Permeation data liquids

Since the early 1980s, the EPA has compiled all the available infoimation form the government laboratories, equipment mannfactnrers, and snppliers into a database. Permeation data for several of the solvent liquids listed in Table 6.5 are available for commoi glove matedals from this database. 

Figure 31. Summary of molecular oxygen self diffusion, including permeation, data in silicate glasses and liquids, redrawn from Lamkin et al.

A comparison of moisture vapor permeation through various polymers can be seen in Table 12.13. Notice that PCTFE is only second to FEP and both are among the most resistant plastics to water vapor permeation. Permeation data for gases and liquids through fluoroplastics are presented in Figs. 12.5 and 12.6. PermeabiUty data can be found in Appendices II-V. 

The interrelationships of the various coefficients associated with fluid uptake (Section 23.4.2) mean that it should be possible to estimate a rate for one of the uptake phenomena from test data for another of them. Campion proposed using this approach to estimate permeation coefficient Q from solubility coefficient s. The form of a liquid absorption plot (Figure 23.6, Section 23.4.4.1) is such that s should be obtainable from it, and inspection showed that this link was via Henry s law with concentration corrected by the polymer density p. The following expression was derived for s ... 

Although relatively unknown, the instrumentation for 2DLC was conceived and implemented by Emi and Frei (1978). They reported the valve configuration presently used in most comprehensive 2DLC systems. However, they automated neither the valve nor the data conversion process to obtain a contour map or 2D peak display. They used a gel permeation chromatography (GPC) column in the first dimension and a reversed-phase liquid chromatography (RPLC) column in the second dimension and studied complex plant extracts. 

Takamatsu et al. studied the diffusion of water into the acid as well as mono-, di-, and trivalent salt forms of 1155 and 1200 EW samples."pj e gravimetric uptakes of membranes immersed in distilled liquid water versus time were determined. Three approximate diffusion formulas were applied to the data, and all yielded essentially the same result. The log D versus 1/7 plots, over the range 20—81 °C, yielded activation energies of 4.9 and 13.0 kcal/mol for the acid and K+ forms, respectively. Diffusion coefficients of various mineral cations that permeated from aqueous electrolytes were considerably smaller than that of water. Also, log Z7was seen to be proportional to the quantity q a, where q is the charge of the cation and a is the center-to-center distance between the cation and fixed anion in a contact ion pair. 

The presence of the term y) makes the permeability coefficient a function of the solvent used as the liquid phase. Some experimental data illustrating this effect are shown in Figure 2.7 [11], which is a plot of the product of the progesterone flux and the membrane thickness, 7, against the concentration difference across the membrane, (cio — cif ). From Equation (2.28), the slope of this line is the permeability, P]. Three sets of dialysis permeation experiments are reported, in which the solvent used to dissolve the progesterone is water, silicone oil and poly(ethylene glycol) MW 600 (PEG 600), respectively. The permeability calculated from these plots varies from 9.5 x 10 7 cm2/s for water to 6.5 x 10 10 cm2/s for PEG 600. This difference reflects the activity term yj/ in Equation (2.28). However, when the driving force across the membrane is... 

Equation (2.79) expresses the driving force in pervaporation in terms of the vapor pressure. The driving force could equally well have been expressed in terms of concentration differences, as in Equation (2.83). However, in practice, the vapor pressure expression provides much more useful results and clearly shows the connection between pervaporation and gas separation, Equation (2.60). Also, the gas phase coefficient, is much less dependent on temperature than P L. The reliability of Equation (2.79) has been amply demonstrated experimentally [17,18], Figure 2.13, for example, shows data for the pervaporation of water as a function of permeate pressure. As the permeate pressure (p,e) increases, the water flux falls, reaching zero flux when the permeate pressure is equal to the feed-liquid vapor pressure (pIsal) at the temperature of the experiment. The straight lines in Figure 2.13 indicate that the permeability coefficient d f ) of water in silicone rubber is constant, as expected in this and similar systems in which the membrane material is a rubbery polymer and the permeant swells the polymer only moderately. 

Equation (9.1) is the preferred method of describing membrane performance because it separates the two contributions to the membrane flux the membrane contribution, P /C and the driving force contribution, (pio — p,r). Normalizing membrane performance to a membrane permeability allows results obtained under different operating conditions to be compared with the effect of the operating condition removed. To calculate the membrane permeabilities using Equation (9.1), it is necessary to know the partial vapor pressure of the components on both sides of the membrane. The partial pressures on the permeate side of the membrane, p,e and pje, are easily obtained from the total permeate pressure and the permeate composition. However, the partial vapor pressures of components i and j in the feed liquid are less accessible. In the past, such data for common, simple mixtures would have to be found in published tables or calculated from an appropriate equation of state. Now, commercial computer process simulation programs calculate partial pressures automatically for even complex mixtures with reasonable reliability. This makes determination of the feed liquid partial pressures a trivial exercise. 

In the characterization of porous membranes by liquid or gaseous permeation methods, the interpretation of data by the hyperbolic model can be of interest even if the parabolic model is accepted to yield excellent results for the estimation of the diffusion coefficients in most experiments. This type of model is currently applied for the time-lag method, which is mostly used to estimate the diffusion coefficients of dense polymer membranes in this case, the porosity definition can be compared to an equivalent free volume of the polymer [4.88, 4.89]. 

Gel permeation chromatographic (GPC) analysis of the molecular weight distribution of the polymers was performed with a Perkin-Elmer series 10 liquid chromatograph equipped with an LC-25 RI detector (25 °C), a 3600 data station, and a 660 printer. A Perkin-Elmer PL 10- xm particle mixed-pore-size cross-linked polystyrene gel column (32 cm by 7.7 mm) was used for the separation. The eluting solvent was reagent-grade tetrahydrofuran (THF) at a flow rate of 0.7 mL/min. The retention times were calibrated against known monodispersed polystyrene standards with MpS of 194,000, 87,000, or 10,200 and for which the ratio Mw/M is less than 1.09. 

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