Note: Descriptions are shown in the official language in which they were submitted.
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ELASTIC CURRENT DISTRIBUTOR FOR PERCOLATING CELLS.
DESCRIPTION OF THE INVENTION
Technical Field
The invention relates to a cell for industrial electrolytic processes, and in
particular to
a cell comprising an anodic compartment and a cathodic compartment separated
by
an ion-exchange membrane, wherein one or both compartments are equipped with
gas-diffusion electrodes and the process electrolyte flows across a percolator
or
equivalent porous element.
Backoround of the Invention
In the following description, reference will be made to a cell suitable for
depolarised
chlor-alkali electrolysis, that is to the process of alkali chloride brine
electrolysis
wherein the hydrogen evolution cathodic reaction is inhibited in favour of the
reaction
of oxygen consumption on a gas-diffusion cathode, for instance as disclosed in
EP
1033419; the invention is nevertheless not limited to chlor-alkali cells,
being
applicable to any industrial electrochemical process making use of gas-
diffusion
electrodes.
There are known in the art depolarised chlor-alkali cells of particularly
advanced type
wherein the process electrolyte flows across a suitable porous planar element
or
percolator under the action of gravity: a cell of such kind is for instance
disclosed in
WO/0157290. In this kind of cell, there are typically present an anodic
compartment
obtained from a titanium shell, fed with an alkali chloride concentrated brine
and
containing a titanium anode provided with a catalytic coating for chlorine
evolution,
and a cathodic compartment delimited by a nickel cathodic shell; the two
compartments are separated by a cation-exchange membrane. The caustic soda
produced in the process flows by gravity across a porous element inserted in
the
cathodic compartment contacting on one side the ion-exchange membrane, on the
other side a gas-diffusion cathode. In other words, while the anode is a stiff
metallic
element which is electrically and mechanically connected to the anodic shell
by
means of a suitable metal structure selected among those known in the art, for
instance an array of ribs, the cathode is a thin porous element obtained from
a silver
net, a carbon cloth or other type of non self-standing equivalent structure.
For this
reason, the current transmission from the back-wall of the cathodic shell to
the gas-
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diffusion electrode must be effected by means of a structure providing a more
delocalised contact and capable of mechanically supporting the electrode. In
order to
improve the electrochemical features, it is also necessary that the cathode be
pushed
against the percolator with a certain pressure, indicatively 0.1 to 0.5
kg/cm2, so as to
allow the electrical continuity while contributing to the confinement of the
circulating
liquid electrolyte. To satisfy all of the above conditions, the cells of the
prior art are
provided with an electric current feed system relying on two distinct
elements: firstly,
a rigid current collector integral to the cathodic shell which may for
instance consist of
a rib array, as in the anodic side; secondly, a metal mattress positioned
between the
rigid current collector and the gas-diffusion electrode, which is capable, in
conditions
of suitable compression, to transmit a sufficient pressure to the gas-
diffusion
electrode thereby ensuring the required electrical continuity. An equivalent
solution is
applied for the retrofitting of chlor-alkali cells of the traditional type, to
adapt the same
to a percolation-type depolarised process, for instance as illustrated in
figure 2 of WO
03/102271: in this case, the original cell cathode, which is a metallic
electrode for
hydrogen evolution made of nickel or steel, as known in the art, takes the
role of the
current collector, while a nickel mattress (elastic current collector) acts as
the
intermediate element for current transmission between the rigid current
collector and
the gas-diffusion electrode.
The above indicated solution entails however a few inconveniences hampering
the
commercialisation of this type of cells: the two-component type current
transmission
system involves in fact excessive costs and thicknesses, difficulties of
installation and
of dimensional control of the mattress (especially in the peripheral zone),
difficulty of
controlling the deformations and the elastic forces, besides of course adding
a
contact interface not particularly favourable in terms of ohmic drop, such as
the one
between mattress and gas-diffusion electrode.
Summary of the Invention
It is one object of the present invention to provide an electrolytic cell
separated by an
ion-exchange membrane and equipped with gas-diffusion electrode and percolator
element for electrolyte circulation overcoming the limitations of the prior
art.
Under another aspect, it is one object of the present invention to provide an
improved
electric current feed system for an electrolytic cell provided with gas-
diffusion
electrode and percolator.
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The invention consists of an electrolysis cell with an anodic compartment and
a cathodic
compartment separated by an ion-exchange membrane, wherein at least one of the
two
compartments is equipped with a gas-diffusion electrode having two major
surfaces, a
first major surface facing the membrane being in contact with a percolator
traversed by
an electrolyte flow, and a second major surface, opposed to the first major
surface,
being in contact with a current distributor comprising a multiplicity of
elastic conductive
protrusions suitable for compressing the gas-diffusion electrode against the
percolator.
As percolator it is intended any porous planar element suitable for being
traversed by
gravity by a liquid flow, as disclosed in WO/0157290. In one preferred
embodiment, the
current distributor, which replaces the rigid current collector-elastic
current collector
assembly of the prior art, is obtained by cutting and shaping of a single
metal sheet, for
instance a nickel sheet in the case of a cathodic collector for chlor-alkali
cells. In this
case, the nickel sheet is a sheet of thickness typically comprised between 0.5
and 1.5
mm, preferably provided with a coating suitable for reducing the contact
resistance. The
nickel material of the sheet may be variously alloyed and for instance
selected from the
assortment of commonly available products; the choice of a nickel material of
grade and
mechanical characteristics suitable for the manufacturing of springs, for
instance with
superior elastic features, will prove particularly advantageous. In one
particularly simple
and effective embodiment, the conductive protrusions capable of imparting a
sufficient
pressure to the electrode are spring tags arranged in couples so that two
adjacent
spring tags protrude in opposite direction from the major plane of the metal
sheet from
which they are obtained. In this way a more effective and homogeneous support
of the
whole electrode surface is obtained. The above indicated solution is suited to
an
optimum cell design in almost every process condition; nevertheless, the use
of the
mattress according to the prior art as a contact element at high current
density has the
advantage of allowing an effective gas circulation (for the case of
depolarised chlor-
alkali electrolysis for example, an effective supply of oxygen to the gas-
diffusion
electrode) which could fall short with a simple lamellar structure. In this
case, a
particularly preferred embodiment provides the conductive protrusions to be in
form of
individual tiles, in their turn comprising one or more spring tags for
providing the
electrical contact but also one or more openings to favour the gas passage.
The
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conductive protrusions may for instance be disposed in parallel rows
distributed along
the whole electrode surface.
The current distributor in accordance with the invention is suitable for
achieving an
efficient electrical contact directly on the gas-diffusion electrode surface,
at a pressure
preferably comprised between 0.1 and 0.5 kg/cm2, thereby getting rid of a
contact
interface with respect to the system of the prior art in which a rigid current
collector is
coupled to an elastic current collector; on the other hand, in one embodiment
of the
invention an additional element for distributing the mechanical compression
force may
be inserted between current distributor and gas-diffusion electrode, for
example
consisting of a thin mesh, or of an expanded or punched sheet. In such case
the
number of contact interfaces is equivalent to that of the prior art,
nevertheless the
corresponding resistance is substantially lower than what would be obtained
with the
scarcely elastic mattress of the prior art directly in contact with a gas-
diffusion electrode.
Moreover, as it will be easily appreciated by one skilled in the art, the
overall thickness
of the cell is substantially lower.
Brief Description of the Drawings
The invention will be described more in detail with the aid of the attached
drawings,
which have a merely exemplifying purpose and are not intended to limit the
invention.
- Figure 1 represents a percolation type depolarised chlor-alkali cell
according to the
prior art.
- Figure 2 represents a percolation type depolarised chlor-alkali cell
according to the
present invention.
- Figure 3 represents a first embodiment of the current distributor according
to the
invention.
- Figure 4 represents a second embodiment of the current distributor
according to the
invention.
- Figure 5 represents a third embodiment of the current distributor according
to the
invention.
Detailed Description of the Preferred Embodiments
In figure 1 it is shown a percolation type depolarised chlor-alkali cell
according to the
prior art, comprising one anodic and one cathodic compartment separated by an
ion-
exchange membrane (500). The cathodic compartment is delimited by a cathodic
back-
wall (101), in contact with an electric current feed system relying on two
distinct
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elements, a rigid current collector (201) integral thereto, and an elastic
current collector
(210) consisting of a mattress, for instance made of nickel. The cathode (301)
consists
of a porous gas-diffusion electrode fed with oxygen, contacting on one side
the mattress
(210), on the other side a percolator (400) consisting of a planar porous
element
traversed by the electrolyte flow under the action of gravity. The ion-
exchange
membrane (500) acting as the separator has a cathodic surface in contact with
the
percolator (400) and an anodic surface facing an anode (302) which may be in
contact
therewith or kept at a small predetermined distance. The anode (302) is
normally
comprised of a titanium substrate consisting of a mesh or of an expanded or
punched
sheet, or optionally of a juxtaposition of two such elements; the anodic
substrate is
provided with a catalytic coating for chlorine evolution as known in the art.
The electrical
continuity between anode (302) and anodic compartment back-wall (102) is
ensured by
a rigid current collector (202). The cathodic (201) and anodic (202) rigid
current
collectors may consist of rib arrays, undulated sheets, sheets provided with
suitably
spaced gophers or other types of current collectors as known by those skilled
in the art.
In figure 2 it is shown a percolation type depolarised chlor-alkali according
to the
present invention, wherein the elements in common with the cell of figure 1
are
indicated by the same reference numerals.
The electric current feed system consists of a multiplicity of conductive
protrusions
(220), for instance an assembly of springs or elastic spring tags suitable for
compressing the gas-diffusion electrode (301) against the percolator (400);
between the
assembly of conductive protrusions (220) and the gas-diffusion electrode (301)
an
optional element for distributing the mechanical compression force (230) is
inserted, for
instance a thin mesh, or an expanded or punched sheet.
Figure 3 shows one embodiment of the multiplicity of conductive protrusions
obtained
from a single metal sheet and consisting in this case of an assembly of
elastic spring
tags (221) disposed in parallel according to a comb-like geometry: the spring
tags are
arranged in couples, so that each two spring tags protrude in opposite
directions from
the major plane of the original metal sheet. Depending on the cell size, a
single row of
spring tags (221) may cover the whole active surface, or more rows may be
arranged
side by side, as will be evident to one skilled in the art.
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Figure 4 shows a preferred embodiment of the multiplicity of conductive
protrusions
obtained from a single metal sheet: in this case the protrusions are
preferably
quadrangular individual tiles (222) obtained by cutting and shaping of a
sheet, optionally
welded directly to the rigid current collector (201), each of them comprising
elements
performing different functions: for example, by means of a suitable folding
step, each tile
is provided with edges with a curvature angle of about 900 (223) in order to
impart the
required stiffness. A multiplicity of suitably spaced apart spring tags (224)
acts as the
contact element with the gas-diffusion electrode (301), and a multiplicity of
holes (225)
favours the gas supply and circulation, in this case with particular reference
to the
oxygen required for the cathodic reaction. The various tiles welded to the
rigid current
collector (201) are preferably arranged on optionally off-set parallel rows.
Figure 5 shows a variation of the preferred embodiment shown in figure 4 of
the
multiplicity of conductive protrusions obtained from a single metal sheet: in
this case the
original metal sheet is a punched sheet, and the multiplicity of holes (225')
extends on
the whole body of the tile (222), including the spring tags (224). In this way
an enhanced
gas supply is obtained, also effective when the spring tags (224) are
compressed until
the end of stroke, coming in contact with the sheet from whence they are
projected. An
albeit marginal saving in the manufacturing phase is also obtained, consisting
of the
independent execution of holes (225) indicated on tile (222) of figure 4. The
tile
configuration also presents a further mechanical advantage: in case of a
sudden high
cathode counterpressure (for instance due to errors in the control of process
conditions,
or to element handling and assembling mistakes), the spring tags do not
undergo a
permanent deformation in view of the abutment of the GDE on the whole tile
surface. In
this case, the fact that the tiles are obtained from a punched sheet is even
more
important to guarantee the correct gas supply in any case, as is it evident to
one skilled
in the art.
EXAMPLE 1
A lab experimental electrolysis cell of 0.16 m2 active area was equipped
according to
the scheme of figure 2 with a titanium DSA anode (302) provided with a
ruthenium and
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titanium oxide-based catalytic coating, a Nafion N982 ion-exchange membrane
(500)
commercialised by Dupont/USA, a nickel foam percolator, a gas-diffusion
electrode
consisting of a silver net activated with a silver-based catalyst.
The electric current feed system was comprised of a multiplicity of elastic
conductive
protrusions each consisting of a tile (222) as illustrated in figure 5,
obtained from a 1
mm thick nickel punched sheet.
The cell anodic compartment was fed with a circulating sodium chloride brine
having a
concentration of 210 g/I, at a current density of 4 kA/m2 and at a temperature
of 90 C.
The cathodic product consisted of 32% by weight caustic soda flowing downwards
across the percolator. In these conditions, after stabilising the process
conditions on the
plant for ten days, a cell voltage comprised between 2.00 and 2.05 V was
detected.
EXAMPLE 2
The test of example 1 was repeated in analogous conditions, making use of a
cell of the
prior art. The only substantial difference consisted therefore in the cathodic
current feed
system, comprising a rigid current collector structure consisting of a nickel
rib array
welded to the cathodic back-wall coupled to a commercial nickel mattress.
In the same process conditions of example 1, after ten days of stabilisation a
cell
voltage comprised between 2.10 and 2.15 V was detected.
The foregoing description is not intended to limit the invention, which may be
used
according to different embodiments without departing from the scopes thereof,
and
whose extent is univocally defined by the appended claims.
Throughout the description and claims of the present application, the term
"comprise"
and variations thereof such as "comprising" and "comprises" are not intended
to
exclude the presence of other elements or additives.
