US2755244A - Temperature control of electrolytic cells - Google Patents

Description

July 17, 1956 c. c. HARVEY TEMPERATURE CONTROL OF ELECTROLYTIC CELLS Filed Oct. 30, 1952 INVENTOR. CLARENCE CI HARVEY ATTORNEY United States Patent TEMPERATURE CONTROL OF ELECTROLYTIC CELLS Clarence C. Harvey, Baton Rouge, La., assignor to Ethyl Corporation, New York, N. Y., a corporation of Delaware Application October 30, 1952, Serial No. 317,699

2 Claims. (Cl. 204-247) This invention relates to the temperature control of electrolytic cells, particularly to cells in which fused salts are clectrolyzed.

Electrolysis, when practiced on a commercial scale, is accompanied by the generation of considerable heat, and provision is usually made for its dissipation. However, these provisions are not too adequate, particularly for large cells where the surface to volume ratio is small and for cells that operate with fused electrolytes at high temperatures. Under these conditions, any slight increases in temperature tend to cause a loss of electrical efiiciency, thereby generating still more heat. Furthermore, excessive volatility tends to cause the volatilizible material, which is generally highly reactive, to escape into the surroundings.

In the electrolytic production of metallic sodium from sodium chloride, for example, electrolysis is conducted at temperatures in the neighborhood of 600- C. by reason of the high melting point of the bath. At these temper.- atures a change of only about C. will cause an appreciable increase in the vaporization of the sodium that is produced and also tends to the extensive formation of a metallic fog within and without the cell. The metallic fog, in turn, reacts with the chlorine being produced to reform sodium chloride that has to be reelectrolyzed. Furthermore, the heat generated by the electrolysis is usually concentrated at one of the electrodes and excessive heating generally causes that electrode to be subjected to intense hot spots that shorten its life. In fact, abnormal temperatures are usually associated with the anode effect, an expression that refers to the formation of a film .of gas about the anode which film appears to insulate part of the anode surface, effectively decreasing the anode surface available for electrolysis. This produces local overheating which in turn tends to maintain the insulating film and perpetuates the increase in temperatures.

Although the overheating can be controlled by merely reducing the electrolyzing current, this reduces the capacity of the cell and is accordingly undesirable.

It is, therefore, the prime object of the instant invention to control the temperature and avoid the above disadvantages. It is a further object of the present invention to provide an eflicient temperature control for the electrolytic production of sodium. Other objects will become apparent from the description and the claims that follow.

Broadly speaking, the invention contemplates the use .of a heat transfer element which may project from within to without the electrolytic cell to introduce a very good heat absorber into the cell and using it to effectively radiate and dissipate the heat it absorbs. This heat transfer unit can be so arranged that it is of very little efiect during the normal operation of the cell but great- 1y increases its effectiveness upon increase of the internal cell temperature to the point where the cell efiiciency might be impaired or damage incurred.

A preferred form of the invention contemplates the use Patented July .17, 19.5.6

of a closed elongated vessel that is inserted through the Cover of ,a fused-electrolyte cell unit and into the molten electrolyte, the vessel leaving a large fraction of its surface projecting through the cell cover into the air there.- above. Within this vessel is .a quantity of a volatilizable material such as metallic sodium under a predetermined pressure selected to equal the vapor pressure of the sodium at or just below the desired maximum cell operating temperature. Thus, when the cell batch temperature approaches this maximum, the liquid sodium, which would normally occupy the lower end of the vessel, would be vaporized and begin to boil. The vapors thus formed rise in the vessel and are rapidly cooled in its external portion, condensing to run back to th lower end, thus establishing rapid thermal circulation and cooling.

If desired, the heat dissipation from the external portion of the vessel can be increased by providing it with cooling fins or otherwise increasing the surface that is exposed to the ambient air whereby the radiation area may be increased to the extent desired. Further, it may be desirable in some instances to provide a separate reflux line to return the condensed liquid sodium from the heat radiation portion to the reboiler portion of the transfer vessel. This reflux line may take the form of a separate conduit or, if desired, may simply constitute a partitioned portion of the principal conduit.

An alternative form of heat control in accordance with the present invention uses the very high heat transfer characteristics .of molten metals. Thus by inserting from the exterior a tubular conduit, open at the lower end, into the molten metal produced in the cell and adjusting the pressure within this conduit so as to lift the metal to a predetermined height within the conduit, a very efiicient heat transfer link of molten metal can be established with the ,ambientair.

Having defined the invention in general terms it will now be described with respect to specific illustrated examples in which:

Fig. 1 is a simplified sectional view of one form of the invention;

Fig. 2 is a fragmentary sectional view of a second form; and

Fig. 3 is a fragmentary partly sectional view of a further modified form.

Reference will now be made to Fig. 1 in which is dis.- closed an electrolytic cell of the knapsack or Downs type wherein a central anode 13 is concentrically surrounded by an annular cathode 14 within an enclosed chamber 10. The chamber 10 is of any conventional design and includes a removable cover portion 11 through which electrolyte may be introduced as .desired, an opening (not shown) being conveniently provided for this purpose. The anode 13 is seated upon a metal suppor-t12 and secured thereto by any conventional means (not shown). In practice, suitable cooling means, such as water lines and electrical bus bar connections (neither of which are illustrated) are provided in the support 12. The cathode 14 is supported by a plurality of bus bars 14a which extends through and are supported on the walls of the chamber 10.

The anode 13 is provided with a plurality of vertical slots 13a which permit and facilitate circulation of molten electrolyte into and from the center of the anode. Thus, the electrolyte can circulate in contact with the lower vaporizer or reboiler portion of the heat transfer unit to accomplish cooling of the molten electrolyte body in the cell 10.

A collector unit 15 is suspended vertically above the anode and cathode and consists of a centrally depending funnel 16 which communicates with an upper cylindrical chamber 18, and has a concentric collector ring 17 integrally formed thereabout. The tunnel 16 is positioned directly over the anode 13 and functions to channel chlorine gases, evolved during the electrolysis, into the collecting chamber 18 and thence through an outlet conduit 19 for further processing. The collector rlng 17 is formed by two radially spaced downwardly depending sidewalls positioned on either side of the cathode annulus l4, and functions to collect the molten sodium, evolved during the electrolysis, as an annular pool above the cathode 14. A wire gauze screen 20 depends from the collector unit between the anode and cathode and segregates the products of electrolysis adjacent their respect ve electrodes. A sodium riser 21, formed integerally with the collector ring 17, is used to syphon the molten metal from within ring 17 out of chamber for further processing.

In accordance with one form of the invention a closed elongated tubular vessel 22 is inserted into the central passageway or core usually provided in the anode 13. The major portion of the vessel projects through chamber 18 freely into the ambient air above the cell. A plurallty of heat radiating structures such as fins 23 can be provided on the vessel 22 externally of the cell. With this construction the vessel 22 can be so adjusted that its lower end will project within the core of anode 13 into close proximity with the base member 12 upon the placing of the collector unit over the anode and cathode members. Thus positioned, the lower portion of the conduit will extend along a considerable portion of the anodes length and thereby rapidly respond to any change in temperature at any localized point or adjacent the anode surface.

A quantity of metallic sodium 35 is placed within the vessel 22 for a purpose hereinafter explained; the quantity being preferably only sufficient to extend to the level of the electrolyte in the cell. The upper end of the closed vessel 22 can be completely sealed off as shown. Before it is sealed, however, the pressure within vessel 22 is adjusted to equal or be slightly lower than the vapor pressure of sodium at the desired cell control temperature.

In operation, the cell chamber 10 is filled in the regular manner to approximately the dotted line 30 with a mixture of sodium chloride and calcium chloride, which is then fused. A potential of between 5 and 8 volts connected to supply a direct current of between 30,000 to 40,000 amperes is then applied across the anode and cathode bus bar connections, generating chlorine gas at the anode and metallic sodium at the cathode by electrolysis. This electrolysis proceeds best at a temperature range between 595 and 605 C., the pressure within vessel 22 being adjusted to about 35 millimeters of mercury so as to approximately equal the vapor pressure of sodium at 595 C. As long as the cell temperature remains in the lower portion of the temperature range, the control mecha nism will effect very little cooling.

However, during electrolysis, unpredictable sudden temperature increases take place. Upon such an occurrence, the sodium pool 35 within vessel 22 vaporizes very rapidly and a column of sodium vapor rises within the vessel to the region of the cooling fins 23 where it condenses and returns by gravity to the lower or reboiler portion of the vessel located within the anode 13. It will be readily appreciated that this vertical vapor column provides a direct and efiicient heat transfer link between the molten pool 35 and the cooling fins 23, so that excess cell heat will be continuously and expediently dissipated. Due to the fact that sodium absorbs a. relatively large amount of heat during vaporization, the vessel 22 is enabled to provide progressively increasing heat transferring efiiciency as the temperature within the cell increases. The temperature control of the cell is particularly effective and convenient since the heat dissipated increases considerably in proportion to the temperature increase above the predetermined operating range. It therefore becomes unnecessary to interrupt the electrolysis process.

In some installations, it may be de i bl to li the width of the lower portion of tube 22 to make sure it i well insulated from the anode. In such instance the tube can have an enlarged chamber of any configuration or size immediately above the anode. This will provide adequate protection against the possibility of sodium vapor bubbles forming beneath the surface of the molten metal in the restricted area of the reboiler portion, pushing up the liquid sodium above it and effectively keeping this sodium out of reach of the reboiler with an attendant loss of heat conducting and absorbing material. The provision of the enlarged chamber effectively precludes such result by allowing the liquid sodium to drain away from the bubble as it rises in the reboiler" portion of the vessel.

Fig. 2 illustrates an alternative form of the invention wherein the sodium evolved during the electrolysis is utilized as the heat transferring medium. In this form of the invention the heat transfer vessel 122 is constructed as an elongated tubular element which is open at its lower end. The tube is inserted through the cover 111 of the cell and its open lower end communicates with the sodium well below the sodium collector ring 117. The upper end of the tube 122 is closed and is connected via a conduit 124 and three-way valve 125 with a pressure adjusting mechanism 126, which functions to control the height of a column of sodium 150 which is sucked into the vessel 122 from the cathode pool.

With this mode of operation the height of the column of sodium 150 and thereby the amount of heat radiated from the vessel may be controlled by simply controlling the pressure within the upper closed end of vessel 122. Increasing the height of the column 150 increases the surface of sodium available to radiate and provides an effective means for controlling the amount of heat dissipated from cell 110 since the liquid sodium is an excellent heat conductor and is in direct contact with the electrolyte. If desired, suitable means such as the fins 23 of Fig. 1 may be placed on the upper portion of vessel 1.22 to increase the heat transferring efficiency.

In order to render the device more effective and automatic, the valve 125 and pressure adjusting mechanism are connected to a servo mechanism via suitable linkages 127 and 128, respectively, and the servo mechanism is controlled in turn by a temperature responsive element positioned to extend into the electrolyte or the sodium adjacent the cathode, as shown, or adjacent the anode. A suitable connection such as electrical wiring 129 between the temperature responsive element 140 and the servo mechanism 130 is provided so that upon an increase in temperature within the cell 110 the servo mechanism 130 is energized to lower the pressure above the sodium column as by starting a suction pump in the pressure adjusting mechanism 126 and suitably setting valve 125 to connect the suction pump to the upper end of tube 122.

Upon a decrease in temperature within the cell, the element 140 initiates a second control operation via connection 129 and causes the servo mechanism 130 to open the valve 125 to atmospheric pressure, and if desired deenergize the suction pump. Further, the temperature responsive element 140 may also be designed so as to provide a third signal initiating condition when the temperature of the cell 110 is in the desired operating range. Under such conditions the third response merely causes the pressure to stop changing.

If desired, the control element 140 can be connected to the lower portion of tube 122 to respond to the temperature of the sodium. Since the liquid sodium is a very efficient heat conductor, it will rapidly reflect thermal changes in the cell.

Fig. 3 shows a further modified form of the invention similar in construction and operation to that of Fig. 2. In this form of the invention the heat transferring unit also consists of an elongated tubular element 222 which is open at its lower end and includes a closed chamber 226 at its upper end, and, as shown in the figure, is positioned directly in the sodium uptake riser 221. A conduit 224 and valve mechanism 225, similar in construction to the conduit and valve units previously described with respect to Fig. 2, is positioned to connect with the upper surface of the chamber 226 for the same pressure controlling purpose.

The operation and function of this form of the invention is similar to that of Fig. 2 with the exception that the sodium level when kept in chamber 226 provides much more surface for vaporization. The cooling effect thereby provided is more efiicient than the mere heat transfer by radiation or conduction since additional heat is absorbed by vaporization. The vapors are in turn cooled by the walls of chamber 226 to condense and return to the body of the sodium in tube 222.

The molten sodium in the heat transfer units of the above examples should be protected from contact with moisture as Well as oxygen in the air. The outlets 125 and 225 can therefore be connected to a supply of inert gas, such as nitrogen kept at atmospheric pressure for example.

Many variations are possible in constructing specific structures according to the invention. Heat transfer materials other than metallic sodium may be used, and examples of such materials are liquid metals such as zinc, potassium, tin, Woods metal, solder, etc., but they are not as efficient as molten sodium. However, alloys of metals, such as sodium-potassium alloys are highly suitable for this purpose. Further, any other type of heat radiating or dissipating structure, such as cooling coils, etc., may be used in lieu of the fins 23 or dome One of the distinguishing features of the instant invcntion is that it provides a heat transfer apparatus wherein the heat transferred increases out of all proportion to increases in temperature. Another feature is that it provides an elfective temperature control structure which is extremely simple and inexpensive in construction. A still further feature of the invention is the fact that the operating temperature which is desired to be retained in the electrolytic bath may be effectively varied at will by the simple expedient of varying the pressure within the heat transfer vessel without interrupting the operation of the cell. Such variation is readily attainable with the construction of Fig. 1 by merely providing the vessel 21 with a pressure controller such as those shown in Figs. 2 and 3.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the above invention is not limited except as defined in the appended claims.

What is claimed is:

1. An electrolytic cell for electrolyzing a fused electrolyte comprising a chamber provided with an anode and cathode, a means to collect molten metal produced in said cell, and a heat transfer unit comprising a closed elongated tubular vessel, a quantity of vaporizable liquid metal within a liquid-containing vaporizer portion of said vessel and partially filling the same, the interior of said vessel being evacuated to a sub-atmospheric pres sure at which said liquid metal boils when heated to about the maximum temperature at which the molten metal is to be kept, said vessel having a condenser portion positioned above said liquid containing portion and externally of the cell, said liquid containing portion being positioned within said cell and being adapted to contact ,said fused electrolyte.

2. The cell of claim 1 in which said liquid metal is sodium.

References Cited in the file of this patent UNITED STATES PATENTS 510,276 Lyte Dec. 5, 1893 2,130,801 Hulse Sept. 20, 1938 2,393,330 McNitt -s Jan. 22, 1946

Claims (1)

1. AN ELECTROLYTIC CELL FOR ELECTROLYZING A FUSED ELECTROYLTE COMPRISING A CHAMBER PROVIDED WITH AN ANODE AND CATHODE, A MEANS TO COLLECT MOLTEN METAL PRODUCED IN SAID CELL, AND A HEAT TRANSFER UNIT COMPRISING A CLOSED ELONGATED TUBULAR VESSEL, A QUANTITY OF VAPORIZABLE LIQUID METAL WITHIN A LIQUID-CONTAINING VAPORIZER PORTION OF SAID VESSEL AND PARTIALLY FILLING THE SAME, THE INTERIOR OF SAID VESSEL BEING EVACUATED TO A SUB-ATMOSPHERIC PRESSURE AT WHICH SAID LIQUID METAL BOILS WHEN HEATED TO ABOUT THE MAXIMUM TEMPERATURE AT WHICH THE MOLTEN METAL IS TO BE KEPT, SAID VESSEL HAVING A CONDENSER PORTION POSITIONED ABOVE SAID LIQUID CONTAINING PORTION AND EXTERNALLY OF THE CELL, SAID LIQUID CONTAINING PORTION BEING POSITIONED WITHIN SAID CELL AND BEING ADAPTED TO CONTACT SAID FUSED ELECTROLYTE.

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