Note: Descriptions are shown in the official language in which they were submitted.
1139264
BIPOLAR ELECTROLYZER HAVING SYNTHETIC SEPARATOR
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Descri tion of the Invention
P
In one commercial manufacture of chlorine and alkali metal
hydroxides, e.g., sodium hydroxide and potassium hydroxide, an electrolytic
cell having an anolyte compartment separated from the catholyte compartment
by an ion permeable separator is utilized. In an electrolytic cell
having an ion permeable separator the anolyte compartment has an acidic
anolyte liquor at a pE~ from about 2.5 to about 5.5, containing from about
125 to about 250 grams per liter of alkali metal chloride, and with chlorine
being evolved at the anode therein. The catholyte compartment has an
alkaline catholyte having an alkali metal hydroxide content in excess of
about one mole per liter, with hydrogen being evolved at the cathode
therein.
The synthetic separator separates the acidic anolyte from the
alkaline catholyte, maintaining differences in pH and concentration
therebetween. The synthetic separator may be a microporous diaphragm, or
it may be a permionic membrane. Microporous diaphragms, e.g., microporous
fluorocarbon films, allow chloride ions to diffuse through the separator,
thereby providing a cell liquor of about lO to L5 weight percent alkali
metal hydroxide and about 15 to 25 weight percent alkali metal chloride.
According to an alternative exemplification, the synthetic
separator may be a permionic membrane, as a cation selective permionic
membrane. Cation selective permionic membranes useful for chlor alkali
electrolysis are fluorocarbon resins with acid groups thereon such as
carboxylic acid groups, sulfonic acid groups, phosphonic acid groups,
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phosphoric acid groups, derivatives thereof, and precursors thereof.
Cation selective permionic membranes are substantially impermeable to the
flow of chloride ions, thereby providing a substantially chloride free cell
liquor containing from about lO to about 50 weight percent alkali metal
hydroxide.
The anticipated long life and stable performance of synthetic
separators encourages the use of bipolar electrolyzers, which also are
reported to offer significant economy of construction and operation.
Bipolar electrolyzers are characterized by a backplate, also known as a
bipolar unit, or as a bipolar electrode, as will be described more fully
hereinafter. The bipolar unit including the bipolar backplate, serves as a
common structural member, supporting cathodes of one cell of a bipolar
electrolyzer and the anodes of the next adjacent cell of the electrolyzer.
An individual cell of a bipolar electrolyzer is formed by the
anode element of one bipolar electrode or bipolar unit, and the cathode
element of the next adjacent bipolar electrode or bipolar unit. The
cathodes are electrolyte permeable and covered with an ion permeable
separator as described hereinabove.
In the operation of the bipolar electrolyzer, brine is fed into
each of the separate cells, and an electric potential is imposed across the
electrolyzer. The electrolytic potential causes current to flow from a
power supply to an anodic end unit, and from the anodic unit of the electro-
lyzer to the individual cells of the electrolyzer, in series, and finally
to a cathodic end unit, and thence back to the power supply or to an
adjacent bipolar electrolyzer.
Typically brine, for example concentrated or even saturated brine
containing from about 300 to about 325 grams per liter of sodium chloride
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or from about 400 to about 450 grams per liter of potassium chloride,is fed
to the anolyte compartments of the individual electrolytic cells, and
chlorine is recovered from the anolyte chambers of the individual cells,
while hydrogen gas and caustic cell liquor is recovered from the individual
catholyte compartments of the electrolyzer.
Where the synthetic separator is an microporous diaphragm, the
catholyte liquor typically contains approximately 120 to 220 grams per
liter of sodium chloride and from about 110 to about 150 grams per liter of
sodium hydroxide, or from about 160 to about 300 grams per liter potassium
chloride and from about 160 to about 220 grams per liter of potassium
hydroxide.
Alternatively, where the synthetic separator is a permionic
membrane rather than a microporous diaphragm, the catholyte liquor may
contain up to 300 or more grams per liter or sodium hydroxide and con-
lS siderably lesser amounts of sodium chloride, for example less than about lO
grams per liter of sodium chloride and preferably less than one gram per
liter of sodium chloride. Alternatively, the catholyte liquor may
contain up to about 450 or more grams per liter of potassium hydroxide
and considerably lesser amounts, e.g., less than about 10 grams per liter
of potassium chloride and preferably less than one gram per liter of
potassium chloride.
While bipolar electrolyzers offer significant economies of
construction and operation, especially with synthetic separators interposed
between an anolyte compartment and a catholyte compartment of an individual
cell, the fluorocarbon material useful in forming the synthetic separators
are difficult to form into the shapes necessary for the banks of fingered
electrodes in a narrow pitch fingered electrode bipolar electrolytic cell.
~39~
The provision of the joints, seams, and convolutions requires high temper-
atures, strong reagents, high pressures or combinations thereof, all of
which have a deleterious effect on the electrodes.
A particularly satisfactory design Eor a bipolar electrolyzer
utilizing a synthetic separator between the anolyte compartment and
catholyte compartment of the individual cells should be one providing an
electrolyte tight seal while avoiding complex post-assembly seaming and
joining of the permionic membrane or microporous diaphragm. It has now
been found that a particularly satisfactory design is one where the cathode
fingers are independently or individually removable from the cathode
backscreen, and where each of the independently removable individual
cathode fingers bears a separate ion permeable synthetic separator sheet.
The ion permeable synthetic separator sheet should be one surrounding the
individual cathode finger, being perforate between the cathode and the
cathodic backscreen, whereby to allow the flow of catholyte therebetween,
and being jointed upon itself at a location remote from the cathodic
backscreen.
The Figures
FIG. 1 is an isometric view of a bipolar electrolyzer which may
have the electrode structure and synthetic separator combination herein
contemplated.
FIG. 2 is a partial cutaway plan view of bipolar electrolyzer
shown in FIG. l.
FIG. 3 is a partial cutaway side elevation of the bipolar electrolyzer
shown in FIGS. 1 and 2.
FIG. 4 is a partial cutaway of an ;ndividual cathode elernent of
the bipolar electrolyzer shown in FIGS. l, 2 and 3.
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~IG. 5 is a partially exploded isometric view of an individual
cathode element of bipolar electrolyæer shown in FIGS. 1 through 4.
Detailed Description of the Invention
FIG. 1 shows a bipolar electrolyzer having a plurality of individual
electrolytic cells 11, 12, 13, and 14 electrically and mechanically in
series through bipolar elements 21, 23, 25 and 27 which are also electrically
and mechanically in series.
The bipolar electrolyzer 1 has brine feed header 91 through
brine-chlorine tank 95 and brine return line 99 into the anolyte compart-
ment of each individual cell, with chlorine coming up through chlorine and
brine line 97, to the brine tank 95 where the chlorine and brine froth is
separated with the chlorine being recovered through brine header 91. In
this way a circulatory motion is provided to the anolyte liquor especially
when the brine return line 99 extends into the anolyte compartment, e.g.,
below the level of electrolyte therein. Additionally, depleted brine is
recovered from the anolyte compartments through brine recovery line 111,
and acid is fed to the individual anolyte compartments through acid feed
header 101 to provide an acidified anolyte liquor.
Water is fed to the catholyte compartments of the individual
electrolytic cells Ll, 12, 13, 14 through water header 103 and individual
water lines 105, with hydrogen being recovered through hydro~en recovery
line 109 to hydrogen header 107. Cell liquor is recovered through cell
liquor recovery line 115 to cell liguor header 113.
The individual bipolar units 21, 23, 25 and 27 incLude backplates
31 which separate the anolyte compartments of one celL, for example cell
11, from the catholyte compartments of the prior celL, for example cell L2,
1139Z6~
in the electrolyzer. The backplate 31 includes an anolyte resistant
surface 41 on one side and a catholyte resistant surface 61 on the opposite
side. The anolyte resistant surface 41 is fabricated of a valve metal
and may be a sheet, plate, coating or lining. Valve metals are those
metals which form an oxide upon exposure to acidic media under anodic
tonditions, for example titanium, tantalum, tungsten, niobium, zirconium
and the like. The opposite surface of the backplate 31, has a catholyte
resistant surface 61 thereon. By a catholyte resistant surface is meant a
surface of iron, or an iron alloy such as steel, stainless steel or low
carbon mild steel.
Additionally, but not shown, there may be a third layer between
the anolyte resistant surface 41 and the catholyte resistant surface
61 of the backplate. The third layer has means for preventing hydrogen
embrittlement of the anolyte resistant surface 41 of the backplate 31.
This may take the form of copper cladding, plate, or sheet, or a sheet of a
platinum group metal interposed between the anolyte resistant surface 41
and the catholyte resistant surface 61. Alternatively, there may be a
sheet of material substantially impermeable to the flow of nascent hydrogen
interposed between the catholyte liquor and the catholyte resistant surface
61 of the backplate 31.
The individual anodes 43 are fabricated of a valve metaL, as
described above, having an electrocatalytic surface thereon. The individual
anodes 43 may be welded to the anolyte resistant surface 41 of the back-
plate 31. Alternatively, the individual anodes 43 may be welded to bars,
not shown, or to extensions of the cathodic conductors.
The individuaL anode blades 43 may be perforated, foraminous,
metal mesh, sheets, plates or the like. They are parallel to each other
1139264
and extend perpendicularly from the anolyte resistant surface 41 of the
backplate 31, whereby the electrodes are fingered and interleaved between
the fingered electrodes of opposite polarity.
The cathode structure includes individual hollow cathode fingers
63 having sidewalls, a top edge, a botto-n edge, and a learling edge or tip.
The cathode fingers are formed of a suitable material, i.e., one that is
electrically conductive, alkali resistant, and in an electrolyte permeable
form. That is, they allow electrolyte to flow between the permionic
membrane 81 and the catholyte compartment. The electrolyte permeable form
may be provided by perforated plate, perforated sheet, metal mesh, or
expanded metal mesh so as to provide an open area of from about 30 percent
to about 70 percent.
The materials of construction of the individual cathode fingers
63 may be iron, or an iron alloy such as steel, mild low carbon steel, or
stainless steel. Additionally, the cathode 63 may have hydrogen overvoltage
reducing catalysts or depolarizing catalysts thereon.
The cathode finger 63 has openings 64 in the base thereof where
the cathode fingers 63 are interposed against the backscreen 67, which has
similar openings 68. By opening is meant that there is a substantial
absence of metal mesh, perforated plate, or the like, so as to allow the
unimpeded flow of catholyte liquor and hydrogen gas between the compartments.
The cathode backscreen 67 is substantially parallel to and spaced
from the cathodic surface 61 of the backplate 31. The cathodic backscreen
67 is substantially coextensive with the cathodic surface 61 of the back-
plate 31. It may be fabricated of the same material as the cathode fingers
63 and be in the same form. That is, it may be formed of an electroconduc-
tive, electrolyte impermeable metal in an electrolyte permeable form,
~1392~4
such as perforated plate, perforated sheet, metal mesh or expanded metal
mesh having from 30 to 70 percent open area and being iron or an iron alloy
such as steel, low carbon mild steel, or stainless steel. Alternatively,
the cathodic backscreen 67 may be fabricated of a nonconductive material,
for example a heavy polymeric material or polymer coated material, and may
be substantially electrolyte impermeable.
The hollow cathode fingers 63 are mounted on, in fluid communica-
tion with, and extend perpendicularly from the cathode backscreen 67. The
volume within the cathode fingers 63 and the volume between the cathode
backscreen 67 and the cathodic surface 61 of the backplate 31 are one
catholyte volume. The feed of electrolyte, for example water, to the
catholyte volume is through water header 103 and water feed line 105, while
the recovery of gas therefrom is through hydrogen line 109 to hydrogen
header 117.
The synthetic separator 71 separates the anolyte compartment
from the catholyte compartment, and bears upon the hollow electrodes 63.
While in the exemplification herein contemplated, the hollow electrodes are
cathodes, it is to be understood the hollow electrodes may also be anodes
and that by merely reversing the choice of materials of construction a cell
may be prepared having a catholyte compartment surrounding hollow anodes,
which hollow anodes bear the permionic membrane or microporous diaphragm
thereon.
The synthetic separator may be a permionic membrane. That is, it
may be substantially impermeable to the flow of anions and substantially
permeable to the flow of cations whereby to allow the flow of alkali metal
ions therethrough while preventing the flow of chloride ions. Alternatively,
the synthetic separator ~1 may be a microporous diaphragm, that is a
:11392~i4
synthetic separator that is substantially permeable to the bulk flow of
electrolyte therethrough including both anions and cations.
Typically, the synthetic separators are fluorocarbon. Where the
synthetic separator 81 is a perrnionic membrane, it is typically a fluoro-
carbon having pendant acid groups thereon, for examp]e, sulfonic groups,
carboxylic groups, phosphonic groups, phosphoric groups, precursors thereof
or reaction products thereof.
Fluorocarbon synthetic separator material is characterized by
difficulties in joining sheets of the fluorocarbon material to itself as
well as in fitting fluorocarbon sheets to complex shapes. This generally
has required ultrasonic, thermal, pressure, or chemical procedures to join
the sheets of the synthetic separator 81 to each other. It has been found
that particularly desirable results are obtained where the sheets are
joined directly to each other with the joining procedure, for example
ultrasonic, chemical, pressure, or thermal, with necessary apparatus on
each side of the joint, e.g., presses and heating elements.
As herein contemplated, the combination of independently removable
individual cathode fingers 63 with each cathode finger bearing its own
synthetic separator sheet 81, which sheet is perforate 82 between the
cathode finger 63 and the backscreen 67 and sealed remotely from the base
of the cathode finger 63, is particularly desirab1e.
Independently removable cathodes are shown, for example in FIG.
5. As there shown, boLt means 71 extend outwardly from the base of the
cathode fingers 63. The bolt means may be threaded boLt means, of a
suitable electro-conductive material such as copper, iron or the like, and
have a diamter of from about 3/16 inch to about 5/16 inch, for example, as
described in U. S. Patent 4,016,064 to Cunningharn et al for Diaphragm Cell
Cathode Structure.
1~39264
As there described, the bolt means 71 are electrically and
mechanically joined to the cathode finger 63. For exampLe, the bolt may be
welded to the cathode walls by tap welding, spot welding or the like.
According to a further exemplification there described, the bolt means 71
may be welded to a stud which is in turn welded to the walls of the cathode
fingers 63.
rhe cathodic backscreen 67 has apertures therein. The apertures
correspond to the bolt means 71 and are of a diarneter greater than the
diameter of the bolt means 71 to allow, for example, the movement such as
the slideable movement of the cathode fingers 63, while being close enough
in size to the diameter of the bolt means 71 to allow the bolt means 71 to
be fastened thereto. As described in the aforementioned patent of Cunningham
et al, the diameter of the apertures adapted to receive the bolt means 71
is from about l/4 inch to about l/2 inch greater than the diameter of the
bolt means 71.
Additionally, the cathodic backscreen 67 includes apertures 68
of sufficient size to allow the unimpeded passage of cell liquor and
hydrogen gas between the hollow interiors of the cathode fingers 63 and the
volume between the cathodic backscreen 67 and the cathodic surface 61 of
the bipolar backplate 31.
The electrical contact between the bipolar backplate 61 and the
individual hollow cathode fingers 63 is provided by a system of flexible
elastic conductor means substantially as described in the aforementioned
patent of Cunningham et al. rhe flexible, elastic conductor means includes
conductors 69 mounted Oll the individual cathode fingers 63, as shown in
FIG. 5, electrically and mechanically connected thereto, and extending
outwardly from the cathode fingers 63 toward the cathodic surface 6l of the
113926~a
bipolar backplate 31. By flexible and elastic is meant that the conductor
means are yieldable to allow moVenlent and positioning of the cathode
fingers 63, while being elastic to allow a tight connection between the
two pairs of electrical conductor means.
One pair of conductor means 71 are joined ~o the individual
cathode fingers 63, for example, by welding. A second pair of conductor
means are joined to the catholyte resistant surface 61 of the bipolar
backplate 31 or directly to the bipolar backplate 31 by bolting, welding or
the like.
~ach of the cathode fingers 63 bears an independent ion permeable
synthetic separator sheet 81. As shown in FIG. 4, the membrane 81 envelopes
an individual cathode finger 63 and is compressed between the base of the
cathode finger 63 and the cathodic backscreen 67 whereby to provide an
electrolyte seal therebetween.
As shown in FIG. 5, there are perforations or openings 64 in the
base of the cathode finger 63 corresponding to openings 82 in the base of
the permionic membrane 81, which further correspond to openings 68 in the
backscreen 67 and where present, openings 66 in gasket 65. These openings
or perforations allow for the passage of the conductor element 71, as well
as for the passage of the ellectrolyte and hydrogen. The synthetic separator
sheet 81 surrounds the cathode finger 63 as described above, and is jointed
upon itself, through overlaps 83. The joining may be accomplished by
ultrasonic means, chemical means or thermal means, after envelopement of
the hollow cathode fingers 63.
According to a further exemplification of the structure herein
contemplated, a resilient layer 65 may be provided between the backscreen
67 and the permionic membrane or microporous diaphragm 81 which further has
1~3926~
perforations 66 corresponding to the perforations or openings 82 and
perforations or openings 64 in the synthetic separator 81 and cathode 63
respectively. The resilient layer 65 provides additional electrolyte
sealing as well as means of increasing the tightness of fit of the permionic
membrane - backscreen seal.
While the invention has been described with certain specific and
illustrated embodiments, it is not intended to be so limited, except
insofar as it appears in the accompanying claims. For example, according
to an alternative exemplification, there may be provided a bipolar electro-
lyzer having a plurality of bipolar elements electrically and mechanically
in series where each of the bipolar elements comprises a backplate having
an anolyte resistant surface on one side and a catholyte resistant surface
on the opposite side where the backplate separates the anolyte compartment
of one cell from the catholyte compartment of the prior adjacent cell in
the electrolyzer and has cathodes extending from the catholyte resistant
surface thereof and anodes extending from the anolyte resistant surCace
thereof. The anodes may include an anodic backscreen parallel to and
spaced from the anolyte resistant surface of the backplate, and hollow
anode fingers mounted on, in fluid communication with, and perpendicularly
extending outwardly from the anodic backscreen whereby tl-e volume within
the hollow anode fingers and between the hollow backscreen and anolyte
resistant surEace of the backplate defines the anolyte volume, with the ion
permeable synthetic separtor being mounted upon the hollow anodes. ~lere a
structure as thus describe is utili~ed, the anode fingers are independently
removable from the anodic backscreen, and each of the holLow anode fingers
bears an ion permeable synthetic separator sheet. As so contemplated, tlle
ion permeable synthetic separator sheet surrounds an individudl hollow
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1~39Z6~L
anode finger, being perforate, that is having openings, between the hollow
anode finger and the anodic backscreen whereby to allow the flow of anolyte
liquor therebetween and being jointed upon itself remote frorn the anodic
backscreen. Additionally, the anodic backscreen may have a layer thereon
adjacent the synthetic separator sheet whereby to provide an electrolyte
tight seal therebetween. In this way with brine being fed into the anolyte
compartment between the anodic backscreen and the anolyte resistant surface
of the backplate, the individual hollow anode fingers are electrically in
communication with each other through the common anolyte compartment,
with means being provided for recovering chlorine and depleted brine from
the anolyte compartment between the anodic backscreen and the anolyte
resistant surface of the backplate. In the alternative exernplification
herein described, the synthetic separator may be a microporous diaphragm or
permionic membrane.
In a further exemplification, non-conductive non-catalytic
spacer means may be inserted on the membrane bearing electrode between the
membrane and the electrode, to space the membrane from the electrode.
Alternatively, or additionally, the spacer means may be placed on the opposite
electrodes.
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