Kinetic sieving diameter

Molecule Molecular weight Mean free path (nm) Kinetic sieving diameter (nm)... 

Fig. 8. Variation of activation energy with kinetic molecular diameter for diffusion in 4A 2eohte (A), 5A 2eohte (0)> carbon molecular sieve (MSC-5A) (A). Kinetic diameters are estimated from the van der Waals co-volumes. From ref. 7. To convert kj to kcal divide by 4.184.

Species separated by molecular sieving effects when kinetic diameters fall iato different zeoHte aperture size categories (standard molecular sieve diameters = 300, 400, 500, 800, 1000,1300 pm. 

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

Table 4.3.2. Kinetic S sieving diameter of a gas/vapor based on the smallest zeolite window where it can fit...

Dehydration of organics (removal of <1% water) generally feasible by molecular sieving, if kinetic diameter of organic >300 pm. 

The more permeable component is called the. st ga.s, so it is the one enriched in the permeate stream. Permeability through polymers is the product of solubihty and diffusivity. The diffusivity of a gas in a membrane is inversely proportional to its kinetic diameter, a value determined from zeolite cage exclusion data (see Table 22-23 after Breck, Zeolite Molecular Sieves, Wiley, NY, 1974, p. 636). 

Molecular sieving effect of the membrane has been evidenced using a mixture of two isomers (i.e. no Knudsen separation can be anticipated), n-hexane and 2-2 dimethylbutane (respective kinetic diameters 0.43 and 0.62 nm). Figure 10 shows the permeate contains almost only the linear species, due to the sieving effect of the zeolite membrane (pore size ca 0.55 nm). This last result also underlines that the present zeolite membrane is almost defect-fi ee. 

Advanced Materials Experimental membranes have shown remarkable separations between gas pairs such as O2/N2 whose kinetic diameters (see Table 20-26) are quite close. Most prominent is the carbon molecular sieve membrane, which operates by ultramicro-porous molecular sieving. Preparation of large-scale permeators based on ultramicroporous membranes has proven to be a major challenge. 

Figure 6.2 illustrates the separation of n-Csis and non-n-Cs/is in CaA molecular sieves or 5A. The separation mechanism is obvious when the kinetic diameter of the molecules and molecular sieve pore size opening are compared. n-Csjc have kinetic diameters of less than 4.4 A which can diffuse freely into the 4.7 A pores of the CaA molecular sieve, while non-n-Cs/ have kinetic diameters of 6.2A. A commercial example of shape-selective adsorption is the UOP Molex process, which uses CaA molecular sieves to separate Cio-C n-paraffins from non- -parafHns (aromatics, branched, naphthenes). 

A good example for reactant shape selectivity includes the use of catalysts with ERI framework type for selective cracking of linear alkanes, while excluding branched alkanes with relatively large kinetic diameters from the active sites within the narrow 8-MR zeolite channels [61, 62]. Here molecular sieving occurs both because of the low Henry coefficient for branched alkanes and because of the intracrystalline diffusion limitations that develop from slow diffusivities for branched alkane feed molecules. 

The use of molecular sieving for the diffusive separation of molecules very close in size (3.64 A vs 3.8 A kinetic diameters) with commercially viable productivities is possible. 

The crystallographic diameter is 7.4A but the kinetic diameter of 8.1A is the more applicable in this situation, Breck, D.W. "Zeolite Molecular Sieves" Wiley Interscience New York, 1974. 

The primary requirement for an economic adsorption separation process is an adsorbent with sufficient selectivity, capacity, and life. Adsorption selectivity may depend either on a difference in adsorption equilibrium or, less commonly, on a difference in kinetics. Kinetic selectivity is generally possible only with microporous adsorbents such as zeolites or carbon molecular sieves. One can consider processes such as the separation of linear from branched hydrocarbons on a 5A zeolite sieve to be an extreme example of a kinetic separation. The critical molecular diameter of a branched or cyclic hydrocarbon is too large to allow penetration of the 5A zeolite crystal, whereas the linear species are just small enough to enter. The ratio of intracrystalline diffusivities is therefore effectively infinite, and a very clean separation is possible. 

To evaluate the applicability of molecular sieves, we compare the kinetic diameter of components. Low-size sieves would filter all, except chloro-ethane and benzene. Then, the remaining H2S may be handled by wet scrubbing. 

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