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.
Highlights
Global warming, a slow increase in the average temperature of the earth’s atmosphere, has a severe effect on world climate change, which has widespread impacts on humans and ecosystems [1]
It is clear that all isotherms exhibited Type I isotherm based on IUPAC (International Union of Pure and Applied Chemistry) classification [38], which is characterized by a rapid increase in the amount of gas adsorbed over relatively low pressures, and followed by a plateau region at higher pressures
This isotherm behavior signifies that the tested activated carbons contain mostly micropores with the pore sizes smaller than 2 nm and the adsorption taking place by the pore-filling mechanism [39]
Summary
A slow increase in the average temperature of the earth’s atmosphere, has a severe effect on world climate change, which has widespread impacts on humans and ecosystems [1]. This atmospheric warming is caused by the increasing concentrations of greenhouse gases (GHGs) in the upper atmosphere that prevents the dissipation of accumulated heat out into the space [2]. Carbon dioxide (CO2), the largest contributor to the global warming effect, has the concentration level in the atmosphere of 414 parts per million (ppm), a 47% increase since the beginning of the industrial revolution era in 1750, and still rising at an alarming rate [4]. It is obvious that CO2 capture at the point source emissions should be the logical means to lower the CO2 level in the atmosphere, albeit the energy penalties involved in the separation processes are relatively high
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