Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a cell for electrolysis
where gas is generated, which cell comprises separate anode
and cathode compartments separated by an ion-selective
5 membrane. The invention also relates to a method for making a -~
cell according to the invention, and to the use of the cell in
electrolysis. ;-
Many electrolytic processes, e.g. the production of
chlorine and alkali, are often carried out in cells having
separate anode and cathode compartments separated by one or
more ion-selective membranes. The life time of the membrane is
drastically shortened owing to mechanical stress and too high
a current density, which may appear locally depending on large
concentration gradients in the electrolyte. For the membrane
to function satisfactory, it must also be in permarlent contact
with liquid, which may be difficult to achieve in electrolytic -
processes with gas generation, especially with the very short
electrode spacings which are aimed at to minimise the consump-
tion of electric current.
Chlorine-alkali electrolysis is often performed in two-
chamber cells with vertical electrodes, in which the membrane
is pressed against the anode by a positive pressure in the
cathode compartment. Normally, the electrode extends in each
chamber as far up as the upper boundary surface of the cell,
and the gas-rich liquid produced during the electrolysis
leaves the cell through an outlet, whereupon the gas is
separated off. Gas bubbles may get stuck between one of the
electrodes, especially the anode, and the membrane, this
increasing the electrical resistance of the cell and, hence,
decreasing current efficiency. Another problem is the migra-
tion of hydroxide ions through the membrane into the anode
compartment, which give rise to undesired side reactions,
unless being quickly rinsed off.
To increase the electrolyte flow between the electrode
and the membrane and to promote-the removal of gas bubbles,
electrodes have been designed which are provided with differ-
ent types of channels on the surface, as described in EP-
A 415 896 and EP-A 533 237
The patent literature also discloses electrolytic cells
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with internal circulation, which are described e.g. in US
Patent 3,647,672, DE-A1 33 23 803 and EP-A2 383 243. None of
these publications is however concerned with the problem of
extending the life time of ion-selective membranes or enhanc-
5 ing the efficiency of cells where an ion-selective membrane is
pressed against one of the electrodes.
Electrolytic cells with internal circulation are also
disclosed in EP-A 99 693, EP-A 311 575, and in US patent
5,130,008.
The invention aims at solving the problem of providing
an efficient membrane cell for electrolysis where gas is
generated, while considering the life time of the cell
membrane. The invention also solves the problem of enhancing
the efficiency of an existing membrane cell by a simple
15 modification.
According to the invention, it has been found that both
the efficiency of the cell and the life time of the membrane
increases if the electrolyte is flowed at a high velocity at
the electrode surface. It has also been found possible to
20 prevent the membrane from being directly contacted with gas if
this is separated in a region adjacent the upper edge of the
electrode.
The invention relates to a cell for electrolysis where
gas is generated, which cell comprises separate anode and
25 cathode compartments separat~d by at least one preferably
substantially vertical ion-selective membrane. At least one of
the anode and cathode compartments of the cell is designed for
electrolysis where gas is generated and comprises at least one
substantially vertical electrode, the front side of which is
30 facing a membrane, and at least one partition, so as to form
at least two substantially vertical channels and a space of
larger cross-sectional area located above the partition, of
which a first channel for upward transport of gas-rich
electrolyte is defined by said partition and the electrode
35 while a second channel for downward transport of gas-deficient
electrolyte, as seen from the electrode, is located behind
said first channel, said partition being arranged such that
gas-rich electrolyte which is transported upwards in said
first channel i9 subjected to a venturi effect so that the
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main part of the gas is separated off in an area adjacent the
upper edge of the electrode and the main part of the electro-
lyte is recycled downwards through said second channel.
Suitably, the partition is arranged such that from about 50%
to about 98~, preferably from about 70% to about 95~, o~ the
electrolyte supplied to the cell chamber is recycled.
Generally, the partition should be placed such that the
separation of gas from the electrolyte takes place as high up
as possible. The upward transport of the gas-rich electrolyte
takes place by the gas which is generated during the electro-
lysis producing an upwardly directed pumping force Phy~aul which
depends on the height of the channel and on the density
difference between gas and liquid, this difference partly
depending on the flow velocity. This pumping force must exceed
the flow resistance PCha~e~ in the channel which depends on the
hydraulic diameter thereof, the friction factor of the
material, and the flow velocity. The level of separation is
determined by the static pressure difference according to the
formula:
P9tat = Phyd~aul Pcha:lnel
The static pressure difference is directly proportional to the
height to which the pumping force is able to transport the
liquid and where the separation starts as foaming. Preferably,
the foaminy area extends substantially as far up as the upper
edge of the electrode, which in most cells means the upper
boundary surface of the cell. As appears from the above, the
optimal location of the partition depends on several para-
meters, and so the proper position must be tried out in the
particular cases by varying the location of the partition both
vertically and laterally, and hence the height and cross-
section of the channel, so that separation takes place as high
up as possible and the desired recirculation is achieved.
The electrode suitably comprises through openings so as
to be permeable to electrolyte. All known electrodes intended
to be arranged vertically can be used. Examples of known
electrodes are perforated plate electrodes, electrodes of
expanded metal or fine-meshed netting, electrodes comprising
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longitudinal or transverse rods or slats, and as well as
electrodes comprising curved or straight lamellae, which may
be vertically or horizontally extended, punched from a common
metal sheet, for example louver electrodes. The electrode
surface may, but need not, comprise channels for promoting the
electrolyte flow on the side facing the membrane. These
electrode types are well known and described e.g. in the
above-mentioned EP-A 415 896 and EP-A 533 237, as well as in
GB Patent Specification 1,324,427.
According to the invention, the anode and cathode
compartments may also comprise one or more auxiliary elec-
trodes, for example in the form of expanded metal or fine-
meshed netting, fixed in and electrically connected to the
partition, optionally via spacer means so as to be disposed
adjacent the main electrode. This is especially advantageous
if the main electrode has a comparatively open structure, for
example is a double-sided louver electrode punched from a
metal sheet.
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The electrode may be manufactured from metals such as
Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Ag, Pt, Ta, Pb, Al or
alloys thereof. Ti or Ti alloys are preferred as anode
material, while Fe, Ni or alloys thereof are preferred as
cathode material. The electrode is suitably activated by a
surface coating of a suitable catalytic material, depending on
what reactions should be catalysed. Suitable catalytic
materials are metals, metal oxides or mixtures thereof from
group 8B in the Periodic table, i.e. Fe, Co, ~i, Ru, Rh, Pd,
Os, Ir or Pt, of which Ir and Ru are particularly preferred.
An optional auxiliary electrode preferably consists of the
same or similar materials as the electrode and is, like the
electrode, provided with a suitable catalytic surface coating.
The partition is preferably arranged substantially
parallel to the electrode and has preferably substantially the
same lateral extension. Further, the partition is suitably
made of an electrically conductive material and is
galvanically connected to the electrode, e.g. by being
connected to the same current source, this promoting the
current distribution in the cell~ An optional auxiliary
electrode may have its main current lead-in through the
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partition. All materials that can be used for the electrode
are also suitable for the partition. Preferably, use is made
of the same material for the partition as for the electrode,
for example titanium. Normally, the partition need not be
activated, although it is of course possible.
The invention can be applied to electrolytic cells where
the anode and the cathode are arranged in separate cell
chambers for anolyte and catholyte, respectively. These cell
chambers may be arranged beside each other and then be
separated only by an ion-selective membrane. A cell may also
comprise one or more chambers located between the anode and
cathode compartments and separated from these and from each
other by ion-selective membranes. Both the anode compartment
and the cathode compartment suitably comprise an inlet for
electrolyte provided in the bottom, and one or more outlets
for gas and non-recycled electrolyte provided in the top. The
cells are suitably arranged as separate units, for example in
an electrolyser of the filter press type. A cell chamber may
comprise one or more channels for ascending gas-rich electro-
lyte and one or more channels for descending gas-deficient
electrolyte.
According to the simplest embodiment, the cell chamber
accommodates an electrode and an partition such that the rear
channel is defined by the partition and the rear vessel wall
of the cell. A cell chamber may also comprise a double-sided
electrode, the two sides of which are facing a respective
membrane. In this case, two partitions are required which
together form a centrally located channel for descending gas-
deficient electrolyte and which define, together with a
respective electrode side, a respective channel for ascending
gas-rich electrolyte. Such a cell chamber is normally of
substantially symmetrical design.
A cell according to the invention may also comprise
means for conducting part of the recycled electrolyte in the
horizontal direction, for the purpose of further reducing the
concentration gradients. This can be achieved by means of
guide vanes disposed in the upper part of the cell or by
varying the level of the upper edge of the partition in the
cell.
According to another embodiment, the partition may
comprise means for localised admixture of gas-deficient
electrolyte to the channel located closest to the electrode
for ascending gas-rich electrolyte, which is advantageous for
large electrode surfaces, for example exceeding about 0.5 m2,
since the concentration gradients in the electrolyte whic~ are
detrimental to the membrane can then be reduced. In this
embodiment, the channel closest to the electrode may be
divided into sections at different levels, which are not in
contact with each other but open in a common channel located
behind for ascending gas-rich electrolyte. The partition may
also quite simply have a number of openings admitting gas-
deficient electrolyte to the channel closest to the electrode.
The upper edge of the partition, i.e. where gas separa-
tion takes place, may be straight or have recesses in the formof different geometric figures. The upper part of the parti-
tion may also comprise openings of different types.
The invention also relates to a method for making an
electrolytic cell as described above. The method comprises the
step of modifying an existing membrane cell having separate
anode and cathode compartments separated by at least one
preferably substantially vertical ion-selective membrane, at
least one of the anode and cathode compartments comprising at
least one substantially vertical electrode, the front side of
which is facing the membrane. This modification is performed
by providing at least one of the anode and cathode compart~
ments with at least one partition so as to form at least two
substantially vertical channels and a space of larger cross-
sectional area located above the partition, of which a first
channel for upward transport of gas-rich electrolyte is
defined by the partition and the electrode while a second
channel for downward transport of gas-deficient electrolyte,
as seen from the electrode, is located behind the first
channel, the partition being arranged such that gas-rich
electrolyte which is transported upwards in the first channel
is subjected to a venturi effect so that the main part of the
gas is separatecl off in an area adjacent the upper edge of the
electrode and the main part of the electrolyte is recycled
downwards through the second channel. If the electrode in the
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existing cell chamber i9 comparatively open, for example like
certain louver electrodes, it is advantageously made more
close by providing its rear side with an auxiliary electrode
of e.g. expanded metal or fine-meshed netting. All newly-added
parts, such as partitions, auxiliary electrodes and the like,
may first be assembled into a unit which is then mounted in
the anode and cathode compartments, respectively. In other
respects, reference is made to the description of the cell
according to the invention.
Finally, the invention relates to a method in electroly-
sis where gas is generated, using a cell according to the
invention.
Thanks to the invention, it has become possible to
considerably increase the flow velocity of the electrolyte
through a membrane cell at the same time as the generated gas
can be separated from the liquid within the cell but in an
area where the e~fect on the sensitive membrane is at a
minimum. The life time of the membrane is extended by reducing
the pH-gradient and the concentration gradient in the electro-
lyte and by raising the liquid level in the electrode gap. Bydisplacing the gas separation zone and shielding it from the
electrode gap, the formation of bubbles in the membrane
induced by uneven current distribution can be reduced, this
solving one of the most serious problems inherent in prior-art
membrane cells. A special advantage is that existing cells can
be easily modified so as to considerably increase their
efficiency and the life time of the membrane.
The invention can be applied to all electrolytic
processes which involve gas generation, usually one or more of
oxygen, hydrogen gas and chlorine gas, and which can be
carried out in membrane cells ha~ing separate anode and
cathode compartments. Conceivable processes are the electroly-
sis of sodium chloride in aqueous solution to chlorine,
hydrogen and alkali, or the electrolysis of sodium sulphate in
aqueous solution to oxygen, hydrogen, sulphuric acid and
alkali. In the last-mentioned case, use is generally made of
a cell where at least one cell chamber without electrodes i9
disposed between the anode and cathode compartments.
The invention is illustrated in the accompanying
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schematic drawings. It is not restricted there~o, but many
other embodiments are conceivable within the scope of the
appended claims.
Fig. 1 is a lateral section of a cell with a double-
sided electrode, while Fig. 2 is a front section along A-A,
and Fig. 3 is a top section along B-B. Fig. 4 is a lateral
section of a cell with a single-sided electrode. Figs 5-7 are
front sections of three other cell designs. Figs 8a-8f show
different designs of a partition. Fig. 9 is a top section
showing a detail of a cell according to another embodiment,
while Figs 10 and 11 show details in a lateral section along
B-B and A-A.
Referring now to Figs 1-3, there is illustrated a cell
chamber in the form of an anode compartment or a cathode
compartment with an inlet lo for electrolyte disposed in the
lower part of one short side, and an outlet 11 for gas and
non-recycled liquid disposed in the upper part of the other
short side. Further, the cell chamber is of substantially
symmetrical design and comprises two main electrodes la, lb of
the same polarity, which are preferably interconnected so as
to form together a double-sided electrode having a common
current lead-in. The electrodes la, lb have openings (not
shown) along the surface so as to be permeable to electrolyte.
The cell chamber is defined by two ion-selective membranes
12a, 12b arranged on the front side of and optionally in
contact with the respective electrode la, lb. The membranes
12a, 12b constitute boundaries to cell chambers (not shown)
with electrodes of opposite polarity, or to cell chambers
disposed between the anode and cathode compartments. Two
partitions 3a, 3b with the same lateral extension as the
e,lectrodes la, lb are arranged in the cell chamber so as to
define two outer channels 4a, 4b for ascending gas-rich
electrolyte, as well as an inner channel 5 for descending gas-
deficient electrolyte. All the channels 4a, 4b, 5 join in a
space 8 above the partitions 3a, 3b, and in a space 9 below
these, the spaces 8, 9 comprising substantially the entire
cross-sectional area of the cell chamber. Spacer members 7 of
substantially circular cross-section are arranged in the
channel 5 between the two partitions 3a, 3b and are preferably
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connected to the same current source as the main electrodes
la, lb. Auxiliary electrodes 2a, 2b, for example in the form
of a fine-meshed netting, are arranged adjacent the main
electrodes la, lb electrically connected to the spacer members
7 via the partitions 3a, 3b and permeable spacer members 6
which may consist e.g. of expanded metal. Preferably, the main
electrodes la, lb as well as the auxiliary electrodes 2a, 2b
are activated by a suitable catalytic surface layer. For
greater clarity, the width of the channels is greatly exagger-
ated in Figs 1 and 3.
Many existing electrolytic cells can be modified byassembling partitions 3a, 3b, spacer members 6, 7 and auxili-
ary electrodes into a unit which is then mounted in an anode
compartment or a cathode compartment to obtain a cell accord-
ing to the illustrated embodiment. All joints or connectionscan normally be achieved by any welding technique suitable for
the material, for example spot-welding.
When using the cell, for example in the electrolysis of
sodium chloride in aqueous solution to chlorine and alkali,
the illustrated cell chamber may advantageously function as
anode compartment, the two membranes 12a, 12b constituting
boundaries to cathode compartments which are located beside
the illustrated anode compartment and designed for internal
circulation according to the invention, or are of conventional
design. The anode compartment is supplied with sodium chloride
solution through the inlet 10, the main part of the electro-
lyte being transported upwards in the outer channels 4a, 4b
while a minor part passes between the anodes la, lb and the
membranes 12a, 12b. In the electrolysis, bubbles of chlorine
gas are formed whose density is lower than that of the liquid,
this giving rise to a pumping force causing the liquid to rise
in the channels 4a, 4b. When the gas-rich electrolyte reaches
the space 8, the cross-sectional area increases considerably
and a foaming area is formed. The chlorine gas is no longer
capable of lifting the liquid but is separated off and leaves
through the outlet 11. The major part of the liquid flows back
through the inner channel 5 to the space 9 where it is mixed
with freshly-supplied electrolyte and is again raised through
the outer channels 4a, 4b. The cell functions correspondingly
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in other electrolytic processes.
It has been found that a membrane cell for chlorine-
alkali electrolysis where the anode compartment is designed
according to the embodiment shown in Figs 1-3, operates
excellently if the anodes la, lb have a width of about 1000 mm
and a height of about 300 mm, the height of the areas 8, 9
above and below, respectively, the partitions 3a, 3b being
about 25 mm, the distance between the two partitions 3a, 3b
being about 3 mm and the distance between the partitions 3a,
3b and the respective anode la, lb being about 2 mm.
Fig. 4 shows a cell chamber with only one electrode 1
which is defined by a membrane 12 and a vessel wall 13. A
partition 3 is arranged so as to define a channel 4 for
ascending gas-rich electrolyte closest to the electrode 1 and
another channel 5 for descending gas-deficient electrolyte at
the vessel wall 13. Otherwise, it operates in the same way as
the cell chamber in Figs 1-3.
Fig. 5 shows a cell having an inlet 10, two outlets 11,
a partition 3, means 7 for supporting the partition 3, and a
space 8 above and a space 9 below the partition 3. The upper
edge of the partition 3 is highest in the middle, i.e.
straight above the inlet 10 of the cell, and lowers stepwise
towards the two outlets. As a result, the horizontal mixing in
the cell is promoted. For example, from about 5% to about 10%
of the electrolyte flow is conducted in the horizontal
direction from the inlet 10 towards the outlets 11. In a
special embodiment, the distance between the upper and lower
edges of the partition 3 may be the same throughout the entire
cell while the position of the wall 3 is varied such that the
height of the space 9 below the partition 3 is small where the
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height of the space ~ above the partition 3 is large. If the
electrode of the cell (not shown) is double-sided, an addi- ;
tional partition should be provided before the illustrated
partition 3, the means 7 serving as spacers between two -
partitions 3.
Fig. 6 shows a cell which differs from that of Fig. 5
only by the location of the inlet 10 and the outlet 11 and, as ~`~
a result thereof, in that the upper edge of the partition 3 is
highest at one end and lowest at the other end of the cell.
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Other features, like the operation, are identical with those
of the cell in Fig. 5, to the description of which reference
is thus made.
Fig. 7 shows a c~ll similar to that in Fig. 6, except
that the partition 3 has the same height along the entire
cell, but instead is provided with guide vanes 20 serving to
conduct part of the circulating electrolyte in the horizontal
direction towards the outlet 11.
Figs 8a-8f illustrate different designs of the upper
part of a partition 3. In Figs 8a-8c, the upper edge of the
partition 3 is formed with recesses of different shapes, while
the partitions 3 shown in Figs 8d-8f are provided with holes
of different shapes.
Figs 9-11 show an embodiment which i6 advantageous for
very large electrode surfaces. None of the Figures shows the
whole cell chamber, but only details thereof. The cell chamber
comprises an electrode 1 which is located just behind a
membrane (not shown), which together with a rear vessel wall
13 constitutes the boundary surfaces of the chamber. Adjacent
the electrode 1, along the entire front side of the cell
chamber, extends a channel 4a for ascending gas-rich electro-
lyte. The channel 4a is divided into sections at different
levels, which are not in direct contact with each other but
which in their upper parts 15 open into common collecting
channels 4c located behind for ascending gas-rich electrolyte.
The number of sections depends on the height of the electrode
and may, for example, be from about 5 to about 10 per metre.
Between the collecting channels 4c are provided channels 5 for
descending gas-deficient electrolyte. The channels 5 comprise
means 14 for supplying part of the electrolyte flow therein to
each section of the channel 4a closest to the electrode. The
boundaries between the channels 4a, 4c, 5 consist of parti-
tions 3a, 3b. The relative positions of the channels 4a, 4c,
5 appear most clearly from Fig. 9 showing a detail of the cell
chamber from above. The number of channels 4b, 4c, 5a, 5b
located behind the channel 4a closest to the electrode depends
on the width thereof and may, for example, be from about 10 to
about 30. The top and the bottom of the cell chamber comprises
corresponding spaces ~not shown) having a larger cross-
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sectional area as in the embodiments shown in Figs 1-4.
When using the cell chamber according to Figs 9~
recycled gas-deficient electrolyte will flow downwards through
the channels 5. Every time the bottom of a section in channel
4a closest to the electrode is passed, part of the gas-
deficient electrolyte will be supplied into said section. When
the gas-rich electrolyte in a section of said channel 4a
reaches the top thereof, the entire flow is fed out into one
of the common collecting channels 4c. When the collecting flow : ~:
reaches the top of the cell chamber, the generated gas is
separated and the main part of the liquid is recycled through
the inner channel 5. ~:
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