(244c) Modifications to the SLD-PR Model for Pure Water Adsorption On Activated Carbons and Coals

The adsorption behavior of water on carbons is fundamentally different from the adsorption of simple, non-polar fluids like nitrogen, methane and organic vapors. The difference arises mainly because the fluid-fluid interactions for water are more dominant than the fluid-solid interactions. In this work, we reviewed the water adsorption behavior on carbonaceous matrices with special interests to coalbed methane and CO2 sequestration applications. Based on our review, we modified the simplified local-density/Peng-Robinson (SLD-PR) adsorption model to more effectively represent water adsorption phenomena on carbons and coals.

The adsorption of water on carbons is bimodal. First it occurs through the formation of hydrogen bonds between the water and the carbon surface, and then the adsorbed water molecules act as secondary adsorption centers giving rise to additional adsorption of water on the carbon surface. Finally, this dual mechanism leads to filling up of the carbon pores at saturation.

Modeling results obtained with the modified SLD-PR model show that traditional characterization information such as the BET surface area cannot be used alone to predict, a priori, the water uptake at saturation or the adsorbed water capacity. This confirms similar observations in the literature that water adsorption does not depend on the extent of the surface; rather, it is dependent on the surface chemistry of the carbon surface.