Adsorbents zeohtes

The rather low concentration of the desired p-xylene component in the Parex unit feed means a large fraction of the feed stock contains other A8 components that are competing for adsorption sites in the adsorbent zeoHte cages. Due to this typically lean feed, a significant hike in the Parex unit capacity can be obtained by even a small increase in the composition of the p-xylene. Techniques to increase the p-xylene feed concentration include greater dealkylation of the ethylbenzene in the Isomar unit by converting from an ethylbenzene isomerization catalyst to... 

The adsorbent typically consists of the selectively adsorbing zeoHte and a binder. The binding material is required to make parHcles of the zeoHtes of a large enough size to allow for practical use in a commercial appHcation. The particle formation method and resulting characteristics of the binder/zeoHte combinaHon is of criHcal importance, especially as it relates to mass transfer through the particle and the characteristics of the zeoHte [30]. The practical aspects of forming and... 

Vb and Vg stand for the wavenumbers of the beginning and the end of the band, respectively, T (v) and T"(v) for the transmittance along the base line and the band contour, respectively. Modern instrumentation usually allows routine base Hne determination and band integration to evaluate Aj, . In IR and Raman spectroscopy of zeoHtes and adsorbate/zeohte samples, the extinction coefficient, or e,(c), is usually unknown. Thus, if knowledge of the absolute concentrations is required, e, has to be determined in separate experiments (cf., e.g., [ 129-135,]). In such experiments, the absorbance has to be measured of zeolite samples covered with a known number of functional surface groups or loaded with well-defined amounts of adsorbate in order to obtain caUbration cmves, Ajn, vs. c. An example is shown in Fig. 7. 

Hydrophilic and Hydrophobic Surfaces. Water is a small, highly polar molecular and it is therefore strongly adsorbed on a polar surface as a result of the large contribution from the electrostatic forces. Polar adsorbents such as most zeoHtes, siUca gel, or activated alumina therefore adsorb water more strongly than they adsorb organic species, and, as a result, such adsorbents are commonly called hydrophilic. In contrast, on a nonpolar surface where there is no electrostatic interaction water is held only very weakly and is easily displaced by organics. Such adsorbents, which are the only practical choice for adsorption of organics from aqueous solutions, are termed hydrophobic. 

Despite the difference ia the nature of the surface, the adsorptive behavior of the molecular sieve carbons resembles that of the small pore zeoHtes. As their name implies, molecular sieve separations are possible on these adsorbents based on the differences ia adsorption rate, which, ia the extreme limit, may iavolve complete exclusion of the larger molecules from the micropores. 

Desiccants. A soHd desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or Hquid) stream. The main requkements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are siHca gel, activated alumina, and the aluminum rich zeoHtes (4A or 13X). The equiHbrium adsorption isotherms for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different appHcations. 

Fig. 3. Adsorption equiHbrium isotherms for moisture on three commercial adsorbents peUetized 4A zeoHte (—), siHca gel (-), and a typical activated...

Isotherms for H2O and / -hexane adsorption at room temperature and for O2 adsorption at Hquid oxygen temperature on 13X (NaX) zeoHte and on the crystalline Si02 molecular sieve siHcaHte are are shown in Figure 8 (43). SiHcaHte adsorbs water very weaMy. Further modification of siHcaHte by fluoride incorporation provides an extremely hydrophobic adsorbent, shown in Figure 9 (44). These examples illustrate the broad range of properties of crystalline molecular sieves. 

Drying. The single most common gas phase appHcation for TSA is drying. The natural gas, chemical, and cryogenics industries all use zeoHtes, siHca gel, and activated alurnina to dry streams. Adsorbents ate even found in mufflers. 

A, 5A, and 13X zeoHtes are the predorninant adsorbents for CO2 removal by temperature-swing processes. The air fed to an air separation plant must be H2O- and C02-ftee to prevent fouling of heat exchangers at cryogenic temperatures 13X is typically used here. Another appHcation for 4A-type zeoHte is for CO2 removal from baseload and peak-shaving natural gas Hquefaction faciHties. 

Zeohte 5A is used because its pores can size-selectively adsorb straight-chain molecules while excluding branched and cycHc species. The normal hydrocarbon fraction has better than 95% purity, and the higher octane isomer fraction contains less than 2% normal hydrocarbons (64). 

A significant advantage of adsorbents over other separative agents Hes in the fact that favorable equiHbrium-phase relations can be developed for particular separations adsorbents can be produced that are much more selective in their affinity for various substances than are any known solvents. This selectivity is particularly tme of the synthetic crystalline zeoHtes containing exchangeable cations. These zeoHtes became available in the early 1960s under the name of molecular sieves (qv) (9). 

Adsorption. Although several types of microporous soHds are used as adsorbents for the separation of vapor or Hquid mixtures, the distribution of pore diameters does not enable separations based on the molecular-sieve effect. The most important molecular-sieve effects are shown by crystalline zeoHtes, which selectively adsorb or reject molecules based on differences in molecular size, shape, and other properties such as polarity. The sieve effect may be total or partial. 

Activated diffusion of the adsorbate is of interest in many cases. As the size of the diffusing molecule approaches that of the zeohte channels, the interaction energy becomes increasingly important. If the aperture is small relative to the molecular size, then the repulsive interaction is dominant and the diffusing species needs a specific activation energy to pass through the aperture. Similar shape-selective effects are shown in both catalysis and ion exchange, two important appHcations of these materials (21). 

During the adsorption or occlusion of various molecules, the micropores fill and empty reversibly. Adsorption in zeoHtes is a matter of pore filling, and the usual surface area concepts are not appHcable. The pore volume of a dehydrated zeoHte and other microporous soHds which have type 1 isotherms may be related by the Gurvitch rule, ie, the quantity of material adsorbed is assumed to fill the micropores as a Hquid having its normal density. The total pore volume D is given by... 

The channels in zeoHtes are only a few molecular diameters in size, and overlapping potential fields from opposite walls result in a flat adsorption isotherm, which is characterized by a long horizontal section as the relative pressure approaches unity (Fig. 6). The adsorption isotherms do not exhibit hysteresis as do those in many other microporous adsorbents. Adsorption and desorption are reversible, and the contour of the desorption isotherm foUows that of adsorption. 

The 2eohte sodium X (type 13X) has a crystallographic aperture of 0.74 nm. This compares well with the adsorbate value of 0.81 nm. ZeoHte calcium X exhibits a smaller apparent pore si2e of 0.78 nm (lOX). This difference is probably due to some distortion of the aluminosihcate framework upon dehydration and calcium ion migration. 

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

ZeoHte-based materials are extremely versatile uses include detergent manufacture, ion-exchange resins (ie, water softeners), catalytic appHcations in the petroleum industry, separation processes (ie, molecular sieves), and as an adsorbent for water, carbon dioxide, mercaptans, and hydrogen sulfide. 

Zeohte ciyst lite diffusivities for sorbed gases range from 10" to lO" " cmVs. These diffusivities generally show a strong increase with the adsorbate concentration that is accounted for by the Darken thermodynamic correction factor... 

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