Breakthrough of shallow activated carbon beds under constant and pulsating flow.

During actual use of a gas mask canister the flow through the activated carbon bed is pulsating. Pulsating flows were found to be less favorable for the breakthrough behavior compared to a constant flow pattern. The present article demonstrates this difference and clarifies how and why the breakthrough time depends on the airflow pattern. Furthermore, it shows the importance of good estimates of both diffusion parameters and mass transfer coefficients to obtain accurate predictions of the initial part of the breakthrough curve. Breakthrough measurements applying continuous and pulsating flows were performed using toluene on shallow activated carbon beds; toluene is a good representative for the type of vapors for which activated carbon forms a suitable adsorbent. Pulsating flow was studied by using the positive halves of a sinusoidal flow pattern, which closely resembles the actual breathing pattern. A two-dimensional mathematical model was used to describe the dynamic behavior. The agreement between measured and simulated breakthrough curves was good for both the constant and pulsating flow experiments. The difference in time between simulation and experiment was 10% at most. The influence of pulsating flow on the breakthrough behavior is well accounted for by the model. The influence of flow rate on the mass transfer from the bulk gas phase to the surface of the adsorbent particles is ultimately responsible for the difference in breakthrough times. Testing under pulsating flow allows for a more realistic assessment of the performance of cartridges and canisters.