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2 Domga et al.: Study of Some Electrolysis Parameters for Chlorine and Hydrogen Production Using a New Membrane Electrolyzer following reactions: At the anode: Cl- → Cl2 + 2e- (1) At the cathode: 2H2O + 2e- → 2OH- + H2 Overall reaction: 2H2O + 2NaCl → Cl2 + 2NaOH + H2 (3) A challenge in the process is parasitic oxygen evolution 8]. The possible pathways for this oxygen evolution, reaction (4) include direct anodic oxidation, competing with reaction 1, direct anodic chlorate formation, reaction (5) as well as decomposition of hypochlorite, reaction (6). Engineers, Eltech Systems Corporation, INEOS Chlor, and Uhde [1]. In addition, there are few studies reported in the scientific journals [14]. In the present work the electrochemical reactor (electrolyzer) was conceived and used to identify the most important parameters of the chlor- alkali membrane process and to investigate their effect on the cell performance. 2. Material and Methods 2.1. Membrane Cell Setup In this study, many monopolar cells design were used to study and optimize various operating parameters (temperature, electrolyte concentration, current density) and engineering parameters (electrode surface area, gap between the electrodes). The setup used was made in PVC to avoid corrosion from chlorine, brine and sodium hydroxide. It has the cylindrical structure (10 cm of diameter) and comprises two cylindrical compartments (one is inside the other). The inside cylindrical with 0.65 L forms the anode side and was fed with sodium chloride. The other cylindrical with 0.75 L forms the cathode compartment and water was introduced in this side. The compartments were separated by an ion exchange membrane (DuPont N324). Electrodes (anode and cathodes) used in this study were composed of graphite tube with an effective surface area of 12,058 cm2. These electrodes are recycled from the used batteries. A more detailed description of the electrolyser and a design of this electrochemical reactor are illustrated in figure 1. Figure 1. Components of the electrochemical cell. 2H2O+2e- →O2 +4H+ +4e- [7- (4) 6ClO- +3H O→ClO - +4Cl- +6H+ +1.5O +6e- (5) 232 2HOCl → 2HCl + O2 (6) The reduction in current efficiency due to these parasitic reactions ranges from 1-5%. Whiles measures such as usage of selective electrodes and anolyte acidification can reduce the oxygen side reaction to enable a current efficiency of close to 99% [9]. All modern cells (since the 1970's) use these so-called “Dimensionally Stable Anodes” (DSA) [5]. The DSA as an industrial electrode made of RuO2 as active component, inert oxide such as TiO2 as stabilizer and coated onto titanium substrate [8, 10-11]. The cathode is typically steel in diaphragm cells, nickel in membrane cells, and mercury in mercury cells [4]. The main technologies applied for chlor-alkali production are diaphragm, mercury and membrane cell electrolysis. The difference in these technologies lies in the manner by which the chlorine gas and the sodium hydroxide are prevented from mixing with each other to ensure generation of pure products. Currently, the membrane cell is the best available technique for the chlor-alkali production, which represents about 61% of the chlorine production capacity in Europe [12]. Two types of electrolyzers were developed, they are monopolar and bipolar. In the monopolar type, all of anode and the cathode elements are arranged electrically in parallel. In the bipolar type, the cathode of a cell is connected to the anode of an adjoining cell, so that the cells are in series. Each type of electrolyzer had advantages and disadvantages [13-14]. The main problem of all these chlor-alkali processes is the high electric energy consumption which usually represents a substantial part of the production cost [6, 15]. The knowledge of the influencing parameters of electrolysis reactions is a need for the improvement of an electrolyzer performance. Despite being a mature technology, the knowledge about the chlor-alkali membrane process is dominated by membrane suppliers [16]. There are only five major suppliers of cell technology and they are: Asahi Kasei Corporation, Chlorine (2)PDF Image | Electrolysis Parameters for Chlorine and Hydrogen Production
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