| Membrane distillation is a novel membrane separation technology for desalination. With the advantages of low operating pressure, high salt rejection and widely application, membrane distillation is a promising technology in desalination, drawing great attentions by many researchers. Of all the factors of restricting its application, membrane fouling is one of the key factors.In this thesis, the key object is calcium and magnesium fouling in the process of membrane distillation. The experiment research force on the temperature polarization and concentration polarization and their effects on membrane fouling systematically. Founding that the polarization in vacuum membrane distillation process was influenced by feed flow rate, feed temperature and the vacuum. The feed flow rate is the main factor which affected the polarization. Increasing the feed flow rate could weaken the polarization phenomenon significantly in vacuum membrane distillation (VMD) process. Another important influencing factor is the pure water flux. Generally speaking, the bigger the flux is, the more serious the polarization is. Therefore, the operating parameters such as temperature and vacuum would influence the polarization too.The fouling mechanism of calcium and magnesium ions which were easy to contaminate the film had been discussed respectively in this thesis. The results showed that the contamination caused by calcium carbonate and calcium sulfate was mainly formed by the attachment of the insoluble substance on the fibers. The adsorptivity of calcium sulfate crystals on the membrane was poor and its contamination was unstable. Many experiment results showed that calcium sulfate produced serious contamination to the membrane. The contamination of magnesium carbonate was caused by the formation of small whisker-like crystals around the membrane pores. The effect of NaCl on the scaling of calcium and magnesium ions had been investigated. The results showed that NaCl would increase the solubility of the calcium and magnesium ions and it would weaken their contamination to the membrane distillation process.Finally, the washing and recovery of membrane had been studied. The results showed that for membrane fouling caused by calcium and magnesium carbonate, the membrane can be well resumed by the use of traditional anti-flush and dilute hydrochloric acid. For membrane fouling caused by calcium sulfate, the effect of anti-flush and dilute hydrochloric acid wash washing not obvious. Citric acid washing will be better.In summary, the mechanism of calcium and magnesium fouling in the process of vacuum membrane distillation had been investigated and the model had been established. These work laid elementary foundation for the systematic research on the membrane fouling in the VMD desalination process. |
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