U.S. patent application number 12/086374 was filed with the patent office on 2010-12-16 for oxygen-consuming zero-gap electrolysis cells with porous/solid plates.
Invention is credited to Michael L. Perry.
Application Number | 20100314261 12/086374 |
Document ID | / |
Family ID | 38163364 |
Filed Date | 2010-12-16 |
United States Patent
Application |
20100314261 |
Kind Code |
A1 |
Perry; Michael L. |
December 16, 2010 |
Oxygen-Consuming Zero-Gap Electrolysis Cells With Porous/Solid
Plates
Abstract
An electrolysis stack (53) with oxygen-depolarized cathodes (31)
employs solid-plate anodes (38) and porous-plate cathodes (42). The
stack (53) of electrolysis cells (29) (e.g, hydrogen-chloride or
chlor-allkali cells) each include an ion exchange membrane (32)
sandwiched between an anode conductor (34) and a permeable cathode
(35); an oxygen-consuming gas diffusion cathode (31) is adjacent
the cathode conductor of each cell. Between the anode conductor of
one cell and the gas diffusion cathode of an adjacent cell there is
a composite bipolar plate (51) including a solid plate (38) having
channels (39) for conducing salt solution and product of the
process; the bipolar plates also include a porous plate (42) having
channels (43) for conducting oxidant adjacent the gas diffusion
cathode and channels (49) connected to a source of liquid (such as
water or dilute sodium hydroxide).
Inventors: |
Perry; Michael L.; ( South
Glastonbury, CT) |
Correspondence
Address: |
M.P. Williams
210 Main Street
Manchester
CT
06042
US
|
Family ID: |
38163364 |
Appl. No.: |
12/086374 |
Filed: |
December 14, 2005 |
PCT Filed: |
December 14, 2005 |
PCT NO: |
PCT/US05/45298 |
371 Date: |
June 11, 2008 |
Current U.S.
Class: |
205/618 ;
204/258; 204/265 |
Current CPC
Class: |
C25B 1/26 20130101; C25B
15/08 20130101; C25B 9/77 20210101 |
Class at
Publication: |
205/618 ;
204/265; 204/258 |
International
Class: |
C25B 1/26 20060101
C25B001/26; C25B 9/10 20060101 C25B009/10 |
Claims
1. A method of operating an oxygen-depolarized electrolysis cell
having an anode and having a cathode including a water permeable
gas diffusion electrode, said method comprising: feeding a solution
to the anode of the cell selected from (a) a salt solution and (b)
a solution of halide acid; applying DC power between the anode and
the cathode of the cell to drive electrochemical reactions in the
cell to produce desired product; and recovering desired product
from the cell; characterized by: flowing oxygen-containing gas
through passageways on a side of a porous hydrophilic plate
adjacent to the gas diffusion electrode of the cathode; and
circulating a water-containing liquid solution through passageways
in the porous hydrophilic plate which are separated from the flow
of oxygen-containing gas.
2. A method according to claim 1 wherein said step of flowing is
further characterized by: flowing non-hydrated oxygen-containing
gas.
3. A method according to claim 1 wherein said step of flowing is
further characterized by: flowing oxygen-containing gas which is
not saturated with water.
4. A method according to claim 1 wherein said step of flowing is
further characterized by: flowing oxygen-containing gas at a
pressure which is lower than the pressure of the water-containing
liquid.
5. A method according to claim 1 further characterized by: removing
substantially all carbon dioxide from the oxygen-containing gas
before flowing the oxygen-containing gas through said cell.
6. A method according to claim 1 further characterized by: said
step of feeding comprising feeding a halide acid solution in water;
and said step of circulating comprises circulating water.
7. A method according to claim 6 further characterized by: said
step of feeding comprises feeding hydrochloric acid.
8. A method according to claim 6 further characterized by: said
step of feeding comprises feeding brine; and said step of
circulating comprises circulating a dilute solution of sodium
hydroxide.
9. An electrolysis cell (29) with an oxygen-depolarized cathode
(31), comprising: a permeable anode conductor (34), a permeable
cathode conductor (35), an ion exchange membrane (32) disposed
between and contacting said conductors, a solid plate (38) having
salt/product channels (39) adjacent to said anode conductor,
configured to receive salt solution and configured to conduct
product of said salt/product channels, and an oxygen consuming, gas
diffusion cathode (31) contacting said cathode conductor;
characterized by the improvement comprising: a porous, hydrophilic
plate (42) having oxidant channels (43), extending from a first
surface thereof contacting said gas diffusion cathode, configured
to receive an oxygen-containing gas, said porous hydrophilic plate
also having liquid channels (44), extending from a second surface
thereof opposite to said first surface, configured to receive a
water-containing liquid.
10. An electrolysis cell (29) according to claim 9 wherein: a noble
metal or noble metal alloy catalyst is disposed in said cathode
conductor (35) adjacent to said membrane (32).
11. A stack (53) of electrolysis cells (29) according to claim
9.
12. An electrolysis cell (29) according to claim 9 wherein: said
salt/product channels (39) are configured to receive brine; said
liquid channels (43) are configured to receive a dilute solution of
water and sodium hydroxide; and said salt/product channels are
configured to provide chlorine as product.
13. An electrolysis cell (29) according to claim 9 wherein: said
salt/product channels (39) are configured to receive halide acid
solution; said liquid channels (43) are configured to receive
water; and said salt/product channels are configured to provide
chlorine as product.
14. An electrolysis cell (29) according to claim 13 wherein: said
salt/product channels (39) are configured to receive hydrogen
chloride solution.
Description
TECHNICAL FIELD
[0001] This invention relates to industrial electrolysis cells,
such as may be used in the production of chlorine gas from hydrogen
chloride or from sodium chloride solutions, employing an
oxygen-consuming cathode, with a zero-gap structure, which employs
solid plates adjacent the anode where chlorine is evolved and
porous plates adjacent the cathode electrode having passageways for
liquid solution and having passageways for oxygen-containing
gas.
BACKGROUND ART
[0002] Traditional production of chlorine employs a process in
which the electrolysis of brine (that is, sodium chloride and
water) to form chlorine gas and sodium hydroxide (also known as
caustic soda) utilizing hydrogen-evolving cathodes. In the year
2003, U.S. production of chlorine was about 13 million tons and
production of caustic soda was about 15 million tons, the
production of which consumed about 10 GW (about 317 trillion BTUs)
of electrical energy; this corresponds to about two percent of the
total electric power generated in the U.S.
[0003] In order to achieve greater energy savings, the
hydrogen-evolving cathodes can be replaced by oxygen-consuming
cathodes to produce an energy savings of as much as 30%, in what
are sometimes called oxygen-depolarized cells.
[0004] Additionally, other industrial electrolysis processes that
employ hydrogen-evolving cathodes can also realize an energy
efficiency benefit by employing oxygen-consuming cathodes. Examples
include the electrolysis of hydrogen chloride to recover chlorine
or the electrolysis of hydrogen bromide solution to produce
bromine.
[0005] The original oxygen-consuming chlor-alkali processes
involves an anode chamber, through which brine is circulated and
from which chlorine gas evolves, a cathode chamber through which
oxygen is circulated, and a solution chamber between the other two
chambers in which water is converted to sodium hydroxide. This is
typically referred to as the finite-gap design. It is thought that
the layer of sodium hydroxide between the oxygen electrode and the
membrane unfavorably increases the cell resistance, and hydrostatic
pressure of the sodium hydroxide solution leads to non-uniform
gas/liquid interfaces within the oxygen electrode, which results in
electrode flooding in some spots, and in leakage of the sodium
hydroxide into the oxygen compartment in other spots.
[0006] To overcome these difficulties, a zero-gap oxygen-consuming
chlor-alkali electrolysis cell was developed, as disclosed in U.S.
Pat. No. 6,117,286 and described with respect to FIG. 1 herein. The
cell 11 is portioned into an anode chamber 13 and a cathode chamber
14 by means of an ion-exchange membrane 12. The cell has a
mesh-form insoluble anode 15, which may comprise a conventional
insoluble titanium electrode known as a dimensionally-stable anode
(DSA), in intimate contact with the ion-exchange membrane 12, on
the side thereof adjacent the anode chamber 13. A sheet-form
hydrophilic material 16 is in intimate contact with the
ion-exchange membrane 12 on the side thereof adjacent the cathode
chamber 14. The cell 11 also has a liquid-permeable oxygen gas
diffusion cathode 17 in intimate contact with the hydrophilic
material 16 on the side thereof adjacent the cathode chamber 14. A
mesh-form cathode collector 18 is connected to the oxygen gas
diffusion cathode 17 so that electricity is supplied through the
anode 15 and the collector 18, as shown by the plus and minus
signs.
[0007] An inlet 20 receives saturated aqueous sodium chloride
solution as well as discharging the chlorine gas which is produced,
and an outlet 19 discharges the aqueous solution of unreacted
sodium chloride. An inlet 21 receives humidified oxygen-containing
gas and an outlet 22 allows discharge of excess oxygen-containing
gas as well as sodium hydroxide formed in the process. Sodium
hydroxide is generated on the surface of the ion-exchange membrane
12 which faces the cathode chamber 14 and descends in a dispersed
fashion, especially due to gravity, within the hydrophilic material
16, which provides less flow resistance to the sodium hydroxide
solution than would the cathode 17 itself. The sodium hydroxide
solution drips from the lower edge of the hydrophilic material 16
and passes through the outlet 22. This avoids having the sodium
hydroxide solution residing in the oxygen gas diffusion cathode and
impeding the oxygen-containing feed gas from smoothly permeating
through the cathode.
[0008] In patent application publication US 2005/0026005, instead
of employing a hydrophilic sheet material 16 adjacent the oxygen
gas diffusion cathode 17, the oxygen gas diffusion cathode is
provided with a composite layer of carbon-supported platinum and
polytetrafluorethylene on the side of the oxygen diffusion cathode
facing the cathode chamber 14. The purpose is stated to avoid
generation of peroxide, the precipitation of which as sodium
peroxide would cause liquid flow maintenance problems and damage to
the membrane 12 and/or the oxygen gas-diffusion cathode.
[0009] The aforementioned zero gap, oxygen consuming electrode
chlor-alkali cells present challenging problematic conditions. For
instance, the relatively high viscosity and strong corrosiveness of
concentrated sodium hydroxide can impede the effective transport of
reactants and products within the cathode and can damage the
cathode. Although the structure of the aforementioned patent
publication tends to avoid local dry out of the cathode which
promotes the formation of harmful peroxide, it is extremely
difficult to maintain the balance of having the cathode fully
saturated but not flooded at practical reactant stoichiometries,
especially under a wide variety of commercial operating conditions
across extended periods of time.
[0010] Additionally, other industrial electrolysis processes, such
as the conversion of hydrogen chloride to chlorine with an
oxygen-depolarized cathode, face analogous problematic issues.
DISCLOSURE OF INVENTION
[0011] Objects of the invention include: maintaining proper liquid
balance in any oxygen-consuming, electrolysis membrane cell;
providing a gas diffusion electrode which will maintain three-phase
boundaries of gas, liquid and solid, that include oxygen,
water/other solutions (such as water/caustic-soda solutions), and
the cathode catalyst/support particles; providing a gas diffusion
electrode which effectively transports oxygen to the cathode
catalyst layer while removing the liquid products away from the
catalyst layer; provision of a gas diffusion electrode in an
electrolysis cell which assures that the ion-exchange membrane
remains well hydrated without local dryout regions, while at the
same time preventing flooding of the cathode catalyst layer; a
chlor-alkali cell which does not require a supply of air that is
humidified; electrolysis cells that are not adversely affected by a
gradient of liquid pressure from the top of the cell to the bottom
of the cell; and improved electrolysis cells.
[0012] This invention is predicated in part on the discovery that
the presence of a dilute solution of sodium hydroxide, in a
relatively uniform concentration and pressure, across the entire
gas diffusion cathode enhances operation of the oxygen-consuming
chlor-alkali cell. The invention is also predicated on the
discovery that the oxygen reactant gas can be provided directly to
the oxygen-consuming cathode without interference from sodium
hydroxide or other liquid flooding, by providing channels for a
recirculating flow of sodium hydroxide, water, or other
water-containing liquid separate from channels for a flow of
oxygen-containing gas, such as air.
[0013] In accordance with the invention, oxygen-containing gas is
flowed across the gas diffusion layer of an oxygen depolarized
cathode of an electrolysis cell, and a liquid solution is flowed
through channels separated from the diffusion layer by a porous
plate to hydrate the membrane and remove excess water.
[0014] According to one form of the present invention, an
electrochemical apparatus includes a composite, bipolar plate
disposed between the gas diffusion cathode of one oxygen consuming
electrolysis cell and the anode of an adjacent cell; the portion of
the bipolar plate adjacent to the anode is solid and contains
passageways which circulate salt solution (such as brine or halide
acid solution) and recover the gaseous product (e.g., chlorine or
bromine) that is produced. The other portion of the bipolar plate
is highly porous and hydrophilic, having reactant air or oxygen
channels in one surface which are disposed in intimate contact with
the oxygen-consuming gas diffusion cathode, and having channels in
the opposite surface of said porous plate, through which liquid
(e.g., sodium hydroxide or water, in some embodiments) is
circulated, the liquid solution entering those channels at a
desired reduced concentration (which may typically be on the order
of 32%). According to the invention, the pressure of the sodium
hydroxide or other water-containing liquid circulating through the
porous plate is lower than the pressure of the air or other
oxygen-containing gas; this pressure differential provides a
driving force for liquid removal from the gas-diffusion electrode,
which prevents the gas diffusion electrode from being flooded,
while at the same time the electrode and the adjacent membrane is
kept well hydrated by the liquid solution circulating through the
plate.
[0015] The invention may be practiced as a mono-polar cell, if
desired.
[0016] An oxygen-depolarized electrolysis cell according to the
present invention provides the proper amount of hydration
throughout the face of the entire gas diffusion cathode, removing
excess liquid product where necessary and providing additional
moisture where necessary, at various locations across the planform
of the gas diffusion cathode. Similarly, since the oxygen, in
accordance with the present invention, is presented to the gas
diffusion cathode through separate air channels, at a pressure
higher than that of the liquid solution, the presence of air at all
areas of the gas diffusion electrode is ensured.
[0017] The invention may be practiced by providing solid and porous
carbon plates in accordance with techniques which are customary in
the production of solid plates and porous, hydrophilic plates for
use in fuel cells, particularly for use in proton exchange membrane
fuel cells.
[0018] Other objects, features and advantages of the present
invention will become more apparent in the light of the following
detailed description of exemplary embodiments thereof, as
illustrated in the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a sectional, side elevation view of a zero-gap,
oxygen consuming chlor-alkali cell known to the prior art.
[0020] FIG. 2 is a sectioned side elevation view of a
hydrogen-chloride electrolysis cell in accordance with the present
invention, with stippling indicating the porous portion of the
bipolar plate and section lines indicating the solid portion of the
bipolar plate.
[0021] FIG. 3 is a sectioned side elevation view of a chlor-alkali
cell in accordance with the present invention, with stippling
indicating the porous portion of the bipolar plate and section
lines indicating the solid portion of the bipolar plate.
[0022] FIG. 4 is a multiple representation of the chlor-alkali cell
of the invention shown in FIG. 3, illustrating the simplicity of
the repetitive structure of a stack of electrolysis cells in
accordance with the invention.
MODE(S) FOR CARRYING OUT THE INVENTION
[0023] Referring to FIG. 2, a hydrogen-chloride electrolysis cell
29 employing an oxygen-consuming, gas-diffusion cathode 31 includes
a conventional ion-exchange membrane 32 flanked by conductive anode
and cathode screens 34, 35. The screens 34, 35 may contain
catalysts that promote the respective electrochemical reactions,
numbered 1-3 hereinafter, at respective electrodes. The anode may
be a conventional DSA. On the anode side, a solid (non-porous)
plate 38 includes passageways 39 for the circulation of hydrogen
chloride solution and for the extraction of chlorine gas which is
produced by the electrolysis process. On the cathode side, a
micro-porous, hydrophilic plate 42 has passageways 43 for
conducting non-hydrated, oxygen-containing gas, such as air, and
passageways 44 for circulating a water-containing liquid such as
sodium hydroxide or simply water.
[0024] The electrode conductors 34, 35 are respectively positive
and negative terminals (as indicated by the plus and minus signs
46, 47) and these are connected to an appropriate source of direct
current electrical (DC) power 48. With a hydrogen chloride
solution, water, and air being provided to the cell 29, and with
the anode and cathode conductors 34 and 35 connected across the DC
power 48, the reactions at the anode are shown by equations 1 and
2, and the reaction at the cathode is shown by equation 3.
[0025] At anode
4HCl.fwdarw.4Cl.sup.-+4H.sup.+ 1.
4Cl.sup.-.fwdarw.+2Cl.sub.2+4e- 2.
[0026] At cathode
O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O 3.
A significant difference of the cell 29 compared to the prior art
is that the oxygen-containing stream (e.g., air) need not be
humidified prior to being fed to the cell 29. This is because the
hydrophilic pores in the porous plate 42 are filled with water (or
a dilute hydrogen chloride solution) and provide a means to fully
hydrate (saturate with water) the gas in the cathode passageways
43. Keeping the cell well-hydrated is critical since the membrane
is a poor ionic conductor if it dries out and the lifetime of the
membrane is significantly reduced under dry conditions.
Additionally, since the water in the passageways 44 is circulated
at a pressure below that of the gas pressure in the cathode
passageways 43, any excess liquid water in the cathode 31 is
removed by this pressure gradient. The removal of excess liquid
water from the cathode is necessary since liquid water is both
produced at the cathode, via reaction 3, and is transported to the
cathode from the anode with the flow of protons via electro-osmotic
drag. If the cathode is flooded with water, even in local spots,
this will prevent the access of oxygen to the cathode catalyst
layer 35, and instead of reaction 3 the following reaction can
occur in these flooded regions:
4H.sup.++4e.sup.-.fwdarw.2H.sub.2 4.
Obviously, this reaction is not desired from a safety perspective.
Additionally, it decreases the efficiency of the oxygen-depolarized
cathode.
[0027] FIG. 3 is described using an HCl solution as the source of
chlorine product; however, other halide acids, such as HBr, may be
used.
[0028] Referring to FIG. 3, a chlor-alkali cell 29 employing an
oxygen-consuming gas diffusion cathode 31 includes a conventional
ion exchange membrane 30 flanked by conductive anode and cathode
screens 34, 35. The screens may have catalysts to promote reactions
5-8, hereinafter, on respective electrodes. On the anode side, a
solid (non-porous) plate 38 includes passageways 39 for the
circulation of brine (sodium chloride solution) and for the
extraction of chlorine gas which is produced by the electrolysis
process. On the cathode side, a porous, hydrophilic plate 42,
passageways 43 for conducting oxygen-containing gas, such as air,
and passageways 44 for circulating sodium hydroxide solution, which
enters the passageways 44 in a desired concentration, such as on
the order of 32%, and leaves the passageways in a more concentrated
solution, due to the formation of sodium hydroxide by the
electrolysis process.
[0029] The electrode conductors 34, 35 are respectively positive
and negative terminals (as indicated by the plus and minus signs
46, 47) and these are connected to an appropriate source of DC
power 48. With brine, sodium hydroxide, and air being provided to
the cell 29, and with the anode and cathode conductors 34 and 35
connected across the source of power 48, the reactions at the anode
are shown by equations 5 and 6, and the reactions on the cathode
are shown by equations 7 and 8.
[0030] At anode
4NaCl.fwdarw.4Cl.sup.-+4Na.sup.+ 5.
4Cl.sup.-.fwdarw.2Cl.sub.2+4e.sup.- 6.
[0031] At cathode
2H.sub.2O+O.sub.2+4e.sup.-.fwdarw.4OH.sup.- 7.
4Na.sup.++4OH.sup.-.fwdarw.4NaOH 8.
[0032] A significant difference of the cell 29 compared to the
prior art is that sodium hydroxide in an appropriate solution
strength is provided to the cell, the solution thereby providing
the desired water (reaction 7) to eventually provide sodium
chloride (reaction 8). By providing a sodium hydroxide solution,
not only is water provided so that air need not be moisturized as
in the prior art, but the concentration of sodium hydroxide
solution, and therefore water, will be substantially uniform across
the planform of the gas diffusion cathode 31. This is achieved by
the porous plate 42 which is hydrophilic and allows passage of
sodium hydroxide solution through the porous plate 42 and air
channels 43 to reach the surface of the gas diffusion cathode 31.
Additionally, the pressure gradient provided across the porous
plate 42 will remove any excess liquid water in the cathode, which
will enable adequate oxygen access to support reaction 7 and
prevent hydrogen evolution from occurring at the cathode via:
4Na.sup.++4H.sub.2O.fwdarw.4NaOH+4H.sup.+ 9.
followed by reaction 4, hereinbefore.
[0033] Even in a low concentration of alkali solution, carbon
dioxide will result in the formation of carbonate salts that will
precipitate out of solution; therefore, the oxidant-containing
source (e.g., air) should first be scrubbed to remove CO.sub.2, as
is conventional. However, the air stream need not be
pre-humidified, or saturated with water, as described hereinbefore.
The invention may be practiced in a mono-polar cell design (a
single cell), or it may be practiced in a stack of cells. The cell
29 of FIG. 3 is shown in FIG. 4 with an additional cell 29 to
illustrate the ease of repeatability so as to form a stack 53 of
chlor-alkali cells 29. As easily seen in FIG. 4, the solid plates
38 and porous plate 42 together comprise a composite bipolar plate
51. If desired, mono-polar cells could also be readily constructed
by those skilled in the art using the porous-plate cathode concept
taught herein.
[0034] The configuration of the porous plate 42 may be very similar
to similar porous, hydrophilic plates which are known in fuel
cells, and typically comprise woven carbon sheets which are
rendered hydrophilic by treating with tin, or by other known
processes. The solid plate 38 may comprise solid carbon, solid
metal, or any other suitable material, such as a plastic with
carbon or glass fibers, metal or the like. The porous plate has to
be conducting in a bipolar plate configuration, and preferably
should be conducting in a mono-polar configuration, since it is the
thickest part and current must flow parallel to the plate
direction. The porous plate 42 can be constructed of a variety of
materials that have been used as porous reactant gas flow field
plates in fuel cell applications or it may be a porous metallic
plate made from powdered metal.
[0035] The gas diffusion electrode 31 may be constructed of
conventional materials used in fuel-cell cathodes, such as porous
carbon papers, cloths, or non-woven materials. Alternatively, the
gas-diffusion electrode could be constructed of a metal screen or a
combination of metal and carbon. The cathode catalyst layer 35 may
be constructed in a manner similar to state-of-the-art proton
exchange membrane (PEM) fuel cell cathodes, which typically are a
combination of catalyst and ion-exchange ionomer. In addition to
the ionomers in the catalyst layer, other polymers (e.g., PTFE) may
be used as binders and/or to control the desired hydrophobicity and
porosity of this layer. The catalyst should be one that promotes
the oxygen-reduction reaction, which typically requires a noble
metal such as platinum or some platinum-based alloy. Preferably,
this catalyst should not be adversely affected by the presence of
chlorine, which may be present in small amounts at the cathode. The
catalyst layer is typically formed by mixing the catalyst with the
polymers in solution and carefully casting the resultant ink onto
the membrane, or some other suitable substrate, to obtain the
desired porous structure after the solvent(s) are removed by
evaporation. The ion-exchange membrane 12 may be the same as those
used in conventional chlor-alkali membrane cells or in PEM fuel
cells. These membranes are typically fluorinated polymers with
sulfonate groups to provide the ionic sites, such as Du Pont
NAFION.RTM.. These membranes are formed into thin films by
extrusion or casting and they can be reinforced with other
materials (e.g., fibers of expanded PTFE) to improve their
mechanical properties. The anodes can be constructed with
conventional materials used in chlor-alkali and hydrogen-chloride
electrolysis cells.
* * * * *