Molecular Dynamics Simulation On The Diffusion Properties Of PEBA Membrane And The Pervaporation Performance Of PEBA-2533/MCM-41Hybrid Membrane For Separating Phenol/Water Mixture

Phenol, as one of the main pollutant of industrial wastewater, itsstrong toxicity and hard biodegradation pose serious threat to environment andhuman health. Considered from its industrial value and environmental hazards,separation and recovery of phenol are of great significance. Pervaporation, as anew type of membrane separation technology, is characterized by easy operation,energy saving and no secondary pollution compared with the traditionalseparation process, as for which it has attracted more attention. By our previouswork, poly(ether-block-amide)(PEBA-2533) membrane showed the bestpervaporation properties for phenol. And pervaporation-crystallization couplingsystem (PVCC) can be used to recover high purity phenol crystals from diluteaqueous solutions, which indicates that it is necessary for us to study the masstransfer characteristics of PEBA-2533membrane and improve the pervaporationperformance. In this article, Materials studio(MS) software was adopted to studythe diffusion characteristics of PEBA-2533membrane. Moreover, PEBA/MCM-41hybrid membrane was prepared to improve the pervaporationperformance.PEBA-2533modules were constructed by MS software. In order totestify the accuracy of the used force field and the reliability of PEBAmembrane, the glass transition temperature Tgand WAXD diagram wereanalyzed. One module containing both feed and membrane molecules was buitto explore the behaviors of feed components at the interfacial region andevaluate the selective adsorption of phenol on the PEBA membrane surfacesfrom molecular scales. PEBA-2533modules under different feed concentrationsand temperatures were constructed to understand the intrinsic relationshipbetween membrane structures and feed diffusion behaviors. Free volumefraction (FAV), pore size distribution (CSD) and mean square displacement(MSD) were calculated to analyze the membrane morphology and diffusivitiesof feed molecules. The simulated and experimental diffusion coefficients ofphenol and water under different temperatures were compared to further confirmthe accuracy of simulation. The results showed that phenol molecules havestronger affinity with PEBA-2533membrane. Compared with the watermolecules, phenol moved to the membrane surface more quickly, which inducedthe motility of film segment and membrane expansion. Phenol molecules couldenter the membrane more easily and swelling occurs along with them. With theincreasing of feed concentration, membrane swelling led to higher FAV valuesand larger cavities, which improves molecular diffusion performance. The degree of membrane swelling decreased with rising feed temperature, whichresulted in lower FAV values, narrower holes in membranes and did’t favormolecular diffusion. However, the molecular thermal motion increased withtemperature increasing, feed molecules’ diffusivities were enhanced. In addition,simulated diffusion coefficients agreed well with experimental values, whichindicate that the simulation results are reliable. The molecular dynamics(MD)simulation technique proved itself as a potential approach to characterize thePEBA-2533membrane performance during the pervaporation separationprocess.Mesoporous MCM-41filled PEBA-2533membranes were preparedfor pervaporation separation of phenol from dilute aqueous solutions. Thestructural morphology and thermal stability of these hybrid membranes werecharacterized by SEM, FT-IR and TGA. The effects of MCM-41content on thesorption, diffusion and pervaporation performances were investigated. Theresults showed that the swelling degree and diffusion separation factor ofPEBA/MCM-41membranes increased. When the MCM-41content was4%, themembrane’s swelling degree increased by43%compared with that of plainPEBA. With the increase of MCM-41content, PEBA/MCM-41membranesdisplayed significantly improved permeation flux and almost equivalentphenol/water separation factor. The hybrid membrane containing4wt%MCM-41showed the best pervaporation performance with increasing phenolflux by21%. The diffusion coefficients of phenol and water were also estimated by Fick’s first law, which suggests that using a mesoporous molecular sieve asthe filler could enhance penetration of the polymer. Effects of operatingconditions on the separation performance have been systematically assessed. Itwas found that the permeation flux increased while the separation factordecreased with the increase of feed concentration. All the hybrid membranesdemonstrated increasing separation factor and permeation flux with increasingtemperature.