Ed Kanegsberg

Liquid filtration is invaluable for extending chemical agent life, whether for a solvent or aqueous process. In cleaning processes, filtration is used for pre-conditioning of cleaning agents prior to a process, maintaining cleaning agent quality during the process, or pre-disposal treatment. If you are going to discharge water from the plant, it has to have a composition that makes it legal to do so. Current filters have limitations:
• Filters have limited specificity both in terms of particle size and of chemical composition
• If filters become clogged, regeneration may be difficult
• Operation at high temperature is often not possible
One promising approach to next-generation filtration is based on carbon nanotube technology. Nanotube-based filters exhibiting chemical selectivity would have the advantage of allowing, for example, complete regeneration of an aqueous/surfactant package while removing all of the soils. Currently, we depend on the skill of the formulation chemist to minimize changes to the cleaning agent during filtration.
The new filters are anticipated to exhibit chemical-species selectivity.
The filters are expected to have higher physical strength and higher temperature tolerance. This will lead to a more rugged process, more rapid filtration, and lowers costs. Regeneration via thermal means rather than physical removal should be possible.
Details:
A collaborative study (1) conducted by researchers from Rensselaer Polytechnic Institute and Banaras Hindu University provides an early demonstration of nanotube filtration and a glimpse into potential applications. One often thinks of nanostructures in terms of microscopic laboratory demonstrations. The researchers devised a method for making comparatively large-scale structures and have made liquid filters from them. Because the pore sizes in the carbon membrane are more uniform than those in conventional membranes, a carbon-nanotube filter could be especially effective at filtering out selected chemicals or microorganisms. What's more, because carbon nanotubes can tolerate much higher temperatures and are stronger than polymers, heat and ultrasound could be used to periodically unclog the membrane without destroying it.
In the recent study, the researchers injected a solution of benzene and ferrocene into a stream of argon gas and then sprayed the mixture into a quartz tube that was located inside a furnace heated to 900°C. A dense forest of carbon nanotubes formed on the inner walls of the quartz tube, yielding a hollow black cylinder. They then removed the cylinder, which measured several centimeters long and up to a centimeter in diameter.
In one experiment, they then capped one end of the tube and let petroleum flow into it. As the oil passed through the cylinder's wall, the membrane caught the large and complex hydrocarbons—a necessary step in making gasoline. This could be used similarly in cleaning agent preparation or re-purification.
In a second experiment, they tested their filter on contaminated water. The researchers had added Escherichia coli, the bacterium responsible for a common intestinal disease, to a sample of water and passed the sample through the filter. Analysis of the filtered water showed that it was devoid of E. coli. More surprising, when the researchers tried water contaminated with the poliovirus, which is much smaller, not one virus made it through the sieve.
According to the researchers practical filters are anticipated to be commercially available within a few years. A necessary step will be to compare their material's performance with that of conventional ceramic or polymer filters to gauge how competitive in performance a carbon-nanotube filter will be.
For more information on this topic, or many more topics in the world of cleaning, contamination control and surface preparation, contact BFK Solutions.
Reference:
(1) Srivastava et al, “Nature Materials, Vol 3, p. 610 – 614, 2004.

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