Supercritical slowing down

Abstract Alkylation of benzene with ethylene over Y-type zeolite has been carried out under supercritical conditions. Two aspects of the reaction have been paid attention to slowing down the deactivation rate and decreasing the by product selectivity. Experiments have revealed the existence of some coke precursors that are partly removed from the catalyst surface. By product xylenes are decreased and are explained due to high diffiisivity in the supercritical fluid. 

Iron, titanium, and zirconium salts can be substituted for the chromium ones.146 Vegetable tannins, such as 4.20 can also be used, but they slow down the tanning process. Similar phenols can be found in the residue from tea leaves left after the manufacture of instant tea. Perhaps, they could be used in making leather to eliminate or reduce the amount of another waste product. Getting reagents to penetrate the hide is a problem. Newer methods, such as ul-trasonication and supercritical fluid extraction, may help reduce the time required to make leather, so that these alternative-tanning agents can be used instead of the chromium. 

A remarkable increase in catalyst efficiency in SCCO2 as compared to conventional solvents was observed for the Ir-catalyzed enantioselective hydrogenation of imines shown in Scheme 12.12 [34]. Imine hydrogenation is a key step in the commercial synthesis of (S)-metolachlor, a commercial herbicide produced by Novartis in Switzerland. The reaction is approximately zero-order with respect to substrate in CO2 whereas it slows down dramatically at higher conversion in the organic solvent (Fig. 12.8). Thus, the time required for quantitative conversion is reduced by a factor of 20 when changing from the conventional to the supercritical solvent ... 

On the other hand, reactions of high molecular mass free radicals, e.g. occurring during pyrolysis, are slowed down by a so-called cage effect caused by solvent molecules at high pressure [5]. In some cases these effects may be reinforced by the special properties of water. A similar effect may be the reason why organometaUic complexes are able to exist and even act as catalysts at rather high temperatures in supercritical water. 

In thermally non-homogeneous supercritical fluids, very intense convective motion can occur [Ij. Moreovei thermal transport measurements report a very fast heat transport although the heat diffusivity is extremely small. In 1985, experiments were performed in a sounding rocket in which the bulk temperature followed the wall temperature with a very short time delay [11]. This implies that instead of a critical slowing down of heat transport, an adiabatic critical speeding up was observed, although this was not interpreted as such at that time. In 1990 the thermo-compressive nature of this phenomenon was explained in a pure thermodynamic approach in which the phenomenon has been called adiabatic effect [12]. Based on a semi-hydrodynamic method [13] and numerically solved Navier-Stokes equations for a Van der Waals fluid [14], the speeding effect is called the piston effecf. The piston effect can be observed in the very close vicinity of the critical point and has some remarkable properties [1, 15] ... 

The pressure level at which the various nonidealities discussed above become important is dependent primarily on the condensability of the penetrant. The most convenient measure of condensability is the penetrant vapor pressure. Even for slightly supercritical components, a hypothetical vapor pressure can be obtained by extrapolation of a semilog plot of vapor pressure versus the inverse absolute temperature up to the system temperature. Water has a low vapor pressure at ambient temperatures and is quite condensable. Only a few mm Hg of water vapor may cause the permeability depression effect shown in the left-hand side of Fig. 20.4-4 for typical less condensable penetrants such as CH4 and Hj. Conversely, high partial pressures of supercritical components such as N2, CH , CO, and Hj would be required to cause noticeable depression of each other s permeabilities. Nevertheless, as noted earlier, measurable examples of this effect have been observed even in the Hj-CO and H2-CH4 cases where the faster permeating H2 appeared to have been "slowed down by the slower CO and CH4. 

The coefficient of sell-diffusion does not appear to have an anomaly near the critical point. For the engineer, however, the mutual dift usion coefficient is the more important property. The binary dilfusion coefficient approaches zero at the mixture critical point ("critical slowing-down"). In dilute mixtures, however, the decrease of the binary dilfusion coefficient is not seen until the critical line is approached very closely. For many practical purposes, such as supercritical extraction and chromatography, the mixture is dilute, and it can be assumed that the coefficient of binary diffusion is intermediate between that in the vapor and that in the liquid. Since the diffusion coefficient decreases roughly inversely proportional to the density, dilfusion in supercritical solvents is much more rapid than in liquid solvents, thus increasing the speed of diffusion-controlled chemical processes. 

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