Commercialization, molecular sieve zeolites

The foregoing discussion has focused on the most important commercial molecular sieves, zeolites. New directions in the preparation of framework stmctures of different chemical composition and of large-pore molecular sieves have also appeared. 

As synthesized, commercial molecular sieve zeolite crystals are quite small (typically 1-10 fim) and to prepare a practically useful adsorbent these crystals must be formed into a macroporous pellet of suitable dimensions, porosity, and mechanical strength. Scanning electron micrographs of two representative commercial pelletized adsorbents are shown in Figure 1.13. The optimal pellet generally represents a compromise between various conflicting requirements and may therefore be different for different process applications. 

Molecular sieve dryers, 10 613 Molecular-sieve effects, 16 821 Molecular sieve membranes, 15 813t Molecular sieve products commercial, 16 838-839t manufacturing processes for, 16 831 Molecular sieves, 16 811-853. See also Carbon molecular sieves Zeolite entries... 

For a fixed-bed operation, zeolite adsorbents should have a reasonable size to avoid an excessive pressure drop. Synthetic zeolites and some natural zeolites produced in a fine size powder have to be formed into spheres, extrudates, or pellets usually with an inert binder. Some commercial molecular sieve adsorbents, however, are called binderless because they contain a much higher (up to 95%) zeolite content than most zeolite adsorbents. 

Commercial molecular sieve pellets consist of small zeolite crystals formed into a macroporous pellet generally with the aid of an inert clay binder. In the present analysis the pellets are considered to be spherical, and each pellet is assumed to contain an assemblage of uniformly sized spherical zeolite crystals. This idealization must be treated with caution since zeolite crystals are not spherical, and the range of crystal sizes present in some commercial molecular sieve pellets may be quite large (3). 

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) ... 

Molecular-sieve zeolites, commercial use, 80 Molybdenum carbide, M02C, sonochemical synthesis, 16-17... 

Molecular sieve zeolites have become established as an area of scientific research and as commercial materials for use as sorbents and catalysts. Continuing studies on their synthesis, structure, and sorption properties will, undoubtedly, lead to broader application. In addition, crystalline zeolites offer one of the best vehicles for studying the fundamentals of heterogeneous catalysis. Several discoveries reported at this conference point toward new fields of investigation and potential commercial utility. These include phosphorus substitution into the silicon-aluminum framework, the structural modifications leading to ultrastable faujasite, and the catalytic properties of sodium mordenite. 

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... 

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

Although many molecular sieve zeolites are known, most of the spectroscopic studies have involved the commercially important large-pore X and Y zeolites. Isolated studies of other zeolites, such as A, L, and mordenite have been reported. Discussions of the application of infrared spectroscopy to zeolites appeared recently (7, 24, 39, 74, 75). In this review, recent studies of the nature of surface structural groups and the interactions of various molecules will be discussed. 

There is a great need for robust, defect-free, highly selective molecular sieve (zeolite) thin film membranes for light gas molecule separations in hydrogen fuel production from CH4 or H2O sources. They contain an inherent chemical, thermal and mechanical stability not found in conventional membrane materials. Our goal is to utilize those zeolitic qualities in membranes for the separation of light gases, and to eventually partner with industry to commercialize the membranes. To date, we have successfully ... 

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. 

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. 

The commercial molecular sieves generally belong to the zeolite class of minerals, i.e., hydrated alkali metal or alkaline earth aluminosilicates, which are activated by heat to drive off the water of crystallization. The crystals have a robust cubic structure, which does not collapse on heating, so that activation results in a geometric network of cavities connected by pores. The pores are of molecular dimensions and cause the sieving action of these materials. 

To-date, the focus of fuel desulfurization research has been on low temperature absorption systems such as activated carbon beds and molecular sieves (zeolite). This process has been selected for initial commercial... 

Zeolite and Molecular Sieve-Based Process. Mobil has commercialized several xylene isomerization processes that are based on ZSM-5. Amoco has developed a process based on a medium-pore borosiUcate molecular sieve. 

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