Limitations of the Zero-Length Column Technique to Measure Diffusional Time Constants in Microporous Adsorbents

Abstract

The zero-length column (ZLC) technique has been developed to measure the intracrystalline diffusivity of strongly adsorbed species in large zeolite crystals above 50 mm in the Henry's law range of sorption equilibrium. The ZLC is a macroscopic technique, and there is a need of large crystallites or pellets to measure the intracrystalline diffusivity D-c of fast diffusion species or the macropore diffusivity D-P of weakly adsorbed species, respectively. Another limitation is that ZLC desorption curves produce similar concentration profiles (linear isotherms) in bidisperse adsorbents (pellets) under macropore or micropore diffusion control. Moreover, the forms of the response curves are very similar in both diffusion- and nonlinear equilibrium-controlled processes, leading to some misinterpretations of ZLC experiments. In this work, two criteria are developed showing that, in order to macroscopically measure the micropore diffusion time constant r(c)(2)/D-c or the macropore diffusion time constant R-p(2)(1 + K)/D-P, the time of the ZLC experiments t should be higher than 7.0 x 10(-2)r(c)(2)/D-c or 7.0 x 10(-2)R(p)(2) (1 + K)/D-P, respectively. The interpretation of the ZLC response curve data is also checked in two completely different regimes, showing that a single ZLC response curve is not enough to conclude if a system is under a kinetic or an equilibrium regime.