EP0097991B1 - 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 influenced by the scatterability of the electrolyte as well as by the homogeneity of the electrodes. The greater the area on the counterelectrode that is affected by the streamlines, the better the scatterability. A lack of scattering capacity can be compensated for by increasing the electrode spacing, but this increases the voltage drop in the cell. Inhomogeneities in the electrode surface cause current distortion. The distance of the electrode plates, i.e. the distance between the anode and cathode is therefore of great importance.

Ideally, the surfaces of both electrodes face each other in parallel. Flat 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. In order to keep the voltage drop as low as possible and thus reduce the energy consumption, the distance between the anode and cathode should also be kept as small as possible. All of these requirements are relatively easy to implement in small laboratory cells, but the construction of large industrial units is difficult if the ideal ideas that are theoretically required are to be realized. In addition, the larger cells are, the more sensitive they are to deviations from plane parallelism and to current distortion. In order to avoid accelerated destruction of the ion exchange membrane of this type, there is generally a requirement to limit the height of the electrodes, to set a considerable distance between the electrodes of the cell and to limit the electrical current density, which at the same time affects the energy yield of the electrolytic cell and its productivity Disadvantage is.

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. In order to ensure that, on the one hand, the required narrow tolerances in the plane-parallel arrangement of the electrodes are maintained, but on the other hand, a large number of such frames can be connected leak-free to an electrolyser according to the filter press principle, the contact surfaces of the frames must also be machined accordingly.

From DE-PS 563 393 an electrolytic cell is known, in which elastic or resilient elements are attached between segmented electrodes and the diaphragm, which prevent the diaphragm from independent vibrations or harmful movements.

The membrane electrolysis cell known from FR-OS 2 486 105 has electrodes divided vertically into several units, and the anode arrangement has flexible spring elements which make the anodes displaceable.

According to a proposal known from DE-AS 20 59 868, an electrode plate consisting of individual plates has already been provided for vertically arranged electrodes in gas-forming diaphragm cells, the individual plates having guide surfaces for the discharge of the gas generated. Due to the intended inclination of the guide plate or surface, there are inevitably different distances between the active surface and the counterelectrode, warps being easily caused, in particular, by local temperature increases in the sensitive partition walls of poor thermal conductivity. 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 getrennte Einheiten horizontal geteilt ist,
• b) die Elektrode der entgegengesetzten Polarität in mehrere getrennte 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 and provided with spring elements. In a cell of the type mentioned, the invention is that

• a) the electrode of one polarity is horizontally divided into several separate units,
• b) the electrode of the opposite polarity is vertically divided into several separate 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, is rigidly connected to the cathode frame in individual horizontally divided plate sections, while the electrode of the opposite polarity, such as the anode vertically divided into several plates or strip units, is designed to be flexible or displaceable. This flexible design is brought about by spring elements. The spring elements are useful on the Power leads attached to the electrodes and cause electrical contact with the individual strip units of the electrode (anode) via contact pressure or welding.

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 for discharging developed and accumulated gas, such as chlorine gas, is used and configured 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 opened towards them. For example, the spring element can 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, horizontal separation points are created between the individual units of the electrode, on which the membrane does not rest, in which spacers are arranged. 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 z. 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 acts as a gas separation unit when arranged horizontally. It then consists, for example, of strip-shaped plates with serrated edges or strips with slot-shaped or circular openings, or of grid-shaped or network-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 a vertically and horizontally divided anode 3.

• 1a is a section along the line I - I in FIG. 1 and shows the horizontally designed cathode plate 2 with spacer 1.
• Fig. 2 is an enlarged view of section "A" in Fig. 1a. 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.
• 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 section "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 electrolytic cell according to the invention has i.a. following advantages. Due to the movable electrode combination with spring elements caused by multiple divisions, the smallest critical electrode spacing can be maintained at any time during the operation of the electrolytic cell. This combination saves a considerable amount of technical production effort for both the electrodes and for the electrode frames with regard to maintaining tight manufacturing tolerances. Furthermore, a limitation of the height design of the electrolysis cell is practically removed, since developed gas is removed from the electrode gap in each division, i. H. 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) Laboratory cell for the production of sodium chlorate.

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 can be determined in a first approximation via the power consumption.

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

At the same current density, the power consumption would be 1 cm ² on the area raised by 1 mm

The power consumption on the non-raised area is then

• d.h. die Spannung reduziert sich auf
  
• die Stromdichte auf der nicht erhabenen Fläche auf
  
• > r die Stromdichte auf der erhabenen Fläche
  
• >r B) Membranzelle zur Erzeugung von C1₂, NaOH, H₂
  

The total power consumption is 1.860,

• ie the tension is reduced to
  
• the current density on the non-raised surface
  
• > r the current density on the raised surface
  
• > r B) membrane cell for the production of C1 ₂ , NaOH, H ₂
  

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:

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

Heat development at 3 kAfm ² in the membrane:

3 x 0.4 x 860 = 1032 kcaljm ² xh

Heat development at 3.24 kAfm ² :

With the same heat dissipation, the temperature difference between the membrane and the electrolyte increases by about 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 electrolytes in the upper part of electrolytic cells.

The following examples illustrate the effect of a shorter distance and higher gas concentrations are explained.

Example 2

Industrial cells

A) Membrane cell for the production of C1 ₂ , NaOH, H ₂ , monopolar

Adoption:

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

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

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. However, the heat development in the membrane (see Example 1, B) rises to 1380 kcal / m ² × h, corresponding to 133% of the normal value.

B) Hydrochloric acid electrolysis with diaphragm to generate C1 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 (4)

1. A membrane-type electralytic cell comprising vertically extending electrodes, which are composed of a plurality of units and provided with spring elements, characterized in that

a) the electrode of one polarity is horizontally divided into a plurality of separate units,

b) the electrode of the opposite polarity is vertically divided into a plurality of separate units, and

c) the units of at least one of the two electrodes are displaceable by spring elements.

2. A membrane-type electrolytic cell according to claim 1, characterized in that horizontal gaps, which contain spacers, are left between the units of that electrode which ia not contacted by the membrane.

3. A membrane-type electrolytic cell according to claim 1 or 2, characterized in that the spacers in the horizontal gaps consist of a strip-shaped plate for diverting gases which evolve at the electrode unit.

4. A membrane-type electrolytic cell according to claims 1 to 3, characterized in that the electrodes are divided into vertical units and the spring elements are concavely curved toward the bottom of the cell or form an angle which is open in that direction to conatitute gas-with-drawing means.

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