Note: Descriptions are shown in the official language in which they were submitted.
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Electric circuit of an electrolyzer with bipolar electrodes and electrolysis
installation with bipolar electrodes
The invention relates to electrolyzers with bipolar electrodes, and
especially to the electric power supply of such electrolyzers.
The invention especially relates to an electric circuit for supplying
rectified
electric power to an electrolyzer with bipolar electrodes.
Electrolyzers with bipolar electrodes, supplied with DC power, are
commonly used in the electrochemical industry. Such electrolyzers are
commonly used to electrolyze aqueous solutions of sodium chloride, in order to
produce chlorine, aqueous sodium hydroxide solutions or aqueous solutions of
sodium chlorate.
Considering the high current densities employed in electrolyzers with
bipolar electrodes, rectified AC power is generally substituted for DC power.
Rectified AC power normally presents pulses of which the frequency and
amplitude depend on the rectifier used. Accordingly, the electromagnetic field
produced by the rectified AC power is likely to generate induced currents
which
may be relatively strong in certain industrial applications, particularly with
bipolar electrolyzers for the continuous production of chlorine and of aqueous
sodium hydroxide solutions.
It is also well known that under extreme conditions, high electromagnetic
fields can have detrimental effects on the human organism, especially those
produced by rectified AC power, because of the induced currents that they are
liable to generate. It is consequently important to take measures to protect
the
personnel in the neighbourhood of the industrial installations or to reduce
the
strength of the electromagnetic fields. Standards have moreover been set in
this
sense, requiring the limitation of the strength of electromagnetic fields in
industrial premises. Among these standards, European standard 89/391/EEC is
particularly stringent.
It is an object of the invention to provide an electric circuit of novel
design,
to supply high strength electric current to an industrial electrolyzer with
bipolar
electrodes.
It is a particular object of the invention to provide an electric circuit with
which the electromagnetic field in the neighbourhood of the electrolyzer is
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reduced to a sufficiently low value to meet the abovementioned European
standard.
It is even more particularly an object of the invention to reduce the strength
of the magnetic field on walkways installed along the side walls of
electrolyzers
with bipolar electrodes.
In consequence, the invention relates to an electric circuit of an
electrolyzer with bipolar electrodes, comprising at least one electric current
line
that is placed outside the electrolyzer, characterized in that the electric
current
line comprises at least one busbar which is placed below and/or above the
electrolyzer.
The invention relates more specifically to electrolyzers with substantially
vertical bipolar electrodes. Such electrolyzers are well known in the art,
where
they are widely used for the electrolysis of aqueous solutions of metal
halides,
particularly sodium chloride. These electrolyzers are generally formed from a
succession of metal frames each comprising a bipolar electrode, these frames
being juxtaposed as in a filter press (Moderne Chlor-alkali technology,
Volume 3, SCI, 1986, chapter 13 "Operating experience gained with the bipolar
Hoechst-Uhde membrane cell" ; Modem Chlor-alkali Technology, Volume 4,
SCI, 1990, chapter 20 "Hoechst-Uhde single element membrane electrolyzer :
concept-experiences-applications"). The frarnes usually have a square or
rectangular profile, so thathwhen they are juxtaposed, as in a filter press,
they
form an upper wall, a lower or bottom wall, and two side walls of the
electrolyzer. The electrolyzer is normally supplied with DC power or, more
generally, with rectified AC power. The DC or rectified AC power flows from a
tenninal of the DC source or of the rectifier, through the bipolar electrodes,
and
then retu.rns to the other terminal of the DC source or the rectifier, via an
electric
current line located outside the electrolyzer. According to the invention, the
said
return electric current line comprises at least one busbar that is placed
below or
above the electrolyzer. The choice of placing the busbar below or above the
electrolyzer is dictated by considerations related to the construction of the
electrolyzer and the method of assembling the bipolar electrodes. As a
variant,
the abovementioned electric current line may comprise a busbar placed below
the electrolyzer and another busbar placed above the electrolyzer. According
to
another variant, the electrolyzer may also comprise a plurality of busbars
below
the electrolyzer and/or a plurality of busbars above the electrolyzer. In
practice,
for considerations related to the assembly and maintenance of the
electrolyzer, it
is generally preferable for the abovementioned electric current line not to
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comprise a busbar above the electrolyzer.
It has been found, all other things remaining equal, that the electric circuit
according to the invention significantly reduces the electromagnetic field in
the
neighbourhood of the electrolyzer with bipolar electrodes, chiefly along its
side
walls, especially on walkways that are normally installed along the side walls
and are used by the operating and maintenance personnel. In the discussion
below, the expression "in the neighbourhood of the electrolyzer" means the
space along the side walls of the electrolyzer, where the walkways used by the
operating and maintenance personnel of the electrolyzer are normally
installed.
In the electric circuit according to the invention, the material of the busbar
is not a critical factor for the definition of the invention. It is generally
made
from copper, aluminium or aluminium alloy.
In the electric circuit according to the invention, the profile of the cross
section of the busbar is not a critical factor for the definition of the
invention. It
may, for example, be square, rectangular, circular or polygonal.
In a first and particular embodinlent of the electric circuit according to the
invention, the busbar has a rectangular profile and is oriented so that its
large
sides are substantially horizontal. It has been observed, all other things
remaining equal, that the selection of a rectangular section busbar, placed
horizontally below and/or above the electrolyzer, minimises the strength of
the
electromagnetic field in the neighbourhood of the electrolyzer. It.1as also
been
observed that the decrease in the electromagnetic field in the neighbourhood
of
the electrolyzer is greater if the ratio of the thickness to the width of the
busbar is
smaller. In practice, it is consequently preferable to use a metal flat for
the
busbar. As a variant, a plurality of metal flats can be used, placed side by
side
below and/or above the electrolyzer.
It has further been observed, all other things remaining equal, that the
strength of the electromagnetic field in the neighbourhood of the electrolyzer
decreases as the busbar is brought closer to the wall of the electrolyzer.
In consequence, in a second embodiment of the electric circuit according to
the invention, the busbar is placed immediately next to a wall of the
electrolyzer.
In this embodiment of the invention, the said wall of the electrolyzer is the
lower
or bottom wall of the electrolyzer or its upper wall, depending on whether the
busbar is positioned below or above the electrolyzer. In this embodiment of
the
invention, the expression "immediately next to the wall of the electrolyzer"
means that the distance between this wall and the busbar is not more than five
times (preferably three times) the thickness of the busbar. Preferably, this
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distance does not exceed the thickness of the busbar.
In a preferred variant of the abovementioned second embodiment of the
invention, the busbar is attached to the said wall of the electrolyzer. In
this
preferred variant of the invention, the busbar is advantageously a metal flat
of
which one of the large sides is attached to the said wall, separated from the
wall
only by the thickness of the necessary electrical insulation. The metal flat
may
be attached to a portion of the surface of the said wall. It is preferable for
the
metal flat to be attached to substantially the entire surface of the said
wall.
In a third particular embodiment of the invention, the abovementioned
electric line further comprises two additional busbars, that are placed
respectively immediately next to two side walls of the electrolyzer. In this
embodiment of the invention, the expression "immediately next to" is identical
to
the definition of this expression given in the second embodiment discussed
above. All other things remaining equal, the presence of the additional
busbars
reduces the strength of the electromagnetic field in the neighbourhood of the
electrolyzer.
In this third embodiment according to the invention, the additional busbars
may have any shape compatible with the construction of the electrolyzer. They
may, for example, have a square, rectangular, polygonal, oval or circular
profile.
The additional busbars may also have the same profile or different profiles
and
they may have the same dimensions or different dimensions. In practice,
however, it is preferable that the additional busbars have the same profile
and the
same dimensions. It is also preferable for the additional busbars to have a
rectangular profile and that they be attached by their large side respectively
to
the two side walls of the electrolyzer.
In the third embodiment of the invention described above, the respective
dimensions of the additional busbars and those of the or each busbar that is
placed below and/or above the electrolyzer are determined according to the way
in which the electric current is to be distributed to all these busbars. In
practice,
it is recommended that the strength of the electric current in the busbar
positioned below and/or above the electrolyzer differ by no more than 30%
(preferably no more than 20%) from the strength of the electric current in
each of
the additional busbars. It is preferable for the strength of the electric
current to
be substantially identical in the busbar that is positioned below and/or above
the
electrolyzer and in each of the additional busbars.
In a fourth embodiment of the invention, which is especially advantageous,
the return electric current line of the electric circuit is positioned so as
to
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generate an electromagnetic field that is substantially symmetrical about the
median vertical plane of the electrolyzer. In this embodiment, the aim (to
generate an electromagnetic field that is substantially symmetrical about the
median vertical plane of the electrolyzer) is achieved by adequately
dimensioning and positioning the or each busbar. The choice of the optimal
dimensions and the optimal position is determined by a person skilled in the
art,
particularly according to the shape and dimensions of the electrolyzer. In
practice, this result can generally be achieved by placing the busbar or the
busbars symmetrically about the median vertical plane of the electrolyzer.
The electric circuit according to the invention significantly reduces the
electromagnetic field in the neighbourhood of the electrolyzer with bipolar
electrodes.
In consequence, the invention further relates to the use of the electric
circuit according to the invention, to reduce the electromagnetic field in the
neighbourhood of the electrolyzer.
The electric circuit according to the invention applies specially to
electrolyzers for the continuous electrolysis of water or of aqueous solutions
such as aqueous solutions of alkali metal halides, especially of sodium
chloride.
In consequence, in a preferred embodiment of the invention, the electrolyzer
comprises a line for the continuous intake of an aqueous electrolyte and a
line for
the continuous removal of an aqueous electrolyte.
The, invention applies in particular to electrolyzers for the production of
sodium chlorate by the electrolysis of aqueous solutions of sodium chloride.
The
invention applies especially to electrolyzers for the production of chlorine
and of
aqueous sodium hydroxide solutions, by the electrolysis of aqueous solutions
of
sodium chloride, these electrolyzers comprising membranes that are selectively
permeable to cations, and which are inserted between the bipolar electrodes.
The electric circuit according to the invention applies to any electrolysis
installation incorporating at least one electrolyzer with vertical bipolar
electrodes.
In consequence, the invention further relates to an electrolysis installation
comprising at least one electrolyzer with bipolar electrodes, connected to an
electric circuit according to the invention. The installation according to the
invention may comprise a single electrolyzer or a plurality of electrolyzers
connected in electrical series.
The invention relates in particular to the use of this installation for the
production of chlorine and of aqueous sodium hydroxide solutions.
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Particular features and details of the invention will appear from the
following description of the figures appended hereto, which show a number of
particular embodiments of the invention.
Figure 1 shows an overall plan view of an electrolysis installation prior to
the invention.
Figure 2 shows a schematic longitudinal elevation view of a particular
embodiment of the electrolysis installation according to the invention.
Figure 3 shows a vertical cross section on the plane III-III of Figure 2.
Figure 4 is a view similar to that of Figure 3, of another embodiment of the
installation according to the invention.
Figure 5 is a preferred variant of the installation in Figure 4. In these
figures, the same reference numerals designate identical elements.
The electrolysis installation shown in Figure 1 is prior to the invention and
does not conform to it. It comprises three electrolyzers 1, 2 and 3 designed
for
the production of chlorine, hydrogen and sodium hydroxide by the electrolysis
of
an aqueous solution of sodium chloride. The electrolyzers 1, 2 and 3 are of
the
vertical bipolar electrode type. They are formed by the juxtaposition of
vertical
rectangular frames 4, each containing a vertical bipolar electrode (not
shown).
The frames 4 are juxtaposed as in a filter press. Membranes that are
selectively
permeable to cations are inserted between the frames 4 to form alternating
anode
and cathode chambers. The anode chambers of the electrolyzers 1, 2 and 3
communicate with a line (not shown) for the continuous intake of an aqueous
solution of sodium chloride. They also communicate with a manifold (not
shown) for the continuous removal of chlorine. The cathode chambers of the
electrolyzers 1, 2 and 3 communicate with two manifolds (not shown) that serve
respectively for the continuous extraction of hydrogen, on the one hand, and
of
an aqueous sodium hydroxide solutions, on the other.
The electrolyzers 1, 2 and 3 are coupled in electrical series with a
rectifier 5 of an AC power source, via an electric circuit comprising, on the
one
hand, busbars 6 inserted between the electrolyzers 1, 2 and 3 and, on the
other, a
return electric current line 7, placed outside the electrolyzers 1, 2 and 3.
The
electric circuit further comprises a bipolar switch 8.
In the electrolysis installation in Figure 1, the return electric current line
7
consists of a long busbar running along a longitudinal side wall of the
electrolyzers 1, 2 and 3.
In the electrolysis installation shown in Figure 1, each of the three
electrolyzers 1, 2, and 3 may, for example, comprise 30 to 40 elementary
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electrolysis cells and the electric power supply comprises, for example, a 520
V
DC rectifier, capable of delivering a current of between 8 and 20 kA.
Depending
on the surface area of the electrodes, this may result in an anodic current
density
of 2.5 to 6 kA/mz of anode area. However, these numerical values are purely
indicative and do not limit the scope of the invention and the claims that
follow.
When the bipolar switch is closed, rectified electric current flows
successively in the electrolyzers 1, 2, and 3, through the bipolar electrodes
and in
the return line 7. This electric current generates an electromagnetic field in
the
environment of the installation.
The installation shown in Figures 2 and 3 conforms to the invention. In
these figures, only the electrolyzer 3 has been shown. In the installation in
Figures 2 and 3, the return electric current line 7 comprises two busbars 9
and 10
that are placed below the bottom wall 11 of the electrolyzer 3. The busbars 9
and 10 are prismatic bars of a metal that is a good electrical conductor
(preferably copper or aluminium). These busbars are placed symmetrically on
each side of the median vertical plane X-X of the electrolyzer. The busbars 9
and 10 are further placed in the neighbourhood of the bottom wall l l of the
electrolyzer 3. The layout of the busbars 9 and 10 in the way shown in Figure
3
has the effect of reducing the strength of the electromagnetic field on the
walkways 12 running along the side walls 13 of the electrolyzer 3 and which
are
intended for the electrolyzer maintenance personnel.
All other things remaining equal, it has been found that the strength of the
electromagnetic field on the walkways 12 is reduced more if the busbars 9 and
10 are closer to the mid-plane X-X and to the bottom wall 11. It has also been
observed that the strength of the electromagnetic field on the walkways 12 is
reduced by decreasing the ratio of the thickness to the width of the busbars 9
and
10. It is accordingly preferable to use flats or horizontal strips for the
busbars 9
and 10.
In the embodiment shown in Figure 4, the return electric current line 7
comprises a metal flat or strip 14 that is attached to the bottom wall 11 of
the
electrolyzer and that substantially covers this entire wall.
In the installation in Figure 5, the electric current line 7 comprises a metal
flat 14 that is applied against the bottom wall 11 of the electrolyzer 3 and
two
additional busbars 15 and 16 positioned respectively along the two side walls
13
of the electrolyzer 3. The two additional busbars 15 and 16 are advantageously
metal flats or strips that are attached to the side walls 13.
