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6 Domga et al.: Study of Some Electrolysis Parameters for Chlorine and Hydrogen Production Using a New Membrane Electrolyzer necessary to wait more than 25 minutes. A similar effect was noticed from catholyte compartment, in which hydrogen production rate almost increased with increased current density (figure 5b). This indicates that, the chlorine and hydrogen production rate are the most dependent upon the applied current density. That confirmed to the previous report; a high current density should result in a high selectivity for hydrogen and chlorine evolution [26]. Others authors [17] reported that current density was the most striking parameters on the cell voltage with contribution percentage of 70%. The maximum current density (i = 41.4 A/dm2) in this study was smaller than the standard because small electrode was used. Its standard normal rate was 1-7 kA.m-2 [1, 18, 27]. 3.5. Effect of Electrode Surface Area The electrode surface can directly affect the kinetics of electrolysis. Thus, the cell performance of different number of electrodes was evaluated. Three experimental setups (monopolar) was developed; N=1 for 1 anode and 1 cathode, N=2 with 2 anodes and 2 cathodes, N=3 with 3 anodes and 3 cathodes. The operating conditions were: 320 g.L-1 NaCl (pH=2), 24% NaOH, T= 80°C. The experimental results are shown in figure 6. Figure 6. Effect of surface area of electrode on the electrolysis kinetic. The volume of chlorine increased with the number of anode (figure 6a). Indeed, to produce 300 mL of chlorine, we need only 10 minutes with 3 anodes, whereas with an anode, it’s necessary to wait for more than 30 minutes to obtain 300 mL under the same operating conditions. Similarly, the production of hydrogen increased with the number of cathode, figure 6b. Results indicate that a larger electrode surface area per unit volume was better for the production of chlorine and hydrogen with the same current density. According to this investigation, the use of several electrodes assembled in series makes it possible to optimize the production of gases, despite for the fact that the current crosses each electrode. In addition, current efficiency drops off due to high current consumption [26]. However, increase the cell with electrodes (reduction of inter-electrodes distance), the density of current becomes too high and could cause short circuits [28]. The choice of the electrode number (3 anodes and 3 cathodes) in this study was for practical reasons. Indeed, the electrodes number lower than three would not be interesting, because gases production would slow and the electrodes number higher than 3 will be difficult to realize taking into account the dimensions of cell. 3.6. Current Efficiency 3.6.1. Current Efficiency for Anodic Reaction The current efficiency for Cl2 production was calculated from the ratio of the obtained values by the thermodynamic values from the Faraday’s law of electrolysis, equation (10). The current efficiency is 81%. Thus the current was not exclusively used for the oxidation of chlorides ions to chlorine gas. This may be accounted for by one or more of a number of factors. The generation of secondary compounds may cause reduction of faradaic efficiency. Chloride ions in the brine are converted into various chlorine compounds during electrolysis and hydrolysis. In addition, according to previous study, the production of oxygen during the NaCl electrolysis, although slow, is inevitable [29]. Thus reactive oxygen species from oxygen evolution also reacts with chloride in water to produce reactive chlorine species such as free chlorine: HOCl, ClO– [27]. According to [26] the presence of chlorate in the effluent is expected due to high cell temperature. The reduction in current efficiency due to these parasitic reactions ranges from 1-5% [9]. In this study, the material anode was graphite and the overpotential for chlorine evolution was as high as 500 mV which increased energy consumption and consequently the current efficiency is affected. There was some attempt to replace these by a dispersed form of a precious metal such as Pt, Pt/Ir, and Ru on a titanium base, and indeed these did reduce the overpotential to about 100 mV. These anodes were, however, expensive and not entirely stable, platinum in thePDF Image | Electrolysis Parameters for Chlorine and Hydrogen Production
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