Preparation Of Anti-wetting Electrospun Membranes For Desalination By Membrane Distillation

Desalination equipment is a necessary life supporting facility for ocean-going vessels.Restricted by energy supply,using waste heat or solar energy is a promising method to obtain fresh water in vessels.Membrane distillation(MD)has great prospect in desalination for its lower operating temperature.The MD separation performance is determined by used membrane.The high porosity endows electrospun membranes high flux,which has been widely studied in MD.Polyvinylidene fluoride(PVDF)has good hydrophobicity and is soluble in many polar solvents.Therefore,it is the most used material for electrospun membranes preparation.However,the PVDF electrospun membranes have moderate anti-wettability,which result in poor stability in MD process.In order to improve the anti-wettability of PVDF electrospun membranes,in this thesis,the inherent hydrophobicity was improved form electrospun membrane surface to the nanofibers.At the same time,the electrospun membrane wetting mechanism was studied,and the regeneration of MD flux was realized.Finally,the MD desalination process based on electrospun membrane was studied to verify the application feasibility on vessels.The contents of this thesis includes five aspects:(1)Surface loading of nano-carbon particles(NCPs)to improve anti-wettability of electrospun membrane.In order to solve the problem of PVDF electrospun membranes wetting phenomenon during the MD long-term operation,surface spraying and heat-pressing methods were used to load hydrophobic NCPs on the electrospun membrane surface.Due to the higher inherent hydrophobicity of the NCPs,the contact angle of the modified membrane increases from 130.6° to 156.0°.MD desalination results showed that the flux and anti-wettability of the modified membrane were greatly improved.When the NCPs loading density was 4 g/m2 and membrane thickness was 160 ?m,the flux reached to 14.6 kg/m2h,which is 33%higher than the pristine membrane.The stable operation time is increased to 41.5 h,which is 34%higher than that of the pristine membrane.(2)Surface coated by superhydrophobic carbon nanotube(CNT)network layer to improve the anti-wettability of the electrospun membrane.In order to prevent the spherical nanoparticles deposited in the electrospun membrane pores,and the increased mass transfer resistance due to the small accumulation gap of NCPs,CNTs were used to modify the electrospun membrane.In order to make the CNTs evenly distribute on membrane surface,ethanol,which volatilizes quickly and completely wets the PVDF membrane surface,was selected as solvent to prepare a low CNT content dispersion.The CNT dispersion was sprayed on the membrane surface,and then heat pressed.The SEM results showed that the CNTs were distributed homogeneously on the membrane surface and interwoven into a thin network layer.Due to the strong inherent hydrophobicity of the CNTs,the modified membrane reached a contact angle of 159.0°.The MD desalination results showed that when the CNTs coated density was 20 g/m2 and membrane thickness was 100 ?m,the flux of the modified membrane was 28.4 kg/m2h,which was 48.4%higher than the pristine membrane.The stable MD operation time was 25 h,which was 78.1%higher than the pristine membrane.(3)Low surface energy polydimethylsiloxane(PDMS)modified nanofibers to prepare high anti-wettability electrospun membranes.Surface modification by nano-particles leaded to a change in the membrane surface pore size.In this case,modification electrospun membrane's nanofibers was proposed.In order to prepare more hydrophobic electrospun membrane,the nanofibers were coated by PDMS through a dip-coating method.Due to the lower surface energy of PDMS,the contact angle of the modified membrane rises to 148.6°.The MD desalination results showed that,the stable MD operation time of the pristine membrane was less than 1 h.While the stable operation time increased to 22 h for the modified membrane.At the same time,due to the decrease of membrane thickness,the mass transfer resistance was lower and the flux was increased to 30.2 kg/m2h.(4)Studying of electrospun membrane wetting mechanism and recovering flux in MD.Based on the electrospun membrane wetting problem in MD,by analyzing the changes of wetted membrane in hydrophobicity and pore size,the electrospun membrane wetting mechanism was explored.In the MD process,the feed solution partially sinks into the membrane pores and the water vapor evaporates on the concave surface continuously.When no fresh feed solution supplemented,the solute was crystallized in the pores of the membrane.The feed solution infiltrated along the crystallized solutes,moving the crystallization interface from feed side to permeate side until the membrane was completely wetted.The SEM results confirmed that NaCl crystallized in the pores of the wetted membrane,resulting in the pore size distribution changes and poor hydrophobicity.According to the wetting mechanism,the NaCl in the pores of the membrane is removed by washing,and the membrane flux can be fully recovered,thereby realizing membrane regeneration.(5)Feasibility analysis of electrospun membrane in desalination process by MD.A 10 t/d desalination process was designed for vessel.In order to avoid membrane wetting,a regular backwashing method was designed.Based on the water production,the required area of the electrospun membrane was calculated,the thermal load was estimated,and the number of ultrafiltration membrane modules required for seawater pretreatment was estimated.The results show that using electrospun membranes and vessel's waste heat were feasible in small-scale seawater desalination systems and have promising prospects.