Molecular Dynamics Simulation of Steady-State Droplet Condensation on A Fiber in Direct Contact Membrane Distillation Settings
Molecular Dynamics Simulation of Steady-State Droplet Condensation on A Fiber in Direct Contact Membrane Distillation Settings
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DOI：
10.1016/j.molliq.2022.120736
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发表时间：
2022-11
影响因子：
6
通讯作者：
S. Raza;Jixiong He;H. Tafreshi;J. Liu
中科院分区：
文献类型：
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作者：
S. Raza;Jixiong He;H. Tafreshi;J. Liu
了解液滴在纤维上冷凝-蒸发行为的动力学对于改善用于水净化应用的纤维膜的性能是重要的，例如，直接接触膜蒸馏（DCMD）。DCMD是一种很有前途的方法净化水时，低品位的废热或可再生能源。然而，DCMD遭受低通量质量通量，并且它也易于发生膜泛。在这项工作中，我们使用分子动力学模拟，因为传统的（实验或计算）方法不具有所需的原子分辨率来揭示纤维表面上的水冷凝-蒸发的动力学。我们的模拟表明，当纤维的杨氏-拉普拉斯接触角（YLCA）大于抑制冷凝的临界值时，穿过膜的蒸汽通量保持恒定（纤维上没有液滴形成）。然而，由于纤维上水滴的形成和生长，在较低的YLCA下，质量通量随时间而降低，这可能最终导致膜泛液。我们还研究了进料温度、渗透温度、纤维直径、纤维位置和区域尺寸对纤维临界YLCA的影响。优化这些参数允许在膜制造中使用广泛的材料，甚至包括亲水性材料，同时防止膜溢流并提高质量通量。在本文中，我们还提出了一种新的方法来模拟稳态液滴冷凝蒸发过程的分子动力学模拟框架，即，模拟时间> 100 ns，与大多数先前研究中报道的准稳态模拟（模拟时间<100 ns）形成对比。我们的工作展示了一个模拟平台来研究纤维上水冷凝蒸发的动力学，并可用于指导DCMD膜的设计。
Understanding the dynamics of droplet condensation–evaporation behavior on fibers is important for improving the performance of fibrous membranes that are used in water purification applications, e.g., Direct Contact Membrane Distillation (DCMD). DCMD is a promising method of purifying water when low-grade waste heat or renewable energies are available. However, DCMD suffers from low throughput mass flux, and it is also prone to membrane flooding. We used molecular dynamics simulations in this work as the conventional (experimental or computational) methods do not have the required atomistic resolution to reveal the dynamics of water condensation–evaporation on the surface of fibers. Our simulations indicate that vapor flux across the membrane remains constant (with no droplet formation on the fiber) when the fibers’ Young–Laplace Contact Angle (YLCA) is greater than a critical value at which condensation is suppressed. However, mass flux decreases with time at lower YLCAs due to the formation and growth of water droplets on the fibers, which could ultimately lead to membrane flooding. We also studied the impact of feed temperature, permeate temperature, fiber diameter, fiber position, and domain size on the fiber critical YLCA. Optimizing these parameters allows the use of a wide array of materials in membrane fabrication, including even hydrophilic materials, while preventing membrane flooding and also enhancing mass flux. In this work, we also present a novel methodology to simulate steady-state droplet condensation–evaporation process in the framework of molecular dynamics simulation, i.e., simulation times >∼10 ns, in contrast to the quasi-steady-state simulations (simulation time <∼2 ns) reported in most previous studies. Our work demonstrates a simulation platform to study the dynamics of the water condensation–evaporation on fibers and can be used to guide the design of DCMD membranes.