Membranes their performance characteristic

Purchased membranes were initially tested for compliance with the specifications designed to assure their suitability for winding operations (thickness, tensile strength and elongation) and for their performance characteristics (flux and salt rejection)... 

Perfluorinated ionomer membranes show considerable promise with respect to their performance characteristics, low resistivity, high permselectivity, and long-term stability. However, the present cost of these membranes is more than 300 per square meter. The relatively high cost limits their application in many electrochemical cells when cost effectiveness is a major concern. 

Development of Alternative Membranes. Nafion membranes show considerable promise with respect to their performance characteristics, low resistivity and long-term stability. However, the present cost (about 30/ft ) of Nafion membranes is rather expensive for the SPE cell to be cost-effective for industrial and utility applications. TJje ultimate goal is to reduce the membrane cost to about 3/ft. Alternative membranes have been evaluated for use in the SPE cell. Only fluorocarbon membranes have been considered in this application (10), since the cell environment is corrosive, and cell tempperature is high (from 80°C to 150°C). <, 3, 3 - trifluorostyrene-sulfonic acid... 

Sulfonated PBIs, other sulfonated polymers, and their blends show great potential for use as membranes in high-temperature fuel cells. The synthesis, conductivity, mechanical properties, and performance still require further development, but results so far are promising. Further investigation remains to determine whether these problems can be overcome and useful chemistries developed to meet the needs of high-temperature membranes with performance characteristics comparable to lower-temperature membranes. 

An important performance characteristic of passive samplers that operate in the TWA regime is the diffusion barrier that is inserted between the sampled medium and the sorption phase. This barrier is intended to control the rate of mass transfer of analyte molecules to the sorption phase. It is also used to define the selectivity of the sampler and prevent certain classes (e.g., polar or nonpolar compounds) of analytes, molecular sizes, or species from being sequestered. The resistance to mass transfer in a passive sampler is, however, seldom caused by a single barrier (e.g., a polymeric membrane), but equals the sum of the resistances posed by the individual media (e.g., aqueous boundary layer, biofilm, and membrane) through which analyte diffuses from the bulk water phase to the sorption phase.19 The individual resistances are equal to the reciprocal value of their respective mass transfer coefficients and are additive. They are directly proportional to the thickness of the barrier... 

Membranes can be divided into two categories according to their structural characteristics which can have significant impacts on their performance as separators and/or reactors (membrane reactors or membrane catalysts) dense and porous membranes. Dense membranes are free of discrete, well-defined pores or voids. The difference between the two types can be conveniently detected by the presence of any pore structure under electron microscopy. The effectiveness of a dense membrane strongly depends on its material, the species to be separated and their interactions with the membrane. 

The preparation and fabrication methods and their conditions described in Chapter 3 dictate the general characteristics of the membranes produced which, in turn, affect their performance as separators or reactors. Physical, chemical and surface properties of inorganic membranes will be described in detail without going into discussions on specific applications which will be treated in later chapters. Therefore, much of this chapter is devoted to characterization techniques and the general characteristics data that they generate. 

Silver membranes are permeable to oxygen. Metal membranes have been extensively studied in the countries of the former Soviet Union (Gryaznov and co-workers are world pioneers in the field of dense-membrane reactors), the United States, and Japan, but, except in the former Soviet countries, they have not been widely used in industry (although fine chemistry processes were reported). This is due to their low permeability, as compared to microporous metal or ceramic membranes, and their easy clogging. Bend Research, Inc. reported the use of Pd-composite membranes for the water-gas shift reaction. Those membranes are resistant to H2S poisoning. The properties and performance characteristics of metal membranes are presented in Chapter 16 of this book. 

For many years, polymeric membranes have been widely utilized in practical appHca-tions without having precise information on their pore size and pore size distribution, despite the fact that most commercial membranes are prepared by the phase inversion technique, and the performance of those membranes is known to be governed by their pore characteristics in a complicated manner [1]. These pore characteristics are influenced both by the molecular characteristics of the polymer and by the preparative method [2]. Crudely, membranes applied for pressure-driven separation processes can be distinguished on the basis of pore diameter as reverse osmosis (RO, < 1 nm), dialysis (2-5 nm), ultrafiltration (UF, 2-100 nm), and microfiltration (MF, 100 nm to 2 J,m). Nanofiltration (NF) membranes are a relatively new class and have applications in a wide range of fields [3]. The pore sizes of NF lie between those of RO and UF membranes. 

An example of a membrane module for assembling in a MBR is reported in Fig. 23.5. It is evident that many factors could affect MBR performance, as summarized in Fig. 23.6 membrane properties, sludge characteristics and operating conditions that are related to capital and maintenance costs, materials and utUities costs as well as service costs. Their optimization is strictly related to the economics of the treatment systems, also compared with traditional ones. 

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