A composite absorbent based on multilayer loading treatment of activated carbon fiber, its preparation method and application

Abstract

This invention discloses a composite absorbent based on multilayer loading of activated carbon fibers, its preparation method, and its application. The preparation method of the composite absorbent includes: using activated carbon fibers as a carrier, impregnating the carrier in an inorganic alkaline solution at room temperature to allow the inorganic alkaline solution to fully penetrate the nanopores of the carrier; draining off excess solution from the carrier; then adding the carrier to an organic amine solution for impregnation; and finally draining off excess solution from the carrier to obtain the composite absorbent. This composite absorbent combines the high absorption capacity of inorganic alkaline solutions with the high absorption efficiency of organic amines, achieving increased amine loading on the solid material and reduced amine loss during cycling, thus significantly improving the carbon dioxide absorption efficiency and cycling efficiency.

Description

Technical Field

[0001] This invention relates to the field of direct air capture of carbon dioxide, specifically to a composite absorbent based on multilayer loading of activated carbon fibers, its preparation method, and its application. Background Technology

[0002] Currently, while traditional liquid-phase absorption and solid-phase adsorption technologies are widely used in the field of air CO2 capture, they still have many limitations. Liquid-phase absorption technology often faces problems such as high energy consumption for absorbent regeneration, equipment corrosion, and secondary pollution during the treatment process. Solid-phase adsorption materials, on the other hand, may be insufficient to meet the requirements for efficient, economical, and sustainable air CO2 capture due to limited adsorption capacity and difficulty in regeneration. In view of this, multilayered loaded composite absorbents have emerged.

[0003] Patent CN118558296A reports a method using one or more ionic liquids as CO2 liquid-phase absorbents. However, although ionic liquids have a large absorption capacity, their large-scale application is hindered by drawbacks such as complex preparation, viscosity after absorption, slow absorption / desorption rates, and long absorption times. Patent CN118179209A reports a method using a compound of organic amines and ionic liquids as CO2 liquid-phase absorbents. While this enhances the absorption effect, these solvents still have a certain vapor pressure, requiring replenishment of organic solvents for long-term use. Using organic amines as absorbents also presents certain toxicity, reduces CO2 purity, and causes problems such as high energy consumption for absorbent regeneration, equipment corrosion, and secondary pollution. Patent CN202410290969.0 reports a method of loading organic amines / amine-functionalized ionic liquids into liquid films for air carbon dioxide capture. However, the unavoidable volatility of amines and the required desorption energy consumption of approximately 95°C can be optimized.

[0004] Based on this, this invention patent employs a composite absorbent in the field of air carbon capture. This technology first pre-treats the absorbent with an alkaline solution to enhance the active sites of the amine, then loads it with amine, combining the high absorption capacity of inorganic alkaline loading with the high absorption efficiency of organic amine loading. Furthermore, by loading the absorbent onto activated carbon fiber, a solid carrier with a high specific surface area and excellent pore structure, a novel absorption system is constructed. The activated carbon fiber not only provides a large adsorption interface for the absorbent, enhancing its contact efficiency with CO2, but also promotes the mass transfer process through its own pore structure, thereby significantly improving the absorption rate and capacity. Moreover, after multiple cycles of regeneration, the amine loss is greatly reduced, resulting in better cycle performance. Summary of the Invention

[0005] To address the aforementioned technical problems in the existing technology, the present invention aims to provide a composite absorbent based on multilayer loading treatment of activated carbon fiber, its preparation method, and its application. The composite absorbent of the present invention can effectively and directly capture low concentrations of carbon dioxide in the air, exhibits better adaptability when treating high humidity gases, operates under relatively mild conditions, and demonstrates high efficiency, energy saving, and environmental protection characteristics.

[0006] To achieve the above objectives, the present invention adopts the following solution: The method for preparing a composite absorbent based on multilayer loading treatment of activated carbon fibers includes the following steps: Step 1: Using activated carbon fiber as a carrier, the carrier is immersed in an inorganic alkaline solution at room temperature for 5-20 minutes to allow the inorganic alkaline solution to fully enter the nanopores of the carrier. The carrier with the liquid film is then removed, and excess solution is drained off. The mass concentration of the inorganic alkaline solution is 1% to 15%, and the impregnation ratio of the inorganic alkaline solution to the carrier is controlled at 5 to 15:1. Step 2: Add the carrier with liquid film obtained in Step 1 to the organic amine solution, impregnate it according to the method described in Step 1, drain off the excess solution on the carrier, control the impregnation ratio of organic amine solution to carrier to be 6~12:1, and the concentration of organic amine solution to be 5%~20%, and the composite absorbent is obtained.

[0007] Furthermore, in step 1, the impregnation ratio of the inorganic alkaline solution of the load to the carrier is controlled at 9-12:1.

[0008] Furthermore, in step 1, the mass concentration of the inorganic alkaline solution is between 10% and 15%.

[0009] Furthermore, in step 1, the inorganic base is one or more of NaOH, Ca(OH)2, and KOH, preferably NaOH.

[0010] Furthermore, in step 2, the organic amine is one or both of tetraethylenepentamine (TEPA) and monoethanolamine (MEA), preferably TEPA.

[0011] Furthermore, in step 2, the concentration of the organic amine solution is 10%~15%, and the impregnation ratio of the loaded organic amine solution to the carrier is controlled at 10~12:1.

[0012] Furthermore, the inorganic alkaline solution in step 1 also contains a liquid film aid, which is lysine. The mass fraction of the liquid film aid in the inorganic alkaline solution is 5-15%, preferably 8-10%.

[0013] Furthermore, the BET specific surface area of ​​activated carbon fiber is 1500 m². 2 The pore size is above / g and below 4nm.

[0014] The present invention also discloses the application of the composite absorbent in the adsorption and capture of low concentrations of carbon dioxide in the air.

[0015] Compared with the prior art, the beneficial effects achieved by the present invention are: 1) The activated carbon fibers of this invention provide a larger adsorption interface for multilayer loading, enhancing the contact efficiency with CO2. The porous structure of the activated carbon fibers themselves promotes mass transfer, significantly improving the absorption rate and capacity. The multilayer loading of activated carbon fibers in this invention can effectively reduce problems such as high energy consumption during absorbent regeneration and equipment corrosion, improving the stability and economy of the system.

[0016] 2) This invention uses a composite absorbent, which is more effective than simple amine loading. After 10% TEPA impregnation of ACF is treated with alkaline solution, the adsorption capacity reaches 11.5 mmol / g, which is 1.9 times that of the untreated product. The alkaline solution treatment enhances its adsorption capacity, and the alkaline solution can more easily penetrate and activate the amine sites, thereby increasing the amine loading, releasing free amine and generating HCO3⁻, which regenerates the amine groups (stoichiometric ratio 1:1), thereby improving the adsorption capacity.

[0017] 3) Composite absorbents can absorb CO2 more effectively in mixed gases, reduce interference from other gases, and have high selectivity; organic amines and alkaline solutions have low corrosivity, making them suitable for absorption scenarios where equipment is sensitive to corrosion. Detailed Implementation

[0018] [Formula for Calculating Absorbent Effectiveness Analysis]: (1) Maximum absorption efficiency (η) The absorption performance of an absorbent is measured by its highest CO2 absorption efficiency (η), calculated as follows: ; In the formula: η: maximum CO2 absorption efficiency, % C in CO2 concentration in the inlet gas, mg / m³ 3 ; C out-min CO2 concentration in the outlet gas during the first 300 seconds of the stable period, mg / m³ 3 .

[0019] (2) Total absorption capacity (q) and absorption capacity (Q) of the absorbent. The total absorption capacity q of the absorbent is measured by the total amount of CO2 absorbed, and the calculation formula is as follows: .

[0020] Absorption capacity Q is the mass of CO2 absorbed per 1g of absorbent, used to evaluate the absorption effect of the absorbent. The formula for calculating absorption capacity Q is as follows: .

[0021] In the formula: q—the amount of CO2 absorbed by the absorbent, in mg; Q—Absorption capacity, mmol / g of effective absorbed component; G—Total gas flow rate, L / min; t s —Time required for the absorbent to become saturated with CO2, in minutes; m—mass of the absorbent in the absorbent, which refers to the mass of the organic amine absorbent loaded on the absorbent (the mass of the organic amine absorbent loaded on the absorbent is measured by the mass difference of the absorbent before and after loading with the organic amine absorbent), in g; C0—Initial inlet CO2 concentration, mg / m³ 3 ; w 有机胺 —mass fraction of organic amine solution; C in —Inlet CO2 concentration, mg / m³ 3 ; C out —Outlet CO2 concentration, mg / m³ 3 .

[0022] (3) Utilization rate of effective components of absorbent (amine-to-carbon ratio) The amount of active ingredient in the absorbent (n) 有机胺 ) and the amount of carbon in the absorbed CO2 (n C The ratio of ).

[0023] For the ACF+TEPA absorption system, when the amine-to-carbon ratio is >0.5, TEPA is not fully used to absorb CO2 or there is a loss of TEPA; when the amine-to-carbon ratio is =0.5, TEPA is fully utilized, which is the theoretical value.

[0024] (4) Impregnation ratio x (calculated based on the feeding ratio) The impregnation ratio refers to the mass ratio of the liquid phase (NaOH solution / TEPA solution) to the absorbent solid phase (ACF).

[0025] [Instrument Characterization]: 1) Specific surface area and pore structure Under different conditions, such as before and after activated carbon fiber absorption and before and after regeneration, data such as its specific surface area, pore volume, and pore size distribution were tested using a physical adsorption instrument. Before starting absorption, to eliminate the possible influence of water, surface adsorbed gases, etc., the sample was first placed in an oven at 150°C and dried continuously for 12 hours to achieve dehydration. Then, activation and desorption treatment was carried out for 12 hours in a vacuum environment at 150°C. In the physical adsorption experiment, nitrogen (N2) was used as the adsorbate, and adsorption and desorption operations were carried out in a constant-temperature liquid nitrogen environment (77K). The DFT method was used for analysis based on the adsorption-desorption isotherms.

[0026] 2) Ion chromatography analysis After the absorption process, the concentration and composition of inorganic ions, such as sodium ions, in the enriched solution were analyzed. The instrument used was an ICS5000+ ion chromatograph manufactured by Thermo Fisher Scientific. During the analysis, KOH solution and methanesulfonic acid solution were used as eluents via an eluent generator, employing isocratic elution at a concentration of 30 mmol / L. An anion / cation suppressor was installed, and the pump flow rate was controlled at 1 mL / min. Before testing and analysis, the sample was diluted, and 25 μL of sample was injected each time. A standard curve was plotted using the external standard method and fitted to accurately determine the concentration of each ion component in the enriched solution.

[0027] 3) Potentiometric titration analysis After the absorption process, the concentration and composition of inorganic ions such as carbonate and bicarbonate in the enriched solution were analyzed. According to the national standard DZ / T0064.49-2021, a Wantong 855 potentiometric titrator was used, with 0.1 mol / L hydrochloric acid as the titrant. 5 mL of sample was taken, accurate to 0.01 μL. A standard curve was plotted using the external standard method to fit the titrant, thereby analyzing the concentration of each ion component in the sample.

[0028] The activated carbon fiber used in this embodiment of the invention has a BET specific surface area of ​​1500 m². 2 With a pore size of less than 4 nm, its carbon tetrachloride adsorption value was determined to be 82.4% according to the national standard GB / T7702.13-2008.

[0029] Example 1: A method for preparing a composite absorbent, comprising the following steps: Step 1: Use two pieces of activated carbon fiber (ACF) with a diameter of 20mm and a thickness of 4mm (the BET specific surface area of ​​activated carbon fiber is 1500m²). 2Cotton pads (with a pore size of less than 4 nm) were used as absorbent carriers, with a mass of approximately 0.15 g. NaOH impregnation solutions of different mass fractions (using water as the solvent, the same below) were prepared. The carrier was added to the NaOH impregnation solution at room temperature, with an impregnation ratio of NaOH impregnation solution to carrier of 12:1. The process was allowed to continue at room temperature for 10 minutes to allow the impregnation solution to fully penetrate the nanopores of the carrier. The carrier with the liquid film was then removed, and excess solution was appropriately drained (the appropriate draining of excess solution mentioned in this invention refers to the operation of leaching excess solution off the carrier, the same below). Step 2: Add the carrier with liquid film obtained in Step 1 to a 10% (w / w) organic amine (TEPA) solution. The impregnation ratio of the organic amine solution to the carrier is 12:1. After waiting for 10 minutes at room temperature, drain the excess solution appropriately to obtain the composite absorbent.

[0030] Example 1: Composite absorbents prepared under different mass concentrations of NaOH impregnation solution were used to adsorb and capture low concentrations of CO2 in the air (the CO2 concentration in the air was about 400 ppm). The amine-to-carbon ratio under adsorption of different composite absorbents was calculated, and the test results are shown in Table 1.

[0031] Table 1. Amine-to-carbon ratio after impregnation with different alkali concentrations .

[0032] Analysis of the amine-to-carbon ratio (ACR) of the absorption curves after impregnation with different alkali concentrations revealed that 1%, 2.5%, and 5% NaOH concentrations had low ACR and high ACR, indicating poor NaOH performance. However, the ACR of the absorption curves for 10% and 15% NaOH concentrations was close to 0.5, with the 15% concentration showing an even closer ACR, indicating better carbon dioxide absorption under these conditions. Since the adsorption capacity and efficiency of the 10% and 15% NaOH concentrations were both high, the optimal concentration was further screened based on absorption rate. Subsequent absorption rate measurements showed that while the 15% NaOH concentration had high adsorption capacity and efficiency, this was achieved at the expense of absorption time; therefore, a 10% concentration might be a better choice.

[0033] Example 2: Preparation of a composite absorbent Example 2: The preparation method of the composite absorbent is the same as that of Example 1, except that "in step 1, a 10% NaOH impregnation solution is used, and the impregnation ratio of the loaded NaOH impregnation solution to the carrier is controlled at 0:1, 6:1, 7:1, 9:1, 12:1, 13:1, and 15:1 respectively", and the other conditions remain unchanged.

[0034] Example 2: Composite absorbents with different NaOH impregnation ratios to carriers were used to adsorb and capture low-concentration CO2 in the air (CO2 concentration in the air is about 400 ppm). The amine-to-carbon ratio under different composite absorbents was calculated, and the test results are shown in Table 2.

[0035] Table 2. Carbon ratio of amines absorbed after NaOH treatment with different impregnation ratios .

[0036] An appropriate impregnation ratio ensures sufficient contact between ACF and the absorbent, maximizing adsorption performance. A lower impregnation ratio results in insufficient absorbent, failing to fully utilize the high specific surface area of ​​ACF for liquid dispersion; while a higher impregnation ratio leads to excessive absorbent, increasing costs and causing the absorption effect to gradually shift towards pure liquid phase absorption, with the absorption capacity decreasing due to excess absorbent. Experiments show that an alkaline solution with an impregnation ratio of 12:1 exhibits higher absorption capacity and peak absorption efficiency after treatment.

[0037] Example 3: Preparation of a composite absorbent Example 3 The preparation method of the composite absorbent is the same as that of Example 1, except that "in step 1, a 10% NaOH impregnation solution is used, and an auxiliary agent with a mass fraction of 10% is added to the NaOH impregnation solution", and the other conditions remain unchanged.

[0038] Example 3 shows the composite absorbents prepared under different conditions, which were used to adsorb and capture low concentrations of CO2 in the air (the CO2 concentration in the air is about 400 ppm). The amine-to-carbon ratio under different composite absorbents was calculated, and the test results are shown in Table 3.

[0039] Table 3. Amine-to-carbon ratio after treatment with different additives in alkali solution .

[0040] The absorption capacity is improved when lysine is added to alkaline solution, so lysine has a certain synergistic promoting effect on absorption.

[0041] Example 4: Preparation of a composite absorbent Example 4 The preparation method of the composite absorbent is the same as that of Example 1, except that "in step 1, different inorganic base solutions with a mass fraction of 10% are used, and the inorganic base is KOH, Ca(OH)2, Ca(OH)2-KOH with a molar ratio of 1:1, Ca(OH)2-NaOH with a molar ratio of 1:1 or NaOH", and the other conditions remain unchanged.

[0042] Example 4 shows the composite absorbents prepared under different conditions, which were used to adsorb and capture low concentrations of CO2 in the air (the CO2 concentration in the air is about 400 ppm). The amine-to-carbon ratio under different composite absorbents was calculated, and the test results are shown in Table 4.

[0043] Table 4. Amine-to-carbon ratio after impregnation with different alkaline solutions for carbon dioxide absorption .

[0044] The effects of three different alkaline solutions and their combinations on carbon dioxide absorption performance were compared, and NaOH and KOH were selected as the preferred choices. NaOH and KOH are also commonly used absorbents in liquid-phase absorption systems for direct capture of carbon dioxide from the air, so the experiment specifically compared their absorption performance. The absorption capacity of NaOH (11.5 mmol / g) was slightly higher than that of KOH (11.1 mmol / g), and the price of KOH is usually about 1,000 yuan higher per ton than that of NaOH, so NaOH was chosen.

[0045] Example 5: Preparation of a composite absorbent Example 5: The preparation method of the composite absorbent is the same as that in Example 1, except that: ① In step 1, a 10% NaOH impregnation solution is used; ② In step 2, TEPA solutions of different mass concentrations were used, while other conditions remained unchanged.

[0046] Example 5 shows the composite absorbents prepared under different conditions, which were used to adsorb and capture low concentrations of CO2 in the air (the CO2 concentration in the air is about 400 ppm). The amine-to-carbon ratio under different composite absorbents was calculated, and the test results are shown in Table 5.

[0047] Table 5. Amine-to-carbon ratio at different amine concentrations .

[0048] At a mass concentration of 10%, the amine-to-carbon ratio is 0.6, which is closest to the theoretical value of 0.5. Using a 10% TEPA mass concentration as the optimal concentration is a reasonable choice. In practical applications, the optimal TEPA concentration can be designed based on the designed single-absorption effective time to meet the requirements for absorption duration.

[0049] Example 6: Preparation of a composite absorbent Example 6 The preparation method of the composite absorbent is the same as that of Example 1, except that "in step 1, the mass fraction of NaOH impregnation solution is 10%; in step 2, the impregnation ratio of organic amine solution to carrier is changed", and the other conditions remain unchanged.

[0050] Example 6: Composite absorbents under different NaOH impregnation ratios to carriers were used to adsorb and capture low-concentration CO2 in the air (CO2 concentration in the air is about 400 ppm). The amine-to-carbon ratio under different composite absorbents was calculated, and the test results are shown in Table 6.

[0051] Table 6. Amine-to-carbon ratio absorbed by different amine solutions .

[0052] The absorption curves for different impregnation ratios showed similar trends, but the highest absorption efficiency and absorption capacity varied with the impregnation ratio. Efficiency and adsorption capacity both increased with increasing impregnation ratio. At an impregnation ratio of 12:1, ACF+TEPA exhibited high absorption capacity and a high highest absorption efficiency, reaching 11.7 mmol·(gTEPA). -1 The results of the amine-to-carbon ratio for different impregnation ratios are shown in Table 6. After loading the TEPA solution onto the activated carbon fiber, the amine-to-carbon ratio decreased significantly. TEPA is the main absorbent component in absorption, and the impregnation ratio of the absorbent directly affects the TEPA content per unit absorbent and the mass transfer effect. Experiments showed that impregnation ratios of 8:1 to 12:1 resulted in higher absorption capacity and peak absorption efficiency. Considering that the viscosity of the TEPA solution is higher than that of the NaOH solution, organic amines are more easily lost than inorganic alkalis during absorption. Organic amines form a miscible system with water, and the organic amines themselves have a certain viscosity, thus generating a large number of bubbles during absorption. These bubbles adhere to the tube wall with the airflow, affecting the absorption capacity. Therefore, an intermediate impregnation ratio of 10:1 was selected as the experimental condition for subsequent tests.

[0053] Example 7: Preparation of a composite absorbent Example 7 Preparation method of composite absorbent: Repeat Example 1, and directly wash the absorbent with 50 mL of deionized water for 5 min to obtain regenerated activated carbon fiber solid carrier, re-impregnate with the loaded absorbent solution, and then conduct the next round of cycle absorption test to observe whether it can restore absorption activity.

[0054] Table 7 Absorption capacity at different cycle numbers .

[0055] This indicates that the regeneration method of direct washing and regeneration with absorbent can indeed regenerate the absorbent and achieve good absorption effect.

Claims

1. A method for preparing a composite absorbent based on multilayer loading treatment of activated carbon fibers, characterized in that, Includes the following steps: Step 1: Using activated carbon fiber as a carrier, immerse the carrier in an inorganic alkaline solution at room temperature for 5-20 minutes to allow the inorganic alkaline solution to fully penetrate the nanopores of the carrier. Remove the carrier with the liquid film and drain off the excess solution on the carrier. The inorganic alkaline solution has a mass concentration of 1% to 15%, and the impregnation ratio of the inorganic alkaline solution to the carrier is controlled at 5 to 15:

1. Step 2: Add the carrier with liquid film obtained in Step 1 to the organic amine solution, impregnate it according to the method described in Step 1, drain off the excess solution on the carrier, control the impregnation ratio of organic amine solution to carrier to be 6~12:1, and the concentration of organic amine solution to be 5%~20%, and the composite absorbent is obtained.

2. The method for preparing a composite absorbent based on multilayer loading treatment of activated carbon fiber as described in claim 1, characterized in that, In step 1, the impregnation ratio of the inorganic alkaline solution of the load to the carrier is controlled at 9-12:

1.

3. The method for preparing a composite absorbent based on multilayer loading treatment of activated carbon fiber as described in claim 1, characterized in that, In step 1, the mass concentration of the inorganic alkaline solution is between 10% and 15%.

4. The method for preparing a composite absorbent based on multilayer loading treatment of activated carbon fiber as described in claim 1, characterized in that, In step 1, the inorganic base is one or more of NaOH, Ca(OH)2, and KOH, with NaOH being preferred.

5. The method for preparing a composite absorbent based on multilayer loading treatment of activated carbon fiber as described in claim 1, characterized in that, In step 2, the organic amine is one or both of tetraethylenepentamine (TEPA) and monoethanolamine (MEA), preferably TEPA.

6. The method for preparing a composite absorbent based on multilayer loading treatment of activated carbon fiber as described in claim 1, characterized in that, In step 2, the concentration of the organic amine solution is 10%~15%, and the impregnation ratio of the loaded organic amine solution to the carrier is controlled at 10~12:

1.

7. The method for preparing a composite absorbent based on multilayer loading treatment of activated carbon fiber as described in claim 1, characterized in that, The inorganic alkaline solution in step 1 also contains a liquid film aid, which is lysine. The mass fraction of the liquid film aid in the inorganic alkaline solution is 5-15%, preferably 8-10%.

8. The method for preparing a composite absorbent based on multilayer loading treatment of activated carbon fiber as described in claim 1, characterized in that, The BET specific surface area of ​​activated carbon fiber is 1500 m². 2 The pore size is above / g and below 4nm.

9. A composite absorbent based on multilayer loading treatment of activated carbon fiber prepared by any one of the methods described in claims 1-8.

10. The application of the composite absorbent as described in claim 9 in the adsorption and capture of low concentrations of carbon dioxide in the air.