U.S. patent application number 14/780317 was filed with the patent office on 2016-02-25 for low-energy electrochemical separation of isotopes.
This patent application is currently assigned to ATOMIC ENERGY OF CANADA LIMITED. The applicant listed for this patent is ATOMIC ENERGY OF CANADA LIMITED. Invention is credited to Hugh BONIFACE, Ian CASTILLO, Nirmal GHANAPRAGASAM, Keith KUTCHCOSKIE, Hongqiang Li.
Application Number | 20160053387 14/780317 |
Document ID | / |
Family ID | 51622307 |
Filed Date | 2016-02-25 |
United States Patent
Application |
20160053387 |
Kind Code |
A1 |
KUTCHCOSKIE; Keith ; et
al. |
February 25, 2016 |
LOW-ENERGY ELECTROCHEMICAL SEPARATION OF ISOTOPES
Abstract
The invention relates to isotope separation methods, and methods
for separating isotopes with low energy consumption, demonstrated
using hydrogen isotopes. To this end, an isotope transfer
electrochemical cell is provided, which comprises an anode plate
and a cathode plate; current carrier plates with flow channels or
mesh layers or porous material; a proton exchange membrane or solid
polymer electrolyte membrane; and gas diffusion layers positioned
on either side of the proton exchange membrane which together with
the proton exchange membrane forms a membrane electrode assembly;
and a housing containing the anode and cathode plates in operable
arrangement with the membrane electrode assembly, and defining a
hydrogen feed inlet on the anode, a product outlet on the cathode,
an outlet for excess hydrogen on the anode, and internal flow paths
for transfer of gases and fluids on either side of the membrane
electrode assembly. Also described are methods for enriching or
depleting the isotope present in the hydrogen gas/vapour feed e.g.
for tritium removal, tritium enrichment and deuterium enrichment,
by arranging a series of cells in a cascaded configuration.
Inventors: |
KUTCHCOSKIE; Keith;
(Petawawa, CA) ; GHANAPRAGASAM; Nirmal; (Deep
River, CA) ; BONIFACE; Hugh; (Deep River, CA)
; CASTILLO; Ian; (Pembroke, CA) ; Li;
Hongqiang; (Deep River, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ATOMIC ENERGY OF CANADA LIMITED |
Chalk River |
|
CA |
|
|
Assignee: |
ATOMIC ENERGY OF CANADA
LIMITED
Chalk River
ON
|
Family ID: |
51622307 |
Appl. No.: |
14/780317 |
Filed: |
March 28, 2014 |
PCT Filed: |
March 28, 2014 |
PCT NO: |
PCT/CA2014/000293 |
371 Date: |
September 25, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61806427 |
Mar 29, 2013 |
|
|
|
Current U.S.
Class: |
205/637 ;
204/258; 204/265 |
Current CPC
Class: |
C25B 15/08 20130101;
C25B 9/18 20130101; C25B 1/02 20130101; C25B 9/10 20130101; B01D
59/50 20130101; B01D 59/40 20130101; B01D 59/38 20130101; B01D
59/30 20130101; B01D 53/326 20130101; B01D 59/42 20130101 |
International
Class: |
C25B 1/02 20060101
C25B001/02; B01D 59/40 20060101 B01D059/40; C25B 9/18 20060101
C25B009/18; C25B 9/10 20060101 C25B009/10; C25B 15/08 20060101
C25B015/08 |
Claims
1. An isotope transfer electrochemical cell (ITEC) for separating
isotopes comprising: an anode plate and a cathode plate; a proton
exchange membrane; gas diffusion layers positioned on either side
of the proton exchange membrane, which together with the proton
exchange membrane form a membrane electrode assembly; and a housing
containing the anode and cathode plates in operable arrangement
with the membrane electrode assembly, and defining a gas feed
inlet, a product outlet, an outlet for excess gas, and internal
flow paths for transfer of gases and fluids on either side of the
membrane electrode assembly.
2. The isotope transfer electrochemical cell of claim 1, wherein
the proton exchange membrane comprises a polymer-based
electrolyte.
3. The isotope transfer electrochemical cell of claim 2, wherein
the gas diffusion layers each comprise catalyst-coated porous
conductors.
4. The isotope transfer electrochemical cell of claim 3, wherein
the catalyst-coated porous conductors are hydrophobic.
5. The isotope transfer electrochemical cell of claim 1, wherein
the housing comprises end plates on the outer sides thereof with a
gas inlet and gas outlet openings.
6. The isotope transfer electrochemical cell of claim 5, wherein
the end plates comprise an anode side flange and a cathode side
flange made of structural material such as aluminum, stainless
steel or fibre-reinforced polymer.
7. The isotope transfer electrochemical cell of claim 1, wherein
the anode and cathode each comprise current connectors to connect
to an external power source.
8. The isotope transfer electrochemical cell of claim 7, wherein
the anode and cathode current connectors are made of titanium or
stainless steel or aluminum and are electrically insulated from the
end-plates.
9. The isotope transfer electrochemical cell of claim 1, further
comprising current carriers, or current carrier plates, positioned
between the gas diffusion layers and the respective anode or
cathode, the current carriers being of a material effective to
carry current to the electrodes and to form a pathway for gas,
vapour and condensed phase accessing the anode and discharging from
the cathode during operation.
10. The isotope transfer electrochemical cell of claim 9, wherein
the current carriers comprise titanium or stainless steel or
aluminum based mesh layers or porous material or plate type flow
channels or grooves of suitable geometry.
11. The isotope transfer electrochemical cell of claim 1, wherein
the membrane electrode assembly is comprised of two layers of
electrically-conductive gas diffusion layer with catalyst and
related material coated on the side that is held up against either
side of a proton exchange membrane.
12. The isotope transfer electrochemical cell of claim 1, wherein
the gas diffusion layer comprises a catalyst comprising a
supported-platinum powder mixed with similar polymer material as in
the membrane held in a porous electrically-conductive, partially
hydrophobic substrate.
13. The isotope transfer electrochemical cell of claim 1, wherein
the proton exchange membrane comprises a membrane made of polymers
with similar functions to sulfonated tetrafluoroethylene.
14. The isotope transfer electrochemical cell of claim 13, wherein
the proton exchange membrane comprises any one of the following:
Nafion.RTM. NR212, N115, N117 or N1110, or sulphonated PEEK or
other proton conducting membrane.
15. The isotope transfer electrochemical cell of claim 13, wherein
the thicknesses of the proton exchange membrane ranges from about
0.05 mm to about 0.25 mm.
16. The isotope transfer electrochemical cell of claim 1, wherein
the housing, hydrogen feed inlet, product outlet, and outlet for
excess hydrogen are effective to carry hydrogen gas and water
vapour.
17. The isotope transfer electrochemical cell of claim 15, wherein
a feed stream travels to the anode through the hydrogen feed inlet
and the current carrier.
18. The isotope transfer electrochemical cell of claim 1, wherein
the hydrogen feed inlet, product outlet, outlet for excess
hydrogen, and internal flow paths are arranged for either
co-current or counter-current feed with respect to the extract flow
directions.
19. The isotope transfer electrochemical cell of claim 18, wherein
the extract contains isotopically enriched or depleted hydrogen gas
and water vapour/liquid condensate and a raffinate contains the
balance of hydrogen gas and water vapour/liquid condensate from the
feed, and the hydrogen gas in the extract can be at elevated
pressure.
20. A system comprising a plurality of isotope transfer
electrochemical cells as defined in claim 1, arranged in series and
configured to pass isotopically depleted hydrogen from subsequent
cells in the series to the feed of previous cells in the
series.
21. The system of claim 20, wherein the system is for removal of
tritium from a hydrogen source.
22. A system comprising a plurality of isotope transfer
electrochemical cells as defined in claim 1, arranged in series and
configured to direct isotopically enriched hydrogen gas from
subsequent cells in the series to the feed of previous cells in the
series.
23. The system of claim 22, wherein the system is for enriching
deuterium in a hydrogen source.
24. A method of separating isotopes in a hydrogen source,
comprising providing at least one isotope transfer electrochemical
cell as defined in claim 1, feeding a hydrogen gas and water vapour
mixture via the feed inlet of the cell to the anode of the cell;
applying a current between the anode and cathode to facilitate
transfer of hydrogen ions from the anode through the membrane
electrode assembly to the cathode, the hydrogen ions recombining
with electrons at the cathode to form gaseous hydrogen, enabling
pressure to the hydrogen gas and water vapour leaving the cathode
of the cell, the anode of the cell, or both, collecting excess
hydrogen gas and water vapour leaving the anode side of the cell,
and collecting extracted hydrogen gas and water vapour leaving the
cathode of the cell, the extracted hydrogen gas being enriched in
deuterium, tritium, or both as compared to the feed gas.
25. The method of claim 24, wherein the excess hydrogen gas and
water vapour collected from the anode, the extracted hydrogen gas
and water vapour collected from the cathode, or both, are dried by
cooling or adsorption.
26. The method of claim 24, wherein the current is applied at a
voltage ranging from about near zero to 0.7 volts.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to U.S. provisional
application Ser. No. 61/806,427, filed on Mar. 29, 2013, the
contents of which are incorporated herein by reference.
FIELD OF INVENTION
[0002] The present invention relates to isotope separation methods,
and particularly, to a method and apparatus for separating isotopes
with low energy consumption. The isotope relevant to a particular
feed element is separable by using an ion-exchange membrane that is
specific to the feed element and its isotope(s).
BACKGROUND OF THE INVENTION
[0003] Isotope separation typically involves the processing of a
particular feed element and concentrating specific isotopes of
interest, usually by removing other isotopes from the feed. The
separation of hydrogen isotopes is discussed below as but one
example of the various isotope separation processes used in the
field.
[0004] Hydrogen has three naturally occurring isotopes, .sup.2H
(deuterium, also labeled as D), .sup.3H (tritium, also labeled as
T), as well as the most highly abundant .sup.1H isotope (sometimes
called "protium"). Other isotopes have been synthesized in the
laboratory, but are not observed in nature.
[0005] Hydrogen and its isotopes have a number of applications. For
instance, hydrogen is an energy carrier, and can be used for fuel
cells. Tritium can also be used, for example, in low power
equipment in remote areas. Deuterium, in the form of heavy water
(D.sub.2O), is useful as a neutron moderator, and is an important
component in the CANDU (short for CANada Deuterium Uranium) nuclear
reactors developed by Atomic Energy of Canada Limited. However, the
high cost of heavy water produced using the Girdler-Sulphide
process, and ammonia-based processes, can affect the economic
attractiveness of heavy-water moderated reactors such as CANDU.
[0006] Several processes are currently available for separating
hydrogen isotopes. These include: electrochemical processes, in
which water electrolysis cells are used to produce hydrogen and
oxygen from water, typically with the heavier hydrogen isotope
enriched within the cell electrolyte and depleted in the hydrogen
gas; chemical exchange processes, such as the Girdler-Sulfide
process, in which hydrogen isotopes are exchanged between hydrogen
sulfide and water with the heavier isotope preferentially
transferring to the water; phase separation processes, in which
fractional distillation or other physical separation processes are
used to separate the hydrogen isotopes based on differences in
volatility between the lighter and heavier isotopes; diffusion
processes in which gas transport, e.g. through a porous membrane,
enables the lighter isotope to diffuse more quickly and be enriched
in the transported gas; and laser activation, whereby
pre-dissociation of formaldehyde with a tuned laser can be used to
separate hydrogen and deuterium.
[0007] A significant drawback of these processes is that they are
energy intensive. In addition, electrolysis cells can be quite
large and costly, whereas water distillation requires very large
equipment because of the very poor separation factor. The
Girdler-sulfide process requires very large equipment and can only
be economical for very large-scale production. Membrane diffusion
also requires very large equipment, and many stages of pumping
which makes maintenance a significant cost. Laser activation uses
rather exotic materials, and equipment that is quite costly and
complex.
[0008] Accordingly, there remains a need for new methods and
equipment for efficiently separating hydrogen isotopes, as well as
isotopes of other elements.
SUMMARY OF THE INVENTION
[0009] It is an object of the invention to provide an improved
apparatus and method for separating isotopes.
[0010] According to an aspect of the present invention, there is
provided an isotope transfer electrochemical cell (ITEC)
comprising:
[0011] an anode plate and a cathode plate;
[0012] an ion exchange membrane positioned between the anode and
cathode plates;
[0013] gas diffusion layers (GDL) positioned on either side of the
ion exchange membrane, which together with the ion exchange
membrane form a membrane electrode assembly (MEA); and
[0014] a housing containing the anode and cathode plates in
operable arrangement with the membrane electrode assembly, and
defining a gas feed inlet, a product outlet, an outlet for excess
feed gas, and internal flow paths for transfer of gases and fluids
on either side of the membrane electrode assembly.
[0015] In certain embodiments of the isotope transfer
electrochemical cell, the ion exchange membrane may comprise a
polymer-based proton-exchange material. In addition, the gas
diffusion layers may each comprise catalyst-coated porous
electrical conductors.
[0016] In further embodiments, the housing may comprise end plates
on the outer sides thereof, the end plates having gas inlet and gas
outlet openings. The end plates may, in further non-limiting
embodiments, also comprise an anode side flange and a cathode side
flange made of suitable structural materials.
[0017] The anode and the cathode may also each comprise connectors
to connect to an external power source. For example, yet without
wishing to be limiting, the anode and cathode connectors may be
made of metals or other electrically-conductive material, and may
be electrically insulated from the end-plates.
[0018] In further embodiments of the invention, the isotope
transfer electrochemical cell may comprise current carriers,
positioned between the gas diffusion layers and the respective
anode or cathode, the current carriers being of a material
effective to conduct current to the electrodes and to form a
pathway for gas accessing the anode and discharging from the
cathode during operation. In particular non-limiting embodiments,
the current carriers comprise titanium or stainless steel or
aluminum based mesh layers.
[0019] The gas diffusion layer, in further non-limiting
embodiments, comprise a layer of porous electrically-conductive
material which has partial water repellent coating on it to allow
gas through it while moisture needs to be present in the
membrane.
[0020] In addition, the gas diffusion layer may comprise a catalyst
layer comprising a supported-platinum powder mixed with a suitable
binder material held in the said porous electrically-conductive
material as a coating.
[0021] In yet further embodiments, the ion exchange membrane may
comprise a polymer membrane that is conductive to protons. For
example, yet without wishing to be limiting, the ion exchange
membrane (specifically, proton exchange membrane, PEM) may comprise
sulfonated tetra-fluoro-ethylene (TFE) based
fluoro-polymer-copolymer (such as Nafion.RTM.), or sulphonated
polyether-ether-ketone (PEEK). The thicknesses of the dry proton
exchange membrane may vary, for instance, in a range from about
0.05 mm to about 0.25 mm.
[0022] The housing, hydrogen feed inlet, product outlet, and outlet
of the cell are, in embodiments, generally effective to contain and
transport hydrogen gas and water vapour. The hydrogen feed inlet,
product outlet, outlet for excess hydrogen, and internal flow paths
may also be arranged for either co-current or counter-current feed
and extract flow directions.
[0023] In yet further embodiments, the extract (product stream) may
contain isotopically enriched or depleted hydrogen gas, water
vapour and condensate. The raffinate (excess of feed stream)
contains the balance of hydrogen gas, water vapour and condensate
from the feed. The hydrogen gas in the extract can be at elevated
pressure with respect to the feed.
[0024] Also provided herein is a system comprising a plurality of
isotope transfer electrochemical cells as described herein,
arranged in series and configured to pass isotopically depleted
hydrogen gas from subsequent cells in the series to the feed of
previous cells in the series. In certain embodiments, the system
may be configured for removal of tritium or protium from a hydrogen
source.
[0025] Also provided herein is a system comprising a plurality of
isotope transfer electrochemical cells as described herein,
arranged in series and configured to direct isotopically enriched
hydrogen gas from subsequent cells in the series to the feed of
previous cells in the series. In certain embodiments, the system
may be configured for enriching deuterium or tritium in a hydrogen
source.
[0026] In addition, there is also provided a method of separating
isotopes in a hydrogen source, comprising:
[0027] providing at least one isotope transfer electrochemical cell
as described herein,
[0028] feeding a hydrogen gas and water vapour mixture via the feed
inlet of the anode side of the cell using said proton exchange
membranes, but only feeding hydrogen gas when using membranes which
require no water vapour to function;
[0029] applying a voltage to the anode to facilitate the formation
of hydrogen and isotopic ions on the anode and the transfer of
these ions through the membrane electrode assembly to the cathode,
the hydrogen and isotopic ions recombining with electrons at the
cathode to form gaseous hydrogen with a different isotopic
concentration,
[0030] applying a pressure to the hydrogen gas and water vapour
leaving the cathode of the cell, the anode of the cell, or
both,
[0031] collecting excess hydrogen gas and water vapour leaving the
anode side of the cell, and
[0032] collecting extracted hydrogen gas and water vapour leaving
the cathode side of the cell, the extracted hydrogen gas being
enriched/depleted in deuterium, tritium, or both as compared to the
feed gas.
[0033] Electrochemical processes for separation of hydrogen
isotopes--such as water electrolysis--typically require about 1.5
volts or higher of electrical potential. Thus, the invention is
particularly advantageous since it enables hydrogen isotope
separation at electrical potentials that are much lower. The
current may be applied at a voltage below one volt (often below 0.5
volts) which depends on the required enrichment ratio and
resistance due to material and assembly of the internal components
of the cell.
[0034] In further embodiments, the present invention can also be
applied to separating isotopes of elements other than hydrogen.
This can be carried out using the methods and apparatus described
herein, by selecting a suitable electrolytic (ion-transfer)
membrane for the element of interest and its isotope(s).
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] These and other features of the invention will become more
apparent from the following description in which reference is made
to the appended drawings, wherein:
[0036] FIG. 1 shows a schematic of an example of a Isotope transfer
electrochemical cell (ITEC), illustrating internal components of
the cell, in accordance with an embodiment of the invention;
[0037] FIG. 2 illustrates schematic diagrams of (A) an example of a
co-current flow arrangement of an ITEC and (B) an example of a
counter-current flow arrangement of an ITEC, in accordance with
further embodiments of the invention;
[0038] FIG. 3 illustrates an example of an embodiment of a cascaded
arrangement of ITECs, useful for deuterium enrichment;
[0039] FIG. 4 illustrates an example of an embodiment of a cascaded
arrangement of ITECs, useful for tritium removal;
[0040] FIG. 5 shows the results of the effect of membrane specific
cell resistance (MSCR) on enrichment ratio for Nafion.RTM. N115,
N117 and N1110 membranes;
[0041] FIG. 6 shows the results of testing the effect of catalyst
loading density on enrichment ratio for Nafion.RTM. N117 and N1110
membranes;
[0042] FIG. 7 shows the results of testing the effect of cell
voltage on enrichment ratio for Nafion.RTM. N115, N117 and N1110
membranes;
[0043] FIG. 8 shows the results of the effect of cell current
density on enrichment ratio for Nafion.RTM. N115, N117 and N1110
membranes;
[0044] FIG. 9 shows the results of the effect of cell electrical
power on two best enrichment ratios for Nafion.RTM. N115, N117 and
N1110 membranes;
[0045] FIG. 10 shows the results of the effect of cell operating
temperature on enrichment ratio for Nafion.RTM. N115, N117 and
N1110 membranes;
[0046] FIG. 11 shows the results of testing the effect of gas
pressure on enrichment ratio for Nafion.RTM. N117 and N1110
membranes;
[0047] FIG. 12 shows the results of testing the effect of in situ
membrane water uptake on enrichment ratio for Nafion.RTM. N115,
N117 and N1110 membranes;
[0048] FIG. 13 shows the effect of membrane thickness on enrichment
ratio for Nafion.RTM. N115, N117 and N1110 membranes;
[0049] FIG. 14 shows the results of testing effect of extract to
raffinate flow ratio on enrichment ratio for Nafion.RTM. N115, N117
and N1110 membranes;
[0050] FIG. 15 shows results of testing differences in enrichment
ratio for Nafion.RTM. N115 membrane due to co-current and
counter-current flow arrangements.
DETAILED DESCRIPTION
[0051] The electrochemical cell described herein can be operated at
low power to transfer hydrogen through a membrane in such a way
that the transferred hydrogen is enriched in the heavier isotope,
and the portion not transferred is enriched in the lighter isotope.
By applying the pressure rise resulting from the transfer a staged
cascade of cells can also be provided to improve the separation,
such that deuterium is enriched to produce heavy water, or tritium
can be removed from hydrogen or deuterium.
[0052] In an embodiment, the electrochemical cell comprises the
following features:
[0053] two sides, with anode and cathode end plates, electrical
connectors and current carriers;
[0054] a proton exchange membrane or polymer electrolyte membrane
(PEM) in the middle, which in preferred embodiments comprises a
solid polymer-based electrolyte;
[0055] gas diffusion layers (GDL) comprising catalyst-coated porous
conductors attached on either side of the PEM membrane, which
together with PEM form a membrane electrode assembly (MEA); and
[0056] a mechanical housing with a hydrogen feed point, a product
outlet and an outlet for excess hydrogen (raffinate), as well as
appropriate internal flow paths for the fluids on either side of
the MEA.
[0057] When a small electric potential (below 1.0 volt) is applied
between the anode and the cathode and hydrogen is supplied to the
electrochemical cell, hydrogen isotope separation occurs producing
one stream of hydrogen enriched in the heavy isotope and one
hydrogen stream depleted in the heavy isotope.
[0058] A cascaded arrangement of a series of such cells is also
provided, which in certain embodiments allows for: enrichment of
deuterium in hydrogen; and removal of tritium from light or heavy
water.
[0059] Without wishing to be limiting in any way, it is envisioned
that using certain configurations of the electrochemical cells and
methods described herein can provide one or more of the following
beneficial features: [0060] i. low electrical energy: unlike a
water electrolysis cell, only a small amount of electrical energy
may be needed to separate deuterium or tritium from protium; [0061]
ii. only hydrogen gas and water are involved: there is no oxygen
production according to the reactions carried out by the described
electrochemical cells, and thus the use of oxygen sensitive and
oxygen safety related materials is reduced or eliminated; [0062]
iii. simultaneous enrichment and depletion: the described
electrochemical cells can enrich one portion of a feed stream while
depleting the other with deuterium or tritium simultaneously, which
makes it easier for the cell to be used in reversible applications.
[0063] iv. low or complete lack of electro-catalyst on the cathode
side allows the cell to operate in isotope depletion mode with
respect to the feed isotope concentration and may reduce
significantly the cost of cell construction.
[0064] The electrochemical cell and methods of the present
invention can, in certain embodiments, be used in the production of
heavy water, e.g. for general use, or for use in the nuclear
industry; be used in the detritiation of light water, for example
as a means for waste remediation; be used in the enrichment or
concentration of tritium, for example, to improve sensitivity in
very low-level sample analysis; and be used in the upgrading of
heavy water, for example in CANDU reactor operations.
[0065] Thus, the electrochemical cell and methods of the present
invention may provide a practical alternative to the commonly used
Girdler-Sulfide heavy water production process, and the various
distillation processes currently used for heavy water upgrading and
detritiation. Moreover, embodiments of the electrochemical cell and
methods of the present invention can be especially advantageous,
since they enable hydrogen isotope separation with low energy
requirements, while still maintaining a good enrichment ratio.
[0066] The electrochemical cell and methods of the present
invention will now be described in further detail with reference to
one non-limiting embodiment of the electrochemical cell, referred
to herein as a Isotope Transfer Electrochemical Cell (ITEC).
[0067] Unlike the water electrolysis cells currently used for
hydrogen isotope separation, the ITEC can operate at low cell
voltages since the hydrogen transfer reaction employed is
relatively more facile than the water decomposition reaction. As
will be described in further detail below, the ITEC can also be
used as an electrochemical compressor to pump a certain isotopic
hydrogen gas to high pressures.
[0068] The principle of operation of the ITEC is that hydrogen is
passed through a proton exchange membrane (PEM) under the influence
of an electric current. The ITEC arrangement thus includes the
cathode half of a PEM water electrolysis cell and anode half of a
PEM fuel cell. The hydrogen is first oxidized on the inlet (anode)
side of the membrane to protons which transfer to the cathode side
through certain transport mechanisms and are reduced to reform
hydrogen gas. In an electrochemical compressor, the objective is
for the electric current to produce the hydrogen at a higher
pressure at the cathode than the anode side. In the ITEC, on the
other hand, the objective is to preferentially transfer one of the
hydrogen isotopes from the anode side to the cathode side of the
cell. In practice, part of the feed stream to the anode passes
through the membrane to the cathode and is enriched (or depleted if
there is no catalyst on the cathode side) in one of the isotopes,
with the remaining hydrogen from the feed stream being depleted in
that isotope. The electrochemical process of transferring hydrogen
through a PEM in this way requires no moving parts, uses materials
that are well-developed and robust, and requires modest voltages
and hence, power. Thus, this method of hydrogen isotope separation
has the potential to be both practical and economical.
Chemical Reactions
[0069] Hydrogen isotopes have three possible transport processes
happening inside the MEA of the ITEC at steady state: (i)
electrochemical reaction, (ii) isotopic separation, and (iii)
transport of hydrogen gas and solvated protons (hydronium
ion--H.sub.3O.sup.+). The electrochemical separation processes
within an ITEC occurs on both anode and cathode when appropriate
cell voltage is applied. In the presence of a catalyst, there are
two primary reactions occurring at anode and cathode of the cell.
The anode reactions can be expressed as Equations 1, 5, and 6. The
cathode reactions can be expressed as Equations 7-9. Only deuterium
based reactions are provided here, tritium follows similar
principles. By careful selection of the catalyst and the cell
voltage, isotopes will be preferentially enriched/concentrated at
the cathode. The hydrogen isotopic separation in general involves
two principles:
a) Equilibrium isotope effect (EIE) and its isotopic analogues:
HD.sub.(g)+H.sub.2O.sub.(1).revreaction.H.sub.2(g)HDO.sub.(1)
(1)
[0070] This effect is the end result of the catalyst present at
both the electrodes. This effect occurs in two consecutive
steps
HD.sub.(g)+H.sub.2O.sub.(v).revreaction.H.sub.2(g)+HDO.sub.(v)
(2)
HDO.sub.(v)+H.sub.2O.sub.(1).revreaction.H.sub.2O.sub.(v)+HDO.sub.(1)
(3)
b) Kinetic isotope effect (KIE) is inherent in electrolytic
hydrogen evolution reaction; here it occurs in the cathode of the
ITEC.
KIE = k H k D ( 4 ) ##EQU00001##
Where k.sub.H and k.sub.D are reaction rate constants for hydrogen
and deuterium reactions, respectively.
[0071] The reactions in the equations above are well known
characteristics in liquid phase catalytic exchange processes for
hydrogen isotope separation. Within the ITEC, the reactions
occurring on anode and cathode are:
[0072] Anode reactions for ITEC:
2H.sub.2O.sub.(v)+H.sub.2(g).fwdarw.2H.sub.3O.sup.++2e.sup.-
(5)
2H.sub.2O.sub.(v)+HD.sub.(g).fwdarw.H.sub.3O.sup.++H.sub.2DO.sup.++2e.su-
p.- (6)
HD.sub.(g)+H.sub.2O.sub.(1).revreaction.H.sub.2(g)HDO.sub.(1)
(1)
[0073] Cathode reactions for the ITEC:
2H.sub.3O.sup.++2e.sup.-.fwdarw.H.sub.2(g)+2H.sub.2O.sub.(v)
(7)
H.sub.3O.sup.++H.sub.2DO.sup.++2e.sup.-.fwdarw.HD.sub.(g)+2H.sub.2O.sub.-
(v) (8)
2H.sub.2DO.sup.++2e.sup.-D.sub.2(g)+2H.sub.2O.sub.(v) (9)
[0074] These reactions occur in sequence as hydrogen passes through
the three zones within the cell where: (i) gas is oxidized into
ions on the anode side, (ii) ions transport through the membrane
with isotopes from either side approaching equilibrium; and (iii)
ions are reduced to form gas on the cathode side. Of these reaction
steps one will typically be the reaction rate-limiting step, which
is the slowest reaction step compared to the other reaction steps.
Material and operating conditions of the ITEC determine this
rate-limiting step. Within the ITEC the reaction steps occur only
in the catalyst layer of the anode and cathode sides while the
kinetic isotope effect may occur simultaneously on either side
depending on local electronic and ionic conduction and
concentration gradients. The ionic forms of the hydrogen isotopes
need to be transferred from the anode to the cathode through the
membrane of the ITEC. Three possible ionic transport mechanisms are
believed to be present within the membrane: [0075] a) surface
conduction via proton hopping, [0076] b) bulk conduction via
Grothuss diffusion, and [0077] c) bulk conduction via en masse
diffusion.
[0078] The driving force for proton transport across the membrane
is mainly due to the existence of electrolyte potential gradient
while the effect of diffusion within the membrane is relatively
small [Y. Wang, K. S. Chen, J. Mishler, S. C. Cho, X. C. Adroher. A
review of polymer electrolyte membrane fuel cells: Technology,
applications, and needs on fundamental research. Applied Energy, 88
(2011) 981-1007]. Thus the proton transport occurs in only one
direction within the electrochemical cell from anode to cathode.
Water in the membrane is essential for proton transport through a
mechanism called `vehicular` diffusion. This happens when protons
form hydronium ions (H.sub.3O.sup.+), which can transport from high
to low proton concentration regions. The vehicular diffusion seems
to occur at a typical water content of about 22% as the channels
between water clusters within the membrane are beginning to form at
a water content of 14%. This vehicular mechanism dominates at high
hydrogen partial pressures while the transport along proton-binding
groups (such as sulfonate), through proton tunneling (or hopping of
protons), dominates at low hydrogen partial pressures. Recent
experimental work in low hydrogen partial pressure range has
revealed two other particularly noteworthy features of the
Nation.RTM. conductivity: (i) its linear increase with membrane
thickness; and (ii) a strong isotope effect with up to fourfold
reversible decrease in the power output of PEM fuel cells when
hydrogen is replaced by deuterium at the anode. This confirms the
higher cell resistance, seen in ITEC operation, as an artifact of
transporting deuterium across Nafion.RTM. membranes. The overall
transport mechanism is governed by both thermodynamics and reaction
kinetic effects.
Single Cell Configuration:
[0079] The schematic of a simple version of an ITEC with internal
components is shown in FIG. 1. The ITEC (10) looks very similar to
other types of PEM electrochemical cells. It has several layers of
square or circular shaped components held together by a set of
bolts (15) along its perimeter. There are two separated sides in
the cell: (i) anode side, where the hydrogen gas is fed and excess
hydrogen leaves; and (ii) cathode side, where hydrogen gas is
produced and possibly pumped to a higher pressure. The components
of the illustrated cell design are described below:
[0080] 1. End-plates and insulator: There are two flanges on the
outer sides of the cell to hold everything together. These flanges
serve as the end plates (16) of the cell with openings for gas
inlet (17) and outlets (18). In the embodiment illustrated, the
anode side flange and the cathode side flange is made of stainless
steel. Other materials capable of withstanding pressure and
electrochemical environment may also be used. There is a thin sheet
(19) in between the end plate and the electrical connector plate
that provides insulation against electrical current from getting to
the end plate.
[0081] 2. Electrical connector plates: Next to the insulated thin
sheet toward the center are the anode and cathode electrical
connector plates (20), as shown in FIG. 1. In the embodiment
illustrated, they both are made of titanium or stainless steel or
aluminum and are electrically insulated from the end-plates. The
ITEC is connected to an external direct current (DC) power source
via these two plates.
[0082] 3. Current carrier (21): These are titanium or stainless
steel or aluminum based mesh, shaped according to the geometry of
the cell active area that help carry current to the electrodes of
the ITEC. The meshing also forms a pathway for humidified gas
accessing the anode or discharging from the cathode during
operation. Design and development of the current carrier is
focussed in reducing the resistance to electronic pathway, while
maintaining adequate pathway for the hydrogen gas-water vapour
mixture that reside behind the gas diffusion layer.
[0083] 4. Electrode assembly: This is the combination of gas
diffusion layer (GDL) and the catalyst layer available for the
reaction. The constituents for this assembly could be the same on
both anode and cathode sides, or different on either side depending
on the nature of the isotopic separation required. [0084] a) Gas
diffusion layer (GDL) (22): This has a layer of material that is
permeable to gas and moisture; is electrically-conductive and; is
partially hydrophobic (either blended or coated with
water-repelling compound such as Teflon.RTM.). Often a type of
carbon paper or carbon cloth is used as a GDL material. Other
materials with similar properties can be used depending on the need
to reduce electronic resistance, improve cell performance and
reduce cost. [0085] b) Catalyst (23): The catalyst in the form of
carbon supported-platinum powder (other similar catalysts may be
used primarily to reduce cost while maintaining performance) is
mixed along with a polymer like Nafion.RTM. and sprayed or printed
or coated on to the GDL (22) to form the electrode assembly.
[0086] 5. Proton exchange membrane or polymer electrolyte membrane
(PEM) (25): In this cell the electrolyte is in the form of a
polymer that creates ionic transport paths when hydrated (brought
in contact with water or water-vapour). Such membranes are
commercially available, including membranes made from the polymer
Nafion.RTM. with varying dry thicknesses available for use. In
certain non-limiting embodiments, membranes made with DuPont
Nafion.RTM. NR212, N115, N117 and N1110, or with sulphonated PEEK
may be used. The membrane thicknesses when dry can vary, in some
instances, from about 0.05 mm to about 0.25 mm. The membrane
thickness changes when hydrated depending on its polymer's
characteristic.
[0087] 6. Membrane electrode assembly (MEA): This is a combination
of the membrane with the anode and cathode electrode assemblies
(GDL and catalyst layer combined), and can be made either as one
integrated assembly by pressing them together at a certain
temperature and pressure for an amount of time or by just arranging
them in layers as shown in FIG. 1 and letting the pressure from the
bolts hold these three layers together.
[0088] 7. Gas and vapour flow inlet and outlets (17,18): There are
three ports (made of plastic or stainless steel fittings) for the
gas and vapour/liquid to enter and leave the cell: [0089] a) FEED:
The feed contains hydrogen gas in isotopic equilibrium with water
vapour or water. The moisture in the hydrogen is necessary to keep
Nafion.RTM.-type membranes wet, which increases the proton
conductivity, of the membranes. The feed stream enters the anode
side of the cell through the inlet port (17) as shown in FIG. 1.
The actual feed flow rate and composition varies depending on the
operating conditions. [0090] b) EXTRACT: The extract contains the
isotopically enriched or depleted hydrogen gas and water
vapour/water. This is the product stream that exits the cell on the
cathode side as shown in FIG. 1. The hydrogen gas in the extract
can be at elevated pressure. [0091] c) RAFFINATE: The raffinate
contains the balance of feed, typically hydrogen gas and water
vapour or water. It will contain the balance of the isotope not
transferred to the extract. The raffinate stream exits the cell on
the anode side as shown in FIG. 1.
[0092] Internal gas and vapour flow management: The isotope
separation on either side of the membrane may be dominated by two
aspects of internal gas/vapour flow arrangement: (i) setting a
gradient in concentration potential difference (CPD) between the
two sides of the membrane in the active area from FEED to
RAFFINATE, and; (ii) controlling the axial dispersion of gas and
vapour in reducing localized CPD between FEED and RAFFINATE ports
on the anode side.
[0093] Setting a gradient in CPD of the heavier isotope between the
two sides of the membrane is done by either by changing the
position of the EXTRACT port on the cathode side as shown in FIGS.
2A and 2B or by reversing the flow direction of FEED stream. There
are two possible arrangements, similar to flow of two fluids in a
mass transfer column: (i) co-current and (ii) counter-current,
based on FEED and EXTRACT flow directions, as depicted in FIG. 2 (A
and B). The counter-current flow arrangement has demonstrated
better isotopic separation than the co-current arrangement, proving
the existence of a gradient that is independent of other functions
of the cell but the flow arrangement and possibly in combination
with axial dispersion (path function between flow ports).
[0094] The axial dispersion of flow occurs due to the presence of
non-plug flow (when using layered mesh current carriers) and
minimizing this dispersion will reduce localized concentration
potential difference (CPD) thus maintaining higher potential
between the two sides of the membrane. On the anode side the GDL
may disperse gas/vapour flow unevenly across its area thus the
isotope concentrations between the FEED and RAFFINATE creates
uneven localized CPD. To avoid the dispersion, linear pathways for
gas/vapour flow may be created using flow channels that are typical
of fuel cells but with modifications needed to suit ITEC
operation.
Multi-Cell/Cascaded Configuration:
[0095] A cascaded arrangement is when several ITECs are arranged in
a manner where the isotopically depleted gas from the next ITEC is
added to the feed of the previous ITEC in such a way that higher
levels of isotope enrichment/depletion can be achieved. FIG. 3 and
FIG. 4 show two cascaded flow system arrangements using a plurality
of ITECs in series. The arrangement in FIG. 3 is for enriching
deuterium where the enriched gas from the first stage becomes feed
for the second stage, second enriched gas becomes feed for the
third stage, and so on. The depleted gas is reconnected to the feed
of a previous stage based on the condition that the concentration
of deuterium in that feed is less than depleted gas. For example,
if D.sub.M (depleted gas from the intermediate stage) is greater or
equal to E.sub.M-2 then M is the stage starting from first that
actually can use the deuterium in the depleted gas stream D.sub.M.
So, all stages before M will have depleted gas streams with lower
concentrations than streams after M.sup.th stage.
[0096] In FIG. 4, the arrangement shown is for removing tritium
from the feed. This is the opposite of that shown in FIG. 3, in
that the depletion happens in the forward direction rather than
enrichment. Thus both cascaded arrangements can be used reversibly
depending on the feed isotope concentration. In FIG. 4 the depleted
gas from the first stage becomes feed for the second stage, second
depletion becomes feed for third stage, and so on. The enriched gas
is reconnected to the feed of a previous stage based on the
condition that the concentration of tritium in that feed is more
than enriched gas. For example, if E.sub.M is less than or equal to
D.sub.M-2 then M is the stage starting from first that actually can
use the tritium in the enriched gas stream E.sub.M. So, all stages
before M can only store the enriched gas streams due to higher
concentrations of tritium than tritium in depleted streams from
previous stages.
Methods of Operation:
[0097] As discussed above, the ITEC can in certain embodiments be
used for: (i) hydrogen isotope separation and (ii) increasing the
product gas pressure, particularly for cascading. To achieve these
two goals, the cell operation may involve the following
processes:
[0098] 1. Humidification of feed gas: Isotope separation occurs
between the source hydrogen gas and water vapour that accompanies
it. So the feed gas is either humidified outside the cell with
water vapour or liquid water is injected into the feed gas and is
humidified inside the cell. In both cases, the isotope
concentration of source water is in equilibrium with the gas. When
injecting water into the feed gas, the water flow rate can be
controlled independent of the gas flow to affect the water content
in the membrane.
[0099] 2. Anode side process: The feed gas and water vapour mixture
flows to the anode side of the cell (FIG. 1). The electric current
available at the anode determines the amount of hydrogen available
at the cathode by proton transfer through the membrane assembly
into the cathode side of the cell. The anode side process of ITEC
is very similar to the anode side process of a PEM fuel cell where
the hydrogen oxidation reaction occurs.
[0100] 3. Excess/raffinate gas and vapour stream: Any excess gas
and water vapour exits the anode side of the cell, which may be
cooled to remove condensate. The deuterium concentration in the
condensate is changed from that of the feed water vapour based on
the enrichment or depletion of the raffinate gas.
[0101] 4. Cathode side process: In the cathode side of the ITEC,
the protons transported through the membrane combine with electrons
to form gaseous hydrogen and a condensed phase that exits as water
vapour with the gas. The cathode side process of ITEC is very
similar to the cathode side process of a PEM water electrolysis
cell where the hydrogen evolution reaction occurs.
[0102] 5. Extract/pumped gas and vapour stream: The gas and water
vapour leaving the ITEC on the cathode side is the extracted
hydrogen, meaning it is pumped up to the set exit pressure with
enrichment or depletion of deuterium/tritium compared to the feed
gas. The stream is cooled to remove condensate and the dry gas is
collected. This gas is analysed to estimate the degree of isotope
exchange due to this electrochemical process. The deuterium
concentration in the condensate is changed from that of the feed
water vapour based on the enrichment or depletion of the extract
gas.
Examples
Optimization of ITEC Characteristics
[0103] Optimization of the ITEC features has been carried out using
deuterium separation. However, it is possible to extrapolate the
optimal features for tritium separation from the deuterium
separation experiments, based on experience in similar
electrochemical separation systems.
[0104] Five membranes were tested. Of these, it was found that
Nafion.RTM. NR212 and SPEEK had lower separation efficiency than
the other three (Nafion.RTM. N115, Nafion.RTM. N117 and Nafion.RTM.
N1110). Thus, these two membranes are generally not included in the
discussions below except for comparative purposes.
[0105] The following parameters were used to characterize the
performance of the ITEC based on the described experimental
configuration:
[0106] 1. Enrichment ratio (ER): This is the ratio of deuterium
concentration in the extract gas stream to that in the raffinate
gas stream. This is also referred to as the `separation factor`--a
term generally used in literature of hydrogen isotope separation
processes. Primary ITEC operating parameters are compared against
this ratio. The maximum enrichment ratio achieved so far using a
single cell configuration was about three, at a cell voltage of 0.5
volts using membrane Nafion.RTM. N117. The maximum enrichment ratio
is subject to improvement upon further modifications to cell
materials and operating conditions.
[0107] 2. Membrane specific cell resistance (MSCR): This is the
ratio of actual cell voltage to current multiplied by the ratio of
membrane area to thickness with a unit of .OMEGA.m. MSCR for a
given membrane increases with electrical power, since the
separation increases with cell voltage. MSCR also was observed to
be affected by the amount of platinum catalyst in the electrodes
and Nafion.RTM.-carbon ratio in the gas diffusion layer. Higher
platinum loading and higher cell temperature reduced the MSCR. The
isotope separation is generally found to have a positive
correlation to the MSCR for all the membranes tested (as shown in
FIG. 5)--higher enrichment ratio is associated with higher cell
resistance.
[0108] 3. Catalyst: Catalyst is necessary to enable the
electrochemical reactions such as the oxidation reaction in the
anode to evolve protons for effective isotope separation and
subsequent transport of protons across the membrane to the cathode
side, where, catalyst is needed to enable reduction reaction to
evolve deuterium enriched hydrogen gas. The following are catalyst
related performance characteristics of ITEC: [0109] Catalyst layer
thickness: At a higher catalyst loading density the catalyst layer
adds a certain thickness to the GDL increasing the electrical
resistance slightly. The higher catalyst loading density the
thicker this catalyst layer becomes. This may not improve the cell
performance due to the poor catalyst utilization, as the additional
catalyst layers may hinder gas to access all the catalyst sites.
This was evident in tests done with catalyst loading densities
higher than that reported in FIG. 6. The catalyst coating procedure
and catalyst layer thickness is subject to further investigation
whose effects may improve cell performance. [0110] Catalyst
loading: Increased catalyst loading seems to have less impact on
the enrichment ratio of membrane N117 while it increased enrichment
ratio by 10% for membrane N1110, as shown in FIG. 6, at catalyst
loading densities of 0.55 mg Pt/cm.sup.2 and 0.84 mg Pt/cm.sup.2.
[0111] Anode catalyst loading: ITEC will not work as designed
without catalyst on the anode but will function even with very low
catalyst loading correspondingly with very low enrichment ratio.
[0112] Cathode catalyst loading: ITEC will work without catalyst on
the cathode but only in isotope depletion mode, that is with an
enrichment ratio less than 1.0. [0113] High and low catalyst
loading: When catalyst is coated onto a gas diffusion layer, a
loading beyond 0.8 mg Pt/cm.sup.2 is found not to improve
separation while all other conditions remain the same. Catalyst
loading below this number reduces the enrichment ratio. [0114]
Different loading on anode and cathode: The enrichment ratio is
different when the catalyst loading is different on the anode and
cathode. Lower catalyst on anode reduces the enrichment ratio
significantly (comparing first and third rectangles in FIG. 6)
compared to the case of lower catalyst on cathode (comparing first
and fourth rectangles in FIG. 6). This suggests that the anode side
has the rate-limiting step with isotope separation occurring mostly
in the anode catalyst.
[0115] 4. Cell voltage: Cell voltage seems to be one of the
important driving mechanisms for better enrichment ratio (isotope
separation factor) based on experimental tests done on ITEC. All
three membranes N115, N117 and N1110 have almost proportional
increase with voltage from 0.1 to 0.5 volts as seen in FIG. 7.
There seems to be a threshold at around the same voltage (between
0.4 and 0.5 volts) where the enrichment ratio is just over 2 for
all three membranes. This has been confirmed for the cell and
vapour temperatures ranging from 20 to 40.degree. C. The enrichment
ratio is lower on either side of this threshold voltage as shown in
FIG. 7 and is the subject of further investigation.
[0116] 5. Cell current density: The current density determines the
rate of hydrogen transferred by the cell) per unit area of the MEA,
with expectations of higher value being better cell performance.
FIG. 8 provides the cell current density data for the three
membranes and the corresponding enrichment ratios. The enrichment
ratio increases with current densities for all three membranes.
Thicker membranes (N1110 and N117) tend to have higher enrichment
ratios.
[0117] 6. Cell power: Better separation at lower power would make
the ITEC more attractive than a water electrolysis cell in terms of
both capital and operating costs. FIG. 9 shows the two best
separations per membrane achieved at the lowest power consumption
yet. Among the three, N117 performed the best with a enrichment
ratio of 2.6 at about 3.4 watts. With further improvements to cell
components and operating conditions, higher separation at lower
power than that reported in FIG. 9 is possible.
[0118] 7. Cell temperature: The relatively modest effect of
temperature on separation factor is evident in the data provided in
FIG. 10, where separation shows a maximum at about 25.degree. C.
for all three membranes.
[0119] 8. Gas pressure: Increasing the extract (product stream)
pressure for compressing hydrogen gas has a small effect on
enrichment ratio. Electrochemical pumping operation at lower
pressure on the feed side and higher pressure on the extract side
reduces the separation marginally (in FIG. 11 comparing 1.sup.st
and 2.sup.nd bars). This separation reduction is more significant
at lower voltages as shown in FIG. 11 (comparing 3.sup.rd and
4.sup.th bars).
[0120] 9. Humidity: Water vapour in the feed is necessary to
hydrate the membrane to conduct protons across the membrane and to
enable isotope separation between the raffinate and extract. There
are two ways to humidify the gas entering the cell: (i) using a
humidifier; and (ii) using direct water injection into the feed. In
the direct water injection, the water becomes vapour at cell
temperature inside the cell. The gas exiting the cell is moist and
the amount of water carried by the raffinate and extract streams
depend on feed gas pressure, cell temperature, voltage, current and
the membrane properties. At high temperatures more vapour is made
available in vapour-saturated gas stream but too much vapour did
not improve separation. Too little vapour might dry up the membrane
thereby increasing the cell electrical resistance. The Nafion.RTM.
membranes used performed well when they remained partially wetted
as shown in FIG. 12, where the best separation is when the membrane
water uptake is lower. With direct water injection to the feed
stream, enrichment ratio was found to be lower when the water flow
rate was increased consistently.
[0121] 10. Membrane: There are two main aspects for a given type of
membrane that affect the ITEC performance in terms of higher
enrichment ratio: (i) membrane water uptake; and (ii) membrane
thickness.
[0122] Membrane water uptake: Proton conductivity of the membrane
(and thus its performance) depends on its water content which is
related to the water uptake percentage. FIG. 10 shows the best
separation for all three membranes in the temperature range of 20
to 25.degree. C. suggesting that the membrane need not be fully
hydrated for the best separation. The level of hydration or water
uptake by the membrane is difficult to measure but can be deduced
from a proton conductivity calculation. From Nafion.RTM. membrane
characteristics, the higher the water uptake the lower is the
membrane resistance. But for the ITEC, better enrichment ratio is
always associated with higher MSCR (FIG. 5) which results from
lower water uptake. Based on the deduced data in FIG. 12, this may
be a distinguishing feature of ITEC when compared with PEM water
electrolysis cell and PEM fuel cell in terms of the hydration
levels needed to operate optimally.
[0123] Membrane thickness: The thicknesses for membranes N115, N117
and N1110 are the only difference between these membranes (ex situ
and dry) and they are 0.127, 0.178 and 0.25 mm, respectively. N1110
has the best separation observed in these studies, and is thicker
than all other membranes tested. The difference in separation for
three membranes is shown in FIG. 13 for the same power, flow
ratios, catalyst loading, cell temperature and pressure. This
suggests that the thicker membrane provides better separation but
at the cost of having higher MSCR.
[0124] 11. Extract to raffinate flow (gas and vapour) ratio: Other
than the cell voltage, the flow ratio between the extract (cathode
side) and the raffinate (anode side) provides the best driving
mechanism for higher enrichment ratio in ITEC. This is evident in
FIG. 14 for the three membranes, especially for the thicker
membranes (N117 and N1110). One possible reason for this is that
the depletion of the feed gas is progressive as it flows through
the cell (a rate-governed process), so the longer residence time in
the cell at higher flow ratios results in greater depletion in the
raffinate. Thus the best enrichment ratio for all membranes
achieved so far (FIG. 14) is when the raffinate flow was about five
times lower than the extract. This flow ratio is not expected to be
the limit.
[0125] 12. Flow arrangements: The gas flow arrangement comprises
two aspects of the cell design: (i) extract flow port arrangement
(or reversing flow of FEED stream); and (ii) axial flow dispersion
inside the cell on the non-catalyst side of the GDL. The flow
arrangement between the two sides of the cell as shown in FIGS. 2A
and 2B does affect the enrichment ratio. The counter-current
arrangement provides higher enrichment ratio (as shown in FIG. 15)
suggesting the existence of varying concentration potential
difference between the two sides of the membrane as the gases flow
through each side of the cell. This is consistent with the
observations on the effect of flow ratio mentioned previously.
Applications:
[0126] The electrochemical cell and methods of the present
invention may be implemented in a variety of configurations for the
enrichment or removal of hydrogen isotopes. Without wishing to be
limiting in any way, the following are possible applications that
may be carried out using ITECs as described above.
[0127] Heavy water production: Looping several ITECs in a cascaded
system (e.g. as shown in FIG. 3) increases deuterium enrichment in
steps to achieve very high concentrations. A simple cascading
system with recycle from a higher enriched stage to a lower stage,
as shown in FIG. 3, was modeled for a fixed enrichment ratio of 2.
The results indicated that it would require about 43 ITECs to
achieve nuclear reactor grade heavy water (99.75-99.98%). A similar
cascaded arrangement (e.g. as shown in FIG. 4) can be used for
removing high levels of tritium from a gas feed humidified with
enriched vapour.
[0128] Tritium decontamination: Any application or process
requiring modest removal of tritium is a possible application. For
example, the ITEC is suitable for lower stages of a tritium removal
process; for drinking water tritium removal; and for
decontaminating low levels of tritium in service water from a
shutdown nuclear reactor.
[0129] Tritium monitoring: To measure low levels of tritium in
water/hydrogen gas exhausts of nuclear power plants, the ITEC may
be coupled with tritium monitors at various tritium testing
locations. A sample can be processed through the ITEC to provide a
tritium enriched stream that would improve sensitivity of tritium
detection.
[0130] Isotope separation: Separation of isotopes of elements other
than hydrogen from the feed stream to the extract (product) stream
is possible. Depending on the element and isotope(s) involved, the
internal components of ITEC are subject to minor changes,
especially the material of the ion exchange membrane that separates
the two sides of the cell and the catalyst used in the MEA.
[0131] One or more currently preferred embodiments have been
described by way of example. It will be apparent to persons skilled
in the art that a number of variations and modifications can be
made without departing from the scope of the invention as defined
in the claims.
* * * * *