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S. P. VAVALIDES', R. E. CABLE, W.
K. HENDERSON, and
C. A. POWELL
Union Carbide Nuclear Co., Paducah, Ky.
High-Capacity, Long-Life Fluorine Cell Construction and operating details are presented for a new, 32-anode fluorine cell which operates at 4000 to 6000 amperes, the maior causes for failure of this type of cell, and efforts to improve and develop it
FLcORIxE REQUIREMENTS have been increasing yearly, especially for use in the atomic energy program; consequently, considerable effort has been expended in developing more efficient individual fluorine-manufacturing units.
the head (see page 183). The gas separation skirt (see page 183), made of Monel, is welded to the head and extends downward 10 inches from the lower side of the head plate. A red rubber gasket maintains the seal between the tank and the head.
The anode assembly consists of eight pairs of 2 X 8 X 205,'s inch carbon blades bolted to an AISI-4140 steel bar and of AISI-4140 steel cap screws and copper plates for maintaining electrical contact at the carbon-metal joint. The cap screws are torqued to 85 foot-
Cell Description The high-capacity, long-life fluorine cell, a larger version of a Union Carbide Nuclear Co. cell described previously ( I ) , is of the medium-temperature type, operating at approximately 200" F. The fused salt bath is potassium bifluoride (melting point, 161 " F.); fluorine is liberated at the carbon anode, and hydrogen is evolved at the steel cathode. The major improvements in the larger cell are increased cooling capacity, 50% greater anode area, and construction materials with increased corrosion resistance to potassium bifluoride and hydrogen fluoride (3). The main components are the cell tank, the cell head, the anode assembly, the cathode assembly, the screen diaphragm, and the packing gland. The cell tank is constructed of welded 3/s-inch Monel plate, contains 12 internal cooling pipes centrally located, and is surrounded by an externally baffled steel jacket for cooling and heating. The cell head is fabricated from a 1inch steel plate and has a separate compartment for fluorine and hydrogen. The outlet-gas manifolds, the hydrogen fluoride feed and purge lines, and the electrical connections are on the top of address, Union Nuclear Co., Oak Ridge, Tenn. 1
Present
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Carbide
INDUSTRIAL AND ENGINEERING CHEMISTRY
Materials Used in
89 X 32 X 39 Inch Cell Material
Component Shell Head Anodes Cathode Diaphragm Anode support bar Anode cap screws Anode pressure plates
Monel liner, steel jacket Steel plate, Monel skirt Carbon, grade YBD Steel Monel Chrome-molybdenum steel (AISI-4 140) Chrome-molybdenum steel (AISI-4140) Copper
E-EC'RODE-
Sectional assembly detail of packing gland Gasket, Anchor No. 415 standard quality red rubber, 80 durometers hardness
pounds, and each cell contains two anode assemblies. Each anode bar contains three ll/s-inch copper conductor posts, which extend through packing glands in the cell head, and serve both as power-supply lead and as support for the anode after the cell is assem bled. The cathode assembly (see page 183) consists of three vertical, '/e-inch, parallel steel plates. T h e plates surround the anode assembly and are supported by three steel posts, which also serve as conductors, The screen diaphragm consists of a Monel frame supporting 6-mesh Monel screens. The screens are placed between the anode and the cathode assemblies to prevent broken carbons from causing a short circuit between the anode and cathode and to prevent hydrogen and fluorine from mixing in the cell. The packing gland consists of a steel tube 3 inches in outside diameter, 3 3 / ~ inches long, welded to the cell head. This packing supports the anode and cathode parts in the cell and electrically insulates the cell head from the anode and cathode. Power supply to the cell ranges from 4000 to 6000 amperes and is supplied from a 6000-ampere ignitron rectifier source.
Cell Operation
Startup. Prior to operation in the plant, all cells are specially conditioned to prevent polarization after installation and to check for proper construction and assembly. Only half the cell is conditioned at one time, to avoid half-cell polarization after installation. The cell is started at a low current
fration of 40% is believed to result in decreased corrosion of the metal parts. Rigid specifications on hydrogen fluoride purity are maintained, and a hydrogen fluoride 99.95% pure is fed to the cell during operation. The electrolyte level is set at approximately 5 inches below the cell head and is used to maintain a seal between the fluorine and the hydrogen compartments in each cell. Temperature Control. The cell cooling system must be able to remove approximately 90,000 B.t.u. per hour to maintain the temperature at 200' F. a t 4000-ampere operation. Cooling water at 170' F. and a flow of 12 to 15 gallons per minute is used. Table I compares the operating characteristics of the 32-blade cell a t 4000- and 6000-ampere operating levels with those of the smaller cell ( 2 ) . The principal physical differences in the new cell are the larger anode area, which gives a decrease in current density a t 4000-ampere operation, and its larger over-all dimensions. A threefold increase in cell life is evident.
density of 24 amperes per sq. foot or a total current of 500 amperes. If it operates satisfactorily, the current density is increased to a maximum of 167 amperes per sq. foot, or a total current of 3500 amperes. After operation a t high current density for 10 to 15 minutes, the cell will polarize. After operation a t high voltage for a few minutes, polarization will cease and the cell voltage will decrqase to the normal level of 8 to 9 volts. All cells are carefully observed, and a maximum cell temperature of 230' F. is maintained during conditioning. If the cell has not depolarized a t this temperature, it is shut down. I n most cases the cell will start u p and operate a t 8 to 9 volts, regardless of initial polarization, without further difficulty. After conditioning is comblete, the cell operates a t 4000 or 6000 amperes continuously, until the cell voltage drop increases to 12 volts, a t which point the cell is considered to have failed. Operation in excess of 12 volts is not recommended, because of increased electrolyte entrainment in the gas and accelerated corrosion of metal components in the cell. Polarization during actual operation is infrequent. On-stream efficiency for many cells has exceeded 97%; most of the off-stream time has been necessitated by routine maintenance work on control instruments and by plugging of the cell cooling lines because of corrosion in the coolingwater system. Hydrogen Fluoride Feed. Hydrogen fluoride is vaporized and introduced into the cell through a feed line, immersed in the electrolyte. Hydrogen fluoride feed rate is controlled by routine sampling of the electrolyte to obtain a hydrogen fluoride concentration of 40 to 42%. A hydrogen fluoride concen-
Cell Failures
When the first large cells were placed in operation, the average cell life was approximately 2,400,000 ampere-hours. After data on failed cells were collected, it was evident that the failure was due primarily to loosening of the carbon anode hanger and subsequent loss of contact a t the carbon-metal joint. In the initial group of 32-blade cells AISI-4140 chrome-molybdenum steel pressure plates and cap screws were used, and the pressure plates were half submerged in the electrolyte. The chromemolybdenum steel pressure plates of the failed cells had corroded rapidly in the
b Failed anode assembly, Note lack of corrosion products in lower half of assembly
Anode hunger showing points of carbon-metal contact VOL. 50, NO. 2
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anodes to break, which results in carbon overheating and burning. Cell Development
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(IElectrolyte! KF-2HF,41 w t :% HF ) CORROSION TESTS OF CONSTRUCTION MATERIALS FOR FLUORINE CELLS IN ELECTROLYTE
Conclusion
Volts
Voltage-amperage curve obtained by using various metals as anodes in electrolyte solution
electrolyte solution. As copper pressure plates had exhibited a greater corrosion resistance, a test cell was built in which the pressure plates in one anode hanger were of copper and those in the other were of chrome-molybdenum steel. The cell failed after 5.2 X lo6 ampere-hours of operation. After the cell was dismantled, the blades connected with copper pressure plates still had good contact; those connected with steel plates were almost a total failure. All cells now use copper pressure plates to maintain the contact between the carbon and the anode hanger; improvement in the anode assembly has
Table 1.
also increased the cell life of the small cell substantially. The primary cause of cell failures is not carbon-anode breakage but increased contact resistance a t the anode joint. Corrosion of the metal parts in the anode assembly-that is, copper pressure plates and steel bolts-decrease contact pressure of the carbon at the carbon-metal joint. This loss of contact exposes the bolts and plates to the electrolyte solution, accelerating bolt corrosion and overheating the carbon at the joint. .4fter the metal parts have failed because of corrosion, the loss in contact causes the carbon
Cell Operating Characteristics Small Cell at 4000 Amp.
Operating voltage Cell operating temperature, O F. Hydrogen fluoride it1 electrolyte, % ’ Effective anode area, sq. ft. Anode current density, amp./sq. ft. Anodes, number Life, amp.-hr.
180
8-12 200-220 40-42 32 125 24 5 x 106
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Efforts were made to improve the anode-joint assembly by finding materials of construction with greater corrosion resistance to potassium bifluoride, determining the effect of temperature on the anode joint, and eliminating metal parts. Several possible construction materials for anode assemblies were compared for corrosion resistance, by measuring the current carried at a given voltage by a cell containing an anode of the material under test. Materials that permitted higher current flows were considered less resistant to corrosion. Test cells have been built in which phosphor bronze and aluminum bronze were used for cap screws and pressure plates. The use of copper anode hangers is also being investigated. T o investigate the effect of temperature on the anode-joint assembly, test cells were built in which an anode assembly was used to obtain a temperature-compensated carbon-metal joint. Work is also being done, on steel cap screwswith larger heads, increased torque (200 foot-pounds maximum) on the cap screws, and bronze alloy metals at the joint. A carbon-carbon joint is being tested in an effort to eliminate metal components from the anode assembly.
New Cell At 4000 amp. At 6000 amp. 8-12 190-220 40-42 42 94 32
9-12 190-220 40-42 42 140 32
16 X 106
15.5 X IO6
T h e larger cell has increased capacity because of its ability to operate at 6000 amperes. The cell life is 16 X lo6 ampere-hours, a threefold increase over that of the smaller cell. Cell on-stream efficiency is in excess of 97%. Cells are considered to have failed when the total cell voltage drop increased to 12. Cell failures have been due to corrosion of metal components in the anode-joint assembly, which caused a loss in electrical contact between the carbon-metal joint and subsequent cell operation at higher voltage. Development work is being directed toward obtaining materials for the anode assembly w7hich have better corrosion resistance in the eIectrolyte. Literature Cited (1) Dykstra, J., others, IND.EXG. CHEM. 47, 883-7 (1955). (2) Ibid., p. 884. (3) Simmons, R. E., others, “Materials for Fluorine Generator Anode Assembly,” Rept. KY-191(Sept. 28,1956).
RECEIVED for review June 4, 1957 ACCEPTED December 16, 1957 Division of Industrial and Engineering Chemistry, Symposium on Nuclear Technology in the Petroleum and Chemical Industries, 131st Meeting, ,4CS, Miami, Fla., -4pril 1957.
