Dynamic of CO2 adsorption in a fixed bed of microporous and mesoporous activated carbon impregnated with sodium hydroxide and the application of response surface methodology (RSM) for determining optimal adsorption conditions

Abstract

Microporous and mesoporous activated carbon produced from longan-seed biomass were impregnated with NaOH and used to capture CO₂ from a simulated flue gas in a fixed-bed column. The process variables that were studied included types of activated carbon as characterized by the volume ratio of micropores and mesopores (V_mic/V_mes), adsorption temperature, NaOH loading, gas feed rate and the adsorbent amount. All five process variables affected the two important breakthrough parameters, namely the breakthrough time (t_B) and CO₂ adsorption capacity at breakthrough time (q_B), with different trends and degrees. However, it was only the NaOH loading that showed a characteristic of an optimum loading that provided the maximum of the breakthrough parameters. It was found that an approximate 45% increase in the adsorbed amount of CO₂ could be achieved with the activated carbon impregnated with around 1 weight % NaOH solution as compared to the case of the non-impregnated carbon. The response surface methodology (RSM) was applied to develop the correlations for both t_B and q_B and the maximum predicted q_B of 33.58 mg/g was derived at the NaOH loading of 76.5 mg/g carbon, V_mic/V_mes of 2.83, adsorption temperature of 20°C, gas feed rate of 156 kg/m²-h and adsorbent amount of 51 kg/m² of column cross-section area. The Klinkenberg's breakthrough model was able to describe the CO₂ breakthrough curves reasonably well for all the tested conditions. The analysis of the two model parameters, the affinity constant (K) and the effective pore diffusivity (D_e), revealed that the optimum V_mic/V_mes that provided the maximum K value was around 2.90, corresponding to the activated carbon that contains 74% and 26% by volume of micropores and mesopores, respectively. The proper volume ratio of micropores and mesopores along with alkali addition into activated carbon can be effectively used for maximizing CO₂ adsorption in a fixed-bed adsorption system.

Keywords: Activated carbon; Alkali impregnation; CO2 adsorption; Fixed-bed adsorption; Response surface methodology.