(382o) Vapor–Liquid Equilibria of Mixtures of Thiophene and n-Hexane Using Monte Carlo Simulations

In this study, [1] the activity fraction expanded ensemble (AFEE) [2] and the grand canonical transition matrix Monte Carlo (GC-TMMC) [3, 4] simulation techniques have been extended to predict the vapor–liquid coexistence behavior of binary mixtures of molecular fluids. The example system of molecular mixtures studied here is the thiophene–n-hexane (a mixture of an aromatic compound with the n-alkane). The pressure–composition (P–x–y), pressure–density (P–ρ), and density–composition (ρ–x–y) diagrams at a constant temperature (338.15 K) are obtained using the GC-TMMC and AFEE algorithms and compared with experimental data. [5] The primary objective of this work is to implement the AFEE algorithm for molecular systems, and study the merits and demerits of the algorithm with respect to the GC-TMMC methodology. We have investigated the computational efficiency of the AFEE technique at a temperature where both compounds exhibit subcritical behavior and, hence, the entire phase envelope can be calculated using both GC-TMMC and AFEE simulations. A secondary objective is to verify the applicability of the force field employed (TraPPE-UA [6]) here in predicting VLE data of mixtures. For the thiophene–n-hexane system at 338.15 K, AFEE simulations are observed to take 19% of the time as entailed by GC-TMMC simulations, a significant improvement in terms of computational efficiency. The results obtained from both the simulation methods show excellent agreement. The GC-TMMC/AFEE simulation results are also found to agree with experimental data close to the pure thiophene limit and also predict a positive deviation of the saturated liquid line from Raoult’s law behavior implying unfavorable interactions between thiophene and n-hexane. There are, however, significant deviations in the predicted value of the pressure in the pure n-hexane limit (a limitation of the TraPPE-UA force field), and our simulations were unable to predict the existence of an azeotropic point obtained from experiments. Canonical Monte Carlo simulations are also performed to obtain the radial distribution functions (RDFs) which have been employed to study the structure of the coexisting liquid phases to gain insights into the molecular origins of the observed macroscopic phase behavior. The maxima of the first peak of thiophene—thiophene RDFs at coexistence liquid conditions also showed an increasing trend with decreasing concentrations of thiophene, thus confirming the unfavorable interactions between thiophene and n-hexane.

References

[1] T. Chakraborti and J. Adhikari,Ind. Eng. Chem. Res., 2018, 57 (36), 12235–12248

[2] Kumar, V. and Errington, J. R., J. Chem. Phys. 2013, 138, 174112

[3] Errington, J. R., J. Chem. Phys. 2003, 118, 9915

[4] Errington, J. R. and Shen, V. K., J. Chem. Phys. 2005, 123, 164103

[5] Sapei, E., et al. J. Chem. Eng. Data 2006, 51, 2203– 2208

[6] Eggimann, et al. Molec. Simul. 2014, 40, 101-105