Removing VOCs

Question:

I am working on a new metal finishing project and have been given the responsibility of removing volatile organic compounds (VOCs) from the air exhaust using activated carbon. Could you give me some guidance on the use and sizing of activated carbon use? M.P.

Answer:

First, I am unable to adequately cover your question in the space of this column; there are books written solely on this subject. Secondly, you did not state what process is emitting the VOCs and the specific compounds in question. However, I will try to give you some guidance.

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Activated carbon has been used for more than 200 years to remove impurities and has proven itself very effective in removing organic compounds from water and air. It works on the principle of adsorption, that is a contaminant is attracted to the surface of the activated carbon and “sticks” to this surface, thereby removing it from the waste stream. To accomplish this, activated carbon has an enormous surface area to weight ratio, typically 1,000 sq meters/gram or 30,000 sq ft/ounce weight. When the activated carbon becomes fully loaded or “spent,” it must be removed and replaced.

The “replaced” activated carbon can be new or regenerated by thermal oxidation. The “spent” carbon must then be disposed of or sent off site for regeneration. Because the “spent” carbon may contain “listed” hazardous waste constituents, it needs to be evaluated, and if disposed, it may need to be handled as a hazardous waste.

The feasibility and sizing of activated carbon units are governed by a multitude of variables such as targeted chemical or chemicals, inlet concentration, outlet concentration, temperature, air moisture content, presence of fouling substances such as oil mists and vapors and particulate matter, operating pressure and desired life of activated carbon between replacement. As an example of the variation of activated carbon’s capacity, let us assume an air inlet concentration of 100 ppmof methanol and methyl ethyl ketone at 25°C (77°F); the amount activated carbon that will hold one lb of methanol has the capability to hold 26 lbs of methyl ethyl ketone. Hence, the chemical makeup of the air stream is critical to size an activated carbon unit.

With the use of activated carbon technology, we strongly recommend an upstream filter to remove solids that will plug the unit and cause premature replacement. Also, if you are treating an air stream containing oil mists and vapors, we also strongly recommend the removal of these materials by filtration or the use of activated clays. Oils coat the pores of the activated carbon also causing premature replacement.

There are a number of reputable suppliers that can assist you. A good source is Products Finishing’s 2004 Directory and Technology Guide under “carbon.