493
and new types are under development. Their high-surface area and
interconnected open pore structure are advantageous for trapping
particles but still retaining high air flow or flux levels. Industrialscale HVAC filtration for large spaces has seen development of filters
that incorporate nanofibers to get finer than the HEPA standard.
Likewise, facemask filters are being developed to capture particles
considerably smaller than the HEPA standard.
In adsorption systems, large airborne particles are captured on the
surface of a bead, granule, or crystal of an adsorbent material. The
adsorbed material is held mechanically on a substrate rather than
chemically and can therefore be released (desorbed) rather easily by
heat or vacuum. Adsorptive materials are typically used to capture
VOCs. The most common adsorbent now in wide use is activated
carbon, typically used against odors, fumes, and chemicals. The
carbon material is not only highly porous but has a high surfacearea-to-volume ratio. Preparation processes for this material have an
impact on the particles it can best adsorb, which typically include
odors, fumes, and chemicals. As mentioned before, nanotubes have
been suggested as sorbents, and other types of nanomaterials can
serve this function as well. A parameter that characterizes the sorption affinity for carbon nanotubes—the Langmuir constant—has
been found to be much higher for carbon nanotubes than for activated charcoal. Costs here for large-scale applications are undoubtedly problematic. Another adsorbent is the aluminosilicate crystal
structure known as zeolite, which has uniformly sized pores that can
be at the nanoscale. All naturally occurring zeolite is hydrophilic
and contains aluminum. Removing the aluminum from natural
zeolite makes it hydrophobic and gives it an affinity for nonpolar substances, such as many volatile organic compounds. Zeolite
has many natural states and can come in nanosized forms having
nanodimension pore sizes that have extremely high relative surface
areas (see Section 10.4).
In chemical absorption systems, a gas is absorbed in a liquid solvent
through the formation of chemically bonded compounds. As mentioned, one application involves carbon sequestration, whereby
CO 2 is typically recovered from combustion exhaust such as that
produced by coal-firing plants. The process, known also as scrubbing, uses amine absorbers, most commonly monoethanolamine
(MEA), diethanolamine (DEA), or methyldiethanolamine (MDEA)
compounds. After removing impurities such as nitric oxide, sulfuric oxide, and hydrocarbons, the gas is channeled into an absorption column, where the amine reacts with the CO 2 and selectively
absorbs it. The CO 2 -rich amine is then heated, whereby the CO 2 is
Environmental Benefits and Impacts
and new types are under development. Their high-surface area and
interconnected open pore structure are advantageous for trapping
particles but still retaining high air flow or flux levels. Industrialscale HVAC filtration for large spaces has seen development of filters
that incorporate nanofibers to get finer than the HEPA standard.
Likewise, facemask filters are being developed to capture particles
considerably smaller than the HEPA standard.
In adsorption systems, large airborne particles are captured on the
surface of a bead, granule, or crystal of an adsorbent material. The
adsorbed material is held mechanically on a substrate rather than
chemically and can therefore be released (desorbed) rather easily by
heat or vacuum. Adsorptive materials are typically used to capture
VOCs. The most common adsorbent now in wide use is activated
carbon, typically used against odors, fumes, and chemicals. The
carbon material is not only highly porous but has a high surfacearea-to-volume ratio. Preparation processes for this material have an
impact on the particles it can best adsorb, which typically include
odors, fumes, and chemicals. As mentioned before, nanotubes have
been suggested as sorbents, and other types of nanomaterials can
serve this function as well. A parameter that characterizes the sorption affinity for carbon nanotubes—the Langmuir constant—has
been found to be much higher for carbon nanotubes than for activated charcoal. Costs here for large-scale applications are undoubtedly problematic. Another adsorbent is the aluminosilicate crystal
structure known as zeolite, which has uniformly sized pores that can
be at the nanoscale. All naturally occurring zeolite is hydrophilic
and contains aluminum. Removing the aluminum from natural
zeolite makes it hydrophobic and gives it an affinity for nonpolar substances, such as many volatile organic compounds. Zeolite
has many natural states and can come in nanosized forms having
nanodimension pore sizes that have extremely high relative surface
areas (see Section 10.4).
In chemical absorption systems, a gas is absorbed in a liquid solvent
through the formation of chemically bonded compounds. As mentioned, one application involves carbon sequestration, whereby
CO 2 is typically recovered from combustion exhaust such as that
produced by coal-firing plants. The process, known also as scrubbing, uses amine absorbers, most commonly monoethanolamine
(MEA), diethanolamine (DEA), or methyldiethanolamine (MDEA)
compounds. After removing impurities such as nitric oxide, sulfuric oxide, and hydrocarbons, the gas is channeled into an absorption column, where the amine reacts with the CO 2 and selectively
absorbs it. The CO 2 -rich amine is then heated, whereby the CO 2 is
Environmental Benefits and Impacts