Note: Descriptions are shown in the official language in which they were submitted.
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TITT.~
ELECTROCHEMICAL CONVERSION OF
ANHYDROUS HYDROGEN HALIDE TO HALOGEN
GAS USING A CATION-TRANSPORTING MEMBRANE
~;O~ lNulNli APP~ICATION DATA
This application is a continuation-in-part of
U.S. Application Serial No. 08/156,196, filed November
22, 1993, and of U.S. Application Serial No.
08/246,909, filed May 20, 1994, which is a
continuation-in-part of U.S. Application Serial No.
08/156,196.
~Ov~:KN~ENT ~1~-~ RIGHTS
The U.S. Government has a paid-up license in a
portion of this invention and the right in limited
circumstances to require the patent owner to license
others on reasonable terms as provided for by the
terms of Reimbursable (Funds-In) Agreement No.
DE-FI-04-94AL73697 awarded by the Department of
Energy.
~G~OVND OF T ~ l~v~lON
1. F; el ~ of ~he Invent;on
The present invention relates to an electro-
chemical cell, system and process for converting
essentially anhydrous hydrogen halide to essentially
dry halogen gas. The process of the present invention
is useful for converting anhydrous hydrogen halide, in
particular, hydrogen chloride, hydrogen fluoride,
hydrogen bromide and hydrogen iodide, to a halogen
gas, such as chlorine, fluorine, bromine, or iodine.
2. Descr;ption of the Related Art
Hydrogen chloride (HCl) or hydrochloric acid is a
reaction by-product of many manufacturing processes
which use chlorine. For e~ample, chlorine is used to
manufacture polyvinyl chloride, isocyanates, and
chlorinated hydrocarbons/fluorinated hydrocarbons,
with hydrogen chloride as a by-product of these
processes. Because supply so exceeds demand, hydrogen
chloride or the acid produced often cannot be sold or
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used, even after careful purification. Shipment over
long distances is not economically feasible.
Discharge of the acid or chloride ions into waste
water streams is environmentally unsound. Recovery
and feedback of the chlorine to the manufacturing
process is the most desirable route for handling the
HCl by-product.
A number of commercial processes have been
developed to convert HCl into usable chlorine gas.
See, e.g., F. R. Minz, "HCl-Electrolysis - Technology
for Recycling Chlorine", Bayer AG, Conference on
Electrochemical Processing, Innovation ~ Progress,
Glasgow, Scotland, UK, 4/21-4/23, 1993.
Currently, thermal catalytic oxidation proces-~es
exist for converting anhydrous HCl and aqueous HCl
into chlorine. Commercial processes, known as the
"Shell-Chlor", the "Kel-Chlor" and the MT-Chlor"
proces-~es, are based on the Deacon reaction. The
original Deacon reaction as developed in the 1870's
made use of a fluidized bed cont~;n;ng a copper
chloride salt which acted as the catalyst. The Deacon
reaction is generally expressed as follows:
Cataly~t
4HCl + ~2 ~ 2C12 + 2H2~ (1)
where the following catalysts may be used, depending
on the reaction or process in which equation (1) is
used.
Catalyst Reaction or Process
Cu Deacon
Cu, Rare Earth, AlkaliShell-Chlor
NO2, NOHSO4 Kel-Chlor
Crm~n MT-Chlor
The commercial improvements to the Deacon reaction
have used other catalysts in addition to or in place
of the copper used in the Deacon reaction, such as
rare earth compounds, various forms of nitrogen oxide,
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and chromium oxide, in order to improve the rate of
conversion, to reduce the energy input and to reduce
the corrosive effects on the processing equipment
produced by harsh chemical reaction conditions.
However, in genera~ these thermal catalytic oxidation
processes are complicated because they require
separating the different reaction components in order
to achieve product purity. They also involve the
production of highly corrosive intermediates, which
necessitates expensive construction mate~ials for the
reaction systems. Moreover, these thermal catalytic
oxidation processes are operated at elevated
temperatures of 250~ C and abcve.
Electrochemical processes exist for converting
lS aqueous HCl to chlorine gas by passage of direct
electrical current through the solution. The current
electrochemical commercial process is known as the
Uhde process. In the Uhde process, aqueous HCl
solution of approximately 22% is fed at 65~ to 80~ C
to both compartments of an electrochemical cell, where
exposure to a direct current in the cell results in an
electrochemical reaction and a decrease in HCl
concentration to 17~ with the production of chlorine
gas and hydrogen gas. A polymeric separator divides
the two compartments. The process requires recycling
of dilute (17~) HCl solution produced during the
electrolysis step and regenerating an HCl solution of
22% for feed to the electrochemical cell. The overall
reaction of the Uhde proce~s is expre~ed by the
equation:
Electrical
Energy
2HCl (aqueou-~) ~ H2twet) ~ C12(wet) (2)
As is apparent from equation (2), the chlorine gas
~ produced by the Uhde process is wet, usually
containing about 1% to 2% water. This wet chlorine
gas must then be further processed to produce a dry,
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usable gas. If the concentration of HCl in the water
becomes too low, it is possible ~or oxygen to be
generated from the water present in the Uhde process.
This possible side reaction of the Uhde process due to
the presence of water, is expressed by the equation:
2H2O ~ ~2 1 4H+ + 4e~
Further, the presence of water in the Uhde system
limits the current densities at which the cells can
perform to less than 500 amps./ft. 2, because of this
side reaction. The side reaction results in reduced
electrical efficiency and corrosion of the cell
components.
Another electrochemical process for processing
aqueous HCl has been described in U.S. Patent No.
4,311,568 to Balko. Balko employs an electrolytic
cell having a solid polymer electrolyte membrane.
Hydrogen chloride, in the form of hydrogen ions and
chloride ions in aqueous solution, is introduced into
an electrolytic cell. The solid polymer electrolyte
mem.brane is bonded to the anode to permit transport
from the anode surface into the membrane. In Balko,
controlling and m; n;m; zing the oxygen evolution side
reaction is an important consideration. Evolution of
oxygen decreases cell efficiency and leads to rapid
corrosion of components of the cell. The design and
configuration of the anode pore size and electrode
thickness employed by Balko m~;m; zes transport of the
chloride ions. This results in effective chlorine
evolution while m; n;m; zing the evolution of oxygen,
since oxygen evolution tends to increase under
conditions of chloride ion depletion near the anode
surface. In Balko, although oxygen evolution may be
minimized, it is not eliminated. As can be seen from
Figs. 3 to 5 of Balko, as the overall current density
is increased, the rate of oxygen evolution increases,
as evidenced by the increase in the concentration of
oxygen found in the chlorine produced. Balko can run
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at higher current densitie~, but is limited by the
deleterious e~fects of oxygen evolution. If the Balko
cell were to be run at high current densities, the
anode would be destroyed.
The conductivity of a membrane is directly
- related to the water content in the membrane and
decreases at low water content. Limiting current
density occurs when the concentration of water within
the membrane reaches a value that will no longer
support additional proton conduction. Therefore,
limiting current density can develop when the
conductivity decreases due to low water
concentrations. When a cell is run above limiting
current, the components of the cell may be destroyed.
The existing electroc~emical processes for
converting hydrogen halides as discussed above are
aqueous processes whi~h require first dissolving the
hydrogen halide in water. Since these electrochemical
cells have water in their anolytes and catholytes, the
membranes of such cells are normally kept hydrated.
There exists a need for directly producing
essentially dry halogen gas without having to first
dissolve the hydrogen halide in water, and for keeping
the membrane hydrated during such a process. This
would allow the limiting current density of the cell
to be increased and/or controlled, so that the cell
components would not be destroyed.
~UMMaRY OF THE l~v~lON
The present invention solves the problems of the
prior art by providing an electrochemical cell, system
and process for directly producing essentially dry
halogen gas from essentially anhydrous hydrogen
halide. This cell, system and process allow for
direct processing of anhydrous hydrogen halide which
is a by-product of manufacturing processes, without
first dissolving the hydrogen halide in water. This
direct production of essentia'ly dry halogen gas, when
done, for example, for chlorine gas, is less capital
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intensive than processes of the prior art, which
re~uire separation o~ water ~rom the chlorine gas.
This direct production of essentially dry halogen ga-
~also requires lower investment costs than the
electrochemical conversions of hydrogen chloride of
the prior art. This advantage can translate directly
into lower power costs per pound of say, chlorine,
generated than in the aqueous electrochemical
processes of the prior art. The direct production of
essentialiy dry halogen gas also provides a process
which produces drier chlorine gas with fewer
processing steps as compared to that produced by
electrochemical or catalytic systems of the prior art,
thereby simplifying processing conditions and reducing
capital costs.
In the anhydrous cell, system and process of the
present invention, the membrane is kept hydrated.
This enables the limiting current density of the cell
to be increased, as well as controlled. In turn, this
allows an electrochemical cell to be de~igned in which
a valuable component, such as the cation-exchange
membrane, is protected from prolonged exposure to
excessive current, which could deteriorate the
membrane, and thus impact the membrane's and the
cell's long-term performance. Moreover, control of
limiting current is especially desirable where it is
necessary to compensate for changes in manufacturing
rates of suppliers of the anhydrous hydrogen halide,
such as hydrogen chloride.
In accordance with the present invention, the
membrane of an electrochemical cell used to convert
essentially anhydrous hydrogen halide to essentially
dry halogen gas is kept hydrated in a variety o~ ways.
The advantages achieved by keeping the membrane
hydrated by these v~rious ways make the process of the
present invention even more practicable and more
economically attractive.
-
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In one particular embodiment of the present
invention, the membrane is kept hydrated by a supply
of oxygen, which provides an excess of oxygen to the
cell. This increases the conversion rate of protons
transported across the membrane and the supplied
oxygen to water, which allows the limiting current
density to be increased. In addition, with this
supply of oxygen, it is possible to take advantage of
this excess of oxygen without the economic penalty of
discarding products from the cell associated with
other ways of hydrating the membrane.
To achieve the foregoing solutions, and in
accordance with the purposes of the invention as
embodied and broadly described herein, there is
provided an electrochemical celi for directly
producing essentially dry halogen gas from essentially
anhydrous hydrogen halide. The electrochemical cell
comprises means for oxidizing molecules of essentially
anhydrous hydrogen halide to produce essentially dry
halogen gas and protons, cation-transporting means for
transporting the protons therethrough, where the
oxidizing means is disposed in contact with one side
of the cation-transporting means, means for reducing
the transported protons, wherein the reducing means is
disposed in contact with the other side of the cation-
transporting means, and means for supplying water to
the cati~n-transporting means at the other side of the
cation-transporting m~n.~,
Further in accordance with the purposes of the
invention, there is provided a system for directly
producing essentially dry halogen gas from essentially
anhydrous hydrogen halide. The electrochemical cell
comprises means for oxidizing molecules of essentially
anhydrous hydrogen halide to produce essentially dry
halogen gas and protons; cation-transporting means for
transporting the protons therethrough, where the
oxidizing means is disposed in contact with one side
of the cation-transporting means; means for reducing
,
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the transported protons, wherein the reducing means i~
disposed in contact with the other side of the cation-
transporting means, and inlet means for supplying
water to the cation-transporting means at the other
side of the reducing means; inlet means for supplying
water to the cation-transporting means at the other
side of the cation-transporting means; outlet means
for releasing a fluid from reducing means at the other
side of the cation-transporting means; and means for
recycling the fluid back to the cation-transporting
means at the other side of the cation-transporting
means.
Further in accordance with the purposes of the
invention, there is provided a process for directly
producing essentially dry halogen gas from essentially
anhydrous hydrogen halide, wherein molecules of
essentially anhydrous hydrogen halide are fed to an
inlet of the electrochemical cell and are transported
to an anode of the cell; the molecules of the
essentially anhydrous hydrogen halide are oxidized at
the anode to produce essentially dry halogen ga-~ and
protons; the protons are transported throu~h a cation-
transporting membrane of the cell; the transported
protons are reduced at a cathode.of the
electrochemical cell; and a humidified gas stream is
supplied to the membrane.
Alternatively, the process of the present
invention may be described as a process for directly
producing essentially dry halogen gas from essentially
anhydrous hydrogen halide, wherein current is supplied
to an electrochemical cell; molecules of essentially
anhydrous hydrogen halide are fed to an inlet of the
electrochemical cell and are transported to an anode
of the cell; the molecules of the essentially
anhydrous hydrogen halide are oxidized at the anode to
produce essentially dry halogen gas and protons; the
protons are transported through a cation-transporting
membrane o~ the cell; the transported protons are
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W O96/34g98 PCTrUS95/16032 reduced at a cathode of the electrochemical cell;
water is supplied to the membrane at the cathode and
the and water is transported by diffusion towards the
anode; the transported protons drag the water present
in the membrane towards the cathode; and the amount of
current required to achieve a balance between the
water transported by diffusion toward the anode and
dragged by the proton transport toward the ca-hode is
controlled by adjusting the amount of water supplied
to the membrane at the cathode.
In either process, a fluid is released from the
cell and may be recycled back to the membrane.
BRIE~ DESC:~IPTION OF T~: DPl~ lN~
Fig. 1 i~ a cross-sectional view of an electro-
chemical cell for producing halogen gas from anhydroushydrogen halide according to any of the ~irst, second,
third or fourth embodiments of the present invention.
Fig. lA is a cut-away, top cross-sectional view
of the anode and cathode mass flow fields as shown in
Fig. 1.
Fig. 2 is a schematic diagram of a system for
producing essentially dry halogen gas from anhydrous
hydrogen halide using the electrochemical cell of
Fig. 1 and for recycling a fluid released from the
cell back to the membrane, where liquid water is added
to the cathode-side inlet of the cell.
Fig. 3 is a schematic diagram of a system for
producing essentially dry halogen gas from anhydrous
hydrogen halide using the electrochemical cell of
Fig. 1 and for recycling a fluid released from the
cell back to the membrane, where a humidified gas
stream comprising hydrogen is added to the cathode-
side inlet of the cell.
Fig. 4 is a schematic diagram of a system for
producing essentially dry halogen gas from anhydrous
hydrogen halide using the electrochemical cell of
Fig. 1 and for recycling a fluid released from the
cell back to the membrane, where a humidified gas
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stream comprising o~ygen is added to the cathode-side
inlet of the cell.
Fig. 5 is a schematic diagram of a system for
producing essentially dry halogen gas from anhydrous
hydrogen halide using the electrochemical cell of
Fig. 1 and for recycling a fluid released from the
cell back to the membrane, where a humidified gas
stream comprising oxygen is added to the cathode-side
inlet of the cell.
DE~CRIPTION OF T~ ~K~-~KK~ ~M~O~IM~N~S
Reference will now be made in detail to the
present preferred embodiments of the invention as
illustrated in the accompanying drawings.
In accordance with a first, second, third and
fourth embodiment of the present invention, there i~
provided an electrochemical cell for the direct
production of essentially dry halogen ga-~ from
anhydrou-~ hydroge~ halide. Such a cell is shown
generally at 10 in Fig. 1. The cell of the pre~ent
invention will be described with respect to a
preferred embodiment of the pre~ent invention, which
directly produces essentially dry chlorine gas from
anhydrous hydrogen chloride. This cell may
alternatively be used to produce other halogen gases,
such as bromine, fluorine and iodine from a respective
anhydrous hydrogen halide, such as hydrogen bromide,
hydrogen fluoride and hydrogen iodide. The term
"direct" means that the electrochemical cell obviates
the need to remove water from the halogen gas produced
or the need to convert essentially anhydrous hydrogen
halide to aqueous hydrogen halide before electro-
chemical treatment. In the first embodiment, chlorine
gas, as well as hydrogen, is produced in this cell.
In a second embodiment, water, as well as chlorine
gas, is produced by this cell, as will be explained
more fully below.
The electrochemical cell of the first through
fourth embodiments comprises means for oxidizing
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molecules of essentially anhydrous hydrogen halide to
produce essentially dry halogen gas and protons. The
oxidizing means is an electrode, or more specifically,
an anode 12 as shown in Fig. 1. On the anode side,
electrochemical cell 10 has an anode-side inlet 14 and
an anode-side outlet 16. Since in the preferred
embodiment, anhydrous HCl is carried through the
inlet, and chlorine gas is carried through the outlet,
the inlet and the outlet may be lined with a
perfluoropolymer, sold as TEFLON~ PFA (hereinafter
referred to as "TEFLOND PFA") by E. I. du Pont
de Nemours and Company of Wilmington, Delaware
(hereinafter referred to as "DuPont").
The electrochemical cell of ,he first through
fourth embodiments also comprises cation-transporting
means for transporting the protons therethrough, where
one side of the oxidizing means is disposed in contact
with one side of the cation-transporting means.
Preferably, the cation-transporting means is a cation-
transporting membrane 18 as shown in Fig. 1. Morespecifically, membrane 18 may be a proton-conducting
membrane. Membrane 18 may be a commercial cationic
membrane made of a fluoro- or perfluoropolymer,
preferably ~ copolymer of two or more fluoro or
perfluoromonomers, at least one of which has pendant
sulfonic acid groups. The presence of carboxylic
groups is not desirable, because those groups tend to
decrease the conductivity of the membrane when they
are protonated. Various suitable resin materials are
a~ailable commercially or can be made according to
patent literature. They include fluorinated polymers
with side chains of the type -CF2CFRSO3H and
-OCF2CF2CF2SO3H, where R is a F, Cl, CF2Cl, or a C1 to
C10 perfluoroalkyl radical. The membrane resin may be,
for example, a copolymer of tetrafluoroethylene with
CF2=CFOCF2CF(CF3)OCF2CF2SO3H. Sometimes those resins
may be in the form that has pendant -SO2F groups,
rather than -SO3H groups. The sulfonyl fluoride groups
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can be hydrolyzed with potassium hydroxide to -SO3K
groups, which then are e~changed with an acid to -SO3H
groups. Suitable cationic membranes, which are made
of hydrated, copolymers of polytetrafluoroethylene and
poly-sulfonyl fluoride vinyl ether-containing pendant
sulfonic acid groups, are offered by DuPont under the
trademark "NAFION" (hereinafter referred to as
NAFION~). In particular, NAEION~ membranes containing
pendant sulfonic acid groups include NAFION~ 117,
NAFION~ 324 and NAFION~ 417. The first type of NAFION~
is unsupported and has an equivalent weight of
1100 g., equivalent weight being defined as the amount
of resin required to neutralize one liter of a lM
sodium hydroxide solution. The other two types of
NAFION~ are bcth supported on a fluorocarbon fabric,
the equivalent weight of NAFION0 417 also being 1100 g.
NAFION0 324 has a two-layer structure, a 125 ~m-thick
membrane having an equivalent weight of 1100 g., and a
25 ~m-thick membrane having an equivalent weight of
1500 g. There also is offered a NAEION~ 117F grade,
which is a precursor membrane having pendant -SO2F
groups that can be converted to sulfonic acid groups.
Although the present invention describes the use
of a solid polymer electrolyte membrane, it is well
within the scope of the invention to use other cation-
transporting me~branes which are not polymeric. For
example, proton-conducting ceramics such as beta-
alumina may be used. Beta-alumina is a class of
nonstoichiometric crystalline compounds having the
general structure Na2Ox A12O3, in which x ranges from 5
(~"-alumina) to 11 (~-alumina). This material and a
number of solid electrolytes which are useful for the
invention are described in the Fuel Cell ~an~hook,
A. J. Appleby and F. R. Foulkes, Van Nostrand
Reinhold, N.Y., 1989, pages 308-312. Additional
useful solid state proton conductors, especially the
cerates of strontium and barium, such as strontium
ytterbiate cerate (SrCeO 95Ybo 05O3_~) and barium
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neodymiate cerate (BaCeO gNdo 01~3-a) are described in a
final report, DOE/MC/24218-2957, Jewulski, Osi~ and
Remick, prepared for the U.S. Department of Energy,
Office of Fossil Energy, Morgantown Energy Technology
Center by Institute of Gas Technology, Chicago,
~ Illinois, December, 1990.
The electrochemical cell of the first through
fourth embodiments also comprises an electrode, or
more specifically, cathode 20, where the cathode is
disposed in contact with the other side (as opposed to
the side which is in contact with the anode) of
membrane 18 as illustrated in Fig. 1. Cathode 20 has
a cathode-side inlet 24 and a cathode-side outlet 26
as shown in Fig. 1. Since in the preferred
embodiment, anhydrous HCl is processed, and since some
chlorides pass through the membrane and consequently,
HCl is present on the cathode-side of the cell, the
cathode inlet and the outlet may be lined with
TEFLON~ PFA.
As known to one skilled in the art, if electrodes
are placed on opposite faces of membrane, cationic
charges (protons in the ~Cl reaction being described)
are transported through the membrane from anode to
cathode, while each electrode carries out a half-cell
reaction. In the first and second embodiments,
molecules of anhydrous hydrogen chloride are
transported to the surface of the anode through anode-
side inlet 14. The molecules of the anhydrous
hydrogen chloride are oxidized to produce essentially
dry chlorine gas and protons. The essentially dry
chlorine gas exits through anode-side outlet 16 as
shown in Fig. 1. The protons, H+, are transported
through the membrane and reduced at the cathode. This
is explained in more detail below.
The anode and the cathode may comprise porous,
gas-diffusion electrodes. Such electrodes provide the
advantage of high specific surface area, as known to
one skilled in the art. The anode and the cathode
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comprise an electrochemically active material disposed
adjacent, meaning at or under, the surface of the
cation-transporting membrane. A thin film of the
electrochemically active material may be applied
directly to the membrane. Alternatively, the
electrochemically active material may be hot-pressed
to the membrane, as shown in A. J. Appleby and E. B.
Yeager, Energy, Vol. 11, 137 (1986). Alternatively,
the electrochemically active material may be deposited
into the membrane, as shown in U.S. Patent No.
4,959,132 to Fedkiw. The electrochemically active
material may comprise any type of catalytic or
metallic material or metallic oxide, as long as the
material can ~upport charge tran~fer. Preferably, the
electrochemically active material may comprise a
catalyst material such as platinum, ruthenium, osmium,
rhenium, rhodium, iridium, palladium, gold, titanium
or zirconium and the oxides, alloys or mixtures
thereof. However, in general, the oxides of these
materials are not used for the cathode. Other
catalyst materials suitable for use with the present
invention may include, but are not limited to,
transition metal macro cycles in monomeric and
polymeric forms and transition metal oxides, including
perovskites and pyrochores.
In a hot-pre~sed electrode, the electrochemically
active material may comprise a catalyst material on a
support material. The support material may comprise
particles of carbon and particles of polytetrafluoro-
ethylene, which is sold under the trademark "TEFLON"(hereinafter referred to as TEFLON~), commercially
available from DuPont. The electrochemically active
material may be bonded by virtue of the TEFLON~ to a
support structure, or gas diffusion layer, of carbon
paper or graphite cloth and hot-pressed to the cation-
transporting membrane. The hydrophobic nature of
TEFLON~ does not allow a film of water to form at the
anode. A water barrier in the electrode would hamper
14
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the diffusion of HCl to the reaction sites. The
electrodes are pre~erably hot-pressed into the
membrane in order to have good contact between the
catalyst and the membrane.
The loadings of electrochemically active material
- may vary based on the method of application to the
membrane. Hot-pressed, gas-diffusion electrodes
typically have loadings of 0.10 to 0.50 mg/cm2. Lower
loadings are possible with other available methods of
deposition, such as distributing them as thin films
from inks onto the membranes, as described in Wilson
and Gottesfeld, "High Performance Catalyzed Membranes
of Ultra-low Pt Loadings for Polymer Electrolyte Fuel
Cells", Los Alamos National Laboratory, J. Electro-
chem. Soc., Vol. 139, No. 2 L28-30, 1992, where the
inks contain solubilized NAEION~ ionomer to enhance the
catalyst-ionomer surface contact and to act as a
binder to the NAEION~ membrane sheet. With such a
system, loadings as low as 0.017 mg active material
per cm2 have been achieved.
The membrane of the present invention serves as
the electrolyte in which the protons constitute the
current. The membrane must be in a sufficient state
of hydration in order to act as a proton conductor.
Electro-osmotic drag occurs in the membrane, where
protons drag water in the direction of current flow.
This leads to the development of a concentration
gradient of water within the membrane. The
conductivity of a membrane is directly related to the
water content in the membrane and decreases at low
water content. Limiting current occurs when the
concentration of water within the membrane reaches a
value that will no longer support additional proton
conduction. Thus, limiting current density can
develop when the conductivity decreases due to low
water concentrations.
Therefore, according to the present invention,
water, in the form of a humidified gas stream or
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liquid water, is supplied to the membrane at the
cathode. This is necessary in order to achieve
e~ficient proton transport. In the first three
embodiments, which have a hydrogen-producing cathode,
S the hydration of the membrane is accomplished by the
introduction of either liquid water in the first
embodiment, or a gas stream comprising hydrogen or
nitrogen, respectively, in the second and third
embodiments, to the membrane at the cathode-side of
the membrane In-the fourth embodiment, which has a
water-producing cathode, membrane hydration is
accomplished by the production of water at the
cathode, as well as the introduction of a humidified
gas stream comprising oxygen at the cathode-side of
the membrane, which produces water. Although
hydrogen, nitrogen and oxygen are described herein, it
is within the scope of the present invention to
humidified gas streams comprising other gases.
The water at the c~thode side of the membrane is
transported by diffusion towards the anode.
Additionally, the transported protons drag the water
i-n the membrane, which includes-water already present
in the membrane to begin with, towards the cathode.
Applicants have found that the amount of current
required to achieve a balance between the water
transported by diffusion toward the anode and dragged
by the proton transport toward the cathode, at which
point limiting current occurs, may be controlled by
adjusting the amount of water supplied to the membrane
at the cathode. Consequently, with the present
invention, limiting current may be controlled. This
is especially desirable where it is necessary to lower
or raise limiting current in order to compensate for
changes in the amount of anhydrous hydrogen halide
which needs to be processed. This may change in
response to changes in manufacturing rates of
manufacturers which produce hydrogen chloride.
16
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In order to adjust the amount of water ~upplied
to the membrane, the electrochemical cell of the first
through fourth embodiments also comprises inlet means
for supplying water to the cation-transporting means
at the other side of the cation-transporting means.
Preferably, the inlet means comprises a cathode-side
inlet 24 as shown in Fig. 1 which supplies water, in
various forms as will be explained below, to the side
of the membrane which is disposed in contact with the
cathode. The electrochemical cell also comprises
outlet means for releasing a fluid from the reducing
means at the other side of the cation-transporting
means. Preferably, the outlet means comprises a
cathode-side outlet 26 as shown in Fig. 1, which
relea~es a fluid from the cathode at the side of the
membrane which is disposed in contact with the
cathode. Since in the preferred embodiment, anhydrous
HCl is processed, and since there may'be some chloride
passing through the'membrane to'the cathode-side of
the cell, the cathode-side inl~t and outlet are
preferably lined with TEFLON0 PFA.
The electrochemical cell of the first through
fourth embodiments further comprises an anode flow
field 28 disposed in contact with the anode and a
cathode flow field 30 disposed in contact with the
cathode. The flow fields are electrically conductive,
and act as both mass and current flow fields. More
-~pecifically, the mass flow fields may include a
plurality of anode flow channels 29 and a plurality'of
cathode flow channels 31 as shown in Fig. lA. Anode
flow field and channels 29 direct reactants, such as
anhydrous HCl, and products, such as essentially dry
chlorine gas, from the anode. Cathode flow field 30
and channels 31 direct catholyte, such as liquid water
in the first embodiment, or a humidified gas stream in
the second through fourth embodiments to the cathode
and products, such as hydrogen vapor, liquid water and
HCl dissolved in the water in the first embodiment,
CA 02219922 1997-10-29
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hydrogen and hydrogen halide, in the form of vapor, in
the second embodiment, hydrogen, nitrogen, water and
hydrogen halide, all in the form of vapor, in the
third embodiment, and oxygen, water and hydrogen
halide. all in the form of vapor, in the fourth
embodiment. The anode and the cathode mass flow
fields may comprise grooved porous graphite paper.
The flow fields may also be made of a porous carbon in
the form of a foam, cloth or matte.
The electrochemical cell of the first through
fourth embodiments may also comprise an anode mass
flow manifold 32 and a cathode mass flow field
manifold 34 as shown in Fig. 1. The purpo~e of such
mani~olds is to bring anolyte to and products from the
anode, and catholyte to and products from the cathode.
In addition, the manifolds form a frame around the
anode mass flow field and the anode, and the cathode
mass flow field and the cathode, respectively. These
manifolds are pre~erably made of a corrosion resistant
material, ~uch TEFLON~ PFA. A gasket 36, 38, also
contributes to forming a frame around the respective
anode and cathode mass flow fields. Theqe gaskets are
pre~erably also made of a corrosion resistant
material, such as polytetrafluoroethylene, sold under
the trademark TEFLON~ PTFE by DuPont.
The electrochemical cell of the first through
fourth embodiments also comprises an anode current bus
46 and a cathode current bus 48 as shown in Fig. 1.
The current buses conduct current to and from a
voltage source (not shown). Specifically, anode
current bus 46 is connected to the positive terminal
of a voltage source, and cathode current bus 48 is
connected to the negative terminal of the voltage
source, so that when voltage is supplied to the cell,
current flows through all of the cell components to
the right of current bus 46 as shown in Fig. 1,
including current bus 48, from which it returns to the
CA 02219922 1997-10-29
W096t34998 PCT~S95/16032
voltage source. The current buses are made of a
conductor material, such as copper.
The electrochemical cell of the first and second
embodiments further comprises a respective current
distributor disposed in contact with a respective flow
~ field. An anode current distributor 40 is disposed in
contact with anode flow field 28, and a cathode
current distributor 42 is disposed in contact with
cathode flow field 30. The anode current distributor
collects current from the anode bus and distributes it
to the anode by electronic conduction. The cathode
current distributor collects current from the cathode
and distributes it to the cathode bus. The anode and
the cathode current distributors preferably each
compri-~e a non-porous layer. The anode current
distributor provides a barrier between the anode
current bus and the anode, as well as between the
current bus and the anhydrous hydrogen halide, such as
hydrogen chloride, and the halogen gas, such as
chlorine. The cathode current distributor provides a
barrier between the cathode current bus and the
cathode, as well as between the cathode current bu~
and the hydrogen halide. This barrier is desirable,
as there is some migration of hydrogen halide through
the membrane. The current distributors of the pre~ent
invention may be made of a variety of materials, and
the material u-~ed for the anode current distributor
need not be the ~ame as the material used for the
cathode current distributor. In one instance, the
anode current distributor is made of platinized
tantalum, and the cathode current distributor is made
of a nickel-based alloy, such as UNSl0665, sold as
HASTELLOY~ B-2, by Haynes, International.
In the first through fourth embodiments, the
electrochemical cell also comprises a conductive
structural support 44 disposed in contact with anode
current distributor 40. The support on the anode side
is preferably made of UNS31603 (316L stainless steel).
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A seal 45, preferably in the form of an O-ring made
from a perfluoroelastomer, sold under the trademark
KALREZ~ by DuPont, is disposed between structural
-~upport 44 on the anode side and anode current
distributor 40. The cathode current distributor acts
as a corrosion-resistant structural backer on the
cathode side. This piece can be drilled and tapped to
accept the TEFLOND PFA fitting, which is used for the
inlet and outlet.
When more than one anode-cathode pair is used,
such as in manufacturing, a bipolar arrangement, as
familiar to one skilled in the art, is preferred. The
electrochemical cell of the present invention may be
used in a bipolar stack. To create such a bi-polar
stack, current distributors 40 and 42 and all the
elements disposed in between as shown in Fig. 1 are
repeated along the length of the cell, and current
buses are placed on the outside of the stack.
In any of the firQt through fourth embodiments,
the electrochemical cell can be operated over a wide
range of temperatures. Room temperature operation is
an advantage, due to the ease of use of the cell.
However, operation at elevated temperatures provides
the advantages of improved kinetics and increased
electrolyte conductivity. In addition, it should be
noted also that one is not restricted to operate the
electrochemical cell of any of the first through
fourth embodiments at atmospheric pressure. The cell
could be run at differential pressure gradients, which
change the transport characteristics of water or other
components in the cell, including the membrane.
The electrochemical cell of any of the
embodiments of the present invention can be operated
at higher temperatures at a given pressure than
electrochemical cells operated with aqueous hydrogen
chloride of the prior art. This affects the kinetics
of the reactions and the conductivity of the NAFION~.
Higher temperatures result in lower cell voltages.
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However, limits on temperature occur because of the
properties of the materials used for elements of the
cell. For example, the properties of a NAFION~
membrane change when the cell is operated above 120~
C. The properties of a polymer electrolyte membr~ne
make it difficult to operate a cell at temperatures
above 150~ C. With a membrane made of other
materials, such as a ceramic material like beta-
alumina, it is possible to operate a cell at
temperatures above 200~ C.
Further in accordance with the first embodiment
of the present invention, there is provided a system
and a process for recycling a fluid released from an
electrochemical cell used for the direct production of
essentially dry halogen gas from essentially anhydrou~
hydrogen halide. The system of the first embodiment
is shown in Fig. 2 and compri~es an electrochemical
cell 10, which is the same a.s electrochemical cell 10
as described above. In the first embodiment, liquid
water is fed to cathode-side inlet 24 aS shown in Fig.
1, and subsequently to the cathode-side of the
membrane. Cathode-side outlet 26 releases a fluid,
which in the first embodiment comprises water (H2O) in
the form of a liquid, hydrogen (H2) in the form of
vapor, and hydrogen halide, such as hydrogen chloride,
which is dissolved in the water.
The system for directly producing es-~entially dry
halogen gas from essentially anhydrous hydrogen halide
further comprises means for recycling the released
fluid back to the cation-transporting means. More
specifically, the recycling means comprises a
recycling loop which recycles the released fluid back
to the membrane at the cathode-side of the membrane.
- The recycling means may comprise a cooler for cooling
the released fluid. As shown in Fig. 2, hydrogen (H2)
in the form of a vapor, water (H2O) in the form of a
liquid, and a hydrogen halide, such as hydrogen
chloride, which is dissolved in the water, which have
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been released from the cell, are carried through a
line 50 to a cooler 52 which cools the released fluid.
The recycling means may also comprise a separator for
removing a portion of the hydrogen halide from the
released fluid. As shown in Fig. 2, after the
released fluid is cooled, it is carried through a line
54 to a separator 56, which removes a portion of the
hydrogen chloride dissolved in water (H2O) through a
line 70 as shown in Fig. 2. The recycling means may
further comprise a scrubber for removing another
portion of the hydrogen halide. Specifically,
hydrogen (H2), hydrogen chloride (HCl) and water (H2O),
all in the form of vapor, are carried through a line
60 to a scrubber 62. An alkali solution is added to
scrubber 62 through a line 64, so that the scrubber
removes another portion of the hydrogen chloride
through a line 68, as a halide alkali salt. Hydrogen
(H2) and wa~er (H2O) both in the form of vapor, which
may be discarded or used in another process, is vented
from scrubber 62 through a line 66.
In the first embodiment of Fig. 2, the recycling
means may also comprise a pump for pumping the
released fluid through the recycling loop back to the
membrane. As shown i~ Fig. 2, liquid water (H2O) and
hydrogen chloride (HCl), dissolved in the water, are
carried through line 70 to a pump 72. The water and
hydrogen chloride are then carried out of pump 72
through a line 74: The recycling means of the first
embodiment may also comprise a temperer, shown as a
heater/cooler 80 in Fig. 2, which tempers the released
fluid. Temperer 80 may either heat or cool the
released fluid, depending on the desired temperature
for the electrochemical cell. The recycling means of
the first embodiment may further comprise a
conditioner 78 for conditioning the released fluid.
Conditioner 78 supplies heat and water through a line
77 to the released fluid. Liquid water (H20), with a
small amount of residual hydrochloric acid, is carried
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through a line 82 back to electrochemical cell 10,
where it is used to continuously supply liquid water
to the membrane.
- Further in accordance with the second embodiment ~--
of the present invention, there is provided a system
and a process for recycling a gas stream from an
electrochemical cell used for the direct production of
essentially dry halogen gas from essentially anhydrous
hydrogen halide. The system of the second embodiment
is shown in Fig. 3 and comprises an electrochemical
cell 10', which is the same as electrochemical cell 10
as described above. In the second embodiment, a
humidified gas stream comprising hydrogen is fed to
cathode-side inlet 24 of the cell as shown in Fig. 1.
The humidified gas stream comprises primarily
hydrog~n. However, in practice, traces of other
gases, except oxygen, may be included in the gas
stream of the second embodiment. -Cathode-side outlet
26 as shown in Fig. l releases a fluid from the
cathode, which in the second embodiment comprises
water (H2O), hydrogen (H2), and-hydrogen halide, such
as HCl, all in the form of vapor.
The system for directly producing essentially dry
halogen gas from essential~y anhydrous hydrogen halide
of the second embodiment further comprises means for
recycling the released fluid back to the cation-
transporting means. More specifically, the recycling
means comprises a recycling loop which recycles the
released fluid back to the membrane at the cathode-
side of the membrane. The recycling means of thesecond embodiment may comprise a cooler for cooling
the released fluid. As shown in Fig. 3, the water
(H2O), hydrogen (H2) and hydrogen halide, all in the
form of vapor, are carried through a line 50' to a
cooler 52' which cools the released fluid. As noted
above, the released fluid comprises hydrogen halide,
specifically hydrogen chloride in a preferred
embodiment, and the recycling means may also comprise
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a separator for removing a portion of the hydrogen
halide from the released fLuid. As shown in Fig. 3,
after the reieased fluid is cooled, it is carried
through a line 54' to a separator 56', which removes a
portion of the hydrogen chloride, as well as water
vapor, through a line~70' as shown in Fig. 3. This
~Cl and water is discarded. The recycling means may
further comprise a scrubber for removing another
portion of the hydrogen halide. Specifically,
hydrogen (H2), hydrogen chloride (HCl) and water (H2O),
all in the form of vapor, are carried through a line
60' to a scrubber 62'. An alkali solution is added to
scrubber 62' through a line 64', so that the scrubber
removes another portion of the hydrogen chloride
through a line 68', as a halide alkali salt. Hydrogen
vapor (H2) and water vapor (H2O), which may be
discarded or used in another process, are vented from
scrubber 62' through a line 66'. Hydrogen vapor (H2)
and water vapor (H2O) are carried away from scrubber
62' by a line 67'.-
In the second embodiment of Fig. 3, the recyclingmeans may also comprise a humidifier for humidifying
the released fluid. A humidifier 65' humidifies the
released fluid, which is hydrogen vapor and water
vapor, with water, in either liquid or vapor form,
through a line 69'. The humidified hydrogen and water
vapor is carried away from line 69' by a line 71'.
The recycling means may also comprise a compressor 72'
for compressing the released fluid. The water and
hydrogen are then carried out of compressor 72'
through a line 74'. The recycling means of the second
embodiment may also comprise a temperer, shown as a
heater/cooler 80' in Fig. 2, which tempers the
released fluid. Temperer 80' may either heat or cool
the released fluid, depending on the desired
temperature of the cell. The water and hydrogen vapor
are then carried through a line 76'. The recycling
means of the second embodiment may further comprise a
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conditioner 78' for conditioning, or supplying heat
and water, to the released fluid, which is still
hydrogen vapor and water vapor, through a line 77''.
In the second embodiment, either a humidifier or a
conditioner are used, but not both. The hydrogen and
water vapor are carried through a line 82' back to
electrochemical cell 10', where they are used to
continuously suppiy a humidi-fied gas stream, which
includes hydrogen, to the membrane.
Further in accordance with the third embodiment
of the present invention, there is provided a system
and a process for recycling a gas stream from an
electrochemical cell used for the direct production of
essentially dry halogen gas from essentially anhydrous
hydrogen halide. The system of the third embodiment
is shown in Fig. 4 and comprise~ an electrochemical
cell lO'', which is the same as electrochemical cell
10 as described above. In the third embodiment, a
humidified gas stream comprising nitrogen is fed to
cathode-side inlet 24 as shown in Fig. l. The
humidified gas comprises primarily nitrogen. However,
in practice, traces of other gases other than oxygen
may be included in the gas stream. Cathode-side
outlet 26 as shown in Fig. 1 releases a fluid from the
cathode thro~gh a line 50'', which in the third
embodiment comprises nitrogen (N2), hydrogen (H2),
water (H2O), and hydrogen halide, such as hydrogen
chloride (HCl), all in the form of vapor, as shown in
Fig. 4-
The system for directly producing essentially dry
halogen gas from essentially anhydrous hydrogen halide
of the third embodiment further comprises means for
recycling the released fluid back to the cation-
transporting means. More specifically, the recycling
means comprises a recycling loop which recycles the
released fluid back to the membrane at the cathode-
side of the membrane. The recycling means of the
third embodiment may comprise a cooler for cooling the
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released fluid. As shown in Fig. 4, the water (H2O),
nitrogen (N2), hydrogen (H2) and hydrogen halide, such
as HCl, all in the form of a vapor, are carried
through line 50'' to a cooler 52'' which cools the
released fluid. ~s noted above, the released ~luid
comprises hydrogen halide, specifically hydrogen
chloride in a preferred embodiment, and the recycling
means may also comprise a separator for removing a
portion of the hydrogen halide from the released
fluid. As shown in Fig. 4, after the released fluid
is cooled, it is carried through a line 54'' to a
separator 56'', which removes a portion of the
hydrogen chloride and the water (H2O) in the form of
liquid through a line 70''. The recycling means may
further comprise a scrubber for removing another
portion of the hydrogen halide. Specifically,
hydrogen (H2), nitrogen (N2) hydrogen chloride (HCl)
and water (H2O), all in the form of vapor, are carried
through a line 60'' to a scrubber 62''. An alkali
solution is added to -~crubber 62'' through a line
64'', so that the scrubber removes another portion of
the hydrogen chloride through a line 68'', as a halide
alkali salt. In this third embodiment, the nitrogen
and the hydrogen and the water vapor cannot be
separated. Thus, a portion of the hydrogen (H2),
nitrogen (N2) and water (H2O), all in the form of
vapor, is vented through a line 66''. Another portion
of the hydrogen (H2), nitrogen (N2) and wat~r (H2O),
all in the form of vapor, is carried away from
scrubber 62'' by a line 67''.
In the third embodiment of Fig. 4, the recycling
means may also comprise a humidifier for humidifying
the released fluid. A humidifier 65'' humidifies the
released fluid, which is hydrogen vapor, nitrogen
vapor and water vapor, with water, in either liquid or
vapor form through a line 69''. The recycling means
may also comprise a compressor 72'' for compressing
the released fluid. The water, hydrogen and nitrogen,
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W O 96/34998 PCT~US95/16032
all in the form of vapor, are then carried out of
compressor 72'' through a line 74ll. The humidified
fluid is carried away from line 69'' by a line 71''.
The recycling means of the third embodiment may also
comprise a temperer, shown as heater/cooler 80'' in
Fig. 4, which tempers the released fluid. The water,
hydrogen and nitrogen are then carried through a line
74'' to temperer 80''. Temperer 80'' may either heat
or cool the released fluid, depending on the desired
temperature of the cell. The recycling means of the
second embodiment may further comprise a conditioner
78'' for supplying heat and water to the released
fluid through a line 77''. As in the second
embodiment above, either a humidifier or a conditioner
is used in the recycle loop of the third embodiment,
but not both. In addition, in the third embodiment,
additional nitrogen is supplied from a source 84'' to
the cell through a line 83'', since nitrogen is being
lost in the loop through line 66. However, it should
be noted that the nitrogen may be added anywhere in
the recycle loop. Nitrogen tN2) and water vapor
(H2G(vapor)) are carried through a line 82'! back to
electrochemical cell lQ'', where they are used to
continuously supply a humidified gas stream, which
include~ nitrogen, to the membrane at the cathode-side
of the membrane.
Further in accordance with the fourth embodiment
of the present invention, there is provided a system
and a process for recycling a gas stream from an
electrochemical cell used for the direct production of
essentially dry halogen gas from essentially anhydrous
hydrogen halide. The system of the ~ourth embodiment
is shown in Fig. 5 and comprises an electrochemical
cell 10''', which is the same as electrochemical cell
10 as described above. In the fourth embodiment, a
humidified gas stream comprising oxygen is fed to
cathode-side inlet 24 as shown in Fig. 1. The
humidified gas comprises primarily oxygen. However,
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in practice, traces of other gases, not including
hydrogen, but including nitrogen, may be in the gas
stream. Cathode-side outlet 26 as shown in Fig. 1
releases a fluid from the cathode through a line
50''', which in the fourth embodiment comprises oxygen
(~2)~ water (H2O) and hydrogen halide, such as hydrogen
chloride (HCl), all in the form of vapor, as shown in
Fig. 5.
The system for directly producing essentially dry
- 10 halogen gas from essentially anhydrous hydrogen halide
of the fourth embodiment further comprises means for
recycling the released fluid back to the cation-
transporting means. More specifically, the recycling
means comprises a recycling loop which recycles the
released fluid back to the membrane at the cathode-
side of the membrane. The recycling means of the
third embodiment may comprise a cooler for cooling the
released fluid. As shown in Fig. 5, water, (H2O),
oxygen (~2) and hydrogen chloride (HCl), all in the
form of vapor, are carried through a line 50''' to a
cooler 52''' which cools the released fluid. As noted
above, the released fluid comprises hydrogen halide,
and preferably, hydrogen chloride, and the recycling
means may also comprise a separator for removing a
portion of the hydrogen halide from the released
fluid. As shown in Fig. 5, after the released fluid
is cooled, it is carried through a line 54''' to a
separator 56''', which removes a portion of the
hydrogen chloride and the water (H2O), in the form of
liquid, through a line 70'''. The recycling means may
further comprise a scrubber for removing another
portion of the hydrogen halide. Specifically, oxygen
(~2)~ hydrogen chloride (HCl) and water (H2O), all in
the form of vapor, are carried through a line 60''' to
a scrubber 62'''. An alkali solution is added to
scrubber 62''' through a line 64''', so that the
scrubber removes another portion of the hydrogen
chloride through a line 68''', as a halide alkali
28
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W 096/34998 PCTnUS95tl6032
~3alt I n this fourth embodiment, a portion o~ the
oxygen vapor (~2) and the water vapor (H2O) is not
vented, but rather all of the oxygen and water vapor
are carried away from scrubber 62''' by a line 67'''.
In the fourth embodiment of Fig. 5, the recycling
means may also comprise a humidifier 65''' for
humidifying the released fluid with water, in either
liquid or vapor form, through a line 69'''. The
humidified fluid is carried away from line 69''' by a
line 71'''. The recycling means may also comprise a
compressor 72''' for compressing the released fluid.
The water vapor and oxygen vapor are then carried out
of compressor 72''' through a line 74'''. The
recycling means of the fourth embodiment may also
comprise an oxygen supply for supplying additional
oxygen to the released fluid. An oxygen supply
supplies additional oxygen to the released fluid
through a line 77--l as shown in Fig. 5, although thi-
~line could be placed anywhere in the recycling loop.
It is neces~ary to add oxygen in the fourth embodiment
since oxygen gets consumed as it reacts with protons
in the cell to make water. A line 79''' carries the
oxygenated released fluid away from oxygen supply line
83'''. The recycling means of the fourth embodiment
may also comprise a temperer, shown as heater/cooler
80''' in Fig. 5, which tempers the released fluid.
Temperer 80''' may either heat or cool the released
~luid, depending on the desired temperature of the
cell. A line 76''' carries the tempered fluid away
from temperer 80'''. The recycling means of the
fourth embodiment may further comprise a conditioner
78''' for supplying heat and water to the released
fluid through a line 77'''. As in the second and
- third embodiments, either a humidifier or a
conditioner, but not both, is used in the fourth
embodiment. Oxygen (~2) and water vapor (H2O(Vapor))
are carried through a line 82''' back to
electrochemical cell 10''', where the oxygen they are
29
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used to continuously supply a humidified gas stream,
which includes oxygen to the membrane.
Further in accordance with the first through
fourth embodiments of the present invention, there is
provided a process for the direct production of
essentially dry halogen gas ~rom essentially anhydrous
hydrogen halide. The anhydrous hydrogen halide may
comprise hydrogen chloride, hydrogen bromide, hydrogen
fluoride or hydrogen iodide. It should be noted that
the production of bromine gas and iodine gas can be
accomplished when the electrochemical cell is run at
elevated temperatures (i.e., about 60~ C and above for
bromine and about 190~ C and above for iodine). In
the case of iodine, a membrane made of a material
other than NAFIOND should be used.
The operation of the electrochemical cell of the
first three embodiments will now be described as it
relates to a preferred embodiment of the process of
the present invention, where the anhydrous hydrogen
halide is hydrogen chloride. In operation, current
flow to the anode bus, and anode current distributor
40 collects current from the anode bus and distributes
it to the anode by electronic conduction. Molecules
of essentially anhydrous hydrogen chloride gas are fed
to an inlet, specifically anode inlet 14 of
electrochemical cell 10, and are transported to the
surface of anode 12 and through the gas channels in
the mass flow fields. In the first embodiment, liquid
water as shown in Fig. 2 is added to the cell at the
cathode. The water is delivered to the cathode
through cathode inlet 24 and through flow channels 31
formed in cathode flow field 30. In the second
embodiment, a humidified gas stream comprising
hydrogen is delivered to the cathode through cathode
inlet 24, and in the third embodiment, a humidified
gas stream comprising nitrogen in delivered through
inlet 24. This hydrates the membrane and thereby
increase the efficiency of proton transport through
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the membrane. Molecules of the anhydrous hydrogen
chloride are oxidized at the anode under the potential
created by the voltage source to produce essentially
dry chlorine gas at the anode, and protons (H+). This
reaction is given by the equation:
Electrical
2Hcl(g) Energy ~ 2~+ + cl2(g) + 2e~
The chlorine gas exits through anode outlet 16 as
shown in Fig. 1. The protons are transported through
the membrane, which acts as an electrolyte. The
transported protons are reduced at the cathode. This
reaction is given by the equation:
Electrical
2H+ + 2e~ ~nergy ~ H2(g) (5)
A fluid is released from the~cell and is recycled back
to the membrane through the respective recycle loops
as described above with respect to Figs. 2 - 4.
Hydrogen which is evolved at the interface between the
cathode and the membrane exits via cathode-side outlet
26. The hydrogen bubbles through the water and is not
affected by the TEFLON0 in the electrode. Cathode
current distributor 4Z collects current from cathode
20 and distributes it to cathode bus 48. In the first
through third embodiments, the amount of current
required to achieve a balance between the water
transported by diffusion toward the anode and dragged
by proton transport toward the cathode is controlled
by adjusting the amount of water supplied to the
membrane. In the second and third embodiments, the
water supplied to the membrane is adjusted by
controlling the feed rate of the humidified gas
stream. Alternatively, the water supplied to the
membrane is adjusted by controlling the water content
of the humidified gas stream. As noted above,
although humidified gas streams comprising either
hydrogen or nitrogen are described for the first
31
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embodiment, it is within the scope o~ the present
invention to humidified gas streams comprising other
gases.
In the fourth embodiment of the present
invention, the electrochemical cell operates as
described above, except that a humidified gas stream
comprising oxygen is supplied to the cell at the
cathode. Oxygen and the transported protons are
reduced at the cathode to water, which is expressed by
the equation:
~O2(g) + 2e~ + 2H+ ~ H2O (g) (6)
The water formed exits via cathode--side outlet 2 6 as
shown in Fig. 1, along with any nitrogen and unreacted
oxygen. The water also helps to maintain hydration of
the membrane. The cathode-side outlet release-~ a
fluid from the cathode, which comprises oxygen (~2)r
water (H2O) and hydrogen halidë, such as hydrogen
chloride (HCl), as described above with respect to
Fig. 5. This fluid released from the cell is recycled
back to the membrane through the recycle loop as
described above with respect to Fig. 5. A-~ in the
first three embodiments, in the fourth embodiment, the
amount of current required to achieve a balance
between the water transported by diffusion toward the
anode and dragged by proton transport toward the
cathode is controlled by adjusting the amount of water
supplied to the membrane. Also, in the fourth
embodiment, as in the second and third embodiments,
the water supplied to the membrane is adjusted by
controlling the feed rate of the humidified gas
stream. Alternatively, the water supplied to the
membrane is adjusted by controlling the water content
of the humidified gas stream. Again, although a
humidified gas stream comprising oxygen is described
for the second embodiment, it is within the scope of
the present invention to humidified gas streams
comprising other gases.
32
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In this fourth embodiment, the cathode reaction
is the formation of water. This cathode reaction has
the advantage of more favorable thermodynamics
relative to H2 production at the cathode as in the
first embodiment. This is because the overall
reaction in this embodiment, which is expressed by the
~ollowing equation:
Electrical
2HCl(~) + ~O2(g) Energy ~ H2O(g) + C12(g) (12)
involves a smaller free-energy change than the free-
energy change for the overall reaction in the first
embodiment, which is expressed by the following
equation:
Electrical
2HCl(g) ~n~rgy ~ H2(g) + Cl2(g) (13)
Thus, the amount of voltage or energy required as
input to the cell is reduced in the fourth embodiment.
Moreover, in the first three embodiments of the
present invention, there is no oxygen in the cathode
reaction. However, in the fourth embodiment, where
oxygen is added to the cell, there is always an excess
o~ oxygen in the cell. This means that almost all the
protons transported across the membrane react with
oxygen to form water. This provides a higher
conversion to water. In addition, the higher the
excess of oxygen, the faster the cathode reaction
proceeds. Thus, the recycle loop of the fourth
embodiment in particular provides a ~aster cathode
reaction. Moreover, with the fourth embodiment, it is
possible to take advantage of this excess of oxygen
without the economic penalty of discarding products
from the cell through a vent 66, 66' or 66'' as
described above for the first three embodiments.
Additional advantages and modifications will
readily occur to those skilled in the art. The
invention, in its broader aspects, is therefore not
33
CA 02219922 1997-10-29
W O 96/34998 PCTrUS95/16032
limited to the specific details, representative
apparatus and illustrative Examples shown and
described. Accordingly, departures may be made from
such details without departing from the spirit or
scope of the general inventive concept as defined by
the appended claims and their equivalents.
34
