EP0097991A1 - Membrane-electrolysis cell with vertically arranged electrodes - Google Patents

Abstract

In an electrolytic cell having a membrane and vertical electrodes composed of a plurality of units, a. the electrode having one polarity is horizontally divided into a plurality of units, b. the electrode having the opposite polarity is vertically divided into a plurality of units, and c. the units of at least one of the two electrodes are adapted to be displaced by spring elements. Spacers are suitably provided between the units of that electrode which is not contacted by the membrane.

Description

The invention relates to a membrane electrolysis cell with vertically arranged electrodes for electrochemical processes.

When carrying out electrochemical processes, it is important to distribute the current evenly over the electrode surface. The uniform distribution is also affected by the _S as throwing power of the electrolyte by the homogeneity of the electrodes. The. Scatterability is better, the larger the area on the counterelectrode that is affected by the streamlines. A lack of scattering capacity can be compensated for by increasing the distance between the electrodes, but this increases the voltage drop in the cell. Inhomogeneities in the electrode surface lead to current distortion. The distance between the electrode plates, ie the distance between the anode and cathode, is therefore of great importance.

Ideally, they stand. Areas of both electrodes in parallel. Plane parallelism of the surfaces is the prerequisite for an efficiently working cell, since this is the only way to ensure an even current distribution and to avoid local overheating. To the voltage drop as possible. to keep small, and thus to reduce power consumption, the distance between anode and cathode moreover possible _l ichst be kept low to. All these claims are re- ativ easy to realize _l in small laboratory cells, prepares the construction of large industrial units but difficulties the ideal ideas to be theoretically demanded should be realized. In addition, cells are more sensitive to deviations from plane parallelism and to current warps, the larger they are. In order to avoid accelerated destruction of the ion exchange membrane of this type, there is generally a compulsion to limit the height of the electrodes and to set a considerable one Distance between the electrodes of the cell and to limit the electrical current density, which is also disadvantageous for the energy yield of the electrolytic cell and its productivity.

To reduce these disadvantages of electrolysis cells with membranes and vertically arranged electrodes, electrodes with openings for the removal of the reaction gases are generally used, for example perforated electrodes, wire mesh or expanded metal. The disadvantages include the reduced active surface, the lack of mechanical stability and the loss of high-quality coating material on the back of the electrodes.

Usually, membrane cells with ion exchange membranes are provided with a frame construction that is as rigid as possible, in which the electrodes are rigid, in the majority of cases by welded connections. To ensure that on one hand adhered to the necessary close tolerances in the plane-parallel arrangement of the electrodes; on the other _r erseits but a plurality of such frames for a electrolytically s _e can be free of leaks related to the filter press principle and-the contact surfaces of the frame according to have can be processed with great effort.

According to a known proposal of DE-AS 20 59 868 it has also-been at electrodes to be arranged vertically in the gas-forming _n - _D iaphragmazellen an existing individual plates electrode plate provided, with the individual plates _F üh- have surfaces for the discharge of the generated gas. Or area yield on the basis of the intended inclination of the guide plate zwangsläüfig different distances from the active surface to the counter electrode, in particular by local temperature elevations in the sensitive _T racing walls poor thermal conductivity slightly distortions are caused. Furthermore, the entire active surface of the electrode cannot be brought into the energetically desirable close distance from the counter electrode.

The object of the invention is therefore to avoid the mentioned and other disadvantages and to provide an electrode arrangement for a membrane electrolysis cell which, under technical operating conditions, ensures a secure plane parallelism of the electrode surfaces and an energetically favorable minimum electrode spacing and ensures safe and rapid gas removal.

• a) die Elektrode der einen Polarität in mehrere Einheiten horizontal geteilt ist,
• b) die Elektrode der entgegengesetzten Polarität in mehrere Einheiten vertikal geteilt ist, und
• .c) die jeweiligen Einheiten mindestens einer der beiden Elektroden durch Federelemente verschiebbar sind.

The invention solves this problem with a membrane electrolysis cell with vertically arranged electrodes composed of several units. In a cell of the type mentioned, the invention is that

• a) the electrode of one polarity is horizontally divided into several units,
• b) the electrode of the opposite polarity is divided vertically into several units, and
• .c) the respective units of at least one of the two electrodes are displaceable by spring elements.

With the arrangement according to the invention, the two geometric reference systems in the cell, namely frame / frame and anode / cathode, are designed independently of one another. For example, the one electrode, such as the cathode, becomes horizontal in individual divided plate sections rigidly connected to the cathode frame, while the electrode of the opposite polarity, such as anode divided vertically into several plate or strip units, is designed to be flexible or displaceable. This flexible design is brought about by spring elements. The spring elements are expediently attached to the current leads to the electrodes and, via contact pressure or welding, bring about the electrical contact with the individual strip units of the electrode (anode).

According to the invention, in the above-mentioned arrangement, the cathode can also be set up flexibly when the anode is rigidly fixed. However, both electrodes, which are divided into individual units, can also be made displaceable by spring elements. In this way, the unevenness of the contact surfaces of the cell frame which is inevitably present and can only be removed with a great deal of work is not transferred to the positioning of the electrode. Rather, the tolerances occurring in the area of the cell frame are bridged by means of the movable connection of the current distributor to the active surface of the electrode.

The spring force of the spring elements is dimensioned so that it allows the relative spatial position of the anode and cathode to be adjusted. Here, the frames can advantageously be made from commercially available, drawn material without substantial post-processing, and the required tight tolerances can be achieved using spacers.

According to a further embodiment of the invention, the movable or displaceable arrangement of the electrode active surfaces is used to discharge developed and accumulated gas, such as chlorine gas, and is designed accordingly. In this case, the spring elements designed as flexible power supply lines form a concave curvature directed towards the cell bottom or an angle which is open there. For example, that Spring element be a leaf spring welded to the power supply. The chlorine gas collected under the individual flexible spring elements or current feeders is discharged upwards at one point by gas discharge elements arranged laterally in the electrolysis room. In this way, partial degassing of the electrode space or anode space takes place. This partial degassing in turn causes convection flows in the electrolyte and an improved electrolyte exchange in the active area of the electrodes, which leads to considerable improvements in the energy yield.

According to the invention, spacers are attached to the horizontal or vertical separation points between the individual units of the electrode on which the membrane is not in contact. Due to the different densities of catholyte and anolyte, the membrane rests on an electrode at the same hydrostatic heights, i. that is, a lateral force acts on the electrode.

This side force is now countered by the spring force of the flexible power supply. Spring strengths and hydrostatic height difference between the anolyte and catholyte circuit are therefore coordinated so that, for. B. Several spacers mounted horizontally on the cathode without great effort, i.e. with the least possible pinching of the membrane, adjust the relative position of the two active surfaces to each other. The spacers are preferably 1 to 5 mm thick.

In a further embodiment of the invention, in the case of gas-developing processes, the spacer is designed as a guide element for discharging the developed gas from the electrode space. The spacer functions as a gas separation unit when arranged horizontally. It then consists of stripes, for example fen-shaped plates with serrated edges or strips with slit or circular openings or from lattice or net-shaped strips. Such spacers bring about a complete gas withdrawal from the electrode gap after each division of the multiple horizontally divided electrode (cathode).

The invention is illustrated in more detail and by way of example in FIGS. 1 to 4 of the drawing.

1 shows a front view of an electrode frame F with a horizontally divided cathode plate 2. FIG. 1b is a similar view of an electrode frame with anode 3 divided vertically and horizontally.

Fig. La is a section along the line I - I in Fig. 1 and shows the horizontally designed cathode plate 2 with spacer 1st

Fig. 2 is an enlarged view of section "A" in Fig. La. In Fig. 2, the spacer 2 illustrates a gas discharge member. The horizontally divided electrode 2 (cathode) and the vertically divided counter electrode 3 (anode) are also shown. The arrows 5 and 6 indicate the electrolyte entry or exit of the gas-electrolyte mixture from the cell.

Fig. 3 shows a plan view of a displaceable electrode combination of horizontally divided cathode 2 and vertically divided anode 3 and spring elements 7, which are connected to the power supply-8.

4 shows a displaceable anode 3 in a top view. This figure is an enlarged view of the detail "B" in FIG. 1c and shows spring elements 7 which are connected to the power supply 8 and the anode 3. In the working position, the anode is pressed against the membrane 4.

The electrolysis cell according to the invention has the following advantages, among others. Due to the movable electrode combination with spring elements caused by multiple divisions, the smallest critical electrode spacing can be changed at any time during loading Drive of the electrolytic cell are observed. This combination saves a considerable amount of technical production effort for the electrodes as well as for the electrode frames with regard to compliance with tight manufacturing tolerances. Furthermore, a limitation of the height construction of the electrolysis cell is practically eliminated, since developed gas is removed from the electrode gap in each division, ie gas accumulation is avoided.

The invention is explained in more detail and by way of example using the examples and calculations below.

example 1 A) _L aboratoriumszelle for the production of sodium chlorate.

• Size: 50 x 50 mm = 0.0025 m ²
• Distance between electrodes: 5 mm
• Current density: 3 kA / m ²
• Voltage drop in ^-
• Electrolytes: 250 mV

• 1 cm² einer der Elektroden sei um 1 mm erhaben. Dann ergibt sich an der erhabenen Stelle eine Stromdichte, die in .erster Näherung über die. Leistungsaufnahme zu ermitteln ist.

Adoption:

• 1 cm ^{2 of} one of the electrodes is raised by 1 mm. Then there is a current density at the raised point, which is in the first approximation over the. Power consumption is to be determined.

With plane-parallel electrodes of uniform spacing, the power consumption is

- With the same current density, the power consumption would be on the 1 mm raised area of 1 c _m ²

The power consumption on the non-raised area is then

die Stromdichte auf der nicht erhabenen Fläche auf

die Stromdichte auf der erhabenen Fläche

The total power consumption is 1.860, ie the voltage is reduced to

the current density on the non-raised surface

the current density on the raised surface

_{B) M} embranzelle for the generation of Cl _2, NaOH, H ₂

Annahme:

• 1 cm² einer der Elektroden sei um 1 mm erhaben.

Adoption:

• 1 cm ^{2 of} one of the electrodes is raised by 1 mm.

The same calculation as under example 1, A then gives the following values:

• Wärmeentwicklung bei 3 kA/m² in der Membran:
  
• Wärmeentwicklung bei 3,24 kA/m²:
  

The membrane as an additional resistance has a stabilizing effect, but the heat development in the membrane increases not insignificantly:

• Heat development at 3 kA / m ² in the membrane:
  
• Heat development at 3.24 kA / m ² :
  

With the same heat dissipation, the temperature difference between the membrane and the electrolyte increases by approx. 20%.

It is obvious that an unevenness of 1 mm is difficult to represent in small laboratory cells.

In contrast, unevenness of 1 mm in industrial-sized cells cannot be avoided without special measures. Economic constraints do not allow working with cells of industrial size with intervals of 5 mm. Distances that allow the lowest voltage drop are aimed for. Depending on the shape of the electrode, this is 1 to 3 mm. The entire anode or cathode area can reach orders of magnitude of 50 m ² , heights normally not exceeding 1.2 m. The reason for the limitation of the height is an unavoidable increase in the gas concentration in the electrolyte in the upper part of electrolysis cells.

The following examples are intended to explain the effect of a shorter distance and higher gas concentrations.

Example 2 Industrial cells _A ) _{Membrane cell} for the production of Cl ₂ , NaOH, H ₂ , monopolar

• 10 cm² beider Elektroden sind um 0,75 mm erhaben und stehen sich gegenüber.

Adoption:

• 10 cm ^{2 of} both electrodes are raised by 0.75 mm and face each other.

• _Gesamtspannungsabfall 550 mV Stromdichte an den erhabenen
• Flächen: 3,47 kA/m²

The same calculation (as in example 1, A) then gives the following values:

• _G esamtspannungsabfall 550 mV current density at the raised
• Areas: 3.47 kA / m ²

Due to the relation of the raised area to the rest of the area there is practically no change in the total voltage drop and no measurable reduction in the current density on the non-raised areas.

The heat development in the membrane (see Example 1, B). however rises to 1380 kcal / m x h corresponding to 133% of the normal value.

_{B) S} alzsäureelektrolyse with diaphragm for the generation of Cl ₂ and H ₂ from waste acid, bipolar.

Example 2 shows the limitations in the construction of large-scale electrolytic cells due to power warps. ± 0.75 mm are tolerances that can just be maintained with reasonable effort. For a 1 m wide or tall cell, this tolerance means an accuracy of 0.075% based on the gauge block. Furthermore, 30 to 50% free area for the gas discharge is the maximum of the tolerable, because otherwise the effective current density increases too much.

Claims (5)

1. Membrane electrolytic cell with vertically arranged electrodes composed of several units, characterized in that a) the electrode of one polarity is horizontally divided into several units, b) the electrode of the opposite polarity is divided vertically into several units, and c) the respective units of at least one of the two electrodes are displaceable by spring elements. 2. Membrane electrolytic cell according to Claim 1, characterized in that spacers are arranged between the units of the electrode to which the membrane is not in contact. 3. Membrane electrolytic cell according to claims 1 to 2, characterized in that the spacer is designed as a guide element for deriving the developed gases from the electrode gap. 4. Membrane electrolytic cell according to claims 1 to 3, characterized in that the spring elements are designed as a gas extraction device when the electrodes are divided into vertical units. 5. Membrane electrolysis cell according to claims 1 to 4, characterized by gas discharge organs arranged laterally in the electrolysis chamber.

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