Thermodynamic And Kinetic Properties Of Confined Fluid Studied By Molecular Simulation And Density Functional Theory

There are significant differences of the thermodynamic and kinetic propertiesbetween the confined and the bulk fluids. In this work, density functional theory andmolecular simulation are used to study the adsorption, phase equilibrium and transportproperties of fluid confined on planar solid, in slit-like pore and in ZSM-5.A new weighted density functional theory is proposed based on a modifiedfundamental measure theory for the hard-core repulsion, a second-order Taylorexpansion around zero-bulk-density for attraction, and a correlation term evaluated bythe weighted density approximation combined with Modified Benedict-Webb-Rubinequation of state. For the Ar/CO₂system，the thin-thick film transition，the wettingtemperature and the surface critical temperature are predicted accurately.The Gibbs Ensemble Monte Carlo（GEMC） simulation is used to investigate thevapor-liquid phase equilibrium of a binary Lennard-Jones（L-J） mixture with azeotropicpoints and the CH₄/N₂/CO₂binary mixtures. With the enhancement of the wallselectivity, the azeotropic point is shifted drastically and almost disappeared, theselectivity of the vapor-liquid phase equilibrium can be improved. The narrowing of thepore width almost has no influence on the location of the azeotropic point, but causesthe decreasing of the vapor-liquid phase selectivity. In unselectively slit-like pores, thestrengthening of wall-fluid interactions brings the up-shifting of thepressure-composition（p-x） phase diagram and the easier reaching of supercritical statefor each CH₄/N₂/CO₂binary mixture, and it also makes the vaporization enthalpy andthe vapor-liquid phase selectivity decrease. The narrowing of the pore width influenceson the phase behavior of confined mixture in different ways according to the wall-fluidinteractions: the p-x phase diagram shifts up and the density of vapor phase decreases instrong attractive slit-like pores, while that changes inversely in weak attractive slit-likepores. Grand Canonical ensemble Monte Carlo （GCMC） simulations are carried outfor supercritical L-J fluid, with a modified equation for the statistic of adsorptionenthalpy, and it is found that: at a reduced temperature T*1.5, a vapor-liquid phasetransition could be observed, which becomes more apparent with the narrowing of thepore width and the strengthening of the wall-fluid interaction.Using Compass force field with charge parameters modified by us, the adsorption and transport properties of NH₃in H-ZSM-5are simulated. The predicted adsorptionisotherm and adsorption enthalpy show good agreements with those from experiment.The mechanism of NH₃adsorption in H-ZSM-5is: locations around H⁺>the cross pointsof the pores>other positions of the pores. Both temperature rising and concentrationincreasing make the diffusion coefficient of NH₃increase and are benefit for thebreaking the strong interaction between H⁺and NH₃.A new force field for ZSM-5has been developed, and the adsorption properties ofNH₃and alkanes in H（Ag,Cu）-ZSM-5are simulated with it. Good agreements areachieved between the simulated adsorption isotherm and adsorption enthalpy andcorresponding experimental data. The adsorption concentration of methane and ethanein pores beyond the cross point is a little higher, and their adsorption enthalpy is about1/6¹/4of that of NH₃. When alkanes and NH₃are adsorbed in H-ZSM-5simultaneously, the promotion of alkanes on the adsorption of NH₃firstly increases andthen decreases as the length of the chains grows. But when the lenght of alkane exceedsheptane, the promotion effect changes into hindrance effect. The lengthening of thecarbon chain and the increasing of the alkane concentration both cause an increasing ofthe adsorption enthalpy of NH₃. The NH₃are mainly adsorbed around H⁺, while thealkanes are adsorbed in other positions of the pores and their adsorption in zigzag poresdecreases continuously with the lengthening of carbon chain until disappears. InAg（Cu）-ZSM-5, NH₃are also mainly adsorbed around metal ions. Several locationregions could be found around a metal ions, and it means that there are more active sitesin Ag-or Cu-ZSM-5when compared to that in H-ZSM-5.