Impurity effects, membrane cells

Three pairs of membranes, each with two different porosities, were installed in the RO cells. The membranes used were PA (92 and 972), CAc (852 and 912), and PBI (892 and 992). Each cell had an effective membrane diameter of 4.1 cm (area of 13.4 cm2). The operating pressure for all runs was 260 10 psig, and the flow rate was adjusted to 410 10 mL/min. The system and membranes were washed by operating with an ethanol/water mixture (1 9 v/v twice) for a 6-8-h period to get rid of any trace organic impurities in the system. The system was then cleaned twice with purified water and equilibrated with purified water (3 X 10 h). During the run, the temperature of the feed solution increased from 20-22 °C to 26-29 °C. 

This is an ideal representation that approximates the real behavior of a membrane cell. A variety of ionic and neutral species besides Na" ", Cl , and OH exist both in the anolyte and in the catholyte, and these impiuities also affect the performance of the cell. Consequently, the effects of these impiuities on membrane-cell behavior have been investigated extensively in order to determine the allowable limits of impurity concentrations and develop industrial procedures for the removal of these impurities from the electrolysis system. 

Before discussing impurity effects, let us examine the pH profile across the membrane during the course of electrolysis. Using experimental data from a laboratory cell, Ogata and coworkers [18,103] and Obanawa and coworkers [104] found the pH in a sulfonate-carboxylate bilayer to be in the range 9-12 over the bulk of the membrane with the exception of narrow regions near the membrane/solution interfaces. On the anode side, the pH decreased steeply to about 3, while on the cathode side, it increased to 14. Thus, the pH of 9-12 in the membrane can indeed force the precipitation of metal hydroxides [105] when the metal ion concentration exceeds the dictates of the solubility product (Fig. 4.8.34). It should be noted that Hg and Fe are electrodeposited on the cathode and oxides of Mn, Pb, and Fe are formed on the anode as a result of oxidation of the relevant ionic species by the active chlorine in the anolyte. 

The exact specifications depend on the membrane, cell design, and operating conditions [125-127], The brine specifications recommended by membrane suppliers are shown in Table 4.8.8, and Table 4.8.9 describes the impurities and possible mechanisms of membrane damage. The reader is referred to the Appendix, where the effects of impurities are summarized along with the recommended analytical methods suggested by the membrane manufacturers. References [128-131] provide a discussion on brine treatment costs vs membrane costs. 

Impurities in brine affect electrode reaction kinetics, cell performance, the condition of some cell components, and product quality. Treatment of brine to remove these impurities has always been an essential and economically significant part of chlor-alkali technology. The brine system typically has accounted for 15% or more of the total capital cost of a plant and 5-7% of its operating cost. The adoption of membrane cells has made brine specifications more stringent and increased the complexity and eost of the treatment process. Brine purification therefore is vital to good electrolyzer performance. This section considers the effects of various impurities in all types of electrolyzer and the fundamentals of the techniques used for their control. Section 7.5 covers the practical details of the various brine purification operations. 

Early membrane cells operated at relatively low current efficiencies and were able to tolerate correspondingly higher concentrations of impurities. As membranes were improved and better results became possible, the requirements for brine purity became stricter. For a time, the addition of phosphate to the brine to sequester the hardness ions and prevent them from entering the membranes mitigated some of the effects of hardness. Finally, it became necessary to devise a process to increase the purity of the brine well beyond that obtained by chemical treatment, and ion exchange is now the standard technique. Several general reviews of the brine ion-exchange process itself are available [119-121]. 

I. Silica. Silica is one of the secondary impurities, but it has synergistic effects in membrane cells when present along with calcium and aluminum [80]. It is difficult to remove silica from brine. The operator s best defense is prevention. Selection of salt, choice of dissolving conditions in order to reject as much silica as possible, and prevention of contamination by foreign substances are all important toward this end. 

Qian G, Benicewicz B (2011) Fuel impurity effects on high temperature PBI based fuel cell membranes. ECS Trans 41 1441-1448... 

Qian G, Benicewicz BC (2011) Fuel impurity effects on high temperature PBI based fuel cell membranes. In Gasteiger HA, Weber A, Narayanan SR, Jones D, Strasser P, Swider Lyons K et al (eds) Polymer electrolyte fuel cells 11. Electrochemical Society, Pennington, pp 1441-1448... 

Finally, there are some miscellaneous polymer-electrolyte fuel cell models that should be mentioned. The models of Okada and co-workers - have examined how impurities in the water affect fuel-cell performance. They have focused mainly on ionic species such as chlorine and sodium and show that even a small concentration, especially next to the membrane at the cathode, impacts the overall fuelcell performance significantly. There are also some models that examine having free convection for gas transfer into the fuel cell. These models are also for very miniaturized fuel cells, so that free convection can provide enough oxygen. The models are basically the same as the ones above, but because the cell area is much smaller, the results and effects can be different. For example, free convection is used for both heat transfer and mass transfer, and the small... 

Fuel cells can be broadly classified into two types high temperature fuel cells such as molten carbonate fuel cells (MCFCs) and solid oxide polymer fuel cells (SOFCs), which operate at temperatures above 923 K and low temperature fuel cells such as proton exchange membrane fuel cells (PEMs), alkaline fuel cells (AFCs) and phosphoric acid fuel cells (PAFCs), which operate at temperatures lower than 523 K. Because of their higher operating temperatures, MCFCs and SOFCs have a high tolerance for commonly encountered impurities such as CO and CO2 (CO c)- However, the high temperatures also impose problems in their maintenance and operation and thus, increase the difficulty in their effective utilization in vehicular and small-scale applications. Hence, a major part of the research has been directed towards low temperature fuel cells. The low temperature fuel cells unfortunately, have a very low tolerance for impurities such as CO , PAFCs can tolerate up to 2% CO, PEMs only a few ppm, whereas the AFCs have a stringent (ppm level) CO2 tolerance. 

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.)...

Uribe FA, Gottesfeld S, Zawodzinski Jr TA (2002) Effect of ammonia as potential fuel impurity on proton exchange membrane fuel cell performance. J Electrochem Soc 149 A293-6... 

Various aspects of the mechanisms of microbial oxidation of sulfur have been referred to earlier, but it is clear that further investigation of such aspects as the nature of the frequently-observed close attachment of the cells to the sulfur surface and the penetration or otherwise of cell membranes by elemental sulfur (Kaplan and Rittenberg, 1962) would be of interest. Elemental sulfur occurs in a number of solid allotropic forms, its chemical activity is profoundly affected by a number of impurities, and it is photosensitive (Meyer, 1968). There is a paucity of information on the effects of variation of these factors on the amenability or otherwise of elemental sulfur to microbial attack. 

Okada, T. Theory for water management in membranes for polymer electrolyte fuel cells part 1. The effect of impurity ions at the anode side on the membrane performances. J. Electro-anal. Chem. 1999, 465 (1), 1-17. 

Almost all cross-flow filtration processes are inherently susceptible to flux decline due to membrane fouling (a time-dependent phenomenon) and concentration polarization effects which reflect concentration buildup on the membrane surface. This means lower flux (i.e., product output) which could drive the capital costs higher due to the requirement of a larger surface area to realize the desired production rate. In some situations, the lower flux could also result in lower selectivity which means reduced recoveries and/or incomplete removal of impurities from the filtrate. For example, removal of inhibitory metabolites such as lactic acid bacterial or separation of cells from broth while maximizing recovery of soluble products. 1 1... 

F. Uribe, S. Gottesfeld and T. Zawodzinski, "The Effect of Ammonia as Potential Fuel Impurity on Proton Exchange Membrane Fuel Cell Performance", J. Electrochem. Soc., 149, A293 (2002). 

Thus, during a cell shutdown, the transport of anions is solely governed by diffusion (Fig. 4.8.27). The flow of anions into the cathode compartment can be minimized by lowering the cell operating temperature before shutdown, as is evident from Eq. (33), via the variations in D with cell temperature, T. Because of this effect of maximal carryover of anions at i -> 0, it is important that the cells be designed so that the ratio of active membrane area to the peripheral inactive area is high in order to achieve low anionic impurity content in the caustic product. 

In conclusion, the alkaline earth metal ions, precipitated in the membrane, affect both current efficiency and cell voltage. At sufficiently high concentrations, they physically distort the membrane structure. The effects of the anions, such as sulfate, and nonionic species, such as silica, are relatively minor, but these impurities often form insoluble compounds with other chemical species entering the membrane, thereby seriously affecting the membrane structure. 

Chloride concentrations in the catholyte are measured in tens of parts per million. These concentrations are not so low as those in the mercury cell and are of concern in certain applications. The same can be said of chlorate concentrations. The concentrations of these impurities in the product are functions of their speed of transport through the membranes relative to the speed of die cation. At lower current densities, these relative rates are higher. Moreover, at shutdown there is still a slow transfer of anions across the membranes. These effects can lead to problems in maintaining tight specifications when production is curtailed. They are particularly troublesome in the production of KOH. 

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