Applications of zeolite adsorbents

Other Liquid and Gas Drying. Applications of zeolite adsorbents in drying other industrial gases and liquids are well known and have been discussed in Refs. 34-36. Therefore, although it is an important application, it is not discussed here. 

TABLE I Some Important Applications of Zeolite Adsorbents... 

New developments for niche applications of zeolite adsorbents are expected in various domains where they can provide an alternative to carbon filters to meet ultra-low VOCs emission requirements in confined environments (such as houses, vehicle habitacles, painting cabins, workshops. ..) [34]. 

Table 2.4 Some important applications of zeolite adsorbents ...

Concerning the application of these adsorbents as thermal energy storages the amount of water, which can be adsorbed is the most important property. Figure 238 shows the maximum water uptake of some commercially available adsorbents. Zeolite A can reach 25% and Zeolite 13X up to 32% of its dry weight. Narrow pore Silicagel can adsorb 38% water. Two special adsorbents Sizeo, which is a mixture of Zeolite and Silicagel, and SWS, which is a wide... 

The major problem of the application of zeolites in alkane-alkene alkylation is their rapid deactivation by carbonaceous deposits. These either strongly adsorb on acidic sites or block the pores preventing the access of the reactants to the active sites. A further problem is that in addition to activity loss, the selectivity of the zeolite-catalyzed alkylation also decreases severely. Specifically, alkene formation through oligomerization becomes the dominant reaction. This is explained by decreasing ability of the aging catalyst to promote intermolecular hydride transfer. These are the main reasons why the developments of several commercial processes reached only the pilot plant stage.356 New observations with Y zeolites reconfirm the problems found in earlier studies.358,359... 

Sorption capacity is one of the major properties used for industrial applications of zeolites. H. Lee reviews the aspects of zeolites used as adsorbents. The other papers in the section deal with the theory of sorption and diffusion in porous systems, the variation of sorption behavior upon modification, and the variation of crystal parameters upon adsorption. NMR and ESR studies of sorption complexes are reported. H. Resing reviews the mobility of adsorbed species in zeolites studied by NMR. 

New applications of zeolite adsorption developed recently for separation and purification processes are reviewed. Major commercial processes are discussed in areas of hydrocarbon separation, drying gases and liquids, separation and purification of industrial streams, pollution control, and nonregenerative applications. Special emphasis is placed on important commercial processes and potentially important applications. Important properties of zeolite adsorbents for these applications are adsorption capacity and selectivity, adsorption and desorption rate, physical strength and attrition resistance, low catalytic activity, thermal-hydrothermal and chemical stabilityy and particle size and shape. Apparent bulk density is important because it is related to adsorptive capacity per unit volume and to the rate of adsorption-desorption. However, more important factors controlling the raJtes are crystal size and macropore size distribution. 

Vne of the major industrial applications of zeolites is in the area of ad-sorption processes. Zeolite adsorbents are not only the most important adsorbents today, but their importance is increasing, mainly because of the following unique adsorptive properties (a) selective adsorption of molecules based on molecular dimensions, (b) highly preferential adsorption of polar molecules, (c) highly hydrophilic surface, and (d) variation of properties by ion exchange. 

The study of zeolites as adsorbent materials began in 1938, when Barrer published a series of papers on the adsorptive properties of zeolites [97], In the last 50 years, zeolites, both natural and synthetic, have become one of the most important materials in modern technology [97-107], Today, the production and application of zeolites for industrial processes is a multimillion dollar industry. 

The study of zeolites as adsorbent materials began in 1938 when Professor Barrer published a series of papers on the adsorptive properties of zeolites [28], In the last 50 years, zeolites, natural and synthetic, have turned out to be one of the most significant materials in modem technology [27-37], Zeolites have been shown to be good adsorbents for H20, NH3, H2S, NO, N02, S02, C02, linear and branched hydrocarbons, aromatic hydrocarbons, alcohols, ketones, and other molecules [2,31,34], Adsorption is not only an industrial application of zeolites but also a powerful means of characterizing these materials [1-11], since the adsorption of a specific molecule gives information about the microporous volume, the mesoporous area and volume, the size of the pores, the energetics of adsorption, and molecular transport. 

Today the melt crystallization can be advantageously replaced by a more challenging separation method known as simulated moving bed (SMB) technology. The method exploits the differences in affinity of zeolitic adsorbents for p-xylene with respect to other A8 components. Despite the name, the adsorbent phase is stationary and only fluid phase is distributed in a cyclic manner by a multivalve system. Operation parameters are temperatures of 125 to 200 °C and pressures up to 15 bar. Lighter (toluene) or heavier solvents (p-diethylbenzene) may be used as a desorbent. The Parex process working on this principle today has many applications. 

Activated charcoal is a finely divided form of amorphous carbon and is manufactured from organic materials (e.g. peat, wood) by heating in the presence of reagents that promote both oxidation and dehydration. Activated charcoal possesses a pore structure with a large internal surface area microporous materials exhibit pores <2nm wide, macroporous refers to activated charcoals with a pore size >50nm, and mesoporous materials fall in between these extremes. The largest internal surface areas are found for microporous materials (>700m g ). The ability of the hydrophobic surface to adsorb small molecules is the key to the widespread applications of activated charcoal. (Comparisons should be made with the porous structures and applications of zeolites see Sections 13.9 and 26.6.)... 

Tn most applications of zeolites, surface properties and reactivity are of major importance. Infrared spectroscopy can give useful information on the constitution and surface properties of zeolites and how these are modified by various treatments. Changes in the spectra of the zeolite and of molecules adsorbed on the surface can yield direct information about the surface, how adsorbed molecules interact, and where molecules adsorb. 

Zeolite could selectively adsorb A -nitrosamines in the solution of methylene chloride or water, and the equilibrium data were fitted to Freundlich-type isotherms. Textural and acid-basic properties of zeolite determined their adsorption capacity. The extraordinary adsorption properties of NaA zeolite for iV-nitrosamines is inferred that the adsorbates inert the channel with the group -N=N-0. Larger amount of A-nitrosamines was adsorbed on ZSM-5 zeolite in water instead in methylene chloride, due to the hydrophobicity of the zeolite. Application of zeolite to remove A-nitrosamines from beer seems successful. Up to 100% of the worst carcinogenic compounds could thus be removed with 1.4 g/L of zeolite which was proven to be better adsorbent than silica or alumina. On Na 3 zeolite and MCM-41 mesoporous material A-nitrosodimethylamine decomposed above 573 K and the liberated NOx could be detected even at 773 K during the TPSR process, indicating the strong adsorption of A-nitrosamines on molecular sieves that makes zeolite become the functional materials for environmental protection. 

Applicability of Quantum Chemistry Methodology to the Investigation of Structural and Electronic Properties of Zeolite-Adsorbate Complexes... 

Usually, natural gas treatment on the basis of thermal process engineering takes place in three steps (see Fig. 7.9). The first step that may consist of partial steps just like all other subsequent steps, serves the preparation of the crude gas for its processing. Here, for example, acid-forming gas components, such CO2, H2S and other sulphuric compounds are removed. Usually, chemical scrubbing with amines (MEA, DEA, MDEA) is applied in which the adsorbent is being regenerated. Then the natural gas is dried. In case of moderate water dew point requirements, glycol is used as wash liquor. The lowest water contents (< 1 ppm) are achieved with the application of zeolitic molecular sieves. Finally, mercury is removed in case aluminium will be used as material of construction for equipment. Mercury in contact with aluminium may lead to catastrophic corrosion. 

Let us illustrate the above with a specific example. Although this example is strongly application driven, the conclusions are equally valid for more fundamental problems in simulations of complex fluids. There is considerable interest in studying the adsorption of alkanes in the pores of a zeolite. Zeolites are microporous materials which are used as catalysts in petrochemical applications (see Zeolites Applications of Computational Methods). A prerequisite for an understanding of the catalytic activity of these zeolites is a knowledge of the behavior of the molecules adsorbed in their narrow pores. Since this type of information is extremely difficult to obtain experimentally, simulations appear to be an attractive alternative. Indeed, over the last decade many simulation studies on the behavior of molecules in zeolites have been published (for a review see Ref. 3). A more careful look at these studies reveals that most simulations concern the adsorption of noble gases or methane, only a few studies of ethane or propane have been published. In petrochemical applications of zeolites, however, we are interested in the behavior of much longer alkanes such as octane and decane. 

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