Adsorption 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. 

Thus, by the mid-1930s the literature described the ion exchange, adsorption, molecular sieving and structural properties of zeolite minerals as well as a number of reported syntheses of zeolites. The early synthetic work remains unsubstantiated because of incomplete characterization and the difficulty of experimental reproducibility. 

Tn presenting the adsorptive properties of molecular sieve zeolites, most authors (1, 2) report isosteric heats. These are obtained from the application of the thermodynamically derived Clausius-Clapeyron type equation to experimentally measured equilibrium data. At a constant... 

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... 

Pure component loadings for CO2, N2 and O2 on commercial pelleted forms of Linde type 4A, 5A and 13X molecular sieve zeolites were derived from various gravimetric and volumetric measurements. The range of pressures and temperatures over which these measurements were made were at least as broad as those encountered in the breakthrough experiments described here, to permit accurate estimations of heats of adsorption in the manner described by equation (6) above. As mentioned above, the pure component data were correlated to the LRC model, and the CO2 loadings predicted by the multicomponent LRC model compared to actual loadings in the breakthrough runs at bed saturation. 

There are only four types of sorbents that have dominated the commercial use of adsorption activated carbon, molecular-sieve zeolites, sihca gel, and activated alumina. Estimates of worldwide annual sales of these sorbents are as follows (Humphry and Keller, 1997) ... 

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. 

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. 

Other Applications. Many applications of adsorption involving radioactive compounds simply parallel similar applications involving the same compounds in nonradio active forms, eg, radioactive carbon-14, or deuterium- or tritium-containing versions of C02, H20, hydrocarbons. For example, molecular sieve zeolites are commonly employed for these separations, just as tor the corresponding nonradio active uses. 

Molecular sieve zeolites constitute a class of stationary phase that combines exclusion with specific adsorption properties. These materials, which are crystalline aluminum silicates (commonly sodium or calcium aluminum silicates), have rigid, highly uniform three-dimensional porous structures containing up to 0.5ml/g of free pore volume, resulting when water of crystallization is removed by heating. Although munerous natural zeolites are known, most practical work is done with... 

Reiss, G. Molecular sieve zeolite for producing hydrogen by pressure variation adsorption technique. U.S. Patent 4,477,267, 1984. 

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. 

Zeolite properties are being studied by nearly every type of modern scientific discipline, and they are being utilized in many new chemical engineering processes. Important advances include detailed basic information on cations in zeolites, more understanding of the mechanism of zeolite formation, the formation and character of structural defects and hydroxyl groups, the role of zeolite structure in adsorption and catalysis, and the increasing technology of the use of molecular sieve zeolites in catalysis and adsorption. 

Since the sublimation rate can be increased by a higher heating rate only to a certain extent, the rate of water vapor removal from the sublimation front becomes the key point in freeze drying. Adsorption of water vapor by sorbents placed inside a drying chamber or by direct contact of the sorbent and drying material (as a mixture) is one of the most promising methods in this area. Molecular sieves, zeolites, or ceramics can be recommended here because their sorption activity under rarefied conditions does not drop with temperature rise due to the heat of sorption. The use of particulate sorbents permits sublimation to take place in a fluidized state under atmospheric pressure [67,68]. 

RO membranes including polymeric (PA-TFC membrane) and molecular sieve zeolite membranes were investigated for ion removal from the water produced at oil field and coal bed methane sites by a cross-flow RO process [78]. Pretreatments including NF and adsorption by active carbon were implemented. The study revealed that (1) most of permeation tests lasted only 3 months due to severe fouling, (2) multistage pretreatment is crucial to extend membrane life, and (3) only NF treatment could extend the membrane life to 6 months. 

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. 

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). 

Cottttnetcial adsorbents are divided irrto four ittajor classes molecular-sieve zeolites, activated alumina, silica gel. and activated carbon. Sittce adsorption is a surfiicfr-related phenoitteaon, the useful adsorbents... 

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. 

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