Method for Producing a Uniform Cross-Flow of an Electrolyte Chamber of an Electrolysis Cell

The invention relates to a method for producing a uniform flow through an electrolyte space of an electrolysis cell, and to an electrolysis cell.

Electrolysis is very important in the chemical industry. Examples of fields in which electrolysis is used are the synthesis of chlorine by chloralkali electrolysis or hydrogen chloride electrolysis, electrolytic generation of chromic acid, electrochemical production of sodium dithionite and electrochemical water purification and metal precipitation to obtain pure metals.

For a large number of electrochemical cells, it is desirable to provide an electrode surface whose active surface area is larger than its purely geometrical dimensions.

The most prominent examples of this are to be found in fuel cell technology. In a polymer electrolyte fuel cell, for example, the active electrode face consists of a gas diffusion layer based on carbon black, which is activated by special methods, saturated with ionomers and hydrophobicized in order to offer a much larger reaction area to the gases than would correspond to the dimensions of the gas diffusion layer.

In organic electrochemistry, for example, electrodes made of felt are used in order to increase the active surface area of the electrodes for mediated processes in particular, that is to say for processes in which there are small amounts of an electro-catalytically active redox system in the reaction solution. Similar arrangements are also used in electro-enzymatics. For example, a multi-cathode cell containing cathodes which consist of a plurality of assembled network layers is used for the electrochemical reduction of vat dyes.

The oxidation of sugars to sugar acids is carried out in a special stirred reactor equipped with anode grids.

Cathodes to which a ribbed structure is imparted to increase the throughput are used for the reduction of phthalic acid to dihydrophthalic acid.

The so-called Swiss roll cell has been developed for nickel oxide-catalyzed reactions. Here, the anode and the cathode are spirally wound.

Electrodes whose active surface area is larger than their purely geometrical dimensions are often referred to as three-dimensional electrodes.

Arrangements in which layers of materials with a large surface area are precoated onto an electrode substrate are also known.

Lamellar designs which are formed from strips of metallic glasses, for example, are also known for organic and inorganic electrolysis.

Such three-dimensional electrodes are used in inorganic electrolysis, for example, in order to precipitate traces of metal from effluents. Felted electrodes or electrodes of particle beds, for example, are used for this purpose.

Electrodes in the form of a networked design, for example, may be used for the production of sodium dithionite.

A disadvantage with the electrolysis cells used at present is the fact that the hydrodynamics on the electrode face, that is to say the 2-phase flow of the liquid/gas mixture, are often defined only insufficiently by the design configuration of the overall electrode and of the electrolyte space. In fuel cells, for example, the gas feed is established accurately by the so-called flow field, but the formation of a liquid phase is a phenomenon to be feared since it can critically interfere with the gas supply as well as the potential distribution and the current density distribution. This interference can lead to destruction of the cell.

The design configuration of the overall electrode and of the electrolyte space using the flow field is relatively uncritical in some cases, for example in chloralkali electrolysis according to the membrane method, in which two grid electrodes that evolve gases face each other while being separated by a membrane. The mammoth pump effect, which is created by the gas bubbles being evolved, ensures sufficient equidistribution in the two electrolyte spaces. Neither strong nor defined recirculation of the electrolyte is required.

For electrolysis cells in which a high selectivity with high throughput is a critical quantity, problems occur in electrolysis cells without defined hydrodynamics. In order to avoid dead spaces in which the uncontrolled formation of secondary components can occur, and in order to achieve optimum use of the electrode surface, it is necessary to ensure a maximally uniform distribution of the reaction liquid in the electrolyte space so as to ensure a maximally homogeneous current density distribution. To that end, it is also necessary to control the liquid flows outside the immediate vicinity of the electrode surface. Examples of dead spaces are gas cushions (that is to say static gas bubbles) or regions through which no liquid flows. Such regions occur, for example, owing to vortex formation, backward flows or stagnation at obstacles in the flow path.

When through-flow porous electrodes are used in membrane electrolysis cells, a nonuniform pressure distribution in the anolyte space and the catholyte space can lead to a bypass, through which the electrolyte flows, being formed between the membrane and the porous electrode. This leads to a reduction of the throughput. In the case of through-flow electrodes, the term bypass is here intended to mean a stream which flows past the electrode rather than through it.

From U.S. Pat. No. 4,204,920, in the case of a membrane electrolysis cell, it is known to set up a higher pressure in the anolyte space than in the catholyte space, so that the membrane is pushed away from the anode towards the cathode.

But a narrow dwell time distribution, and therefore a uniform flow through the cross section, which is necessary for uniform conversion in the electrolyte spaces, is not achieved by setting different backpressures for the anolyte space and the catholyte space.

It is an object of the present invention to provide a method which ensures a uniform flow through an electrolyte space of an electrolysis cell and therefore a narrow dwell time distribution.

The object is achieved by a method for producing a uniform flow through an electrolyte space of an electrolysis cell, in which a maximum deviation of less than 1% to 25% from the average flow rate is achieved by suitable design measures.

An electrolysis cell is preferably formed by at least two electrolyte spaces. In this case, at least one electrolyte space is an anolyte space and at least one electrolyte space is a catholyte space. An anolyte space and a catholyte space are respectively adjacent and separated from each other by at least one membrane.

The maximum deviation from the average flow rate is preferably achieved by setting up an additional pressure reduction. This is preferably from 1 to 10 times the pressure difference in the inlet region of the electrolyte space (that is to say the pressure reduction in the inlet region between the feed to the inlet region and the electrode in the electrolyte space, if no additional pressure reduction is applied). The calculation is carried out according to a equation (1):
$\begin{matrix} Δ p_{DV} = \frac{\langle p_{dyn} + Δ p_{V} \rangle}{{(A + 1)}^{2} - 1} - Δ p_{E}, & (1) \end{matrix}$

when the feed into the inlet region of the electrolyte space is such that the incoming volume flow is distributed approximately uniformly into two sub-flows with opposite principal flow directions in the inlet region. Here, the width of the electrolyte space is the dimension which extends perpendicularly to the principal flow direction in the electrolyte space and perpendicularly to the principal direction of the electric field (gap width).

When the feed is organized in a different way from the type described above, the calculation is carried out according to Equation (2):
$\begin{matrix} Δ p_{DV} = \frac{\langle p_{dyn} + Δ p_{V} \rangle}{{(A + 1)}^{2} - 1} - Δ p_{E}, & (2) \end{matrix}$

This applies, in particular, when the feed is organized laterally to the electrolyte space with respect to the width of the electrolyte space.

- Here:
- Δp_DV=additional pressure reduction,
- Δp_V=frictional pressure reduction in the inlet region,
- p_dyn=dynamic pressure in the inlet region,
- Δp_E=overall pressure reduction in the electrolyte space, and
- A=maximum deviation from the average flow rate, 0 denoting no deviation, and 1 denoting 100% deviation.

Here, centrally with respect to the electrolyte space means in the middle of the cross section perpendicular to the flow direction on the influx side of the electrode.

In a preferred embodiment, the additional pressure reduction is produced by pressure reducing elements (that is to say design measures by which an additional pressure reduction is obtained) in the inlet and/or outlet region of the electrolyte space. Here, the inlet region is the region between the feed to the electrolyte space and the electrode. In general, the flow cross section is widened there to the cross section of the electrolyte space and the stream is deviated in order to flow through the electrolyte space, if the feed is not aligned flush with the electrolyte space in the flow direction. Correspondingly, the outlet region is the region between the electrode and the discharge from the electrolyte space. For example, the inlet region may be designed as a distributor and the outlet region as a collector. The pressure reducing elements preferably produce a reduction of the flow cross section. In a preferred embodiment, the pressure reducing elements are fixtures in the inlet region and/or outlet region of the electrolyte space.

The pressure reducing elements in the inlet region and/or outlet region compensate for differences in the flow rate which, for example, occur owing to pressure gradients in the inlet region or in the outlet region. For example, the pressure gradients result from the feed to the inlet region being arranged perpendicularly to the flow direction in the electrode. The liquid is therefore deviated in the inlet region. The inlet region is closed on the opposite side from the feed. The liquid first flows in the direction which is dictated by the feed. The liquid stagnates on the opposite side from the feed, which increases the pressure. The liquid is then deviated into the electrode owing to the increased pressure. The effect achieved by using the at least one pressure reducing element is that the pressure is uniformly distributed after flowing through the pressure reducing element. This leads to a uniform flow rate.

Other components which contribute to non-equidistribution of the pressure in the inlet region are inertial effects of the liquid and frictional losses in the inlet region.

Pressure gradients in the outlet region result, for example, if the liquid accumulates at the outlet from the electrolyte spaces or the gas formed during the electrolysis accumulates in the outlet region. The outlet region preferably extends parallel to the efflux side of the electrolyte space. If the cross-sectional area of the outlet region remains the same, the velocity increases in the flow direction owing to the increasing amount of liquid or gas. Like the inlet region, the outlet region is preferably closed on one side. Since the amount of liquid or gas in the flow direction increases in the outlet region, the pressure changes here as well. Other factors influencing the pressure distribution in the outlet region are inertial effects and friction, as in the case of the inlet region. In a preferred embodiment, therefore, pressure reducing elements are arranged in the outlet region for equidistribution in the electrolyte spaces.

A uniform flow rate can also be achieved if the feed into the inlet region lies opposite the feed of the electrolyte space and the inlet region widens in the form of a diffuser cell. Owing to the small aperture angle of diffusers, however, this requires a great deal of space which is often unavailable for installation of the electrolysis cell. The slow transition from one cross section to another in the diffuser also leads to long dwell times and a correspondingly large hold-up. By arranging the feed at an arbitrary point of the inlet region and the discharge at an arbitrary point of the outlet region, the use of pressure reducing elements in the inlet region and/or in the outlet region permits a significantly reduced requirement for space compared with the use of diffusers. At the same time, the smaller volumes of the inlet region and of the outlet region reduce the hold-up.

In the context of the invention, the terms “in the inlet region” or “in the vicinity of the outlet region” mean that the pressure reducing element is arranged between the feed and the electrolyte space, or between the electrolyte space and the discharge, respectively.

For many applications a plurality of electrolysis cells, each comprising an anolyte space and a catholyte space, are joined together as cells in order to achieve higher throughputs. The liquid is fed into the individual electrolysis cells via a distribution system, which preferably comprises a channel from which a feed respectively branches off at the inlet region to each electrolyte space. On the outlet side of the electrolyte spaces, the outlet region is respectively connected to a discharge which leads into a discharge channel.

Fixtures which can be used as pressure reducing elements owing to their design features are known to the person skilled in the art. Perforated metal sheets are an example of a pressure reducing element. The openings in the perforated metal sheets may be provided with any cross section. Bores are preferred openings in the perforated metal sheets.

Plates containing at least one channel are also suitable as pressure reducing elements. When there are a plurality of channels, these are preferably arranged parallel to one another. The channels have a circular cross section in a preferred embodiment, since this is the simplest to produce with conventional tools. The channels may, however, also be designed elliptically or in the form of a polygon with at least three vertices. Any other cross-sectional geometry known to the person skilled in the art may also be envisaged for the channels contained in the plates. There is preferably also a gap in the pressure reducing element.

In another embodiment, the pressure reducing elements are designed as fabrics or as a foam structure or as a plate containing capillaries.

In particular when perforated metal sheets or plates containing channels are being used as the pressure reducing elements, the flow may emerge in the form of a jet from the pressure reducing element. This jet should not continue directly into the working electrode which is connected downstream of the pressure reducing element, since the jet would then produce a large pressure reduction in the working electrode. For this reason, in a preferred embodiment, a settling section for distribution of the emerging jet is provided between the pressure reducing element and the working electrode.

Since the outlet region is essentially configured in a similar way to the inlet region, the configuration may essentially be the same as for the inlet region. In the outlet region, however, the frictional effects often dominate. It has also been found that uniform efflux from the electrolyte spaces often requires greater pressure reductions for homogenization of the flow.

When porous electrodes are used, the pressure reduction due to the flow through the electrode likewise needs to be taken into account when dimensioning the pressure reducing elements.

When a porous electrode is used, uniform electrolytic conversion requires that the electrolyte should flow uniformly through the electrode. This is achieved by fixing the membrane between the anolyte space and the catholyte space against the porous electrode. In a preferred variant of the method, this is done by keeping the pressure in the electrolyte space with the porous electrode at a lower level than the pressure in the other electrolyte space. The electrolyte space with the porous electrode may in this case be the anolyte space or the catholyte space, depending on how the electrolysis cell is being used. The pressure level required in the electrolyte spaces, in order to press the membrane onto the porous electrode, is preferably achieved by setting up a backpressure in the outlet region.

The backpressure in the outlet region should in this case be selected such that the pressure at any point in the electrode space with the porous electrode is lower than the pressure in the other electrolyte space.

In another embodiment, in particular when fabrics or foam structures are being used as the pressure reducing elements, these are additional electrodes.

When fabrics or foam structures, or fillers or structured packing are being used as the pressure reducing elements, a settling section behind the pressure reducing elements may be obviated since a uniform velocity profile is already obtained in the pressure reducing element because of transverse flows.

The invention will be described in more detail below with reference to a drawing, in which:

FIG. 1 shows a section through an electrolysis cell,

FIG. 2 shows a section through a catholyte space of an electrolysis cell,

FIG. 3 shows a section through a cell stack,

FIG. 4 shows a detail of a catholyte space having a distributor and pressure reducing elements contained therein,

FIG. 5 shows a detail of a catholyte space having a distributor and a pressure reducing element with capillaries.

FIG. 1 shows a section through an electrolysis cell.

An electrolysis cell 1 comprises an anolyte space 2 and a catholyte space 3. In the embodiment represented here, the anolyte space 2 contains an anode 4 in the form of a plate. Besides the anode 4 designed as a plate in the anolyte space 2, the wall 14 of the anolyte space 2 may also be designed as a bipolar plate so as to fulfill the function of the anode 4.

The catholyte space 3 contains a cathode 5, which has a porous structure and fills the entire catholyte space 3.

The catholyte space 3 is separated from the anolyte space 2 by a membrane. In order to achieve a uniform flow through the cathode 5 in the catholyte space 3, the membrane 6 is fixed against the cathode. To that end, preferably, the pressure at any point in the anolyte space 2 is higher than in the catholyte space 3. The membrane 6 is thereby pressed onto the cathode 5. Bypasses between the cathode 5 and the membrane 6 are avoided in this way, and all of the catholyte flows through the cathode 5 which is designed as a porous structure.

In the embodiment represented in FIG. 1, the anolyte is delivered to the anolyte space 2 via a pressure reducing element 9.1 from an inlet region, which is designed as an anolyte distributor 10. The anolyte flows via another pressure reducing element 9.3 into an outlet region, which is designed as a collector 12. The flow direction of the anolyte is indicated by an arrow with the reference numeral 7.

The catholyte flows into the catholyte space 3 via a pressure reducing element 9.2 from an inlet region, which is designed as a catholyte distributor 11, then flows through the electrode 5 and finally flows via a pressure reducing element 9.4 into an outlet region, which is designed as a catholyte collector 13.

FIG. 2 shows a section through a catholyte space of an electrolysis cell. The catholyte space is rotated through 90° here, compared with FIG. 1.

The catholyte enters the catholyte distributor 11 through either a central feed 15 or a lateral feed 17. From there, the catholyte flows via the pressure reducing element 9.2 into the catholyte space 3, which is entirely filled by the porous cathode 5. The catholyte flows through the porous cathode 5 and enters the catholyte collector 12 via the pressure reducing element 9.4. The catholyte is removed from the catholyte collector 12 via a central discharge 16 or a lateral discharge 18.

FIG. 3 shows a section through a cell stack.

A cell stack 19 comprises at least two electrolysis cells 1. Depending on the required throughput, however, any number of electrolysis cells 1 may be joined together as a cell stack 19.

Anolyte spaces 2 and catholyte spaces 3 respectively alternate in a cell stack 19. The anolyte space 2 and the catholyte space 3 in an electrolysis cell 1 are separated by the membrane 6. Two electrolysis cells are separated by the wall 14 which, for example, may be designed as a bipolar plate.

FIG. 3 shows that each anolyte space 2 and each catholyte space 3 of the cell stack 19 is supplied via a distributor 10, 11 with a corresponding electrolyte, that is to say catholyte or anolyte. To that end, the electrolyte flows through the pressure reducing element 9.1, 9.2 and thus enters the anolyte space 2 or catholyte space 3, respectively. On the outlet side, the electrolyte flows through the pressure reducing elements 9.3, 9.4 and thus enters the collector 12, 13 assigned to each anolyte space 2 or catholyte space 3. The flow direction of the electrolyte is indicated here by the arrows 7, 8.

Besides the flow direction represented in FIGS. 1 to 3, according to which the electrolyte flows upwards through the electrolysis cell 1, the electrolyte may also flow in the opposite direction downwards through the electrolysis cell 1. The electrolysis cell 1 may furthermore be arranged such that the distributors 10, 11 and the collectors 12, 13 are at the same level. The electrolysis cell 1 may also be inclined at any desired angle.

FIG. 4 shows a detail of a catholyte space with distributor and pressure reducing element.

It can be seen from FIG. 4 that the catholyte in the catholyte distributor 11 flows transversely to the flow direction in the catholyte space 3. Some of the catholyte flows through openings 23 in the pressure reducing element 9.2. This leads to a reduction of the amount of liquid and therefore to a reduction of the flow rate in the distributor 11. If the distributor has only one feed 15, 17 and no discharge, the liquid stagnates in the distributor 11 and thus leads to a pressure that decreases as the distance from the feed 15, 17 increases. The effect of a higher pressure is that more liquid flows into the catholyte space 3 at this position. A uniform flow rate over the entire width of the cathode 5 can be achieved by the pressure reducing element 9.2, which has a pressure reduction calculated according to Equation (1) or Equation (2). So that the liquid jet flowing in through the openings 23 in the pressure reducing element 9.2 does not strike the cathode 5 directly, a settling section 21 is formed behind the pressure reducing element 9.2. In the settling section, the liquid jet passing through the opening 23 widens according to the flow direction indicated by the arrow 22. In the settling section 21, a uniform liquid distribution is achieved with a virtually constant pressure and therefore with a consistent entry velocity into the cathode 5.

The structure when using a pressure reducing element 9.1 in the distributor 10 to the anolyte space 2 corresponds to that represented in FIG. 4 for the catholyte space 3.

On the outlet side as well, a settling section 21 is preferably interconnected between the porous cathode 5 and the pressure reducing element 9.4. This ensures that stagnation of the liquid at the impermeable regions of the pressure reducing element 9.4 does not lead to stagnation in the porous cathode 5, but instead a uniform flow rate is maintained in the cathode 5 as far as the settling section 21.

When a porous anode 4 is used, a settling section 21 should also be provided between the porous anode 4 and the pressure reducing element 9.3 in a similar way to the porous cathode 5.

The openings 23 in the pressure reducing element 9.1, 9.2, 9.3, 9.4 may, for example, be bores in a perforated metal sheet. Besides the usual round cross section of bores, the openings 23 may also be provided with any other cross section.

For example, the opening 23 may also be a gap over the entire length of the electrolyte space. Here, the term “length” is intended to mean the larger extent of the electrode perpendicular to the flow direction of the electrolyte.

Furthermore—as represented in FIG. 5—the pressure reducing element 9.1, 9.2, 9.3, 9.4 may also contain capillaries 24. Here, the pressure reduction in the pressure reducing element 9.1, 9.2, 9.3, 9.4 is primarily produced by friction forces.

Besides the openings 23 or the capillaries 24 in the pressure reducing element 9.1, 9.2, 9.3, 9.4, fabrics or foam structures as well as fillers or structured packing are also suitable as pressure reducing elements 9.1, 9.2, 9.3, 9.4.

EXAMPLE

A plate electrolysis cell has a through-flow cross section of 5 mm×500 mm. A distributor measuring 20×20×500 mm is provided for distribution of the electrolyte. The volume flow rate of the electrolyte is 720 l/h with an electrolyte density of 1000 kg/m³. The homogenization of the flow is intended to be achieved by a pressure reducing element with bores. The maximum deviation from the average flow rate should then be 5%.

The distribution error should be determined by inertia.

A maximum flow rate v of
$v = \frac{V}{A} = \frac{720 l / h}{20 20 {mm}^{2}} = 0.5 m / s$

is obtained from the volume flow rate and the cross section of the distribution channel.

This gives a dynamic pressure of

ρ_dyn=0.5·ρ·v²=1.02 mbar

with an electrolyte density ρ of 1000 kg/m³.

For the intended 5% deviation, Equation (1) then gives a required pressure reduction of 12.2 mbar across the pressure reducing elements. Taking the relevant pressure reducing parameter into account, such a pressure reduction is only obtained with a flow rate v_Oof
$v_{\overset{..}{O}} = \sqrt{\frac{2 Δ p_{DV}}{ζ \cdot ρ}} = 1.626 \frac{m}{s}$

in the opening, with a pressure reducing parameter ζ=1.5 for the openings.

Taking the volume flow rate of 720 l/h into account, a necessary maximum overall flow cross section A_Qof
$A_{Q} = \frac{V}{v_{\overset{..}{O}}} = 123 {mm}^{2}$

is obtained.

With bore holes each measuring 3 mm in diameter, this corresponds to 17.4 bore holes. A pressure reducing element with 17 bore holes should accordingly be selected.

LIST OF REFERENCES

1 electrolysis cell

2 anolyte space

3 catholyte space

4 anode

5 cathode

6 membrane

7 flow direction of the anolyte

8 flow direction of the catholyte

9.1, 9.2, 9.3, 9.4 pressure reducing element

10 anolyte distributor

11 catholyte distributor

12 anolyte collector

13 catholyte collector

14 wall

15 central feed

16 central discharge

17 lateral feed

18 lateral discharge

19 cell stack

20 flow direction in the distributor 11

21 settling section

22 flow direction in the settling section 21

23 opening

24 capillary

Method for Producing a Uniform Cross-Flow of an Electrolyte Chamber of an Electrolysis Cell

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