Note: Descriptions are shown in the official language in which they were submitted.
1221657
DESCRIPTION
CHA~NEL FLOW CATHODE ASSEMBLY A~l~ ELECTROLYZER
BACKGROVND OF TEIE INVENTION
This invention relates to an electrode
assembly design to be employed in a permselective
membrane electrolyzer cell useful for electrolysis of
brine for production of chlorine and alkali metal
hydroxide, and more particularly to a cathode assembly
design which provides a cathode assembly with channels
to produce an aqueous alkali metal hydroxide electrolyte
composition having a maximum electrical conductivity and
thereby to reduce electrolyzer cell voltage. A process
of employing the cathode assembly in an electrolytic
cell or bank thereof is also contemplated.
The electrolysis of alkali metal chloride
brine, for example sodium chloride, is by far the most
lS important commercial process for producing chlorine and
alkali metal hydroxide, especially caustic soda.
Recently, there has been tremendous commercial interest
in electrolysis cells incorporating metallic anodes
rather than graphite anodes used theretofore in this
ZO process. Further, there is evolving a clear trend
toward the use of cationic permselective membranes
rather than conventional permeable deposited asbestos
diaphragms in these cells. The permselective membranes
differ substantially from the permeable diaphragms in
that no hydraulic flow from anode to cathode compart-
ments is permitted. The permselective membranes,
--2--
typically ion exchange resins cast in the forM of very
thin sheet, consist of a perfluorinated organic polymer
matrix to which inorganic sulfonate groups are attached.
Cationic permselective membrane cells for
electrolysis of aqueous alkali metal halide solution to
form alkali metal hydroxide and diatomic halide gas are
comprised of a housing, an anode and a cathode located
within the housing, a cationic permselective membrane
separating the anode and the cathode and dividing the
housing into an anode compartment and a cathode compart-
ment. In operation, an aqueous alkali .netal halide
solution is fed to the anode compartment, and water or
aqueous alkali metal hydroxide solution is fed to the
cathode compartment. A direct electric current is made
to flow from the cathode to the anode. It is the
primary function of the cationic permselective membrane
to permit passage of only positively charged alkali
metal ions from the anode compartment to the cathode
compartment; negatively charged ions are substantially
inhibited from passing through the membrane, as a con-
sequence of the nature of the membrane.
Thus, during electrolysis of sodium chloride
brine, the negatively charged groups permit transference
of current-carrying sodium ions across the membrane
while excluding chloride ions. Consequently, it is now
possible to produce caustic soda of a predetermined con-
centration and nearly free of chloride within the
cathode compartment.
Maximum utility of a system incorporating
metallic anodes and permselective membranes is achieved
by a multi-cell design wherein cells are arranged in
serial fashion. An anode mounted on one cell frame faces
the cathode mounted on the adjoining cell frame.
Between the two cell frames is interposed a cationic
permselective membrane. In a configuration such as
this, it is important to have the paired anode and
cathode parallel to each other. This permits one to
minimize the interelectrode gap and the cell voltage
lZ21657
--3--
drop due to the fluid paths in the cathode and anode
chambers.
U.5. Patent No. 4,115,236 discloses an inter-
cell connector which provides direct electrical communi-
cation and secure mechanical connection between cells ofan electrolyzer.
U.S. Patent No. 4,115,236 discloses a design
I for a cathode assembly for a plural cell electrolyzer
¦ which provides a cathode with an essentially flat
surface for use in the electrolysis of brine for produc-
tion of chlorine and caustic soda.
Just as there are factors which cause the
actual current drawn by a cell to exceed the current
theoretically corresponding to the amount of product
actually produced, some of which have been discussed
above, there are factors which cause the voltage
requirement to exceed the theoretical decomposition
voltage for the anode and the cathode reactions. The
voltage efficiency of the cell is the theoretical
decomposition voltage for the desired overall reaction,
divided by the actual voltage across the cell, expressed
as percent.
For example, for a cell for electrolysis of
aqueous sodium chloride to form sodium hydroxide,
chlorine and hydrogen, the actual voltage across a cell
is determined by the following relationship:
RT aNaOH C12 H2
E = Eo + F ln aNaC12aH20 + kI
wherein "Eol' is the theoretical decomposition voltage
(2.3 volts in the case of NaCl), "RT/F" is a factor
variable only with the temperature of the electrolyte
(T), "a" denotes the activities (concentrations times
the activity coefficients) of the pro~ucts and the
reactants, and "k" is the sum of all the ohmic resis-
tances in the cell, that is of all the resistances
which are at least approximately proportional to the
current, "I".
'
i221657
--4--
A typical cell for electrolysis of sodium
chloride solution, whether diaphragm cell or perm-
selective rnembrane cell, may operate at a total
voltage of about 4 volts. The difference between this
practical operating voltage and the theoretical decom-
position voltage resides in the last two terms of the
above equation. The activity term (the term in the
middle of the right hand side of the above equation)
reflects the effect of product and reactant concentra-
tions. The last term includes the electrical resis-
tance across the electrolyte and the membrane separating
the electrodes, the resistance througil the electrodes,
and the resistance through electrode connections. For
the sake of convenience, one may also include herein the
effect of electrode polarization due to, for example,
accumulation of evolved gas on the surface on the
electrodes and local concentration gradients.
The overall efficiency with which a cell con-
verts electrical energy to useful products is measured
by the power efficiency, which is simply the product of
the current and the voltage efficiencies.
Current efficiency as used herein denotes the
-~ fraction, expressed as percent by weight, of the amount
of alkali metal hydroxide actually produced in a cell
or a bank of a plurality of cells, divided by the
theoretical amount of alkali metal hydroxide that should
have been produced in the cell or the bank of cells for
a given amount of electrical current actually passed
therethrough.
In order to reduce the expense of subsequent
evaporation of the alkali metal hydroxide solution as
obtained from the electrolysis process to obtain more
concentrated solution containing in the order of about
50 percent alkali metal hydroxide, the usual form in
which such solutions are sold, it would be desirable
to operate the cells with the highest possible hydroxide
ion concentration in the cathode compartment. However,
with increasing hydroxide ion concentration the current
1221657
--5--
efficiency is reduced due to increased back-migration of
hydroxide ion. Increasing hydroxide ion concentration
also tends to decrease the voltage efficiency (due to
the influence of the product and reactant activity term
in above equation), although this effect is reduced by
the concurrent decrease in the electrical resistance of
the catholyte, thus reducing the contribution of the
ohmic term to the total voltage.
¦ While it is true that a higher cell current
efficiency as well as, perhaps, higher voltage effi-
ciency can be achieved by maintaining a lower overall
hydroxide ion concentration in the sum total catholyte
compartments of a bank of permselective membrane cells,
it has been found that the influence of changes in the
current efficiency becornes far more significant than the
influence of changes in the cell voltage which occur in
response to changes in the catholyte hydroxide ion con-
centration. Under these circumstances, total cell
operating efficiency, which is the power efficiency,
will be determined predominantly by the current
efficiency.
U.S. Patents 4,057,474 (~urtz et al.) and
4,181,587, (Kurtz) describe a technique known as "series
catholyte flow" wherein one or more cells are fed water,
the resultant caustic product is then fed to one or more
cells in sequence, and the product continuing to be fed
in a sequential manner until the desired caustic
strength is reached.
U.S. Pat. No. 4,057,474 (Kurtz et al.)
describes a process for electrolyzing sodium chloride
brine in membrane cells in which current efficiency is
improved. This improvement is accomplished by operating
a bank of a plurality of cells and causing the catholyte
to pass from the cathode compartment of a first cell to
the cathode compartment of one or more succeeding cells
in the bank, i.e., by operating in series catholyte flow.
U.S. Patent No. 4,181,587 (Kurtz) describes a
process for producing chlorine and caustic soda
1221657
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involving a bank of electrolytic membrane cells arranged
for series catholyte flow wherein power efficiency is
improved by maintaining at least two of the initial
cells in the bank in parallel catholyte flow cornbining
the catholyte streams from such initial cells and intro-
ducing the combined catholyte into the cathode compart-
ment of one or more succeeding cells in the bank.
I While the series catholyte flow technique dis-
¦ closed in u.S. Patent Nos~ 4 057 474 and 4 181 587
achieved a goal of operating at electrolyte near the
conductivity maximum with improved current efficiency
this technique requires additional hydraulic complexity
in the external piping system to achieve said goal.
Other prior art patents of interest include
U.S. Patents 4,142,950; 4,108,756; 4,101,410; and
3 297,561; these patents all deal with the design of
electrodes to enhance gas flow in catholyte or anolyte
compartments.
Since the principal economic factor for pro-
cesses which produce chlorine and caustic soda iselectric energy, attempts are constantly being made to
improve the efficiency of the use of this energy.
Accordingly, there is still a need for an
improved electrode design which improves the power
efficiency in the production of chlorine and alkali
metal hydroxide without employing extensive external
piping.
It is an object of this invention to provide
an electrode design for a permselective membrane
electrolyzer cell to improve the cell's power
efficiency.
It is another object of this invention to pro-
vide a cathode assembly design for use in a permselec-
tive membrane electrolyzer cell to establish within at
least a cathode compartment of said electrolyzer cell at
least a partial zone which operates at the conductivity
maximum for the aqueous alkali metal hydroxide electro-
lyte and thereby improves said cell's power efficiency.
~Z~G57
--7--
It is still another object of this invention
to provide an electrolytic process for production of alkali
metal hydroxide, hydrogen and chlorine by employing the
cathode assembly in a permselective membrane electrolyzer
cell.
These an~ other objects and advantages will be
evident from the description herein.
SUMMARY OF THE INVENTION
In accordance with the objects and advantages
of the present invention, there is provided a cathode
assembly which retards vertical mixing of the catholyte in
an electrolyzer cell having a permselective membrane between
the anode and cathode compartments, the assembly comprising
a vertical substantially flat foraminous metal cathode,
one or more fluid impervious baffles between the cathode
and the permselective membrane dividing the cathode compart-
ment into two or or more vertically disposed regions,
each baffle extending diagonally upward from one side
of the cathode compartment to a point short of the
opposite side of the cathode compartment thereby
providing a passage-around the end of the baffle for
fluid from the region below the baffle to the region
above the baffle, the baffles being positioned such
that the fluid passages for adjacent baffles are at
opposite sides of the cathode compartment whereby
catholyte injected at the bottom of the cathode
compartment and hydrogen evolved at the cathode flow
upward through the regions of the cathode compartment
without any backflow of catholyte from an upper region
to a lower region and with the path of catholyte and
hydrogen flow being generally upward and from side to
side through the regions of the cathode compartmen~
defined by the sides of the compartment and the baffles,
and means for withdrawing the catholyte and hydrogen at
the top of the cathode compartment.
,
32,653-F -7-
--8--
In accordance with the objects and advantages
of the present invention, there is also provided a method
for reducing vertical mixing of the catholyte and incresing
the energy efficiency of an electrolysis cell having a
permselective membrane separating anode and cathode com-
partments in the electrolysis of an alkali metal chloride
solution wherein an aqueous alkali metal chloride solution
is fed to the bottom of the anode compartment and water
or a dilute.alkali metal hydroxide solution is fed to the
bottom of the cathode compartment, chlorine gas and
depleted brine are withdrawn from the top of the anode
compartment and alkali metal hydroxide solution and
hydrogen gas are withdrawn from the top of the cathode
compartment, the improvement comprising re~ucing the
vertical mixing of the alkali metal hydroxide solution
in the cathode compartment with a baffle between the
cathode and the permselective membrane attached perpen-
dicularly to the face of the cathode and extending
diagonally upward from one side of the cathode compart-
ment to a point short of the opposite side of the
cathode compartment providing a passage for fluid flow
in the cathode compartment between the end of the
baffle and said opposite side of the cathode compartment.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood and
additional advantages will become apparent when reference
is made to the following description and accompanying
drawings wherein:
Figure l is an elevation view of an embodiment
of a cathode assembly of this invention employing a single
separating means and a single baffle.
Figure 2 is an elevation view of an alternate
embodiment of a cathode assembly employing two separating
means and no baffles.
. 32,653-F -8-
~ ' .
. . . ..
: " :
" ~' ~ , ' ' ~ '-
.:
.
6S~
-8a-
Figure 3 is an elevation view of an alterna-
tive embodiment of a cathode assembly of the invention
employing four separating means and three baffles.
Figure 4 illustrates a permselective membrane
electrolyzer employing the cathode assembly of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The cathode assembly of the present invention
is designed for use as part of the cathode compartment
in conjunction with a plural cell, bipolar permselective
membrane electrolyzer. The cathode assembly is
32,653-F -8a-
12Zi657
especially adapted for use in an electrolyzer which
receives an input to anode compartrnent of alkali metal
halide brine for the conversion thereof to halogen and
alkali metal hydroxide. water or dilute alkali rnetal
hydroxide is fed to inlet disposed in the bottom of the
cathode compartment, and hydrogen gas and more
concentrated alkali metal hydroxide are removed via an
I outlet at the top thereof.
¦ Prior art permselective membrane electrolyzers
used for the production of chlorine gas and alkali metal
hydroxide operate as idealized stirred tank reactors
wherein each cell operates at substantially product
stream conditions. Accordingly, if a prior art cell is
producing 25 wgt. ~ alkali metal hydroxide, the
operating conditions within this cell will be those of a
25 wgt % alkali metal hydroxide. R. H. Fitch et al. in
"Chlorine/Caustic Soda Production in a Permselective
Membrane Electrolyzer Employing Series Catholyte Flow",
Proceedin~s of the 155th Meeting of the Electrochemical
Society, Boston, MA (May 9, 1979) disclose that the
conductivity maximum of an electrolyte and corresponding
cell voltage minimum occur at conditions other than
product stream conditions. Specifically, R. H. Fitch
et al. disclose that for 18 wgt % product sodium
hydroxide stream, the conductivity maximum occurs at 13
wgt % sodium hydroxide.
The cathode assembly of the present invention
employs at least one separating means adjacent to a
face of said cathode assembly and comprises a fluid
impervious material and extends diagonally upwards
with a positive monotonic slope to separate the cathode
assembly into two interconnected regions. When the
cathode assembly of the present invention having at
least one separating means is used in a cathode compart-
ment, at least two interconnected chambers are formed:one lower and a first upper. The composition of alkali
metal hydroxide produced in the lower chamber so formed
is in the range of about 12 to 15 wgt % alkali metal
~Z~65~
-10~
hydroxide, which substantially approximates the conduc-
tivity maximum for alkali metal hydroxide. Surprising-
ly, a permselective electrolyzer cell employing the
cathode assembly of the present invention operates at a
cell voltage lower by approximately 0.07 volts thereby
increasing the cell's power efficiency by about 2%
in comparison to prior art permselective cells employing
a cathode assembly as disclosed in us Patent No.
4,115,236.
The alkali metals of commercial importance are
sodium and potassium. Accordingly, the components of
the cathode assembly and permselective membrane elec-
trolyzer are chosen from a design and material viewpoint
with the highly corrosive chemicals such as sodium
hydroxide, potassium hydroxide and chlorine in mind.
The cathode assembly of the present invention
cornprises at least one, preferably one to four, separat-
ing means. A cathode assembly comprising two separating
means is more preferred.
The separating means are positioned adjacent
to, preferably affixed to, a face of the cathode
assembly.
Referrin9 to the drawings in more detail,
Fig. 1 shows a rigid cathode support 10 joined to
cathode 12 by connecting member 14. The materials from
which the cathode support, cathode, and connecting
members are fabricated should be electrically conduc-
tive and resistant particularly to hydroxyl ions.
Typically, these elements of the cathode assembly are
fashioned from metal selected from the group consisting
of iron, steel, cobalt, nickel, manganese and the like,
iron and steel being preferred. ~lthough it is not
essential that the elements all be fabricated from the
same metal, some corrosion problems can be avoided by
doing so. The cathode must be of foraminous material
to allow release of gas from front surface of cathode.
The connecting members serve both to ensure that the
: cathode maintains a flat surface and to provide
.
:
il2z~657
--11--
electrical communication between cathode and support.
The connectors must be of foraminous material to permit
the hydrogen evolved on the cathode to rise to the
surface of the catholyte. The foraminous material of
the cathode and connectors may be expanded metal or,
preferably, perforated metal sheet. Most preferably,
these elements comprise perforated low-carbon steel
sheets. Instead of sheet, the connectors may
alternatively be either angle or channel.
A purpose of the cathode support is to ensure
that the paired anode/cathode elements are parallel. To
accomplish this purpose, the support must be rigid and
have an accurately flat faceO Adequate rigidity may-be
achieved with a support area about 1~ of the cathode
area; however, preferably the support area covers at
least about 25% of the cathode area. The support should
comprise a metal plate at least about 4.5 mm thick.
Precision surface grinding of the support faces is the
preferred method for achieving the required flat face.
Figure 1 also shows other elements of the
cathode assembly, including through bores 16 in the
cathode support through which the intercell connectors
join the cathode support to the anode in an adjacent
cell. Through bores 18 in the cathode provide access to
the heads of the intercell connectors. To ensure a
smooth edge for the holes 18~ there are no perforations
punched in the cathode on the perimeter of said holes.
In the preferred embodiment, the cathode 12 is cut at
the corners and folded at about a 90 angle around the
edges to assist in achieving flatness after the
punching step. Where reference is made herein to the
flat surface of the cathode and to the requirement that
anode and cathode surfaces be parallel, these folded
edges are obviously excluded.
Referring again to Figure 1, separating means
24 is positioned adjacent, preferably affixed via a
first edge 23 of separating means 24, to a face of
cathode 12 and extends diagonally upwards from a point
.
21 on a first side 30 of said cathode 12 to a point 27
short of second side 34'opposite the first side 30 of
cathode 12. The separating means 24 has a positive
monotonic slope with reference to said first side 30 and
to third side 32 adjacent to said first side 30 and
thereby separating said face of cathode 12 into two
interconnecting regions: one lower, designated A and one
upper, designated B.
Separating means 24 is preferably affixed to a
face of cathode 12 and more preferably is disposed sub-
stantially perpendicular to said face of said cathode
12.
While it is preferred that separating means 24
be affixed to the face of cathode 12 opposite from the
permselective membrane, it is considered within the
- scope of the present invention to affix separating means
24 to either one or both faces of cathode 12. In the
most preferred embodiment of the present invention, the
separating means 24 is affixed to the same face of
cathode 12 having cathode support 10.
While separating means 24 may be displaced as
in Figures 1-3, it is understood that any configuration
or design such as snake-like may be employed so long as
the slope thereof is monotonic so as to prevent forma-
tion of dead zones for gas and/or liquid along thelength of the separating means.
By the term "monotonic slope" as used herein,
it is meant that the sign of the first derivative of a
mathematical function or curve which defines a sepa-
rating means never changes in the region of interest,i.e., the face of the cathode.
Preferably, a downwardly disposed flange 26 is
affixed to a second edge 25 opposite the first edge 23
and extending along substantially the full length of
separating means 24.
Both separating means 24 and flange 26 are
comprised of any fluid impervious material such as
plastic or mild steel; preferably the same material,
'~, ' ,
',
,~ - , .
l~Z~657
-13-
though fluid impervious, as used for fabrication of
cathode support 10 and cathode 12.
Baffle 28 is disposed adjacent, preferably
affixed to, a face of said cathode 12 in upper region B.
Preferably, baffle 28 comprises a fluid permeable or
fluid impervious material and is disposed in a direction
substantially parallel with first side 30 of cathode 12.
More preferably, said baffle 28 is affixed substantially
¦ perpendicular to said face of said cathode 12. When
I 10 baffle 28 is fabricated of fluid impervious material,
said baffle should be positioned in region B so as not
to intersect with any side, e.g. side 36 of said cathode
12 or separating means 24.
The purpose of baffles is to restrict axial
lS dispersion of alkali metal hydroxide electrolyte in at
least one upper region such as B in Figure 1 and to
force region B to operate like a plug flow reactor
wherein only the electrolyte concentration at the exit
port is at the high, undesirable product concentration.
The alkali metal hydroxide electrolyte concentration in
the lower region A separated from upper region B by
separating means 24 substantially approximates the con-
ductivity maximum for alkali metal hydroxide.
Figs. 2 and 3 show elevation views of alterna-
tive embodiments of the cathode assembly. Preferably,as shown in the Figures, the center of cathode support
10 is positioned substantially over the center of
cathode 12, with the two elements having the same
orientation, i.e., the edges of the cathode are parallel
to the corresponding edges of the support. Figs. 2 and
3 also show the preferred embodiment of the cathode
support 10 of this invention, in which a substantially
rectangular cutout 11 yields a picture frame configura-
tion. The center of the cutout substantially coincides
with the center of the support, and the cutout and sup-
port have substantially the same orientation. The
` primary advantage of the cutout is a substantial weight
reduction. The cutout must, however, not be so large
,
lZ2~657
-14-
that the s~pport lacks rigidity; th~s the area of the
cutout ~us~ be no greater than about 50% of the area
enclosed by the outer perimeter of the support.
Figure 2 shows a preferred embodiment of the
cathode assembly of the present invention employing two
separating means 24 and 34, each equipped with downward-
ly disposed flanges 26 and 36, respectively, thereby
separating the face of cathode 12 into three intercon-
¦ nected regions: a lower, designated A; a first upper,
designated B and a second upper, designated C, with theproviso that said first and second separating means 24
and 34 do not intersect.- The flanges 26 and 36 are
affixed to edges 25 and 35 of separating means 24 and
34, respectively, and extend along substantially the
full length of said separating means.
The preferred embodiment of the present
invention shown in Figure 2 does not comprise baffle;
the use of baffle depends on the individual operating
condition, e.g. current density.
Figure 3 displays an alternate preferred
embodiment of the present invention wherein four sepa-
rating means, 24, 34, 44 and 64, equipped with down-
wardly disposed flanges 26, 36, 46 and 66, respectively,
are employed thereby separating the face of said cathode
into five interconnected regions, designated: A (lower),
B (first upper), C (second upper), D (third upper) and E
(fourth upper) with the proviso that said first (24),
second (34), third (44) and fourth (64) separating means
do not intersect. Each of the separating means is dis-
posed adjacent to, preferably affixed to, a face ofcathode 12 and extends diagonally upwards with monotonic
slopes of alternating signs.
Baffles 38, 48, and 58 are adjacent to a face
of cathode assembly and are disposed in upper regions B,
C and D, respectively, substantially parallel to sides
30 and 34 of cathode 12. For convenience, baffles 38,
48 and 58 are comprised of fluid-impervious material and
are incorporated as part of connecting members 14.
~2%i6S7
-15-
The cathode assembly of the present invention
need not be rectangular or comprise a rigid cathode
support 10 or picture window 11. In addition, while the
Figures 1-3 show separating means adjacent to the same
face of cathode as said cathode support, it i8 under-
stood that said separating means and said cathode sup-
port may be on opposite faces of said cathode. Further,
it is considered within the scope of the present inven-
tion that the separating means may be affixed to webbing
means (not shown) that may be disposed adjacent to
either face of cathode. In addition, the cathode assem-
bly of this invention may form a cathode compartment
incorporated in a permselective membrane electrolyzer
cell in a bank of a plurality of cells adapted to
operate in series catholyte flow as disclosed in U.S.
Patent 4,057,474, or modified series parallel catholyte
as disclosed in U.S. Patent 4,181,587.
The present invention also contemplates a
process of using the cathode assembly of the present
invention to produce alkali metal hydroxide, hydrogen
and chlorine gas by an electrolysis of alkali metal
chloride brine in electrolysis cell or bank of elec-
trolysis cells each having a cathode compartment con-
taining a cathode assembly separated from an anode
compartment containing an anode assembly by a permselec-
tive membrane, preferably a cation permselective
membrane. Aqueous alkali metal chloride brine of any
convenient concentration is introduced into the anode
compartment and water or dilute alkali metal hydroxide
is introduced into the cathode compartment of each cell.
Under the influence of electric current, sodium ions,
but not chloride ions, migrate through the preferred
cation permselective membrane from the anode compartment
into the cathode compartment wherein alkali metal
hydroxide and hydrogen gas are formed.
t Prior art permselective membrane cells for
electrolysis of alkali metal chloride brine suffer from
the disadvantage that each cell operates like an
~ ,.......
.
~ZZ~657
-16-
idealized stirred tank reactor at substantially product
stream conditions. Thus, if a given cell is producing
25 weight percent alkali metal hydroxide by electrolysis
of alkali metal chloride brine, the operating conditions
within the cell are approximately those of 25 weight
percent alkali metal hydroxide. These prior art stirred
tank reactors may operate at conditions equivalent to
the product streams conditions which are different from
¦ the conductivity maximum for the alkali metal hydroxide
electrolyte and thereby consume more electrical energy.
The process of the present invention employs
the cathode assembly of the present invention to provide
at least two interconected zones, one lower and at least
one upper zone, within the cathode compartment containing
the cathode assembly. The concentration of alkali metal
hydroxide produced in the lower zone is maintained at
substantially the conductivity maximum for said concen-
tration. The alkali metal hydroxide produced in the
lower zone is then passed to at least one upper zone
wherein the concentration of alkali metal hydroxide is
increased to the desired concentration.
The number of interconnected zones found use-
ful in the process of the present invention is at least
two, preferably two to five, more preferably two. The
fluid impervious separating means described herein-
above are employed to provide the interconnected zones.
While permselective electrolysis using the
process of the present invention operates to produce
alkali metal hydroxide and chlorine gas at increased
cell power efficiency so long as at least a portion of
the alkali metal hydroxide produced in the lower zone
is maintained at the conductivity maximum for that
concentration of alkali metal hydroxide, it is preferred
to maximize the volume of alkali metal hydroxide having
the concentration that provides the conductivity maximum.
When 18 weight percent alkali metal hydroxide
is the desired product concentration and a water feed for
cathode is utilized, the conductivity maximum occurs
~Zz~657
-17-
At 13 weight percent alkali metal hydroxide. To
maximize the volume of the cell's cathode cornpartment
provided with two interconected chambers that operates
at 13 weight percent alkali metal hydroxide, the
separating means should be disposed so that about 13/18
or 72% of the cathode compartment is preferably occupied
by the lower zone.
The separating means comprise a fluid
¦ impervious material and have a monotonic slope. The
precise slope is not critical. Slopes found useful are
in the range of about 0.1 to about 0.5.
When 25 weight percent alkali metal hydroxide
is the desired product concentration for a cathode
compartment having three interconnected zones, the
conductivity maximum occurs at about 15 weight percent
alkali metal hydroxide. To maximize the volume of the
cell's cathode compartment that operates at this con-
ductivity maximum, about 55~ of the cathode compartment
is preferably occupied by the lower zone, about 23% by
the first upper and about 22% by the second upper zone.
Of course baffles may be provided in the first and
second upper zones in the preferred embodiment of the
process of the present invention.
The following examples are intended to illu-
strate, but not to limit, the present invention compared
to the broader scope set forth in the claims that
follow.
EXAMPLE 1
An electrolyzer 100 of five electrochemical
cells (101, 201, 301, 401 and 501) in a bipolar filter
¦ press arrangement illustrated in Figure 4 is used. Each
cell, e.g. 101, contains an anode 102, preferably ruthe-
nium dioxide coated titanium expanded metal with current
collection means; a cell divider constructed of mineral
filled polypropylene; a cathode 112 as described in
Figure 3; a perfluoro sulfonic acid cation exchange
permselective membrane 106, e.g. DuPont's NAFION~ 390
membrane, and a means for feeding brine 108 and water 110
1221657
-18-
in a parallel fashion to the anode and cathode
compartments, respectively. (See Figure 4). In
operation, face B of cathode 112 is pushed against
facing side of cation permselective membrane 102 and
separating means are affixed to face A of cathode 112.
The cell is operated in a continuous Eashion
with a current load of 2500 amperes; each cell is fed
0.8 liters/min of 280 g/l sodium chloride brine and 0.35
l/min water to the anode and cat'node compartments
respectively of each cell. The cell is allowed to
operate until steady state conditions are achieved. The
solution leaving compartments A, B, C, D and E of the
cathode compartments is found to average 12.5, 14.6,
16.5, 18.5 and 20.3 weight percent sodium hydroxide
respectively. The cell voltage is found to average 3.7
volts. The power efficiency is found to be 0.53.
EXAMPLE 2
The electrolyzer of Example 1 is modified by
replacing the cathodes of Figure 3 with cathodes identi-
cal except that no baffle or seperator means areattached to the cathode. The resulting electrolyzer is
then operated under the conditions of Example 1.
The resulting solution leaving the cathode
chambers then has an average concentration of 19.6
weight percent sodium hydroxide and the cell voltage is
3.9 volts per cell. The power efficiency is 0.47.
Other changes and modifications in the
specifically described embodiments can be carried out
without departing from the scope of the invention which
is intended to be limited only by the scope of the
appended claims.
