Prediction of Binary Gas Adsorption of CO2/N2 and Thermodynamic Studies on Nitrogen Enriched Nanostructured Carbon Adsorbents

Pure component adsorption isotherms for CO2 and N2 on the prepared nitrogen enriched carbon were evaluated using a static volumetric analyzer at four different adsorption temperatures ranging from 30–100 °C and were then correlated with three pure component adsorption isotherm models, namely, Langmuir, Sips, and dual-site Langmuir (DSL) models. Adsorption equilibria of binary CO2–N2 adsorption was then predicted by extending Sips and DSL equations empirically along with the usage of ideal adsorbed solution theory and was compared with experimental data obtained from the breakthrough curves through various phase diagrams. Breakthrough data for binary system were obtained at four different CO2 feed concentrations (5–12.5% by volume) and four adsorption temperatures (30–100 °C) using a fixed-bed reactor. Among three adsorption isotherms models used to investigate the equilibrium data of pure component system, Sips and DSL adsorption isotherm models fitted well, indicating energetically heterogeneous adsorbent surface. However, for the binary adsorption system, their extended forms highly under-predicted the amount of CO2 adsorbed over the complete temperature and feed concentration range because of the difference in adsorptive strengths of CO2 and N2 molecules. Also, ideal adsorbed solution theory was not able to describe the mixed-gas adsorption equilibria. Total adsorbed amounts were found to increase with CO2 gas phase molar fraction implying positive deviations from Raoult's law with asymmetric x–y diagrams. Thermodynamic functions such as molar Gibbs free energy change, entropy change, and enthalpy change were evaluated numerically for pure component system. They confirmed the feasibility of adsorption process and indicated the formation of more ordered configuration of CO2 molecules on adsorbent surface and hence exhibited higher heats of adsorption as compared to N2.