JP2000109311A - Separation of gaseous carbon dioxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separating method of gaseous carbon dioxide capable of easily obtaining highly concentrated gaseous carbon dioxide and applicable for various kinds of fermentation gases, a by-produced gas from a petrochemical process, a by-produced gas from a steel making process, a boiler combustion gas and the like by using activated carbon having high selective adsorptivity to gaseous carbon dioxide. SOLUTION: In a pressure swing adsorption system, a gaseous mixture of gaseous nitrogen or/and gaseous oxygen with gaseous carbon dioxide is used as a gaseous starting material, the activated carbon obtained by holding carbon material or activated carbon under an activating atmosphere containing oxygen and in a condition that combustion does not occur at <=600 deg.C and having high selective adsorptivity to carbon dioxide against gaseous nitrogen or gaseous oxygen is used as an adsorbent, an adsorption vessel 2 after the completion of the adsorption is cleaned with gaseous carbon dioxide separated in the process to expel gaseous nitrogen or gaseous oxygen and after that, gaseous carbon dioxide is taken out. In the process, an adsorbent obtained by treating the activated carbon with an oxidizing agent can be also used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for separating carbon dioxide gas. More specifically, the present invention relates to a method for separating nitrogen gas, oxygen gas and carbon dioxide gas from a system containing mainly nitrogen gas, oxygen gas and carbon dioxide gas. This method separates high-concentration carbon dioxide gas by pressure fluctuation adsorption method (hereinafter referred to as PSA method) using activated carbon with high selective adsorption of carbon dioxide gas.

[0002]

2. Description of the Related Art It is well known that activated carbon has a property of adsorbing a large amount of water vapor and gases such as carbon dioxide gas, oxygen gas and nitrogen gas. However, ordinary activated carbon has a lower selective adsorption of carbon dioxide gas than oxygen and nitrogen gas. Therefore, PSA method is more effective than gas containing carbon dioxide such as fermentation gas or boiler combustion gas. There was no known method of economically separating carbon dioxide gas at a concentration.

When high-purity carbon dioxide gas is separated from a system containing nitrogen gas, oxygen gas and carbon dioxide gas by the PSA method, the selective adsorption of carbon dioxide gas to oxygen and nitrogen gas is as high as that of ordinary activated carbon. When the agent is used, a large amount of carbon dioxide gas obtained by this method is required to drive out nitrogen and oxygen gas adsorbed in the adsorption tank after adsorption. For this reason, high concentrations (99%
) Was difficult to economically separate carbon dioxide gas.

[0004]

In view of the above problems, an adsorbent having a high selective adsorption of carbon dioxide gas to nitrogen and oxygen gas and a large adsorption capacity has been developed and suitable for this adsorbent. It aims to provide a high concentration of carbon dioxide gas by developing a PSA method.

[0005]

Activated carbon is usually produced by activating a carbon material at a high temperature in an atmosphere containing steam and carbon dioxide gas. However, it is said that this atmosphere must not contain oxygen gas, and must not be brought into contact with oxygen gas even after the activation is completed until it is cooled. This is because the carbon material burns and the yield is significantly reduced. However, the present inventors have found that the selective adsorption of carbon dioxide gas is remarkably improved as compared with oxygen and nitrogen gas by contacting with oxygen-containing gas under such conditions that the activated carbon at high temperature after activation is not burned. I found to do.

In the case of activating the carbon material, the same effect can be obtained by treating and activating the carbon material in an atmosphere containing oxygen gas in a gas containing water vapor and carbon dioxide gas so as not to cause combustion. The knowledge that it can be obtained was obtained. Alternatively, it has been found that a similar effect can be obtained by treating activated carbon with an oxidizing agent. Further, the present inventors have studied a PSA process that can economically separate a high-concentration carbon dioxide gas using these adsorbents, and reached the present invention.

That is, in the pressure fluctuation adsorption system, nitrogen gas or / and a mixed gas containing oxygen gas and carbon dioxide gas as main components is used as a raw material gas, and a carbon material or activated carbon is used as an adsorbent. In the atmosphere
Using activated carbon, which has high selective adsorption of carbon dioxide gas to nitrogen gas or oxygen gas, obtained by holding it under the condition that combustion does not occur below 600 ° C, the adsorption tank where adsorption is completed is separated by this process A method for separating carbon dioxide gas, which comprises removing carbon dioxide gas after flushing with nitrogen gas or oxygen gas after washing with carbon dioxide gas.

In this step, an adsorbent obtained by treating activated carbon with an oxidizing agent can also be used. Here, "obtained by maintaining the conditions so as not to burn" means that the combustion is maintained at a substantially constant temperature without a sudden increase in temperature due to a rapid oxidation reaction. Meaning.

As the carbon material used in the present invention, a wide range of carbon materials such as coconut shell charcoal, lignite, anthracite, charcoal, and coke can be used, but coconut shell charcoal is particularly preferable in terms of adsorptivity after activation. In addition, a binder such as pitch or coal tar may be added to these carbon materials to form a uniform shape such as a sphere or a column. When used as an adsorbent for PSA equipment, cylindrical granulated coal with a diameter of 2 to 4 m / m is preferred. Ordinary activated carbon is made from these carbon materials in an atmosphere containing no oxygen gas.
It is prepared by processing at a high temperature of 700 ° C to 900 ° C.

In the present invention, when treating a carbon material or activated carbon, it is necessary to treat the carbon material or the activated carbon in an activated atmosphere containing oxygen under conditions that do not cause combustion at 600 ° C. or lower. Usually, the activation atmosphere is a mixed gas composed of a large amount of water vapor, carbon dioxide gas, inert gas and the like. Each of these components can be varied over a fairly wide range, but oxygen gas must not be included. This is because a combustion reaction occurs concurrently with the activation reaction, and the yield of the activation reaction is significantly reduced.

The activation atmosphere in the present invention refers to the same atmosphere as that for preparing activated carbon, and includes water vapor, carbon dioxide gas, nitrogen gas and other inert gases, but their composition is not particularly limited and may vary considerably. You may. In the present invention, it is necessary to further add oxygen gas to this atmosphere. The oxygen content is not particularly limited.
When the treatment is carried out in such an atmosphere at a temperature of not more than ° C., the composition must be such that combustion does not occur.

At this time, conditions that do not cause a combustion state are determined by the heat balance in the reaction system, and are considered to depend on the particle size and shape of the carbon material or activated carbon, the gas flow rate, and the degree of heat removal by heat transfer. The biggest factor is the oxygen gas content. Therefore, in order to keep the condition such that combustion does not occur, it is possible to make the oxygen gas content considerably high at a relatively low temperature, and depending on the condition, about 20
%.

However, when the treatment is carried out at a high temperature, the oxygen gas content is preferably about 3 to 6% in order to keep the inside of the system from burning. The processing temperature is about 600
It is necessary to be below ° C. If it is more than this, the selective adsorption of carbon dioxide gas decreases compared to nitrogen gas,
This is because it is difficult to increase the separation efficiency of high carbon dioxide gas when applied to a PSA device. On the other hand, the lower the treatment temperature, the higher the selective adsorptivity, but the lower the reaction rate.

Accordingly, the lower limit of the processing temperature is not particularly limited, but in consideration of these points, the temperature is most preferably 350 ° to 450 ° C. The selective adsorption was measured by measuring the amount of carbon dioxide gas and nitrogen gas adsorbed for 10 minutes at which the equilibrium adsorption amount was reached at 1 atm. 25 ° C., and expressed as the ratio of the former to the latter.

In the present invention, the activated carbon is treated with an oxidizing agent such as concentrated nitric acid or an aqueous solution of hydrogen peroxide or a strong oxidizing agent such as ozone to prepare activated carbon having high selective adsorption of carbon dioxide gas to nitrogen gas. can do. The concentration of the aqueous hydrogen peroxide solution is preferably about 30%. The treatment condition is that the selective adsorption property can be improved by immersing the activated carbon at room temperature for 3 to 5 hours.

Since the adsorbent prepared by the method of the present invention has a property of selectively adsorbing carbon dioxide gas contained in nitrogen and oxygen gas, it can be applied to many uses by utilizing its characteristics. When is used as an adsorbent in the PSA system, carbon dioxide gas can be continuously and efficiently separated from these gas mixtures.

[0017] The PSA method usually uses two adsorbents alternately filled with an adsorbent having selective adsorption, and continuously separates a target gas by utilizing a pressure difference between adsorption and desorption. It is a method. After concentrating by adsorbing gas with high selective adsorption at higher concentration during adsorption, gas with low selective adsorption and loosely adsorbed by adsorbent and gas existing between particles of adsorbent were expelled Thereafter, the pressure in the adsorption tank is reduced to desorb and selectively separate the adsorbed gas.

In the present invention, however, a mixed gas containing nitrogen gas and / or oxygen gas and carbon dioxide gas as main components is preferable as the source gas. Note that the source gas may contain a small amount of other components. As the adsorbent, it is necessary to use activated carbon having a higher carbon dioxide gas adsorptivity than nitrogen gas and oxygen gas. The purpose of concentrating and separating carbon dioxide gas can be achieved only when the composition of the raw material gas and the properties of the adsorbent are combined. As the adsorbent, activated carbon produced by the above-described method is suitable, but activated carbon having a high selective adsorption property of carbon dioxide gas as compared with nitrogen gas and oxygen gas may be used. Applicable.

Next, in the PSA method of the present invention, it is necessary to clean the adsorption tank in which the adsorption of the raw material gas has been completed by using a part of the high-concentration carbon dioxide gas obtained by the present PSA apparatus. Thus, it is considered that the gas remaining between the particles of the adsorbent and the nitrogen gas and the oxygen gas which are weakly adsorbed on the activated carbon as compared with the carbon dioxide gas are expelled. As shown in Example 5, when the amount of the cleaning gas was reduced, the purity of the product gas was significantly reduced.

By treating a carbon material or activated carbon at 600 ° C. or lower in an activated atmosphere containing oxygen gas, the selective adsorption of carbon dioxide gas can be remarkably improved as compared with nitrogen gas and oxygen gas. This is apparent from measuring the adsorption amounts of these gases before and after the treatment of activated carbon and comparing the selective adsorption coefficients of carbon dioxide gas with respect to nitrogen gas. It can be seen that the selective adsorption coefficient is greatly increased as a result of the nitrogen gas adsorption amount decrease being significantly larger than the adsorption amount decrease. Also, when activated carbon is immersed in an oxidizing agent at normal temperature, substantially the same effect is exhibited. This is clear from the results of Example 4.

When an adsorbent having higher selective adsorption than a gas mixture is used to separate its components, the PSA method is considered to be one of the most effective methods. The method of the present invention is a method for efficiently separating carbon dioxide gas having a higher concentration than carbon dioxide gas and a mixed gas containing nitrogen and / or oxygen gas as a main component, which is apparent from the results of Example 5.
Here, in addition to the activated carbon adsorbent obtained by the method of the present invention,
As a comparative example, a test was performed under the same conditions using ordinary activated carbon. As a result, when ordinary activated carbon was used, the amount of carbon dioxide gas obtained was significantly reduced.

The selective adsorption coefficient of carbon dioxide gas with respect to nitrogen gas is 11.1 for activated carbon obtained by the method of the present invention, and
6.9, indicating that the difference in adsorbability of the adsorbent has a very large effect when applied to the PSA method. The mixed gas to which the method of the present invention can be applied is a fermentation gas, a by-product gas from a petrochemical process, a by-product gas from an iron making process, a boiler combustion gas, and the like.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described more specifically with reference to examples.

(Example 1) Activated carbon prepared from coconut shell charcoal by a conventional method. The nitrogen gas adsorption amount and carbon dioxide gas adsorption amount at 1 atm, 25 ° C for 3 minutes, 10.9 m ² / g and 74.5 m, respectively.
² / g (selection coefficient of carbon dioxide gas over nitrogen gas: 6.
82) is treated with a gas obtained by burning liquefied petroleum gas in an atmosphere containing 3% of oxygen gas at 400 ° C for 2 hours.
The adsorption amount of nitrogen gas and carbon dioxide gas is 7.42 m each
² / g and 67.5 m ² / g (selective adsorption coefficient 9.1). Hereinafter, the method of the present example was applied as a method for measuring the gas adsorption amount.

(Example 2) When coconut shell charcoal is treated at 400 ° C. for 5 hours in an atmosphere having the same composition as in Example 1, the adsorbed amounts of nitrogen gas and carbon dioxide gas are 3.6 m ² / g and 40 m ² / g, respectively. m ² / g (selective adsorption coefficient 11.1), and the gas adsorption capacity as activated carbon was slightly low, but the selectivity was much higher than that of ordinary activated carbon.

(Example 3) A gas obtained by burning liquefied petroleum gas from activated carbon prepared from coconut shell charcoal, and changing the processing temperature in the range of 300 ° C to 700 ° C in an atmosphere containing 4% of oxygen gas. Treatment was performed for 1 to 2 hours so that the yield was about 90%. Table 1 shows the treatment temperature and the adsorption amounts of nitrogen gas and carbon dioxide gas before and after the treatment.

[0027]

[Table 1]

From the above, it can be seen that the selective adsorptivity is significantly affected by the treatment temperature and is preferably 400 ° C. or lower, and the effect of the present invention cannot be obtained at 600 ° C. or higher.

Example 4 After immersing a nitrogen gas adsorption amount of 9.90 m ² / g and carbon dioxide adsorption amount of 67.5 m ² / g (selection coefficient 6.82) prepared by a conventional method in concentrated nitric acid at normal temperature for 2 hours, , Washed with water
Dried at 130 ° C. Each resulting nitrogen gas and carbon dioxide gas adsorption amount of activated carbon 8.25 m ² / g and 70.0 m ² / g
(Selection coefficient 8.48).

(Example 5) Using a mixed gas of nitrogen gas 75% and carbon dioxide gas 25% as a raw material, a single-column PSA apparatus having a 1-liter adsorbent charge as shown in FIG. Using an adsorbent prepared by the method of the present invention and activated carbon obtained by a conventional method, a product gas having a carbon dioxide gas concentration of 99% was separated.

In FIG. 1, the source gas 1 enters the adsorption tank 2 through the valve 6, and the unadsorbed gas is released through the check valve 3. When the adsorption is completed, the valve 6 is closed, the valve 7 is opened, and the carbon dioxide gas in the product tank is discharged into the adsorption tank and discharged through the check valve 3 to wash the activated carbon in the adsorption tank.

When the cleaning is completed, the valve 7 is closed, the valve 8 is opened, and the vacuum pump 4 is operated to maintain a predetermined degree of reduced pressure, so that the gas in the adsorption tank is suctioned and desorbed and sent to the product tank 5. When the desorption is completed, the valve 8 is closed, the vacuum pump is stopped, and then the valve 6 is opened to start the next cycle by introducing the raw material gas into the adsorption tank.

The gas stored in the product tank is taken out by opening the valve 9 as required. Using activated carbon prepared by the method of the present invention (Example 1) and activated carbon prepared by a conventional method (Comparative Example 1) as an adsorbent,
Table 2 shows the results of searching for the conditions under which the largest amount of carbon dioxide gas with a purity of 99% can be obtained.

[0034]

[Table 2]

The cleaning gas amount in Comparative Example 1 was
When the gas concentration decreases to 10.0 liters, the product gas concentration and amount become 89.5% and 44.1 l / h, respectively. It can be seen that the product gas amount increases slightly or the gas concentration decreases significantly.

The adsorbent obtained according to the present invention and
It is recognized that the PSA system can efficiently obtain high-concentration carbon dioxide gas with relatively simple equipment using a mixed gas of nitrogen and carbon dioxide.

[0037]

According to the present invention, a high-concentration carbon dioxide gas is separated from a mixed gas mainly composed of nitrogen gas, oxygen gas and carbon dioxide gas by using an activated carbon adsorbent having a high selective adsorption to carbon dioxide gas. Is the way. High-concentration carbon dioxide gas can be easily obtained by using activated carbon with high selective adsorptivity, and can be applied to various fermentation gases, by-product gases from petrochemical processes, by-product gases in steelmaking processes, boiler combustion gases, etc. is there.

[Brief description of the drawings]

FIG. 1 shows one embodiment of a carbon dioxide gas separation step by the PSA method of the present invention.

[Explanation of symbols]

 1 Raw material gas 2 Adsorption tank 3 Check valve 4 Vacuum pump 5 Product gas tank 6, 8, 9 valve

Claims (2)

[Claims] In a pressure fluctuation adsorption system, nitrogen gas or / and a mixed gas containing oxygen gas and carbon dioxide gas as main components are used as a raw material gas, and a carbon material or activated carbon containing oxygen as an adsorbent is activated. Use activated carbon, which has a high selective adsorption of carbon dioxide gas to nitrogen gas or oxygen gas, obtained by holding it in an atmosphere at a temperature not exceeding 600 ° C so that combustion does not occur. A method for separating carbon dioxide gas, comprising removing nitrogen gas or oxygen gas by washing with carbon dioxide gas separated in the process, and then removing carbon dioxide gas. 2. In the pressure fluctuation adsorption method, nitrogen gas or / and a mixed gas containing oxygen gas and carbon dioxide gas as main components are used as source gases, and activated carbon is treated with an oxidizing agent as an adsorbent. In addition, using activated carbon having a high selective adsorption of carbon dioxide gas with respect to nitrogen gas or oxygen gas, the adsorption tank in which the adsorption is completed is washed with the carbon dioxide gas separated in this step to remove nitrogen gas or oxygen gas. A method for separating carbon dioxide gas, comprising removing carbon dioxide gas after being expelled.