Preparation Of PVDF Hydrophobic Membrane And Study On Membrane Distillation Integrated Technique

Membrane distillation (MD) is a separation technique, which depends on the vapor pressure difference between two sides of the membrane as the driving force of the transport. Compared to others separation technique, MD has the following advantages: higher rejection, higher concentrating multiple and lower operating temperature. MD has a bright future on the applications of sea water desalination, high salt concentration waste water treatment and concentration of temperature sensitive materials. The research of MD is more necessary at the present conditions that the pollution and shortage of fresh water resource becomes more and more serious. But now, there are some problems need be resolved for the commercial application of MD: (1) MD flux of the membrane is still need to be improved. (2) Only integrated with the traditional unit operations, and utilize the regeneration energy and low quality heat resource well, could MD show its advantage.Aimed to these problems, PVDF membrane used for MD were prepared via immersion precipitation phase inversion technique in the thesis. Membrane formation mechanism and influence of various parameters on membrane morphology and its MD performance were discussed. Based on the study of MD process, applications of MD integrated systems were carried out. Mainly research of the thesis is focuses on the following aspects.(1) Discussion on membrane formation mechanismBased on the Flory-Huggins theory, Phase equilibrium equations of PVDF/DMAc/water ternary system were wrote out, and the binodal and spinodal lines in the phase diagrams of the system were obtained via numerical calculation. Combined the kinetic description and the thermodynamic phase diagram, membrane formation process were analyzed, which prepared the theoretical foundation for the further membrane formation mechanism research.Gelation boundary of PVDF/DMAc/(Additive)/water system at certain temperature was determined by the "cloud point" method. According to the data processing of experiment,it was found that a linearized cloud point curvecorrelation,In(φ₁)/(φ₃) = bln(φ₂)/(φ₃)+a, satisfied the system. Moreover, a theoretical verification of the relation was carried out based on Flory-Huggins theory.(2) Analyse of PVDF membrane formation conditions and influencing parametersInfluences of solvent, molecule weight of polymer, dissolving temperature and maturation time of dopes, additives and gelation bath temperature on the morphology, crystallization and performance of prepared membranes were examined. Experiment results shown that DMAc was the best choice to used as solvent to prepare PVDF flat membrane for MD. Among the examined PVDF materials with various molecule weight, PVDF with molecule weight of 431000 is more suitable to prepare membrane for MD.Cross section morphologies of membranes prepared by dopes dissolving at different temperature are composed of interconnected globule-like particulates. Moreover, the surfaces of the globules are bi-continuous network structures. Combined the membrane morphology and the ternary system phase diagram, it is considered that, which is the results of the cooperation of binodal liquid-liquid demixing and spinodal liquid-liquid sub-demixing during the membrane formation process.With the increasing of the dissolving temperature of the dopes, the diameter of the globules becomes larger. Morphologies difference of the membranes is attributed to the solvolytic reaction of the system. The membranes performance results show that lower dope dissolving temperature (50-70℃) is comparatively suitable to prepare membrane for MD.Maturation time of dopes has great influence on the morphologies of the prepared membranes. For the dope prepared in the initial day, the liquid-liquid demixing is ahead of the solid-liquid demixing in the process of the membrane formation. The morphology of the prepared membrane is fingerlike structures with sponge substrate beneath the porous skin in cross section. For the cases of long time matured dopes, solid-liquid micro-phase separation results in the existence of the micro-liquid phase area and micro-solid phase crystallite area in the dopes. In the process of the membrane formation, the micro-solid phase crystallites are connected by the polymer chains together, which results in three-dimensional network gelation morphology. With the maturation time lasting of the dopes, the porosity, the mean size of the porous and the flux of the prepared membranes are increase in general.According to the research of the influence of additives on membrane morphology and performance, it is found that, the prepared membranes using mixed additives of glycerol (2wt%) and water (3wt%) has higher MD water flux and salt rejection. Moreover, the prepared membranes have less defect (example pinhole), and the shrinkage phenomena in the drying process of membrane is improved greatly.Based on the phase diagram analysis and experiment results, it is found that gelation is the dominate membrane formation mechanism at low gelation bath temperature for the examined system (PVDF: 15 wt%; DMAc: 79 wt%; water: 6 wt%), and the prepared membranes shrink seriously in drying process. While the bicontinuous structure of the formed membrane at 60℃gelation bath temperature is attribute to spinodal decomposition. The different membrane formation mechanisms are validated by the light transmission measurements during membrane formation process. Higher gelation bath temperature (50℃～60℃) is more suitable to prepared PVDF flat membrane for MD.(3) Study on vacuum membrane distillation processMechanism of vacuum membrane distillation (VMD) process is discussed. VMD experiments were performed to evaluate the influence of various operating parameters on MD flux for brine desalination process using PVDF membrane self-prepared. The experiment results show that, the flux increase exponentially with the increase of the inlet temperature; With the flow rate increase of the feed, the flux increase linearly, but the increase extent is not much; A reduction of the permeate flux occurred with the increase of salt concentration in feed solution, which attributes to both the decrease of the activity of the water and the effect of concentration polarization at the surface of the membrane. Vacuum degree of penetration side influence the flux greatly after the vacuum degree is higher than 0.08Mpa. Compared to the influence of salt concentration and flow rate, feed temperature and vacuum degree of penetration side have more obviously influence on the VMD flux.(4) Applications of MD integrated techniquesA set of Solar/VMD Integrated System (SVMDIS) of for potable water production from underground water was designed. The experiment results shown that the largest permeate flux of the pure water is amount to 32.19 kg/m²h, and the largest cumulated flux for one day is up to 173.5 kg/m². The energy analysis shown that the energy consume of the solar/VMD system is only 3.88% that of the VMD operation separately to obtain the same quantity of pure water in one day.A set of Membrane distillation/Evaporation/Crystallization Integrated (MDECI) System (10 ton/day) was designed for treatment of epoxy resin wastewater. The technical design and engineering calculation of the system were performed based on the experiment data on spot. The treatment Engineering installation and operation of the system was performed in the factory. The system resolved the tough problem that wastewater with high COD and high salts can't be treat directly via traditional biochemical method, and high energy consume problem for traditional technology. The MDECI system realized the synthesis utilization of energy source, and obtained the target of energy saving and green production.