Study On The Molecular Simulation Of The Association Characteristics And Vapor-liquid Phase Equilibrium Properties For Acetic Acid Systems

The strong association between acetic acid molecules leads to serious non-ideality of the thermodynamic properties,which affects the prediction accuracy of thermodynamic data.GEMC method has become an effective way to obtain phase equilibrium data.However,there is hardly accurate description on the association between acetic acid molecules in the existing force field.In this paper,aiming at the associating properties between acetic acid molecules,a new method for descripting the associating interaction between acetic acid molecules by the polarized acetic acid molecule model was proposed based on the formation mechanism of intermolecular hydrogen bonds between acetic acid molecules.The vapor-liquid phase equilibrium properties of pure acetic acid component and binary systems containing acetic acid were investigated by GEMC method,verifying the applicability of the model to the simulation of vapor-liquid phase equilibrium properties,which could provide guidance to obtain vapor-liquid equilibrium data in industrial production.In this thesis,the molecular dynamics methods were used to study the association conformers and their distribution of acetic acid molecules under different conditions.The proportions of acetic acid monomer and multimers were obtained,which provide the basis for the vapor-liquid equilibrium simulations.Quantum chemistry methods were used to study the association mechanism between acetic acid molecules.By comparing the structure and non-bonded interaction energies of different dimers,it is found that the cyclic dimer containing two OC---HO hydroge n bonds had the most probability to exist when acetic acid molecules approached.The electronic properties of dimers showed that the electrons around the carbonyl oxyge n atoms were deflected by the attraction of hydrogen atoms in the other molecule,which polarized the acetic acid molecules when the hydrogen bonds between acetic acid molecules were formed,providing theoretical basis for the polarized acetic acid molecular model.The polarized acetic acid molecular model was proposed for the first time,and the TraPPE-UA-Polar force field describing the association of acetic acid molecules was constructed.The missing force field parameters were supplemented by quantum chemical methods.The vapor-liquid equilibrium simulation of the pure component acetic acid was carried out and compared with the experimental values.The results showed that the statistical average error of acetic acid liquid phase density was 0.97%,and that of gas phase density was 3.88%,which was more accurate than the results calculated by TraPPE-UA force field.The vapor-liquid equilibrium data of binary systems containing acetic acid were simulated by TraPPE-UA-Polar force field with the molecular model of polarized acetic acid,and compared with experimental values.The results show ed that compared with the TraPPE-UA force field,the average statistical error of the mole fractions in liquid and that in gas phase of methyl acetate-acetic acid system were reduced from 4.80%and 10.10%to 3.73%and 4.15%,respectively.The average statistical error of the mole fraction in liquid and that in gas phase of ethanol-acetic acid system were reduced from 18.15%and 17.49%to 6.36%and 7.39%,respectively.The average statistical error of CO₂ solubility in acetic acid solution was 6.86%.The applicability and accuracy of the proposed to the vapor-liquid equilibrium simulation of different binary systems were verified.It could also provide a new method to obtain the vapor-liquid phase equilibrium data in industrial production.