Fabrication Of PVDF Hollow Fiber Membrane And The Mathmatical Simulation Of Heat Pump Coumpled With Direct Contact Membrane Distillation

Membrane distillation has not been commercialization,the main reason is that lower flux,lower efficiency,easy to hydropholicity and so on.The membrane is required to have good hydrophobicity,higher porosity,favorable pore size and good stability and thermal stability.In this study,membrane structure and membrane module requirements for the membrane distillation process were investigated and the following three aspects were included:?1?PVDF hydrophobic hollow fiber membranes were fabricated via dry-jet wet spinning process.LiCl and PEG-400 were used as non-solvent additives in the cast solution.The effects of PVDF concentration,PEG-400/LiCl mass ratio,dope extrusion pressure and bore liquid flow rate on the fiber dimension and membrane microstructure and performance in DCMD were investigated.The cross section presented a sponge-cavity-sponge-finger structure.With the increase of PVDF concentration,finger-like structure shrank,sponge-like structure became denser,and the whole cross section of membrane became denser.The increase of the polymer concentration contributed to the shrinkage of porosity,membrane thickness,mean pore diameters and the maximum pore diameter,which resulted in low distillate flux and thermal efficiency.LiCl contributed to improving the viscosity of dope and delayed the phase separation.PEG increased the thermodynamic instability of dope in contact with water due to its non-solvent effect and acted as a phase separation enhancer.Increasing PEG-400/LiCl ratio resulted in cavities in the middle of cross section,but more PEG-400 resulted in a denser inner skin and delayed the phase separation at lumen-side.Extrusion pressure had little effect on membrane structure.Dimension increased whereas thickness,mean pore diameter and the maximum pore diameter were reduced by increasing extrusion pressure.Bore fluid flow rate had little effect on membrane structure.The inner and outer diameter,porosity,the mean pore diameter and the maximum pore diameter increased whereas thickness and LEP_w decreased with the increase of bore fluid flow rate.?2?The commercial PVDF hollow fiber membranes were modified by dipping method.The addition of CuSO₄/H₂O₂ in the dopamine solution accelerates the preactivation rate of the pristine membrane.The contact angel increased with the increase of the SiO₂ concentration in the coating solution and the coating time.When the SiO₂ concentration is 3 wt.%,the contact angel increased to 140°,and the modified membranes presented favorable hydrophobicity with 24h coating time.So the homemade PVDF hollow fiber memrbanes were modified with the condition of 1wt.%SiO₂,2 wt.%PFTS and 24h coating time.The distillate and electricity conductivity didn`t show a clear rise during 56h DCMD process for the 3.5 wt.%NaCl solution.?3?Based on the direct contact membrane distillation system with single effect coupled with the heat exchanger,the mathematical model of the two-effect DCMD system was established.The water consumption energy?GOR?were used as the index of the system.On this basis,the mathematical model of the heat pump coupled with two effect DCMD system was established,and the relevant performance indexes were calculated,including SHEC?GOR?COP_H.The calculation flow of the heat pump coupled with two effect DCMD system is given.