FAQ – Oximax Electrochlorination System
Q. What is Electrochlorination?
A. Electrochlorination is an electrolysis process where either : natural seawater, or, an artificial brine solution (fresh water + salt) is converted into sodium hypochlorite solution.

Salt is composed of sodium and chloride. When in solution and DC (direct current) is passed through titanium electrodes, the chlorides will disassociate to form chlorine. The process is basically as follows:
  • Electrolysis occur in an undivided cell when a DC current is passed through a saline water solution. (e.g. seawater or brine solution)
  • At the anode: Oxidation of chloride ions to produce chlorine (Cl2).
  • At the cathode: Reduction of water to produce sodium hydroxide (NaOH) and hydrogen (H2).
  • Liberated chlorine reacts with the sodium hydroxide to produce sodium hypochlorite (NaOCl) instantaneously.
  • The overall reaction, sustained by electrical energy, is as follows:
    2NaCl (salt) + H2O (water) -> NaOCl (hypo) + NaCl (salt) + H2 (hydrogen)
Q. What is Sodium Hypochlorite?
A. Sodium hypochlorite, “NaOCl” (also known as “hypo”, “bleach”, or “liquid chlorine”), is a powerful oxidant that is used world-wide for the chlorination of water for :
  • anti-biofouling (e.g. to prevent sliming, mussel/barnacle growth, etc)
  • disinfection (e.g. for drinking water, general anti-bacterial purposes in process water, etc)
  • chemical treatment (e.g. bleaching, removal of iron, manganese, etc)

Typical applications are at :
  • Power stations
  • Oil & gas installations
  • Off-shore platforms
  • Desalination plants
  • Waste water plants
  • Potable water plants
  • Marine vessels
  • Swimming Pools
Q. What are the advantages of Electrochlorination?
A. Electrochlorination is the only 100% safe chlorine technology as Cl2, in the form of a low concentrated hypo solution, is generated:
  • on-site (at the point of application)
  • on demand (only when required and always "fresh")
  • no bulk storage (because it is generated on demand)
  • only basic/safe inputs are used (seawater + electricity, or, fresh water + salt + electricity)

When using an electrochlorinator, there is no need for any commercial chlorine products. It is a safe, economical and independent chlorine alternative to hazardous, high concentrated commercial chlorine products e.g. gaseous chlorine, commercial hypo and dry chlorine (granular calcium hypochlorite).
 
It is also safe for the environment :
  • no heavy metal pollution (e.g. copper or lead), and,
  • chlorine decomposes back into common salt (sodium chloride) again
Q. What is the raw material requirement & consumption?
A. The generation of sodium hypochlorite requires only:

seawater + electricity or fresh water + salt + electricity

The following table shows the consumption of a typical plate electrolyser when generating hypo containing 1kg chlorine:

SEA WATER SYSTEM (2,000 PPM) FRESH WATER + SALT SYSTEM (6000 to 8,000 PPM)
  • 4.2 kWh (AC)
  • 500 litres seawater
  • no salt required
  • 4.5 kWh (AC)
  • 125 litres fresh water
  • 3.8 kg salt
Q. What is an electrolyser?
A. The electrolyser consist of bi-polar anodes and cathodes that are arranged to form multiple cells. The number of cells depends on the specific design parameters and will vary from case to case.

 All anodes and cathodes are made of titanium. Anodes require a coating of SC12 mixed metal oxides (MMO) of the platinum group of metals. This coating is essential for the generation of chlorine and can last between 5 to 8 years depending upon proper cleaning and maintenance.
Q. What is scaling and de-scaling?
A. All water contains dissolved calcium and magnesium. During electrolysis, calcium carbonate and magnesium hydroxide will deposit on the cathode, a process commonly referred to as “scaling”. It is a universal phenomenon with all electrochlorinators, without exception.

Scale is easily removed by flushing the electrolyser with 5% diluted hydrochloric acid. The process takes approx. 1 hour and is required as follows:
  • Seawater systems: After approx 10 days of continuous electrochlorination. The average down-time is therefore only about 0.4% of the running time.
  • Artificial brine systems: between 10 to 50+ days, depending on the hardness level of the supply water and salt supply.  (NOTE: If soft water and high purity salt is used, de-scaling may be required only once every 1 or 2 years)
The consumption of hydrochloric acid is very low and it is always re-used several times before additional acid is required to restore the acid concentration.

HES WATER ENGINEERS is offering manual and automatic acid washing systems. There is also a range of acid-free self-cleaning electrochlorination systems e.g. tubular, and, reverse polarity automatic self-cleaning plate electrolysers.
Q. I want to know more about hydrogen and hypo de-gassing tank?
A. Hydrogen is always generated during electrochlorination and is directly proportional to the amount of generated chlorine. Hydrogen is an inert by-product and does not participate in the overall chemical reaction. It is discharged along with the hypo into the de-gassing tank where it will disengage. This is the primary function of the hypo de-gassing tank. Hydrogen is then safely diluted and removed. In larger units, blower fans are used to vent hydrogen.

In order to ensure 100% safety, hydrogen is diluted 4 times below the LEL (lower explosion limit) of H2.

The second purpose of the hypo de-gassing tank is to act as a retention tank for the dosing system to offer a steady supply of hypo and to prevent pump dry-running.

The third purpose of the hypo de-gassing tank is to control the start/stop of the hypo generation process. During hypo dosing the hypo level will drop and as it reaches the low level (LSL) the electrochlorination process will start and the hypo de-gassing tank will refill again. As the level reaches the high level (LSH), hypo generation will stop.

Hypo dosing and hypo generation are therefore two completely independent processes that are only interphased with the hypo tank’s low and high level switches.
Q. What is hypo dosing?
A. The dosing of chlorine in the form of hypo is always expressed in:
  • PPM (parts per million) or mg/litre chlorine. Both represent exactly the value e.g. 2 PPM = 2 mg/litre chlorine, and,
  • total and residual chlorine, where total chlorine is the dosing at the point of application, and, residual chlorine is the remaining chlorine level after a certain time period of oxidatio.
The level of chlorine dosing depends on the client’s needs and will vary according to specific conditions and objectives.

Typical hypo dosing and control methods:
  • pump types: metering (piston or diaphragm), or, centrifugal magdrive dosing pumps
  • quantity of dosing pumps: 1x 100%, or, 2x 100% hypo (a duty and standby pump)
  • dosing control: proportional flow, or, programmable on-line residual Cl2 control
  • continuous and shock dosing (required for anti-biofouling i.e. in sea water applications)
Q.  What is  bio-fouling, continuous and shock dosing?
A. Micro-fouling:
Comes in the form of a layer of slime over the surface of heat exchanger plates and tubes and can reduce heat transfer efficiency by as much as 25%. It can also accelerate corrosion and result in a situation where maintenance is required more often than usual with subsequent increases in running costs.

Macro-fouling:
Barnacle and mussel growth inside seawater pipes and cooling systems can gradually restrict seawater flow and reduce the flow and heat transfer efficiencies. In extreme cases, pipes and valves can become completely blocked, resulting in shut-down of systems, damage to equipment, and, consequent loss of production time.

Continuous and shock dosing:
Continuous and shock dosing are usually considered for the effective control of micro and macro-fouling in seawater applications. During continuous dosing, a low chlorine dosing is maintained. It provides protection against the majority of biofouling. Unfortunately macro organisms can become resistant to this low chlorine dosing and require a much higher chlorine dosing. During shock dosing, a much higher chlorine dosing is applied only for a short time and repeated at certain fixed intervals. This will ensure complete protection against all micro and macro-fouling.
Q. What are the main components of an electro chlorination system?
A. A typical electrochlorinator is made up of the following main components and usually comes as a complete self-contained skid-mounted package:
  • Control panel with /without microprocessor or PLC (optional)
  • DC power supply with thyristor/rectifier controller
  • Filter, pressure/flow regulators, etc
  • General field instruments, sensors and control interlocks
  • Electrolyser Transfer pumps
  • Hypo dosing pumps
  • Acid washing system
  • Blower fans
  • Hypo tank
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