Adsorption extension

Equation 2 has been used with some success by Bangham (I), McIntosh (5), Yates (20), and others to correlate adsorption extension data. Equation 3 has also been used with considerable success by McIntosh (15) and Dacey (6), and in some of my own work (8, 15). 

Adsorption-extension and permeability measurements provide fairly searching tests of the validity of the assumption that in physical adsorption reactions the adsorbate and adsorbent behave essentially as bodies of matter in mechanical equilibrium. 

Figure 1. Adsorption extension data for water vapor on carbon...

These equations in fact describe a good deal of adsorption-extension data remarkably well, and also describe some permeability data (Figures 1 to 4). 

The value of this approach is to differentiate between the structure sensitive adsorption, of Stage 1, which involves the initial high heats of adsorption extensive adsorption, at pressures above 1(T mm. with an undetermined upper limit and a low heat of adsorption, which is responsible for the parahydrogen conversion and the slow sorption process with an appreciable heat of sorption which causes poisoning of the parahydrogen conversion. The work can give no indication of the physical processes involved in the three types of adsorption. 

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

As stated in the introduction to the previous chapter, adsorption is described phenomenologically in terms of an empirical adsorption function n = f(P, T) where n is the amount adsorbed. As a matter of experimental convenience, one usually determines the adsorption isotherm n = fr(P), in a detailed study, this is done for several temperatures. Figure XVII-1 displays some of the extensive data of Drain and Morrison [1]. It is fairly common in physical adsorption systems for the low-pressure data to suggest that a limiting adsorption is being reached, as in Fig. XVII-la, but for continued further adsorption to occur at pressures approaching the saturation or condensation pressure (which would be close to 1 atm for N2 at 75 K), as in Fig. XVII-Ih. 

This is useful since c can be estimated by means of the BET equation (see Section XVII-5). A number of more or less elaborate variants of the preceding treatment of lateral interaction have been proposed. Thus, Kiselev and co-workers, in their very extensive studies of physical adsorption, have proposed an equation of the form... 

The oxidation of CO to CO2, whieh is essential to eontrolling automobile emissions, has been extensively studied beeause of the relative simplieity of this reaetion. CO oxidation was the first reaetion to be studied using the surfaee seienee approaeh and is perhaps the most well understood heterogeneous eatalytie reaetion [58]. The simplieity of CO oxidation by O2 endears itself to surfaee seienee studies. Both reaetants are diatomie moleeules whose adsorption... 

M. S. Tswett, the Polish botanist, in 1906 used adsorption columns in his investigations of plant pigments. It was not untU about 1930 that the method was used extensively by chemists. The most startling results have been obtained in the fields of plant pigments and natural products, but... 

For practical reasons, the application of the adsorption method to the study of surface area and porosity has to be limited to bodies which are either very finely divided or possess an extensive pore system. It is helpful to consider the case of finely divided bodies first. 

Fig. 4.29 Adsorption isotherms of water vapour on caldte, after being balt-milted for different periods (A, B, C) and on precipitated calcium carbonate (D). Period of milling (A) 1000h (B) ISOh (C) 22h outgassing temperature 2S°C. Isotherms A, B and C (but not D) all showed extensive low-pressure hysteresis, but for clarity the desorption branch is omitted. The amount adsorbed is referred to 1 m of BET-nitrogen area. ...

The incorporation of the new material without any increase in the overall length of the book has been achieved in part by extensive re-writing, with the compression of earlier material, and in part by restricting the scope to the physical adsorption of gases (apart from a section on mercury porosimetry). The topics of chemisorption and adsorption from solution, both of which were dealt with in some detail in the first edition, have been omitted chemisorption processes are obviously dependent on the chemical nature of the surface and therefore cannot be relied upon for the determination of the total surface area and methods based on adsorption from solution have not been developed, as was once hoped, into routine procedures for surface area determination. Likewise omitted, on grounds of... 

Adsorption Dynamics. An outline of approaches that have been taken to model mass-transfer rates in adsorbents has been given (see Adsorption). Detailed reviews of the extensive Hterature on the interrelated topics of modeling of mass-transfer rate processes in fixed-bed adsorbers, bed concentration profiles, and breakthrough curves include references 16 and 26. The related simple design concepts of WES, WUB, and LUB for constant-pattern adsorption are discussed later. 

The discovery (92) that the graphite coating of molecular sieves can dramatically improve their attrition resistance without significantly impairing adsorption performance should allow the extension of moving-bed technology to bulk gas separations (93). 

Surface Area. Surface area is measured by determining the quantity of nitrogen gas that adsorbs on the particle/crystal surfaces of a dry sample. Determination of surface area by measuring adsorption at gas—soHd interfaces is covered extensively in the Hterature (84). Instmments such as the FlowSorb 2300 are used to control the adsorption/desorption within specific conditions of temperature and pressure. 

The technological appHcations of molecular sieves are as varied as their chemical makeup. Heterogeneous catalysis and adsorption processes make extensive use of molecular sieves. The utility of the latter materials Hes in their microstmctures, which allow access to large internal surfaces, and cavities that enhance catalytic activity and adsorptive capacity. 

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