Molecular Modeling of Double Retrograde Vaporization Using Monte Carlo Simulations and Equations of State

Vapor-liquid equilibria of binary systems consisting of a low-boiling (i.e., more volatile) and a high-boiling compound may exhibit unexpected behavior near the critical point of the low-boiling compound. Near the critical temperature of the low-boiling compound and for compositions rich in the low-boiling compound, increasing the pressure may result in multiple crossings of the dew-and bubble-point curves. This phenomenon is often called double retrograde vaporization (or condensation) and may play a role in oil field operations and gas transport through pipelines, but the microscopic driving forces for the unusual shape of the dew-point curve are not well understood. Monte Carlo simulations in the constant-pressure, constant-temperature Gibbs ensemble using the united-atom version of the TraPPE force field were carried out for the methane/n-butane mixture at temperatures ranging from 0.95 to 1.05 of the reduced (T/Tc) temperature of methane. The simulations predict a wealth of additional thermodynamic data (densities and free energies of transfer) and structural data that are used to provide much needed molecular-level insights into the fluid properties associated with double retrograde vaporization. Simulated thermodynamic data are also compared with calculations using the Peng-Robinson and PC-SAFT equations of state.

Automated summary

The behavior of binary systems containing a low-boiling compound and a high-boiling compound near the critical point of the low-boiling compound can be unexpected. Increasing the pressure near the critical temperature of the low-boiling compound and for compositions rich in the low-boiling compound may result in the crossing of dew-point and bubble-point curves multiple times. This phenomenon, known as double retrograde vaporization, has implications in oil field operations and gas transport through pipelines, but the underlying reasons for its occurrence are not well understood. Monte Carlo simulations were conducted to provide molecular insights into the fluid properties associated with double retrograde vaporization using the united-atom version of the TraPPE force field for a methane/n-butane mixture.