Carbon dioxide recovery method

Abstract

To provide a carbon dioxide recovery method which is increased in the purity and recovery rate of product CO2 without increasing the consumption of product CO2 or energy consumption.SOLUTION: A carbon dioxide recovery method according to the present invention comprises a primary concentration step of concentrating the carbon dioxide concentration in a carbon dioxide-containing gas to 40-60%, and a secondary concentration step of performing secondary concentration of the primary concentration gas using an adsorption separation apparatus composed of three vessels. The secondary concentration step includes: an adsorption step in which two of the vessels are connected in series to adsorb carbon dioxide; a pressure equalization, concentration, and capture step in which the two vessels are disconnected, and the vessel filled with a breakthrough adsorbent is connected with the third vessel that has undergone desorption and depressurization, thereby performing pressure equalization, concentration of carbon dioxide inside the breakthrough vessel, and capture of the carbon dioxide flowing out from the breakthrough vessel in the third vessel; an adsorption continuation step performed using the vessel other than the two vessels used in the pressure equalization, concentration, and capture step; and a recovery step. The adsorption step, the pressure equalization step, the adsorption continuation step, and the recovery step are repeatedly executed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide recovery method for recovering carbon dioxide (hereinafter sometimes referred to as "CO2") from a raw material gas containing carbon dioxide using a carbon dioxide adsorbent (hereinafter simply referred to as "adsorbent"). [Background technology]

[0002] A well-known method for recovering carbon dioxide using an adsorbent is the pressure swing adsorption (PSA) method, which uses two vessels filled with the adsorbent and alternately repeats adsorption and desorption. When adsorption and desorption are repeated alternately in two containers, in order to increase the CO2 capture rate, it is necessary to stop adsorption before the amount of CO2 that cannot be captured and leaks increases, but in this case, the purity of the captured CO2 will be low.

[0003] On the other hand, if the amount of adsorbed CO2 is increased by extending the adsorption time in order to increase the purity of the captured CO2, the amount of CO2 that cannot be captured and leaks increases, resulting in a decrease in the CO2 capture rate. In this way, in the case of the two-tower (vessel) system, there is a trade-off between increasing the purity of the captured CO2 and increasing the CO2 capture rate.

[0004] Therefore, Patent Document 1 discloses a method for obtaining high-purity CO2 without reducing the CO2 recovery rate, in which product CO2 is passed through a container after adsorption to increase the purity of the adsorbed CO2, and the container with the increased CO2 purity is depressurized to recover high-purity CO2. In this case, since desorption cannot be performed immediately after adsorption, three vessels are used to achieve the three processes of adsorption, purification, and desorption, allowing for continuous processing.

[0005] Furthermore, as another method for obtaining high-purity CO2, for example, Patent Document 2 discloses a method in which two containers filled with packing material are connected in series, adsorption is performed after breakthrough in the first column and before breakthrough in the second column, and then the two columns are separated, and a container that has been pressurized in advance is connected to the second column before breakthrough, and adsorption is performed therein.

[0006] Furthermore, Patent Document 3 discloses a method for recovering carbon dioxide at a high concentration from exhaust gas by performing primary concentration using a membrane separation device and then secondary concentration using an adsorption separation device. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 2681894 [Patent Document 2] Patent No. 2579179 [Patent Document 3] Japanese Patent Application Publication No. 6-99035 Summary of the Invention [Problem to be solved by the invention]

[0008] If it were possible to concentrate CO2 to 99% or more from CO2-containing gases with low concentrations of carbon dioxide, such as combustion exhaust gas, it is expected that the sales channels for the captured CO2 would expand for applications such as the production of dry ice for refrigeration. In particular, if it were possible to satisfy the quality standards for industrial gases stipulated in JIS standards by concentrating it to 99.5% or more, it is expected that the sales channels would expand even further. In this regard, PSA is a device that can concentrate CO2-containing gas containing a moderate concentration of carbon dioxide to a high concentration, but it is not suitable for concentrating CO2-containing gas containing a low concentration of carbon dioxide of around 10%, and there are problems such as limited processing capacity and a failure to increase the concentration of the recovered CO2.

[0009] As in Patent Document 3, by combining a membrane separation device with PSA, it is possible to concentrate CO2-containing gas that contains carbon dioxide at a low concentration. However, Patent Document 3 does not provide a detailed description of the operating conditions of the PSA, and the recovered CO2 concentration is 95.2%, which is not necessarily satisfactory.

[0010] The present invention has been made to solve such problems, and has an object to provide a method for recovering carbon dioxide with high purity and high recovery rate using PSA. [Means for solving the problem]

[0011] (1) The carbon dioxide recovery method according to the present invention includes a primary concentration step of concentrating the carbon dioxide concentration in a carbon dioxide-containing gas to 40 to 60% using a primary concentration device; a secondary concentration step of secondarily concentrating the primary concentrated gas, the carbon dioxide concentration of which has been concentrated in the primary concentration step, using N (N≧3) containers filled with an adsorbent that adsorbs carbon dioxide, using an adsorption / separation device in which the N containers repeatedly perform adsorption, desorption, and pressure equalization, The secondary concentration step includes: an adsorption step of connecting (N-1) containers in series and flowing the raw material gas through the container at the most upstream side until the adsorbent in the container at the most downstream side breaks through and before the adsorbent in the container at the most downstream side breaks through, thereby adsorbing carbon dioxide; After the adsorption step, a pressure equalization, concentration, and capture step is performed in which the connected most upstream container filled with the breakthrough adsorbent is disconnected from the connection, the disconnected container is connected to a container whose pressure has been reduced by desorption for carbon dioxide recovery, and the pressure of the two connected containers is equalized, the carbon dioxide concentration in the container filled with the breakthrough adsorbent is concentrated, and the carbon dioxide flowing out from the container filled with the breakthrough adsorbent is captured in the decompressed container; an adsorption continuing step in which the raw material gas is passed through (N-2) series-connected vessels other than the two vessels in which pressure equalization is performed in the pressure equalization, concentration, and capture step, and adsorption of carbon dioxide is continued; a recovery step of recovering carbon dioxide by desorption of carbon dioxide due to reduced pressure in a container filled with the breakthrough filler after the pressure equalization, concentration, and capture step; an adsorption step in which, after the pressure equalization, concentration, and capture step, the container that captured carbon dioxide is connected to the most downstream side of the container that is continuing adsorption in the adsorption continuation step, and adsorption is performed in a series of containers, The method is characterized in that the adsorption step, the pressure equalization / concentration / capture step, the adsorption continuation step, and the recovery step are repeatedly performed.

[0012] (2) In addition, in the above-mentioned (1), the pressure equalization, concentration, and capture process is characterized in that the outlet of the container filled with the breakthrough adsorbent is connected to the inlet of the container whose pressure has been reduced by desorption.

[0013] (3) In addition, in the above (1) or (2), a membrane separation device is used as the primary concentration device. [Effects of the Invention]

[0014] According to the present invention, the purity and recovery rate of product CO2 can be increased without increasing the consumption of product CO2 or energy consumption. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is an explanatory diagram of an apparatus configuration for carrying out a carbon dioxide recovery method according to an embodiment. [Figure 2] 3A to 3C are diagrams illustrating the operating states of each vessel in all steps of the carbon dioxide recovery method according to the embodiment. [Figure 3] FIG. 1 is a diagram (part 1) illustrating the state of each container and the gas flow in each step of the carbon dioxide recovery method according to the embodiment. [Figure 4] FIG. 2 is a diagram (part 2) illustrating the state of each container and the gas flow in each step of the carbon dioxide recovery method according to the embodiment. [Figure 5] FIG. 3 is a diagram illustrating the state of each container and the gas flow in each step of the carbon dioxide recovery method according to the embodiment (part 3). [Figure 6] FIG. 4 is a diagram illustrating the state of each container and the gas flow in each step of the carbon dioxide recovery method according to the embodiment (part 4). [Figure 7]FIG. 5 is a diagram illustrating the state of each container and the gas flow in each step of the carbon dioxide recovery method according to the embodiment (part 5). [Figure 8] FIG. 6 is a diagram illustrating the state of each container and the gas flow in each step of the carbon dioxide recovery method according to the embodiment (part 6). [Figure 9] 1 is a graph showing the relationship between elapsed time and exhaust gas concentration (part 1). [Figure 10] 1 is a graph showing the relationship between the CO2 concentration on the membrane separation concentration side and the purity of recovered CO2. [Figure 11] 10 is a graph showing the relationship between elapsed time and exhaust gas concentration (part 2). [Figure 12] FIG. 10 is an explanatory diagram of an apparatus configuration for carrying out a carbon dioxide recovery method according to another aspect of the embodiment. [Figure 13] 10A to 10C are diagrams illustrating the operating states of each vessel in all steps of a carbon dioxide recovery method according to another aspect of the embodiment. [Figure 14] FIG. 10 is a diagram (part 1) illustrating the state of each container and the gas flow in each step of a carbon dioxide recovery method according to another aspect of the embodiment. [Figure 15] FIG. 10 is a diagram (part 2) illustrating the state of each container and the gas flow in each step of a carbon dioxide recovery method according to another aspect of the embodiment. [Figure 16] FIG. 10 is a diagram (part 3) illustrating the state of each container and the gas flow in each step of a carbon dioxide recovery method according to another aspect of the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] Before describing the embodiments of the present invention, the background to the invention will be described. When CO2-containing gas containing about 50% CO2 is passed through a container filled with a CO2 adsorbent such as zeolite, the CO2 concentration in the emitted gas changes as shown in Figure 9. The adsorbent in the vessel gradually adsorbs CO2 from the inlet to the outlet, and the breakthrough point is when the CO2 that cannot be adsorbed leaks out of the vessel outlet. The adsorbent is made of a material that is particularly good at adsorbing CO2, but each component gas contained in the supply gas is adsorbed according to its respective partial pressure. Therefore, the more CO2 is adsorbed, the higher the partial pressure of CO2.

[0017] In a conventional pressure swing adsorption (PSA) device, in order to prevent an increase in the CO2 concentration in the exhaust gas, adsorption is terminated before the breakthrough point is reached, and the adsorbed CO2 is recovered by depressurizing it. If CO2 is adsorbed sufficiently beyond this breakthrough point, the CO2 concentration that can be recovered by reducing the pressure increases, but the CO2 concentration in the exhaust gas that passes through the container and is released increases, reducing the CO2 recovery rate.

[0018] To solve this problem, similar to the method disclosed in Patent Document 2, two containers (container A, container B) filled with CO2 adsorbent are connected in series to allow CO2-containing gas to flow through them, and when the upstream container A exceeds the breakthrough point (ii) (see Figure 9) and the downstream container B does not exceed the breakthrough point (i) (see Figure 9), they are separated, and only the upstream container A is depressurized to recover CO2 and obtain high-concentration CO2. Then, the downstream vessel B is moved to the upstream side and the raw gas is directly introduced, and a new third vessel C is connected to the outlet.

[0019] Here, by utilizing a pressure equalization operation to alleviate pressure fluctuations when switching between containers, impure gases in the containers after adsorption can be effectively discharged. If the purpose is simply to equalize the pressure, the upper parts (outlets) and lower parts (inlets) of the positive pressure container A and the negative pressure container C can be connected to quickly equalize the pressure.

[0020] However, the inventor realized that in order to not only equalize the pressure but also to preferentially transfer impure gases in order to increase the purity of CO2, it would be effective to extract the impure gas from the outlet of container A after adsorption and then introduce the impure gas from the inlet of container C, which is at negative pressure after CO2 recovery. Before breakthrough, a large amount of impurity gas is adsorbed on the outlet side of the vessel, so it is recommended to perform a pressure equalization operation to extract the impurity gas from the outlet side.

[0021] However, when comparing vessel A, which had adsorbed CO2 until breakthrough, and the impure gas was extracted from the inlet of vessel A during the pressure equalization operation with the impure gas extracted from the outlet, it was found that the volume of the impure gas in the recovered gas remained almost the same. That is, it was considered that the amount of impurity gas did not change depending on the withdrawal direction after breakthrough. On the other hand, it was found that the amount of CO2 entrained in the impure gas emissions varies depending on the direction of extraction, with less CO2 being extracted from the outlet side. By minimizing the release of adsorbed CO2, it is possible to increase the amount of CO2 that can be recovered and also increase the purity of the CO2.

[0022] The reason why the amount of CO2 accompanying the discharge of impure gas changes depending on the direction of extraction is thought to be as follows. As shown in Figure 9, even after the CO2 concentration in the exhaust gas increases sharply after the breakthrough point, the CO2 concentration does not yet reach the CO2 concentration in the input gas and continues to rise slowly. This is because the adsorbent continues to adsorb CO2 at a slow rate even after it has reached the point where it can adsorb CO2 at a fast adsorption rate. As a result, the adsorbent near the inlet holds a large amount of CO2 that was adsorbed at a slow rate, but because this CO2 near the inlet that was adsorbed at this slow rate is easily desorbed, more CO2 desorbs from the adsorbent on the inlet side when the pressure is reduced through the pressure equalization operation.By extracting from the outlet side during the pressure equalization operation, the CO2 that desorbed from the adsorbent on the inlet side passes through the adsorbent on the outlet side that still has CO2 adsorption capacity, causing re-adsorption, which is thought to have reduced the amount of CO2 emitted.

[0023] This adsorbent is made of a material that is particularly good at adsorbing CO2, but each component gas contained in the supply gas is adsorbed according to its respective partial pressure. Even when CO2 is adsorbed until breakthrough, the more CO2 is adsorbed, the more CO2 is adsorbed as the partial pressure of CO2 increases. At the same time, the amount of gas components other than CO2 adsorbed decreases as the partial pressure decreases. In other words, increasing the CO2 concentration in the gas supplied to the PSA effectively increases the purity of the CO2 recovered after adsorption.

[0024] Typical combustion exhaust gas contains about 10% CO2, but when concentrating this CO2 using PSA to obtain high-purity CO2, it is advantageous to concentrate the CO2 to a certain extent in the stage prior to PSA. The first-stage concentrator does not need to be high performance. A membrane separation device is advantageous as a simple device that consumes little power and can moderately concentrate CO2. A membrane separation device is a device that sets up a membrane that selectively allows CO2 to pass through a container, dividing it into two compartments. The raw gas is passed through the front side of the membrane, and the pressure is reduced on the back side of the membrane with a vacuum pump, allowing the CO2 to selectively pass through, obtaining a gas enriched in CO2. The separation membrane can be a polymer membrane such as a rubber-like polymer or polyimide, or an inorganic membrane such as zeolite.

[0025] When a PSA packed with a zeolite CO2 adsorbent is used in the second-stage concentrator, the CO2 concentration required at the outlet of the first stage was investigated. Each of the three adsorption vessels was filled with 3.5 kg of zeolite-based CO2 adsorbent (F-9HA manufactured by Tosoh Corporation). N2 gas containing CO2 at various concentrations was prepared using a membrane separation device, and this gas was supplied to the adsorption separation device at 20 L / min. Two vessels were connected in series to perform adsorption. The outlet of the second vessel was at atmospheric pressure, and pressure equalization after adsorption in the first vessel was performed only on the outlet side. The purity of the CO2 recovered from the first vessel after pressure equalization is shown in the table below, and the relationship between the CO2 concentration on the membrane separation concentration side and the recovered CO2 purity is shown in the graph in Figure 10.

[0026] [Table 1]

[0027] It was found that if the CO2 concentration is increased to 40% or more using membrane separation, the purity of the recovered CO2 can be increased to 99% or more, and if the CO2 concentration is increased to 50% or more, the purity of the recovered CO2 can be increased to 99.5% or more.

[0028] Here, to ensure stable membrane separation and a constant CO2 concentration on the enrichment side, it is effective to minimize fluctuations in the vacuum pump that reduces pressure on the back side of the membrane. When switching between PSA vessels, the input gas is often stopped for a short period of time, but this is undesirable because the vacuum pump exhaust gas has nowhere to go. It is particularly important to avoid temporarily stopping the PSA input gas during the pressure equalization process. It is necessary to set up a sequence so that the PSA input gas always flows into one of the three vessels.

[0029] The present invention is based on the above findings, and the configuration of an apparatus for carrying out the present invention will be described with reference to FIG. The carbon dioxide recovery device 1 of this embodiment has a membrane separation device 4 as a primary concentration device, a vacuum pump 6 for membrane separation, and three containers, namely, container A, container B, and container C (number of containers N=3). The separation membrane of the membrane separation device 4 is made of a polymer membrane such as a rubber polymer or polyimide, or an inorganic membrane such as zeolite, and each container is filled with a zeolite-based CO2 adsorbent. Each container is connected to four lines: a primary concentrated gas inlet line 3 for introducing the raw gas; an off-gas line 5 for discharging the off-gas after adsorption to the outside; an other-container connection line 7 connecting the container's own outlet to the inlet of another container; and a CO2 recovery line 9 for desorbing and recovering the adsorbed CO2.

[0030] Each line is provided with a valve for opening and closing the line, as follows: Gas inlet valves 11A, 11B, and 11C are provided on the primary concentrated gas inlet line 3 of each container, off-gas exhaust valves 13A, 13B, and 13C are provided on the off-gas line 5 of each container, and other-container connection inlet valves 15A, 15B, and 15C are provided on the inlet side of the other-container connection line 7 of each container.

[0031] Further, CO2 capture valves 17A, 17B, and 17C are provided on the CO2 capture line 9 of each container, and other-container connection outlet valves 19A, 19B, and 19C are provided on the outlet side of the other-container connection line 7 of each container, respectively. Furthermore, the CO2 capture line 9 is provided with a desorption vacuum pump 21 that operates when desorbing CO2, and a CO2 tank 23 for storing the desorbed CO2.

[0032] Next, the operation of the device configured as above will be described with reference to FIGS. FIG. 2 shows the operating state of each container over time, and indicates that steps I to VI are repeated. For example, in process I, container A is in the main adsorption state, container B is in the post-adsorption state, and container C is in the recovery state. It is assumed that the primary concentrated gas that has been primarily concentrated by the membrane separation device 4 is introduced into each vessel.

[0033] 3 to 8 show the paths and valves that are functioning in each process, with functioning paths indicated by thick lines, open valves indicated by white outlines, and closed valves indicated by black outlines. In addition, when switching the valves in each process, the valves are not all switched at the same time, but are switched in order to prevent backflow of the off-gas or short-circuiting between the circulating gas and the vacuum pump if there is a possibility of these occurring. In the following description, the state in which the containers are connected to each other due to the valves being open is referred to as a connected state, and the state in which the containers are not connected to each other due to the valves being closed is referred to as a disconnected state.

[0034] <Process I> In step I, as shown in FIG. 3, containers A and B are connected in series, and container C is separated from the other containers. The primary concentrated gas is then introduced into container A, where CO2 is adsorbed. The gas that passes through container A enters the inlet of container B, where CO2 is also adsorbed, and the remaining gas is discharged to the off-gas line 5. On the other hand, in vessel C, only the CO2 capture valve 17C is open, and CO2 is captured by desorption. After capture, the vessel waits to become a capture vessel in step II. Alternatively, even after the flow rate of captured CO2 reaches almost zero, capture continues using the entire time until moving to the next step II in order to capture as much CO2 as possible that is gradually desorbed under reduced pressure. Step I is carried out until breakthrough occurs in container A and before breakthrough occurs in container B, and corresponds to the adsorption step of the present invention.

[0035] <Process II> Next, in step II, as shown in Figure 4, vessels A and B are separated, and the outlet of vessel A where the breakthrough has occurred is connected to the inlet of vessel C whose pressure has been reduced by desorption for carbon dioxide capture, and a pressure equalization, concentration, and capture operation is carried out. This pressure equalization, concentration, and capture operation corresponds to the pressure equalization, concentration, and capture step of the present invention. On the other hand, the primary enriched gas is directly supplied to vessel B to continue the adsorption of CO2, which corresponds to the adsorption continuation step of the present invention. In the CO2 recovery operation, it is necessary to stably process a constant amount of primary concentrated gas so as not to cause fluctuations in the operation of the CO2 generation facility. In this regard, in this embodiment, by supplying the primary concentrated gas directly to vessel B, the same amount of primary concentrated gas as in process I is processed in process II, and no fluctuations in the operation of the CO2 generation facility are caused.

[0036] The effects of the pressure equalization, concentration and capture operations are as follows: First, the high-pressure vessel A and the low-pressure vessel C are connected to equalize the pressure difference between the two vessels. Furthermore, by connecting the breakthrough vessel A with the recovered vessel C, the impure gas is discharged from vessel A and the CO2 in vessel A is concentrated. At this time, because the outlet of container A is connected to the inlet of container C, some of the CO2 adsorbed in container A will desorb and move toward container C. Because the inlet side of container A holds a large amount of CO2 that is adsorbed at a slow rate and easily desorbed, CO2 is likely to desorb from the inlet side of container A, but the CO2 that desorbs from the inlet side is re-adsorbed within container A as it moves toward the outlet side of the container, thereby suppressing the amount of CO2 emitted from container A. A small amount of CO2 is still emitted from vessel A, but this small amount of CO2 is captured in vessel C.

[0037] Container C has the role of adsorbing the CO2 in the primary concentrated gas in the next process and beyond, but if the amount of CO2 discharged from Container A increases, the amount of CO2 that can be adsorbed in Container C from the primary concentrated gas will decrease. In other words, an increase in the amount of CO2 discharged from Container A means an increase in the amount of CO2 circulating within the device, and the amount of CO2 recovered from the primary concentrated gas will decrease. Therefore, in addition to increasing the concentration of CO2 that can be recovered from vessel A, it is also effective to connect the outlet of vessel A with the inlet of vessel C to reduce the amount of CO2 emitted from vessel A, in order to increase the recovery rate of CO2 in this primary concentrated gas.

[0038] <Process III> In step III, as shown in Figure 5, after the pressure equalization, concentration, and capture operations are completed, vessel A and vessel C are separated, vessel C is connected to the downstream of vessel B, and adsorption is carried out until vessel B breaks through and before vessel C breaks through. On the other hand, vessel A captures CO2.

[0039] <Process IV> In step IV, as shown in Figure 6, vessels B and C are separated, and the outlet of vessel B after breakthrough is connected to the inlet of vessel A after recovery treatment, and pressure equalization, concentration, and capture operations are carried out. On the other hand, primary concentrated gas is directly supplied to vessel C, where CO2 is adsorbed.

[0040] <Process V> In step V, as shown in Figure 7, after the pressure equalization, concentration, and capture operations are completed, vessel B and vessel A are separated, vessel A is connected to the downstream of vessel C, and adsorption is carried out until vessel C breaks through and before vessel A breaks through. On the other hand, vessel B captures CO2.

[0041] <Process VI> In step VI, as shown in Figure 8, vessel C and vessel A are separated, and the outlet of vessel C after breakthrough is connected to the inlet of vessel B after recovery treatment, and pressure equalization, concentration, and capture operations are carried out. On the other hand, primary concentrated gas is directly supplied to vessel A, and CO2 adsorption is carried out.

[0042] After that, the process returns to step I and the same process is repeated.

[0043] In this embodiment, pressure equalization, concentration, and capture are performed effectively in the pressure equalization, concentration, and capture operation, so the purity of the product CO2 can be increased without consuming product CO2 or increasing energy consumption.

[0044] If the volume of the space in the piping from each valve on the inlet side of the adsorption vessel to the adsorbent layer cannot be ignored, it is effective to remove the primary concentrated gas that fills this space before CO2 capture in order to increase the captured CO2 concentration. In this case, it is effective to extract a minimum amount of gas not only from the outlet of the adsorption vessel but also from the inlet side to purge the piping.

[0045] Furthermore, the above is an explanation of the state in which the device continues to operate, but when starting up the device, the following operations are performed. The pressure in vessels A and B is set to the same as the pressure in the off-gas line during operation. Normally, it is sufficient to leave it at atmospheric pressure. For vessel C, the desorption vacuum pump 21 is started and the pressure is completely reduced. The exhaust gas at this time is not sent to the CO2 tank 23 but is discharged to the outside. Thereafter, step I shown in FIG. 3 is started, and the gas that has been primarily concentrated in the membrane separation device 4 and discharged from the membrane separation vacuum pump 6 is introduced into the A container. Since there is no CO2 pre-adsorbed in vessel A, the duration of this startup can be until vessel A breaks through, or, since a steady state is reached by repeating the process cycles, the duration can be the same as that of process I, which is performed after process VI. The gas inside vessel B is purged by the gas discharged from vessel A and is discharged to off-gas line 5. After that, if the process returns to process I via process II and process VI, the process will be in almost normal operating condition.

[0046] Furthermore, although the above description is for the case where the number of containers N=3, the number of containers N of the present invention is not limited to 3, and may be 4 or more. Therefore, the following will explain another embodiment in which the number of containers N=4. First, the advantages of setting N=4 will be explained. In the state (ii) shown in Figure 9, the breakthrough point has been reached but there is still room for CO2 adsorption. Therefore, as shown in Figure 11, by extending the adsorption time to the state (iii), the concentration of recovered CO2 can be increased. Furthermore, when shortening the adsorption time to reduce the CO2 concentration of the gas discharged into the off-gas line, if four or more vessels are used, it will be possible to continue adsorption until the most upstream vessel finally exceeds the breakthrough point.

[0047] The device configuration when N=4 is shown in Fig. 12. In Fig. 12, the same parts as those in Fig. 1, which shows the device configuration when N=3, are given the same reference numerals. In Figure 1, there are three vessels, so when two vessels are connected and adsorption or pressure equalization, concentration, and capture operations are being performed, the other vessels are not connected to any of the vessels. Therefore, the line connecting to other vessels for adsorption and pressure equalization, concentration, and capture operations can be used as other vessel connecting line 7.

[0048] However, when there are four containers, two containers can be connected while the other two are being connected and performing the pressure equalization, concentration, and capture operations. Therefore, instead of the other container connection line 7 in Figure 1, a pressure equalization line 25 is provided to be used when performing the pressure equalization, concentration, and capture operations, and an other container connection line 27 is provided to be used when connecting to other containers during adsorption. In conjunction with the provision of these lines, there are also provided pressure equalizing outlet valves 29A, 29B, 29C, and 29D, pressure equalizing inlet valves 31A, 31B, 31C, and 31D, connecting outlet valves 33A, 33B, 33C, and 33D, and connecting inlet valves 35A, 35B, 35C, and 35D.

[0049] <Process I> In step I shown in Figure 13, three containers A, B, and C are connected as shown in Figure 14, and container D collects CO2. Step I is carried out until container A passes the breakthrough point and adsorbs a sufficient amount of CO2 (state (iii) in Figure 11), container B also passes the breakthrough point (state (ii) in Figure 11), and container C continues until just before the breakthrough point (state (i) in Figure 11). Note that container B may or may not pass the breakthrough point (state (ii) in Figure 11).

[0050] <Process II> In step II shown in Figure 13, the outlet of vessel A is connected to the inlet of vessel D to perform pressure equalization, concentration, and capture operations, as shown in Figure 15. At the same time, while vessels B and C remain connected, the primary concentrated gas is directly supplied to vessel B and discharged from vessel C.

[0051] <Process III> In step III shown in Figure 13, as shown in Figure 16, after the pressure equalization, concentration, and capture operations are completed, vessel A and vessel D are separated, vessel D is connected to the downstream of vessel C, and adsorption is continued until vessel B passes the breakthrough point and a sufficient amount of CO2 is adsorbed, but before vessel D breaks through.

[0052] Steps IV to VIII in FIG. 13 are the same as steps I to III described above, but are carried out in different vessels.

[0053] When the number of containers N is 5, the piping configuration for the most upstream container and the most downstream container should be the same as that for container A and container D shown in Figure 12, and a container unit with the same piping and valve configuration as that for container B and container C in Figure 12 should be inserted just before the most downstream container. The container unit referred to here, for example, in the case of container B, includes container B, part of the off-gas line 5, part of the primary concentrated gas introduction line 3, part of the CO2 recovery line 9, part of the equalization line 25, the line 27 connecting to other containers during adsorption, the off-gas discharge valve 13B, the gas introduction valve 11B, the CO2 recovery valve 17B, the equalization outlet valve 29B, the equalization inlet valve 31B, the connecting outlet valve 33B, and the connecting inlet valve 35B. By using such a vessel unit, it becomes possible to change the number of adsorption vessels by increasing or decreasing the number of vessel units with the same piping and valve configuration. This makes it possible to add more adsorption vessels to improve performance and increase the purity and recovery rate of the product CO2, or to detach one vessel for maintenance while continuing operation. Therefore, if the number of containers is to be six or more, this can be achieved by adding the above-mentioned container units.

[0054] In the above explanation, an example was shown in which the outlet side of the vessel filled with the breakthrough adsorbent was connected to the inlet side of the desorption vessel whose pressure had been reduced in the pressure equalization, concentration, and capture operation. However, the present invention is not limited to the above embodiment, and also includes an embodiment in which the inlet side of the vessel filled with the breakthrough adsorbent is connected to the inlet side of the desorption vessel whose pressure has been reduced during the pressure equalization, concentration, and capture operation.

[0055] In addition, in the above embodiment, a membrane separation device is exemplified as the primary concentrator, but the primary concentrator of the present invention may be any device that can concentrate carbon dioxide-containing gas to 40 to 60%, and may be a membrane separation device or a PSA. [Example]

[0056] An experiment was conducted to confirm the effects of the present invention, which will be described below. First, an outline of the experiment will be described. In addition to the examples of the invention, comparative examples 1 to 6 were also carried out.

[0057] Example 1 In Example 1, as shown in FIG. 1, the primary concentration step was carried out using a membrane separation device using a polymer membrane, and the secondary concentration step was carried out using an adsorptive separation device using three adsorption vessels. 3.5 kg of zeolite CO2 adsorbent (F-9HA manufactured by Tosoh Corporation) was placed in each of the three adsorption vessels of the adsorption separation device. A raw gas containing 10% CO2 in N2 was introduced into the membrane separation device at 167 L / min, and the pressure was reduced with a vacuum pump to pass through the membrane, yielding a primary enriched gas with a CO2 concentration of 50% at 20 L / min. This primary concentrated gas with a CO2 concentration of 50% was fed into the adsorption separation device at a rate of 20 L / min. In the adsorption separation device, two vessels were connected in series to perform adsorption, and the outlet of the second vessel was set to atmospheric pressure. After adsorption was completed, the pressure was equalized by drawing out only from the inlet side. After 11 minutes of recovery, the amount of CO2 emitted reaches 0 L / min, and the recovery pump is now recovering CO2 to the limit of its capacity (the same applies to the following comparative examples).

[0058] <Example 2> Example 2 differs from Example 1 in that pressure equalization after the end of adsorption is performed only from the downstream side, but other conditions are the same as those of Example 1.

[0059] <Comparative Example 1> This is an embodiment in which membrane separation is not performed and only an adsorption separation device is used. 3.5 kg of zeolite-based CO2 adsorbent was placed in each of the two adsorption vessels, and raw gas containing 10% CO2 in N2 was introduced at 20 L / min. Adsorption was carried out in only one vessel, and no pressure equalization operation was performed after adsorption was completed.

[0060] <Comparative Example 2> This is an embodiment in which membrane separation is not performed and only an adsorption separation device is used. 3.5 kg of zeolite CO2 adsorbent was placed in each of the three adsorption vessels. Two vessels were connected in series, and raw gas containing 10% CO2 in N2 was introduced at 20 L / min. The outlet of the downstream vessel was set to atmospheric pressure. After adsorption was completed, pressure was equalized by drawing out only from the outlet.

[0061] <Comparative Example 3> This is an embodiment using a membrane separation device and an adsorption separation device. 3.5 kg of zeolite CO2 adsorbent was placed in each of the three adsorption vessels. 167 L / min of raw gas containing 10% CO2 in N2 was concentrated in a membrane separator, and 20 L / min of gas with a CO2 concentration of 50% was fed into the adsorption separator. Adsorption was carried out for 9 minutes using only one vessel, which corresponds to the condition just before breakthrough, and the pressure equalization operation after adsorption was completed was carried out only on the outlet side.

[0062] <Comparative Example 4> This is an embodiment using a membrane separation device and an adsorption separation device. 3.5 kg of zeolite CO2 adsorbent was placed in each of the three adsorption vessels. 167 L / min of raw gas containing 10% CO2 in N2 was concentrated in a membrane separator, and 20 L / min of gas with a CO2 concentration of 50% was fed into the adsorption separator. Adsorption was carried out in only one vessel for 12 minutes, which was the time after breakthrough, and pressure equalization after adsorption was performed only on the outlet side.

[0063] <Comparative Example 5> This is an embodiment using a membrane separation device and an adsorption separation device. 3.5 kg of zeolite CO2 adsorbent was placed in each of the three adsorption vessels. 167 L / min of raw gas containing 10% CO2 in N2 was concentrated in a membrane separator, and 20 L / min of gas with a CO2 concentration of 50% was fed into the adsorption separator. In the adsorption separation device, two vessels were connected in series and adsorption was performed. The downstream vessel adsorbed for 12 minutes before breakthrough, and the upstream vessel adsorbed for a further 12 minutes before breakthrough. No pressure equalization was performed after the end of adsorption.

[0064] The experiment outline and results are summarized in Table 1 below. [Table 2]

[0065] Example 1 The entire amount of gas recovered from one of the adsorption vessels during the recovery process was recovered, and the volume and composition were measured, resulting in 113 L of CO2 and 0.9 L of N2. The recovery rate of the membrane separation device is (0.5×20) / (0.1×167)=0.59, or 59%. The recovery rate of the adsorption separation device is 0.5 L x 20 L / min x 12 min = 120 L, which is 113 / 120 x 100% = 94%. The decrease in recovery rate in the adsorption separation device was kept to a minimum, and the CO2 recovery rate of the entire device was 0.59 x 94% = 55%. The purity was 113 / (113+0.9)=0.992, or 99.2%.

[0066] At the end of the operation, the gas extracted in the pressure equalization process and transferred to the recovery vessel was depressurized and the entire volume was recovered, and the volume and composition were measured, revealing that it was 8.6 L of CO2 and 4.6 L of N2. Therefore, the amount of N2 removed during the pressure equalization process was approximately five times the amount of N2 (=0.9 L) released into the product.

[0067] As described above, in Example 1, a high purity of 99.2% and a high recovery rate of 55% were achieved. However, under the conditions of inlet side extraction, the pressure equalization operation could not exceed 99.5%, and the amount of N2 removed during the pressure equalization operation was about five times that discharged into the product.

[0068] <Example 2> The total amount of gas recovered from one adsorption vessel during the recovery process was recovered, and the volume and composition were measured, resulting in 114 L of CO2 and 0.3 L of N2. The recovery rate of the membrane separation device is (0.5×20) / (0.1×167)=0.59, or 59%. The recovery rate of the adsorption separation device is 0.5 x 20 L / min x 12 min = 120 L, which is 114 / 120 x 100% = 95%. The decrease in recovery rate in the adsorption separation device was kept to a minimum, and the CO2 recovery rate of the entire device was 59% x 0.95 = 56%. The purity was 114 / (114+0.3)=0.997, or 99.7%.

[0069] At the end of the operation, the gas that had been extracted by the pressure equalization operation and transferred to the recovery vessel was depressurized and all of it was recovered, and the volume and composition were measured, revealing that it was 5.9 L of CO2 and 4.8 L of N2. The amount of N2 removed by the pressure equalization operation was 16 times the amount of N2 (=0.3 L) emitted into the product.

[0070] As described above, in Example 2, in which pressure equalization was performed as a condition for withdrawal from the outlet side, a purity of 99.7%, which is even higher than in Example 1, and a high recovery rate of 56% were achieved. Furthermore, the amount of N2 removed from the product was reduced to about one-third of that in Example 1.

[0071] <Comparative Example 1> The purity of the captured CO2 was low at 55.2%. This is due to the low concentration efficiency of the adsorption separation device for low-concentration CO2. This shows that with simple operation, the performance of the adsorption separation device is not significantly different from that of membrane separation, which can concentrate CO2 with a concentration of 10% to 50%.

[0072] <Comparative Example 2> The CO2 recovery rate was 52%, which was the same as in Comparative Example 1, but the CO2 purity was improved to 81.7% compared to Comparative Example 1 by carrying out the pressure equalization operation. Furthermore, since Comparative Example 2 processes the same flow rate of gas as that after membrane separation in Example 1, it can be seen that the raw gas flow rate that can be processed in Example 1 is increased by more than eight times by installing the membrane separation device, and the recovered CO2 purity increases from 81.7% to 99.7%.

[0073] <Comparative Example 3> By concentrating CO2 using membrane separation, PSA adsorption using only one vessel under conditions just before breakthrough, and pressure equalization operation withdrawing from the outlet side, the purity of recovered CO2 was improved to 96.8%. Furthermore, the decrease in recovery rate in the adsorption separation unit was kept to a minimum, and the overall CO recovery rate was 55%.

[0074] <Comparative Example 4> The purity of recovered CO2 was improved to 99.6% through CO2 concentration by membrane separation, breakthrough, and pressure equalization. However, because only one vessel was used for adsorption up to the breakthrough stage, the recovery rate in the adsorption separation device fell to 79%, and the overall CO2 recovery rate of the entire device was 47%.

[0075] <Comparative Example 5> The recovered CO2 purity was 93.4% through CO2 concentration and breakthrough by membrane separation, but because there was no pressure equalization operation, it was not as good as in Examples 1 and 2. The decrease in recovery rate in the PSA was kept to a minimum, and the overall CO2 recovery rate was 56%, the same as in Example 1.

[0076] The above experiments demonstrated that by feeding gas concentrated to 50% using a membrane separation device into a two-vessel connected three-vessel PSA with pressure equalization operation, which is suitable for capturing high-concentration CO2, it is possible to significantly increase the processing volume from gas containing low concentrations of CO2 by adding a simple device, and to capture high-concentration CO2 of 99% or more while maintaining the CO2 capture rate. [Explanation of symbols]

[0077] 1. Carbon dioxide capture device 3 Primary concentrated gas introduction line 4 Membrane separation device 5 Offgas Line 6. Vacuum pump for membrane separation 7. Other container connection line 9 CO2 capture line 11A, 11B, 11C Gas introduction valve 13A, 13B, 13C Off-gas discharge valve 15A, 15B, 15C Other container connection inlet valve 17A, 17B, 17C CO2 capture valve 19A, 19B, 19C Other container connection outlet valve 21 Desorption vacuum pump 23 CO2 tank 25 Pressure Equalization Line 27 Other container connection line during adsorption 29A, 29B, 29C, 29D Equalizing outlet valve 31A, 31B, 31C, 31D Equalizing press-fit side valve 33A, 33B, 33C, 33D Connecting outlet valve 35A, 35B, 35C, 35D Connection inlet valve

Claims

1. a primary concentration step of concentrating the carbon dioxide concentration in the carbon dioxide-containing gas to 40 to 60% using a primary concentration device; a secondary concentration step of secondarily concentrating the primary concentrated gas, the carbon dioxide concentration of which has been concentrated in the primary concentration step, using N (N≧3) containers filled with an adsorbent that adsorbs carbon dioxide, using an adsorption / separation device in which the N containers repeatedly perform adsorption, desorption, and pressure equalization, The secondary concentration step includes: an adsorption step of connecting (N-1) containers in series and flowing the raw material gas through the series until the adsorbent in the most upstream container breaks through but before the adsorbent in the most downstream container breaks through, thereby adsorbing carbon dioxide; a pressure equalization, concentration, and capture step in which, after the adsorption step, the connected most upstream container filled with the breakthrough adsorbent is disconnected from the connection, and the disconnected container is connected to a container whose pressure has been reduced by desorption for carbon dioxide recovery, to equalize the pressure of the two connected containers, concentrate the carbon dioxide concentration in the container filled with the breakthrough adsorbent, and capture the carbon dioxide flowing out from the container filled with the breakthrough adsorbent in the decompressed container; an adsorption continuing step in which the raw material gas is passed through (N-2) series-connected vessels other than the two vessels in which pressure equalization is performed in the pressure equalization, concentration, and capture step, and adsorption of carbon dioxide is continued; a recovery step of recovering carbon dioxide by desorption of carbon dioxide due to reduced pressure in a container filled with the breakthrough filler after the pressure equalization, concentration, and capture step; an adsorption step in which, after the pressure equalization, concentration, and capture step, the container that captured carbon dioxide is connected to the most downstream side of the container that is continuing adsorption in the adsorption continuation step, and adsorption is performed in a series of containers, A carbon dioxide recovery method characterized by repeatedly performing the adsorption step, the pressure equalization / concentration / capture step, the adsorption continuation step, and the recovery step.

2. 2. The carbon dioxide recovery method according to claim 1, wherein the pressure equalization, concentration, and capture steps are carried out by connecting the outlet of a container filled with the breakthrough adsorbent to the inlet of a container whose pressure has been reduced by desorption.

3. 3. The carbon dioxide recovery method according to claim 1, wherein a membrane separation device is used as the primary concentration device.

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