The Gas-liquid Extraction Method Is Used In The Pretreatment Research Of Membrane Distillation

With progress in membrane technology,hydrophobic porous membranes have been extensively research in separation processes,such as the ocean water desalination,industrial wastewater treatment,and chemical concentration.However,hydrophilic leakage of hydrophobic porous membrane channels is one of the challenges that must be faced in the long-term application of hydrophobic porous membrane,which seriously impedes the industrial application of hydrophobic porous membrane.Surface active substances(SAS)present in the feed solution have been commonly considered as the "ccelerator" of such wetting processes.Thus,developing a simple and efficient pretreatment method,which removes the SAS before hydrophobic membrane application,might be an effective strategy to retard membrane pore channel wetting.In this work,a gas-liquid interface extraction method was employed as a pretreatment process to remove the surface-active substances(SAS)before commencing hydrophobic membrane application.Vacuum membrane distillation(VMD),a typical hydrophobic membrane application example,was used as a representative process to investigate the influence of this pretreatment method on the pore channel wetting behavior in a hydrophobic membrane application.Sodiulm dodecyl benzene sulfonate(SDBS)was used to represent a highly wetting substance in the feed solution.The aim is to retard pore channel wetting in hydrophobic membrane applications.Accordingly,based on the results of single-factor experiments,the effects of hydraulic retention time(HRT),bubble intensity(BI),and gas-liquid interface extraction height(h)on the pore channel hydrophilic wetting were investigated using response surface methodology(RSM).The experimental results indicated the optimal conditions were an HRT of 30 min,BI of 0.5 NL/min,and h of 15 cm;thereafter,the time before hydrophilic leakage time(HLT)could be prolonged from 15 min to 590 min,which meant that the stable running time was increased forty-fold.In addition,while effectively extending the effective operation time of the hydrophobic membrane separation and the recovery rate of feed solution in the pretreatment process was effectively increased by using RSM was used to optimize the process parameters.The experimental results indicated the optimal conditions were an HRT of 24 min,BI of 1.5 NL/min,and h of 25 cm;Under the optimal conditions the time before hydrophilic leakage time(HLT)could be prolonged from 15 min to 400 min,which meant that the stable running time was increased twenty-seven-fold,meanwhile the recovery rate R,can be improved to 95%.Finally,the air flotation flocculation/VMD coupling process was proposed to explore the effects of the electricity property of flocculants,the type of flocculants and amount of flocculants on the hydrophilic leakage time(HLT)of the hydrophobic membrane separation.Adding flocculants in the pretreatment process can significantly prolong the time of hydrophobic membrane reaching hydrophilic leakage and effectively prolong the effective operation time of vacuum membrane distillation;The pretreatment effects was related to the electricity property of flocculants.The pretreatment effect of cationic organic flocculants was better than that of anionic organic flocculants.The pretreatment effect was related to the type of flocculants.The pretreatment effect of organic flocculants was better than that of inorganic flocculants.The pretreatment effect was related to the amount of flocculants,and the best pretreatment effect was achieved when the amount of CPAM was 12 mg/L.