Commercialization, molecular sieve

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. 

Dehydration is by far the largest industrial separation of interest here. Removal of water was the first commercial application of molecular sieves. Dehydration and related fixed-bed adsorptive separations in the process industries account for more than half of the commercial molecular sieve business volume. 

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). 

Pmcesses. Manufacturing processes for commercial molecular sieve products may be classified into three groups, as shown in Table 3. 

Analysis of the as plots can be used to obtain useful information concerning the pore (or pore entrance) size and distribution of commercial molecular sieve and superactivated carbons. A more complete characterisation of some of the sanq)les would require the use of larger molecules than those used here. Reference data for the adsorption of neopentane has already been published [9] and this would appear to be the best choice for extending the range of molecular size in future work. 

The choice of SO2 as the adsorbed component was made in view of the successful operation of a commercial unit for separating SO2 from the tail gas of a sulfuric acid unit using Zeolon molecular sieve (3). In this work, however, use was made of molecular sieve 5A, which unlike Zeolon is not acid-resistant, due to the existence of fairly accurate equilibrium adsorption data (4) for SO2 adsorption on this specific adsorbent. It should be noted that additional equilibrium adsorption data for SO2 on a variety of natural and commercial molecular sieves are available (5-12). 

TABLE 12.2-1 Major Commercial Molecular-Sieve Adsorbent Products and Some Physical Properties... 

Molecular sieves arc aluminum silicate ion exchangers, whose pore size depends on the kind of cation present. Commercial preparations of these materials arc available in particle sizes of 40 to 60 mesh to 00 to 120 mesh. The sieves are classified according to the maximum diameter of molecules that can enter the pores. Commercial molecular sieves come in pore sizes of 4, 5, 10, and 1,1 A, Molecules smaller than Ihese dimensions penetrate into the interior of the panicles where adsorption takes place. For such molecules, the surface area is enormous when compared with the area available to larger molecules. Thus, molecular sieves can be used to separate small molecules from large. For... 

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. 

FIGURE 1,15, Result. of accelerated aging test,s carried out with six different commercial molecular sieve adsorbents (4A and 13X). 

The conductivity tests were carried out for a number of P(VDF-CTFE)/ Nafion/Inorganic membranes 3D phase zirconium phosphate, 3D porous titanosili-cate, amorphous zirconium phosphate (AZP), mesoporous alumina, and a silica-based commercial molecular sieve (MS). Figure 12.12 shows the conductivity of various PEM composite membranes under various humidity conditions. The inorganic phase... 

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. 

Table 12-8 Basic Types of Commercial Molecular Sieves ... 

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