Direct contact membrane distillation (DCMD) is a thermal process in which warm feed and cool distilled water flow on opposite sides of a hydrophobic membrane. The temperature difference causes water to evaporate from the feed, travel through the membrane, and condense in the distillate. Because DCMD is insensitive to osmotic pressure, it has emerged as a promising means of concentrating brines to their saturation limit. Studies have shown that temperature and concentration polarization are the two crucial factors affecting
DCMD performance in the treatment of hypersaline brines. Temperature polarization refers to a reduction in the transmembrane temperature difference due to heat transfer through the membrane. Concentration polarization describes the accumulation of solutes adjacent to the feed side of the membrane. To date, computational fluid dynamics (CFD) studies of DCMD focus primarily on the challenge of temperature polarization. For high concentration brines, however, concentration polarization is another major challenge that reduces system efficiency and leads to mineral scaling. Temperature and concentration polarization are further complicated by spacers, a mesh-like material that separates and supports tightly packed membrane sheets. These interactions are not well understood, because they are difficult to study experimentally and numerically, and the flow regimes are not fully charted. We consequently develop a tailored in-house CFD code that simulates unsteady two-dimensional heat and mass transport in plate-and-frame DCMD systems with cylindrical spacers. The code uses an efficient combination of finite-volume methods in space, projection methods in time, and recent advances in immersed boundary methods for the spacer surfaces.
For DCMD systems without spacers, we perform a comprehensive parametric study of polarization phenomena for a wide range of feed and distillate operating conditions, system length, and co-current versus counter-current operation. We also investigate the system-level performance by measuring the average permeate flux, single-pass water recovery, maximum concentration polarization coefficient, and gained output ratio of DCMD systems with heat recovery. Though the transmembrane vapor flux is small, we observe dramatic increases in solute concentration at the membrane surface, exceeding 1.6 times the feed value. The temperatures, concentration, and vapor flux vary considerably in the downstream direction, and are poorly approximated by common Nusselt and Sherwood correlations.
For DCMD systems with spacers, we investigate the impact of the Reynolds number, spacer diameter, and spacer position on polarization and system performance. We show that the impact of spacers can be explained by examining the various steady and unsteady vortical flow structures generated in the bulk and near the bounding plates and membranes. Overall, we show that though unsteady vortex structures tend to mix temperature polarization layers with the bulk, they are not similarly able to mix the thin concentration layers. Rather, vortical structures tend to create regions of preferential salt accumulation. In the vortex shedding regime, the net result is that spacers often increase vapor production at the expense of increasing the risk of mineral scaling.
|Commitee:||Cath, Tzahi Y., Decaluwe, Steven C., Ganesh, Mahadevan|
|School:||Colorado School of Mines|
|School Location:||United States -- Colorado|
|Source:||DAI-B 82/4(E), Dissertation Abstracts International|
|Subjects:||Mechanical engineering, Engineering|
|Keywords:||Cmputational fluid dynamics, Concentration polarization, Immersed boundary method, Membrane distillation, Temperature polarization, Vortex shedding|
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