Note: Descriptions are shown in the official language in which they were submitted.
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REVERSE POLARITY CLEANING AND ELECTRONIC FLOW CONTROL SYSTEMS FOR
LOW INTERVENTION ELECTROLYTIC CHEMICAL GENERATORS
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to and the benefit of filing of U.S.
Provisional Patent
Application Serial No. 61/056,718, entitled "Reverse Polarity Cleaning for
High Current
Density Electrolytic Cells," filed on May 28, 2008. This application is also a
continuation-in-
part application of U.S. Patent Application Serial No. 11/946,772, entitled
"Low Maintenance
On-Site Generator", filed on November 28, 2007, which application claims
priority to and the
benefit of filing of U.S. Provisional Patent Application Serial No.
60/867,557, entitled "Low
Maintenance On-Site Generator", filed on November 28, 2006. The specification
and claims
of all of these applications are incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention (Technical Field):
The present invention relates to an electrolytic on-site generator which is
nearly free
of maintenance.
Background Art:
Note that the following discussion refers to a number of publications and
references.
Discussion of such publications herein is given for more complete background
of the scientific
principles and is not to be construed as an admission that such publications
are prior art for
patentability determination purposes.
Electrolytic technologies utilizing dimensionally stable anodes have been
developed to
produce mixed-oxidants and sodium hypochlorite solutions from a sodium
chloride brine solution.
Dimensionally stable anodes are described in U.S. Patent No. 3,234,110 to
Beer, entitled
"Electrode and Method of Making Same," wherein a noble metal coating is
applied over a
titanium substrate. Electrolytic cells have had wide use for the production of
chlorine and mixed
oxidants for the disinfection of water. Some of the simplest electrolytic
cells are described in
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U.S. Patent No. 4,761,208, entitled "Electrolytic Method and Cell for
Sterilizing Water", and U.S.
Patent No. 5,316,740, entitled "Electrolytic Cell for Generating Sterilizing
Solutions Having
Increased Ozone Content."
Electrolytic cells come in two varieties. The first category comprises divided
cells that
utilize membranes to maintain complete separation of the anode and cathode
products in the
cells. The second category comprises undivided cells that do not utilize
membranes, but that
also do not suffer nearly as much from issues associated with membrane
fouling. However, it is
well accepted that one of the major failure mechanisms of undivided
electrolytic cells is the
buildup of unwanted films on the surfaces of the electrodes. The source of
these contaminants is
typically either from the feed water to the on-site generation process or
contaminants in the salt
that is used to produce the brine solution feeding the system. Typically these
unwanted films
consist of manganese, calcium carbonate, or other unwanted substances. If
buildup of these
films is not controlled or they are not removed on a fairly regular basis, the
electrolytic cells will
lose operating efficiency and will eventually catastrophically fail (due to
localized high current
density, electrical arcing or some other event). Typically, manufacturers
protect against this type
of buildup by incorporating a water softener on the feed water to the system
to prevent these
contaminants from ever entering the electrolytic cell. However, these
contaminants will enter the
process over time from contaminants in the salt used to make the brine. High
quality salt is
typically specified to minimize the incidence of cell cleaning operations.
Processes are well
known in the art for purifying salt to specification levels that will avoid
contaminants from entering
the cell. However, these salt cleaning processes, although mandatory for
effective operation of
divided cells, are considered too complicated for smaller on-site generation
processes that utilize
undivided cells.
U.S. Patent Application Serial No. 11/287,531, which is incorporated herein by
reference,
is directed to a carbonate detector and describes one possible means of
monitoring an
electrolytic cell for internal film buildup. Other possible means for
monitoring carbonate buildup
in cells that utilize constant current control schemes is by monitoring the
rate of brine flow to the
cell. As brine flow increases, it is usually, but not always, indicative of
carbonate formation on
the cathode electrode which creates electrical resistance in the cell. Other
than these methods
and/or visual inspection of the internal workings of a cell, there currently
is not an adequate
method of monitoring the internal status of the buildup on an electrolytic
cell.
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The current accepted method of cleaning an electrolytic cell is to flush it
with an acid
(often muriatic or hydrochloric acid) to remove any deposits which have
formed. Typically,
manufacturers recommend performing this action on a regular basis, at least
yearly, but
sometimes as often as on a monthly basis. Thus there is a need for a more
reliable method for
insuring cleanliness of the electrolytic cell is to perform a cleaning process
on an automated
basis that does not require the use of a separate supply of consumables such
as muriatic or
hydrochloric acid, and that does not require operator intervention.
U.S. Patent No. 5,853,562 to Eki, et al. entitled "Method and Apparatus for
Electrolyzing
Water" describes a process for reversing polarity on the electrodes in a
membraneless
electrolytic cell for the purpose of removing carbonate scale and extending
the life of the
electrolytic cell. This method of electrolytic cell cleaning is routinely used
in flow through
electrolytic chlorinators that convert sodium chloride salt in swimming pool
water to chlorine via
electrolysis. However, currently used flow through electrolytic cells are
constructed of electrodes
(anode and cathode) that both have common catalytic coatings. As electrical
polarity is changed,
the old cathode becomes the anode, and the anode becomes the cathode. Special
catalytic
coatings have been developed for these applications. For instance, Eltech
Corporation has
developed the EC-600 coating specifically for the swimming pool chlorination
market. Sodium
chloride is typically added to the pool water raising the total dissolved
solids (TDS) content to
approximately 4 to 5 grams per liter. At these TDS values, the current density
in the swimming
pool electrolytic cells is relatively low. The special anode coatings for pool
applications are
designed to tolerate these low current densities for extended periods with
polarity applied in
either direction. However, most dimensionally stable anodes for chlorine
production in
membraneless electrolytic cells producing chlorine at 8 gram per liter (8,000
mg/L) concentration
of free available chlorine (FAC) cannot tolerate high current densities
(greater than approximately
1 amp per square inch) in reverse polarity mode. Thus, although simply
reversing the polarity
works for low current density electrolytic cells, it will not work for
electrolytic cells which normally
operate at a high current density, since the anode will be damaged if high
current density is
applied during the reverse polarity cleaning operation.
One of the other maintenance items for electrolytic generators is the
requirement that
operators occasionally measure and set water flow into the system. The flow
through the
generator can vary greatly with incoming and outgoing water pressure and/or
contaminant
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buildup in the system or electrolytic cells. Typically, measurements are made
with either
flowmeters or with timed volume measurements, and adjustments to the flow are
performed with
manual valves. Keeping the electrolytic generator operating within flow
specifications is
important, as it ensures reliable long term operation the generator within its
efficiency
specifications.
SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)
The present invention is a method for operating an electrolytic cell, the
method
comprising the steps of supplying brine to an electrolytic cell, producing one
or more oxidants
in the electrolytic cell, detecting a level of contaminant buildup,
automatically stopping the
brine supply after an upper contaminant threshold is detected, automatically
cleaning the
electrolytic cell, thereby reducing contaminants in the electrolytic cell, and
automatically
continuing to produce the one or more oxidants after a lower contaminant
threshold is
detected. The cleaning step preferably comprises providing brine to an acid
generating
electrolytic cell, generating an acid in the acid generating electrolytic
cell, and introducing the
acid into the electrolytic cell. The acid preferably comprises muriatic acid
or hydrochloric acid.
The method preferably further comprises the step of diluting the brine. The
detecting step
preferably comprises utilizing a carbonate detector. The detecting step
preferably comprises
measuring the rate of brine consumption in the electrolytic cell, optionally
by measuring a
quantity selected from the group consisting of flow meter output, temperature
of the
electrolytic cell, brine pump velocity, and incoming water flow rate. The
method preferably
further comprises comparing the rate of brine consumption to the rate of brine
consumption in
a clean electrolytic cell. The cleaning step optionally comprises using an
ultrasonic device
and/or using a magnetically actuated mechanical electrode cleaning device, or
reversing the
polarity of electrodes in the electrolytic cell, thereby lowering the pH at a
cathode.
The present invention is also an apparatus for producing an oxidant, the
apparatus
comprising a brine supply, an electrolytic cell, an acid supply, and a control
system for
automatically introducing acid from the acid supply into the electrolytic
cell. The acid supply
preferably comprises a second electrolytic cell, and the brine supply
preferably provides brine
to the second electrolytic cell during a cleaning cycle. The apparatus
preferably further
comprises a variable speed brine pump, a carbonate detector, one or more
thermowells for
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measuring a temperature of said electrolytic cell, and/or one or more
flowmeters for
measuring the brine flow rate.
The present invention is also an apparatus for producing an oxidant, the
apparatus
comprising a brine supply, an electrolytic cell, a cleaning mechanism in the
electrolytic cell,
and a control system for automatically activating the cleaning mechanism. The
cleaning
mechanism preferably is selected from the group consisting of ultrasonic horn,
magnetically
actuated electrode mechanical cleaning device, and acidic solution at a
cathode surface. The
apparatus preferably further comprises a device selected from the group
consisting of a
carbonate detector, at least one thermowell for measuring a temperature of
said electrolytic
cell, and a flowmeter for measuring a brine flow rate.
The present invention is also a method for cleaning an electrolytic cell
comprising
electrodes, the method comprising the steps of reversing polarities of two or
more of the
electrodes and providing a cleaning current density to the electrodes which is
lower than an
operational current density used during normal operation of the electrolytic
cell. During
normal operation the electrolytic cell preferably produces a concentration of
free available
chlorine greater than approximately four grams per liter, more preferably
greater than
approximately five grams per liter, and most preferably approximately eight
grams per liter.
The operational current density is preferably greater than approximately one
amp per square
inch. The cleaning current density is preferably less than approximately 20%
of the
operational current density, and more preferably between approximately 10% and
approximately 15% of the operational current density. The providing step is
preferably
performed for less than approximately thirty minutes, and more preferably for
between
approximately five minutes and approximately ten minutes. The reversing step
optionally
comprises using at least one power supply relay or other switching device. The
operational
current density is preferably provided by an operational power supply and the
cleaning current
density is preferably provided by a separate cleaning power supply. The power
producing
capacity of the cleaning power supply is preferably smaller than the power
producing capacity
of the operational power supply. The method preferably further comprises the
step of
monitoring a flow rate of electrolyte through the electrolytic cell. The
monitoring step is
preferably performed using a flowmeter, a rotameter, or a pressure transducer,
or monitoring
a temperature difference across the electrolytic cell via a first thermocouple
or thermowell
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disposed at an inlet of the electrolytic cell a second thermocouple or
thermowell disposed at
an outlet of the electrolytic cell. The method preferably further comprises
the step of
automatically adjusting the flow rate, and preferably further comprises the
step of initiating a
cleaning cycle at a predetermined flow rate.
The present invention is also method for cleaning an electrolytic cell
comprising
electrodes, the method comprising the steps of reversing polarities of two or
more of the
electrodes and providing a cleaning voltage potential difference to the
electrodes which is
lower than an operational voltage potential difference used during normal
operation of the
electrolytic cell. During normal operation the electrolytic cell preferably
produces a
concentration of free available chlorine greater than approximately five grams
per liter. The
providing step is preferably performed for a time between approximately five
minutes and
approximately ten minutes. The reversing step preferably comprises using at
least one power
supply relay or other switching device. The operational voltage potential
difference is
preferably provided by an operational power supply and the cleaning voltage
potential
difference is preferably provided by a separate cleaning power supply. The
method
preferably further comprises the steps of monitoring a flow rate of
electrolyte through the
electrolytic cell and automatically adjusting the flow rate.
The present invention is also an apparatus for producing electrolytic
products, the
apparatus comprising an electrolytic cell comprising electrodes; a first power
supply for
providing a first current density to the electrodes, a second power supply for
providing a
second current density to the electrodes, the second power supply having an
opposite polarity
to the first power supply, wherein the second current density is smaller than
the first current
density. The electrolytic cell preferably produces a concentration of free
available chlorine
greater than approximately five grams per liter. The second current density is
preferably
between approximately 10% and approximately 15% of the first current density.
The
apparatus preferably further comprises at least one power supply relay or
other switching
device, and preferably comprises a flow monitoring device for monitoring a
flow rate of
electrolyte through the electrolytic cell. The flow monitoring device is
preferably selected from
the group consisting of a flowmeter, a rotameter, a pressure transducer, a
pair of
thermocouples, and a pair of thermowells. If a pair of thermocouples or
thermowells is used,
one thermocouple or thermowell is preferably disposed at an inlet of the
electrolytic cell and
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another thermocouple or thermowell is preferably disposed at an outlet of the
electrolytic cell.
The apparatus preferably further comprises an electronically operated valve
for adjusting the
flow rate.
Objects, advantages and novel features, and further scope of applicability of
the
present invention will be set forth in part in the detailed description to
follow, taken in
conjunction with the accompanying drawings, and in part will become apparent
to those
skilled in the art upon examination of the following, or may be learned by
practice of the
invention. The objects and advantages of the invention may be realized and
attained by
means of the instrumentalities and combinations particularly pointed out in
the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawing, which is incorporated into and form a part of the
specification, illustrates an embodiment of the present invention and,
together with the
description, serves to explain the principles of the invention. The drawing is
only for the
purpose of illustrating a preferred embodiment of the invention and is not to
be construed as
limiting the invention. In the drawings:
FIG. 1 is a diagram of one embodiment of a low maintenance on-site generator
unit.
FIG. 2 is a schematic of a reverse polarity system for electrolytic cell
cleaning.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(BEST MODES FOR CARRYING OUT THE INVENTION)
Embodiments of the present invention are methods and devices whereby an on-
site
generator electrolytic cell is preferably monitored automatically for buildup
of contaminants on the
electrode surfaces, and when those contaminants are detected, the electrolytic
cell is cleaned
automatically (i.e, without operator intervention), thereby providing a
simple, low cost, and
reliable process for achieving a highly reliable, low maintenance, on-site
generator which does
not require the typical operator intervention and/or auxiliary equipment (such
as a water softener)
now required for long life of electrolytic cells.
The internal status of the electrolytic cells can be monitored automatically
by monitoring
cell inputs and performance. It is known that how much brine a cell consumes
is dependent on
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the amount and type of film buildup on that given cell. If brine flow is
continuously monitored,
any dramatic change in brine flow to reach a given current at a given voltage
is indicative of a
potential problem with film buildup within a cell. The invention preferably
monitors the flow
characteristics of the brine, incoming water, temperature, etc., to determine
whether or not there
has been contaminant buildup within the electrolytic cell. When potential film
buildup is detected
in the cell by the control system, the cell is preferably automatically acid
washed.
A carbonate detector integrated with an electrolytic cell, automatic acid
washing, and
device controls may be utilized. A separate electrolytic cell from the one
used to create the
mixed oxidant or sodium hypochlorite is preferably used to create the acid on
site and on
demand and to provide the acid for removing of contaminants in the
electrolytic cell used for
creating the sodium hypochlorite or mixed oxidants. Alternatively a reservoir
is used to store
concentrated acid onsite for cleaning the cell, and monitoring that acid
reservoir and alarming
operators when that acid reservoir would need to be refilled, as well as
optionally diluting the acid
to a desired concentration prior to washing the cell. An ultrasonic cleaning
methodology for
automatically removing unwanted contaminants when the contaminants are
detected by the
methods described above may also be integrated into the present invention.
An embodiment of the present invention is shown in FIG. 1. All of the
components of
this device are preferably mounted to back plate 15. The controls and power
supplies for all
the separate components shown in this embodiment are all preferably contained
within
control box 5, but may alternatively be located wherever it is convenient,
preferably as long as
there are master controls for the overall operation of the apparatus.
Control box 5 preferably shows the status of the unit via display 10, and the
master
controls as well as electrical power and/or component signals are preferably
carried via
electrical connections 50 between control box 5 and the various individual
components.
Water preferably enters the system through water entrance pipe 30, and brine
preferably
enters the system through brine entrance pipe 25. Brine, preferably stored in
a saturated
brine silo or tank, is preferably pumped via variable speed brine pump 20,
which is preferably
controlled and powered by electrical connection 50. The brine then preferably
passes
through flow meter 35, which can be electrically monitored via electrical
connection 50. The
control system can control the flow rate of the brine by increasing the speed
of variable speed
brine pump 20.
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When the electrolytic generator is in normal operation mode and is at target
current and
target voltages, the total flow through the electrolytic cell 55 can be
monitored, for example by a
flowmeter, rotameter, or pressure transducer, or by monitoring the change in
temperature across
the electrolytic cell 55 by monitoring inlet thermowell 65 and exit thermowell
70. When control
box 5 determines that flow is off target, for example in response to
fluctuations in incoming
pressure and/or flow to the electrolytic generator, it preferably
automatically adjusts flow by
changing electronically controlled cell inlet valve 6. In this way, the cell
can always operate near
target flow levels and will not routinely require measurement or adjustment of
incoming flows.
Data from any of the following sources (or combinations of data from any of
these
sources) is preferably used to determine the volumetric flow rate of brine:
flow meter 35,
carbonate detector 60, electrolytic cell 55, acid generating electrolytic cell
45, and/or
thermowells 65, 70. Valve 40 can direct flow either to electrolytic cell 55 or
to acid generating
electrolytic cell 45. Valve 40 typically flows an electrolyte comprising
diluted brine (as both
the concentrated brine and water inflows have preferably been plumbed together
and the
brine has been diluted before it reaches valve 40) to electrolytic cell 55. In
this standard
operating configuration, the system produces, for example, mixed oxidants or
sodium
hypochlorite.
As contaminants build up on carbonate detector 60, which may be located
elsewhere
according to the present invention, carbonate detector 60 sends a series of
signals to control
box 5, preferably via electrical connections 50, which indicate whether or not
a contaminant
film is building up on electrolytic cell 55. When carbonate detector 60
indicates that there is
contaminant film, control box 5 preferably begins an acid cleaning cycle in
the device, wherein
valve 40 is actuated via electrical connection 50 to force diluted brine
through acid generating
cell 45, which is also preferably energized by control box 5 via electrical
connections 50. The
system preferably runs brine pump 20 to flow at a rate (as measured by flow
meters 35)
which has been optimized for optimal acid creation in acid generating
electrolytic cell 45. In
this embodiment, the acid created in acid generation cell 45 preferably flows
through
electrolytic cell 55, where it preferably cleans the contaminants, then flows
through carbonate
detector 60. The system preferably runs in this acid cleaning mode until
carbonate detector
60 sends a signal to control box 5 indicating that the system is clean and can
begin running
again in standard mixed oxidant or sodium hypochlorite production mode. The
acid used to
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clean electrolytic cell 55 is preferably dumped to a separate waste drain
after flowing through
carbonate detector 60 instead of dumping it to the oxidant storage tank.
Electrolytic cell 55
may optionally be cleaned with an ultrasonic horn and/or a magnetically
actuated electrode
mechanical cleaning apparatus in addition to or in place of using an acid
generating cell.
In an alternative embodiment, concentrated acid is stored in a reservoir.
During the
acid cleaning cycle, control box 5 preferably activates a pump or valve to
allow flow of the
acid to electrolytic cell 55. The reservoir is preferably large enough to
accommodate many
different acid wash cycles. Some of that acid may potentially be diluted with
standard
incoming water to clean electrolytic cell 55.
If carbonate detector 60 (or any other contaminant detecting component) is not
used,
electrolytic cell 55 preferably may be cleaned on a predetermined cleaning
schedule to
ensure contaminants do not ruin electrolytic cell 55. Typically this cleaning
schedule would
be based upon the number of hours that the electrolytic cell had been running
since the last
cleaning was completed, and is preferably frequent enough to ensure that there
is no
excessive contaminant buildup on the electrolytic cell.
The rate of brine consumption may optionally be used to determine the presence
of
contaminants in electrolytic cell 55. In normal operation in a clean cell, the
rate of brine
consumption is steady and measurable. As carbonate scale builds up within
electrolytic cell
55, the carbonate layer acts as an electrical insulator between the anode and
cathode within
electrolytic cell 55. To compensate for this insulating effect, and to
maintain the amperage
within electrolytic cell 55, the rate of brine consumption increases to
increase the conductivity
within electrolytic cell 55. This increased rate of brine consumption is
compared to the normal
rate of brine consumption. Flow through electrolytic cell 55 can also be used
to measure
contaminant buildup within electrolytic cell 55. Flow can be measured
indirectly by measuring
the temperature rise through electrolytic cell 55, for example by comparing
the temperature
difference between two thermocouples or inlet thermowell 65 and cell discharge
thermowell
70. When carbonate buildup is detected by any of these means, electrolytic
cell 55 can be
cleaned by any of the methods or components described above. Brine consumption
may be
measured using brine flow rate, tachometer rates of brine pump 20, or incoming
water flow
rates.
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In addition to (or instead of) the cleaning methods described above, the
electrolytic cell
may optionally be cleaned by reversing the polarity of the electrodes in
electrolytic cell, while
flowing electrolyte through the electrolytic cell or not, and preferably for a
very short duration.
Reversing the polarity of the electrodes, preferably at low current densities,
lowers the pH at the
cathode, which dissolves and removes the contaminants. However, the
dimensionally stable
anode in the chlorine (4 to 8 gm/L) producing electrolytic cell described
herein typically operates
well at high current densities (up to 2 amps per square inch), but would fail
quickly if polarity were
reversed at the same current density. Thus it is preferable to use a separate
power source at
lower current density and/or lower plate to plate voltages to clean the cell
in reverse polarity
mode, which is only operated when the normal chlorine production operational
mode is in
standby, so that the primary anode coating remains undamaged. Under these
conditions,
cleaning cycles of less than 30 minutes can be achieved, preferably ranging
between
approximately 5 minutes and 10 minutes. Industry experience indicates that
cell cleaning
intervals of less than a week would represent an unfavorable situation where
the feed water to
the electrolytic cell, or the salt used to make the brine solution, would
typically be poor quality.
Intervals between cleaning of greater than one week are clearly the industry
norm. Under the
worst case condition of cleaning once per week, the loss of system duty cycle
(production
operation mode) would still be negligible.
In any embodiment using reverse polarity to clean the electrolytic cell, both
the anode
and cathode surfaces of both primary and bi-polar electrodes are preferably
coated with an
appropriate dimensionally stable anode coating.
FIG. 2 is a schematic of an embodiment of a system for implementing reverse
polarity
cleaning. Electrolytic cell 130 comprises anode 134 and cathode 132 with
electrolyte flowing
in at the bottom and oxidant flowing out at the top of the cell. In normal
operation, electrolytic
cell 130 has electrical energy applied to anode 134 and cathode 132 via main
power supply
136. Periodically, electrolytic cell 130 will be cleaned by reversing the
polarity on anode 134
and cathode 132, effectively making anode 134 the cathode, and cathode 132 the
anode. In
the normal mode of production where the system is producing a chlorine based
disinfectant,
the current density on anode 134 is preferably between approximately 1 and 2
amps per
square inch. To avoid damage to anode 134 during the reverse polarity cleaning
step, the
current density is preferably less than approximately twenty percent of the
normal operating
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current density range, and more preferably between about 10% and 15% of the
normal
operating current density range. Because the reverse polarity cleaning
operation operates at
much lower power settings, power is preferably supplied by cleaning power
supply 138, which
can be much smaller than main power supply 136. Power from main power supply
136 is
transferred to electrolytic cell 130 preferably via main power cables 144.
Power from
cleaning power supply 138 is transferred to electrolytic cell 130 preferably
via cleaning power
cables 146. The power supplies are preferably isolated via main power supply
relay 140 and
cleaning power supply relay 142. In normal operation when chlorine is being
produced within
electrolytic cell 130, main power supply 136 is energized and main power
supply relay 140 is
closed. To avoid backflow of current to cleaning power supply 138 with the
wrong polarity,
cleaning power supply relay 142 is open. Likewise, when electrolytic cell 130
is operating in
cleaning mode, cleaning power supply 138 is energized, main power supply 136
is de-
energized, main power supply relay 140 is open, and cleaning power supply
relay 142 is
closed. By utilizing less current density and/or lower potentials on anode 134
during the short
cleaning cycle, damage to anode 134 or cathode 132 due to the cleaning cycle
is negligible.
An alternative embodiment to the one shown in Fig 2 uses the main power supply
136 to provide power for normal operation as well as the cleaning cycles. This
approach
preferably employs the use of power supply relays 142 or other switching
devices to reverse
the polarity. Typically this approach requires the electrolytic cell brine
concentrations during
the cleaning cycle to be much less than in normal operation. With this
approach, however, it
is still preferable that the cleaning cycle be performed at lower current
densities and/or lower
potentials for short periods of time.
Although the invention has been described in detail with particular reference
to these
preferred embodiments, other embodiments can achieve the same results.
Variations and
modifications of the present invention will be obvious to those skilled in the
art and it is
intended to cover all such modifications and equivalents. The entire
disclosures of all patents
and publications cited above are hereby incorporated by reference.
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