U.S. patent application number 15/739736 was filed with the patent office on 2018-07-12 for electrolytic system and reduction method for electrochemical carbon dioxide utilization, alkali carbonate preparation and alkali hydrogen carbonate preparation.
This patent application is currently assigned to Siemens Aktiengesellschaft. The applicant listed for this patent is Siemens Aktiengesellschaft. Invention is credited to Maximilian Fleischer, Philippe Jeanty, Ralf Krause, Erhard Magori, Anna Maltenberger, Sebastian Neubauer, Christian Reller, Bernhard Schmid, Gunter Schmid, Elena Volkova, Kerstin Wiesner-Fleischer.
Application Number | 20180195184 15/739736 |
Document ID | / |
Family ID | 56296812 |
Filed Date | 2018-07-12 |
United States Patent
Application |
20180195184 |
Kind Code |
A1 |
Fleischer; Maximilian ; et
al. |
July 12, 2018 |
Electrolytic System And Reduction Method For Electrochemical Carbon
Dioxide Utilization, Alkali Carbonate Preparation And Alkali
Hydrogen Carbonate Preparation
Abstract
The present disclosure relates to electrolysis. The teachings
thereof may be embodied in a reduction process and/or an
electrolysis system for electrochemical carbon dioxide utilization
wherein carbon dioxide is introduced into an electrolysis cell and
reduced at a cathode. For example, an electrolysis system for
carbon dioxide utilization may comprise: an electrolyzer including
an anode in an anode space and a cathode in a cathode space. The
cathode space has an entrance for carbon dioxide. The cathode space
comprises a catholyte including alkali metal cations. The anode
space has an entrance for an anolyte. The anode space comprises an
anolyte comprising chlorine anions.
Inventors: |
Fleischer; Maximilian;
(Hohenkirchen, DE) ; Jeanty; Philippe; (Munchen,
DE) ; Krause; Ralf; (Herzogenaurach, DE) ;
Magori; Erhard; (Feldkirchen, DE) ; Maltenberger;
Anna; (Leutenbach, DE) ; Neubauer; Sebastian;
(Breitengussbach, DE) ; Reller; Christian;
(Minden, DE) ; Schmid; Bernhard; (Erlangen,
DE) ; Schmid; Gunter; (Hemhofen, DE) ;
Volkova; Elena; (Erlangen, DE) ; Wiesner-Fleischer;
Kerstin; (Hohenkirchen-Siegertsbrunn, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens Aktiengesellschaft |
Muenchen |
|
DE |
|
|
Assignee: |
Siemens Aktiengesellschaft
Muenchen
DE
|
Family ID: |
56296812 |
Appl. No.: |
15/739736 |
Filed: |
July 30, 2016 |
PCT Filed: |
July 30, 2016 |
PCT NO: |
PCT/EP2016/065277 |
371 Date: |
December 24, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 1/00 20130101; C25B
1/14 20130101; C25B 15/08 20130101; C25B 3/04 20130101; C25B 1/26
20130101 |
International
Class: |
C25B 3/04 20060101
C25B003/04; C25B 1/14 20060101 C25B001/14; C25B 15/08 20060101
C25B015/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 3, 2015 |
DE |
10 2015 212 504.1 |
Claims
1. An electrolysis system for carbon dioxide utilization, the
system comprising: an electrolyzer including an anode in an anode
space and a cathode in a cathode space; the cathode space has an
entrance for carbon dioxide; the cathode space comprises a
catholyte including alkali metal cations; the anode space has an
entrance for an anolyte; the anode space comprises an anolyte
comprising chlorine anions.
2. The electrolysis system as claimed in claim 1, further
comprising a deposition tank configured for crystallization of an
alkali metal hydrogencarbonate and/or alkali metal carbonate out of
the catholyte; wherein the deposition tank includes a product
outlet.
3. The electrolysis system as claimed in claim 2, further
comprising a cooling apparatus for the deposition tank.
4. The electrolysis system as claimed in claim 2, further
comprising a reservoir connected to the cathode space or the
deposition tank to buffer the catholyte.
5. The electrolysis system as claimed in claim 1, wherein the
catholyte comprises a solvent.
6. The electrolysis system as claimed in claim 1, wherein the
anolyte includes at least one water-soluble alkali metal salt.
7. The electrolysis system as claimed in claim 1, further
comprising the anode space connected to a gas separation unit for
separation of chlorine gas from the anolyte.
8. The electrolysis system as claimed in claim 1, further
comprising a cation-conducting membrane separating the anode space
and cathode space from one another.
9. A reduction process for carbon dioxide utilization, the process
comprising: introducing a catholyte and carbon dioxide into a
cathode space with a cathode; reducing carbon dioxide at the
cathode; introducing an anolyte including chloride anions into an
anode space and brought into contact with an anode; wherein the
anolyte includes alkali metal cations that migrate into the
catholyte; oxidizing chloride anions at the anode to chlorine;
separating chlorine from the anolyte as chlorine gas using a gas
separation unit; and introducing at least a portion of the
catholyte volume into a deposition tank, where an alkali metal
hydrogencarbonate and/or alkali metal carbonate crystallizes
out.
10. The reduction process as claimed in claim 9, further including
reducing, at the cathode, carbon dioxide (CO.sub.2) to carbon
monoxide (CO), ethylene (C.sub.2H.sub.4), methane (CH.sub.4),
ethanol (C.sub.2H.sub.5OH), and/or monoethylene glycol
(OHC.sub.2H.sub.4OH).
11. The reduction process as claimed in claim 10, further
comprising converting the hydroxide ions (OH.sup.-) formed in the
carbon dioxide reduction to hydrogencarbonate ions
(HCO.sub.3.sup.-) with carbon dioxide (CO.sub.2) present in
excess.
12. The reduction process as claimed in claim 9, further including
introducing at least a portion of the catholyte into a deposition
tank, where it is cooled down by at least 15 kelvin.
13. The reduction process as claimed in claim 9, further comprising
introducing at least a portion of the catholyte volume into a
deposition tank, where the pH thereof is lowered from above 8 to a
pH of 6 or less by blowing in carbon dioxide (CO.sub.2).
14. The reduction process as claimed in claim 9, further comprising
introducing at least a portion of the catholyte volume into a
deposition tank, where an alkali metal hydrogencarbonate is
crystallized and is subsequently converted to an alkali metal
carbonate by heating.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a U.S. National Stage Application of
International Application No. PCT/EP2016/065277 filed Jun. 30,
2016, which designates the United States of America, and claims
priority to DE Application No. 10 2015 212 504.1 filed Jul. 3,
2015, the contents of which are hereby incorporated by reference in
their entirety.
TECHNICAL FIELD
[0002] The present disclosure relates to electrolysis. The
teachings thereof may be embodied in a reduction process and/or an
electrolysis system for electrochemical carbon dioxide utilization
wherein carbon dioxide is introduced into an electrolysis cell and
reduced at a cathode.
BACKGROUND
[0003] At present, about 80% of the global energy requirement is
provided by the combustion of fossil fuels, the combustion
processes of which cause global emission of about 34 000 million
metric tons of carbon dioxide into the atmosphere per annum. This
release into the atmosphere comprises the majority of carbon
dioxide released, which can be up to 50 000 metric tons per day in
the case of a brown coal power plant, for example. Carbon dioxide
is one of the gases known as greenhouse gases, the adverse effects
of which on the atmosphere and the climate are a matter of some
dispute. Since carbon dioxide exists at a very low thermodynamic
level, it can be reduced to reutilizable products only with
difficulty, which has left the actual reutilization of carbon
dioxide in the realm of theory or in the academic field to
date.
[0004] Natural carbon dioxide degradation proceeds, for example,
via photosynthesis. This involves conversion of carbon dioxide to
carbohydrates in a process subdivided into many component steps
over time and, at the molecular level, in terms of space. As such,
this process cannot easily be adapted to the industrial scale. No
copy of the natural photosynthesis process with photocatalysis on
the industrial scale to date has had adequate efficiency.
[0005] An alternative is the electrochemical reduction of carbon
dioxide. Systematic studies of the electrochemical reduction of
carbon dioxide are still a relatively new field of development.
Only in the last few years have there been efforts to develop an
electrochemical system that can reduce an acceptable amount of
carbon dioxide. Research on the laboratory scale has shown that
electrolysis of carbon dioxide may be accomplished using metals as
catalysts. The publication "Electrochemical CO.sub.2 reduction on
metal electrodes" by Y. Hori, published in: C. Vayenas, et al.
(eds.), Modern Aspects of Electrochemistry, Springer, New York,
2008, p. 89-189, discloses Faraday efficiencies at different metal
cathodes; see table 1. If carbon dioxide is reduced, for example,
at silver, gold or zinc cathodes, what is formed is almost
exclusively carbon monoxide.
TABLE-US-00001 TABLE 1 Electrode CH.sub.4 C.sub.2H.sub.4
C.sub.2H.sub.5OH C.sub.3H.sub.7OH CO HCOO.sup.- H.sub.2 Total Cu
33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7
10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0
79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0
0.0 0.0 23.2 0.0 79.0 102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4
Hg 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9
3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6 100.1 Cd 1.3 0.0 0.0 0.0
13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.0 0.0 95.1 6.2 101.3 Ni 1.8
0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8
Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.0 0.0 0.0 0.0 0.0
99.7 99.7
[0006] The table gives Faraday efficiencies [o] of products that
form in the carbon dioxide reduction at various metal electrodes.
The values reported apply to a 0.1 M potassium hydrogen-carbonate
solution as electrolyte and current densities below 10
mA/cm.sup.2.
[0007] At a silver cathode, for example, predominantly carbon
monoxide and only a little hydrogen form. The reactions at anode
and cathode can be represented by the following reaction
equations:
Cathode: 2 CO.sub.2+4 e.sup.-+4 H.sup.+.fwdarw.2 CO+2 H.sub.2O
Anode: 2 H.sub.2O.fwdarw.O.sub.2+4 H.sup.++4 e.sup.-
[0008] As can also be inferred from table 1, at a copper cathode
for instance, a multitude of hydrocarbons are formed as reaction
products. One aspect of particular economic interest is, for
example, the electrochemical production of methane or ethylene,
ethanol or monoethylene glycol. These are higher-energy products
than carbon dioxide.
Ethylene:
2CO.sub.2+12e.sup.-+8H.sub.2O.fwdarw.C.sub.2H.sub.4+12OH.sup.-
Methane: CO.sub.2+8e.sup.-+4H.sub.2O.fwdarw.CH.sub.4+4OH.sup.-
Ethanol: 2CO.sub.2+12e.sup.-+9H.sub.2O
C.sub.2H.sub.5OH+12OH.sup.-
Monoethylene glycol:
2CO.sub.2+10e.sup.-+8H.sub.2O.fwdarw.HOC.sub.2H.sub.4OH+10OH.sup.-
[0009] With a chloride-containing electrolyte, the following
reaction can proceed at the anode:
2 Cl.sup.-.fwdarw.Cl.sub.2+2 e.sup.-
[0010] In the electrochemical conversion of matter of carbon
dioxide to a higher-energy product, there is an interest in
increasing the economic viability, and in improvement with regard
to the continuous operability of the electrolysis systems.
SUMMARY
[0011] Consequently, an improved solution for the electrochemical
utilization of carbon dioxide would avoid the disadvantages known
from the prior art. More particularly, the solution may enable
continuous carbon dioxide conversion. The teachings of the present
disclosure may provide an improved reduction process and
electrolysis system for carbon dioxide utilization.
[0012] For example, some embodiments may include electrolysis
systems for carbon dioxide utilization, comprising an electrolyzer
(E1-E5) having an anode (A) in an anode space (AR) and a cathode
(K) in a cathode space (KR). The cathode space (KR) has at least
one entrance for carbon dioxide (CO.sub.2) and is configured to
bring the carbon dioxide (CO.sub.2) that has entered into contact
with the cathode (K). The cathode space (KR) comprises or can
accommodate a catholyte which can enter the cathode space (KR)
through the same entrance or a separate entrance and which includes
alkali metal cations. The anode space (AR) has at least one
entrance for an anolyte and comprises an anolyte or can accommodate
it via this entrance, wherein the anolyte includes chlorine
anions.
[0013] In some embodiments, there is a deposition tank (AB),
wherein the deposition tank (AB) is configured for crystallization
of an alkali metal hydrogencarbonate and/or alkali metal carbonate
out of the catholyte and has a product outlet (PA3).
[0014] In some embodiments, the deposition tank (AB) has a cooling
apparatus.
[0015] In some embodiments, at least one reservoir (PR) is
configured and arranged with connection to the cathode space (KR)
and/or the deposition tank (AB) such that it serves to buffer the
catholyte.
[0016] In some embodiments, the catholyte comprises at least one
solvent, especially water.
[0017] In some embodiments, the anolyte includes at least one
water-soluble alkali metal salt.
[0018] In some embodiments, the anode space (AR) is connected to a
gas separation unit for separation of chlorine gas from the
anolyte.
[0019] In some embodiments, anode space (AR) and cathode space (KR)
are separated from one another by a cation-conducting membrane
(M).
[0020] As another example, some embodiments may include a reduction
process for carbon dioxide utilization by means of an electrolysis
system as described above. In some embodiments, a catholyte and
carbon dioxide (CO.sub.2) are introduced into a cathode space (KR)
and brought into contact with a cathode (K). Carbon dioxide
(CO.sub.2) is reduced at the cathode (K). An anolyte including
chloride anions (Cl.sup.-) is introduced into an anode space (AR)
and brought into contact with an anode (A). Chloride anions
(Cl.sup.-) are oxidized at the anode (A) to chlorine (Cl.sub.2) and
the latter is separated from the anolyte as chlorine gas by means
of a gas separation unit. The anolyte includes alkali metal cations
that migrate into the catholyte. At least a portion of the
catholyte volume is introduced into a deposition tank, where an
alkali metal hydrogencarbonate and/or alkali metal carbonate
crystallizes out.
[0021] In some embodiments, there is reduction at the cathode (K)
of carbon dioxide (CO.sub.2) to carbon monoxide (CO), ethylene
(C.sub.2H.sub.4), methane (CH.sub.4), ethanol (C.sub.2H.sub.5OH)
and/or monoethylene glycol (OHC.sub.2H.sub.4OH).
[0022] In some embodiments, the hydroxide ions (OH.sup.-) formed in
the carbon dioxide reduction are converted to hydrogencarbonate
ions (HCO.sub.3.sup.-) with carbon dioxide (CO.sub.2) present in
excess.
[0023] In some embodiments, at least a portion of the catholyte
volume is introduced into a deposition tank, where it is cooled
down by at least 15 kelvin, preferably at least 20 kelvin.
[0024] In some embodiments, at least a portion of the catholyte
volume is introduced into a deposition tank, where the pH thereof
is lowered from above 8 to a pH of 6 or less by blowing in carbon
dioxide (CO.sub.2).
[0025] In some embodiments, at least a portion of the catholyte
volume is introduced into a deposition tank, where an alkali metal
hydrogencarbonate is crystallized and is subsequently converted to
an alkali metal carbonate by heating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] Examples and embodiments of teachings of the present
disclosure are described in an illustrative manner with reference
to FIGS. 1 to 6 of the appended drawing:
[0027] FIG. 1 shows a schematic diagram of an electrolysis system
with a carbon dioxide reservoir and deposition tank, according to
teachings of the present disclosure;
[0028] FIG. 2 shows a schematic diagram of an electrolysis system
with a gas diffusion electrode, according to teachings of the
present disclosure;
[0029] FIG. 3 shows a schematic diagram of a PEM setup of an
electrolysis cell, according to teachings of the present
disclosure;
[0030] FIG. 4 shows a schematic diagram of a PEM half-cell coupled
to a gas diffusion electrode, according to teachings of the present
disclosure;
[0031] FIG. 5 shows a schematic diagram of a PEM half-cell coupled
to a cathode with backflow, according to teachings of the present
disclosure; and
[0032] FIG. 6 shows a Hagg diagram.
DETAILED DESCRIPTION
[0033] The electrolysis system of the present disclosure may allow
improved carbon dioxide utilization. Some embodiments include at
least one electrolyzer having an anode in an anode space and a
cathode in a cathode space. The cathode space has at least one
entrance for carbon dioxide and is configured to bring the carbon
dioxide that has entered into contact with the cathode. In
addition, the cathode space comprises a catholyte or is configured
to be able to accommodate a catholyte. The catholyte can access the
cathode space through the same entrance as the carbon dioxide or
via a separate second entrance. At least the anode space also
includes alkali metal cations in the operation of the cell anode
and cathode space.
[0034] Catholyte refers to an electrolyte which directly affects
the cathode in the electrolysis. Correspondingly, reference is also
made hereinafter to anolyte when referring to an electrolyte
directly affecting the anode in an electrolysis. Alkali metal
cations refer to positively charged ions having at least one
element of the first main group of the Periodic Table.
[0035] The anode space of the electrolyzer has at least one
entrance for an anolyte and comprises an anolyte or is at least
configured to accommodate an anolyte via this entrance, said
anolyte including chlorine anions.
[0036] In some embodiments, in the electrode system, the anode
space and the cathode space are separated from one another by a
membrane. The membrane here may include at least one mechanically
separating layer, for example a diaphragm, which separates the
electrolysis products formed in the anode space and cathode space
from one another. They could then also be referred to as separator
membrane or separation layer.
[0037] Since the electrolysis products are in many cases gaseous
substances, the membrane may have a high bubble point of 10 mbar or
higher. The "bubble point" is a defining parameter for the membrane
used, which describes the pressure difference .DELTA.P between the
two sides of the membrane from which gas flow through the membrane
would set in. The membrane may also be a proton- or
cation-conducting or -permeable membrane. While molecules, liquids
or gases are being separated, proton or cation flow from the anode
space to the cathode space is assured. In some embodiments, the
membrane comprises sulfonated polytetrafluoroethylene, e.g.
Nafion.
[0038] In some embodiments, the electrolysis system further
comprises at least one deposition tank for crystallization of an
alkali metal hydrogencarbonate and/or alkali metal carbonate out of
the catholyte. In some embodiments, this deposition tank has a
product outlet. According to the product, whether an alkali metal
hydrogencarbonate and/or alkali metal carbonate is to be taken from
the catholyte, and according to the alkali metal, a second
deposition tank may also be provided for a crystallization process.
The latter is then typically arranged downstream of the first
deposition tank in catholyte circulation direction.
[0039] According to the cathode material used, the reduction of
carbon dioxide gives rise to different products: for example,
carbon monoxide, ethylene, methane, ethanol, or monoethylene may be
formed. In all these cases, hydroxide ions also form, which may be
neutralized to hydrogencarbonate by excess carbon dioxide. The
source of the alkali metal cations is in the anode space. A cation
stream through the membrane compensates for the electrical current
resulting from the voltage applied.
[0040] For example, the alkali metal cations and the chloride
anions may be metered into the anolyte in the form of a chloride
salt. While the chloride anions are oxidized at the anode to
chlorine and leave the anolyte circuit as chlorine gas, the alkali
metal cations migrate through the membrane into the catholyte
circuit, where they react in the cathode space with the carbonate
or hydrogencarbonate formed there to give an alkali metal carbonate
or alkali metal hydrogencarbonate and may leave the catholyte
circuit via the separate product outlet of the deposition tank.
[0041] The electrolysis systems of the present disclosure may have
produce not only chlorine but at least one alkali metal carbonate
and/or alkali metal hydrogencarbonate as a chemical material of
value. Whether alkali metal carbonate or alkali metal
hydrogencarbonate is formed depends, for example, on the alkali
metal and the utilization method. In aqueous solution, for
instance, the solubility is crucial. The sparingly soluble
carbonate or hydrogencarbonate crystallizes out. In the case of
sodium and potassium it is the hydrogencarbonate that is more
sparingly soluble than the carbonate, and then has to be calcined
in a subsequent step. The combustion of sodium in carbon dioxide is
an example in which carbon monoxide and, in a direct manner, sodium
carbonate Na.sub.2CO.sub.3 are produced.
[0042] Furthermore, the electrolysis system may utilize carbon
dioxide and it is thus also typically possible to provide at least
one third material of value, for example carbon monoxide, ethylene,
methane, ethanol or monoethylene glycol. The exploitation of the
compensating current of the cations thus creates an electrolysis
system which enables continuous hydrogencarbonate production.
[0043] As already described, the actual cathode reaction in which
the carbon dioxide is reduced is followed by a subsequent reaction,
namely the neutralization of the hydroxide ions (OH--). These are
especially neutralized by excess carbon dioxide to
hydrogencarbonate (HCO.sub.3.sup.-). This firstly has the effect
that the pH in the cathode space is thus buffered within a pH range
from 6 to 8. It also has the effect that the electrolyte
concentration rises considerably. But if the catholyte is conducted
into a catholyte circuit, i.e. pumped into the cathode space and
led out of it again, the hydrogencarbonate formed in the cathode
space can be taken from the catholyte. For this purpose, more
particularly, at least one pump in each case may be arranged in the
catholyte circuit, or else, for example, in the anolyte circuit,
and this ensures electrolyte circulation.
[0044] Subsequent to the neutralization reaction of the hydroxide
ions (OH.sup.-) with excess carbon dioxide to give
hydrogencarbonate ions (HCO.sub.3.sup.-), these may react further
with alkali metal cations to give alkali metal hydrogencarbonates.
The alkali metal cations present in the cathode space come from the
anode space, into which they were initially introduced especially
in the form of alkali metal chloride as oxidation reactant or in
the form of another alkali metal salt, for increasing the
conductivity for example. The alkali metal cations in the anode
space may be replenished in the form of alkali metal chloride. The
membrane between the anode space and cathode space may be chosen
such that the cation flow from the anode space toward the cathode
in the electrical field of the electrolyzer is assured. The effect
of the temperature and also pH dependence of the solubility of
alkali metal hydrogencarbonates is then that different processes
for crystallization or for withdrawal from the catholyte are
undertaken:
[0045] Firstly, it is possible to utilize the temperature
dependence of the solubility of the alkali metal hydrogencarbonates
desired as electrolysis product. For this purpose, the deposition
tank may comprise a cooling apparatus, by means of which the
catholyte is cooled down by several degrees Kelvin compared to the
temperature range that prevails in the electrolyzer. In some
embodiments, the temperature difference set from the deposition
tank to the electrolyzer is at least 15 K, especially at least 20
K. According to the electrolyte concentration in the catholyte and
according to the alkali metal cations with which the
hydrogencarbonate is formed, a temperature difference between 30 K
and 50 K may also be particularly suitable. The temperature
difference between the electrolyzer and deposition tank may be
within a temperature range between 5 K and 70 K.
[0046] The lowering of the temperature in the deposition tank may
provide cooling in the catholyte circuit precedes the recycling of
the catholyte into the cathode space. Thus, excessively high
systemic evolution of heat, specifically in the electrolyzer, is
avoided. But it is also possible to dispense with any cooling unit
provided specially for this purpose.
[0047] In the case of the deposition method of crystallization of
the alkali metal hydrogencarbonate by means of cooling of the
catholyte too, preference is given to using pH buffers provided,
for example, in a buffer reservoir to the deposition tank and/or to
the catholyte circuit and/or to the cathode space, to
correspondingly buffer the catholyte volume.
[0048] The pH of the catholyte can also be employed as such for the
control of the operation of deposition of the alkali metal
hydrogencarbonate out of the electrolyte. For this purpose, more
particularly, the pH in the cathode space is at first kept at a
higher value, for example 8 or higher. This can shift the
equilibrium in favor of the alkali metal carbonate and away from
the alkali metal hydrogencarbonate. For crystallization in the
deposition tank, the pH is then lowered, e.g., to a value of 6 or
less, which leads to formation and crystallization of the alkali
metal hydrogencarbonate. The lowering of the pH is typically
accomplished by blowing carbon dioxide into the deposition
tank.
[0049] According to the alkali metal cation with which the
hydrogencarbonate reacts, and depending on the pH in the cathode
space, it is at first possible to form an alkali metal
hydrogencarbonate or an alkali metal carbonate. More particularly,
the two procedures described for withdrawal of the desired product
from the catholyte can also be combined. In some cases, for example
in the case of formation of sodium hydrogencarbonate NaHCO.sub.3,
it is also possible, for example, to obtain the sodium carbonate
Na.sub.2CO.sub.3 subsequently from the sodium hydrogencarbonate
NaHCO.sub.3 that has crystallized out by heating. In that case,
hydrogencarbonate may be first produced and deposited, and
subsequently the desired proportion thereof is processed further to
give carbonate.
[0050] The pH dependence of the hydrogencarbonate or carbonate ions
is shown, for example, in FIG. 6 in a Hagg diagram for a sodium
carbonate solution.
[0051] In the electrolysis system, a buffer reservoir may be
provided in the anolyte circuit, which can especially also serve
for introduction or replenishment of alkali metal chloride into the
electrolyte, to maintain the salt content in the anolyte.
[0052] In some embodiments, the catholyte includes at least one
solvent, especially water. Typically, aqueous electrolytes and
correspondingly water-soluble conductive salts may be employed. The
conductive salt content can be increased by the addition of further
carbonates, hydrogencarbonates, but also sulfates or other
conductive salts, to increase the conductivity of the electrolyte
in the catholyte circuit and also in the anolyte circuit, which
leads to an increase in the conversion of matter in the overall
system. According to which and what amounts of additional
conductive salts are present in the catholyte circuit, the
crystallization process is adjusted correspondingly to extract the
desired product with maximum purity. Conductive salts used may be
chosen such that the solubility thereof differs significantly from
that of the alkali metal hydrogencarbonate or the alkali metal
carbonate.
[0053] Typically, the electrolysis system has a gas separation unit
on the anolyte side, which is configured to undertake the
separation of chlorine gas from the anolyte. In the catholyte
circuit too, a gas separation unit may be provided, for example
when it is directed to carbon monoxide gas production via use of a
silver-containing cathode. In the anolyte circuit and in the
catholyte circuit, additional units for inlets or outlets from the
system or additional buffer reservoirs may be provided.
[0054] The nature and quality of the membrane used in the
electrolyzer ultimately makes a significant contribution to how
pure the crystallized product is. If the membrane used is merely a
separator, it is also possible, for example, for chloride anions to
diffuse into the cathode space, even counter to the electrical
field in the electrolyzer, such that not only hydrogencarbonate but
possibly also chlorides are formed. Therefore, in some embodiments,
there is a cation-conducting membrane through which virtually
exclusively cations can pass. A purely anion-conducting membrane
may be less useful.
[0055] In some embodiments, the reduction process described for
carbon dioxide utilization by means of an electrolysis system as
described above comprises the following steps: a catholyte and
carbon dioxide are introduced into a cathode space, where they are
contacted with a cathode. Within the cathode space, this catholyte
includes alkali metal cations which migrate through the membrane
that separates anode space and cathode space. At least a portion of
the catholyte volume may be introduced into a deposition tank,
where an alkali metal hydrogencarbonate and/or an alkali metal
carbonate crystallizes out.
[0056] In some embodiments, an anolyte including chloride anions,
is brought into contact with an anode. The chloride anions are
oxidized at the anode to chlorine and the latter is separated from
the anolyte as chlorine gas by means of a gas separation unit.
Typically, this reduction process is effected such that anolyte and
catholyte are each conducted into a separate circuit, meaning that
two pumps are provided in the electrolysis system, which bring
about transport of the catholyte through the cathode space and
transport of the anolyte through the anode space at least at one
point in the circuit.
[0057] The circuits are separated from one another by the membrane
in the electrolyzer, which may permit exclusively transport of
cations from the anode space into the cathode space. More
particularly, the alkali metal cations required in the cathode
space may be obtained from the anode space. For this purpose, the
anolyte may include an alkali metal chloride; the latter may be
used as conductive salt, or else likewise as electrolysis reactant.
In some embodiments, the alkali metal chloride in the anolyte can
be used as electrolysis reactant, and an additional conductive
salt, for example a sulfate, a phosphate et cetera, e.g, an alkali
metal sulfate, can be used. In some embodiments, it is also
possible to use ammonium salts or homologs thereof. Imidazolium
salts or other ionic liquids can have a positive effect on the
selectivity of the electrode, particularly the cathode.
[0058] In some embodiments, in the reduction process, the reduction
of the carbon dioxide at the cathode produces carbon monoxide,
ethylene, methane, ethanol and/or monoethylene glycol. For this
purpose, an appropriate cathode may be used as catalyst for these
reactions. For this purpose, the cathode may include copper. In
some embodiments, this reduction process produces, in addition to
carbon dioxide utilization, chemical substances of value.
[0059] In some embodiments, the hydroxide ions formed in the carbon
dioxide reduction can be converted to hydrogencarbonate ions with
carbon dioxide present in excess. Hydrogencarbonate production
directly in the cathode space allows these to react further
directly with alkali metal cations present in the cathode space to
give a further material of value which is of interest, which would
otherwise have to be produced in separate production processes. In
some embodiments, to withdraw this material of value from the
system, at least a portion of the catholyte volume is introduced
into a deposition tank, where it is cooled down by at least 15 K,
and/or by at least 20 K. Here, the temperature dependence of the
carbonate solubility is thus exploited to withdraw the material of
value from the catholyte circuit. The temperature differential from
deposition tank to electrolyzer may also be more than 30 K,
especially also more than 50 K, according to the present alkali
metal hydrogencarbonate to be extracted and also depending on which
further salts are present in the circuit. The temperature
differential between electrolyzer and deposition unit may be
between 5 K and 70 K. In some embodiments, for extraction of the
hydrogencarbonate product from the catholyte volume, the dependence
of the solubility on the pH is exploited. This process can be
combined with the temperature-dependent process.
[0060] In some embodiments, for this purpose, at least a portion of
the catholyte volume is introduced into a deposition tank, where
the pH thereof is lowered, especially by means of blowing in carbon
dioxide, from above 8 to a pH of 6 or less. Specifically the
buffering of the pH to a value of more than 8 in the cathode space
prevents the precipitation of the alkali metal hydrogencarbonate in
the cathode space itself.
[0061] In some embodiments, the reduction process can be undertaken
such that the precipitated alkali metal hydrogencarbonate is
converted to alkali metal carbonate by heating. This can be
effected directly after the crystallization of the
hydrogencarbonate in the deposition tank or separately from the
electrolysis system described.
[0062] In some embodiments, as an alternative to the temperature
method of crystallization or to the temperature-assisted
crystallization, or else in combination therewith, the process can
also be run such that the pH in the cathode space is kept at the
upper limit of the reaction of around 8 or higher, such that the
equilibrium is at first shifted in favor of sodium carbonate:
2 NaHCO.sub.3.fwdarw.Na.sub.2CO.sub.3+H.sub.2O+CO.sub.2.
[0063] For this purpose, the carbon dioxide supply to the system
must be very well controlled, to arrive at and maintain this basic
regime. In the deposition tank, the pH would then be lowered for
optimal deposition of the sodium hydrogencarbonate by blowing in
carbon dioxide, and hence the equilibrium reaction would again be
shifted in favor of sodium hydrogencarbonate.
[0064] However, the process is not restricted to sodium
hydrogencarbonate. For example, it is also possible to prepare
potassium hydrogencarbonate in this process. Analogously to the
deposition process described for sodium hydrogencarbonate, it is
also possible to crystallize the potassium hydrogencarbonate out of
a pure potassium hydrogencarbonate electrolyte by lowering the
temperature in the deposition tank. At 20.degree. C. the solubility
of potassium hydrogencarbonate is 337 g/l, and at 60.degree. C. it
is 600 g/l.
[0065] A somewhat different procedure is necessary if an additional
conductive salt, for example potassium sulfate (K.sub.250.sub.4),
is to be used. This has a lower solubility of 111.1 g/l at
20.degree. C. and 250 g/l at 100.degree. C., which means that the
potassium sulfate would always precipitate out first in the mixed
electrolyte. In order to obtain the potassium hydrogencarbonate
(KHCO.sub.3) from an electrolyte containing both potassium sulfate
and potassium hydrogencarbonate, it is necessary to proceed as
follows: in the deposition tank AB, potassium sulfate
K.sub.250.sub.4 preferentially crystallizes out and can be fed back
to the electrolyte subsequently, i.e. downstream of the deposition
tank AB in circulation direction. The electrolyte volume from which
the potassium sulfate K.sub.2SO.sub.4 has already been removed is
then concentrated, preferably in a further deposition tank, meaning
that the water is removed from the potassium hydrogencarbonate
solution, for example by cooling, to obtain the crystalline
material.
[0066] In principle, this process is also applicable to other
cations or mixtures of cations. The migration of the cations
results in concentration of the catholyte to such an extent that
the most sparingly soluble salt or double salt separates out. It is
important here that the process of concentration and deposition
does not proceed in the cathode space, i.e. not in the electrolysis
cell itself, but that the catholyte is transported for the purpose
into a deposition tank integrated within the electrolysis system.
By means of a further additional physical or chemical difference
between the electrolysis cell and deposition tank, i.e., for
example, by means of a temperature, pH or pressure gradient, the
deposition in the deposition tank is achieved or promoted. A
suitable pressure differential between electrolysis cell and
deposition tank may be up to 100 bar. A pressure differential
between 2 bar and 20 bar would preferably be chosen. An elevated
pressure in the deposition tank would promote hydrogencarbonate
formation.
[0067] On the anode side, in principle, alternative anode reactions
are also conceivable, but coupling to chlorine production is the
most economically viable, since the chlorine market is about 75
million metric tons per year. Current production of sodium
hydrogencarbonate (NaHCO.sub.3) is about 50 million metric tons per
year, which have to date been produced via the energetically
unfavorable Solvay process.
[0068] With the electrolysis system and reduction process
described, it is possible to electrochemically, continuously and
simultaneously produce three materials of value: at the cathode, a
material of value such as carbon monoxide, ethylene, methane,
ethanol or monoethylene glycol is obtained from the carbon dioxide
reduction, sodium hydrogencarbonate and/or sodium carbonate is
co-produced as a conversion product of this reduction reaction
formed in the cathode space, and on the anode side chlorine is
produced.
[0069] FIGS. 1 and 2 show, in a schematic representation, examples
of electrolysis systems for carbon dioxide reduction, which can
equally be read as flow diagrams for the reduction process
described. Shown on the left-hand side in each case is the anolyte
circuit AK, and on the right-hand side the catholyte circuit KK.
These two circuits AK, KK are connected via the electrolyzer E1,
E2, the anode space AR and cathode space KR of which are connected
to one another and separated from one another by means of a
membrane M. The membrane M used may be a cation-conducting membrane
M. In the anode space AR is disposed an anode A, and in the cathode
space KR a cathode K, which are electrically connected by a voltage
source U.
[0070] Each of the circuits AK, KK may include a pump P1, P2, which
pump the electrolytes through the electrolyzer. In addition, units
N1, N2, N3 in the two circuits AK, KK may be present at different
points in the flow direction, which may be additional inlets or
outlets or in the form of buffer reservoirs. In the anolyte circuit
AK, at least one gas separation unit G2 with a product outlet PA2
is provided, by means of which the chlorine gas product Cl.sub.2
can be withdrawn. Likewise provided in the catholyte circuit KK is
at least one gas separation unit G1 with a product outlet PA1, by
means of which, for example, the carbon monoxide electrolysis
product CO, and, for example, hydrogen H.sub.2 as well can be
withdrawn. But it is also possible for further electrolysis
products, such as ethylene, methane, ethanol, monoethylene glycol,
to be withdrawn from the system via this or, for example, via a
further product outlet. The electrolyzer E1, E2 has, for example, a
gas diffusion electrode GDE for the carbon dioxide inlet.
[0071] In the case of the electrolyzer E1 shown in FIG. 1, a
two-chamber setup is chosen and the carbon dioxide CO.sub.2 is
introduced into the electrolyte via a reservoir CO.sub.2--R and
upstream of the cathode space KR in circulation direction. The
catholyte circuit KK, in both cases shown, has a deposition tank AB
which may be incorporated directly into the circuit or through
which just a portion of the catholyte volume is conducted. For this
purpose, as shown in FIGS. 1 and 2, a branch in the circuit KK may
be provided. The deposition tank AB or a plurality of
series-connected deposition tanks may be connected, for example, to
a cooling unit or to a buffer reservoir PR, such that the
crystallization of the hydrogencarbonate is promoted by
establishing a temperature differential, pressure differential or
pH differential with respect to the electrolyzer E1, E2. In
addition, the deposition tank AB may include a product outlet PA3.
Multiple series-connected deposition tanks would each have a
product outlet.
[0072] FIGS. 1 and 2 thus show electrolysis systems usable for the
methods described herein. In this setup, it is ensured that there
are separate anolyte circuits AK and catholyte circuits KK. The
electrolytes used are then pumped continuously through the
electrolysis cell E1, E2, i.e. through the anode space AR and
through the cathode space KR. For this purpose, in the setup, one
pump P1, P2 is provided in each of the two circuits AK, KK. The
setup may include materials made of plastic, plastic-coated metal
or glass. Reservoir vessels used may be glass flasks; the cell
itself is made, for example, of PTFE, and the hoses of
neoprene.
[0073] The electrolyzer E1, E2, as constructed in the electrolysis
systems shown, may also have a different setup as shown, for
example, in FIGS. 3 to 5. An alternative electrolysis cell is that
according to the polymer electrolyte membrane setup (PEM setup). In
this case, at least one electrode directly adjoins the polymer
electrolyte membrane PEM. Correspondingly, the electrolysis cell
can be configured as a PEM half-cell, as shown in FIGS. 4 and 5, in
which the anode side is configured as a PEM half-cell, i.e. the
anode A is arranged in direct contact with the membrane PEM and the
anode space AR is arranged on the side of the anode A facing away
from the membrane.
[0074] In the cases as shown in FIGS. 4 and 5, the cathode K is
porous and at least partly gas-permeable and/or
electrolyte-permeable. In FIG. 4, the anode PEM half-cell is
combined with a gas diffusion electrode GDE for introducing the
carbon dioxide CO.sub.2 into the cathode space KR. Also shown in
FIG. 5 is a cathode K with backflow, the cathode space KR of which
is connected to a gas reservoir via the cathode K. The gas
reservoir here, for its part, has at least one gas inlet GE and
optionally a gas outlet GA. Such an embodiment has been used to
date, for example, as an oxygen-depolarized electrode, for example
in the production of sodium hydroxide solution. In that case, there
would be oxygen backflow through the cathode K. The
oxygen-depolarized cathode can be used, for example, to avoid
hydrogen formation H.sub.2 in the cathode space KR in favor of a
reaction to give water H.sub.2O. The energy of water formation here
lowers the necessary system voltage U and thus brings about lower
energy consumption of the electrolysis system. Since the cathode K
of an oxygen-depolarized electrode consists primarily of silver, it
can also catalyze carbon dioxide reduction. If no oxygen is
provided, the oxygen-consuming reaction cannot proceed. Instead,
carbon dioxide reduction to carbon monoxide CO takes place with a
certain degree of hydrogen formation.
[0075] If, for example, sodium is chosen as alkali metal, in the
case of use of a copper-containing cathode K, the following
reactions proceed in the cathode space KR:
Ethylene: 12 NaCl+14 CO.sub.2+8 H.sub.2O.fwdarw.C.sub.2H.sub.4+12
NaHCO.sub.3+6 Cl.sub.2
Methane: 8 NaCl+9 CO.sub.2+4 H.sub.2O.fwdarw.CH.sub.4+8
NaHCO.sub.3+4 Cl.sub.2
Ethanol: 12 NaCl+14 CO.sub.2+9 H.sub.2O.fwdarw.C.sub.2H.sub.5OH+12
NaHCO.sub.3+6 Cl.sub.2
[0076] Monoethylene Glycol:
10 NaCl+12 CO.sub.2+8 H.sub.2O.fwdarw.HOC.sub.2H.sub.4OH+10
NaHCO.sub.3+5 Cl.sub.2.
[0077] In the case of a silver-containing cathode K, the following
reactions would proceed at the cathode:
[0078] Carbon Monoxide:
2 NaCl+3 CO.sub.2+H.sub.2O.fwdarw.CO+2 NaHCO.sub.3+Cl.sub.2.
[0079] These equations describe the cumulative process in the
electrolysis cell. The chlorine gas Cl.sub.2 forms, as described,
through oxidation of the chloride anions Cl.sup.- at the anode A;
the other electrolysis products form at the cathode K or through
conversion reactions in the cathode space KR.
[0080] The example of sodium may be suitable since sodium
hydrogencarbonate can be deposited very efficiently from the
electrolyte. Moreover, sodium hydrogencarbonate and sodium
carbonate are important chemical materials of value that are
frequently required. Global annual sodium carbonate production is
about 50 000 000 metric tons, as can be inferred for example from
the Roskill market report "Soda Ash: Market Outlook to 2018",
available from Roskill Information Services Ltd, E-Mail:
info@roskill.co.uk, www.roskill.co.uk/soda-ash.
[0081] The solubility of sodium hydrogencarbonate NaHCO.sub.3 in
water H.sub.2O is comparatively low and also shows strong
temperature dependence; see table 2.
TABLE-US-00002 TABLE 2 Molecular formula KHCO.sub.3 K.sub.2SO.sub.4
K.sub.3PO.sub.4 KI KBr KCl NaHCO.sub.3 Na.sub.2SO.sub.4 Molar mass
100.1 174.3 212.3 166.0 119.0 74.6 84.01 142.04 in g/mol Solubility
in H.sub.2O at 20.degree. C.: In g/l 337 111 900 1400 678 344 96
170 In mol/l 3.37 0.64 4.24 8.43 5.70 4.61 1.19 1.14 Conductivities
.sigma. in mS/cm: At 0.05M 4.8 9.9 17.3 7.2 7.7 7.4 5.8 14.8 At
0.1M 9.1 19.2 30.1 14.0 14.3 13.8 28.1 51.6 At 0.5M 38.9 (69.9) 108
65.2 67.5 62.8
[0082] Table 2 lists further salts, potassium hydrogencarbonate
KHCO.sub.3, potassium sulfate K.sub.2SO.sub.4, potassium phosphate
K.sub.3PO.sub.4, potassium iodide KI, potassium bromide KBr,
potassium chloride KCl, sodium hydrogencarbonate NaHCO.sub.3,
sodium sulfate Na.sub.2SO.sub.4, which can be used with preference.
But other sulfates, phosphates, iodides or bromides can also be
used to increase the conductivity in the electrolyte. By constantly
supplying the carbon dioxide, it is not necessary to supply
carbonates or hydrogencarbonates; instead, they are formed in
operation in the cathode space KR.
[0083] The solubility of sodium hydrogencarbonate NaHCO.sub.3 in
water is 69 g/l at 0.degree. C., 96 g/l at 20.degree. C., 165 g/l
at 60.degree. C. and 236 g/l at 100.degree. C. Sodium carbonate
NaCO.sub.3, by contrast, has comparatively good solubility; the
solubility thereof is 217 g/l at 20.degree. C. With continuing
electrolysis, the sodium hydrogencarbonate NaHCO.sub.3 thus has a
tendency to crystallize out in the electrolysis cell E1, E2. This
can be counteracted via an elevated temperature as arises as a
result of the operation of the system, and also via corresponding
buffering of the pH.
[0084] The sodium hydrogencarbonate NaHCO.sub.3 is not supposed to
crystallize out of the electrolyte until within the deposition tank
AB. As a result of the pumped circulation of the electrolyte in a
circuit KK, the sodium hydrogencarbonate NaHCO.sub.3 formed in the
cathode space KR is conducted out of it and the catholyte circuit
KK can run through a deposition tank AB, or a part-volume of the
catholyte is branched into a deposition tank AB in which, for
example, the sodium hydrogencarbonate NaHCO.sub.3 crystallizes out
as a result of the cooling of the electrolyte and can thus be
recovered. Since the electrolysis cells E1, E2 are in any case
heated significantly in operation as a result of process losses,
there can be effective crystallization at temperature differentials
of up to 70 K between cathode space KR and deposition tank AB.
Preference is given to working within a range between temperature
differential 30 K and 50 K. Especially with a temperature
differential of at least 15 K or even at least 20 K.
[0085] If the catholyte also contains further additions for
enhancing conductivity and hence increasing the energy efficiency,
and an additional conductive salt thus minimizes the ohmic losses
in the electrolyte, this has to be taken into account in the
crystallization of the sodium hydrogencarbonate NaHCO.sub.3, in
order to obtain a product of maximum purity. In some embodiments, a
hydrogensulfate HSO.sub.4.sup.- or sulfate SO.sub.4.sup.2- is
included as a conductive additive. This may, for example, be sodium
sulfate Na.sub.2SO.sub.4 or sodium hydrogensulfate NaHSO.sub.4. The
solubility of sodium hydrogensulfate NaHSO.sub.4 is 1080 g/l at
20.degree. C. and that of sodium sulfate Na.sub.2SO.sub.4 is 170
g/l at 20.degree. C.; see table 2. Given this great difference in
solubility from sodium hydrogencarbonate NaHCO.sub.3, it is assured
that sodium hydrogencarbonate NaHCO.sub.3 will crystallize out
preferentially in the deposition tank.
[0086] This variant of the reduction process may basically replace
the Solvay process that has been used as standard to date for
sodium hydrogencarbonate production. This is because the Solvay
process for sodium hydrogencarbonate production has a great
disadvantage, namely that it consumes very large amounts of water.
Moreover, for every kilogram of soda, i.e. sodium carbonate
Na.sub.2CO.sub.3, about one kilogram of unusable calcium chloride
CaCl.sub.2 is also produced, which is usually released into the
wastewater and hence into rivers and seas. Given an annual
production of 50 million metric tons of sodium carbonate
Na.sub.2CO.sub.3, this is thus about 50 million metric tons of
calcium chloride CaCl.sub.2.
[0087] The natural sources for soda Na.sub.2CO.sub.3 that are
available aside from the Solvay process are by no means sufficient.
Sodium hydrogencarbonate NaHCO.sub.3 occurs as the natural mineral
nahcolite in the United States of America. It usually occurs in
fine distribution in oil shale and can then be produced as a
by-product of oil production. Particularly rich nahcolite horizons
are being mined in the state of Colorado. However, annual
production in 2007 was only 93 440 metric tons. It also occurs, for
example, in soda lakes in Egypt, in Turkey in Lake Van, in East
Africa, for example in Lake Natron and other lakes in the East
African rift, in Mexico, in California (USA), and as trona
(Na(HCO.sub.3).Na.sub.2CO.sub.3.2H.sub.2O) in Wyoming (USA),
Mexico, East Africa and in the southern Sahara.
[0088] FIG. 6 shows, for illustration of the dependence on the
concentration and pH parameters, an example of a Hagg diagram of a
0.05 molar solution of carbon dioxide CO.sub.2. Within a moderate
pH range, carbon dioxide CO.sub.2 and salts thereof are present
alongside one another. While carbon dioxide CO.sub.2 under strongly
basic conditions preferentially takes the form of carbonate
CO.sub.3.sup.2- and preferentially takes the form of
hydrogencarbonate HCO.sub.3.sup.- in the moderate pH region, the
hydrogencarbonate ions are driven out of the solution in the form
of carbon dioxide CO.sub.2 at low pH values in an acidic
medium.
* * * * *
References