A theory-based simple extension of Peng–Robinson equation of state for nanopore confined fluids

Abstract

In a recent publication (Islam et al. in J Nat Gas Sci Eng 25:134–139, 2015), the van der Waals equation of state (EOS) was modified to assess phase behavior of nanopore confined fluids. Although the changes of critical properties were well captured, it was limited to only subcritical conditions. Peng–Robinson EOS showed inconsistent critical shifts. Here, we develop a simple extension of Peng–Robinson (PR) derived similarly from the Helmholtz free energy function by applying the same energy and volume parameter relations. This modified PR reproduces experimental and molecular simulation results satisfactorily. It shows that there is pore proximity effect also in supercritical condition which, however, diminishes as temperature increases. The proposed model can show heterogeneous density or layered distribution of molecules inside nanopore. We have tested common shale (natural) gas molecules and the condition of Haynesville plays where temperature and pressure can be very high. This simple model can offer alternatives to more computationally expensive molecular simulations to study the pore proximity phenomenon.