Page 93 - Adsorption Technology & Design, Elsevier (1998)

90 Rates of adsorption of gases and vapours by porous media

4.3.2 Gravimetric measurements
As gas or vapour is adsorbed by an adsorbent so its weight gradually
increases until the adsorbent is saturated. Thus if an adsorbate is admitted to
an adsorbent and the increase in weight of the adsorbent is measured as a
function of elapsed time, the uptake curve can be used to measure the
diffusion coefficient by matching the curve obtained with the theoretical
uptake curve described by equation (4.22). Provided the experimental
conditions are such that isothermal conditions are maintained and the total
quantity of gas adsorbed up to the time when the adsorbent is saturated in
comparison with the amount of adsorbate remaining in the gas phase is small
(essentially constant adsorbate concentration), then equation (4.22) may be
used to estimate the effective diffusion coefficient for the adsorbate
-adsorbent pair. The gravimetric balance described by Gunn et al. (1974) for
the adsorption of water vapour by porous polyurethane materials is a
suitable description of the construction and operation of the apparatus and
also the method of curve fitting used to extract the diffusion coefficient from
the experimental uptake curve. The form of the curve in the example cited
differs from equation (4.22) because of the hollow cylindrical shape of the
sample used which results in radial as well as longitudinal diffusion
coefficients being a property of the system. Commercially available
gravimetric balances are also available which are suitable for experiments of
this type. When interpreting results from gravimetric measurements involv-
ing crystalline adsorbents, the portion of the uptake curve which is most
suitable for matching with equation (4.22) is when 0.2 < mt/m~ < 0.5.
This is because the initial uptake is sensitive to interparticle transport resis-
tance while portions of the uptake curve at values of mt/m~ < 0.5 may be
affected by heat transfer resistances.

4.3.3 Nuclear magnetic resonance measurements
Application of nuclear magnetic resonance (NMR) to the study of diffusion
of liquids in adsorbents has evolved through the use of the pulsed field
gradient (PFG) method originally developed by Stejskal and Tanner (1965
and 1968) who measured self-diffusion coefficients in liquids. In this
technique a sample of the material to be investigated is placed in a pulsed
magnetic gradient field. Nuclear spins of the sample are then excited by
means of a radio frequency pulse. Reversing the magnetic gradient field
pulse following a known interval of time produces an attenuation of the
signal which is a direct measurement of the mean square distance travelled
by the diffusing species in the time interval between the gradient pulses. The
diffusion of n-hexane in zeolite crystals has been successfully studied using