Electro chlorination 4

Sodium Hypochlorite Generators are the heart of each Electro-chlorination Package. Each Generator matches with a dedicated Transformer/Rectifier unit. The modules of each Generator are connected in electrical series to the Rectifier. The modules are also hydraulically connected in series. In fact, feed seawater passes through the modules, where it is electrolyzed.

Each generator work with a constant feed seawater flow-rate. This flow-rate is kept constant by the flow loop controller (magnetic flow transmitter + flow control valve) installed on the inlet line of each generator. Electrolytic cells are bipolar in design. This means that the cathodes of every cell are directly connected to the anodes of the next cell.

The electrolyzers operate under constant seawater flow rate while the D.C. current is adjusted so that the generation of chlorine instantaneously matches the water treatment demand. Finally, sodium hypochlorite solution and by-product hydrogen, in mixed phase, are sent through Generators outlet header to the sodium hypochlorite storage and hydrogen dilution system. 

Chlorine is generated at the anode:

2 Na+ + 2 Cl- → Cl2 + 2 Na+ + 2e-

Hydrogen is evolved at the cathode with the corresponding formation of hydroxyl ion:

2 H2O + 2e- → 2 OH- + H2

The overall electrochemical reaction is:

2 Na+ + 2 Cl- + 2 H2O →Cl2 + H2 + 2 Na+ + 2 OH-

Then chlorine and hydroxyl ions react chemically producing sodium hypochlorite and sodium chloride, as follows:

2 Na+ + 2 OH- + Cl2→ Na+ + ClO- + Na+ + Cl- + H2O

The overall electrochemical reaction can be expressed as follows:

NaCl + H2O → NaClO + H2

Inside electrolyzer

Side reactions, both chemical and electrochemical, take place simultaneously with the basic reactions, such as the decomposition of hypochlorite to chloride, the anodic oxidation of hypochlorite to chlorate (traces only) the cathodic reduction of hypochlorite to chloride and the anodic evolution of oxygen. All these side reactions affect the current efficiency so that D.C. power required to produce hypochlorite is actually higher than the theoretical one.

Moreover, some cations which are present in seawater (e.g. Calcium, Magnesium) form hydroxides and carbonates resulting in suspended solids, which are mostly carried out of the electrolyzers by the chlorinated seawater stream. However, due to deposit formation on the electrode surface, the electrolyzers must undergo a periodic cleaning with diluted hydrochloric acid (6 % w/w). Best frequency of acid washing, for a given plant site, can be validated only on the basis of the field experience.


Electro chlorination 3

In the chemical literature, hypochlorite concentrations are commonly referred to in term of “available” or “active chlorine”, i.e. the quantity of chlorine having the same oxidizing effect as the hypochlorite when analyzed by standard methods. 


Acid Washing System

It is well known that the generators of active chlorine by electrolysis of untreated brine or seawater are fouled by precipitation on the cathodes of hydroxides and carbonates of cations, such as magnesium and calcium, present in the feed solution.

The fouling rate mainly depends on:

• cathodic current density;

• seawater chemical composition, temperature and flow-rate through the electrolyzers;

• material and surface finishing of the cathodes.


Generally, low current density together with low seawater flow-rate favors the fouling of the electrolyzers. The optimum operating conditions to minimize the fouling vary for each specific design, so that a comparison of operating data of various electrolyzers disregarding the design characteristics should not be taken into consideration.

It should be noted that it may not be feasible to maintain constantly the optimum operating conditions during the whole life of the electrolyzers, and that fouling usually takes place as soon as the electrolyzers operate, even for a limited period of time.

The cleaning of the electrolyzers does not require any dismantling and it is done by circulating through the electrolyzers a dilute hydrochloric acid solution (normally 6% HCl by weight). The system shall be acid washed on a routine basis, after 700 hours of generator working. The duration and frequency of acid washing is dependent on the seawater chemical composition and temperature and on the current density. Notwithstanding, optimum cleaning frequency is always related on how the Sodium Hypochlorite Generator is operated and maintained.


If the duration of each acid cleaning, the number of replacements given by the acid circulating pump and/or the cleaning time (which should be at least 1.5 hours), or acid concentration are not sufficient, salt deposits will lead to hard scaling problems of the electrodes. Therefore, subsequent cleaning cycles could not be effective, if path between adjacent electrodes is completely blocked. In conclusion, the acid cleaning at plant site is validated only on the basis of the field experience.

In fact, during acid cleaning performed on site, the operator could determine the concentration of the circulating acid every 10-15 minutes, by chemical titrimetric analysis: a constant acid concentration (not decreasing anymore) will indicate that all the precipitates have been removed and the circulation can be stopped. The best washing efficiency is obtained by using HCl between 4% (minimum) and 6% by weight, because the lower is the concentration; the longer is the time to dissolve precipitates.





Contact with us                            
connect with us