Microfibrous Media (Nonwoven) Technology
Microfibrous media is a highly-porous, sintered, nonwoven support structure capable of entrapping catalysts sorbents, and other desirable materials in a fixed-fluidized bed configuration. Microfibrous media can be made of polymers, ceramics, glasses, metals, and alloys by IntraMicron's proprietary wet-lay and sintering process. The material selected for the microfibrous matrix depends on the desired application. Polymers are typically used for low cost applications while ceramic/glass fibers are best for highly corrosive environments. Metal and alloy microfibers are ideal for cases where enhanced heat and/or electrical conductivity are desired. Sintering is a necessary step to stabilize the microfibrous structure by mechanically welding its component fibers. In the case of metal microfibrous media, this sintering step also puts the metal fibers in intimate thermal and electrical contact, enhancing the media's electrical and thermal properties.
MICROFIBROUS MEDIA FABRICATION
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Visit the Sintering Technology page to learn more about microfibrous media fabrication.
Visit the Entrapment Page to learn more about microfibrous entrapped catalysts and microfibrous entrapped sorbents.
Microfibrous Media Properties
Microfibrous media structures are unique because the volumetric loading of the metal fibers and the active phase are mostly independent of one another. This allows the relative amounts of each component to be adjusted over a wide range of parameters compared with most current materials.
When particles are included in the microfibrous media, they are much smaller than those typically used in packed bed systems. This allows mass transfer resistances to be significantly reduced or eliminated. The sintered microfibrous structure separates and supports the small particulates, allowing a high void fraction to be maintained to keep microfibrous media pressure drop low relative to a packed bed of similar particles. In addition to acting as a support structure, metal microfibrous media provides a heat and charge transfer network that enhances heat and/or electrical transfer between the catalyst particles. When Cu (copper) microfibrous media is used, effective thermal conductivities at least 50 times that of an alumina packed bed can be achieved with a 10-fold improvement in the inside-the-wall heat transfer coefficient.
The versatility of the microfibrous media structure allows the heat and mass transport properties of the media to be varied by changing the intrabed voidage. This characterisic allows the microfibrous media structure to be tailored to give the optimal heat, mass, and charge transport characteristics for a numerous catalytic and sorptive processes. These improved characteristics result in microfibrous media-based systems having enhanced performance compared with traditional systems.
Nonwovens refer to paper-like structures made from non-cellulose fibers (such as polymer, ceramic, glass, metal, alloy, and carbon fibers).
During the wet-lay process, the fibers are dispersed in liquid, and the liquid is drained through a screen (on a handsheet mold or continuous paper machine). As the stock or slurry passes through the scree, the fibers are deposited on the screen. Then as more stock runs through the process, it is directed by fluid dynamic forces to the places where the least amount of fiber had been previously deposited. This fills in light spots and prevents heavy spots from forming. This action is referred to as "self-healing", and as a result, the wet-lay method produces sheets with better than three times the uniformity of air-laid sheets.
Since no hydrogen bonding takes place with noncellulosic fibers, an alternative bonding method is necessary. The method most commonly employed by IntraMicron is to sinter the nonwoven at high temperature, which results in very strong bonds. Sintering takes place below the melting point of the fiber, but close enough to that point to cause solid phase welding at the points where fiber crosses fiber. An alternative bonding method is using a chemical binder, but for IntraMicron's products this method is not typically used because of its interference with chemical reactions, sorbents, and catalysts.
The typical wet-lay process produces a sheet with a 95% void volume before calendaring (pressing). IntraMicron has developed a method to produce 98% void thermally conductive media that can be sintered to entrap catalysts and sorbents. This allows for the manufacture of extremely active (endothermic and exothermic) catalyst structures that have the ability to maintain remarkably consistent temperature profiles.