Selective diffusion

In all of these tests, flame acceleration was minimal or absent. Acceleration, when it occurred, was entirely due to intrinsic flame instability, for example, hydrodynamic instability (Istratov and Librovich 1969) or instability due to selective diffusion (Markstein 1964). To investigate whether the flame would accelerate when allowed to propagate over greater distances, tests were carried out in an open-sided test apparatus 45 m long (Harris and Wickens 1989). Flame acceleration was found to be no greater than in the balloon experiments (Table 4.1a). 

Figure 10.5 shows the basic concept of the particle-level MR that gives (i) selective addition of reactants to the reaction zone and (ii) selective removal of products from the reaction zone. In the first case, if the diffusivity of one reactant (A) is much higher than that of the other components (B), the reactant (A) selectively diffuses into a catalyst particle through a membrane. Undesired reactions or the adsorption of poisons on the catalysts can be prevented. In the second case, the reaction has a hmited yield or is selectivity controlled by thermodynamics. The selective removal of the desired product from the catalyst particle gives enhancement of selectivity when the diffusivity of one product (R) is much greater than that of the other products (S). 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

Kameyama, T.. K. Fukuda, M. Fujishige, H. Yokokawa and M. Dokiya and Y. Kotera. 1981. Production of hydrogen from hydrogen sulfide by means of selective diffusion membranes Adv. Hydrogen Energy Prog. 2 569 19. 

In silicate minerals, typical D values at 1200°C are of the order m /s, and typical activation energy is about 300kJ/mol. The diffusivity of cations depends on the charge of the cations. Highly charged cations (such as Si and Zr ) diffuse more slowly and have higher activation energy than univalent or divalent cations (such as Fe +-Mg + interdiffusion). Tables l-3b, l-3c, and Appendix 4 list selected diffusivities in silicate minerals. 

Compound Sorption coefficient S (mg-mg ) Diffusion coefficient D (m -s ) Sorption selectivity Diffusion selectivity Overall permselectivity... 

Hashimoto, A., Kobayashi, M., Kamijoh, T., Takano, H. and Sakuta, M. Properties of PECVD SiOxNy films as selective diffusion barrier. Journal of the Electrochemical Society 133, 1464— 1467 (1986). 

In freeze drying, upon water crystallization, the non-frozen solution is viscous and the diffusion of flavors is retarded. Upon beginning of freeze drying, the surface of this solution becomes an amorphous solid in which selective diffusion comes into play. 

At the earlier conferences on molecular sieves, in London in 1967 and in Worcester, Mass. (U.S.) in 1970, attention was focused exclusively on the zeolites. In an etymological sense (separation of molecules according to size by selective diffusion through pores of appropriate diameter) the field of molecular sieves must not be restricted to the tectosilicates with porous framework. This point is developed by R. M. Barrer in Chapter 1, where he gives a broad review of those compounds which can exhibit molecular sieve properties. 

King. C.J. "Spray Drying Food Liquids and the Retention of Volatiles (Selective Diffusion)," Client. Tint Pmgn ss. 33 [June 1990). 

There are several future trends for the development of passive sampling techniques. The first is the development of devices that can be used to monitor emerging environmental pollutants. Recently, attention has shifted from hydrophobic persistent organic pollutants to compounds with a medium-to-high polarity, for example, polar pesticides, pharmaceuticals, and personal care products.82 147148 Novel materials will need to be tested as selective receiving phases (e.g., ionic liquids, molecularly imprinted polymers, and immunoadsorbents), together with membrane materials that permit the selective diffusion of these chemicals. The sample extraction and preconcentration methods used for these devices will need to be compatible with LC-MS analytical techniques. 

For more realistic modeling, it is necessary to consider the surface relaxation of the growing film due to the diffusion of the deposited particles. This process is not a specific feature of ALD and can be considered independently in the model in the same way as in PVD modeling similarly to the surface tension in liquids, selective diffusion [95] is introduced leading to the relaxation of the surface and the reduction of its area. 

Figure 16.7 Schematic representation illustrating the selective diffusion of dopamine over glutamic acid and tyrosine into the mesopores of PLA-coated MSN-based fluorescence sensor system.

A liquid membrane can be prepared by emulsifying an aqueous solution in an organic liquid, then adding the emulsion to another aqueous solution. In this way, the organic liquid segregates the solutions but allows selective diffusion of solutes across it. Similarly, oil/water/oil type emulsions can be formed in which the liquid membrane is the water encapsulation layer. Very high rates of mass transfer can be achieved because of the large effective membrane surface area represented by the emulsion droplets. 

FIGURE 38 Selective diffusion across ion-exchange membranes. (a) Anion exchange, and (b) cation exchange. Metal cations are designated by M+, anion A-, proton H+, and the fixed charges in the membrane by + and -. 

TABLE 5.7 Selected Diffusion Coefficients at 25 °C in Water and Acetonitrile... 

T. Kameyama, M. Dokiya, M. Fujihige, H. Yokokawa and K. Fukuda, Production of Hydrogen from Hydrogen Sulfide by Means of Selective Diffusion Membranes , Int. J. Hydrogen Energy, 8 [1] 5-13 (1983). 

Monsanto and Ube (Japan) developed membrane processes for purification of hydrogen gas mixtures. This process is based on the selective diffusion of hydrogen through semi-permeable membranes in the form of hollow fibers. The Monsanto PRISM separator process (owned by Air Products as of 2004) uses a polysulfone fiber whereas Ube uses an aromatic polyimide fiber.46... 

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