The Sal Process
The saline water passes through an electrolytic cell(s) utilising titanium electrodes coated with precious metals which provide high selectivity towards chlorine production. A DC current at the electrodes results in a number of electrolytic reactions taking place at the anode and cathode.
In a sodium chloride solution, chlorine gas will be created at the anode (positive electrode) by the oxidation of the chloride ion Cl⁻.
At the cathode (negative electrode), water is hydrolyzed producing hydroxyl ions and hydrogen:
1. Anode: 2CI⁻ → Cl2 + 2e⁻²
2. Cathode: 2H2O + 2e⁻ → 2OH⁻+ H2
These gases in turn will react with the water to give hypochlorous acid and a hypochlorite ion, in proportions which depend on the pH of the water.
3. In Neutral to Acidic Conditions: Cl2 + H2O → Cl⁻ + HCIO +H⁺
4. In Alkaline Conditions: Cl2 + 2OH⁻ → CIO⁻ + Cl⁻ + H2O
The hydrolysis of chlorine into hypochlorous acid, reaction 3, occurs rapidly and is complete in pH values ranging from 7 to 9.
The partial dissociation of hypochlorous acid, reaction 4, to a great extent is prevented by a lower pH. The undissociated form is desirable, as it is a better disinfectant than the hypochlorite ion.
These chlorine derived compounds are the primary disinfectants produced by the electrolytic process.
A number of other competing reactions also take place in the electrolytic cell.
The amount of chlorine produced is directly proportional to the amperage drawn at each electrolytic cell. The amount of electric current required to generate one mole of chlorine, (Cl2, -70.906g), or its equivalent in chlorine compounds after hydrolysis, is calculated as follows:
a) One mole chlorine Cl2 requires 2 faradays of electrons flowing.
2 faradays = 2 x 96,500 coulombs.
b) 1 coulomb = 1 Amp/second.
2 faradays equals: 2 x 96,500A/sec = 193.000 = 53.6111 A/hr 3,600c)
1 gram Cl2 is generated by 53.6111 = 0.756087 A,
1A = 1.3226 g Cl2/hr 70.906
These calculations assume a 100% cell efficiency. In reality a number of competing reactions take place. Cell losses, varying electrolyte conductivity, impurities etc., reduce cell efficiencies to in the region of 70% to 90%. A number of wide ranging variables such as water chemistry, cell design, ohmic losses and other factors ultimately dictate the chlorine production in each particular application.
Hydrogen Gas Evolution
Hydrogen gas is also produced at the cathode. Hydrogen is generated at the rate of 18.656 moles per 1000 Amps/hr.
I mole of H2 = 2.0158g.
18.656 moles H2= 37.606764
gH2 Gas density = 0.0898 g/1 at 0°C at 1 atm
Volumetric measurements of H2 can be calculated by:
V= Vo 273+t Po / 273 P
Where t=temp in °C
Po, P = pressure in atm273 = temperature in 0Kelvin relating to 0°C.
Assuming an ambient temperature of 25°C and atmospheric pressure, the volume generated by 1,000Alhr is: 273+25 x 418.83= 457.18 litres/hr273
Hydrogen air mixtures can be hazardous in a wide range of hydrogen/air ratios:
Hydrogen/Air content % volume (25°C)
0 – 4 Not flammable
4- 18 Flammable1
8 – 59 Detonative
59 – 75 Flammable
75 – 100 Not flammable
In water, hydrogen oxygen mixtures do not ignite. However care must be taken in ensuring that hydrogen levels do not build up in air or piping to flammable levels. Accordingly, the SAL plant design and installation incorporates the following design factors:
a) Cell and plumbing design to avoid the accumulation of gas build.
b) Automatic cessation of power supply ( and generation of hydrogen) in the event of interruption of water flow.
c) Well ventilated rooms for electrolytic cells.
d) Incorporation of gas loop in plumbing as a secondary preventative measure to avoid continuation of electrolysis in the event of interrupted water flow.
e) Straight line plumbing after electrolytic cells. Placement of cells after receptacles such as filters tanks etc.
f) Strategic venting of hydrogen to the atmosphere on larger units
Is The Salt Corrosive?
Water containing salt in concentrations of 5,500 to 6,500ppm is mildly corrosive, but as all chlorine is made from salt, conventionally chlorinated water is corrosive too. Whereas saline water has no harmful effects at all, impurities in chemical chlorine can damage pool surfaces. Most leading brands of filters and pumps are not affected by mildly saline water, and carry warranties to that effect. Modern corrosive resistant materials such as UPVC should be used in plumbing, as is now customary with any water treatment systems. Sea water has substantially higher salinity levels, and in such cases, the usual special precautions need to be taken against corrosivity