CO SHIFT CONVERSION DEVICE AND SHIFT CONVERSION METHOD

Abstract

The present invention provides a CO shift conversion device and a CO shift conversion method which improves CO conversion rate without increasing usage of a shift conversion catalyst. A CO shift conversion device includes: a CO shift converter 10 having a catalyst layer 5 composed of a CO shift conversion catalyst and performing CO shift conversion process on a gas flowing inside; and a CO.sub.2 remover 51 removing CO.sub.2 contained in a gas introduced. The catalyst layer 5 is composed of a CO shift conversion catalyst having a property that a CO conversion rate decreases with an increase of the concentration of CO.sub.2 contained in a gas flowing inside. The concentration of CO.sub.2 contained in a gas G0 to be processed is lowered by the CO.sub.2 remover 51 and, after that, the resultant gas is supplied to the CO shift converter 10 where it is subjected to the CO shift conversion process.

Claims

1. A CO shift conversion method in which CO and H.sub.2O contained in a gas to be processed are reacted and thereby converted into CO.sub.2 and H.sub.2, the method comprising the steps of: lowering a concentration of CO.sub.2 contained in the gas to be processed to 5% or less in volume ratio; and subsequently performing a CO shift conversion process on the gas by allowing the gas to 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 CO.sub.2 contained in the gas flowing inside the catalyst layer due to a CO.sub.2 poisoning action.

2. The CO shift conversion method according to claim 1, wherein the concentration of CO contained in the gas to be processed is 2% or less in volume ratio.

3. The CO shift conversion method according to claim 1, wherein the CO shift conversion catalyst composing the catalyst layer includes a copper-zinc-based catalyst.

4. The CO shift conversion method according to claim 1, wherein the step of lowering a concentration of CO.sub.2 contained in the gas to be processed to 5% or less in volume ratio is performed by using a membrane which selectively passes CO.sub.2.

5. A CO shift conversion method in which CO and H.sub.2O contained in a gas to be processed are reacted and thereby converted into CO.sub.2 and H.sub.2 by allowing the gas to pass through a catalyst layer composed of a CO shift conversion catalyst and divided into a plurality of stages, the method comprising the steps of: performing a CO shift conversion process on the gas to be processed by allowing the gas to pass through an upstream stage of the catalyst layer; subsequently lowering the concentration of CO.sub.2 contained in the gas to be processed to 5% or less in volume ratio; and subsequently performing a CO shift conversion process on the gas to be processed by allowing the gas to pass through an downstream stage of the catalyst layer, wherein the catalyst layer has a property that a CO conversion rate decreases with an increase of the concentration of CO.sub.2 contained in the gas flowing inside the catalyst layer due to a CO.sub.2 poisoning action.

6. The CO shift conversion method according to claim 5, wherein the concentration of CO contained in the gas to be processed is 2% or less in volume ratio.

7. The CO shift conversion method according to claim 5, wherein the CO shift conversion catalyst composing the catalyst layer includes a copper-zinc-based catalyst.

8. The CO shift conversion method according to claim 5, wherein the step of lowering a concentration of CO.sub.2 contained in the gas to be processed to 5% or less in volume ratio is performed by using a membrane which selectively passes CO.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 is a conceptual diagram schematically illustrating the configuration of a shift conversion device.

[0038] FIG. 2 is a conceptual diagram illustrating the configuration of an experiment device for the present invention.

[0039] FIG. 3 is a diagram illustrating a list of compositions of gases to be processed for use in the experiment device of FIG. 2.

[0040] FIGS. 4A and 4B are graphs illustrating comparison of CO conversion rates of gas #1 and gas #2.

[0041] FIGS. 5A and 5B are graphs illustrating comparison of CO conversion rates of gas #3 and gas #4.

[0042] FIGS. 6A and 6B are graphs illustrating comparison of CO conversion rates of gas #5 and gas #6.

[0043] FIGS. 7A and 7B are graphs illustrating comparison of CO conversion rates of gases #1, #7, and #8.

[0044] FIGS. 8A and 8B are graphs illustrating comparison of CO conversion rates of gas #1 and gas #3.

[0045] FIGS. 9A and 9B are graphs illustrating comparison of CO conversion rates of gases #5, #9, and #10.

[0046] FIG. 10 is a conceptual diagram of a CO shift conversion device of the present invention.

[0047] FIG. 11 is a conceptual diagram illustrating another configuration of the CO shift conversion device of the present invention.

[0048] FIG. 12 is a conceptual diagram illustrating another configuration of the CO shift conversion device of the present invention.

DESCRIPTION OF EMBODIMENTS

[0049] FIG. 1 schematically illustrates the configuration of a CO shift converter. A CO shift converter 10 has a catalyst layer 5 charged with a predetermined CO shift conversion catalyst in a cylindrical reaction tube 3. When a gas (gas to be processed) G0 as an object to be subjected to a shift conversion process is supplied from an inlet 7 of the reaction tube 3 to the shift converter 10, the gas G0 is led into the catalyst layer 5 and a shift conversion reaction occurs while the gas G0 passes through the catalyst layer 5. A gas (processed gas) G1 after the shift conversion reaction is taken from an outlet 9 of the reaction tube 3.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] By earnest studies, the inventors of the present invention have found that CO.sub.2 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 CO.sub.2 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.

[0054] FIG. 2 schematically illustrates the configuration of an experiment device used for experiments by the inventors of the present invention. An experiment device 20 has gas supply pipes 11, 13, and 15. Gases flowing in from the pipes are mixed in a mixing pipe 21 and, after that, supplied to the inlet of a steam generator 23. At some midpoints in each of the pipes 11, 13, and 15, a stop valve, a pressure reducing valve, an electromagnetic valve, a mass flow controller, a check valve, a pressure gauge, and the like which are communicated with a gas source are provided as necessary (not illustrated).

[0055] 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.

[0056] The purified water injected to the steam generator 23 is vaporized at a temperature of about 200 C., thereby becoming water vapor (H.sub.2O gas). Therefore, by passing the Hz gas from the pipe 11, the CO.sub.2 gas from the pipe 13, and CO gas from the pipe 15, a mixed gas of H.sub.2, CO, CO.sub.2, and H.sub.2O 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 FIG. 1.

[0057] At the time of causing a shift conversion reaction by using the experiment device 20, first, only the water vapor (H.sub.2O) 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 H.sub.2, CO, and CO.sub.2 is started.

[0058] 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).

[0059] 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.

[0060] 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.

[0061] 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 FIG. 3 were prepared and properly used according to experiments. The mixture ratio of the component gases of each of the ten kinds of the gases G0 to be processed is adjusted by controlling the supply amount of each of the component gases from the pipes 11, 13, and 15 and the supply amount of the purified water (H.sub.2O) to the steam generator 23.

[0062] 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.

[0063] Gases #1 and #2 belong to group A.

[0064] Gases #3 and #4 belong to group B.

[0065] Gases #5 and #6 belong to group C.

[0066] Gases #1, #7, and #8 belong to group D.

[0067] Gases #5, #9, and #10 belong to group E.

[0068] The mixing ratio of CO, CO.sub.2, H.sub.2O, and H.sub.2 of the gas #1 is 10:5:30:55. The gas #2 has a composition obtained by replacing CO.sub.2 of the gas #1 with N.sub.2 without changing the mixing ratio and the mixing ratio of CO, N.sub.2, H.sub.2O, and H.sub.2 of the gas #2 is 10:5:30:55.

[0069] The mixing ratio of CO, CO.sub.2, H.sub.2O, and H.sub.2 of the gas #3 is 4:14:23:59. The gas #4 has a composition obtained by replacing CO.sub.2 of the gas #3 with N.sub.2 without changing the mixing ratio and the mixing ratio of CO, N.sub.2, H.sub.2O, and H.sub.2 of the gas #4 is 4:14:23:59.

[0070] The mixing ratio of CO, CO.sub.2, H.sub.2O, and H.sub.2 of the gas #5 is 1:14:21:64. The gas #6 has a composition obtained by replacing CO.sub.2 of the gas #5 with N.sub.2 without changing the mixing ratio and the mixing ratio of CO, N.sub.2, H.sub.2O, and H.sub.2 of the gas #6 is 1:14:21:64.

[0071] 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 CO.sub.2 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.

[0072] The effect of preparing the gas obtained by replacing CO.sub.2 with N.sub.2 of the same volume ratio, not simply removing CO.sub.2 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, H.sub.2O, and H.sub.2) in the gas G0 to be processed. As a gas for comparison, N.sub.2 which is a stable gas and can be obtained at a low cost was used.

[0073] The mixing ratio of CO, CO.sub.2, H.sub.2O, and H.sub.2 of the gas #7 is 4:5:25:66. The mixing ratio of CO, CO.sub.2, H.sub.2O, and H.sub.2 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 CO.sub.2 to the same as the gas #1 (5%).

[0074] 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.

[0075] The mixing ratio of CO, CO.sub.2, H.sub.2O, and H.sub.2 of the gas #9 is 1:5:24:70. The mixing ratio of CO, CO.sub.2, H.sub.2O, and H.sub.2 of the gas #10 is 1:1:24:74. Those gases correspond to gases each obtained by varying the concentration of CO.sub.2 from the gas #5 while keeping the concentration of CO to the same as the gas #5 (1%).

[0076] 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 CO.sub.2 existing in the gas G0 to be processed can be performed.

[0077] 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/CeO.sub.2 catalyst (platinum-based catalyst) obtained by preparing a nitric acid solution having a predetermined concentration of dinitrodianmine platinum crystal (Pt(NO.sub.2).sub.2(NH.sub.3).sub.2), carrying it on cerium oxide (CeO.sub.2), 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 H.sub.2 reducing process for one hour at 200 C. were used. FIGS. 4A and 4B to FIGS. 9A and 9B illustrate results of the experiment.

[0078] FIGS. 4A and 4B are graphs illustrating, by the catalysts used for the catalyst layer 5, the relationship between the temperature (reaction temperature) in the reaction tube 3 and the ratio of CO converted (CO conversion rate) in the case of using the gases #1 and #2 in the group A as the gases G0 to be processed. FIG. 4A is a graph illustrating the case where the Cu/Zn catalyst is used as the catalyst layer 5, and FIG. 4B is a graph illustrating the case where the Pt/CeO.sub.2 catalyst is used as the catalyst layer 5.

[0079] Similarly, FIGS. 5A and 5B are graphs illustrating, by the catalysts, the relationship between the reaction temperature and the CO conversion rate in the case of using the gases #3 and #4 in the group B as the gases G0 to be processed. FIGS. 6A and 6B are graphs illustrating, by the catalysts, the relation of the reaction temperature and the CO conversion rate in the case of using the gases #5 and #6 in the group C as the gases G0 to be processed.

[0080] It is understood from FIGS. 4A and 4B to FIGS. 6A and 6B that the CO conversion rate of the gas (#2, #4, and #6) obtained by replacing CO.sub.2 with N.sub.2 in each of the groups is higher. It is also understood that the difference of the CO conversion rate appears conspicuously when the Cu/Zn catalyst is used as compared with the case of using the Pt/CeO.sub.2 catalyst.

[0081] FIGS. 7A and 7B are graphs illustrating, by the catalysts used for the catalyst layer 5, the relationship between the temperature (reaction temperature) in the reaction tube 3 and the ratio of CO converted (CO conversion rate), in the case of using the gases #1, #7, and #8 in the group D as the gases G0 to be processed. Like FIGS. 4A and 4B to FIGS. 6A and 6B, FIG. 7A is a graph illustrating the case where the Cu/Zn catalyst is used as the catalyst layer 5, and FIG. 7B is a graph illustrating the case where the Pt/CeO.sub.2 catalyst is used as the catalyst layer 5.

[0082] As illustrated in FIG. 3, in the group D, the concentration of CO.sub.2 is fixed and the CO concentration is varied to 10% (gas #1), 4% (gas #7), and 2% (gas #8). In the case of using the Cu/Zn catalyst as illustrated in FIG. 7A, the tendency that the decrease of the CO conversion rate appears conspicuously as the CO concentration becomes high. Also in the case of using the Pt/CeO.sub.2 catalyst as illustrated in FIG. 7B, the CO conversion rate in the case of using the gas #1 whose CO concentration is 10% is largely lower than that in the case of using the gas #7 whose CO concentration is 4% and that in the case of using the gas #8 whose CO concentration is 2%.

[0083] To examine the effect of fixing the CO.sub.2 concentration, FIGS. 8A and 8B illustrate graphs comparing CO conversion rates in the cases of using the gases #1 and #3 having different CO.sub.2 concentration and different CO concentration. The gas #3 has lower CO concentration and higher CO.sub.2 concentration as compared with the gas #1. FIG. 8A illustrates that, in the case of using the Cu/Zn catalyst, the CO conversion rate of the gas #1 having higher CO concentration is higher than that of the gas #3 having lower CO concentration, which is different from the graph of FIG. 7A.

[0084] It is determined that the difference between the data indicated by the graph of FIG. 7A and that indicated by the graph of FIG. 8A comes from the point whether the CO.sub.2 concentration is fixed or not. Although the CO concentration of the gas #3 is lower than that of the gas #1, the CO.sub.2 concentration of the gas #3 is higher than that of the gas #1. It is, therefore determined that, in the case of using the gas #3, since the concentration of CO.sub.2 contained is higher as compared with the case of using the gas #1, the CO conversion rate decreases, the degree of decrease is higher than the increase amount of the CO conversion rate because of the low concentration of CO contained and, as a result, the CO conversion rate decreases.

[0085] In the case of using the Pt/CeO.sub.2 catalyst, as illustrated in FIG. 8B, the CO conversion rate of the gas #1 still having higher CO concentration is lower than that of the gas #3 having lower CO concentration also in the case where the CO.sub.2 concentration is varied.

[0086] That is, it is determined that, in the case of using the Pt/CeO.sub.2 catalyst, although the CO conversion rate of the gas #3 is lower because the concentration of contained CO.sub.2 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 FIG. 7B in which the CO.sub.2 concentration is fixed, the CO conversion rate of the gas #3 whose contained CO concentration is lower is higher than that of the gas #1.

[0087] That is, the graphs of FIGS. 7A and 7B and FIGS. 8A and 8B suggest that the Cu/Zn catalyst is more sensitive to a change in the CO.sub.2 concentration than the Pt/CeO.sub.2 catalyst. When the example is regarded that the presence/absence of CO.sub.2 causes conspicuous change in the concentration of contained CO.sub.2, the above description matches the description made with reference to the graphs of FIGS. 4A and 4B to FIGS. 6A and 6B.

[0088] FIGS. 9A and 9B are graphs illustrating, by the catalysts used for the catalyst layer 5, the relationship between the temperature (reaction temperature) in the reaction tube 3 and the ratio of CO converted (CO conversion rate), in the case of using the gases #5, #9, and #10 in the group E as the gases G0 to be processed. The method of forming the graphs is similar to that of FIGS. 4A and 4B to FIGS. 8A and 8B.

[0089] As illustrated in FIG. 3, in the group E, the concentration of CO is fixed and the CO.sub.2 concentration is varied to 14% (gas #5), 5% (gas #9), and 1% (gas #10). It is understood from both FIGS. 9A and 9B that the tendency that the decrease of the CO conversion rate appears conspicuously as the CO.sub.2 concentration becomes high. More specifically, the behavior of the change in FIG. 9A is larger than that in FIG. 9B.

[0090] In the graphs of FIGS. 9A and 9B, when the changes in the value of the CO conversion rate under the same reaction temperature is watched in the order of the gas #10, the gas #9, and the gas #5, transition of the changes in the CO conversion rate in the case of changing the concentration of CO.sub.2 contained in the gas to be processed to 1%, 5%, and 14% can be obtained.

[0091] In the case of the Cu/Zn catalyst illustrated in FIG. 9A, only by changing the CO.sub.2 concentration from 1% to 5%, large decrease in the CO conversion rate can be seen. On the other hand, in the case of the Pt/CeO.sub.2 catalyst illustrated in FIG. 9B, when the CO.sub.2 concentration is changed from 1% to 5%, although the CO conversion rate decreases, it is understood that the degree of decrease is very small.

[0092] It is understood from FIGS. 9A and 9b, when the CO.sub.2 concentration is changed from 1% to 14%, the CO conversion rate decreases conspicuously in the case of the Cu/Zn catalyst. Also in the case of the Pt/CeO.sub.2 catalyst, when the CO.sub.2 concentration is changed from 1% to 14%, the CO conversion rate decreases more largely than when the CO.sub.2 concentration is changed from 1% to 5%. However, the degree of the change in the case of the Pt/CeO.sub.2 catalyst is smaller than that in the case of the Cu/Zn catalyst.

[0093] Therefore, FIGS. 9A and 9B also suggest that the Cu/Zn catalyst is more sensitive to a change in the CO.sub.2 concentration than the Pt/CeO.sub.2 catalyst.

[0094] It is understood from the graphs of the above-described drawings that the higher the concentration of CO.sub.2 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 CO.sub.2 in the gas to be processed and, as a result, the CO conversion rate decreases. In the case of setting the concentration of CO.sub.2 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/CeO.sub.2 catalyst. It is consequently understood that there is also a difference in the magnitude of the influence of poisoning by CO.sub.2 in accordance with the kinds of the catalysts.

[0095] From the above-described experiment results, it is understood that by decreasing the concentration of the CO.sub.2 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.

[0096] FIG. 10 illustrates schematic configuration of a CO shift conversion device of the present invention. A CO shift conversion device 50 has CO shift converters (CO shift conversion units) 10 and 10a and a CO.sub.2 remover (CO.sub.2 removing unit) 51.

[0097] 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, CO.sub.2, H.sub.2, and H.sub.2O.

[0098] 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 CO.sub.2 concentration increases as compared with G0. The gas Ga in which the CO.sub.2 concentration increases is introduced to the CO.sub.2 remover 51 via a pipe.

[0099] The CO.sub.2 remover 51 can be realized by using the existing CO.sub.2 separating technique. For example, a chemical absorption method of using an alkaline solution such as amine as an absorbing solution and removing CO.sub.2 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.

[0100] In the CO shift conversion device 50, it is also preferable to use a membrane absorption method as a technique of separating CO.sub.2 from a mixed gas by using the difference in permeation speeds of gases by a membrane as the CO.sub.2 remover 51. The applicants of the present invention also developed a membrane technique of selectively passing CO.sub.2 from a mixed gas containing H.sub.2 (refer to, for example, JP 2008-036463 A and WO 2009/093666).

[0101] Each of the membranes disclosed in the documents has high CO.sub.2/H.sub.2 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 CO.sub.2 remover 51 and supplying the mixed gas Ga obtained from the CO shift converter 10 to the membrane, the concentration of CO.sub.2 contained in mixed gas Gb obtained from the CO.sub.2 remover 51 can be largely decreased.

[0102] In the case of using the membrane absorption method, obviously, the membrane used as the CO.sub.2 remover 51 is not limited to the membranes disclosed in the documents. Another membrane can be also used if it can realize high CO.sub.2/H.sub.2 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.

[0103] A gas Gb released from the CO.sub.2 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.

[0104] As described above, the CO shift conversion catalysts used for the catalyst layers 5 and 5a are poisoned by CO.sub.2 in the passing gas. Since the CO.sub.2 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).

[0105] In the CO shift conversion device 50, after the contained CO.sub.2 is removed by the CO.sub.2 remover 51 to decrease the contained CO.sub.2 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.

[0106] Although the CO shift conversion device 50 illustrated in FIG. 10 has the configuration that the CO shift converters are provided in two stages and the CO.sub.2 remover 51 is provided between them, it is also possible to provide CO shift converters in a plurality of stages which are three or more stages and provide a CO.sub.2 remover between the respective shift converters. FIG. 11 illustrates the case of a three-stage configuration. In a CO shift conversion device 50a illustrated in FIG. 11, 51a indicates a CO.sub.2 remover, 10b indicates a CO shift converter, and 5b indicates a catalyst layer.

[0107] 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 CO.sub.2 remover is provided on the upstream of the CO shift converter (FIG. 12). In the case of assuming a reformed gas as a gas to be subjected to the converting process, since CO.sub.2 is mixed inevitably, the concentration of the contained CO.sub.2 is preliminarily lowered by removing CO.sub.2 in the CO.sub.2 remover 51 before the gas to be processed is introduced into the CO shift converter 10 (gas Gb), which can improve the CO conversion rate as compared with the case of FIG. 1. In FIG. 12, 7b indicates the inlet of the CO.sub.2 remover 51.

[0108] Obviously, also in the configurations of FIGS. 10 and 11, it is also possible to mount a CO.sub.2 remover on the upstream side of introducing the gas G0 to be processed to the CO shift converter 5 to remove CO.sub.2 in advance.

[0109] With the configuration as described above, the CO conversion rate can be further improved than the general shift converter illustrated in FIG. 1.

[0110] Hereinafter, other embodiments will be described.

[0111] <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/CeO.sub.2 catalyst are described above as examples, obviously, catalysts made of materials other than those materials can be also used.

[0112] 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/CeO.sub.2 catalyst, the Cu/Zn catalyst is more sensitive to a change in the CO.sub.2 concentration, that is, has a larger CO.sub.2 poisoning action. In the case of preparing two kinds of materials having the difference in CO.sub.2 poisoning actions, the CO conversion rate in the shift converter can be also improved by the use of a material having a larger CO.sub.2 poisoning action in a part near the inlet and the use of a material having a smaller CO.sub.2 poisoning action in a part near the outlet as catalyst layers in the same shift converter.

[0113] <2> Although the CO shift device in which processors (CO shift converter and CO.sub.2 remover) are connected via a pipe is assumed in the configurations illustrated in FIGS. 10 to 12, an integrated device in which an area for performing CO shift conversion process and an area for performing CO.sub.2 removing process may be continuously configured in series in a single casing may be configured.

[0114] <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 CO.sub.2 and CO.

EXPLANATION OF REFERENCES

[0115] 3 reaction tube [0116] 5, 5a, 5b catalyst layer [0117] 7, 7a, 7b inlet [0118] 9, 9a outlet [0119] 10, 10a, 10b CO shift converter [0120] 11 gas supply pipe [0121] 13 gas supply pipe [0122] 15 gas supply pipe [0123] 20 experiment device [0124] 21 mixing pipe [0125] 23 steam generator [0126] 25 water supply pipe [0127] 27 water tank [0128] 29 mantle heater [0129] 31 electric furnace [0130] 35 exhaust pipe [0131] 37 drain tank (cooler) [0132] 39 exhaust pipe [0133] 41 gas chromatography analysis device [0134] 50, 50a, 50b CO shift conversion device of the present invention [0135] 51, 51a CO.sub.2 remover [0136] G0 gas (gas to be processed) [0137] G1, G1 gases (processed gases)