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interesting "ECF" ClO2 data
at: http://www.clo2.com/index.html you'll find an interesting
description of a new membrane/electrode based method to produce chlorine
dioxide (ClO2) w/ far less imporities and only one input chemical. the
text version follow, but the doc. has diagrams (tho not critical) and other
interesting documents. it's sterling pulp chemicals, inc.; and they are
moving into the water disinfenction field w/ their new ClO2 generating
method, because it not only produces less Cl2 as a byproduct, but also less
chlorate (apparantly disinfection byproducts such as chlorform & other
trihalomethanes largely result from chlorate formation).
they claim a conventional dioxide generator produces 31 mg/L of chlorine
gas for every 1,000 mg/L of dioxide created (3.1%), yet in the (omitted)
graphics it also states 100-1,000 ppm (ie mg/L) of Cl2 gas formed. in
either case that's a lot *AND OF INTEREST TO PULP & PAPER CLUSTER RULE
ACTIVISTS*. hope this is new info for folks, i believe i saw the reference
to the web site on a post to the usenet group sci.environment, so it should
looking up us patent # 468039 (twardowsky & mcgillvray), _awwa j._ nov.
'92, or the contact for sterling may elucidate matters further.
[Chlorite to ClO2 and NaOH]
The Chlorine Dioxide Generation System
of the Future.
Presently, nearly 4.5 million pounds per day of chlorine
dioxide are produced worldwide using Sterling developed
technologies. The rapid growth in chlorine dioxide use
over the last twenty years is primarily due to its
proven effectiveness as an environmentally friendly
bleaching and disinfectant agent.
ECF� technology research began in 1985(1) and has
continued since that time in Sterlings' Research
Laboratories in Toronto, Canada. The primary aim was to
develop a process with the following attributes:
* Single chemical feed (no chlorine, sodium
hypochlorite, or hydrochloric acid, and potential
reduction in process safety management and SARA
* High Efficiency (low operating cost).
* High Purity Chlorine Dioxide (minimal chlorine,
sodium chlorite, sodium chlorate, or sodium
chloride in the product).
* Ease of modulation and control (simple remote
method of tracking and controlling production
* No Elemental Chlorine (single chemical feed).
ECF� Development History
A typical chlorite based generator using hydrochloric
acid/sodium hypochlorite operates essentially as
Conventional Small Scale Sodium Chlorite Based Chlorine
[Conventional Small Scale Sodium ChloriteBased ClO2 Generator]
A two chemical generator uses gaseous chlorine injected
with sodium chlorite in a similar fashion.
These conventional chlorine dioxide processes using
sodium chlorite require chlorine, or sodium hypochlorite
and hydrochloric acid to convert sodium chlorite to
NaClO2 + 1/2 Cl2 -> ClO2 + NaCl.
5 NaClO2 + 4HCl -> 4 ClO2 + 5 NaCl + 2H2O
2 NaClO2 + HOCl + HCl -> 2 ClO2 + H2O + 2 NaCl
These systems are reliable and can be efficient but for
continuous high efficiency/low by-product/low residual
chlorine operation they require attention. In operations
that require frequent capacity changes they are
difficult to monitor for yield, efficiency and product
These conventional processes require an excess of
chlorine or acid to maximize sodium chlorite conversion.
The amount of excess is dependent upon the generator
design and can be 10-15% of the stoichiometric amount.
The control and proportioning of two or three chemical
feeds in these "once-through" systems is difficult, and
if not carefully monitored, can lead to untreated
chlorite or excessive amounts of chlorine leaving the
system, which, in turn can lead to the formation of
sodium chlorate via other side reactions or to the
formation of chlorine related disinfection by-products
in the finished water.
ECF� System Development
In the 1980's, significant strides were made in the
development of membranes for a variety of applications.
One such development was the hydrophobic gas pore
During this time period Sterling began working with
membrane materials which could withstand a chlorine
dioxide environment and developing technology to take
advantage of the potential selectiveness which could be
achieved using such materials to allow some components
of a solution to pass through the membranes while
keeping the remaining components on the other side.
Gas Pore Membrane
[Gas Pore Membrane]
In the 1990's the need to develop disinfecting systems
which could deliver pure agents to water, with minimal
or strictly limited disinfection by-products, presented
the opportunity for Sterling to combine its
electrochemical chlorine dioxide generating experience
with its potential gas pore membrane technology and
develop its new ECF� systems for on-site generation of
pure chlorine dioxide from sodium chlorite.
The chemical reaction involved in Sterling's ECF�
process is as follows:
NaClO2 + H2O --> ClO2 +NaOH + 1/2 H2
(Sodium Chlorite Chlorine Caustic Hydrogen
Solution) Dioxide Soda (Vented from
The system is represented as follows:
ECF� System for On-site Production of Pure Chlorine
Dioxide from Sodium Chlorite
[ECF System for On_site Production of PureClO2 from Sodium
Sodium chlorite solution is recirculated by a seal-less
pump through the anode compartment of a cell. Chlorite
ion is converted to chlorine dioxide gas in solution,
and sodium passes through the cathode compartment, where
it is discharged to form caustic soda and hydrogen. The
hydrogen is separated and vented.
The anolyte solution, which contains dissolved chlorine
dioxide gas, then flows to the perstraction module,
where chlorine dioxide and water vapor are transferred
to the acceptor solution waterstream. The depleted
solution then returns via the pump to the cell and is
further fortified with sodium chlorite feed. Not shown
on the diagram, is a small controll cell which maintains
a pH of approximately 5-5.5 in the anolyte circuit.
The perstraction membrane isolates the reaction system
from the water being disinfected. It will only pass
chlorine dioxide and other gases (oxygen and water
vapor) to the acceptor solution. An excess of sodium
chlorite is maintained in the reaction mixture which is
recycled and cannot leave the system in any form but
chlorine dioxide. Any sodium chlorate formed via side
reactions, is either reconverted to chlorine dioxide or
purged periodically from the circuit.
By using a closed reaction system, the electrochemical
conversion efficiency of chlorite to chlorine dioxide is
high. This is because the concentration of sodium
chlorite can greatly exceed chlorine.
The cell is shown schematically in the following
The chlorine dioxide purity using ECF� technology
compared to conventional systems is shown in the
PRODUCT SOLUTION (normalized to 1,000 mg/l)
ECF� Generator Conventional Generator
Chlorine dioxide1,000 (min'm) 1,000 (min'm)
Chlorine 3 31
Chlorate 3 86
Chlorite 10 66
M.H. Griese et al (3) studied the reaction by-products
in finished water from conventional generators,
producing chlorine dioxide in the liquid and phases, and
then compared the chlorite and chlorate ion
concentrations in the treated water (see graphs).
Surprisingly it was found that the gas stripped chlorine
dioxide led to less chlorite and considerably less
chlorate in the finished water. Analysis to date
suggests that gas treatment of the water to be
disinfected results in a 10-15% improvement in
disinfection efficiency. This result is thought to be
due to the absence of chlorine.
Ease of Modulation and Control
The ECF� system has a single set point adjustment for
modulating capacity. The actual online production rate
is available for the control room operator to confirm
that the expected rate equals the actual rate of
[Control Room/Feild Diagram]
Thus a field unit can be controlled from a few hundred
yards to several miles without any post-adjustment of
other feeds to compensate for changes in quality etc. in
the feedstocks. There is only ONE feedstock.
The ECF� system as planned and developed, will allow
municipalities and other users to realize the benefits
of chlorine dioxide, such as its effectiveness in
deactivating cryptosporidium and giardia, without
exceeding EPA guidelines in disinfection by-products
(chlorite, ion, etc.) and also to produce and apply the
chlorine dioxide dose in an effective and economic
US Pat# 4683039 Twardowski & McGillvray
Can Pat. # Appl. 2162471 Cowley, Lipsztajn & Ranger
M.H. Greise et al AWWA Journ. Nov. 1992
for further information on the ECF� System contact:
G. CowleyTel: 416-234-7522
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