Adsorption scaling

Figure 6.14. General adsorption scale for separations by silica gel thin-layer chromatography.

A comparison of the adsorption energies of the complexes formed by ammonia, pyridine and isopropylamine with a simple potential-energy model, which assumes that the heat of adsorption scales linearly with gas-phase proton affinities, suggests that proton transfer dominates the interaction between the adsorbate and the acid site [56],... 

Fig. 9 Adsorption scaling diagram shown on a log-log plot for (a) strongly charged surfaces, E = > 1, and for (b) weakly charged surfaces, E < 1. We find a...

It is clear that by permutation many more models can be constructed. In fact the situation is still worse. Fig. 7 presents a possible model. The parameters to be filled in have not been invented by the author, but most of them are under discussion in literature euid different proposals, backed by experimental evidence, are given for them. Even this model is of course not complete radial distribution of parameters, adsorption, scaling up factors for parameters, etc, have not been specified. 

For a further interpretation of the adsorption curves of Fig 31 and for a comparison widi the results on the mercury surface it is necessary to derive the differential capacity of the double layer by differentiation of the adsorption (charge) with respect to the double layer potentiaL The capacities obtained arc given in Fig 33a and the capacities of the double layer on mercury against NaF solutions in Fig 33b The two sets of curves resemble each other very much especially for dilute solutions. As in these solutions the diffuse parts of the double layer are preponderant and there is every reason to expect that the diffuse double layers will be identical for the two cases the adsorption scale in Fig 31 and therewith the capacity scale in Fig 33 have so been adjusted that the capacities for 0 001 N solutions at the ero point of charge are identical ... 

Adsorption is a different separation process from absorption, distillation, and extraction discussed in earlier chapters. Adsorption takes place at unsteady state with nonlinear isotherms. Because of these complexities, adsorption depends more on experiments than other separations. For the common case of a favorable isotherm, these experiments are used to determine the length of unused bed, a measure of the separation s efficiency. This length is independent of the total bed length and so serves as the basis of adsorption scale-up. 

On the atomic level, etching is composed of several steps diflfiision of the etch molecules to the surface, adsorption to the surface, subsequent reaction with the surface and, finally, removal of the reaction products. The third step, that of reaction between the etchant and the surface, is of considerable interest to the understanding of surface reactions on an atomic scale. In recent years, STM has given considerable insight into the nature of etching reactions at surfaces. The following discussion will focus on the etching of silicon surfaces [28]. 

The SHG and SFG teclmiques are also suitable for studying dynamical processes occurring on slower time scales. Indeed, many valuable studies of adsorption, desorption, difhision and other surface processes have been perfomied on time scales of milliseconds to seconds. 

Fig. 4.11 (o) Adsorption isotherm for (i) a powder made up of nonporous particles (ii) a solid which is wholly microporous (iii) a powder with the same external surface as in (i) but made up of microporous particles having a total micropore volume given by the plateau of isotherm (ii). The adsorption is expressed in arbitrary units, (b) t-Plots corresponding to isotherms (i) and (iii). The o,-plots are similar, except for the scale of... 

Fig. 5.12 (a) Water adsorption isotherms at 20°C on Graphon activated to 24-9 % burn-off, where its active surface was covered to varying extents by oxygen complex. (b) The results of (a) plotted as amount adsorbed per of active surface area (left-hand scale) and also as number of molecules of water per atom of chemisorbed oxygen (right-hand scale). (After Walker.)... 

Column Si. Size-exclusion chromatography columns are generally the largest column on a process scale. Separation is based strictly on diffusion rates of the molecules inside the gel particles. No proteins or other solutes are adsorbed or otherwise retained owing to adsorption, thus, significant dilution of the sample of volume can occur, particularly for small sample volumes. The volumetric capacity of this type of chromatography is determined by the concentration of the proteins for a given volume of the feed placed on the column. 

Gas-phase adsorption is widely employed for the large-scale purification or bulk separation of air, natural gas, chemicals, and petrochemicals (Table 1). In these uses it is often a preferred alternative to the older unit operations of distillation and absorption. 

Nuclear Waste Management. Separation of radioactive wastes provides a number of relatively small scale but vitally important uses of gas-phase purification appHcations of adsorption. Such appHcations often require extremely high degrees of purification because of the high toxicity of... 

In contrast to trace impurity removal, the use of adsorption for bulk separation in the liquid phase on a commercial scale is a relatively recent development. The first commercial operation occurred in 1964 with the advent of the UOP Molex process for recovery of high purity / -paraffins (6—8). Since that time, bulk adsorptive separation of liquids has been used to solve a broad range of problems, including individual isomer separations and class separations. The commercial availability of synthetic molecular sieves and ion-exchange resins and the development of novel process concepts have been the two significant factors in the success of these processes. This article is devoted mainly to the theory and operation of these Hquid-phase bulk adsorptive separation processes. 

Industrial-scale adsorption processes can be classified as batch or continuous (53,54). In a batch process, the adsorbent bed is saturated and regenerated in a cychc operation. In a continuous process, a countercurrent staged contact between the adsorbent and the feed and desorbent is estabhshed by either a tme or a simulated recirculation of the adsorbent. 

Liquid adsorption processes hold a prominent position ia several appHcations for the production of high purity chemicals on a commodity scale. Many of these processes were attractive when they were first iatroduced to the iadustry and continue to iacrease ia value as improvements ia adsorbents, desorbents, and process designs are made. The UOP Parex process alone has seen three generations of adsorbent and four generations of desorbent. Similarly, Hquid adsorption processes can be applied to a much more diverse range of problems than those presented ia Table 3. 

A surprisiagly large number of important iadustrial-scale separations can be accompHshed with the relatively small number of zeoHtes that are commercially available. The discovery, characterization, and commercial availabiHty of new zeoHtes and molecular sieves are likely to multiply the number of potential solutions to separation problems. A wider variety of pore diameters, pore geometries, and hydrophobicity ia new zeoHtes and molecular sieves as weU as more precise control of composition and crystallinity ia existing zeoHtes will help to broaden the appHcations for adsorptive separations and likely lead to improvements ia separations that are currently ia commercial practice. 

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