Note: Descriptions are shown in the official language in which they were submitted.
1 155792
AIR-DEPOLARIZED C~ILOR-ALKALI CELL OPERATION METHODS
BACKGROUl~iD OF THE INVENTION
The present invention relates generally to the operation of an oxygen
electrode for use in an electrolytic cell and particularly for the production ofchlorine and caustic (sodium hydroxide) in such a manner as to significantly reduce
the voltages r.ecessary for the operation of such electrolytic cells and to increase
substantially the power efficiencles available from such electrolytic cells utilizing
oxygen eiectrodes. More particularly, the present disclosure relates to improvedmethods of operation of oxygen electrodes which include utilizing a positive air to
liquid pressure drop on the air feed side of the oxygen electrode to improve
performance, control of the total flow of the gas feed stream to improve the mass
transfer within the air feed side of the oxygen electrode at the reaction sites,humidification of the gas feed to the oxygen electrodes to reduce the drying out and
delamination of the oxygen electrode so that it might function at a higher current
density over a Ionger lifetime, and the elimination of certain gases such as carbon
dioxide to increase the lifetime of the oxygen electrodes by elimination of salts
lS which might be formed upon the porous structure of the oxygen electrode during the
use thereof~ These rnethods of operation may be utilized singularly or preferably in
combination to produce higher power efficiencies at lower voltages so as to produce
a more energy-efficient oxygen electrode in an electrolytic cell especially suitable
for the production of chlorine and caustic (sodium hydroxide).
Chlorine and caustic are essential large volume commodities which are
basic chemicals required by all industrial societies. They are produced almost
entirely electrolytically from aqueous solutions of alkaline metal halides or more
particularly sodium chloride with a major portion of such production coming fromdiaphragm type electrolytic cells. In the diaphragm electrolytic cell process, brine
(sodium chloride soiution) is fed continuously to the anode compartment to flow
through a diaphragm usually made of asbestos particles formed over a cathode
structure of a foraminous nature. To minimize back migration of the hydroxide ions,
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1155792
the flow rate is always maintained in excess of the conversion rate so that the
resulting catholyte solution has unused or unreacted sodium chloride present. The
hydrogen ions are discharged from the solution at the cathode in the form of
hydrogen gas. The catholyte solution containing caustic soda (sodium hydroxide),
S unreacted sodium chloride and other impurities, must then be concentrated and
purified to obtain a marketable sodium hydroxide commodity and sodium chloride
which is to be reused in electrolytic cells for further production of sodium hydroxide
and chlorine. The evolution of the hydrogen gas utilizes a higher voltage so as to
reduce the power efficiency possible from such an electrolytic cell thus creating an
10 energy inefficient means of producing sodium hydroxide and chlorine gas.
With the advent of technological advances such as dimensionally stable
anodes and various coating compositions therefor which permit ever narrowing gaps
between the electrodes, the electrolytic cell has become more efficient in that the
power efficiency is greatly enhanced by the use of these dimensionally stable
lS anoaes. Also, the hydraulically impermeable membrane has added a great deal to
the use of tbe electrolytic cells in terms of selective migration of various lons
across the membrane so as to excludc c~ntaninants frcm the resultant product
thereby eliminating some of the costly purification and concentration steps of
processing. Thus, with the great advancements that have tended in the past to
20 improve the efficiency of the anodic side and the membrane or separator portion of
the electrolytic cells, more attention is now being directed to the cathodic side of
the electrolytic cell in an effort to improve the power efficiency of the cathodes to
be utilized in the electrolytic cells Shusto create a sign~ficant ener~y ~avin~s in the
resultant production of chlorlne and caustic. Looking more spedfically at the
2S problem of the cathodic side of a conventional chlorine and caustic cell, it may be
seen that in a cell employing a conventional anode nd a cathode and a diaphragm
therebetween, the electrolytic reaction at the cathode may l~e represented as
2H20 ~ 2e yields H2 + 20H
~ he potential of this reaction versus a standard H2 electrode is ~.8~
30 volts. The desired reaction under ideal circumstances to be prornoted at the cathode
--3--
1 155792
would be
2H20 ~ 2 + 4e yields 40H
The potential for this reaction is +0.40 volts which would result in a theoretical volt-
age savings of 1.23 volts. The electrical energy necessarily consumed to produce
S the hydrogen gas which is an undesirable reaction of the cathode of the conventional
electrolytic cells has not been counter balanced efficiently in the industry by the
utilization of the resultant hydrogen since it is basical~y an undesired product of the
reaction. While some uses have been made of the excess hydrogen gas those uses
have not made up the difference in the expenditure of electrical energy necessary to
10 evolve the hydrogen thus if the eYolution of a hydrogen could be eliminated it would
save electrical energy and thus make production of chlorine and caustic a more
energy efficient reaction.
The oxygen electrode presents one possibility of elimination of this
reaction since it consumes electrochemically activated oxygen to combine with
15 water and the electrons available at the cathode in accordance with the follou.ing
e~uation
2H20 + 2 + 4e yields 40H
It is readily apparent that this reaction is more energy efficient by the very absence
of the production of any hydrogen at the cathode, and the reduction in potential as
20 shown above. This is accomplished by feeding an oxygen rich fluid such as air or
oxygen to an oxygen side of an oxygen electrode where ~he oxygen has ready access
to the electrolytic surface so as to be consumed in the fashion according to the
equation above. This does, however, require a slightly different structure for the
electrolytic cel~ itself so as to provide for an oxygen compartment on the cathodic
35 side of the cathode so that the oxygen rich substance may be fed thereto.
lhe oxygen electrode i~self is well known in the art since the many
NASA proJects utilized to promote space travel during the 1960s also provided funds
for the development of a fuel cell utilizing an oxygen electrode and a hydrogen
anode such that the gas feeding of hydrogen and oxygen would produce an electrical
30 current for utilization in a space craft. While this major government-financed
115~792
research effort produced many fuel cell components including an oxygen electrode
the circumstances and the environment in which the oxygen electrode was to
function were quite different from that which would be experienced in a chlor-alkali
cell. Thus while much of the technology gained during the NASA projects is of value
5 in the chlor-alkali industry with regard to development of an oxygen electrode, much
further development is necessary to adapt the oxygen electrode to the chlor-alkali
cell environment.
Some attention has been given to the use of an oxygen electrode in a
chlor-alkali cell so as to increase the efficiency in the manner described to be
10 theoretically feasible, but thus far the oxygen electrode has failed to receive
significant interest so as to produce a commercially effective or economically viable
electrode for use in an electrolytic cell to produce chlorine and caustic. While it is
recognized that a proper oxygen electrode will be necessary to realize the
theoretical efficiencies to be derived therefrom, the chlor-alkali cell will require
lS operational methodology significantly different from that of a fuel cell since an
electrical potential will be applied to the chlor-alkali cell for the production of
chlorine and caustic in addition to the supply of an oxygen-rich fluid to enhance the
electrochemical reaction to be promoted. lherefore, it would be advantageous to
develop the methodology for the operation of an oxygen electrode directed
20 specifically toward the maximization of the theoretical electrical efficiencies
possible with such an oxygen electrode in a chlor-alkali electrolytic cell for the
production of chlorine and caustic.
SUMMA~Y OF T~E INYENTION
It is therefore an object of the present invention to provide a
methodology of operation of an oxygen electrode which will enhance and maximize
25 the energy efficiencies to be derived from an oxygen electrode within the
environment of a chlor-alkali electrolytic cell.
It is another object of the present invention to provide an adjusted
pressure of the gas feed ~o the oxygen electrode to promote this maximization.
1155792
It is ano.her object of the present invention to control the total flow of
the gas feed to the oxygen electrode to maximize its efficiencies.
It is still another object of the present invention to provide a humidified
gas feed to the oxygen electrode to maximize its efficiencies. and lifetimes.
It is a further object of the present invention to eliminate contaminating
substances such as CO2 from the gas feed to maximize the lifetime and efficiencyof the oxygen electrodes.
These and other objects that present invention, together with the
avantages thereof over existing and prior art forms which will become apparent to
those skilled in the art from the detailed disclosure of the present invention as set
forth herein and below, are accomplished by the improvements herein shown,
descri~ed and claimed.
It has been found that a chlor-alkali electrolytic cell having an anode
compartment, a cathode compartment divided from the anode compartment by a
lS separator and an oxygen compartment divided from the cathode compartment by an
oxygen electrode can be operated by a method comprising the steps of: feeding analkali metal halide solution to the interior of the anode compartment; feeding an
aqueous solution to the interior of the cathode compartment; feeding a molecularoxygen containing fluid to the interior of the oxygen compartment at a positive
gauge pressure; so as to accomplish a total flowrate in excess of the theoretical
stoichiometric amount of oxygen necessary for the reaction; applying an electrical
potential between the cathode and anode of the electrolytic cell; removing halogen
gas from the anode compartment; removing alkali metal hydroxide from the cathodecompartment; and removing an oxygen depeleted fluid from the oxygen compart-
2S ment while maintaining the positive gauge pressure upon the interior of the oxygen
ccmpartment.
lt has also been found that a chlor-alkali electrolytic cell having an
anode compartment, a cathode compartment divided from the anode compartment
by a separator and an oxygen compartment divided from the cathode compartment
3~ by an oxygen electrode can ~e operated by a method comprising the steps of:
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feedin~ an alkali metal halide solution to the interior of the anode compartment;
feeding an aqueous solution to the interior of the cathode compartment; feeding a
molecular oxygen containing carbon dioxide depleted gas having a high humidity
content to the interior of the oxygen compartment; applying an electrical potential
S between the cathode and anode of the electrolytic cell; removing the halogen gas
from the anode compartment; removing the alkali metal hydroxide from the cathodecompart:nent; and removing the oxygen depleted humidified gas from ~he oxygen
compartment.
It has also been found that a chlor-alkali electrolytic cell for the produc-
tion of chlorine and alkali metal hydroxide comprising: an anode compartment
adapted to contain an anolyte containing an alkali metal chloride; a cathode
compartment adapted to contain a catholyte containing an alkali metal hydroxide
and divided from said anode compartment by a separator; a separator; an oxygen
compartment adapted to receive an oxygen eontaining fluid, free of carbon dioxide,
humidified, at a posit.ve gauge pressure, and at a positive total flow of from 1.' to
10 times the stoichiometric amount of oxygen; an oxygen electrode dividing said
cathode compartment from said oxygen compartment; means for controlling the
moisture content of the oxygen containing substance; means for controlling the
pressure of the oxygen containing fluid within said oxygen compartment; means for
controlling the total flowrate of the oxygen containing fluid within said oxygencompartment; means for removing chlorine from said anode compartment; means for
removing alkali metal hydroxide from said cathode compartment; means for
supplying alkali metal chloride to said anode compartment; and means for supplying
an electroly-ing electrical energy to said anode and said cathode.
2S The preferred embodiments of the subject invention are shown and
described by way of example in this disclosure without a~tempting to show all of the
various forms and modifications in which the subject invention might be embodied;
the invention being measured by the appended claims and not by the details of this
disclosure.
~ 155792
In accordance with the present teachings, a method
is provided for operating a chlor-alkali electrolytic cell which
has an anode compartment, a cathode compartment divided from the
anode compartment by a separator and an oxygen compartment divided
from the cathode compartment by an oxygen electrode. The method
comprises the steps of feeding an alkali metal halide solution to
the interior or the anode compartment; feeding an aqueous solu-
tion to the interior of the cathode compartment; removing sub-
stantially all carbon dioxide from air, thereafter saturating the
air with water at a temperature in the range of 40-70C. and
feeding the air at a higher temperature in the range of 40 to 90C
to the interior of the oxygen compartment at a positive gauge
pressure in the range of 0.25 to 250 grams per square centimeter
~0.1 to 100 inches of H2O); providing a total flow rate in the
range of 1.5 to 5 times the theoretical stoichiometric amount
of oxygen necessary for the reaction; applying an electrical
potential between the cathode and anode of the electrolytic cell;
removing halogen gas from the anode compartment; removing alkali
metal hydroxide from the cathode co~partment; and removing an
oxygen depleted air from the oxygen compartment while maintaining
the positive gauge pressure upon the interior of the oxygen
compartment.
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1155792
I~RIEF DESCRIPTION OF THE DRAWING
Figure 1 is a schematic view of an electrolytic cell for the production of
halogen gas and alkali metal hydroxides according to the concepts of the present in-
~rention.
Figure 2 is a graphical representation of the relationships between total
S flow, pressure differential, and measured potential of the cathode.
DESCRIPTION OF THE PREFERREI) EMBODIMENTS
~eferring to Figure 1, numeral 12 refers to a monopolar divided electro-
lytic cell whlch is suitable for use according to the concepts of the present
invention. The applicants recognize that various other designs for electrolytic cells
could incorporate the methods according to the concepts of the present invention,
but that for illustration purposes the applicants choose the present schematic to
more amply describe the details of the applicants' invention. Electrolytic cell 12, as
shown in Figure ~, would generally have some environmental supporting structure or
fountation to maintain each electrolytic cell 12 in correct alignment so as to build a
bank of electrolytic cells for production purposes. The details of this environmental
lS structure have not been shown for ease of illustrating the concepts of the present
invention. The cell itself could be manufactured from various materials either
metallic or plastic in nature as long as these materials resist the caustic
surrountings of the chlorine environment, and temperature characteristics duringthe operation of the basic chlor-alkali cell which are well known in the ~rt. Such
materials generally include but are not limited to metallic materials such as steel,
nickel, titanium and other valve metals in addition to plastics such as polyvinylchlo-
ride, polyethylene, polypropyltene, fiberglass and o~chers too nur~ous to n~ntioa~.
The ~alve metals include aluminum, molybdenum, niobium, titanium, tungsten,
zirconium and alloys thereof.
2S it can be observed from the drawing that the electrolytic cell 12 shown
has an anode 14, a separator 16, and a cathode 18 such that three individual
--8 --
.,,
, . .
,,, ~ ,
~ 155792
compartments are formed within the electrolytic cell being mainly the anode
compartment 20, the cathode compartment 22, and the oxygen compartment 24.
The anode 14 will generally be constructed of a metallic substance,
although graphitic carbon could be used as in the old electrodes which have lar~ely
S been discarded by the industry presently. These anodes, particularly if they are to
be used in a chlor-alkali cell 12, would generally be active material resistant to the
anolyte such as a valve metal. A preferred val~e metal based upon cost, availability
and electrical chemical properties is titanium. There are a number of forms a
titanium substrate may take in the manufacture of an electrode, including for
exarnple, solid metal sheet material, expanded metal mesh material with a large
percentage open area, and a porous titanium with a density of 30 to 70 percent pure
titanium which can be produced by cold compacting titanium powder. Porous
titanium seems to be the preferred substance presently for its long life character-
istics along with its relative structural integrity. If desired, the porous titanium can
lS be reinforced ~ith titanium mesh in the case of large electrodes.
Usually ~hese substrate materials will have a surface coating to protect
the material against passivation such as to make same what is generally known inthe ar~ as a dimensionally stable anode. Most of these coatings contain a noble
metal, a noble metal oxide either alone or in combination with a valve metal oxide
or other electro~talytically acti-~ ~rr~si~tan~ materials. ffle~e 80 called
dimensionally stable anodes are well-lcnown and are widely used in the industry. One
type of coatin~ for instance would be a Beer-type coating which can be seen fromU.S. Paten~ Numbers: 3,236,7S6; 3,623,498; 3,711,38S; 3,751,~96; and 3,g33,616.
Another type of coating utilized is one which tin, titanium and ruthenium oxides are
used for surface coating as can be seen in U.S. Patent Numbers 3,776,834 and
3,85~,092. Two other examples of surface coatings include a tin, antimony with
titanium and ruthenium oxides as found in U.S. Patent Number 3,875,043 and a
tantal um iridium oxide coatin~ as found in U.S. Patent Number 3,878,083. There
are, of course, other coatings which are available to those skilled in the art for use
in chlor-alkali cells as well as other types of applications in which electrodes would
1 155792
~e necessary for elec~lytic reactions.
~ here are a nurnber of materials which may be utilized for the
separator 16 as s~ in the drawing. ane ~pe of material such as
Naficn, atrademarkforaperfl~rocarbonmaterial, of o~urse, anticipates
t~e use of s~nething substantially hydraulically in~enreable or a cation
exchange m~[ibrane as it is known in the art. ~ne type of hydralll;ca11y
erneable cation exchan~e ~rane, which can be used in the apparatus
of the present i~tion, is a 'chin film of fluorinated cc~o1y~ having
p~a~nt su1fonic acid gra~s. The f1ut~rinat~ c~oly~[~ is derived fmn
m;~rs of the formul~:
1) CF2 = CF-( R ~ nS02F
lO in which the pendant -SO2F groups are converted to -S03H groups, and monomers of
the formula
(2) CF2 = CXXl Rl
wherein R represents the group -CF- CF2 ~- ~CFY-CF2 ~ ~)m in which the R is
fluorine or fluoroalkyl of 1 thru lO carbon atoms; Y is fluorine or trifluoromethyl; m
lS is 1, 2 or 3; n lS O or l; X is fluorine, chlorine or trifluoromethyl; and Xl is X or CF3
~CF2~ a~ wherein a is O or an integer from 1 to 5.
This results in copolymers having the repeating structural units
(3) -CF2 -CF-
,)n
SO3H
and (4) -C~2-CXXl-
In the copolymer there should be sufficient repeating units, according to
formula t3) above, to provide an -SO3H equivalent weight of about 800 to l600.
Materials having a water absorption of about 2S percent or greater are preferredsince higher cell voltages at any given current density are required for materials
having less water absorption. Similarly, materials having a film thickness
2S ~unlaminated) of about 8 mils or more, require higher cell voltages resulting in a
lower power efficiency.
Typically, because of large surface areas of the membrane in commercial
- 10- .
,... .
1 155792
cells, the substrate film material will be laminated to and impregnated onto a
hydraulically permeable, electrically non-conductive, inert, reinforcing member such
as a woven or non-woven fabric made of fibers of asbestos, glass, TEFLON, or thelike. In film/fabric composite materials, it is preferred that the laminating produce
S an unbroken surface of the film resin on at least one side of the fabric to prevent
leakage through the substrate film material.
The materials of this type are further described in the following patents
U.S. Patent Numbers 3,û41,317;
3,282,875; 3,624,053; 3~784,~99 and aritish Patent Number 1,184,321. Substrate
materials as aforedescribed are available from E. 1. duPont deNemours and Co.
wlder the trademark NAFION.
Polymeric materials, according to formulas 3 and 4, can also be made
wherein the ion exchange group instead of being a sulfonic acid exchange group
could be many other types of structures. One particular type of structure is a
carboxyl group ending in either an acid, and ester or a salt to form an ion exchange
group similar to that of the sulfonic acid. In such a group instead of having SO2F
one would find COOR2 in its place wherein R2 may be selected from the group of
hydrogen, an alkali metal ion or an organic radical. These polymeric materials are
a~ailable presently from E. I. duPont deNemours ~ Co. Furthermore, it has been
foùnd that a substrate material such as NAFION having any ion exchange group or
function group capable of being converted into an ion exchange group or a function
group in which an ion exchange group can easily be introduced would include suchgroups as oxy acids, salts, or esters of carbon, nitrogen, silicon, phosphorus sul~ur
chlorine, arsenic, selenium, or tellurium.
2S A second type of substrate material has a backbone chain of copolymers
of tetrafluoroethylene and hexafluoropropylene and, grafted onto this backbone, a
fifty-fifty mixture of styrene and alpha-methyl styrene. Subsequently, these grafts
may be sulfonated or carbonated to achieve the ion exchange characteristic. Thistype of substrate while having different pendant groups has a fluorinated backbone
chain so that the chemical resistivit es are reasonably high.
- 11 - .
., .
1 155792
Another type of substrate film material would be polymeric substances
having pendant carboxylic or sulfonic acid groups wherein the polymeric backbone is
derived from the polymerization of a polyvinyl aromatic component with a mono-
vinyl aromatic component in an inorganic solvent under conditions which prevent
solvent evaporation and result in a generally copolymeric substance although a 100
percent polyvinyl aromatic compound may be prepared which is satisfactory.
The polyvinyl aromatic component may be chosen from the group
including: divinyl benzenes, divinyl toluenes, divinyl n~hchalenesdivinyl diphenyls,
divinyl-phenyl vinyl ethers, the substituted alkyl derivatives thereof such as
dimethyl divinyl benzenes and similar polymerizable aromatic compounds which arepolyfunctional with respect to vinyl groups.
The monovinyl aromatic component which will generally be the im-
purity present in commercial grades of polyvinyl aromatic compounds include:
styrene, isomeric vinyl toluenes, vinylr~aphthalenes, vinyl ethyl benzenes, vinyl
lS chlorobenzenes, vinyl xylenes, and alpha substituted alkyl derivates thereof, such as
alpha methyl vinyl benzene. In cases where highpurity polyvinyl aromatic
compounds are used, it may be desirable to add monovinyl aromatic compounds so
that the polyvinyl aromatic compound will constitute 30 to 80 mole percent of
polymerizable material.
Suitable solvents in which the polymerizable material may be dissolved
prior to polymerization should be inert to the polymerization (in that they do not
react chemically with the monomers or polymer), should also possess a boiling point
greater than 60C, and should be miscible with the sulfonation medium.
Polymerization is effected by any of the well known expedients, for in-
2S stance, heat, pressure, and ~atalytic accelerators, and is continued until an
insoluble, infusible gel is formed substantially throughout the volume of solution.
The resulting gel structures are then sulfonated in a solvated condition and to such
an extent that there are not more than four equivalents of sulfonic acid groups
formed for each mole of polyvinyl aromatic compound in the polymer and not less
than one equivalent of sulfonic acid groups formed for each tenm~Les of poly and
- 12-
1 1 55792
monovinyl aromatic compound in the polymer. As with the NAFION type material
these materials may require reinforcing of similar materials.
Substrate film materials of this type are further described in the
following patents which are hereby incorporated by reference: U.S. Patent Numbers
S 2,731,408; 2,731,411 and 3,887,499. These materials are available from Ionics, Inc.
under the trademark IONICS CR6.
Various means of improving these substrate materials have been sought,
one of the most effective of which is the surface chemical treatment of the
substrate itself. Generally these treatments consist of reacting the pendant ~roups
with substances which will yield less polar bonding and thereby absorb fewer water
molecules by hydrogen bonding. This has a ~endency to narrow the pore openin~s
through which the cations travel so that less water of hydration is transmitted with
the cations through the membrane. An example of this would be to react the
ethylene diamine with the pendant ~roups to tie two of the pendant groups to~ether
lS ~y two nitrogen atoms in the ethylene diamine. Generally, in a film thickness of 7
mils, the surface treatment will be done to a depth of approximately 2 mils on one
side of the film by controlling the time of reaction. This will result in good
electrical conductivity and cation transmission with less hydroxide ion and
associated water reverse migration.
The separator 16 could also be a porous diaphragm which may be made of
any material compatible with the cell liquor environment, the proper bubble pressure
and electrical conductivity characteristics. One example of such a material is
asbestos which can be used either in paper sheet form or be vacuum-deposited
fibers. A further modifkation can be effected by adding polymeric substances,
23 generally fluorinated, to the slurry from which the diaphragm is deposited. A~so
polymeric materials themselves can be made porous to the extent that they show
operational characteristics of a diaphragm. Those skilled in the art will readily
recognize the wide variety of materials that are presently available for use as
separators ih chlor-alkali cells.
The third major component of these subject cells to be utilized according
1 155792
.o the mcthods of the present invention is a cathode 18 as seen in the drawing. The
cathode 18, in order to be utilized according to the methods of the present
invention, will necessarily be an oxygen cathode. An oxygen electrode or oxygen
cathode may be defined as an electrode which is supplied with a molecular oxygen
5 containing fluid to lower the voltage below that necessary for the evolution of
hydrog~n. The basic support for an oxygen cathode will generally include a current
collector which could be constructed of a base metal although carbon black might
also be used. The expression base metal is used herein to refer to inexpensive
m~tals which are commercially available for common construction purposes. Base
I0 metals are characterized by low cost, ready availability and adequate resistances to
chemical corrosion when utllized as a cathode in electrolytic cells. Base metals
would include, for instance, iron, nickel, lead and tin. Base metals also include
alloys such as mild steels, stainless steel, bronze, ~nel and cast ir~n. me h~ce
metal preferablyisd~rdcal~resistant to the catholyte and has a high electrical
15 conductivity. Furthermore, this material will generally be a sligl tly porous material
such as a mesh when used in the construction of an oxygen cathode. A preferred
metal, based upon cost, resistance to the cathclyte and voltages available, is nickel.
Other current collectors would include: tantalum, titanium, silver, gold, and plated
base metals. Upon one side of this basic support material will be a coating of a
20 porous material either compacted in such a fashion as to adhere to the nickel
support or held together with some kind of binding substance so as to produce a
porous substrate material. A preferrcd porous material based upon cost is carbon.
Anchored within the porous portion of the oxygen cathode is a catalyst to catalyze
the reaction wherein molecular oxygen combines with water molecules to produce
25 hydroxide groups. These catalysts are generally based upon a silver or a platinum
group metal such as palladium, ruthenium, gold, iridium, rhodium, osmium, or
rhenium but also may be based upon semiprecious or nonprecious metal, alloys,
metal oxides or organometal complexes. Generally such electrodes will contain a
hydrophobic material to wetproof the electrode structure. Of course, those skilled
30 in the art will realize that the porosity of the carbon material, the amount and the
*Trad~oark fc~r a nic~ c~ allc~y -
- ~
1 1 55792
type of catalytic material used will affect the voltages and
current efficiencies of the resultant electrolytic cell as well
as their lifetimes. A preferred cathode 18 may be constructed
according to U.S. Patent No. 3,423,247.
As seen in the drawing, utilizing an anode 14, a
separator 16 and oxygen cathode 18 as described above will
result in an electrolytic cell 12 having three compartments,
basically an anode compartment 20, a cathode compartment 22 and
an oxygen compartment 24. A brine of an alkali metal halide
solution is introduced into the anode compartment 20 using
inlet 26. The alkali metal halide solution preferably would
be one which would evolve chlorine gas, such as sodium chloride
or potassium chloride. An aqueous solution is introduced into
the cathode compartment 22 through an inlet 28. The aqueous
solution must contain sufficient water molecules to be broken
down to form the required hydroxide groups necessary for the
reaction. A gaseous fluid containing oxygen is introduced into
oxygen ~ompartment 24 using,an;o~ygen~inlet 30, the quantity
of oxygen in t~e fluid and th~ volume of the fluid being
sufficie~t to sustain the chemical reaction occurring at the
oxygen cathode, and preferably being in a substantial
stoichiometric excess for such reaction. Such a substance
would generally be a gas and most preferably would be air
with carbon dioxide removed and humidified or pure molecular
oxygen which had been humidified. The reaction products such
as chlorine gas would be removed from the anode compartment 20
through the halogen outlet 32 and aqueous NaOH or KOH would be
removed from the cathode compartment 22 through the alkali
metal hydroxide outlet 34 and an oxygen depleted fluid either
in the form of residual pure oxygen or air most preferably would
be removed from the depleted fluid outlet 36.
1 155792
In the three compartment cells 12 according to
the above description, an oxidizing gas depolarized chlor-
alkali cell, a pressure differential is applied across the
porous cathode 18 so that the pressure in the oxygen
compartment 24 is higher than that in the cathode compartment
22. The pressure of gaseous fluid introduced into the
oxygen compartment can be zero gauge and thereby just
sufficient to trigger some bubbling through the gases into
the oxygen cathode, or may be greater to compensate for the
hydraulic head o~ liquid catholyte present in the cathode
compartment on the opposite side of the oxygen cathode from
the oxygen compartment.
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1 155792
Elevated pressure assists in mass transfer of the axi~izing gas such as
o~ygen into the cathode 18 thereby preventing oxyqen depletion in the
reaction zone within the cathode 18 ar~ leading to a longer cathode 1ifetime.
lhis pressure differential it shc>uld be rem~bbered is based upon the partial
S pressure of the axygen present if less than 100% axygen is used.
Increasing the total flow of the depolarizing gas in the oxygen
compartment 24 also enhances the mass transfer of oxy~en into the reaction siteswithin the cathode 18. This is particularly important where less than 100% pure 2
is used. Molecular oxygen is consumed by the reaction taking place at the catalytic
10 sites within the porous material of the oxygen cathode 18. As oxygen is consumed,
additional quantities must be available continously and must, therefore, be fed on a
continuous basis into the oxygen compartment 24. The preferred total flows are
between 0 and 10 times the theoretica~ stoichiometric amount of oxy~en necessaryfor the reaction with a flow of about 2.5 times being the best.
lS Pure oxygen gas may be supplied to the oxygen compartment 24, however
air may also be used since it contains approximately 23% free molecular oxy~ell by
weight. In the case of air though, carbon dioxide must be removed from the air
before it is delivered to the oxygen compartment 24. It has been found that car~on
dioxide will promote a formation of certain carbonate deposits upon the cathode
20 which sharply reduces its liietime and power efficiency while increasing the
volta~e. By eliminating the major portion of carbon dioxide this problem was also
largely eliminated.
llle applicants have noticed that the presence of nitrogen in the air
creates problems since it acts as a diluent so as to there}~y decrease the
2S concentration of the o~ygen present within the oocygen canpartment 24 of the
electrolytic cell 12. me nitrogen m)1ecules enter the pores of the cathode
18 and r~st be diffused back out of the pores since they are not used in the
reaction. If not removed they would cause a lack of activity within
the porous catalytic areas of the oxygen c~thode 18 su~h as to reduce the pc~er
30 efficiency possible and increase the ~701tage necessary for the operation of such
a oell. The applicants have fur~er found that this may be redu~d
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1 ~55792
.o a minimum by increasing the total flow so as to provide ample oxygen supply to
the oxygen compartment 24, thus reducing to a minimum the voltage necessary to
operate the cell while increasing to a maximum the possible power efficiency from
such an electrolytic cell 12.
S Applicants have furthermore discovered that evaporation and mass
transfer pose a problem with oxygen cathodes 18 as shown in the electrolytic cells
12. This problem they found could be eliminated by increasing the relative humidity
of the oxygen or air to be supplied to the oxygen compartment 24 by bubbling thegas through water at a temperature of 40 to 70C. so as to produce a relative
lû humidity in the range of 85 percent. This in turn reduces the evaporation and
reduces the drying out of the cathodes which can cause delamination of the porous
material from the solid support material of the oxygen cathode 18 and further
enhances the mass transfer across the porous surfaces. The gas temperature as itwas actually fed to the oxygen compartment 24 was generally in the range of 40 to
90C. and therefore saturated. Furthermore, the humidification seems to have
another effect. Mainly, the evaporative driving force, which causes the mass
transfer of the water from the cathode compartment 22 into the cathode struc.ure18, causes the crystallization of electrolyte to form solids which reduce seriously
the lifetime of a given cathode 18 because the solids plug up the pores. ay the
humidification of the feed gas to the oxygen compartment 24 this is drastically
reduced by eliminating the evaporative driving force involved in transferring the
liquid electrolyte from the cathode compartment 22.
It has been noted though that if the dew point of the gas feed is higher
than the cathode skin temperature, condensation occurs on the cathode surface.
2S When this happens, sites of oxygen mass transfer are occluded so as to decrease
seriously the performance of a given oxygen cathode 18. Therefore, a gas stream
dew point was adjusted to balance the two deleterious effects described above,
specifica~ly to maintain the dew point a few degrees below the cathode skin
temperature while maintaining the relative humidity wlthin a range to eliminate the
evaporative driving force in~olved. It should be noted that higher operation
temperatures lower the voltage of the cell but may shorten the life of the cathode
18. A temperature in the range of 60 to 85C. is ~onsidered optlmum.
~ li ~
1 155792
In order that those skilled in the art may more readily understand the
present invention and certain preferred aspects by which it may be carried into
effect, the following specific examples are afforded.
EXAMPLE 1
~ n oxy~en cathode according to U. S. I'atent No. 3,423,245, was installed
into a nE~rbrane type electrolytic oell so that the ~rbon side faced the
o~ygen canparbnent and the nickel side faced the cathode arpar~t in
which an electrolyte was placed. A dimensionally stable anode, having a
catalytic layer ca~posed of tantalum and iridiun o~ides, was installed
apprc~cimately 7 centimeters away and parallel to the oxygen cathode. A
flaw of carbon dioxide free air was passed into the axygen c~ar~t
of the oell at a fl~ rate of ap~rodumately 790 cubic centimeters per
minutc which is approximately 21 times the theoretical stoichiometric amount
needed when the cell is operated at 1 ampere per square inch current density. lhe
pressure in the oxygen compartment was adjusted to approximately 110 grams per
square centimeter (44 inches of water) above atmospheric pressure by restricting the
lS flow exiting from the outlet 36. The pressure was maintained at that level during
the test. Electrolyte consisting of approximately 400 grams NaOH per liter was
then added to the cathode compartment 22 and agitated continuously by magnetic
stirring aparatus.
The cathode was then conditioned by operating tl-e cell at 60C. and a
20 current density of approximately 0.0S amperes per square centimeter (one-third
ampere per square inch) for about one day. After conditioning was completed, the
current density was increased to approximately 0.1S amperes per square centimeter
(one ampere per square inch~. The air flow, pressure, temperature and current
der~ity were held constant during the rcmainder of the test. It should be noted that
2S these tests were calducted with sodiun hydra~ide electrol~te only h~ever
the results from these tests should correlate closely with those th~
e ~ta~ned ~y using c~ ional ~hlor-aL'cali oells s~ce the type of ar~ode
or the spacing of the anode to the cathode are not critical factors, although the
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1 155792
anode must be stable in sodium hydroxide solution.
The cathodes were conditioned at the reduced current density because it
was thought that the catalytic platinum layer becomes partially oxidized, during the
period when the cathodes are stored before use. The conditioning process restores
5 the catalytic layer to high activity without causing deterioration in the quality of
the cathode. Slow break-in for less noble catalysts however may be harmful.
The electrical connection was made on the nickel side of the cathode be-
cause it was easier to make a good electrical contact on nickel rather than on the
carbon. The cathode reference voltage measured versus a mercury/mercuric oxide
reference electrode cell, changed from -0.31 on day number 1 to -1.03 on day
number 98 when the .est was considered completed. The lifetime of this particular
cathode under these test conditions was 2350 hours.
EXAMPLE 2
A cathode test was done as described in Example 1, except that the air
flow rate was reduced from 7~0 cubic centimeters per minute to 220 cubic
lS centimeters per minute (approximately 6 times the theoretical stoichiometric
amount necessary for reactionj. The reference vol~age changed from -0.43 on day
number I to -2.27 on day number 52. The cathode lifetime was 1240 hours in this
test as compared to 2350 when increased air flow was used in Example 1. This shows
that when the total flow increases, the potentials are lower and the lifetimes are
20 ex~ended.
EXAMPLE 3
A cathode test was done as describ~ed in Example 1, except that an
oxy~en flowrate of 150 cubic centimeters per minute was used instead of an air
flowrate of 790 cubic centimeters per minute. This oxygen flow rate was about 19
times the theoretical stoichiometric flow of oxygen required at one ampere per
25 square inch current density for operation of a cell. For this test, the electrical
connection was made on the ca. bon side of the cathode. The cathode was
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1 1 55792
conditioned by running the cell at a current density of 0.0S amperes per square
centimeter (1/3 ampere per square inch) for about 24~ hours, increasing it to 0.1
ampere per square centimeter (2/3 ampere per square inch) for about 24 hours, and
finally increasing it to about 0.15 ampere per square centimeter (1 ampere per
S square inch) at which level it was held for the remainder of the test. The reference
voltage changed from -0.38 on day number 2 to-0.42 on day number 293. ~he test
was discontinued on day number 293 because delamination of the cathode
lamination occurred. The cathode lifetime was a~out 7030 hours.
This again shows that when the total flow is increased in terms of stoichiometric
10 amounts of available oxygen, the life is extended at lower potentials.
EXAMPLE 4
An oxygen cathode test was done according to Example 1 with an
operating temperature of approximately 85C at a current density of approximately
2 amperes per square inch and a 300 grams per liter NaOH solution. Furthermore,
the membrane utilized in the subject tes- was a standard NAFION. This
lS experimental cell was operated using various types of cathodes. Comparative cell
voltages for the different cathodes were obtained as follows:
Avg. Volt.
Saving Over
Hydrogen Evol.
Average In Volts
Cathode Tvpe Cell Volt. - (% Difference)
Steel Mesh 4.33S
Oxygen Electrode
with platinium catalyst 3.039 1.296 (30%)
using pure oxygen
Oxygen Electrode
with silver catalyst3.306 1.02g (2496)
using pure oxygen
Oxygen Electrode
with silver catalyst3.536 0.799 (18%)
using air feed
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.,, ~. .
1 155792
As can be seen from the results above each of these subject cathodes
when compared to a standard hydrogen evoluting steel mesh cathode shows superior
performance utilizing the methods of the present invention.
EXAMPLE 5
An oxygen cathode test was done accordin~ to Example 1 wherein the run
5 was made on air which was not scrubbed of carbon dioxide. The cathode was broken
in on oxygen and then switched to air, and failure occurred less than 48 hours after
switchlng to air. This performance was typical of cathodes supplied with air which
contained carbon dioxide, thus, showing the necessity of removing carbon dioxide for
the lifetime of an oxygen cathode. The basic conditiors were as those contained in
10 runs according to Examples 1 - 3 and the table below shows the cell voltages and
reference voltage along with remarks.
SEP vs Hgtll~O
Reference
Cell VoltageVoltage Remarks
1.168-.255 at 1/3 asion oxygen for
break in
1.044 -.124
0.995 -. 1 1 2
1.790-.222 at 1 asi
1.768 -.212
1.944 -.354 switch to
compressed air
1 .940 -.347
2.034 -.4~1
2.120 -.490
2.745 -1.066 cathode failed
EXAMPLES 6 to 12
_ _
Oxygen cathode tests were done according to Example 1 wherein the total
air flow rate was varied according to Figure 2 of the drawings and with the pressure
also varied. As seen in Figure 2, the cathode potential decreased with increasing
flowrates and also decreased with increasing pressures. In each case the air supplied
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1 1557~2
was free of c~rbon dioxide and humidified.
The cathode tests as illustrated by the examples above were ended in each
case when the reference voltage reached -1.00 or when delamination of the various
layers of the cathode occurred. The air (or oxygen) flow was recorded on the cathode
5 life cell test data sheets as the height (in millimeters) of the steel ball on the
Matheson number 601 flow meter (except for Example 3 for a which a Matheson
number 602 flow meter was used). These readings were then converted into cubic
centimeters per minute by referring to the appropriate calibration curves. The ex-
amples give results of the cathode tests where the pressure differentials in the range
of 100 grams per square centimeter (40 inches of water). The term pressure
differential means the net pressure exerted between the two sides of the cathode. In
this case the difference between the pressure and the oxidizing gas compartment (100
~srams per square centimeter above atmospheric pressure) and the average hydrostatic
pressure exerted by the electrolyte on the other side of the cathode (10 grams per
15 square meter) is approximately 100 grams per square centimeter. The hydrostatic
pressure is calculated by multiplying the density of the electrolyte (1.33) by average
height above the cathode which averaged 3 inches. According to an estimate, the
useful range of pressure differential probably is in the range of 0.25 through 500 grams
per square centimeter (0.1 to 20~ inches of water column). It is expected that those
20 cathodes utilizing atmospheric pressure or where the gas compartment pressure is not
allowed to exceed atmospheric pressure would be iess than 1240 hours of lifetime
obtained in Example 2 for instance. It should be noted, however, that all the examples
give the results of tests using NaOH electrolyte~only,)chlor-alkali cells were not used
since no porous cathodes are in commercial use to date. All the above described tests
~5 were conducted at 1 ampere per square inch which was selected for test purposes only
for standardization and should not be considered a maximum possible value. It is
expected that current densities of the range of 2 amperes per s~uare inch or higher
could be used. The tests as illustrated by Example 1 - 3 were conducted at 60C, this
temperature being chosen simply as a convenient temperature for ~vhich standardiza-
30 tion can be established.
~ 155792
Thus, it shouid be apparent from the foregoing description of the preferredembodiments that the methods for operation of an oxygen air cathode in an elec-
trolytic cell herein shown and described accomplish the objects of the invention and
solve the problems attendant to such methodology for use in a production chlor-alkali
S electrolytic cell utilizing an oxygen cathode.
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