(436e) Monte Carlo Simulation of Near- and Super-Critical Hexane Fluid and Physisorption Phase Behavior

Authors: 
Benjamin, K. M., South Dakota School of Mines and Technology
Asiaee, A., South Dakota School of Mines & Technology
Veer, C., South Dakota School of Mines & Technology
Losinski, C., South Dakota School of Mines & Technology
Gunderson, S., South Dakota School of Mines & Technology

The fluid phase and physisorption behaviors of near- and super-critical hexane have been studied through a combination of Gibbs (GEMC), canonical (NVT), and grand canonical (GCMC) ensemble Monte Carlo simulations.  GEMC simulations were conducted to determine the critical point of the united atom TraPPE model for hexane.[1]  These simulations predict critical properties of Tc = 509.3 K, Pc = 35.8 bar, and ρc = 0.219 g/ml, in excellent agreement with published experimental values.[2]  Further, canonical ensemble (NVT) simulations explored single-phase supercritical hexane behavior, again in good agreement with experimental values, and superior to predictions from common cubic equations of state such as Peng-Robinson.[2,3]  These simulations indicate the suitability of TraPPE for modeling near- and super-critical hexane fluid phase behavior.

Physisorption of near- and super-critical hexane on cobalt was also studied, with GCMC simulations.  Hexane was modeled with the united atom TraPPE model mentioned above, while cobalt was modeled both with the Universal Force Field and with the ReaxFF force field parameterized by Labrosse.[4,5]  The hexane loading near the catalyst surface was computed, allowing for resolution of the adsorbed layer during supercritical fluid (SCF) catalytic Fischer-Tropsch (FT) synthesis in supercritical hexane.  These loadings provide much needed detail for future atomistic simulations of SCF hexane solvent effects on heterogeneous FT catalysis on cobalt.

References: 

[1] Martin, M. G. and Siepmann, J. I., “Transferable potentials for phase equilibria. 1. United- atom description of n-alkanes”, J. Phys. Chem. B, 102, 2569 (1998).

[2] Span, R., “Multiparameter equations of state – An accurate source of thermodynamic property data”, Springer, Berlin, pg. 367 (2000).

[3] Peng, D. Y. and Robinson, D. B., “A new two-constant equation of state”, Ind. Eng. Chem. Fund., 15, 59 (1976).

[4] Rappe, A. K., Casewit, C. J., Colwell, K. S., Goddard III, W. A., and Skiff, W. M., “UFF, a full periodic table force field for molecular mechanics and molecular dynamics simulations”, J. Am. Chem. Soc., 114, 10024 (1992).

[5] LaBrosse, M., Statistical mechanical and quantum mechanical modeling of condensed phase systems, Ph.D. Thesis, University of Pittsburgh (2009).