| Membrane distillation technology,as one of the new membrane separation technology with good prospects for the development,has attracted more and more attention.However,the scarcity of membrane materials is a major factor restricting its development.It is still difficult to be applied in industrial scale.The polyvinylidene fluoride?PVDF?is a kind of hydrophobic,heat resistance and good chemical stable material,and the price is reasonable.It is one of membrane materials for membrane distillation.This paper selected PVDF as membrane material to prepare microporous membranes by employing the non-solvent induced phase separation?NIPS?method.Micro nano granular structure in the membrane surface was built by adjusting the casting solution structure and film forming conditions,which improved the surface roughness,and then improved the hydrophobicity of microporous membrane.With ethanol aqueous solution or fermentation liquid of Jerusalem artichoke hydrolyzates,vacuum membrane distillation?VMD?was carried out to test the separation performance for ethanol of the prepared membranes.Firstly,PVDF was used as membrane material to prepare superhydrophobic plate microporous membrane by NIPS method with N-methyl pyrrolidone?NMP?as solvent and small molecular compound as additive.The effects of coagulation bath composition,pre-evaporation time and ambient humidity and temperature on the membrane structure were studied.The phase exchange rate and the membrane structure were adjusted by changing the ratio of NMP solvent in the coagulation bath.The effects of pre-evaporation time on the surface roughness,pore size and porosity were investigated.The membrane was characterized by scanning electron microscopy?SEM?,water contact angle?WCA?,pore size distribution,porosity and gas penetrability.The results showed that when the volume fraction of NMP in coagulation bath increased from 0%to 68.5%,the water contact angle of the membrane increased from 100°to 152°.Considering the comprehensive practical application conditions,when the optimum pre evaporation time is 8 s,the porosity of the membrane is about 55.72%,the water penetration pressure is about 1.93 bar and the contact angle is 141°.With 5 wt%ethanol water solution,fermentation liquid of20 wt%Jerusalem artichoke,and fermentation liquid of 20 wt%inulin,respectively,VMD experiments were performed on the PVDF membranes.The results showed that the separation factor and flux of PVDF membranes with special micro nano rough structure were higher.For 5 wt%ethanol water feedstock,separation factor and flux were 7.23 and 5.24 kg m-2 h-1 at feed temperature of 50 ? and permeate vacuum degree of 88 kPa.Furthermore,in order to improve the water penetration pressure and anti wettability of membranes,the above-prepared micro nano rough PVDF film was coated on PTFE microporous membrane to prepare superhydrophobic PVDF-PTFE composite membrane.The experimental results showed that the water contact angle increased from 137°to about 153°due to the existence of micro nano rough structure,which made the pore size distribution narrowed,the maximum pore size decreased,and the water penetration pressure increased.With 20 wt%inulin fermentation liquid as raw material liquid,VMD experiments were carried out using composite membrane.When the PVDF concentration of the coating was 8 wt%,the separation factor increased from 5.8 of PTFE original membrane to 10.1.The PVDF-PTFE composite membrane prepared by this method has good performance in separating ethanol and has good application prospect. |
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