Adsorbents particular

McCabe-Thiele diagrams for nonlinear and more practical systems with pertinent inequaUty constraints are illustrated in Figures 11 and 12. The convex isotherms are generally observed for 2eohtic adsorbents, particularly in hydrocarbon separation systems, whereas the concave isotherms are observed for ion-exchange resins used in sugar separations. 

Concerning consecutive reactions, a typical example is the hydrogenation of alkynes through alkenes to alkanes. Alkenes are more reactive alkynes, however, are much more strongly adsorbed, particularly on some group VIII noble metal catalysts. This situation is illustrated in Fig. 2 for a platinum catalyst, which was taken from the studies by Bond and Wells (45, 46) on hydrogenation of acetylene. The figure shows the decrease of... 

Many adsorbents, particularly the amorphous adsorbents, are characterized by their pore size distribution. The distribution of small pores is usually determined by analysis, using one of several available methods, of a cryogenic nitrogen adsorption isotherm, although other probe molecules are also used. The distribution of large pores is usually determined by mercury porisimetry [Gregg and Sing, gen. refs.]. 

Adsorption of cations or anions may be greatly favored or inhibited when they occur as complexes rather than as free (uncomplexed) ions. For example, the hydroxide complexes of uranyl ion (UOl ) are strongly adsorbed by oxide and hydroxide minerals, whereas urany) carbonate complexes are poorly adsorbed by these minerals (Hsi and Langmuir 1985). In fact, carbonate, sulfate, and fluoride complexes of metals are often poorly adsorbed in general, whereas OH and phosphate complexes are usually readily adsorbed, particularly by oxide and hydroxide solids. Metal adsorption is considered at some length in Chap. 10. 

There is a more or less generalized agreement that the isosteric adsorption heat is strongly affected by the microstructure of the adsorbent, particularly in the case of porous solids. This magnitude is better suited for structural analysis than other thermodynamic quantities. The use of the Clausius—Clapeyron equation to determine the isosteric adsorption heat has several limitations both theoretical and experimental, that are well known. 

As is known [24] the hysteresis loop may persist to the lowest pressure in absence of a hysteresis closing point. This phenomenon referred to, as low-pressure hysteresis (LPH) is more frequent for microporous adsorbents particularly for active carbon. LPH has been the subject of some purposeful experimental investigations and theoretical hypotheses [76—80]. 

The preceding discussion emphasizes the applicability of Eq. (6-6) for adsorption onto polar adsorbents, particularly adsorbent samples deactivated by varying amounts of water. However, the derivation of Eq. (6-6) does not preclude its application to nonpolar adsorbents. Claesson has in fact observed 14) the existence of a similar relationship for adsorption onto various charcoals. For the homologous carboxylic acids as samples and ethanol as solvent, the Langmuir isotherm [Eq. (3.6-)] was found to describe the adsorption of each acid. The derived values of the Langmuir coefficient k [or K° in Eq. (3-6)] could be expressed as a function of the number n of carbon atoms in the sample molecule ... 

Imperfect combustion of rich mixtures leads to soot formation. The basic substance is carbon, and on its surface various harmful substances are adsorbed, particularly carcinogenic hydrocarbons. Petrol engines produce as much as 0.4 g m diesel engines as much as 1.1 g m , i.e. 17 kg of soots per 1 t of fuel [26-28]. 

Polymeric adsorbents, particularly macroporous styrene—divinylbenzene (DVB) copolymers, are free from these drawbacks of activated carbons. The heat of adsorption onto the polymeric adsorbents is significandy lower and, accordingly, the regeneration of polymers proceeds under much milder conditions. As a rule, the copolymers have no functional groups (apart from those specially introduced) that are capable of catalyzing chemical transformation of an adsorbate. However, in contrast to activated carbons, the specific surface area of most polymeric adsorbents is not very high therefore, their sorption capacity is lower, often making the use of macro-porous polystyrene-type adsorbents unprofitable. This circumstance impelled scientists to develop new types of polymeric materials with an enhanced... 

Since only analytes attached to or close to metal surface are enhanced, the need for effective surface attachment is crucial if reproducible results are to be obtained. In many early experiments this was widely ignored. Commercial dyes were simply added to colloid or placed on surfaces and the results, where favourable, have been reported in many studies. The problem with this approach is that the surface chemistry of an element such as silver is complex. Little is understood about the surfaces on which the analyte is adsorbed, particularly in aqueous solution or, as is often done, with particles prepared in aqueous solutions and dried out on a surface for investigation. Further, where colloidal suspensions are used, it is common practice to aggregate the colloid in order to shift the frequency of the plasmon to a value that would place it in resonance with the laser. In fact, it is clear from ultraviolet-visible absorption that a range of clusters are made in most conditions and only a few are likely to be in resonance with the laser. However, this gives a bigger SERS enhancement than for single particles. 

The feasibility of using adsorbents, particularly activated carbon, as an alternative to steam stripping or chemicals injection techniques, for the removal of volatile organic compounds from an SBR latex is demonstrated using batch... 

The intended use of the product has to be taken into account during the scale up and design of large-scale processes. For example, it may be necessary to show that any components of the adsorbent are not present in the final product if this is to be for human use. The affinity adsorbent, and other adsorbents used in the process, has to be controlled strictly to ensure that the product is sufficiently pure. This may preclude use of a particular adsorbent, particularly if the ligand leaches, due to the activation chemistry used in its manufacture. Alternatively it may be necessary to demonstrate that the ligand leakage into the product is of an acceptable level. 

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