Hydrophobic Modification Of PVDF Hollow Fiber Membrane And Its Application In VMD

Membrane distillation technology provides a simple and effective approach for not only utilizing the chemical waste water&heat but also solving the lack of freshwater resources.The main cause holding back MD application is the relative shortage of membranes with high performance.Therefore,the development and hydrophobic modification of membrane distillation materials is crucial for the performance of MD and its mass industrialization.In this paper,Memsino PVDF hollow fiber membrane by TIPS method was used as substrate,two surface modification methods,which reduce the surface free energy and increase the surface roughness with crosslinked PDMS,were proposed to improve the hydrophobicity of PVDF hollow fiber membrane substrate.In addition,a specially-designed 10-inch membrane module was used to investigate the membrane distillation separation performance of sodium chloride aqueous solution systematically.Firstly,the PVDF hollow fiber membrane was treated with anhydrous ethanol to remove the organic solvents in the membrane.In this paper,Polydimethylsiloxane(PDMS),Tetra ethyl orthosilicate(TEOS)and Dibutyl tin dilaurate(DBTDL)catalyst were employed as surface coating agents on polyyinylidene fluoride(PVDF)hollow fiber membrane to prepare PVDF/organosilicon hydrophobic composite membrane.Some major factors on coating result were investigated,including the cross-linking agent molecular weight,coating liquid ratio,coating time,curing temperature,etc.Secondly,the membranes were characterized by scanning electron microscopy(SEM),Fourier transform infrared spectroscopy(FTIR),water contact angle(WCA),pore size distribution to study the changes of surface morphology,the chemical composition,the hydrophobicity property and the pore size.The analysis results revealed that,compared to the original PVDF membrane substrate,the modified composite membrane shows improved MD flux running time loss and hydrophobic property(contact angle from 81°to 115°).Thirdly,based on the hydrophobic PVDF hollow fiber research,sodium chloride solution desalination experimental work was carried out with membrane distillation installation,using specially-disigned(10-inch PVDF membrane module with effective membrane area of single membrane module up to 4m~2).The variation tendency of membrane flux with different parameters,including solution temperature,linear velocity of the feed,concentration,vacuum degree was studied systematically.It was shown that VMD permeation flux increased significantly with the increasing of feed temperature and vacuum degree.With the increasing of flow rate of feed,the permeation flux increased gently.With the increasing of concentration of brine solution,the water flux decline slightly.And under the designed conditions the membrane water flux reached 8.24LMH,desalinization rate achieved above 99.9%,while the salinity of the rejected sodium chloride solution reached over 20%.During the experiment process,the membrane and the membrane distillation system demonstrated good anti-fouling property,strong membrane reproducibility,and simple cleaning requirement.Further,the correlation of VMD flux and other experimental conditions,including length and filling quantity,the feeding direction,N/A of center pipe,was investigated.It was shown that the VMD flux of sodium chloride solution increased with decreasing membrane length or packing fraction of the membrane modules.The membrane distillation flux of the shell side is 26%-40%higher than that of the tube side.The VMD flux with center tube is 30%higher than that without center tube.In conclusion,the hydrophobic process for PVDF hollow fiber membrane in this thesis possesses important advantages,such as simple and flexible procedures,low-cost,and environment-friendly,these make the PVDF hydrophobic membrane a good candidate for membrane distillation applications.Systematic vacuum membrane distillation brine desalination experimental study results show that the vacuum membrane distillation is a outstanding brine desalination technique,with large potential for large-scale industrial applications.