CARBON NANOTUBE TECHNOLOGY PROMISES to lead to the next generation of filters. Filters are critical elements of both liquid and gas phase processes. In cleaning processes, filtration may remove contaminants from cleaning agents prior to a process, maintain cleaning agent quality during the process, and treat prior to disposal. High Efficiency Particulate Air (HEPA) filters are critical components of cleanrooms.

Currently available filters have limitations:

• Specificity and discrimination are limited in terms of particle size and chemical composition of the contaminants
• Regeneration may be difficult or impossible
• Operation at high temperature is often not possible

Nanotube-based filters have the potential to address these limitations.

Physical and conductive properties give nanotube-based filters the potential for enhanced chemical selectivity. Nanotube fibers have much smaller diameters (20 to 50 nm) than cellulose filter fibers (typically 1-10 microns). With smaller, uniform fibers, there is the potential to design specific pore size and shape. Such filters can trap very small particles, including viruses. Also, because carbon is electrically-conductive, there is more control than with a passive pore; so one could distinguish between polar and non-polar molecules. For instance, it might become possible to obtain complete regeneration of an aqueous/surfactant package while removing all of the soils or to select specific contaminants to target.

The hexagonal arrangement of carbon in carbon nanotubes (similar to “Buckyballs”), makes them more temperature tolerant and stronger than polymers; stronger even than steel. Therefore, heat and ultrasound could be used to regenerate a filter membrane without destroying it. Attributes of nanotube filters should lead to more rugged processes, more rapid filtration, and lower costs. Potential applications include an array of critical processes including medical, pharmaceutical, and security applications.

A Scanning Electron Microscope (SEM) image of a
Carbon nanotube HEPA filter configuration.
(Photo Courtesy of NanoTechLabs, Inc.)

Researchers from Rensselaer Polytechnic Institute and Banaras Hindu University have reported a method for making comparatively large-scale nanotube structures and liquid filters [1]. They injected a solution of benzene and ferrocene into a stream of argon gas and then sprayed the mixture into a quartz tube inside a furnace heated to 900°C. A hollow black cylinder of carbon nanotubes formed on the inner walls of the tube. The cylinder, measuring several centimeters long and about a centimeter in diameter, was then removed. A filter was formed by capping one end of the tube.

As a demonstration, the membrane was used to trap large, complex hydrocarbons from petroleum. The high filtration specificity could lead to improved methods of gasoline production, and similarly to more effective means of cleaning agent preparation or re-purification.

In terms of actual commercialized product, suppliers of nanotubes are providing material that is being developed into liquid filters and HEPA filters. The initial application for the liquid filter is for the production of potable water [2]. Testing shows that both bacteria and water borne viruses are removed, to the extent that the output meets or exceeds EPA standards for drinking water. By controlling the way in which thin membranes of the nanotubes are configured, other specific contaminants, including arsenic, can also be effectively removed. Testing of HEPA filters [3] indicates that the nanotube based filters (see Figure 1) are more efficient by a factor of one hundred compared to cellulose based filters. Several filter variations meet guidelines [4] for maximum efficiency HEPA class 14 filters which require 99.975% efficiency at the most penetrating particle size.

Acknowledgements:
The authors thank Rich Czerw from NanoTechLabs., and Alan Cummings from SeldonTechnologies, for supplying helpful comments.

Reference:
1 Srivastava et al. “Nature Materials,” Vol 3, (2004) p. 610 – 614.

2 A. Cummings. Personal communication.

3 R. Czerw. Personal communication; also June 2005 newsletter at http: //www.nanotechlabs.biz/newsroom/newsletter/index.htm

4 Comite European de Normalization (CEN), Standard, EN 1822-1:1998.

 

Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press.Contact them at BFK Solutions LLC., 310-459-3614; [email protected]; www.bfksolutions.com.