Chlorite – HCl Acid and Electrochemical ClO2 production:
5ClO2- + 4H+ → 4ClO2 + Cl- + 2H2O
The stoichiometry of this reaction shows that 20% of the original starting
chlorite does not produce ClO2. This means that when all chlorite ion is
consumed, only 80% is converted to ClO2. In this process, high conversion
efficiency does not result in high yield. This also shows why chlorite ion
consumption should not be used for yield calculations.
In practice, 25% liquid sodium chlorite is reacted with 15% Hydrochloric
acid in a two precursor generator to produce ClO2. The ratio of precursor
chemicals is as follows:
Lbs of ClO2
1 lb
Gal. of 25% Sodium Chlorite
0.81 Gal. (#8.40)
Gal. of 15% HCl
1.12 Gal. (#9.8)
The precursors are drawn by vacuum created by an eductor into water where
they react to form a solution containing ClO2. Solutions ranging in strength
from 500ppm to 2000ppm are typical. Since the kinetics of this reaction are
slow compared to other methods, a large tank is required after the precursors
are drawn into solution to allow sufficient time for ClO2 to form. The ClO2
solution is fed from the reaction tank to the application point with a chemical
feed pump.
Electrochemical ClO2 production
2NaClO2 + 2H2 O + Electricity → 2 ClO2 + 2NaOH + H2
Electrochemical chlorine dioxide generators employ the very same
chemistry and are subject to the same drawbacks as the acid chlorite method.
The only difference is that electrochemical generators use H+ ions formed by
electrolytically splitting H2O into H+ and OH- ions rather than feeding HCl
acid. A consequence of using the H+ ions from electrolytically splitting
water is that the generator produces a waste stream of highly alkaline water
containing the waste OH- ions. This waste stream must be disposed of if the
plant does not have a need for alkaline pH adjust somewhere in its treatment
processes where this waste product can be used. This process also produces
a stream of hydrogen gas which must be properly vented to prevent the
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possibility of a hydrogen gas explosion. Typically, the cost for electricity
and ongoing cost for maintenance and replacement of the electrolytic cell
components by far out ways the expense for HCl usage in a traditional acid
chlorite generator. Higher operating expense and upkeep requirements
combined with the significant additional capital expense for the generator
itself has limited the use of this technology to smaller applications where
chemical handling and storage concerns outweigh cost considerations.
Lbs of ClO2
1 lb
Gal. of 25% Sodium Chlorite
0.65 Gal. (#6.72)
Electricity/Cell$/Waste$
Varies
Since these systems are typically used for small volume applications,
precursors are usually supplied and stored in 55 gallon drums or totes.
Liquid sodium chlorite & chlorine gas ClO2 Production:
2ClO2- + Cl2(g) → 2ClO2 + 2ClThe most common process for converting sodium chlorite into chlorine
dioxide involves educting chlorine gas into a flow of water then adding
liquid sodium chlorite. Typically, chlorine gas is drawn into a flow of water
via a venturi feeder. Liquid sodium chlorite is then fed either by pump or
eductor to this chlorine/water solution in a reaction chamber where chlorine
dioxide is formed. Because the conversion of chlorite to ClO2 is highly
dependent upon the relative concentration of the precursors in solution and
the pH in the reaction chamber (pH between 2.8-3.5 w/ minimum 500ppm
Cl2 concentration is critical), high concentrations of unreacted precursors in
the generator effluent are common. This also makes changes in production
rates (feed rates) complicated because besides maintaining the correct ratio
of precursors, proper pH and chlorine concentrations in the reactor must be
maintained to achieve good conversion efficiency and product purity. In
many cases, chlorine gas is intentionally overfed so that the hydrolysis of
chlorine gas in water will drive the pH in the reaction chamber below 3.5,
the pH required for good conversion. ClO2 solutions ranging in strength
from 500ppm to 2000ppm are typical.
The ratio of precursor chemicals without Cl2 gas overfeed for this process is
as follows:
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Lbs of ClO2
1 lb
Gal. of 25% Sodium Chlorite
0.65 Gal. (#6.72)
Lbs of Chlorine Gas
0.53 lbs
Liquid sodium chlorite for these systems is supplied in drums, totes or bulk
depending upon the usage rate for the application. Chlorine gas cylinders are
sold as a commodity item by various local suppliers.
Solid sodium chlorite & chlorine gas ClO2 Production:
2ClO2- + Cl2(g) → 2ClO2 + 2ClA relative new comer to the market is a solid sodium chlorite based system.
This process employs a packed bed of solid sodium chlorite pellets
imbedded with inert stabilizing components. A dilute chlorine-in-air gas
blend is drawn through a bed of chlorite pellets via eductor where it comes
in intimate contact with thermally stable solid sodium chlorite pellets. The
reaction between the chlorine gas and the sodium chlorite produces chlorine
dioxide gas. This gas is then pulled through a second cartridge to ensure that
no residual chlorine remains in the chlorine dioxide gas. The chlorine
dioxide gas produced is then drawn into solution through the eductor and
carried to the application point. The benefit of this process is that chlorine
production rates (feed rates) can be varied without effecting conversion
efficiency or product purity by simply changing the feed rate of the chlorine
gas.
The ratio of precursor chemicals for this process is as follows:
Lbs of ClO2
1 lb
Lbs of Sodium Chlorite
1.68 lbs
Lbs of Chlorine Gas
0.56 lbs
The solid sodium chlorite is supplied in pre-packaged metal drums with
quick connects sold exclusively by the system’s manufacturer and its
distributors. Chlorine gas cylinders are sold as a commodity item by various
local suppliers.
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Liquid sodium chlorite, bleach & acid ClO2 Production:
In this process bleach is used as the source for chlorine. The pH of liquid
bleach is > 11 to prevent decomposition to chlorate ion. To lower the pH,
hydrochloric acid and bleach are combined to produce a mix of hypochlorite
and Cl2-. Liquid sodium chlorite is then added and ClO2 is produced in a
second reaction zone. An eductor draws the reacted solution into a stream of
motive water and the resultant solution containing ClO2 is carried to the
application point. The drawback of this process is that it requires operators
to balance three separate feed streams to achieve good conversion efficiency.
Since the reaction is highly dependent upon both the relative concentration
of chlorine and chlorite in the final reaction chamber plus the pH (pH
between 2.8-3.5 w/ minimum 500ppm Cl2 concentration is critical), high
concentrations of unreacted precursors are common with this method. The
need to balance three separate feed streams also makes it difficult to change
production rates (feed rates) while maintaining good conversion efficiency
and product purity.
The ratio of precursor chemicals for this process is as follows:
Lbs of ClO2
1 lb
Gal of 25%
Sodium Chlorite
0.65 Gal. (#6.72)
Gal of 12.5%
Bleach
0.45 Gal. (#4.5)
Gal of 15% HCl
0.40 Gal. (#3.5)
Precursors are supplied in drums, totes and bulk depending upon the usage
rate for the application.
Liquid Sodium Chlorate/peroxide (Purate) and Sulfuric acid process:
2NaClO3 + H2 O2 + H2SO4 → 2ClO2 + O2 + Na2SO4 + 2H2 O
In this process a blend of hydrogen peroxide, sodium chlorate and water
(called Purate) is reacted with 78% sulfuric acid in a reaction chamber under
vacuum. The chemicals are fed undiluted to the reaction chamber via
chemical metering pumps where they combine to form pure chlorine dioxide
gas, oxygen and a small amount of sodium sulfate. An eductor on top of the
reaction chamber creates a vacuum which draws the gas out of the reaction
chamber and into solution in the motive water leaving the eductor. This
solution containing ClO2 is then typically carried directly to the application
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point. Draw down columns on the suction side of the chemical metering
pumps are used to measure the exact feed rate of each pump and this data is
entered into a PLC on the generator. The PLC can then control the feed of
precursors to the reaction chamber to precisely maintain the correct ratio of
precursors thereby optimizing conversion efficiency and product purity. The
generator PLC will also take a 4-20ma input from the plant’s flow meter to
automatically flow pace ClO2 feed. This allows plant operating personnel to
simply enter the desired feed rate in parts per million on the generator touch
screen and the machine will automatically maintain that feed rate regardless
of plant flow. There are numerous safety interlocks including low vacuum
alarms, low motive water flow alarms and with the optional OPTEK
continuous ClO2 analyzer, low efficiency alarms (the PLC continuously
monitors precursor conversion efficiency). Since this process uses neither
chlorine (liquid/gaseous) nor chlorite, the generator cannot produce
unreacted chlorine or chlorite in the product stream.
Lbs of ClO2
1 lb
Gal of Purate
0.36 Gal (4.1 lbs)
Gal 78% Sulfuric
0.37 (5.0 lbs)
Precursors are supplied in either totes or in bulk depending upon the usage
rate for the application.
Chlorine Dioxide Solution Storage
When it is desirable to feed chlorine dioxide to multiple feed points, it may
not be economically practical to install a separate generator for each feed
point. In this case a chlorine dioxide solution storage tank must be installed
with feed pumps for each application point. The pumps will draw solution
from a central tank which is filled by the ClO2 generator. Acceptable
materials of construction for the bulk storage tank are HDPE and FRP. To
prevent ClO2 fumes from collecting in the head space of the storage tank an
eductor “gas sweep” and vacuum breaker must be incorporated into the
design of the tank. Automatic level controls for filling the tank should
include a low level sensor to begin fill, a high level sensor to turn off fill and
a secondary “high, high level” sensor to turn off fill should the primary high
level sensor fail. The preferred material of construction for centrifugal or
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gear pumps is titanium. Diaphragm pumps should use Teflon coated
diaphragms and PVC heads with viton seals.
Common chlorine dioxide applications:
Manganese removal:
Manganese can be responsible for a variety of aesthetic problems. Elevated
color in the finished water has been observed with Mn concentrations as low
as 0.02ppm. For this reason, Mn concentrations in the finished water should
be maintained below 0.02ppm. Although chlorine and permanganate will
also oxidize Mn from solution, neither permanganate nor chlorine will
reliably reduce Mn concentrations to below 0.02ppm. Mn reduction with
Chlorine and permanganate will be further hampered if portions of the
manganese present is organically bound which is usually the case. Chlorine
dioxide is the only oxidizer proven to reliable remove manganese and
organically bound manganese to below 0.02ppm.
For each ppm of manganese, 2.45ppm of chlorine dioxide is required to
oxidize the Mn from solution. In practice, more chlorine dioxide must be fed
to overcome any background demand for ClO2. The ClO2 should be fed at a
point where good mixing is available. A minimum reaction time of two
minutes (ideally five minutes) prior to coagulation or filtration should be
allowed.
Iron Removal:
Iron can be responsible for a variety of aesthetic problems such as elevated
color in the finished water. Although chlorine and permanganate will also
oxidize Mn from solution, neither permanganate nor chlorine will reliably
remove organically bound iron and the reaction time can be as long as 24
hours. Chlorine dioxide is the only oxidizer proven to quickly and reliably
remove iron and organically bound iron to below 0.02ppm.
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For each ppm of iron, 1.2ppm of chlorine dioxide is required to oxidize the
Fe from solution. In practice, more chlorine dioxide must be fed to
overcome any background demand for ClO2. The ClO2 should be fed at a
point where good mixing is available. A minimum reaction time of two
minutes (ideally five minutes) prior to coagulation or filtration should be
allowed.
THM/HAA reduction:
Rather than forming substitution reactions that lead to THM/HAA
formation, chlorine dioxide reacts to oxidize background organics thereby
reducing THM/HAA formation in the finished water. To reduce THM/HAA
formation, ClO2 should be fed at least five minutes prior to coagulation or
filtration. The use of ClO2 combined with chlorine has also been shown to
successfully reduce THM/HAA formation. Dosages for THM/HAA
reduction range from as low as 0.25ppm to over 3.0ppm depending on the
organic and inorganic (Fe & Mn) loading of the raw water. Coagulation,
sedimentation & filtration are also critical factors in successfully reducing
THM/HAA formation.
Finished water chlorite reduction:
When chlorine dioxide is used in potable water, finished water chlorite
levels must be monitored to insure that chlorite levels are maintained below
the 1.0ppm MCL. There are several strategies that can be employed to lower
chlorite levels in the finished water. Ferrous iron salts such as ferrous
chloride or ferrous sulfate can be fed to reduce chlorite levels to essentially
zero. The iron salt should be fed prior to coagulation or filtration to insure
the removal of the resultant precipitate. The dosage of ferrous salt required is
1ppm of ferrous iron will reduce 0.3ppm of Chlorite. The reaction is almost
instantaneous so reaction time is usually not a critical consideration. GAC
filtration with either deep bed GAC filters or by capping existing filters with
18” of GAC will also significantly reduces finished water chlorite levels,
usually by 50 to 70%. We should point out that this strategy does not rely
upon the adsorption of chlorite onto the GAC but uses the reductive
environment of the GAC bed to act as a catalyst that drives the reduction
reaction of chlorite to chloride. Another successful strategy to lowering
finished water chlorite levels is to replace some of the ClO2 being fed with
chlorine.
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