outdoor air or indoor plant processes. Controlling gaseous contaminants with methods
such as gas-phase air purification will not be
as effective in an environment where the
room construction isn’t tightly sealed and
positive pressurization and humidity control techniques aren’t used. Room sealing is
paramount because the smallest unsealed
cable penetration or leaky door will revert
a room’s pressurization to zero and expose
the protected environment.
Precision air conditioning systems, which
are designed to maintain a 2°± temperature,
5± percent humidity and a maximum rate of
change of six percent humidity per hour, can
reduce humidity and subsequently galvanic
reactions. Mixing high humidity with gases,
such as H2S or HCL, creates highly corrosive
copper sulfide (Cu2S) or copper chloride
(CuCL), respectively, during the components’
passing of electrons. Even salt air alone is
enough to cause failures in coastal areas.
Controlling Gases with Gas-Phase
Air Purification
Gas-phase air purification has been used
for decades in a variety of heavy industries.
In the last few years however, the technology
has advanced as indoor air quality (IAQ) air
standards have mandated increased outdoor air in occupied buildings. Today, the
technology is popular for HVAC systems at
airport terminals to eliminate vehicle emission gases from outdoor intake as defined
by ASHRAE standards. Laboratories have
found it useful for testing accuracy. Even in
vitro fertilization clinics have proven that the
purification of gaseous contaminants in their
laboratories will increase their conception
success rate.
In industrial processes, gas-phase air purification is vital as a life support system for
electronics. Typically, gas-phase air purification consists of carbon-activated pellet
media, typically made from coal, or more
recently greener materials such as coconut
shells. The media lies in a tray where conditioned air from an HVAC system passes
through it. While the solid matrix carbon
adsorbs gaseous contaminants and acts as a
holding device, impregnated media chemically alter the gas so that it’s no longer present in its original form.
A gas-phase system can consist of a tray(s)
of media built into a new or retrofitted HVAC
system. Or it can be a standalone system such
as a deep bed scrubber located outside the
protected room to purify incoming outdoor
air before it enters its room destination. It
can also be combined with an HVAC system
further down line. There are also smaller
stand-alone systems that can be positioned
inside the electronics room. These stand-alone systems operate independently of
the HVAC system and purify room air by re-circulating it through its media. Stand-alone
systems also add additional protection from
gases resulting from door openings, human
off-gassing, or failed room seals.
An example of both HVAC and stand-alone methods used on the same site is
the Riau Andalan Pulp & Paper (RAPP) Mill,
Kerinci, North Sumatra, Indonesia (see
Figure 1)—one of the most technologically
advanced pulp and paper facilities in the
world. RAPP has 73 electrical, 29 computer
rack and 23 control rooms. RAPP maintains
a G1 Standard by using a deep bed scrubber gas-phase system for outdoor air purification and self-contained re-circulating
gas-phase units in the main control rooms
and in all the satellite control rooms.
While many gases are adsorbed in RAPP’s
custom formulated media mix, it uses a
chemisorption process where the media
is impregnated with potassium hydroxide
(KOH) to react with H2S, CL2 and other gaseous contaminants.
Chemisorption and the aforementioned
solid matrix carbon adsorption are two
popular media types. Each carbon adsorption pellet has an ultra capacity and must
be replaced when it can no longer adsorb
gases. Some gas-phase filtration equipment manufacturers operate laboratories
that can determine if the media needs
replacement.
Chemisorption uses carbon pellets
that are impregnated with chemicals to
remove contaminant gases and destroy
them via chemical reactions (see Figure 2).
Chemisorption also uses alumina, silicon
gel or other methods that react with the
adsorbed gases typically through an acid,
base, or a photo-catalytic oxidation reaction.
If ammonia (NH3) is the targeted chemical
gas needing reduction, the carbon-based
pellets are impregnated with an acid such
as sulfuric acid (H2SO4) or phosphoric acid
Figure 2. Two examples of the many kinds
of media formulations that are custom
formulated to adsorb specific gaseous
contaminants that are known or predicted
to be present at the work site.
(H2PO4) to create a base/acid reaction. In
another example, the gas H2S is reduced by
impregnating the media with a base such
as potassium hydroxide (KOH).
Photocatalytic Oxidation
Recent advances in photocatalytic oxidation are very promising for air purification.
Media and surfaces are impregnated with
titanium dioxide and when combined with
254-nanometer ultra-violet (UV) light rays,
OH- hydroxyl molecules are produced—
as long as humidity is present—which in
turn neutralizes and purifies the air of contaminant gases. The reaction is continually
regenerating itself so replacing the media
isn’t required. For example, titanium dioxide is found naturally in sand, therefore
the constant bombardment of the sun’s
UV rays on a beach forms the OH- hydroxyl
molecules that are continually neutralizing
all the gases from nearby vehicle emissions
and other air pollution sources. Photocatalytic oxidation creates a clean-smelling
odorless air quality during the day. At night,
when the sun’s UV rays are absent, the photocatalytic oxidation stops and odors from
the ocean are more prevalent.
Photocatalytic oxidation technology
is still in its infancy. Currently it’s going
through many tests to solve challenges and
refine its efficacy. Hopefully in the future, it
will be adapted for industrial uses.
A G1 rating (see Sidebar) for gases might
have 40 percent RH, which is an acceptable combination. But if the RH jumps to
50 percent, then the rating might progress
to G2 even though the gas concentrations
