Process Description

Pressurized seawater is delivered to the SEACLOR® system where it is strained to 0.8 mm to remove suspended solids. The seawater passes through a flow control assembly, which may include a flow control valve, and a flow transmitter with local indication and low flow shut down protection. The seawater then passes through the electrolyzer cells and exits the cell as sodium hypochlorite solution and byproduct hydrogen gas. The solution is piped to a tank or cyclone where hydrogen is removed from the solution. The hydrogen is typically diluted with air using a set of redundant blowers to a safe level (typically less than 1%). Finally, the sodium hypochlorite solution is injected at required continuous and shock-dose rates.

The process is based on the electrolysis of seawater as it flows through an unseparated electrolytic cell. The resulting solution exiting the cell is a mixture of seawater, hypochlorite, and hypochlorous acid. Electrolysis of sodium chloride solution (seawater) is the passage of direct current between an anode (positive pole) and a cathode (negative pole) to separate salt and water into their basic elements. Chlorine generated at the anode immediately goes through chemical reactions to form hypochlorite and hypochlorous acid. Hydrogen and hydroxide are formed at the cathode, the hydrogen forms a gas and the hydroxide aids in the formation of hypochlorite and increases the exit stream pH to approximately 8.5.

This overall chemical reaction can be expressed as follows:
(Salt + Water + Energy = Sodium Hypochlorite + Hydrogen)
NaCl + H2O + 2e = NaOCl + H2

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 SEACLOR systems is in the range of 1000 to 2500 ppm.

Producing sodium hypochlorite onsite makes it possible to shockdose while storing a minimal amount of chemicals. Each shock treatment administered at regularly spaced intervals during the day, must correspond to the renewal of the hypochlorite solution in the storage tank. Long storage periods, such as two days or more, should not be considered as a design criteria.

Byproducts
Hydrogen gas is produced in the electrolyzer at the rate of about 0.35m3/kg chlorine. Dilution of hydrogen with air is effected in order to reduce the hydrogen concentration to less than 2 percent (v/v) immediately as it disengages from the liquid effluent in the hypochlorite collecting tank. In fact, the release of hydrogen to the atmosphere as an undiluted gas may create hazardous conditions.