CO2 adsorbent

Abstract

A CO2 adsorbent that enhances the CO2 recovery rate in the CO2 separation and recovery process is provided. The CO2 adsorbent is made of active carbon and/or molecular sieve carbon with a large specific surface area. The active carbon and/or molecular sieve carbon has micropores and/or mesopores 200 Å or less in size and penetrating pores formed by the micropores and/or mesopores connecting with one another. All or part of surfaces of inner passages through which a gas flows and of terminal pores are covered directly with a polar liquid or polar solid without a reaction layer there between.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a CO2 adsorbent used to separate and recover CO2.

[0002] Attempts have been made in recent years to separate and recover CO2 from exhaust emissions. Separation and recovery of CO2 normally involves having a porous structure with a large specific surface area, such as active carbon, adsorb a gas to be recovered, such as CO2, reducing a pressure a predetermined time later to selectively desorb CO2, and repeating this process to extract and recover CO2.

[0003] Among the methods for separating CO2 are an adsorption method (Japanese Patent Laid-Open Nos. 5-116915, 6-99015, 7-80246, 7-277718 and 8-239206), a membrane separation method (Japanese Patent Laid-Open No. 9-70521) and a chemical reaction separation method (Japanese Patent Laid-Open No. 8-257355). These methods, however, require large amounts of energy for separating CO2, have low recoveries, and slow separation speeds.

[0004] For example, when an unprocessed active carbon is used, CO2 is only adsorbed to the surface of the active carbon by the van der Waals attraction, as shown in FIG. 5. Its adsorptive attraction is not large and there are not many active points, so the amount of CO2 adsorbed is not large.

[0005] When zeolite is used, its surface polarity is not large. The adsorptive attraction therefore is not sufficiently large and the adsorbed amount is not large.

[0006] The present invention has been accomplished to overcome these problems experienced with the conventional technologies and to provide a CO2 adsorbent which can separate and recover CO2 with a high recovery rate.

SUMMARY OF THE INVENTION

[0007] As a means for effectively solving the above problems, the present invention provides a CO2 adsorbent having a porous structure with a large specific surface area.

[0008] According to one aspect of the present invention, there is provided a CO2 adsorbent having a porous structure, the porous structure comprising: micropores and/or mesopores and penetrating pores formed by the micropores and/or mesopores connecting with one another; wherein all or part of surfaces of inner passages through which a gas flows and of terminal pores are covered directly with a polar liquid without a reaction layer there between.

[0009] According to another aspect of the invention, there is provided a CO2 adsorbent having a porous structure, the porous structure comprising: micropores and/or mesopores and penetrating pores formed by the micropores and/or mesopores connecting with one another; wherein all or part of surfaces of inner passages through which a gas flows and of terminal pores are covered directly with a polar solid without a reaction layer there between.

[0010] According to still another aspect of the invention, there is provided a CO2 adsorbent having a porous structure in which the polar liquid uses phosphoric acid (H3PO4) and contains 10-30% by weight of phosphoric acid.

[0011] According to a further aspect of the invention, there is provided a CO2 adsorbent having a porous structure in which the polar liquid uses phosphoric acid, sulfuric acid, a mixed liquid of phosphoric acid and sulfuric acid, or a water solution of 50% or more of the mixed liquid.

[0012] According to a further aspect of the invention, there is provided a CO2 adsorbent having a porous structure in which the polar solid uses a salt, such as phosphate, sulfate, nitrate and halide, or a mixture or composite of these salts.

[0013] According to a further aspect of the invention, there is provided a CO2 adsorbent having a porous structure in which the porous structure uses active carbon and/or molecular sieve carbon, and in which the surfaces of inner passages through which a gas flows and of terminal pores are covered with a polar liquid layer which has a thickness smaller than one-half the radius of a circle having an area identical to the cross sectional area of the corresponding inner passage or terminal pore, or the walls of the penetrating pores are covered with a polar liquid layer which has a thickness of 10 Å or more.

[0014] According to a further aspect of the invention, there is provided a CO2 adsorbent having a porous structure in which the porous structure uses active carbon and/or molecular sieve carbon, and in which the surfaces of inner passages through which a gas flows and of terminal pores are covered with a polar solid layer of fine particles smaller in grain diameter than one-half the radius of a circle having an area identical to the cross sectional area of the corresponding inner passage or terminal pore, or the walls of the penetrating pores are covered with a polar solid layer of fine particles which has a thickness of 10 Å or more.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is an enlarged cross section showing an inner construction of the CO2 adsorbent of an embodiment in which a polar liquid layer is formed over the surface of micropores and mesopores.

[0016] FIG. 2 is an enlarged cross section showing an inner construction of the CO2 adsorbent of the above embodiment in which a polar solid layer is formed over the surface of micropores and mesopores.

[0017] FIG. 3 is an explanatory diagram showing an electrostatic attraction acting between phosphoric acid of the CO2 adsorbent and carbon dioxide in the above embodiment.

[0018] FIG. 4 is a schematic diagram showing a CO2 recovery device as one embodiment of the invention.

[0019] FIG. 5 is an explanatory diagram showing the van der Waals attraction acting between active carbon of a conventional CO2 adsorbent and carbon dioxide.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] Now, embodiments of the invention will be described in detail. It should be noted, however, that the following description is only by way of example for better understanding of the essence of the invention, and is not intended to limit the invention unless otherwise specified.

[0021] The porous structure of this embodiment has minute pores distributed almost evenly throughout the entire structure, with the pores connecting with each other to form penetrating pores.

[0022] The pores in porous structures such as active carbon and CMS (molecular sieve carbon) generally include micropores (20 Å or less), mesopores (20-1000 Å) and macropores (1000 Å or more). The porous structure in the embodiment has micro-pores and meso-pores, which combine to form penetrating pores through which a gas can pass. These penetrating pores are preferably 200 Å in effective diameter.

[0023] A polar liquid or polar solid that generates an electrostatic attraction is added to the porous structure. By adding the polar liquid or polar solid to the porous structure, the surfaces of even the smallest pores or micropores are wet by the polar liquid or attached with the polar solid. That is, the surfaces of inner passages through which a gas flows and of terminal pores are covered with a polar liquid layer or polar solid layer of fine particles which has a thickness smaller than one-half the radius of a circle having an area identical to the cross sectional area of the corresponding inner passage or terminal pore, or the walls of the penetrating pores are covered with a polar liquid layer or polar solid layer of fine particles which has a thickness of 10 Å or more.

[0024] In a state in which the pores of the porous structure are wetted with the polar liquid, all the surfaces of micropores 2 and mesopores 3 formed in the porous structure 1 are covered with the polar liquid layer 4, as shown in FIG. 1.

[0025] In a state in which the pores of the porous structure are attached with the polar solid, all the surfaces of micropores 2 and mesopores 3 formed in the porous structure 1 are covered with the polar solid layer 5, as shown in FIG. 2.

[0026] An example polar liquid includes phosphoric acid, sulfuric acid, their mixed liquid, and water solution containing 50% or more of the mixed liquid.

[0027] An example polar solid includes salts such as phosphate, sulfate, nitrate and halides, and a mixture or composite of these salts.

[0028] Because electric charges are given to the porous structure 1, such as active carbon and CMS, when a polar liquid or polar solid is added, an electrostatic attraction is generated between the polar liquid or polar solid and CO2, increasing the adsorptive attraction and therefore the amount of gas adsorbed (see FIG. 3).

[0029] Because the pores of the porous structure 1, to which the polar liquid or polar solid is added, are wetted with the polar liquid or attached with the polar solid on the surfaces of even the smallest micropores 2, the repetition of the adsorption and desorption of CO2 to and from the porous structure 1 can recover CO2 with a high recovery rate.

[0030] Now, embodiments of this invention will be described in more detail.

[0031] [Embodiment 1]

[0032] The CO2 adsorbent according to the embodiment 1 uses active carbon as the porous structure and phosphoric acid as the polar liquid.

[0033] When CO2 is introduced into the active carbon to which phosphoric acid was added, CO2 and phosphoric acid (H3PO4) contact each other, as shown in FIG. 3.

[0034] At this time, P of a P—O bond in the phosphoric acid is given a positive charge and O a negative charge. C of C═O bond in CO2 has a positive charge and O a negative charge. So, an electrostatic attraction is generated between the positive and negative charges in the phosphoric acid and those in the CO2, thus increasing the adsorptive attraction and the amount of CO2 adsorbed and enhancing the recovery rate of CO2.

[0035] A CO2 recovery device has, as shown in FIG. 4, an inlet valve 12 installed at one end of a column 11 which measures 83 mm in diameter and 1,000 mm in length, and an outlet valve 13 installed at the other end. A cutoff valve 15 for closing the line to a vacuum pump 14 that depressurizes the interior of the column is provided at the same end of the column where the inlet valve 12 is installed. The active carbon 16 to which phosphoric acid was added is filled into the column 11 as the CO2 adsorbent in a predetermined amount.

[0036] Into the column 11 which is filled with the phosphoric acid-added active carbon 16 is flowed a mixed gas of 8% CO2 and 92% N2 at 1 NL/min (normal liter/minute).

[0037] Adsorption is carried out for 10 minutes at 196 kPa, after which the vacuum pump 14 is operated to depressurize the column 11 to desorb the CO2 from the active carbon 16 to recover the CO2. The evaluation of the recovered CO2 is performed at 0.98 kPa. The recovery rate is calculated using the following formula.

Recovery rate (%)={(amount of recovered gas)×(CO2 concentration %) }/amount of introduced CO2

[0038] The results for various amounts of phosphoric acid added are shown in Table 1 along with the results when sulfuric acid is added.

TABLE 1
CO2 recovery rate (%) of adsorbent with phosphoric acid added
(along with recovery rate when sulfuric acid is added)
(CO2 recovery rate of adsorbent with no polar liquid is 38%)
	Amount of polar liquid
	10%	20%	30%
	Polar liquid
	Phosphoric	56	63	59
	acid
	Sulfuric acid	50	59	55

[0039] As shown in Table 1, the CO2 recovery rate is 56% for the adsorbent with 10% of phosphoric acid, 63% for the adsorbent with 20% of phosphoric acid, and 59% for the adsorbent with 30% of phosphoric acid. These rates are 47% to 66% higher than the CO2 recovery rate when the adsorbent with no phosphoric acid is used. For the adsorbent with sulfuric acid, the CO2 recovery rate is 50% when 10% of sulfuric acid is added, 59% when 20% of sulfuric acid is added, and 55% when 30% of sulfuric acid is added. The addition of sulfuric acid has increased the CO2 recovery rate by 32% to 55% from that of the adsorbent with no sulfuric acid.

[0040] Table 2 shows the CO2 recovery rate as the size (range) of the pores and the amount of phosphoric acid added (%) are changed.

TABLE 2
CO2 recovery rate (%) of adsorbent with phosphoric acid added
	Size of pores
	100 Å	150 Å	200 Å	300 Å
	or less	or less	or less	or less
	10%	25	48	56	36
	phosphoric
	acid added
	20%	32	56	63	46
	phosphoric
	acid added

[0041] As shown in Table 2, the CO2 recovery rate is 25% for the pore size of 100 Å or less when 10% of phosphoric acid is added to the adsorbent; 48% for 100-150 Å; 56% for 150-200 Å; and 56% for 200-300 Å. When 20% of phosphoric acid is added to the adsorbent, the CO2 recovery rate is 32% for the pore size of 100 Å or less; 56% for 100-150 Å; 63% for 150-200 Å; and 46% for 200-300 Å. The maximum recovery rate exists in the pore size range of 100-200 Å.

[0042] [Embodiment 2]

[0043] Tests were made similar to those of the embodiment 1, except that the polar liquid includes 50% or more of a mixed water solution of phosphoric acid and sulfuric acid. The results are shown in Table 3.

TABLE 3
CO2 recovery rate (%) of adsorbent
	Amount of polar liquid
	10%	20%	30%
	Mixture of
	polar liquid
	20%	46	50	48
	phosphoric
	acid and
	35% sulfuric
	acid
	35%	43	48	47
	phosphoric
	acid and
	20% sulfuric
	acid

[0044] As shown in Table 3, the CO2 recovery rate when using the water solution of 20% phosphoric acid and 35% sulfuric acid is improved 21-32% over that obtained by the adhesive with no polar liquid. When the water solution of 35% phosphoric acid and 20% sulfuric acid is used, the CO2 recovery rate is improved by 13-26%.

[0045] [Embodiment 3]

[0046] In this case, tests were conducted in ways similar to the embodiment 1, except that the adsorbent uses phosphate (NaPo3), sulfate (Na2SO4), nitrate (NaNO3) or halide salts (KF, KBr) as the polar solid. The results are shown in Table 4.

TABLE 4
CO2 recovery rate (%) of adsorbent
	Amount of polar solid added
	10%	20%	30%
	Polar solid
	NaPO3	52	58	57
	(phosphate)
	Na2SO4	50	55	54
	(sulfate)
	NaNO3	48	52	51
	(nitrate)
	KF (halide)	47	50	51
	KBr (halide)	45	49	50

[0047] As shown in Table 4, compared with the adsorbent with no polar liquid, the adsorbent with phosphate increased the CO2 recovery rate by 37-53%, the adsorbent with sulfate increased the CO2 recovery rate by 32-45%, the adsorbent with nitrate improved the CO2 recovery rate by 26-37%, the adsorbent with KF (halide salt) improved CO2 recovery rate by 24-34%, and the adsorbent with KBr (halide salt) improved the CO2 recovery rate by 18-32%.

[0048] [Embodiment 4]

[0049] In this embodiment, CMS is used as the porous structure and phosphoric acid or sulfuric acid is added to the CO2 adsorbent. In other respects, the tests were conducted in the similar manner to that of the embodiment 1. The results are shown in Table 5.

TABLE 5
CO2 recovery rate (%) of adsorbent
	Amount of polar liquid
	10%	20%	30%
	Polar liquid
	Phosphoric	58	63	57
	acid
	Sulfuric acid	53	59	53

[0050] As shown in Table 5, when phosphoric acid is added to the adsorbent, the CO2 recovery rate is 58% for the adsorbent with 10% phosphoric acid, 63% for the adsorbent with 20% phosphoric acid, and 57% for the adsorbent with 30% phosphoric acid. When sulfuric acid is added to the adsorbent, the CO2 recovery rate is 53% for the adsorbent with 10% sulfuric acid, 59 for the adsorbent with 20% sulfuric acid, and 53% for the adsorbent with 30% sulfuric acid.

[0051] As a result, the use of CMS, when a polar liquid was added, produced almost the same CO2 recovery rate as obtained with active carbon.

[0052] As described above, according to claim 1 of the invention, the CO2 adsorbent has a porous structure with a large specific surface area, the porous structure comprising: micropores and/or mesopores and penetrating pores formed by the micropores and/or mesopores connecting with one another; wherein all or part of surfaces of inner passages through which a gas flows and of terminal pores are covered directly with a polar liquid without a reaction layer there between. With this CO2 adsorbent, the polar liquid covering the surfaces of the pores in the porous pore structure adsorbs a greater amount of CO2 than can be adsorbed by conventional techniques, thus enhancing the CO2 recovery rate.

[0053] According to claim 2 of the invention, the CO2 adsorbent has a porous structure with a large specific surface area, the porous structure comprising: micropores and/or mesopores and penetrating pores formed by the micropores and/or mesopores connecting with one another; wherein all or part of surfaces of inner passages through which a gas flows and of terminal pores are covered directly with a polar solid without a reaction layer there between. With this CO2 adsorbent, the polar solid covering the surfaces of the pores in the porous pore structure adsorbs a greater amount of CO2 than can be adsorbed by conventional techniques, thus enhancing the CO2 recovery rate.

[0054] According to claim 3 of the invention, the polar liquid uses phosphoric acid and contains 10-30% by weight of phosphoric acid. This CO2 adsorbent can realize a high CO2 recovery rate of 55% or more.

[0055] According to claim 4 of the invention, the polar liquid uses phosphoric acid, sulfuric acid, a mixed liquid of phosphoric acid and sulfuric acid, or a water solution of 50% or more of the mixed liquid. This CO2 adsorbent can have the surfaces of even micropores covered with the polar liquid layer, increasing the CO2 adsorbing attraction.

[0056] According to claim 5 of the invention, the polar solid uses a salt, such as phosphate, sulfate, nitrate and halide, or a mixture or composite of these salts. This CO2 adsorbent can have the surfaces of even micropores covered with the polar solid layer, increasing the CO2 adsorbing attraction.

[0057] According to claim 6 of the invention, the porous structure uses active carbon and/or molecular sieve carbon, and the surfaces of inner passages through which a gas flows and of terminal pores are covered with a polar liquid layer which has a thickness smaller than one-half the radius of a circle having an area identical to the cross sectional area of the corresponding inner passage or terminal pore, or the walls of the penetrating pores are covered with a polar liquid layer which has a thickness of 10 Å or more. When active carbon is used, the gas adsorbing surface area is increased, thus enhancing the CO2 recovery rate to 2.5 times that of the conventional porous structure. When molecular sieve carbon is used, the gas adsorbing surface area is further increased, enhancing the CO2 recovery rate to 5 times that of the conventional porous structure.

[0058] According to claim 6 of the invention, the porous structure uses active carbon and/or molecular sieve carbon, and the surfaces of inner passages through which a gas flows and of terminal pores are covered with a polar solid layer of fine particles smaller in grain diameter than one-half the radius of a circle having an area identical to the cross sectional area of the corresponding inner passage or terminal pore, or the walls of the penetrating pores are covered with a polar solid layer of fine particles which has a thickness of 10 Å or more. When active carbon is used, the gas adsorbing surface area is increased, thus enhancing the CO2 recovery rate to 2 times that of the conventional porous structure. When molecular sieve carbon is used, the gas adsorbing surface area is further increased, enhancing the CO2 recovery rate to 4 times that of the conventional porous structure.

Claims

1. A CO2 adsorbent having a porous structure with a large specific surface area, the porous structure comprising: micropores and/or mesopores and penetrating pores formed by the micropores and/or mesopores connecting with one another; wherein all or part of surfaces of inner passages through which a gas flows and of terminal pores are covered directly with a polar liquid without a reaction layer there between.

2. A CO2 adsorbent having a porous structure with a large specific surface area, the porous structure comprising: micropores and/or mesopores and penetrating pores formed by the micropores and/or mesopores connecting with one another; wherein all or part of surfaces of inner passages through which a gas flows and of terminal pores are covered directly with a polar solid without a reaction layer there between.

3. A CO2 adsorbent according to claim 1, wherein the polar liquid uses phosphoric acid and contains 10-30% by weight of phosphoric acid.

4. A CO2 adsorbent according to claim 1, wherein the polar liquid uses phosphoric acid, sulfuric acid, a mixed liquid of phosphoric acid and sulfuric acid, or a water solution of 50% or more of the mixed liquid.

5. A CO2 adsorbent according to claim 2, wherein the polar solid uses a salt, such as phosphate, sulfate, nitrate and halide, or a mixture or composite of these salts.

6. A CO2 adsorbent according to any one of claim 1, 3 and 4, wherein the porous structure uses active carbon and/or molecular sieve carbon, and wherein the surfaces of inner passages through which a gas flows and of terminal pores are covered with a polar liquid layer which has a thickness smaller than one-half the radius of a circle having an area identical to the cross sectional area of the corresponding inner passage or terminal pore, or the walls of the penetrating pores are covered with a polar liquid layer which has a thickness of 10 Å or more.

7. A CO2 adsorbent according to claim 2 or 5, wherein the porous structure uses active carbon and/or molecular sieve carbon, and wherein the surfaces of inner passages through which a gas flows and of terminal pores are covered with a polar solid layer of fine particles smaller in grain diameter than one-half the radius of a circle having an area identical to the cross sectional area of the corresponding inner passage or terminal pore, or the walls of the penetrating pores are covered with a polar solid layer of fine particles which has a thickness of 10 Å or more.