Technical Overview
INDEX
1.0 Chemistry of Hypochlorination
1.1 Electrolyte
1.2 Reactions
2.0 Hypochlorination in Industry
2.1 Power and Desalination
2.2 Water, Wastewater, Sewage
2.3 Oil & Gas
2.4 Shipping and Marine
3.0 Models
4.0 Modules
5.0 Cells
6.0 Degassing
7.0 Cathode Deposits
8.0 Acid Cleaning
9.0 System Treatment, Pulp & Paper
10.0 Warranty
11.0 Process Guarantee
1.0 Chemistry of
Hypochlorination
1.1 Electrolyte
Chloride is the essential ion found in the electrolyte solution that is
used in the production of sodium hypochlorite.
Seawater can be used as the source of the chloride ion that is required in making electrolytic hypochlorite. Normal seawater has a chloride concentration of around 19,841 milligrams of Cl- per liter. This is equivalent to a NaCl concentration of 31 grams per liter, which is very suitable for use as the electrolyte. Further modification is not required other than straining the seawater to remove any entrained solids.
Frequently seawater sources are from estuaries or other locations, which
may dilute the saltwater. Such sources are usable provided:
(a) Dilution with fresh water is not excessive.
(b) Only a low concentration of hypochlorite is required.
(c) Objectionable substances are not introduced with the dilution water.
A sample of seawater should be taken to determine whether it is suitable
for use as an electrolyte. Generally, salts other than sodium chloride
are not objectionable.
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1.2 Reactions
The principle reactions occurring in the electrolytic cell that produces
sodium hypochlorite are quite simple, as shown in the following:
Oxidation of the chloride ion occurs at the anode:
2Cl- 
2Cl2 + 2e-
Followed by a rapid hydrolysis of the chlorine:
Cl2 + H2OHOCl
+ HCl
Reduction of the sodium ion occurs at the cathode:
Na+ + e-Na
Followed by a rapid reaction of the sodium with water:
Na+ + H2O1/2H2 + NaOH
The acids (HCl and HOCL) produced at the anode react with the base (NaOH) produced at the cathode:
HCl + NaOHNaCl
+ H2O and,
HOCl + NaOHNaOCl
+ H2O
The net reaction of electrolysis is:
NaCl + H2Oe-NaOCl
+ H2
The amount of hypochlorite produced is related directly to the amount of direct electrical current passed through the cell.
The Hypochlorite produced may be:
(a) Oxidized further due to higher oxychlorine compounds.
(b) Reduced at the cathode back to chloride.
(c) Decomposed to NaCl + 1/2O2
Cell design and the prevailing operating conditions are critical
factors for the efficient production of sodium hypochlorite (NaOCl) at
the lowest overall cost. The system and the mode of operation may vary,
depending on the source of the seawater and the target concentration in
the final product. It should be understood that since seawater is high
in hardness ions, scale can form on the cathode walls. When the adherent
scale forms, it must be removed by interrupting the current to the cells
and circulating a dilute solution of hydrochloric acid (HCl). The scale
is converted back to its original form and flushed out with the solution.
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2.0 Hypochlorination in Industry
2.1 Power and Desalination
On-site sodium hypochlorite generation has proved to be the most economical
method to chlorinate cooling water systems in the power and desalination
industries to keep cooling water systems free from pathogens, algae and
mussel growth. On-site hypochlorination requires no special handling procedures,
is classified non-hazardous at the hypochlorite levels generated, and
does not require transport or storage facilities.
Generally, sodium hypochlorite is generated close to the seawater intake and fed from the cooling water discharge manifold. The pressures are usually moderate at less than 15 Bar, so an off-take to supply the hypochlorite generators can be taken directly from either the manifold or the main cooling water pipe line.
After conversion, the hypochlorite is piped back to either the pump suction or directly into the suction pit, usually before the band screens in seawater applications. Diffusers are placed over the discharge points to ensure that the whole of the incoming seawater is treated. By installing the hypochlorite discharge at the pump suction, the whole of the cooling water system can be treated, including condensers, heat exchangers and associated pipe work.
Usually in clean seawater, where the chlorine demand is low, the cooling water system can be successfully chlorinated with a constant 1mg/l of available chlorine on a 24hr/day basis. Electrichlor hypochlorite generators are sized for the operation required; however, they can be operated up to 125% of their rating for short periods. This feature allows for an intermittent super chlorination to positively kill any algae, mussels or pathogens, which may become resistant to the low dosage of chlorine. Studies have shown that some mussels can survive for several weeks in chlorinated water. Therefore, it is imperative that constant chlorination be carried out with an intermittent high dosage at regular intervals. The Electrichlor system allows for a high dosage output to be automatically provided if required.
Where industries are located inland and cooled with fresh water, saltwater
from a brine mixer can be used. Like the seawater system, water is taken
directly from the cooling water intake manifold or pipework into a brine
mixer tank. The process water should be softened through an ion exchange
unit and then mixed in the brine tank with good quality sea salt. The
brine is piped into the hypochlorite generator and the hypochlorite introduced
back into the cooling water intake. This type of system is suitable for
both cooling tower applications as well as large storage systems. The
amount of hypochlorite usage in cooling tower systems can be considerably
less than storage systems; therefore, brine systems can be very effective
in these applications.
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2.2 Water, Wastewater, Sewage
Treatment and Pulp & Paper
Hypochlorite generation for these industries is almost exclusively brine
systems. Salt water is mixed in a brine tank with softened water and introduced
into the process stream where required. Generally, the user defines the
requirement of the system and Electrichlor will engineer the hypochlorite
generators to suit the need.
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2.3 Oil & Gas
For on-shore refineries and gas pumping plants, cooling water systems
are similar to power stations, although they can be considerably larger,
with the resultant up scaling of hypochlorite equipment. Refinery applications
include process and fire water systems, which can be dosed with hypochlorite.
Electrichlor can provide hypochlorite generators, both seawater and brine
systems, for these applications.
In offshore service such as oilrigs and production platforms, Electrichlor
special application hypochlorite generators can be provided. These generally
fulfill a multipurpose task, providing hypochlorination for cooling, fire,
process water and sewage treatment from the same source.
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2.4 Shipping and Marine
Hypochlorination for the shipping and marine industries is similar to
hypochlorite applications in Oil & Gas offshore systems, namely, the hypochlorination
of cooling, fire, process water and sewage treatment.
Treatment of Ballast Water
The transport of pathogens, algae and mussels in ships' ballast water
has become a worldwide problem. The marine regulatory body, the International
Marine Organization (IMO) based in London, under Resolution A.868(20)
has issued "Guidelines for the control and management of ships' ballast
water to minimize the transfer of harmful aquatic organisms and pathogens".
Under this resolution, MEPC the (Marine Environmental Protection Committee)
as part of the IMO "is working towards the completion of legally binding
provisions on ballast water management, together with guidelines for their
uniform and effective implementation with a view to their consideration
and adoption in the year 2000".
Uncontrolled dumping of ballast water has led to the proliferation of
foreign water borne pathogens, algae and mussel growth invading coastal
and port environments around the world. Ballast water can be successfully
treated with hypochlorite either on board or in ports where on-land holding
basins are installed. In some situations, dechlorination systems may be
necessary. It has been suggested in literature that large amounts of chlorinated
water may disturb the local marine life. While this is improbable, de-chlorination
can be effected before ballast water dumping. Generally, ballast water
would be treated with hypochlorite during the loading of ballast water,
although it can be treated during passage. Ballast water can be drawn
from the ballast tanks and piped through an Electrichlor hypochlorite
generator and returned. This methodology requires a smaller type of Electrichlor
generator. However, for shorter routes or where the ballast water cannot
be held for more than 24 hours, the ingoing seawater to the ballast tanks
should be treated during filling operations. This requires larger hypochlorite
generators.
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Electrichlor manufacture once-through sodium hypochlorite generators
producing from 50 to 2400kg/day of equivalent chlorine from seawater or
a brine-mixing unit. Electrichlor hypochlorite generators can convert
both normal seawater and moderately estuarine waters to hypochlorite.
For the Water, Wastewater, Sewerage Treatment and Pulp and Paper Industries,
a brine mixer tank is used to provide the necessary salty water. Generally
for brine systems, fresh or potable water is treated through an ion exchange
softener to lower the TDS. Two softeners are generally required, with
one in service and one in regeneration mode.
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Electrichlor hypochlorite generators are arranged in modules with 3 cells per module. Each module is capable of producing up to 15kg of equivalent chlorine per hour with a maximum of 8 modules and 100kg/hr.
The modules are manufactured from 100x50 pultruded FRP channel and fitted
together with stainless steel bolts and fastenings. The transformer/rectifier
unit is oil immersed and naturally cooled. The outgoing dc current is
controlled by SCR technology, using Monarch Industries' proven control
system. The current output from the T/R unit can be controlled either
by a hand-adjusted set point or by a chlorine analyzer in the hypochlorite
output. Generally, the hypochlorite generator would be sized with a companion
seawater pump. The hypochlorite output from the hypochlorite generator
can either be matched to the pump discharge capacity or, for a varying
discharge, a flow meter or a pump motor power transducer can be used.
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Electrichlor electrolytic cells are highly efficient due to the patented monopolar, multipass circular configuration that allows a progressively longer residence time for the adsorption of Cl- into the outgoing hypochlorite solution. Electrichlor electrodes have an even current density over all anode and cathode surfaces due to the unique physical and electrical design which further enhances the overall efficiency of the Electrichlor electrolytic cell, promoting long anode life. All wetted parts of the Electrichlor cell are MMO (Mixed Metal Oxide) coated grade 2 titanium on anode active surfaces.
Cells are designed such that the anode assembly can be removed easily, without the use of special tools; only a wrench is necessary for the complete disassembly. Should anodes need inspection or replacement they can be dismantled in place and only the anode assembly returned for replating. Generally, spare anodes are not necessary to be held in stock; however, should they need replating, they can be returned to Electrichlor.
All Electrichlor electrolytic cells are guaranteed against defective parts for 18 months from startup and for 5 years for the MMO anode coating under normal operating conditions.
Each Electrichlor cell is guaranteed to produce a minimum of 1500mg/liter
of sodium hypochlorite in normal seawater at less than 4.5kW/kg of equivalent
chlorine. The flow rate through each cell in seawater is set to deliver
the guaranteed 1500mg/liter of equivalent chlorine.
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Hydrogen gas is a by-product of the sodium hypochlorite process. Hydrogen
can be disengaged by either holding the gaseous hypochlorite in a degas
tank for a minimum of 6 minutes or passing the hypochlorite through a
hydro-cyclone. Both methods are equally effective. However, the second
method is more cost effective as it doesn't require product dosing pumps,
level controls or air blowers. It is also physically smaller, has a smaller
footprint and requires no maintenance. Electrichlor can supply either
system, although the hydro-cyclone is recommended.
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The chemical process involved in converting saltwater to hypochlorite
in seawater can cause a buildup of calcareous deposits on the cathodes
of electrolytic cells. In water where the TDS is high, deposits can inhibit
the flow through the cell as well as the current flow across the cell.
This can lead to disastrous results. Due to the longitudinal water flow
through Electrichlor cells and the smooth surface of the electrodes, Electrichlor
cells are practically self-cleaning, so they require less maintenance.
However, to keep the Electrichlor cells at peak performance and to produce
the maximum amount of hypochlorite, acid cleaning should be carried out
from time to time. To facilitate this, each individual Electrochlor module
is provided with a flow meter to visually indicate the amount of saltwater
flowing though the module, and a switch for remote indication of "Low
Flow" on the control panel.
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8.0 Acid Cleaning
Electrichlor hypochlorite generators are provided with a portable acid
cart for the cleaning of the electrolytic cells. Generally, only one cart
is supplied, as it can clean a multiplicity of hypochlorite generators.
To clean the cells, the generator must be shut down and the incoming seawater
and outgoing hypochlorite valves shut and the module drain valves opened.
The acid cart, holding approximately 1000 liters of 10% diluted Hcl, is
connected to the incoming seawater and outgoing hypochlorite headers with
the module drain valves closed. Dilute Hcl is then circulated through
the system and, on completion, the generator is drained and can be returned
to service. The frequency of cleaning can only be determined by experience.
However, for harder waters where the TDS are high, a once weekly acid
clean is recommended. For water where the TDS are low, the cleaning frequency
can be monthly. Acid cleaning operating times may vary from 30 to 60 minutes,
depending on the size of the generator and the deposit build up.
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The Electrichlor system has been developed from the experience gained
and lessons learned worldwide over the past 25 years, from the manufacture
and installation of several hundreds of thousands of hypochlorite generators
manufactured by Monarch Industries for the domestic and commercial saltwater
swimming pool market. Monarch Industries have emerged as the world leader
in this field, exporting to more than 17 countries worldwide. Monarch
Industries have developed the new Electrichlor hypochlorite generator
for Electrichlor in their factory facility in Perth, Western Australia.
Should site erection, commissioning, start up and training be required
by the Purchaser, Electrichlor personnel are available to carry this out.
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Electrichlor hypochlorite generators are warranted against any defects in material or workmanship for a period of eighteen (18) months from the date of delivery, or twelve (12) months from system start up, whichever occurs first. However, the obligations of Electrichlor under the warranty shall be waived should the equipment be improperly installed, operated or maintained during the warranty period. The warranty does not include incidental job site labor or delivery costs associated with items repaired or replaced under warranty.
The life of the electrolytic cells are warranted for a period of five
(5) years from the date of startup. Any anode that fails within this period
will be recoated or replaced on a prorated basis, provided the cells are
operated and maintained in accordance with Electrichlor operation and
maintenance procedures. Any cell subjected to current reversal, high current
density, low water flow or other conditions detrimental to the operation
of the cell, shall be cause for rejection of any claims under the warranty.
The warranty covers the anode performance only.
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Electrochlor provides a process guarantee with each hypochlorite generator,
guaranteeing the performance of the generator. The performance guarantee,
guarantees the sodium hypochlorite output from the generator under specific
conditions. Conditions may vary from site to site.
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