Permeation with a double mode of sorption.
It may be the case of gas molecules such nitrogen or oxygen in olefinic polymers. Strong interaction sites may be induced by some impurities. The kinetic permeation curve likes a fickian kinetics. The concentration of trapped molecules on Langmu´r sites is low, the stationary total concentration profile is linear and decreases down to the downstream face. The time-lag gives a good estimation of the diffusion coefficient using Fick law.
Example 1, case of low concentration of Langmu´r sites and weak interactions :