Sodium ions catholyte concentration

The membrane cell (Fig. 1) uses a cation exchange membrane in place of an asbestos diaphragm. It permits the passage of sodium ions into the catholyte but effectively excludes chloride ions. Thus the concept permits the production of high-purity, high-concentration sodium hydroxide directly. 

X 550 X 3 mm) suspended into the electrolyzer. Each cathode is fixed in a frame which is enveloped by a thick linen cloth which acts as a diaphragm. The catholyte is a solution of 4 per cent sodium acetate and 3—5 per cent sodium carbonate with a small content of sodium hydroxide. The anolyte contains 4 per cent sodium acetate, 0,06-0.2 per cent sodium carbonate and some 0.05 per cent sodium hydrogen carbonate. It will be seen that the anolyte contains only a very small amount of sodium carbonate the concentration of the latter is maintained constant during electrolysis by the migration of CO ions from the catholytb. 

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.)...

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... 

The measurement and control of transport properties for ion exchange membranes is the key element in optimizing the operating conditions for modern chlor-alkali membrane cells. Ideally, a membrane should allow a large anolyte-catholyte sodium ion flux under load, while at the same time the hydroxide ion and water fluxes are kept minimal. Under these conditions, high current efficiency and low membrane resistance can be realized simultaneously in a cell producing concentrated caustic and chlorine gas. 

Three to four molecules of water of hydration, depending on the operating conditions, are transported with each sodium ion firom the anode side to the cathode side. Hence, only a relatively small amount of water needs to be added to the catholyte loop in order to adjust NaOH concentration in a chlor-alkali cell. In KCl electrolysis, each cation carries only two to three molecules of water. The water balance of a cell alters, and both anolyte and catholyte flows must respond to the change. 

It will be seen from this equation that the dissolving component of the electrolyte, i. e. sodium acetate or chlorate, is continually regenerated in the course of the process. The sodium carbonate and hydroxide consumed in the reaction are supplemented by the migration of C(K and OH- ions from the catholyte. At the cathode made of lead or of iron hydrogen is liberated whereby the concentration of hydroxyl ions in the catholyte increases. In order to maintain their concentration within suitable limits and to replace the carbonate ions consumed in the production of white lead the eleotrolyte is continuously saturated by carbon dioxide thus converting the hydroxide to carbonate. 

In the case of NEOSEPTA-F C-2000, the current efficiency increases with increase of sodium hydroxide concentration in catholyte. It is thought that the water in the membrane surface portion of cathode side is dehydrated and the concentration of fixed ion in the membrane increases. The presumption that the cathode side of the membrane surface would shrink with the increase of sodium hydroxide concentration is obviously proved in the relationship between the sodium hydroxide concentration in catholyte and sodium chloride concentration in the product ( Figure 6 ), The diffused amount of sodium chloride decreased remarkably with increase of sodium hydroxide concentration. 

Figure 4.8 Current efficiency versus fixed ion concentration of a cation exchange membrane in the electrolysis of a sodium chloride solution. Cation exchange membrane sulfonated styrene—divinylbenzene type. Anolyte saturated NaCl catholyte 3.0 N NaOH current density 10Adm 2 at 70 °C.

A diaphragm is porous and cannot discriminate between species. All will diffuse through its pores where there is a concentration difference. For this reason the caustic soda produced in a diaphragm cell is always contaminated with chloride ion and the catholyte leaving the cell cannot contain more than 10% sodium hydroxide since otherwise hydroxide ion diffusion to the anode becomes significant and oxygen as well as chlorine is evolved. Thus prior to sale, the sodium hydroxide produced in a diaphragm cell must be concentrated by evaporation to a 50% solution. 

Factor 2. Decreasing brine flow rate to a cell increases the conversion of sodium chloride to sodium hydroxide and raises the hydroxide concentration in the catholyte because of reduced overflow from the cell. The decreased flow rate of brine through the diaphragm allows increased migration of hydroxide ions into the anolyte. These factors decrease cell efficiency. 

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