Investigation into adsorption equilibrium and thermodynamics for water vapor on montmorillonite clay

Abstract

The isothermal adsorption curves for water vapor on montmorillonite were measured by a gravimetric adsorption system. Dent's model was employed to estimate the adsorption behaviors of water vapor on primary adsorption sites and secondary adsorption sites. The thermodynamics analysis of water vapor adsorption was performed. At low vapor pressure region, primary adsorption predominates, and with increasing vapor pressure, secondary adsorption becomes notable. Primary adsorption sites have an evidently stronger adsorption affinity than secondary adsorption sites. With increasing vapor pressure, Gibbs free energy variation rapidly increases and then reduces slowly. Although increasing vapor pressure raises adsorption spontaneity on primary adsorption sites, the enhancement in vapor pressure decreases the spontaneity of water vapor adsorption on secondary adsorption sites. As adsorbed loading increases, isosteric heat of adsorption and entropy loss decrease first and then increase quickly. The gradually growing water clusters are responsible for the increase of entropy loss at late stage.

Comparative Study for Water Vapor Adsorption Thermodynamics in Different Clays

“…Water adsorption isotherms in shale also illustrate the characteristics for type II isotherm. 22 As reported by Thommes et al, 31 the type II isotherm is attributed to the unrestricted monolayer adsorption−multilayer adsorption up to large P/P 0 . In initial stage, the rapid increase of adsorption quantity is an indication of monolayer adsorption on preferential adsorption sites.…”

“…Primary adsorption is affected by the concentration and distribution of surface functional groups, and secondary adsorption is closely connected with the inner pore structure system. 22 The higher amount for secondary adsorption than that for primary adsorption at high vapor pressure condition indicates that the influence of the inner pore structure on water adsorption is more significant than that of the surface function groups for the now-swelling clays of chlorite and kaolinite. The greater quantity for primary adsorption than secondary adsorption uptake during the whole process reveals that the surface functional groups have a more remarkable effect on water vapor adsorption than the pore structure for the swelling clay of montmorillonite.…”

“…The adsorption quantities on primary and secondary adsorption centers can be gained using eqs 4 and 5, respectively. 22…”

Effects of Sub- and Supercritical CO2 on Coal Diffusivity and Surface Thermodynamics

“…The surface potentials Ω for CO 2 and CH 4 of raw and CO 2 -treated coal samples can be determined from the adsorption isotherms (Figure S3), as shown in Figure . The surface potentials Ω showed a downward decrease trend with the increase of pressure, which is related to the different isothermal work loading molecules onto the coal surface at different pressure stages . Also, the absolute value of Ω for CO 2 is significantly higher than that of CH 4 This is due to the fact that CO 2 has a higher adsorption capacity leading to the greater surface potential .…”

“…The surface potentials Ω showed a downward decrease trend with the increase of pressure, which is related to the different isothermal work loading molecules onto the coal surface at different pressure stages. 58 Also, the absolute value of Ω for CO 2 is significantly higher than that of CH 4 This is due to the fact that CO 2 has a higher adsorption capacity leading to the greater surface potential. 59 Moreover, a small increase in surface potentials is observed after CO 2 treatment.…”