Intracrystalline fluid

Relaxation times Tt and T2 have been determined as a function of temperature and surface coverage in various zeolites, particularly of the faujasite type. The early experiments have been troubled by the very strong dependence of relaxation rates on the concentration of paramagnetic impurities. In order for the relaxation values to be meaningful, such impurities expressed as Fe content must be below ca. 6 ppm. Figure 38 shows the variation of Tt and T2 for water adsorbed in a particularly pure sample of zeolite Na-X (248). The authors (248) account for the experimental results using a model of the intracrystalline fluid, which is about 30 times as viscous as bulk water at room temperature. It shows a broad distribution of molecular mobilities (the ratio T,/T2 at the minimum in Tt is much larger... 

Here Ca is the concentration of isotopically labelled species at a point where the concentration of unlabelled species is Ca. La a and La a are the straight and cross phenomenological coefficients of the irreversible thermodynamic formulation of diffusion. The original relation, Equation 1, assumes a zero cross coefficient, which in dense intracrystalline fluids certainly is not likely to be true. 

The nuclear magnetic resonance relaxation times for the protons of water adsorbed to saturation in a high-purity specimen of zeolite 13-X have been measured between 200° and 500°K. The data can be accounted for by the model of an intracrystalline fluid which is about 30 times as viscous as bulk water at room temperature, shows a broad distribution of molecular mobilities, and is about as dense as liquid water. The median correlation time (time between molecular flights) is... 

If a NS monocrystal takes up a single component from a fluid phase and Intercrystalline transport does not influence the uptake rate, one should be aware of the possibility that, besides intracrystalline diffusion, the following processes may either contribute or even govern the uptake rate ... 

Rare two-phase fluid inclusions e found in anker-ite and in calcite (Fig. 6H). They occur as isolated inclusions or in groups of a few inclusions with a random distribution. Owing to the scarcity of inclusions and to the lack of visible intracrystalline growth features in the carbonate cements, relationships between fluid inclusions and carbonate growth could not be evaluated. As a result, the primary or secondary origin of the inclusions studied cannot be ascertained. Microthermometric measurements are summarized in Table 2 and Fig. 7. 

The intergranular carbonate cement is always iron-rich dolomite (or ankerite). Electron probe microanalysis gives between 7 and 18 mol% FeCOj (and 0.2-0.8 mol% MnCOj) (Fig. 11). The calcium content of the dolomites is relatively stoichiometric, with an average of 51 mol% CaCOj (range 50.0-53.3 mol%). The cement is generally dull to very dull red-brown in CL, with subtle concentric zonation. It is relatively homogeneous when examined in BSEM, suggesting that intracrystalline chemistry is reasonably consistent within and between individual fractures. The dolomite crystals have a variable fluid inclusion density, with a vast preponderance of monophase aqueous inclusions. Any two-phase inclusions identified in thin section... 

A few studies reported considerable intracrystalline isotopic variations within millimetre-sized saddle dolomite crystals. For example, a 1.4%o variation in 6 0 and a 0.7%o variation in 5 C were found in isotopically zoned saddle dolomite crystals from Cretaceous carbonates of south Texas (Woronick Land, 1985). In another study, a 5.2%o decrease in 6 0 (-6 to -11.2%o) and a systematic increase in fluid inclusion homogenization temperature (127-146°C) were reported for a single saddle dolomite crystal in Devonian rocks of Canada (Kaufman et al., 1990). Further, Spangen-berg et al. (1995) reported preliminary stable isotope results for intracrystalline heterogeneities in white sparry dolomite (largely equivalent to saddle dolomite) in samples from a Peruvian MVT... 

Mass transfer from the void space between the individual pellets of a bed of granulated zeolites into the crystallites proceeds in a sequence of three transport processes (1) mass transfer through the intercrystalline space, (2) penetration through the crystallite surface, and (3) intracrystalline diffusion 11-5]. Depending on the given system, each of these processes may be rate determining for the overall adsorption/desorption process. An analysis of the relative importance of each of these processes may therefore be of substantial relevance for ensuring optimum transport conditions for the application of zeolites in both fluid separation and catalysis. 

The model of Santacesaria et is an extension of the linear driving force model, with fluid side resistance, for a nonlinear multicomponent Langmuir system. It includes axial dispersion, and the combined effects of pore diffusion and external fluid film resistance are accounted for throu an overall rate coefficient. Intracrystalline diffusional resistance is neglected and equilibrium between the fluid in the macfopores and in the zeolite crystals is... 

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