At higher pressure, the absolute sorption and excess sorption are different.
The absolute sorption refers to the actual amount of adsorbate present in the simulation box.
In Fig. 5, we show the absolute sorption results for the temperature of 370 K.
Based on our porosity result of 17%% and the absolute sorption, we calculate the excess sorption which is the relevant physical observable in experiments.
In Fig. 10, we show the temperature effect on the absolute sorption on volume basis for CH4, C2H6, and CO2.
The absolute sorption inhibition of the primary sorbate (ΔQpri) was compared with the amount (Qsec) of competitor sorbed.
At 308 K, the initial slope of the absolute sorption of C2H6 is higher than that of CH4 (Fig. 3a and c) due to the stronger affinity between C2H6 and coal and the amount of C2H6 adsorbed in coal is higher than that of CH4 in the low-pressure region.
The absolute sorption (expressed on a mass basis, Figs. 3b and 5b) increases in the order of CH4, C2H6, and CO2.
Interestingly, we found that the absolute sorption expressed on a volume basis (the absolute sorption in terms of kilogram (kg) per ton (t) of coal, shown in Figs. 3b and 5b, divided by bulk density, presented in Fig. 8a, b) for CH4, C2H6, and CO2 merges with increasing pressure at 308 K (Fig. 9a) and 370 K (Fig. 9b).
Open image in new window Fig. 5 Absolute sorption isotherms of CH4, C2H6, and CO2 on coal at a temperature of 370 K in the unit of a cm3 STP/g coal; b kg/t coal; c mol/kg coal Open image in new window Fig. 6 Temperature effect on the absolute sorption isotherms of a CH4; b C2H6; c CO2.
