JP2017176986A - Carbon dioxide separation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon dioxide separation method in which degradation of separation permeation performance is suppressed even if a gas separation membrane is used over a long term.SOLUTION: The carbon dioxide separation method is provided in which a gaseous mixture including carbon dioxide is contacted with one surface of a polymer gas separation membrane to separate the carbon dioxide permeated the membrane from the other surface thereof. The gaseous mixture including carbon dioxide includes moisture of 0.001-0.2 vol%. It is preferable that the polymer gas separation membrane is composed of a polymer having a joint by dehydration condensation, and is also preferable that the membrane is composed of aromatic polyimide.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a separation method from a mixed gas containing carbon dioxide.

  As a method for separating a mixed gas containing two or more kinds of different gases into each gas, a membrane separation method using a difference in gas permeation rate with respect to the membrane is known. In this method, by collecting the permeation gas and / or the non-permeation gas, a high purity high permeability gas and / or a high purity low permeability gas as the target gas can be obtained.

Patent Document 1 describes an apparatus for selectively separating carbon dioxide from a raw material gas in which carbon dioxide and water vapor are contained in a predetermined main component gas. In this apparatus, a CO ₂ facilitated transport membrane formed by adding 2,3-diaminopropionic acid to a polyvinyl alcohol-polyacrylic acid copolymer gel membrane is used as a separation membrane. And in the state which heated source gas to the temperature of 100 degreeC or more, the pressure of this source gas is adjusted so that the water vapor saturation in this source gas may become a specific range, and it supplies to the said apparatus.

  By the way, regarding the separation of carbon dioxide, Non-Patent Document 1 reports that when a polyimide membrane is used as a carbon dioxide separation membrane, the carbon dioxide permeation performance of the polyimide membrane is reduced by 20% in two years.

JP 2008-036464 A

Y. Iwakami, Abstracts (Oral Presentations) of International Congress on Membranes, and Membrane Processes (August, 1993, Heiderberg).

The technique described in Patent Document 1 is a gas separation method using a CO ₂ facilitated transport membrane. In general, the CO ₂ facilitated transport membrane causes water vapor to accompany the raw material gas introduced into the membrane, because the carrier of the facilitated transport membrane reacts with CO ₂ and further water vapor, thereby promoting permeation. Usually, the higher the water vapor concentration, the more the permeation is promoted. In Patent Document 1, a large amount of water vapor of 50% is entrained in the source gas at a high temperature of 100 ° C. or higher, which is greatly different from the idea of the present invention. Further, it is necessary to precisely adjust the supply pressure of the raw material gas, and it is also necessary to adjust the supply temperature. Thus, the technique described in this document is not economically advantageous for long-term implementation on an industrial scale. Moreover, as described in Non-Patent Document 1, when a gas separation membrane made of polyimide is used, it is not easy to maintain the carbon dioxide permeation performance over a long period of time.

  Accordingly, an object of the present invention is to provide a carbon dioxide separation method that can eliminate the above-mentioned drawbacks of the prior art.

  As a result of intensive studies by the present inventors to solve the above problems, surprisingly, if a specific amount of water vapor is allowed to coexist in the mixed gas to be separated, the carbon dioxide permeability of the gas separation membrane is long. It was found that it was maintained over time.

The present invention has been made on the basis of the above-mentioned knowledge. A mixed gas containing carbon dioxide is brought into contact with one surface of the polymer gas separation membrane, and the carbon dioxide that has permeated the gas separation membrane is converted into the gas separation membrane. A method for separating carbon dioxide, which is separated from the other surface,
The said mixed gas containing a carbon dioxide solves the said subject by providing the separation method of the carbon dioxide containing 0.001 volume% or more and 0.2 volume% or less of water vapor | steam.

  ADVANTAGE OF THE INVENTION According to this invention, even if it uses for a long period of time, the fall of the separation-permeability of a carbon dioxide is suppressed, and the separation method of a carbon dioxide very useful on actual use is provided.

FIG. 1 is a graph showing the relationship between carbon dioxide permeation rate and treatment time in Examples and Comparative Examples. FIG. 2 is a graph showing the relationship between the separation factor (P ′ _{CO 2} / P ′ _{CH 4} ) and the processing time in Examples and Comparative Examples.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. In the gas separation method of the present invention, a gas containing carbon dioxide is used as a mixed gas to be separated. Various gases other than carbon dioxide contained in this mixed gas (however, excluding water vapor) can be used. For example, hydrocarbon gas such as methane, nitrogen gas, air or the like can be used. These gases other than carbon dioxide can be contained in the mixed gas in one or more kinds. When the mixed gas is, for example, natural gas or biogas, the mixed gas preferably contains a hydrocarbon gas such as methane and carbon dioxide.

  For separation of the mixed gas, for example, a gas separation membrane made of a polymer material is used. The gas separation membrane can be used in the form of, for example, a film-like flat membrane or a hollow fiber-like hollow fiber membrane. In particular, it is preferable that a hollow fiber membrane is used as a gas separation membrane, and a hollow fiber membrane element in which a plurality of these are bundled is used.

  As the polymer material constituting the gas separation membrane, a material having a selective permeability of carbon dioxide is preferably used. Examples of such a polymer material include polyimide, polyamide, cellulose acetate, polysulfone, polyethersulfone, polyphenylsulfone, silicon rubber, and perfluorosilicon rubber. In particular, the effects of the present invention become more prominent when a polymer material that has a dehydration condensation reaction during synthesis and has a bond due to dehydration condensation is used as a gas separation membrane. From this viewpoint, it is preferable to use polyimide as the gas separation membrane.

  When polyimide is used as the gas separation membrane, it can be obtained by synthesizing a polyamic acid using a tetracarboxylic acid component and a diamine component, and subjecting the polyamic acid to heat treatment for dehydration and imidization.

  As the tetracarboxylic acid component, an aliphatic tetracarboxylic dianhydride or an aromatic tetracarboxylic dianhydride can be used. On the other hand, aliphatic diamine or aromatic diamine can be used as the diamine component.

  Examples of the aliphatic tetracarboxylic dianhydride include cyclobutane-1,2,3,4-tetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, dicyclohexyl-3, 3 ′, 4,4′-tetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic acid-1,2: 4,5-dianhydride, 1,2,3,4-cyclobutanetetra Carboxylic dianhydrides, bicyclo [2.2.2] oct-7-ene-2,3; 5,6-tetracarboxylic dianhydrides and the like. The aromatic tetracarboxylic dianhydride preferably has 2 to 3 aromatic rings. For example, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, 4,4 ′ -(Hexafluoroisopropylidene) -bis (phthalic anhydride) (this compound is also referred to as 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride), 2,3,3 ', 4'-biphenyltetracarboxylic dianhydride, 2,2', 3,3'-biphenyltetracarboxylic dianhydride, pyromellitic dianhydride, benzophenone tetracarboxylic dianhydride, 4,4 ' -Oxydiphthalic dianhydride, diphenyl sulfone tetracarboxylic dianhydride, p-terphenyl tetracarboxylic dianhydride, m-terphenyl tetracarboxylic dianhydride, etc. And the like.

  Examples of the aliphatic diamine include trans-1,4-diaminocyclohexane, cis-1,4-diaminocyclohexane, 1,6-hexamethylenediamine, 1,10-decamethylenediamine, 1,3-bis ( Aminomethyl) cyclohexane, 1,4-bis (aminomethyl) cyclohexane, polyoxypropylenediamine having a weight average molecular weight of 500 or less, and the like. On the other hand, examples of the aromatic diamine include paraphenylenediamine, metaphenylenediamine, 4,4′-oxydianiline, 3,4′-oxydianiline, 4,4′-diaminodiphenylmethane, 2,4-toluenediamine, 3 , 3′-dihydroxy-4,4′-diaminobiphenyl, bis (4-amino-3-carboxyphenyl) methane, 2,4-diaminotoluene, 3,5-diaminobenzoic acid, 3,7-diamino-2, Isomers 3,7-diamino-2,6-dimethyldibenzothiophene = 5,5-dioxide, 3,7-diamino-, which is mainly composed of 8-dimethyldibenzothiophene = 5,5-dioxide and has different methyl group positions Examples thereof include a mixture containing 4,6-dimethyldibenzothiophene = 5,5-dioxide.

  In the present invention, it is particularly preferable to use an aromatic polyimide as the polyimide. The aromatic polyimide can be obtained using an aromatic tetracarboxylic acid component and an aromatic diamine component.

  When a gas separation membrane made of polyimide is used, an asymmetric membrane may be used as the gas separation membrane. In general, the asymmetric membrane has a two-layer structure of a dense skin layer that separates the target gas and a porous layer that supports the skin layer.

  One feature of the present invention is that when a gas separation membrane is used to selectively separate carbon dioxide from a mixed gas containing carbon dioxide, water vapor is contained in the mixed gas. When a polymer material having a bond by dehydration condensation is used as a gas separation membrane, generally, if water vapor is present in the mixed gas to be processed, the above bond is cleaved by hydrolysis due to the water vapor. It has been considered that various properties of the polymer material are deteriorated. Therefore, it has been usual that water vapor is not included in the mixed gas to be processed as much as possible. Contrary to such conventional technical common sense, the present inventors surprisingly, when a specific amount of water vapor coexists in the mixed gas to be treated without causing deterioration of the characteristics of the polymer material. It has been found that the decrease in the separation and permeability of carbon dioxide is suppressed even when used over a long period of time. The inventor believes that this reason is as follows. Carbon dioxide is known to be a highly soluble gas for polymer materials. In the selective permeation separation of carbon dioxide from a mixed gas using a gas separation membrane, the mixed gas acts on the gas separation membrane at a high pressure in order to increase efficiency. Due to this and the fact that carbon dioxide is a highly soluble gas, the polymer material is plasticized or a phenomenon close thereto, and the structure of the separation layer of the gas separation membrane changes. For this reason, when carbon dioxide is separated over a long period of time, it is considered that the carbon dioxide separation permeability of the gas separation membrane gradually decreases. In contrast, when water vapor is present together with carbon dioxide in the mixed gas according to the present invention, the structural change of the separation layer caused by carbon dioxide is suppressed by water vapor, and as a result, the carbon dioxide separation permeability of the gas separation membrane is reduced. It is thought that the decrease of the is suppressed.

  From the viewpoint of making the above-described advantages more remarkable, the proportion of water vapor present in the mixed gas is preferably 0.001% by volume or more and 0.2% by volume or less, and 0.005% by volume or more and 0.000% by volume or less. It is more preferable to set it as 2 volume% or less, and it is still more preferable to set it as 0.01 volume% or more and 0.2 volume% or less.

  The ratio of water vapor in the mixed gas can be adjusted by various methods. For example, (a) the mixed gas containing 0.2% by volume or more of water vapor and carbon dioxide can be dehumidified to control the water vapor to 0.001% by volume or more and 0.2% by volume or less. Alternatively, (b) the water vapor concentration is 0.001% by volume or less, and the mixed gas containing carbon dioxide is humidified to control the water vapor to 0.001% by volume or more and 0.2% by volume or less. Note that “the concentration of water vapor is 0.001% by volume or less” includes the case where the mixed gas does not substantially contain water vapor. The mixed gas with the water vapor ratio adjusted in this way may be supplied to the gas separation membrane described above to separate carbon dioxide. In the case of (a), there is no particular limitation on the dehumidifying method, and a conventionally known method can be appropriately employed. In the case of (b), there is no restriction | limiting in particular in a humidification method, The method known until now can be employ | adopted suitably.

  There is no particular limitation on the temperature of the mixed gas when the mixed gas containing carbon dioxide is brought into contact with one surface of the polymer gas separation membrane. In general, a mixed gas having a temperature range of −30 ° C. to 100 ° C., particularly 0 ° C. to 80 ° C. can be contacted.

  There is no particular limitation on the pressure of the mixed gas when the mixed gas containing carbon dioxide is brought into contact with one surface of the polymer gas separation membrane. In general, a mixed gas having a pressure range of 0.1 MPaG or more and 20 MPaG or less, particularly 1 MPaG or more and 10 MPaG or less can be contacted in terms of gauge pressure.

  As described above, according to the present invention, even when a mixed gas containing carbon dioxide is separated over a long period of time using a gas separation membrane, a decrease in the separation and permeability of carbon dioxide is suppressed. As a result of further investigation by the present inventor, even when water vapor is contained in the mixed gas in a specific ratio according to the present invention, the separation factor α (that is, carbon dioxide and other gases contained in the mixed gas) It has been found that the transmission rate ratio) does not decrease. This means that carbon dioxide can be separated and permeated stably over a long period of time.

  As mentioned above, although this invention was demonstrated based on the preferable embodiment, this invention is not restrict | limited to the said embodiment, In the range which does not impair the effect of this invention, various embodiment is possible.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples.

[Example 1]
(1) Preparation of hollow fiber membrane element An asymmetric membrane made of aromatic polyimide was used as the hollow fiber membrane. This aromatic polyimide was synthesized according to a conventional method using 60 mol% of s-BPDA as an aromatic tetracarboxylic acid component, 40 mol% of 6FDA, and 60 mol% of TSN and 40 mol% of DABA as an aromatic diamine component. Is. The hollow fiber membrane had an inner diameter of 130 μm and an outer shape of 280 μm. The hollow fiber membrane had an asymmetric structure. Three hollow fiber membranes were bundled, and both ends thereof were fixed with an epoxy resin to obtain a hollow fiber element having a length of 5 mm.
The formal names of the abbreviations are as follows.
s-BPDA: 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride 6FDA: 4,4 ′-(hexafluoroisopropylidene) -bis (phthalic anhydride) (note that this compound is 2,2 -Also referred to as bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride.)
TSN: 3,7-diamino-2,8-dimethyldibenzothiophene = 5,5-dioxide as a main component, and isomers 3,7-diamino-2,6-dimethyldibenzothiophene = 5 having different methyl group positions DABA mixture containing 5-dioxide, 3,7-diamino-4,6-dimethyldibenzothiophene = 5,5-dioxide: 3,5-diaminobenzoic acid

(2) Separation of carbon dioxide from mixed gas The hollow fiber membrane element obtained in (1) above was housed in a casing to obtain a hollow fiber membrane module. Using this hollow fiber membrane module, carbon dioxide was separated from a mixed gas containing carbon dioxide and methane. The volume ratio of carbon dioxide and methane in the mixed gas was set to 40 vol%: 60 vol%. Saturated steam at 15 ° C. was brought into contact with this mixed gas so that the mixed gas contained water vapor. The ratio of water vapor in the mixed gas was 0.021 vol%. This mixed gas was supplied to the hollow fiber membrane module under the conditions of 60 ° C. and 8 MPaG, and the gas was permeated from the outside to the inside of the hollow fiber membrane. The supply amount of the mixed gas was 75 ml / min. The permeation rates P ′ _CO2 and P ′ _CH4 were calculated for carbon dioxide and methane, respectively, from the permeation flow rate, supply pressure, and effective area of the gas that had permeated the hollow fiber membrane. Further, the separation coefficient α, that is, P ′ _{CO 2} / P ′ _{CH 4} was calculated. The measurement was performed over 1000 hours. The results are shown in FIGS. In the graph of the permeation speed P ′ _CO2 shown in FIG. 1, the permeation speed P ′ _CO2 is standardized so that the results of this example can be easily compared with the results of the comparative examples described later. .

[Comparative Example 1]
In this comparative example, water vapor was not included in the mixed gas containing carbon dioxide and methane. Other than this, carbon dioxide was separated in the same manner as in Example 1. The results are shown in FIGS.

[Comparative Example 2]
In this comparative example, 60 ° C. saturated water vapor was brought into contact with a mixed gas containing carbon dioxide and methane, and water vapor was contained in the mixed gas. The ratio of water vapor in the mixed gas was 0.24 vol%. Other than this, carbon dioxide was separated in the same manner as in Example 1. The results are shown in FIGS.

[Discussion]
As apparent from the comparison results of Example 1 and Comparative Example 1 shown in FIG. 1, in the initial stage of separation, towards the Comparative Example 1 than in Example 1, permeation rate P _'CO2 is that slightly higher I understand. On the other hand, it can be seen that when the separation process exceeds 600 hours, the permeation speed P ′ _CO2 of Comparative Example 1 starts to decrease and becomes lower than the permeation speed P ′ _CO2 of Example 1. It should be noted that the permeation speed P ′ _CO2 of Example 1 does not change even when the processing time passes 1000 hours. In Comparative Example 2, a rapid decrease in the permeation rate P ′ _{CO 2} was observed in the initial stage of the treatment. Further, before the treatment time reached 1000 hours, the film was deteriorated, and the measurement could not be continued.

  On the other hand, as is apparent from the results shown in FIG. 2, it can be seen that in both Example 1 and each Comparative Example, the separation coefficient between carbon dioxide and methane does not change even after 1000 hours.

Claims (6)

A carbon dioxide separation method in which a mixed gas containing carbon dioxide is brought into contact with one surface of a polymer gas separation membrane, and carbon dioxide permeated through the gas separation membrane is separated from the other surface of the gas separation membrane. And
The method for separating carbon dioxide, wherein the mixed gas containing carbon dioxide contains water vapor in an amount of 0.001 vol% to 0.2 vol%.   The carbon dioxide separation method according to claim 1, wherein the polymer gas separation membrane is composed of a polymer having a bond by dehydration condensation.   The carbon dioxide separation method according to claim 1, wherein the polymer gas separation membrane is made of aromatic polyimide.   The method for separating carbon dioxide according to any one of claims 1 to 3, wherein the mixed gas contains carbon dioxide and hydrocarbon gas.   Claims comprising supplying the polymer gas separation membrane after dehumidifying a mixed gas containing 0.2% by volume or more of water vapor and carbon dioxide to control the water vapor to 0.001% or more and 0.2% by volume or less. Item 5. The method for separating carbon dioxide according to any one of Items 1 to 4.   After the water vapor concentration is 0.001% by volume or less and the mixed gas containing carbon dioxide is humidified to control the water vapor to 0.001% by volume or more and 0.2% by volume or less, the polymer gas separation membrane is formed. The method for separating carbon dioxide according to any one of claims 1 to 4, wherein the carbon dioxide is supplied.

Images