Article as printed in Pollution Engineering Magazine March 2012
Contributors: Jim Kearns, Anthony Mazzoni & Tom Bourke

Activated Carbon Adsorption Equipment
Fabric enclosure structures increase public safety during MGP remediation projects. TIGG air purifiers cleanse air-borne contaminates.
Tasked with the excavation, removal and solidification of soil containing coal-tar by-products generated by a former manufactured gas plant in New York, an experienced environmental remediation firm decided to use a method that proved successful in previous projects.

"The remediation is intended to remove and reduce the mobility of contaminants that were deposited at the site by the former manufactured gas facility during its operations from the early 1900s to around 1950," said Lech Dolata, project manager of the site for the N.Y. State Department of Environmental Conservation. "If left unchecked, contaminants at the site could be harmful to local residents and they must be treated and/or removed."

By erecting a large fabric structure to enclose the area where contaminated soil is being excavated, the remediation contractor increased public safety, preventing harmful vapors and organic contaminants in the soil from escaping into the air. To capture and cleanse air-borne contaminants released during digging, this remediation enclosure structure will make use of activated carbon adsorber systems that act as air purifiers.

Two fully-integrated activated carbon air purification systems, each with a 100 horsepower blower, enable a cross airflow zone by creating negative pressure inside the fabric structure. This cross airflow zone provides workers inside the enclosure with a safer environment to carry out their duties

"The activated carbon filtration systems are being used to protect the local community and help safeguard the workers who are transferring contaminated soil to vehicles for transport and consolidation," said Dolata. "The intent is to reduce the discharge of airborne contaminants into the atmosphere and to control the environment inside and outside the structure."

Running in tandem, the filtration systems process all of the air in the remediation enclosure several times an hour. As the contaminated air flow is drawn into the air purifier, fresh air enters the edifice via louvers on the opposite end of the structure from the blowers. Each blower requires three, four-foot by four-foot makeup inlet air vents.

"Some contaminants that may be released during the digging include naphthalene, benzene, toluene, ethylbenzene, and xylenes," said Jim Kearns, TIGG Corporation Director of National Accounts. "If these compounds are present, your nose is the best way to tell. You will definitely smell something."

TIGG designs, manufactures, sells, and provides rental equipment for vapor and liquid phase applications in the environmental remediation market. The company supplied the activated carbon adsorption filtration equipment for the work at the site. TIGG has provided equipment and solutions for similar remediation projects more than 50 times over the past several years.

Moving air at a combined 40,000 cubic feet per minute, the two blowers draw air into two activated carbon box adsorbers. Each box adsorber contains approximately 16,000 pounds of activated carbon.

Granular CarbonOften used as a filtration media for environmental remediation projects, activated carbon has submicroscopic pores that accommodate organic contaminants of various molecular sizes. The inner walls of the pores provide the surface layer molecules essential for adsorption.

"One pound of carbon provides a surface area equivalent to six football fields," said former TIGG Vice President Anthony Mazzoni. "That is about as much carbon as you can fit in the palm of your hand.

As air moves from the blower into the box adsorber filtration unit, it enters a chamber in the bottom of the 30-foot-long container, where it approaches zero velocity. Pressure buildup pushes the process stream upward through a bed of activated carbon.

Kearns explained that contact time with the carbon and the velocity of flow through the bed are very important to the adsorption process.

"The primary contaminant removal takes place as the air moves through the first several inches of carbon," Kearns said. "The area of carbon where adsorption takes place is known as the mass transfer zone."

Over time, the submicroscopic pores in the carbon bed accumulate as much adsorbate as they can handle and become spent. The two modular activated carbon box adsorbers used in the project have carbon beds approximately 36 inches deep.

This means that as lower layers become spent, the mass transfer zone moves up higher in the carbon bed, leaving spent layers beneath. The length of time it takes carbon to become spent is dictated by the concentration of organic compounds present in the airstream entering the unit and the airflow rate.

"As a remediation project progresses, the contents of the entire box could become spent and require a change out," Kearns said. "We can run prediction models on how long a carbon bed will last based on static contaminate levels. However, conditions in the field are never static."

Sample ports placed on the side of the box adsorber in strategic spots to allow measurements to be taken to determine how far the mass transfer zone has traveled. Using these sample ports, the movement of the mass transfer zone can be tracked.

The duration of the project is expected to be two years. The 16,000 pound bed will be closely monitored, and if the effluent levels exceed a certain limit, the unit's carbon may need changed out.

Once the air has traveled through the bed and the organic compounds are adsorbed, the polished air flows out stacks located at the top of the unit. Verifiable testing procedures ensure the effluent air exiting the system meets stringent air quality standards

"The remediation contractor works under the requirements of the Community Air Monitoring Plan," said Dolata. "The company will continuously verify that the filtration system is working properly. Local residents and workers at the site can be assured that they are safe and that the air being reintroduced into the environment is contaminant free."