A one-dimensional carbon canister adsorption model (CANMOD) has been developed to assist in the prediction of the performance of carbon bed canisters in vehicle evaporative emissions control systems. The model accounts for mass transfer and transient thermal phenomena, both of which are found to be essential in accurately describing canister behavior. The model assumes the vapor above the carbon to be in equilibrium with the adsorbed mass while the local temperature is determined by the dynamic balance between the heat of adsorption, carbon heat capacity and heat loss to ambient. The results of the model compare well with laboratory data on a 1L canister under load and purge conditions typical of vehicle operation. Variables investigated include: load level, feedgas concentration, and purge rate.
The model accurately predicts the hydrocarbon mass adsorbed by the canisters, as well as breakthrough times, and hydrocarbon removal rates. Less accurate predictions are obtained for details of the breakthrough curve and local bed temperatures at the highest loading and purge rates, indicating a need for refinement of the model by including transient vapor-carbon mass transfer, and two-dimensional effects.