Permeability coefficients, helium

In these experiments, the measured helium flux through the membrane was less than the flux predicted on the basis of the average bulk concentrations. Consequently, the helium permeability coefficients calculated from observed membrane flux and the bulk partial pressures are lower than the pure gas values obtained by the membrane supplier or independently by us. At the same time, observed nitrogen coefficients are higher than predicted. 

The values of permeability coefficients for He, O2, N2, CO2, and CH4 in a variety of dense (isotropic) polymer membranes and the overall selectivities (ideal separation factors) of these membranes to the gas pairs He/N2,02/N2, and CO2/CH4 at 35°C have been tabulated in numerous reviews (Koros and Heliums, 1989 Koros, Fleming, and Jordan et al., 1988 Koros, Coleman, and Walker, 1992). Moreover, several useful predictive methods exist to allow estimation of gas permeation through polymers, based on their structural repeat units. The values of the permeability coefficients for a given gas in different polymers can vary by several orders of magnitude, depending on the nature of the gas. Thevalues oftheoverall selectivities vary by much less. Particularly noteworthy is the fact that the selectivity decreases with increasing permeability. This is the well-known inverse selectivity/permeability relationship of polymer membranes, which complicates the development of effective membranes for gas separations. 

The dependence of permeability, diffusion, and solubility coefficients on penetrant gas pressure (or concentration in polymers) is very different at temperatures above and below the glass transition temperature, Tg, of the polymers, i.e., for mbbery and glassy polymers, respectively. Thus, when the polymers are in the rubbery state the pressure dependence of these coefficients depends, in turn, on the gas solubility in polymers. For example, as mentioned in Section 61.2.4, if the penetrant gases are very sparsely soluble and do not significantly plasticize the polymers, the permeability coefficients as well as the diffusion and solubility coefficients are independent of penetrant pressure. This is the case for supercritical gases with very low critical temperatures (compared to ambient temperature), such as the helium-group gases, Ha, Oa, Na, CH4, etc., whose concentration in rubbery polymers is within the Heruy s law limit even at elevated pressures. 

Figure 8 shows that experimental and computed diffusion coefficients are in satisfoctory agreement over the entire temperature range studied of about 200 K. Henry s constant S of Helium sorption in the PC micro-structures were evaluated through Eq. (23) and the equation P = S D was employed to evaluate the permeability coefficients P, see Fig. 9. 

Fig. 9. Permeability of Helium in Bisphenol-A-polycarbonate. Empty symbols are experimental permeability coefficients P, filled squares are computed...

PC and the diffusion and permeability coefficients of Helium have been calculated. The transport coefficients in these 50 A microstructures agreed with those of 33 A micro-structures to within 30% it seems, therefore, that a length scale of several dozen angstroms is sufficient for evaluating the solute s transport coefficients. Similar conclusions can also be drawn for the MD simulations Changing from a 20 A to a 30 A simulation cell did not significantly alter diffusion coefficients of O2 in PIB [49]. 

In our calculations, we used the value of 1.22 A for the Van der Waals radius of Helium. It came out that the point corresponding to the value of 1.22 A clearly goes out of general dependences both of permeability coefficient and of diffusion coefficient for all four polymers. That point remains on the general dependence orrly in case when the Van der Waals radius of Helium is taken equal to 1.68 A. Similar results were obtained for Helium previously [39]. The value of 1.68 A obtained by us for the Van derWaals radius of Helium is close to the effective diameter (1.78 A) calculated by other authors [58]. 

FIGURE 3.35 Dependence of permeability coefficients P (left) and diffusion coefficients D (right) on the fractional accessible volume (FAV) for polymer 5 of the fourth series of polyimides (He - when the Van der Waals radius of Helium atom was taken equal to 1.68 A). 

In a small cell of the type shown in Figure 6.3.32, feed gas mixture containing 50% helium and 50% methane (mole %) is introduced at 17 atm. The helium permeability coefficient through the nonporous membrane has the value 240 (units) and that for methane is 3 (units). The permeate pressure is quite low. Calculate the composition of the permeate side gas emerging at the beginning of the process. (Ans. x ep = 0.987.)... 

Refractive index dn/dt Below Fg Above Tg Weatherability Optical transmission Stress optical coefficient Infrared spectra Abbe number Permeability coefficients Oxygen Nitrogen Carbon dioxide Water vapor Helium Hydrogen Methane Etbane Ethylene... 

The studies where the permeability coefficients were measured at gas pressures changing in a reasonably wide range revealed them to be pressure dependent, typically decreasing upon the pressure increase. The exceptions are helium and hydrogen where no pressure dependence has been reported. For some other permanent gases (e.g. nitrogen and oxygen) pressure dependencies of permeabilities are also rather weak. 

Verhallen P.T.H.M., Oomen L.J.P., v.d.Elsen A.J.J.M., Kruger A.J., and Fortuin J.M.H. (1984) The diffusion coefficients of helium, hydrogen, oxygen and nitrogen in water determined from the permeability of a stagnant liquid layer in the quasi-steady state. Chem. Eng. Sci. 39, 1535-1541. 

Polymers Mixed by Milling. The effect of EVA concentration in the blends on gas permeation and light transmission through the film was studied. The permeability and the diffusion coefficients at 50 °C for the penetrants helium, argon, and carbon dioxide are shown in Figures 1, 2,... 

Figure I. Effect of EVA content in physical blends of PVC /EVA on permeability and diffusion coefficients (P and D) at 50°C for helium. P in (ml STPY (cm)(cmY2(secYl(cm HgY1, D in (cmfisecY1...

A very large area of application for the glass ceramic Zerodur is the technology for modern navigation of space rockets, aircraft, and ships or the positioning of telescopes, antennas, or satellites. The negligibly small coefficient of thermal expansion and the extremely small helium permeability of the material are of particular importance for this application. 

Permeabilities and binary diffusion coefficients for helium/nitrogen mixtures are reported for samples of carbon materials. These are prepared from primary particle sizes between 50 and 250 /im by a novel technique which allows the pore size distribution of the carbon to Ito tailor-o de for particulsu duties. The reported diffusivities correspond to tortuosity factors between 1.7 and 3.0. Permeabilities fall in the range of 2 to 16 x 10 m . 

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