Molecular Dynamics Simulations of Retrograde Condensation in Narrow Oil-Wet Nanopores.

Molecular dynamics (MD) simulations were performed for a 70/30 wt % ethane/heptane mixture unconfined and confined to 60 nm oil-wet nanocapillaries with square cross sectional widths of 4 nm. Large pressure ranges along both prograde (310 K) and retrograde (365 K) isotherms were examined. For the unconfined fluid at 310 K, compression resulted in steady increases in density of both ethane and heptane up to the predicted condensation point where a sudden phase transition was observed. At 365 K, pressure increases caused increased density in ethane but reduced density in condensed heptane droplets and retrograde phase behavior was observed as a gradual collapse of the denser phase with increased pressure above 60 bar. In nanopores, surfaces with strictly repulsive walls greatly favored association with ethane at all pressures and promoted exclusion of heptane from the narrow pore at pressures inside the saturation curve. At the oil-wet surface, preferential adsorption of heptane was seen, the extent of which depended on both temperature and pressure. At 310 K, capillary condensation, primarily of heptane, was observed at all pressures spanning the saturation curve, maximizing at low pressures. At 365 K, heptane accumulation in the pore peaked at intermediate pressures. At lower pressures, two distinct phases were observed: an adsorbed phase composed largely of the heavier molecule and a fluid phase composed mainly of the lighter component. While both equilibrium adsorption and accumulation of fluid inside the pore was significantly reduced at pressures below 30 bar along the retrograde isotherm, when pressure gradients were induced across the narrow pore, significant clogging was observed at the low pressure end for pressures as low as 10 bar.