In sorption and diffusion

As the mobile phase moves through the capillary containing the sorbent under the effect of this electro-osmotic flow (EOF), sample components partition between the two phases in sorption and diffusive mechanisms characteristic of liquid chromatography. Ions in the sample move both under the influence of EOF and by their added attraction toward the oppositely charged electrode (electrophoresis). Uncharged components, on the other hand, move only under the influence of EOF. Thus, sample components, in general, separate by chromatographic and, sometimes, electrophoretic processes. 

The excellent chemical resistance has suggested applications for ultra highly drawn LPE in the form of insulating tapes for electrical cables, or as pipes produced by hydrostatic extrusion or die drawing. Table 4 summarises the behaviour of oriented pipes with regard to toluene and tetrahydrofuran (THF) at 50 C. There are clearly dramatic improvements in sorption and diffusion resistance, and this has been confirmed by limited studies of other solvents, including methanol, 4 methyl pentan-2-one and xylene. 

Fig. 38. Permeability as a function of molar volume for a mbbery and glassy polymer, illustrating the different balance between sorption and diffusion in these polymer types. The mbbery membrane is highly permeable the permeability increases rapidly with increasing permeant size because sorption dominates. The glassy membrane is much less permeable the permeability decreases with increasing permeant size because diffusion dominates (84).

However, it has been concluded from sorption and diffusion experiments that plutonium exists largely in the tetravalent state (53) and clearly not as Pu(V), in the intermediate pH-range under oxic conditions and at low carbonate concentration. This would be representative of many groundwaters and also in agreement with the calculated curves of Figure 2. 

For gas and vapor systems, by combining the laws of sorption and diffusion in the sequence (l)-(3), general permeation equations are obtained. For sheet membrane samples of polymers above Tg, if the definition is made that permeation coefficient Q = Ds,... 

The influence of the CD content in the membrane and the n-PrOH respectively p-xylene content in the feed mixture on the separation factors and sorption and diffusion selectivities of the CD/PVA membranes for the n-PrOH/I-PrOH and p-xylene and o-xylene mixtures by evapomeation are presented in tables 12 and 13. 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

Whereas batch equilibrium tests are designed to study equilibrium sorption of solid phase particles with various pollutants, singly or in combination with other pollutants, solid phase column-leaching tests study both sorption and diffusion of organic pollutants through the subsurface environment [10,11,127, 141,142]. 

Hietala, S., Maunu, S. L. and Sundholm, E. 2000. Sorption and diffusion of methanol and water in PVDE-y-PSSA and Nafion 117 polymer electrolyte membranes. Journal of Polymer Science Part B Polymer Physics 38 3277-3284. 

Peterson MS, Lion LW, Shoemaker CA (1988) Influence of vapor phase sorption and diffusion on the fate of trichloroethylene in an unsaturated aquifer system. Environ Sci Technol 22 571-578 Petersen LW, Moldrup P, El-Farhan YH, Jacobsen OH, Yamaguchi Y, Rolston DE (1995) The effect of moisture and soil texture on the adsorption of organic vapors. J Environ Qual 24 752-759 Pignatello JJ (1989) Sorption dynamics of organic compounds in soils and sediments. In Sawhney BL, Brown K (eds) Reactions and movement of organic chemicals in soils. Soil Sci Soc Amer Spec Publ 22 45- 81... 

Tachi, Y., Shibutani, T., Sato, H. Yui, M. 2001. Experimental and modeling studies on sorption and diffusion of radium in bentonite. Journal of Contaminant Hydrology, 47, 171 -186. 

From Fig. 19.3a-c, and as opposed to purely sorption controlled processes, it can be seen that during pervaporation both sorption and diffusion control the process performance because the membrane is a transport barrier. As a consequence, the flux 7i of solute i across the membrane is expressed as the product of both the sorption (partition) coefficient S, and the membrane diffusion coefficient Di, the so-called membrane permeability U, divided by the membrane thickness f and times the driving force, which maybe expressed as a gradient of partial pressures in place of chemical potentials [6] ... 

In comparison with adsorptive/absorptive techniques for aroma recovery from bioconversions, the disadvantage of pervaporation is the fact that both sorption and diffusion determine the overall selectivity. While the sorption selectivity is very high (equal to that of adsorptive/absorption), the diffusion selectivity favours water owing to the simple fact that water is a smaller molecule than aroma compounds and thus sterically less hindered during diffusion (Table 19.1). The overall (perm)selectivity P=SD) is therefore lower than in strictly sorption controlled processes, although it is still favourable compared with that for evaporation. This shortcoming compares, however, with operational advantages of pervaporation as outlined before. 

In this review, we focus on the information at an atomic/molecular level that is obtainable via the different techniques. The precise methods and techniques used are not extensively discussed instead we summarize the relevant details and direct the reader toward key references. Nor do we review the potentials that are used in the classical simulations of sorption and diffusion. Derivation and evaluation of these parameters require extensive comparison with detailed spectroscopic data and are beyond the scope of this work. Similarly, the volume of experimental results that may be used in comparison to the calculations is vast. We use representative data taken largely from reviews or books. 

In an MD study of methane sorption and diffusion in silicalite, Nicholas et al. (67) identified favorable sites for sorption. From the MD calculations, the time-averaged position of the center of mass of the methane molecule was plotted. Energy minimization calculations were then performed, locating the methane molecule at positions where the MD calculations predicted they spent the most time. Each channel intersection region was found to contain two sites that are minima for methane-zeolite interactions. These two sites are separated by a translation parallel to the straight channel... 

Scow, K. M. (1993)- Effect of sorption-desorption and diffusion processes on the kinetics of biodegradation of organic chemicals in soil. In Sorption and Degradation of Pesticides and Organic Chemicals in Soil, ed. D. M. Linn, T. H. Carski, M. L. Brusseau F-H. Chang, pp. 73-114. Madison, WI Soil Science Society of America, American Society of Agronomy. 

Sorption capacity is one of the major properties used for industrial applications of zeolites. H. Lee reviews the aspects of zeolites used as adsorbents. The other papers in the section deal with the theory of sorption and diffusion in porous systems, the variation of sorption behavior upon modification, and the variation of crystal parameters upon adsorption. NMR and ESR studies of sorption complexes are reported. H. Resing reviews the mobility of adsorbed species in zeolites studied by NMR. 

In the case of sorption and diffusion, the two-dimensional channel systems of the Cmmm and Immm structures should provide higher rates of diffusion than would the one-dimensional channel systems of the Cmcm and Imcm structures. This should be true also for mixtures containing the Cmmm and Immm structures as separate crystals or as intergrown regions or stacking faults in the Cmcm structure. This effect of such stacking faults was first noted by Gard (18). 

Sorption and Diffusion of Light Hydrocarbons and Other Simple Nonpolar Molecules in Type A Zeolites... 

The results of experimental studies of the sorption and diffusion of light hydrocarbons and some other simple nonpolar molecules in type-A zeolites are summarized and compared with reported data for similar molecules in H-chabazite. Henry s law constants and equilibrium isotherms for both zeolites are interpreted in terms of a simple theoretical model. Zeolitic diffusivitiesy measured over small differential concentration steps, show a pronounced increase with sorbate concentration. This effect can be accounted for by the nonlinearity of the isotherms and the intrinsic mobilities are essentially independent of concentration. Activation energies for diffusion, calculated from the temperature dependence of the intrinsic mobilitieSy show a clear correlation with critical diameter. For the simpler moleculeSy transition state theory gives a quantitative prediction of the experimental diffusivity. 

Although the theoretical models presented in this paper are simple idealizations of complex systems, the theory provides a useful understanding of many aspects of the sorption and diffusion of simple nonpolar molecules in type-A zeolites and in H-chabazite. The extent to which such theories are applicable to other systems has not yet been investigated. 

The discussion above explains why basic information on sorption and diffusion under the reaction conditions, especially at elevated pressures, is required for kinetic and mass- and heat- transfer modelling of catalytic polymerization reactors. If such information is sufficiently available, one should be able, for example, to compare the kinetics of gas-phase and slurry-processes directly by taking into account both gas solubilities in swollen polymers and the hydrocarbons used in slurry processes. 

Dual-Mode Gas Sorption and Diffusion in Glassy Polymer Membranes. 97... 

Dual-Mode Ionic Sorption and Diffusion in Charged Polymer Membranes.. 109... 

Sorption and diffusion of a given penetrant in a homogeneous membrane may, in general, be described by l)... 

Membrane-penetrant systems, whose sorption and diffusion properties can be described by Eqs. (5)-(7) with N = 2 ( dual mode sorption and diffusion models ) have attracted much interest. The most important examples of such systems are considered in the next two sections. 

Earlier work on the application of the concept of dual mode sorption and diffusion to glassy polymer-gas systems has been reviewed in detail 6) and important aspects of more recent work have been dealt with in more recent reviews 7 10). Eq. (5) was first applied by Michaels et al U). Sorption in the polymer matrix and in the specific sorption sites was represented by linear (Henry s law) and Langmuir isotherms respectively so that Sj in Eq. (5) is given by... 

The sorption and diffusion behaviour of gas mixtures is of particular interest from the point of view of membrane gas separation, which is steadily gaining in importance by virtue of its low energy requirements. On the basis of the dual mode sorption model, one may reasonably expect that sorption of a binary gas mixture A, B in the polymer matrix will exhibit little gas-gas interaction and hence will tend to occur essentially additively. In the Langmuir-like mode of sorption, on the other hand, there will be competition between A and B for the limited number of available sites. These considerations led 67) to the following reformulation of Eqs. (8) and (9)... 

It is particularly interesting and instructive to note that application of Henry + Langmuir dual-mode sorption and diffusion models is not confined to glassy polymer-gas systems. Sorption and transport of high affinity ionic species, exemplified by anionic dyes, in charged polymers, exemplified by polyamides at low pH, has been treated in the same way. These systems are of considerable importance both from the bio-mimetic and from the textile processing point of view, but have received limited atten-... 

Table 3. Sorption and diffusion parameters for Dyes I—III in Nylon 6 following the treatment of Eq. (22)...

If the component phases denoted by A and B are of sufficient, but still microscopic, size (cf. introductory section) and do not interact appreciably, their individual sorption and diffusion properties may be deduced from measurements on the pure bulk phases. Then, the overall solubility coefficient is given by an additive relation analogous to Eq. (5), except that the volume fractions vA, vB( = 1 — vA) of the respective components in the membranes must be taken into account ... 

The discussion given in this section shows that non-homogeneity of membrane electrical properties is widespread and markedly influences ionic sorption and diffusion behaviour. Proper understanding of these effects is, therefore, important and may be expected to contribute materially to the design of more highly permselective membranes. 

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