WO2005103336A2

WO2005103336A2 - Method for producing a uniform cross-flow of an electrolyte chamber of an electrolysis cell

Info

Publication number: WO2005103336A2
Application number: PCT/EP2005/004074
Authority: WO
Inventors: Harald BÖHNKE; Hermann Pütter; Torsten Mattke
Original assignee: Basf Aktiengesellschaft
Priority date: 2004-04-22
Filing date: 2005-04-18
Publication date: 2005-11-03
Also published as: US20070221496A1; CN1973062A; DE102004019671A1; EP1743051A2; WO2005103336A3; JP2007533855A

Abstract

The invention relates to a method which is used to produce a uniform cross-flow of an electrolyte chamber of an electrolysis cell, whereby a maximum deviation from the central flow speed is less than 1 % - 25 % which is produced by suitable constructive measures. The invention also relates to an electrolysis cell comprising at least two electrolyte chambers, wherein at least one electrode is arranged and which respectively comprises an inlet and outlet area. The cross-flow cross-section in the inlet and/or outlet area reduces in such a manner that the pressure is reduced again.

Description

Method for generating a uniform flow through an electrolyte space of an electrolysis cell

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

Electrolysis is of great importance in the chemical industry. Areas of application for electrolysis are, for example, the synthesis of chlorine by chlor-alkali electrolysis or hydrogen chloride electrolysis, electrolytic chromic acid regeneration, electrochemical production of sodium dithionite as well as electrochemical wastewater treatment and metal separation to obtain pure metals.

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

The most prominent examples of this can be found in fuel cell technology. For example, in the case of a polymer electrolyte fuel cell, the active electrode surface consists of a soot-based gas diffusion layer, which is activated with special methods, saturated with ionomers and hydrophobized in order to offer the reaction gases a reaction surface many times larger than would correspond to the dimensions of the gas diffusion layer.

In organic electrochemistry, in order to enlarge the active surface of the electrodes, especially for mediatized processes, i.e. for processes in which small amounts of an electrocatalytic redox system are present in the reaction solution, for example electrodes made of felt. Similar arrangements are also used in electroenzymatics. For the electrochemical reduction of vat dyes, for example, a multi-cathode cell is used that contains cathodes that consist of several interconnected network layers.

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

For the reduction of phthalic acid to dihydrophthalic acid, cathodes are used, which are imprinted with a groove structure to increase sales. The so-called Swiss Roll Cell was developed for reactions catalyzed by nickel oxide. The anode and cathode are wound in a spiral.

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

Furthermore, arrangements are also known in which layers with materials with a large surface area have been washed onto an electrode base body.

For organic and inorganic electrolysis, lamellar constructions are also known which have been composed, for example, of strips with metallic glasses.

In inorganic electrolysis, such three-dimensional electrodes are used, for example, to separate metal traces from waste water. For this purpose, felt-like electrodes or electrodes made of particle beds are used, for example.

For example, electrodes in the form of a mesh-like construction can be used for the production of sodium dithionite.

The fact that the hydrodynamics on the electrode surface, i. H. the 2-phase flow of the liquid / gas mixture, which is often insufficiently defined by the design of the entire electrode and the electrolyte space. For example, in the case of fuel cells, the gas flow is precisely defined by so-called flow fields, but the formation of a liquid phase is a feared phenomenon, since it severely disturbs the gas supply and the distribution of potential and current density. This disruption can lead to cell destruction.

The design of the overall electrode and the electrolyte space based on the flow field is relatively uncritical in some cases, such as for example in chlor-alkali electrolysis using the membrane process, in which two gas-developing grid electrodes separated by a membrane face each other. The mammoth pump effect created by the developing gas bubbles ensures that the two electrolyte compartments are sufficiently evenly distributed. Neither a strong nor a defined circulation of the electrolytes is necessary. In electrolysis, in which high selectivity with high turnover is a critical variable, problems occur with electrolysis cells without defined hydrodynamics. In order to avoid dead spaces in which uncontrolled formation of secondary components can occur and to achieve optimum utilization of the electrode surface, the reaction liquid must be distributed as evenly as possible in the electrolyte space and in this way the most homogeneous current density distribution possible. For this purpose, it is necessary to influence the liquid flows outside the immediate vicinity of the electrode surface. Dead spaces are, for example, gas cushions (ie stuck gas bubbles) or areas in which there is no liquid flow. Such areas arise, for example, from eddies, backflow or accumulation at obstacles in the flow path.

If porous electrodes with flow through them are used in membrane electrolysis cells, an uneven pressure distribution in the anolyte space and catholyte space can lead to a bypass being formed between the membrane and the porous electrode, through which the electrolyte flows. This leads to a reduction in sales. In the case of electrodes with flow through, bypass is understood to mean a flow that flows past the electrode and not through it.

From US 4,204,920 it is known to set a higher pressure in a membrane electrolysis cell in the anolyte compartment than in the catholyte compartment, so that the membrane is pressed away from the anode towards the cathode.

However, a narrow residence time distribution and thus a uniform flow through the cross section, which is necessary for the uniform implementation in the electrolyte spaces, is not achieved by setting different back pressures in the anolyte space and catholyte space.

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

The solution to the problem consists in a method for producing a uniform flow through an electrolyte space of an electrolysis cell, in which a maximum deviation from the average flow rate of less than 1% to 25% is generated by suitable design measures. At least two electrolyte spaces preferably form an electrolysis cell. 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 adjacent and separated from one another by at least one membrane.

The maximum deviation from the mean flow velocity is preferably obtained by building up an additional pressure drop. This is preferably 1 to 10 times the pressure difference in the entry area of the electrolyte space (i.e. the pressure loss in the entry area between the inlet to the entry area and the electrode in the electrolyte space if no additional pressure loss is applied). The calculation is carried out using equation (1):

if the inflow into the inlet area of the electrolyte space takes place in such a way that the volume flow entering is approximately uniformly divided into two partial flows with opposite main flow direction in the inlet area. This is particularly the case when the inflow is centered on the electrolyte space in relation to the width of the electrolyte space. The width of the electrolyte space is the dimension that is perpendicular to the main flow direction in the electrolyte space and perpendicular to the main direction of the electric field (gap width).

If the inflow takes place in a manner other than that described above, the calculation is carried out according to equation (2):

This is particularly the case when the inlet to the side of the electrolyte space is based on the width of the electrolyte space.

Here mean:

Δpov = additional pressure loss, Δpv = friction pressure loss in the inlet area, p _d y _n = dynamic pressure in the inlet area, ΔpE = total pressure loss in the electrolyte space and A = maximum deviation from the mean flow velocity, where 0 means no deviation and 1 means a deviation of 100%.

The center of the electrolyte space refers to the center of the cross section perpendicular to the direction of flow on the upstream side of the electrode.

In a preferred embodiment, the additional pressure loss is generated by pressure loss elements (i.e. constructive measures by means of which an additional pressure loss is generated) in the entry and / or exit area of the electrolyte space. The entry area is the area between the inlet to the electrolyte space and the electrode. In general, the flow cross section there extends to the cross section of the electrolyte space and the flow is deflected to flow through the electrolyte space, provided the inlet is not aligned with the electrolyte space in the direction of flow. The exit area is accordingly the area between the electrode and the outlet from the electrolyte space. The entry area can be designed, for example, as a distributor and the exit area as a collector. The pressure loss elements preferably produce a reduction in the flow cross section. In a preferred embodiment, the pressure loss elements are internals in the inlet area and / or outlet area of the electrolyte space.

Due to the pressure loss elements in the inlet area and / or in the outlet area, differences in the flow velocity, e.g. caused by pressure gradients in the entry area or in the exit area. The pressure gradients result e.g. B. from the fact that the inlet to the inlet area is arranged perpendicular to the flow direction in the electrode. As a result, the liquid is deflected in the entry area. The entrance area is closed on the side opposite the inlet. The liquid initially flows in the direction specified by the inlet. The liquid builds up on the side opposite the inlet, which increases the pressure. Due to the increased pressure, the liquid is then deflected into the electrode. The use of the at least one pressure loss element ensures that the pressure is evenly distributed after flowing through the pressure loss element. This leads to an even flow rate.

Other components that contribute to an uneven distribution of pressure in the inlet area are inertia effects of the liquid and friction losses in the inlet area. Pressure gradients in the outlet area result, for example, from the fact that the liquid or the gas formed during the electrolysis collects in the outlet area at the outlet from the electrolyte spaces. The exit area preferably runs parallel to the outflow side of the electrolyte space. If the cross-sectional area of the outlet area remains the same, the speed increases in the direction of flow due to the increasing liquid or gas quantity. The exit area, like the entry area, is preferably closed on one side. Due to the increasing amount of liquid or gas in the outlet area in the direction of flow, the pressure also changes here. Other influencing factors on the pressure distribution in the outlet area, like in the inlet area, are inertia effects and friction. In a preferred embodiment, pressure loss elements are therefore arranged in the outlet area for uniform distribution in the electrolyte spaces.

A uniform flow rate can also be achieved if the inlet into the inlet area is opposite the inlet of the electrolyte space and the inlet area widens in the form of a diffuser. Due to the small opening angle of diffusers, however, a lot of space is required for this, which is often not available for installing the electrolysis cell. The slow transition from one cross-section to the other in the diffuser leads to long dwell times and a correspondingly large hold-up. The use of pressure loss elements in the inlet area and / or in the outlet area enables a space requirement that is significantly reduced compared to the use of diffusers by arranging the inlet at any point in the inlet area and the outlet at any point in the outlet area. At the same time, the smaller volume of the entrance area and the exit area reduces the hold-up.

In the inlet area or in the area of the outlet area in the sense of the invention means that the pressure loss element is arranged between the inlet and the electrolyte space or between the electrolyte space and the outlet.

For many applications, several electrolysis cells, each comprising an anolyte space and a catholyte space, are connected to form stacked cells in order to achieve higher sales. The liquid is fed into the individual electrolysis cells via a distribution system, which preferably comprises a channel, from each of which an inlet to the entry area branches off to each electrolyte space. On the outlet side of the electrolyte compartments, the outlet area is connected to an outlet which opens into an outlet channel. Internals are known to the person skilled in the art which, on account of their structural conditions, can serve as pressure loss elements. Perforated sheets are an example of a pressure loss element. The openings in the perforated plates can have any cross section. Preferred openings in the perforated plates are bores.

Furthermore, plates with at least one channel accommodated therein are also suitable as pressure loss elements. If there are several channels, these are preferably arranged parallel to one another. In a preferred embodiment, the channels have a circular cross section, since this is easiest to manufacture with conventional tools. The channels can also be elliptical or in the form of a polygon with at least three corners. Any other cross-sectional geometry known to the person skilled in the art is also conceivable for the channels accommodated in the plates. A gap in the pressure loss element is also preferred.

In a further embodiment, the pressure loss elements are designed as a fabric or as a foam structure or as a plate with capillaries accommodated therein.

In particular when using perforated plates or plates with channels accommodated therein as pressure loss elements, the flow can emerge from the pressure loss element in the form of a jet. This beam should not pass directly into the working electrode which is connected downstream of the pressure loss element, since otherwise a high pressure loss is generated in the working electrode by the beam. For this reason, a calming section for distributing the emerging jet is provided between the pressure loss element and the working electrode.

Since the exit area is essentially designed like the entry area, the design can be carried out essentially as for the entry area. In the exit area, however, the friction effects often dominate. It has also been shown that the uniform outflow from the electrolyte spaces often requires greater pressure losses in order to make the flow more even.

When using porous electrodes, the pressure loss resulting from the flow through the electrode must also be taken into account when dimensioning the pressure loss elements. When using a porous electrode, it is necessary for the electrolytic conversion to be uniform that the electrolyte flows through the electrode evenly. This is achieved in that the membrane between the anolyte and catholyte against the porous Electrode is fixed. In a preferred method variant, 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 can be the analysis space or the catholyte space, depending on the use of the electrolysis cell. The pressure level required in the electrolyte spaces for pressing the membrane against the porous electrode is preferably achieved by setting a counterpressure in the outlet area.

The back pressure in the outlet area should be selected so that the pressure in the electrode space with the porous electrode is lower at every point than the pressure in the other electrolyte space.

In a further embodiment, in particular when using fabrics or foam structures as pressure loss elements, these are additional electrodes.

When using fabrics or foam structures or fillers or structured packings as pressure loss elements, there is no need for a calming section behind the pressure loss elements, since a uniform speed profile is already established in the pressure loss element due to cross flows.

The invention is described in more detail below with reference to a drawing.

Show here:

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 section of a catholyte space with distributor and pressure loss elements accommodated therein,

FIG. 5 shows a section of a catholyte compartment with distributor and a pressure loss element with capillaries,

Figure 1 shows a section through an electrolysis cell. An electrolytic cell 1 comprises an anolyte space 2 and a catholyte space 3. In the embodiment shown here, an anode 4 in the form of a plate is accommodated in the anolyte space 2. In addition to the plate-shaped anode 4 in the anolyte space 2, the wall 14 of the anolyte space 2 can also be designed as a bipolar plate and thus take over the function of the anode 4.

A cathode 5 is accommodated in the catholyte space 3, 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 5. For this purpose, the pressure in the anolyte compartment 2 is preferably higher at every point than in the catholyte compartment 3. As a result, the membrane 6 is pressed against the cathode 5. In this way, bypasses between the cathode 5 and the membrane 6 are avoided and the entire catholyte flows through the cathode 5 designed as a porous structure.

In the embodiment shown in FIG. 1, the anolyte is supplied to the anolyte space 2 from an inlet area designed as an anolyte distributor 10 via a pressure loss element 9.1. The anolyte flows via a further pressure loss element 9.3 into an outlet area designed as a collector 12. The direction of flow of the anolyte is identified by an arrow with reference number 7.

The catholyte flows from an inlet area designed as a catholyte distributor 11 via a pressure loss element 9.2 into the catholyte space 3, flows through the electrode 5 there and finally flows through a pressure loss element 9.4 into an outlet area designed as a camolyte collector 13.

FIG. 2 shows a section through a catholyte space of an electrolysis cell. The catholyte space is rotated by 90 ° in comparison to FIG. 1.

The catholyte either passes through a central inlet 15 or a side inlet

17 into the catholyte distributor 11. From there, the catholyte flows via the pressure loss element 9.2 into the catholyte space 3, which is completely filled by the porous cathode 5. The catholyte flows through the porous cathode 5 and reaches the catholyte collector 12 via the pressure loss element 9.4. Via a central outlet 16 or a lateral outlet

18 the catholyte is withdrawn from the catholyte collector 12. Figure 3 shows a section through a cell stack.

A cell stack 19 comprises at least two electrolysis cells 1. However, depending on the required throughput, any number of electrolysis cells 1 can be connected to form a cell stack 19.

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

From Figure 3 it can be seen that each anolyte space 2 and each catholyte space 3 of the cell stack 19 via a distributor 10, 11 with the corresponding electrolyte, i.e. Catholyte or anolyte. For this purpose, the electrolyte flows through the pressure loss element 9.1, 9.2 and thus reaches the anolyte space 2 or catholyte space 3. On the outlet side, the electrolyte flows through the pressure loss elements 9.3, 9.4 and thus reaches the collector 12, 13 assigned to each anolyte space 2 or catholyte space 3. The direction of flow of the electrolyte is indicated here by arrows 7, 8.

In addition to the flow direction shown in FIGS. 1 to 3, in which the electrolyte flows through the electrolytic cell 1 from bottom to top, the electrolyte can also flow through the electrolytic cell 1 in the opposite direction from top to bottom. Furthermore, the electrolysis cell 1 can also be arranged such that the distributors 10, 11 and the collectors 12, 13 are at one level. The electrolysis cell 1 can also be inclined at any angle.

Figure 4 shows a section of a catholyte compartment with distributor and pressure loss element.

It can be seen from FIG. 4 that the catholyte in the catholyte distributor 11 flows transversely to the direction of flow in the catholyte space 3. Part of the catholyte flows through openings 23 in the pressure loss element 9.2. This leads to a decrease in the amount of liquid and thus to a decrease in the flow velocity in the distributor 11. If the distributor has only one inlet 15, 17 and no outlet, the liquid builds up in the distributor 11 and thus leads to an increasing distance from the inlet 15, 17 increasing pressure. A higher pressure means that more liquid flows into the catholyte space 3 at this point. Due to the pressure loss element 9.2, which is a according to equation (1) or equation (2) calculated pressure loss, a uniform flow rate can be achieved over the entire width of the cathode 5. So that the liquid jet flowing through the openings 23 in the pressure loss element 9.2 does not strike the cathode 5 directly, a calming section 21 is formed behind the pressure loss element 9.2. In the calming section, the liquid jet passing through the opening 23 widens in accordance with the flow direction indicated by the arrow 22. In the calming section 21, a uniform liquid distribution with an almost constant pressure and thus with the same entry speed into the cathode 5 is achieved.

When using a pressure loss element 9.1 in the distributor 10 to the anolyte compartment 2, the structure corresponds to that shown in FIG. 4 for the catholyte compartment 3.

Also on the outlet side, a calming section 21 is preferably interposed between the porous cathode 5 and the pressure loss element 9.4. This ensures that a build-up of the liquid at the impermeable areas of the pressure loss element 9.4 does not lead to a build-up in the porous cathode 5, but rather that a uniform flow rate is obtained in the cathode 5 as far as the calming section 21.

When using a porous anode 4, a calming section 21 between the porous anode 4 and the pressure loss element 9.3 must also be provided here analogously to the porous cathode 5.

The openings 23 in the pressure loss element 9.1, 9.2, 9.3, 9.4 can be holes in a perforated plate, for example. In addition to the usual circular cross section of bores, the openings 23 can also have any other cross section.

The opening 23 can, for example, also be a gap over the entire length of the electrolyte space. Here, length is to be understood as the greater extent of the electrode perpendicular to the flow direction of the electrolyte.

Furthermore, as shown in FIG. 5, capillary 24 can also be accommodated in the pressure loss element 9.1, 9.2, 9.3, 9.4. Here the pressure loss in the pressure loss element 9.1, 9.2, 9.3, 9.4 is primarily generated by frictional forces. In addition to the openings 23 or the capillaries 24 in the pressure loss element 9.1, 9.2, 9.3, 9.4, tissue or foam structures as well as fillers or structured packings are also suitable as pressure loss elements 9.1, 9.2, 9.3, 9.4.

example

A plate electrolysis cell has a cross-section of 5 mm x 500 mm. A distributor of 20 x 20 x 500 nm is provided for the distribution of the electrolyte. The volume flow of the electrolyte is 720 17h with an electrolyte density of 1000 kg / m ³ . The flow should be evened out by a pressure loss element with holes. The maximum deviation from the mean flow velocity should be 5%. The mis-distribution is said to be determined by inertia.

A maximum flow velocity v of results from the volume flow and cross-section of the distribution channel

. = ^y . ™ lL_ _{= V} , A 20-20mm ²

This results in a dynamic pressure of py _n = 0.5 'p ^• v ² = 1.25 mbar

with an electrolyte density p of 1000 kg / m ^3. '

Equation (1) then results in a required pressure loss across the pressure loss elements of 12.2 mbar for the desired deviation of 5%. Such a pressure loss only takes into account the given pressure loss coefficient at a flow velocity in the opening V _Ö of

v _ö = 24P, DV m: 1.626 P

with a pressure loss coefficient ζ = 1.5 of the openings.

Taking into account the volume flow of 720 1 / h, a necessary maximum total flow cross section A _Q of A _Q = - = 123mm ² . ^v o

For boreholes with a diameter of 3 mm, this corresponds to 17.4 boreholes. A pressure loss element with 17 holes should therefore be selected.

LIST OF REFERENCE NUMBERS

1 electrolytic cell

2 anolyte space

3 catholyte space

4 anode

5 cathode

6 membrane

7 Direction of flow of the anolyte

8 Direction of flow of the catholyte

9.1, 9.2, 9.3, 9.4 pressure loss element

10 anolyte distributors

11 catholyte distributors

12 anolyte collectors

13 catholyte collectors

14 wall

15 central inlet

16 central drain

17 side inlet

18 side drain

19 cell stacks

20 flow direction in the distributor 11

21 calming section

22 Flow direction of the calming section 21

23 opening

24 capillaries

Claims

claims

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

2. The method according to claim 1, characterized in that the maximum deviation from the average flow rate is obtained by building up an additional pressure loss.

3. The method according to claim 2, characterized in that the additional pressure loss is 1 to 10 times the pressure difference in the inlet area of the electrolyte space, calculated according to one of the following equations:

if the inflow into the inlet area of the electrolyte space takes place in such a way that the volume flow entering is approximately evenly divided into two partial flows with opposite main flow direction in the inlet area, or

if the inlet is not evenly divided into two partial flows with opposite main flow direction in the inlet area, where

Pdy _n = dynamic pressure in the inlet area, Δp _v = friction pressure loss in the inlet area, A = maximum deviation from the mean flow velocity, where 0 is no deviation and 1 is a deviation of 100%, Δpov = additional pressure loss and Δp _E = total pressure loss in the electrolyte space mean.

Method according to one of claims 1 to 3, characterized in that the additional pressure loss is generated by pressure loss elements in the entry and / or exit area of the electrolyte space.

Method according to one of claims 1 to 4, characterized in that the additional pressure loss is generated by reducing the flow cross-section.

6. Electrolysis cell with at least two electrolyte spaces, in each of which at least one electrode is arranged and each has an entry and an exit area, and at least one electrolyte space is an anolyte space and one electrolyte space is a catholyte space and an anolyte space and a catholyte space are adjacent and are separated from one another by at least one membrane, characterized in that the flow cross section is reduced in the inlet and / or outlet region in such a way that an additional pressure loss is generated.

7. Electrolytic cell according to claim 6, characterized in that the additional pressure loss is generated by the installation of at least one pressure loss element.

8. Electrolytic cell according to claim 7, characterized in that the at least one pressure loss element has a porous structure or is a perforated plate or is a plate with channels accommodated therein.

9. Electrolysis cell according to claim 7, characterized in that the at least one pressure loss element is designed as a fabric, foam structure or plate with capillaries accommodated therein.

10. Electrolytic cell according to claim 7, characterized in that fillers or structured packings are used as the pressure loss element.

11. Electrolysis cell according to one of claims 7 to 10, characterized in that the at least one pressure loss element is an electrode.

12. Electrolytic cell according to one of claims 6 to 11, characterized in that the electrode has a porous structure.

13. Electrolysis cell according to one of claims 6 to 12, characterized in that the inlet area is aligned parallel to the flow direction of the electrolyte space.

14. Electrolysis cell according to one of claims 6 to 13, characterized in that the outlet area is aligned parallel to the outflow side of the electrolyte space.