Adsorption systems cases

Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

Reaction kinetics at phase houndaiies. Rates of adsorption and desorption in porous adsorbents are generally controlled by mass transfer within the pore network rather than by the kinetics of sorption at the surface. Exceptions are the cases of chemisorption and affinity-adsorption systems used for biological separations, where the kinetics of bond formation can be exceedingly slow. 

Adsorption-Control Equipment If a gas stream must be treated for a short period, nsnally only one adsorption unit is necessary, provided, of course, that a sufficient time interval is available between adsorption cycles to permit regeneration. However, this is usually not the case. Since an nninternipted flow of treated gas is often required, it is necessary to employ one or more units capable of operating in this fashion. The units are designed to handle gas flows without interruption and are charac terized by their mode of contact, either staged or continuous. By far the most common type of adsorption system used to remove an objectionable pollutant from a gas stream consists of a number of fixed-bed units operating in such a sequence that the gas flow remains nninternipted. A two- or three-bed system is nsn ly... 

As for equilibrium values of as and P they are mainly dependent on relations between such parameters of the systems as initial electric conductivity of adsorbent, concentration of chemisorbed particles, reciprocal position of the energy levels of absorbate and adsorbent. Thus, during acceptor adsorption in case of small concentration of adsorption particles one can use (1.82) and (1.84) to arrive to expressions for equilibrium values of ohmic electric conductivity and the tangent of inclination angle of VAC ... 

None of these is found to be true experimentally for any adsorption system of interest. Therefore, we do not expect LH rate expressions to be quantitative for any catalytic reaction system. The real situation will always be much more complicated, and in most cases we do not yet know what rate expressions might describe catalytic reactions with more than empirical accuracy. 

The most common adsorption systems consist of silica gel or alumina adsorbents in association with an organic solvent system. The adsorbent can exert a considerable influence on the separation of compounds. Alumina and silica gel, for example, have significantly different properties and can result in quite different separations. Activation of the adsorbent also influences sample retention. The presence of water on the adsorbent decreases the adsorbent activity due to blockage of active sites. If large quantities of water are present, a partition system may be set up which may extensively change the retention times due to the different chromatographic principle involved. Table 2.1 compares results obtained for the separation of the insecticide carbaryl (Sevin) and its hydrolysis product 1-naphthol on alumina and silica gel. Comparisons between activation and deactivation are made. The results show that separation of the two components is reversed with the two adsorbents examined. In most cases, activation of the plates caused the/ f values to increase relative... 

The polarization of the organic molecule is more significant in the case of adsorption on the octahedral side (see Figs. 13.5 and 13.6). The adsorption energies obtained at the ONIOM(B3LYP/6-31G(d,p) PM3) level of theory and using large models of the mineral for the adsorption systems of GB and GD on the octahedral surface of dickite, are about 16 and 15 keal/mol, respectively. In the case of... 

However, activated carbons are the most extensively applied industrial adsorbents for the removal of pollutants from gaseous and aqueous and nonaqueous streams, because of their exceptionally powerful adsorption properties and their readily modifiable surface chemistry [217,218], Carbon is the primarily applied adsorbent in the case of liquid-solid adsorption systems. 

And, in the case of the adsorption system of a methylene group with a support, Eq. (74) can be transformed... 

Equation (2.72) is not easy to use in the general case in which the spreading pressure is unknown. But in the particular case of stepwise isotherms where there are two adsorbed phases in equilibrium with the gaseous adsorptive (i.e. in the case of a univariant adsorption system), Larher (1968, 1970) showed that the isosteric method may be used with the transition pressure p" to give integral molar energies un and entropies sn of the quasi-layer ... 

If, in an ideal case, the probability of desorption of an adsorbed molecule from the surface is independent of the surface coverage (i.e. there are no lateral interactions between the adsorbed molecules), then the value of E is constant for a particular adsorption system. Equation (4.10) is then applicable over the complete range of monolayer coverage. By rearrangement and simplification of Equation (4.10), we arrive at the familiar Langmuir isotherm equation,... 

In Chapter 2 we have introduced a number of thermodynamic surface excess quantities (Equations (2.11)—(2.14)) in the case of a simple gas adsorption system involving a single adsorptive. These quantities were expressed as a function of the surface excess amount, na. In the case of the process of immersion of a solid in a pure liquid, the same surface excess quantities can still be defined and it is useful to express them as a function of the surface area. Thus ... 

Figure 5.10b represents another adsorption system, in which the molar volumes of species 1 and 2 are different, so that adsorption also affects the total concentration c in the solution. Although these are representative of real systems, it must be stressed that in most cases these concentration profiles are unknown. 

Solid-solution adsorption systems have been found to give a variety of isotherm types, as classified by Giles et al. (1960). Many exhibit the characteristic Type I shape (see Figure 1.7), with the equilibrium concentration, c, replacingp[p°. In these cases, over a limited range of c, it is usually possible to apply an empirical equation of Langmuir form... 

We are drawn to the conclusion that log-log fractal plots are useful for the correlation of adsorption data - especially on well-defined porous or finely divided materials. A derived fractal dimension can also serve as a characteristic empirical parameter, provided that the system and operational conditions are clearly recorded. In some cases, the fractal self-similarity (or self-affine) interpretation appears to be straightforward, but this is not so with many adsorption systems which are probably too complex to be amenable to fractal analysis. 

According to the data of analysis of many adsorption systems, the first term in Equation 9 corresponding to the second order appears only v hen considering adsorption of relatively small molecules. They include molecules of linear shape, such as the diatomic gases, carbon dioxide, carbon monoxide, etc. Experimentally realizable orders, n, are integers from 3 to 6 in the general case. With larger polyatomic molecules, no adsorption space remains in the zeolite voids for final adsorption under the effect of dispersion forces. Then Equation 9 retains only the second term, and Uon is expressed by Equation 12. 

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