Carbon Nanotube Filters
Barbara Kanegsberg and Ed Kanegsberg, September 2005
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 Seldon Technologies, 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 cleaning, 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; info@bfksolutions.com; www.bfksolutions.com.
