Adsorption of Carbon Dioxide, Water Vapor, Nitrogen, and Sulfur Dioxide on Activated Carbon for Capture from Flue Gases: Competitive Adsorption and Selectivity Aspects

Abstract

There is extensive interest in postcombustion flue gas treatment for mitigating CO2 emissions and removal of acid gases. In this study we investigate the adsorption of the main flue gas components (CO2, N2, SO2, and water vapor) on Filtrasorb 400 activated carbon to understand adsorption characteristics of the main components and competitive adsorption effects. The adsorption isotherms of the pure components of flue gas, CO2 (273.15–318.15 K and 0–50 bar), N2 (298.15–313.15 K and 0–150 bar), SO2 (273.15–303.15 K and 0–3.6 bar), and water vapor (293.15–303.15 K and 0–41 mbar), were investigated. The isosteric enthalpies of adsorption were determined to be a function of surface excess. The enthalpies at zero surface coverage have the order SO2 > H2O > CO2 > N2. However, the SO2 isosteric enthalpy decreases with increasing surface excess and is lower than that of water vapor at high surface excess uptake values. The temperature range for CO2 adsorption covers the subcritical to supercritical gas transition. There was no evidence for isosteric enthalpy differences over this temperature range. The adsorption kinetics for SO2 (290.65–303.15 K) and H2O (293.15–303.15 K) adsorption were measured for each isotherm pressure increment. In both cases the adsorption kinetics followed the linear driving force model. The adsorption mechanisms for both SO2 and H2O kinetic trends are discussed in terms of the adsorption mechanisms. The water vapor adsorption kinetics showed a minimum in the region where water molecules form clusters around functional groups, which merge in the pores. The SO2 adsorption kinetics also show a minimum with increasing surface coverage, and this is attributed to dipole–dipole interactions. The activation energies for diffusion of both SO2 and H2O into F400 were very low. Both the N2 and CO2 adsorption kinetics were too fast to be measured accurately by the gravimetric method used in this study. Ideal adsorbed solution theory (IAST) was used to calculate competitive adsorption of SO2/CO2 and CO2/N2 from the isotherms of the pure components. The competitive adsorption of CO2/N2 was investigated by using the integral mass balance (IMB) experimental method, and this was used for validation of the IAST. The results provide an insight into the role of competitive adsorption in the capture of CO2 and SO2 from flue gases by adsorption from both thermodynamic and kinetic perspectives.