Pure gas permeability

Transport properties have been studied before and after Si deposition using a rig similar to the one for catalytic testings (Figure 2). Pure gas permeabilities (H2, He, N2, normal and isobutane) were studied by measuring the flux passing though the membrane as a function of temperature and pressure for a constant transmembrane differential pressure (no sweep gas). 

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... 

Table 8.1 are measured with pure gases the selectivity obtained from the ratio of pure gas permeabilities gives the ideal membrane selectivity, an intrinsic property of the membrane material. However, practical gas separation processes are performed with gas mixtures. If the gases in a mixture do not interact strongly with the membrane material, the pure gas intrinsic selectivity and the mixed... 

When the gas or vapor feed stream contains a component that is highly soluble in the polymer membrane and causes plasticization, then the selectivity as defined by Equation 4.6 will depend on the partial pressure or the amount of the plasticizing component sorbed into the membrane. Furthermore, pure-gas permeation measurements are generally not a good indicator of the separation performance, and mixed-gas permeation measurements will be needed [21-23]. Often, the mixed-gas selectivity is less than predicted from pure-gas measurements [8] however, the opposite has been observed [24], Competitive sorption effects can also compromise the prediction of mixed-gas behavior from pure-gas measurements [25], For gas pairs where each component is less condensable than C02, like 02/N2, it is generally safe to conclude that the selectivity characteristics can be accurately judged from pure-gas permeabilities at all reasonable pressures. When the gas pair involves a component more condensable than C02, plasticization is likely to be a factor and pure-gas data may not adequately reflect mixed-gas selectivity. When C02 is a component, the situation depends on the partial pressures and the nature of the polymer. 

The problem with use of polymeric membranes in this application is plasticization, leading to much lower selectivities with gas mixtures than the simple ratio of pure-gas permeabilities would suggest. For this type of separation, a Robeson plot based on the ratio of pure-gas permeabilities has no predictive value. Although membranes with pure-gas propylene/propane selectivities of 20 or more have been reported [43, 44], only a handful of membranes have been able to achieve selectivities of 5 to 10 under realistic operating conditions, and these membranes have low permeances of 10 gpu or less for the fast component (propylene). This may be one of the few gas-separation applications where ceramic or carbon membranes have an industrial future. 

P/l is also referred to as permeability flux and expressed as (m (STP)/(m bar h)). Ap, is the partial pressure difference of i across the membrane measured in pascals or bars. This equation shows that the flux through the membrane is proportional to the pressure difference across the membrane and inversely proportional to the membrane thickness. Por selectivity between gas components the Equations 4.3 and 4.4 are referred to. The ideal separation factor, a (Equation 4.3), may be expressed by the ratio of the pure gas permeabilities for the individual components i and j. 

FIGURE 9.15 Pure-gas permeabilities of an isotropic PDMS film as a function of reciprocal temperature. Feed pressure 4.4 atm permeate pressure 1 atm. (From Pinnau, I. and He, Z., J. Membr. Sci., 244, 227, 2004. With permission.)... 

Table IV Pure gas permeability rates of commercial reverse osmosis membranes at 25°C and 40 bar...

The existence of a methane peak is not considered a phenomenon that will always occur with intermediately permeable gases in multicomponent mixtures. Rather, the peak is thought to be the result of a combination of factors. These factors include composition of the feed mixture, pure-gas permeabilities, and the internal reflux ratio. For instance. Figure 3 indicates that the intermediate-gas composition profile will steadily decrease in a stripper 1.0 m long, but otherwise identical to the column used in this study, fed with a 63.6% N2 - 32.3% CH - 4.1% CO2 mixture under similar total reflux conditions. The presence of an intermediate peak, however, is reminiscent of multicomponent distillation profiles and raises the possibility of withdrawing a side stream enriched with an intermediate gas. 

Cas Pure gas permeability (Barrer) Mixed gas permeability Mixed gas selectivity... 

Olefin/paraffin selectivities of 5 to 10 appear quite modest compared to the claims of some reports [32, 33]. However, much of the literature selectivity data has been calculated from the ratio of the permeabilities of pure olefin to pure paraffin. Olefin/paraffin selectivities measured with gas mixtures under conditions likely in a real process show that using pure gas permeabilities overestimates the membrane selectivity by a factor of 2 to 10. Therefore, it will be some time before olefin/paraffin-selective membranes are used in ethylene plants, although some nearer-term applications exist in petrochemical and refinery operations. 

The pure gas (99.99% or higher purity) transport properties were tested with an instrument (GKSS, Geesthacht, Germany) with eonstant permeate volume described elsewhere [14], The short response time of the instrument allows one to record transient permeation behaviours of less than 1 second. The pure gas permeability is the amount of gas permeating in the unit time, multiplied by the thickness of the membrane and normalized for the membrane surface and the pressure gradient. The recorded pressure vs. time plots were used to derive diffusion coefficients from the initial transient permeation, and permeability at the steady state. The time-lag 9 is the intercept of the linear part of the pressure vs. time curve with the axis of time. In a homogeneous membrane in which the solubility of a gas obeys Henry s law, its diffusion coefficient D can be calculated from the ratio ... 

Polymer Pure Gas Permeabilities (Barrer) Ideal Select vltles (a A/s) ... 

D.F. Sanders, R. Guo, Z.P. Smith, K.A. Stevens, Q. Liu, J.E. McGrath, D.R. Paul, B.D. Freeman, Influence of polyimide precursor synthesis route and ortho-position functional group on thermally rearranged (TR) polymer properties pure gas permeability and selectivity, J. Membr. Sci. 463 (2014) 73-81. 

In this equation the permeabilities of gases are measured with the gas mixture. The selectivity calculated from the ratio of pure gas permeabilities is sometimes called the ideal gas selectivity and is often higher than the actual selectivity value. Permeability Pi can be expressed as the product of two terms. One, the diffusion coefficient Z), reflects the mobility of the individual molecules in the membrane material the other, the Henry s law sorption coefficient ki, reflects the number of molecules dissolved in the membrane material. Thus, equation (6) can also be written as... 

Simple Dual-Mode Transport Models. As a direct extension of the dualmode sorption model, it is convenient to treat pure-gas permeability as the siun of two terms, which are characteristic of the two sorbed penetrant populations. Petropoulos first developed such an expression in terms of the chemical potential gradient of the sorbed gas (100). For the case where there is a vacuum downstream and a pressure pa2 of component A upstream, the following expression results ... 

This model results in a system of nonlinear differential equations that must be solved numerically. Figure 25 (82) shows pure-gas permeability data for CO2... 

TABLE 16.8 Pure Gas Permeabilities and Selectivities of Selected Amorphous Perfluorinated Glassy Polymers at 35°C... 

The unusual solubility of gases and vapors in perfluoropolymers has several applications relevant to membrane gas separations. Perfluoropolymers have solubility selectivities that are significantly different from those of hydrocarbon-based polymers. The amorphous perfluoropolymers can be fabricated into thin, high-flux composite membranes, which possess the excellent chemical and thermal stability. Typical reported pure gas permeabilities and selectivities of these fine amorphous perfluoropolymers are shown in Table 16.8 [33]. 

Table I. Pure Gas Permeability and Selectivity Properties of Selected Polymers...

Pure gas permeability and selectivity data at 25°C for a 225 pm thick, unfilled, PDMS film. Pfeed = 4.4 atm, Ppermeatc 1 utm for all penetrants except -C4H,o. -C4Hjo permeability reported at 25°C, pfeed = 1-34 atm, Ppermeate = 1 atm. 

The journal and patent literature was searched to identify the 87 polymers used in this work. They include members from the polymers, their structure, and gas transport properties are provided in the Appendix. Carbon dioxide is reported to permeate many polymers by a dual mode transport mechanism (13). It has also been noted that COj has a plasticizing effect on some polymers leading to decreased permselectivity at high pressure (14). For these reasons we have preferentially used pure gas permeability data obtained at 10 atm CO2 and 35°C. 

After steady state was reached, the pure gas permeabilities of these films were measured. Independent measurement of nitrogen and oxygen resulted in values that were identical to those reported above. Further, these values are consistent with those reported in the product literature for this polymer (75). 

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