Carbon dioxide absorber and carbon dioxide separation/recovery method using the absorber

Abstract

The present invention provides a carbon dioxide absorber capable of efficiently and stably removing carbon dioxide in a gas or solution. This carbon dioxide absorber contains an amine compound, a weakly acidic compound and water, the pKb value of the amine compound in an aqueous solution at 30° C. is 4.0 to 7.0, the pKa value of the weakly acidic compound in an aqueous solution at 30° C. is 7.0 to 10.0, and the weakly acidic compound is present in an amount within the range of 0.01 equivalents to 1.50 equivalents with respect to amino groups of the amine compound.

Description

[0001]This application is a Divisional of copending application Ser. No. 14 / 125,048, filed on Dec. 9, 2013, which was filed as PCT International Application No. PCT / JP2012 / 064843 on Jun. 8, 2012, which claims the benefit under 35 U.S.C. §119(a) to Patent Application No. JP 2011-129327, filed in Japan on Jun. 9, 2011, Patent Application No. JP 2011-129438, filed in Japan on Jun. 9, 2011, and Patent Application No. JP 2011-171263, filed in Japan on Aug. 4, 2011, all of which are hereby expressly incorporated by reference into the present application.[0002]The present invention relates to a carbon dioxide absorber for absorbing and removing carbon dioxide contained in a gas, and more particularly, to a carbon dioxide absorber which can separate and recover carbon dioxide from a gas while saving energy. In addition, the present invention relates to a method for separating and recovering carbon dioxide from a gas containing carbon dioxide, such as combustion exhaust gas. Moreover, the pr...

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Benefits of technology

[0065]According to the carbon dioxide absorber of the present invention, carbon dioxide in a gas or solution can be removed efficiently and stably. In addition, according to the carbon dioxide separation and recovery method of the present invention, carbon dioxide can be separated and recovered from gas in the manner of combustion exhaust gas while saving energy. In addition, according to the present invention, a carbon dioxide absorber can be provided that is able to prevent volatilization of amine compounds and corrosion of metal, and can be used stably. In addition, the present invention is able to provide a carbon dioxide removal method and separation / recovery method that demonstrate favorable efficiency during continuous use.

[0066]Without being bound by a particular theory, the reason why the carbon dioxide absorber of the present invention demonstrates the aforementioned effects is thought to be as indicated below. First, an explanation of the first configuration is provided. The absorption reaction of carbon dioxide by an amine compound is thought to proceed in the form of an equilibrium reaction between the following two reactions. One reaction is a reaction that goes through carbamic acid formed by a direct reaction between the nitrogen of an amino group and the carbon of carbon dioxide, while the other reaction is a reaction that goes through bicarbonate mediated by water molecules. Many primary and secondary amines are thought to form carbamic acid and subsequently form a carbonate. Since carbamic acid is unstable in water, it either reacts with another molecule of an amine compound to form carbamate or undergoes hydrolysis to form bicarbonate. Bicarbonates are further converted to carbonates corresponding to the base strength of the amine compound. However, the formation rate of bicarbonates is greater than the formation rate of carbonates in the case of almost all amine compounds. Tertiary amines are thought to form bicarbonates and carbonates since the reaction is unable to go through carbamic acid.

[0067]In other words, all absorption reactions of carbon dioxide by an amine compound are thought to be acid-base neutralization reactions, excluding the carbamic acid formation reaction. Thus, the formation of carbamates and the formation of bicarbonates go through a neutralization reaction between an amine compound and a weakly acidic compound. Accordingly, the majority of these reactions go through a single neutralization reaction each time a molecule of carbon dioxide is absorbed.

[0068]Since the carbon dioxide absorber of the present invention contains a weakly acidic compound, a portion of the amine compound and the weakly acidic compound are neutralized prior to absorption of carbon dioxide. When a compound having stronger acidity than the weakly acidic compound enters a system containing this carbon dioxide absorber, it is thought to replace the weakly acidic compound, and the compound having stronger acidity is thought to enter a neutral state with the amine compound. In other words, by neutralizing a compound having weaker acidity than carbamic acid or carbonic acid with the amine compound in the carbon dioxide absorber, an exchange reaction can take place during the carbon dioxide absorption reaction. This exchange reaction brings about a reduction in the heat of reaction, and in turn, a reduction in the amount of energy required during carbon dioxide desorption. This is the result of a decrease in the heat of reaction (enthalpy difference) corresponding to the neutralization reaction between the amine compound and the weakly acidic compound since this exchange reaction uses an amine compound in a neutralized state.

[0069]Thus, a first effect obtained by containing a weakly acidic compound in the carbon dioxide absorber of the present invention is the aforementioned reduction in the heat of reaction, and the reduction of an equal amount of energy from energy used during regeneration of the carbon dioxide absorber (desorption of carbon dioxide). In addition, there are other significant effects as well. The first is the relatively low level of metal corrosion due to the low basicity (pH) of the carbon dioxide absorber as a result of being in a neutralized state. Corrosion of metal has an effect on the materials of the reactor used in the carbon dioxide separation and recovery process, and has an effect on the durability of the overall process.

[0070]Another effect brought about by the weakly acidic compound is a reduction in the volatility of the amine compound. This is because an amine compound that has been neutralized and put into an ionized state has lower volatility than that in a non-ionized state. Since there is contact between liquid and gas during absorption of carbon dioxide, the amine compound present in the liquid volatilizes into the gas and is lost from the absorber depending on the degree of the vapor pressure. Consequently, it is necessary to remove the volatilized amine compound present in the gas as well as replenish the amine compound corresponding to the amount that has been lost. Therefore, lowering volatility of the amine compound is extremely significant.

Problems solved by technology

Since conventional technologies for separating and recovering carbon dioxide, such as a chemical absorption process, require a large amount of additional energy for separating carbon dioxide from absorbers, they have extremely serious economic shortcomings.

However, since alkaline metals are strongly basic, it has been difficult to save energy despite the strong acidity of acidic compounds.

However, since this is used only as an intermediate substance for forming a cosmetic compound, its chemical properties relating to absorption of carbon dioxide are not known.

However, although this use of a heat pump achieves a certain degree of energy savings, when the equipment costs of this system are taken into consideration, the effects cannot be said to be adequate.

In this apparatus as well, the thermal energy able to be used from the flue gas is unable to achieve adequate energy savings.

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