Study On Vapor-Liquid Equilibrium Of Ethylene-Acetic Acid System With Gibbs Ensemble Monte Carlo Simulation

Fluid phase equilibrium data makes up an important part of data-base in thechemical engineering design and industrial process. Besides experimentalmeasurement and semi-empirical calculation, molecule simulation is an importantway to get fluid phase equilibrium data. Nowadays, Gibbs ensemble Monte Carlomethod (GEMC) is one of the most widely used molecule simulation methods. In thismethod, it does not need to consider phase interface, and calculate chemical potentialdirectly. Force field has an important effect on the accuracy of the simulation results.The data of vapor-liquid equilibrium of ethylene-acetic acid is essential for theproduction of vinyl acetate; however, it is difficult to obtain related experimental data.The acetic acid associating needs to be considered as it can affect the phaseequilibrium of the ethylene-acetic acid system. In this paper, the vapor-liquidequilibrium of binary mixtures ethylene-acetic acid is calculated by GEMC method.In order to obtain reliable vapor-liquid equilibrium data, quantum chemical methodwas used to study the associated acetic acid model. The approach of modifying theforce field parameters for the associating system was explored.Firstly, the simulation details of GEMC were confirmed. TraPPE-UA force fieldwas employed to calculate the solubility of ethylene in acetic acid. By comparing thesimulation results with experimental data in reference, the set up of potential functions,mixing rules, molecule models and simulation details were correct.Secondly, it is found that TraPPE-UA force field can not reflect associationcharacteristics of the acetic acid, which cause simulation results deviated with theexperimental value. The acetic acid dimer model and three-dimensionaldetection-particle model were built through quantum chemical method, andintermolecular energy was calculated. Lennerd-Jones12-6potential function waschosen to fit the intermolecular energy, and the potential function parameters from thefitting were used to modify force filed parameters. The TraPPE-UA force field wasmodified twice. It can be seen from the simulation results that both of the twomodified force fields are better than that under unmodified TraPPE-UA force field. Itis indicated that the two modified force fields can give a better description of thethermophysical properties of the ethylene-acetic acid system, which leads to more accurate vapor-liquid equilibrium data in GEMC simulation.Finally, Acetic acid dimer model and three-dimensional detection particle modelwere compared. It is found that the latter was better, which is just about12%ofdeviation comparing to experimental value. Based on all of the results, originalTraPPE-UA force field and two modified force fields were used to predict thevapor-liquid equilibrium of the ethylene-acetic acid system in P=101.325kPa,T=285-395K.