Modeling and calculation of temperature-concentration polarisation in the membrane distillation process (MD)
ABSTRACT A model for a membrane distillation process in a plate-and-frame unit has been developed. It is based on a mass and energy balance equation for hydrodynamic, temperature and concentration boundary layers. The model takes into account energy interdependence between flow in feed and in permeate channels. A model taking into consideration temperature-concentration polarisation (TCP) predicts temperature and concentration values at the membrane surface. The model consists of an analytical equation and permits simulation or analysis of the influence of various factors to permeate flux.
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ABSTRACT: Recent work showed that carbon nanotubes could be fabricated into bucky-paper structures as self-supporting membranes and their hydrophobic property used for water desalination by membrane distillation. Carbon nanotube bucky-papers possess very interesting properties such as natural hydrophobicity, high porosity and very high specific surface area, making of them promising candidates for separation applications. However, to inhibit crack formation associated with poor mechanical stability, improved durability structures that retain a high degree of hydrophobicity need to be developed. In this work we chemically modified high purity carbon nanotubes by (i) UV/ozone treatment to create hydroxyl groups and (ii) reacting those groups by substitution with alkoxysilane based groups. The resulting nanotubes presented a silicon content up to 2.5% and the bucky-papers produced had larger contact angle (140° compared to 125°) and liquid entry pressures. We report here on their fabrication and use as membranes in direct contact membrane distillation.Highlights► Fabrication of carbon nanotubes based membranes. ► Alkoxysilane grafting on nanotubes for enhanced hydrophobicity. ► The membranes were characterised and tested in a membrane distillation rig. ► The alkoxysilane modified CNT BP exhibited higher performance and lifespan than the non treated samples.Journal of Membrane Science 07/2011; 376(1-2):241-246. DOI:10.1016/j.memsci.2011.04.024 · 4.91 Impact Factor
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ABSTRACT: New membrane distillation configurations and a new membrane module were investigated to improve water desalination. The performances of three hydrophobic microporous membranes were evaluated under vacuum enhanced direct contact membrane distillation (DCMD) with a turbulent flow regime and with a feed water temperature of only 40 °C. The new configurations provide reduced temperature polarization effects due to better mixing and increased mass transport of water due to higher permeability through the membrane and due to a total pressure gradient across the membrane. Comparison with previously reported results in the literature reveals that mass transport of water vapors is substantially improved with the new approach. The performance of the new configuration was investigated with both NaCl and synthetic sea salt feed solutions. Salt rejection was greater than 99.9% in almost all cases. Salt concentrations in the feed stream had only a minor effect on water flux. The economic aspects of the enhanced DCMD process are briefly discussed and comparisons are made with the reverse osmosis (RO) process for desalination.Journal of Membrane Science 01/2004; DOI:10.1016/j.memsci.2003.09.006 · 4.91 Impact Factor
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ABSTRACT: The method of Ilias and Govind for modeling ultrafiltration was modified to model a flat sheet air gap membrane distillation module with empty channels. In the membrane distillation process, there is mutual dependence of the momentum and heat transport processes. It complicates the solution of the governing equations and often leads to using simplifications. The developed method allows solving hydrodynamic and heat transport equations without additional assumptions for decoupling the equations with permeation taken into account. Velocity and temperature distributions inside the membrane feed channel were obtained, as well as the concentration profiles of a sparingly soluble salt (barium sulfate). Severe temperature polarization, occurring under poor mixing conditions, has been shown. Supersaturation profiles of barium sulfate are shown to be dependent on the inlet temperature of the membrane channel, and at certain conditions a maximum value of the barium sulfate concentration lies in the bulk of the channel, rather than at membrane surface.Desalination 08/2003; 157(1-3):315-324. DOI:10.1016/S0011-9164(03)00411-9 · 3.96 Impact Factor