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首页> 外文学位 >Osmotically driven membrane processes: Characterization of water transport phenomena through asymmetric polymeric membranes.
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Osmotically driven membrane processes: Characterization of water transport phenomena through asymmetric polymeric membranes.

机译：渗透驱动的膜过程：通过不对称聚合物膜的水传输现象的表征。

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摘要

Freshwater availability is one of the most critical issues facing humanity today. With our available freshwater resources continuing to dwindle, mankind must turn to the ocean and brackish groundwater as sources of freshwater. Desalination technologies like reverse osmosis (RO), while capable of producing high quality potable water, are, however, expensive, energy intensive, and environmentally unsustainable due to a high volume of concentrated brine discharge. Forward osmosis (FO) might be a sustainable alternative to reverse osmosis and thermal desalination technologies. The major obstacle to the further development of FO and other osmotically driven membrane processes is the poor water flux performance of the membrane. The primary culprit of this poor water flux performance is concentration polarization, a membrane boundary layer phenomenon which afflicts forward osmosis processes to a much greater extent than their RO counterparts.;In this dissertation, the influence of asymmetric membrane support layer structure and chemistry on water transport in osmotically driven membrane processes is elucidated and discussed. Using a custom designed benchtop forward osmosis system, asymmetric membranes used for pressure driven RO separation were found to perform very poorly under osmotically driven flow conditions with several draw solutions (or osmotic agents). The poor water flux performance of these membranes was attributed to the prevalence of internal concentration polarization (ICP), a phenomenon contained within the porous support structure of asymmetric membranes. A commercially available forward osmosis membrane was found to perform far better. Higher water fluxes coupled with higher salt rejections and feedwater recoveries were obtained using this membrane tailored for forward osmosis. The superior performance of this membrane was attributed to a thinner, more porous support layer, which resulted in a less severe ICP. Even so, the membrane was itself asymmetric and ICP was found to have a profound effect on water flux considering the very large osmotic pressure driving forces utilized in these experiments.;Subsequent investigations presented in this dissertation examined the severity of ICP using various solutes and characterize the severity of ICP within the membrane structure. The culmination of this aspect of the work resulted in the successful development of a predictive water flux model, which incorporated both internal and external CP effects. This model was tested against experimental data and was used to predict improved flux behavior based on reduced ICP effects due to hypothetical improvements in membrane structural design. Furthermore, a new finding presented as part of this dissertation showed that support layer hydrophobicity may critically hinder water flux for all osmotically driven membranes processes and, therefore, must be considered when explaining poorer than expected flux performance. While not affecting water transport in pressure driven separation processes, the degree of water saturation of these various support layers was found to play a critical role in water transport through the membrane during osmosis. It was concluded that improving membranes by designing asymmetric membrane support layers with thinner, more porous, and more hydrophilic support structures will be essential to the further development of osmotically driven membrane processes. The implications for improved membrane design based on the findings in this dissertation are discussed.
机译：淡水供应是当今人类面临的最关键问题之一。随着我们现有淡水资源的不断减少，人类必须转向海洋和咸淡的地下水作为淡水来源。然而，诸如反渗透（RO）之类的脱盐技术虽然能够产生高质量的饮用水，但由于大量的浓盐水排放，因此价格昂贵，能耗大且对环境不可持续。正向渗透（FO）可能是反渗透和热淡化技术的可持续替代方案。 FO和其他渗透驱动膜工艺进一步发展的主要障碍是膜的水通量性能差。这种不良水通量性能的主要原因是浓度极化，这是一种膜边界层现象，与反渗透膜相比，膜边界层现象对正向渗透过程的影响要大得多；本论文中，不对称膜支撑层结构和化学性质对水的影响渗透驱动的膜过程中的运输进行了阐明和讨论。使用定制设计的台式正向渗透系统，发现用于压力驱动的RO分离的不对称膜在渗透驱动的流动条件下，具有多种抽吸溶液（或渗透剂）的性能非常差。这些膜的差的水通量性能归因于内部浓度极化（ICP）的普遍存在，该现象包含在不对称膜的多孔载体结构中。发现可商购的正向渗透膜的性能要好得多。使用专为正向渗透量身定制的膜，可获得更高的水通量，以及更高的除盐率和给水回收率。该膜的优异性能归因于更薄，更多孔的支撑层，从而降低了ICP的严重性。即便如此，考虑到这些实验中使用的非常大的渗透压驱动力，该膜本身还是不对称的，并且发现ICP对水通量具有深远的影响。;本论文随后进行的研究使用各种溶质检查了ICP的严重程度并表征了ICP在膜结构中的严重程度。这项工作的最终成果是成功开发了预测水通量模型，该模型结合了内部和外部CP效应。该模型针对实验数据进行了测试，并基于膜结构设计的假设性改进，基于降低的ICP效应用于预测通量性能的提高。此外，作为本论文一部分的新发现表明，支撑层的疏水性可能会严重阻碍所有渗透驱动膜工艺的水通量，因此，在解释比预期的通量性能差时必须考虑。虽然在压力驱动的分离过程中不影响水的输送，但发现这些不同支撑层的水饱和度在渗透过程中通过膜的水输送中起着关键作用。结论是，通过设计具有更薄，更多孔和更具亲水性的支撑结构的不对称膜支撑层来改善膜，对于渗透驱动膜工艺的进一步发展必不可少。基于本文的发现，讨论了改进膜设计的意义。

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