Study Of Air-gap Membrane Distillation Of NaCl Solution

Membrane distillation(MD) is a membrane separation process which is derived by the vapor pressure difference led by the existing temperature difference across the micro-porous hydrophobic membrane.MD is a promising technology which is being investigated worldwide as a high rejection,low cost,energy saving alternative to conventional separation processes such as distillation and evaporation.According to the type of condensation methods used in a MD process,MD systems may be classified into four different categories:(â…°) direct contact membrane distillation(DCMD), (â…±) air gap membrane distillation(AGMD),(â…²) vacuum membrane distillation(VMD),and (â…³) sweeping gas membrane distillation(SGMD).The AGMD is being investigated worldwide for its advantages of high purity of the product,high evaporation efficient and that wetting of the membrane pores on the permeate side is avoided..So far,the porous membranes used in MD process are prepared from different hydrophobic polymeric materials such as poly(vinylidene fluoride),poly(tetrafluoroethylene) or poly(propylene).A few researches about inorganic membrane have been reported in MD for most of the inorganic membrane is hydrophilic.Recently,the great increase of interest in the different applications of inorganic membranes is observed.The carbon membrane as a kind of inorganic membrane has been widely investigated for its benefits such as hydrophobicity,low cost,good chemical durability,low flux decay,homogenous pore-size distribution.However,few studies and investigation about carbon membrane used in MD and the relevant mechanism model of the process has been rarely reported.The purpose of this thesis is to present a study of the AGMD NaCl aqueous process using carbon membrane.Firstly,an air-gap module with built-in condensing surface was used for conducting experiments using a tubular hydrophobic carbon membrane firstly to study the effect of the feed temperature,flowrate and concentration on the permeate flux,the rejection of the process was calculated.The feed was an artificially prepared NaCl aqueous with different concentration.Secondly,a fully predictive mathematical model based on the principles of heat transfer was developed,the mass transfer phenomena was studied with the heat transfer model and validated by the experimental permeate flux.The heat transfer equations were solved by the search algorithm and chase algorithm.The correlation of mass transfer coefficient of the membrane and membrane average temperature was obtained. Thirdly,the mass and heat transfer phenomenon of the AGMD was studied with the developed mass and heat transfer model.The effect of operation parameters such as feed temperature,feed flowrate,feed concentration,cooling water temperature,cooling water flowrate and conductivity of the carbon membrane on the permeate flux and evaporation coefficient were investigated.The temperature polarization and concentration polarization phenomenon of the AGMD were also analyzed.Besides,computational fluid dynamics(CFD) simulations coupled the heat and mass transfer model of the process were conducted to predict the heat and mass flux of the AGMD process with the application of FLUENT 6.3 Software.The temperature profile,temperature polarization factor adjacent to the membrane surface along feed fluid direction were computed by the post processor of CFD package FLUENT 6.3,which provide a scientific basis for optimization of the process.