Ideal selectivities

Equipment Selection Ideally, selection of equipment to produce a gas-in-hquid dispersion should be made on the basis of a complete economic analysis. The design engineer and especially the pilot-plant engineer seldom have sufficient information or time to do... 

The conclusion above is valid for ideally selective membranes. Real membranes in most cases have limited selectivity. A quantitative criterion of membrane selectivity for an ion to be measured, relative to another ion M +, is the selectivity coefficient The lower this coefficient, the higher the sefectivity wifi be for ions relative to ions An electrolyte system with an imperfectly selective membrane can be described by the scheme (5.16). We assume, for the sake of simplicity, that ions and have the same charge. Then the membrane potential is determined by Eq. (5.17), and the equation for the full cell s OCV becomes... 

Permselectivity predicted by ideal selectivity at high temperature and low concentration... 

In Chapter 1, we began the discussion of selectivity. In brief review, when an ideal selective layer is exposed to a mixture of molecules, it interacts with those for which the layer is selective and rejects the other, interfering molecules. The selective layer itself can be homogeneous or can contain specific binding sites embedded in a matrix. An outline of the thermodynamics governing the equilibrium binding was given in Section 1.1. 

The subscript 0 indicates that this flux is for solvent A without the presence of rejected solute B. This model is appropriate for membranes that have straight pores however, such membranes are not typical of most industrial membranes. Not only are the rate limiting pores in the actual working skin layer tortuous, but a complex porous support layer is generally present that supports the working skin layer. In well-made sieving membranes, the porous support is ideally invisible to the transport process, as is illustrated in Fig. 2. The support, therefore, simply serves as a scaffold for the ideal selective layer. Formation of such structures requires some care, but technology exists to achieve this requirement as is described elsewhere (Koros and Pinnau, 1994). 

This equation shows that we should ideally select a stationary phase with a polarity that is very different from that of the solute. Indeed, the recommendation to use normal phase chromatography (high 5,) for non-polar solutes (low 5() and reversed phase chromatography (low 8) for the separation of polar solutes (high 8 is not new. However, this rule of thumb is much too simple. A complication is caused by the availability of appropriate mobile phases. For instance, to satisfy eqn.(3.30) for the elution of non-polar solutes (5, 7) from a silica column (5, 16), a mobile phase with 8mx -2 would be required. 

Nanocrystalline CuO-Ce02 catalysts show nearly ideal selectivity and promising activity for the removal of CO from reformed fuels by selective oxidation (Avgouropoulos and loannides, 2003). The ultrafine nanocrystalline CuO-Ce02 catalysts were made by a urea-nitrate combustion method. CuO-Ce02 catalysts are inactive for H2 oxidation at temperatures up to ca. 120 °C and CO does not influence the rate of hydrogen oxidation. The addition of small amount of H2O shows adverse influence... 

In summary, one can see that separation selectivity for gas and vapor molecules depends on the category of pores (mesopores, supermicropores, and ultramicropores) and on the related transport mechanisms. Either size effect or preferential adsorption effect (irrespective of molecular dimension) is involved in selective separation of multicomponent mixtures. The membrane separation selectivity for two gases is usually expressed either as the ratio between the two pure gas permeation fluxes (ideal selectivity) or between each gas permeation flux measured from the mixture of the two gases (real selectivity). More detailed information on gas and vapor transport in porous ceramic membranes can be found in Ref. [24]. 

Permeability to Vapors of Propylene and Propane It can be seen from the data shown in Figure 9.19 as well as Tables 9.6 and 9.7 that sufficiently high permeability to propylene and the highest values of ideal selectivity p CsHs/CsHg are found in... 

Permeability Coefficients of Ethylene and Propylene, as well as Values of Olefin/Paraffin Ideal Selectivity for Several Polyimides... 

In the ideal molecular sieving regime the interaction of the permeating species with the zeolite is minimal, and the separation takes place simply because the size and shape of some of the molecules prevent them from entering the membrane pores or move across them with substantial velocity [2]. In that regime adsorption does not play an important role, and, therefore, separation selectivity (ratio of permeances between species in the mixture) should coincide with ideal selectivity (ratio of single-gas permeances). 

Without knowledge of the actual global optimum value, it seems reasonable to initially set p = 3.0 and (Xgjjp) = 1.00 for ideal selectivity. Because the actual global optimum SEL = 0.5804, this initial estimate of 4>(x ) causes nearly all... 

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