Reversible phase change absorbent for carbon dioxide capture, carbon dioxide gas separation method and regenerated reversible phase change absorbent

Abstract

The invention provides a reversible phase change absorbent for capturing carbon dioxide, a carbon dioxide gas separation method and a regenerated reversible phase change absorbent. The reversible phase change absorbent comprises a solid phase and a liquid phase, the solid phase is a ZIF-8 framework material, and the liquid phase is a mixed solution of 2-methylimidazole and water; based on the total mass of the reversible phase change absorbent being 100%, the content of the ZIF-8 framework material is 5%-30%, the content of the 2-methylimidazole is 10%-45%, and the content of the water is 25%-85%; wherein the mass ratio of the ZIF-8 framework material to the 2-methylimidazole is 1 to (1.25 to 3). The reversible phase change absorbent provided by the invention can be converted into a uniform and transparent solution phase from a solid-liquid two-phase after absorbing CO2, can effectively reduce the risk of solid-phase deposition, and has the characteristics of high CO2 circulation capacity and high absorption speed.

Description

Technical Field

[0001] The present invention belongs to the technical field of CO2 capture and gas separation, and particularly relates to a reversible phase change absorbent for carbon dioxide capture, a carbon dioxide gas separation method, and the regenerated reversible phase change absorbent. Background Art

[0002] At present, the chemical absorption method represented by the amine method is the most widely used CO2 capture method in industrial applications. However, this method has problems such as high operating costs, easy degradation of absorbents, and strong volatility. Therefore, finding a low-energy carbon capture method has become the focus of current research.

[0003] Metal-organic frameworks (MOFs) formed by metal ions and organic ligands are a new type of porous material and have attracted much attention in the field of carbon capture. These emerging solid adsorbents have advantages such as large specific surface area, adjustable structure, and high porosity. However, they still face challenges such as difficult material forming, complex processes, and poor cycle stability in continuous industrial separation.

[0004] At present, the porous liquid technology combines the porosity of solid materials and the good fluidity of liquids, enabling MOF materials (such as ZIF-8) to achieve continuous separation and thermal integration in large-scale carbon capture schemes. CN110479044A discloses an absorption-adsorption hybrid method for carbon capture using a non-aqueous porous liquid based on ZIF-8. However, this technical solution still has deficiencies in terms of flow or mass transfer, and there are still problems such as solid deposition or even blockage due to uneven dispersion of solid materials, which will affect the CO2 capture effect of the absorbent. In practical applications, the deposition and blockage of solids will also increase the pumping power consumption, resulting in additional operating costs, and the cost of amide solvents is high.

[0005] Therefore, how to further reduce the risk of solid phase deposition in the absorbent to obtain more excellent CO2 adsorption performance and improve the separation effect is an urgent problem to be solved at present. Summary of the Invention

[0006] To solve the above technical problems, the purpose of the present invention is to provide a reversible phase change absorbent for carbon dioxide capture, a carbon dioxide gas separation method, and the regenerated reversible phase change absorbent.

[0007] To achieve the above purpose, the present invention provides a reversible phase change absorbent for carbon dioxide capture, wherein the reversible phase change absorbent includes a solid phase and a liquid phase; the solid phase is a ZIF-8 framework material, and the liquid phase is a mixed solution of 2-methylimidazole and water;

[0008] Based on the total mass of the reversible phase change absorbent being 100%, the content of the ZIF-8 framework material is 5% - 30%, the content of 2-methylimidazole is 10% - 45%, and the content of water is 25% - 85%; wherein, the mass ratio of the ZIF-8 framework material to 2-methylimidazole is 1:(1.25 - 3).

[0009] In some specific embodiments, preferably, the content of ZIF-8 is 5% - 20%, the content of 2-methylimidazole is 10% - 30%, wherein, the mass ratio of the ZIF-8 framework material to the 2-methylimidazole is 1:(1.25 - 2); more preferably, the content of ZIF-8 is 15% - 20%, the content of 2-methylimidazole is 18.75% - 30%, wherein, the mass ratio of the ZIF-8 framework material to the 2-methylimidazole is 1:(1.25 - 1.5).

[0010] According to the specific embodiments of the present invention, preferably, before capture, the average particle size of the ZIF-8 framework material is 25 - 60 μm.

[0011] According to the specific embodiments of the present invention, preferably, the preparation method of the above-mentioned reversible phase change absorbent for carbon dioxide capture comprises the following steps:

[0012] Mix 2-methylimidazole and water to obtain a mixed solution, and add the ZIF-8 framework material to the mixed solution, and stir to obtain a reversible phase change absorbent for carbon dioxide capture.

[0013] The present invention also provides a method for separating carbon dioxide gas, wherein, this separation method uses the above-mentioned reversible phase change absorbent for carbon dioxide capture to capture CO2; the reversible phase change absorbent changes from a solid-liquid two-phase to a solution phase after absorbing more than 40% of the CO2 saturation concentration.

[0014] According to the specific embodiments of the present invention, preferably, the separation method is to separate CO2 in the mixed gas, and the volume ratio (initial gas-liquid ratio) of the mixed gas to the reversible phase change absorbent is (10 - 150):1.

[0015] According to the specific embodiments of the present invention, preferably, in addition to CO2, the mixed gas further comprises one or more combinations of N2, H2, and CH4. For example, a mixed gas of CO2 and N2, a mixed gas of CO2 and H2, a mixed gas of CO2 and CH4, a mixed gas of CO2, N2 and H2, a mixed gas of CO2, N2 and CH4, a mixed gas of CO2, H2 and CH4, a mixed gas of CO2, N2, H2 and CH4.

[0016] According to a specific embodiment of the present invention, preferably, the absorption temperature during CO2 capture is 0°C - 50°C.

[0017] According to a specific embodiment of the present invention, preferably, the absorption pressure during CO2 capture is 0.1 MPa - 10 MPa.

[0018] According to a specific embodiment of the present invention, preferably, the separation method further includes a step of desorbing the CO2-rich liquid obtained after capturing CO2 under the conditions of vacuum heating to obtain a regenerated lean liquid that has changed from a solution phase to a solid-liquid two-phase state for recycling.

[0019] According to a specific embodiment of the present invention, preferably, the temperature of the heating is 60°C - 80°C.

[0020] According to a specific embodiment of the present invention, preferably, the absolute pressure of the vacuum is below 90 kPa.

[0021] The present invention also provides a regenerated reversible phase change absorbent, which is regenerated by the steps in the above carbon dioxide gas separation method.

[0022] According to a specific embodiment of the present invention, preferably, in the regenerated reversible phase change absorbent, the average particle size of the ZIF-8 framework material is 1 - 10 μm. In the present invention, after the reversible phase change absorbent is regenerated, the particle size of the obtained ZIF-8 framework material decreases compared with that before capture.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) The reversible phase change absorbent for CO2 capture provided by the present invention can regulate the phase behavior of the absorbent by controlling the ratio between the solid phase and 2-methylimidazole, solves the problem of poor stability of MOFs materials in water, and forms a uniform porous slurry for capturing carbon dioxide; this reversible phase change absorbent has a larger CO2 adsorption capacity, and has the advantages of high CO2 cycle capacity, fast absorption rate, low cycle desorption heat, and low regeneration cost, and has a wide application prospect in the removal of CO2 from flue gas, biogas, IGCC syngas, and other mixed gases.

[0025] (2) Before absorbing CO₂, the reversible phase change absorbent for CO₂ capture provided by the present invention is in a solid-liquid two-phase state. After absorbing CO₂, it can change from a solid-liquid two-phase state to a homogeneous transparent solution phase, which can effectively reduce the risk of solid-phase deposition and blockage of the third type of porous liquid (i.e., porous framework material-based porous liquid). Thus, it not only plays a role in strengthening the thermodynamic performance, but also enables the absorbent to have a lower viscosity and better mass transfer effect. In addition, after the rich liquid captured with CO₂ is desorbed, the regenerated lean liquid can change back to a solid-liquid two-phase state, realizing reversible phase change. As a result, the porous ZIF-8 framework material can be effectively recovered after desorption, maintaining a high cyclic loading capacity while having a low absorption heat.

[0026] (3) After regeneration, the solid phase in the reversible phase change absorbent for CO₂ capture provided by the present invention has a finer particle size, which can eliminate the agglomeration between the crystal particles of the material, is more conducive to the dispersion of the adsorbent material in the liquid phase, increases the adsorption specific surface area of the reversible phase change absorbent, and at the same time, can further avoid problems such as solid deposition and even blockage caused by uneven dispersion of MOFs materials.

[0027] (4) The CO₂ gas separation method provided by the present invention can save the energy consumption of transportation and regeneration by virtue of the heat matching between the "endothermic and exothermic of the phase change behavior" in the reversible phase change absorbent and the "process of CO₂ being absorbed and released", thus providing a new type of absorbent for energy-saving CO₂ capture. Description of the Drawings

[0028] Figure 1 It is an evaluation diagram of the CO₂ solubility and absorption rate of the absorbents prepared in Examples 1-2 and Comparative Examples 1-2.

[0029] Figure 2 It is an evaluation diagram of the CO₂ solubility and absorption rate of the absorbents prepared in Example 2 and Comparative Example 3.

[0030] Figure 3 It is a phase separation diagram of the absorbents prepared in Examples 1-2 and Comparative Examples 1-3 after CO₂ absorption.

[0031] Figure 4 It is a schematic diagram of the reversible phase transition process of the absorbent prepared in Example 2.

[0032] Figure 5 It is the result of the particle size change of the solid phase of the absorbent prepared in Example 2 before and after capturing CO₂ and after regeneration.

[0033] Figure 6 It is an evaluation diagram of the CO₂ absorption heat of the absorbents prepared in Examples 1-2 and Comparative Examples 1-2.

[0034] Figure 7Thermodynamic and kinetic experimental results of CO2 absorption by the regeneration solution of the absorbent prepared in Example 2.

[0035] Figure 8 CO2 cyclic absorption heat results of the absorbent prepared in Example 2.

[0036] Figure 9 Cyclic adsorption test results of the absorbent prepared in Comparative Example 5 for CO2 solubility. Detailed implementation manners

[0037] For a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the technical solutions of the present invention are described in detail below, but it should not be construed as a limitation on the scope of implementation of the present invention.

[0038] The small-scale experimental devices of the present invention are all the devices described in paragraphs 23 and 24 of the specification of CN102389686A (ZL201110284360.5), and are used for thermodynamic and kinetic experiments of pure gases and mixed gases.

[0039] The gas composition in the adsorption phase is obtained by material balance of components during the absorption process. The data processing and calculation processes of relevant gas solubility experiments and separation experiments are as follows:

[0040] In the following calculation results, z1 refers to the mole fraction of CO2 in the raw gas before separation; y1 refers to the mole fraction of CO2 in the mixed gas phase at the adsorption time; x1 refers to the mole fraction of CO2 absorbed by the absorbent. Similarly, z2, y2, and x2 refer to the mole fractions of N2 / H2 / CH4 in the raw gas before separation, in the mixed gas phase at the adsorption time, and absorbed by the absorbent, respectively. T refers to the system temperature; P E refers to the equilibrium pressure in the sapphire autoclave; P1 and P2 refer to the initial pressure of the equilibrium autoclave and the pressure after inlet gas, respectively; R is the gas constant;

[0041] The initial inlet gas mole number n of the sapphire autoclave t is calculated by the following formula:

[0042]

[0043] In the formula, V t is the total effective volume of the equilibrium autoclave and the connecting pipeline; the compression factors Z1 and Z2 corresponding to the initial pressure of the equilibrium autoclave and the pressure after inlet gas are calculated by the BWRS equation of state (Benedict-Webb-Rubin-Starling).

[0044] The total amount of substances in the equilibrium gas phase in the sapphire autoclave n E is calculated by the following formula:

[0045]

[0046] Wherein, V g is the gas phase volume in the sapphire autoclave after absorption equilibrium; Z E is the compression factor corresponding to the temperature and pressure in the sapphire autoclave.

[0047] The total gas moles of CO2 (n1) and N2 / H2 / CH4 (n2) absorbed by the absorbent are calculated by the following formula:

[0048] n1 = n t × z1 - n E × y1; n2 = n t × z2 - n E × y2;

[0049] The mole fraction (x1) of CO2 and the mole fraction (x2) of N2 / H2 / CH4 absorbed by the absorbent are calculated by the following formulas respectively:

[0050]

[0051] S V refers to solubility, and the solubility S of CO2 V1 is defined as:

[0052]

[0053] Wherein, V S is the volume of the absorbent, which is calculated from the height h of the absorbent in the sapphire autoclave; r is the inner diameter of the sapphire autoclave; h is the height of the absorbent in the sapphire autoclave;

[0054] The separation efficiency of the slurry for the mixed gas can be evaluated by the separation factor β and is calculated by the following formula:

[0055]

[0056] The calculation method of the CO2 removal rate R is as follows:

[0057]

[0058] The endothermic heat of the absorbent is calculated from the absorption isotherm of the pure component according to the Clausius - Clapeyron equation, and its calculation method is as follows:

[0059]

[0060] The gas composition analysis is all carried out by using an HP7890B type chromatograph.

[0061] Example 1

[0062] This example provides a reversible phase change absorbent for carbon dioxide capture, specifically as follows:

[0063] Mix 2-methylimidazole and water to obtain a mixed solution. Add the ZIF-8 framework material to the mixed solution and stir at a rotation speed of 150 rpm for 5 min to uniformly disperse the ZIF-8 framework material in the mixed solution, obtaining a reversible phase change absorbent, denoted as absorbent III.

[0064] Among them, the mass ratio of ZIF-8 to 2-methylimidazole is 1:1.25. Based on the total mass of the reversible phase change absorbent being 100%, the composition of absorbent III is: 20% ZIF-8 + 25% 2-methylimidazole + the balance water.

[0065] Example 2

[0066] This example provides a reversible phase change absorbent for carbon dioxide capture, which is prepared according to the steps in Example 1 to obtain a reversible phase change absorbent, denoted as absorbent IV. The difference is only in changing the composition of the absorbent: the mass ratio of ZIF-8 to 2-methylimidazole is 1:1.5. Based on the total mass of the reversible phase change absorbent being 100%, the composition of absorbent IV is: 20% ZIF-8 + 30% 2-methylimidazole + the balance water.

[0067] Comparative Example 1

[0068] This comparative example provides an absorbent for carbon dioxide capture, which is prepared according to the steps in Example 1 to obtain an absorbent, denoted as absorbent I. The difference is only in changing the composition of the absorbent: based on the total mass of the absorbent being 100%, the composition of absorbent I is: 20% ZIF-8 + 10% 2-methylimidazole + the balance water.

[0069] Comparative Example 2

[0070] This comparative example provides an absorbent for carbon dioxide capture, which is prepared according to the steps in Example 1 to obtain an absorbent, denoted as absorbent II. The difference is only in changing the composition of the absorbent: based on the total mass of the absorbent being 100%, the composition of absorbent II is: 20% ZIF-8 + 20% 2-methylimidazole + the balance water.

[0071] Comparative Example 3

[0072] This comparative example provides an absorbent for carbon dioxide capture, specifically as follows:

[0073] Using N,N-dimethylacetamide as a solvent, add ZIF-8 thereto to make the mass concentration of ZIF-8 40%, obtaining an absorbent, denoted as absorbent V.

[0074] Comparative Example 4

[0075] This comparative example provides an absorbent for carbon dioxide capture, specifically as follows:

[0076] Using the traditional alkanolamine solution N-methyldiethanolamine (MDEA) directly as the absorbent, denoted as absorbent VI.

[0077] Comparative Example 5

[0078] This comparative example provides an absorbent for carbon dioxide capture, specifically as follows:

[0079] Adding the ZIF-8 framework material to water and stirring at a rotation speed of 150 rpm for 5 min to uniformly disperse the ZIF-8 framework material in water, obtaining the absorbent, denoted as absorbent VII.

[0080] Among them, based on the total mass of the absorbent being 100%, the composition of absorbent VII is: 25% ZIF-8 + 75% water.

[0081] Test Example 1

[0082] This test example uses the absorbents prepared in Examples 1-2 and Comparative Examples 1-3 as the test objects, and measures the absorption capacity (CO2 solubility) and absorption rate of each absorbent for CO2, specifically as follows:

[0083] Under the condition that the absorption temperature is 30 °C, using pure CO2 gas as the feed gas for testing, the results are as Figure 1 and Figure 2 shown, where Figure 1 (a) in is the test result of the absorption capacity, Figure 1 (b) in is the test result of the absorption rate. The abscissa is the absorption pressure, and the ordinate is the absorption amount value under the corresponding pressure.

[0084] From Figure 1 it can be seen that as the pressure continuously increases, CO2 shows higher solubility in absorbent III and absorbent IV, and the absorption rate of absorbent III and absorbent IV for CO2 is faster. Further, at isothermal (30 °C), comparing the CO2 solubility curve of absorbent IV with absorbent V using N,N-dimethylacetamide as the solvent, the results are as Figure 2 shown. It can be seen that absorbent IV can achieve a higher CO2 adsorption capacity at a lower pressure. For example, when P E is 400 kPa, the solubility of CO2 in absorbent IV is 2.75 mol / L, while in absorbent V it can only reach 1.2 mol / L. Therefore, at this pressure, the solubility of CO2 in absorbent IV is more than twice that of absorbent V, far higher than absorbent V of amide solvents; when P EWhen the pressure is 1000 kPa, the solubility of CO2 in absorbent IV can reach 3.5 mol / L.

[0085] Test Example 2

[0086] In this test example, the absorbents prepared in Examples 1-2 and Comparative Examples 1-2 were used as the test objects, and the phase diagrams of each absorbent in Test Example 1 after absorbing CO2 were measured. The results are as Figure 3 shown, Figure 3 where a, b, c, d, and e in

[0087] From Figure 3 it can be seen that after absorbing CO2, absorbent I, absorbent II, and absorbent V are still in an obvious solid-liquid two-phase state, and the risk of solid-phase deposition cannot be avoided; in contrast, absorbent III and absorbent IV show a uniform solution state after absorbing CO2, and a phase change occurs, thus solving the problem of poor stability of MOFs materials in water and reducing the risk of solid-phase deposition blockage.

[0088] Figure 4 shows a schematic diagram of the phase change of the reversible phase change absorbent provided by the present invention. Taking absorbent IV as an example, before absorbing CO2, absorbent IV is composed of a solid phase of ZIF-8 framework material and a liquid phase of 2-methylimidazole and water, presenting a solid-liquid two-phase state ( Figure 4 a) in Figure 4 As the absorption process proceeds, the CO2 loading gradually increases. From Figure 4 b in

[0089] Test Example 3

[0090] In this test example, the particle size change of the solid phase of absorbent IV prepared in Example 2 before and after CO2 capture was explored. The temperature for CO2 capture was 30 °C, the pressure was 0.3 MPa, and the regeneration conditions were desorption for 20 min at a temperature of 80 °C and a desorption pressure of 80 kPa. The results are as Figure 5 shown.

[0091] From Figure 5It can be seen that the average particle size of the fresh ZIF-8 crystals before capture is 30.79 μm, while the average particle size of the regenerated ZIF-8 crystals is only 6.34 μm. Therefore, the regenerated MOFs material has a smaller particle size and can effectively avoid the risk of blockage.

[0092] Therefore, by means of the reversible phase change process, the present invention can cause the "dissolution-recrystallization" of the MOFs adsorbent material. Through the absorption-desorption cycle, a higher-purity and more complete crystal structure of the MOFs material can be obtained, thereby reducing the purity requirement for the MOFs adsorbent material. This process largely eliminates the agglomeration between the crystal particles of the material and is more conducive to the dispersion of the adsorbent material in the liquid phase, so that the absorbent has a higher adsorption specific surface area.

[0093] Test Example 4

[0094] In this test example, the heat of desorption of CO2 in the above absorbents I, II, III, IV, and VI was investigated. The regeneration conditions were desorption for 20 min at a temperature of 80 °C and a desorption pressure of 80 kPa. The results are shown in Table 1 and Figure 6 as follows.

[0095] Table 1. Average heat of desorption of different absorbents

[0096]

[0097]

[0098] As can be seen from Table 1 and Figure 6 it can be seen that absorbents I, II, III, and IV as a whole have relatively low heats of desorption. Although the heats of desorption of absorbents III and IV are higher than those of absorbents I and II, their heats of desorption still have great advantages compared with absorbent VI using the traditional alkanolamine solution, belonging to low heats of desorption. And according to Test Example 1, the adsorption capacities of absorbents III and IV that have undergone phase change are much higher than those of absorbents I and II that have not undergone phase change. Therefore, the absorbents III and IV provided by the present invention can have both high adsorption capacity and relatively low heat of desorption.

[0099] Test Example 5

[0100] In order to verify whether the absorbent IV prepared in Example 2 can be reused for CO2 capture, in this test example, pure CO2 gas was used as the feed gas, and the above absorbent IV was used for multiple CO2 absorption-desorption operations to investigate its reusability.

[0101] CO2 capture was carried out under the conditions of a temperature of 30 °C and a pressure of 0.5 MPa. After the CO2 absorption experiment was completed, the CO2-rich solution was desorbed for 20 min under the conditions of a temperature of 80 °C and a desorption pressure of 80 kPa, and the obtained lean solution was continued for the CO2 capture experiment. The experimental results are as Figure 7 shown, where Figure 7 (a) in it is the test result of the absorption capacity, Figure 7 (b) in it is the test result of the absorption rate.

[0102] Under these desorption conditions, although the separation performance of absorbent IV decreased slightly after multiple absorption-desorption cycle tests compared with the fresh solution, however, the absorption capacity and absorption rate had tended to be stable after the second regeneration and there was no obvious decrease after multiple cycles of reuse, showing relatively excellent reuse performance.

[0103] The cyclic desorption heat of CO2 in absorbent IV was further tested, and the test results are shown in Table 2 and Figure 8 as follows.

[0104] Table 2. Cyclic desorption heat of absorbent IV

[0105]

[0106]

[0107] From Table 2 and Figure 8 it can be seen that the cyclic desorption heat of CO2 in absorbent IV also tended to be stable after the second regeneration.

[0108] From the perspective of energy consumption analysis, the reversible phase change absorbent provided by the present invention has the following two heat transfer paths to reduce the overall capture energy consumption:

[0109] (1) During the absorption process, the "quasi-dissolution" of the solid-phase MOFs material is an endothermic reaction, while the absorption of CO2 by the special chemical agent 2-methylimidazole is an exothermic reaction. The coupling of the two can achieve the spontaneous integration of heat in the system, thereby saving the cooling water power consumption;

[0110] (2) During the desorption process, the recrystallization of the solid-phase MOFs material is an exothermic reaction, while the special chemical agent 2-methylimidazole releases CO2 by absorbing heat. The two can have a good heat match, thereby further saving the regeneration energy consumption.

[0111] Test Example 6

[0112] This test example uses absorbent IV for the separation of CO2 / N2. In a 59.85 mL fully transparent sapphire autoclave, 10 g of the absorbent was added. At 30 °C, a CO2 / N2 mixed gas (molar ratio 0.14:0.86, z1 / z2) was injected into the autoclave, with an initial gas-liquid ratio of 25:1 and an equilibrium pressure of 501 kPa. After absorption to equilibrium, the composition of the equilibrium gas was analyzed by gas chromatography, and the equilibrium gas composition was CO2 / N2: 0.0023 / 0.9977 (y1 / y2). Finally, the separation factor β of CO2 / N2 and the CO2 removal rate R (%) were obtained by component calculation. The values of β and R (%) for this system were 3543 and 99%, respectively.

[0113] Test Example 7

[0114] This test example uses absorbent IV for the separation of CO2 / H2. In a 59.85 mL fully transparent sapphire autoclave, 10 g of the absorbent was added. At 30 °C, a CO2 / H2 mixed gas (molar ratio 0.45:0.55, z1 / z2) was injected into the autoclave, with an initial gas-liquid ratio of 140:1 and an equilibrium pressure of 2012 kPa. After absorption to equilibrium, the composition of the equilibrium gas was analyzed by gas chromatography, and the equilibrium gas composition was CO2 / H2: 0.0836 / 0.9164 (y1 / y2). Finally, the separation factor β of CO2 / H2 and the CO2 removal rate R (%) were obtained by component calculation. The values of β and R (%) for this system were 653 and 89%, respectively.

[0115] Test Example 8

[0116] This test example uses absorbent IV for the separation of CO2 / CH4. In a 59.85 mL fully transparent sapphire autoclave, 10 g of the absorbent was added. At 30 °C, a CO2 / CH4 mixed gas (molar ratio 0.40:0.60, z1 / z2) was injected into the autoclave, with an initial gas-liquid ratio of 39:1 and an equilibrium pressure of 520 kPa. After absorption to equilibrium, the composition of the equilibrium gas was analyzed by gas chromatography, and the equilibrium gas composition was CO2 / CH4: 0.0208 / 0.9792 (y1 / y2). Finally, the separation factor β of CO2 / CH4 and the CO2 removal rate R (%) were obtained by component calculation. The values of β and R (%) for this system were 457 and 97%, respectively.

[0117] Test Example 9

[0118] This test example uses absorbent VII prepared in Comparative Example 5 to test the cyclic stability of its CO2 adsorption performance (CO2 solubility), as follows:

[0119] Under the condition that the absorption temperature was 30 °C, pure CO2 gas was used as the feed gas for the test, and the results are as Figure 9As shown, the results indicate that when 2-methylimidazole is not added to absorbent VII, the cyclic adsorption performance of the slurry will gradually decay because ZIF-8 undergoes hydrolysis; and in the absence of 2-methylimidazole, ZIF-8 is unstable in an environment of water and CO2, and its structure will collapse.

[0120] Test Example 10

[0121] This test example explored the minimum amount of 2-methylimidazole to be added for the phase change behavior of absorbents with different ZIF-8 contents, with the balance being water. The results are shown in Table 3.

[0122] Table 3.

[0123] ZIF-8 (wt%) 5.00 10.00 15.00 20.00 2-Methylimidazole (wt%) 10.00 15.00 18.75 25.00

[0124] As can be seen from Table 3, by controlling the ratio between ZIF-8 and 2-methylimidazole, the present invention can regulate the phase behavior of the absorbent, solve the problem of poor stability of MOFs materials in water, and thus improve the CO2 adsorption performance.

Claims

1. A reversible phase change absorbent for carbon dioxide capture, wherein, The reversible phase change absorbent includes a solid phase and a liquid phase; the solid phase is a ZIF-8 framework material, and the liquid phase is a mixed solution of 2-methylimidazole and water; Based on the total mass of the reversible phase change absorbent being 100%, the content of the ZIF-8 framework material is 5%-30%, the content of 2-methylimidazole is 10%-45%, and the content of water is 25%-85%; Among them, the mass ratio of the ZIF-8 framework material to the 2-methylimidazole is 1:(1.25 - 3).

2. The reversible phase change absorbent according to claim 1, wherein, The average particle size of the ZIF-8 framework material is 25 - 60 μm.

3. A method for separating carbon dioxide gas, wherein, This separation method uses the reversible phase change absorbent for CO2 capture described in claim 1 or 2; after the reversible phase change absorbent absorbs more than 40% of the CO2 saturation concentration, it changes from a solid-liquid two-phase state to a solution phase.

4. The separation method according to claim 3, wherein, This separation method is to separate CO2 in the mixed gas, and the volume ratio of the mixed gas to the reversible phase change absorbent is (10 - 150):

1.

5. The separation method according to claim 4, wherein In addition to CO2, the mixed gas also includes one or more combinations of N2, H2, and CH4.

6. The separation method according to claim 3, wherein, The absorption temperature during CO2 capture is 0°C - 50°C.

7. The separation method according to claim 3, wherein The absorption pressure during CO2 capture is 0.1 MPa - 10 MPa.

8. The separation method according to claim 3, wherein This separation method further includes a step of desorbing the CO2-rich liquid obtained after capturing CO2 under the conditions of vacuum heating to obtain a regenerated lean liquid that changes from a solution phase to a solid-liquid two-phase state for recycling; Preferably, the heating temperature is 60°C - 80°C; Preferably, the absolute pressure of the vacuum is below 90 kPa.

9. A regenerated reversible phase change absorbent, which is regenerated by the steps in the carbon dioxide gas separation method described in claim 8.

10. The regenerated reversible phase change absorbent according to claim 9, wherein, In the regenerated reversible phase change absorbent, the average particle size of the ZIF-8 framework material is 1 - 10 μm.

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