Preparation Of Zeolite Composite Membranes And Applications In DMF/H₂O Mixture Separation

Dimethylformamide （DMF） is a common industrial chemicals and anexcellent solvent，which is widely used in polyurethane, acrylic, medicine,dyes, electronics industries, etc. In order to eliminate the impact of waste onthe environment and reduce production costs, the DMF wastewater must berecycled，recovered and sold as a byproduct. The traditional separationmethods for DMF/H₂O mixture have many problems, including high energyconsumption, operation complexity and second pollution. Pervaporationtechnology is an effective, energy-efficient and environmental separationmethod, which is considered to be potentially useful for the separation ofDMF/H₂O mixture. In this study, the preparation of zeolite compositemembranes were characterized by pervaporation （PV） performance, and theseparation process of the DMF/H₂O mixture passing through zeolites wereinvestigated by molecular simulation.The hydrophilic and dense NaA composite membranes were synthesizedon α-Al₂O₃and polytetrafluoroethylene （PTFE） film. The crystal sizes ofNaA/PTFE composite membrane prepared by situ hydrothermal synthesismethod, hydrothermal secondary growth method and scratching method wereabout1⁸μm,1-7μm and1²μm. The PV results of DMF/H₂O mixtureindicated that the best cristallization times for NaA/PTFE compositemembranes prepared by hydrothermal secondary growth method andscratching method were4and2, the optimum operating temperature was80℃and the downstream pressure were10kPa,20kPa, respectively. At the bestpervaporation operating conditions, the separation factor and flux aboutNaA/PTFE composite membrane prepared by secondary growth synthesiswere23.5and1.25kg/（m2·h）, and that by scratching method were8.71,1.16kg/（m2·h）. The maximum separation factor of membrane prepared byhydrothermal secondary growth method and scratching method were23.62and23.78when the DMF mass fraction in feed were30%and50%, respectively. The large scale preparation of NaA/PTFE composite membranewas conducted by scratching method, and the PV results of compositemembrane showed good separation selectivity for DMF/H₂O mixture. It setfoundation for further enlarge and industrialization.The ZSM-5and Sn-ZSM-5zeolite membranes were synthesized onα-Al₂O₃and PTFE films. The PV results indicated that the Sn-ZSM-5/PTFEcomposite membrane was hydrophilic. The PV performance of Sn-ZSM-5composite membranes prepared by hydrothermal secondary growth was muchbetter than that by situ hydrothermal synthesis method. The best cristallizationtimes of Sn-ZSM-5/α-Al₂O₃and Sn-ZSM-5/PTFE composite membrane were2and3. The DMF mass fraction in permeates side for Sn-ZSM-5/α-Al₂O₃composite membranes was up to33.36%. The optimum operatingtemperature of Sn-ZSM-5/PTFE was80℃, and the DMF mass fraction inpermeates side was up to31.42%. The Sn-ZSM-5composite membranesachieved the aim of DMF recovery, and the separation performancedegradated with the increasing of DMF mass fraction in feed solution.The grand canonical ensemble Monte Carlo （GCMC） method,Equilibrium Molecular Dynamics （EMD） and Non-equilibrium MolecularDynamics （NEMD） had been utilized to investigate the diffusion of DMF andwater in zeolite NaA and ZSM-5zeolite membranes. The calculation resultsshowed that the mechanism of DMF/water mixture passing through thezeolite membrane could be described by sorption-diffusion model, and thewater and DMF molecules diffused to the permeation side with aback-and-forth moving. In addition, the average energy barrier values ofwater molecules permeating through NaA and ZSM-5zeolite membrane insimulation system with DMF mass fraction of10%and20%were1.327kcal/mol,0.908kcal/mol and1.716kcal/mol,1.830kcal/mol. The effects oftemperature and feed concentration on water flux were simulated, and thetrends of that were consistent well with the experiment dates.