Comparative Study for Water Vapor Adsorption Thermodynamics in Different Clays

Abstract

In this paper, the adsorption behaviors of water vapor in nonswelling clays (chlorite and kaolinite) and swelling clay (montmorillonite) were researched, and the thermodynamics characteristics about the adsorption for water vapor in three clays were systematically analyzed. The results display that montmorillonite has the highest adsorption affinity and adsorption ability for water vapor, while the adsorption affinity and uptake for water vapor on chlorite are the lowest. In montmorillonite, the uptake for secondary adsorption is smaller than the primary adsorption uptake during the entire pressure range. In chlorite and kaolinite, the secondary adsorption center possesses a smaller adsorption capacity at low vapor pressure condition, and the primary adsorption quantity is lower in the high vapor pressure region. With improving pressure, the water vapor adsorption spontaneity in chlorite and kaolinite first increases and then decreases. Contrarily, the spontaneity degree for adsorption of water vapor in montmorillonite can be constantly improved by enhancing the vapor pressure. The values for entropy loss and isosteric heat of adsorption in the three clays for water vapor are as follows: montmorillonite > kaolinite > chlorite. Compared with chlorite and kaolinite, the montmorillonite surface possesses the strongest interaction with water molecules, and the water molecules adsorbed in montmorillonite are the most stable. The trapping of water molecules within shale sediments is mainly resulting from swelling clay. Attention should be paid to mineral composition and content before the hydraulic fracturing process.