Multiscale simulation of enhanced water flow in nanotubes
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Introduction Drinking water scarcity is a major and increasing problem in both developing and developed nations. However, seawater and brackish water are abundant. Reverse osmosis (RO) desalination—the process of forcing seawater through a semipermeable membrane to remove salt ions and other contaminants—is one of the most promising routes toward large-scale freshwater reclamation. Current commercial thin-film composite RO membranes are made from polymeric materials (typically polyamide), but there is great scope to develop advanced nanostructured RO membranes to mitigate energy and capital costs. These include nanoporous membranes built on ceramic–inorganic or metal–organic frameworks (such as zeolites), graphene membranes with precisely controlled porosity, biological membranes (e.g., aquaporin proteins), and membranes comprising aligned nanotubes (NTs).1 In this article, we focus on water flow through NTs, because of their high reported pure water permeabilities and controllable selectivity. The high flow rates arise from the smooth (low friction) inner surfaces of the NT material. For example, the internal surface/water friction of a carbon nanotube (CNT) has been found to decrease with decreasing diameter: the friction is graphene-like for CNT diameters D > ~ 20 nm, but falls to almost zero for D = 0.8 nm.2 The selectivity of ions and other dissolved material is also sensitive to the NT diameter,
with near 100% salt ion rejection occurring for CNTs with a diameter of ∼0.8 nm, decreasing rapidly to 0% when the diameter increases to ∼2 nm.3 Research into flows through NTs of different materials—such as carbon, boron nitride, and silicon carbide—is now just more than a decade old, but commercial membranes of aligned NTs are still not available. This is mainly due to the difficulty in manufacturing these membranes inexpensively in as close to their ideal form as possible—aligned, pristine NTs encapsulated in a robust, defect-free matrix, with a strictly controlled pore entrance chemistry and pore-size distribution, and a large active pore density.
Flows through NT membranes: A multiscale modeling challenge Computational modeling of flows through NT membranes is a challenging problem; however, when combined with experiments, it can play a vital role in this materials research area. Simulations can provide useful scientific insights, for instance, into the large disparity in experimental results. They can also be used to quickly explore a much larger parametric space than would be feasible using experiments (e.g., various materials, operating pressures, NT diameters, and membrane thicknesses). Additionally, membrane simulations enable new ideas and concepts to be tested before launching complex experimental campaigns (e.g., modifying the NT inlet/outlet pore chemistry to control performance).
Matthew K. Borg, The University of Edinburgh, UK; [email protected] Jason M. Reese, The University of Edinburgh, UK; [email protected] doi:10.1557/mrs.2017.59
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• VOLUME 42 • APRIL 2017 • www.mrs.org/bulletin © 2017 Materi
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