Adsorbents molecular sieve zeolites

The lower pressure sub-region is characterized by a considerable enhancement of the interaction potential (Chapter 1) and therefore of the enthalpy of adsorption consequently the pore becomes completely full at very low relative pressure (sometimes 0 01 or less), so that the isotherm rises steeply from the origin. This behaviour is observed with molecular sieve zeolites, the enhancement of the adsorption energy and the steepness of the isotherm being dependent on the nature of the adsorbent-adsorbate interaction and the polarizability of the adsorbate. -... 

In these processes, a solid with a high surface area is used. Molecular sieves (zeolites) are widely used and are capable of adsorbing large amounts of gases. In practice, more than one adsorption bed is used for continuous operation. One bed is in use while the other is being regenerated. 

In a sorption pump, the gas is trapped within the adsorbing material (zeolites or active charcoal) called molecular sieve. Zeolites are porous aluminium silicates which adsorb large amount of gas when cooled to low temperature (usually 77K). The pump is filled with zeolite and put in a bucket containing liquid nitrogen (see Fig. 1.11). 

Crystal lattice packing, 12 249-250 Crystal lattice vibrations, 14 236 Crystalline adsorbents, 1 586, 589. See also Molecular sieves Zeolites for gas separation, 1 631 properties and applications, l 588t Crystalline alkali silicates, atomic structure of, 22 454-455 Crystalline cellulose, 5 373-379 Crystalline epoxy resins, 10 373-374 Crystalline flake graphite, 12 793 manufacture and processing of, 12 781-784... 

Selectivity. Selectivity in a physical adsorption system may depend on differences in either equilibrium or kinetics, but the great majority of adsorption separation processes depend on equilibrium-based selectivity. Significant kinetic selectivity is. in general, restricted to molecular sieve adsorbents—carbon molecular sieves, zeolites, or zeolite analogues. 

Tlie growth in both variety and scale of gas-phase adsorption separation processes, particularly since 1970. is due in part to continuing discoveries of new porous, high surface-area adsorbent materials (particularly molecular sieve zeolites and. especially, to improvements in tlie design and modification of adsorbents. These advances have encouraged parallel inventions of new process concepts. Increasingly, the development of new... 

As knowledge concerning diffusion processes in molecular sieve zeolites broadens, it becomes increasingly clear that development of a truly generalizable model for diffusivity prediction in such adsorbents is not likely in the immediate future ... 

Another Russian scientist who played a leading role in the advancement of the understanding of adsorption mechanisms was A.V. Kiselev. With the help of a large team of co-workers and by making a systematic investigation of various well-defined adsorbents (notably oxides, carbons and zeolites), Kiselev was able to demonstrate that certain specific interactions were involved in the adsorption of polar molecules on polar or ionic surfaces. At the same time, in the UK the specificity of physisorption was under investigation by Barrer - especially in the context of his pioneering work on the properties of the molecular sieve zeolites. 

As we have already seen, an ideal Type I isotherm has a long, almost horizontal plateau, which extends up to p/p0 — 1, as in Figure 8. la. In this case, the micropote capacity, np(mic), is registered directly as the amount adsorbed at the plateau. Such well-defined Type I isotherms are given by large crystals of a molecular sieve zeolite. 

As the shortcomings of the traditional preparative methods outlined above became apparent, it was realized that alternative procedures were required to produce uniform or tailor-made adsorbents and shape-selective catalysts. As we saw in Chapter 11, one major route was opened up by the Linde synthesis in 1956 of the crystalline molecular sieve zeolite A. The search for new microporous crystalline materials has continued unremittingly and has resulted in the synthesis of novel zeolitic structures including the aluminophosphates, which are featured in this chapter. 

Activated carbons contain chemisorbed oxygen in varying amounts unless special care is taken to eliminate it. Desired adsorption properties often depend upon the amount and type of chemisorbed oxygen species on the surface. Therefore, the adsorption properties of an activated carbon adsorbent depend on its prior temperature and oxygen-exposure history. In contrast, molecular sieve zeolites and other oxide adsorbents are not affected by oxidizing or reducing conditions. 

Table 7.1 lists the typical sorbents used their uses as well as strengths and weaknesses. The four major commercial adsorbents are the following zeolite molecular sieves (zms), activated alumina, silica gel, and activated carbon. The surfaces of activated alumina and most molecular-sieve zeolites are hydrophilic, and will preferentially adsorb water over organic molecules. Silicalite, which is a hydrophobic zeolite, is the main exception. Activated carbon, on the other hand, preferentially adsorbs organic and non-polar or weakly polar compounds over water. The surface of silica gel is somewhere in between these limits and has affinity for both water and organics. Detailed information about each of these classes of adsorbents can be found in Refs. [1,4, 6, 7]. 

Commercial adsorbents ate divided into four major classes molecular-sieve zeolites, activated alumina, silica gel. and activated carbon. Since ndsorptlon is a surface-tela ted phenomenon, the useful adsorbents... 

The molecular-sieve zeolites are distiact from other three major npore size. Although other microporous solids are used as adsorbents for the separation of vapor or liquid mixtures, the distribution of pore diameters does not enable separations based on the ssolecular-sieve effect, that is. sepurations caused by difference in the molecular size of the materials to be separated. The most impurtanr molecular-sieve effects are shown by dehydrated crystalline zsoliles. Zeolites selectively adsorb or reject molecules based on differences in molecular size, shepe. and other properties such as polarity. Daring the ndsorption of various molecules, the micropores fill and empty reversibly. Adsorption in zeolites is a matter of pore filling, and the usual surface-area concepts are not applicable. 

Typical properties of commercially available molecular-sieve zeolite adsorbents are presented in Table... 

Figure 3. Spectra of dehydrated Co "-exchanged Type A molecular sieve zeolite with adsorbed ammonia at temperatures between 20 and...

During the last sixteen years the crystalline aluminosilicates, named zeolites, have attracted the attention of industrialists and scientists, because of their possible uses as catalysts, adsorbents and molecular sieves. Zeolite minerals were first identified in 1756 by Baron Crostedt, a Swedish mineralogist (Occelli and Robson 1989). They are natural adsorbents, and for this use they are efficient and more cost-effective than synthetic zeolites. However, since naturally occurring zeolites very frequently contain metal impurities, which can strongly modify their catalytic behaviour, their use as catalysts is limited and synthetic zeolites are preferred for catalytic applications. 

Until very recently, the use of adsorption systems (18) was generally limited to the removal of components present only in low concentrations. Recent progress in materials and engineering techniques has greatly extended the applications, as attested by Table 1.2, which lists only those applications that have been commercialized. Adsorbents used in effecting these separations are activated carbon, aluminum oxide, silica gel, and synthetic sodium or calcium aluminosilicate zeolite adsorbents (molecular sieves). The sieves differ from the... 

For commercial applications, an adsorbent must be chosen carefully to give the required selectivity, capacity, stability, strength, and regenerability. The most commonly used adsorbents are activated carbon, molecular-sieve carbon, molecular-sieve zeolites, silica gel, and activated alumina. Of particular importance in the selection process is the adsorption isotherm for competing solutes when using a particular adsorbent. Most adsorption operations are conducted in a semicontinuous cyclic mode that includes a regeneration step. Batch slurry systems are favored for small-scale separations, whereas fixed-bed operations are preferred for large-scale separations. Quite elaborate cycles have been developed for the latter. 

Adsorbers used for air pollution control and solvent recovery predominantly employ activated carbon. Molecular sieve zeolites are also used. Polymeric adsorbents can be used but are seldom seen. 

Molecular sieve zeolites " are hydrated, crystalline aluminosilicates which give off their crystal water without changing their crystal structure so that the original water sites are free for the adsorption of other compounds. Activation of zeolites is a dehydration process accomplished by the application of heat in a high vacuum. Some zeolite crystals show behavior opposite to that of activated carbon in that they selectively adsorb water in the presence of nonpolar solvents. Zeolites can be made to have specific pore sizes that impose limits on the size and orientation of molecules that can be adsorbed. Molecules above a specific size cannot enter the pores and therefore cannot be adsorbed (steric separation effect). 

This picture is useful but does not match all adsorbents. Gel-type ion-exchange resins have no permanent pores. Instead they consist of a tangled network of interconnected polymer chains into which the solvent dissolves. In effect, Cp = 0. Macroporous ion-exchange resins have permanent pores and Cp > 0, but often < 1.0 for large molecules. Many activated carbons have both macropores and micropores thus, there are two internal porosities. Molecular sieve zeolite adsorbents are used as pellets that are a omerates of zeolite crystals and a binder such as clay. In this case, there is an interpellet porosity (typically, 0.32), an intercrystal porosity ( pi 0.23) and an intracrystal porosity ( p2 0.19), which has < 1.0... 

Cl. A molecular sieve zeolite adsorbent consists of pellets that are agglomerates of zeolite crystals with density p ystai scattered in a continuous phase of clay binder with a density p gy. In this case, there is an interpellet porosity 8g (between pellets—this is normal Eg) an intercrystal porosity Sp, (which is the porosity in the binder), and an intracrystal porosity p2, (inside the crystals). If the fraction of the particle volume that is crystals (including porosity within the crystals) is f jy, derive formulas for the total porosity, Vavajjabjg, and the particle and bulk densities. 

The applications for these adsorbents depend on their particular adsorptive properties. The surface selectivities can be broadly classed as hydrophilic or hydro Atobic. For example, activated alumina and a majority of the molecular-sieve zeolites possess hydrophilic surfoces, and as such adsorb water stroitgly in preference to organic molecules. The surfaces of the vapor-phase-activated carbon products are hydrophobic and prefer organics to water. The surface of silica gel lies between these extremes and has a reasonable affinity for both water and organics. The terms otganophilic and otganophobic are also used. 

ZMlites were first recognized as a new type of mineral in 1756. Studies of the gas-adsorpdon properties of dehydrated natural zeolite crystals more than 60 years ago led to the discovery of their molecular-sieve behavior. As microporous solids with uniform pore sizes that range from 0.3 to 0.8 nm, these materials can selectively adsorb or reject molecules based on their molecular size. This effect, with obvious commercial overtones leading to novel processes for separadon of materials, inspired attempts to duplicate the natural materials by synthesis. Many new crystalline zeolites have been synthesized, and several fulfill important functions in the cherrtical and petroleum industries. Mote than 150 synthetic zeolite types and 40 zeolite minerals ate known. The most irnportam molecular sieve zeolite adsorbents ate the synthetic Type A, Type X, synthetic mordenite, and their ion-exchanged variations, and the mineral zeolites, cha-buite and mordenite. 

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