TITLE OF THE INVENTION
“METHOD AND PLANT FOR THE ELECTROCHEMICAL PRODUCTION
OF OXYGEN”
5
DESCRIPTION
[0001] The present invention relates to a method and to a plant for the
electrochemical production of oxygen according to the preambles of the
10 independent claims.
PRIOR ART
[0002] In general, there are various possibilities for providing oxygen as a gas.
15 Air separation is very widespread, for example, wherein the air is first liquefied
and then fractionally distilled.
[0003] The electrochemical reaction of various oxygen-containing compounds,
such as, for example, water or carbon dioxide, is also known, and provides
20 oxygen. In most cases, however, the oxygen formed is not utilized as a product,
but is discharged from the process and discarded.
[0004] Depending upon the application, stringent requirements may need to be
met with respect to the purity of the oxygen, so that it may be necessary to
25 separate foreign substances from the oxygen as quantitatively as possible. In
addition, a general problem when dealing with oxygen is that plant parts, which
are subjected to increased oxygen concentrations, have to be designed to be
resistant to corrosion.
30 [0005] Moreover, in gas mixtures in which high oxygen concentrations are
present, the formation of explosive mixtures, which constitute a safety risk, is also
possible, depending upon the composition of the constituents. This is the case in
3
particular when the oxygen comes from an electrolysis process in which hydrogen
is formed.
[0006] Although the hydrogen is typically formed on the cathode side in
5 electrolysis processes, the high mobility of the small hydrogen molecule makes it
impossible to completely prevent the oxygen formed on the anode side of the
electrolysis from being contaminated with hydrogen which passes through the
membrane separating the anode side and the cathode side, e.g., a proton exchange
membrane (PEM), an anion exchange membrane (AEM), or a solid oxide high10 temperature membrane of a solid oxide electrolysis cell (SOEC).
[0007] In principle, the following reactions occur during electrolysis.
[0008] In the case of electrolysis with a PEM:
15
At the anode: H2O → ½ O2 + 2 H+
+ 2 eAt the cathode: 2 e-
+ 2 H+
→ H2
[0009] In the case of electrolysis with an AEM:
At the anode: 2 OH-
→ ½ O2 + 2 H2O + 2 eAt the cathode: 2 e-
+ 2 H2O → H2 + 2 OH20 [0010] In the case of electrolysis with an SOEC:
At the anode: 2 O2-
→ O2 + 4 eAt the cathode: H2O + 2 e-
→ H2 + O2-
[0011] As already mentioned above, other compounds containing oxygen can also
be subjected to electrolysis in order to obtain oxygen. If the reactants used are not
4
anhydrous, the reactions described above can occur as side reactions, so that the
formation of hydrogen must always be expected.
[0012] Before the present invention is described in more detail, some of the terms
5 used herein shall first be explained.
[0013] All compositions, concentrations, and proportions of mixtures specified in
the context of the present application refer to the volumetric composition or
concentration or the volume fraction, in each case based upon the dry, i.e., water10 free, mixture, unless explicitly stated otherwise.
[0014] In the language used in the present patent application, a gas mixture is rich
in one or more components when it has a proportion of more than 50%, 60%,
70%, 80%, 90%, 95%, 98%, 99%, 99.9%, or 99.99% of said one or more
15 components, wherein, in the case of several components, the proportion is
understood to be the sum of the individual proportions.
[0015] Accordingly, a mixture is low in one or more components when it is not
rich in said component or components; the proportion of these in the total mixture
20 is therefore below 50%, 40%, 30%, 20%, 10%, 5%, 2%, 1%, 0.1%, or 0.01%.
[0016] In the language used here, a gas or mixture free of one or more
components is very low in said component and has a proportion of less than 1,000
ppm, 100 ppm, 10 ppm, 1 ppm, 100 ppb, 10 ppb, or 1 ppb. In particular, the
25 proportion of the components in which the gas or mixture is free is below a
detection limit of the components.
[0017] A gas or mixture enriched in one or more components denotes a gas or
mixture which has a higher concentration of the one or more components in
30 relation to a starting gas or mixture. In particular, a gas enriched in a component
has at least a 1.1, 1.3, 2, 3, 10, 30, 100, 300, or 1,000-fold proportion of said
component compared to the corresponding source gas.
5
[0018] Accordingly, a gas depleted of a component has at most a 0.001, 0.003,
0.01, 0.03, 0.1, 0.3, 0.5, or 0.9-fold proportion of said component in comparison
to the corresponding source gas.
5 [0019] When it is stated below that a part of a gas or a mixture is used, this can
mean either that a volume fraction of the gas or mixture up to 100% of the total
standard volume of the original gas or mixture with the same composition as the
latter is used, or that a gas or mixture is used which was formed using only certain
components of the original gas or mixture. The part of the gas or mixture can thus
10 have the same composition as the original gas or mixture or a different
composition.
[0020] An explosion is understood in the language used here to mean a
deflagration or detonation.
15
[0021] The object of the present invention is to release oxygen, which is obtained
in an electrolysis reaction, by the catalytic oxidation of hydrogen and thereby to
prevent an excessive temperature increase during the catalytic reaction.
20 DISCLOSURE OF THE INVENTION
[0022] To achieve this object, the present invention proposes a method and a plant
for the electrochemical production of oxygen having the features of the
independent claims. Embodiments are the subject matter of the dependent claims
25 and of the following description.
[0023] Oxygen in a gas from an electrolysis (cathode raw gas), which is rich in
hydrogen, can be converted to water with hydrogen and removed from the gas,
which is rich in hydrogen, together with the water which was not converted during
30 electrolysis. In the context of the present invention, this technology that is
conventionally used to remove oxygen is used to remove hydrogen impurities in a
gas which is rich in oxygen and formed at the anode (raw anode gas). In contrast
to the usual oxygen contents in typical raw cathode gases, the hydrogen content in
6
the raw anode gas is usually higher, since the cathode side is often operated at a
higher pressure.
[0024] According to the invention, the aforementioned object is achieved in that,
5 downstream of an electrolysis unit, a raw anode gas, which is obtained in the
electrolysis unit from a feedstock and contains oxygen and a part hydrogen, is at
least partially subjected to a catalytic conversion of hydrogen with oxygen to
water to obtain an intermediate mixture depleted in hydrogen, wherein a first part
of the intermediate mixture is returned, downstream of the electrolysis and
10 upstream of the catalytic conversion, to the raw anode gas. A gas product
containing oxygen is formed using a second part of the intermediate mixture with
depleted hydrogen content. As mentioned, the “first part of the intermediate
mixture” can be a purely quantitative proportion of the intermediate mixture, but it
can also be a proportion which has a different material composition and is
15 obtained in subsequent steps. The same applies to the “second part of the
intermediate mixture.”
[0025] By using the measures proposed according to the invention, the
concentration of hydrogen in the corresponding plant sections is reduced. In
20 particular, it is provided that the hydrogen concentration be lowered such that an
adiabatic temperature increase in the catalytic conversion is limited to a desired
value. This has the advantage that a conventional adiabatic reactor with low
investment costs can be used. There is, therefore, no need for a cost-intensive use
of an isothermal reactor concept.
25
[0026] As a result of the return, according to the invention, of the first part of the
intermediate mixture with depleted hydrogen content, which can in particular be
free of or low in hydrogen, to the raw anode gas downstream of the electrolysis,
even with high concentrations of hydrogen in the raw anode gas, the hydrogen
30 content can be diluted there to unproblematic values, and the oxygen present in
the raw anode gas can thus be purified and utilized. Downstream of the hydrogen
depletion, hydrogen is also present in unproblematic concentrations anyway, due
to a corresponding removal.
35 [0027] In order to ensure that undesirably high hydrogen concentrations are not
attained, sensors can, particularly advantageously, be provided at certain locations
7
in a production plant - for example, at an output from the electrolysis unit or in the
catalytic conversion unit. These can, for example, directly detect the hydrogen
concentration and, if a predetermined threshold value is exceeded, a dilution
according to the invention of the raw anode gas with the first part of the
5 intermediate mixture with depleted hydrogen content can be effected - for
example, by opening a valve, or by increasing the returned amount of the
intermediate mixture with depleted hydrogen content.
[0028] A further advantageous embodiment of such sensors can facilitate
10 temperature monitoring and, consequently, a controlled or regulated switching off
of the catalytic conversion, or, in turn, a controlled or regulated dilution of the raw
anode gas can be effected. This has the advantage that the catalytic conversion is
only operated when an undesired temperature increase is excluded, and thus an
excessively high thermal load on the catalytic converter, which could lead to the
15 damage or destruction thereof, is avoided.
[0029] Particularly advantageous is a method in which the threshold value for the
maximum hydrogen concentration is variable as a function of other detected
parameters, such as, for example, pressure and/or temperature in the relevant plant
20 section, and thus an efficient process control is facilitated in the respect that gases
which are free of hydrogen or low in hydrogen are returned to the raw anode gas
only to a required extent, and thus, for example, compaction energy can be
conserved downstream of the catalytic conversion.
25 [0030] In all variants of the methods and plants according to the invention, it is,
particularly advantageously, provided that only gas streams that originally come
from the electrolysis are used to reduce the hydrogen concentration. This makes it
possible to prevent contaminants, such as, for example, nitrogen or argon, which
would be difficult to remove from the product gas, from being introduced into the
30 process.
[0031] In one embodiment of the method, the intermediate mixture explained
above is, advantageously, at least partially subjected to condensation to obtain an
intermediate mixture fraction with depleted water content, and a condensate,
35 which is rich in water. The intermediate mixture fraction or a part thereof can be
subjected to drying to obtain the gas product, which contains oxygen, and a
8
residual gas with a depleted oxygen content and an enriched water content, and
the residual gas can be returned partially or completely in the manner explained.
In this embodiment, the residual gas or the returned part thereof thus represents
the first part of the intermediate mixture that has been explained several times,
5 whereas the second part is provided in the form of the gas product, which contains
oxygen. This has the advantage that water, which could potentially interfere in the
gas product, is not carried over into the gas product.
[0032] In a further advantageous embodiment of the method, the first part of the
10 intermediate mixture, which is returned to the raw anode gas, is formed using at
least a part of the intermediate mixture and/or of the intermediate mixture fraction
and/or of the residual gas and/or of the gas product. This has the advantage that
only gases which are present in the process anyway are used to reduce the
hydrogen concentration. This prevents interfering impurities, which consist of
15 gases that can be removed from the gas product only with difficulty, from being
introduced into the process.
[0033] The drying, advantageously, comprises at least one temperature swing
adsorption (TSA), since this can be combined particularly efficiently with the
20 other method steps. However, it is also conceivable for a different form of drying
to be used - for example, a pressure swing adsorption (PSA) or a membrane
method.
[0034] Advantageously, the mentioned intermediate mixture fraction, which
25 remains after the condensation, is subjected to a compression upstream of the
subsequent drying and, to obtain a further intermediate mixture fraction and a
further condensate, to a further condensation, wherein the further intermediate
mixture fraction is at least partially supplied to the drying. As a result, a pressure
that is advantageous for drying can be configured, and water can be separated off
30 even before the drying, so that the drying unit can have smaller dimensions.
[0035] In particular, each of the stated condensates or both condensates together
can, if formed, be partially or completely returned to the electrolysis together with
the feedstock. As a result, a method according to this embodiment can be carried
35 out particularly efficiently in terms of material.
9
[0036] In an advantageous embodiment, one or more process parameters, which
comprise a hydrogen concentration and/or a gas temperature and/or a gas
pressure, are detected downstream of the electrolysis and/or in the catalytic
conversion. The first part of the intermediate mixture is returned to the raw anode
5 gas when the one or more process parameters is/are above a respectively
predetermined threshold value. It is also particularly advantageous to carry out
continuous regulation of the returned quantity of intermediate mixture based upon
the one or more process parameters. As a result, it can be ensured on the one hand
that no potentially dangerous situation arises when the method is being carried
10 out, and, on the other, that an unnecessary additional load on the plant due to
excessive recycle streams is avoided.
[0037] Furthermore, it can advantageously be provided that, when a
predetermined limit value of the one or more process parameters is exceeded - in
15 particular, an increase in temperature in the catalytic conversion - raw anode gas
be discharged from the process - for example, blown off. This allows the plant to
be protected if the reduction in hydrogen concentration is not sufficient to limit
the temperature development.
20 [0038] An operation of the catalytic conversion at an adsorption pressure and/or
an operation of the electrolysis at a pressure at which the drying is also operated -
in particular, at an adsorption pressure and/or an increase in the pressure of the
recycle to the pressure level of the catalytic conversion - can also be
advantageous, since the entire process can thus substantially be operated at a
25 uniform pressure level.
[0039] The raw anode gas or raw oxygen can, advantageously, be heated against
the first mixture by heat exchange before the catalytic conversion, in order to
conserve process heat. In this case, an at least partial condensation of the water
30 contained in the product stream can also occur, which in turn conserves energy
during operation of the condensation.
[0040] According to the invention, a plant for producing a gas product containing
oxygen is also provided with an electrolysis unit, which is designed to subject a
35 feedstock containing water to electrolysis to obtain a raw anode gas, which is rich
in oxygen and contains hydrogen, and a raw cathode gas, which is low in oxygen
10
and rich in hydrogen. A catalytic conversion unit is provided, which is designed to
subject the raw anode gas at least partially to a catalytic conversion of hydrogen to
water to obtain an intermediate mixture with depleted hydrogen content. Means
are provided which are designed to return a first part of the intermediate mixture,
5 downstream of the electrolysis and upstream of the catalytic conversion, to the
raw anode gas. Furthermore, the plant has means configured to form the gas
product containing oxygen using a second part of the intermediate mixture.
[0041] Advantageously, the plant is further equipped with means which enable a
10 method to be carried out according to one of the advantageous embodiments
described above.
DESCRIPTION OF THE FIGURES
15 [0042] Further advantages, embodiments, and further details of the present
invention are described in more detail below with reference to the accompanying
figures, wherein
[0043] Figure 1 shows, in the form of a schematic block diagram, an
20 advantageous embodiment of a method according to the invention, and
[0044] Figure 2 shows, in the form of a schematic block diagram, a further
advantageous embodiment of a method according to the invention - in particular,
using high-pressure electrolysis.
25
[0045] In the exemplary embodiment of a method according to the invention
shown in Figure 1, a feedstock 1, the predominant proportion of which consists of
water, is subjected to electrolysis E. In this case, a raw cathode gas 14, which is
low in oxygen and rich in hydrogen, and a raw anode gas 2, which is rich in
30 oxygen and contains hydrogen, are formed.
[0046] The raw anode gas is at least partially subjected to a catalytic conversion C
as feedstock 3, wherein an intermediate mixture 4 with depleted hydrogen content
11
compared to the raw anode gas is formed. In the catalytic conversion C, hydrogen,
which is contained in the raw anode gas 2 in a certain proportion of, for example,
0.1% to 2%, is converted to water with a part of the oxygen which makes up the
main proportion of the raw anode gas 2. This effectively reduces the concentration
5 of the hydrogen downstream of the catalytic conversion C.
[0047] The intermediate mixture 4 leaving the catalytic conversion C is subjected
in the exemplary embodiment shown here to a first condensation K1, wherein an
intermediate mixture fraction 5, with depleted water content compared to the
10 intermediate mixture 4, and a condensate 6, which is rich in water, are formed.
The intermediate mixture fraction 5 is compressed and cooled to an adsorption
pressure level. After cooling, the compressed intermediate mixture fraction 5 is
subjected to a further condensation K2, wherein a further intermediate mixture
fraction 8, again with depleted water content compared to the intermediate
15 mixture fraction 5, and a further condensate 9 are formed. The condensates 6, 9
are at least partially returned to the electrolysis E together with the feedstock 1.
[0048] In the exemplary embodiment shown in Figure 1, the further intermediate
mixture fraction 8 is subjected to a drying T in the form of a temperature swing
20 adsorption (TSA), wherein residual water contained in the dryer feedstock is
adsorbed on an adsorbent during an adsorption phase. The oxygen contained in
the further intermediate mixture fraction 8 does not adsorb substantially on the
adsorbent and is carried over into a gas product 10. During a desorption phase, the
outlet is closed in the direction of the gas product 10, and the temperature of the
25 TSA device or drying T is increased by overflowing with warm purge gas or by
directly heating the adsorber. As a result, previously adsorbed molecules - in
particular, water molecules - are desorbed on the adsorbent and can be carried
over to a residual gas 11, 12 with a purge gas (not shown), which is formed, for
example, using the product stream 10. If the adsorbent is largely free of adsorbed
30 water and other impurities, the temperature is reduced again, and a further
adsorption phase is introduced.
[0049] Several TSA devices are, advantageously, operated in parallel with one
another, so that at least one of the several TSA devices is in the adsorption phase
35 at any point in time. This allows a continuous stream of the gas product 10 to be
provided.
12
[0050] In particular, it can be ensured that the predominant part of the adsorbed
species has again desorbed by maintaining the elevated temperature over a
predetermined time, or by taking a concentration measurement downstream of the
TSA device or drying T in the residual gas 11, 12. In the event that a period of
5 time is predetermined, the method can, advantageously, be controlled such that the
several TSA devices can be operated alternately in the drying T, while the
concentration-dependent control has the advantage that the desorption phase can
be measured as required, and is not unnecessarily drawn out. As a result, the
efficiency of the overall method can be increased.
10
[0051] At least a part of the residual gas 12 can, upstream of the catalytic
conversion C, be returned to the raw anode gas or the feedstock 3, in order to
regulate the temperature increase in the catalytic conversion by lowering the
hydrogen concentration. For the same purpose, upstream of the drying T, a part of
15 the further intermediate product fraction 8 can also be returned as control stream
13 to the raw anode gas 2 or the feedstock 3.
[0052] Optionally, a further part of the residual gas 11, downstream of the
catalytic conversion C, can be returned to the intermediate mixture 4 (not shown)
20 or to the intermediate mixture fraction 5. As a result, product used as a purge gas
can still be returned to the process to increase the process yield, even if it is not
used for temperature control in the catalytic conversion C.
[0053] In the exemplary embodiment shown in Figure 1, a series of sensors is
25 integrated into the plant in order to be able to retrieve information about the status
of the individual method steps and to thereby regulate the temperature increase in
the catalytic conversion C by configuring the returned streams 12 or 13. For
example, hydrogen sensors 15 detect the hydrogen concentration of the various
gas streams, such as, for example, of the raw anode gas 2. Of course, hydrogen
30 concentrations can also be detected at other points (not shown) - in particular, in a
gas stream downstream of the catalytic conversion C - to quantify the degree of
conversion.
[0054] A temperature sensor 16 can additionally detect the temperature in the
35 catalytic conversion. With the aid of this information, the supply of raw anode gas
or feedstock 3 to the catalytic conversion C can, advantageously, be reduced or
13
stopped if the temperature rises so much as a result of the catalyzed reaction that
there is a risk of catalyst degradation. In the case of such a temperature rise in the
catalytic conversion, raw anode gas can temporarily be discharged from the
process until the temperature has again stabilized to a level that is acceptable for
5 the process. However, the temperature detected by the temperature sensor 16 can
also be used as a control variable for configuring the control current 13.
[0055] An advantageous embodiment of a method according to the invention is
shown schematically in Figure 2. In this exemplary embodiment, the electrolysis
10 E is designed in the form of a high-pressure electrolysis, in which the raw anode
gas 2 already occurs at the adsorption pressure level. Advantageously, there is thus
no need for the compression downstream of the catalytic conversion C, so that the
need for the further condensation K2 is also superfluous. In this case, only one
compressor is necessary for the return of the residual gas from the drying T and
15 for the return of a part of the gas stream 8, upstream of the drying, which is used
as control stream 17. In order to save on a separate compressor for the control
stream 17, the residual gas from the drying T can be returned together with the
control stream 17 via a compressor and fed downstream of the compressor
according to the temperature control upstream (stream 12) or downstream (stream
20 11) of the catalytic conversion C. Otherwise, the procedure can be identical to the
method that was described with reference to Figure 1.

14
CLAIMS
We Claim:
5 1. Method for producing a gas product (10) containing oxygen, wherein a
feedstock (1) containing water is subjected to electrolysis (E) to obtain a raw
anode gas (2), which is rich in oxygen and contains hydrogen, and a raw cathode
gas (14), which is low in oxygen and rich in hydrogen, characterized in that the
raw anode gas (2) is at least partially subjected to a catalytic conversion (C) of
10 hydrogen to water to obtain an intermediate mixture (4) with depleted hydrogen
content, that a first part of the intermediate mixture (4) is returned to the raw
anode gas (2) downstream of the electrolysis (E) and upstream of the catalytic
conversion (C), and that the gas product containing oxygen is formed using at
least a second part of the intermediate mixture (4).
15
2. Method according to claim 1, wherein the intermediate mixture (4) is at
least partially subjected to condensation (K1) to obtain an intermediate mixture
fraction (5) with depleted water content, and a condensate (6), which is rich in
water.
20
3. Method according to claim 2, wherein at least a part of the intermediate
mixture fraction (5) is subjected to drying (T) to obtain the gas product (10),
which contains oxygen, and a residual gas (12) with depleted oxygen content and
enriched water content.
25
4. Method according to one of the preceding claims, wherein the first part of
the intermediate mixture (4), which is returned to the raw anode gas (2), is formed
using at least a quantitative proportion of the intermediate mixture (4) and/or of
the intermediate mixture fraction (5) and/or of the residual gas (12) and/or of the
30 gas product (10).
5. Method according to one of the preceding claims, wherein the first part of
the intermediate mixture is returned to the raw anode gas (2) or the anode-side
feedstock (1) in an amount which is measured such that a hydrogen concentration
15
in the raw anode gas (2) downstream of the return is at most 0.1%, 0.2%, 0.3%,
0.5%, 1%, or 2%.
6. Method according to claim 3, wherein the drying (T) comprises a
5 temperature swing adsorption (TSA).
7. Method according to claim 3 or 6, wherein the intermediate mixture
fraction (5) is subjected to a compression upstream of the drying (T) and, to obtain
a further intermediate mixture fraction (8) and a further condensate (9), to a
10 further condensation (K2).
8. Method according to one of the preceding claims, wherein at least one of
the condensates (6, 9) together with the feedstock (1) is partially or completely
returned to the electrolysis (E).
15
9. Method according to one of the preceding claims, wherein, downstream of
the electrolysis (E) and/or in the catalytic conversion (C), one or more process
parameters, comprising a hydrogen concentration and/or a gas temperature and/or
a difference between two gas temperatures and/or a gas pressure, are detected, and
20 wherein the first part of the first mixture (4)
i) is returned to the raw anode gas (2) when the one or more process
parameters are above a predetermined threshold value; or
ii) is returned in an amount controlled continuously using the detected
process parameters.
25
10. Method according to claim 9, wherein raw anode gas (2) is discharged
from the process when the one or more process parameters - in particular, the
difference between two gas temperatures - exceed a predetermined limit value.
30 11. Method according to one of the preceding claims, wherein the electrolysis
(E) and/or the catalytic conversion (C) is operated at a pressure level at which the
drying (T) is also operated; and/or wherein the part of the first mixture (4)
16
returned to the raw anode gas (2) is compressed to the pressure level at which the
catalytic conversion (C) is operated.
12. Method according to one of the preceding claims, wherein the raw anode
5 gas (2) is heated in a heat exchanger against the first mixture (4).
13. Plant for producing a gas product (10) containing oxygen with an
electrolysis unit configured to subject a feedstock (1) containing water to
electrolysis (E) to obtain a raw anode gas (2), which is rich in oxygen and
10 contains hydrogen, and a raw cathode gas (14), which is low in oxygen and rich in
hydrogen, characterized by a catalytic conversion unit configured, using at least
a part of the raw anode gas (2), to subject to a catalytic conversion (C) of
hydrogen to water to obtain an intermediate mixture (4) with depleted hydrogen
content by means configured to return a first part of the intermediate mixture (4)
15 to the raw anode gas (3) downstream of the electrolysis (E) and upstream of the
catalytic conversion (C), and to form the gas product (10) containing oxygen
using a second part of the intermediate mixture (4).
14. Plant according to claim 13, further comprising means configured to
20 perform all steps of a method according to one of claims 2 through 12.