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Nano Technology

Even at sub-micron removal ratings, conventional filters rely on the physical structure of the filter media for capture and retention of contaminants. The principal...
mechanisms of conventional filtration include, direct interception, inertial impaction, filter cake and bridging.
The following image is a SEM micrograph of the conventional resin impregnated, charge modified glass fiber filter media in use in most reactor coolant systems in North America (scale is in lower left hand corner).

Note the compact, dense structure of the filter media. This leads to low permeability, high differential pressure losses and low dirt holding capacity. For the operator these characteristics, translate into more frequent filter change-outs and increased cost of filtration.
Nano-fiber filters do not rely on their physical structures to capture and retain sub-micron particles. Nano-fiber filters principal mechanism of retention for particles size less than 0.3 microns is electroabsorption. The electroadsorptive properties of the alumina monohydrate – AlOOH - nano fibers (active ingredient in nano-fiber filters) take advantage of charge characteristics of small particles (< 0.3 µm) in water.

The following image is an SEM micrograph of a single nano-fiber filter pore. Unlike the conventional resin impregnated glass fiber media, the nano-fiber filter media has mean pore size of approximately 2.7 – 3 µm.

The electroadsorptive properties of the nano-fibers make the filter highly efficient in removal of waterborne contaminants in the sub 0.1 µm range, while the open pore structure results in higher permeability, lower pressure losses and higher dirt holding capacity. For the operator these characteristics, translate into fewer filter change-outs and decreased cost of filtration.
For the most part conventional filters rely on direct interception (also known as sieve filtration) for the capture and retention of contaminants. As filter removal ratings become tighter, it becomes increasingly impractical to simply continue to decrease the pore size of the filter, which results in fewer voids/lower dirt holding capacity and significantly higher pressure losses which equates to more frequent filter change-outs.

Mechanisms of Filtration - Direct Interception/Sieve Filtration

Courtesy of Jonathan Brant, University of Wyoming, Dep’t of Civil & Architectural Engineering
0.1 µm is the current removal standard for RCS filtration. The bulk amount of the remaining activity in the RCS is believed to be in the colloidal form and may not be easily captured and retained by conventional filters, even with removal ratings in the range of 0.1 µm.

It has been recognized that in polar liquids (water), as particles become smaller (< 0.3 µm) they tend to take on an electrical charge. Conventional resin impregnated glass fiber filters rely on this characteristic, however the epoxy-like resin used to induce charge modification is unable to maintain a high charge density over the 0 - 14 range of pH.

Ref. U.S. Patent # 4523995
Chart details from Pall patent application – October 19, 1981
Nano-fiber based filters are different from conventional filters. Nano-fiber filters rely on electroadsorption rather than physical structure for retention of sub-0.1 µm contaminants.

Image 2: Electroabsorption

Courtesy of Jonathan Brant, University of Wyoming Dep’t of Civil & Architectural Engineering

The scientific name of the ceramic alumina nano-fiber needle is alumina monohydrate – AlOOH. It is also known as boehmite. The boehmite needles or whiskers are very small, approximately 2 nm (diameter) by 300 nm (long). In water at pH = 7.2 the zeta potential attributable to the boehmite fiber has been measured at 54 millivolts (mV).

Photo courtesy of R. Ristau, IMS, University of Connecticut
One gram of boehmite nano-fibers has a surface area greater than 500 m2. One square meter of the nano-fiber filter media (the equivalent filter area of one pleated industrial filter cartridge 30” long by 2.5” diameter) has greater than 42,000 m2 of nano-fiber surface area.

Photo courtesy of R. Ristau, IMS, University of Connecticut

Configured for the RCS filtering application, the nano-fiber media is approximately 800 µm thick and on average has 400 vertical pores. Each pore has more than 550,000 nano-fibers. The computed number of charges attributed to the boehmite whisker’s Al3+ is 2 x1012 per pore. The high zeta potential and high charge density results in strong electroabsorption properties.

Calculations show that the flow path of a small particle (< 0.3 µm) can be influenced by a local field affecting particle trajectory as far as 1 µm from the pore wall. This field overlaps the capture cross section in 2 µm average pore, causing attraction and ultimate retention as the particle traverses a tortuous path of 400 pores.

Photos courtesy of R. Ristau, IMS, University of Connecticut