Subject permeation coefficient

A potential large scale application for gas separation emerges in natural gas purification. For this reason, the carbon dioxide/methane system is subject to extensive research. Sulfonated PPE and a blend of PPE with heteropolyacids (HPA)s has been compared with respect to their separation efficiency for mixtures of carbon dioxide and methane. The permeation coefficients and the selectivity are shown in Table 4.8. 

Katoh and Yoshida evaluated the oxygen permeability (membrane resistance) of various membranes in the presence of water using this equipment. Table 2 lists the results obtained [277]. The membranes used are mainly hydrophobic. A silicone membrane that is supported by a mesh or a composite of a porous polypropylene membrane and homogeneous membrane are the subject of the permeation measurement. For example, the permeation coefficient of a silicone membrane (Silastic 500-1) with a thickness of 125 pm has a permeation coefficient of 1.04cm/min (the membrane resistance is 0.96min/cm). One of the advantages of this method is that not only can the membrane resistance be measured, but... 

Concentration polarization may be a possible explanation for this erratic permeability coefficient behavior however, the influence of concentration polarization in gaseous exchange regimes such as the one herein reported is quite doubtful and subject to dispute(7 ). Alternative possibilities include (a) permeator functional loss due to on-going membrane compaction,... 

Correlation of the permeation properties of a wide variety of polymers with their free volume is not possible [32], But, within a single class of materials, there is a correlation between the free volume of polymers and gas diffusion coefficients an example is shown in Figure 2.24 [33], The relationship between the free volume and the sorption and diffusion coefficients of gases in polymers, particularly glassy polymers, has been an area of a great deal of experimental and theoretical work. The subject has recently been reviewed in detail by Petropoulos [34] and by Paul and co-workers [35,36],... 

Analogously, correlations exist between the coefficient of water permeabihty k and that of capillary absorption S, but these lose their validity if the surface of the concrete is subjected to a hydrophobic treatment, which will reduce considerably capillary absorption but not permeation. For concrete obtained with Portland cement, correlations between the coefficient of water permeability (k) and conductivity (o = 1/p) measured for a given value of relative humidity are available. On the other hand, conductivity varies greatly in concrete made with blended cements or carbonated concrete, while there is no significant change in water permeability. 

The second of Tick s law (Equation 9.2) can be used as a sort of the reference point for describing a permeation process of one component gas (N2,02) through a dense polymeric membrane with no external field. Functional dependence of the diffusion coefficient on coordinate, time or concentration can result from different types of circumstances, i.e. when the membrane is heterogeneous [28], is subjected to some relaxation phenomena [29,30], or membrane transport is accompanied by some other processes, like adsorption, etc. [9,12]. 

Extended immersion of the PIM-PI membrane in alcohol obviously affects the properties of permeation. After immersion in alcohol, the residual solvents can be removed and polymer chain relaxation may be achieved in the swollen state. It was observed that both ethanol and methanol treatment have similar effects on gas permeability. PIM-PI membranes treated with methanol or ethanol for several days show higher gas permeabilities than the films cast directly from chloroform, even though these films are subjected to removal of the chloroform in a vacuum at room temperature for a few days until a constant weight of the film is achieved (Table 5.3). These effects were also observed in PIM-1, though PIM-1 has the higher permeability coefficient of the films. These phenomena indicate local interactions between low molecular weight alcohol and some binding sites on the PIM structure. It should be noted that an increase of permeability is always accompanied by a decrease in permselectivity. 

Various methods are used to improve the membrane properties. One major factor is the relationship between membrane thickness and permeation properties. Figure 4 shows the relationship for cuprammonium rayon hollow fiber which has been subjected to various post-spinning treatments. Cuprammonium rayon hollow fibers can now be produced to meet a broad range of property requirements, by modification of the hollow fiber dimensions and post-spinning treatment. Figure 5 shows that the ultrafiltration ratio and the mass transfer coefficient are virtually independent. 

Experimentally, sedimentation or permeation allow the effective frictional coefficient per monomer to be measured. In a sedimentation experiment measuring the drift velocity of monomers subjected to a centrifugal acceleration y, the sedimentation coefficient, defined as sq = Um/y, is determined. Each monomer experiences a forced = m y, where m is the apparent mass of the monomer immersed in the solution using Eq. (D-1), we have... 

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