Molecular Simulations On The Separation Of Fluid Mixtures

Adsorption separation of the fluid mixture is an operating unit of chemical process andhas an important influence on seawater desalination, purification, wastewater treatment,environmental protection etc. The molecular simulation methods (DFT, MC and MD) andGaussian, Materials Studio, MuSiC and RASPA softwares can be employed to studymembrane properties (free volume, degree of polymerization, functional groups), the natureof adsorbate (molecular size, viscosity, fugacity, polarity), the interactions between membraneand adsorbate (van der Waals force, hydrogen bonds, electrostatics), the external conditions(temperature, pressure, humidity, feed concentration) on the adsorption separation behavior.The optimized adsorption loading and separation factor can be found for the actual adsorptionprocess. The thesis includes five parts, as follows.1. Grand canonical Monte Carlo (GCMC) simulation is used to investigate theperformance of poly (vinyl alcohol)(PVA) membrane in separating the azeotropicwater/ethanol mixture (95.57wt%ethanol) over a wide range of pressures (10–1000kPa),temperatures (298–338K) and PVA polymerization degrees (100–1000). By calculating thesorption isotherms and the ethanol-to-water separation factors, we observe that thewater/ethanol adsorption amount and separation factor decline slowly with the increase oftemperature; as the polymerization degree rises, both of adsorption amounts first increase andthen decrease, while the separation factor changes adversely. Concepts such as fractional freevolume (FFV) and hydrogen bonding interactions were analyzed to explain the observation.As the polymerization degree increases, the FFV changing trend is similar to the onementioned in the discussion of adsorption amount, but their inflexions are different. Hydrogenbonding interaction successfully explains this variation. We further deduce that the fact thatthe change of adsorption amount results from a transition from cooperation to competitionbetween FFV and hydrogen bonding interactions. The optimal operating conditions forseparation are298K and101.325kPa. Under this condition, the PVA membrane(polymerization degree1000) has a separation factor of～80for the water/ethanol azeotropicmixture, which means that ethanol can be refined to99.96wt%and anhydrous ethanol ispossible to be obtained by PVAmembrane evaporation.2. Adsorption and separation of1:99(volume ratio) H2S/N2mixture by single wallcarbon nanotubes are studied using the GCMC method at a range of nanotube diameters,pressures and temperatures. It is demonstrated that the selectivity towards H2S increases andthen decreases with increasing nanotube diameter and the selectivity is highest for (11,0) carbon nanotube, which is due to the synergy of geometry effect and energy effect. It is shownthat under different operation conditions, the adsorption isotherm and selectivity can varysignificantly. At100kPa, the amount of adsorbed of H2S in (11,0) carbon nanotube and theselectivity towards H2S firstly increase and then decrease with increasing temperatures.Moreover, at300K, with increasing pressures, the adsorbed amount of H2S and the selectivitytowards H2S decrease. The simulation findings in this work would be helpful for the designand development of sulfur removal processes.3. Parallel tempering and parallel mol-fraction grand canonical Monte Carlo simulationswith configurational bias are used to study the enantioselective adsorption of four alkanols ina homochiral metal-organic framework (MOF), known as HOIZA-1. Conventional GCMCsimulations are not able to converge satisfactorily for this system due to the tight fit of thechiral alkanols in the narrow pores. Parallel tempering and parallel mol-fraction simulationsovercome this problem because of the improvement of acceptance ratios of Monte Carlomoves, the results of infinite dilution enthalpy of adsorption also prove that they are superiorto conventional GCMC. The simulations show that the enantioselective adsorption of thedifferent (R,S)-alkanols is due to the specific geometry of the chiral molecules relative to thepore size and shape.4. GCMC simulations are employed to study the adsorption and separation of carbondioxide/methane gas mixture by five different metal organic frameworks (MOFs) includingthe unmodified MIL-53(Al) and four amine functionalized (-NH2,-(NH2)4,-NHCO,-CH2CONH2) MIL-53(Al) MOFs. It is found that although original MIL53had the bestadsorption amount, its separation efficiency is not very high. The carbon dioxide/methaneseparation factor of-(NH2)4amine functionalized MIL-53is the best in five MOFs. Moreover,the predicted separation performance of-NH2and-NHCO functionalized MIL-53alsosurpass that of the original one. However, the predicted separation performance of-CH2CONH2modified MIL-53is not so good; i.e., both its carbon dioxide/methaneseparation factor and adsorption amount are lower than those of the original one. Thegeometric effect and energetic effect are analyzed to explain the difference of separationefficiency. This work shows that a rational design of functionalized MOF is a feasible way toimprove the carbon dioxide/methane separation efficiency and to provide helpful informationfor future MOF preparation and applications.5. A novel3-step in-silica screening method for a large number of MOFs and theirfunctionalized ones is offered. The improvement of CO2separation capability from theCO2/CH4mixtures using different amine functionalized MOFs are discussed and analyzed carefully. In the first step, plenty of classical types of original MOFs are screened. In thesecond step, the various amine functional groups are screened. In the third step, differentnumbers of amine functional groups are screened. The results show that the separationefficiencies of (NHCOH)4-MIL-53and (NH2CH2CH2NH2)9-Co-MOF-74reach maximum inabout two hundred screened MOFs; amine functional group saturation degree is also proposed.The novel screening method and the concept of functional group saturation degree could beapplied in cost-effective experiments and reduce the period of the development of newfunctionalized materials in future researches.