419543 Molecular Simulation Study of n-Alkane Mixtures Using Transition Matrix Monte Carlo Methods

Wednesday, November 11, 2015: 10:43 AM
255C (Salt Palace Convention Center)
Tamaghna Chakraborti, Chemical Engineering, IIT Bombay, Mumbai, India

Molecular Simulation Study of n-Alkane Mixtures using Transition Matrix Monte Carlo Methods

Tamaghna Chakraborti1, Jhumpa Adhikari1

1Department of Chemical Engineering, IIT Bombay,

Powai, Mumbai – 400076.

(E-mail : tamaghna.chakraborti@gmail,com, adhikari@che.iitb.ac.in )

Abstract – Fluid phase equilibrium is one of the very important aspects of the chemical industry. With the advent and improvement of computational resources molecular simulation techniques have become an important means to obtain the desired equilibrium properties. Pertinent to molecular simulation techniques is the question of obtaining reliable and reproducible data in the least computationally expensive way possible. Molecular simulation techniques require as input a potential model which governs the way different molecules in a system interact with one another. The accuracy and the reliability of data obtained from molecular simulations depend to a considerable extent on the different potential models which are available in literature like TraPPE1, NERD2 and COMPASS3 with TraPPE being more successful than others in the realm of fluid phase equilibrium. Different simulation techniques have been tried in order to obtain macroscopic equilibrium properties in the least computationally expensive way possible. One of the first techniques to compute equilibrium properties without the use of an interface (which is computationally intensive due to the large number of molecules involved) was by using the Gibbs ensemble Monte Carlo technique4 by Panagiotopoulos. With the advent of transition matrix Monte Carlo methods, new methods for determination of fluid phase equilibria have evolved. Errington5 had successfully incorporated the transition matrix technique to the grand canonical and the isothermal-isobaric ensemble simulation algorithm and used it to determine the phase diagram of Lennard-Jones fluid. Shen and Errington6,7 had extended the technique to the determination of phase diagrams of monoatomic mixtures. It is the endeavor of this work to extend the work of Shen and Errington to molecular mixtures starting with the n-alkane homologous series. The n-alkane series is probably the most widely studied of all molecular groups8,9,10 mainly because of the simplicity of its implementation. The systems being studied here are the methane-ethane, ethane-propane and propane-butane molecular mixtures. The main reason for studying these binary mixture systems is that the critical points of the respective components are close to one another the molecules being adjacent in the same homologous series. All the systems are well studied both using experiments as well as molecular simulations. This makes it amenable to comparison with data available from literature. The whole point of this exercise is to check the validity of the TraPPE forcefield as we deal with molecular mixtures as well as extend the method of Shen and Errington7 to molecular mixtures. The results indicate that TraPPE does well in accounting for the equilibrium densities of the vapor and the liquid phases, it misses the mark by a wide margin when it comes to calculation of pressure.


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