Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1998-11-13
2001-04-03
Gorgos, Kathryn (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S244000, C204S286100, C204S288000
Reexamination Certificate
active
06210549
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the configuration of various components in electrochemical cells for the generation of fluorine by electrolysis of a fused potassium fluoride—hydrogen fluoride electrolyte. In a further aspect, this invention relates to a process for the operation of an electrochemical fluorine cell.
2. Description of the Related Art
In the electrolytic production of fluorine gas, the reaction vessel in which the reaction occurs is commonly referred to as a “cell”. The major components of a cell usually comprise the following six elements. First, an electrolyte resistant container (case) normally jacketed with a temperature control system. Second, an electrolyte, operated in the fluid state (melt), typically comprising about 39 to 42% hydrogen fluoride, although concentrations outside this range are acceptable.
Third, in some cells, a cathode is made an integral part of the cell case, while fourth, an anode is typically made of ungraphitized carbon. The carbon can have either low-permeability or high-permeability, and may be composed of a monolithic structure or a composite structure. Nickel anodes are also occasionally used. Fifth, a gas separation assembly, which captures a cathode produced gas, H
2
, in a chamber and an anode produced gas, F
2
, in a chamber above the melt and separated by a metal wall or skirt. The skirt extends from the top of the cell (cell top) below the normal operating surface of the melt. Some cells also have a sixth (6) component which is a separation diaphragm that extends from the bottom of the skirt below the melt surface to below the end of the anode. The diaphragm is made of a porous media. The diaphragm provides a means for the separation of gas bubbles of hydrogen (formed at the cathode) and the generated fluorine (formed at the anode), to prevent spontaneous and often violent reforming of hydrogen fluoride, as disclosed in U.S. Pat. No. 4,602,985 issued to Hough. The configuration of each of these components and the characteristics of the materials used therefor determines the efficiency and life of each.
In the majority of the commercially operated fluorine cells the anodes are ungraphitized carbon blades, having planar or flat surfaces, are approximately 8 inches wide by 2 inches thick and hang down vertically from 10 to 29 inches in length. These blades are normally bolted to a copper buss bar inside the cell, or are suspended individually through the cell head and fastened to a hanger assembly, as in U.S. Pat. No. 5,688,384 Hodgson. Both these methods and others connect the power supply posts (rods), by penetrating the carbon blade either by bolting the blade to the buss through a drilled horizontal hole in the carbon, or by another method of drilling and tapping a vertical hole into the carbon and the power supply rod is screwed into the hole. These carbon to metal connections are frequently the source of high cell maintenance and short cell life cycles. The large flat face of these anodes are mounted parallel to the flat surface faces of the cathode plates.
The production capacity of a fluorine gas generator cell is commonly understood to be a factor of the quality of the carbon in the anode, and of its ability to withstand a passage of a given electrical current density (as measured in amperes per square inch of interactive surface area) between the parallel interactive surfaces of the anodes and the cathodes. Operating at higher current densities can cause the anodes to degrade and burn away as CF
4
gas. Therefore the total interactive surface area for each anode in a cell times the number of anodes in that cell, will determine the maximum amperage that can be applied to the cell safely. Thus, the fluorine production capacity of the cell is determined by the surface area of the anode.
In the majority of the commercially operated fluorine cells the cathode plates are mounted in a fixed position parallel to the two large anodes faces of each anode blade. The cathode is suspended from posts that penetrate the cell head through isolating packing couplings. This configuration is frequently a factor in poor cell performance. The configuration of the cell head chamber separating skirts, their position between the anodes and cathodes and the depth below the varying electrolyte melt level individually and collectively effect production capacity, product quality, and cell life cycle time.
The configuration and location of the anodes, cathode and skirts with respect to each other all effect the circulation of the electrolyte melt in the cell. The most commonly used fluorine cells today do not have a designed melt circulation path providing beneficial melt temperature control, gas bubble separation into proper chambers, and proper mixing of the hydrogen fluoride feed into the melt. All these factors result in poorer than optimal performance.
The anode hanger support is a carbon to metal connection, one of the primary keys to a long fluorine cell cycle life. In order to maximize the cycle life, there are three major problems which must be overcome.
1. The anode-support connection is subject to contamination by melt creeping into the joint.
2. The fluorine cells commonly in use today have a high current density at the carbon to metal interface. This connection is normally placed under the surface of the electrolyte melt to help dissipate the heat, but this results in the melt creeping into the joint thereby degrading the electrical connection, creating hot spots, and shortening the cycle life.
3. In the majority of the commercially operated fluorine cells present day, an individual cell has banks of anodes that operate in parallel to each other on each bank, but in series to anodes on the other bank. The failure of one anode can have a dynamic shift in current density to the other anodes on that bank of anodes, leading to early failure of the anodes forced to carry the extra load. The fluorine cell components are normally located inside the cell case. The cell case is normally a rectangular box shaped container with a top flange so everything nests inside of the case and supported at the case flange.
The case normally rests on support legs or wheels and has electrical isolation pads between each support to prevent current flow to ground. The case is normally used in maintaining a controlled temperature of the melt inside of the case. The cell case walls are normally jacketed with heat exchanger panels, so heating or cooling fluid media may pass through the heat exchange panels, regulating the melt temperature. In some cells, the heat exchange media is passed through tubes inside the cell case to assist in controlling the melt temperature. Heating temperature control occasionally is applied to the bottom of the case with electrical heating elements. In some cases, the cell case itself is used as the cathode for the cell.
An electrical isolation barrier (such as a sheet of plastic material like PTFE) is placed over the bottom of the cell so as to prevent cathodic interaction with the cell floor and the anode blade(s). Such a component prevents electrolytic interaction from the bottom of the cell up to the anode blades. Such an interaction risks producing both hydrogen and fluorine gases proximate one another. Such cathodic interaction would result in gases which could not be separated, potentially resulting in uncontrolled recombination of the gases, both a potentially hazardous condition and at best a waste of energy.
In prior art the cathodes are supplied power by way of posts that pass through the head plate or through the cell case. Prior art only utilizes two parallel anode surfaces for interactive current flow, not fully utilizing the all available anode surface area. However, the prior art does not supply power through a flange plate that is electrically isolated from the head plate and the case as in the instant invention.
Some cell cases are equipped with special sight glass port windows to allow visual observation of the melt levels and any other activity in the hydrog
Gorgos Kathryn
Litman Richard C.
Maisano J.
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