Mechanism And Experimental Investigation Of Heat And Mass Transfer Mechanism Based On Janus Membrane During Membrane Distillation Process

Membrane distillation（MD）can be widely used in seawater desalination,high-concentration brackish-water treatment,industrial wastewater treatment and other applications.PVDF electrospinning membranes will cause membrane fouling and pore wetting during membrane distillation,leading to a decrease in permeation flux.Janus membranes are an emerging class of materials with opposing wetting properties at interfacial layers because of the different multi-level structures on both sides.Janus membrane can sustain stable MD performance even with harsh environment,and can thus be used for desalinating high-concentration brackish-water and seawater using low-grade thermal energy.The existing plate-and-frame membrane module has the disadvantages of large heat loss,low water production efficiency,and easy contamination,resulting in high operating cost of MD system and low service life of membranes.Installing baffles in flow channel of membrane module is of great significance for enhancing the heat and mass transfer of MD process and improving MD performance.Firstly,Janus membrane is prepared with polyvinylidene fluoride（PVDF）electrospun nanofiber membrane as the matrix.One side of the matrix is electrosprayed with polydimethylsiloxane（PDMS）to reduce the surface energy of the membrane,Si O₂nanoparticles are then loaded to construct a rough surface,and thus Janus-O layer with superhydrophobic property is successfully prepared.The static contact angle on Janus-O layer is 160°.The other side of the matrix is soaked in hydrophilic polyvinyl alcohol,and the glutaraldehyde is cross-linked through acid catalysis to obtain a stable super-hydrophilic Janus-I layer.The contact angle can be reduced to 5°within 10 milliseconds.The prepared Janus membrane is characterized by SEM,FTIR infrared spectroscopy,pore size distribution,porosity and thickness measurement.The average pore size of the Janus membrane was 684.7nm and the porosity was 80.2%.Secondly,baffles were installed in the feed channel of membrane module,and a direct contact membrane distillation test was performed using the Janus membrane.The permeation flux and rejection rate under the operating conditions of different inlet flow rates,temperatures and concentrations were studied.The experimental results show that increasing the inlet temperature and flow rate will increase the flux of the Janus membrane,and increasing the inlet concentration will decrease its flux.The Janus membrane and PVDF membrane were tested for a long time under the conditions of inlet flow rate,temperature and concentration of 0.20 m/s,70℃ and 3.5%respectively.The Janus membrane maintains a stable flux within 24 hours test,reaching a high flux of 26.96 kg/（m²h）,and its salt rejection is as high as 99.995%,but the permeation flux of the PVDF membrane gradually decreases from the beginning,at the end of the test,the flux is 13.50 kg/（m2h）,and its rejection falls to 99.965%.Finally,on the basis of the experiment,a two-dimensional direct contact membrane distillation CFD numerical simulation study of Janus membrane was carried out to study the distribution of the velocity field,temperature field,and concentration field in the module,as well as the temperature polarization and concentration polarization under different operating conditions.The simulation results are in good agreement with the experimental results.The simulation results show that the velocity boundary layer,temperature boundary layer and concentration boundary layer of the feed channel with baffles are thinner than those of the permeate side without baffles,which greatly improves the heat and mass transfer of MD process.Increasing the inlet flow rate,inlet temperature and inlet concentration can reduce the temperature polarization,and the effect of increasing the flow rate is more significant.Increasing the inlet flow rate can reduce the concentration on the membrane surface on the feed channel and reduce concentration polarization.