U.S. patent number 3,926,769 [Application Number 05/493,811] was granted by the patent office on 1975-12-16 for diaphragm cell chlorine production.
This patent grant is currently assigned to The Dow Chemical Company. Invention is credited to Gerhard Gritzner.
United States Patent |
3,926,769 |
Gritzner |
December 16, 1975 |
Diaphragm cell chlorine production
Abstract
Improved apparatus and process to electrolytically produce
chlorine gas and an alkali metal hydroxide in a diaphragm cell. The
improved process comprises employing a cation exchange diaphragm
and contacting a foraminous cathode with an oxidizing gas having a
regulatably controlled moisture content, while substantially
simultaneously regulating the anolyte and catholyte
compositions.
Inventors: |
Gritzner; Gerhard (Midland,
MI) |
Assignee: |
The Dow Chemical Company
(Midland, MI)
|
Family
ID: |
27001409 |
Appl.
No.: |
05/493,811 |
Filed: |
August 1, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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361744 |
May 18, 1973 |
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Current U.S.
Class: |
204/237; 204/265;
204/261; 204/266 |
Current CPC
Class: |
C25B
11/095 (20210101); C25B 1/46 (20130101) |
Current International
Class: |
C25B
1/00 (20060101); C25B 11/00 (20060101); C25B
1/46 (20060101); C25B 11/04 (20060101); C25B
009/00 () |
Field of
Search: |
;204/98,237,261,265,266,DIG.3 ;136/136 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Williams; Howard S.
Assistant Examiner: Solomon; W. I.
Attorney, Agent or Firm: Selby; R. W.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This is a division of application Ser. No. 361,744 filed May 18,
1973.
Claims
What is claimed is:
1. An electrolytic cell to produce chlorine and an 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, said cathode compartment being spaced apart
from said anode compartment by a cation exchange diaphragm;
a cation exchange diaphragm suited to pass alkali metal ions from
said anode compartment to said cathode compartment, said diaphragm
adapted to separate said anode compartment from said cathode
compartment;
an anode positioned in said anode compartment;
a cathode positioned in said cathode compartment and spaced apart
from said diaphragm, said cathode having a wall portion adapted to
be in contact with the catholyte and another wall portion
substantially simultaneously adapted to be in contact with an
oxidizing gas;
an oxidizing gas compartment at least partially defined by a
surface portion of said cathode and adapted to contain an oxidizing
gas;
a means to control the moisture content of the oxidizing gas in
combination with an oxidizing gas supply means and said
cathode;
a means to circulate the catholyte at least within said cathode
compartment in combination with said cathode compartment;
a means to control at least the alkali metal ion concentration of
the anolyte;
a means to control the alkali metal hydroxide concentration of the
catholyte;
a catholyte head control means adapted to maintain the catholyte
upper surface level at least equal to the anolyte upper surface
level;
a means to remove the chlorine from said anode compartment;
a means to remove the alkali metal hydroxide from said cathode
compartment; and
a means to supply electrical energy to said anode and said
cathode.
2. The electrolytic cell of claim 1 wherein said cathode is a
foraminous body having at least the surface composed of a
substantially inert material selected from the group consisting of
Ru, Rh, Pd, Ag, Os, Ir, Pt, Au and Ni with a coating of a mixture
of particulate inert metal and an organic component selected from
the group consisting of polytetrafluoroethylene and fluorinated
copolymers of hexafluoropropylene and tetrafluoropropylene.
3. The electrolytic cell of claim 2 wherein the foraminous body is
a screen.
4. The electrolytic cell of claim 2 wherein the foraminous body is
nickel or an alloy thereof.
5. The electrolytic cell of claim 1 wherein said cathode is a
silver coated, woven copper screen with a mesh size of about 20 to
about 50 with an overcoating of a mixture of platinum, silver,
carbon or nickel particulate and an organic component selected from
the group consisting of polytetrafluoroethylene, and fluorinated
copolymers of hexafluoropropylene and tetrafluoroethylene.
6. The electrolytic cell of claim 1 wherein said cathode is at
least partially nickel.
7. The electrolytic cell of claim 1 wherein the catholyte head
control means is adapted to control the upper catholyte surface
level at from about 1 inch to about 3 feet higher than the upper
surface of the anolyte.
8. The electrolytic cell of claim 1 wherein the catholyte
circulation means is a pump spaced apart from said cathode
compartment and communicating with said cathode compartment.
9. The electrolytic cell of claim 1 wherein said moisture control
means is adapted to control the moisture within the range of from
about 50 to about 100 percent of saturation.
10. The electrolytic cell of claim 9 wherein the catholyte
circulation means is a pump spaced apart from said cathode
compartment and communicating with said cathode compartment.
Description
BACKGROUND OF THE INVENTION
This invention pertains to the electrolytic production of chlorine
in a diaphragm cell and more in particular to an electrolytic cell
containing an oxidizing gas depolarized cathode and a method of
producing chlorine and an alkali metal hydroxide in such
electrolytic cell.
Gaseous chlorine has long been produced from sodium chloride in an
electrolytic cell having an anode positioned within an anode
chamber and a cathode in a cathode chamber spaced apart from the
anode chamber by an ion and liquid permeable diaphragm, such as one
at least partially formed of asbestos. In such an electrolytic cell
chlorine is released at the anode and sodium hydroxide is formed in
the cathode chamber.
Various methods to conserve electrical power in electrolytic cells
have been developed using porous cathodes in combination with an
oxidizing gas to depolarize the electrode; see for example, Juda,
U.S. Pat. No. 3,124,520. It is desired to provide an improved
apparatus and process to reduce the electrical consumption of
chlorine producing electrolytic diaphragm cells.
SUMMARY OF THE INVENTION
An improved electrolytic cell to produce chlorine and an alkali
metal hydroxide has been developed. The electrolytic cell comprises
an anode compartment suited to contain an anolyte such as an
aqueous solution or mixture of an alkali metal chloride, for
example, sodium chloride. A cathode compartment adapted to contain
a catholyte containing the hydroxide of the alkali metal is spaced
apart from the anode compartment by a diaphragm. The diaphragm
separating the anode and cathode compartments is a cation exchange
membrane adapted to pass ions of the alkali metal from the anode
compartment to the cathode compartment. The diaphragm is suitably
positioned in the electrolytic cell to substantially entirely
separate the anode compartment from the cathode compartment.
An anode is suitably positioned within the anode compartment and a
cathode is suitably positioned within the cathode compartment to be
spaced apart from the diaphragm, that is substantially all of the
catholyte is contained within a space or opening at least partially
defined by the diaphragm and at least partially by an outer surface
of the cathode. The cathode is further adapted to have at least one
wall portion in contact with the catholyte and at least one other
wall portion substantially simultaneously in contact with an
oxidizing gas.
Means to circulate the catholyte at least within the cathode
compartment and to control the catholyte composition are in
operative combination with the cathode compartment. A means to
control the moisture content of the oxidizing gas in contact with
the cathode is in operative combination with the cathode.
A means to supply a direct current to the anode and the cathode is
suitably electrically connected to these electrodes. The
electrolytic cell further includes means to control the anolyte
composition, means to remove the chlorine produced from the anode
compartment and a means to remove the alkali metal hydroxide formed
from the cathode compartment.
The described electrolytic cell is advantageously used in an
improved process to produce chlorine and an alkali metal hydroxide.
In the improved process sufficient alkali chloride brine is fed
into and circulated through the anode compartment to maintain the
anolyte at a desired alkali metal chloride concentration.
Substantially simultaneously the catholyte is maintained at a
desired alkali metal hydroxide concentration. Sufficient electrical
energy is supplied to the anode and cathode to release gaseous
chlorine at the anode and to form the alkali metal hydroxide in the
cathode compartment. The gaseous chlorine and alkali metal
hydroxide are suitably recovered by means known to those skilled in
the art.
The efficiency of the cell is improved by maintaining the catholyte
head at least equal that of the anolyte and substantially
simultaneously contacting different wall portions of the cathode
with the catholyte and with an oxidizing gas. The moisture content
of the oxidizing gas is suitably controlled to minimize drying and
deposition of materials such as sodium chloride, sodium hydroxide
and the like on the cathode surface. The catholyte is circulated
within the cathode compartment to maximize contact between the
catholyte and the cathode to thereby further improve the electrical
efficiency of the cell.
DESCRIPTION OF THE DRAWING
The accompanying drawing further illustrates the invention:
In FIG. 1 is depicted a cross sectional view of one embodiment of
the invention.
In FIG. 2 is a cross sectional view of another embodiment of the
invention.
Identical numbers, distinguished by a letter suffix, within the
several figures represent parts having a similar function within
the different embodiments.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An electrolytic cell 10 of FIG. 1 includes an anode compartment 12
with an anode 14 positioned therein juxtaposed and spaced apart
from a cathode compartment 16 with a depolarized cathode 18
positioned therein. The anode compartment 12 is spaced apart from
the cathode compartment 16 by a cation exchange membrane or
diaphragm 20 adapted to pass alkali metal ions from the anode
compartment 12 to the cathode compartment 16. The anode 14
optionally acts as a supporting member for the diaphragm 20. Cation
exchange membranes are well-known to contain fixed anionic groups
that permit intrusion and exchange of cations, and exclude anions,
from an external source. Generally the resinous membrane or
diaphragm has as a matrix a cross-linked polymer, to which are
attached charged radicals such as --SO.sub.3 .sup.-, --COO.sup.-,
--PO.sub.3 .sup.=, --HPO.sub.2 .sup.-, --AsO.sub.3 .sup.= and
--SeO.sub.3 .sup.-. Vinyl addition polymers and condensation
polymers may be employed. The polymer can be, for example, styrene,
divinylbenzene, polyethylene and fluorocarbons. Condensation
polymers are, for example, phenol sulfuric acid and formaldehyde
resins. A method of preparing such resinous materials is described
in U.S. Pat. No. 3,282,875.
The electrolytic cell 10 further includes a source of alkali metal
chloride brine (not shown) and a means 22 to introduce or feed the
brine into the anode compartment 12 and maintain the anolyte at a
predetermined desired alkali metal chloride concentration. A
gaseous chlorine removal means such as a pipe 24 is suitably
connected to the anode compartment 12 to afford removal of gaseous
chlorine without substantial loss of chlorine to the ambient
atmosphere.
A means, such as an ultrasonic vibrator, turbine type impeller or
pump 26, to circulate the catholyte at least within the cathode
compartment 16 is optionally and preferably in combination with the
cathode compartment 16. The pump 26 together with appropriate
conduits extending into the cathode chamber 16 are provided to
afford effective circulation of the catholyte during operation of
the cell 10. Generally the catholyte will be pumped in a manner to
enter at the upper portion of the cathode chamber 16 and be
withdrawn at the lower portion of the chamber; however, pumping can
be carried out to remove catholyte at the upper portion of the
cathode chamber.
During operation of the electrolytic cell 10 the catholyte contains
increasing concentrations of an alkali metal hydroxide, such as
sodium hydroxide, which for efficient operation should be removed
from the cathode compartment 16 to reduce the hydroxide
concentration. For this purpose an alkali metal hydroxide removal
means such as pipe 28 is in combination with the cathode
compartment 16. The hydroxide concentration of the catholyte can be
regulatably controlled by, for example, adding water to a portion
of the catholyte and recirculating it into the cathode compartment
16 through a recirculating means 30. When the flow through the
recirculating means is insufficient to minimize stagnant catholyte
portions within the cathode compartment 16, the pump 26 can be used
to supplement the circulatory effect of the recirculating means
30.
The cathode 18 is spaced apart from a side portion or wall 31 of
the cell 10 to form an opening or gas compartment 32 between the
cathode 18 and the inner surface of the wall 31. An oxidizing gas,
for example air and oxygen, with the moisture content suitably
controlled by a moisture control means 34 is pumped into,
preferably, the upper portion of the gas compartment 32 and passed
in intimate contact with an outer surface 33 of the cathode 18 and
withdrawn through removal means 40 for disposal. The cathode 18 is
formed of a material adapted to transmit or pass an oxidizing gas
from the gas compartment 32 to an inner portion or surface 36 of
the cathode 18. Preferably, formation of oxidizing gas bubbles on
the inner surface 36 of the cathode 18 is minimized and more
preferably the inner surface of the cathode is substantially free
of oxidizing gas bubbles. An oxidizing gas moisture control means
34 is provided to regulatably control the dew point of the
oxidizing gas introduced into the gas compartment 32 to minimize
and preferably substantially entirely eliminate accumulation of
liquid water within the gas compartment 32. The moisture control
means 34 is further adapted to maintain the oxidizing gas moisture
content at a concentration adequate to minimize and preferably
entirely prevent removal of sufficient moisture from the catholyte
within the cathode compartment 16 to result in deposition of solid
materials such as sodium chloride or sodium hydroxide in, for
example, the pores of the cathode 18. Preferably the moisture
control means 34 is adapted to regulate the moisture content of the
oxidizing gas within the range of from about 50 to 100 percent of
saturation.
The cathode 18, which is used in combination with the oxidizing gas
control means 34, is preferably a foraminous body, such as a
screen, expanded metal or a sheet with holes extending
therethrough, having at least the surface thereof composed of a
material substantially inert to the catholyte. For example, the
cathode can be a substrate with at least a surface of Ru, Rh, Pd,
Ag, Os, Ir, Pt, Au and Ni. The inert material is then coated with a
mixture of at least one of the aforementioned inert metals in
particulate form and for example, polytetrafluoroethylene,
polyhexafluoropropylene and other polyhalogenated ethylene or
propylene derivatives. Preferably the particulate inert metal is
what is known in the art as platinum black, silver black, carbon
black, nickel black or nickel oxide black. Particulate designated
as "black" generally and preferably has a U.S. Standard Mesh size
range of less than about 300. Preferably the cathode 18 is a screen
at least partially woven from or adherently coated with metallic
platinum, silver, gold or nickel with a mesh size of about 30 to
about 60.
A source of electrical energy 36 is electrically connected to an
energy transmission or carrying means such as aluminum or copper
conduit as bus bar or cables 38 to transmit direct electrical
current to the anode 14 and the cathode 18.
In operation of the electrolytic cell 10 an alkali metal chloride
containing brine, such as sodium chloride, is supplied or fed
through the brine feed means 22 into the anode chamber 12 wherein,
through electrolytic processes known to those skilled in the art,
gaseous chlorine is formed and removed through pipe 24 and thence
to a chlorine condensing and storage system (not shown). Preferably
substantially only sodium ions pass through the cation exchange
diaphragm 20 into the cathode compartment 16 wherein sodium
hydroxide is formed. An oxidizing gas, preferably oxygen, is fed
into the gas compartment 32 within the cathode 18 substantially
simultaneously with formation of the sodium hydroxide. The presence
of the oxidizing gas and the physical contact thereof with the
outer surface 33 of the cathode 18, while the inner surface 36 of
the cathode 18 is simultaneously in contact with the sodium
hydroxide containing catholyte, is believed to minimize and
preferably prevent formation of gaseous hydrogen in the cathode
compartment 16 to thereby reduce the electrical consumption and
improve the electrical efficiency of the cell. Excess oxidizing gas
is removed from the gas compartment 32 through the oxidizing gas
removal means or conduit 40.
Operation of the cell is even further improved by regulatably
controlling the catholyte head (i.e., the vertical difference, if
any, between the upper surfaces of the anolyte and the catholyte)
at a higher level than the anolyte surface. Preferably the upper
surface of the catholyte is about 1 inch to about 3 feet higher
than that of the anolyte.
To minimize what is believed to be formation of hydrogen at the
cathode 18 it is desirable that substantially all of the catholyte
comes into contact with the cathode. To promote such contact and
reduce the occurrence of stagnant portions of catholyte within the
cathode compartment 16 where little movement of the catholyte
occurs, the catholyte is preferably circulated at a rate sufficient
for substantially all of the catholyte to contact the cathode 18
and insufficient to result in physical injury to the cation
exchange diaphragm 20.
FIG. 2 is illustrative of an electrolytic cell 10a having therein
an anode compartment or chamber 12a spaced apart from a cathode
compartment or chamber 16a by a cation exchange diaphragm 20a. An
anode 14a is suitably attached in the anode chamber 12a. Likewise,
a cathode 18a is suitably attached in the cathode compartment 16a.
The anode is constructed of a material such as carbon or what is
known in the art as dimensionally stable anode such as titanium or
tantalum coated or plated with materials including, for example, at
least one metal or oxide of the platinum group metals including Ru,
Rh, Pd, Ag, Os, Ir, Pt and Au.
The cathode 18a is preferably a silver plated foraminous copper
substrate such as a copper screen or sheet with a thickness of
about 0.01 to about 0.02 inch and sufficient pores or holes with a
diameter of about 0.015 to about 0.03 inch extending therethrough
to provide a total hole or open area equivalent to about 20 to
about 40 percent of that portion of the copper sheet having the
greatest surface area. The foraminous copper sheet is preferably
coated or plated with sufficient metallic silver to provide a
substantially continuous silver layer with a thickness of up to
about 0.002 inch. Plating of the copper substrate is carried out in
a manner known to those skilled in the plating art. A screen woven
from about 0.005 to about 0.02 inch diameter wire into a screen
having a U.S. Standard Mesh size of about 20 to about 50 is
satisfactory when plated with silver as described above. More
preferably the cathode is nickel or a nickel base alloy resistant
to the corrosive effects of the catholyte. The metal substrate is
coated with a mixture of platinum black, silver, black, nickel
black or carbon black and, for example, polytetrafluoroethylene or
a fluorinated copolymer of hexafluoropropylene or
tetrafluoroethylene. The mixture preferably contains from about 30
to about 70 weight percent carbon black with a mesh size of less
than about 300 admixed with up to about 10 weight percent carbon
fibers. The balance of the mixture is essentially the organic
material and impurities generally found in the carbon and the
organic material. The organic mixture coated, silver plated copper
is preferably substantially impervious to passage of the catholyte.
The term copper includes commercially pure copper and copper base
alloys.
A diaphragm support such as member 39 is adapted to retain the
resinous diaphragm in an upstanding position and still permit
effective flow of catholyte through the cathode chamber 12a.
The alkali metal hydroxide, such as sodium hydroxide, concentration
of the catholyte is controlled at a predetermined desired level by
appropriate means (not shown) attached to pipes 28a and 30a. The
alkali metal chloride, such as sodium chloride, concentration is
controlled at a predetermined desired level by appropriate means
(not shown) attached to pipes 22a and 22b. Such anolyte and
catholyte control means can include, for example, recirculatory
systems to add water, sodium chloride, or remove sodium
hydroxide.
An oxidizing gas is pumped through a moisture control means 34a
into a gas compartment 32a at least partially defined by wall
portions of the cathode 18a.
Operation of the electrolytic cell 10a, is substantially the same
as that described for the embodiment of FIG. 1 except that the
catholyte is circulated within the cathode chamber by pumping
through the recirculating and analysis control means (not shown)
attached to the pipes 28a and 30a at a rate effective to minimize
stagnant portions of catholyte.
The following examples further illustrate the invention.
EXAMPLE 1
An electrolytic cell similar to that shown in FIG. 1 with a
ruthenium oxide coated titanium anode spaced apart from an oxygen
gas depolarized cathode by a du Pont Nafion 12V6Cl cation exchange
membrane was operated to produce chlorine gas at the anode and
sodium hydroxide in the cathode compartment. Each electrode had a
surface area of 3 square inches. The cathode was formed by admixing
7 grams of carbon black with 0.2 grams of carbon fiber, 3.3
milliliters of du Pont Teflon 30B latex and about 20 to 30
milliliters of water to form a dough-like mixture. The mixture was
rolled to about 0.05 inches thick and then pressed together with a
40 mesh woven silver screen using a force of about 15 tons. The
pressed composite was heated in a nitrogen atmosphere for about 2
to 3 minutes at a temperature of about 350.degree. to 360.degree.C.
After cooling in a nitrogen atmosphere the composite was heated to
about 100.degree. to 120.degree.C and sprayed on a single surface
with sufficient Teflon 30B latex (diluted one part latex to eight
parts water) to form a coating of about 2 to 10 milligrams Teflon
per square centimeter of surface. The sprayed composite was then
heated for about 2 minutes at about 350.degree. to 360.degree.C in
a nitrogen atmosphere. The sprayed Teflon surface was positioned in
the cell to form a wall portion of a depolarizing gas
compartment.
An aqueous sodium chloride brine was circulated through the anode
compartment, with sodium chloride additions for composition
control, and a sodium hydroxide containing catholyte was
circulated, with water additions for composition control. Oxygen
gas was pumped through the gas compartment at a rate of 66
milliliters per minute after first saturating the oxygen with
water. During operation the anolyte had an acidity (pH) of 5.5 and
contained about 260 to 290 grams per liter sodium chloride. The
catholyte contained 79.6 grams per liter sodium hydroxide and 4.1
grams per liter sodium chloride. The electrolyte temperature was
about 70.degree.C. The catholyte head was 1 1/2 inches higher than
the anolyte. Operating voltage was 1.901 and the amperage was 1.5.
Cell operation was satisfactory without production of hydrogen gas
in the cathode compartment.
EXAMPLE 2
A cell substantially as described in Example 1 was operated as
described in Example 1 with 66 milliliters per minute of water
saturated oxygen depolarizing gas. The aqueous anolyte had a pH of
6.25 and about 260 to 290 grams per liter sodium chloride. The
aqueous catholyte contained 102.4 grams per liter sodium hydroxide.
Catholyte head was one-quarter inch higher than the anolyte. Cell
voltage was 1.888 and the amperage was 1.5.
EXAMPLES 3-30
An electrolytic cell substantially as described in Example 1 was
operated with a woven nickel screen cathode. The cathode was
prepared substantially as described in Example 1. The anolyte was
maintained at a concentration of about 260 to 290 grams per liter
NaCl and the catholyte maintained at about 80 to 100 grams per
liter NaOH. A water saturated oxygen depolarizing gas was pumped
through the gas compartment adjacent to the cathode at a rate of 43
milliliters per minute. Cell operation was satisfactory without
hydrogen production in the cathode compartment. Operating currents
and voltages are shown in Table I.
TABLE I ______________________________________ Example
Temp.(.degree.C) Voltage(volts) Current(Amp.)
______________________________________ 3 25 1.600 0.5 4 70 1.820
1.2 5 25 1.647 0.5 6 25 1.815 0.5 7 70 2.032 1.0 8 70 2.230 1.5 9
70 1.502 0.2 10 70 1.655 0.4 11 70 1.799 0.6 12 70 1.945 0.8 13 70
2.085 1.0 14 70 2.218 1.2 15 70 2.349 1.4 16 70 2.485 1.6 17 70
2.610 1.8 18 70 2.737 2.0 19 70 2.863 2.2 20 70 2.981 2.4 21 70
3.090 2.6 22 70 3.196 2.8 23 70 3.210 3.0 24 70 3.290 3.2 25 70
3.384 3.4 26 70 3.477 3.6 27 70 3.572 3.8 28 70 3.660 4.0 29 70
3.740 4.2 30 70 3.825 4.4
______________________________________
* * * * *