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Membranes 2022, 12, 602 2 of 18 seawater. In all cases, a purification phase will be necessary to avoid fouling problems due to precipitation of insoluble salts. Many elements can vary between the different installations; including the presence or absence of a separator, the nature of the separator, the nature of the electrodes, and, finally, the several possibilities of electrical connection—in series or in parallel—and of hydraulic connection—with or without recycling the products. Whatever the variant of the process used, the choice of electrodes and membranes that can be used is greatly reduced in order to have a minimum longevity of several years due to the oxidizing or basic nature of the reaction products. The nature of the materials used for the anode and cathode also plays an important role in the energy balance due to the voltage surge required for the reactions envisaged. In each case, operating parameters such as salt concentration, solution flow rate, cell dimensions and geometry, and voltage/current parameters must be carefully optimized. As chlorine and caustic soda are important raw materials and are mass-produced, many recommendations have been published by the European Commission [1] and Smith [2] concerning the process choice, and much research is still ongoing to improve yields and energy costs. 1.1. Electrolysis Cells 1.1.1. Reactions The expected reaction at the anode is the oxidation of chloride to chlorine, but due to the oxidation potentials, water will also oxidize to form hydrogen. At the cathode there will be reduction of water to form OH− ions and hydrogen. Depending on the different parameters of the cell constitution or its operation, other parasitic side reactions may also occur, such as chlorate formation [3,4]. 12HClO + 6H2O → 4ClO3− + 8Cl− + 24H+ + 3O2 + 12e− 2HClO + ClO− → ClO3− + 2Cl− + 2H+ These side reactions are repressed by lowering the pH value [5]. Reaction Step I: Anode: 2H2O → 4H+ + O2 + 4e− 2Cl− → Cl2 + 2e− Cathode: 2H2O + 2e− → H2 + 2OH− Reaction Step II: 2Na+ +2OH− +Cl2 →NaOCl+H2O+Na+ +Cl− H2O+Cl2 →HOCl+H+ +Cl− (I-a1) (I-a2) (I-c1) (II-1) (II-2) At the anode, chloride ions are converted into gaseous chlorine: 2Cl− Cl2 + 2e− E0(Cl2/Cl−) = 1.358 V The cathode compartment is fed with a water solution of sodium hydroxide, i.e., pH of 14. At the cathode, water is reduced to gaseous hydrogen and hydroxyl ions: 2H2O + 2e− H2 + 2OH− E0 (H2O/H2) = 0.828 V The hydrogen gas produced at the cathode together with caustic solution (concentra- tion 35 wt%) from chlor-alkali cells is normally used for the production of hydrochloric acid or as a fuel to produce steam and energy [6]. For disinfection purposes, hypochlorous acid is produced by the reaction of chlorine which is disproportionate in the presence of water Cl2 + H2O HClO + H+ + Cl−.PDF Image | Zero Gap Electrolysis Cell for Producing Bleach
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