| Membrane distillation,as a new membrane separation technology,can be used for desalting process using with plant waste heat and other cheap energy,which can remove non-volatile pollutants or recover other components.In the membrane distillation process,the volatile components in the feed liquid are condensed and collected at the condensation side through the porous hydrophobic membrane,realizing the recovery of non-volatile solute and high-purity distillate.Porous hydrophobic membrane plays an important role in the distillation process as the isolation barrier between hot liquid and condensation side.The hollow fiber membrane prepared by thermally induced phase separation(TIPS)has the advantages of high permeability,suitable pore size,narrow pore size distribution and high mechanical properties.Hollow fiber membrane has been widely used in membrane separation engineering due to its high specific surface area and high membrane recovery.Based on the TIPS principle,a continuous production system of hollow fiber membrane for membrane distillation is designed iPP hollow fiber membrane and PP/POE hollow fiber membrane were prepared continuously,and were applied in the vacuum membrane distillation(VMD).The effects of POE content,diluent ratio and solid content on the structure of hollow fiber membrane were studied,including morphology,contact angle,porosity,average pore size and pore size distribution,air permeability and mechanical properties.The operation time,feed temperature and feed flow for VMD process were also explored.The main experiments and conclusions were described as follows:(1)iPP hollow fiber membrane was prepared via continuous TIPS method,and the effect of diluent mixture ratio on membrane performance was explored.With the increase in DBP content in the mixed diluent,the interaction between diluent and polymer is gradually weakened.During the L-L phase separation,the growth rate of polymer lean phase droplets is accelerated,and the rich-phase of polymer forms a continuous honeycomb-like pore structure in the membrane.With the increase in DBP content,the average pore diameter of iPP hollow fiber membrane increased from 0.1896 μm to 0.2243μm;the permeability increased from 0.4120 L/(cm2·min)to 0.5318 L/(cm2·min);the porosity increased from 69.6%to 74.0%;the tensile strength increased from 3.1 MPa to 6.5 MPa;the maximum elongation increased from 15%to 140%.The flux gradually increased from 13.40 kg·m-2·h-1 to 15.35 kg·m-2·h-1.(2)In the iPP hollow fiber membrane,with the increase in the solid content of the casting solution,the cell membrane becomes denser,and the porosity,pore size and permeability gradually decrease,but its mechanical properties and maximum elongation increase.The flux decreased from 14.95 kg·m-2-h-1 to 1.54 kg·m-2·h-1.In the VMD experiment using iPP hollow fiber membrane,the flux was in a dynamic equilibrium state after nearly 55 h of operation without wetting or leakage.When,the best VMD performance of iPP hollow fiber membrane was achieved at DBP:soybean oil ratio of 7:3.(3)The PP/POE hollow fiber membrane was prepared via continuous TIPS method.The optimum.The water contact angle,the average pore diameter,the permeability and the porosity of the blend membrane were increased by 8.5%,24.83%and 8.5%at the POE content of 15%,respectively.The flux of the blend membrane increased from 8.36 kg·m-2·h-1 to 12.60 kg·m-2·h-1,increasing by 50.72%.The toughness of the PP hollow fiber membrane was increased with blending POE,,and the mechanical properties of the blend hollow fiber membrane were improved.The tensile strength and the maximum elongation of the blending film increased by 12.82%and 51.89%,respectively.(4)PP/POE hollow fiber membranes were prepared via continuous TIPS method.PP/POE hollow fiber membrane with DBP:soybean oil of 7:3 and solid content of 28%exhibited the best performance.The water contact angle of the blend membrane is 117°,the average pore diameter is 0.2175 μm,the permeability was 0.4477 L/(cm2·min),the porosity was 74.08%,the tensile strength was 8.49 MPa,the maximum force was 11.79 N,and the maximum elongation was 122%.The VMD experiment showed that the flux was in a dynamic equilibrium state after nearly 45 h of operation,the permeate flux was 16.59 kg·m-2·h-1,and the retention rate was above 99.9%,which indicated that PP/POE hollow fiber membrane exhibited good performance and long service life.The effect of solid content on PP/POE hollow fiber membrane was also studied.With the increase in solid content,the average pore size,permeability,porosity and permeability decreased,but the mechanical strength increased.(5)The influence of operation parameters on VMD process was explored.With the increase in feed flow rate from 6 L/h to 30 L/h,the inner wall of the membrane will form violent turbulent flow,which will weaken the influence of concentration polarization and temperature difference polarization on the mass transfer and heat transfer process.The VMD flux of the blend membrane gradually increases from 11.31 kg·m-2·h-1 to 17.86 kg·m-2·h-1.With the increase in feed temperature from 30℃ to 80℃,the vapor pressure difference between the two sides of the membrane gradually increased,and the driving force of VMD process increased.VMD flux increased from 0.35 kg·m-2·h-1 to 18.36 kg·m-2·h-1. |
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