Many different gas-phase air-filtration
devices are available; however, comparing and rating the effectiveness of
installed sorbent filters is difficult because there is no standard test
method. ASHRAE Standard Project Committee 145 is developing a standard method
for evaluating the effectiveness of gas-phase filtration devices installed in
the ductwork of residential HVAC systems, but not in portable air cleaners.30
Gas-phase air filters remove gases and
odors by either physical or chemical processes. These filters typically are
designed to remove one or more of the gaseous pollutants present at low concentrations
in the airstream that passes through them. They are not, however, designed to
eliminate all gaseous pollutants. Air cleaners that do not contain sorbent
materials or photocatalytic oxidation technology, discussed
on page 20,
will not remove gaseous pollutants.
A sorbent filter’s behavior depends on many
factors that can affect the removal of gaseous contaminants:
·
Airflow rate and velocity through the
sorbent.
·
Concentration of contaminants.
·
Presence of other gaseous contaminants.
·
Total available surface area of the
sorbent. (Some manufacturing techniques can significantly reduce a filter’s
total surface area.)
·
Physical and chemical characteristics of
the pollutants and the sorbent (such as weight, polarity, pore size, shape,
volume, and the type and amount of chemical impregnation).
·
Pressure drop.
·
Removal efficiency and removal capacity.
·
Temperature and relative humidity of the
gas stream.
The limited lifetime of gas-phase filters
may contribute to their less frequent use in home HVAC systems.
Gas-phase filters are much less common than
particle air-cleaning devices in homes because a properly designed and built
gas-phase filtration system is too big for a typical residential HVAC system or
portable air cleaner. Other factors that may contribute to the less frequent
use of gas-phase filters in home HVAC systems are the filters’ limited useful
life, the fact that the sorbent material must be targeted to specific
contaminants, the purchase price of the filters, and the costs of adapting them
to residential applications, when possible, and of operating them once they
have been installed.
Types of Sorbents Used for Gaseous
Pollutant Removal
There are two main processes that remove
gaseous contaminants: a physical process known as adsorption and a chemical
reaction called chemisorption.
Adsorption
results from the physical attraction of gas or vapor molecules to a surface.
All adsorbents have limited capacities and thus require frequent maintenance.
An adsorbent will generally adsorb molecules for which it has the greatest
affinity and will allow other molecules to remain in the airstream. Adsorption
occurs more readily at lower temperatures and humidity. Solid sorbents such as
activated carbon, silica gel, activated alumina, zeolites, synthetic polymers,
and porous clay minerals are useful because of their large internal surface
area, stability, and low cost.
Activated carbon is the most common
adsorbent used in HVAC systems and portable air cleaners to remove gaseous
contaminants. It has the potential to remove most hydrocarbons, many aldehydes,
and organic acids. However, activated carbon is not especially effective
against oxides of sulfur, hydrogen sulfide, low molecular weight aldehydes,
ammonia, and nitrogen oxide.
Chemisorption occurs when gas or vapor
molecules chemically react with sorbent material or with reactive agents
impregnated into the sorbent. These impregnates react with gases and form
stable chemical compounds that are bound to the media as organic or inorganic
salts, or are broken down and released into the air as carbon dioxide, water
vapor, or some material more readily adsorbed by other adsorbents. Many
different chemicals may be impregnated on activated carbon; potassium
permanganate is a common chemisorbent impregnated into activated alumina. It
reacts with many common air pollutants, including formaldehyde and sulfur and
nitrogen oxides. Because a chemisorbent will react with only one or a limited
number of reactive pollutants, it should not be expected to reduce others.
Applications of Sorbents for Gaseous
Pollutant Removal
Gas-phase filters that contain sorbents may
be installed in HVAC systems or in portable air cleaners. They are usually
located downstream of particle air filters. The air filter reduces the amount
of particulate matter that reaches the sorbent, and the sorbent collects vapors
that may be generated from liquid particles that collect on the particle
filter.
Some gas-phase filters may remove, at least
temporarily, a portion of the gaseous pollutants in indoor air. Although some
gas-phase air filters—if properly designed, used, and maintained—may
effectively remove specific pollutants from indoor air, none is expected to remove
adequately all of the gaseous pollutants in a typical home. For example, carbon
monoxide is not readily captured by adsorption or chemisorption. In addition,
gaseous-pollutant-removal systems usually have a limited lifetime before the
sorbent must be replaced. There is also a concern that saturated sorbent
filters may release trapped pollutants back into the airstream.31
Tests of gaseous pollutant removal by
activated carbon generally have been conducted using only high concentrations
of pollutants, so little information is available on carbon’s effectiveness in
removing chemicals present in the low concentrations (parts per billion [ppb])
normally found in indoor air. Tests performed at EPA measured the adsorption
isotherms for three volatile organic compounds (VOCs) at concentrations of 100
ppb to 200 ppb using three samples of activated carbon. The bed depth needed to
remove the compounds was estimated assuming a 150 ppb concentration in the air,
an exit concentration of 50 ppb, and a flow rate of 100 cfm across a 2’ x 2’
filter. The results of the study suggest that breakthrough of these chemicals
would occur quickly in 6-inch deep carbon filters used for odor control.32
Because of their compact design, particle
air filters that use impregnated media are available for residential HVAC
systems and portable air cleaners. They use sorbent particles of carbon,
permanganate alumina, or zeolite incorporated into fibrous filter media. Such
filters generally range from 1/8 inch to 2 inches thick. They provide a
combination of particulate and gas-phase filtration with a minor increase in
pressure drop across the filter. Their use in an existing HVAC system does not
require extensive or expensive modifications to the system. However, their
useful service life varies according to indoor pollution concentrations and
exposure time. Breakthrough of the contaminants back into the room takes place
very quickly in the thin layer impregnated with sorbents, resulting in a much
shorter service life for the filter, which must be replaced frequently. Thus,
these devices usually have limited effectiveness in removing odors.
