Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-10-12
2003-01-07
Valentine, Donald R. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S258000, C204S269000, C204S270000, C204S283000, C204S284000
Reexamination Certificate
active
06503377
ABSTRACT:
The invention relates to an electrolysis apparatus for producing halogen gases from aqueous alkali halide solution, having a number of plate-like electrolysis cells which are arranged beside one another in a stack and are in electrical contact and which each have a housing comprising two half-shells of electrically conductive material with external contact strips on at least one housing rear wall, the housing having devices for feeding the electrolysis current and the electrolysis starting materials and devices for discharging the electrolysis current and the electrolysis products, and in each case having two essentially flat electrodes (anode and cathode), the anode and cathode being provided with apertures like venetian blinds for the electrolysis starting materials and the electrolysis products to flow through, being separated from one another by a dividing wall and arranged parallel to one another and being electrically conductively connected to the respective associated rear wall of the housing by means of metal reinforcements.
The individual electrolysis cells are produced in such a way that the respective housings are assembled from two half-shells in each case with the interposition of the necessary devices and the cathode and anode as well as the dividing wall, and by fixing the same by means of metal reinforcements, and that the anode and housing and cathode and housing are, respectively, electrically conductively fixed to each other, the plate-like electrolysis cells produced in this way then being arranged electrically conductively beside one another in a stack and braced against one another in the stack for the purpose of providing a lasting contact.
The electrolysis current is fed to the cell stack at one outer cell of the stack, it passes through the cell stack in the essentially vertical direction to the central planes of the plate-like electrolysis cells, and it is discharged at the other outer cell of the stack. As referred to the central plane, the electrolysis current reaches average density values of at least 4 kA/m
2
.
Such an electrolysis apparatus is disclosed by DE 196 41 125 A1 from the same applicant. In the case of this known electrolysis apparatus, the anode and the cathode are connected to the respective rear wall of the halves of the housing via vertical, web-like metal reinforcements. On the rear side of the anode and cathode half-shells, a vertical contact strip for the electrical contact to the adjacent, identically constructed electrolysis cell is fitted in each case. The current flows via the contact strip through the rear wall into the vertical, web-like metal reinforcements and, from there, starting from the metal contact points, (reinforcement/anode), it is distributed over the anode. After the current has passed through the dividing wall (the membrane), it is picked up by the cathode, in order to flow via the vertical, web-like reinforcements into the rear wall on the cathode side and again into the contact strip and, from there, to enter the next electrolysis cell. In this case, the connection between the current-carrying components is performed by welding. At the weld points, the electrolysis current forms peak current densities.
The vertical, web-like metal reinforcements are designed as webs which are aligned with the contact strips and whose lateral edges, over the entire height of the rear wall and the anode or cathode, rest on the rear wall and the anode or cathode.
The vertical webs subdivide the electrode rear space within the respective half of the housing into individual electrolyte-carrying segments. In order that a completely non uniform distribution of the concentration in the electrolyte along the depth of the respective half of the housing does not occur, in each half of the housing, at the bottom, an inlet distributor is provided, via which the electrolysis starting materials can be fed into the individual segments formed by the webs in the half-shells.
By means of an electrolyzer configured in this way, gas-producing electrolysis processes, such as chloralkali electrolysis, hydrochloric acid electrolysis or alkaline water electrolysis, are carried out. In chloralkali electrolysis, aqueous alkali halide solutions, for example sodium and potassium chloride, are decomposed in the electrolysis cell, under the influence of the electrical current, into an aqueous alkali hydroxide solution, for example sodium or potassium hydroxide solution, and into a halogen gas, for example chlorine and hydrogen. In the electrolysis of water, water is decomposed and hydrogen and oxygen are formed at the electrodes.
The physical separation of the electrode spaces is carried out by means of the dividing wall mentioned at the beginning, in general a diaphragm or a so-called ion exchange membrane. The diaphragm consists of a porous material which is chemically, thermally and mechanically stable with respect to the media, temperatures and pressures occurring in the cell. The ion exchange membrane is generally a perfluorated hydrocarbon. These membranes are gas-tight and virtually liquid-tight, but permit the transport of ions in the electrical field.
A particular characteristic of this electrolysis process is the fact that the diaphragm or the ion exchange membrane is pressed against at least one of the two electrodes. This is necessary since, as a result, the dividing wall is fixed and is therefore largely unloaded mechanically. It is often the case that the dividing wall may rest only on one of the two electrodes, since only in this way can the longest possible service life of all the components (electrodes and dividing wall) be achieved. In the event of direct contact between the dividing wall and both electrodes, in some cases a chemical reaction may take place between the dividing wall and the electrodes or the gases developed at the electrodes. For example, a distance between the membrane and the cathode is established in chloralkali electrolysis, since otherwise the electrolysis catalyst or, in the case of inactivated nickel cathodes, nickel is dissolved out of the electrode. Another example is provided by nickel oxide diaphragms, which are used in alkaline water electrolysis. If there is too small a distance from the hydrogen-developing electrode, the nickel oxide is reduced to nickel and therefore becomes conductive, which eventually leads to a short circuit.
Contact between the membrane or diaphragm and at least one electrode, in the case of gas-developing processes, leads to a build-up of gas in the electrolyte boundary layer between the electrode and the membrane or the diaphragm. This even affects the electrodes mentioned at the beginning, which are configured in such a way that the electrolysis starting materials and the electrolysis products can flow through them. Such electrodes are preferably provided with apertures (perforated sheet metal, expanded metal, woven mesh or thin metal sheets with apertures like venetian blinds), so that in spite of their flat arrangement in the electrolysis cell, the gases formed in the boundary layer during the electrolysis can more easily enter the rear space of the electrolysis cell.
The gas bubbles rising in the electrolyte agglomerate in particular in the edges or borders of the apertures, which edges or borders are oriented downward in the cell, and remain there in the interstices between the contacting dividing wall (membrane) and the edges of the apertures. These bubbles disrupt the transport flow, that is to say the transport of materials through the dividing wall, since they block the membrane exchange surface and therefore make it impassable, that is to say inactive.
In the case of an electrode configuration which has been provided by the applicant to reduce this build-up of gas and which is described in German patent specification DE 44 15 146 C2, the electrodes are profiled by being provided with grooves and holes, for example. In this way, firstly the gas can escape more easily and secondly fresh electrolyte can get into the electrolytically active boundary layer between the electrode
Borucinski Thomas
Dulle Karl-Heinz
Gegner Jürgen
Wollny Martin
Katten Muchin Zavis & Rosenman
Krupp UHDE GmbH
Valentine Donald R.
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