Diffusivity selectivity

Plasticization Gas solubility in the membrane is one of the factors governing its permeation, but the other factor, diffusivity, is not always independent of solubility. If the solubility of a gas in a polymer is too high, plasticization and swelhng result, and the critical structure that controls diffusion selectivity is disrupted. These effects are particularly troublesome with condensable gases, and are most often noticed when the partial pressure of CO9 or H9S is high. H9 and He do not show this effect This problem is well known, but its manifestation is not always immediate. 

Approaches to make a polymeric membrane selective to C02 attempt to enhance the solubility selectivity of the polymer material for C02 and reduce the diffusivity selectivity of the polymer that favors smaller hydrogen molecule. The permeability of a polymer membrane for species A, PA, is often expressed as (Ghosal and Freeman, 1994)... 

It follows from Equation 8.13 that aA/B can be expressed as the product of the diffusivity selectivity, DA/DB, and the solubility selectivity, SA/SB. Diffusion (or mobility) selectivity is governed primarily by the size difference between gas molecules and always favors smaller gas molecules. Solubility selectivity is controlled by the relative condensability of the gases in the polymer and their relative affinity for the polymer. Solubility selectivity typically favors larger, more condensable molecules. From Equation 8.13, it is seen that the product of gas mobility and solubility selectivity determines the overall membrane selectivity. It is clear that for a membrane to be C02 selective, it must have high diffusivity selectivity based on the affinity for C02 but it should be flexible enough to permeate larger molecules such... 

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 prerequisites of the evaluation of data characteristic of intracrystalline processes in the case of zeolite sorbents are discussed, along with the conditions under which diffusion can be compared to self-diffusion. Selected results of investigations carried out in the author s laboratory are given in order to demonstrate the consistency of sorption kinetic data with intracrystalline mobility data of single components on molecular sieves (HS). Various types of surface barrier which may influence the uptake rate are also described. 

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. 

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

The literature indicates that selectivity often can be improved, particularly with Ni and Pd catalysts by the use of promoters such as amines (ref. 34). Presumably, the amine competes for reactive sites with the alkenes and is effective if its adsorption constant lies between the constants of the competing alkenes. The effectiveness of the promoter is not diminished with the depletion of the more reactive alkene and is most useful with a supported catalyst where the concentration of molecules near a reactive site may be limited by pore diffusion. Selectivity would also improve if the promoter increases the rate of desorption of the alkenes (ref. 35). 

In many cases, but not all, the condensability of penetrants increases as size increases. This is the case for the gas pair i and j suggested in Figure 4.1. In the case shown there, the diffusion selectivity favors i over/ but the solubility selectivity favors j... 

Table 4.2 illustrates the various selectivity factors for some typical rubbery polymers, that is, silicone rubber, poly(dimethyl siloxane), and natural rubber, polyiso-prene, and a glassy polymer, polysulfone. Here, we consider the important 02/N2 pair and several pairs involving C02 that will be our focus later. In all the cases, the solubility selectivity is greater than unity and there is not a large difference between rubbery and glassy polymers. For most of these pairs, the diffusion selectivity is greater than unity, but there are some exceptions for C02/02 and C02/N2 that reflect... 

Table 4.3 shows the permselectivity characteristics of pure, semicrystalline PEO films [76]. The selectivity characteristics for 02/N2 are rather similar to those for silicone rubber and natural rubber shown in Table 4.2. However, the values of permselectivity for C02 relative to the various light gases shown are all much higher than Table 4.2 shows for the rubbery polymers listed there and even for polysulfone except for C02/CH4. Comparison of the data in Tables 4.2 and 4.3 makes it clear that this high permselectivity of PEO stems from its high solubility selectivity for C02 versus other gases this is augmented by modest values of diffusivity selectivity. Data in Table 4.4 for the C02/N2 pair illustrate that this effect can be translated into various block-copolymer structures when the PEO content is high enough to ensure it is the continuous phase. In fact, nearly all these materials have higher permselectivity and solubility selectivity for C02/N2 than does pure PEO (see Table 4.3) however, the diffusion selectivity for these copolymers is much closer to, or even less than, unity than seen for pure PEO. Furthermore, the copolymers all have much higher absolute permeability coefficients than does PEO.

Hg is diffused selectively into a p+-type substrate 11 by the use of a SiC>2 mask to from p-type islands 13. Next, boron ions are injected to form n+-type regions in the p-type islands. 

Even as an analytical instrument applied to flow systems and used for analysis of species present in abundance, the mass spectrograph may involve errors due to diffusive selection of lighter molecules and discrimination by the ion source of the more easily formed ions. ... 

Effects of coke on adsorption, diffusion, selectivities of catalytic reactions, and rates of catalytic reactions. 

During their technical application, molecular sieve catalysts are generally used under the conditions of multicomponent adsorption and diffusion. Selective measurement of the diffusivity of individual components is therefore of both theoretical and practical relevance. The traditional way to perform... 

Robeson [4] showed that there exists a trade-off relationship between selectivity and permeability for dense polymer membranes. This plot was later updated by Singh and Koros [9] (see Figure 4.1). Molecular transport of light gases in such membranes typically occurs by a solution diffusion mechanism (as discussed in Section 4.2.1). For a polymer membrane to be commercially considered for the removal of CO2 from H2, CH4, or air, both the CO2 permeability and selectivity must be competitively high. Since the gases in the mixture with CO2 often are smaller (H2) or about the same size as CO2, they may diffuse more rapidly through the polymers, and it follows that the diffusion selectivity will be <1. The only way... 

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