Solubility selectivity

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... 

In the tabulated data presented in this handbook the aqueous solubilities selected are generally those estimated to be of the non-ionic form unless otherwise stated. 

Xenon + water. The solubility data of Potter and Clynne ((5) and of Stephan, Hatfield, Peoples and Pray (10 ) were used to estimate the mole fraction solubility at one atmosphere xenon pressure at the higher temperatures. The two sets of data were combined with the 20 solubilities selected from five papers by Battino (2) in a linear regression. Figure 6 shows the data, and Battino s equation and the three constant equation. The three constants for the tentative equation for use between the temperatures of 350 and 600 K are in Table V. The Stephan et al. solubility value at 574 K was not included in the regression. 

The method of protein hydrolysis was important acid hydrolysis caused destruction of tryptophan but alkaline treatment gave even greater losses of other amino acids especially cystine. The amino acids were usually separated by then standard chemical procedures based on differences in solubility, selective precipitation by agents such as Reinecke salt (proline and hydroxyproline), or flavianic acid (arginine). 

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.

The solubility selectivity of a membrane for a specific gas pair could be increased (in principle) by inducing specific interactions between the polymer and the more soluble component of the gas pair. For example, the substitution of certain polar groups in some rubbery polymers has been found to increase their solubility selectivity for CO2 relative to CH4 (Story and Koros, 1991 Koros, 1985). Unfortunately, the increase in the polarity of a polymer also tends to increase its chain packing density, and as a result, decreases the gas diffusivity in membranes made from that polymer. 

Supercritical fluids can be combined with polar cosolvent, which enhances the solubility selectivity and sensitivity of the environment to manipulated variables such as temperature and pressure. This... 

Equation 8 demonstrates that the ideal separation ac tor can be separated into a so-called "s lu illty selectivity", [S /Sg], and a "mobility selectivity", ID /Db]. These two ratios are also reported in Table I. Evidently, the contribution of the "mobility selectivity" is the dominant factor for all of the polymers considered except cellulose acetate in which the opposite is observed. The CO2 plasticization tendency of cellulose acetate may, in fact, be related to this polymer s apparent high "solubility selectivity". Clearly, the available data do not justify more than a tentative suggestion at this point that high "solubility selectivity" such as that seen in cellulose acetate may be associated with a tendency to be plasticized with a subsequent loss in glass-like selectivity. 

Poly(phenylene oxide) is an example of a material that might benefit from such a blending approach if an appropriate miscible "solubility selective" polymer could be discovered. As shown in Figure 2 and Table I, PPO is highly permeable to C02 and has a respectable mobility selectivity equal to 6.9 for the CO2/CH4 system. 

The overall selectivity of PPO for this system is, however, rather low due to the low solubility selectivity (the lowest in Table I). 

If blending could raise the solubility selectivity from two to four without a serious loss in CO2 permeability, the resultant material would be quite attractive as a membrane for CO2/CH4 separations. 

If suitable nonmigrating additives can be found to provide sufficiently high values of solubility selectivity, one could, in principle. 

The unique behavior of cellulose acetate was supported recently by Koros (42). He found that of all glassy polymers Investigated, cellulose acetate had the highest solubility selectivity, defined as the ratio of the solubilities of two gases C0 and CH. Its extremely high solubility value of 8.9 measured at 35°C and 20 atm exceeds the values of the other polymers listed below by factors of... 

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