of pressures and temperatures using the thermodynamic-based Clausius-Clapeyron Equation from the Sips Isotherm model. The results revealed an energetically heterogeneous surface of MWCNTs in natural gas adsorption. Also the natural gas adsorption process was kinetically studied through pseudo-second order and intra-particle diffusion models, which indicated the intra-particular diffusion is the rate limiting step in adsorption of methane on MWCNTs.