Note: Descriptions are shown in the official language in which they were submitted.
3S~
This invention relates to ~ process for the electro-
lytic production of an alkali metal hydrox1~e and more parti-
cularly it relates to a process ~or the electrolyt;c product;on
of alkali metal hydroxides which permits variation in the type
of anodic reaction products which are concurrently prorluced.
Alkaline materials, and particularly sodium hydroxide
and potassium hydroxide, are frequently produced by the electro-
lysis of aqueous alkali metal chloride solutions. In this process,
the alkali is formed at the cathode while ch10rine is formed at the
lO anode. Additionally, hydrogen is also forme~ at the cathode but
this material is generally considered to be a waste product.
Unfortunately, the commercial demands for chlorine and
alkali are generally not in the same proportions as the quantities
in which these chemicals are formed by this electrolytic decom-
; 15 position of the alkali metal chloride solutions. Thus, where
the demand for chlorine is less than the demand for alkali, some
other use must be found for the excess chlorine which is produced.
In some instances, it has been proposed to react this excess
chlorine with hydrogen to form hydrogen chloride gas which is
20 then absorbed in water to form hydrochloric acid. This technique, ~-
however, is not extremely attractive since the electrochemical
decomposition of the alkali chloride requires large quantities
of electrical energy whereas the energy recovery at the hydrogen-
chlorine reaction is expensive and, ~urther, involves corrosion
25 problems.
The present invention involves a process for electrolyzing
an aqueous solution of an alkali metal chloride to form an alkali
metal hydroxide at the cathode and chlorine at the anode by the
.
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oxidation of chloride ions in the anolyte. The electrolysis is
carried out in a cell having disposed therein a cathode and an
anode, said cathode and anode being separated by a fluid perme-
able diaphragm, preferably a cation permeable ;on exchange
membrane, to form an anode compartment containing said anode and
a cathode compartment containing said cathode. The anode includes
.
a catalyst for the electrochemical oxidation of hydrogen gas to
hydrogen ions, and the cell includes means for the introduct;on
of hydrogen gas into the anode compartment in contact with sa;d
10 catalyst. The anode is formed in two layers: a first layer being
a porous sintered coarse layer of platinized titanium, and a second
layer being a porous sintered finer layer of titanium. Additionally,
the anode is positioned so that hydrogen gas is passed initially
through said first layer and then through said second layer into
` 15 the anode compartment. By the process of this invention, it is
possible to adjust the production of chlorine and alkali so as to
meet the commercial requirement of these chemicals while minimi~ing
the consumption of electrical energy.
In the process of this invention, components may be
20 used in the electrolytic process which are known to those in the
art. In this connection, however, a new and improved anode is of
particular advantage. The negative electrode used may be of the
type which is used in vertical chlorine-alkali cells or, it may
be an air cathode, such as that used in fuel cells. The anode
25 and the cathode may be separated by permeable diaphragms, such as
asbestos diaphragms, as well as by semipermeable ion exchange
membranes9 such as those described in U.S. patent 3,262,868.
Additionally, the anodes used may be hydrogen anodes of the type
used in acid hydrogen air fuel cells. The llfe of anodes of this
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9~353
type is, however, quite short when they are used for the production
of chlorine in accordance with the method of the present invention.
To overcome this difficulty, it has b~en found that in
the cells of the present invention, the anode may be formed of the
same type of electrode materials which are presently used for
metallic anodes in chlorine-alkali cells. These electrode mate-
rials, for example, contain titanium metal activated ~ith various
; catalysts, such as platinum metal, ruthenium oxide, and the like.
These anodes must, however, as will be described in detail here-
10 inafter, be fabricated in a different way, as compared to the
known metallic anodes, in order to make it possible to carry out
the alternative electrolysis in accordance with the present method.
In this regard, it is to be noted that the anodes used in the
method of the present invention must be capable of working effi- ~ -
15 ciently both as a hydrogen anode and as a chlorine anode. In
this manner, it is possible, depending upon the particular market
situation to produce hydrogen chloride in the anolyte at a very
low cell voltage or, alternatively, to produce chlorine at the
anode. In the latter case, the cell will be operated as a con-
20 ventional chlorine-alkali cell~
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by interrupting the hydrogen supply to the anode and venting the
hydrogen which is developed at the cathode in the conventional manner.
While this conversion to chlorine production is, in principle, possible
with all type of hydrogen anodes, including carbon anodes, the
metallic anodes, such as titanium anodes have been found to be
particularly suitable for this purpose.
A par~icularly advantageous embodiment of the invention is
the use of an air-cathode ;n the cell. Such a cell may, when hydrogen
and air are ;ntroduced, be described as a hydrogen air cell with an
0 acid anolyte and an alkaline catholyte in which the alkali metal ions
becomes concentrated in the catholyte and the chloride ion is present
in the anolyte. In the alternative mode of operation, the cell may
be described as a chlorine-alkali cell with an air-cathode.
It is to be appreciated that the electrolysis cell used
in the method of the present invention may include many different
embodiments. Thus, there are many different types of primary cells
for chlorine-alkali electrolyses as well as many different types
of hydrogen electrodes for use in acid-water solutions. Typical of
the primary cells for chlorine-alkali electrolysis are those described
in the Kirk-Othmer "Encyclopedia of Chemical Technology", Second
Edition, Volume 1, pages 668-707. Similarly, typical hydrogen
` electrodes which may be used are those described in H. A. Liebhafsky
and E. J. Cairns: "Fuel Cells and Fuel Batteries", John Wiley & Sons,
Inc., New York 1968.
In the operation of the cells of the present invention, it
has frequently been found that, during the operation with hydrogen
supplied to the anode, the current density on the anode is a limiting
factor.
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Accordingly, it has been found to be advan-tayeous to use planar
electrodes which furnish a large surface area in a yiven cell
volume. Additionally, it has been found that the electrodes
should desirably have their current conducting surfaces vertically
oriented so as to avoid the development of gas pockets which
further hinder the electrolytic current. Thus, the preferred
cells of the present invention are those in the class of
vertical diaphragm cells, having mainly planar electrodes. A
particularly suitable cell configuration has been found to be
the filter press concept, which has heretofore been used in
units for both chlorine and -alkali electrolysis and in fuel
cell systems. In the drawings which are attached hereto and
form a part hereof, reference is made to cells of this type
with both monopolar electrodes and bipolar electrodes.
The invention is further illustrated in particular
and preferred embodiments by reference to the accomp~nying draw-
ings in which -
FIGURE 1 is a schematic representation of a cell of
the filter press type having a monopolar electrode;
FIGURE 2 is a schematic representation of a cell of
the filter press type employing two layer air cathodes
FIGURE 3 illustrates schematically a modification
of the cell of Figure 1 employing bipolar electrodes;
FIGURE 4 illustrates schematically a modification
of the cell of Figure 2 employing bipolar electrodes,
FIGURE 5 illustrates the anolyte of a cell according
to one embodiment of the invention, and
FIGURE 6 illustrates the catholyte of a cell accord-
ing to one embodiment of the invention,
Referring now to F'igure 1, this is a schematic
representation of a cell of the filter press type having a mono-
polar electrode. ~he cathodeq (1) and the anodes(2) where
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chlorine is developed or alternatively hydroyen ions are formed are
vertically arranyed. The cathode spaces (3) are separated from the
anode spaces (~) by means of diaphragms (5) wh;ch may also be cation
permeable membranes. The anode spaces contain an acid solution of
alkali chloride in water (the anolyte) which is supplied through the
pipes (~) and let out through the pipes (9). In a similar ~lay the
cathode spaces are supplied with a solution of the corresponding
alkali hydroxide and water (the catholyte) through the pipes (6).
The catholyte is let out through the pipes (7). The hydrogen in the ~,
10 catholyte is separated and conducted to a system for hydrogen supply
which is not shown in the figure. This system contains means for
control of pressure and flow of the hydrogen gas to the anode. The
catholyte is also supplied with water which is consumed in the
cathode reaction. The catholyte which is let out of the sys~em is
processed further, for instance by evaporation or the like.
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When the cell is operated on hydrogen the hydrogen yas is
supplied to the gas spaces (10) through the pipes (11) and let out
through the pipes (12). The chlorine which is developed at the anode
when the cell is used for chlorine production is let out through the
pipes (9) together with the anolyte, but it may also leave to the gas
space (10). The chlorine gas is separated by separating me~ns that are
not shown and is thereafter piped for further processing or storage.
The electrical connection takes place by means of the current conductors
(13) and (14).
The anolyte, which has lost part of its content of alkali
chloride in the electrolytic process is recycled to the anode spaces
after addition of alkali chloride in a special dissolver when the cell
is used for chlorine production. When the cell is operated on hydrogen
the anolyte may be processed further depending on the market situation.
15 It is sometimes advisable to produce hydrogen chloride and sodium sul- -
fate by reaction with sulfuric acid. Sodium chloride may also be
separated by evaporation and crystallization for recycle to the process.
In the description that follows below, for simplicity, only the operation
with hydrogen anodes is set forth.
It is frequently not possible to keep anolyte and catholyte
completely separated and then the catholyte gets a certain content of
alkali chloride. The alkali chloride may be removed in a known manner by
crystallization whereafter the alkali chloride is supplied to the dissolver
in the anolyte circuit. Water is consumed in the cathode reaction and
therefore water is supplied to the catholyte. The net quantity of alkali
hydroxide formed is recovered by means of a flow of a circulating
catholyte which is piped away for further processing for instance evapor-
ation to dryness.
53
The functional cell elements, that is cathodes, anodes and
separators, are arran~ed in electrically -isolat;ng frames (15) which
are pressed together to form a pile or a filter press like structure.
This pile can also contain elements for other functions than those
mentioned above, for instance cooiing or heating. Tightness between
the frames can be obtained by means of special gaskets of rubber or
the like. Supporting elements are frequently required to distribute
the pressure from the end-plates and to support the electrodes. The
frames can also be welded, glued or joined together in other ways where-
by special end-plates for the pile in certain cases can be dispensed
with. These frames may advantageously be made of a polymer material
whjch is resistant to the chemicals present. Penton (trademark), a
chlorinated polyether derived from 3,3-bis(chloromethyl) oxetane,
Te~lon (trademark), a tetrafluoroethylene fluorocarbon polymer, PVC,
Kel-F (trademark), a fluorocarbon polymer derived from chlorotrifluoro-
ethylene, polypropylene, chlorine and alkali resistant rubber are ex-
amples of such materials. The cross-section of the pile can be circular,
square, rectangular or have another shape. The pipes for the process
media can be completely or partly arranged as channels in the frames
with connections to each cell space by means of so called bi-channels
which end directly in the space around the pile. For obvious reasons,
`~ howeYer, in such cases the simultaneous out-Flow of process media is
limited to those which are inert towards each other. For an embodiment
of this kind it is apparently desirable and sometimes necessary (for
instance when there is a free out-let of chlorine) to arrange the pile
in a special container which in such a case must be equipped with the
necessary couplings, joints, conductors and other auxiliary functions.
Such a vessel may contain several piles arranged in seYeral decks.
~ - 7 -
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The electrodes rnay be joined in parallel in groups which
in their turn may be joined in series. The current conductors which
are necessary for this purpose and auxiliary electrical connections
are not shown in the figures. These current conducting elements
may be contained in special ducts in the frames or may be arranged
outside the pile.
The anodes (2) contain a porous sintered coarse layer
composed of for instance platinized titanium metal (16) on which
layer a finer layer is arranged which contains non-activated
titanium which exposes its surface to the electrolyte (17). The
coarse layer is partly filled with hydrogen during operation, the
hydrogen gas passing initially through said coarse layer and then
through said finer layer into the anolyte, where the hydrogen ions
formed thereby are reacted with chloride ions.
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The cell accordiny to Fiyure I rnay use a conventlonal catho~e,
made ~or instance of iron, where hydrogen is developed. It ls, however,
also possible to use ~wo layer air cathodes. This embodiment of the
invention is shown in Figure 2. The air space (18) is supplied with
air by the pipes (19), the excess air leaves through the pipe (20)~
The air cathodes (21) contain a coarser active layer (22) and a finer
protective layer (23), which layer thanks to the capillary forces
prevents air leakage into the catholyte spaces.
The separating means may also, particularly when both elect-
10 rodes are gas diffusion electrodes, be made of a porous body of asbestos,or of some other kind of compacted or sintered chemically resistant
material, which completely fills up the space between the anodes and
the cathodes. Special fine electrode layers may therefore be dispensed
with if the pores in the separating means in this particular embodiment
15 are-made so small that the gas pressures in the air and the hydrogen
spaces do not overcome the capillary forces in the separating means
filled up wtih electrolyte. In another embodiment, the diaphragm may
be arranged directly on the electrodes, as in Swedish patent 21~,659,
where the alkali hydroxide seeps out into the air space to be conducted
20 away at the bottom of this space. In this particular case it is of
advantage to supply the water which is consumed in the air cathode in
the form of water vapour in the supplied air.
The carbon dioxide present in the air which is carried to the
air spaces in the cell in Figure 2, can be removed for instance in an
alkaline scrubber. This is, however, not necessary when carbonate can
be tolerated in the product. The in-going air can with advantage be
preheated by heat exchange with out-going air. It may also be of
advantage in certain cases when cheap oxygen is available to increase
the oxygen content of the air by addition of oxygen or to substitute
an oxygen flow for the air flow. With certain types of cathodes this
will produce a considerable improvernent of electrode performance.
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Figures 3 and 4 show modifications with bipolar electrodes of
the cell concepts in Figure 1 and Figure 2. The rnain difference between
the embod;ments accord;ng to F-igure 1 and F;gure 3 is that the current
conductors (13) and (14) in Figure 1 are replaced with an electr;cally
conducting separating wall which connects electrically the active elec-
trode material in the anode and the cathode. This known principle For
bipolar electrodes can be used for instance by having the separating wall
made of a thin plate of titanium (23) which is activated with an iron
layer (1) on its catholyte side. The anodes and the cathodes are welded
together peripherically by means of for instance electron beam welding
to the element (24) wh;ch has the shape of a ring with ~langes. The
element (24) which can be formed by pressing a nickel plate to this shape
contains connections for air, (11) and (12), welded to the element.
Figure 4 shows a bipolar arrangement with an anode of the same
kind as in Figure 2. The gas spaces (10) and (18) are arranged between
the separating wall (25) and the porous anode (2) with the coarse layer
(16? and the fine layer (17) respectively the porous air cathode (21) with
` the layer (223 and the fine layer (23). The separating wall (25) contains
`~ flanges to whlch anode and cathode are welded according to the example in
Figure 3. The element is preferably fabricated in titanium.
Bipolar electrodes with the necessary connections for air, and
the like are stacked in the same manner as has been indicated above for
the monopolar electrodes to a pile which is kept together by end-plates
and bolts in the known manner. Electrically isolating auxiliary elements
(26~ are arranged between the electrode elements which together with the
electrode elements separate the electrolyte spaces. These auxiliary
elements are preferably equipped with the connections for anolyte, catho-
lyte, and the like.
The cathode material in Figure 1 and Figure 3 may be iron, e.g.
an iron plate. The air cathode for Figure 2 and 4 can be made of many
different materials such as actiYe carbon, n~ckel etc. according to the
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state of art. It may be hydrophobic with part1c1es of Teflon~D or o~her
hydrophobic materials present in the porous cathode structure. One type
of air cathode works with two porous layers, one fine layer exposed to
the electrolyte and one coarse active layer. The catalyst is present in
the coarse layer wh;ch when in operation ;s partly filled up with air.
Silver in a concentration of 1-10 mg silver/cm2 may serve as a catalyst.
The electrode can be made of a mixture of catalyst carbonyl-nickel powder
and particulate hydrophobic material, such as Teflon~, which are pressed
and sintered to the desired porosity, frequently about 50-70%, at a -
sintering temperature of say 800C. The fine layer which does not
contain catalyst and hydrophobic material is first put into the mold.
The powder for the coarser layer is then put on top of this 1ayer and
pressed for instance at 1 ton/cm2. The corrosion resistance can be
improved considerably in the known manner by developing a semi-conducting
lithiated nickel oxide on the cathode surfaces. These air cathodes
may also operate as conventional electrodes by interruption of the air
supply. It is then possible to fill up the air space with catholyte
or to permit the hydrogen gas formed to build up a suitable differential
pressure so as to keep the air spaces fille~ with hydrogen.
Porous titanium anodes of the two-layer type are made in a
completely analogous manner with methods which are known in the art of
making porous bodies of titan~um (Kirk-Othmer "Encyclopedia of Chemical
Technology", 2~d Edition, Volume 20, pages 347-379). It is also suitable
to use hydrophobic hydrogen electrodes of the kind used in fuel cells
operating with phosphoric acid as electrolyte at 100-150C. The hydrogen
electrode may also be a thin sheet of Pd-Ag alloy permeable to hydrogen.
In order to further improve the corrosion resistance o~ the
titanium structure, when the anode is operated with the addition of
` hydrogen, a protective layer of a tungsten bronze may be deposited on
- 30 the structure. The corrosion resistance of the titanium may also be
further improved by alloying with nickel or palladium.
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A very efficient separating means comprises a cation permeable
ion exchange membrane which contains a condensate of phenol sulfonic acid
and formaldehyde For instance according to the Swedish patent 156,526. As
was mentioned above a cation permeable membrane may be combined with an
anion permeable one and an intermediate brine space. It is also possible
to use diaphragms of the kind which are used in vertical electrolysis cells
according to the state o~ art. These separating means can be built up
directly on the electrodes for instance as has been described in the
Swedish patent 216,659.
In order to describe the present invention further, one embodi-
ment which is related to one of the cell concepts described in the Figures
1, 2, 3 and 4 will be described in somewhat more detail. The invention
is, however, not limited to the particular embodiments that are described
in the Figure 1-4 and therefore the following description does not indi-
15 cate limitations in the invention. The purpose is simply to exemplify
one out of several possible embodiments of the invention. The following
embodiment relates to Figure 1 and makes use of activated titanium anodes,
two-layer air cathodes and ion exchange membranes according to the Swedish
patent 156,526. The cell has further a free outlet of anolyte and chlorine.
The titanium anode for this cell has the dimension 111 x 225 mm
which gives the total anode surface of 250 cm2. The thickness is 0.8 mm
out of which 0.3 mm serves as a fine layer with 50% porosity and pores
with a size of about 10 mm. The coarse layer which thus is 0.5 mm thick
contains coarser pores with an average size of 25 mm and a catalyst con-
; 25 centration of one milligram of noble metal per square centimeter. The
noble metal catalyst may be formed of oxides of platinum, iridium, ruthe-
nium or the like, either alone or as mixtures of these. Activation may
take place by vapor phase deposition. Prior to activation, a protective
layer of sodium tungstenate may be formed on the titanium surface, such
` 30 as described in the published German patent application 2,054,963. This
anode operates at 0.2 bar pressure difference in the hydrogen mode.
,~
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Under these con~JItlons -the electrode opera~es at a potentlal
of about ~0.1 volt against the hydrogen reference electrode ~n 1-N hy-
drogen chloride at 110 mA/cm2. The anodes are connected in a gas tight
manner to frames of Penton~ according to Figure 5. The anode may be
connected to a ~rame ~or instance by direct welding o~ the polymer
against the fine layer of the anode at its periphery. Current conducting
titanium plates are contact-welded towards the coarse layer of the anode
in its upper edge. Two such frames are welded together by melting the
plastic frames in their periphery. This ~orms a gas space between the
two anodes. This gas space has a thickness of 0.8 mm and contains a
support which conducts the hydrogen gas from its inlet to its outlét in
such a manner that the gas comes into contact with the whole surface of
the anode. The current conductors are also carried through the frame in
a gas tight manner. This can be accomplished for instance by means of
high frequency heating of the current conductor which melts the polymer
material in contact with the titanium plate which makes it adhere to the
plate.
Figure 5 shows a hydrogen element according to the above des-
cription seen from an anolyte space. We are using the same notation as
earlier and thus the anode is designated (2~, the two current conductors,
one for each anode (14), the polymer frame (15) the gas ducts (11) and
(12) and the channels for in-going anolyte (8) and out-going anolyte (9)
and the channel for in-going catholyte (6). The anolyte channels (8)
and (9) are connected to the catholyte space by means of the bi-channels
(27) and (28) which are made as grooves in the polymer frame. The
channel (29) made as a groove in the frame serves to drain the space
between the gas channels.
; The cathode elements have basically the same shape as the anode
elements. The cathode (1) in the form of an 0.8 mm iron plate and with
the current conductor (13) is built into its frame when the latter is
fabricated by injection molding whereby a gas-tight bond is formed towards
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the space between the channels. The ca-tholyte is supplied to the catho1yte
space from the channel (6) through the bi-channel (30) formed as a groove
in the polymer frame. Catholyte and hydroyen leave direct1y to the
surroundings v;a the bi-channels (31).
S The cation permeable membrane has the same outer shape as the
cathode element according to Figure 5 with holes punched for the various
channeis. Membranes are arranged between all anode and cathode elements.
The membranes serve simultaneously as gaskets. The membrane can advant-
tageously be supported by supporting structures in the two electrolyte
spaces, which may be formed from suitable materials such as Penton~.
Anode elements, membranes and cathode elements are stacked in
a pile containing 80 anodes and 160 cathodes, electrically joined in 20
series coupled groups with four parallel anodes and eight parallel
cathodes in each group. Each group is separated by a blind cathode
frame with a polymer disc instead of the cathode. By minimization of
channel dimensions the current leakage may be kept at a level of only
a few %. The pile ends in two end plates of polymer coated metal. The
plates are drawn together by means of bolts which are arranged in an
electrically isolated manner outside the pile. Variations in the length
of the pile because of temperature variations are taken care of by
feather discs, etc. All components outside the pile are painted with a
chlorine resistant polymer. The same is the case with current conductors
and electrical couplings. All connections are arranged at one end plate.
The channels end blindly in the other end plate.
An alternative embodiment with regard to the transport-ways
for the process media is to arrange the anolyte with a free outlet in
the vessel and instead to carry the hydrogen gas in pipes together with
the catholyte. All process media may also be completely encapsulated.
The total electrode area exposed to current is 0.2 m2/group
and 4 m2/pile With the current density oP llO mA/cm2 this gives a total
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current of 220A. The hy~rogen flow is then 0.00005 kg/sec. A 20~
solution of sodium chlaride in water is ~ed to the anode space in a
flow amount;ng to 0.02 kg/sec. counted on the who1e pile. About 75%
of the added chloride is decomposed. The cathode spaces are supplied
with a flow of weak sodium hydroxide solution for instance 0.071 kg/sec.
of solution with 4% sodium hydroxide. The outflow may be 0.085 kg/sec.
of a 20% solution corresponding to 0.017 kg/sec. produced sodium hydr-
oxide per pile or slightly more than 1 ton/day of sodium hydroxide for
a battery of eight piles.
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853
The cell voltage ~mounts to ~bout 1 volt per group ln the
above example whereby the energy consumption for production of 1 ton
alkali amounts to about 1000 kwh, which is consider~bly below electro-
lysis accord;ng to state of art. The cell temperature in this case
amounts to about 50 C. With a higher cell temperature, say 70-90C.
The energy consumption is reduced further mainly because the lower
polarization of the air cathodes.
Piles of the kind described above are arranged in closed
vessels. These closed vessels or boxes may contain several piles which
are electrically joined in series or in parallel. The process flows are,
however, in general coupled in parallel. Such a vessel may ~hus contain
16 electrically parallel coupled piles whereby the current amounts to
3500 A. Since the voltage for one pile amounts to about 20 volt the
power requirement in ~his is 70 kw direct current for the production of
15 2 ton/day of hydrogen chloride and 2 ton/day of sodium hydroxide whereby
about 4 ton sodium chloride is consumed. A vessel containing these 16 ;
piles arranged in four decks with 4 piles in each deck has a bottom area
of lm x 1 m. and a total height of 1,5 m. All connections are made via
the cover of the vessel.
Floor area requirements are therefore of the magnitude of about
1 m2/day ton alkali which is several times less than the requirement for
vertical diaphragm cells and of course much less than with horizontal
mercury cells. These circumstances and the fact that the cells are built
of a large number of similar elements which can be mass-produced means a
25 smaller total investment than with the chlorine alkali electrolysers of
the state of art. The low investment cost and the low energy requirements
contribute to an extremely good process economy.
An embodiment according to Figure 2 can easily be developed
from that shown in Figure 5 and 6. The cathode element according to
Figure 6 is then made in principle in the same way as the anode element
according to Figure 5 but with two layer alr cathodes instead of the
anodes. Furthermore channels for ln- and out-going air have to be added
which may be placed in the lower part of the frame.
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