【作者】 陈颖；

【导师】 徐象国； 张绍志；

【作者基本信息】 浙江大学 ， 制冷及低温工程， 2025， 博士

【摘要】 膜蒸馏分离技术应用于吸收式制冷系统代替传统发生器不仅有利于低品位能源的利用,还有望减小系统体积、扩大应用范围。然而膜蒸馏在吸收式制冷中的应用还面临不少挑战,高浓度溶液导致的膜通量下降问题以及膜污染问题都亟待解决。本文运用分子动力学模拟对膜蒸馏过程进行分子层面的微观研究,揭示膜蒸馏过程中水分子的微观传质机理,提出提升膜蒸馏性能的膜改性方向以及工质对选择指导。首先,将溴化锂(LiBr)溶液模型与聚偏氟乙烯(PVDF)膜材料模型组合构建了膜蒸馏溶液侧分子模型,通过对不同温度及浓度下水分子在纯溶液模型以及膜蒸馏溶液侧模型中的传质特性进行分子动力学模拟,揭示了温度与浓度影响膜蒸馏性能的微观机理。基于此,提出通过调控膜-溶液相互作用来提高膜蒸馏传质效率的策略,并对不同膜材料影响下水分子在膜蒸馏过程的传质特性进行比较分析。揭示了膜-溶液相互作用不仅会影响到膜蒸馏的溶液侧传质性能,而且涉及复杂的竞争关系。相比PVDF与聚四氟乙烯(PTFE)膜材料,30%表面羟基浓度的二氧化硅改性材料提高了LiBr溶液中水分子的扩散系数,有利于溶液侧传质效率的提升。其次,为防止膜表面结晶并保证膜蒸馏的性能,探究了六种传统吸收式制冷工质对以及三种离子液体工质对的膜蒸馏传质特性,并分析溶质组分对膜蒸馏溶液侧传质性能的影响机理。在六种传统工质对的纯溶液中,氯化锂(LiCl)溶液含有的自由水分子数量最多,LiBr溶液中水分子的扩散系数最高。溶质离子与水分子之间以及溶质离子与膜材料之间较强的相互作用会降低水分子的扩散系数并减少自由水分子数。与传统吸收剂相比,三种咪唑类离子液体溶液侧传质效率明显低于传统工质对溶液,难以用于膜蒸馏吸收式制冷。与PVDF膜材料相比,采用PTFE膜材料对LiBr溶液和LiCl溶液的膜蒸馏溶液侧的传质性能有所提升。在二氧化硅膜材料的影响下,LiBr溶液中自由水分子的数量和扩散系数都提升了,而LiCl溶液中自由水分子的数量和扩散系数都降低了。然后,采用非平衡分子动力学模拟方法,模拟得到不同工况下膜蒸馏跨膜传质过程500ps内的跨膜水分子数,通过微观跨膜传质通量反映膜通量的相对大小。结果表明,PVDF膜材料的微观跨膜传质通量要高于PTFE膜材料,而二氧化硅改性材料存在膜孔湿润的风险。各工质对溶液的微观跨膜传质通量与溶液侧的水分子传质效率的模拟结果相对应,但采用PTFE膜的模拟中六种工质对的跨膜水分子数都低于PVDF膜材料。为完善膜蒸馏微观传质机理,模拟了三种工质对溶液中水分子分别穿越PVDF与PTFE膜孔的跨膜传质过程,通过伞状抽样和直方图分析得到自由能曲线并计算得到跨膜能垒。结果表明,PTFE膜材料影响下的水分子跨膜能垒高于PVDF膜材料,跨膜传质效率较低。最后,设计并搭建了真空膜蒸馏实验台,测试了PTFE中空纤维膜组件在不同工况下的膜通量,并与PVDF真空纤维膜组件的性能进行比较。在海水淡化应用场景下,探究温度、流速以及真空度对氯化钠(NaCl)溶液真空膜蒸馏膜通量的影响。在吸收式制冷应用场景下,通过实验对52.4%LiBr溶液膜蒸馏分离的可行性进行了验证,发现在真空度85kPa以上的真空膜蒸馏中能够有效产生膜通量,具备应用于吸收式制冷系统的可行性。在95 kPa真空度、60℃出口温度下测试三种工质对溶液的膜蒸馏性能,三种工质对溶液中膜通量最高的是LiBr溶液。膜通量的实验测量结果与跨膜传质通量模拟的结果相符合,膜通量的相对大小与跨膜水分子数的相对大小一致。说明分子动力学模拟可以在相同的工况下通过溶液侧传质特性以及跨膜能垒共同表征不同溶液的膜蒸馏传质性能。更多还原

【Abstract】 The application of membrane distillation separation technology in absorption refrigeration systems as a replacement for traditional generators not only facilitates the utilization of low-grade energy but also holds promise for reducing system volume and expanding application scope.However,the integration of membrane distillation into absorption refrigeration still faces significant challenges,including the decline in membrane flux caused by high-concentration solutions and membrane fouling.This study employs molecular dynamics simulations to conduct a microscopic investigation of the membrane distillation process at the molecular level,aiming to reveal the underlying mechanisms affecting water molecule mass transfer efficiency.First,a molecular model of the solution side in membrane distillation was constructed by combining the LiBr solution model with the PVDF membrane model,using a force field suitable for concentrated solutions and polymeric materials.Molecular dynamics simulations were then performed to investigate the mass transfer characteristics of LiBr solutions at different temperatures and concentrations,thereby revealing the microscopic mechanisms by which temperature and concentration influence membrane distillation performance.The key solution-side transport properties affecting membrane distillation include the number of free water molecules and the water diffusion coefficient.Since the number of free water molecules cannot be obtained quantitatively from simulations,it was characterized through the coordination structure and hydrogen-bond network of the solution.Based on this,a strategy was proposed to enhance mass transfer efficiency by tuning membrane–solution interactions,and a comparative analysis was conducted on the transport behavior of water molecules under different membrane materials.The results reveal that membrane–solution interactions not only affect solution-side transport but also involve complex competitive relationships.Specifically,Br^-ions and H₂O molecules compete both in coordination with Li⁺ions and in interactions with the membrane.By forming hydrogen bonds with water molecules,silica-modified membranes with 30%surface hydroxyl groups not only strengthened their interactions with water molecules in LiBr solution but also enhanced the diffusion coefficient of water.Second,to prevent membrane surface crystallization and maintain membrane distillation performance,this study investigated the mass transfer characteristics of six conventional absorption refrigerant pairs and three ionic liquid refrigerants,as well as the mechanisms by which solute composition influences solution-side transport properties.Among the six conventional working pairs,LiCl solutions contained the highest number of free water molecules,while LiBr solutions exhibited the largest water diffusion coefficient.In contrast,divalent cations or hydrophilic organic anions negatively affected solution-side transport efficiency.Strong interactions between solute ions and water molecules,as well as between solute ions and the membrane,reduced both the water diffusion coefficient and the number of free water molecules.Compared with traditional absorbents,the three imidazolium-based ionic liquid solutions showed significantly lower transport efficiency on the solution side,making them unsuitable for MDAR systems.Using PTFE membranes instead of PVDF improved solution-side transport performance for both LiBr and LiCl solutions.Under silica modification,the number and diffusion coefficient of free water molecules increased in LiBr solutions but decreased in LiCl solutions.Thus,the most suitable membrane material for membrane distillation depends strongly on the specific feed solution.Third,non-equilibrium molecular dynamics（NEMD）simulations were employed to evaluate transmembrane water transport over 500 ps under different operating conditions.The microscopic transmembrane water flux was used to compare relative membrane performance.Under absorption refrigeration conditions,PVDF membranes exhibited significantly higher microscopic flux than PTFE membranes,although silica-modified membranes posed a risk of pore wetting.Among the six working pairs studied using the PVDF model,LiBr solutions yielded the highest number of transmembrane water molecules,while KCOOH solutions yielded the lowest.The simulated transmembrane fluxes were consistent with solution-side transport efficiencies.In contrast,all six working pairs showed lower transmembrane water counts with PTFE membranes than with PVDF,and the three ionic liquid solutions exhibited extremely low transmembrane transport with both membrane types.To further refine the microscopic mechanism,umbrella sampling and histogram analysis were used to derive free energy profiles for water molecules crossing PVDF and PTFE pores in three representative solutions.In PVDF membranes,LiBr solutions had the highest transmembrane energy barrier,while LiCl solutions had the lowest,though the differences were small.In PTFE membranes,however,LiBr solutions showed the lowest barrier,and the energy barriers for all three solutions were markedly higher compared with PVDF.Finally,a vacuum membrane distillation（VMD）test bench was designed and constructed to experimentally evaluate the performance of PTFE hollow-fiber membranes under various operating conditions,and results were compared with PVDF hollow-fiber modules.In seawater desalination scenarios,the effects of temperature,flow rate,and vacuum level on NaCl solution flux were investigated.In absorption refrigeration scenarios,the feasibility of water production by concentrating 52.4%LiBr solution through VMD was validated.At vacuum levels above 85kPa,effective flux was achieved,demonstrating the applicability of VMD in absorption refrigeration systems.At 95 kPa vacuum and 60℃ outlet temperature,three working pairs were tested,all maintaining stable water production over 30 min of operation.Among these,CaCl₂ solutions exhibited much lower flux than LiBr and LiCl solutions,with LiBr showing the highest flux.Experimental flux results closely matched simulation predictions:the relative order of membrane flux corresponded to the relative number of transmembrane water molecules.This confirms that molecular dynamics simulations can reliably characterize membrane distillation transport performance under identical conditions through a combined assessment of solution-side transport properties and transmembrane energy barriers.更多还原