AN633 – CO2 sorption on activated carbon in presence of water


Controlling the CO2 emission in the atmosphere is a major challenge of this century. Therefore high research effort is needed to find ways to collect efficiently the CO2. One task is to capture CO2 from emissions, and the other task is to sequestrate the captured CO2 safely. CO2 hydrates are supposed to form in the sea deeper than 400m and could be seen as potential way to store the CO2. However, the large concentration difference of CO2 in hydrates and in the surrounding natural seawater renders the naked CO2 hydrates unstable. This potential dissolution of CO2 hydrates could be reduced or at least made to take place over a longer time frame if the carbon dioxide were injected into porous media where the fluid flow was restricted. Cheap support media are characterised in this study with a volumetric equipment to assess the CO2 absorption properties.



Activated carbon sample were made from bamboo and coconut shells. Sorption studies are carried out with wet and dry sample. The quantity of water loaded in samples is expressed as water ratio, Rw, the weight ratio of water to dry carbon.
The experimental condition covered 275–281K and 0–3.6MPa. The sorption equilibrium was firstly measured on dry sample, and then on wet samples with different water ratios.
Finally, sorption equilibrium was measured for 277–283K with 2K increment at a definite water ratio. The  experimental setup needs be vacuumed before sorption measurement. The wet sample was cooled down to 253K before vacuuming and the temperature was recovered to the specified value afterwards.


Fig. 1: Sorption isotherms of CO2 on the wet bamboo activated carbon at 275 K. (1) Rw = 0; (2) Rw = 1.35; (3) Rw = 1.80; (4) Rw = 2.15; (5) Rw = 2.36.


Fig. 2: Sorption isotherms of CO2 on coconut shells carbon BY1 at 275K. (1) Rw = 0; (2) Rw =1.74; (3) Rw = 1.32; (4) Rw = 1.65.



The Curves 1 of the figures 1 and 2 are the CO2 adsorption isotherm on the dry bamboo carbon and coconut shell carbon. They belong to different types of isotherm. The difference in isotherm types is originated in the different adsorption mechanism of gases. CO2 is condensable and multilayer adsorption or condensation in micro or mesopores might take place. The type II isotherm (fig.1 curve 1) is the sign of a multilayer adsorption. On the contrary, because micropores dominate the pore structure of the coconut shells carbon, the volume filling mechanism of adsorption functioned and type I isotherm was observed in the curve 1 of the fig. 2.

The collected sorption CO2 isotherms on wet samples at 275K are shown in Figs. 1 and 2. The two sets of isotherms have a common feature, i.e., an inflection pressure is observed at about 1.5–2.0MPa. One realized that pore dimension of the activated carbon exerted considerable effect on the sorption behavior of CO2. Although considerable amount was absorbed before the inflection pressure, a plateau is shown on isotherms to mark saturation. Saturation is reached when all pore spaces have been filled with hydrates of/or condensed CO2.
Only the pores with a definite size are suitable for hydrate formation. The inflection pressure of isotherms indicates a phase transition of CO2 (fig 3.), and the enthalpy change of phase transition was calculated from the Clausius–Clapeyron plot drawn with the inflection pressure for different temperatures as shown in fig. 4. Pressure was converted to fugacity in the calculation, and the result of Hf = −54.0 kJ/mol was obtained.


Fig. 3: Sorption isotherms of CO2 at different temperatures (Rw = 2.13): (1) 275 K; (2) 277 K; (3) 279 K; (4) 281 K.


Fig. 4: Clausius–Clapeyron plot for the formation of CO2 hydrates in pure water and carbon pores. (1) In wet carbon and (2) in pure water.



Volumetric technique and in particular the PCTPro-2000 are well suited to characterize any type of CO2 sorption on solid media. Its wide ranges of temperature and pressure permit to access to a large number of applications in carbon capture and sequestration.


For more information see: Comparative studies of CO2 and CH4 sorption on activated carbon in presence of water; Y. Wang, Y. Zhou, C. Liu, L. Zhou, Phys. Colloids and Surfaces A: Physicochem. Eng. Aspects 322
(2008) 14–18.