What is Electrochlorination?

Electrochlorination is an electrolysis process where natural seawater or a salt solution is converted into a sodium hypochlorite solution. The dissolved salt is sodium chloride (NaCl), where one sodium atom (Na) is bound to one chlorine atom (Cl). When direct current is passed by titanium electrodes through a sodium chloride solution in an undivided cell, an oxygen atom is attached to the sodium chloride to produce sodium hypochlorite (NaOCl).

• Oxidation of chloride ions at the anode produces liberated chlorine (Cl2).
• Reduction of water at the cathode produces sodium hydroxide (NaOH).
• The liberated chlorine Cl2 immediately reacts with the sodium hydroxide NaOH to produce sodium hypochlorite (NaOCl).

The overall electro chemical reaction is as follows:
2NaCl (salt) + H2O (water) => NaOCl (sodium hypochlorite) + NaCl (salt) + H2 (hydrogen)

An old chlorination system: hazardous buckets of hypochlorite.	A new flow-through electrochlorination system: automated continuous generation of hypochlorite.

Sodium Hypochlorite Applications

Sodium hypochlorite, also known as “hypo”, “bleach” or "liquid chlorine," is a powerful oxidant. It is used worldwide for:

• Marine growth anti-biofouling (e.g., to prevent slime, mussel, barnacle growth, etc.)
• Disinfection (e.g., for drinking water, general anti-bacterial purposes, etc.)
• Chemical treatment (e.g., bleaching, removal of iron, manganese, etc.)

Typical uses and applications:

• Marine Vessels
• Power Stations
• Oil & Gas Installations
• Off-shore platforms
• Desalination Plants
• Wastewater plants
• Potable water plants

Advantages of Electrochlorination

Electrochlorination is a 100% safe chlorine technology, which generates Cl2 as needed in the form of a low-concentration hypo solution. The process only uses seawater plus electricity, or fresh water and salt plus electricity. Advantages include:

• On-site, on-demand generation.
• No need to purchase expensive chemicals.
• No handling or bulk storage of hazardous chemicals.
• Automatic operation to minimize human error/

Electrochlorination is the safe, economical alternative to hazardous, high concentrations of commercial chlorine products, e.g., gaseous chlorine, commercial hypo, or dry chlorine. It is also safe for the environment, as there are no heavy metal byproducts, such as copper or lead, and the hypochlorite (NaOCl) reverts back to common salt (NaCl) after giving up its oxygen atom.

Electrochlorination to Avoid Bio-Fouling

Micro-fouling refers to a layer of slime over surface. On heat exchanger plates and tubes, micro-fouling can reduce heat transfer efficiency by as much as 25% and also accelerate corrosion. Periodic maintenance to clean the surfaces is required, creating downtime and increasing running costs.

Macro-fouling refers to barnacle and mussel growth. Inside seawater pipes and cooling systems, macro-fouling can gradually restrict seawater flow and consequently reduce flow and heat transfer efficiencies. In extreme cases, pipes and valves can become completely blocked, resulting in equipment damage and major system shutdowns.

Hypochlorite continuous and shock dosing are used to effectively control micro and macro-fouling in seawater applications. In continuous dosing, a low level of chlorine is continually supplied. This protects against the majority of bio-fouling, but unfortunately certain macro organisms can become resistant to low level dosing. These macro organisms can be eliminated by shock dosing, where much higher levels of chlorine are applied for a short time at regular intervals.

Electrochlorination Systems Hardware

Electrochlorination electrolizer flow-through electrode systems.

Electrolyzer Subsystem

The electrolyzer consists of anodes and cathodes that are arranged to form multiple cells in series. The number of cells is determined by the application and the amount of chlorine to be generated. The anodes and cathodes are arranged in a concentric tubular assembly, where seawater is passed in the annular space between the outer and inner tubes. High-velocity seawater reduces scaling and acts as a continuous self -leaning mechanism.

Farwest anodes and cathodes are made of titanium. The anodes are coated with mixed metal oxides (MMO), which is essential for the generation of chlorine and can last from 5 to 8 years. Farwest electrolyzers are guaranteed for 5 years continuous duty.

Electrolyzer Maintenance

All water contains dissolved calcium and magnesium. During electrolysis, calcium carbonate and magnesium hydroxide will deposit on the cathode, a process referred to as “scaling.”  Scale is easily removed by flushing the electrolyzer with 5% diluted hydrochloric acid. This process takes about an hour and should be done after 10 days continuous electrochlorination. The consumption of hydrochloric acid is very low .