Study On Microscopic Mechanism Of Solvent In Extractive Distillation With Molecular Simulation

There are many compounds which are difficult to be separated,such as benzene,thiophene and cyclohexane.The extractive distillation is a common method to separate these close-boiling compounds or azeotropes.In the extractive distillation,the vapor liquid equilibrium data is the basic data for design and optimization of the separation process.And selecting appropriate solvents is the key to achieve low energy consumption and high efficiency.The traditional solvent selection methods are usually based on experiences or experiments,which are not only hard to screen solvents,but also can not study the extraction mechanism of solvents.However,studying the extraction mechanism is very important for the selection of solvents and molecular design.Therefore,in this paper,Monte Carlo method was used to calculate the vapor-liquid equilibrium of mixtures which are difficult to be separated.And based on this,the extraction mechanism of solvent was studied.Meanwhile,the selectivity of solvents was compared by using the quantum chemistry method,and exploring the extraction mechanism of solvents,which provides theoretical guidance for the selection of solvents and molecular design.The accuracy of the simulations for vapor-liquid equilibrium depends on the force field used in the calculations.Due to lack of the force field parameters of solvents,such as N-Methyl-2-pyrrolidone(NMP),an extensional TraPPE-UA force field of NMP was constructed by combining Monte Carlo simulation and the quantum chemistry method.The new force filed was used to calculate the vapor-liquid phase equilibrium of NMP.The results showed that this new force field is able to accurately predict the vapor-liquid equilibrium of NMP,which lays the foundation for the simulations of vapor-liquid phase equilibrium of mixtures containing NMP.Based on the new force field,the Gibbs ensemble Monte Carlo(GEMC)method was used to calculated the vapor-liquid phase equilibrium of solvent and compounds which are hard to be separated,such as NMP-benzene,NMP-thiophene and NMP-benzene-thiophene.The simulation results were in agreement with the experiments,which further confirmed the practicability of the new force field.And the radial distribution functions of benzene-thiophene-NMP ternary liquid mixture were calculated to explore the mechanism of extracting thiophene from benzene by NMP.The results showed that the oxygen atom of NMP forms stronger hydrogen bonding with the hydrogen atom of thiophene than with that of benzene.Perhaps it was one of the most important reasons that NMP could be used as an extracting agent to separate benzene and thiophene in the extractive distillation.The quantum chemistry method was used to calculated the interaction energies between solvents,such as DMF,NMP,DMSO,Sulfolane and compounds which are difficult to be separated,such as benzene,cyclohexane.The selectivity of solvents was compared through calculating the interaction energy difference between complexes-solvents-benzene and complexes-solvents-cyclohexane.And the selectivity of solvents in the benzene-cyclohexane system followed this order DMSO>DMF>NMP>Sulfolane.Through the AIM and RDG analysis,it could be found that C-H…? interactions only exist the complexes-solvents-benzene,and solvents form stronger hydrogen bonding with cyclohexane than with benzene.Moreover,CH…? interactions were found to be dominant when comparing with hydrogen bonding effect.It also could be discovered that solvents with excellent performance in benzene-cyclohexane system should have strong electron withdrawing group and small molecular volume by the ESP analysis.