Vapor−Liquid Phase Coexistence, Critical Properties, and Surface Tension of Confined Alkanes

Abstract

Configurational-bias grand-canonical transition-matrix Monte Carlo simulations are conducted to investigate various thermophysical properties, such as phase coexistence, critical properties, density and orientation profiles of liquid and vapor phases, and vapor-liquid surface tension of methane, ethane, propane, n-butane, and n-octane in bulk and slit pores of graphite and mica surfaces. An exponential-6 (exp-6) model is used for the investigation of normal alkanes with a cutoff radius, 15 Å. It is found that the surface tension of the bulk n-alkane, C 1 -C 4 , based on a truncated exp-6 model, agrees reasonably well with the experimental data. Critical properties are reported by means of the rectilinear diameter approach and least-squares technique. The shift in the critical temperature under confinements follows more than two linear regimes with an inverse in the slit width, as the slit width approaches the two-dimensional limit. This is contrary to what has been previously reported in the literature. The behavior of the critical temperature shift is sensitive to the nature of the surface. The critical density, on the other hand, fluctuates with a decrease in the slit width. The shift in the critical vapor pressure continuously increases with a decrease in the slit width toward the two-dimensional value and becomes constant for pore sizes less than 5 Å for the fluid studied in this work. Corresponding state plots suggest that the deviation of the saturation vapor pressure from the bulk saturation pressure under confinement is positive for large pores and negative for smaller pores. Vapor-liquid surface tension values for n-alkanes, in both types of slit pores, are computed via the finite-size scaling method of Binder and compared with their bulk values. Our investigation reveals that under slit-pore confinement the vapor-liquid surface tension decreases many fold.

Vapor-liquid critical and interfacial properties of square-well fluids in slit pores

“…We have also recently shown that such two linear regimes for the shift in the critical temperature in slit pores generally holds for normal alkanes. 60 We further studied the structural behavior of the coexisting phases to understand the behavioral change in the critical temperature shift. Figures 5͑a͒ and 5͑b͒ present the density profile of the coexistence vapor and liquid phases for the fluid interaction range = 3.0 at a reduced temperature, T R = T / T c,pore = 0.7, for various slit pores.…”

Fast mass transport of oil and supercritical carbon dioxide through organic nanopores in shale

“…A few studies dealing with the adsorption and static properties of confined hydrocarbon liquid films have been reported [20][21][22][23]; however, there is only limited research on the mass transfer of alkanes inside the carbonaceous substrate, in spite of its immediate relevance to applications. The MD simulations conducted by Supple and Quirke [24] show a rapid decane imbibition (6445 m/s) along the inner surface of a (13,13) carbon nanotube (CNT) at 298 K, and the penetration length is linearly dependent on time.…”

Oil adsorption in shale nanopores and its effect on recoverable oil-in-place

“…Later, Severson and Snurr (2007) studied the adsorption isotherms of linear alkanes (ethane, pentane, decane, and pentadecane) on activated carbon and evaluated the functions of slit size, chain length, and temperature on adsorption. Furthermore, it was disclosed by Singh et al (2009) that the thermophysical properties of alkanes under confinement of nanopores, including vapor-liquid phase coexistence, critical properties, and surface tension, show dramatic distinction from bulk values. Most recently, to understand the effect of carbonaceous deposits on the performance of a spark-ignition engine, Harrison et al (2014) studied the singlecomponent adsorption of normal and branched alkanes in slits with different apertures (1, 2, and 4 nm) at 390 K. They concluded that the preferential adsorption of i-octane was enhanced substantially in smaller pores due to packing effects.…”

“…(2) the temperatures and pressures in previous studies were either too high (1500 K, 600 MPa (Severson and Snurr, 2007); 560 K (Singh et al, 2009)) or too low (293 K, 10 kPa (Do and Do, 2005); 300 K, 100 kPa (Severson and Snurr, 2007); 3 MPa (Harrison et al, 2014)) to represent the conditions in shale reservoirs; and (3) the adsorption of alkanes in carbonaceous slits with apertures greater than 5 nm\ \the common sizes in shale\ \has not been reported.…”