Membrane materials studies

There are many different zeolite structures but only a few have been studied extensively for membrane applications. Table 10.1 lists some of these structures and their basic properties. One of the most critical selection criterion when choosing a zeolite for a particular application is the pore size exhibited by the material. Figure 10.1 compares the effective pore size of the different zeolitic materials with various molecule kinetic diameters. Because the pores of zeolites are not perfectly circular each zeolite type is represented by a shaded area that indicates the range of molecules that may stiU enter the pore network, even if they diffuse with difficulty. By far the most common membrane material studied is MFI-type zeolite (ZSM-5, Al-free siUcahte-l) due to ease of preparation, control of microstructure and versatility of applications [7]. 

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. 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

Porous glass structures have not been marketed for membrane separation applications until recently, despite having been studied as a membrane material for a long time. A number of glass companies such as Asahi Glass... 

The major chemical processes of membrane deteriorations are hydrolysis and oxidation. Cellulose acetate is most stable at the level of around pH 4.7, and at the pHs lower or higher than that value, membrane hydrolysis is accelerated. In practical applications of cellulose acetate membranes, feed water pH is usually controlled between 5 to 6. But it is Impossible to control the pH of demineralized pure water for electronic and pharmaceutical uses, i.e. for ultrapure water polishing. In such cases feed water pH 7 should be supplied to cellulose acetate material. Studies of membrane behaviour under such conditions will give good information for estimating the membrane life. 

The above discussion indicates that only one poly(aryl ether) has been explored as a membrane material (the commercial, amorphous, homopolymer P-1700 produced by Union Carbide). The present program expands these studies to Include other poly(aryl ethers). 

This paper has provided the reader with an introduction to a class of polymers that show great potential as reverse osmosis membrane materials — poly(aryl ethers). Resistance to degradation and hydrolysis as well as resistance to stress Induced creep make membranes of these polymers particularly attractive. It has been demonstrated that through sulfonation the hydrophilic/hydrophobic, flux/separation, and structural stability characteristics of these membranes can be altered to suit the specific application. It has been Illustrated that the nature of the counter-ion of the sulfonation plays a role in determining performance characteristics. In the preliminary studies reported here, one particular poly(aryl ether) has been studied — the sulfonated derivative of Blsphenol A - polysulfone. This polymer was selected to serve as a model for the development of experimental techniques as well as to permit the investigation of variables... 

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V205/Si02 catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 

Problems of desorption and loss of activity encountered with natural heparin have led numerous workers to explore synthetic heparin-like polymers or heparinoids, as reviewed by Gebelein and Murphy [475, 514, 515]. The blood compatibility of 5% blended polyelectrolyte/polyfvinly alcohol) membranes was studied by Aleyamma and Sharma [516,517]. The membranes were modified with synthetic heparinoid polyelectrolytes, and surface properties (platelet adhesion, water contact angle, protein adsorption) and bulk properties such as permeability and mechanical characteristics were evaluated. The blended membrane had a lower tendency to adhere platelets than standard cellulose membranes and were useful as dialysis grade materials. 

Determination of Pore Size Distributions on the Surface of CA Membranes and Aromatic PAH Membranes. The organic solutes listed in Table IV were chosen as reference solutes, and then D and B values with respect to CA-398 and PAH materials were obtained by step 4. The results are listed in Table IV. Then, by using these B and D values, the average pore size and the pore size distribution on surfaces of membranes under study were calculated by following step 5. In these calculations, B and D values for CA-400 material were assumed to be equal to those of CA-398 material because of the closeness of acetyl content. The results are listed in Table II. 

Emission rate studies are currently underway with a number of membrane materials. Early tests were very encouraging a number of more recent test results show variations in emission rate from lot to lot and as a function of ambient relative humidity and temperature. Tests have not yet started on long-term emission behavior,... 

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