Composite absorbent for improving CO2 absorption rate of N-methyldiethanolamine aqueous solution and application thereof

Abstract

The invention discloses a composite absorbent for improving the CO2 absorption rate of an N-methyldiethanolamine aqueous solution and application of the composite absorbent, AFIL in the composite absorbent can rapidly react with CO2 and generate hydrogen ions, so that the AFIL can be rapidly combined with MDEA to improve the CO2 absorption rate. The composite absorbent for improving the CO2 absorption rate of the N-methyldiethanolamine aqueous solution is prepared from the following raw materials in percentage by mass: 5-25wt% of N-methyldiethanolamine, namely MDEA, serving as an absorption main component, and the balance of a solvent, wherein the mass fraction range of the N-methyldiethanolamine, namely MDEA, is 5-25wt%; the mass fraction range of the amino functionalized ionic liquid, namely AFIL, serving as an activator component is 5-25wt%; the mass fraction of the water serving as an auxiliary absorption and dilution component is 50-70wt%; wherein the amino-functionalized ionic liquid consists of an alkyl polyamine cation and a cyclic nitrogen-containing anion, and is synthesized by directly carrying out equimolar addition reaction on the cation and the anion on the basis of an acid-base neutralization reaction.

Description

Technical Field

[0001] The present invention relates to a composite absorbent for absorbing CO2 and its application, and more specifically to a composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution and its application. Background Art

[0002] At present, excessive CO2 emissions from industrial processes have led to frequent extreme weather. At the same time, CO2, as a common impurity, exists in energy gases such as synthesis gas and natural gas, which reduces its utilization efficiency and needs to be further removed. Therefore, the development of new CO2 absorption technology has received widespread attention (Nature, 2016, 534, 631). Chemical absorbents represented by organic amine solutions have been widely used in CO2 absorption and separation due to their large CO2 absorption capacity and high chemical stability (Chemical Engineering Journal, 2023, 466, 143279). Among them, MDEA, as an important representative of organic amines, has excellent CO2 absorption performance. However, the rate at which MDEA aqueous solution absorbs CO2 is slow, which usually requires the addition of an activator to increase the CO2 absorption rate (Journal of Cleaner Production, 2023, 433, 139816).

[0003] In order to improve the CO2 absorption rate of MDEA aqueous solution, the Chinese patent with publication number CN 108722115A discloses a special absorbent for removing CO2 from mixed gas, wherein the concentration of MDEA is 10-35wt%, the concentration of piperazine is 5-10wt%, and the rest is water. The absorbent has the advantages of large absorption capacity, small circulation volume, and low energy consumption. The Chinese patent with publication number CN114015486A discloses an activation enhancer for natural gas decarbonization based on MDEA method, wherein the absorbent is 40-80wt% MDEA+5-20wt% tetramethylguanidine+10-40wt% N-ethylenecyclohexylamine+5-25wt% diisopropylbenzene peroxide, which can be used for natural gas decarbonization. However, the above absorbent does not have a good effect on enhancing the absorption rate of MDEA. At the same time, the absorbent has high volatility and desorption energy consumption, which limits its wide industrial application.

[0004] Functional ionic liquids (FIL) are considered to be potential activators of MDEA aqueous solutions due to their high CO2 absorption capacity, low vapor pressure, high thermal stability and structural controllability (Chemical Reviews, 2017, 117, 9652). For example, when 1-butyl-3-methylimidazolium tetrafluoroborate activated MDEA aqueous solution to absorb CO2, the CO2 absorption rate increased by 2 times (Applied Energy, 2019, 235, 311). For example, a Chinese patent with publication number CN 101804292 B discloses a FIL-activated MDEA aqueous solution for CO2 absorption, wherein the cation of the FIL is tetraalkylammonium, and the anion is an amino acid root or an organic hydroxy acid root, and the composite absorbent effectively improves the CO2 absorption rate. In addition, a Chinese patent with publication number CN 104415642A discloses a diamino ionic liquid-MDEA composite absorbent for CO2 absorption. When the content of diamino ionic liquid is 5-20wt%, the content of MDEA is 5-50wt%, and the water content is 30-90wt%, the CO2 absorption rate of the composite absorbent can be increased by about 5 times relative to a 30wt% MDEA aqueous solution. At the same time, the CO2 absorption capacity is 1.35molCO2 / mol absorbent and the desorption temperature is 343.2K. However, the above composite absorbent is still restricted by the low CO2 absorption rate and high desorption enthalpy, resulting in the need for larger equipment and higher energy consumption in the CO2 absorption process. Therefore, it is necessary to develop a composite absorbent with a higher absorption rate and its application for CO2 absorption and removal. Summary of the invention

[0005] The purpose of the present invention is to provide a composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution in order to solve the problem that the existing MDEA aqueous solution has a slow CO2 absorption rate. The AFIL in the composite absorbent can react quickly with CO2 and produce hydrogen ions, so that it can quickly combine with MDEA to improve the CO2 absorption rate.

[0006] Another object of the present invention is to provide the application of the composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution in CO2 absorption. On the premise of maintaining the low desorption enthalpy and CO2 absorption capacity of the composite absorbent, the CO2 absorption rate of MDEA aqueous solution can be significantly improved, and it is suitable for other CO2-containing mixed gases such as synthesis gas, natural gas, and refinery gas.

[0007] The present invention is achieved through the following technical solutions:

[0008] The composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution of the present invention is composed of the following raw materials in mass fraction:

[0009] The mass fraction of N-methyldiethanolamine (MDEA) as the main absorption component is in the range of 5 to 25 wt %;

[0010] The mass fraction of the amino-functionalized ionic liquid, i.e., AFIL, as the activator component is in the range of 5 to 25 wt %;

[0011] The mass fraction of water as auxiliary absorption and dilution component is: 50-70wt%;

[0012] The amino-functionalized ionic liquid is composed of an alkyl polyamine cation and a cyclic nitrogen-containing anion, and is synthesized by an equimolar addition reaction between the cation and the anion directly based on an acid-base neutralization reaction.

[0013] The composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution of the present invention has a further technical solution in which the alkyl polyamine cation is tetraethylenepentamine or pentaethylenehexamine, and the cyclic nitrogen-containing anion is 2-methylimidazole or piperazine.

[0014] The composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution of the present invention has a further technical solution in which the synthesis steps of the amino-functionalized ionic liquid are as follows:

[0015] S1. Weigh cations and slightly excess anions and put them into a reaction bottle, then add a sufficient amount of deionized water as a water solvent, mix, and stir at room temperature for more than 24 hours to allow them to react fully to obtain an AFIL aqueous solution;

[0016] S2, placing the AFIL aqueous solution prepared in step S1 into a rotary evaporator, and performing rotary evaporation under vacuum conditions at a temperature of 65 to 75° C. to remove unreacted components and deionized water to obtain an amino-functionalized ionic liquid;

[0017] S3. The trace amount of deionized water contained in the amino-functionalized ionic liquid obtained in step S2 is dried in a vacuum oven at a temperature of 65-75° C. for more than 24 hours to obtain AFIL.

[0018] The composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution of the present invention can be used in absorbing CO2. A further technical solution is that the composite absorbent is applicable to a temperature of 40-70°C and an absorption pressure, i.e., a CO2 partial pressure of 0.1-1.0 MPa.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The CO2 absorption rate of the amino-functionalized ionic liquid, i.e., AFIL, as the activator component in the composite absorbent of the present invention is compared with that of other activators. The CO2 absorption rate of the 20wt% MDEA-10wt% [TMTD][2-IM] system is 3 times that of the 25wt% MDEA-5wt% PZ aqueous solution (the prior art with publication number CN 108722115 A), and is 10 times that of the 30wt% MDEA aqueous solution.

[0021] 2. The composite absorbent of the present invention, whose 20wt% MDEA-10wt% [TMTD][2-IM] system has a CO2 absorption capacity equivalent to the 25wt% MDEA-5wt% PZ system commonly used in industry, and its desorption enthalpy is 23% lower than that of the MDEA-PZ system.

[0022] 3. Compared with the MDEA-PZ system, the composite absorbent of the present invention has the advantages of high synthesis gas purification, good recycling capacity, low corrosiveness, low solution degradation and low consumption, and can be widely used for CO2 absorption in synthesis gas, natural gas, refinery gas and other CO2-containing mixed gases.

[0023] In the present invention, the absorption capacity is measured and calculated as follows:

[0024] The gas-liquid phase equilibrium data was obtained using a static method. The apparatus consisted of two tanks of different sizes, whose temperatures were regulated by a water bath with an uncertainty of ±0.1K. The larger chamber was used as a gas storage tank, while the smaller chamber equipped with a magnetic stirrer was a balance tank. Two pressure sensors (Rosemount 2088G) were used to monitor the pressure of the two chambers with an uncertainty of ±0.5% (relative to full scale). The absorbent (10.0 ml) was placed in the balance tank and degassed with a vacuum pump. The remaining pressure in the balance tank was recorded as P0. CO2 was pumped from the gas storage tank into the balance tank through the valve between the two tanks. When the pressure in the balance tank remained constant for at least 4 hours, it was considered that absorption equilibrium was reached. The pressures before and after absorption are represented as P1 and P2, respectively. Based on decompression, the CO2 absorption was calculated using the following equations (1 to 3):

[0025]

[0026] Z=PV m / RT (3)

[0027] In the formula, R is the gas constant, 8.314 J / (mol·K), V C and V L denote the volumes of the balance tank and absorbent, respectively. Z1 and Z2 are the compression factors corresponding to the pressures before and after CO2 absorption. The solubility determination should be repeated at least three times and the average value reported.

[0028] The apparent absorption rate is calculated as follows:

[0029] The absorbent (10.0 ml) was placed in a phase equilibrium device with an online pressure acquisition system. The system temperature was set to 313.2 K. 150 kPa of CO2 was introduced. The acquisition system recorded the system pressure every 2 seconds. The apparent absorption rate k of CO2 in the composite absorbent can be calculated by formula (4):

[0030] Ln[(n0-n e ) / (nn e )]=k ·t (4)

[0031] Where n represents the molar amount of CO2 in the composite absorbent at time t. n0 and n e represent the molar amounts of CO2 at the beginning and at absorption equilibrium, respectively, and k represents the apparent absorption rate constant.

[0032] The desorption enthalpy is calculated as follows:

[0033] Place the absorbent (10.0 ml) into the gas-liquid phase equilibrium device and measure the equilibrium partial pressure of CO2 at a specific loading amount of CO2 in the absorbent. When the system pressure is stable at 1.0 MPa for 4 hours, it can be considered that the CO2 absorption has reached equilibrium. Record the absorption temperature T (K). As shown in formula (5), the desorption enthalpy of CO2 in the composite absorbent can be calculated by the formula:

[0034]

[0035] Where P CO2 is the equilibrium CO2 partial pressure of the solution at a specific loading, kPa; T is the reaction temperature, K; ΔH is the enthalpy change of the absorption reaction, J / mol. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Molecular structures of AFIL prepared in Examples 1-4: a) AFIL1: [TEPA][2-IM]; b) AFIL2: [TMTD][2-IM]; c) AFIL3: [TEPA][PZ]; d) AFIL4: [TMTD][PZ].

[0037] Figure 2 The four AFILs prepared in Examples 1-4 are 1 H NMR spectrum.

[0038] Figure 3This is a comparison chart of the CO2 absorption rate of the composite absorbent AFIL2 of the present invention and the MDEA and MDEA-PZ systems in Example 6 (T=313.2K): ■, 30wt% MDEA-H2O; ○, 25wt% MDEA-5wt% PZ-H2O; □, 20wt% MDEA-10wt% AFIL2-H2O; ●, 45wt% MDEA-5wt% PZ-H2O; ▲, 30wt% MEA-H2O.

[0039] Figure 4 This is a comparison chart of the absorption capacity of the composite absorbent AFIL (20wt% MDEA-10wt% AFIL-H2O) of the present invention in Example 5 with 30wt% MDEA-H2O and 25wt% MDEA-5wt% PZ-H2O systems (T=313.2K, P=1MPa).

[0040] Figure 5 This is a comparison chart of the desorption enthalpy of the composite absorbent AFIL (20wt% MDEA-10wt% AFIL-H2O) of the present invention in Example 6, 30wt% MDEA-H2O and 25wt% MDEA-5wt% PZ-H2O systems. DETAILED DESCRIPTION

[0041] The present invention is further described below with reference to the following examples, but these examples do not limit the protection scope of the present invention.

[0042] Example 1

[0043] 30.123 g (0.159 mol) of TEPA and 15.345 g (0.186 mol) of 2-IM were weighed separately using an electronic balance and placed in a conical flask. 460.235 g of deionized water was added as a solvent, and the mixture was stirred and reacted at room temperature for 24 h to prepare a [TEPA][2-IM] aqueous solution.

[0044] The unreacted raw materials in the above-mentioned [TEPA][2-IM] aqueous solution were removed by filtration. At the same time, the filtered [TEPA][2-IM] aqueous solution was subjected to a rotary evaporator to remove the water solvent under vacuum conditions at a temperature of 348.2K for 4 hours.

[0045] Then, the prepared [TEPA][2-IM] containing trace water was placed in a vacuum oven, P2O5 was added as a desiccant, and dried at 338.2K for 24 hours to remove trace water in [TEPA][2-IM], and finally 38.127g of [TEPA][2-IM] was obtained, which was referred to as AFIL1. The water content of AFIL1 was determined by Karl Fischer titration, and its value was less than 0.2wt%.

[0046] Finally, AFIL1 was placed in a nuclear magnetic resonance tube, and 0.6 ml of deuterium oxide was added to prepare the sample. The sample was analyzed using a Bruker advanced (500 MHz) nuclear magnetic spectrometer. 1 H NMR spectroscopy analysis yielded 1 H NMR spectra, such as Figure 2 shown.

[0047] Example 2

[0048] 30.208 g (0.130 mol) of TMTD and 12.233 g (0.149 mol) of 2-IM were weighed separately using an electronic balance and placed in a conical flask. 460.325 g of deionized water was added as a solvent, and the mixture was stirred and reacted at room temperature for 24 h to prepare a [TMTD][2-IM] aqueous solution.

[0049] The unreacted raw materials in the above-mentioned [TMTD][2-IM] aqueous solution were removed by filtration. At the same time, the filtered [TMTD][2-IM] aqueous solution was subjected to a rotary evaporator to remove the water solvent under vacuum conditions at a temperature of 343.2K for 4 hours.

[0050] Then, the prepared [TMTD][2-IM] containing trace water was placed in a vacuum oven, P2O5 was added as a desiccant, and dried at 348.2K for 24 hours to remove trace water in [TMTD][2-IM], and finally 37.167g of [TMTD][2-IM] was obtained, which was referred to as AFIL2. The water content of AFIL2 was determined by Karl Fischer titration, and its value was less than 0.2wt%.

[0051] Finally, AFIL2 was placed in a nuclear magnetic resonance tube, and 0.6 ml of deuterium oxide was added to prepare the sample. The sample was analyzed using a Bruker advanced (500 MHz) nuclear magnetic spectrometer. 1 H NMR spectroscopy analysis yielded 1 H NMR spectra, such as Figure 2 shown.

[0052] Example 3

[0053] 25.556 g (0.135 mol) of TEPA and 13.093 g (0.152 mol) of PZ were weighed using an electronic balance and placed in a conical flask. 460.325 g of deionized water was added as a solvent, and the mixture was stirred and reacted at room temperature for 24 h to prepare a [TEPA][PZ] aqueous solution.

[0054] The unreacted raw materials in the above-mentioned [TEPA][PZ] aqueous solution were removed by filtration. At the same time, the filtered [TEPA][PZ] aqueous solution was subjected to a rotary evaporator to remove the water solvent under vacuum conditions at a temperature of 338.2K for 4 hours.

[0055] Then, the prepared [TEPA][PZ] containing trace water was placed in a vacuum oven, P2O5 was added as a desiccant, and dried at 348.2K for 24 hours to remove trace water in [TEPA][PZ], and finally 33.201g of [TEPA][PZ] was obtained, which was referred to as AFIL3. The water content of AFIL3 was determined by Karl Fischer titration, and its value was less than 0.2wt%.

[0056] Finally, AFIL3 was placed in a nuclear magnetic resonance tube, and 0.6 ml of deuterium oxide was added to prepare the sample. The sample was analyzed using a Bruker advanced (500 MHz) nuclear magnetic spectrometer. 1 H NMR spectroscopy analysis yielded 1 H NMR spectra, such as Figure 2 shown.

[0057] Example 4

[0058] 30.672 g (0.132 mol) of TMTD and 12.748 g (0.148 mol) of PZ were weighed separately using an electronic balance and placed in a conical flask. 460.325 g of deionized water was added as a solvent, and the mixture was stirred and reacted at room temperature for 24 h to prepare a [TMTD][PZ] aqueous solution.

[0059] The unreacted raw materials in the above-mentioned [TMTD][PZ] aqueous solution were removed by filtration. At the same time, the filtered [TMTD][PZ] aqueous solution was subjected to a rotary evaporator to remove the water solvent under vacuum conditions at a temperature of 348.2K for 4 hours.

[0060] Then, the prepared [TMTD][PZ] containing trace water was placed in a vacuum oven, P2O5 was added as a desiccant, and dried at 343.2K for 24 hours to remove trace water in [TMTD][PZ], and finally 38.264g of [TMTD][PZ] was obtained, which was referred to as AFIL4. The water content of AFIL4 was determined by Karl Fischer titration, and its value was less than 0.2wt%.

[0061] Finally, AFIL4 was placed in a nuclear magnetic resonance tube, and 0.6 ml of deuterium oxide was added to prepare the sample. The sample was analyzed using a Bruker advanced (500 MHz) nuclear magnetic spectrometer. 1 H NMR spectroscopy analysis yielded 1H NMR spectra, such as Figure 2 shown.

[0062] Example 5

[0063] 1g of AFIL of Examples 1-4, 2g of MDEA, and 7g of deionized water were weighed and put into a beaker by an electronic balance, and mixed and stirred for 5 minutes to prepare a 20wt% MDEA-10wt% AFIL aqueous solution. The solution was added to a gas-liquid phase equilibrium device, and the absorption capacity of CO2 in the four MDEA-AFIL aqueous solutions was obtained by measuring the absorption capacity and calculating the method (the system temperature was 313.2K). The results are shown in Table 1.

[0064] Table 1.

[0065]

[0066] Example 6

[0067] 1g of AFIL of Examples 1-4, 2g of MDEA, and 7g of deionized water were weighed by an electronic balance and put into a beaker, and mixed and stirred for 5 minutes to prepare a 20wt% MDEA-10wt% AFIL aqueous solution. It was placed in a phase equilibrium device with an online pressure acquisition system, the temperature of the system was set to 313.2K, 150kPa of CO2 was introduced, and the acquisition system recorded the system pressure every 2 seconds. The apparent absorption rate k of CO2 in the composite absorbent was obtained by measuring the apparent absorption rate and calculating the method, and the results are shown in Table 2.

[0068] Table 2.

[0069]

[0070] Example 7

[0071] 1g of AFIL of Examples 1-4, 2g of MDEA, and 7g of deionized water were weighed by an electronic balance and put into a beaker, and mixed and stirred for 5 minutes to prepare a 20wt% MDEA-10wt% AFIL aqueous solution. Four MDEA-AFIL aqueous solutions were added to a gas-liquid phase equilibrium device, and the equilibrium CO2 partial pressure of the solution at a specific loading amount of CO2 in the four MDEA-AFIL aqueous solutions was measured, and the absorption temperature T (K) was recorded. The desorption enthalpy of CO2 in the composite absorbent was obtained by measuring the desorption enthalpy and calculating the method, and the results are shown in Table 3.

[0072] Table 3.

[0073] <![CDATA[20wt%MDEA-10wt%AFIL-H2O]]> ΔH(kJ / mol) C(mol / kg) AFIL1 63.23 2.62 AFIL2 57.41 2.46 AFIL3 59.63 2.45 AFIL4 61.82 2.36

[0074] Example 8

[0075] 1g of AFIL of Examples 1-4, 2g of MDEA, and 7g of deionized water were weighed into a beaker by an electronic balance, and mixed and stirred for 5 minutes to prepare a 20wt% MDEA-10wt% AFIL aqueous solution. The solution was added to a gas-liquid phase equilibrium device to measure the regeneration performance of CO2 in four MDEA-AFIL aqueous solutions (system temperature was 313.2K). After 7 cycles, the results are shown in Table 4.

[0076] Table 4.

[0077]

[0078] Comparative Example 1

[0079] 3.043 g of MDEA and 7.015 g of deionized water were weighed and placed in a beaker using an electronic balance, and mixed and stirred for 5 minutes to prepare a 30 wt% MDEA aqueous solution. The solution was added to a gas-liquid phase equilibrium device, and the absorption capacity, absorption rate, and desorption energy consumption of CO2 in the MDEA aqueous solution were calculated and determined using the above method. The results are shown in Table 5.

[0080] Comparative Example 2

[0081] 2.504g of MDEA, 0.502g of PZ, and 7.124g of deionized water were weighed and put into a beaker by an electronic balance, and mixed and stirred for 5 minutes to prepare a 25wt% MDEA-5wt% PZ aqueous solution. The solution was added to a gas-liquid phase equilibrium device, and the absorption capacity, absorption rate, and desorption energy consumption of CO2 in the MDEA-PZ aqueous solution were calculated by the above method. The results are shown in Table 5.

[0082] Comparative Example 3

[0083] 4.512g of MDEA, 0.501g of PZ, and 5.018g of deionized water were weighed and put into a beaker by an electronic balance, and mixed and stirred for 5 minutes to prepare a 45wt% MDEA-5wt% PZ aqueous solution. The solution was added to a gas-liquid phase equilibrium device, and the absorption capacity, absorption rate, and desorption energy consumption of CO2 in the MEA aqueous solution were calculated by the above method. The results are shown in Table 5.

[0084] Comparative Example 4

[0085] 3.108g of MDEA, 2.012g of AFIL2, and 5.015g of deionized water were weighed into a beaker by an electronic balance, and mixed and stirred for 5 minutes to prepare a 30wt% MDEA-20wt% AFIL2 aqueous solution. The solution was added to a gas-liquid phase equilibrium device, and the absorption capacity, absorption rate, and desorption energy consumption of CO2 in the MDEA-AFIL aqueous solution were calculated by the above method. The results are shown in Table 5.

[0086] Table 5.

[0087]

Claims

1. A composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution, characterized in that: The composite absorbent is composed of the following raw materials by mass fraction: composition: The mass fraction of N-methyldiethanolamine (MDEA) as the main absorption component is in the range of 5 to 25 wt %; The mass fraction of the amino-functionalized ionic liquid, i.e., AFIL, as the activator component is in the range of 5 to 25 wt %; The mass fraction of water as auxiliary absorption and dilution component is: 50-70wt%; The amino-functionalized ionic liquid is composed of an alkyl polyamine cation and a cyclic nitrogen-containing anion, and is synthesized by an equimolar addition reaction between the cation and the anion directly based on an acid-base neutralization reaction.

2. The composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution according to claim 1, characterized in that: The alkyl polyamine cation is tetraethylenepentamine or pentaethylenehexamine, and the cyclic nitrogen-containing anion is 2-methylimidazole or piperazine.

3. The composite absorbent for improving the CO2 absorption rate of N-methyldiethanolamine aqueous solution according to claim 1, characterized in that: The synthesis steps of the amino-functionalized ionic liquid are as follows: S1. Weigh cations and slightly excess anions and put them into a reaction bottle, then add a sufficient amount of deionized water as a water solvent, mix, and stir at room temperature for more than 24 hours to allow them to react fully to obtain an AFIL aqueous solution; S2, placing the AFIL aqueous solution prepared in step S1 into a rotary evaporator, and performing rotary evaporation under vacuum conditions at a temperature of 65 to 75° C. to remove unreacted components and deionized water to obtain an amino-functionalized ionic liquid; S3. The trace amount of deionized water contained in the amino-functionalized ionic liquid obtained in step S2 is dried in a vacuum oven at a temperature of 65-75° C. for more than 24 hours to obtain AFIL.

4. Use of a composite absorbent for increasing the CO2 absorption rate of an aqueous solution of N-methyldiethanolamine as claimed in any one of claims 1 to 3 in absorbing CO2.

5. The use according to claim 4, characterized in that: The applicable temperature of the composite absorbent is 40-70°C, and the absorption pressure, i.e., the CO2 partial pressure, is 0.1-1.0MPa.

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