Catholyte transport number

Some of these features are illustrated in Figures 14-18. A rather typical literature plot of current efficiency vs, sodium hydroxide concentration for perfluorosulfonate membranes is shown in Fig. 14. Nation 427 is a 1200-EW sulfonate membrane with fabric reinforcement. Poor hydroxide rejection occurs at catholyte concentrations above 10 wt % but a minimum is seen at higher concentrations, wtih increasing current efficiency from 28 to 40% caustic (9-14 M). The current efficiency of a 1200-EW homogeneous perfluorosulfonate film is shown in more detail over this concentration region in Fig. 15. Sodium ion transport number niol F ), which is equivalent to caustic current efficiency, is plotted vs. both brine anolyte and caustic catholyte concentration. These values were determined using radiotracer techniques, which have proven to be rapid and accurate methods for the determination of membrane performance. " " " A rather sharp maximum is seen at 14 M NaOH, and the influence of brine con-... 

Figure 15. Sodium ion transport number for Nafion 120 us. brine anolyte and caustic catholyte concentrations (Ref. 170).

In Figure 17, sodium ion transport number is plotted vs, catholyte concentration for a homogeneous perfluorocarboxylate film. The current efficiency is now higher than 90% over the entire caustic concentration region studied, although a minimum and maximum in performance is again observed. These features are shifted to lower concentration compared to perfluorosulfonate behavior though. Finally, the performance of a sulfonate-carbox-ylate bilayer membrane, Nafion 901, is plotted in Fig. 18. For such... 

Figure 17. Sodium ion transport number vs. caustic catholyte solution for a perfluorinated carboxylate membrane ( ) anolyte is 5 M NaCl and (O) anolyte and catholyte are identical concentrations of NaOH. (Ref. 149 reprinted by permission of the publisher, The Electrochemical Society, Inc.)...

Electroosmotic effects also influence current efficiency, not only in terms of coupling effects on the fluxes of various species but also in terms of their impact on steady-state membrane water levels and polymer structure. The effects of electroosmosis on membrane permselectivity have recently been treated through the classical Nernst-Planck flux equations, and water transport numbers in chlor-alkali cell environments have been reported by several workers.Even with classical approaches, the relationship between electroosmosis and permselectivity is seen to be quite complicated. Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in Fig. 17. The different results for the two types of experiments here can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte to catholyte. Another frequently observed consequence of water transport is higher membrane conductance, " " which is an important factor in the overall energy efficiency of an operating cell. 

Figure 6. Three dimensional plot of sodium transport number versus anolyte-catholyte concentrations.

A laboratory membrane brine electrolysis cell, designed for automated operation, was constructed ( 1,2). This system enables the measurement of the sodium ion transport number of a membrane under specific sets of conditions using a radiotracer method. In such an experiment, the sodium chloride anolyte solution is doped with 22Na radio-tracer, a timed electrolysis is performed, and the fraction of current carried by sodium ion through the membrane is determined by the amount of radioactivity that has transferred to the sodium hydroxide catholyte solution. The voltage drop across the membrane during electrolysis is simultaneously measured, so that the overall performance of the material can be evaluated. 

A block diagram of the apparatus is shown in Figure 1. The system is constructed to use three sodium chloride anolyte and four sodium hydroxide catholyte concentrations. The starred anolyte compartments refer to separate solutions which have been doped with radiotracer. These solutions are used only for determinations of transport number the nonradioactive brine solutions are used for system flushing and membrane equilibrations. Solutions are selected and pumped into the cell, under computer control, through an all-Teflon pump-valve system. The solutions are heated during these transfers to ensure rapid attainment of experimental temperature in the cell. The brine system is designed to enable the return of radiotracer solutions to their storage vessels after each use. This serves to reduce consumption of radioactive solutions. 

For the application of these membranes to the electrolytic production of chlorine-caustic, other performance characteristics in addition to membrane conductivity are of interest. The sodium ion transport number, in moles Na+ per Faraday of passed current, establishes the cathode current efficiency of the membrane cell. Also the water transport number, expressed as moles of water transported to the NaOH catholyte per Faraday, affects the concentration of caustic produced in the cell. Sodium ion and water transport numbers have been simultaneously determined for several Nafion membranes in concentrated NaCl and NaOH solution environments and elevated temperatures (30-32). Experiments were conducted at high membrane current densities (2-4 kA m 2) to duplicate industrial conditions. Results of some of these experiments are shown in Figure 8, in which sodium ion transport number is plotted vs NaOH catholyte concentration for 1100 EW, 1150 EW, and Nafion 295 membranes (30,31). For the first two membranes, tjja+ decreases with increasing NaOH concentration, as would be expected due to increasing electrolyte sorption into the polymer, it has been found that uptake of NaOH into these membranes does occur, but the relative amount of sorption remains relatively constant as solution concentration increases (23,33) Membrane water sorption decreases significantly over the same concentration range however, and so the ratio of sodium ion to water steadily increases. Mauritz and co-workers propose that a tunneling process of the form... 

Figure 8. Sodium ion transport number vs. NaOH catholyte molarity for Nafion membranes, 80° C. Key O,, Nafion 295 , , 1150 EW A, 1100 EW. Ano-lyte solution is NaOH for light symbols and 5M NaCl for dark symbols.

Figure 2.1 Change in current efficiency (transport number) with concentration of sodium hydroxide in catholyte. (1) Perfluorocarbon membrane NEOSEPTA-F C-1000 (2) Perfluorocarbon membrane NEOSEPTA-F C-2000. Electrolysis at 20 A dm 2 at 80 °C using 3.5 N sodium chloride solution as anolyte and sodium hydroxide solution of various concentrations as catholyte.

During the beginning of the transient period, the concentration in the membrane remains unchanged, except in its two diffusion layers, which remain very thin. This means that the major part of the membrane is not a seat for diffusion phenomena. Based on this outcome, it is possible to show that in the transient period, as long as the concentration in the membrane remains constant, the anolyte concentration varies linearily with time, and the corresponding slope is characteristic of the ion transport numbers (see demonstration below Hittorf s mass balance). The same applies for the catholyte concentration. Therefore, if it is possible to find experimental means to monitor the change in the ion concentrations in the anolyte (and/or the catholyte) over time, or after a given duration, then one can determine the mass transport parameters of both ions. 

It will be demonstrated that current efficiency also decreases with an increased concentration of the caustic. Should the transport of current be performed by hydroxide ions alone (the transference number of OH- is t ), the formation of 1 gram-equivalent of hydroxide would be accompanied by the simultaneous emigration of t gram-equivalents of OH- from the cathode compartment. Actually, however, only an a -th part of the current is transported by the caustic, thus merely (l. x) gram-equivalent of OH are transferred from the catholyte. The current efficiency 7)i in per cent is, therefore, given by the equation ... 

The electrodes and the separator are the only components in an electrolytic cell which are not to be found in other chemical reactors. Electrode materials have been discussed thoroughly in earlier sections but some comments should be made about separators. In the first place it is clear that a cell should only have a separator if one is entirely necessary. Besides their cost, the inclusion of a separator restricts the electrode geometry and mass transport conditions which are possible, increases the cell resistance substantially and certainly makes the cell design more complex the separator must be gasketed to avoid leaks, there must be separate anolyte and catholyte chambers and therefore twice the number of pipe connections to the cell. 

Crude salt, whether it be rock or sea salt, contains a number of chemical constituents (see Table 5.1). The saturated NaCl solution prepared from these salts must be treated to remove impurities before going to a cell. Brine specifications depend on the type of cell used (diaphragm, mercury, or membrane) and the operating conditions. Water is transported from the anolyte to the catholyte through the membrane, as stated above, but more is required to maintain the water balance in the cathode compartment. 

The different transport mechanisms for cations and anions enable the permselectivity. However, it is insufficient for sulfonic acid fixed ions. The water absorption is too high and the diameter of clusters and especially of channels is too large. An improvement was achieved by decreasing the ion exchange capacity of the membrane material, i.e., with an expanded content of inactive PTFE material (increasing of number n at top of Fig. 2). Then less water is absorbed, the size of clusters and channels is diminished and their number is enlarged. Hence, the described mechanism of permselectivity operates more effectively. But above 80 % Na" ion permselectivity for a catholyte with 20 wt% NaOH is not attainable using this method. 

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