A Molecular Insight On The Vapor-Liquid Equilibrium Based On GEMC Method

In recent 10 years, the application of Monte Carlo simulation approach in the region of physics and chemistry is gradually becoming a research focus. Gibbs ensemble Monte Carlo(GEMC) simulation approach is commonly used to solve problems of molecular thermodynamics, and has been proved to show better adaption than other molecular simulation methods to predict the behavior of phase equilibrium. As azeotrope shows particular phase behavior, causes of this phenomenon and its structural characters haven't been reasonably reported in detail, which greatly limits the separation design of this kind of mixture in industrial engineering. GEMC and TraPPE-UA force field is used in this article to calculate the vapor-liquid equilibrium(VLE) of polar-nonpolar azeotrope for evaluating the adaption of TraPPE-UA model and explore the particular microstructure of azeotrope and reasons leading to azeotropic phenomenon in detail, thus represents model assumptions and important inferences. Except for azeotrope, our study also evaluates the results of VLE for mixtures that hold the adjacent boiling point from GEMC method and Tra PPE-UA or OPLS-AA force field. Likewise, the equilibrium liquid microstructure and distribution of hydrogen bonding association are also briefly explored so that some rules can be summarized.Firstly, taking ehanol/benzene as the study system, the article calculate the VLE of this azeotrope by utilizing GEMC simulation and TraPPE-UA force field, and the results are compared with the data from experiments and Wilson EOS. It reveals that a good description of VLE behavior can be achieved by the simulation. Moreover, the article proposes a hypothetical model ‘azeotropic cluster' to help understand the formation of azeotrope and their structural properties, in which the hypothesis are preliminarily proved by analyzing the radial distribution functions(RDFs) and its integral. Besides, analysis in the distribution of association cluster demonstrates that the formation of ethanol/benzene azeotrope correlates to the large amount of ring structures in the system. Therefore, it can be deduced that ring structures are the necessary condition for the azeotropic behavior.GEMC simulation method is also conducted to calculate the VLE of three systems: ethylene glycol/1,2-Butanediol(EG/1,2-BDO)?ethylene glycol/1,3-propan-ediol(EG/1,3-PDO) and ethylene glycol/1,4-butanediol(EG/1,4- BDO). It shows that when OPLS-AA force field is applied to EG/1,2-BDO system, and TraPPE-UA force field is applied to the other EG/1,3-PDO and EG/1,4-BDO system, simulation results are basically in accordance with the experimental data. Furthermore, the RDFs analysis in liquid phase of three system reveals the order of interaction between different molecules is: EG/1,2-BDO > EG/1,3-PDO > EG/1,4-BDO. On the other hand, the weaker interaction between EG and 1,4-BDO molecules also can be demonstrated by the analysis of distribution of hydrogen bond association for EG/1,4-BDO and EG/1,3-PDO system, which exposes the reason why EG is likely easier to be separated in EG/1,4-BDO.