Microfibrous Entrapped Sorbents (MFESs)
MFESs allow sorption processes to occur with enhanced contacting efficiency and decreased pressure drop compared with traditional packed bed adsorption systems.
Sorbents can be entrapped in microfibrous media when enhanced contacting is required to improve sorption efficiency. Traditional packed sorbent beds have high adsorption capacities, but often suffer from short breakthrough times due low apparent adsorption rates. The apparent adsorption rate is a function of molecular adsorption kinetics as well as internal and external diffusion resistances. A common approach to enhancing the capacity utilization before breakthrough is decreasing adsorbent particle size to lower diffusion resistances; however, this approach can significantly increase pressure drop within the bed.
Microfibrous entrapment of the sorbent particles offers a solution to achieve enhanced adsorption kinetics (contacting efficiency) and a low pressure drop. The highly porous nature of MFESs allows the microfibrous beds to have 1/8 to 1/2 the pressure drop of a packed bed made of the entrapped particles. Furthermore, the micron-diameter fibers dominate the flow pattern producing a uniform-velocity flow profile with no channeling. MFESs have lower volumetric saturation capacity than traditonal packed sorbent beds because they have lower volumetric sorbent loadings (MFES:0-35% vs. PB:60-70%). This decrease in capacity, however, is overcome with their enhanced adsorption efficiency and lower pressure drop. This allows them to be especially useful as a polishing layer on a traditional packed bed, allowing the capacity of the packed bed and effective contacting of the MFES to be utilized simultaneously, as shown below.
The enhanced sorption characteristics of microfibrous entrapped sorbents (MFESs) enable processes improvements that require high sorbent utilization and contact efficiency. Some common applications include use as polishing sorbents, gas mask elements, active air filters, and in collective protection equipment:
The microfibrous media (MFM), which functions as the sorbent carrier, is prepared by a robust, scalable wet-lay process. This process results in a highly porous structure (~94%) that consists of independently-oriented microfibers that are micro-welded via sintering. The random orientation of the microfibers provides a uniform flow profile throughout the bed which minimizes channeling and assists with mixing. MFESs are prepared using a proprietary method that locks small sorbent particles (0-35 vol.%) within the microfibrous media. The microfibrous structure can be formed from a variety of materials including metals (Cu, Ni, ect.), alloys (stainless steel, brass), polymers, and glass, allowing the support structure to be tailored to a given reaction system. Metals are typically used when enhanced heat transfer is needed. Polymeric entrapment can be performed for low-temperature processes where minimizing mass transfer resistance is the primary concern, and glass fibers are the ideal material when enhanced mass transfer and corrosion resistance are required.
MFES-containing Composite Bed
One important application of MFESs is as thin polishing layers at the downstream end of a typical packed bed. This combined approach is referred to as the composite bed. Without significantly increasing bed length, the breakthrough time of the composite bed can be three-times longer than that of the packed bed.