U.S. patent application number 11/587056 was filed with the patent office on 2007-09-27 for method for producing a uniform cross-flow of an electrolyte chamber of an electrolysis cell.
This patent application is currently assigned to BASF AKTIENGESELLSCHAFT. Invention is credited to Harald Bohnke, Torsten Mattke, Hermann Putter.
Application Number | 20070221496 11/587056 |
Document ID | / |
Family ID | 35124701 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070221496 |
Kind Code |
A1 |
Bohnke; Harald ; et
al. |
September 27, 2007 |
Method for Producing a Uniform Cross-Flow of an Electrolyte Chamber
of an Electrolysis Cell
Abstract
The invention relates to 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. The invention also relates
to an electrolysis cell with at least two electrolyte spaces, in
each of which at least one electrode is arranged and each of which
has an inlet region and an outlet region, the flow cross section
being reduced in the inlet and/or outlet region so as to produce an
additional pressure reduction
Inventors: |
Bohnke; Harald; (Speyer,
DE) ; Putter; Hermann; (Neustadt, DE) ;
Mattke; Torsten; (Freinsheim, DE) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
BASF AKTIENGESELLSCHAFT
Ludwigshafen
DE
67056
|
Family ID: |
35124701 |
Appl. No.: |
11/587056 |
Filed: |
April 18, 2005 |
PCT Filed: |
April 18, 2005 |
PCT NO: |
PCT/EP05/04074 |
371 Date: |
October 20, 2006 |
Current U.S.
Class: |
204/242 ;
205/799 |
Current CPC
Class: |
C02F 2201/4611 20130101;
C02F 2201/4618 20130101; C25B 15/08 20130101; C02F 2001/46161
20130101; C02F 2201/46115 20130101; C25B 15/02 20130101; C02F
1/46109 20130101; C25B 9/19 20210101 |
Class at
Publication: |
204/242 ;
205/799 |
International
Class: |
C25B 9/00 20060101
C25B009/00; C25B 15/02 20060101 C25B015/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 22, 2004 |
DE |
10 2004 019 671.0 |
Claims
1-14. (canceled)
15: 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, wherein the maximum deviation from the average
flow rate is achieved by setting up an additional pressure
reduction, wherein the additional pressure reduction is from 1 to
10 times the pressure difference in the inlet region of the
electrolyte space, calculated according to one of the following
equations: .DELTA. .times. .times. p DV = p dyn + .DELTA. .times.
.times. p V ( A + 1 ) 2 - 1 - .DELTA. .times. .times. p E , ( 1 )
##EQU6## 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, or .DELTA. .times. .times. p
DV = p dyn + .DELTA. .times. .times. p V ( A + 1 ) 2 - 1 - .DELTA.
.times. .times. p E , ( 2 ) ##EQU7## when the feed is not
distributed uniformly into two sub-flows with opposite principal
flow directions in the inlet region, in which p.sub.dyn=dynamic
pressure in the inlet region, .DELTA.p.sub.v=frictional pressure
reduction in the inlet region, A=maximum deviation from the average
flow rate, 0 being no deviation and 1 being 100% deviation,
.DELTA.p.sub.DV=additional pressure reduction, and
.DELTA.p.sub.E=overall pressure reduction in the electrolyte
space.
16: The method according to claim 15, wherein the additional
pressure reduction is produced by pressure reducing elements in the
inlet or outlet region or both the inlet and outlet region of the
electrolyte space.
17: The method according to claim 15, wherein the additional
pressure reduction is produced by reducing the flow cross
section.
18: An electrolysis cell with at least two electrolyte spaces, in
each of which at least one electrode is arranged and each of which
has an inlet region and an outlet region, with at least one
electrolyte space being an anolyte space and one electrolyte space
being a catholyte space, with an anolyte space and a catholyte
space respectively being adjacent and separated from each other by
at least one membrane, wherein the flow cross section is reduced in
the inlet or outlet region or both the inlet and outlet region so
as to produce an additional pressure reduction.
19: The electrolysis cell according to claim 18, wherein the
additional pressure reduction is produced by the incorporation of
at least one pressure reducing element.
20: The electrolysis cell according to claim 19, wherein the at
least one pressure reducing elements has a porous structure or is a
perforated metal sheet or a plate containing channels.
21: The electrolysis cell according to claim 19, wherein the at
least one pressure reducing element is designed as a fabric, foam
structure or plate containing capillaries.
22: The electrolysis cell according to claim 19, wherein fillers or
structured packing are used as the pressure reducing element.
23: The electrolysis cell according to claim 19, wherein the at
least one pressure reducing element is an electrode.
24: The electrolysis cell according to claim 18, wherein the
electrode has a porous structure.
25: The electrolysis cell according to claim 18, wherein the inlet
region is aligned parallel with the influx direction of the
electrolyte space.
26: The electrolysis cell according to claim 18, wherein the outlet
region is aligned parallel with the efflux side of the electrolyte
space.
Description
[0001] The invention relates to a method for producing a uniform
flow through an electrolyte space of an electrolysis cell, and to
an electrolysis cell.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] The oxidation of sugars to sugar acids is carried out in a
special stirred reactor equipped with anode grids.
[0007] Cathodes to which a ribbed structure is imparted to increase
the throughput are used for the reduction of phthalic acid to
dihydrophthalic acid.
[0008] The so-called Swiss roll cell has been developed for nickel
oxide-catalyzed reactions. Here, the anode and the cathode are
spirally wound.
[0009] Electrodes whose active surface area is larger than their
purely geometrical dimensions are often referred to as
three-dimensional electrodes.
[0010] Arrangements in which layers of materials with a large
surface area are precoated onto an electrode substrate are also
known.
[0011] Lamellar designs which are formed from strips of metallic
glasses, for example, are also known for organic and inorganic
electrolysis.
[0012] 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.
[0013] Electrodes in the form of a networked design, for example,
may be used for the production of sodium dithionite.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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): .DELTA. .times. .times. p
DV = p dyn + .DELTA. .times. .times. p V ( A + 1 ) 2 - 1 - .DELTA.
.times. .times. p E , ( 1 ) ##EQU1## 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).
[0024] When the feed is organized in a different way from the type
described above, the calculation is carried out according to
Equation (2): .DELTA. .times. .times. p DV = p dyn + .DELTA.
.times. .times. p V ( A + 1 ) 2 - 1 - .DELTA. .times. .times. p E ,
( 2 ) ##EQU2##
[0025] This applies, in particular, when the feed is organized
laterally to the electrolyte space with respect to the width of the
electrolyte space. [0026] Here: [0027] .DELTA.p.sub.DV=additional
pressure reduction, [0028] .DELTA.p.sub.V=frictional pressure
reduction in the inlet region, [0029] p.sub.dyn=dynamic pressure in
the inlet region, [0030] .DELTA.p.sub.E=overall pressure reduction
in the electrolyte space, and [0031] A=maximum deviation from the
average flow rate, 0 denoting no deviation, and 1 denoting 100%
deviation.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] In another embodiment, the pressure reducing elements are
designed as fabrics or as a foam structure or as a plate containing
capillaries.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] In another embodiment, in particular when fabrics or foam
structures are being used as the pressure reducing elements, these
are additional electrodes.
[0049] 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.
[0050] The invention will be described in more detail below with
reference to a drawing, in which:
[0051] FIG. 1 shows a section through an electrolysis cell,
[0052] FIG. 2 shows a section through a catholyte space of an
electrolysis cell,
[0053] FIG. 3 shows a section through a cell stack,
[0054] FIG. 4 shows a detail of a catholyte space having a
distributor and pressure reducing elements contained therein,
[0055] FIG. 5 shows a detail of a catholyte space having a
distributor and a pressure reducing element with capillaries.
[0056] FIG. 1 shows a section through an electrolysis cell.
[0057] 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.
[0058] The catholyte space 3 contains a cathode 5, which has a
porous structure and fills the entire catholyte space 3.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] FIG. 2 shows a section through a catholyte space of an
electrolysis cell. The catholyte space is rotated through
90.degree. here, compared with FIG. 1.
[0063] 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.
[0064] FIG. 3 shows a section through a cell stack.
[0065] 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.
[0066] 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.
[0067] 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.
[0068] 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.
[0069] FIG. 4 shows a detail of a catholyte space with distributor
and pressure reducing element.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] 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
[0078] A plate electrolysis cell has a through-flow cross section
of 5 mm.times.500 mm. A distributor measuring 20.times.20.times.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/m.sup.3. 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%.
[0079] The distribution error should be determined by inertia.
[0080] A maximum flow rate v of v = V A = 720 .times. .times. l
.times. / .times. h 20 .times. .times. 20 .times. .times. mm 2 =
0.5 .times. .times. m .times. / .times. s ##EQU3## is obtained from
the volume flow rate and the cross section of the distribution
channel.
[0081] This gives a dynamic pressure of
.rho..sub.dyn=0.5.rho.v.sup.2=1.02 mbar with an electrolyte density
.rho. of 1000 kg/m.sup.3.
[0082] 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.sub.O of v O .. = 2 .times. .DELTA. .times. .times. p
DV .zeta. .rho. = 1.626 .times. .times. m s ##EQU4## in the
opening, with a pressure reducing parameter .zeta.=1.5 for the
openings.
[0083] Taking the volume flow rate of 720 l/h into account, a
necessary maximum overall flow cross section A.sub.Q of A Q = V v O
.. = 123 .times. .times. mm 2 ##EQU5## is obtained.
[0084] 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
[0085] 1 electrolysis cell [0086] 2 anolyte space [0087] 3
catholyte space [0088] 4 anode [0089] 5 cathode [0090] 6 membrane
[0091] 7 flow direction of the anolyte [0092] 8 flow direction of
the catholyte [0093] 9.1, 9.2, 9.3, 9.4 pressure reducing element
[0094] 10 anolyte distributor [0095] 11 catholyte distributor
[0096] 12 anolyte collector [0097] 13 catholyte collector [0098] 14
wall [0099] 15 central feed [0100] 16 central discharge [0101] 17
lateral feed [0102] 18 lateral discharge [0103] 19 cell stack
[0104] 20 flow direction in the distributor 11 [0105] 21 settling
section [0106] 22 flow direction in the settling section 21 [0107]
23 opening [0108] 24 capillary
* * * * *