Gas Permeation Studies

The driving force of a membrane for gas separation is the pressure difference across the membrane. The yield of the separated gas can be expressed in terms of membrane permeance, which can be characterised by the amount of permeated gas that passes through a certain membrane area in a given time at a definite pressure difference. The values of permeability are often quoted in Barter (1 Barter=10 cm s cm cm Hg = 3.35 x 10 mol m m s Pa STP, standard temperature and pressure). Gas permeation phenomena can be described by a simple solution diffusion model, which involves (l) sorption or dissolution of the permeating gas in the membrane at the higher pressure side, (2) diffusion through the membrane and (3) desorption or dissolution at the lower pressure side. Thus, the permeability coefficient P can be determined by the product of the solubility coefficient S and the mutual diffusion coefficient D [eqn (5.1)]  

The selectivity (a) of an ideal mixture of two gases, A and B, is the difference between the permeability coefficients of the two gases and is equal to the product of the solubility selectivity S/JS and the mobility selectivity 

Reprinted with permission from B. S. Ghanem, N. B. McKeown, P. M. Budd, N. M. Al-Harbi, D. Fritsch, K. Heinrich, L. Starannikova, A. Tokarev and Y. Yampolskii, Macromolecules, 2009, 42, 7881. Copyright 2009, American Chemical Society. 

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. 

The simple solution-diffusion model was often used to explain the gas permeation of a polymer membrane. The permeability coefficient (P) is the product of the solubility coefficient (S) and the diffusion coefficient (D). For a given gas molecule, the main factor for the diffusion coefficient of a certain 

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We report here on the structure and gas transport properties of asymmetric membranes created by the Langmuir-Blodgett deposition of ultra-thin polymeric lipid films on porous supports. Transmission and grazing angle FTIR spectroscopy provide a measure of the level of molecular order in the n-alkyl side-chains of the polymeric lipid. The level of orientational order was monitored as a function of the temperature. Gas permeation studies as a function of membrane temperature are correlated to the FTIR results. 

Gas permeation studies of the diethoxy phosphazene are also available ... 

An alternative approach to solving stability problems with ILMs is presented by Bhave and Sirkar (114). Aqueous solutions are immobilized in the pore structure of hydophoblc, polypropylene hollow fibers by a solvent exchange procedure. Gas permeation studies are reported at pressures up to 733 kPa with the high pressure feed both on the shell and lumen sides of the laboratory scale hollow fiber permeator. No deformation of the hollow fibers is observed. Immobilizing a 30 weight % KjCO, solution in the hollow fibers greatly improved the separation factor, a(C02/Na). from 35.78 with pure water to 150.9 by a facilitated transport mechanism. Performance comparisons with commercial COj separation membranes are made. 

Thin-film MIP composite membranes (cf. Scheme lb), imprinted for theophyUin and caffein, had been prepared by Hong et al. [99], using photo-copolymerization of a MAA/EDMA mixture on top of an asymmetric 20 nm pore size alumina membrane. Additional gas permeation studies suggested that the membranes were defect ( pinhole ) free. 

The kinetic diameter of penetrant molecules used in the gas permeation study are of the order CO2 (3.3 A) < CO (3.76 A) < CH4 (3.8 A). The selectivities of pure PDMS 2vol% Au/PDMS 3 vol% Au/PDMS are reported in Table. 1. As expected the pure polymer shows permeation to gas molecules in the order of their kinetic diameter, i.e., selectivity is based on the size exclusion of gas molecules or diffusion selective. Thus CO2 is more permeable in pure polymer and the selectivity of CH4 and CO is close to unity, which shows they are equally permeable. But when the filler loading of 2 3 vol% of Au NPs in PDMS, there is a reverse selective phenomenon of CO2, i.e., now CO2 is less permeable compared to higher kinetic diameter counterparts CO CH4. This could be CO2 has a quadrupole moment and the inter-connectivity of free volume elements enabled by Au NPS must have restricted CO2 movement. Also CO2 is expected to be more soluble in polymer with polar group, but the interaction of PDMS and Au NPs could have reduced the polarity of hybrid membrane. 

As the pore size is reduced to 1 nm or less, gas permeation may exhibit a thermally activated diffusion phenomena. For example, in studies at Oak Ridge National Laboratory, for a certain proprietary membrane material and configuration, permeation of helium appeared to increase much faster than other gases resulting in an increase in Helium to C02 selectivity from 5 at 25°C to about 48.3 at 250°C (Bischoff and Judkins, 2006). Hydrothermal stability of this membrane in the presence of steam, however, was not reported. 

Permeability measurements for polymer blends prepared by mixing different latices have been reported by Peterson (14). Interpreting transport data for such heterogeneous systems as polymer blends is extremely difficult, however (3, 9,15). The main purpose of the present investigation is, therefore, to study the applicability of gas permeation measurements to characterize polymer blends and not to evaluate the different theoretical models for the permeation process in heterogeneous polymer systems. 

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,... 

We report here on the structure and gas transport properties of asymmetric membranes produced by the LB deposition of a polymeric lipid on porous supports. The effects of temperature on the structure and gas transport is described. The selectivity of CO2 over N2 permeation through the LB polymer films is determined. The polymerized lipid used in this study contains tertiary amines which may influence the CO2 selectivity over N2. The long term objective of our work is to understand how structure and chemistry of ultrathin films influence the gas permeation. 

Nylon-6,6 membrane was formed at the solution interface of adipoyl chloride (0.01 M in 1,2-dichlorocthane solution) and hexamethylenediamine (0.1 M in NaOH solution) within a Pyrex glass microchip (treated with APTES) (see Figure 3.22). The membrane was used in a permeation study to examine diffusion of dissolved NH3 gas through the membrane to a phenolphthalein-containing solution [435]. The membrane can also be modified with horseradish peroxidase on only one side for carrying out an enzymatic reaction. H202 permeates through the membrane and enzymatically reacts with N-ethyl-N-(2-hydroxy-3-sulfopro-pyl)-m-toluidine and 4-AAP (4-aminoantipyrine.) to form a dye [435]. 

Example 9-5. When measuring the gas permeation through a film one obtains a time-axis intercept of the steady-state permeation asymptote of 0 = 254 min using the time-lag method. The thickness of the film being studied is 75 pm. The pressure difference (Ap) between the two sides of the film remains constant at 0.2 bar and the flux through the film is 2 cwW h. Calculate the value of the solubility coefficient 5. 

Since the product Dk is known from the steady state rate of permeation, kp can also be obtained. This time lag method is the basis of most of the gas and some of the vapor transport studies made today. Little application of the time lag method was made until Barter introduced the use of vacuum on the downstream side of the membrane and measured the gas permeation rate by monitoring the increase in pres-arre in a fixed downstream receiving volume Recently the original isobaric method has been reintroduced in a number of commercial permeability instruments. 

Our main concern here is to present the mass transfer enhancement in several rate-controlled separation processes and how they are affected by the flow instabilities. These processes include membrane processes of reverse osmosis, ultra/microfiltration, gas permeation, and chromatography. In the following section, the different types of flow instabilities are classified and discussed. The axial dispersion in curved tubes is also discussed to understand the dispersion in the biological systems and radial mass transport in the chromatographic columns. Several experimental and theoretical studies have been reported on dispersion of solute in curved and coiled tubes under various laminar Newtonian and non-Newtonian flow conditions. The prior literature on dispersion in the laminar flow of Newtonian and non-Newtonian fluids through... 

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