Adsorber Types

The adsorber type is essentially governed by the chosen operating conditions. Discontinuous adsorption operations distinguish themselves by  

Discontinuous adsorption is mainly carried out in a vessel absorber with an adsorbent fixed bed, an adsorbent of honeycomb structure, or an adsorbent ring layer, and sometimes in a one stage fluidized bed 

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This description is traditional, and some further comment is in order. The flat region of the type I isotherm has never been observed up to pressures approaching this type typically is observed in chemisorption, at pressures far below P. Types II and III approach the line asymptotically experimentally, such behavior is observed for adsorption on powdered samples, and the approach toward infinite film thickness is actually due to interparticle condensation [36] (see Section X-6B), although such behavior is expected even for adsorption on a flat surface if bulk liquid adsorbate wets the adsorbent. Types FV and V specifically refer to porous solids. There is a need to recognize at least the two additional isotherm types shown in Fig. XVII-8. These are two simple types possible for adsorption on a flat surface for the case where bulk liquid adsorbate rests on the adsorbent with a finite contact angle [37, 38]. 

AH practical adsorbents have surfaces that are heterogeneous, both energetically and geometrically (not all pores are of uniform and constant dimensions). The degree of heterogeneity differs substantially from one adsorbent type to another. These heterogeneities are responsible for many nonlinearities, both in single component isotherms and in multicomponent adsorption selectivities. 

Principal Adsorbent Types. Commercially useful adsorbents can be classified by the nature of their stmcture (amorphous or crystalline), by the sizes of their pores (micropores, mesopores, and macropores), by the nature of their surfaces (polar, nonpolar, or intermediate), or by their chemical composition. AH of these characteristics are important in the selection of the best adsorbent for any particular appHcation. 

Advances in fundamental knowledge of adsorption equihbrium and mass transfer will enable further optimization of the performance of existing adsorbent types. Continuing discoveries of new molecular sieve materials will also provide adsorbents with new combinations of useflil properties. New adsorbents and adsorption processes will be developed to provide needed improvements in pollution control, energy conservation, and the separation of high value chemicals. New process cycles and new hybrid processes linking adsorption with other unit operations will continue to be developed. 

Adsorption enthalpies vary with surface coverage and adsorbent type but are usually exothermic - at least during initial stages of the adsorption process. 

ZeoHte adsorbents play a dominant role in purifications owing to their ability to both adsorb large quantities of material and to achieve extremely low mole fractions of these adsorbed these compounds in product gas. Zeolites are the preferred adsorbent types for dehydration to low levels, purification and in several bulk separations. Zeolites also are employed in a significant portion of the PSA hydrogen purification market segment where they add value to bulk separations by achieving particularly high purity specifications. 

Other innovations include PLE, MAE (see Section 1.3.1), and solid-phase microextraction (SPME). SPME is a sampling method applied to GC, HPLC, and CE. It is based on adsorbent- or adsorbent-type fibers and lends itself well to miniaturization. ... 

If the numbers of A s and B s are fixed, then a structural change affects only the first term. If minimal energy is obtained if Nab is maximal. If i ab > ,repulsion between an A and a B is larger than the average of the repulsion between two A s and the repulsion between two B s, then the A s and B s will form islands. If not, they will mix. If there are more than two adsorbate types more complicated situations can arise. An example is given in Section 4.1. ... 

Common Name of Adsorbent Type Area (m2/g) Pore Size (A) Method Refs. Supplier... 

Adsorption Selectirities. For a given adsorbent, the relative strength of adsorption of different adsorbate molecules depends on the relative magnitudes of Ihe polarizability a. dipole moment p. and quadrupole moment Q of each. Often, just die consideration of the values of a. //, and Q allows accurate qualitative predictions to be made of die relative strengths of adsorption of given molecules on an adsorbent or of the best adsorbent type (polar nr nonpolar) for a particular separation. 

Figure 4.15 The six types of adsorption isotherms (V = volume adsorbed) Type I shows a monolayer (Langmuir isotherm) types II and III show multilayer adsorption type IV shows first a monolayer, followed by filling of mesopores. The knee in isotherms I, II, and IV, indicated bya blackdot, indicates the point of monolayer formation (point B ).

Three types of solvent or solute delocalization have now been examined, as summarized in Table III for three different adsorbent types (four, if we distinguish Cig-deactivated silica from silica). The theoretical requirements on the configuration and density of adsorption sites were discussed earlier (Section II,B) for a given type of localization/delocalization to be possible. In each case the nature of adsorption sites is fairly well understood for the four adsorbents of Table III, as disucssed in Ref. / and 17 and shown in Fig. 14. Thus, in the case of alumina, surface hydroxyls do not function as adsorption sites. Although surface oxide atoms are capable of interacting with acidic adsorbate molecules (see below), in most cases the adsorbate will interact with a cationic center (either aluminum atom or lattice defect) in the next layer. As a result, we can say that in most cases adsorption sites on alumina are buried within the surface, rather than being exposed for covalent site-adsorbate interaction. These sites are also rigidly positioned within the surface. Finally, the... 

Figure 6. (plates A-D) Scanning electron micrographs of the major adsorbent types at 10,000— 15,000 magnification. 

Surface Characteristics of Different Sorbent Types Figures 8-11 present reasonably probable representations for surface active sites associated with the three major adsorbent types used in fats and oils processing. 

Figure 2 presents results for pure crystal pellets (CPS) at 73.8°C, which are typical of all the data obtained. Clearly, the model provides an excellent fit to these data, with the same diffusivity applying to both adsorption and desorption runs. There is some slight deviation at early times, but this is well within limits of experimental error. Results at other temperatures and with all 3 adsorbent types show the same kind of model fit. 

Tetanus toxoid is prepared from Clostridium tetani and can be given either in a fluid form (plain) or an adsorbed form. The slight local reactions that tend to occur (induration, erythema, tenderness) are more common with the adsorbed type. Intramuscular injection of tetanus toxoid... 

There are numerous commercial processes available to remove the sulfur content from syngas, ranging from throwaway regenerable adsorbent-type to regen-erable solvent-type processes. Solvent-type processes are of primary interest for sulfur removal from syngas. The solvent-type processes can be divided into three types ... 

Adsorbent type Flushing solvent Storage solvent... 

Before starting extensive experiments, a procedure recommended by Kaiser and Oel-rich (1981) to rule out adsorbents by fast experiments should be employed. Each elution experiment takes about 20 s. For this purpose samples are applied on a 50 X 50 mm TLC plate at 9 points, which are exactly 10 mm apart. Five microlitres of methanol are drawn into a micro-capillary with a Pt-Ir point. By applying the point of the filled capillary on one of the sample points on the plate, methanol is introduced onto the plate. A miniature radial chromatogram of ca. 7 mm diameter is produced. If the sample components remain at the point of application, the use of this adsorbent type is ruled out for HPLC usage. To make sure, the procedure is repeated with 5 pi of acetonitrile and tetrahydrofuran, respectively. If the products still remain at the point of application, the situation will not be changed by using any other mobile phase that is suitable for preparative chromatography work. 

Adsorbent Type Chermcal Form Granule Size Capacity Useable pH Range... 

Table 7.1 Adsorbent types [1]. Reproduced with permission of the American Institute of...

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