Impurity-resistant membranes

Impurity-Resistant Membranes. Section 4.8 discusses two fundamental issues related to membranes, the possibility of achieving nearly 100% current efficiency and the development of impurity-resistant membranes. Total rejection of the hydroxyl ion by the membrane is possible if the only anode reaction is the discharge of the chloride ion. However, if oxygen evolution, which is favored thermodynamically, also takes place, the principle of electroneutrality makes it impossible for the membrane to exclude the back-migration of the hydroxyl ion. 

S.1B. Formation of Hydroxides. The effects of impurities on membranes are classified as chemical or physical in nature. When the micropores are plugged by insolubles, the ionic and water fluxes become limited, resulting in increased resistance in those regions. As a result, heat is generated and the vapor pressure of the water increases. 

Once the plate starts to corrode, many problems appear to affect performance and durability, even serious failure, of fhe fuel cells. For example, fhe interface contact resistance between the corroded metal plates and GDL will increase to reduce the power output. The corrosion products (mainly various cations) will contaminate the catalyst and membrane and affect eir normal functions because the polymer membrane essentially is a strong cation exchanger and the catalyst is susceptible to the ion impurity. Hence, adding a corrosion-resistant coating to the metal plate will almost inevitably assure the performance and long-term durability of a sfack. 

The synthesis of silica membranes has only recently been described. Silica forms sols and gels very easily both by the colloidal suspension and by the polymeric gel route. Its chemical resistance and its thermal stability in the presence of water vapor or metal impurities are not very good however. Larbot et al. (1989) have described the synthesis of silica membranes starting with a commercially available silica sol (Cecasol Sobret) in an aqueous solution at pH 8. 

Dialysis operates by the diffusion of selected solutes across a nonporous membrane from high to low concentration. An early industrial application of dialysis was caustic soda recovery from rayon manufacturing. It had been a viable process because inexpensive but alkali-resistant cellulose membranes were available that were capable of removing polymeric impurities from the caustic. Gradually however, dialysis is being replaced by dynamic membrane technology for caustic soda recovery because of the latter s much higher productivity. 

Another approach to achieve purification of rinses and recovery in one step, electrodialysis has been suggested for chromic acid recovery and removal of metallic impurities [108]. As the authors point out there are two main process limitations first, the poor stability of most anion-exchange membranes against the oxidative chromic acid solution and secondly the increase in membrane resistance due to the formation of polychromates in the membrane. 

A novel type of membrane reactor, emerging presently, is the pervaporation reactor. Conventional pervaporation processes only involve separation and most pervaporation set-ups are used in combination with distillation to break azeotropes or to remove trace impurities from product streams, but using membranes also products can be removed selectively from the reaction zone. Next to the polymer membranes, microporous silica membranes are currently under investigation, because they are more resistant to chemicals like Methyl Tertair Butyl Ether (MTBE) [23-24], Another application is the use of pervaporation with microporous silica membranes to remove water from polycondensation reactions [25], A general representation of such a reaction is ... 

Polyallylamine (Mw = 70 000 g/mol) and polystyrene sulfonate (M, = 70 000 g/mol) were obtained from Aldrich. PAH was used as received, while PSS was purified from low molecular weight impurities by dialysis (Polyether-sulfone membranes, MW cut off 10 000 g/mol, Millipore) against ultra pure water and freeze dried. All water used for preparing solutions or for dialysis was purified by a Purelab Plus UV/UF, Elga Lab Water system and had a resistivity smaller than 0.055 xS/cm and a total organic content between 2 and 12 ppb (parts per billion). 

Figure 5.36. Effects that long-term NH3 exposure has on H2-air fuel cell high-frequency resistance at 80°C. 30 ppm NH3 (g) was injected into the anode feed stream [39], (Reproduced by permission of ECS—The Electrochemical Society, from Uribe FA, Gottesfeld S, Zawodzinski Jr. TA. Effect of ammonia as potential fuel impurity on proton exchange membrane fuel cell performance.)...

Since the electrical resistance of the effiuent and parasitic currents are minimal at high level of impurities, specihc interest in electrically assisted membrane processes could increase due to more strict laws and legislation around effluents. The depletion of freshwater resources and the necessity to process brackish or seawater to produce potable water could promote the use of electrically assisted membrane processes in the future. Electrodialysis will have to compete with pressure-driven membrane processes such as reverse osmosis. The growing awareness of the unique cleaning ability of electrically ionized water (EIW) [47], a byproduct of electrodialysis, may be a factor to consider in the choice between ED and RO systems. NMR relaxation measurements were used to determine the water cluster size of electrically ionized water EIW. It is known that the water cluster size of EIW is signihcantly smaller than that of tap water. The smaller water cluster size is believed to enhance the penetration and extractive properties of EIW. Recently, EIW has been produced and used in several cleaning processes [47] in industry. 

The enzymes responsible for the conversion of lanosterol to cholesterol, as were those for the conversion of farnesyl pyrophosphate to squalene and lanosterol, are all integral membrane-bound proteins of the endoplasmic reticulum. Many have resisted solubiUzation, some have been partially purified, and several have been obtained as pure proteins. As a consequence, much of the enzymological and mechanistic studies have been done on impure systems and one would anticipate a more detailed and improved understanding of these events as more highly purified enzymes become available. Many approaches have been taken to establish the biosynthetic route that sterols follow to cholesterol. Some examples are synthesis of potential intermediates, the use of inhibitors, both of sterol transformations and of the electron transfer systems, and by isotope dilution experiments. There is good evidence that the enzymes involved in these transformations do not have strict substrate specificity. As a result, many compounds that have been found to be converted to intermediates or to cholesterol may not be true intermediates. In addition, there is structural similarity between many of the intermediates so that alternate pathways and metabolites are possible. For example, it has been shown that side-chain saturation can be either the first or the last reaction in the sequence. Fig. 21 shows a most probable series of intermediates for this biosynthetic pathway. 

Membrane filters (Whatman 40, Whatman 41) have exceedingly low contents of impurities and ash. They can be easily mineralized and they exert a low filtration resistance. Their disadvantages are their high hygroscopicity and lower efficiency for the capture of particles of sizes below 1 /im. 

Membrane filters (Sartorius SM, Synpor, Millipore) made of cellulose derivatives contain varying amounts of impurities and thus, before the sample analysis the given component should be determined in unexposed filters. Such membrane filters are very resistant to water, weak acids and bases, however, they are soluble in ketones, esters and alcohols. Their ashing is often carried out after solution in isopropyl alcohol. 

The use of adherent refractory mixed conducting catalysts on the partial oxidation surface of OTMs permits gasification of carbonaceous soUd feedstocks such as coal fines and biomass to be realized. Refractory catalysts can be selected which not only mediate oxygen to the reaction site from the membrane bulk, but more importantly can be tailored to have high resistance to potentially poisoning impurities, typified by sulfur-containing materials, present in the gasified feedstock. 

Section 4.8 and its appendix discuss the action of iodine in more detail. It interacts with other elements to form other precipitates in either NaCl or KO service. These precipitates include barium paraperiodate, and so there may be a synergistic effect when both barium and iodine are present in the brine. Table 4.8.8 lists commercial brine specifications for some of the common membranes [202]. The allowable concentrations of barium and iodine may be related to each other or to operating current density. Table 4.8.9 lists the adverse effeets of various brine impurities. There are reports of physically distinct forms of Ba-I precipitates, with some very fine particles that form in regions away from the main current paths through the membrane [203]. These tend to have relatively little effect on membrane performance [204], and Section 4.8.S.3 also discusses the development of membranes with enhanced resistance to the effects of iodine. 

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