Page 435 - Air Pollution Control Engineering

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Gas Phase Activated Carbon Adsorption 407

Fig. 3. Pressure drop versus carbon bed depth at various air velocities (Sources: Union Carbon
Corporation and US EPA).

that it is one of the most widely used control technologies, and subsequently, much tech-
nical information is available from numerous vendors. Design considerations for GAC
systems include characteristics of the emission stream, molecule weight of the adsorbate,
and the pretreatment requirements.
An important limitation of the GAC system is that only compounds with molecu-
lar weights in the 50–200 g/g-mol range are effectively adsorbed by the system.
Another characteristic of the GAC system is that the pollutants are not destroyed,
only transferred from one medium to another, inevitably leaving solid or liquid waste
after treatment with the GAC system. As a result of this transfer, GAC systems are
often used in industry to capture and recycle valuable pure VOCs. At remediation pro-
jects using GAC systems, VOCs usually are not of sufficient purity or high value to
warrant recycling, and as a result, the disposal of the adsorbed VOCs is almost always
the final step.
Condensers, incinerator, or combustion engines become competitive in cost-effec-
tiveness with GAC systems when VOC concentration in the air emission stream
exceeds 1000 ppmv. Effectiveness of GAC systems depends on temperature, pressure,
and moisture content of the emission stream. At high temperature and pressure, GAC
systems are less effective. Additionally, GAC systems require low humidity in the emis-
sion stream because water binds to the active sites in the carbon, reducing the system’s