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4 Domga et al.: Study of Some Electrolysis Parameters for Chlorine and Hydrogen Production Using a New Membrane Electrolyzer conductivity, different solutions of NaCl and NaOH at different concentration were used. The experimental results are shown in figure 2. Figure 2. Evolution conductivity at 25°C according to NaCl and NaOH concentration. In figure 2, the conductivity of brine and sodium hydroxide is shown to increase with concentration. Indeed, the conductivity of 0.1% NaCl is 5825 μS/cm and by increasing the concentration to 1% NaCl, the conductivity also increased to 17600 μS/cm. Similarly to the brine, the conductivity of sodium hydroxide increases with the concentration. These results are in agreement with the literature. The chlorine reaction is concentration dependent. Increasing the chloride concentration is one of the main ways through which the selectivity for the desired products can be maximized. Authors [17] reported that, in the case of current efficiency, the highest contribution percentage is due to brine concentration with 33%. Decreasing the brine concentration in systems results in slow, reaction and decrease in system voltage and electrolysis pause. However, too high caustic or brine concentration can lead to the formation of blisters on membrane accelerating the degradation of the whole membrane [16]. The presence of blisters on the membrane structure causes the ohmic resistance to increase. A lower sodium hydroxide (26% NaOH) concentration has a positive effect on the cell voltage. In others hands, Oxygen evolution by water oxidation, reaction (4), is an important anodic side reaction. Anolyte acidification (pH = 2 - 2.5) can reduce the oxygen side reaction to enable a current efficiency of close to 99% in the chlor-alkali process [18]. Others authors also reported the same effect [19]. In this study, to optimize the chlorine, hydrogen production and reduce oxygen side reaction, we recommend these values: 320 g.L-1 NaCl (pH = 2) and 24% NaOH. 3.2. Influence of Temperature on the Conductivity To demonstrate the effect of temperature on conductivity, different solutions were used: 0.5 g/L NaCl and 0.5 g/L NaOH. Conductivity measurements were taken for each solution set at 23, 28, 33, 38, 43 and 50°C. The experimental conductivity values for each solution are shown in figure 3. Figure 3. Effect of temperature on the conductivity of 0.5 g/L NaCl and 0.5 g/L NaOH. The temperature profile has a great impact on the electrolyte conductivity. For example, the brine conductivity at 23°C is 973.8 μS/cm and by increasing the temperature at 28°C, its conductivity increases to 1077.54 μS/cm. This could be related to the mobility of ions. As the matter of fact, by increasing the temperature of solution, the mobility of ions (Na+, Cl- for NaCl and Na+, NaOH- for NaOH) in solution also increases, and consequently this will lead to an increase in its conductivity. Additionally, different ionic species show different effects with temperature which is due to the size of the ion and its charge density. This study shows the conductivity response to temperature of 0.5 g/L NaCl compared to 0.5 g/L NaOH. The ion OH-, being smaller than Cl-, has a higher charge density and hence the conductivity of a solution of the same concentration is greater. From figure 3 it can be concluded that the optimum electrolyte conductivity value was observed for higher temperatures. The cell performance improved with electrolyte temperature [16]. This positive contribution could be related to the internal kinetic processes, which are normally exponentially temperature dependent, and to the NaCl and NaOH conductivity increase. Authors [17] reported that cell temperature was one of the most striking parameters on the cell voltage with contribution percentage of 23%. However, the increase in the temperature is limited by the behavior of electrolyzer materials and at temperatures higher than 90°C the amount of water vapor increases tremendously, while the membrane stability decreases [20]. In addition, the membrane stability may decrease due to decarboxylation for temperatures higher than 90°C [21]. At industrial scale, physical damage to the membrane (blistering) becomes more likely to occur at temperatures lower than 75°C. It is usually recommended to operate in the range 80 - 90°C [16]. In these experiments, the value of 80°C was retained forPDF Image | Electrolysis Parameters for Chlorine and Hydrogen Production
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