Study of the optimal electrolysis mode while producing sodium hypochlorite from Mediterranean Sea water

Summary

The theoretical substantiation, as well as the research results of developing the technology for producing low-concentration sodium hypochlorite by direct electrolysis of Mediterranean water for the biocidal treatment of the cooling circuit of nuclear and thermal power plants are presented. The optimal process parameters have been determined. Methods for preventing the formation of cathode deposits have been studied. It has been established that during the electrolysis of seawater with an anode current density of 1000 A/m² the maximum concentration of active chlorine 7.9 g/dm³ can be achieved; whereas at 600 A/m² – up to 6.8 g/dm³ is achieved on iridium-ruthenium-titanium oxide anodes. The current chlorine yield is within 10%. It has been shown that the current chlorine yield and specific power consumption per 1 kg of generated active chlorine reach their optimum values (85–90% and 3–3.5 kWh/kg) at an active chlorine concentration in the electrolysis product of 2 ± 0.2 g/dm³. This determines the process and economic feasibility of producing sodium hypochlorite from sea (ocean) water.

Key words

disinfection , decarbonization , low-concentrated sodium hypochlorite , membraneless electrolysis , inhibition , acid-washing , electrolysis of Mediterranean water

Engineering-economic analysis and study of acid washing of electrolyzer cathodes with solutions of hydrochloric and sulfamic acids

Summary

Experience in operating electrolysis plants shows that the main problem with the reliability of their operation is associated with insoluble compounds – CaCO₃, CaSO₄, Mg (OH)₂ – buildup on the surface of the cathodes that cause abnormal electrolysis mode, overheating and warping of the electrode system, destruction of the oxide coating of the anodes, sudden reduction in the plant efficiency etc. The greatest effect on the liquid salts and gypsum buildup during the electrolysis of salt solutions is produced by the chemical composition of water that is associated with the presence of Ca²⁺, Mg²⁺, HCO^-₃, SO₄ and dissolved CO₂ ions. The method used in practice to remove carbonate deposits by regular washing the electrode system with 3–5% solution of hydrochloric acid is not always economically justified, particularly, for the water sources characterized by increased carbonate hardness. An engineering-economic comparison of two methods of acid washing of electrode plates of an electrolysis plant with solutions of hydrochloric and sulfamic acids while dissolving in them the accumulated buildup resulted from the preparation of sodium hypochlorite from a 3% solution of sodium chloride is presented. With a slightly higher cost of 1 liter of 5% sulfamic acid (6.9 rubles/l) compared to 5% hydrochloric acid (5.36 rubles/l), the cost of one washing with sulfamic acid (2.9 rubles) is 6 times higher than the cost of one washing with hydrochloric acid (0.48 rubles), which is associated with different possibilities for reusing acids for the next washing of the electrolysis plant. In other words, if 5% hydrochloric acid (HCl) can be used 11.1 times before its neutralization, then 5% sulfamic acid (NH₂SO₃H) can be used only 2.4 times, or 11.1/2.4 = 4.6 times less, which is illustrative of the obvious economic preference of hydrochloric acid over sulfamic acid.

Key words

water disinfection , membraneless electrolysis , acid-washing , low-concentrated sodium hypochlorite