Pretreatment method for an air separation apparatus, and pretreatment apparatus therefor, and carbon dioxide recovery method

Abstract

To propose a pre-treatment device that supplies raw material gas to an air separation device that separates oxygen and nitrogen from the raw material gas, a pre-treatment method for the air separation device, the pre-treatment device for the same, and a carbon dioxide recovery method for recovery of carbon dioxide adhering to an absorbent.SOLUTION: A pre-treatment method for an air separation device comprises: an adsorption process which uses at least two adsorption towers to adsorb and remove carbon dioxide gas and moisture mainly in raw air with the one adsorption tower; and a regeneration process which has a first-stage desorption process that desorbs the carbon dioxide gas and moisture adsorbed by the other adsorption tower by using the carbon dioxide gas. Additionally, the regeneration process has a second-stage desorption process that performs desorption by using impure nitrogen derived from the air separation device instead of the carbon dioxide gas. The carbon dioxide gas and impure nitrogen are preferably heated gases. The adsorption process duration is preferably longer than the regeneration process duration. It is preferable that the adsorption process and the regeneration process are alternately executed between the one adsorption tower and the other adsorption tower.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an air separation device, and particularly to a pretreatment method and a pretreatment device for removing moisture and carbon dioxide gas contained in raw air, and a carbon dioxide gas recovery method.

Background Art

[0002] In blast furnaces and converters in steelworks, a large amount of oxygen is consumed. In places where such a large amount of oxygen is required, generally, an oxygen production facility is installed in the steelworks, and the oxygen generated by the oxygen production facility is supplied to facilities such as blast furnaces and converters.

[0003] In order to separate and extract air components such as oxygen and nitrogen from the air and commercialize them, there are air separation devices using a cryogenic separation method, a TSA (Temperature Swing Adsorption) method, a PSA (Pressure Swing Adsorption) method, etc. In order to avoid solidification of moisture and carbon dioxide gas in the raw air in the main body of the air separation device, or to avoid deterioration of the adsorbent in the main body, as a pretreatment, moisture and carbon dioxide gas in the raw air are removed before introducing the raw air into the main body of the air separation device. The pretreatment is generally performed by flowing raw air through an adsorber filled with an adsorbent. For example, an adsorber filled with alumina gel (a moisture adsorbent) and zeolite (a carbon dioxide gas adsorbent) is provided, and the raw air is flowed through the adsorber in the order of alumina gel and zeolite, so that moisture and carbon dioxide gas are removed in this adsorber.

[0004] Generally, a pair of such adsorbers is provided. While raw air is flowed through one adsorber to adsorb moisture and the like, a high-temperature regeneration gas is introduced into the other adsorber to regenerate the other adsorber (for example, Patent Document 1, Patent Document 2). The regeneration gas is obtained by heating nitrogen gas after air separation with a heater. A plurality of adsorption towers are arranged, and some of the adsorption towers are subjected to an adsorption operation, and the other adsorption towers are subjected to a desorption operation.

[0005] Furthermore, in air separation equipment, a two-tower alternating adsorption removal system of the PSA or TSA type is used as a pretreatment device to remove impurities (moisture, carbon dioxide, etc.) from the raw air. Among these methods, the TSA method is adopted in most equipment because it involves minimal process pressure fluctuations and long switching times. The TSA method removes impurities (moisture, carbon dioxide, etc.) by utilizing the difference in adsorption capacity of the adsorbent at various temperatures. Therefore, the greater the difference between the operating temperature and the regeneration temperature, the greater the adsorption capacity per unit weight of the adsorbent. Naturally, when designing a device, the larger the temperature difference, the less total amount of adsorbent can be used. Furthermore, the higher the pressure, the lower the saturated moisture content in the raw air, thus reducing the total adsorption capacity, and consequently, the total amount of adsorbent can be reduced. Additionally, during desorption, heat is applied to the adsorbent at a pressure lower than the adsorption pressure, and specific components of the adsorbent are desorbed by heating and cooling the adsorbent.

[0006] Pretreatment devices for air separation systems are predominantly of the type that adsorb moisture and carbon dioxide onto adsorbents. For example, Patent Document 3 discloses a pretreatment method for an air liquefaction separation system that continuously purifies raw air by using multiple adsorption towers filled with adsorbents that adsorb and remove moisture and carbon dioxide from raw air and are switched between. It describes optimizing the use of adsorption towers filled with moisture adsorbents such as activated alumina and carbon dioxide adsorbents such as X-type zeolite. Furthermore, used adsorption towers are regenerated by desorbing adsorbed substances using high-temperature purge gas and reused repeatedly. Patent documents 4 and 5 describe techniques for predicting the usage limits and the amount of energy required for regeneration of adsorption towers.

[0007] Various efficient methods are being considered for the pretreatment of air separation equipment. Patent Document 1 discloses a method for pre-treating raw air without using a fluorocarbon refrigerator. Specifically, in a pre-treatment method that removes moisture and carbon dioxide from raw air by passing it through an adsorbent before introducing it into the main body of the air separation device, the raw air is passed through a moisture adsorption section that adsorbs and removes moisture, then the raw air is cooled, and then passed through a carbon dioxide adsorption section that adsorbs and removes carbon dioxide.

[0008] Patent Document 2 discloses a pretreatment method and apparatus for an air separation apparatus that can eliminate the refrigerator. Specifically, it is an air separation apparatus comprising a first adsorption tower that adsorbs and dehumidifies moisture in the raw air, a heat exchanger that cools the raw air heated by the adsorption and dehumidification by the first adsorption tower, and a second adsorption tower that adsorbs and removes carbon dioxide and moisture from the raw air that has passed through the heat exchanger.

[0009] Patent Document 3 discloses a pretreatment method and apparatus for an air liquefaction separation apparatus that directly introduces pressurized raw material air at a high air temperature, which has exited the air compressor's aftercooler using air cooling or water cooling, into the adsorbent of a pretreatment device, without the need for special equipment such as precooling equipment or dehumidifiers to replace fluorocarbon chillers or water-washing cooling towers. Subsequently, it discloses a pretreatment method and apparatus for an air liquefaction separation apparatus that removes moisture and carbon dioxide that can be operated at high air temperatures. Specifically, it is a pretreatment method for an air liquefaction separation apparatus that is filled with an adsorbent that adsorbs and removes impurities such as moisture and carbon dioxide from raw material air, and is equipped with multiple adsorption cylinders that are used in a switchable manner, and continuously purifies raw material air, characterized by an adsorption step, a depressurization step, a heating step, a cooling step, a heating regeneration step, and a charging and pressurizing step.

[0010] Patent Document 4 discloses a pretreatment device that effectively utilizes the adsorbent so as not to switch between adsorption and regeneration while the adsorption tower still has sufficient adsorption capacity, thereby suppressing the consumption of excessive regeneration energy, and can also handle cases where the CO2 in the air significantly exceeds the design value. Specifically, it is a pretreatment device that supplies a raw material gas to an air separation device that separates oxygen and nitrogen from the raw material gas, and is characterized by comprising at least two adsorption towers that pass air through to remove H2O, CO2, and N2O from the air, and an N2O sensor that measures the concentration of N2O in the gas that has passed through one of the adsorption towers, and switching the adsorption tower when the concentration of N2O measured by the N2O sensor reaches a specified value.

[0011] Patent Document 5 describes a pretreatment apparatus that effectively utilizes the adsorbent so as not to switch between adsorption and regeneration while the adsorption tower still has sufficient adsorption capacity, thereby suppressing the consumption of excessive regeneration energy, and can also handle cases where the amount of CO2 in the air significantly exceeds the design value. The following is disclosed: Specifically, a pretreatment device for supplying a raw material gas to an air separation device that separates oxygen and nitrogen from the raw material gas, comprising: at least two adsorption towers through which the gas passes to remove at least CO2 from the gas; a CO2 sensor for measuring the concentration of CO2 in the gas before it passes through the adsorption towers; a gas meter for measuring the amount of gas that has passed through the adsorption towers; a CO2 amount integrating unit for integrating the total amount of CO2 that has passed through one of the adsorption towers based on the CO2 concentration and the amount of gas; and switching the adsorption towers before the total amount of CO2 reaches a predetermined value. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2002-66245 [Patent Document 2] Japanese Patent Publication No. 2004-209474 [Patent Document 3] Japanese Patent Application Publication No. 9-38446 [Patent Document 4] Japanese Patent Publication No. 2014-113593 [Patent Document 5] Japanese Patent Publication No. 2014-113594 [Overview of the Initiative] [Problems that the invention aims to solve]

[0013] In recent years, with the growing societal demand for CO2 emission reductions, CCU (Carbon dioxide Capture and Utilization) technology has been attracting attention. However, conventional technology does not recover the adsorbed carbon dioxide, which poses a problem from the perspective of reducing carbon dioxide emissions.

[0014] The present invention has been made in view of the above circumstances, and aims to propose a pretreatment method for supplying raw material gas to an air separation device that separates oxygen and nitrogen from raw material gas, a pretreatment device for an air separation device, and a carbon dioxide recovery method. [Means for solving the problem]

[0015] The pretreatment method for an air separation apparatus according to the present invention, which advantageously solves the above problems, is configured as follows. [1] A pretreatment method for an air separation apparatus, comprising: an adsorption step in which at least two adsorption towers are used to adsorb and remove mainly carbon dioxide and moisture from raw air in one adsorption tower; and a regeneration step in which the adsorbed carbon dioxide and moisture are desorbed in the other adsorption tower using heated carbon dioxide in a first stage desorption step. [2] In the above [1], the flow of carbon dioxide discharged from the other adsorption tower is divided into a discharge line and a heating line, a portion of the carbon dioxide is discharged, the remainder of the carbon dioxide flows for heating and is heated in a heater, and the heated carbon dioxide is supplied to the other adsorption tower in the regeneration process, which is a pretreatment method for an air separation apparatus. [3] In the above [1], the regeneration step includes a second desorption step of desorbing using impure nitrogen introduced from an air separation device instead of the carbon dioxide gas, and it is a pretreatment method of an air separation device. [4] In the above [1] or [3], in the regeneration step, it is a pretreatment method of an air separation device that uses a heated gas. [5] In the above [1] or [3], it is a pretreatment method of an air separation device where the time of the adsorption step is longer than the time of the regeneration step. [6] In any one of the above [1] to [3], by switching a switching valve in a gas flow path, an adsorption step and a regeneration step are alternately performed in the one adsorption tower and the other adsorption tower, and it is a pretreatment method of an air separation device.

[0016] The pretreatment device of an air separation device according to the present invention that advantageously solves the above problems is configured as follows. [7] It includes at least two adsorption towers, and each adsorption tower is filled with an adsorbent that mainly adsorbs and removes carbon dioxide gas and moisture in raw air. The filler has a function of desorbing the adsorbed carbon dioxide gas and moisture with a heated gas. When mainly adsorbing and removing carbon dioxide gas and moisture in raw air in one adsorption tower, in the other adsorption tower, it is configured to desorb the adsorbed carbon dioxide gas and moisture with heated carbon dioxide gas, and it is a pretreatment device of an air separation device. [8] In the above [7], the carbon dioxide gas flow path where the flow of the carbon dioxide gas derived from the other adsorption tower is divided into a discharge and heating use, and the carbon dioxide gas flow path where the heated carbon dioxide gas flows and is supplied to the other adsorption tower, and it is a pretreatment device of an air separation device. [9] In the above [7], it further includes a switching valve that can switch the supply of raw air to the one adsorption tower and the flow path of the processed air, and the supply of the heated gas to the other adsorption tower and the flow path of the carbon dioxide gas, and it is a pretreatment device of an air separation device.

[0017] The carbon dioxide gas recovery method according to the present invention that advantageously solves the above problems is configured as follows.

[10] A carbon dioxide gas recovery method for recovering carbon dioxide gas flowing for discharge, which is derived from an adsorption tower, by the pretreatment method according to any one of [1] to [3] above.

Advantages of the Invention

[0018] According to the present invention, in order to remove moisture and carbon dioxide gas contained in raw material air, a pretreatment method and a pretreatment device for an air separation device are provided, and hitherto uncollected carbon dioxide gas can now be recovered. In addition, clean carbon dioxide gas from the atmosphere can be utilized and used as a material for a clean CCU.

Brief Description of the Drawings

[0019] [Figure 1] It is a configuration diagram for explaining an air separation facility in an embodiment of the present invention. [Figure 2] It is a configuration diagram for explaining a conventional air separation facility. [Figure 3] It is an explanatory diagram of the regeneration of an adsorbent when the present invention is applied as compared with the prior art.

Modes for Carrying Out the Invention

[0020] Hereinafter, a pretreatment method for an air separation device, which is an embodiment of the present invention, and a pretreatment device used therefor will be described in detail together.

[0021] <Pretreatment Device for Air Separation Device> The pretreatment device for an air separation device according to the present embodiment includes at least two adsorption towers, and each adsorption tower is filled with an adsorbent that mainly adsorbs and removes carbon dioxide gas and moisture in raw material air. The filler has a function of desorbing the adsorbed carbon dioxide gas and moisture by heated gas. Further, when mainly adsorbing and removing carbon dioxide gas and moisture in raw material air in one adsorption tower, the pretreatment device is configured such that in the other adsorption tower, the adsorbed carbon dioxide gas and moisture are desorbed by heated carbon dioxide gas.

[0022] Figure 1 shows an example of an air separation system. The arrows indicate the direction of gas flow. This pretreatment device 1 supplies raw material gas to an air separation device 2 that separates oxygen and nitrogen from the raw material gas compressed by a gas compressor 4. It also includes at least two adsorption towers (11, 12) through which gas is passed to remove at least carbon dioxide from the gas, an air separation device 2 into which purified air 8 cleaned by the adsorption tower 11 is introduced, and a heater 3 for heating the regenerated gas 7. It is preferable to install a control device (not shown) as part of the carbon dioxide recovery system.

[0023] A regenerative gas supply route may include a heat exchanger (not shown) placed in the flow path where a portion of the nitrogen gas or carbon dioxide discharged from the air separation unit is used as regenerative gas, and the regenerative gas heated by a heater is supplied to the adsorption tower. Alternatively, the regenerative gas may be heated by a heat exchanger instead of a heater.

[0024] In the gas compressor 4, the gas (e.g., air) taken in through a filter is compressed, then cooled by a cooler such as an aftercooler (not shown), and the gas is supplied to the pretreatment device 1 as a raw material gas.

[0025] In the pretreatment device 1, the compressed raw material gas supplied by the gas compressor 4 is supplied to the adsorption towers (11, 12) for purification. In the adsorption towers, moisture and impurities such as hydrocarbons and carbon dioxide in the passing raw material gas are removed by adsorption onto an adsorbent. The purified raw material gas (purified air) 8 is cooled to near the dew point in a heat exchanger (not shown) located on the outlet side of the pretreatment device 1, and then supplied to the air separation device 2.

[0026] In an air separation system, it is preferable to install two-way and three-way switching valves in the flow path to switch the gas flow. In Figure 1, the white outline indicates the open state of the switching valve, and the black outline indicates the closed state. Therefore, in the state of the switching valve in Figure 1, the air 9 is compressed by the compressor 4 and, following the flow indicated by the arrows, is then separated into exhaust carbon dioxide 5 and circulating carbon dioxide 6 after passing through the adsorption tower 1, air separation device 2, heater 3, and adsorption tower 2.

[0027] In the air separation unit 2, for example, when using a cryogenic separation unit, the cleaned raw material air (purified air 8) supplied by the pretreatment unit is further cooled, and nitrogen, oxygen, and argon are separated by rectification using the difference in liquefaction temperature. The oxygen and nitrogen separated in the rectification column are supplied to the end users via the gas supply routes for each product. For example, in a steel mill, oxygen is supplied to end-user plants such as blast furnaces and converters.

[0028] <Pretreatment method for air separation equipment> The pretreatment method for the air separation apparatus according to this embodiment uses at least two adsorption towers and includes an adsorption step in which one adsorption tower adsorbs and removes mainly carbon dioxide and moisture from the raw air, and a regeneration step which includes a first-stage desorption step in which the adsorbed carbon dioxide and moisture are desorbed using carbon dioxide in the other adsorption tower.

[0029] The carbon dioxide flow from other adsorption towers is divided into discharge and heating channels. The heated carbon dioxide flows through the heating channel and is then introduced into other adsorption towers, forming the first stage. Escape A separation process is carried out. The carbon dioxide discharged from other adsorption towers is again separated and circulated for discharge and heating, and the carbon dioxide circulating for heating forms a circulation path.

[0030] Specifically, while one adsorption tower, for example, adsorption tower 1, is used to adsorb and remove impurities from the air, another adsorption tower, for example, adsorption tower 2, is regenerated with heated gas. Furthermore, by alternately switching the flow path switching valve and using the removal and regeneration adsorption towers alternately, it becomes possible to continuously purify the raw material gas. Alternatively, three or more adsorption towers, including at least one removal tower and at least one regeneration tower, may be used by switching the flow path switching valve.

[0031] In this embodiment, it is preferable to use nitrogen of lower purity than the product nitrogen (waste nitrogen) after carbon dioxide regeneration and carbon dioxide recovery for the regeneration treatment of the adsorbent in the adsorption tower by supplying it through the regeneration gas discharge route.

[0032] For heating the regenerated gas, it is preferable to use a heater. Examples of heaters include electric heaters and heat exchangers. When using a heat exchanger (not shown), it is placed in the same flow path as the heater.

[0033] A portion of the nitrogen gas discharged from the air separation unit, i.e., waste nitrogen, is supplied to a heat exchanger via a distribution channel, where it is heated and converted into regenerated gas. The generated regenerated gas is then supplied to an adsorption tower for regeneration treatment via a regenerated gas supply distribution channel, where carbon dioxide and moisture adsorbed onto the used adsorbent are removed, regenerating the adsorption tower so that it can be used repeatedly.

[0034] Regenerated gas is supplied to adsorption tower 1(11) via the regenerated gas supply distribution route, and regeneration is performed to remove carbon dioxide and moisture adsorbed on the spent adsorbent. Furthermore, it is preferable that the flow path for the regenerated gas supply is switched alternately at the same time that the removal and regeneration adsorption towers 1(11) and 2(12) described above are switched alternately by the switching valve, thereby supplying and stopping the regenerated gas. In Figure 1, the switching valve position in the flow path represents a gas flow path where adsorption tower 1(11) performs the removal function and adsorption tower 2(12) performs the regeneration function. It is preferable to arrange two adsorption towers, 1(11) and 2(12), and alternately switch the flow path switching valve to alternately perform the removal and regeneration functions of the adsorption towers.

[0035] Figure 3 shows an image of the regeneration flow of an adsorbent to which the present invention is applied, compared to the conventional method.

[0036] The carbon dioxide recovery system of this embodiment may also be equipped with a control device (not shown). The control device is composed of a computer, for example, equipped with a CPU and memory, and can control each device and equipment. For example, it is preferable to incorporate a concentration meter for measuring the concentration of carbon dioxide, a flow meter for measuring the amount of gas, and a control device that makes a decision on carbon dioxide recovery based on the gas concentration and amount, thereby controlling the concentration and amount of carbon dioxide to be recovered. The carbon dioxide recovery method of this embodiment has not been performed conventionally and can be applied to air separation processes that include a pretreatment step. [Examples]

[0037] The effects of this embodiment will be described in detail below based on the examples, but the present invention is not limited to these examples.

[0038] [Example 1] The following air separation device pretreatment was performed using the equipment configuration shown in Figure 1. In an air separation apparatus equipped with an adsorption device that adsorbs and removes mainly carbon dioxide and moisture from raw air, a pretreatment method was implemented in which the adsorption tower was regenerated in the first desorption step, which uses carbon dioxide for desorption, during the regeneration process of the adsorption device. In this case, carbon dioxide was circulated and discharged. Out Carbon dioxide was recovered from the flow path. In addition, two adsorption towers, 1(11) and 2(12), were set up, and the flow path switching valve was alternately switched to alternately perform the removal and regeneration processes of the adsorption towers. According to the embodiment of the present invention, carbon dioxide adsorbed in the adsorption process can be recovered, and the adsorption tower can be regenerated. Furthermore, in the regeneration process, using heated regenerated gas allowed for more efficient recovery of carbon dioxide compared to using unheated gas.

[0039] [Example 2] The following air separation apparatus pretreatment was performed using the equipment configuration shown in Figure 1. In the regeneration process of the adsorption device, a pretreatment method was implemented to regenerate the adsorption tower by performing a first-stage desorption process using carbon dioxide and a second-stage desorption process using impure nitrogen introduced from an air separation device. According to the embodiment of the present invention, carbon dioxide adsorbed in the adsorption process can be recovered, and the adsorption tower can be regenerated to a state with a higher degree of cleanliness than in Example 1.

[0040] [Example 3] Using the equipment configuration shown in Figure 1, and based on the pretreatment conditions shown in Example 1, we investigated the effect of heating the regenerated gas on carbon dioxide recovery. As a result, using heated regenerated gas allowed for more efficient carbon dioxide recovery compared to using unheated gas.

[0041] [Example 4] Using the equipment configuration shown in Figure 1, and based on the pretreatment conditions shown in Example 1, we investigated the effect of the time difference between the adsorption and regeneration processes on carbon dioxide recovery. As a result, when the adsorption process time was longer than the desorption (regeneration) process time, carbon dioxide could be recovered without reducing the productivity of air separation, compared to when the adsorption process time was shorter. Here, 60,000 Nm 3 This air separation system can produce oxygen at a rate of 1 / h and can recover up to 2000 tons of CO2 per year.

[0042] In the conventional equipment configuration shown in Figure 2, air separation can be performed without problems, but the carbon dioxide adsorbed in the adsorption process is released into the atmosphere during regeneration. [Industrial applicability]

[0043] The air separation pretreatment method of the present invention is applicable not only to specific processes but also to the manufacturing processes of all oxygen-utilizing products. [Explanation of Symbols]

[0044] 1. Pre-treatment device 11 Adsorption tower 1 12 Adsorption tower 2 2. Air separation device 3 Heater 4. Compressor 5. Carbon dioxide (including water) 6. Circulating carbon dioxide 7. Regenerative gas (nitrogen) 8. Purified air 9 Air 10 Gas after air separation 13. Nitrogen (including water and carbon dioxide)

Claims

1. A pretreatment method for an air separation device, Using at least two adsorption towers, In the first adsorption tower, an adsorption process is performed to adsorb and remove mainly carbon dioxide and moisture from the raw air. A regeneration process including a first desorption step in which heated carbon dioxide, which is a portion of the carbon dioxide discharged from the other adsorption tower, is used to desorb the adsorbed carbon dioxide and water in another adsorption tower, Includes, A pretreatment method for an air separator, wherein the regeneration process includes a second desorption step in which impure nitrogen introduced from an air separator is used for desorption instead of carbon dioxide.

2. The flow of carbon dioxide discharged from the other adsorption tower is divided into discharge and heating. A portion of the aforementioned carbon dioxide is released for recovery. The remaining carbon dioxide gas is circulated for heating and heated in a heater. The heated carbon dioxide is supplied to other adsorption towers in the regeneration process. A pretreatment method for an air separation apparatus according to claim 1.

3. In the regeneration process, the heated impurity nitrogen is used in the second desorption step. A pretreatment method for an air separation apparatus according to claim 1.

4. The pretreatment method for an air separation apparatus according to claim 1, wherein the time of the adsorption step is longer than the time of the regeneration step.

5. A pretreatment method for an air separation apparatus according to any one of claims 1 to 3, wherein the adsorption process and the regeneration process are alternately performed in the first adsorption tower and the other adsorption tower by switching a switching valve in the gas flow path.

6. Equipped with at least two adsorption towers, Each adsorption tower is filled with an adsorbent that primarily adsorbs and removes carbon dioxide and moisture from the raw air. The adsorbent has the function of releasing the adsorbed carbon dioxide and moisture when heated. When adsorbing and removing mainly carbon dioxide and moisture from the raw air in the first adsorption tower, Other adsorption towers are configured to desorb adsorbed carbon dioxide and water using heated carbon dioxide, which is a portion of the carbon dioxide discharged from other adsorption towers. A pretreatment device for an air separation device, wherein the aforementioned adsorption tower is configured to use impurity nitrogen introduced from an air separation device instead of carbon dioxide to desorb adsorbed carbon dioxide and moisture.

7. The flow of carbon dioxide discharged from the aforementioned other adsorption tower is divided into a carbon dioxide flow path for discharge and a path for heating, A carbon dioxide flow path through which heated carbon dioxide is supplied to the other adsorption towers, A pretreatment device for an air separation apparatus according to claim 6, comprising:

8. The supply path for raw material air to the aforementioned adsorption tower and the flow path for treated air, The pretreatment device for an air separation apparatus according to claim 6, further comprising a switchable valve for supplying heated impure nitrogen to the other adsorption tower and for the flow of carbon dioxide.

9. A carbon dioxide recovery method for recovering carbon dioxide that is discharged from an adsorption tower and is flowing for use in the pretreatment method described in claim 2.

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