Study On The Separation Of Methanol-DMC Azeotropic System By Extractive Distillation With Molecular Simulation

Phase equilibrium data serve as important data of basic properties in the unit operation of distillation.Experimental and semi-empirical methods are used to acquire the phase equilibrium data traditionally.With the development of calculation level,molecular simulation has become a significant way to acquire the phase equilibrium data.As an azeotrope formed in the system of methanol and dimethyl carbonate?DMC?,and it could be separated by extractive distillation.Ionic liquids?ILs?are economic,environmental friendly and green extractants,especially imidazole-based ILs are favored by researchers owing to their good effects on the separation of this azeotrope.As the interaction mechanism is not clear between the extractants and components,it cannot provide a convincing explanation for the mechanism of separation process.So how to choose an optimal ILs extractant becomes a problem of industrial application.Firstly,the mechanism of DMC and low carbon alcohol azeotropes formed was explored by the Gibbs Ensemble Monte Carlo?GEMC?simulation method at the molecular level.And we attempted to search for the azeotrope clusters in order to reveal the mechanism of azeotrope formed.The missing parameters for bond,angle and dihedral angle of DMC molecule in TaPPE-UA force field were supplied based on the method of quantum chemical calculation.The GEMC method in combination with NVT ensemble was used to calculate the properties of vapor-liquid coexistence curve,saturated vapor pressure and the enthalpy of evaporation for the pure DMC,which conformed the accuracy of new parameters and the applicability of the force field.Then the phase equilibria of four azeotropic systems methanol-DMC,ethanol-DMC,n-propanol-DMC and n-butanol-DMC at atmospheric pressure were simulated by NPT-GEMC method,and the simulation results were in good agreement with experimental data.Furthermore,the equilibrium liquid phase microstructures of these systems were analyzed by radial distribution functions.It was found that self-association of hydrogen bonds between alcohol molecules would weaken when the mole fraction of the alcohol increased,and the interaction between alcohol molecules and DMC molecules also attenuated.And with the growth of carbon chain in the alcohol molecules,the lipophilicity of the hydroxyl would be strengthened,which resulted in stronger interactions between alcohol molecules and DMC molecules.Next,based on the mechanism of azeotrope formation,a DFT quantum chemical calculation method was used to explore the interaction of methanol/DMC and ILs com-posed of different cations and anions.We tried to reveal the separation mechanism of methanol and DMC azeotrope by extractive distillation with ILs and develop a scientific method for the screening of the extractants.In this paper,four different ILs[BMIM][OAc],[BMIM][Cl],[EMIM][Br],and[OMIM][BF₄]were chosen.And the strength and nature of weak interactions were explored by the analysis of interaction energies,AIM and RDG function.The calculated results demonstrated that the interaction between methanol and DMC molecule was weaker than that between the ion pairs and methanol or DMC molecule.A decreasing trend was presented for the interaction energies:complex[BMIM][OAc]-methanol>[BMIM][Cl]-methanol>[EMIM][Br]-methanol>[OMIM][BF₄]-methanol and the complexes ILs-DMC shared the same order.The differences in the interactions decreased in proper order.The interactions between ILs and methanol were much stronger than that between ILs and DMC.