Patent Application
20170066649
The present invention relates to a device and a method for carbon monoxide (CO) shift conversion, in which carbon monoxide and water vapor contained in a reaction gas are reacted and thereby converted into carbon dioxide and hydrogen.
As a hydrogen source for a fuel cell and the like, a reformed gas obtained by reforming hydrocarbon, alcohol, or the like is used. The reformed gas contains therein about 10% of carbon monoxide and carbon dioxide in addition to hydrogen. In the following, carbon monoxide will be referred to as CO and carbon dioxide will be referred to as CO2.
In the case of a polymer electrolyte fuel cell which operates at a low temperature of 100° C. or less, it is known that a platinum catalyst for use in an electrode is poisoned with CO contained in the reformed gas. When the platinum catalyst is poisoned, the reaction of hydrogen is inhibited, and the power generation efficiency of the fuel cell decreases considerably. To realize high power generation efficiency, it is required to suppress the concentration of CO in the reformed gas to 100 ppm or less, and preferably 10 ppm or less.
To lower the CO concentration in the reformed gas, it is necessary to remove CO to be contained. Usually, to remove CO contained in a mixed gas, shift conversion reaction is used. Specifically, in a shift converter in which a shift conversion catalyst is placed, a CO shift conversion reaction (water gas shift reaction) is generated in which CO and water vapor (H2O) contained in a mixed gas (in this case, reformed gas) are reacted, and thereby converted to CO2 and hydrogen (H2). By the shift conversion reaction, the CO concentration in the reformed gas can be reduced to a range from several thousands ppm to about 1%.
Subsequently, in a selective oxidation device in which a platinum-based selective oxidation catalyst is placed, the mixed gas whose CO concentration is lowered is reacted with a trace amount of oxygen (may be air) (selective oxidation reaction). By the reaction, the concentration of CO contained in the mixed gas can be reduced to about 10 ppm or less at which an adverse effect is not exerted on the power generation efficiency of the fuel cell.
At the time of execution of the selective oxidation reaction, an oxidation reaction inevitably occurs not only with CO contained in the mixed gas but also hydrogen. When the concentration of CO in the mixed gas to be supplied to a selective oxidation device is high, the amount of oxygen necessary to oxidize CO increases, so that the amount of hydrogen to be oxidized also increases. As a result, the hydrogen generation amount decreases relative to a source gas amount, and the efficiency as a whole decreases. It is therefore understood that, to improve the hydrogen production efficiency, the concentration of CO in the mixed gas needs to be sufficiently reduced in a shift converter on the upstream side.
CO+H2OH2+CO2 (Chemical Formula 1)
The CO shift conversion reaction is an equilibrium reaction as represented by Chemical Formula 1, and the reaction to the right-hand side is an exothermic reaction. The sign “” indicates that the reaction is in chemical equilibrium.
In the case where the reaction temperature is low, the composition is moved to the right-hand side (product side) of Formula. Therefore, from the viewpoint of lowering the concentration of CO in the mixed gas, the low reaction temperature is advantageous, but has another problem of a decrease in reaction rate.
When the conversion of CO (the reaction to the right-hand side as represented by Chemical Formula 1) progresses to a certain degree, the progress of the shift conversion reaction is inhibited due to restriction on chemical equilibrium. Therefore, to sufficiently lower the CO concentration, a large amount of shift conversion catalyst is required. However, a long time is needed for heating such a large amount of shift conversion catalyst. The above problems are disincentive to the reduction in shift converter size and the demand for saving start-up time, and are problematic, in particular, in a reforming system for a hydrogen station, a fuel cell system for household, and the like.
Methods for sufficiently lowering the concentration of CO in a mixed gas by the CO shift conversion reaction have been studied and developed so far.
Patent Document 1 discloses a configuration of performing the CO shift conversion reaction in two or more stages. The technique uses the fact that the CO shift conversion reaction is an exothermic reaction and, as described above, when the reaction temperature is low, the composition is moved to the right-hand side (product side) of Chemical Formula 1. Specifically, a reaction in the first stage is performed on the higher temperature side, and a reaction is performed in the low temperature range which is advantageous for equilibrium in the second stage.
As the shift conversion catalysts to be used, an iron-chromium-based catalyst or the like, which functions at 300° C. or higher, is used in the shift converter on the high-temperature side, and a copper-zinc-based catalyst, a copper-chromium-based catalyst or the like, which functions at 150° C. to 300° C., is used in the shift converter on the low-temperature side. The copper-based shift conversion catalyst, in particular, the copper-zinc-based catalyst is more advantageous than the catalyst for higher temperatures in that the shift conversion reaction is possible at a low temperature of 150° C. to 300° C., and in terms of CO conversion rate, and advantageous in cost in that expensive materials such as noble metals are not used, and thus used widely in not only fuel cells but also hydrogen production processes.
The active species of the copper-based shift conversion catalyst is a reduced metal copper, which contains approximately 30 to 45% of copper oxide in the shipment of the catalyst, and therefore the catalyst is needed to be reduced with a reducing gas such as hydrogen for activation before use. In Patent Documents 2 and 3 below, it has been proposed that the reduction treatment is carried out in a short period of time with the use of a highly heat-resistance noble metal catalyst.
Patent Document 1: JP 2004-075474 A
Patent Document 2: JP 2000-178007 A
Patent Document 3: JP 2003-144925 A
As described above, while there are various compositions as the shift conversion catalyst, there has been a need to use a large amount of catalyst which is highly active at low temperatures that is advantageous in terms of CO conversion rate, in order to sufficiently lower the CO concentration to 1% or less. Conventionally, the inhibition of the reaction by restriction on chemical equilibrium with the progress of the CO shift conversion reaction has been considered as a main factor.
The present invention has been achieved in view of the problems with the shift conversion catalyst described above, and an object of the invention is to provide an apparatus and a method for CO shift conversion, which improves the conversion rate of CO without increasing usage of a shift conversion catalyst.
To achieve the object, the present invention provides a CO shift conversion device in which CO and H2O contained in a gas to be processed are reacted and thereby converted into CO2 and H2, the device including:
a CO shift conversion unit having a catalyst layer composed of a CO shift conversion catalyst and performing a CO shift conversion process on a gas flowing inside; and
a CO2 removing unit removing CO2 contained in a gas introduced and transmitting a processed gas whose CO2 concentration is lower than that of the introduced gas to a downstream side, wherein
the catalyst layer is composed of a CO shift conversion catalyst having a property that a CO conversion rate decreases with an increase of the CO2 concentration contained in the gas flowing inside, and
the device is configured so that the gas to be processed is supplied to the CO shift conversion unit after the concentration of CO2 contained in the gas to be processed is lowered by the CO2 removing unit.
In addition, the CO shift conversion device according to the present invention has the CO shift conversion unit provided in a plurality of stages, and is configured so that
the gas to be processed is subjected to the CO shift conversion process in the CO shift conversion unit on an upstream side, and subsequently introduced to the CO2 removing unit where the concentration of contained CO2 is lowered, and subsequently supplied to the CO shift conversion unit on the downstream side.
The catalyst layer may contain a copper-zinc-based catalyst or a platinum-based catalyst. This configuration is similarly applied to the following methods.
To achieve the object, the present invention provides a CO shift conversion method in which CO and H2O contained in a gas to be processed are reacted and thereby converted into CO2 and H2, the method including the steps of: lowering a concentration of CO2 contained in the gas to be processed; and subsequently performing a CO shift conversion process on the gas by allowing the gas pass through a catalyst layer composed of a CO shift conversion catalyst, wherein the catalyst layer has a property that a CO conversion rate decreases with an increase of the concentration of CO2 contained in a gas flowing inside.
In addition, the CO shift conversion method according to the present invention has the catalyst layer divided in a plurality of stages, wherein
in arbitrary catalyst layers in two successive stages, the method comprises the steps of:
performing a CO shift conversion process on the gas to be processed by allowing the gas pass through the catalyst layer on an upstream side;
subsequently lowering the concentration of contained CO2; and
subsequently performing a CO shift conversion process on the gas to be processed by allowing the gas pass through the catalyst layer on the downstream side.
By earnest studies, the inventors of the present invention have found that a CO shift conversion catalyst is poisoned by CO2 contained in a mixed gas as a gas to be processed, which deteriorates the efficiency of the CO shift conversion reaction. On the basis of the study results, the inventors propose a method of preliminarily lowering the concentration of CO2 contained by removing CO2 contained in the gas to be processed and, after that, performing a CO shift conversion process using the CO shift conversion catalyst. According to the present invention, as compared with the conventional methods of performing the CO shift conversion process without lowering the CO2 concentration, the influence of CO2 poisoning on the CO shift conversion catalyst is suppressed and, as a result, the CO conversion rate can be largely improved.
In the CO shift conversion reaction, CO2 is inevitably generated. Consequently, if contained CO2 is removed to lower its concentration after the CO shift conversion process on a gas to be processed is performed once and then the CO shift conversion process is performed again, the concentration of the contained CO can be reduced considerably as compared with that in the conventional method.
Therefore, according to the present invention, without introducing a large amount of CO shift conversion catalyst, the CO conversion rate can be largely improved. Thus, for example, with the CO shift conversion process on a reformed gas by using the method of the present invention, a hydrogen gas suitable as a fuel for a fuel cell, in which the concentration of CO contained is conspicuously lowered, can be produced.
As described above in BACKGROUND ART, to decrease the CO concentration in a reformed gas in order to obtain hydrogen gas as a fuel for a fuel cell, conventionally, the reformed gas as the gas G0 to be processed is supplied to the CO shift converter 10, and the processed gas G1 whose concentration of contained CO is decreased to thousands ppm to about 1% is taken from the outlet 9 of the reaction tube 3. Subsequently, the gas G1 is supplied to a selective oxidation device (not illustrated) to be subjected to a selective oxidation reaction. The gas taken from the selection oxidation device has extremely low concentration of CO contained (about 10 ppm or less), so that it can be used as a fuel gas for a fuel cell.
As described above, to improve the hydrogen production efficiency, it is requested to sufficiently reduce the concentration of CO contained in the gas in the upstream of the selective oxidation device, that is, in the CO shift converter 10.
One of methods for sufficiently decreasing the concentration of CO contained in the gas in the CO shift converter 10 is a method of simply increasing the amount of a shift conversion catalyst composing the catalyst layer 5. In this case, the size of the reaction tube 3 itself becomes large.
By earnest studies, the inventors of the present invention have found that CO2 contained in the mixed gas decreases the efficiency of the shift conversion reaction. The inventors also have found that since the degree of decrease of the efficiency varies when the kinds of shift conversion catalysts used as the catalyst layer 5 are changed, the shift conversion catalysts are poisoned by CO2 and, as a result, the efficiency of the shift conversion reaction decreases. In the following, the details will be described with reference to experiment results.
To the inlet of the steam generator 23, purified water is injected from a water tank 27 via a water supply pipe 25. At some midpoints in the pipe 25, a pump, a check valve, a resistor, and the like are provided as necessary.
The purified water injected to the steam generator 23 is vaporized at a temperature of about 200° C., thereby becoming water vapor (H2O gas). Therefore, by passing the Hz gas from the pipe 11, the CO2 gas from the pipe 13, and CO gas from the pipe 15, a mixed gas of H2, CO, CO2, and H2O is generated in the steam generator 23, and the mixed gas is led to the reaction tube 3. The mixture gas is a gas to be subjected to shift conversion process and corresponds to the gas G0 to be processed illustrated in
At the time of causing a shift conversion reaction by using the experiment device 20, first, only the water vapor (H2O) is introduced from the steam generator 23 into the reaction tube 3. After the water vapor sufficiently reaches the catalyst layer 5, supply of the mixture gas of H2, CO, and CO2 is started.
During the gas G0 to be processed passing through the catalyst layer 5, a shift conversion reaction occurs, and the gas G0 to be processed is converted to the processed gas G1. When the processed gas G1 flows out from the outlet of the reaction tube 3 via an exhaust pipe 35, the processed gas G1 passes through a drain tank (cooler) 37 in which purified water is contained, and is cooled to remove moisture. A processed gas G1′ from which the moisture is removed is supplied to a gas chromatography analysis device 41 via an exhaust pipe 39. At some midpoints in the pipe 39, a pressure gauge, a back pressure valve, a three-way electromagnetic valve, and the like are provided as necessary (not illustrated).
The reaction tube 3 is housed in an annular-shaped electric furnace 31 and each of an inlet and an outlet is covered with a mantle heater 29. The catalyst layer 5 is provided in the central part in the reaction tube 3, and front and rear sides of the catalyst layer 5 are filled with glass wool so that the catalyst layer 5 is fixed and is not be moved. In the reaction tube 3, a sheath pipe is inserted from the outlet to a position close to the outlet-side end of the catalyst layer 5, and a thermocouple is inserted in the sheath pipe (not illustrated). With such a configuration, the reaction temperature in the reaction tube 3 is measured by the thermocouple, and the heating state of the electric furnace 31 and the mantle heater 29 is adjusted based on the measured temperature, so that the reaction temperature in the reaction tube 3 can be controlled to a predetermined range.
In the experiment device 20, the tube body part, plugs of the inlet and outlet, a reducer part, and the like of the reaction tube 3 are made of a metal such as stainless steel. The structure, size, material, and the like of the reaction tube 3 may be appropriately determined depending on the treatment amount of the CO shift conversion reaction and the like.
Next, the gas composition of the gas G0 to be processed used for experiments will be described. In the experiment, ten kinds of gases G0 to be processed #1 to #10 shown in the gas composition table of
The ten kinds of the gases G0 to be processed are classified to groups A to E having certain common rules on the composition ratios. In the following experiment, comparison and examination are carried out on the basis of data obtained by using the gases to be processed belonging to the same group.
Gases #1 and #2 belong to group A.
Gases #3 and #4 belong to group B.
Gases #5 and #6 belong to group C.
Gases #1, #7, and #8 belong to group D.
Gases #5, #9, and #10 belong to group E.
The mixing ratio of CO, CO2, H2O, and H2 of the gas #1 is 10:5:30:55. The gas #2 has a composition obtained by replacing CO2 of the gas #1 with N2 without changing the mixing ratio and the mixing ratio of CO, N2, H2O, and H2 of the gas #2 is 10:5:30:55.
The mixing ratio of CO, CO2, H2O, and H2 of the gas #3 is 4:14:23:59. The gas #4 has a composition obtained by replacing CO2 of the gas #3 with N2 without changing the mixing ratio and the mixing ratio of CO, N2, H2O, and H2 of the gas #4 is 4:14:23:59.
The mixing ratio of CO, CO2, H2O, and H2 of the gas #5 is 1:14:21:64. The gas #6 has a composition obtained by replacing CO2 of the gas #5 with N2 without changing the mixing ratio and the mixing ratio of CO, N2, H2O, and H2 of the gas #6 is 1:14:21:64.
By comparing results of experiments performed by using the gases #1 and #2 belonging to the group A, examination regarding the influence on a shift conversion reaction given by the presence/absence of CO2 in the gas G0 to be processed can be performed. Further, with comparison between the gases #3 and #4 belonging to the group B and comparison between the gases #5 and #6 belonging to the group C, more rigorous examination can be performed.
The effect of preparing the gas obtained by replacing CO2 with N2 of the same volume ratio, not simply removing CO2 from the gas G0 to be processed in each of the groups A, B, and C, is to eliminate the influence on the shift conversion reaction of the change in the ratio of the other gases (CO, H2O, and H2) in the gas G0 to be processed. As a gas for comparison, N2 which is a stable gas and can be obtained at a low cost was used.
The mixing ratio of CO, CO2, H2O, and H2 of the gas #7 is 4:5:25:66. The mixing ratio of CO, CO2, H2O, and H2 of the gas #8 is 2:5:25:68. Those gases correspond to gases each obtained by varying the concentration of CO from the gas #1 while keeping the concentration of CO2 to the same as the gas #1 (5%).
That is, by comparing results of the experiments performed by using the gases #1, #7, and #8 belonging to the group D, examination regarding the influence on a shift conversion reaction given by the concentration of CO existing in the gas G0 to be processed can be performed.
The mixing ratio of CO, CO2, H2O, and H2 of the gas #9 is 1:5:24:70. The mixing ratio of CO, CO2, H2O, and H2 of the gas #10 is 1:1:24:74. Those gases correspond to gases each obtained by varying the concentration of CO2 from the gas #5 while keeping the concentration of CO to the same as the gas #5 (1%).
That is, by comparing results of experiments performed using the gases #5, #9, and #10 belonging to the group E, examination regarding the influence on a shift conversion reaction given by the concentration of CO2 existing in the gas G0 to be processed can be performed.
In the experiment, by changing the two kinds of catalysts used for the catalyst layer 5 for the ten kinds of the gases G0 to be processed (#1 to #10), the characteristics of the CO conversion rates in respective states were examined. As CO shift conversion catalysts, two kinds of catalysts were used for the examination; a commercially-available copper-zinc-based catalyst (Cu/Zn catalyst) which is prepared by a general preparation method (coprecipitation method) and whose composition is made of copper oxide, zinc oxide, and alumina (carrier), and a Pt/CeO2 catalyst (platinum-based catalyst) obtained by preparing a nitric acid solution having a predetermined concentration of dinitrodianmine platinum crystal (Pt(NO2)2(NH3)2), carrying it on cerium oxide (CeO2), drying the resultant, and reducing it in hydrogen stream. The two catalysts each having a granular shape with 0.85 to 1 mm in a grain diameter and subjected to an H2 reducing process for one hour at 200° C. were used.
Similarly,
It is understood from
As illustrated in
To examine the effect of fixing the CO2 concentration,
It is determined that the difference between the data indicated by the graph of
In the case of using the Pt/CeO2 catalyst, as illustrated in
That is, it is determined that, in the case of using the Pt/CeO2 catalyst, although the CO conversion rate of the gas #3 is lower because the concentration of contained CO2 is higher than that of the gas #1, the degree of decrease is below the increase amount of the CO conversion rate because of the low concentration of CO contained. That is, it is determined that the influence of the low CO concentration on the CO conversion rate is strong and, as a result, like the case of
That is, the graphs of
As illustrated in
In the graphs of
In the case of the Cu/Zn catalyst illustrated in
It is understood from
Therefore,
It is understood from the graphs of the above-described drawings that the higher the concentration of CO2 contained in the gas G0 to be processed is, the more the influence that the CO conversion rate decreases occurs. It suggests that the catalyst used for the catalyst layer 5 is poisoned by CO2 in the gas to be processed and, as a result, the CO conversion rate decreases. In the case of setting the concentration of CO2 contained in the gas G0 to be processed to the same, the CO conversion rate of the Cu/Zn catalyst decreases more than that of the Pt/CeO2 catalyst. It is consequently understood that there is also a difference in the magnitude of the influence of poisoning by CO2 in accordance with the kinds of the catalysts.
From the above-described experiment results, it is understood that by decreasing the concentration of the CO2 gas contained in the gas G0 to be processed as a shift conversion target, the CO conversion rate can be improved, and a hydrogen gas having low concentration of contained CO can be generated.
From the inlet 7 of the CO shift converter 10, the gas G0 to be processed as a shift conversion target is supplied. As described above, when it is assumed to use the present invention at the time of generating hydrogen gas as a fuel for a fuel cell from a reformed gas, the gas G0 to be processed corresponds to the reformed gas and usually contains CO, CO2, H2, and H2O.
The gas G0 to be processed causes a shift conversion reaction represented by Chemical Formula 1 while it passes through the catalyst layer 5. In a gas Ga which completely passed through the catalyst layer 5, the contained CO concentration decreases and the CO2 concentration increases as compared with G0. The gas Ga in which the CO2 concentration increases is introduced to the CO2 remover 51 via a pipe.
The CO2 remover 51 can be realized by using the existing CO2 separating technique. For example, a chemical absorption method of using an alkaline solution such as amine as an absorbing solution and removing CO2 by chemical reaction and a physical absorption method of physically absorbing carbon dioxide at high pressures and low temperatures using an absorbing solution such as methanol, polyethylene glycol, or the like can be used.
In the CO shift conversion device 50, it is also preferable to use a membrane absorption method as a technique of separating CO2 from a mixed gas by using the difference in permeation speeds of gases by a membrane as the CO2 remover 51. The applicants of the present invention also developed a membrane technique of selectively passing CO2 from a mixed gas containing H2 (refer to, for example, JP 2008-036463 A and WO 2009/093666).
Each of the membranes disclosed in the documents has high CO2/H2 selectivity under conditions of high temperature of 100° C. or higher and high pressure of about 100 to 500 kPa. Therefore, by using the membrane as the CO2 remover 51 and supplying the mixed gas Ga obtained from the CO shift converter 10 to the membrane, the concentration of CO2 contained in mixed gas Gb obtained from the CO2 remover 51 can be largely decreased.
In the case of using the membrane absorption method, obviously, the membrane used as the CO2 remover 51 is not limited to the membranes disclosed in the documents. Another membrane can be also used if it can realize high CO2/H2 selectivity under mounting conditions. The applicants of the present invention are developing other membranes of different materials and different structures, and some of the membranes have been already developed.
A gas Gb released from the CO2 remover 51 is transmitted into the CO shift converter 10a on the downstream side via a pipe. The CO shift converter 10a causes a shift conversion reaction using the gas Gb as a gas to be processed. Specifically, in a manner similar to the case of the gas G0 to be processed, the shift conversion reaction represented by Chemical Formula 1 occurs while the gas Gb to be processed passes through the catalyst layer 5a. The concentration of CO contained in a gas G1 which completely passed through the catalyst layer 5a and released from an outlet 9a further decreases as compared with that in the gas Gb.
As described above, the CO shift conversion catalysts used for the catalyst layers 5 and 5a are poisoned by CO2 in the passing gas. Since the CO2 concentration in the gas rises toward the downstream side by the shift conversion reaction, the CO conversion rate decreases while the gas passes through the same catalyst layer. Specifically, in the CO shift converter 10, the CO conversion rate decreases toward the downstream (the outlet 9 side).
In the CO shift conversion device 50, after the contained CO2 is removed by the CO2 remover 51 to decrease the contained CO2 concentration, the gas to be processed is introduced into the CO shift converter 10a. Consequently, when the gas passes through the catalyst layer 5a in a position close to the inlet 7a of the CO shift converter 10a on the downstream side, the poisoning action is considerably lowered as compared with the case that the gas passes through the catalyst layer 5 in a position close to the outlet 9 of the CO shift converter 10 on the upstream side, and thus the CO conversion rate improves. Therefore, also in the CO shift converter 10a on the downstream side, the contained CO concentration can be lowered. As a result, the concentration of CO contained in the processed gas G1 obtained by the CO shift conversion device 50 can be made conspicuously lower than that of CO contained in the gas Ga.
Although the CO shift conversion device 50 illustrated in
The effects of the present invention can be realized also by a configuration in which a CO shift converter has a one-stage configuration and a CO2 remover is provided on the upstream of the CO shift converter (
Obviously, also in the configurations of
With the configuration as described above, the CO conversion rate can be further improved than the general shift converter illustrated in
Hereinafter, other embodiments will be described.
<1> In the case of the configuration of providing CO shift converters in a plurality of stages, the CO shift conversion catalysts used for catalyst layers of the shift converters may be made of the same material or different materials. Although the Cu/Zn catalyst and the Pt/CeO2 catalyst are described above as examples, obviously, catalysts made of materials other than those materials can be also used.
It is beneficial to employ a configuration that the catalyst material of a catalyst layer near the inlet of a CO shift converter and that of a catalyst layer near the outlet of the CO shift converter are different. It is understood from the above-described experiment results that, in the case of comparing the Cu/Zn catalyst and the Pt/CeO2 catalyst, the Cu/Zn catalyst is more sensitive to a change in the CO2 concentration, that is, has a larger CO2 poisoning action. In the case of preparing two kinds of materials having the difference in CO2 poisoning actions, the CO conversion rate in the shift converter can be also improved by the use of a material having a larger CO2 poisoning action in a part near the inlet and the use of a material having a smaller CO2 poisoning action in a part near the outlet as catalyst layers in the same shift converter.
<2> Although the CO shift device in which processors (CO shift converter and CO2 remover) are connected via a pipe is assumed in the configurations illustrated in
<3> Although the gas to be processed which is introduced to the inlet of the CO shift conversion device is a reformed gas in the above description, obviously, the invention is not limited to the reformed gas as long as the gas is a mixed gas containing CO2 and CO.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-282112 | Dec 2011 | JP | national |
This application is a continuation of U.S. patent application Ser. No. 14/366,661, filed 18 Jun. 2014.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14366661 | Jun 2014 | US |
| Child | 15356298 | US |
