【作者】 刘军；

【导师】 李保安；

【作者基本信息】 天津大学 ， 化学工程， 2018， 博士

【摘要】 高分子疏水膜材料具有一定的憎水性和自清洁性能,在化工、环保、食品等领域中具有广泛的应用潜力,比如脱盐淡化领域中具有传质功能的疏水多孔膜,以及仅有传热功能的导热高分子膜等。本课题采用理论推导、模拟计算和实验研究相结合的方法,研究了高分子疏水多孔膜中的热质传递机理及表面润湿性能对热质传递特性的影响,优化设计了中空纤维膜蒸馏组件,探究了高分子无孔膜在蒸发换热应用中的传热特性。膜蒸馏过程中疏水多孔膜的传质过程可以用努森扩散和粘滞流动混合机理模型进行描述,膜孔径及膜厚度参数对传质通量影响显著,理论推导及实验结果显示,当膜孔径变化范围较大时采用多层膜模型(非均态膜模型)描述膜孔中的传质过程更为准确,且其传质阻力主要集中在孔径较小的膜层内;膜表面的液固接触状态由膜疏水能力决定,接触角较低时为Wenzel接触模型,接触角较高时为Cassie-Baxter接触模型;提高膜表面的疏水性可以显著增加MD过程的实际蒸发面积并提高膜的抗污染能力,进而提高膜通量。采用FLUENT软件对错流式膜蒸馏组件中中空纤维膜的排列优化进行了研究,分析了管间距和管间夹角对组件综合性能的影响。当中空纤维膜管间距为2.5倍管径、管间夹角为90°时组件具有佳通量性能,其温差极化系数(TPC)和浓差极化系数(CPC)值均高于其他组件,表明组件内部的流场得到优化。通过实验手段对导热中空纤维管在蒸发换热过程中的传热研究表明,沸点进料(95℃)、管内流速为0.03 m/s时蒸发器可获得最大产水通量为20.76 kg/(m~2·h),过程总传热系数在800—1000 W/(m~2·K)左右,传热热阻主要集中在管壁及管程侧,且管壁热阻约占65%;模拟计算结果显示,若管壁材料的导热系数由0.3W/(m·K)提高到1.0 W/(m·K)以上、壁厚在0.10 mm以下时,管壁热阻不再是过程的主要热阻,且总传热系数可提高到2000 W/(m~2·K)以上,与金属换热器传热性能相当,这些结论为高分子换热器的进一步研究提供了突破方向。以上研究结果丰富和完善了高分子疏水膜中的热质传递理论,为其在脱盐淡化领域中的广泛应用提供了理论支撑和数据参考。更多还原

【Abstract】 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.更多还原