用网络模型揭示分离膜形态对油滴截留的影响

Revealing The Effects of Membrane Morphology on The Rejection of Oil Droplets Via Network Modeling

李卓

11749138

硕士

环境科学与工程

李炜怡

2019-05-28

2019-07-13

哈尔滨工业大学

深圳

含油废水若未经充分处理而直接排放，会对生态环境造成严重的危害。与传统方法（如离心法、重力分离法和化学破乳法）相比，基于膜分离的含油废水处理技术作为一种高效、灵活的处理手段，不仅可以达到除油的目的，而且可以将水从水包油乳液中回收并再次利用。尽管膜分离技术有着诸多优点，油滴与多孔膜结构间复杂相互作用的潜在机理却仍不清晰，而此机理的解读对于开发新型分离膜以更有效的处理含油废水具有重要价值。因此，本研究将探讨油滴截留和分离膜的形态的关联，以此为分离膜的设计与发展提供新思路。本研究采用基于网络模型的方法，从理论上研究了分离膜形态对油滴截留的影响。将多孔分离膜在数值上离散为一个由立方单元格组成的体系，并利用其坐标方向建立节-点结构。此离散化处理为数值分析提供了一个具有各种拓扑结构的局部线性空间域。进而将基于有限元法的网络分析和“迷宫”求解器作为互补手段相结合，以探索网络的拓扑结构。本研究对网络的横向尺寸（单元格的数量）进行了优化，以尽量降低可能由网络次级结构的不确定性带来的负面影响；对有限元分析的数值求解方法进行了迭代容差值和预处理方面的优化来提高模型模拟效率。此外，还建立起基于归一化流量的标准来精确判定网络中的活跃单元组（即拥有非零流量的连通单元格群）。若改变各方向上的闭合几率，网络的结构会随之发生改变（如从高度连通型变为直通型）。基于此，以统计方式模拟单个油滴的在网络表面的沉积来研究跨油滴压随横向闭合几率的变化；探究了多油滴沉积（即变化网络表面覆盖率）情况下跨油滴压的分布规律；依据活跃单元组中拓扑特性的相关变化解释了跨油滴压与闭合几率间的依存关系。本研究讨论了一个在过去工作中建立的临界压力模型，并将其应用于网络模型中，以预测在含油废水处理中油滴截留率随跨膜压的变化。通过比较具有不同连通性的网络的模拟结果，确定出油滴截留率与网络拓扑结构的关系。此外，在不同表面覆盖率的情况下，讨论了废水含油量对油滴截留率的影响。所有的模拟结果均证实，当分离膜具有高度连通的膜孔结构时，跨油滴压可能明显小于跨膜压，从而产生相对较高的油滴截留率。此外，分离膜表面较高的油滴覆盖率可以显著改变其内部结构中压力的分布，从而使油滴截留率降低。有理由相信，此网络模型将会为提升基于膜分离的含油废水处理的性能做出贡献。

Oily wastewater without adequate treatment could cause negative impacts on the environment. In comparison with conventional methods (like centrifugation, gravity separation, and chemicals-based demulsification), membrane-based treatment is emerging as an effective and flexible way to removing the oil and reclaiming the water from an oil-in-water emulsion. Despite the advantages, it is still challenging to reveal the underlying mechanisms accounting for the complex interactions between the oil droplets and the porous membrane substructures, which are of great value in designing novel membranes for better performance of the oily wastewater treatment via membrane separations. Therefore, the objective of the current study is to establish a correlation between the rejection of oil droplets and the membrane morphology, which could be exploited to derive a set of design heuristics for developing improved membranes (as a rejector or a coalescer).The current study employed a network-based approach to theoretically investigate the effects of membrane morphology on the rejection of oil droplets. The porous membrane was numerically discretized into a system of cubic unit cells, whose coordinate directions were exploited to derive a bond-node structure. This discretization provided a locally linear spatial domain with various topological structures for the numerical analysis. Methods based on the finite element analysis and ‘maze’ solver were developed to determine, in a complementary sense, the active groups that are composed of connected unit cells with a nonzero flow rate.The size (i.e., number of the unit cells) of the network in the lateral direction was optimized to minimize the negative effects resulting from the uncertainty of the network substructures. In order to enhance the modeling efficiency, the numerical solver for the finite element analysis was also optimized in terms of the setting of iterative tolerance and the use of preconditioning. Moreover, a criterion based the normalized flux was successfully established to accurately determine the active groups in the network.The network was varied from a highly connected structure to a straight-through one by continuously changing the lateral blockage probability while fixing the transverse blockage probability at different values. The deposition of a single oil droplet was then simulated in a statistical fashion to evaluate the trans-droplet pressure (TDP) as a function of the lateral blockage probability. The distribution of the TDPs was also studied for the deposition of multiple oil droplets (i.e., varied surface coverage). The corresponding variations in the topological characteristics on the active groups were exploited to interpret the dependence of the TDP on the blockage probability.A critical pressure model developed in prior work was discussed and adapted to the network model for predicting the initial rejection of oil droplets as a function of the trans-membrane pressure (TMP) during a process of oily wastewater treatment. The modeling results for networks with a different connectivity were compared to correlate the initial rejection of oil droplets with the topological structures of the network. The effects of the emulsion concentration on the initial rejection were also discussed in the context of varied initial surface coverage. All the modeling results confirm the hypothesis that the TDP could be considerably less than the TMP when the membrane has a highly connected structure and thereby give rise to a relatively high rejection of oil droplets. Moreover, a high surface coverage by the oil droplets could substantially change the distribution of pressure within the membrane structure, which might decrease the rejection. It is desirable that the network model would offer a powerful tool for enhancing the performance of membrane-based oil wastewater treatment.

含油废水处理 膜过滤 网络模型 分离膜形态优化 油滴截留

oily wastewater treatment membrane filtration network modeling membrane-morphology optimization rejection of oil droplets

中文

联合培养

南方科技大学

李卓. 用网络模型揭示分离膜形态对油滴截留的影响[D]. 深圳. 哈尔滨工业大学,2019.