Description:

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 considerations.

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 -150^oF 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 organics.

Synonyms:

Applicability:

Limitations:

• 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 capacity.
• Biological growth on carbon or high particulate loadings can reduce flow through the bed.
• Some compounds, such as ketones, may cause carbon bed fires because of their high heat release upon adsorption.

Data Needs:

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.

Performance Data:

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.

Cost:

References:

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.

• Soil Vapor Extraction System at Commencement Bay, South Tacoma Channel (Well 12A), Phase 2, Tacoma, Washington

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.

• Commencement Bay, South Tacoma Channel (Well 12), Phase 2, Tacoma, WA

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