Zero length column

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. 

Diffusion Measurements- Zero Length Column (ZLC) 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.

It is of interest that similar ideas have been applied in the conception of a flow system for measuring the diffusion coefficients for gases in porous or microporous solids. Ruthven and Eic (20,21) use a zero-length column (ZLC) to suppress concentration gradients along the bed in the gas phase. As in the differential reactor described previously, a high gas flow rate is used so that the fixed bed acts as if it were very short. A preadsorbed adsorbate is removed by an inert gas stream. The diffusion inside the solid is very close to the classical solution for zero concentration on the surface, but the small concentration actually present in the gas leaving the bed (column) can be measured accurately. 

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. 

In the case of n-hexane adsorption on a powder sample, the concentration profiles (Figure 6) become perfectly rectangular after only t = 0.03 h. The value obtained for 0, (wlO m s ) is of the same order of magnitude as that reported Time(h) in the literature using other techniques (zero length column, fioquency response, etc.) [13]. 

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. 

Hufton JR, Brandani S, Ruthven DM (1994) Measm-ement of intracrystalline diffusion by zero length column tracer exchange. In J Weitkamp, Karge HG, Pfeifer H, Holderich W (eds) Zeolites and related microporous materials state of the art 1994. Proc 10th Int Zeolite Conf, Garmisch-Partenkirchen, Germany, 17-22 July 1994. Elsevier, Amsterdam, p 1323 Stud Surf Sci Catal 84 1323... 

Zero length column Coordinate along the reactor axis... 

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... 

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