Transient regime of gas diffusion-physisorption through a microporous barrier

A chemically passive porous barrier of finite thickness is exposed to the air polluted with a target gas (TG) of low concentration, at t=0. The TG diffuses through the barrier while being physically adsorbed onto the effective surface of the porous solid. At the early stages of the process, the calculation of the net flow rate of the TG through the barrier is of considerable complexity, as it is strongly influenced by the TG diffusion, adsorption, and desorption processes. In this paper, the problem is considered quantitatively. A nonlinear partial differential equation has been derived for the process, which relates the TG concentration at any point and time to the concentration and molecular parameters of the polluting TG. The equation is numerically solved. The results predicted a delay in the molecular diffusion of the TG through the barrier due to the interactions between the TG and the effective surface of the porous solid. The delay depends on the nature of the TG and is analytically predictable. The results were experimentally verified by the measurement of the TG concentration build up at the opposite side of a highly porous 1.5-mm-thick slab of aluminum silicate fiberboard. It is shown that such measurements are of potential importance in the design of diagnostic gas sensors.