U.S. patent application number 13/816580 was filed with the patent office on 2013-09-26 for mixed gas generating device.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is Yasushi Ito. Invention is credited to Yasushi Ito.
Application Number | 20130248359 13/816580 |
Document ID | / |
Family ID | 46206719 |
Filed Date | 2013-09-26 |
United States Patent
Application |
20130248359 |
Kind Code |
A1 |
Ito; Yasushi |
September 26, 2013 |
MIXED GAS GENERATING DEVICE
Abstract
Disclosed is a mixed gas generating device and has an object to
provide a mixed gas generating device capable of generating CO and
H.sub.2 with favorable energy efficiency. An energy efficiency
characteristic line 10 of electrolysis with respect to a generation
ratio between CO and H.sub.2 (CO/H.sub.2) becomes a curve
projecting downward. Thus, electrolysis is executed not at an
operation point A where CO/H.sub.2=1/2 is given but in a divided
manner at an operation point B and an operation point C. However,
the electrolysis time is divided into electrolysis time periods
such that the generation ratio of a mixed gas becomes
CO/H.sub.2=1/2 after the electrolysis is executed with the
respective generation ratios. As a result, energy efficiency in the
vicinity of an operation point D higher than the operation point A
can be obtained.
Inventors: |
Ito; Yasushi; (Susono-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ito; Yasushi |
Susono-shi |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi, Aichi-ken
JP
|
Family ID: |
46206719 |
Appl. No.: |
13/816580 |
Filed: |
December 8, 2010 |
PCT Filed: |
December 8, 2010 |
PCT NO: |
PCT/JP2010/072033 |
371 Date: |
February 12, 2013 |
Current U.S.
Class: |
204/229.5 ;
204/230.2 |
Current CPC
Class: |
C01B 32/40 20170801;
C25B 15/02 20130101; C25B 1/04 20130101; Y02E 60/366 20130101; C25B
1/00 20130101; Y02E 60/36 20130101 |
Class at
Publication: |
204/229.5 ;
204/230.2 |
International
Class: |
C25B 15/02 20060101
C25B015/02; C25B 1/04 20060101 C25B001/04 |
Claims
1. A mixed gas generating device for generating a mixed gas
composed of hydrogen and carbon monoxide with a predetermined
mixing ratio by electrolyzing water and carbon dioxide, comprising:
at least one electrolytic tank provided with water and carbon
dioxide therein; a pair of electrodes provided in said electrolytic
tank; and potential control means for applying, instead of a
predetermined potential difference which achieves said
predetermined mixing ratio, a plurality of potential differences
including a potential difference larger than said predetermined
potential difference and a potential difference smaller than said
predetermined potential difference between said electrodes.
2. The mixed gas generating device according to claim 1, wherein
said plurality of potential differences are determined so that an
energy efficiency (a calorific value of a generated product to
input energy) obtained in the case where each of said plurality of
potential difference is applied between said electrodes is higher
than that obtained in the case where said predetermined potential
difference is applied between said electrodes.
3. The mixed gas generating device according to claim 2, wherein
the order of applying said plurality of potential differences
between said electrodes is determined in accordance with a degree
of temporal change of an energy efficiency obtained in the case
where each of said potential differences is applied between said
electrodes.
4. The mixed gas generating device according to any one of claims 1
to 3, wherein said electrolytic tank is provided with an
electrolytic solution having carbon dioxide absorbing
characteristics therein; and the order of applying said plurality
of potential differences between said electrodes is determined to
be an order from the larger potential difference.
5. The mixed gas generating device according to claim 4, further
comprising: pH changing means for changing pH of said electrolytic
solution from the alkali side to the acid side, when said plurality
of potential differences are to be applied between said
electrodes.
6. The mixed gas generating device according to any one of claims 1
to 5, wherein each potential difference on the side smaller than
said predetermined potential difference is set to a potential
difference larger than a predetermined potential difference at
which a generation speed of hydrogen to be generated becomes the
lowest speed determined in advance.
Description
TECHNICAL FIELD
[0001] The present invention relates to a mixed gas generating
device. More particularly, the present invention relates to a mixed
gas generating device for generating a mixed gas composed of
H.sub.2 and CO by electrolyzing water and CO.sub.2.
BACKGROUND ART
[0002] Fossil fuels including petroleum, coal, natural gas and the
like are used as material for generation of heat and electricity
and as fuels for transportation and support the modern energy
consumption society. However, such fossil fuels are exhaustible
fuels and their reserves are limited. Thus, it is needless to say
that preparation is needed for depletion of the fossil fuels.
Moreover, emissions of CO.sub.2 caused by combustion of the fossil
fuels into the atmosphere are known to be one of the factors which
cause global warming. Thus, reduction of CO.sub.2 emissions has
become a recent object.
[0003] As a means for solving these problems, an alternative fuel
using CO.sub.2 as a material has been examined. For example, Patent
Document 1 discloses a system for manufacturing hydrocarbon fuel
(HC) using CO.sub.2 as a material. This system includes an
electrolytic cell having an oxygen ion conductive film composed of
a solid oxide electrolyte and a cathode and an anode arranged on
the both surfaces thereof, respectively, and HC is synthesized by a
material gas generated by using this electrolytic cell.
[0004] A specific manufacturing method of HC in the above system is
as follows. First, while electric power and heat are supplied to
the electrolytic cell, a CO.sub.2 gas and steam are supplied to the
cathode, and a carbon monoxide (CO) gas and a hydrogen (H.sub.2)
gas which become material gases are generated on this cathode,
respectively. Subsequently, the generated material gas is recovered
from the electrolytic cell and made to react in a known
manufacturing device using Fischer-Tropsch reaction (FT reaction)
so as to obtain HC.
RELATED ART LITERATURE
Patent Documents
[0005] Patent Document 1: JP-A-2009-506213
[0006] Patent Document 2: JP-A-9-085004
[0007] Patent Document 3: JP-A-2008-214563
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
[0008] According to the system of the above-described Patent
Document 1, CO and H.sub.2 which become materials of HC can be
generated simultaneously. However, if time for executing
electrolysis is set to a given period, energy (electric power)
input for electrolysis takes a different value depending on a
potential difference applied between the anode and the cathode.
Moreover, this potential difference is correlated with a generation
ratio of CO and H.sub.2. Thus, calorific values of CO and H.sub.2
are varied depending on the potential difference. From these facts,
it was likely that energy efficiency (a calorific value of a
generated product to input energy. The same applies to the
following) lowers depending on the potential difference during
electrolysis.
[0009] The present invention was made in order to solve the
above-described problem and has an object to provide a mixed gas
generating device which can generate CO and H.sub.2 with favorable
energy efficiency.
Means for Solving the Problem
[0010] To achieve the above mentioned purpose, a first aspect of
the present invention is a mixed gas generating device for
generating a mixed gas composed of hydrogen and carbon monoxide
with a predetermined mixing ratio by electrolyzing water and carbon
dioxide, comprising:
[0011] at least one electrolytic tank provided with water and
carbon dioxide therein;
[0012] a pair of electrodes provided in said electrolytic tank;
and
[0013] potential control means for applying a plurality of
potential differences including a potential difference larger than
a predetermined potential difference which achieves said
predetermined mixing ratio and a potential difference smaller than
said predetermined potential difference between said
electrodes.
[0014] A second aspect of the present invention is the mixed gas
generating device according to the first aspect, wherein
[0015] said plurality of potential differences are determined on
the basis of a model which creates an association between energy
efficiency specified by a calorific value of a generated product
with respect to input energy and said plurality of potential
differences.
[0016] A third aspect of the present invention is the mixed gas
generating device according to the second aspect, wherein
[0017] the order of applying said plurality of potential
differences between said electrodes is determined in accordance
with a degree of temporal change of said energy efficiency obtained
in the case where each of said potential differences is applied
between said electrodes.
[0018] A fourth aspect of the present invention is the mixed gas
generating device according to any one of the third aspects,
wherein
[0019] said electrolytic tank is provided with an electrolytic
solution having carbon dioxide absorbing characteristics therein;
and
[0020] the order of applying said plurality of potential
differences between said electrodes is determined to be an order
from the larger potential difference.
[0021] A fifth aspect of the present invention is the mixed gas
generating device according to the forth aspect, further
comprising:
[0022] pH changing means for changing pH of said electrolytic
solution from the alkali side to the acid side, when said plurality
of potential differences are to be applied between said
electrodes.
[0023] A sixth aspect of the present invention is the mixed gas
generating device according to any one of the first to the fifth
aspects, wherein
[0024] each potential difference on the side smaller than said
predetermined potential difference is set to a potential difference
larger than a predetermined potential difference at which a
generation speed of hydrogen to be generated becomes the lowest
speed determined in advance.
Advantages of the Invention
[0025] According to the first invention, when a mixed gas is to be
generated with a predetermined mixing ratio, a plurality of
potential differences including a potential difference larger than
a predetermined potential difference which achieves this
predetermined mixing ratio and a potential difference smaller than
this predetermined potential difference can be applied between
electrodes. Since the mixing ratio of CO and H.sub.2 is correlated
with a potential difference to be applied between the electrodes,
it is only necessary to apply the predetermined potential
difference between the electrodes in order to achieve the
predetermined mixing ratio. However, considering energy efficiency,
the energy efficiency obtained by applying the predetermined
potential difference is not necessarily the best. In this regard,
the predetermined mixing ratio can be achieved even if the
potential difference is divided into the plurality of potential
differences and applied. Moreover, by applying the potential
difference divided into the plurality of potential differences, a
combination that can improve the energy efficiency better than the
application of the predetermined potential difference can be
realized. Therefore, according to the first invention, CO and
H.sub.2 can be generated with a predetermined mixing ratio with
favorable energy efficiency.
[0026] According to the second invention, a combination of the
potential differences that can improve the energy efficiency better
than the application of the predetermined potential difference can
be determined easily by a model which relates the energy efficiency
determined by a calorific value of a generated product to input
energy with the plurality of potential differences.
[0027] If the potential difference is applied between the
electrodes, electrolysis progresses and water and CO.sub.2 in the
electrolytic tank decrease, and thus, energy efficiency lowers.
According to the third invention, the order of applying these
potential differences is determined in accordance with a degree of
temporal change of the energy efficiency obtained in the case where
each of the potential differences is applied. Thus, it becomes
possible to apply these potential differences in the order from
that with a higher degree of decrease in the energy efficiency, for
example. Therefore, an influence of the temporal change of the
energy efficiency can be minimized.
[0028] If the plurality of potential differences are applied
between the electrodes, electrolysis progresses, and a temperature
in the electrolytic tank rises. If the temperature in the
electrolytic tank rises, CO.sub.2 absorbed in an electrolytic
solution evaporates and its absorbed amount decreases. If CO.sub.2
decreases, it becomes difficult to achieve the predetermined mixing
ratio. According to the fourth invention, the order of applying the
plurality of potential differences between the electrodes is
determined from that with a larger potential difference. As
described above, the mixing ratio of CO and H.sub.2 is correlated
with the potential difference to be applied between the electrodes.
In more detail, if the potential difference to be applied between
the electrodes becomes large, a CO generation amount increases.
Thus, by applying the potential differences in the order from that
with a larger potential difference according to the fourth
invention, CO.sub.2 can be electrolyzed before CO.sub.2 decreases.
Therefore, reduction in the CO generation amount caused by the
temperature rise in the electrolytic tank can be favorably
suppressed.
[0029] According to the fifth invention, pH in the electrolytic
tank can be changed by pH changing means from the alkali side to
the acid side. Therefore, when the potential differences are
applied in the order from that with a larger potential difference,
pH can be changed from the alkali side where CO.sub.2 rarely
evaporates to the acid side where H.sub.2 can be easily generated.
Therefore, the reduction in the CO generation amount in pH on the
alkali side can be favorably suppressed, while H.sub.2 generation
speed in pH on the acid side can be improved.
[0030] According to the sixth invention, each of the potential
differences on the side smaller than the predetermined potential
difference can be set to a potential difference larger than the
predetermined potential difference at which the generation speed of
H.sub.2 to be generated becomes the lowest speed determined in
advance. Therefore, since the H.sub.2 generation speed can be
ensured, reduction in the work efficiency during electrolysis can
be decreased.
BRIEF DESCRIPTION OF DRAWINGS
[0031] FIG. 1 is a diagram illustrating a relationship between the
generation ratio between CO and H.sub.2 and the energy efficiency
during electrolysis.
[0032] FIG. 2 is a diagram illustrating the lateral axis in FIG. 1
is replaced by the generation ratio of CO to the total generation
amount.
[0033] FIG. 3 is a diagram illustrating a temporal change of the
energy efficiency during electrolysis.
[0034] FIG. 4 is a diagram for explaining the order of electrolysis
in this embodiment.
[0035] FIG. 5 is a diagram for explaining an outline of the control
in this embodiment.
[0036] FIG. 6 is a diagram illustrating the generation speeds
(mol/min) of CO and H.sub.2 and the energy efficiency with respect
to the generation ratio between CO and H.sub.2.
MODE FOR CARRYING OUT THE INVENTION
Embodiment 1
Configuration of Electrolysis Apparatus
[0037] First, configuration of an electrolysis apparatus of an
embodiment 1 of the present invention will be described in brief.
The electrolysis apparatus of this embodiment includes an
electrolytic tank filled with an electrolytic solution in which
CO.sub.2 is dissolved, a working electrode (WE), a counter
electrode (CE), and a reference electrode (RE) provided in this
electrolytic tank, and a potentiostat configured capable of
changing a voltage of the WE to the RE. Detailed explanation of
these constituents will be omitted since these constituents are in
common with a known electrolysis apparatus.
[0038] The electrolysis apparatus of this embodiment also includes
a controller which holds a voltage of the WE to the RE at a set
value by having a predetermined current value flow between the WE
and the CE by controlling the potentiostat. An internal memory of
this controller stores a set value of the voltage corresponding to
an operation point determined by a method which will be described
later, operation time and the like. The controller controls the
potentiostat on the basis of them.
[0039] Subsequently, an electrolytic reaction in the
above-described electrolysis apparatus will be described. By
applying an electric current flow between the WE and the CE by
controlling the potentiostat, electrochemical reactions of the
following formulas (1) to (3) occur at the WE and the CE:
WE: CO.sub.2+2H.sup.++2e.sup.-.fwdarw.CO+H.sub.2O (1)
2H.sup.++2e.sup.-.fwdarw.H.sub.2 (2)
CE: 2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4e.sup.- (3)
[0040] As illustrated in the above-described formulas (1) and (2),
CO and H.sub.2 are generated simultaneously in the WE. Thus, by
collecting the generated CO and H.sub.2 and applying the FT
reaction to them, HC as an alternative fuel for the fossil fuel can
be manufactured. It is needless to say, here, that HC is preferably
manufactured efficiently from a material stage. In this connection,
it has been known that high energy efficiency of the FT reaction is
achieved when the mixing ratio between CO and H.sub.2 is set to
CO/H.sub.2=1/2.
[0041] In this embodiment, considering the energy efficiency of the
FT reaction, the generation ratio between CO and H.sub.2 to be
generated at the WE simultaneously is set to CO/H.sub.2=1/2. In
general, in order to set the mixing ratio between CO and H.sub.2 to
CO/H.sub.2=1/2, the ratio needs to be adjusted from CO and H.sub.2
prepared independently. In that regard, according to the
above-described electrolysis apparatus, CO and H.sub.2 can be
generated simultaneously on the WE. Thus, by setting the generation
ratio to CO/H.sub.2=1/2 in advance, the material generation and the
ratio adjustment can be proceeded with at the same time. If the
material generation and the ratio adjustment can be proceeded with
at the same time, the generated CO and H.sub.2 can be put into the
FT reaction as they are. Therefore, HC can be manufactured with
high energy efficiency from the material stage.
Characteristics of Embodiment 1
[0042] Incidentally, the generation ratio between CO and H.sub.2
largely depends on a current value made to flow between the WE and
CE, that is, a set value of a voltage of the WE to the RE. Thus, in
order to set the generation ratio to CO/H.sub.2=1/2, it is only
necessary to set this voltage to an appropriate value. However, if
CO and H.sub.2 are to be generated at the same time, the energy
efficiency with the other generation ratios might become higher
than the energy efficiency at a point when CO/H.sub.2=1/2 is given.
This will be described by using FIG. 1. FIG. 1 is a diagram
illustrating a relationship between the generation ratio between CO
and H.sub.2 (=voltage of the WE to the RE) and the energy
efficiency during electrolysis.
[0043] A curve in FIG. 1 is an energy efficiency characteristic
line 10 of electrolysis to CO/H.sub.2. Points A to C on the
characteristic line 10 indicate operation points, respectively.
Specifically, the operation point A corresponds to a case where the
generation ratio between CO and H.sub.2 is set to 1/2 (the voltage
of the WE to the RE is set to a set value V.sub.A) and electrolysis
is executed. The operation point B and the operation point C
correspond to a case where the generation ratio between CO and
H.sub.2 is set to rb (>1/2) and rc (<1/2), respectively (the
voltage of the WE to the RE is set to set values
V.sub.B(>V.sub.A) and V.sub.C(<V.sub.A)) and electrolysis is
executed.
[0044] As illustrated in FIG. 1, the characteristic line 10 is a
curve projecting downward. One of the reasons is that a generation
potential of H.sub.2 in an absolute value is 0.11 V and is lower
than the generation potential of CO. If the generation potential of
H.sub.2 is lower than the generation potential of CO, when the
voltage of the WE to the RE is set low, a H.sub.2 generation amount
becomes relatively large, and CO/H.sub.2 becomes smaller. Here, the
voltage of the WE to the RE is low is synonymous with the fact that
the input energy during electrolysis is small. Therefore, the
smaller CO/H.sub.2 is, the higher the energy efficiency becomes.
That is, by sliding the operation point in the direction of the
operation point C from the operation point A, the energy efficiency
becomes higher.
[0045] Another reason is that the calorific value of CO (283
kJ/mol) is larger than the calorific value of H.sub.2 (242 kJ/mol).
As described above, the H.sub.2 generation potential is lower than
the CO generation potential. Thus, by setting the voltage of the WE
to the RE higher, the CO generation amount can be increased, and
CO/H.sub.2 becomes large. Here, since the calorific value of CO is
larger than the calorific value of H.sub.2, if CO/H.sub.2 becomes
larger, the calorific value of the generated product increases.
Therefore, the larger CO/H.sub.2 becomes, the higher the energy
efficiency becomes. That is, by sliding the operation point in the
direction of the operation point B from the operation point A, the
energy efficiency is improved.
[0046] In this embodiment, in view of such characteristics of the
characteristic line 10, electrolysis is executed not at the
operation point A but at the operation point B and the operation
point C in a divided manner. That is, instead of electrolysis for a
given period of time at the set value V.sub.A, the execution of
electrolysis is shared by the set value V.sub.B and the set value
V.sub.C without changing the total time. However, the electrolysis
time is divided into electrolysis time periods in such a manner
that the generation ratio of a mixed gas becomes CO/H.sub.2=1/2
after the electrolysis is executed with the respective generation
ratios. As a result, energy efficiency in the vicinity of an
operation point D higher than the operation point A can be
obtained.
[0047] A specific method of determining the operation points B and
C will be described. For convenience of calculation, the lateral
axis in FIG. 1 is replaced by the generation ratio of CO to the
total generation amount as illustrated in FIG. 2
(=CO/(CO+H.sub.2)). Then, CO/H.sub.2=1/2 can be converted to
CO/(CO+H.sub.2)=1/3. As illustrated in FIG. 2, each of coordinates
of the operation points A, B, and C is set at (Ra, .eta.a), (Rb,
.eta.b), and (Rc, .eta.c). Moreover, assume that the total
generation amount of CO and H.sub.2 at the operation point A is Ma
(mol), input energy is Ea, and the calorific value of the generated
product is Ha. For the operation points B and C, too, assume that
the total generation amounts of CO and H.sub.2 are Mb and Mc, input
energies are Eb and Be, and the calorific values of the generated
product are Hb and Hc. Moreover calorific values per unit mol of CO
and H.sub.2 are assumed to be H.sub.CO and H.sub.H2.
[0048] The energy efficiency .eta.a at the operation point A can be
expressed by the following formula (4):
.eta.a=Ha/Ea={RaMaH.sub.CO+(1-Ra)MaH.sub.H2}/Ea (4)
[0049] The energy efficiency .eta.b at the operation point B and
the energy efficiency .eta.c at the operation point C can be also
expressed similarly to the above formula (4).
[0050] The generated product amount has relationship of the
following formulas (5) and (6):
Ma=Mb+Mc (5)
RaMa=RbMb+RcMc (6)
[0051] By organizing the above formula (6) using the above formula
(5), the following formula (7) can be derived:
Mb/Mc=(Ra-Rc)/(Rb-Ra) (7)
[0052] The total energy efficiency .eta.b+c when execution of
electrolysis is divided by the operation point B and the operation
point C is expressed by the following formula (8) using the above
formulas (4) and (5) and Hb+Hc=Ha:
.eta. b + c = ( Hb + Hc ) / ( Eb + Ec ) = ( Mb + M c ) { RaH CO + (
1 - Ra ) H H 2 } / [ Mb / .eta. b { R bH CO + ( 1 - Rb ) H H 2 } +
M c / .eta. c { R cH CO + ( 1 - Rc ) H H 2 } ] ( 8 )
##EQU00001##
[0053] By dividing the upper and lower sides in the formula (8) by
Mc, the following formula (9) can be derived:
.eta.b+c=(Mb/Mc+1){RaH.sub.CO+(1-Ra)H.sub.H2}/[(1/.eta.b)(Mb/Mc){RbH.sub-
.CO+(1-Rb)H.sub.H2}+(1/.eta.c){RcH.sub.CO+(1-Rc)H.sub.H2}] (9)
[0054] By multiplying the upper and lower sides of the above
formula (9) by (Rb-Ra) using the above formula (7), the following
formula (10) can be derived:
.eta.b+c=(Rb-Rc){RaH.sub.CO+(1-Ra)H.sub.H2}/[{(Ra-Rc)/.eta.b}{RbH.sub.CO-
+(1-Rb)H.sub.H2}+{(Rb-Ra)/.eta.c}{RcH.sub.CO+(1-Rc)H.sub.H2}]
(10)
[0055] By organizing the above formula (10) using Ra=1/3, the
following formula (11) can be derived:
.eta.b+c=(Rb-Rc)(Hc+2H.sub.H2)/[{(1-3Rc)/.eta.b}{RbH.sub.CO+(1-Rb)H.sub.-
H2}]+{(3Rb-1)/.eta.c}{RcH.sub.CO+(1-Rc)H.sub.H2}] (11)
[0056] Here, H.sub.CO is 283 kJ/mol
(CO+1/2O.sub.2.fwdarw.CO.sub.2+283 kJ/mol) and H.sub.H2=242 kJ/mol
(H.sub.2+1/2O.sub.2.fwdarw.H.sub.2O+242 kJ/mol).
[0057] Therefore, by obtaining a plurality of (Rb, .eta.b) and (Rc,
.eta.c) in advance under actual electrolysis conditions (type of
electrolytic solution, set values V.sub.B and V.sub.C of the
voltage of the WE to the RE and the like) and by applying the (Rb,
.eta.b) and (Rc, .eta.c) to the above formula (11), the total
energy efficiency .eta.b+c can be calculated. It is only necessary
that two points corresponding to the best among the total energy
efficiency .eta.b+c calculated as above are determined as the
operation points B and C.
[0058] As described above, according to this embodiment, since
execution of electrolysis is divided by the operation point B and
the operation point C determined by the above-described method, the
energy efficiency in the vicinity of the operation point D higher
than the operation point A can be obtained. Therefore, the energy
efficiency during electrolysis can be improved. Thus, the energy
efficiency during a series of HC manufacture from electrolysis to
the FT reaction can be improved.
[0059] In this embodiment, electrolysis is executed at the
operation points B and C, but electrolysis may be executed at three
or more operation points. In this case the electrolysis can be
improved if each of the electrolysis time is divided in such a
manner that the generation ratio of a mixed gas becomes
CO/H.sub.2=1/2 after the electrolysis is executed at each operating
point. This variation can be also applied similarly to embodiments
3 to 6 which will be described later.
Embodiment 2
Configuration of Electrolysis Apparatus
[0060] Subsequently, an electrolysis apparatus of Embodiment 2 of
the present invention will be described. The electrolysis apparatus
of this embodiment includes two electrolytic tanks, which is a
point of difference from the configuration of the electrolysis
apparatus in the above-described Embodiment 1. Since each
constituent element is basically in common with those in the
above-described Embodiment 1 except this point, the description
will be omitted.
[0061] In the above-described Embodiment 1, one electrolytic tank
is used and electrolysis is executed at the operation points B and
C. However, it is required to control to switch the operation point
from B to C (or from C to B) since the operation points B and C
cannot be set at the same time. In this regard, according to this
embodiment in which two electrolytic tanks are included,
electrolysis at the operation point B and electrolysis at the
operation point C can be executed in the respective electrolytic
tanks. Therefore, advantageous effects substantially similar to the
above-described Embodiment 1 can be obtained easily without the
switching control of the operation point.
[0062] In this embodiment, two electrolytic tanks are included, but
the number of electrolytic tanks may be three or more. If
electrolysis is to be executed at three or more operation points,
as described in the variation of the above-described Embodiment 1,
electrolysis can be executed in various forms by combining the
number of the operation points and the number of electrolytic
tanks. For example, in the case of electrolysis at three operation
points, it may be so configured that three electrolytic tanks are
included, and electrolysis is executed at each corresponding
operation point and tank. Moreover, in the case of electrolysis at
three operation points, for example, it may be so configured that
two electrolytic tanks are provided, in which only electrolysis at
the operation point C is executed in one of them, while control of
switching the operation point from C to B is executed in the other.
As described above, in the case of the electrolysis by combining
the number of operation points and the number of electrolytic
tanks, too, advantageous effects similar to this embodiment can be
obtained.
Embodiment 3
[0063] Subsequently, by referring to FIG. 3, Embodiment 3 of the
present invention will be described. This embodiment uses the
configuration of the above-described Embodiment 1 and is
characterized in that the order of electrolysis is determined on
the basis of a degree of deterioration of energy efficiency as the
progress of the electrolysis. Since each constituent element is
basically in common with those in the above-described Embodiment 1,
the description will be omitted.
[0064] As described in the above-described Embodiment 1, by having
an electric current flow between the WE and the CE by controlling
the potentiostat, the reactions in the above formulas (1) to (3)
progress. If the reactions in the above formulas (1) to (3)
progress, a reaction substance in the electrolytic solution
decreases. For example, if the reaction in the above formula (1)
progresses, CO.sub.2 in the electrolytic tank decreases, while if
the reaction in the above formula (2) progresses, protons decrease.
Therefore, concentration of the reaction substances in the
electrolytic tank lowers over time. If the concentration of the
reaction substance lowers, more input energy is required in order
to generate certain amounts of CO and H.sub.2. Therefore, the
energy efficiency during electrolysis decreases over time.
[0065] FIG. 3 is a diagram illustrating a temporal change of the
energy efficiency during electrolysis. If electrolysis progresses,
the concentration of CO.sub.2 or protons in the electrolytic
solution lowers. And thus, the energy efficiency decreases over
time. For example, the operation point of the electrolysis is
sifted to an operation point C.sub.2 from an operation point
C.sub.1, which was the start operation point of the electrolysis.
Similarly, the operation point of the electrolysis is sifted to an
operation point B.sub.2 from an operation point B.sub.1, which was
the start operation point of the electrolysis. The other operation
points are shifted to in the similar way, the characteristic line
10 may change to a characteristic line 20 with elapse of time.
[0066] Here, when attention is paid to a lowering width of the
energy efficiency in FIG. 3, a relationship between a lowering
width (B.sub.1-B.sub.2) of the energy efficiency from the operation
point B.sub.1 to the operation point B.sub.2 and that from the
operation point C.sub.1 to the operation point C.sub.2
(C.sub.1-C.sub.2) is (B.sub.1-B.sub.2)<(C.sub.1-C.sub.2). Thus,
in this embodiment, electrolysis is executed as: a voltage of the
WE to the RE is set to the value V.sub.C at first and then is
changed to the set value V.sub.B after the expiration of
predetermined time. That is, the electrolysis is started at the
operation point where the lowering width of the energy efficiency
is larger. As a result, the energy efficiency at the operation
point C.sub.1 and the energy efficiency at the operation point
B.sub.2 can be obtained. Therefore, an influence of a temporal
change of the energy efficiency can be minimized.
[0067] Incidentally, in electrolysis of this embodiment, the
voltage of the WE to the RE is set to the set value V.sub.C, and
then is changed to the set value V.sub.B after the expiration of
predetermined time. However, this order may be switched. Depending
on the electrolysis conditions, this lowering width might become
opposite. In that case, electrolysis is executed as: a voltage of
the WE to the RE is set to the value V.sub.B at first and then is
changed to the set value V.sub.B after the expiration of
predetermined time. As a result, advantageous effects substantially
similar to this embodiment can be obtained.
[0068] Moreover, depending on the electrolysis conditions, the
energy efficiency might be improved over time. In that case, too,
the order of electrolysis can be determined by the method similar
to the above. That is, a temporal change width of the energy
efficiency is compared, and if it is a lowering width, electrolysis
is executed in the order from that with the larger value, while if
it is an increasing width, electrolysis is executed in the order
from that with the smaller value so that the effect substantially
similar to this embodiment can be obtained.
Embodiment 4
[0069] Subsequently, by referring to FIG. 4, Embodiment 4 of the
present invention will be described. This embodiment uses the
configuration of the above-described Embodiment 1 and is
characterized in that the order of electrolysis is determined by
considering a temperature rise of an electrolytic solution by
progress of the electrolysis. Since each constituent element is
basically in common with those in the above-described Embodiment 1,
the description will be omitted.
[0070] As described above, by setting the mixing ratio between CO
and H.sub.2 after electrolysis to CO/H.sub.2=1/2, energy efficiency
in the FT reaction is favorable. However, since the temperature of
the electrolytic solution rises by progress of the electrolysis,
CO.sub.2 dissolved in the electrolytic solution evaporates and
CO.sub.2 concentration lowers. If the CO.sub.2 concentration
lowers, the CO generation amount by the reaction in the
above-described (1) also lowers. And thus, the generation ratio
between CO and H.sub.2 becomes smaller than 1/2.
[0071] Thus, in this embodiment, electrolysis mainly for generating
CO is executed at first and then, electrolysis mainly for
generating H.sub.2 is executed. FIG. 4 is a diagram for explaining
the order of electrolysis in this embodiment. The operation point
B.sub.1 in FIG. 4 is an operation point corresponding to the
operation point B in FIG. 1. In this embodiment, electrolysis is
executed at first by setting the voltage of the WE to the RE to the
set value V.sub.B. As a result, the electrolysis mainly for
generating CO is executed at the operation point B.sub.1. As
described in the above-described Embodiment 1, by setting the
voltage of the WE to the RE higher, the CO generation amount can be
made relatively large. That is, the electrolysis mainly for
generating CO can be executed. Subsequently, in this embodiment,
electrolysis is executed by setting the voltage of the WE to the RE
to the set value V.sub.C. As a result, the electrolysis mainly for
generating H.sub.2 is executed. The reason why H.sub.2 is mainly
generated is described in the above-described Embodiment 1.
[0072] By executing the electrolysis mainly for generating CO prior
to the electrolysis mainly for generating H.sub.2, CO can be
generated more before the CO.sub.2 concentration lowers by a rise
of the temperature of the electrolytic solution. Therefore, the
generation ratio between CO and H.sub.2 can be prevented from
becoming smaller than 1/2.
[0073] In addition to the above-described suppression effect,
H.sub.2 generation during electrolysis mainly for generating
H.sub.2 can be promoted according to this embodiment. If the
temperature of the electrolytic solution rises by progress of the
electrolysis, a moving speed of protons or the reaction speed of
the above-described formula (2) increases. Thus, the characteristic
line 10 in FIG. 4 changes to a characteristic line 30 with elapse
of time. Therefore, as compared with the electrolysis at the
operation point C.sub.1 (corresponding to the operation point C in
FIG. 1), H.sub.2 can be generated in a shorter time. That is, the
energy efficiency in the vicinity of the operation point C.sub.3
can be obtained. Thus, the energy efficiency during the
electrolysis can be further improved.
Embodiment 5
[0074] Subsequently, by referring to FIG. 5, Embodiment 5 of the
present invention will be described. This embodiment is
characterized in that an adding device for adding a pH adjusting
agent into the electrolytic tank is added to the configuration of
the above-described Embodiment 1, and this adding device is
controlled during electrolysis. Since each constituent element is
basically in common with those in the above-described Embodiment 1,
the description will be omitted.
[0075] As described in the above-described Embodiment 4, since the
temperature of the electrolytic solution rises by progress of the
electrolysis, CO.sub.2 dissolved in the electrolytic solution
evaporates and CO.sub.2 concentration lowers. Thus, in this
Embodiment, the electrolysis is executed in the order similar to
the above-described Embodiment 4, and moreover, the adding device
is controlled so that pH of the electrolytic solution decreases in
accordance with the progress of the electrolysis.
[0076] FIG. 5 is a diagram for explaining an outline of the control
in this embodiment. In this embodiment, similarly to the
above-described Embodiment 4, the electrolysis mainly for
generating CO is executed prior to the electrolysis mainly for
generating H.sub.2. In addition, in this embodiment, the
above-described adding device is controlled so that pH of the
electrolytic solution becomes slightly higher at which CO.sub.2 is
difficult to evaporate during the electrolysis mainly for
generating CO. Therefore, the generation ratio between CO and
H.sub.2 can be further prevented from becoming smaller than
1/2.
[0077] In this embodiment, the above-described adding device is
controlled so that pH of the electrolytic solution becomes slightly
lower at which H.sub.2 can be generated easily during the
electrolysis mainly for generating H.sub.2. Thus, the
characteristic line 10 in FIG. 5 changes to a characteristic line
40 with elapse of time. Therefore, as compared with the
electrolysis at the operation point C.sub.1 (corresponding to the
operation point C in FIG. 1), H.sub.2 can be generated in a shorter
time. That is, the energy efficiency in the vicinity of an
operation point C.sub.4 can be obtained. Thus, the energy
efficiency during the electrolysis can be further improved.
Embodiment 6
[0078] Subsequently, by referring to FIG. 6, Embodiment 6 of the
present invention will be described. This embodiment uses the
configuration of the above-described Embodiment 1 and is
characterized in that the operation points B and C are determined
by considering the generation speed of CO or H.sub.2. Since each
constituent element is basically in common with those in the
above-described Embodiment 1, the description will be omitted.
[0079] As described in the above-described Embodiment 1, the
operation points B and C can be determined as two points
corresponding to the best among the total energy efficiency
.eta.b+c calculated by using the above formula (11) and the like,
and by executing electrolysis at the operation points B and C
determined as above, the energy efficiency during the electrolysis
can be improved. However, if the generation speed of CO or H.sub.2
is too slow, electrolysis requires long time and deterioration of
work efficiency of HC manufacture is concerned.
[0080] As described in the above-described Embodiment 4, on the
side of the operation point smaller than 1/2, that is, on the
operation point C side, the electrolysis mainly for generating
H.sub.2 can be executed. Thus, if the generation speed of H.sub.2
on the operation point C side is too slow, an influence by
deterioration of the work efficiency becomes large. Similarly, on
the side of the operation point larger than 1/2, that is, on the
operation point B side, the electrolysis mainly for generating CO
can be executed. Thus, if the generation speed of CO on the
operation point B side is too slow, the influence by deterioration
of the work efficiency becomes large.
[0081] Thus, in this embodiment, lower limit values of the
generation speeds of CO and H.sub.2 are set, and the operation
points B and C are determined in a range where the generation
speeds are faster than the lower limit values. FIG. 6 is a diagram
illustrating the generation speeds (mol/min) of CO and H.sub.2 and
the energy efficiency with respect to the generation ratio between
CO and H.sub.2.
[0082] A characteristic line 50 in FIG. 6 indicates the generation
speed of H.sub.2 and a characteristic line 60 indicates the
generation speed of CO, respectively. As described in the
above-described Embodiment 1, the generation ratio between CO and
H.sub.2 depends on the voltage of the WE to the RE. Moreover, if
the voltage of the WE to the RE is set higher, the CO generation
amount increases as well as the H.sub.2 generation amount.
Moreover, since the generation amounts of CO and H.sub.2 increase
if their generation speeds are fast, there is correlation between
the generation speeds and the generation amounts of CO and H.sub.2.
Therefore, as illustrated in FIG. 6, the higher the generation
ratio between CO and H.sub.2 is set, the higher the generation
speeds of CO and H.sub.2 rise. Here, the lower limit values of the
generation speeds of CO and H.sub.2 are set as illustrated in FIG.
6. Then, an operation point corresponding to the lower limit value
of the generation speed of H.sub.2 is an operation point E in FIG.
6. Moreover, an operation point corresponding to the lower limit
value of the generation speed of CO is an operation point F in FIG.
6.
[0083] As described above, on the operation point side smaller than
1/2, the electrolysis mainly for generating H.sub.2 can be
executed, while on the operation point side larger than 1/2, the
electrolysis mainly for generating CO can be executed. Therefore,
in this embodiment, the operation point C is determined within a
range in which CO/H.sub.2 is higher than the operation point E and
the operation point B is determined within a range in which
CO/H.sub.2 is higher than the operation point F. Thus, the
generation speeds of CO and H.sub.2 can be ensured.
[0084] As described above, according to this embodiment, the lower
limit values of the generation speeds of H.sub.2 and CO are set in
advance, and the operation points B and C can be determined within
a range satisfying them, respectively. Therefore, the energy
efficiency can be improved while deterioration of the work
efficiency during electrolysis is reduced.
DESCRIPTION OF REFERENCE NUMERALS
[0085] 10, 20, 30, 40, 50, 60 energy characteristic line
* * * * *