Molecular sieve adsorbents

C. Adsorbents. Water can be removed from solvents by means of adsorbents. Molecular sieves and alumina are the most common adsorbents used to attain very low moisture levels. These adsorbents should be activated prior to use, as described in Chapter 3. The purification can be carried out in a batch manner with the molecular sieves or alumina introduced into a solvent container. After standing and allowing the adsorbent to settle, the solvent may be drawn off. Much lower levels of impurities can be achieved by taking advantage of the multiple-stage purification with a column of adsorbent (see Fig. 1.27). The column is filled with activated adsorbent, then purged with inert gas The... 

Controlled porosity glasses (CPGs) as adsorbents, molecular sieves, ion-exchangers and support matericils... 

SBA-15-type materials are novel well-ordered mesoporous molecular sieves with high surface area and narrow distribution of pores of about 60-100 A in diameter [1]. These materials have already been tested in a variety of applications, such as adsorbents, molecular sieves, catalysts and catalytic supports. However, up to now only purely siliceous SBA-15 materials were... 

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... 

Poly(m-phenylene isophthalamide) and other aramids have been proposed as precursor for activated carbon materials. These materials exhibit a very homogeneous nucropore size. This property makes them usable as adsorbents, molecular sieves, catalysts or electrodes. 

Thermal desorption Volatile compounds in gases such as pollutants in air can be trapped in a small adsorption tube, either by pumping the gas through or by passive diffusion. The packing in the trap can be chosen from a wide variety of adsorbents (molecular sieves, graphitized carbon blacks, organic polymers). After sample collection the adsorption tube is rapidly heated in a stream of purge gas which transports the released analytes to the GC column where the separation runs. 

Column 22 m X 1 mm adsorbent molecular sieve 5A deposited on the inner wall ... 

Ryckebosch et al. [6] reported polymer beds (Tenax TA 60/80 mesh and Amberlite XAD n 20/60 mesh), inorganic adsorbents (Molecular sieve 13X 45/60 mesh en silica gel particle size 1-3 mm), and two adsorbents with carbon as the major component (activated charcoal, particle size 2.5 mm en Carbopack B 60/80 mesh). 

C2H4 selectivities up to 100% and total C2H4 yields up to 85%, when the zirconia reactor is used in a gas-recycle loop containing a molecular sieve adsorbent (molecular sieve 5A) for C2 hydrocarbon trapping. This is by far the highest ethylene or C2 hydrocarbon yield obtained so far with the OCM reaction. 

Zeolites are an important class of materials because of their industrial use as catalysts, adsorbents ( molecular sieves ), and ion-exchangers. They can also serve as a nanoporous matrix for studying host-guest interactions. Originally the term zeolite was used for naturally occurring aluminosilicate minerals of the composition... 

A mixture of hydrocarbons can be separated by three selective adsorbents molecular sieves which retain the n-paraffins, mercuric perchlorate which adsorbs the unsaturated compounds, and a stationary liquid phase which retains the aromatic hydrocarbons. Boric acid on a stationary phase such as Chromosorb P is used for the removal of alcohols from a mixture of organic compounds, the products are nonvolatile esters. A similar technique was used for removal of terpene alcohols from a mixture of terpenoids. Acids can be adsorbed on potassium hydroxide deposited on quartz powder. This technique was found to be suitable for the analysis of compounds having active hydrogens such as fluorene, indene, carbazole, indole, and pyrrole as well as steroids such as estrogens and ketosteroids. o-Dianisidine quantitatively subtracts aldehydes, ketones, and epoxides, and phosphoric acid subtracts epoxides. 

Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

Fig. 4.20 DR plots for carbon dioxide adsorbed at 293 K on Linde molecular sieves. O, powder SA , powder 4A. (Reduced from the original diagram of Lamond and Marsh. )...

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. -... 

Adsorption Processes. Adsorption represents the second and newer method for separating and producing high purity PX. In this process, adsorbents such as molecular sieves are used to produce high purity PX by preferentially removing PX from mixed xylene streams. Separation is accomphshed by exploiting the differences in affinity of the adsorbent for PX, relative to the other Cg isomers. The adsorbed PX is subsequendy removed... 

A degree of control over the kinetic selectivity of molecular sieve adsorbents can be achieved by controUed adjustment of the pore si2e. In a carbon sieve this may be accompHshed by adjusting the bum-out conditions or by controUed deposition of an easily crackable hydrocarbon. In a 2eoHte, ion... 

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. 

In certain adsorbents, notably partially coked 2eohtes and some carbon molecular sieves, the resistance to mass transfer may be concentrated at the surface of the particle, lea ding to an uptake expression of the form... 

Fig. 1. Water isotherms for various adsorbents (1). Activation conditions Linde molecular sieves, 350°C and <1.33 Pa activated alurnina, 350°C and...

Typical pore size distributions for these adsorbents have been given (see Adsorption). Only molecular sieve carbons and crystalline molecular sieves have large pore volumes in pores smaller than 1 nm. Only the crystalline molecular sieves have monodisperse pore diameters because of the regularity of their crystalline stmctures (41). 

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. 

Activated carbons contain chemisorbed oxygen in varying amounts unless special cate 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 2eohtes and other oxide adsorbents are not affected by oxidi2ing or reducing conditions. 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

To prevent such release, off gases are treated in Charcoal Delay Systems, which delay the release of xenon and krypton, and other radioactive gases, such as iodine and methyl iodide, until sufficient time has elapsed for the short-Hved radioactivity to decay. The delay time is increased by increasing the mass of adsorbent and by lowering the temperature and humidity for a boiling water reactor (BWR), a typical system containing 211 of activated carbon operated at 255 K, at 500 K dewpoint, and 101 kPa (15 psia) would provide about 42 days holdup for xenon and 1.8 days holdup for krypton (88). Humidity reduction is typically provided by a combination of a cooler-condenser and a molecular sieve adsorbent bed. 

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