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Carbon Adsorption Technology Review |
Adsorption is the process by which organic molecules are attracted to the surface of an adsorbent. Adsorbents include activated carbon, silica gel, zeolites and polymers. The most common and widely used adsorbent is activated carbon. A requirement of a good adsorbent is for there to be a high surface area to mass ratio as it assures a large amount of sites where molecules can attach to the surface of the adsorbent. This high surface area typically results in a higher adsorption capacity which is usually defined as the pounds of adsorbate that can be adsorbed per pound of adsorbent.
Adsorption is not a chemical process but a physical process. This allows the adsorption process to be reversible. Heat and pressure fluctuations can be used to remove organic molecules from the surface of adsorbents. In this manner, adsorbents can be regenerated for reuse and the organic molecules recovered for reuse or destroyed. The most common approach for regeneration is through heat introduction typically using steam.
Activated Carbon
Activated carbon is a form of carbon made from many different raw materials including coal, coconut, wood, etc. The most common raw materials are coal and coconut. The activation process takes place at elevated temperatures in the presence of steam or gas to create a large quantity of molecular size pores throughout the carbon. Through the activation process, the carbon attains a very high surface area to mass ratio. This ratio is on the order of 500 – 1400 m2/g.
Activated carbon adsorption capacity and efficiency relies on the surface area, pore size distribution, adsorbate molecule size, temperature, pressure, moisture content, and concentration of adsorbate. Typical removal efficiencies for activated carbon are 95 to 99 wt%.
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Surface Area: |
The higher the surface area the higher the adsorption capacity of a particular carbon. |
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Pore Sizes: |
During the activation process, the use of steam or gas and the selection of raw material dictate the pore sizes and size distribution of macropores and micropores. The size distribution dictates how readily different size molecules adsorb on the surface (i.e., larger molecules require larger pores). |
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Molecule Size: |
Molecules must be matched with proper size distribution of adsorbent pores to achieve high efficiencies. |
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Temperature: |
The lower the temperature the higher the adsorption capacity. |
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Pressure: |
The higher the pressure the higher the adsorption capacity. |
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Moisture Content: |
Water competes with organic molecules for activated sites on carbon. Therefore, the lower the moisture content the higher the adsorptive capacity. |
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Concentration: |
Adsorption capacity is proportional to the concentration of the adsorbate (i.e., higher concentration higher capacity). |
Carbon Adsorption Applications
As stated previously, adsorption can be reversed through temperature and pressure fluctuations. Once the capacity of activated carbon has been reached the carbon must be replaced, regenerated or reactivated. Replacement and reactivation require that replacement carbon be brought on site which can become costly if this is a high frequency event. In those cases and in instances where the recovery and reuse of solvent is possible, steam regeneration can be implemented.
Offsite Regenerable Carbon Adsorption Systems
Activated carbon can be provided in either a bulk basis or on a service basis. In both of these cases, the carbon upon saturation is taken offsite to either a landfill or to a reactivation facility. In the case of landfill, the carbon is replaced with virgin product. In reactivation applications, the carbon can be brought back onsite for reuse once reactivated. The reactivation process uses a high temperature kiln to remove the organics from the surface of the carbon to "reactivate" the existing adsorption sites. There is some loss of capacity and mass during these operations which must be taken into account when using reactivated material.
Onsite Regenerable Carbon Adsorption Systems
In cases where offsite regeneration is not practical, onsite regenerable carbon adsorption systems can be utilized. These systems include process equipment such as condensers, automated valves, pumps, and tanks designed to automatically regenerate carbon onsite via steam injection.
Upon saturation of the carbon, steam is introduced to remove the organic molecules from the pores and to regenerate the carbon. This process is automatically initiated typically through a timer sequence programmed within the equipment's control system. The steam along with the released organic vapors are routed to a condensation system where the steam and organics are recovered in liquid form allowing for reuse or disposal. After regeneration, the carbon is cooled and dried after which it can then be utilized for adsorption. While these systems have a much higher capital expense than offsite regeneration, the carbon typically only requires replacement every 8 to 10 years which may justify this additional upfront expense.
This application of both offsite and onsite regenerable carbon adsorption technologies are suitable for VOC/HAP abatement. Onsite steam regenerable carbon adsorption systems can also be used in solvent recovery applications. The removal efficiencies that can be accomplished with these types of system are 90 to 99 wt%.
Each of these adsorption technologies has advantages and disadvantages over one another. When reviewed along with other air pollution control equipment the decision process can become confusing. Fusion Environmental can assist by conveying all the advantages/disadvantages of each technology for a particular application.
For more detailed information on selecting the right technology, you may Contact Us or you may provide us your application specific details through our Information Request Form. Fusion Environmental Corporation will review your particular application and provide a recommendation on the best possible solution.