Nanoscale-extended alpha functions for pure and mixing confined fluids

Abstract

In this paper, two new nanoscale-extended attractive (alpha) functions in Soave and exponential types are developed for the first time, which are applied and evaluated for the calculations of the thermodynamic and phase properties of confined fluids coupled with a modified equation of state (EOS). Moreover, a novel method is proposed and verified to determine the nanoscale acentric factors. The behaviour of several important parameters, i.e., minimum reduced temperature, nanoscale acentric factor, alpha function and its first and second derivatives, are specifically analyzed at different temperatures and pore radii. The newly-developed alpha functions are validated to accurately calculate the thermodynamic and phase properties in bulk phase (rp = 1000 nm) and nanopores. The minimum reduced temperature from the Soave alpha function occurs at the acentric factor of ω = −0.295211 while the exponential function is monotonically related to the temperatures without any minimum conditions. Moreover, the acentric factors and intermolecular attractivities are found to be increased with the pore radius reductions at most temperatures, wherein they remain constant or slightly increase by reducing the pore radius at rp ≥ 50 nm while become quickly increased at rp < 50 nm. It should be noted that the alpha functions are decreased with the pore radius reduction at the critical temperature (Tr = 1). The intermolecular attractivities are found to be stronger for the heavier or high carbon number components. Furthermore, the first and second derivatives of the Soave and exponential alpha functions to the temperatures are continuous at T ≤ 4000 K. Overall, the two original (Soave and exponential) and two nanoscale-extended alpha functions are proven to be accurate for the thermodynamic and phase calculations in bulk phase and nanopores.