JP2021154237A - Carbon dioxide absorption liquid, carbon dioxide separation and recovery method and biogas treatment method - Google Patents

Abstract

To provide a carbon dioxide absorption liquid which is excellent in the separation and recovery performance of carbon dioxide in the continuous steps of the absorption and diffusion of carbon dioxide and suitable for biogas treatment separating methane gas and carbon dioxide in biogas, concentrating methane gas and recovering carbon dioxide.SOLUTION: A non-aqueous carbon dioxide absorption liquid contains: a carbon dioxide chemical absorption amine having a nitrogen-hydrogen bond; and a tertiary multidentate amine having an oxygen atom and/or a nitrogen atom through a hydrocarbon group having a main chain carbon number of two or more and having hydrogen bond acceptability where the total amount of oxygen atom and nitrogen atom is two or more. The carbon dioxide chemical absorption amine having the nitrogen-hydrogen bond is a secondary amine having a hydrocarbon group with a hydroxy group, has high polarity, has low volatility and contains a non-aqueous diluent reducing viscosity in both of before absorption of carbon dioxide and after absorption of carbon dioxide in the carbon dioxide absorption liquid.SELECTED DRAWING: Figure 5

Description

The present invention relates to a carbon dioxide absorbing liquid, a carbon dioxide separation and recovery method, and a biogas treatment method.

Technology for separating and recovering carbon dioxide is necessary for the production of methane from natural gas and biogas, the maintenance of closed living environments such as in space and underwater, and the emission of greenhouse gases. From the viewpoint of reduction, we are actively researching various concentrations of carbon dioxide sources, such as those targeting large-scale emission sources such as thermal power plants and steel mills, and those targeting carbon dioxide fertilization from the atmosphere to the agricultural field. Has been done.
Among them, a chemical absorption method using an aqueous solution of an amine compound as an absorption liquid for carbon dioxide has been put into practical use. In the process of this chemical absorption method, a gas containing carbon dioxide is brought into contact with the absorption liquid in the absorption tower near room temperature to selectively chemically absorb carbon dioxide into the absorption liquid, and the gas and dioxide having a reduced carbon dioxide concentration are used. The absorption liquid that has absorbed carbon is separated into gas and liquid, and in the regeneration tower, the absorption liquid that has absorbed carbon dioxide is heated to dissipate and recover the carbon dioxide, and at the same time, the absorption liquid is regenerated and the regenerated absorption liquid is absorbed. It circulates in the tower.

However, in such a carbon dioxide separation and recovery method using an aqueous amine solution, a large amount of water as a solvent evaporates in the regeneration process of heating the absorption liquid, so that latent heat of vaporization must be excessively input as regeneration energy. .. In addition, the aqueous solution has a large specific heat, and sensible heat is more than twice that of the organic solvent. Furthermore, since the evaporation of water as a solvent promotes the inclusion of amine, which is a reaction substrate, care must be taken in controlling the mass balance in controlling the separation and recovery process. Therefore, extra cooling energy is required, such as equipping the absorption tower and the regeneration tower with a condenser for recovering amines, which is a factor that complicates the process. Further, since the deterioration of amine progresses in the heat regeneration process at high temperature, it is necessary to periodically replenish the absorption liquid due to the disappearance of the reaction substrate, and there is a concern that the running cost will increase.
In order to solve such a problem, a non-aqueous solution of an amine compound is being studied.

For example, Patent Document 1 describes dioxide containing a carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and a hydrogen bond acceptor solvent which is an ionic liquid or an amide compound having a carbonyl group or a phosphinyl group as an electron attracting group. A carbon absorbent is described (claim 1).

Patent Document 2 describes a carbon dioxide absorbing solution containing a carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and a tertiary polydentate amine solvent having a high hydrogen bond acceptability and no nitrogen-hydrogen bond. (Claim 1).

Patent Document 3 describes an absorption unit that absorbs carbon dioxide under the conditions of pressure P1 and temperature T1 and a discharge unit that releases carbon dioxide under the conditions of pressure P2 (P1 <P2) and temperature T2 (T1 <T2). It is an amine-containing absorption liquid used in a carbon dioxide absorption / release device having a recovery unit for separating carbon dioxide obtained in the release unit, and the absorption liquid has a lower viscosity and a lower vapor pressure (higher) for adjusting the viscosity. It is stated that a solvent of (both boiling point) may be included (claim 1, paragraph [0033]).

Non-Patent Document 1, the solvent effect on CO ₂ absorption amines compounds in non-aqueous solvent, an organic solvent is CO ₂ solubility increases, and the temperature dependency of the CO ₂ solubility increases at medium to high temperature range It is stated that CO _{2 emission is promoted.}

JP-A-2017-104775 JP-A-2017-1047776 Japanese Unexamined Patent Publication No. 2019-181401

Mitsuo Kanakubo et al., "Solvent Effects of Amines Compounds on CO2 Absorption in Non-Aqueous Solvents", 40th Solution Chemistry Symposium, October 18, 2017

In order to avoid energy loss and equipment complexity in the conventional carbon dioxide separation and recovery method using an amine aqueous solution, the temperature difference between the carbon dioxide absorption temperature near room temperature and the emission temperature of the absorbed carbon dioxide is small. It is required to separate and recover carbon dioxide under temperature conditions where there is little volatilization or loss of the absorption liquid.
In particular, the technology that separates methane gas in biogas from carbon dioxide and concentrates it is excellent in environmental friendliness in terms of extracting energy and useful substances from waste, and by concentrating methane with more energy saving, the latter stage. It is expected that the power generation efficiency will be improved by the combustion of methane.

Under the condition that the temperature difference between absorption and emission is small, the emission amount is often smaller than the absorption amount, and the magnitude of the emission amount contributes to the carbon dioxide separation and recovery performance. The non-aqueous carbon dioxide absorbing solution having a large emission amount can be selected from, for example, conventional absorbing solutions containing amine compounds described in Patent Documents 1 to 3 and Non-Patent Document 1.
On the other hand, since the absorption rate at the absorption temperature is generally slower than the emission rate at the emission temperature, the absorption rate is a rate-determining process in the carbon dioxide separation and recovery step. In the actual carbon dioxide separation and recovery step, the absorption liquid and the processing gas are brought into contact with each other for a predetermined time in the absorption tower to absorb carbon dioxide. Therefore, if the absorption rate of carbon dioxide is slow, carbon dioxide cannot be absorbed up to the saturated absorption amount, and the carbon dioxide separation and recovery efficiency is lowered.
The absorption rate of carbon dioxide depends on the rate of chemical reaction between the amine compound and carbon dioxide, the rate of dissolution of carbon dioxide in the absorption solution, and the substance transport of the reaction product of carbon dioxide and the amine compound in the absorption solution. .. The dissolution rate of carbon dioxide in the absorption liquid and the substance transport of the reaction product with carbon dioxide and amine compounds in the absorption liquid strongly depend on the viscosity of the absorption liquid. The conventional non-aqueous absorption liquid has a higher viscosity than the water-based absorption liquid, and the transport of substances in the absorption liquid is hindered, so that the absorption rate cannot be sufficiently increased.

Therefore, an object of the present invention is to provide a carbon dioxide absorbing liquid capable of efficiently absorbing and dissipating in the carbon dioxide separation and recovery step.
Further, the present invention is a carbon dioxide absorbing liquid suitable for biogas treatment that separates methane gas and carbon dioxide in biogas, concentrates methane gas, and recovers carbon dioxide under the condition that the temperature difference between absorption and emission is small. The challenge is to provide.

The present invention employs the following means in order to solve the above problems.
[1] A carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and an oxygen atom and / or a nitrogen atom via a hydrocarbon group having two or more carbon atoms in the main chain, and the sum of the oxygen atom and the nitrogen atom. Is a non-aqueous carbon dioxide absorbing solution containing a tertiary polydentate amine having two or more hydrogen bond accepting properties, and the carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond is a hydrocarbon group having a hydroxyl group. A secondary amine having a high polarity, low volatility, and a non-aqueous diluent that reduces the viscosity of the carbon dioxide absorbing solution both before and after carbon dioxide absorption. A carbon dioxide absorber characterized by containing.
[2] The carbon dioxide absorbing solution according to [1], wherein the carbon dioxide chemically absorbing amine is at least one of dialkanolamine and N-alkylmonoalkanolamine.
[3] The carbon dioxide-absorbing amines include diethanolamine, dipropanolamine, dibutanolamine, N-methylethanolamine, N-ethylethanolamine, 3-methylamino-1-propanol, and N-butylethanolamine. The carbon dioxide absorbing solution according to the above [2], which is at least one of N-propylethanolamine.
[4] The carbon dioxide absorbing solution according to the above [3], wherein the carbon dioxide chemically absorbing amine is diethanolamine.
[5] The diluent used is dimethyl sulfoxide, hexamethylphosphoric acid triamide, 1,3-dimethyl-2-imidazolidinone, N, N'-dimethylpropylene urea, tetramethylurea, N, N-dimethylformamide, N. , N-Dimethylacetamide, N-methylformamide, N-methylacetamide, N-methyl-2-pyrrolidone, sulfolane, the carbon dioxide absorbing solution according to any one of the above [1] to [4].
[6] The tertiary polydentate amine is a tertiary polydentate amine in which two hydrocarbon groups having a hydroxyl group and one hydrocarbon group having no hydroxyl group are bonded to one nitrogen atom. ] To [5].
[7] The carbon dioxide absorbing solution according to any one of [1] to [6] above, wherein the tertiary polydentate amine is at least one of N-methyldiethanolamine, N-ethyldiethanolamine, and N-butyldiethanolamine. ..
[8] The ratio of the carbon dioxide chemically absorbing amine is 1/100 to the above carbon dioxide chemically absorbing amine / (the carbon dioxide chemically absorbing amine + the tertiary amine solvent + the diluent) (mass ratio). The carbon dioxide absorbing solution according to any one of the above [1] to [7], which is 50/100.
[9] The content ratio of the diluent is 1/100 to 50/100 in terms of the diluent / (the carbon dioxide chemically absorbing amine + the tertiary amine solvent + the diluent) (mass ratio). The carbon dioxide absorbent according to any one of [1] to [8].
[10] By contacting the carbon dioxide absorbing solution according to any one of [1] to [9] with a mixed gas containing carbon dioxide at 10 ° C. or higher and 40 ° C. or lower, carbon dioxide is absorbed by the carbon dioxide absorbing solution. Then, an absorption step of selectively separating carbon dioxide from the mixed gas and a carbon dioxide absorbing liquid that has absorbed the carbon dioxide are heated to a temperature higher than the absorption temperature to dissipate and recover the absorbed carbon dioxide. A method for separating and recovering carbon dioxide, which comprises a heating and regeneration step of regenerating the carbon dioxide absorbing solution.
[11] By bringing the carbon dioxide absorbing solution according to any one of [1] to [9] into contact with a mixed gas containing carbon dioxide at 10 ° C. or higher and 40 ° C. or lower, carbon dioxide is absorbed. An absorption step of absorbing carbon dioxide in a liquid to selectively separate carbon dioxide from the mixed gas, and carbon dioxide absorbed by heating the carbon dioxide absorbing liquid that has absorbed the carbon dioxide to 60 ° C. or higher and 100 ° C. or lower. A method for separating and recovering carbon dioxide, which comprises a heating and regenerating step of dissipating and recovering the carbon dioxide and regenerating the carbon dioxide absorbing liquid.
[12] A biogas treatment method for concentrating methane gas in biogas using the carbon dioxide separation and recovery method according to the above [10] or [11].

According to the present invention, in the carbon dioxide separation and recovery step, it is possible to provide a carbon dioxide absorbing solution capable of continuously and efficiently performing absorption and emission, and various conditions are such that the temperature difference between absorption and emission is small. It is possible to provide an energy-saving carbon dioxide separation and recovery method for a carbon dioxide source having a concentration. In particular, it is possible to provide a carbon dioxide absorbing liquid suitable for biogas treatment that separates methane gas and carbon dioxide in biogas, concentrates methane gas, and recovers carbon dioxide.

The figure which shows the carbon dioxide absorption and emission test apparatus The figure which shows the time change of the carbon dioxide absorption amount in 1 to 4 cycles of the carbon dioxide absorption test (absorption: 30 degreeC, regeneration: 60 degreeC) of the absorption liquid which concerns on Example of this invention. The figure which shows the time change of the carbon dioxide absorption amount in 1 to 4 cycles of the carbon dioxide absorption test (absorption: 30 degreeC, regeneration: 60 degreeC) of the absorption liquid which concerns on the comparative example of this invention. The figure which shows the time change of the carbon dioxide absorption amount from the 1st cycle and the 2nd cycle of the carbon dioxide absorption test (absorption: 30 degreeC, regeneration: 60 degreeC) of each absorption liquid. The figure which shows the time change of the carbon dioxide absorption amount after the 2nd cycle of the carbon dioxide absorption test (absorption: 30 degreeC, regeneration: 60 degreeC) of each absorption liquid (partially enlarged view of FIG. 4).

In order to improve the carbon dioxide separation and recovery efficiency, the amount of carbon dioxide absorbed per predetermined time at an absorption temperature near room temperature, that is, the carbon dioxide absorption rate is large and higher than the absorption temperature, and the emission temperature is relatively mild for a predetermined time. The amount of carbon dioxide emitted per unit, that is, the rate of carbon dioxide emission is required to be large. Therefore, the present inventors focused on the transport of substances in the absorption liquid, which is one of the controlling factors of the carbon dioxide absorption rate. Then, it has a carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and an oxygen atom and / or a nitrogen atom via a hydrocarbon group having two or more carbon atoms in the main chain, and the total of the oxygen atom and the nitrogen atom is A non-aqueous carbon dioxide absorbing solution containing a tertiary polydentate amine having two or more hydrogen bond accepting properties has high polarity, low volatility, and suppresses an increase in viscosity both before and after absorption of carbon dioxide. We came up with the idea of adding a diluent to be used, and came up with the present invention.
That is, by adding the diluent, it is possible to improve the absorption and emission rates by suppressing the increase in viscosity before and after the absorption of carbon dioxide. Further, since the polarity of the diluent is high, the solubility of the amine compound or the reaction product of the amine compound and carbon dioxide can be improved, and the precipitation of solid components and the liquid-liquid phase separation in the absorption liquid can be avoided. .. Further, since the diluent has low volatility, it is possible to suppress the latent heat due to absorption and emission of carbon dioxide, particularly evaporation during regeneration heating, and to reduce the energy required for raising the temperature. In addition, the diluent does not inhibit the chemical reaction between the amine compound and carbon dioxide, and does not adversely affect the amount of carbon dioxide absorbed or emitted by the chemically absorptive amine.

Hereinafter, embodiments of the present invention will be described, but these embodiments are for explaining the present invention and do not limit the scope of the present invention.
The present invention includes various embodiments and modifications thereof, and the scope of the present invention is indicated by the scope of claims, and is applied within the scope of claims and the equivalent meaning of the invention. Various modifications are considered to be within the scope of the present invention.

One embodiment of the present invention has a carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and an oxygen atom and / or a nitrogen atom via a hydrocarbon group having two or more carbon atoms in the main chain. A non-aqueous carbon dioxide absorbing solution containing, and a tertiary polydentate amine having a total of 2 or more hydrogen bond acceptors, the carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond is a hydroxyl group. It is a secondary amine having a hydrocarbon group having, and further has high polarity and low volatility, and reduces the viscosity of the carbon dioxide absorbing solution both before and after absorbing carbon dioxide. The present invention relates to a carbon dioxide absorber characterized by containing a non-aqueous diluent.

In another embodiment of the present invention, carbon dioxide is absorbed by the carbon dioxide absorbing liquid by contacting the carbon dioxide absorbing liquid with a mixed gas containing carbon dioxide at 10 ° C. or higher and 40 ° C. or lower, and the mixing is performed. The absorption step of selectively separating carbon dioxide from the gas and the carbon dioxide absorbing liquid that has absorbed the carbon dioxide are heated to a temperature higher than the absorption temperature to dissipate and recover the absorbed carbon dioxide, and the carbon dioxide is recovered. It is a carbon dioxide separation and recovery method including a heating and regeneration step of regenerating a carbon absorption liquid.

Yet another embodiment of the present invention is a biogas treatment method for concentrating methane gas in biogas using the carbon dioxide separation and recovery method described above.

Hereinafter, the details of each element according to the embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in order.

[Carbon dioxide chemically absorbent amine]
The carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond according to the present embodiment is a secondary amine having a hydrocarbon group having a hydroxyl group.

A carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond (hereinafter, may be referred to as "absorbable amine") chemically absorbs carbon dioxide near room temperature and releases the absorbed carbon dioxide by heating. , Can be played repeatedly. Further, the hydroxyl group contained in the absorbent amine can control the basicity of the absorbent amine, improve the carbon dioxide separation and recovery efficiency, reduce the volatility, and prevent the loss due to evaporation at the time of dissipation. In order to control the basicity of the absorptive amine, reduce the volatility, and have a high boiling point, the number of carbon atoms in the main chain of the absorptive amine is preferably 2 or more.

（［式１］中、Ｒ７は、無置換若しくは置換基（水酸基を除く）を有していてもよい炭化水素基、ｎ７は、２以上の整数である。括弧内の炭素原子は置換基を有していてもよく、炭素原子の一部がヘテロ原子で置換されていてもよい。）

（［式２］中、ｎ８及びｎ９は、２以上の整数である。括弧内の炭素原子は置換基を有していてもよく、炭素原子の一部がヘテロ原子で置換されていてもよい。） The carbon dioxide chemically absorbing amine according to the present embodiment is preferably any one or more of the amines represented by [Formula 1] or [Formula 2].

(In [Formula 1], R7 is a hydrocarbon group which may have an unsubstituted or substituent (excluding a hydroxyl group), and n7 is an integer of 2 or more. The carbon atom in parentheses is a substituent. It may have, and a part of the carbon atom may be replaced with a hetero atom.)

(In [Equation 2], n8 and n9 are integers of 2 or more. The carbon atom in parentheses may have a substituent, or a part of the carbon atom may be substituted with a heteroatom. .)

Here, in the present specification, unless otherwise specified, examples of the hydrocarbon group include an alkyl group having no substituent or a substituent such as a halogen group and a hydroxyl group, an alkenyl group, and an alkynyl group, which are cyclic. May also be acyclic and may have a heteroatom in the skeleton. Among them, the hydrocarbon group having a hydroxyl group is preferably an alkyl group having a hydroxyl group. Further, the hydrocarbon group having no hydroxyl group is preferably an alkyl group.

The amine represented by the formula 1 is preferably any one or more of N-alkylmonoalkanolamines such as 3-methylamino-1-propanol, N-butylethanolamine, and N-propylethanolamine. ..

Further, the amine represented by the formula 2 is preferably any one or more of dialkanolamines such as diethanolamine, dipropanolamine and dibutanolamine, and particularly preferably diethanolamine.

[Level 3 polydentate amine]
The tertiary polydentate amine according to the present embodiment does not have a nitrogen-hydrogen bond, is rich in hydrogen bond acceptability, stabilizes the three-dimensional structure, and promotes the reaction between the carbon dioxide chemically absorbing amine and carbon dioxide. As such, it is a tertiary polydentate amine having an oxygen atom and / or a nitrogen atom via a hydrocarbon group having 2 or more carbon atoms in the main chain and having a total of 2 or more oxygen atoms and nitrogen atoms, and is absorbed. Functions as a solvent for sex amines. Here, as shown in, for example, [Equation 3]), "rich in hydrogen bond acceptability, stable in three-dimensional structure, and promoted reaction between carbon dioxide chemically absorbing amine and carbon dioxide" is 3 The nitrogen and oxygen atoms of the class polydentate amine interact with the hydrogen of the carbon dioxide chemically absorbing amine at the polydentate to stabilize the reaction product with carbon dioxide.

In [Formula 3], the ^{compound represented by HN (R 1} ) R ² represents a carbon dioxide chemically absorbing amine according to the present embodiment, and in Formula 1, X ₁ R ⁵ N (R ³ ) R. the compound represented by the ⁴ represents a tertiary polydentate amine according to the present invention, R ³ and R ⁴ are unsubstituted or substituted optionally may hydrocarbon group having a, R ⁵ is an unsubstituted or It is a hydrocarbon group having 2 or more carbon atoms in the main chain, which may have a substituent, and X ¹ has a nitrogen atom or an oxygen atom and hydrogen or an unsubstituted or substituent bonded to them. It is a hydrocarbon group which may be used. In the present specification, the hydrocarbon group having two or more carbon atoms in the main chain means that the shortest basic skeleton between the nitrogen atom of the tertiary amine and the nitrogen atom or oxygen atom is an ethylene group or a propylene group. , Butylene group, etc., which has 2 or more carbon atoms. Therefore, for example, amines having a main chain hydrocarbon of 1 such as HO-C (H) (CH ₃ ) N (R ³ ) R ⁴ are not included in the tertiary polydentate amine according to the present invention. As the hydrocarbon group having 2 or more carbon atoms in the main chain, an ethylene group, a propylene group, or a butylene group constituting an acyclic skeleton having a high degree of freedom is preferable, and an ethylene group is more preferable.

The electron-donating oxygen atom and nitrogen atom have high hydrogen bond acceptability and can interact with hydrogen of carbon dioxide chemically absorbing amine to stabilize the reaction product with carbon dioxide. Then, two or more hydrocarbon groups having a carbon number of main chain represented by R ⁵ and curve wherein 1 has a high degree of freedom, a nitrogen atom and / or nitrogen atoms bonded to both ends thereof, the carbon dioxide It can form hydrogen bonds with the hydrogen of chemically absorptive amines. Such a tertiary polydentate amine can be stabilized with carbon dioxide by forming a hydrogen bond with hydrogen of the carbon dioxide chemically absorbing amine on a relatively low temperature side such as near room temperature where carbon dioxide is absorbed. Promote the reaction. In addition, ^{hydrogen bonded to X 1} can form a hydrogen bond with carbon dioxide that has reacted with carbon dioxide chemically absorptive amine, further stabilize the reaction product, and promote the reaction with carbon dioxide. On the other hand, on the high temperature side where carbon dioxide is emitted, the degree of this hydrogen bond decreases, so that the emission of carbon dioxide can be promoted.

In the present specification, when a plurality of nitrogen atoms exist in one molecule and the nitrogen atoms have different series, the series of amine is the higher series. For example, a tertiary nitrogen atom (three hydrocarbon groups are bonded to a nitrogen atom) and a secondary nitrogen atom (two hydrocarbon groups are bonded to a nitrogen atom, and one hydrogen atom is bonded) in one molecule. If), the amine is a tertiary amine. Therefore, in the case of this example, it is a tertiary amine having a hydrogen-nitrogen bond, and is classified as a carbon dioxide chemically absorbing amine rather than a tertiary amine solvent. The nitrogen-carbon double bond is classified as having two hydrocarbon groups bonded to the nitrogen atom.

（［式４］中、Ｒ^３及びＲ^４は、無置換又は置換基（水酸基を除く）を有していてもよい炭化水素基、ｎ１は、２以上の整数であり、括弧内の炭素原子は置換基を有していてもよく、炭素原子の一部がヘテロ原子で置換されていてもよい。）

（［式５］中、Ｒ^３は、無置換又は置換基（水酸基を除く）を有していてもよい炭化水素基、ｎ１及びｎ２は、２以上の整数であり、括弧内の炭素原子は置換基を有していてもよく、炭素原子の一部がヘテロ原子で置換されていてもよい。） As the tertiary polydentate amine according to the present embodiment, the carbon of the main chain is rich in hydrogen bond acceptability, is structurally stable, and promotes the reaction between the carbon dioxide chemically absorbing amine and carbon dioxide. It is not particularly limited as long as it has an oxygen atom and / or a nitrogen atom via a hydrocarbon group having 2 or more, and the total of the oxygen atom and the nitrogen atom is 2 or more, and is not particularly limited. For example, [Equation 4]. It is a tertiary polydentate amine in which two hydrocarbon groups having a hydroxyl group and one hydrocarbon group having no hydroxyl group are bonded to one nitrogen atom represented by. It is preferably a tertiary polydentate amine in which one hydrocarbon group having a hydroxyl group and two hydrocarbon groups having no hydroxyl group are bonded, which are represented by [Formula 5]. ;

(In [Formula 4], R ³ and R ⁴ are hydrocarbon groups which may have an unsubstituted or substituent (excluding a hydroxyl group), n1 is an integer of 2 or more, and a carbon atom in parentheses. May have a substituent, and a part of the carbon atom may be substituted with a heteroatom.)

(In Expression 5], R ³ is an unsubstituted or substituted group (excluding a hydroxyl group) may have a hydrocarbon group, n1 and n2 is an integer of 2 or more, carbon atoms in parentheses It may have a substituent, and a part of the carbon atom may be substituted with a heteroatom.)

Examples of the tertiary polydentate amine represented by [Formula 4] include 2-dimethylaminoethanol ((Me) 2N (EtOH)) and 2- (diethylamino) ethanol ((Et) 2N (EtOH)), 2 -(Diisopropylamino) ethanol ((i-Pr) 2N (EtOH)), 2- (dibutylamino) ethanol ((n-Bu) 2N (EtOH)), 3-dimethylamino-1-propanol ((Me) 2N) (N-PrOH)), 4- (dimethylamino) -1-butanol ((Me) 2N (n-BuOH)) and the like.

Among them, in [Formula 4], R ³ and R ⁴ are tertiary amines which are hydrocarbon groups having 2 or more carbon atoms, that is, the main chain having a hydroxyl group in one nitrogen atom has 2 or more carbon atoms. A tertiary amine in which one hydrocarbon group and two hydrocarbon groups having 2 or more carbon atoms are bonded, or a tertiary amine in which n1 is an integer of 3 or more in Formula 2, that is, a hydroxyl group is added to one nitrogen atom. A tertiary amine in which one hydrocarbon group having 3 or more carbon atoms and two unsubstituted hydrocarbon groups are bonded is preferable. Specifically, 2- (diethylamino) ethanol, 2- (diisopropylamino) ethanol, and 2- (dibutylamino) ethanol, and 3-dimethylamino-1-propanol and 4- (dimethylamino) -1-butanol, etc. Can be mentioned.

Examples of the tertiary polydentate amine represented by [Formula 5] include N-methyldiethanolamine (MDEA), N-ethyldiethanolamine ((Et) N (EtOH) 2), and N-butyldiethanolamine ((n-Bu). ) N (EtOH) 2).

[Diluent]
The diluent according to the present embodiment is a non-aqueous solvent having high polarity and low volatility, and which reduces the viscosity of the carbon dioxide absorbing liquid both before and after carbon dioxide absorption.
The conventionally known non-aqueous absorption solution is a mixture of the absorbable amine and the tertiary polydentate amine according to the present embodiment, but this absorption solution increases the amount of carbon dioxide absorbed. The viscosity of the solution increased and the absorption rate decreased.

By adding the diluent according to the embodiment of the present invention to a solution containing an absorbable amine and a tertiary polydentate amine, it is possible to reduce the viscosity before carbon dioxide absorption and of course to reduce the viscosity of the absorption liquid after carbon dioxide absorption. The increase in viscosity can be suppressed. Therefore, the carbon dioxide absorption rate can be improved, the absorption amount per predetermined time can be increased, the carbon dioxide emission rate can be improved, and the emission amount per predetermined time can be increased.

Further, since the diluent according to the present embodiment is highly polar, it is an absorbable amine stabilized by an absorbable amine, a tertiary polydentate amine, and a tertiary polydentate amine represented by [Formula 3]. It has high solubility in reaction products of carbon dioxide and carbon dioxide, can accelerate the absorption rate of carbon dioxide near room temperature, and promotes the emission of carbon dioxide even under temperature conditions where the temperature difference between absorption and emission is small. The regeneration efficiency of the sex amine can be improved. That is, the diluent according to the present embodiment does not inhibit the chemical reaction between the absorbent amine and carbon dioxide, and does not adversely affect the amount of carbon dioxide absorbed or emitted by the absorbent amine.

Further, since the diluent according to the present embodiment has low volatility, it is possible to suppress the absorption and emission of carbon dioxide, particularly the loss due to volatility in the step of regenerating the absorbing liquid.
The absorption liquid to which the diluent according to the present embodiment has been added has a lower viscosity than the conventional non-aqueous absorption liquid, and can suppress an increase in viscosity due to absorption of carbon dioxide near room temperature, so that the absorption rate and emission can be suppressed. The speed is high. By increasing the absorption rate and the emission rate, the amount of gas processed per unit time can be improved. In addition, carbon dioxide separation and recovery can be efficiently performed even under temperature conditions where the temperature difference between absorption and emission is small. Therefore, the absorption liquid according to the present embodiment can provide an energy-saving carbon dioxide separation and recovery method for carbon dioxide generation sources having various concentrations. In particular, it is suitable for carbon dioxide separation and recovery from biogas, which continuously absorbs and dissipates under mild temperature conditions.

Diluting agents that satisfy the above conditions include dimethylsulfoxide, hexamethylphosphoric acid triamide, 1,3-dimethyl-2-imidazolidinone, N, N'-dimethylpropylene urea, tetramethylurea, N, N-dimethylformamide, N, N-dimethylacetamide, N-methylformamide, N-methylacetamide, N-methyl-2-pyrrolidone, sulfolane and the like are preferable, and in particular, highly polar dimethylsulfoxide, hexamethylphosphoric acid triamide, 1, 3-Dimethyl-2-imidazolidinone, N, N'-dimethylpropylene urea, and tetramethylurea are preferred.

[Carbon dioxide absorbent]
The carbon dioxide absorbing solution according to the present embodiment contains the above-mentioned carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and the above-mentioned tertiary polydentate amine having no nitrogen-hydrogen bond, and further, the above-mentioned dilution. Contains agents. The carbon dioxide chemically absorbing amine according to the present embodiment, the tertiary polydentate amine, and the diluent are usually liquids at room temperature, and the carbon dioxide absorbing liquid according to the present embodiment is a carbon dioxide chemically absorbing amine and. It is obtained by mixing a tertiary polydentate amine and adding a diluent.

The combination of the carbon dioxide chemically absorbing amine, the tertiary polydentate amine, and the diluent is not particularly limited. For example, the carbon dioxide chemically absorbing amine includes diethanolamine, dipropanolamine, dibutanolamine, and N-methylethanolamine. , N-ethylethanolamine, 3-methylamino-1-propanol, N-butylethanolamine, or N-propylethanolamine, as one or more tertiary polydentate amines, N-methyldiethanolamine, N- One or more of ethyldiethanolamine or N-butyldiethanolamine, and as diluents, dimethylsulfoxide, hexamethylphosphate triamide, 1,3-dimethyl-2-imidazolidinone, N, N'-dimethylpropylene urea, Alternatively, a combination of tetramethylurea is preferable.

The ratio of the carbon dioxide chemically absorbing amine in the carbon dioxide absorbing solution is not particularly limited, and is appropriately selected depending on the carbon dioxide chemically absorbing amine, the tertiary polydentate amine, and the type of the diluent, but the carbon dioxide chemically absorbing amine. The amine / (carbon dioxide chemically absorbing amine + tertiary polydentate amine + diluent) (mass ratio) is preferably 1/100 to 50/100, more preferably 10/100 to 40/100. When the ratio of carbon dioxide chemically absorptive amine is in this range, the amount of carbon dioxide absorbed near room temperature and the rate of carbon dioxide absorption can be increased, and carbon dioxide easiness can be achieved under conditions where the regeneration temperature is mild. ..

The ratio of the diluent in the carbon dioxide absorbing solution is not particularly limited, and is appropriately selected depending on the carbon dioxide chemically absorbing amine, the tertiary polydentate amine, and the type of the diluent. The amine + tertiary polydentate amine + diluent) (mass ratio) is preferably 1/100 to 50/100, more preferably 10/100 to 40/100. When the ratio of the diluent is in this range, the carbon dioxide absorption amount and the carbon dioxide absorption rate near room temperature can be increased, and carbon dioxide easiness can be achieved under the condition that the regeneration temperature is mild.

The carbon dioxide absorbing liquid according to the present embodiment is a non-aqueous carbon dioxide absorbing liquid and does not substantially contain water. Specifically, the water content of the carbon dioxide absorbing liquid of the present invention is preferably less than 10% by mass, more preferably less than 5% by mass, and particularly preferably less than 3% by mass.

The carbon dioxide absorbing liquid according to the present embodiment can be applied to a method for separating and recovering carbon dioxide gas from a mixed gas containing carbon dioxide. The mixed gas is not particularly limited as long as it is a gaseous mixture containing carbon dioxide, and may contain other components. Examples of other components include hydrocarbon gas, acid gas other than carbon dioxide, nitrogen gas, oxygen gas, water, soot and dust, and the carbon dioxide absorbing liquid according to the present embodiment is particularly methane gas contained in biogas. Suitable for the method of separating and recovering carbon dioxide and carbon dioxide. Examples of acidic gases other than carbon dioxide include hydrogen sulfide; sulfur oxides such as sulfur monoxide, sulfur dioxide (sulfur dioxide gas), and sulfur trioxide; nitric oxide, nitrogen dioxide, and nitric oxide (dinitrogen monoxide). , Nitric oxides such as dinitrogen trioxide, dinitrogen tetroxide, dinitrogen pentoxide; inorganic acids such as hydrochloric acid, nitric acid, phosphoric acid and sulfuric acid; organic acids such as carboxylic acid, sulfonic acid and carbonic acid. The carbon dioxide absorbing liquid of the present invention has little effect on carbon dioxide recovery even if the mixed gas contains a saturated amount of water as another component. Further, the carbon dioxide absorbing liquid of the present invention has little effect on the recovery of carbon dioxide even if the mixed gas contains dust as another component.

[Carbon dioxide separation and recovery method]
Next, a carbon dioxide separation and recovery method using the carbon dioxide absorbing liquid according to the present embodiment will be described.
In the carbon dioxide separation and recovery method of the present invention, carbon dioxide is absorbed by the carbon dioxide absorbing liquid by contacting the above-mentioned carbon dioxide absorbing liquid with a mixed gas containing carbon dioxide, and carbon dioxide is separated from the mixed gas. Heating that selectively separates the absorption step and heats the carbon dioxide absorbing solution that has absorbed the carbon dioxide to a higher temperature than the absorption step to dissipate and recover the absorbed carbon dioxide and regenerate the carbon dioxide absorbing solution. Including the regeneration process. As the contact method between the carbon dioxide absorbing liquid and the mixed gas containing carbon dioxide, for example, an absorption tower method or a scrubber method is used, but the method is not limited to these embodiments, and the contact efficiency of gas and liquid is increased. It would be good if the absorption rate of carbon dioxide could be improved. Further, as a method for heating and regenerating the carbon dioxide absorbing liquid that has absorbed carbon dioxide, for example, a regeneration tower method or a flash drum method is used, but the method is not limited to these embodiments, and the heat transfer efficiency of heating and air are not limited. It suffices if the contact efficiency of the liquid can be increased and the emission rate of carbon dioxide can be improved.

In the carbon dioxide separation and recovery method according to the present embodiment, the absorption rate is high near room temperature, carbon dioxide is easily dissipated with a small temperature rise, the evaporation loss of the absorption liquid is small, the specific heat is low, and the heat of reaction is high. Is small, so that the energy required for regeneration of the carbon dioxide absorbing liquid per carbon dioxide to be recovered can be reduced, and the separation and recovery efficiency of carbon dioxide can be improved. Therefore, the carbon dioxide separation and recovery method according to the present embodiment can separate and recover carbon dioxide with energy saving targeting carbon dioxide sources of various concentrations, and in particular, separate and recover methane gas and carbon dioxide from biogas. , Used in a biogas treatment method for concentrating methane gas.

The temperature of the absorption step is preferably 10 ° C. or higher and 40 ° C. or lower near room temperature (25 ° C. ± 15 ° C.). If it is near room temperature, it is not necessary to excessively cool the carbon dioxide absorbing liquid or the target processing gas, the amount of carbon dioxide absorbed and the absorption rate can be improved, and energy saving can be achieved.
In the carbon dioxide separation and recovery method of the present invention, the pressure in the absorption step is not particularly limited. When the processing gas in the vicinity of normal pressure is targeted, if the absorption step is performed in the vicinity of normal pressure as it is, no extra compression energy of the processing gas is applied, which is preferable from the viewpoint of energy saving. On the other hand, in order to improve the amount of carbon dioxide absorbed into the carbon dioxide absorbing liquid and the absorption rate, high pressure conditions of normal pressure or higher, for example, 1 MPaG to 6 MPaG, can be used.

The temperature of the heat regeneration step is higher than the temperature of the absorption step, but is not particularly limited. However, if the temperature of the regeneration step is remarkably raised, the amount of carbon dioxide absorbing liquid emitted increases, but the energy required for heating becomes large, and the carbon dioxide separation and recovery efficiency decreases. Therefore, it is preferable to carry out the regeneration step under mild temperature conditions, preferably 100 ° C. or lower, more preferably 80 ° C. or lower, and particularly preferably 60 ° C. or lower.
The pressure in the heat regeneration step is preferably equal to or lower than the pressure in the absorption step, but is not particularly limited. Since the carbon dioxide absorbing liquid of the present invention has a low vapor pressure and can suppress volatilization, it can be treated under reduced pressure. It is expected that the amount of carbon dioxide emitted from the carbon dioxide absorbing liquid can be improved by appropriately reducing the pressure under the condition that the energy required for depressurization does not become large. On the other hand, carbon dioxide emitted from the carbon dioxide absorbing liquid in the regeneration process can be recovered at high pressure. By recovering carbon dioxide at high pressure, compression energy can be reduced when high pressure carbon dioxide is required in the subsequent stage.

In the carbon dioxide separation and recovery method according to the present embodiment, when the absorption step and the heat regeneration step are continuously performed, if the carbon dioxide absorbed in the heat regeneration step remains, that is, the regeneration of the absorbing liquid is insufficient. In general, the absorption amount and absorption rate of carbon dioxide in the second cycle are smaller than the absorption amount and absorption rate of carbon dioxide in the first cycle. On the other hand, if a predetermined amount of carbon dioxide always remains in the heating regeneration step, that is, if the regeneration rate of the absorbing liquid is constant, the absorption amount and absorption rate after the third cycle are almost the same as those in the second cycle and are stable. (See FIG. 2).

The lower the carbon dioxide recovery rate in the first cycle (the rate at which carbon dioxide absorbed in the absorption step is recovered by emission in the heating and regeneration step), the more the amount and rate of carbon dioxide absorbed in the second and subsequent cycles decrease, and the carbon dioxide The recovery rate also decreases. The higher the carbon dioxide recovery rate in the first cycle, the higher the carbon dioxide separation and recovery efficiency. Therefore, the energy required for heating becomes large. When the temperature difference between the absorption step and the heat regeneration step is 50 ° C., it is preferably 50% or more, and when the temperature difference is 30 ° C., it is preferably 30% or more. The smaller the temperature difference, the lower the recovery rate, but the smaller the temperature difference, the smaller the heating energy for carbon dioxide recovery and absorption liquid regeneration.

The carbon dioxide absorbing liquid according to the present embodiment has a lower viscosity than the conventional non-aqueous absorption liquid, can suppress an increase in viscosity due to absorption of carbon dioxide in the absorption step, and has a high absorption rate and emission rate. By increasing the absorption rate and the emission rate, the amount of gas processed per unit time can be improved. Further, even if the temperature difference in the heat regeneration step is smaller than that in the absorption step, the amount of carbon dioxide emitted and the carbon dioxide recovery rate are high, and the loss due to the volatilization of the absorption liquid can be reduced.

The carbon dioxide absorbing solution of the present invention and the carbon dioxide separation and recovery method using the same can separate and recover carbon dioxide with energy saving targeting carbon dioxide sources of various concentrations, and in particular, methane gas in biogas. Suitable for biogas treatment methods for concentration treatment.

Hereinafter, the present invention will be described based on examples, but the present invention is not limited to these examples. The measurement was carried out by the following carbon dioxide absorption and emission test.

[Carbon dioxide absorption and emission test]
Using the carbon dioxide absorption / emission test apparatus shown in FIG. 1, a carbon dioxide absorption / emission test was conducted at normal pressure. The carbon dioxide absorption / emission test apparatus analyzes the reaction vessel 115 made of stainless steel, the controller 110 for controlling the temperature of the reaction vessel, the introduction part of the mixed gas consisting of carbon dioxide and methane, and the absorption / release of carbon dioxide in the absorption liquid. It consists of a carbon dioxide concentration meter 124 and a gas flow meter 125 for the purpose.
The mixed gas composed of carbon dioxide and methane is a mixture of gases whose flow rates are controlled by mass flow controllers (103 and 104) from carbon dioxide cylinders 101 and methane cylinders 102 via valves A (105) and valves B (106), respectively. , It was blown from the gas introduction pipe 116 into the absorption liquid 121 in the reaction vessel 115.
The reaction vessel 115 was installed in the constant temperature bath 117, and the temperature of the absorbing liquid 121 was measured by the thermometer 113, and controlled by the controller 110 by the cooling water circulation device 118 and the heater 114. During the experiment, the absorption liquid phase and the gas phase in the reaction vessel 115 were stirred by the motor 111 by the stirring blade 112. Moreover, the pressure in the reaction vessel was measured with a pressure gauge 122. A cooling tower 123 cooled to a predetermined temperature by a cooling water circulation device 119 was provided at the outlet of the reaction vessel.

The procedure for measuring the amount of carbon dioxide absorbed using this carbon dioxide absorption / emission test apparatus is described below.
1) The inside of the reaction vessel 115 was replaced with methane gas, and carbon dioxide was expelled from the system.
2) 100 ml of the absorption liquid prepared in advance under a nitrogen atmosphere was introduced into the reaction vessel 115 from the syringe 120 via the valve D (108).
3) The temperature of the absorbing liquid 121 was measured with a thermometer 113, controlled to 30 ° C. by the controller 110 by the cooling water circulation device 118 and the heater 114, and waited for stabilization. At that time, the stirring blade 112 installed in the gas phase and the liquid phase in the reactor was rotated at 800 rpm by the motor 111, and the cooling tower 123 was cooled to 5 ° C. by the circulating water cooling device 119.
4) With the three-way valves (107 and 109) as the bypass 126 side, the gas supplied from the carbon dioxide cylinder 101 and the methane cylinder 102 is sent to the mass flow controllers (103 and 104) via the valves A (105) and B (106), respectively. ), And adjusted so that the indication of the carbon dioxide concentration meter 124 was 46%. At that time, the total flow rate of the mixed gas was adjusted to 400 ml / min. Since the gas flow rate depends on the temperature and pressure, a thermometer and a pressure gauge were installed in the gas flow meter 125 to correct the gas flow rate.
5) The mixed gas was allowed to flow for about 30 minutes, and it was confirmed that the carbon dioxide concentration was stable at 46%.
6) The carbon dioxide absorption test of the first cycle at 30 ° C. was started with the three-way valves (107 and 109) on the reaction vessel 115 side. The time of switching the three-way valve was set to 0 minutes, and the time change of the carbon dioxide concentration and the flow rate of the gas discharged from the reaction vessel 115 was continuously recorded by the carbon dioxide concentration meter 124 and the gas flow meter 125.
7) After conducting a carbon dioxide absorption test at 30 ° C. for 1 hour, the cooling water circulation device 118 was stopped, and the temperature of the absorption liquid was raised to 60 ° C. by the heater 114. The temperature rise from 30 ° C. to 60 ° C. was performed in less than 5 minutes.
8) A carbon dioxide emission test is performed at 60 ° C. for 30 minutes, and as in the absorption test, the carbon dioxide concentration meter 124 and the gas flow meter 125 discharge the carbon dioxide from the reaction vessel 115, with the time set at 60 ° C. as 0 minutes. The time course of carbon dioxide concentration and flow rate of the gas was continuously recorded.
9) After conducting a carbon dioxide emission test at 60 ° C. for 30 minutes, the cooling water circulation device 118 is operated with the three-way valves (107 and 109) as the bypass 126 side, and the temperature of the absorbent liquid 121 is returned to 30 ° C. by the controller. rice field.
10) After confirming that the temperature of the absorption liquid 121 returned to 30 ° C. and was stable, the absorption test of the second cycle was started. The procedure is the same as after 5).
11) The above absorption and emission tests were repeated for 4 cycles.
12) From the concentration of carbon dioxide and the gas flow rate measured at the outlet of the reaction vessel 115, calculate the amount of substance of carbon dioxide discharged from the reaction vessel at each time, subtract it from the amount of substance of carbon dioxide introduced, and absorb it in the absorption liquid. The amount of carbon dioxide absorbed was determined. The amount of carbon dioxide absorbed per 100 ml of the amount of the absorbing liquid charged was converted into per ^{1000 ml (= per 1 dm 3).}

(Example)
Diethanolamine (DEA, manufactured by Wako Pure Chemical Industries, Ltd., purity 99.0 +%) as a carbon dioxide chemically absorbent amine, N-methyldiethanolamine (MDEA, manufactured by Aldrich, purity ≥99%) as a tertiary polydentate amine, and dilution. Dimethyl sulfoxide (DMSO, manufactured by Wako Pure Chemical Industries, Ltd., purity 99.0 +%) was mixed as an agent to obtain a carbon dioxide absorbent solution according to an example (in terms of mass fraction, DEA: MDEA: DMSO = 30:50). : 20). The water content is 1% or less.

(Comparison example)
In the above example, DEA and MDEA were mixed at a mass fraction of 30:70 without adding DMSO to obtain a carbon dioxide absorbent according to a comparative example (in mass fraction, DEA: MDEA = 30:: 70).

According to the procedure of the carbon dioxide absorption and emission test described above, 400 ml of the mixed gas was supplied per minute, and the time change of the carbon dioxide absorption amount in 1 to 4 cycles of the absorption liquids according to Examples and Comparative Examples was determined.
2 and 3 show changes in the amount of carbon dioxide absorbed in the 1st to 4th cycles of the absorbing liquids according to Examples and Comparative Examples, respectively.
It can be seen that the time change of the absorption amount in the 2 to 4 cycles is significantly different from the time change of the absorption amount in the first cycle, but there is almost no change between the 2 to 4 cycles and it is stable. Hereinafter, the cycle after the second cycle is referred to as "multi-cycle".

The results of comparing the examples and the comparative examples are shown with reference to FIGS. 4, 5 and Tables 1 and 2 below.
FIG. 4 corresponds to the one in which FIGS. 2 and 3 are superimposed, and FIG. 5 is an enlarged view of the time change of the absorption amount in the multi-cycle.
Table 1 shows the time change of the carbon dioxide absorption amount in the absorption step of the first cycle for the absorption liquids according to Examples and Comparative Examples, and Table 2 shows the carbon dioxide absorption amount in the multi-cycle absorption step of the absorption liquid. Shows the time change of.
For reference, FIGS. 4 and 5 and Tables 1 and 2 also show data of an aqueous solution containing 30% by mass of monoethanolamine (MEA), which is a conventionally used aqueous absorption liquid.

As is clear from the above results, when a highly polar diluent is added to a mixed solution of a carbon dioxide chemically absorbing amine and a tertiary polydentate amine, the amount of carbon dioxide absorbed and the absorption rate in the first cycle are increased. It can be seen that the absorption amount and the absorption rate increased even in the multicycle in which absorption and emission were repeated.
The MEA aqueous solution has a large absorption amount and absorption rate in the first cycle, but the absorption amount and absorption rate are significantly reduced in the multicycle, so that it is not suitable for the carbon dioxide separation and recovery step in which absorption and emission are continuously repeated. You can see that.

Next, the viscosities of the carbon dioxide absorbing liquids according to Examples and Comparative Examples were compared before and after carbon dioxide absorption at 30 ° C. The viscosity was measured using SVM3000 manufactured by AntonioPaar. The results are shown in Table 3 below.

It was found that when DMSO as a diluent was added to the carbon dioxide absorbing solution, the viscosity before carbon dioxide absorption could be reduced to about 1/3 of that without the diluent. It was also found that the viscosity after carbon dioxide absorption was reduced to about 1/4.
Therefore, by adding a highly polar and low volatility solvent such as DMSO to the carbon dioxide absorbing solution as a diluent, the carbon dioxide absorbing solution can be made low in viscosity both before and after the carbon dioxide absorption, and carbon dioxide can be reduced. It was found that the carbon dioxide separation and recovery performance in the continuous process of carbon absorption and emission was excellent.
As diluents having similar performance, hexamethylphosphoric acid triamide, 1,3-dimethyl-2-imidazolidinone, N, N'-dimethylpropylene urea, tetramethylurea, N, N-dimethylformamide, N, N-dimethylacetamide, N-methylformamide, N-methylacetamide, N-methyl-2-pyrrolidone, sulfolane and the like are preferably used, but particularly highly polar dimethylsulfoxide, hexamethylphosphoric acid triamide, 1,3-. Dimethyl-2-imidazolidinone, N, N'-dimethylpropylene urea, and tetramethylurea are preferred.

According to the present invention, in the carbon dioxide separation and recovery step, by using the carbon dioxide absorbing solution of the present invention, absorption and emission can be continuously and efficiently performed, so that carbon dioxide separation and recovery can be performed with energy saving. .. In particular, it can be used for biogas treatment that separates methane gas and carbon dioxide in biogas, concentrates methane gas, and recovers carbon dioxide.

101 Carbon dioxide cylinder 102 Methane cylinder 103 Mass flow controller for carbon dioxide 104 Mass flow controller for methane 105 Valve A
106 Valve B
107 Three-way valve C
108 Valve D
109 Three-way valve E
110 Controller 111 Motor 112 Stirring blade 113 Thermometer 114 Heater 115 Reaction vessel 116 Gas introduction pipe 117 Constant temperature bath 118 Cooling water circulation device 119 Cooling water circulation device 120 Liquid sample inlet 121 Absorbent liquid 122 Pressure gauge 123 Cooling tower 124 Carbon dioxide concentration meter 125 Gas flow meter 126 bypass

Claims (12)

Carbon dioxide chemically absorptive amine with nitrogen-hydrogen bond,
A tertiary polydentate amine having an oxygen atom and / or a nitrogen atom via a hydrocarbon group having 2 or more carbon atoms in the main chain, and having a total of 2 or more hydrogen bond acceptability of the oxygen atom and the nitrogen atom. It is a non-aqueous carbon dioxide absorber that contains
The carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond is a secondary amine having a hydrocarbon group having a hydroxyl group.
Further, carbon dioxide having high polarity and low volatility, and containing a non-aqueous diluent that reduces the viscosity of the carbon dioxide absorbing liquid both before and after carbon dioxide absorption. Carbon absorption liquid. The carbon dioxide absorbing solution according to claim 1, wherein the carbon dioxide chemically absorbing amine is at least one of dialkanolamine and N-alkylmonoalkanolamine. The carbon dioxide-absorbing amines are diethanolamine, dipropanolamine, dibutanolamine, N-methylethanolamine, N-ethylethanolamine, 3-methylamino-1-propanol, N-butylethanolamine, N-propyl. The carbon dioxide absorbing solution according to claim 2, which is at least one of ethanolamine. The carbon dioxide absorbing liquid according to claim 3, wherein the carbon dioxide chemically absorbing amine is diethanolamine. The diluents are dimethyl sulfoxide, hexamethylphosphoric acid triamide, 1,3-dimethyl-2-imidazolidinone, N, N'-dimethylpropylene urea, tetramethylurea, N, N-dimethylformamide, N, N-. The carbon dioxide absorbing solution according to any one of claims 1 to 4, which is dimethylacetamide, N-methylformamide, N-methylacetamide, N-methyl-2-pyrrolidone, or sulfolane. The tertiary polydentate amine is
The dioxide according to any one of claims 1 to 5, which is a tertiary polydentate amine in which two hydrocarbon groups having a hydroxyl group and one hydrocarbon group having no hydroxyl group are bonded to one nitrogen atom. Carbon absorber. The carbon dioxide absorbing solution according to any one of claims 1 to 6, wherein the tertiary polydentate amine is at least one of N-methyldiethanolamine, N-ethyldiethanolamine, and N-butyldiethanolamine. The ratio of the carbon dioxide chemically absorbing amine is 1/100 to 50/100 of the carbon dioxide chemically absorbing amine / (the carbon dioxide chemically absorbing amine + the tertiary amine solvent + the diluent) (mass ratio). The carbon dioxide absorbing solution according to any one of claims 1 to 7. The content ratio of the diluent is 1/100 to 50/100 in terms of the diluent / (the carbon dioxide chemically absorbing amine + the tertiary amine solvent + the diluent) (mass ratio), claims 1 to 1. Item 8. The carbon dioxide absorbing solution according to any one of 8. By contacting the carbon dioxide absorbing solution according to any one of claims 1 to 9 with a mixed gas containing carbon dioxide at 10 ° C. or higher and 40 ° C. or lower, carbon dioxide is absorbed by the carbon dioxide absorbing solution. An absorption step that selectively separates carbon dioxide from the mixed gas, and
Carbon dioxide separation including a heating and regeneration step of heating the carbon dioxide absorbing liquid that has absorbed the carbon dioxide to a temperature higher than the absorption temperature to dissipate and recover the absorbed carbon dioxide and regenerate the carbon dioxide absorbing liquid. Collection method. By contacting the carbon dioxide absorbing solution according to any one of claims 1 to 9 with a mixed gas containing carbon dioxide at 10 ° C. or higher and 40 ° C. or lower, carbon dioxide is absorbed by the carbon dioxide absorbing solution. An absorption step that selectively separates carbon dioxide from the mixed gas, and
Carbon dioxide including a heating and regeneration step of heating the carbon dioxide absorbing liquid that has absorbed the carbon dioxide to 60 ° C. or higher and 100 ° C. or lower to dissipate and recover the absorbed carbon dioxide and regenerate the carbon dioxide absorbing liquid. Separation and recovery method. A biogas treatment method for concentrating methane gas in biogas using the carbon dioxide separation and recovery method according to claim 10 or 11.

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