Typical Vapor-Phase Carbon Adsorption System
Vapor-phase carbon adsorption is a remediation technology in which
pollutants are removed from air by physical adsorption onto activated carbon grains.
Carbon is "activated" for this purpose by processing the carbon to create porous
particles with a large internal surface area (300 to 2,500 square meters or 3,200 to
27,000 square feet per gram of carbon) that attracts and adsorbs organic molecules as well
as certain metal and inorganic molecules.
Commercial grades of activated carbon are
available for specific use in vapor-phase applications. The granular form of activated
carbon is typically used in packed beds through which the contaminated air flows until the
concentration of contaminants in the effluent from the carbon bed exceeds an acceptable
level. Granular-activated carbon (GAC) systems typically consist of one or more vessels
filled with carbon connected in series and/or parallel operating under atmospheric,
negative, or positive pressure. The carbon can then be regenerated in place, regenerated
at an off-site regeneration facility, or disposed of, depending upon economic
Carbon can be used in conjunction with steam reforming. Steam reforming is a technology
designed to destroy halogenated solvents (such as carbon tetrachloride, CCl4,
and chloroform, CHCl3) adsorbed on activated carbon by
reaction with superheated steam (steam reforming).
VOC Recovery and Recycle
Another more recent technology related to vapor phase carbon absorption is the
Brayton-cycle heat pump (BCHP). This technology created by Idaho National Engineering
Laboratory offers a method for VOC recovery and recycling. A Brayton-cycle heat pump can
condense volatile organic compounds (VOCs) from an air stream, which offers the potential
for both recovery and either on-site or off-site recycle of a wide range of VOCs. The
VOC-laden air stream can come from either vapor vacuum extraction of soil or air stripping
of contaminated ground water.
The technology consists of activated carbon adsorbers located at each extraction well,
plus a truck-mounted BCHP to regenerate the adsorbers on a periodic basis. The VOC-laden
air from the well is passed through the carbon bed, adsorbing the VOCs. When the bed
becomes saturated, hot nitrogen from the regenerator is used to desorb the VOCs from the
bed. The nitrogen passes through a chiller, is compressed, and is then cooled in a
recuperator, where 50% to 80% of the organics are recovered. The partially depleted
nitrogen stream is then expanded through a turbine, lowering the temperature to as low as
-150oF and condensing the remaining organics. The now-clean nitrogen passes
through the recuperator to cool the VOC-laden nitrogen before returning to the carbon bed.
The only outputs will be the clean off-gas from the well and a small amount of recovered
DSERTS Code: F20 (Carbon Absorption)
Vapor-phase carbon adsorption is not recommended to remove high
contaminant concentrations from the effluent air streams. Economics favor pretreatment of
the VOC stream, followed by the use of a vapor-phase GAC system as a polishing step.
Factors that may limit the effectiveness of this process include:
- Spent carbon transport may require hazardous waste handling.
- Spent carbon must be disposed of and the adsorbed. contaminants must be destroyed, often
by thermal treatment.
- Relative humidity greater than 50% can reduce carbon capacity.
- Elevated temperatures from SVE pumps (greater than 38° C or 100° F) inhibit adsorption
- Biological growth on carbon or high particulate loadings can reduce flow through the
- Some compounds, such as ketones, may cause carbon bed fires because of their high heat
release upon adsorption.
A detailed discussion of these data elements is provided in Subsection 2.2.3. (Data Requirements for Air
Factors that affect adsorption are temperature,
type and pore
size of the carbon, the type and concentration of the contaminant, residence time in the
bed; and in gas phase adsorption, temperature and humidity. At high temperatures, the
volatility of compounds increases, thus reducing their affinity for carbon. Basic
compounds are adsorbed better at high pH. Activated carbon is available from manufacturers
in a variety of grades with different properties and affinities for adsorption of
contaminants. Thus, it is often necessary to conduct adsorption tests with a particular
contaminated stream on a variety of activated carbons from several manufacturers to
identify a carbon that will be most effective for a particular application.
For gaseous systems, linear bed velocities typically range between 8 and
100 feet per minute, although velocities as high as 200 feet per minute have been used,
and residence times range from one tenth of a second to a minute.
If only one or two
contaminants are of concern in the wastestream and there is little or no contamination, a
batch isotherm test is usually sufficient to design the system (i.e., determine system
size and carbon usage). It is also possible to use historical column test data that are
available from vendors for a wide assortment of contaminants to obtain initial design
estimates and to corroborate test results. Isotherm tests can also be used to compare
different carbons and to investigate the effect of temperature on carbon performance. If
the use of regenerated carbon is planned, tests should be performed with regenerated
carbon to obtain a more realistic estimate of the average adsorptive capacity that can be
expected during operation. Regenerated carbon costs less but tends to have a lower
adsorptive capacity than virgin carbon.
Equipment costs range from less than $1,000 for a 100-scfm unit to $40,000
for a 7,000-scfm unit. Carbon cost is $2 to $3 per pound.
DOE, 1994. Technology Catalogue, First Edition.
EPA, 1991. Granular Activated Carbon Treatment, Engineering
Bulletin, EPA, OERR, Washington, DC, EPA/540/2-91/024.
Federal Remediation Technologies Roundtable, 1995. Remediation Case
Studies: Soil Vapor Extraction, EPA/542/R-95/004.
Hinshaw, G.D., C.B. Fanska, D.E. Fiscus, and S.A. Sorensen, Midwest Research Institute,
Undated. "Granular Activated Carbon (GAC) System Performance Capabilities and
Optimization", Final Report, USAEC, APG, MD, MRI Project No. 81812-S, Report
No. AMXTH-TE-CR87111. Available from NTIS, Springfield, VA, Order No. ADA179828.
Points of Contact:
General FRTR Agency Contacts
Technology Specific Web Sites:
Government Web Sites
A list of vendors offering Air
Emission/Off-Gas Treatment is available from EPA
REACH IT which combines information from three established EPA databases,
the Vendor Information System for Innovative Treatment Technologies (VISITT),
the Vendor Field Analytical and Characterization Technologies System (Vendor
FACTS), and the Innovative Treatment Technologies (ITT), to give users access to
comprehensive information about treatment and characterization technologies and
Health and Safety: