Zero-length column method

Fig. 18. Self-diffusion coefficients of benzene in NaX at 458 K PFG NMR, O (97) and (92) (JENS, A (13) deduced from NMR lineshape analysis, (10). Comparison with nonequilibrium measurements T, sorption uptake with piezometric control (93) , zero-length column method (96) o, frequency-response and single-step frequency-response technique (98). The region of the results of gravimetric measurements with different specimens (92) is indicated by the hatched areas. Asterisked symbols represent data obtained by extrapolation from lower temperatures with an activation energy confirmed by NMR measurements.

Diffusion Measurements- Zero Length Column (ZLC) Method... 

The Zero-length Column Chromatography (ZCC) method was used (5). Between 10-50 mg of catalyst, 150 cm /min He flow and a concentration of C2H4of 500 ppm were employed. Ethylene was adsorbed at different temperatures till the saturation of the sample was achieved. The hydrocarbon flow was then stopped and its concentration was measured during... 

More recently, a detailed study of diffusion of the xylene isomers in large crystals of NaX and natural faujasite was undertaken by both sorption rate and tracer exchange.(11-14) The data obtained by both these techniques using several different crystal sizes were entirely consistent but the diffusivities were much smaller than the values derived for the same systems by NMR PFG measurements. In an attempt to resolve this discrepancy we have developed a new chromatographic technique (zero length column or ZLC) which is less sensitive than conventional sorption methods to the intrusion of external heat and mass transfer resistances and which is therefore useful for following relatively rapid diffusion processes. The method has now been applied to study the diffusion of a range of different hydrocarbons in both A and X zeolite crystals and the results of these studies are summarized here. 

Deposition of silane on a zeolite s external surface is a well-established method of increasing its shape selective properties by increasing diffusion resistances. In this work, the intracrystalline diffusivities of both parent and silanised ZSM-5 samples are measured by the zero length column technique. It is found that the apparent intracrystalline diffusivity does decrease in the modified samples. This change is either the result of a surface barrier caused by pore mouth narrowing or an increase in intracrystalline tortuosity as a result of pore blockage. It was attempted to clarify the dominant mechanism by considering various mathematical models. 

A variant of the zero-length column (ZLC) method has also been developed to permit rapid measurement of both Henry constants and complete isotherms [4]. This method works well provided the curvature of the isotherm is moderate but it breaks down for highly favorable (rectangular) isotherms. 

The zero length column (ZLC) method is based on a very short fixed bed which is treated as an ideal stirred vessel with an infinite dispersion coefficient. With the gas volume Vq in the adsoiber, the carrier gas flow rate Vc, the mass of the adsorbent, the overall concentration C, and the molar concentration y, the loading results from... 

Macroscopic, such as the analysis of uptake curves, Wicke-Callanbach methods based on steady-state or transient diffusion cell, time lag method, chromatographic methods, zero length column (ZLC) method, and FR method... 

The zero length column (ZLC) technique has become a common tool to measure mass transfer kinetics in microporous adsorbents. The partial loading experiment is a variant of the traditional ZLC method in which the adsorbent is not allowed to reach full equilibration with the gas phase. Even though this variant of the ZLC experiment was introduced over 10 years ago, it has been applied only by few researchers. In this contribution we review the basic theory of the partial loading experiment and show that it can be used to establish the contributions of different mass transfer mechanisms. A detailed numerical model that includes the effects of nonlinearity of the isotherm and combined diffusion and surface barrier effects is presented to allow the correlation of complex sorbate-sorbent systems. 

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