CA1091618A - Method of operating a chlor-alkali cell - Google Patents
Method of operating a chlor-alkali cellInfo
- Publication number
- CA1091618A CA1091618A CA294,004A CA294004A CA1091618A CA 1091618 A CA1091618 A CA 1091618A CA 294004 A CA294004 A CA 294004A CA 1091618 A CA1091618 A CA 1091618A
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- CA
- Canada
- Prior art keywords
- power supply
- power source
- cathode
- main
- direct
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
Abstract
Abstract of the Disclosure Chlor-alkali cell operation is improved by provid-ing, in addition to the main power supply and in parallel thereto, a standby power circuit including an auxiliary power source capable of maintaining a cathodic potential on the cathodes of the cell and a diode means for protecting the auxiliary power supply.
Description
1. Field of the Invention:- This invention relates to the ~ .
operation of electrolytic cells for the electrolysis of alkali-metal halides to produce hydrogen, chlorine, and alkali-metal hydroxide, and in particular, it relates to avoiding the problems which arise when the main power supply to such a cell is interrupted for any reason.
operation of electrolytic cells for the electrolysis of alkali-metal halides to produce hydrogen, chlorine, and alkali-metal hydroxide, and in particular, it relates to avoiding the problems which arise when the main power supply to such a cell is interrupted for any reason.
2. Description of the Prior Art:- The use of electrolytic cells for the production of chlorine and caustic from brine has been known for about seventy years and is widely practiced in the United States, to the extent of producing several thousand tons of chlorine per day.
Such diaphragm cells usually contain an anode of graphite or of a titanium-base alloy coated with platinum or ruthenium oxide, alone or with other oxides, a cathode made of steel, and a diaphragm made of asbestos or a suitable syn-thetic porous material which separates the anode chamber from the cathode chamber.
As is appreciated in U. S. Patent No. 3,4~5,730, particular problems will arise if the supply oE electrical power to such a cell is interrupted for any reason. In particular, when this happens, the steel cathode is attaclced and ions of iron enter the solution in the cathode chamber.
Any ions of iron which make their way to the vicinity of the diaphragm Eorm insoluble iron hydroxides which tend to p1ug the diaphragm and made the cell inoperable. An interruption oE the main power supply for a period of time as short as two or three minutes has a considerable deleterious effect on the operation of the cell. A longer interruption, such as one half hour or more, will certainly render the cell inoperable.
The above-mentioned U. S. patent is concerned with the problem of providing diaphragm cells which may be operated intermittently, i.e., being run at night or at other times when electrical power is not in great demand and thus can be purchased at lower "off-peak" rates. According to the patent, the problem of protecting the cathode is solved by measures which include washing the cell with an alkaline brine during the time that it is shut down. Those skilled in the art will appreciate that it would be advantageous to be able to avoid any such operation.
The present invention is also of particular use-fulness in respect to the operation of a chlor-alkali cell which has an "activated-surface" cathode. It is known in the art that it is possible to use, in place of the usual steel-mesh cathode, one having upon its surface a continuous or discontinuous coating of a particular nature which will provide numerous sites of enhanced activity. In the preparation of such a cathode, there may be used materials such as nickel or platinum or any of a variety of alloys. These coatings are frequently porous. If the cathode is allowed to be in contact with the electrolyte solution in the cell under conditions when no current is applied, a galvanic-cell effect develops between the activated site and the underlying steel or other base material forming the bulk o~ the cathode. This causes the base metal to corrode and causes a loosenin~ or separation of the activated coating on the cathode material.
The present invention is especially valuable because it prevents such separation or loosenin~.
To our knowledge, the problem of protecting the cathodes in diaphragm cells in the event of a scheduled or unscheduled interruption of the main supply of electrical power is one that has been met only with such expedients as rapidly draining or flushing the cell, or providing an aux-iliary power supply which is itself capable of operating the cell at about its usual rate of production.
l~g~
Summary of the Invention In accordance with the invention, there is provided an auxiliary power supply which is connected in parallel to the main power supply and provides a potential which is suf-ficient to prevent the liquid in the cathode chamber from attacking the steel cathode, and in series with such auxiliary power supply, there is provided a diode means, i.e., a device permitting flow of current only in one direction, such as a diode or a suitably wired silicon-controlled rectifier (SCR), so that the main power supply does not, during ordinary operation, i tend to force current into the auxiliary power supply. The auxiliary power supply is desirably not one that is capable o~ supplying the considerable amperage that is required to permit continued operation of the cell on a production-as-usual basis. Adding such means to a commercial electrolytic diaphragm cell is relatively inexpensive. It affords protec-tion against the consequences of an inadvertent and unscheduled interruption of the power supply. It also makes the equipment capable o being operated in an on-of~ mode to take advantage oE low electric-power rates for off-peak power. The invention is of especial use when a cathode having an activated surf~ce coatinq is used.
Description of the Drawinq A complete understanding of the invention may be o~tain~d ~rom the Eoregoing and ~ollowing description thereof, taken together with the attendant drawing, the sole FIGURE
o~ which is a schematic illustration of a diaphragm cell provided with equipment in accordance with the present invention.
Description of the Preferred Embodiments In the drawing, there is indicated an electrolytic cell 2 having an anode chamber ~ and a cathode chamber 6 which are separated by a separator member 8. The member 8 may be a diaphragm of asbestos or of polychlorotrifluoroethylene or other suitable synthetic material. The present invention is also of use, moreover, in the so-called "membrane cells", in which the separator member 8 comprises a membrane of some suitably permselective material, such as the "hydrolyzed copolymer of perfluorinated hydrocarbon containing 2 to 5 carbon atoms and a fluorosulfonated perfluorovinyl ether"
mentioned in U. S. Patent No. 3,852,135, or the material of U. S. Patent No. 3,282,875. The anode is indicated at 10, and the cathode at 12. Brine is supplied to the anode chamber 4 through a line 14, and weak cell llquor, a dilute solution o~ sodium hydroxide and sodium chloride in the case o~ a diaphragm cell, and a sodium hydroxide solution in the case of a membrane cell, is withdrawn from the cathode chamber 6 through a line 16. Chlorine is withdrawn from the top of the anode chamber 4 through a line 18, and hydrogen i9 withdrawn from the top of the cathode chamber 6 through a line 20. There is provided a main power supply 22, which is connected through a line 24 to the anode 10 and through a line 26 to the cathode 12. As can be seen, in a diaphragm cell the liquid in the anode chamber 4 is maintained at a relatively high level 28, in comparison to the level 30 o~ the liquid in the cathode chamber 6, thereby providing the driving force to cause the liquid in the anode chamber 4 to migrate through the diaphragm 8 and into the cathode chamber 6. The portion of the equipment described above in entirely conventional. In the case of a membrane cell, the liquid-level differential is not absolutely required.
In accordance with the invention, we also provide an auxiliary direct-current power supply 50, which is connected by lines 52 and 54 in parallel to the main power supply 22.
The line 54 contains, in series with the auxiliary power supply 50, a suitable device 56 for permitting flow of electrical current only in one direction, such as a diode or a suitably wired SCR, i.e., a diode means.
In ordinary operation of the cell 2 with the use of the main power supply 22, current is passed through the cell 2 at a substantial rate, such as one affording a cur-rent density of 16.14 amperes per square decimeter (150 amperes per square foot), a typical value of the voltage for the main power supply being 3.6 volts. The auxiliary power supply 50 generates a potential difference of some suitable lesser value, such as 3.0 volts. Thus, the point 58 is more negative than the point 60, and the device 56 is reverse-biased, so that there is no conducting of current in the lines 52 and 54.
If the main power supply 22 is shut down, the voltage across lines 24 and 26 decreases, and when the potential at point 58 becomes less negative than the potential at point 60, the device 56 becomes Eorward-biased, and a flow of current is established from the auxiliary power supply 50 through the lines 54 and 26 and through the cell 2, with a return to the power supply 50 throu~h the lines 24 and 52. Even though this flow oE current is at a far lower amperage than that which passes through the cell in its normal mode oE operation in production, the establishment of such a small current density, i.e., about 0.22 amperes per square decimeter t2 amperes per square foot) is sufficient to protect the cathode 12 against corrosive attack by the Iiquid in the cathode chamber 6.
This provides a convenient and inexpensive way of protecting the cathode 12 in the event of an unscheduled inter-ruption of power from the main power supply 22. Moreover, it makes the equipment capable of being operated purposely on an intermittent basis. Whenever it is desirable to interrupt the operation of the cell, it is possible to close the valve 15 in the line 1~ as soon as the main power supply 22 has been interrupted or shortly before, and then, after the level 28 has decreased to approximately the level 30, to close the valve 17 in the line 16. Later, when power ~rom the main power supply 22 is about to be restored, the valve 15 is opened to cause the liquid in the anode chamber 4 to build up its former level 28, and then the valve 17 in the line 16 is opened and power from the main power supply 22 may then be used, as before, to operate the cell in its production mode.
Those skilled in the art will appreciate that this method of cathode protection may equally well be applied to a plurality of cells connected in series, provided that the auxiliary power supply and the diode means are suitably chosen.
While we have shown and described herein certain embodiments of our invention, we intend to cover as well any change or modification therein which may be made without de-parting from its spirit and scope.
Such diaphragm cells usually contain an anode of graphite or of a titanium-base alloy coated with platinum or ruthenium oxide, alone or with other oxides, a cathode made of steel, and a diaphragm made of asbestos or a suitable syn-thetic porous material which separates the anode chamber from the cathode chamber.
As is appreciated in U. S. Patent No. 3,4~5,730, particular problems will arise if the supply oE electrical power to such a cell is interrupted for any reason. In particular, when this happens, the steel cathode is attaclced and ions of iron enter the solution in the cathode chamber.
Any ions of iron which make their way to the vicinity of the diaphragm Eorm insoluble iron hydroxides which tend to p1ug the diaphragm and made the cell inoperable. An interruption oE the main power supply for a period of time as short as two or three minutes has a considerable deleterious effect on the operation of the cell. A longer interruption, such as one half hour or more, will certainly render the cell inoperable.
The above-mentioned U. S. patent is concerned with the problem of providing diaphragm cells which may be operated intermittently, i.e., being run at night or at other times when electrical power is not in great demand and thus can be purchased at lower "off-peak" rates. According to the patent, the problem of protecting the cathode is solved by measures which include washing the cell with an alkaline brine during the time that it is shut down. Those skilled in the art will appreciate that it would be advantageous to be able to avoid any such operation.
The present invention is also of particular use-fulness in respect to the operation of a chlor-alkali cell which has an "activated-surface" cathode. It is known in the art that it is possible to use, in place of the usual steel-mesh cathode, one having upon its surface a continuous or discontinuous coating of a particular nature which will provide numerous sites of enhanced activity. In the preparation of such a cathode, there may be used materials such as nickel or platinum or any of a variety of alloys. These coatings are frequently porous. If the cathode is allowed to be in contact with the electrolyte solution in the cell under conditions when no current is applied, a galvanic-cell effect develops between the activated site and the underlying steel or other base material forming the bulk o~ the cathode. This causes the base metal to corrode and causes a loosenin~ or separation of the activated coating on the cathode material.
The present invention is especially valuable because it prevents such separation or loosenin~.
To our knowledge, the problem of protecting the cathodes in diaphragm cells in the event of a scheduled or unscheduled interruption of the main supply of electrical power is one that has been met only with such expedients as rapidly draining or flushing the cell, or providing an aux-iliary power supply which is itself capable of operating the cell at about its usual rate of production.
l~g~
Summary of the Invention In accordance with the invention, there is provided an auxiliary power supply which is connected in parallel to the main power supply and provides a potential which is suf-ficient to prevent the liquid in the cathode chamber from attacking the steel cathode, and in series with such auxiliary power supply, there is provided a diode means, i.e., a device permitting flow of current only in one direction, such as a diode or a suitably wired silicon-controlled rectifier (SCR), so that the main power supply does not, during ordinary operation, i tend to force current into the auxiliary power supply. The auxiliary power supply is desirably not one that is capable o~ supplying the considerable amperage that is required to permit continued operation of the cell on a production-as-usual basis. Adding such means to a commercial electrolytic diaphragm cell is relatively inexpensive. It affords protec-tion against the consequences of an inadvertent and unscheduled interruption of the power supply. It also makes the equipment capable o being operated in an on-of~ mode to take advantage oE low electric-power rates for off-peak power. The invention is of especial use when a cathode having an activated surf~ce coatinq is used.
Description of the Drawinq A complete understanding of the invention may be o~tain~d ~rom the Eoregoing and ~ollowing description thereof, taken together with the attendant drawing, the sole FIGURE
o~ which is a schematic illustration of a diaphragm cell provided with equipment in accordance with the present invention.
Description of the Preferred Embodiments In the drawing, there is indicated an electrolytic cell 2 having an anode chamber ~ and a cathode chamber 6 which are separated by a separator member 8. The member 8 may be a diaphragm of asbestos or of polychlorotrifluoroethylene or other suitable synthetic material. The present invention is also of use, moreover, in the so-called "membrane cells", in which the separator member 8 comprises a membrane of some suitably permselective material, such as the "hydrolyzed copolymer of perfluorinated hydrocarbon containing 2 to 5 carbon atoms and a fluorosulfonated perfluorovinyl ether"
mentioned in U. S. Patent No. 3,852,135, or the material of U. S. Patent No. 3,282,875. The anode is indicated at 10, and the cathode at 12. Brine is supplied to the anode chamber 4 through a line 14, and weak cell llquor, a dilute solution o~ sodium hydroxide and sodium chloride in the case o~ a diaphragm cell, and a sodium hydroxide solution in the case of a membrane cell, is withdrawn from the cathode chamber 6 through a line 16. Chlorine is withdrawn from the top of the anode chamber 4 through a line 18, and hydrogen i9 withdrawn from the top of the cathode chamber 6 through a line 20. There is provided a main power supply 22, which is connected through a line 24 to the anode 10 and through a line 26 to the cathode 12. As can be seen, in a diaphragm cell the liquid in the anode chamber 4 is maintained at a relatively high level 28, in comparison to the level 30 o~ the liquid in the cathode chamber 6, thereby providing the driving force to cause the liquid in the anode chamber 4 to migrate through the diaphragm 8 and into the cathode chamber 6. The portion of the equipment described above in entirely conventional. In the case of a membrane cell, the liquid-level differential is not absolutely required.
In accordance with the invention, we also provide an auxiliary direct-current power supply 50, which is connected by lines 52 and 54 in parallel to the main power supply 22.
The line 54 contains, in series with the auxiliary power supply 50, a suitable device 56 for permitting flow of electrical current only in one direction, such as a diode or a suitably wired SCR, i.e., a diode means.
In ordinary operation of the cell 2 with the use of the main power supply 22, current is passed through the cell 2 at a substantial rate, such as one affording a cur-rent density of 16.14 amperes per square decimeter (150 amperes per square foot), a typical value of the voltage for the main power supply being 3.6 volts. The auxiliary power supply 50 generates a potential difference of some suitable lesser value, such as 3.0 volts. Thus, the point 58 is more negative than the point 60, and the device 56 is reverse-biased, so that there is no conducting of current in the lines 52 and 54.
If the main power supply 22 is shut down, the voltage across lines 24 and 26 decreases, and when the potential at point 58 becomes less negative than the potential at point 60, the device 56 becomes Eorward-biased, and a flow of current is established from the auxiliary power supply 50 through the lines 54 and 26 and through the cell 2, with a return to the power supply 50 throu~h the lines 24 and 52. Even though this flow oE current is at a far lower amperage than that which passes through the cell in its normal mode oE operation in production, the establishment of such a small current density, i.e., about 0.22 amperes per square decimeter t2 amperes per square foot) is sufficient to protect the cathode 12 against corrosive attack by the Iiquid in the cathode chamber 6.
This provides a convenient and inexpensive way of protecting the cathode 12 in the event of an unscheduled inter-ruption of power from the main power supply 22. Moreover, it makes the equipment capable of being operated purposely on an intermittent basis. Whenever it is desirable to interrupt the operation of the cell, it is possible to close the valve 15 in the line 1~ as soon as the main power supply 22 has been interrupted or shortly before, and then, after the level 28 has decreased to approximately the level 30, to close the valve 17 in the line 16. Later, when power ~rom the main power supply 22 is about to be restored, the valve 15 is opened to cause the liquid in the anode chamber 4 to build up its former level 28, and then the valve 17 in the line 16 is opened and power from the main power supply 22 may then be used, as before, to operate the cell in its production mode.
Those skilled in the art will appreciate that this method of cathode protection may equally well be applied to a plurality of cells connected in series, provided that the auxiliary power supply and the diode means are suitably chosen.
While we have shown and described herein certain embodiments of our invention, we intend to cover as well any change or modification therein which may be made without de-parting from its spirit and scope.
Claims (6)
1. In combination with an electrolytic-cell apparatus for the electrolysis of alkali-chloride brine comprising a chamber having first and second parts, an anode in said first part of said chamber, a cathode in said second part of said chamber, a diaphragm between said first and second parts of said chamber, means for leading brine to said first part, means for withdrawing liquid from said second part, means for withdrawing chlorine gas from said first part and means for withdrawing hydrogen gas from said second part, and a main direct-current power source connected to said anode and to said cathode to cause production of hydrogen, chlorine, and caustic liquid, the improvement which consists in a cathode-protection means which comprises:
- an auxiliary direct-current power source developing a potential difference less than that of said main power source but sufficient ot protect said cathode against corrosive attack by liquids in said second chamber when power from said main direct-current power source is interrupted, - a diode means connected in series with said aux-iliary direct-current power source, and - means connecting said auxiliary direct-current power source and said diode means in parallel with said main direct-current power source, said diode means being connected with said auxiliary power source in such sense that when said main power source is operated, said diode means is reverse-biased and substantially no current flows in said means connecting said auxiliary direct-current power source and said diode means in parallel with said main direct-current power source.
- an auxiliary direct-current power source developing a potential difference less than that of said main power source but sufficient ot protect said cathode against corrosive attack by liquids in said second chamber when power from said main direct-current power source is interrupted, - a diode means connected in series with said aux-iliary direct-current power source, and - means connecting said auxiliary direct-current power source and said diode means in parallel with said main direct-current power source, said diode means being connected with said auxiliary power source in such sense that when said main power source is operated, said diode means is reverse-biased and substantially no current flows in said means connecting said auxiliary direct-current power source and said diode means in parallel with said main direct-current power source.
2. A combination as defined in claim 1, wherein said cathode is one of ferrous metal.
3. The method of protecting a cathode of brine-electrolysis cell against corrosive attack during times of interruption of the main power supply to such an electrolytic cell, said method comprising:
connecting in parallel to said main power supply an auxiliary power supply generating a potential difference less than that of said main power supply but sufficient to guard said electrode against corrosive attack, said auxiliary power supply having in series with it a diode means serving to prevent forcing of electric current into said auxiliary power supply by said main power supply when said main power supply is operative.
connecting in parallel to said main power supply an auxiliary power supply generating a potential difference less than that of said main power supply but sufficient to guard said electrode against corrosive attack, said auxiliary power supply having in series with it a diode means serving to prevent forcing of electric current into said auxiliary power supply by said main power supply when said main power supply is operative.
4. A method as defined in claim 3, characterized in that use of said main power supply and the flow of liquid through said cell are cylically and purposely interrupted and re-established.
5. A method as defined in claim 4, characterized in that use of said main power supply is interrupted at on-peak times and re-established at off-peak times.
6. A method as defined in claim 3, characterized in that said cathode is one having an activated surface.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US75413576A | 1976-12-27 | 1976-12-27 | |
| US754,135 | 1976-12-27 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1091618A true CA1091618A (en) | 1980-12-16 |
Family
ID=25033596
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA294,004A Expired CA1091618A (en) | 1976-12-27 | 1977-12-28 | Method of operating a chlor-alkali cell |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1091618A (en) |
-
1977
- 1977-12-28 CA CA294,004A patent/CA1091618A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |