| Membrane distillation is a promising separation technology,can help reducing the worldwide water-energy stress in a sustainable way.MD uses low-grade thermal energy to drive desalination,to remove nonvolatile contaminants or to recover other components.In MD,the volatile components of a feed solution evaporate and cross a microporous hydrophobic membrane;then,these volatile components condensate in a permeate solution.This process results in a highly pure distillate solution and a concentrated feed solution where the non-volatile solutes are retained.Although the porous membrane serves as a physical barrier between hot feed and cold permeate,it plays a crucial role during MD process.In this paper,PP/POE blend hydrophobic microporous membranes were prepared by thermally induced phase separation(TIPS).The effect of polymer and POE concentration on the formation of blend membrane were investigated by single and mixed diluents.The physical and chemical properties such as morphology,porosity,pore size distribution,hydrophobicity,liquid penetration pressure and mechanical properties of the blend membranes were investigated.The characteristics of the separation performance like air permeability were characterized.And the scope of the membrane preparation conditions was reduced.The PP/POE blend membrane was applied to the vacuum membrane distillation process of simulated seawater,and it was investigated from the vacuum degree,the feed temperature,the feed rate and the operating time.The main research contents and conclusions were as follows.(1)The performance of PP membrane was improved by POE.It was found that the PP/POE/soybean oil system had better compatibility with the thermodynamic phase diagram.With the increase of the polymer concentration of the casting solution,the surface pore of the blend membrane tends to be thinning.The section of the blend membrane forms a branch shaped membrane structure,and the porosity,pore size and gas permeability gradually decrease.The liquid entry pressure gradually increases,and both of which are larger than 0.2 MPa.The water contact angles are above 115°,and the pore size distribution is more uniform in the 30%?40%polymer concentration.(2)The addition of POE reduces the crystallinity of the blend polymer.With the increase of POE concentration in the casting solution from 5%to 6%,the porosity of the blend membrane decrease from 71.8%and 69.8%.The pore size distribution is more uniform at 6%to 8%,and the water contact angle is higher than 120°.The mechanical strength increases by 41.01%when the POE concentration is 6%.(3)The addition of DBP in the mixed diluent reduces the interaction between the polymer and the diluent,and the L-L phase separation is more fully,which forming a well connected cellular structure.With the increase of polymer concentration,the porosity and pore size of PP/POE blend membranes are gradually reduced and the pore size distribution is uniform,the porosity is from 54.0%to 70.0%,the pore size is from 0.0533 ?m to 0.1842?m.The LEP of the blend membranes increase from 0.25 MPa to 0.76 MPa,and the permeability are reduced from 0.53 L·cm-2·min-1 to 0.11 L·cm-2·min-1.(4)With the increase of DBP ratio in the mixed diluent,the solubility parameter difference of polypropylene and diluent gradually increased,and the interaction gradually weakened.The membrane surface changed from petal irregular pore to regular round hole,and the section of the membrane structure changed from the gap of the polymer branch into the regular cellular.Simultaneously,the average flow pore size of the blend membrane decreases gradually from 0.7333?m to 0.06400?m,the uniform pore size distribution is obtained between the soybean oil:DBP is from 7:3 to 4:6.LEP is increased from 0.19 MPa to 0.38 MPa,and the permeability is reduced from 4.93 L·cm-2·min-1 to 0.063 L·cm-2·min-1.When the soybean oil:DBP was 5:5,the mechanical properties of the blend membrane reached the best.Compared with the pure PP membrane,the tensile strength of the blend membrane increased by 8.133%and the elongation at break increased by 11.78%,(5)The performance of vacuum membrane distillation was tested on the prepared PP/POE blend hydrophobic microporous membrane.The effects of vacuum degree,feed rate,feed temperature and operation time on permeation flux and reject ratio were investigated.With the increase of vacuum degree,the permeation flux increased from 0.64 kg·m-2·h-1 to 9.72 kg·m-2·h-1.The higher vacuum degree can improve the reject radio of the blend membrane.The change of permeation flux is small with the feed rate,and the best feed rate is 18L/h.While the feed temperature rises from 50 degrees Celsius to 80 degrees Celsius,the permeation flux of the blend membrane increases from 1.34 kg·m-2·h-1 to 13.25 kg·m-2·h-1 and the permeation flux has a linear correlation with the feed temperature.The reject rate of PP/POE blend membrane is more than 99.9%when the feed rate was 18L/h,the vacuum degree was 0.09MPa and the feed temperature at 80 degrees Celsius,and no wetting or leakage occurred after running nearly 60 hours.. |
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