ZLC measurements

NMR PFG measurements determine the tracer or self-diffusivity (D ) under equilibrium conditions with no concentration gradient. n any sorption rate measurement it is the transport diffusivity under the influence of a concentration gradient which is measured. In general these two quantities are not the same but the relationship between them can be established from irreversible thermodynamics. (17,18) In the low concentration limit the thermodynamic correction factor vanishes and the transport and self diffusivities should approach the same limit. Since ZLC measurements are made at low concentrations within the Henry s Law region the diffusivity values should be directly comparable with the NMR self-dif fusivities. ... 

Transient uptake rate measurements are subject to intrusion of heat transfer hmitations, especially in batch measurements at low pressures. Membrane permeation, frequency response, and ZLC measurements should not be subject to serious heat transfer limitations but, especially in frequency response and ZLC, there is always a danger of intrusion of extracrystalline resistances to mass transfer, although in principle these can be eliminated by reducing the sample size and ensuring that the crystals within the sample are dis-... 

It is clear that comparative ZLC measurements can provide useful information concerning the transport properties of even such structurally complex materials as the mesoporous sihcas. 

First tracer ZLC measurements [113], however, are in contrast to this finding. In these studies, tracer exchange is found to be rather fast, so that there is no indication of single-file behavior. Moreover, it appears that the (transport) diffusivities resulting from ZLC measurements are in complete agreement with the concentration dependence of the (self-) diffusivities resulting from... 

Fig. 1. Schematic diagram of system for vapor phase ZLC or tracer ZLC measurements. 

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... 

This approach is illustrated in fig. 2 which shows a family of ZLC desorption curves for CO2 from a sample of a carbon monolith, measured over a wide range of flow rates. The isotherm is linear so at low flow rates (equilibrium control) the response is given by ... 

The ZLC method offers advantages of speed and simplicity and requires only a very small adsorbent sample thus making it useful for characterization of new materials. The basic experiment using an inert carrier (usually He) measures the limiting transport difiiisivity (Do) at low concentration. A variant of the technique using isotopically labeled tracers (TZLC) yields the tracer diffiisivity and counter diffusion in a binary system may also be studied by this method. To obtain reliable results a number of preliminary experiments are needed, e.g. varying sample quality, nature of the purge gas, the flow rate and, if possible, particle size to confirm intracrystalline diffusion control. 

The difiiisional behavior of p-xylene is complicated. The FR measurements reveal two different diffrisivities corresponding to movement through the straight and sinusoidal channels. The ZLC method increases only the average diffusivity which is similar to the value for benzene but it is possible that the difference between the self and transport diffrjsivity results from the two channel behavior revealed by the FR data [33]. 

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 ZLC method recently developed by Eic and Ruthven (5) is an important new technique for measuring the intracrystalline diffusion in Zeolites since only a very small sample of... 

A new experimental technique (ZLC) has been developed and applied to study the diffusion of a range of hydrocarbons (xylene, benzene, cyclohexane and linear paraffins) in unaggregated crystals of zeolites A and X. The validity of the method was confirmed by varying the crystal size and the nature and flowrate of the purge gas. The method has advantages of speed and simplicity but the major advantage is that the intrusion of extraneous heat and mass transfer resistances is much less significant than in conventional uptake rate measurements. As a result, the new method can be applied to systems in which diffusion is too rapid to follow in a conventional sorption experiment. 

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. 

ZLC desorption curves were measured over a wide range of conditions for several different hydrocarbon sorbates in a range of different sizes of NaX and 5A zeolite crystals. In general He was used as the purge gas but numerous checks were made with an Ar purge to confirm the absence of any extracrystalline resistance. The form of the desorption curves was consistent with the theoretical model outlined above and consistent diffusivity values were obtained at different flowrates and with different crystal sizes. A few representative curves are shown in figure 1. 

Arrhenius plots showing the temperature dependence of diffusivity for n-butane in various 5A samples are shown in figure 4 while figure 5 shows the trend of activation energy with carbon number. For linear paraffins higher than n-butane, diffusion, even in the large 5A crystals, is too slow to measure by the NMR PFG method so it is only for butane that a direct comparison between ZLC and PFG NMR data is possible. 

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. 

Measurement of binary isotherms by traditional methods is tedious and time-consuming. For systems in which the two components are adsorbed with comparable strength the ZLC approach has been shown to provide a relatively rapid and straightforward measurement of the separation factor, but determination of the complete isotherm by this method is still somewhat labor intensive. 

Fig. 16 Comparison of diffusivities for n-alkanes in silicalite measured by different experimental methods o, , MD simulations +, QENS V, single crystal membrane A, PEG NMR A, ZLC. Data are from various sources. From Jobic [96] with permission...

Although the ZLC technique was originally developed for the measurement of intracrystalline diffusion, it may also be appHed to measure macropore diffusion in composite adsorbent particles [56]. 

In tracer ZLC (TZLC) [28,51,58] the experiment is similar to the standard method, but the monitored species is the deuterated form of the sorbate. This introduces an additional cost for the material and the requirement for an online mass spectrometer. The advantages are the eUmination of all possible heat effects, strict Unearity of the equiUbrium between the fluid phase and the adsorbed phase, and the possibility of measuring directly the tracer diffusivities (which shoifld be the same as the microscopically measured self-diffusivity) over a wide range of loading. To reduce the costs the carrier is prepared with a mixture of pure and deuterated hydrocarbons. It has been shown that small imbalances in the concentration of the carrier and the purge streams do not affect the desorption dynamics [58]. 

The consistency between sorption rate measurements and PFG NMR measurements in large crystals of 5A zeolite was noted many years ago. In more recent studies a similar pattern of consistency between sorption rate, ZLC, and PFG NMR data has been observed for Xe, CO2, and C3H8 in 5A zeoHte (Fig. 13). 

This system has been studied by ZLC, tracer ZLC, and infrared temperature-rise measurements as well as by PFG NMR, with striking consistency between... 

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