Problems raised by the application of the usual methods of process identification to the determination of diffusion coefficient of gases in molecular sieves

A method is presented for measuring diffusion coefficient in molecular sieves where gas adsorption in the sieve does not appear because the experimental temperature pressure conditions are far from equilibrium liquid-vapour conditions. The method determines the relation which connects the pressure variation of gas in a system formed by a cell, including the sieve, to the volume variation applied to the cell. The value of the diffusion coefficient for the molecular sieve can be obtained by the analysis of the pressure in terms of time. The method of obtaining the diffusion coefficient is related to a problem of process identification, and thereby the authors have naturally been led to make use of the usual inputs of the technique: i.e., unit step response, harmonic and random analysis. The authors consider the study of theoretical problems raised in the measurement of diffusion coefficient by the use of these three experimental techniques; the description of the apparatus, the experimental results and their synthesis.