Separation factor for gases

Pores Even porous membranes can give very high selectivity. Molecular sieve membranes exist that give excellent separation factors for gases. Their commercial scale preparation is a formidable obstacle. At the other extreme, UF,3 separations use Knudsen flow barriers, with aveiy low separation factor. Microfiltration (MF) and iiltrafiltra-tion (UF) membranes are clearly porous, their pores ranging in size from 3 nm to 3 [Lm. Nanofiltration (NF) meiTibranes have smaller pores. 

The separation factor for gases in mixture ay (Equation 4.4) is expressed by the mole fractions of the components in the feed (x) and the permeate (y), respectively ... 

In mesoporous membranes the maximum obtainable separation factor for non-condensable gases is limited to the Knudsen separation factor. For adsorbing gases below their critical point, surface flow can play an important role and high values of the permeation and of the separation factor can be obtained in some cases up to temperatures of 300°C. 

Experimental measurements of permeability and selectivity for a large number of gas-polymer systems indicate that the mobility selectivity controls the intrinsic separation factor for nonpolar gases [27, 38]. To impart mobility selectivity, the polymer matrix must possess molecular size gaps sufficiently small and uniform in size to discriminate between molecules that may differ in size by less than 1 A, for example oxygen and nitrogen differ in size by 0.2 A. 

The permeability, diffusion, and solubility coefficients for the gases as well as the ideal separation factors for gas pairs have been determined. A poly(imide-siloxane) with 20% siloxane content shows the best compromise with very high permeability and a still high permselectivity for the CO2/CH4 gas pair [12]. 

In the case of the PDMS gas, the membrane permeability of CO2 decreased, but the selectivity of CO2 over CH4 was found to be remarkably improved irrespective of the plasma gas used (NH3, Ar, Nj, O2). The nitrogen plasma treatment seemed to give better selectivity than the ammonia plasma (Matsuyama et al. 1995). The NH3 and N2 plasma treatment of the dense PE (Nakata and Kumazawa 2006) and PP (Teramae and Kumazawa 2007) membranes increased both the permeation coefficient for CO2 and the ideal separation factor for CO2 relative to N2. The effects of both plasma gases are very similar. 

The separation factor for pure water membrane was 29.9. This value agreed with 29.0 which was calculated from the solubilities and the diffusivities of these gases in water (DtH4.w = l.VOxlO cm /s (77), Hch4w = 1.34x10-3 mol/(dm3atm) (78)). 

Scheme (2) is very effective in the absenee of noneondensable gases. Figure 11.10 shows how the heat transfer area ean be ehanged by flooding the condenser tubes with liquid. The reflux flow essentially sets the separation factor for the tower. If vapor flow into the condenser exceeds liquid flow out, condensate will rise to eover more heat transfer surface. This will cause a pressure rise, whieh in turn will reduce the heat input through the pressure controller. Beeause of the rapid response of vapor flow to heat input, this is a fairly fast loop. 

Fig. 10.9 Effect of number of carbon layers on permeance and separation factor for single gases. (From [6])...

Gas permeability measurements were performed on the blended films as presented in Fig. 33.14. The blend shows a definite increase in permeability for all gases tested, relative to polyimide and the base form of polyaniline [60J. Ordinarily, there is an inverse relationship between permeability of two gases and the separation factor for those gases [61-64]. However, this blend does not hold to this axiom. Figure 33.15 illustrates the separation factors for the blend and the homopolymers. The separation factors for the blend are comparable to those of polyaniline (as-cast) for H2/N2 (a = 200) and O2/N2 (a = 9) and closer to that of polyimide for CO2/CH4 (a = 58). Thus, this blend appears to have achieved an improved combination of properties compared to its parent polymers, with enhanced permeability and good selectivity. 

Membrane separation of gases has developed into a unit operation of importance for various separations of gas pairs (O2/N2 CO2/CH4 H2/N2 H2/CH4, He/air). It has been observed that limitations exist for these separations with polymeric membranes, where an upper bound exists for the separation factor versus the permeability of the more permeable gas [193,194]. This is illustrated in Fig. 6.20 for O2/N2, where the literature data of log of the separation factor for O2/N2 is plotted versus the log of the O2 permeability. 

Table 4.1.1. Henry s law constants and separation factors for acid gases in absorbents ...

Table 25-1 Claimed gas separation factors for industrially important gases,...

Kruczek and Matsuura in their studies on characterization of gas separation properties of SPPO films have reported similar trends for permeabilities and permselectivities for O2 and N2. They have also reported CO2/CH4 permeability ratio of 43 corresponding to CO2 permeability of 11 Barrer for SPPO. The authors have conducted a detailed study on the effect of mono-, di- and trivalent cation substitutions of SPPO membranes on their gas separation performances. The thus substituted polymers were more permeable to gases than the hydrogen form of SPPO without any loss in the permeability ratios. The improved gas transport properties (separation factor for O2/N2 of 7.65 corresponding to 67.3% of O2 in the permeate when the membrane was used for oxygen enrichment of air) of SPPO with a degree of substitution of 18.5% and in the Mg " form for O2/N2 gas pair placed the polymer above the upper-bound line. 

The steady state separation factor (a) for the two gases (C02 and CH4) is defined as the ratio of their individual permeabilities. 

There are few studies in literature reporting pure gas permeabilities as well as separation factors of mixtures. Vuren et al. (1987) reported Knudsen diffusion behavior of pure gases for y-alumina membranes with a mean pore radius of 1.2 nm. Separation experiments with a 1 1 H2/N2 mixture showed, that the theoretical Knudsen separation factor [of 3.7, Equation 6.4)j for this mixture could be obtained (Keizer et al. 1988 see also Figure 6.2). In Figure 6.2, the effect of process parameters is also demonstrated. The separation factor is a function of the pressure ratio over the membrane, which is the ratio of the pressure on the permeate-side to that on the feed-side. For pressure ratios approaching unity, which means the pressure on both sides of the... 

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