Molecular Transport in Granulated Zeolite NaCaA

NMR study of mass transfer in granulated molecular sieves

“…It has been shown (Richter et al, 1986a) that in this case, by N M R tracer desorption technique a time constant &,, is obtained which in the limit of small values coincides with the intracrystalline lifetime 7inl,o (under the chosen experimental conditions practically for c. 5 ms), and which increases faster than only linearly with increasing values of T~~~,~. Hence, apart from a simplification of the experimental procedure, differences in the kinetics of intercrystalline molecular exchange in different samples may more easily be determined.…”

“…After 10 cycles, the retention volume V,, the N M R tracer desorption time constant T;,,,,~, and the intracrystalline self-diffusion coefficient Dinfro were measured. Table 2 shows the results obtained with hydrocarbons of different boiling ranges (Richter et al, 1986b). As in the case of hydrothermal deterioration, from the large difference in the values of and 7%,, one has to conclude that molecular exchange is controlled by surface barriers rather than by the intracrystalline self-diffusion.…”

“…In order to investigate the mechanisms of molecular sieve deterioration under hydrothermal conditions, molecular sieves 5A Z were introduced into a column (45 mm dia., filling height 200 mm) and subjected to a nitrogen stream of extreme humidity (p,,, I I 90 kPa) at different temperatures (373-873 K) and at atmospheric pressure (Richter et al, 1986b). Figure 2 gives a comparison between the macroscopically observed transport characteristics of these sieves with the parameters of molecular transport as determined by NMR.…”