膜蒸馏系统中蒸发潜热的调控与高效膜蒸馏系统构建

Membrane distillation (MD) is a promising technique for the desalination of seawater and preparation of pure water by combining hydrophobic membrane and thermal distillation process. The central component of MD is a stable porous hydrophobic membrane with a large liquid-vapor interface for efficient water vapor transport. A key challenge for MD is the high energy consumption causing by the high latent heat of water evaporation. At operational level, the performance of MD system is negatively affected by “temperature polarization”, a phenomenon intrinsically related to the removal of latent heat associated with water evaporation, which causes the decrease of the temperature at the membrane surface with respect to its bulk value. In this work, latent heat of water evaporation is reduced by polyvinyl alcohol (PVA) hydrogel and efficient desalination process is achieved with Janus membranes of PVA hydrogel and hydrophobic membrane. A membrane distillation system is demonstrated with photo-thermal/electric-heating PVA hydrogel to increase the feed temperature at the membrane surface to withdraw temperature polarization. This work will be a complement to current MD technologies and has important theoretical value and great industrial application prospect.

膜蒸馏技术是一种将膜技术和热蒸馏过程结合起来的分离技术，可应用于海水淡化、超纯水的制备等。目前膜蒸馏的研究主要集中在多孔疏水膜的制备方面，以提高膜通量和耐用性为主要目的。但在实际应用中，水极高的蒸发潜热导致热法海水淡化的能耗较高。另外，在膜蒸馏系统中，水在膜的一面蒸发吸热，另一面冷凝放热，导致温度极化现象，降低了膜蒸馏过程的产水效率。本项目拟从蒸发潜热调控出发，包括利用聚乙烯醇水凝胶降低水的蒸发潜热，以及复合水凝胶和疏水膜，实现膜蒸馏过程中能量的高效利用。最终本项目将耦合太阳能/电能，制备同时可实现光热效应和焦耳热效应的水凝胶，以提高蒸发面温度抑制温度极化现象，构建高效的太阳能海水淡化系统。本项目提出方案可与目前大多数的膜制备技术结合，是主流膜蒸馏技术研究的有效补充，具有重要的理论价值和巨大的工业应用前景。

膜蒸馏技术可以利用低品位热能处理超高浓度的盐溶液，在全球水、能危机背景下，表现出巨大潜力。开发高效稳定的膜蒸馏水处理技术，需要调控蒸发过程的潜热提高其产水量，需要对传热传质过程进行优化，提高热利用效率以及稳定性。基于此，本项目研究水蒸发状态对水相变焓的影响，提出了毛细态水降低蒸发潜热的机制，实现了水蒸发潜热由2450 J g-1调控到1565 J g-1，并在多级膜蒸馏体系中验证了其效果；本项目从传热传质角度出发，重新设计了太阳能被动式多级蒸馏系统，从空间角度重新分配膜蒸馏体系中盐析出位置，使得蒸馏区内盐溶液保持较低水平，浓缩后的盐溶液在边缘结晶析出，解决了被动式蒸馏系统中盐堵塞问题，并实现了对高浓盐水（170 g L-1）的零排放高效处理。本项目的实施有利于缓解水资源短缺压力并避免了产水过程对环境造成压力。