How crucial is activated carbon RESIDENCE TIME ?

In the designing mechanism of Local Exhaust ventilation System which has a chimney, Clean Air Regulations requires a System Notification to be done. One of the standard rule of thumb is for the system to have an air cleaning device. 

As this requirement has been clearly known, a sudden surge in installation of activated carbon filter has risen since later 2018 just to fulfil the permitting process. However, are these carbon filters adequately designed ? Are they suitable for the emission of the specific airborne hazard ? At this point we need to understand, CARBON FILTERS are not universal air cleaning device.

In industrial hygiene and air purification, the residence time of air in activated carbon is critical for effective contaminant removal, especially when dealing with VOCs like xylene and MEK. Residence time refers to the length of time the air (or any other stream) remains in contact with the activated carbon bed. Here’s why it matters:

1. Adsorption Efficiency: Longer residence times improve adsorption efficiency. VOCs need sufficient time to diffuse into the carbon’s porous structure and bind to the internal surface. Insufficient time may lead to incomplete removal, allowing some contaminants to pass through the filter.

2. Contaminant Type: Heavier or more complex VOCs often require longer residence times due to their molecular structure, which may take more time to interact with and adsorb onto the carbon.

3. Carbon Bed Saturation: With too short a residence time, the carbon bed can saturate more quickly because the contaminant molecules don’t have adequate contact time for effective adsorption. This leads to faster breakthrough and requires more frequent carbon replacement or regeneration.

4. Airflow Rate: Residence time is inversely related to airflow rate. Higher airflow reduces residence time, so balancing these parameters is essential. If residence time is sacrificed to maintain high airflow, the overall efficiency may suffer, requiring a design adjustment.

5. Temperature and Humidity Impact: Higher temperatures can reduce adsorption capacity, which means you may need to increase residence time to maintain efficiency. Similarly, high humidity can compete with VOCs for adsorption sites on the carbon, affecting performance.

For effective VOC removal, residence times typically range from 0.1 to 0.5 seconds for general air streams. However, optimizing for specific contaminants and conditions is ideal.

Excerpts of Carbon Filter Design Calculation Which is Crucial to obtain highest cleaning efficiency of the pollutant