Preparation Of Electrospun Nanofibrous Composite Membranes And Their Application In Oil-in-water Emulsions Coalescence Separation

With the development of society,the demand for petroleum energy has increased sharply,and the consequent oil pollution has caused great harm to the global ecological environment and threatened human health seriously.Therefore,effective disposal of the oily water is becoming an urgent global problem.According to the difference of the droplet size,the oil existing in water can be divided into four types:suspended oil,dispersed oil,emulsified oil and dissolved oil,among which emulsified oil is the most difficult to be disposed for its high stability.Traditional oil-water separation technologies such as sedimentation,membrane separation technology,and biological method have been used to solve this problem and achieved certain separation effects.However,these methods have the disadvantages of high energy consumption and low efficiency.Coalescence technology is a kind of physical method that relies on the affinity difference of oil and water for the separate materials,in which the tiny oil droplets grow up to large ones.Finally,oil is separated by gravity sedimentation.The coalescence separation has many advantages such as no secondary pollution,easy operation,and low cost,which has attracted more and more attention.The key of this technology is the effective interception of the emulsified droplets and their aggregation and growth on the surface of the material,which is closely relative to the diameter and the surface wettability of the fiber materials used.However,the fiber used in traditional coalescence materials have large diameter,small specific surface area,and uncontrollable surface wettability,which results in some defects of the coalescence bed such as large bed height,low stacked bed density,and poor oil-water separation efficiency.In order to improve the oil droplet interception efficiency of the emulsion and adjust the wettability of the materials,electrospun nanofibrous materials were used to improve the oil-water separation efficiency in this work.Furthermore,inorganic nanoparticles were added to improve the roughness of the nanofibers,which was favorable to oil-water separation.The main research work is as follows:（1）Preparation of electrospun polyvinylidene fluoride（PVDF）nanofibers.The effects of molecular weight of PVDF,solvent,solution concentration,voltage and tip-to-collector distance on the morphology of nanofibers were studied in detail.The rough and homogenous PVDF nanofibers were obtained under the following conditions:the relative molecular mass of PVDF 7 million,DMF as the solvent,the solution concentration 15%,the electrospinning voltage 15 k V and the tip-to-collector distance 12 cm.（2）Preparation of PVDF nanofibrous composite membranes and their application in emulsified oil-water separation.Composite PVDF nanofiber membranes with different fiber deposition densities on polyester scrim were fabricated by controlling electrospinning time.Then,a coalescing unit using the above composite membrane was prepared by layer-by-layer method to separate emulsified oil and water.The effects of different layers,different flow rates,different initial concentrations,and different oils on oil-water separation performance were studied in detail.When the number of layers of the composite membrane was 4 and the flow rate was 0.594 m min^-1,the separation efficiency of the hexadecane-in-water emulsion reached99.5%.The separation efficiency remained above 98%when the initial hexadecane concentration was 2000 ppm.In addition,the coalescing material had very good separation efficiency on n-octane,diesel and rapeseed oil emulsions.（3）Improvement of the roughness of nanofibers coalescence material.In order to improve the roughness of the nanofiber,SiO₂nanoparticles were added to the spinning solution and used to prepare the nanofibrous membranes.The effect of the SiO₂nanoparticle size,content of SiO₂nanoparticles and the spinning process parameters on the morphology of nanofibers was studied,and their application performance in emulsified oil-water separation was investigated.The optimized operation condition was obtained when the particle size of SiO₂ was 200-400 nm with 4 wt%and the electrostatic spinning voltage 15 k V and the distance 12 cm.The presence of SiO₂ nanoparticles improved the roughness of the PVDF nanofibrous membrane,which significantly improved the separation efficiency of hexadecane emulsion.