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Chemistry
The chemistry of hypochlorite production is based on the electrolysis of sodium chloride. Seawater (normally about 28 grams/liter NaCl) or other water containing NaCl may be used to generate sodium hypochlorite by passing a direct electrical current between an anode and a cathode through the salt solution. In a commercial situation to boost production there are multiple electrodes contained in one body called an electrolyzer. Also in a commercial situation the brine solution flows through the electrolyzer and is only partially depleted. The electrolysis of NaCl solutions to form sodium hypochlorite is shown in the overall reaction as equation (1)
Equation (1)
NaCl + H2O + 2e- à NaOCl + H2
Chlorine is generated at the anode according to equation (2)
Equation (2)
2Cl- à Cl2 + 2e-
The chlorine is hydrolyzed in solution to form hypochlorous acid as in equation (3)
Equation (3)
Cl2 + 2H2O à 2HOCl + 2H+
The hypochlorous acid dissociates to hypochlorite in the bulk solution at alkaline pH as in equation (4)
Equation (4)
HOCl à OCl- + H+
Hydrogen and hydroxide ion are formed at the cathode per equation (5)
Equation (5)
2H2O + 2e- à H2 (g) + 2OH-
Because the anode and cathode compartments are not segregated (un-separated electrolytic cells) the species formed at the anode and cathode are free to react in the solution to form the respective end products shown in the overall electrochemical and chemical reactions. This includes side reactions which lead to the decomposition of hypochlorite to chloride and chlorate and oxygen as in equation (6)
Equation (6)
6ClO- + 3H2O à
2ClO3- + 4Cl- + 6H+ + 3/2O2 + 6e-
Also hypochlorite can be decomposed at the cathode to chloride as shown in equation (7)
Equation (7)
ClO- + H2O + 2e- à Cl- + 2OH-
All the reactions above are for a basic un-separated hypochlorite cell. The side reactions reduce the production efficiency; therefore, the actual amount of electric power required to produce hypochlorite is approximately 12 percent higher than the theoretical amount.
Seawater also contains approximately 60 parts per million (ppm) bromine. The bromine reacts with the chlorine and displaces it. This alters the above chemical products to form hypobromous acid and hypobromite as shown in the following equations (8) and (9). These then become the biocide and disinfectant. The following reactions are unique to seawater systems. Seawater contains large amounts of magnesium, calcium, and other metals. These react with the hydroxide produced at the cathode to form metal hydroxides and carbonates that form deposits on the cathode surface and precipitates in the solution. The suspended solids are easily flushed from the electrolyzer during normal operation. The tenacious cathode deposits require periodic cleaning with an acid to dissolve the solids, but the unique BALPURE® electrodes eliminate buildup and required cleaning.
Equation (8)
HOCl + Br- à HOBr + Cl-
Equation (9)
HOBr à OBr- + H+
The BALPURE® ballast water treatment system has the added feature of oxidant neutralization prior to discharge overboard. The reaction uses sulfite to react with the oxidant to create harmless sulfate at approximately 10 ppm. Sulfate is present in the ocean at concentrations of 4,000 ppm. The reaction is shown in equation (10)
Equation (10)
Na2SO3 + HOBr à Na2SO4 + HBr
Product Characteristics
In chemical literature, hypochlorite concentrations are commonly referred to in terms of available or active chlorine (e.g. the quantity of chlorine having the same oxidizing effect as the hypochlorite, when analyzed by standard methods). The available chlorine concentration in hypochlorite solutions produced by SANILEC and BALPURE® Systems is in the range of 500 to 2500 ppm.
Byproducts
Hydrogen gas is produced in the electrolyzer at the rate of approximately 0.35 m3/kg chlorine. Dilution of hydrogen with air is effected in order to reduce the hydrogen concentration to less than 1 percent (v/v) immediately as it disengages from the liquid effluent in the hypochlorite collecting tank. The LEL for hydrogen in air is 4 percent.
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