U.S. patent application number 15/321102 was filed with the patent office on 2018-07-05 for narrow gap, undivided electrolysis cell.
The applicant listed for this patent is Chemetics Inc.. Invention is credited to Ian Bailey, David Summers.
Application Number | 20180187316 15/321102 |
Document ID | / |
Family ID | 53525276 |
Filed Date | 2018-07-05 |
United States Patent
Application |
20180187316 |
Kind Code |
A1 |
Summers; David ; et
al. |
July 5, 2018 |
NARROW GAP, UNDIVIDED ELECTROLYSIS CELL
Abstract
An undivided electrolysis cell for electrolyzing a liquor is
disclosed which has a narrow gap between the electrodes and
improved energy efficiency. The electrolysis cell comprises a
porous anode, a porous cathode, and an electrically insulating
separator therebetween which are all permeable to the liquor.
Electrolysis is performed while directing the liquor through the
porous anode, the electrically insulating separator, and the porous
cathode. Gas products generated during electrolysis are carried out
with the liquor and do not remain between the electrodes thereby
reducing "gas blinding". The electrolysis cell is particularly
suitable for chlorate electrolysis.
Inventors: |
Summers; David; (Vancouver,
CA) ; Bailey; Ian; (Vancouver, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chemetics Inc. |
Vancouver |
|
CA |
|
|
Family ID: |
53525276 |
Appl. No.: |
15/321102 |
Filed: |
June 20, 2015 |
PCT Filed: |
June 20, 2015 |
PCT NO: |
PCT/US2015/036845 |
371 Date: |
December 21, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62016647 |
Jun 24, 2014 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 9/02 20130101; C25B
9/063 20130101; C25B 9/203 20130101; Y02E 60/36 20130101; C25B
1/265 20130101; Y02E 60/366 20130101 |
International
Class: |
C25B 1/26 20060101
C25B001/26; C25B 9/02 20060101 C25B009/02; C25B 9/06 20060101
C25B009/06; C25B 9/20 20060101 C25B009/20 |
Claims
1. An undivided electrolysis cell for electrolyzing a liquor
comprising: a porous anode permeable to the liquor; a porous
cathode permeable to the liquor; an electrically insulating
separator between the anode and cathode and permeable to the
liquor; an anode compartment whose surface comprises the porous
anode and an anode current carrier electrically connected to the
porous anode; a cathode compartment whose surface comprises the
porous cathode and a cathode current carrier electrically connected
to the porous cathode; a liquor inlet at the bottom of an inlet
compartment wherein the inlet compartment is one of the anode
compartment and the cathode compartment; and a liquor and gas
product outlet at the top of an outlet compartment wherein the
outlet compartment is that one of the anode compartment and the
cathode compartment other than the inlet compartment.
2. The electrolysis cell of claim 1 wherein the inlet compartment
is the anode compartment and the outlet compartment is the cathode
compartment.
3. The electrolysis cell of claim 1 wherein the electrolysis cell
is a chlorate electrolysis cell and the liquor comprises NaCl and
water.
4. The electrolysis cell of claim 1 wherein the porous anode is
made of expanded metal or louvered metal.
5. The electrolysis cell of claim 4 wherein the anode metal is
titanium.
6. The electrolysis cell of claim 1 wherein the porous cathode is
made of expanded metal or louvered metal.
7. The electrolysis cell of claim 6 wherein the cathode metal is a
nickel-free stainless or mild steel.
8. The electrolysis cell of claim 1 wherein the electrically
insulating separator is selected from the group consisting of
porous polymer sheet, polymer buttons, and a gap.
9. The electrolysis cell of claim 8 wherein the electrically
insulating separator is a porous fluoropolymer sheet.
10. The electrolysis cell of claim 1 wherein the distance between
the anode and the cathode is less than about 2 mm.
11. The electrolysis cell of claim 1 wherein the flow resistance of
at least one of the separator and the cathode is greater than the
flow resistance of the anode.
12. The electrolysis cell of claim 1 wherein the anode current
carrier is pan shaped and comprises a set of formed features.
13. The electrolysis cell of claim 1 wherein the porous anode is
electrically connected to the anode current carrier by a set of
electrically conductive fingers.
14. The electrolysis cell of claim 1 wherein the cathode current
carrier is pan shaped and comprises a set of formed features.
15. The electrolysis cell of claim 1 wherein the porous cathode is
electrically connected to the cathode current carrier by a set of
electrically conductive fingers.
16. The electrolysis cell of claim 1 wherein the cathode
compartment comprises features for directing gas product upwards
towards the gas product outlet.
17. The electrolysis cell of claim 1 comprising no other inlet in
either the anode or the cathode compartments and no outlet in the
anode compartment.
18. The electrolysis cell of claim 1 comprising a vent in the anode
compartment.
19. The electrolysis cell of claim 1 comprising a drain in the
cathode compartment.
20. An electrolyzer comprising a plurality of the electrolysis
cells of claim 1 stacked in a bipolar arrangement.
21. The electrolyzer of claim 20 wherein the anode current carriers
in the electrolysis cells are pan shaped and comprise a set of
formed features, the cathode current carriers in the electrolysis
cells are pan shaped and comprise a set of formed features, and the
set of features in the anode current carriers nests with the set of
features in adjacent cathode current carriers in the bipolar
arrangement.
22. The electrolyzer of claim 20 wherein the electrolysis cells are
stacked such that the anodes and cathodes in the cells are oriented
at an angle greater than zero with the vertical.
23. The electrolyzer of claim 22 wherein the electrolysis cells are
stacked such that the anodes and cathodes in the cells are oriented
perpendicular to the vertical.
24. The electrolyzer of claim 20 wherein the electrolysis cells are
modules and the electrolyzer is modular.
25. A method for the undivided electrolysis of a liquor comprising:
providing the electrolysis cell of claim 1; directing the liquor to
the liquor inlet in the inlet compartment and into the inlet
compartment; directing the liquor in a direction through the porous
anode, through the electrically insulating separator, and through
the porous cathode; directing the liquor into the outlet
compartment; applying an electrolysis voltage and current across
the anode and cathode current carriers during the directing of the
liquor; and directing the liquor and gas product out from the
liquor outlet in the outlet compartment.
26. The method of claim 25 comprising recirculating the liquor from
the liquor outlet in the outlet compartment to the liquor inlet in
the inlet compartment.
27. The method of claim 25 for the production of sodium chlorate
wherein the liquor comprises NaCl and water, and wherein the gas
product is hydrogen.
Description
TECHNICAL FIELD
[0001] The present invention pertains to undivided cells for energy
efficient electrolysis, and particularly to undivided chlorate
electrolysis cells.
BACKGROUND
[0002] Numerous chemicals are prepared on an industrial scale using
electrolysis cells to electrolyze a liquor containing suitable
reactants and thereby generate various products of electrolysis. In
this regard, there are two basic types of electrolysis cells,
namely divided cells and undivided cells. In both cell types,
certain ions may travel between the electrodes. However, in a
divided cell, the cathode and anode sides of the cell are separated
(divided) in some manner such that most or all of the reactants and
products on each side of the cell are kept separate. On the other
hand, in an undivided cell, the cathode and anode sides of the cell
are not separated (undivided) and the reactants and products on
each side of the cell can mix.
[0003] For example, sodium hydroxide, along with hydrogen and
chlorine, is typically produced by chlor-alkali electrolysis. The
chlor-alkali electrolysis process employs divided electrolysis
cells to electrolyze a solution of sodium chloride. Various types
of divided cells have been used commercially and include mercury,
diaphragm, and membrane cell types. In a mercury cell, liquid
mercury provides separation in the cell. During electrolysis,
chlorine gas is generated at the anode. Sodium is formed at the
mercury cathode and dissolves in the liquid mercury to create an
amalgam which is then transported to another chamber where the
sodium reacts with water to create sodium hydroxide and hydrogen.
In a diaphragm cell, a partially hydraulically permeable diaphragm
(typically made of asbestos fibres) provides separation in the
cell. A solution of sodium chloride is continually introduced to
the anode compartment and flows through the diaphragm to the
cathode compartment. During electrolysis, chlorine is again
generated at the anode while sodium hydroxide and hydrogen are
generated at the cathode. The product sodium hydroxide solution is
continually removed from the cathode compartment. The diaphragm
prevents the chlorine gas from crossing over to the cathode
compartment and the continual flow of solution through the
diaphragm prevents the sodium hydroxide from flowing back to the
anode compartment, thereby preventing the chlorine and sodium
hydroxide from reacting. In a membrane cell, an ion-selective
permeable membrane provides separation in the cell. During
electrolysis, chlorine is again generated at the anode and positive
sodium ions pass through the membrane to the cathode side where
sodium hydroxide and hydrogen are generated.
[0004] Numerous refinements and improvements have been made to
chlor-alkali electrolysis cells over the decades. For instance,
U.S. Pat. No. 3,242,059 disclosed diaphragm type electrolysis cells
in which the anode-cathode gap could be eliminated except for the
thickness of the diaphragm itself and a thin anode supporting
member. Also for instance, U.S. Pat. No. 4,279,731 disclosed
bipolar electrolyzers comprising a series of divided electrolysis
cells of the diaphragm or membrane type. The cells comprised screen
or expanded metal electrodes which can be close to or pressed
against the dividing diaphragm or membrane. The design provided
internal recirculation of the electrolyte and uniform distribution
over the electrode surface. And further, an improvement to the
electrical connection of the electrodes in each bipolar element
through a bipolar separator was provided.
[0005] In another example, sodium chlorate is typically produced by
a chlorate electrolysis process which also involves electrolyzing a
solution of sodium chloride but uses undivided electrolysis cells.
(Sodium chlorate is primarily produced as a precursor for the
subsequent production of chlorine dioxide. Since chlorine dioxide
is relatively unstable and is difficult to transport and store, it
is usually produced only as needed onsite.) The chlorate process
however involves a complicated set of both electrochemical and
chemical reactions and tight control of various operating
parameters including pH, temperature, composition and concentration
of electrolyte is required. As in the chlor-alkali process,
chlorine gas is produced at the anode and hydrogen and hydroxyl
ions are produced at the cathode during electrolysis. However, in
the chlorate process, the produced chlorine is immediately reacted
chemically with the hydroxyl ions present in the cell liquor so as
to primarily produce hypochlorite which in conjunction with the
sodium ions present in the solution produces sodium hypochlorite.
The hypochlorite is an intermediate in the process and is
subsequently converted to chlorate via chemical reactions with
other species in the electrolyte liquor. Usually this conversion is
accomplished in a chemical reactor operating at elevated
temperature and located downstream of the chlorate electrolyzer. A
thorough review of sodium chlorate electrolysis can be found for
instance in "Electrolytic Sodium Chlorate Technology: Current
Status", B. V. Tilak, ECS Proceedings Vol. 99-21, Page 8,
Chlor-alkali and Chlorate Technology: R. B. Macmullin Memorial
Symposium.
[0006] In the chlorate electrolysis process, it is intended that
various species in the cell liquor readily react chemically with
each other and thus undivided electrolysis cells are employed here.
Numerous refinements and improvements have also been made to such
undivided electrolysis cells over the years. In particular, it has
been highly desirable to improve the energy efficiency of chlorate
electrolyzers and thus reduce the power consumption and operating
cost required for production. Despite all the advances in design
and in complex controls introduced to date, there are still several
sources of resistive losses in present day chlorate electrolyzers
that undesirably affect energy efficiency.
[0007] Resistive losses arise in the numerous electrical
connections appearing between the various metal components in the
electrolyzer. Where possible, it is thus desirable to reduce the
path length that current is required to travel. Bipolar
electrolyzer constructions can therefore be preferred. Further,
losses arise due to electrolyte resistance and this resistance
generally increases with the path length or gap between the anode
and cathode electrodes. Thus, the minimum gap possible between
anodes and cathodes would seem preferred. However, during
operation, hydrogen gas is continually generated at the cathode at
a rate which is a function of the electrolysis current. This
generated hydrogen displaces electrolyte between the cell
electrodes and increases the effective resistance of the
electrolyte remaining between the electrodes (known as "hydrogen
blinding"). The volume of hydrogen generated, and hence the volume
of displaced electrolyte, is essentially independent of the gap
between anodes and cathodes. Thus for narrow gap cells, any
improvement in resistive losses resulting from reducing the gap
between anodes and cathodes is at least partially offset by the
adverse effect of hydrogen blinding. At some point, any further
reduction in gap actually increases the resistance between the
electrodes while in operation. A trade-off is thus required and
conventional chlorate electrolysis cells typically have a
significant non-zero gap between the anodes and cathodes where the
losses due to the combined effects of gap and hydrogen displacement
have been minimized
[0008] There is a continuing desire to improve the energy
efficiency of electrolyzers in general for the industrial
production of chemicals. And specifically, there is a desire to
improve the energy efficiency of electrolyzers comprising undivided
electrolysis cells for the production of sodium chlorate and other
related chemicals. This invention addresses that desire and
provides other advantages as discussed below.
SUMMARY
[0009] The present invention provides new designs for and methods
for operating an undivided electrolysis cell to improve energy
efficiency in the electrolyzing of a liquor. The cells are
characterized by a narrow gap between the electrodes and the porous
design employed (for both electrodes and for an insulating
separator between them) allows for generated gas to be removed
without "blinding" the electrodes.
[0010] Specifically, the undivided electrolysis cell comprises a
porous anode, a porous cathode, and an electrically insulating
separator therebetween which are all permeable to the liquor. In
addition, the electrolysis cell comprises an anode compartment
whose surface comprises the porous anode and an anode current
carrier electrically connected to the porous anode, and a cathode
compartment whose surface comprises the porous cathode and a
cathode current carrier electrically connected to the porous
cathode. Further, there is an inlet for the liquor at the bottom of
the cell, and an outlet for the liquor and any gas product at the
top of the cell. The liquor inlet may be provided in either of the
anode or cathode compartments while the liquor outlet is then
provided in the other of these two compartments. Herein, the inlet
compartment refers to whichever of these two compartments comprises
the inlet, and the outlet compartment refers to whichever of the
two compartments comprises the outlet. In a preferred embodiment,
the inlet compartment is the anode compartment and the outlet
compartment is the cathode compartment.
[0011] Electrolysis cells of the invention are particularly
suitable for chlorate electrolysis in which the primary components
in the liquor to be electrolyzed are NaCl and water. Typically
however, the liquor is recirculated and thus can also comprise a
certain amount of the products of electrolysis and also products of
subsequent chemical reaction (e.g. sodium hypochlorite, sodium
chlorate, dissolved chlorine and hydrogen gases). Further, as is
known in the art, additives may also be included in the liquor to
enhance chlorate formation (e.g. sodium dichromate).
[0012] The porous anode in the electrolysis cell can conveniently
be made of expanded metal or louvered metal which has an
appropriate coating. And a conventional preferred metal can be
employed for the anode, e.g. titanium. In a like manner, the porous
cathode can also be made of an expanded metal or louvered metal.
And here too, a conventional preferred metal can be employed for
the cathode, e.g. a nickel-free stainless or mild steel.
[0013] The electrically insulating separator in the electrolysis
cell can be of various constructions as long as it is suitably
porous to the liquor and provides adequate electrical insulation
between the electrodes. For instance, the electrically insulating
separator can be a porous polymer sheet, a set of polymer buttons,
or merely a suitable gap. In particular, a suitable electrically
insulating separator for achieving a very narrow gap is a porous
fluoropolymer sheet. Alternatively similar embodiments made of
ceramic may be employed as the electrically insulating
separator.
[0014] As mentioned, the energy efficiency of such an electrolysis
cell can be improved. In part, this is because a narrower gap can
be employed between the electrodes than the gap employed in
conventional undivided electrolysis cells. For instance, the
distance between the anode and the cathode can be less than about 2
mm.
[0015] In an exemplary embodiment, the cell components can be
selected such that the flow resistance of at least one of the
separator and the cathode is greater than the flow resistance of
the anode. This results in an increase in the pressure difference
across the electrodes which can be beneficial for preventing
backflow of hydrogen from the cathode compartment into the
electrochemically active area of the cell.
[0016] In certain embodiments, both the anode current carrier
and/or the cathode current carrier can be pan shaped and comprise a
set of formed features. Also in certain embodiments, the porous
anode and porous cathode can be electrically connected to the anode
and cathode current carriers respectively by appropriate sets of
electrically conductive fingers.
[0017] The present invention also includes electrolyzers comprising
a plurality of the aforementioned electrolysis cells in a bipolar
arrangement. In embodiments in which the anode and cathode current
carriers are both pan shaped and comprise a set of formed features,
the features can be configured such that the set of features in the
anode current carriers nest with the set of features in adjacent
cathode current carriers in the bipolar arrangement.
Advantageously, the electrolysis cells can be made as modules. The
electrolyzes can thus be modular and individual cells may thus be
readily removed and exchanged as desired.
[0018] In such electrolyzers, it may be advantageous (e.g. for
purposes of gas removal) to stack the electrolysis cells such that
the anodes and cathodes in the cells are oriented at an angle
greater than zero with the vertical (e.g. perpendicular to the
vertical).
[0019] The electrolysis cells of the invention are for the
undivided electrolysis of a liquor. The method of operating such
cells comprises directing the liquor to the liquor inlet in the
inlet compartment and into the inlet compartment. The liquor is
then directed in a direction through the porous anode, through the
electrically insulating separator, and through the porous cathode.
Depending on the locations of the liquor inlet and liquor outlet,
the direction can be from anode to cathode or from cathode to
anode. An electrolysis voltage and current is applied across the
anode and cathode current carriers during the directing of the
liquor, thereby electrolyzing the liquor and then the liquor and
gas product are directed out from the liquor outlet in the outlet
compartment. In a preferred embodiment, the liquor inlet and liquor
outlet are located in the anode and cathode compartments
respectively, and thus the liquor is directed from anode to cathode
inside the cell. In order to direct the liquor appropriately
throughout the cell, the electrolysis cell may comprise no other
inlet in either the anode or the cathode compartments and no outlet
in the anode compartment. Alternatively however, the electrolysis
cell may comprise a suitable drain in the cathode compartment
and/or a vent in the anode compartment. Further, the cathode
compartment may advantageously comprise features for directing gas
product upwards towards the gas product outlet.
[0020] In a typical embodiment for chlorate electrolysis, the
operating method also comprises recirculating the liquor from the
liquor outlet in the cathode compartment to the liquor inlet in the
anode compartment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 a shows a schematic of a cell unit for a bipolar
chlorate electrolyzer in the prior art. The cell unit comprises a
number of undivided monopolar chlorate electrolysis cells.
[0022] FIG. 1b illustrates the primary reactions and the hydrogen
gas product generated in the electrolysis cells of FIG. 1a.
[0023] FIG. 2a shows a schematic of a chlorate electrolysis cell of
the invention.
[0024] FIG. 2b illustrates the primary reactions and the hydrogen
gas product generated in the electrolysis cell of FIG. 2a.
[0025] FIG. 3 shows a schematic of a bipolar electrolyzer
comprising a series stack of the inventive chlorate electrolysis
cells of FIG. 2a in which the anodes and cathodes in the cells are
oriented vertically.
[0026] FIG. 4 shows a schematic of the bipolar electrolyzer of FIG.
3 in which the anodes and cathodes in the cells are oriented
perpendicular to the vertical.
[0027] FIG. 5 shows a schematic of a chlorate production system
comprising a chlorate electrolyzer of the invention.
[0028] FIG. 6 plots the polarization results obtained for the
inventive and comparative electrolysis cells of the Examples.
DETAILED DESCRIPTION
[0029] Unless the context requires otherwise, throughout this
specification and claims, the words "comprise", "comprising" and
the like are to be construed in an open, inclusive sense. The words
"a", "an", and the like are to be considered as meaning at least
one and not limited to just one.
[0030] In addition, the following definitions are intended. In a
numerical context, the word "about" is to be construed as meaning
plus or minus 10%.
[0031] As mentioned before, an electrolysis cell may either be a
divided type or an undivided type of cell. In both, certain ions
may travel between the electrodes. A divided cell however is one in
which the cathode and anode sides of the cell are separated
(divided) in some manner such that most or all of the reactants and
products on each side of the cell are kept separate. An undivided
cell is one in which the cathode and anode sides of the cell are
not separated (undivided) and the reactants and products on each
side of the cell can mix.
[0032] Herein, the term "electrically insulating separator" is used
in the context of an undivided electrolysis cell and refers to any
of the various materials, components, or means for providing
electrical separation between the anode and cathode electrodes. It
thus can include porous sheet materials, spacers such as buttons,
or simply a gap (i.e. a space) between the electrodes.
[0033] The term "louvered" is used in the context of the porous
cell electrodes and refers to an electrode comprising a series of
angled openings (thereby rendering the electrode porous) which are
typically spaced at regular intervals (e.g. suggestive of window
blinds).
[0034] The present invention relates to improved designs and
methods for operating undivided electrolysis cells in order to
obtain better energy efficiency. It is particularly suitable for
use in chlorate electrolysis.
[0035] A schematic of a cell unit for an exemplary bipolar chlorate
electrolyzer in the prior art is shown in FIG. 1a. Cell unit 1
comprises anode carrier plate 2 and cathode carrier plate 3. A
plurality of solid anode plates 4 are mounted perpendicularly to a
side of anode carrier plate 2. In a like manner, a plurality of
solid cathode plates 5 are mounted perpendicularly to a side of
cathode carrier plate 3. These assemblies are then configured as
shown such that anode plates 4 interleave with cathode plates 5.
Each adjacent pair of anode plate 4 and cathode plate 5 forms an
electrolysis cell. The desired gap between each anode and cathode
plate is obtained by mounting plates 4, 5 with an appropriate
spacing to their respective carrier plates 2, 3.
[0036] A bipolar electrolyzer is then made by combining two or more
cell units 1 in a series stack, i.e. the flat side of anode carrier
plate 2 from one cell unit is attached to the flat side of cathode
carrier plate 3 from a second cell unit. (Typically, the carrier
plates are welded together, for instance by explosive bonding
because dissimilar metals are used for each polarity.) This kind of
bipolar arrangement is known in the art as a multi-monopolar
configuration.
[0037] The liquor for chlorate electrolysis primarily contains
sodium chloride in aqueous solution. However, the liquor is usually
recirculated and thus comprises a certain amount of the products of
electrolysis and also products of subsequent chemical reaction
(e.g. sodium hypochlorite, sodium chlorate, chlorine and hydrogen
gases). Further, certain additives (e.g. sodium dichromate) are
also usually included to enhance chlorate formation. In operation,
this liquor is supplied at the bottom of the numerous electrolysis
cells (indicated by arrow 6 in FIG. 1a) and flows upwardly between
the anode and cathode plates 4, 5. Electrolysis takes place along
with certain chemical reactions as the liquor travels between anode
and cathode plates 4, 5. The collected liquor comprising the
products of the electrolysis and these chemical reactions is
removed at the top of the electrolysis cells (indicated by arrow 7
in FIG. 1b).
[0038] FIG. 1b illustrates the primary electrochemical and chemical
reactions occurring in the chlorate electrolysis process. At anode
plate 4, electrochemical reaction 1) occurs:
2NaCl.fwdarw.Cl.sub.2+2e.sup.-+2Na.sup.| 1)
[0039] At cathode plate 5, electrochemical reaction 2) occurs:
2H.sub.2O+2e.sup.-.fwdarw.H.sub.2.uparw.+2OH.sup.- 2)
[0040] Because the electrolysis products and liquor are not kept
apart or divided, while the liquor travels between the cell
electrodes, the chlorine produced by electrolysis reacts quickly
with components in the liquor according to chemical reaction
3):
2Na.sup.++2OH.sup.-+Cl.sub.2.fwdarw.Na.sup.+Cl.sup.-+Na.sup.+OC.sup.-+H.-
sub.2O 3)
[0041] As illustrated in FIG. 1b, the hydrogen gas produced
according to reaction 2) forms bubbles 8 and displaces the liquor
electrolyte between anode plate 4 and cathode plate 5. Hydrogen 8
and collected liquor are then removed from the top of the
electrolysis cell (arrow 7). The NaOCl produced according to
reaction 3) is converted to sodium chlorate in a chemical reactor
at elevated temperature at a later stage in the production process.
The overall reaction 4) in a chlorate production system is
thus:
NaCl+3H.sub.2O.fwdarw.NaClO.sub.3+3H.sub.2.uparw. 4)
[0042] To a certain extent, hydrogen bubbles 8 interfere with the
electrolysis process by increasing the effective resistance of the
liquor electrolyte between the electrodes and/or by "gas blinding"
(wherein access to the cathode surface is blocked). Thus, the gap
between anode plate 4 and cathode plate 5 cannot be made too small
without experiencing a substantial negative effect due to these
effects.
[0043] In a chlorate electrolysis cell of the invention however,
the generated hydrogen gas is quickly removed from between the
anode and cathode electrodes and out of the electrolysis region.
Any increase in electrolyte resistance and gas blinding is
significantly reduced and the electrodes can be brought much closer
together, thereby reducing the liquor electrolyte resistance
between the electrodes and increasing energy efficiency. In order
to further reduce any negative effects arising from the presence of
hydrogen bubbles between the electrodes, it can be beneficial to
increase the pressure drop across the electrolysis cell which in
turn can advantageously prevent back migration of hydrogen into the
spaces between the electrodes. Such an increased pressure drop can
be engineered into the electrolysis cell generally by selecting the
cell components such that the flow resistance of at least one of
the separator and the cathode is greater than the flow resistance
of the anode. As will be readily apparent to those in the art, this
can be achieved for instance via appropriate modifications to the
relative pore characteristics in these various components.
[0044] An improved chlorate electrolysis cell of the invention is
shown in the schematic of FIG. 2a. Electrolysis cell 10 comprises
porous anode 14 and porous cathode 15 separated by porous and
electrically insulating separator 11. Porous anode 14 is
electrically connected to anode pan 12 by several sets 18 of
electrically conductive fingers. Anode pan 12 serves as an anode
current carrier for the cell. In a like manner, porous cathode 15
is electrically connected to cathode pan 13 by several sets 19 of
electrically conductive fingers and posts 20. Cathode pan 13 serves
as a cathode current carrier for the cell.
[0045] Further, electrolysis cell 10 comprises seals 21 at both top
and bottom which seal to anode pan 12, anode 14, cathode 15, and
cathode pan 13. The sealed anode 14 and anode pan 12 thus form the
surface of an anode compartment for the chlorate liquor. In a like
manner, the sealed cathode 15 and cathode pan 13 form the surface
of a cathode compartment for the chlorate liquor. In the embodiment
shown in FIG. 2a, the anode compartment comprises liquor inlet 22,
while the cathode compartment comprises liquor and hydrogen gas
outlet 23. (In actual embodiments, a distributor may also be
employed to distribute the liquor at the inlet, but is not shown in
FIG. 2a. Further, it should be noted that, in principle, the liquor
inlet and liquor outlet may instead be located in the opposite
compartments, namely the cathode and anode compartments
respectively. While such an arrangement may not be optimal in most
applications, it nonetheless is expected to be functional and can
be contemplated.) The cathode compartment may optionally contain
features (not shown in FIG. 2a) which assist in directing hydrogen
gas product upwards towards the hydrogen gas outlet 23. Such
features may include shaped baffles and the like for this
purpose.
[0046] In electrolysis cell 10, the liquor is forced through porous
anode 14 and then porous cathode 15 instead of travelling upwards
between anode plate 4 and cathode plate 5 as in the prior art cell
of FIG. 1b. Here, liquor is supplied (indicated by arrow 16) to
liquor inlet 22, enters the anode compartment, is forced through
porous anode 14, through porous separator 11, through porous
cathode 15, and into the cathode compartment. The collected liquor
and hydrogen gas are then directed out (indicated by arrow 17) from
outlet 23. In order to ensure that the liquor is completely
directed through porous anode 14 and porous cathode 15,
electrolysis cell 10 may comprise no substantial outlet in the
anode compartment and no other inlet in either the anode or cathode
compartment (although a small bleed outlet may typically be
provided to discharge hydrogen collecting at the top at the anode
compartment).
[0047] Both anode 14 and cathode 15 are desirably quite permeable
to the liquor. As mentioned previously however, it may be
advantageous for the former to be more permeable than the latter to
introduce a desirable pressure drop in the cell. While various
design options are available, use of expanded metals for either or
both of these electrodes is a convenient choice. Alternatively, and
as more clearly shown for both anode 14 and cathode 15 in FIG. 2b,
louvered metal electrodes may be employed. The louvered metal/s can
have a plurality of angled openings or slots to assist in guiding
the liquor and generated hydrogen gas upwards, through, and out of
the electrolysis region. Many other design options are possible
though for either or both electrodes including plates in which
numerous holes have been drilled, sheets made of sintered metal
fibres, etc. In chlorate electrolysis, the anode and other anode
hardware is typically made of a coated titanium and the cathode and
other cathode hardware is typically made of nickel-free stainless
steel or a type of mild steel.
[0048] Porous, electrically insulating separator 11 is also
desirably quite permeable to the liquor. Further, it should be
compatible with the chemical reactants and products. And any means
for achieving a reliable, insulating separation may be employed.
For instance, separator 11 can simply be a gap between the
electrodes. However, to obtain narrow gaps while still maintaining
reliable electrical separation between the electrodes, it is
generally desirable to employ some porous, electrically insulating
spacing material. For instance, a number of polymer buttons may be
used to space anode 14 and cathode 15 apart. Or, in one desirable
embodiment, a porous polymer sheet is used because such sheets can
be made quite thin yet robust and allow for distances between the
anode and cathode of less than about 2 mm (a typical minimum in
conventional chlorate electrolysis cells relying on a gap). A
variety of fluoropolymers can be used as suitable materials for
separator 11 because they are capable of withstanding the
corrosive, demanding environment in the electrolyzes.
Alternatively, certain ceramics can be used as suitable materials
for separator 11. Materials may have hydrophobic surface
properties. Alternatively, hydrophilic materials may further
improve performance by reducing hydrogen disengagement.
[0049] In FIG. 2a, the main current carriers for the electrolysis
current are anode pan 12 and cathode pan 13. Current is distributed
to anode 14 from anode pan 12 via set 18 of electrically conductive
fingers. And current is directed from cathode 15 to cathode pan 13
via set 19 of electrically conductive fingers and connecting posts
20. Again, this anode and cathode hardware is typically made of a
coated titanium and nickel-free stainless steel or mild steel
respectively.
[0050] Anode pan 12 and cathode pan 13 can comprise sets of
complementary features which have been formed therein for purposes
of providing structural strength and of locating and electrically
connecting cells during assembly into a multi-cell bipolar
electrolyzer. In FIG. 2a, anode pan 12 and cathode pan 13 comprise
complementary sets of cone shaped features, namely 12a and 13a
respectively, which have been formed therein for this purpose.
(Exemplary bipolar electrolyzers made with such cells appear later
in FIGS. 3 and 4.)
[0051] FIG. 2b illustrates the primary reactions and the hydrogen
gas product generated in the electrolysis cell of FIG. 2a. The same
electrochemical and chemical reactions occur here as those in FIG.
1a. Here however, the supplied liquor (indicated by arrow 16) flows
through porous anode 14, separator 11, and porous cathode 15.
Electrolysis takes place during this transit and collected liquor
and hydrogen gas product 24 are removed as indicated by arrow 17.
Unlike in the cell of FIG. 1b, the generated hydrogen bubbles 24
are quickly carried out with the liquor from the region between
anode 14 and cathode 15 and into the cathode compartment. Gas
blinding is thus substantially reduced. Note that the chemical
reaction of generated chlorine with components in the liquor can
take place within separator 11, porous cathode 15, and/or in the
cathode compartment.
[0052] Operation of the chlorate electrolysis cells of FIGS. 2a and
2b is generally similar to that of conventional chlorate
electrolysis cells. For instance, the reactants, concentrations,
temperatures and pressures can all be the same or similar. Again
though, as mentioned previously, it may be advantageous to provide
for an increased pressure drop across the cell in order to prevent
backflow of hydrogen from the cathode compartment into the
electrochemically active area of the cell.
[0053] Electrolysis cells of the type shown in FIG. 2a can be
readily used to construct electrolyzers with a bipolar
configuration. FIG. 3 shows a schematic of such a bipolar
electrolyzer which comprises a stack of four chlorate electrolysis
cells 10 of FIG. 2a in series.
[0054] In bipolar electrolyzer 30, four chlorate electrolysis cells
10 are stacked such that the anodes 14 and cathodes 15 in the cells
therein are oriented vertically. Additionally, electrolysis cells
10 are stacked such that complementary features 12a in anode pans
12 nest with features 13a in adjacent cathode pans 13 and thereby
ensure that the cells are located well with respect to each other
and are well connected electrically. An advantage of the invention
is that the electrolysis cells can be made as modules and thus may
be readily removed and exchanged as desired.
[0055] In the embodiment shown in FIG. 3, bipolar electrolyzer 30
comprises a series of liquor feed headers 31 (one for each
electrolysis cell 10) which distribute liquor to liquor inlets 23
in individual electrolysis cells 10. Feed headers 31 are fed with
liquor by a series of liquor feed tubes 32. Bipolar electrolyzer 30
also comprises a series of collector/degassifier headers 33 (again,
one for each electrolysis cell 10) which serve as headers to
collect liquor and hydrogen gas from outlets 23 from individual
electrolysis cells 10. The collected liquor comprising hypochlorite
and other dissolved products of electrolysis is removed from
electrolyzer 30 via outlet tube 34. The collected hydrogen gas is
then separated off from outlet tube 34 via branch tube 35. If
desired, electrolyzer 30 may also comprise a suitable drain in the
cathode compartment and/or a vent in the anode compartment.
[0056] A different exemplary orientation for the cells in a bipolar
electrolyzer of the invention is shown in FIG. 4. Therein, bipolar
electrolyzer 40 is oriented such that anodes 14 and cathodes 15 in
the cells are oriented perpendicular to the vertical. Specifically,
electrolyzer 40 is oriented such that cathodes 15 are above anodes
14 in the individual electrolysis cells. Depending on the
components used in the cells and other details of construction,
this orientation may improve the removal of hydrogen gas
by-product.
[0057] Electrolyzers of the type shown in FIGS. 3 and 4 may then be
employed to obtain improved efficiency in otherwise conventional
chlorate production systems. In that regard, FIG. 5 shows a
schematic of a merchant chlorate production system comprising a
chlorate electrolyzer of the invention.
[0058] Merchant chlorate production system 50 is relatively complex
and in sequence comprises salt and water feed 51, brine saturator
52, guard filter 53, ion exchange subsystem 54, brine line 55,
chlorate reactor 56, liquor inlet line 57, chlorate electrolyzer 58
of the invention, collected liquor outlet line 59, chlorate
crystallization subsystem 61 and line 62 which are interconnected
as shown. Brine for electrolysis is prepared in brine saturator 52.
A suitable source of salt (e.g. evaporated salt) and a supply of
demineralised water is provided at salt and water feed 51. From
there, brine is directed via brine line 55 to guard filter 53, then
to ion exchange subsystem 54 and finally to chlorate reactor 56
where it is mixed with the product from chlorate electrolyzer 58 to
maintain the salt content in the electrolyzer feed. Chlorate
reactor 56 directs an electrolyte solution for electrolysis
comprising both chlorate and brine to chlorate electrolyzer 58 via
liquor inlet line 57. And electrolyzed chlorate solution from
chlorate electrolyzer 58 is directed back to chlorate reactor 56
via collected liquor outlet line 59. The hydrogen gas produced is
removed via line 60.
[0059] Concentrated product chlorate solution from chlorate reactor
56 is directed to chlorate crystallization subsystem 61 where
chlorate product is crystallized out from the more concentrated
chlorate solution and removed at 62. The leftover solution after
crystallizing is recirculated back to chlorate reactor 56 via
recirculation line 63. Over time, impurities can accumulate in
recirculation line 56. These impurities may be removed in a variety
of ways. For instance, chlorate electrolysis system 50 in FIG. 5
includes side stream subsystem 64 connected in parallel to
recirculation line 63. The configuration of components within side
stream subsystem 64 and their operation are fully described in
WO2014/029021.
[0060] Electrolyzers of the invention may also be employed in
chlorate production systems or subsystems other than merchant
chlorate production systems like that shown in FIG. 5. For
instance, a chlorate electrolyzer of the invention may desirably be
employed in a chlorate production subsystem which forms part of an
integrated chlorine dioxide production system. An exemplary such
system is the Chemetics integrated chlorine dioxide system which
consists of three plant areas to produce the two intermediate
products, sodium chlorate (NaClO3) and hydrochloric acid (HCl), and
the final product, chlorine dioxide (ClO2). In such a system,
sodium chlorate can desirably be produced by electrolysis of a
sodium chloride solution using an electrolyzer of the invention to
make strong sodium chlorate liquor. The salt for this reaction is a
recycled by-product from the chlorine dioxide production area.
Hydrogen gas is co-produced with the sodium chlorate, and is used
as a feedstock for hydrochloric acid production. Hydrochloric acid
is produced by burning chlorine gas and hydrogen gas. The hydrogen
gas comes from the sodium chlorate electrolysis area. Make-up
chlorine gas comes from the plant battery limits. Weak chlorine
gas, a recycled by-product of the chlorine dioxide generation area,
is combined with this chlorine make-up stream prior to being burned
with the hydrogen gas. Chlorine dioxide gas is produced, along with
chlorine gas and sodium chloride (salt), by combining strong
chlorate liquor and hydrochloric acid in the chlorine dioxide
generator. The chlorine dioxide gas is absorbed in chilled water
and then stripped with air to remove residual chlorine, to produce
a high-purity chlorine dioxide solution for commercial use;
typically in an ECF pulp mill bleach plant. The liquor leaving the
generator contains unreacted sodium chlorate and the by-product
salt. This solution, called weak chlorate liquor, is recycled back
to the sodium chlorate electrolysis area for reconcentration. The
chlorine by-product (weak chlorine), which is not absorbed, is
recycled for hydrochloric acid production. Present Chemetics
integrated chlorine dioxide systems are described in more detail
in, for instance, "Adopting the Integrated Chlorine Dioxide Process
for Pulp Bleaching to Comply with CREP Regulations"; A. Barr et
al., IPPTA, J. Vol. 21, No. 1, January-March, 2009 121-127.
[0061] The method of the invention is particularly suitable for use
in chlorate electrolysis. However, any process involving undivided
electrolysis may benefit from the designs and methods of the
invention, e.g. hypochlorite production, perchlorate production,
potassium permanganate production. Further, use of the invention
can provide greater flexibility in the design and/or configuration
of the electrolyzers employed. For instance, conventional chlorate
electrolyzers have practical limits on cell height arising from the
amount of generated hydrogen gas. However, such limitations may not
apply to electrolyzers of the invention, and thus higher cells may
be considered in practical embodiments.
[0062] The following Examples have been included to illustrate
certain aspects of the invention but should not be construed as
limiting in any way.
EXAMPLES
[0063] Laboratory type chlorate electrolysis cells were made and
operated to compare the characteristics of a cell of the invention
to those of a conventional cell.
[0064] An inventive electrolysis cell was made comprising a
pan-shaped anode compartment and a pan-shaped cathode compartment.
Both compartments were made of grade 2 titanium and had dimensions
of about 38 cm in width, 23 cm in height and 2.2 cm in depth. A
cell assembly comprising a porous anode, a porous cathode, and an
electrically insulating separator therebetween was sandwiched
together between the lips of the pan-shaped anode and cathode
compartments. The lips of the anode and cathode compartments
directly contacted the edges of the anode and cathode respectively
and thus were electrically connected thereto to serve as anode and
cathode current carriers respectively. The separator extended
slightly beyond the edges of the anode and cathode. The
electrolysis cell was sealed by bolting together suitably shaped
gaskets between the extended separator edge and the lips of the
anode and cathode compartments.
[0065] Both the anode and cathode employed substrates which were
made of expanded titanium mesh (approximately 1 mm thick, with
diamond shaped openings in which the diamond dimensions were about
3 mm by 1.5 mm, and an open area .about.50%) thus rendering both
anode and cathode permeable to cell liquor. Prior to assembly, both
substrates were treated by sandblasting and an acid etching
process, and then coated with catalyst comprising mixed metal
oxides (mainly RuO.sub.2, IrO.sub.2, and TiO.sub.2).
[0066] An expanded PTFE mesh was used as the cell separator, thus
rendering the separator permeable to cell liquor. The PTFE mesh was
about 0.95 mm thick and had diamond shaped openings whose
dimensions were about 1/2 the size of those of the anode and
cathode substrates. In addition however, to improve flow and
performance, additional larger openings were cut out from the
separator. Specifically, nine evenly-spaced rectangular openings
(about 32 mm by 83 mm in size) were cut out from the separator
surface.
[0067] The active surface of the cell assembly (i.e. the surface
within the seal of the electrolysis cell) had dimensions of about
10 cm by 25 cm. The electrolysis cell was oriented vertically and
had three liquor inlets at the bottom of the anode compartment, and
three liquor and gas product outlets at the top of the cathode
compartment. These liquor inlets and outlets were evenly spaced and
about 13 mm in diameter. Liquor distributors were employed within
each of the anode and cathode compartments to achieve a more even
flow of cell liquor into and out from the electrolysis cell. The
anode and cathode distributors were located in the compartments
immediately adjacent the liquor inlets and outlets respectively.
Both distributors were about 16 mm.times.22 mm.times.25 cm in size
and each comprised 8 evenly spaced distribution holes about 6 mm in
diameter.
[0068] A comparative electrolysis cell was constructed in a similar
manner to a conventional chlorate electrolysis cell (e.g. to
operate as represented by FIG. 1b) and which had been optimized for
commercial use. Here, the anode employed a substrate comprising
solid titanium plate (approximately 2 mm thick) which had been
prepared and coated with mixed oxide catalyst in a similar manner
to the preceding inventive cell. The cathode was a solid steel
plate (approximately 3 mm thick) with no catalyst coating. The
anode and cathode were separated via use of a gasket at the
periphery with a gap of about 2 mm between anode and cathode. The
comparative electrolysis cell had a liquor inlet accessing the
bottom of the gap between anode and cathode, and a liquor outlet at
the top of the gap.
[0069] The electrolyte used for test purposes was a convention
chlorate solution having a composition of
NaClO.sub.3/NaCl/Na.sub.2Cr.sub.2O.sub.7 in concentrations of
450/110/5 gpl. In the inventive electrolysis cell, electrolyte
flowed in the liquor inlets at the bottom of the anode compartment,
through the liquor distributor therein, through the porous anode,
the porous separator, and the porous cathode, and then out through
the liquor distributor in the cathode compartment, and finally out
the top of the cathode compartment. In the comparative cell,
electrolyte flowed in from the liquor inlet at the cell bottom,
through the gap between anode and cathode, and then out the liquor
outlet at the cell top. The cells were initially conditioned for
4.5 days by operating at a current density of 3 kA/m.sup.2 (based
on active area of electrodes), a pH of 6, and a temperature of
80.degree. C. Thereafter, the temperature of the electrolyte was
raised to 90.degree. C. and polarization results (i.e. voltage
versus current density) were obtained for each cell over a range of
current densities from 0.5 to 5 kA/m.sup.2. The electrolyte flow
rate used was about 100 l/h.
[0070] FIG. 6 plots the polarization results obtained for the
inventive and comparative electrolysis cells. The cell voltage of
the inventive electrolysis cell is markedly better than that of the
conventional electrolysis cell over the full range of current
densities tested and is, for instance, about 620 mV lower at 4
kA/m.sup.2.
[0071] All of the above U.S. patents, U.S. patent applications,
foreign patents, foreign patent applications and non-patent
publications referred to in this specification, are incorporated
herein by reference in their entirety.
[0072] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art without
departing from the spirit and scope of the present disclosure,
particularly in light of the foregoing teachings. Such
modifications are to be considered within the purview and scope of
the claims appended hereto.
* * * * *