Composite method of trapping carbon dioxide in gas mixture

Abstract

A hybrid method for capturing CO.sub.2 from a gas mixture is provided, comprising a step of contacting the CO.sub.2 containing gas mixture with a slurry consisting of a liquid medium, imidazole or imidazole derivative(s), and a metal-organic framework material (MOFs). For the slurry system, the mass fraction of the imidazole or imidazole derivative(s) in it ranging from 2 to 50% and the mass fraction of the metal-organic framework material in it ranging from 5 to 25%. In the technical solution provided in the present invention, through combining absorptive separation by the liquid solution in which the imidazole or imidazole derivative(s) is dissolved, adsorption separation by the MOF material suspended in the solution, and selective permeation separation by a liquid medium film forms on the outside surface of the suspended MOFs, an absorption-adsorption hybrid separation effect for CO.sub.2 gas mixtures is efficiently achieved. In the CO.sub.2 capture method provided in the present invention, conventional absorption separation and adsorptive separation technologies are effectively combined, furthermore, the addition of imidazole or imidazole derivative(s) substantially increases both the CO.sub.2 capture ability and capture amount of the MOFs/liquid slurry, showing a great potential in industrial applications.

Claims

1. A hybrid method for capturing CO.sub.2 from a gas mixture, the method comprising: contacting the CO.sub.2 containing gas mixture with a slurry mixture consisting of: (1) a liquid medium, (2) imidazole or benzimidazole, and (3) a metal-organic framework material, wherein the content of the imidazole or benzimidazole ranges from 2% to 50%, wherein the content of the metal-organic framework material ranges from 5% to 25% by weight with respect to the slurry mixture, and wherein the liquid medium is ethylene glycol or triethylene glycol.

2. The method according to claim 1, wherein the metal-organic framework material has a diameter of pore window that ranges from 0.25 to 0.4 nm.

3. The method according to claim 2, wherein the metal-organic framework material is ZIF-8, ZIF-65, ZIF-67, ZIF-71, ZIF-20, ZIF-21, or ZIF-77.

4. The method according to claim 1, wherein the liquid medium molecules have a diameter that is larger than the diameter of pore window of the metal-organic framework material.

5. The method according to claim 1, wherein the step of contacting the CO.sub.2 containing gas mixture with the slurry mixture is conducted under the conditions of a temperature that ranges from 273.15 K to 353.15 K and a pressure that ranges from 0.1 MPa to 15.0 MPa.

6. The method according to claim 1, further comprising: forming a CO.sub.2 captured slurry mixture after the CO.sub.2 containing gas mixture contacted with a slurry mixture, and recycling the CO.sub.2 captured slurry mixture after the absorbed gas is released under the condition of ambient temperature through vacuuming or under the condition of heating at an absolute pressure that ranges from 0.5 atm to 1.0 atm.

7. The method according to claim 6, wherein the vacuum refers to an absolute pressure that ranges from 0.0002 atm to 0.5 atm, and the heating temperature ranges from 323.15 K to 363.15 K.

8. The method according to claim 1, wherein the CO.sub.2 containing gas mixture includes one or a combination of more of flue gas, biogas, integrated gasification combined cycle (IGCC) gas mixture and natural gas.

9. The method according to claim 1, wherein the volume ratio between the gas mixture containing CO.sub.2 and the slurry mixture is from 5 to 200:1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic of the hybrid absorption-adsorption separation process for CO.sub.2 gas mixtures provided in the present invention.

(2) FIG. 2 is the schematic diagram of the experimental apparatus used in Example 1.

REFERENCE NUMBERS OF MAIN COMPONENTS

(3) TABLE-US-00001 Air bath 1 Sapphire cell 2 Stirring device 3 Magnet 4 Piston 5 Cut-off valves 6, 8, 11, 13, 14 Hand pump 7 Heise pressure meter 9 High-pressure gas cylinder 10 Three-way valve 12 Equilibrium cell 15

DETAILED DESCRIPTION OF THE INVENTION

(4) For a clearer understanding of the technical features, purposes and beneficial effects of the present invention, detailed description of the technical solutions of the present invention are hereinafter provided, which cannot be construed as limitation to the implementable scope of the present invention.

Example 1

(5) In this example, a hybrid method for capturing CO.sub.2 from a gas mixture is provided, which employs an apparatus as shown in FIG. 2, which has been also described in CN 102389686A patent before. In this example, ethylene glycol was used as the liquid medium, methylimidazole was used as an accelerator, ZIF-8 was used as the MOF material, and the corresponding gas mixture was a CO.sub.2/N.sub.2 (20.65/79.35 mol %) gas mixture.

(6) Prior to the experiment, the sapphire cell 2 was first unloaded, washed with distilled water, rinsed with ethylene glycol three times and wiped dry; then, ethylene glycol, methylimidazole, and ZIF-8 were added into the sapphire cell 2 with a mass ratio of 51:34:15 and evenly; subsequently, the sapphire cell 2 was installed back into the air bath 1. The sapphire cell 2, equilibrium cell 15, and the whole high pressure piping system were vacuumed. Afterwards, enough amount of synthetic gas was discharged from the high-pressure cylinder 10 to the equilibrium cell 15. After the temperature of the air bath 1 and the pressure within the equilibrium cell 15 were stabilized, the pressure of the equilibrium cell 15 was recorded, which is used to calculate the mole number of gases in the equilibrium cell 15 (detailed calculation method is described below). The top valve of the sapphire cell was opened slowly then, letting the desired amount of synthetic gas flow into the sapphire cell 2 from the equilibrium cell 15. Afterwards, this valve was closed and the magnetic stirrer 3 was turned on. With the absorption of gas mixture by the slurry, the pressure in the sapphire cell decreased gradually. When the system pressure remained as a constant for at least 2 hours, we considered the equilibrium of system was achieved. The pressure in the equilibrium cell 15 and the sapphire cell 2 were recorded. Gas mixture in the equilibrium gas phase of the sapphire cell was sampled under constant pressure by pushing the connected hand pump and analyzed by a HP 7890 gas chromatograph.

(7) The gas composition in the equilibrium slurry phase was determined through mass balance as described below.

(8) In the following calculation procedure, z.sub.1, y.sub.1, and x.sub.1 are the molar fraction of N.sub.2 in the initial (feed) gas, the equilibrium gas phase, and the equilibrium absorption phase, respectively; z.sub.2, y.sub.2, and x.sub.2 are the molar fraction of CO.sub.2 in the initial (feed) gas, the equilibrium gas phase, and the equilibrium absorption phase, respectively; T is the experimental temperature; P.sub.1 and P.sub.2 are the initial pressure and the separation equilibrium pressure in the equilibrium cell, respectively, and P.sub.E is the equilibrium pressure of the sapphire cell. The total mole number of gas mixture that was injected into the sapphire cell (n.sub.0) and the total gas amount (n.sub.E) in the equilibrium gas phase of the sapphire cell after absorption and adsorption equilibrium are calculated by the following equations:

(9) $n_0=\frac{P_1{V}_E}{Z_1\mathrm{RT}}-\frac{P_2{V}_E}{Z_2\mathrm{RT}}$$n_E=\frac{P_E{V}_A}{Z_E\mathrm{RT}}$
where V.sub.E is the volume of the equilibrium cell, V.sub.A is the volume of equilibrium gas phase in the sapphire cell at the end of each experimental run; Z.sub.1, Z.sub.2, and Z.sub.E respectively correspond to the gas compressibility factors at pressures P.sub.1, P.sub.2 and P.sub.E, and are calculated by the BWRS state equations:
Z.sub.1=Z(T,P.sub.1,z.sub.i)
Z.sub.2=Z(T,P.sub.2,z.sub.i)
Z.sub.E=Z(T,P.sub.E,y.sub.i)

(10) The mole number of N.sub.2 having entered the sapphire cell (n.sub.1) and that of CO.sub.2 (n.sub.2) are calculated as:
n.sub.1=n.sub.0z.sub.1 n.sub.2=n.sub.0z.sub.2

(11) The mole number of N.sub.2 and that of CO.sub.2 in the equilibrium gas phase after separation equilibrium are calculated as:
n.sub.E.sup.1=n.sub.Ey.sub.1 n.sub.E.sup.2=n.sub.Ey.sub.2

(12) The molar fraction of N.sub.2 and CO.sub.2 in the equilibrium slurry phase can be obtained by the following formulas:

(13) $x_1=\frac{n_1-n_E^1}{n_0-n_E}$$x_2=\frac{n_2-n_E^2}{n_0-n_E}$

(14) Both the separation factor () of CO.sub.2 over N.sub.2 and the solubility coefficient of CO.sub.2 in the slurry (S.sub.c mol.Math.(L bar).sup.1) are used to characterize the CO.sub.2 capture ability of the slurry system:

(15) $=\frac{x_2}{y_2}/\frac{x_1}{y_1}$$s_c=\left(n_2-n_{E2}\right)/\left(S_v{P}_E{y}_2\right)$

(16) Where S.sub.v is the volume of slurry.

(17) To demonstrate the excellent CO.sub.2 capture ability of the method provided in the present invention, separation experiments for a CO.sub.2/N.sub.2 (20.65/79.35 mol %) gas mixture at 293.15 K in dry ZIF-8, pure ethylene glycol, an ZIF-8/glycol slurry, and an ZIF-8/glycol-methylimidazole slurry were performed, the corresponding experimental results are summarized in Table 1.

(18) As Table 1 shows, compared to dry ZIF-8, pure ethylene glycol, and the ZIF-8/glycol (85:15) slurry, much smaller CO.sub.2 mole fraction (y.sub.2) in the equilibrium gas phase, higher CO.sub.2 selectivity (), and higher CO.sub.2 solubility coefficient (S.sub.c) are obtained in the ZIF-8/glycol-methylimidazole slurry, in which the mass ratio among ZIF-8, glycol and methylimidazole was equaled to 15:51:34, demonstrating the excellent promoting effect of methylimidazole on the whole separation system. Furthermore, in order to verify the reusability of the slurry system provided in the present invention, CO.sub.2 captured ZIF-8/glycol-methylimidazole slurry was then regenerated at room temperature through vacuuming and was used to separate the same CO.sub.2/N.sub.2 gas mixture again, the relevant experimental results was also listed in Table 1. We observed no loss of CO.sub.2 separation ability of the slurry.

(19) TABLE-US-00002 TABLE 1 CO.sub.2/N.sub.2 (20.65/79.35 mol %) gas mixture separation results in four different separation media containing ZIF-8 at 293.15 K .sup.aP.sub.0/ P.sub.E/ y.sub.2/ x.sub.2/ S.sub.c/mol .Math. Separation media (MPa) (MPa) % % (L bar).sup.1 Dry ZIF-8 1.90 1.63 16.59 51.20 5.28 Pure ethylene glycol 1.79 1.64 17.03 72.12 13 0.06 ZIF-8/glycol slurry 1.75 1.5 10.10 89.54 76 0.15 ZIF-8/glycol- 1.65 1.33 1.73 86.47 362 1.29 methylimidazole slurry Recycled ZIF-8/glycol- 1.64 1.33 1.74 86.37 358 1.28 methylimidazole slurry .sup.aP.sub.0 is the initial pressure in the sapphire cell

Example 2

(20) In this example, a ZIF-8/triethylene glycol-methylimidazole slurry was used to separate the CO.sub.2/N.sub.2 (20.65/79.35 mol %) gas mixture at 293.15 K, in which the mass ratio among ZIF-8, triethylene glycol and methylimidazole was specified to 15:51:34. The same experimental setup and data processing are used in this example have been detailed in example 1. And the corresponding separation results are shown in Table 2.

(21) As Table 2 shows, similar to ZIF-8/glycol-methylimidazole shown in example 1, high CO.sub.2 selectivity and solubility coefficient have also been obtained by using this ZIF-8/triethylene glycol-methylimidazole slurry. It should be noted that attributing to the relative higher viscosity of triethylene glycol than that of ethylene glycol, CO.sub.2 capture rate in the ZIF-8/triethylene glycol-methylimidazole slurry is some slower than that in ZIF-8/glycol-methylimidazole slurry.

(22) TABLE-US-00003 TABLE 2 Results of separation of CO2/N2 by a triethylene glycol/methyl imidazole/ZIF-8 slurry mixture P.sub.0/ P.sub.E/ y.sub.2/ x.sub.2/ S.sub.c/mol .Math. Separation medium (MPa) (MPa) % % (L bar).sup.1 ZIF-8/triethylene 1.65 1.35 1.88 87.15 354 1.23 glycol-methyl imidazole slurry

Example 3

(23) In this example, a ZIF-8/glycol-imidazole slurry was used to separate the CO.sub.2/N.sub.2 (20.65/79.35 mol %) gas mixture at 293.15 K, in which the mass ratio between ZIF-8, glycol and imidazole was specified to 15:51:34. The same experimental setup and data processing are used in this example have been detailed in example 1. And the corresponding separation results are shown in Table 3.

(24) As Table 3 shows, both the CO.sub.2 selectivity and solubility coefficient obtained in this ZIF-8/glycol-imidazole slurry are even some higher than that in the ZIF-8/glycol-methylimidazole slurry shown in example 1, demonstrating the better CO.sub.2 capture ability of the former. This effect should be attributed to the fact that compared to methylimidazole, imidazole has a smaller molecule mass, which suggests under the same condition, more imidazole molecules may exist in the slurry to react with CO.sub.2 than that of methylimidazole.

(25) TABLE-US-00004 TABLE 3 Results of separation of CO.sub.2/N.sub.2 by an ethylene glycol/imidazole/ZIF-8 slurry mixture P.sub.0/ P.sub.E/ y.sub.2/ x.sub.2/ S.sub.c/mol .Math. Separation medium (MPa) (MPa) % % (L bar).sup.1 ZIF-8/glycol- 1.65 1.30 1.50 85.68 393 1.33 imidazole slurry

Example 4

(26) In this example, a ZIF-8/glycol-benzimidazole slurry was used to separate the CO.sub.2/N.sub.2 (20.65/79.35 mol %) gas mixture at 293.15 K, in which the mass ratio between ZIF-8, glycol and benzimidazole was specified to 15:51:34. The same experimental setup and data processing are used in this example have been detailed in example 1. And the corresponding separation results are shown in Table 4.

(27) As Table 4 shows, attributing to the much larger molecular mass of benzimidazole than that of methylimidazole, under the same experimental conditions, both the CO.sub.2 selectivity and solubility coefficient in the ZIF-8/glycol-benzimidazole slurry is some smaller than that in ZIF-8/glycol-methylimidazole slurry shown in example 1. However, it should be noted that the CO.sub.2 separation ability of the ZIF-8/glycol-benzimidazole slurry is still much better than that of pure ethylene glycol, ZIF-8 and ZIF-8/glycol slurry.

(28) TABLE-US-00005 TABLE 4 Results of separation of CO.sub.2/N.sub.2 by an ethylene glycol/benzimidazole/ZIF-8 slurry mixture P.sub.0/ P.sub.E/ y.sub.2/ x.sub.2/ S.sub.c/mol .Math. Separation medium (MPa) (MPa) % % (L bar).sup.1 ZIF-8/glycol- 1.66 1.38 2.21 87.83 319 1.18 benzimidazole slurry

Example 5

(29) In this example, a ZIF-67/glycol-methylimidazole slurry was used to separate the CO.sub.2/N.sub.2 (20.65/79.35 mol %) gas mixture at 293.15 K, in which the mass ratio between ZIF-67, glycol and methylimidazole was specified to 15:51:34. The same experimental setup and data processing are used in this example have been detailed in example 1. And the corresponding separation results are shown in Table 5.

(30) As can be seen from Table 5, both the CO.sub.2 selectivity and solubility coefficient obtained in ZIF-67/glycol-methylimidazole slurry are even some higher than that in ZIF-8/glycol-methylimidazole slurry shown in example 1, demonstrating ZIF-67 can also be used to realize the absorption-adsorption hybrid method provided in this invention.

(31) TABLE-US-00006 TABLE 5 Results of separation of CO.sub.2/N.sub.2 by an ethylene glycol/methyl imidazole/ZIF-67 slurry mixture P.sub.0/ P.sub.E/ y.sub.2/ x.sub.2/ S.sub.c/mol .Math. Separation medium (MPa) (MPa) % % (L bar).sup.1 ZIF-67/glycol- 1.64 1.28 1.32 85.13 428 1.80 methylimidazole slurry

Example 6

(32) In order to broaden the application range of the absorption-adsorption hybrid separation method provided in this invention, in this example, a ZIF-8/glycol-methylimidazole slurry was used to separate a CO.sub.2/CH.sub.4 (21.93/78.07 mol %) gas mixture at 293.15 K. In this experimental run, the mass ratio among ZIF-8, glycol and methylimidazole was specified to 15:51:34. The same experimental setup and data processing are used in this example have been detailed in example 1. The corresponding separation results are shown in Table 6.

(33) TABLE-US-00007 TABLE 6 Results of separation of CO.sub.2/CH.sub.4 by an ethylene glycol/methyl imidazole/ZIF-8 slurry mixture P.sub.0/ P.sub.E/ y.sub.2/ x.sub.2/ S.sub.c/mol .Math. Separation medium (MPa) (MPa) % % (L bar).sup.1 ZIF-8/glycol- 0.65 0.49 2.05 74.24 138 1.12 methylimidazole slurry

(34) As Table 6 shows, CO.sub.2 can be effectively separated from the CO.sub.2/CH.sub.4 gas mixture by using the ZIF-8/glycol-methylimidazole slurry. The obtained CO.sub.2 over CH.sub.4 selectivity is much higher than that reported in the literature by using other separation media. Furthermore, the CO.sub.2 solubility coefficient (S.sub.C) in the slurry is also much higher than those obtained from the water-based technology and in the ionic liquids reported in the literature. Demonstrating the proposed absorption-adsorption hybrid separation method can also be used to separate CO.sub.2/CH.sub.4 gas mixtures.

Example 7

(35) In order to broaden the application range of the absorption-adsorption hybrid separation method proposed in this invention, in this example, a ZIF-8/glycol-methylimidazole slurry was used to separate a CO.sub.2/H.sub.2 (23.6/76.4 mol %) gas mixture at 293.15 K. In this experimental run, the mass ratio among ZIF-8, glycol and methylimidazole was specified to 15:51:34. The same experimental setup and data processing are used in this example have been detailed in example 1. And the corresponding separation results are shown in Table 7.

(36) TABLE-US-00008 TABLE 7 Results of separation of CO.sub.2/H.sub.2 by an ethylene glycol/methyl imidazole/ZIF-8 slurry mixture P.sub.0/ P.sub.E/ y.sub.2/ x.sub.2/ S.sub.c/mol .Math. Separation medium (MPa) (MPa) % % (L bar).sup.1 ZIF-8/glycol- 4.13 3.34 2.02 95.16 951 1.21 methylimidazole slurry

(37) As Table 7 shows, CO.sub.2 can also be effectively separated from CO.sub.2/H.sub.2 gas mixtures by using the ZIF-8/glycol-methylimidazole slurry, the obtained CO.sub.2 over H.sub.2 selectivity is much higher than that reported in the literature. Meanwhile, the obtained CO.sub.2 solubility coefficient (S.sub.c) is also much higher than those reported in ionic liquids. Demonstrating the proposed absorption-adsorption hybrid separation method can also be used to separate CO.sub.2/H.sub.2 gas mixtures.

(38) All results mentioned above demonstrate that the hybrid method proposed in the present invention has high CO.sub.2 capture ability and capture amount.