Chemistry
The chemistry of electrochlorination is based on the partial
electrolysis of sodium chloride contained in raw seawater.
As the seawater flows between anodic and cathodic electrodes
energized by a direct current in an cell (electrolyzer),
chemical reactions take place in the seawater between the
products of electrolysis.
By passing direct current through an aqueous solution of
sodium chloride sodium ion (NaCL), which is completely
dissociated into sodium ion Na+ and chloride ion CL-
1. Free chlorine is generated at
the anode:
2Cl - à Cl 2 + 2e-
2. Hydrogen is evolved at the cathode with the
corresponding formation of hydroxide ions ( OH - ):
2H 2 O + Cl 2 à NaClO + NaCl + H 2 O
3. OH - ions migrate from the cathode area
and react with sodium (Na+) and chloride (Cl 2 ).
Near the Anode, thus producing sodium hypochlorite NaClO;
4. This overall chemical reaction can be expressed as follows:
2NaOH + Cl 2 à NaCl+ H 2 O
Side reactions, both chemical and electrochemical,
take place simultaneously with basic reactions, such
as the decomposition of hypochlorite to chloride,
the anodic oxidation of hypochlorite to chlorate (
traces only) and the cathodic reduction of
hypochlorite to chloride and the anodic evolution of oxygen.
Some reactions that are present in seawater (e.g.,
calcium, magnesium and other metals) form hydroxides
and carbonates resulting in suspended solids that are
taken out of the electrolyzer by the seawater stream.
Cathodic deposits, however, tend to deposit on the
cathodic surfaces and therefore a periodic chemical
cleaning procedure is recommended.
These side reactions reduce the current efficiency;
therefore, the actual amount of electric power
required to produce hypochlorite is approximately
10 percent higher than the theoretical foreseen.
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 SANILEC Systems is in the
range of 500 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.
Process Description
Seawater is delivered to SANILEC Systems at a constant flow rate.
Sodium hypochlorite is taken downstream to the hypochlorite injection point,
as it is essential that the seawater delivery system contain
some active chlorine to protect pipelines and other equipment
(such as screens) from forming organic fouling.
Before entering a SANILEC System, seawater is strained through
0.8 mm screens to remove suspended solids that would otherwise
affect the proper functioning of the installation.
The required seawater pressure varies with the capacity of the
SANILEC System and the circuit characteristics from 4 to 7 bar g.
Electrolyzer Cell
Severn Trent De Nora offers three different mechanical
configurations of electrolyzer cells. We select the best
cell configuration for the application. When selecting a
cell configuration we consider the application, seawater
characteristics, cost, size, weight, pressure, maintenance and historic preference
SANILEC Tube Type |
SANILEC Plate Type |
SEACLOR Cells |
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SANILEC® Seawater Plate Cell Specifications
Design: Each cell is monopolar in design and comes standard as 1, 3 or 6 packs per cell. This cell arrangement provides maximum flexibility for circuit configuration.
Cell Body: PVC with ultraviolet stabilizers for
good corrosion resistance and outdoor stability. Flanged
inlet and outlet for positive sealing to seawater piping.
Anodes: Dimensionally stable, of expanded titanium
metal with precious metal oxide coating (DSA®). Anode
mesh is supplied with PVDF spacers to maintain a 2.5 mm
gap between anode and cathodes.
Cathodes: Nickel alloy for excellent corrosion
resistance to the seawater/hypochlorite solution produced
in the cell and for 10% lower power consumption as compared
to standard titanium cathodes. Not subject to hydrogen
embrittlement as are titanium cathodes.
Cell Cover: Clear acrylic for visual inspection
of the cell internal components during normal operations.
Gaskets: Viton O-ring seals around conductors and a
silicon rubber O-ring seals the cell cover to the body. These
gasket types have demonstrated long life and excellent sealing properties.
Hardware: All internal fasteners and hardware are titanium; external fasteners are
stainless steel.
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