What is a Carbon Adsorber and how does it work?
One of today’s most widely accepted air pollution control technologies, across many industries, is Carbon Adsorption. A Carbon Adsorber is a type of pollution control equipment that is used to abate Volatile Organic Compounds (VOCs) in low concentration gas streams. Adsorption is a filtration method used to remove volatile organic compounds (VOCs) from waste gas streams. A Carbon Adsorber can be used when you are trying to accomplish one or all of these three things: VOC abatement, no NOx, and solvent recovery.
The gas flows through an activated carbon bed that traps the VOC chemical on its surface. Various types of activated carbon are often used in adsorption systems. The carbon is thermally or chemically activated, with the objective of providing a large surface area. One way of considering carbon’s potential is to consider that 1 pound of carbon has more than 100 acres of surface, which enables activated carbon to have the tremendous ability to adsorb certain VOCs. Initially for carbon adsorber system design, isotherms are studied for particular VOCs. Other considerations include air volume, pressure, elevation, relative humidity, temperature, and the potential effects of multiple compounds in the airstream. Adsorption is a heat-sensitive and relative humidity sensitive process. Additional considerations in the process design are particulate, oils, or other contaminants that may prohibit the carbon from adsorbing VOCs at optimal rates.
While the initial layers of the packed carbon bed may attain high working saturation during the adsorption mode approaching those of the isotherm, the last third of the bed is basically a polishing zone. For the removal of multiple compounds, different types of activated carbon are used in separate canisters, connected in series. As the process gas flows into each canister, the targeted contaminate will be removed by the selected type of media.
There are several types of Carbon Adsorbers with the most commonly used being Fixed Beds and Fluid Bed Adsorbers.
Fixed Bed:
Fixed Bed Adsorbers are a widely used option, because they can control continuous, VOC laden streams, over a wide range of flow rates. These flows can range from several hundred to several hundreds of thousands CFM. The VOC concentration of the exhaust stream can vary greatly that a Carbon Adsorber might be treating. Another reason a Fixed Bed Adsorber is so widely used is because it is able to run intermittently or continuously. When a Fixed Bed is running intermittently and can be Regenerated on site, the desorption cycle happens offline; this is when the Solvent (or VOCs) previously caught by the carbon are removed. After this process is completed, the equipment will sit idle, until it is started back up, again.
Desorbing or Removing the Solvent from a Fixed Bed Carbon Adsorber
There are two commonly used desorption agents – steam and hot gas via air/nitrogen mixture. Steam, as it is used in many manufacturing facilities, is readily available at most industrial sites. Steam has two advantages; it has high energy content per pound and is easily condensed using cooling tower water and a closed condenser to return the solvent and steam vapor to a liquid state. (See Diagram 1) The other advantage is that steam rapidly fills the spent carbon vessels and its properties supply a quick and even distribution of the heat.
When the bed has been saturated with VOCs and the mass transfer zone begins to see breakthrough into the polishing lower section, it is regenerated by the counter flow introduction of saturated steam. The steam fills the carbon, which releases most of the VOCs, and then is migrated away to a condenser/decanter for separation.
Systems are typically provided with three (3) modes of operation.
Mode 1 (Optional): Analyzer Driven Desorption Cycles
The system can be operated or cycled by measuring the effluent toluene levels and subsequently offer the best in utility consumption and carbon life.
Mode 2: Timer Driven Desorption Cycles
They have a PLC capable of adjusting the program timers to control the length of time for each of the basic cycles. (Adsorption and Desorption).
Mode 3: Manual Override
They come with a manual override allowing the operator to cycle the system manually via the touch screen controls provided.
Fluidized Bed Adsorber:
A Fluidized Bed Adsorber also removes VOCs from industrial streams, but it does it in a slightly different way. A Fluid Bed Carbon Adsorber works by forcing the contaminated stream through several trays of activated carbon beads, using varying velocity air streams. This allows the carbon beads to fluidize. The saturated beads then move from the Adsorption tower to the Desorption tower, where the beads are heated and the VOCs are released.
The first section of the system contains a series of perforated plat adsorption trays (A). Contaminated process exhaust enters from the bottom, passing upward through the adsorption trays, and fluidizing the carbon media which is adsorbing the VOCs. Becoming saturated with VOCs as the carbon flows to the bottom of the adsorber vessel (B), the carbon adsorbent is removed at a slow, steady rate and transferred to the desorber (C). Meanwhile, regenerated carbon adsorbent is continuously fed into the top of the adsorber vessel (D), providing counter-current VOC removal.
In the desorber (C), the temperature of the carbon adsorbent material is increased, (E) causing it to release the VOC contaminants into a low volume gas stream, usually air, steam, or nitrogen. The cleaned carbon adsorbent is then returned to the top of the adsorption vessel for reuse.
The concentrated contaminant gas stream (F) is so small it can be easily treated with a simple thermal oxidizer (G), or the solvent(s) can be recovered for reuse or disposal through a cooler and condenser / decanter system (H).
Carbon Adsorption Advantages:
- Handles streams with varying VOC concentrations
- Accepts streams with variable flow rates
- No onsite chemical storage
- Lower maintenance costs
- Carbon has tremendous adsorption capability
- High DRE capabilities
- Custom built, turnkey systems
- Our In-house produced control panels set us apart!
- UL Rated
- Remote access for monitoring and controls
- Remote service support
Options:
- Several carbon types available
- Granular Activated Carbon
- Powdered Activated Carbon
- Pelletized Activated Carbon
- Activated Carbon Cloth
- Integration into existing systems
- Media options available
- Designs available for smaller or restricted spaces
Gulf Coast Environmental Systems has the experience and knowledge to help you with your Carbon Adsorber needs. Our unique approach is to analyze your process stream and business objectives, from which we propose an ideal solution or range of options to help achieve and optimize those objectives. To speak with one of our pollution control solution experts, click here.