Equilibrium and Kinetics of CO2 Adsorption by Coconut Shell Activated Carbon Impregnated with Sodium Hydroxide

Abstract

The equilibrium and kinetics of CO2 adsorption at 273 K by coconut-shell activated carbon impregnated with sodium hydroxide (NaOH) was investigated. Based on nitrogen adsorption isotherms, porous properties of the tested activated carbons decreased with the increase of NaOH loading, with the decrease resulting primarily from the reduction of pore space available for nitrogen adsorption. Equilibrium isotherms of CO2 adsorption by activated carbons impregnated with NaOH at 273 K and the pressure up to 100 kPa displayed an initial part of Type I isotherm with most adsorption taking place in micropores in the range of 0.7–0.9 nm by pore-filling mechanisms. The amount of CO2 adsorbed increased with the increase of NaOH loading and passed through a maximum at the optimum NaOH loading of 180 mg/g. The CO2 isotherm data were best fitted with the three-parameter Sips equation, followed by Freundlich and Langmuir equations. The pore diffusion model, characterized by the effective pore diffusivity (De), could well describe the adsorption kinetics of CO2 in activated carbons impregnated with NaOH. The variation of De with the amount of CO2 adsorbed showed three consecutive regions, consisting of a rapid decrease of De for CO2 loading less than 40 mg/g, a relatively constant value of De for the CO2 loading of 40–80 mg/g and a slow decrease of De for the CO2 loading of 80–200 mg/g. The maximum De occurred at the optimum NaOH loading of 180 mg/g, in line with the equilibrium adsorption results. The values of De varied from 1.1 × 10−9 to 5.5 × 10−9 m2/s, which are about four orders of magnitude smaller than the molecular diffusion of CO2 in air. An empirical correlation was developed for predicting the effective pore diffusivity with the amount of CO2 adsorbed and NaOH loading.