Calculation of self-diffusion coefficients in supercritical carbon dioxide using mean force kinetic theory

Area of Science:

• Physical Chemistry
• Chemical Engineering
• Thermodynamics

Background:

• Supercritical fluids exhibit unique properties valuable in chemical processes.
• Accurate prediction of transport properties like self-diffusivity is crucial for designing supercritical fluid applications.
• Existing theories often face limitations when applied to molecular systems like CO2.

Purpose of the Study:

• To adapt and apply mean force kinetic theory (MFT) for calculating the self-diffusivity of CO2 in the supercritical fluid regime.
• To introduce modifications to MFT for handling molecular species and their correlations.
• To validate the theoretical predictions against molecular dynamics simulations.

Main Methods:

• Application of mean force kinetic theory (MFT) with modifications for molecular species.
• Utilizing the molecule center-of-mass pair correlation function (C-C pair correlation function for CO2).
• Introducing a novel definition for the Enskog factor to account for surface correlations.
• Employing molecular dynamics (MD) simulations with the FEPM2 CO2 model to obtain pair correlation functions.

Main Results:

• The modified MFT successfully calculates the self-diffusivity of CO2 in supercritical conditions.
• Theoretical results show good agreement with MD simulation data, particularly near the Frenkel line.
• The developed model performs comparably to existing methods but offers enhanced applicability.

Conclusions:

• The adapted MFT provides a reliable method for predicting CO2 self-diffusivity in supercritical fluids.
• The modifications enhance MFT's capability for systems with molecular complexity and long-range interactions.
• This approach holds potential for broader applications in mixed-species systems and those involving electrostatic interactions.