Vapor Phase Activated Carbon Regeneration with Solvent Recovery
Granular Activated Carbon (GAC) is used to remove Volatile Organic Compounds (VOC’s) from a number of dilute vapor streams. In most cases, the GAC is disposed of once it becomes saturated with contaminant. Depending on the nature of the contaminant, this used carbon may be considered a hazardous waste. Even if the GAC is not hazardous, it must be disposed of and replaced. Due to the costs associated with carbon change-out and disposal, GAC systems are only used for dilute streams.
The current technology used to treat more concentrated vapor streams is catalytic oxidation. These systems are very well established, but have a number of drawbacks. First, these systems generate secondary air pollutants, including Nitrogen Oxides (NOx) and dioxin. Due to these secondary pollutants, it is becoming more difficult to permit these systems in some locals. The other major drawback is the need to use supplemental fuel for most of these contaminant streams. The supplemental fuel (typically natural gas) is used to maintain a high temperature in the catalyst bed, ensuring complete conversion of the organic compounds to carbon dioxide and water. With the volatility and rising cost of natural gas, this supplemental fuel becomes a significant economic disadvantage to catalytic oxidation.
Under SBIR Phase I and II grants sponsored by the National Institute of Environmental Health Sciences, CHA Corporation has developed a technology to regenerate GAC on site and recover the adsorbed contaminants. This allows the GAC to be reused and valuable chemicals to be recycled. Even if the recovered chemicals have little value, the volume of waste is significantly reduced, simplifying disposal. The system is compact enough to mount on a trailer, making it simple to regenerate carbon at any site. This makes it feasible to use GAC as an alternative to catalytic oxidation for concentrated streams because we can reduce the cost of carbon change out and the need for supplemental fuel is eliminated.
Under the Phase I NIEHS grant, a prototype unit was installed and demonstrated at McClellan Business Park, CA (formerly McClellan AFB). The unit uses a 3kW microwave generator to heat the GAC moving through the regeneration reactor at a mass flow rate of 25 lbs/hr. This rapidly heats the carbon, which desorbs the contaminants from the GAC. The contaminants are removed from the reactor with a nitrogen sweep stream. The sweep gas stream passes through a dual stage condensing system, which recovers the organic vapors, and allows the nitrogen stream to be recycled through the system. The system was operated successfully for two months that completed 13 adsorption and regeneration cycles using a slip stream taken from an active Soil Vapor Extraction (SVE) system at McClellan.
Under the phase II NIEHS SBIR grant, we constructed a trailer mounted mobile version of the system used at McClellan. The unit regenerates carbon at a rate of 100 lb/hr. The system was field tested at McClellan Park in 2006 and will be used for conducting a field demonstration of biogas upgrading in 2009. Figure 2 is a structural drawing of the unit. The derrick on the end of the unit is designed to lower during transportation.