Desalination Process-based Heat And Mass Transfer Characteristics Of Hydrophobic Membrane/film

Hydrophobic polymeric membrane/film has promising applications in chemical engineering,environment protection and food industry fields for its intrinsic hydrophobicity and self-cleaning properties.For example in the desalination field,the porous membrane with mass transfer function and the nonporous polymeric conduction film only with heat conduction function.In this project the methods including theoretical derivation,numerical simulation and experimental studies were applied to investigate the heat and mass transfer mechanisms in the hydrophobic membrane pores and the influence of the surface hydrophobicity on the heat and mass transfer process,to design and optimise the hollow fiber membrane distillation modules,and to explore the heat transfer characters of the nonporous polymeric film in evaporation/heat exchanger applications.The mass transfer process of the membrane distillation in the hydrophobic polymeric membrane pores can be described by the combination mechanism with Knudsen diffusion and viscous flow,and the permeate flux is sensitive to the membrane pore size and the membrane thickness.The theoretical deduction and simulation results show that when the pore size across the membrane layer changes significantly,it is more accuracy by applying the multi-layer model(heterogeneous model)to describe the mass transfer in the membrane pores,and the mass transfer resistance are focus on the layers with smaller pore size.The surface liquid-solid contact state is decided by the membrane surface hydrophobicity,when the water contact angle is low it can be described by the Wenzel contact model,and when the water contact angle is high it can be described by the Cassie-Baxter contact model.When improving the membrane surface hydrophobicity,the actual evaporation area in MD process increases and the antifouling ability is improved,which also can improve the membrane permeability.The investigations on the cross-flow hollow fiber membrane distillation configuration optimization were carried out through the FLUENT software and the factors of fiber row space and intersection angle were anlysized.The hollow fiber MD configuration can obtain the highest flux performance when the fiber row space is 2.5times of the fiber diameter and the intersection angle is 90°,the TPC and CPC values are higher than the other arrangements,which means the flowing field inside the configuration is optimised.The experimental study of the hollow fiber heat transfer characteristics in evaporation/heat exchanger application indicate that,the module can gain the highest water flux of 20.76 kg/m~2·h when the inlet feed is at boiling point(95?)with lumen side velocity of 0.03 m/s.The total heat transfer coefficient(HTC)of this process is between 800 to 1000 W/m~2·K,the major heat transfer resistance is focus on the fiber walls and the lumen side,and the fiber wall heat transfer resistance can be as high as65%of the total heat transfer resistance alone.The further simulation investigations showed that if the conductivity of the fiber wall material can be improved from 0.3W/(m·K)to higher than 1.0 W/(m·K)and the wall thickness is thinner than 0.10 mm,the fiber wall is no longer the main heat transfer resistance in this process and the total HTC value could be higher than 2000 W/(m~2·K),which is equivalent to heat transfer performance of the traditional metal heat exchangers.These results provide new breakthrough directions for the future researches of the polymeric heat exchangers.The results in this project as stated above enriched and improved the heat and mass transfer theories with the hydrophobic polymeric membrane/film processes,which would provide theoretical supports and data references for the widely applications in desalination industries by using hydrophobic polymeric membranes/films.