JP7539632B2 - Carbon dioxide absorbing liquid, carbon dioxide separation and capture method, and biogas treatment method - Google Patents

Description

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

Technologies for separating and capturing carbon dioxide are necessary for the production of methane using natural gas or biogas, and for maintaining closed living environments such as in outer space or under the sea. In addition, from the perspective of reducing greenhouse gas emissions, research is being actively conducted on sources of carbon dioxide of various concentrations, such as those targeting large emission sources such as thermal power plants and steelworks, and those targeting the fertilization of agricultural carbon dioxide from the atmosphere.
Among these, a chemical absorption method using an aqueous solution of an amine compound as an absorbing liquid for carbon dioxide has been put to practical use. In this chemical absorption process, a gas containing carbon dioxide is brought into contact with an absorbing liquid at around room temperature in an absorption tower to selectively chemically absorb the carbon dioxide into the absorbing liquid, the gas with a reduced carbon dioxide concentration and the absorbing liquid that has absorbed the carbon dioxide are separated into gas and liquid, and in a regeneration tower, the absorbing liquid that has absorbed the carbon dioxide is heated to release and recover the carbon dioxide, while at the same time regenerating the absorbing liquid, and the regenerated absorbing liquid is circulated to the absorption tower.

However, in such carbon dioxide separation and capture methods using an aqueous amine solution, a large amount of the water solvent evaporates during the regeneration process in which the absorbing solution is heated, and the latent heat of evaporation must be input in excess as regeneration energy. In addition, aqueous solutions have a large specific heat, and require more than twice the sensible heat compared to organic solvents. Furthermore, since the evaporation of the water solvent promotes the entrainment of the amine, which is the reaction substrate, careful control of the material balance is required in managing the separation and capture process. Therefore, extra cooling energy is required, such as by equipping the absorption tower and regeneration tower with a condenser for amine recovery, which is a factor that leads to the complication of the process. Furthermore, since the amine deteriorates during the high-temperature heating and regeneration process, it is necessary to periodically replenish the absorbing solution due to the disappearance of the reaction substrate, and there is a concern that this will increase running costs.
In order to solve such problems, non-aqueous solutions of amine compounds have been investigated.

For example, Patent Document 1 describes a carbon dioxide absorbing liquid that contains a carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and a hydrogen bond acceptor solvent that is an ionic liquid or an amide compound having a carbonyl group or a phosphinyl group as an electron-withdrawing group (Claim 1).

Patent document 2 describes a carbon dioxide absorbing liquid that contains a carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and a tertiary polydentate amine solvent that is highly hydrogen bond accepting and does not have a nitrogen-hydrogen bond (claim 1).

Patent document 3 describes an amine-containing absorption liquid used in a carbon dioxide absorption and release device having an absorption section that absorbs carbon dioxide under conditions of pressure P1 and temperature T1, a release section that releases carbon dioxide under conditions of pressure P2 (P1<P2) and temperature T2 (T1<T2), and a recovery section that separates the carbon dioxide obtained in the release section, and that the absorption liquid may further contain a solvent with low viscosity and low vapor pressure (high boiling point) to adjust the viscosity (Claim 1, paragraph [0033]).

Non-Patent Document 1 describes that, regarding the solvent effect on _CO2 absorption by amine compounds in non-aqueous solvents, organic solvents have high _CO2 solubility and are highly temperature-dependent, and that _CO2 emission is promoted in the medium to high _temperature range.

JP 2017-104775 A JP 2017-104776 A JP 2019-181401 A

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

In order to avoid the energy loss and complex equipment involved in conventional carbon dioxide separation and capture methods using an aqueous amine solution, it is necessary to separate and capture carbon dioxide under temperature conditions where the temperature difference between the carbon dioxide absorption temperature, which is near room temperature, and the temperature at which the absorbed carbon dioxide dissipates is small and where there is little volatilization or loss of the absorption solution.
In particular, the technology of separating the methane gas in biogas from the carbon dioxide and concentrating it is highly environmentally friendly in that it allows energy and useful materials to be extracted from waste. It is also expected that concentrating methane in a more energy-efficient manner will lead to improved power generation efficiency through subsequent methane combustion.

Under conditions where the temperature difference between absorption and dissipation is small, the dissipation amount is often smaller than the absorption amount, and the magnitude of the dissipation amount is one factor that determines the carbon dioxide separation and capture performance. A non-aqueous carbon dioxide absorbing liquid having a large dissipation amount can be selected from conventional absorbing liquids containing amine compounds, for example, as 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 diffusion rate at the diffusion temperature, the absorption rate is the rate-limiting step in the carbon dioxide separation and capture process. In an actual carbon dioxide separation and capture process, the absorption liquid is brought into contact with the treated gas for a certain period of time in an absorption tower to absorb carbon dioxide. Therefore, if the carbon dioxide absorption rate is slow, carbon dioxide cannot be absorbed to the saturated absorption amount, and the carbon dioxide separation and capture efficiency decreases.
The absorption rate of carbon dioxide depends on the rate of chemical reaction between the amine compound and carbon dioxide, the dissolution rate of carbon dioxide in the absorbing liquid, and the mass transport of the reactants with carbon dioxide and the amine compound in the absorbing liquid. The rate of chemical reaction between the amine compound and carbon dioxide is high when the basicity of the amine compound is high and there is little steric hindrance in the reaction between the amino group and carbon dioxide. However, when the basicity of the amine compound is high, it generally becomes difficult to dissipate the absorbed carbon dioxide with a small temperature difference, and it is necessary to increase the dissipation temperature. On the other hand, the dissolution rate of carbon dioxide in the absorbing liquid and the mass transport of the reactants with carbon dioxide and the amine compound in the absorbing liquid are strongly dependent on the viscosity of the absorbing liquid. Conventional non-aqueous absorbing liquids have a higher viscosity than aqueous absorbing liquids, and the mass transport in the absorbing liquid is hindered, making it difficult to sufficiently increase the absorption rate.
Therefore, in order to improve the efficiency of carbon dioxide separation and capture, it is necessary to increase the amount of carbon dioxide emitted and the amount of carbon dioxide absorbed per given time, that is, the carbon dioxide absorption rate.

Therefore, an object of the present invention is to provide a carbon dioxide absorbing liquid capable of efficiently performing absorption and desorption in a carbon dioxide separation and capture process.
Another object of the present invention is to provide a carbon dioxide absorption liquid suitable for biogas treatment, which separates methane gas and carbon dioxide in biogas, concentrates methane gas, and recovers carbon dioxide under conditions where the temperature difference between absorption and dissipation is small.

In order to solve the above problems, the present invention employs the following means.
[1] A non-aqueous carbon dioxide absorbing liquid containing a carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and a tertiary multidentate amine having an oxygen atom and/or a nitrogen atom via a hydrocarbon group having a carbon number of 2 or more in the main chain and having a hydrogen bond acceptability of 2 or more in total of the oxygen atom and the nitrogen atom, wherein the carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond is a secondary amine having a hydrocarbon group having an ether group and not having a hydroxyl group, and the tertiary multidentate amine is any one or more of 2-(diethylamino)ethanol, 2-(diisopropylamino)ethanol, 2-(dibutylamino)ethanol, 3-dimethylamino-1-propanol, 4-(dimethylamino)-1-butanol, N-methyldiethanolamine, N-ethyldiethanolamine, and N-butyldiethanolamine, characterized in that the carbon dioxide absorbing liquid.
[2] The carbon dioxide absorbing solution according to [1], wherein the carbon dioxide chemically absorbing amine is one or more of bis(2-methoxyethyl)amine, bis(2-ethoxyethyl)amine, and bis(2-isopropoxyethyl)amine.
[3] The carbon dioxide absorbing liquid according to [1] or [2], wherein a ratio of the carbon dioxide chemically absorbing amine to the tertiary multidentate amine is 1/100 to 50/100 in terms of a mass ratio of the carbon dioxide chemically absorbing amine/(the carbon dioxide chemically absorbing amine+the tertiary multidentate amine).
[4] A method for separating and recovering carbon dioxide, comprising: an absorption step of contacting the carbon dioxide absorbing liquid of any of [1] to [3] above with a mixed gas containing carbon dioxide at 10°C or higher and 40°C or lower to absorb carbon dioxide into the carbon dioxide absorbing liquid and selectively separate carbon dioxide from the mixed gas; and a heating regeneration step of heating the carbon dioxide absorbing liquid that has absorbed carbon dioxide to a temperature higher than the absorption temperature to dissipate and recover the absorbed carbon dioxide, thereby regenerating the carbon dioxide absorbing liquid.
[5] A method for separating and recovering carbon dioxide, comprising: an absorption step of contacting the carbon dioxide absorbing liquid according to any one of [1] to [ 3 ] above with a mixed gas containing carbon dioxide at 10°C or higher and 40°C or lower to absorb carbon dioxide into the carbon dioxide absorbing liquid and selectively separating carbon dioxide from the mixed gas; and a heating regeneration step of heating the carbon dioxide absorbing liquid that has absorbed carbon dioxide to 60°C or higher and 100°C or lower to dissipate and recover the absorbed carbon dioxide, and regenerating the carbon dioxide absorbing liquid.
[6] A biogas treatment method for concentrating methane gas in biogas using the carbon dioxide separation and recovery method according to [4] or [5].

According to the present invention, it is possible to provide a carbon dioxide absorption liquid capable of efficiently absorbing and discharging in a carbon dioxide separation and capture process, and to provide an energy-saving carbon dioxide separation and capture method for carbon dioxide sources of various concentrations under conditions where the temperature difference between absorption and discharging is small. In particular, it is possible to provide a carbon dioxide absorption liquid suitable for biogas processing in which methane gas and carbon dioxide in biogas are separated, the methane gas is concentrated, and carbon dioxide is captured.

Diagram showing carbon dioxide absorption test equipment Graph showing the temperature dependence of carbon dioxide absorption amount for each absorbing solution (CO ₂ (mol)/absorbing amine (mol)) Graph showing the temperature dependence of carbon dioxide absorption amount for each absorption solution (CO ₂ (mol)/absorption solution (kg)) A graph showing the temperature dependence of viscosity of each absorbing liquid

To improve the efficiency of carbon dioxide separation and capture, it is necessary to have a large amount of carbon dioxide absorption per given time at an absorption temperature near room temperature, i.e., a high carbon dioxide absorption rate and a large amount of carbon dioxide emission at a emission temperature with a small temperature change from the absorption temperature. Therefore, the inventors focused on the molecular structure of amine compounds that have excellent carbon dioxide absorption and release performance in a narrow temperature range, and also on the viscosity related to the mass transport in the absorption liquid, which is one of the controlling factors of the carbon dioxide absorption rate, and arrived at the present invention.

Hereinafter, embodiments of the present invention will be described. However, these embodiments are merely for the purpose of 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 claims. Various modifications made within the scope of the claims and the meaning of the invention equivalent thereto are considered to be within the scope of the present invention.

One embodiment of the present invention relates to a non-aqueous carbon dioxide absorbing liquid that contains a carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond and a tertiary multidentate amine having oxygen atoms and/or nitrogen atoms via a hydrocarbon group having a carbon number of 2 or more in the main chain and having hydrogen bond acceptance with a total of 2 or more oxygen and nitrogen atoms, characterized in that the carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond is a secondary amine having a hydrocarbon group with an ether group and not having a hydroxyl group.

Another embodiment of the present invention is a carbon dioxide separation and recovery method including an absorption step in which the carbon dioxide absorbing liquid is brought into contact with a mixed gas containing carbon dioxide at 10°C or higher and 40°C or lower to absorb carbon dioxide into the carbon dioxide absorbing liquid and selectively separate carbon dioxide from the mixed gas, and a heating and regeneration step in which the carbon dioxide absorbing liquid that has absorbed carbon dioxide is heated to a temperature higher than the absorption temperature to dissipate and recover the absorbed carbon dioxide, thereby regenerating the carbon dioxide absorbing liquid.

Yet another embodiment of the present invention is a biogas processing method that uses the carbon dioxide separation and capture method described above to concentrate methane gas in biogas.

The details of each element of this embodiment of the present invention (hereinafter referred to as "this embodiment") will be described in order below.

[Carbon dioxide chemically absorbing amine]
The carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond according to this embodiment is a secondary amine having a hydrocarbon group with an ether group and no hydroxyl group.

As carbon dioxide chemically absorbing amines having nitrogen-hydrogen bonds (hereinafter sometimes referred to as "absorbing amines"), dialkanolamines having two hydroxyl groups and N-alkylmonoalkanolamines having one hydroxyl group have been known so far. However, absorbing solutions containing these absorbing amines generally have high viscosity due to the presence of hydrogen bonds derived from the hydroxyl groups.
Therefore, the inventors searched for an absorbing amine that exhibits a lower viscosity than conventional ones and has excellent carbon dioxide absorption and release performance in a narrow temperature range, and discovered a new secondary amine that has no hydroxyl group and has a hydrocarbon group having an ether group.
The absorbent amine according to the present embodiment reacts with carbon dioxide at about room temperature to chemically absorb it, and is regenerated by heating it to a temperature higher than the absorption temperature to release the absorbed carbon dioxide. The absorbent amine has an ether bond, and thus has a basicity equivalent to that of an absorbent amine having a hydroxyl group, and has a lower viscosity than conventional absorbent amines, and can have a lower viscosity than conventional absorbing liquids even after absorbing carbon dioxide.

In the absorbent amine according to the present embodiment, the number of carbon atoms in the hydrocarbon group having an ether group is preferably 2 or more in order to reduce volatility and prevent loss due to evaporation during diffusion. In addition, the chemical structure may be either symmetrical or asymmetrical, but a symmetrical structure is preferred in terms of lower production costs.
Among these, at least one of bis(2-methoxyethyl)amine (BMEA) represented by the following formula 1, bis(2-ethoxyethyl)amine (BEEA) represented by the following formula 2, and bis(2-isopropoxyethyl)amine (BIPEA) represented by the following formula 3 is preferred.

[Tertiary polydentate amine]
The tertiary multidentate amine according to this embodiment is a tertiary multidentate amine having oxygen atoms and/or nitrogen atoms via a hydrocarbon group having a main chain carbon number of 2 or more, and the total of oxygen atoms and nitrogen atoms is 2 or more, so as to have no nitrogen-hydrogen bonds, have high hydrogen bond acceptance, be stable in terms of three-dimensional structure, and promote the reaction between the carbon dioxide chemically absorbent amine and carbon dioxide. Here, "high hydrogen bond acceptance, be stable in terms of three-dimensional structure, and promote the reaction between the carbon dioxide chemically absorbent amine and carbon dioxide" means that, for example, as shown in [Formula 4], the nitrogen atoms and oxygen atoms of the tertiary multidentate amine interact with the hydrogen of the carbon dioxide chemically absorbent amine in a multidentate manner to stabilize the reaction product with carbon dioxide.

In [Formula 4], the compound represented by H-N(R ¹ )R ² represents the carbon dioxide chemically absorbing amine according to this embodiment, and in [Formula 4], the compound represented by X ₁ R ⁵ N(R ³ )R ⁴ represents the tertiary multidentate amine according to the present invention, in which R ³ and R ⁴ are hydrocarbon groups which may be unsubstituted or substituted, R ⁵ is a hydrocarbon group having 2 or more carbon atoms in the main chain which may be unsubstituted or substituted, and X ¹ is a nitrogen atom or an oxygen atom and hydrogen bonded thereto, or a hydrocarbon group which may be unsubstituted or substituted. In this specification, the hydrocarbon group having 2 or more carbon atoms in the main chain refers to a basic skeleton having 2 or more carbon atoms in the shortest length between the nitrogen atom of the tertiary amine and the nitrogen atom or oxygen atom, such as an ethylene group, a propylene group, or a butylene group. Therefore, for example, an amine having 1 hydrocarbon in the main chain, such as HO-C(H)(CH ₃ )N(R ³ )R ⁴ , is not included in the tertiary multidentate amine according to the present invention. As the hydrocarbon group having two or more carbon atoms in the main chain, an ethylene group, a propylene group, or a butylene group which constitutes a non-cyclic skeleton having a high degree of freedom is preferred, and an ethylene group is more preferred.

The electron-donating oxygen atom or nitrogen atom has high hydrogen bond acceptance and can interact with the hydrogen of the carbon dioxide chemically absorbent amine to stabilize the reaction product with carbon dioxide. The hydrocarbon group having 2 or more carbon atoms in the main chain represented by R ⁵ and a curve in formula 4 has a high degree of freedom, so the nitrogen atom and/or oxygen atom bonded to both ends of the hydrocarbon group can form a hydrogen bond with the hydrogen of the carbon dioxide chemically absorbent amine. Such tertiary multidentate amines can form hydrogen bonds with the hydrogen of the carbon dioxide chemically absorbent amine to stabilize the reaction with carbon dioxide at a relatively low temperature side such as near room temperature where carbon dioxide is absorbed, thereby stabilizing the reaction. In addition, the hydrogen bonded to X ¹ can form a hydrogen bond with the carbon dioxide that has reacted with the carbon dioxide chemically absorbent amine, further stabilizing the reaction product and promoting the reaction with carbon dioxide. On the other hand, at a high temperature side where carbon dioxide is released, the degree of this hydrogen bond decreases, which can promote the release of carbon dioxide.

The tertiary multidentate amine according to this embodiment is not particularly limited as long as it has oxygen and/or nitrogen atoms via a hydrocarbon group having two or more carbon atoms in the main chain so as to be rich in hydrogen bond acceptance, is three-dimensionally stable, and promotes the reaction between the carbon dioxide chemically absorbing amine and carbon dioxide, and the total of the oxygen and nitrogen atoms is two or more; however, for example, a tertiary multidentate amine represented by [Formula 5] in which one hydrocarbon group having a hydroxyl group and two hydrocarbon groups not having a hydroxyl group are bonded to one nitrogen atom, or a tertiary multidentate amine represented by [Formula 6] in which two hydrocarbon groups having a hydroxyl group and one hydrocarbon group not having a hydroxyl group are bonded to one nitrogen atom, is preferred.
(In formula 5, ^R3 and ^R4 are hydrocarbon groups which may be unsubstituted or substituted (excluding a hydroxyl group), n1 is an integer of 2 or more, and the carbon atom in the parentheses may have a substituent, or a part of the carbon atom may be substituted with a heteroatom.)
(In formula 6, ^R3 is a hydrocarbon group which may be unsubstituted or substituted (excluding a hydroxyl group), n1 and n2 are integers of 2 or more, and the carbon atom in the parentheses may have a substituent, or a part of the carbon atom may be substituted with a heteroatom.)

Examples of tertiary polydentate amines represented by formula 5 include 2-dimethylaminoethanol ((Me)2N(EtOH)), 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)), and 4-(dimethylamino)-1-butanol ((Me)2N(n-BuOH)).

Among these, tertiary amines in which R ³ and R ⁴ in [Formula 5] are hydrocarbon groups having 2 or more carbon atoms, i.e., tertiary amines in which one hydrocarbon group having 2 or more carbon atoms in the main chain and one hydrocarbon group having 2 or more carbon atoms are bonded to one nitrogen atom, or tertiary amines in which n1 in [Formula 5] is 3 or more, i.e., tertiary amines in which one hydrocarbon group having 3 or more carbon atoms in the main chain and one hydrocarbon group having 2 or more carbon atoms are bonded to one nitrogen atom, are preferred. Specific examples include 2-(diethylamino)ethanol, 2-(diisopropylamino)ethanol, and 2-(dibutylamino)ethanol, as well as 3-dimethylamino-1-propanol and 4-(dimethylamino)-1-butanol.

Examples of tertiary polydentate amines represented by formula 6 include N-methyldiethanolamine (MDEA), N-ethyldiethanolamine ((Et)N(EtOH)2), and N-butyldiethanolamine ((n-Bu)N(EtOH)2).

[Carbon dioxide absorbing liquid]
The carbon dioxide absorbing liquid according to this embodiment contains the carbon dioxide chemically absorbing amine having the above-mentioned nitrogen-hydrogen bond and the tertiary multidentate amine not having the above-mentioned nitrogen-hydrogen bond. The carbon dioxide chemically absorbing amine and the tertiary multidentate amine according to this embodiment are usually liquid at room temperature, and the carbon dioxide absorbing liquid according to this embodiment is obtained by mixing the carbon dioxide chemically absorbing amine and the tertiary multidentate amine.

As the carbon dioxide chemically absorbing amine, one or more of the above-mentioned bis(2-methoxyethyl)amine, bis(2-ethoxyethyl)amine, and bis(2-isopropoxyethyl)amine are preferred, and as the tertiary multidentate amine, it is preferred to combine it with one or more of N-methyldiethanolamine, N-ethyldiethanolamine, and N-butyldiethanolamine.

The carbon dioxide absorbing liquid according to this embodiment has a lower viscosity than conventional non-aqueous absorbing liquids, and the increase in viscosity due to the absorption of carbon dioxide at around room temperature is suppressed, so that the absorption rate is high. In addition, even if the temperature rise from the absorption process to the heating regeneration process is small, a large amount of dissipation can be obtained, the carbon dioxide recovery rate and the regeneration efficiency of the absorbing liquid can be improved, and the loss due to the volatilization of the absorbing liquid can be reduced. Therefore, the absorbing liquid according to this embodiment can provide an energy-saving carbon dioxide separation and recovery method for carbon dioxide sources of various concentrations. In particular, it is suitable for biogas processing in which methane gas and carbon dioxide are separated and recovered from biogas, and the methane gas is concentrated.

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 types of carbon dioxide chemically absorbing amine, tertiary multidentate amine, and diluent, but the carbon dioxide chemically absorbing amine/(carbon dioxide chemically absorbing amine+tertiary multidentate amine) (mass ratio) is preferably 1/100 to 50/100, and more preferably 10/100 to 40/100. If the ratio of the carbon dioxide chemically absorbing amine is within this range, the amount and speed of carbon dioxide absorption near room temperature can be increased, and easy carbon dioxide desorption can be achieved under mild regeneration temperature conditions.

The carbon dioxide absorbing liquid according to this 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, acidic gases other than carbon dioxide, nitrogen gas, oxygen gas, water, soot, and the like, but the carbon dioxide absorbing liquid according to the present embodiment is particularly suitable for a method for separating and recovering methane gas and carbon dioxide contained in biogas. Examples of acidic gases other than carbon dioxide include hydrogen sulfide; sulfur oxides such as sulfur monoxide, sulfur dioxide (sulfurous acid gas), and sulfur trioxide; nitrogen oxides such as nitric oxide, nitrogen dioxide, nitrous oxide (dinitrogen monoxide), dinitrogen trioxide, dinitrogen tetroxide, and dinitrogen pentoxide; inorganic acids such as hydrochloric acid, nitric acid, phosphoric acid, and sulfuric acid; and organic acids such as carboxylic acid, sulfonic acid, and carbonic acid. The carbon dioxide absorbing liquid of the present invention has little effect on the recovery of carbon dioxide even if the mixed gas contains a saturated amount of water as another component. In addition, the carbon dioxide absorbing liquid of the present invention has little effect on the recovery of carbon dioxide even if the mixed gas contains soot and dust as another component.

[Carbon dioxide separation and capture method]
Next, a carbon dioxide separation and capture method using the carbon dioxide absorbing liquid according to this embodiment will be described.
The carbon dioxide separation and recovery method of the present invention includes an absorption step in which the carbon dioxide absorbing liquid is brought into contact with a mixed gas containing carbon dioxide, thereby absorbing carbon dioxide into the carbon dioxide absorbing liquid, and selectively separating carbon dioxide from the mixed gas, and a heating regeneration step in which the carbon dioxide absorbing liquid that has absorbed carbon dioxide is heated to a temperature higher than that in the absorption step to dissipate and recover the absorbed carbon dioxide, and the carbon dioxide absorbing liquid is regenerated. The method for contacting the carbon dioxide absorbing liquid with the mixed gas containing carbon dioxide may be, for example, an absorption tower method or a scrubber method, but is not limited to these embodiments, as long as the gas-liquid contact efficiency is increased and the absorption rate of carbon dioxide is improved. In addition, the method for heating and regenerating the carbon dioxide absorbing liquid that has absorbed carbon dioxide may be, for example, a regeneration tower method or a flash drum method, but is not limited to these embodiments, as long as the heat transfer efficiency of heating and the gas-liquid contact efficiency are increased and the dissipation rate of carbon dioxide is improved.

The carbon dioxide separation and capture method according to this embodiment has a fast absorption rate near room temperature, and carbon dioxide dissipates easily with a small temperature rise. Moreover, the evaporation loss of the absorbing liquid is small, the specific heat is low, and the reaction heat is small, so that the energy required to regenerate the carbon dioxide absorbing liquid per unit of carbon dioxide captured can be reduced, and the carbon dioxide separation and capture efficiency can be improved. Therefore, the carbon dioxide separation and capture method according to this embodiment can separate and capture carbon dioxide with energy savings from carbon dioxide sources of various concentrations, and is particularly used in a biogas processing method that separates and captures methane gas and carbon dioxide from biogas and concentrates the methane gas.

The temperature in the absorption step is preferably in the vicinity of room temperature (25° C.±15° C.), ie, 10° C. to 40° C. In the vicinity of room temperature, there is no need to excessively cool the carbon dioxide absorbing solution or the target gas to be treated, and the amount and rate of carbon dioxide absorption can be improved, thereby achieving energy savings.
In the carbon dioxide separation and capture method of the present invention, the pressure in the absorption step is not particularly limited. When the target gas is a gas under near normal pressure, it is preferable from the viewpoint of energy saving to carry out the absorption step at near normal pressure, since no extra energy is required to compress the gas under treatment. On the other hand, in order to improve the amount and rate of absorption of carbon dioxide into the carbon dioxide absorbing solution, high pressure conditions of, for example, 1 MPaG to 6 MPaG or higher, can also be used.

The temperature of the heating regeneration step is higher than that of the absorption step, but is not particularly limited. However, if the temperature of the regeneration step is significantly increased, the amount of carbon dioxide absorbing solution dissipated increases, but the energy required for heating increases, 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 less, more preferably 80° C. or less, and particularly preferably 60° C. or less.
The pressure in the heating regeneration step is preferably equal to or lower than the pressure in the absorption step, but is not particularly limited. The carbon dioxide absorbing liquid of the present invention has a low vapor pressure and can suppress volatilization, so that it can be treated under reduced pressure. By carrying out a moderate reduced pressure treatment under conditions in which the energy required for decompression is not too large, it is expected that the amount of carbon dioxide released from the carbon dioxide absorbing liquid can be improved. On the other hand, the carbon dioxide released from the carbon dioxide absorbing liquid in the regeneration step can also be recovered under high pressure. By recovering carbon dioxide under high pressure, it is possible to reduce compression energy when high-pressure carbon dioxide is required in a later stage.

In the carbon dioxide separation and capture method according to this embodiment, the proportion of carbon dioxide absorbed in the absorption step that is captured in the heating regeneration step is preferably 50% or more, and more preferably 60% or more, when the temperature difference between the absorption step and the heating regeneration step is 30°C (30°C to 60°C). When the temperature difference is 50°C (10°C to 60°C), it is preferably 60% or more, and more preferably 70% or more. The smaller the temperature difference, the lower the capture rate, but the smaller the temperature difference, the less heating energy is required for capture.

The carbon dioxide absorption liquid of the present invention and the carbon dioxide separation and capture method using the same can separate and capture carbon dioxide from carbon dioxide sources of various concentrations in an energy-saving manner, and are particularly suitable for biogas processing methods that concentrate and process methane gas in biogas.

The present invention will be described below based on examples, but the present invention is not limited to these examples. Measurements were performed using the following carbon dioxide absorption test.

[Carbon dioxide absorption test]
A carbon dioxide absorption test was performed at normal pressure using the carbon dioxide absorption test apparatus shown in Fig. 1. The carbon dioxide absorption test apparatus includes a carbon dioxide cylinder 101 for introducing carbon dioxide into a glass reaction vessel 112, a pressure reducing valve 102, a flowmeter 103, a valve 104, a coil-shaped preheater 105, and a valve 106, as well as a thermostatic bath 108 for containing a heat medium 107, a resistance indicator 110 connected to a platinum temperature measuring element 109 for measuring the temperature of the heat medium 107 in the thermostatic bath 108, a cooling water circulator 111 for adjusting the temperature of the heat medium 107 in the thermostatic bath 108 to a constant value, and a magnetic stirrer 114 for rotating a rotor 113 placed in the reaction vessel 112.
A stopper 115, a gas inlet pipe 116, and a valved discharge pipe 117 can be attached to the reaction vessel 112. The valve 106 can be connected to the gas inlet pipe 116 attached to the reaction vessel 112. The preheater 105 and the reaction vessel 112 are immersed in a heat medium 107 in a constant temperature bath 108, and are kept at a constant temperature by a cooling water circulator 111. A rotor 113 is placed in the reaction vessel 112, and the carbon dioxide absorbing liquid in the reaction vessel 112 can be stirred by a magnetic stirrer 114. The heat medium 107 is water or silicone oil.

The procedure for measuring the amount of carbon dioxide absorption using this carbon dioxide absorption test device is described below.
1) Under a nitrogen atmosphere, a predetermined amount (about 6 to 7 g) of the carbon dioxide absorbing liquid is dispensed into a glass reaction vessel 112, and the mouth of the reaction vessel 112 is sealed with a stopper 115. The mass of the entire reaction vessel is measured with an analytical balance, and the mass of the tare (reaction vessel 112, rotor 113, and stopper 115) is subtracted from this to obtain the mass W _total of the carbon dioxide absorbing liquid.
2) Attach the gas inlet pipe 116 and the outlet pipe 117 to the reaction vessel 112, and measure the mass again to obtain the mass _W2 of the entire reaction vessel.
3) The reaction vessel 112 is placed in the thermostatic chamber 108. The gas introduction pipe 116 is connected to the valve 106.
4) The temperature of the thermostatic chamber 108 is kept at a predetermined absorption temperature, carbon dioxide is circulated through the reaction vessel 112, and the carbon dioxide is absorbed in the carbon dioxide absorbing liquid for 15 to 60 minutes. After the predetermined time has elapsed, the mass of the entire reaction vessel is measured with an analytical balance and is designated as _W3 .
5) The mass of carbon dioxide absorbed in the carbon dioxide absorbing liquid, W _CO2 , is calculated based on the following formula.
W _CO2 = W ₃ - W ₂
In addition, the carbon dioxide absorption amount α _CO2 per mole of the carbon dioxide chemically absorbing amine in the carbon dioxide absorbing solution is determined based on the following formula.
α _CO2 = (W _CO2 /M _CO2 )/(W ₁ /M _chem )
In the above formula, M _CO2 is the molar mass of carbon dioxide, M _chem is the molar mass of the carbon dioxide chemically absorbing amine, and the mass W ₁ of the carbon dioxide chemically absorbing amine is calculated by multiplying the mass W _total of the carbon dioxide absorbing liquid by the mass fraction x ₁ .
6) The amount of carbon dioxide absorbed in the low temperature range is subtracted from the amount of carbon dioxide absorbed in the high temperature range to calculate the amount of carbon dioxide emitted (amount of carbon dioxide emitted), and then the amount of carbon dioxide absorbed in the low temperature range is divided to obtain the carbon dioxide recovery rate.

Example 1
Bis(2-methoxyethyl)amine (BMEA, manufactured by Tokyo Chemical Industry Co., Ltd., purity >98.0%) as a carbon dioxide chemically absorbing amine and N-methyldiethanolamine (MDEA, manufactured by Aldrich, purity ≥99%) as a tertiary polydentate amine were mixed to prepare an MDEA solution containing 30 wt % BMEA in mass fraction, which was used as the carbon dioxide absorbing liquid in Example 1. The water content was 1% or less.

Example 2
A carbon dioxide absorbing solution according to Example 2 was obtained in the same manner as in Example 1, except that bis(2-ethoxyethyl)amine (BEEA, manufactured by Tokyo Chemical Industry Co., Ltd., purity>98.0%) was used as the carbon dioxide absorbing amine (mass fraction of BEEA=30 wt%).

Example 3
A carbon dioxide absorbing liquid according to Example 3 was obtained in the same manner as in Example 1, except that bis(2-isopropoxyethyl)amine (BIPEA, manufactured by Tokyo Chemical Industry Co., Ltd., purity>97.0%) was used as the carbon dioxide absorbing amine (mass fraction of BIPEA=30 wt%).

(Comparative Example 1)
A carbon dioxide absorbing solution according to Comparative Example 1 was obtained in the same manner as in Example 1, except that diethanolamine (DEA, manufactured by Wako Pure Chemical Industries, Ltd., purity 99.0+%) was used as the carbon dioxide chemically absorbing amine (mass fraction of DEA=30 wt%).

(Comparative Example 2)
A carbon dioxide absorbing solution according to Comparative Example 2 was obtained in the same manner as in Example 1, except that N-ethylethanolamine (EEA, manufactured by Tokyo Chemical Industry Co., Ltd., purity>98.0%) was used as the carbon dioxide chemically absorbing amine (mass fraction of EEA=30 wt%).

(Comparative Example 3)
A carbon dioxide absorbing solution according to Comparative Example 3 was obtained in the same manner as in Example 1, except that N-butylethanolamine (BEA, manufactured by Kanto Chemical Co., Ltd., purity >99.0%) was used as the carbon dioxide chemically absorbing amine (mass fraction of BEA = 30 wt%).

[Carbon dioxide absorption test]
A carbon dioxide absorption test was carried out on the carbon dioxide absorbing solutions according to the respective Examples and Comparative Examples under the following conditions.
Absorption temperature: 10°C, 30°C, 60°C, 80°C

FIG. 2 shows the temperature dependency of the carbon dioxide absorption amount α _CO2 (per mole of absorbing amine) of each absorbing solution at each temperature, and FIG. 3 shows the temperature dependency of the carbon dioxide absorption amount α _CO2 converted into a value per kg of absorbing solution.
Table 1 shows the carbon dioxide absorption amount converted per 1 kg of absorbing amine at absorption temperatures of 10° C., 30° C., and 60° C., and the carbon dioxide recovery rate.

From the results in FIG. 2, it was found that the absorbing solutions according to Examples 1 to 3 containing absorbent amines having no hydroxyl group and an ether bond (BMEA, BEEA, BIPEA) had a larger carbon dioxide absorption amount α _CO2 in the low temperature range and a smaller carbon dioxide absorption amount α _CO2 in the high temperature range (larger emission amount) than the absorbing solutions containing absorbent amines having a hydroxyl group (DEA, EEA, BEA) according to Comparative Examples 1 to 3.
On the other hand, from the carbon dioxide absorption amount per unit mass of the absorbent amine shown in Figure 3, it was found that the absorbent amines according to Examples 1 to 3 have a larger molecular weight than the absorbent amine of the comparative example, and therefore the absolute value of the carbon dioxide absorption amount is smaller than that of Comparative Examples 1 to 3. However, the carbon dioxide absorption amount per unit mass of the absorbent amines of Examples 1 to 3 is highly temperature dependent and there is a large difference in the absorption amount between the low temperature range and the high temperature range, so that as shown in Table 1, it was found that the carbon dioxide recovery rate was excellent.
A high carbon dioxide recovery rate means that the absorbing liquid can be efficiently regenerated and a larger amount of carbon dioxide can be separated and recovered by circulating a predetermined mass of the absorbing liquid. Therefore, the absorbing liquid according to this embodiment is suitable for an actual carbon dioxide separation and recovery process in which the absorbing liquid is regenerated and used repeatedly, and is particularly expected to be applied to biogas treatment in which the absorbing liquid is regenerated under narrow temperature conditions.

[Viscosity measurement]
The viscosities of the absorbing solutions according to the respective Examples and Comparative Examples at 10° C., 30° C., and 60° C. before carbon dioxide absorption were measured using a viscometer (SVM3000 manufactured by Anton Paar).
The results are shown in Table 2 below and in FIG.

From the results in Table 2, it was found that the absorbing solutions according to Comparative Examples 2 and 3, which contain DEA and EEA with a reduced number of hydroxyl groups to one, before carbon dioxide absorption, were able to reduce the viscosity at 10°C to 60°C to half or less compared to the absorbing solution according to Comparative Example 1, which contains DEA, an absorbent amine with two hydroxyl groups. On the other hand, it was found that the absorbing solutions according to Examples 1 to 3, which contain absorbent amines (BMEA, BEEA, BIPEA) that do not have hydroxyl groups and have ether bonds, were able to further reduce the viscosity at 10°C to 60°C compared to the absorbing solutions according to Comparative Examples 1 to 3.

In addition, the viscosity of each absorption liquid was measured after it was saturated with carbon dioxide at an absorption temperature of 30°C. The results are shown in Table 3 below, along with the viscosity before carbon dioxide absorption at 30°C and the ratio of the viscosity after absorption to the viscosity before absorption.

From the results in Table 3, it was found that the absorbing solutions of Examples 1 to 3 not only had low viscosity before absorbing carbon dioxide, but also had low viscosity after absorbing carbon dioxide, and the increase in viscosity before and after absorption was small.
Therefore, in the absorption liquid according to Absorption Liquid Examples 1 to 3 of this embodiment, the dissolution rate of carbon dioxide into the absorption liquid and the mass transport of the reaction product with carbon dioxide and the amine compound in the absorption liquid occur quickly, and it is considered that absorption and desorption can be performed efficiently in the carbon dioxide separation and capture process.

According to the present invention, by using the carbon dioxide absorption liquid of the present invention in the carbon dioxide separation and capture process, it is possible to efficiently absorb and release carbon dioxide within a narrow temperature range, and to perform carbon dioxide separation and capture with reduced energy. In particular, it can be used in biogas processing to separate methane gas and carbon dioxide in biogas, concentrate methane gas, and capture carbon dioxide.

REFERENCE SIGNS LIST 101 Carbon dioxide cylinder 102 Pressure reducing valve 103 Flow meter 104 Valve 105 Preheater 106 Valve 107 Heat medium 108 Thermostatic bath 109 Platinum temperature detector 110 Resistance indicator 111 Cooling water circulation device 112 Reaction vessel 113 Rotor 114 Magnetic stirrer 115 Stopper 116 Gas inlet pipe 117 Discharge pipe

Claims (6)

a carbon dioxide chemisorbing amine having a nitrogen-hydrogen bond;
A non-aqueous carbon dioxide absorbing liquid containing a tertiary multidentate amine having an oxygen atom and/or a nitrogen atom via a hydrocarbon group having a main chain carbon number of 2 or more and having a hydrogen bond acceptability of 2 or more in total of oxygen atoms and nitrogen atoms,
The carbon dioxide chemically absorbing amine having a nitrogen-hydrogen bond is a secondary amine having a hydrocarbon group having an ether group and having no hydroxyl group,
The carbon dioxide absorbing solution is characterized in that the tertiary polydentate amine is one or more of 2-(diethylamino)ethanol, 2-(diisopropylamino)ethanol, 2-(dibutylamino)ethanol, 3-dimethylamino-1-propanol, 4-(dimethylamino)-1-butanol, N-methyldiethanolamine, N-ethyldiethanolamine, and N-butyldiethanolamine . The carbon dioxide absorbing liquid according to claim 1, wherein the carbon dioxide chemically absorbing amine is one or more of bis(2-methoxyethyl)amine, bis(2-ethoxyethyl)amine, and bis(2-isopropoxyethyl)amine. The carbon dioxide absorbing liquid according to claim 1 or 2, wherein the ratio of the carbon dioxide chemically absorbing amine to the tertiary multidentate amine is 1/100 to 50/100 in terms of the carbon dioxide chemically absorbing amine/(the carbon dioxide chemically absorbing amine+the tertiary multidentate amine) (mass ratio). An absorption step of contacting the carbon dioxide absorbing liquid according to any one of claims 1 to 3 with a mixed gas containing carbon dioxide at 10 ° C. or more and 40 ° C. or less to absorb carbon dioxide into the carbon dioxide absorbing liquid, and selectively separating carbon dioxide from the mixed gas;
a heating regeneration step of heating the carbon dioxide absorbing liquid that has absorbed carbon dioxide to a temperature higher than the absorption temperature to dissipate and recover the absorbed carbon dioxide, thereby regenerating the carbon dioxide absorbing liquid. A carbon dioxide separation and recovery method comprising: an absorption step of contacting the carbon dioxide absorbing liquid according to any one of claims 1 to 3 with a mixed gas containing carbon dioxide at 10°C or higher and 40°C or lower to absorb carbon dioxide into the carbon dioxide absorbing liquid and selectively separating carbon dioxide from the mixed gas; and a heating and regeneration step of heating the carbon dioxide absorbing liquid that has absorbed carbon dioxide to 60°C or higher and 100°C or lower to dissipate and recover the absorbed carbon dioxide, thereby regenerating the carbon dioxide absorbing liquid. A biogas processing method for concentrating and processing methane gas in biogas using the carbon dioxide separation and capture method according to claim 4 or 5.