Water Retention in the Membranes

For the PEMFC, high water content is required in the electrolyte to maintain high ionic conductivity. The water content should not be overflooding to prevent blocking of pores of the electrodes or gas difiusion layer. The reaction layers at anode and cathode are given as follows  

water is formed at the cathode and it will keep the electrolyte hydrated. Any excess water would be dried by blowing air over the cathode. The water permeates through the electrolyte towards the anode side, hence maintaining the water requirement of the electrolyte. Three processes can cause problems in water management of membranes  

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These were presumably due to two reasons (a) higher water content on the sweep side raised the water retention in the membrane, thus increased the mobility of both mobile and fixed carriers and enhanced the reaction rates of C02 with the carriers (Eqs. 9.12-9.14) and (b) higher water content also increased the driving force for C02 transport by diluting C02 in the permeate. The increase of C02 permeability with increasing gas water content was also reported in the literature.28,29... 

The water content on the feed side also had significant effects on C02 permeability and C02/H2 selectivity. As shown in Table 9.2 for 120°C, both C02 permeability and C02/H2 selectivity increased as the feed water content increased. This might be caused by the higher water retention in the membrane with the higher water content on the feed side. Consequently, the C02 transport was enhanced by the increased mobility of both mobile and fixed carriers and the C02-carrier reaction rates, while the transport of H2 was not affected significantly. 

Early reports and patent applications of Stonehart and Watanabe [22], Antonucci et al. [23], and Antonucci and Arico [24] claim the advantage of the introduction of small amounts of sihca particles to Nafion to ino-ease the retention of water and improve the membrane performance above 100°C. The effect is believed to be a result of water adsorption on the oxide surface. As a consequence the back-diffusion of the cathode-produced water is enhanced and the water electro-osmotic drag from anode to cathode is reduced [3]. A recent report of the group of Arico et al. [25] confirms the effect of water retention with the inclusion of oxide particles in Nafion and the importance of the acidity of the particle surface. An increase in both strength and amount of add surface functional groups in the fillers enhances the water retention in the membrane SiO -PWA (modified with phosphotungstic acid) > SiOj > neutral-AljOj > basic-AI2O3 > ZrO. ... 

The three streams and associated variables of the RO membrane process are shown in Figure 2b the feed the product stream, called the permeate and the concentrated reject stream, called the concentrate or retentate. The water flow through the membrane is reported in terms of water flux, J. ... 

Membrane-retained components are collectively called concentrate or retentate. Materials permeating the membrane are called filtrate, ultrafiltrate, or permeate. It is the objective of ultrafiltration to recover or concentrate particular species in the retentate (eg, latex concentration, pigment recovery, protein recovery from cheese and casein wheys, and concentration of proteins for biopharmaceuticals) or to produce a purified permeate (eg, sewage treatment, production of sterile water or antibiotics, etc). Diafiltration is a specific ultrafiltration process in which the retentate is further purified or the permeable sohds are extracted further by the addition of water or, in the case of proteins, buffer to the retentate. 

A bottleneck in all membrane processes, applied in practice, is fouling and scaling of the membranes. These processes cause a decrease in water flux through the membrane and a decrease in retention. Much attention is paid, especially in case of nanofiltration and hyperfiltration, to prevent fouling of the membrane by an intensive pretreatment and the regular removal of fouling and scaling layers by means of mechanical, physical or chemical treatment. 

NF is used when high molecular weight solutes have to be separated from a solvent. It is effective in the production of drinking water, especially in the case of water softening. Compared to RO, a lower retention is found for monovalent ions. But very recently [9], it has been found that NF separates the ions of the same valency for a selective defluorination of brackish water. RO and UF have shown, respectively, solution-diffusion and convection mass transfers. In NF, a synergism between both can be observed but strongly depends on the operational conditions (pH, ionic strength, flow rate, transmembrane pressure) and on the membrane material used. 

Table 12 shows the typical LRV values obtained using a polymeric and ceramic microfilter. Sterile filtration requires 100% bacteria retention by the membrane, whereas in many industrial bacteria removal applications the presence of a small quantity of bacteria in the filtrate may be acceptable. For example, drinking water obtained by microfiltration may contain nominal counts of bacteria in the filtrate which is then treated with a disinfectant such as chlorine or ozone. The use of ceramic filters may allow the user to combine the sterile filtration with steam sterilization in a single operation. This process can be repeated many times without changing filters due to their long service life (5 years or longer). 

Charge alone is not sufficient to induce ionic conductivity. Ion exchange capacity (lEC), water uptake, and water retention capabilities help to ensure good electrochemical properties such as membrane conductivity. As water uptake and water retention properties increase in the bulk membrane, the conductivity tends to increase proportionally. lEC provides information regarding the density of ionizable hydrophilic groups in the membrane matrix, which are responsible for the conduction of protons and thus lEC is an indirect and reliable approximation of the proton conductivity [8]. 

Most of the water is removed as bottoms in colitrtm C-1 (Fig. 11.4-6). The liquid azeotropic overhead fraction is pressurized to approximately 4 bar and fed irrto a membrane stack (5-7 stages). Water preferably penetrates the membrane. Due to the high pressure difference the water-rich permeate is flashed in the merrrbrane. After eorrdensation it is recycled into eolrrrtm C-1. The retentate is very rieh in orgarric compounds and, in turn, often meets the product specification. If not, the retentate is fed into column C-2 for further purificatioa Decisive for the process is the availabihty of efficient membranes with both a high capacity and a high selectivity. 

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