CO2 RECOVERY UNIT AND CO2 RECOVERY METHOD

Abstract

A CO2 recovery unit includes a CO2 absorber that brings a gas having a low CO2 concentration into countercurrent contact with a CO2 absorbent to remove CO2 from the gas. The CO2 recovery unit further includes a first absorbent circulation line that supplies a CO2 absorbent from a first CO2 absorption section as a first circulation solution to an upper side of a first CO2 absorption section; a second absorbent circulation line that supplies a CO2 absorbent from a second CO2 absorption section as a second circulation solution to an upper side of a second CO2 absorption section; and an absorbent discharge line that discharges a part of the first circulation solution from the first absorbent circulation line and supply the part of the first circulation solution as a discharged solution to the second absorbent circulation section.

Description

FIELD

The present invention relates to a CO₂ recovery unit and a CO₂ recovery method.

BACKGROUND

Various methods have been proposed so far to recover and remove acid gases, especially CO₂, contained in flue gases from boilers, for example. One of such methods is to bring CO₂ into contact with a CO₂ absorbent made of an amine aqueous solution to remove and recover CO₂ contained in the flue gases.

For example, the method involves use of the CO₂ absorbent in a CO₂ absorber to absorb CO₂ contained in the flue gases for removal of CO₂ from the flue gases, followed by regeneration of the CO₂ absorbent by releasing CO₂ absorbed in the CO₂ absorbent and regenerate the CO₂ absorbent in a regenerator, and circulation of the regenerated CO₂ absorbent into the CO₂ absorber for reuse of the CO₂ absorbent to remove CO₂ from the flue gases again. The CO₂-absorbing CO₂ absorbent is heated in the regenerator with use of steam flowed from a reboiler to release CO₂ for recovery of CO₂ at a high purity. In this method, it is proposed that a part of the CO₂ absorbent is discharged at a lower part and returned to at least one portion(s) between an upper CO₂ absorbent supply part and a lower gas supply part in the CO₂ absorber to increase an amount of CO₂ absorbed from the flue gas while reducing a reboiler heat duty required for the regeneration of the CO₂ absorbent (for example, Patent Literature 1). It is proposed that the CO₂ absorber is provided at its absorption section with a plurality of stages to allow a part of a discharged solution to be returned to an upper side above a position for discharging a circulated solution at the lower stage in the absorption section to increase the amount of CO₂ absorbed from the flue gas while reducing an energy required for the regeneration of the CO₂ absorbent, as well (for example, Patent Literature 2).

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Translation of PCT Application Publication No. 2013-512088

Patent Literature 2: International Publication No. 09/104744

SUMMARY

Technical Problem

However, the current CO₂ recovery unit has a problem of requiring a large amount of heat in reboiling for reuse of a CO₂ absorbent because of difficulties in CO₂ release at a CO₂ regenerator and reduction of steam energy required for regeneration of the CO₂ absorbent.

Furthermore, although the CO₂ content in a flue gas is 10 to 15% by volume (represented simply by “%”, hereinafter), there is a demand for a CO₂ recovery unit and a CO₂ recovery method that are capable of reducing the amount of heat in reboiling for the reuse of the CO₂ absorbent even when CO₂ is recovered from gases under nearly atmospheric pressure containing a subtle amount of CO₂ other than the flue gas as well as a variety of gases with low CO₂ contents in gases having CO₂ concentrations of 10% or less flowed from gas turbine gas and the like, for example.

In light of the aforementioned circumstances, the present invention has an object to provide a CO₂ recovery unit and a CO₂ recovery method that are capable of reducing an amount of heat in reboiling for reuse of a CO₂ absorbent to recover CO₂ from a variety of gases with low CO₂ contents.

Solution to Problem

A CO₂ recovery unit according to a first aspect of the present invention includes: a CO₂ absorber that brings a gas having a low CO₂ concentration into countercurrent contact with a CO₂ absorbent to remove CO₂; an absorbent regenerator that performs a heat exchange between a rich solution containing absorbed CO₂ and water vapor flowed from a reboiler to regenerate the CO₂ absorbent; a rich solution supply line that discharges a rich solution containing absorbed CO₂ from a bottom part of the CO₂ absorber and supplies the rich solution to an upper side of the absorbent regenerator; and a lean solution supply line that discharges a lean solution obtained by releasing CO₂, from a bottom part of the absorbent regenerator and supplies the lean solution to an upper side of the CO₂ absorber. The CO₂ absorber includes a CO₂ absorption section with at least two or more stages. The CO₂ recovery unit further includes: absorbent circulation lines that each discharge the CO₂ absorbent from a lower side of one of the stages in the CO₂ absorption section and supply the CO₂ absorbent as a circulation solution to an upper side of the corresponding stage used for the discharge in the CO₂ absorption section; and an absorbent discharge line that discharges a part of the circulation solution from the absorbent circulation line and supply the part of the circulation solution as a discharged solution to a stage immediately below the stage used for the discharge.

A CO₂ recovery method according to a second aspect of the present invention includes: a CO₂ absorption step of bringing a gas having a low CO₂ concentration into countercurrent contact with a CO₂ absorbent to remove CO₂; an absorbent regeneration step of performing a heat exchange between water vapor and a rich solution containing the absorbed CO₂ to regenerate the CO₂ absorbent; and a step of discharging a rich solution containing CO₂ that is absorbed at the CO₂ absorption step, supplying the rich solution to the absorbent regeneration step, discharging a lean solution that is obtained by releasing CO₂ from a tower bottom part at the absorbent regeneration step, and supplying the lean solution to the CO₂ absorption step to recover CO₂ in the gas. The CO₂ absorption step includes an absorbent circulation step of, in a CO₂ absorption section with at least two or more stages, discharging the CO₂ absorbent from lower sides of the stages in the CO₂ absorption section and supplying the CO₂ absorbent as a circulation solution to upper sides of the corresponding stages used for the discharge in the CO₂ absorption section; and an absorbent discharge step of discharging a part of the circulation solution from the absorbent circulation step and supplying the part of the circulation solution as a discharged solution to a stage immediately below the stage used for the discharge.

Advantageous Effects of Invention

According to the present invention, an amount of heat can be reduced in reboiling for CO₂ recovery to recover CO₂ from a variety of gases with low CO₂ contents.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view representing a configuration of a CO₂ recovery unit according to a first embodiment.

FIG. 2 is a schematic view representing a configuration of a CO₂ recovery unit according to a second embodiment.

FIG. 3 is a schematic view representing a configuration of a CO₂ recovery unit according to a third embodiment.

FIG. 4 is a graph representing CO₂ concentration ratios in a rich solution in test example 1.

FIG. 5 is a graph representing CO₂ recovery amount ratios in test example 1.

FIG. 6 is a graph representing reduction percentages (%) of reboiler heat duty in an absorbent regenerator in test example 1.

FIG. 7 is a graph representing CO₂ concentration ratios in a rich solution in test example 2.

FIG. 8 is a graph representing CO₂ recovery amount ratios in test example 2.

FIG. 9 is a graph representing reduction percentages (%) of reboiler heat duty in an absorbent regenerator in test example 2.

FIG. 10 is a graph representing CO₂ concentration ratios in a rich solution in test example 3.

FIG. 11 is a graph representing CO₂ recovery amount ratios in test example 3.

FIG. 12 is a graph representing reduction percentages (%) of reboiler heat duty in an absorbent regenerator in test example 3.

FIG. 13 is a graph representing CO₂ concentration ratios in a rich solution in test example 4.

FIG. 14 is a graph representing CO₂ recovery amount ratios in test example 4.

FIG. 15 is a graph representing reduction percentages (%) of reboiler heat duty in an absorbent regenerator in test example 4.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention are described in detail with reference to the accompanying drawings. The present invention is not limited to these embodiments, and modifications, additions, and omissions can be made by a person skilled in the art without departing from the spirit and scope stated in the claims.

First Embodiment

FIG. 1 is a schematic view representing a configuration of a CO₂ recovery unit according to a first embodiment. As illustrated in FIG. 1, a CO₂ recovery unit 10A is a unit for recovering CO₂ in an introduced gas (e.g., air) 11A under nearly atmospheric pressure with a low concentration of CO₂, for example. The CO₂ recovery unit 10A is provided with: a CO₂ absorber 14 configured to bring the introduced gas 11A into countercurrent contact with a lean solution 13A of a CO₂ absorbent for allowing CO₂ contained in the introduced gas 11A to be absorbed in the CO₂ absorbent and then removed; and an absorbent regenerator 15 that is formed at a subsequent stage of the CO₂ absorber 14 to release CO₂ from a rich solution 13C of CO₂ absorbent containing absorbed CO₂ and then regenerate the lean solution 13A.

In this CO₂ recovery unit 10A, the CO₂ absorbent is circulated between the CO₂ absorber 14 and the absorbent regenerator 15. The lean solution 13A of the CO₂ absorbent obtained as a result of releasing CO₂ absorbs CO₂ from the gas in the CO₂ absorber 14 to become the rich solution 13C. This rich solution 13C is supplied to the absorbent regenerator 15. The supplied rich solution 13C release CO₂ in the absorbent regenerator 15, and is regenerated as the lean solution 13A to be subsequently supplied to the CO₂ absorber 14. Here, the CO₂ absorbent is a generic term referring to: the lean solution 13A obtained by releasing CO₂; a semi-rich solution 13B that absorbs a part of CO₂ in the gas; and the rich solution 13C that contains CO₂ absorbed from the gas and is discharged from the CO₂ absorber 14. Each of the names is selected depending on circulating positions in the CO₂ recovery unit 10A and the CO₂ content ratio.

The CO₂ absorbent usable in the present invention is not particularly limited. Examples of the CO₂ absorbent include amine compounds such as alkanolamines and hindered amines with alcoholic hydroxyl groups. Examples of such alkanolamines include monoethanolamine, diethanolamine, triethanolamine, methyl diethanolamine, diisopropanolamine, and diglycolamine. Among them, monoethanolamine (MEA) is generally preferred. Examples of such hindered amines with alcoholic hydroxyl groups include 2-amino-2-methyl-1-propanol (AMP), 2-(ethylamino)-ethanol (EAE), 2-(methylamino)-ethanol (MAE), and 2-(diethylamino)-ethanol (DEAE).

A rich solution supply line L₁ is provided to supply the rich solution 13C containing CO₂ absorbed in the CO₂ absorber 14 to an upper side of the absorbent regenerator 15 between a bottom 14b of the CO₂ absorber 14 and a top 15a of the absorbent regenerator 15. The rich solution supply line L₁ is provided with: a rich solution pump 51 configured to supply the rich solution 13C containing CO₂ absorbed in the CO₂ absorber 14 to the absorbent regenerator 15; and a rich/lean solution heat exchanger 52 configured to heat the rich solution 13C using the lean solution 13A that is obtained as a result of releasing CO₂ by heating in the absorbent regenerator 15.

A CO₂ absorption section 141 includes: a first CO₂ absorption stage 141A (referred to as “first absorption stage”, hereinafter) arranged to absorb CO₂ from the introduced gas 11A; and a second CO₂ absorption stage 141B (referred to as “second absorption stage”, hereinafter) located below the first absorption stage 141A that are disposed inside the CO₂ absorber 14 in its height direction.

A solution reservoir 143A and a chimney tray 143B are provided between the first absorption stage 141A and the second absorption stage 141B. The solution reservoir 143A is arranged to store the semi-rich solution 13B that is flowed down from upper portions of the first absorption stage 141A to reach lower portions of the first absorption stage 141A.

The solution reservoir 143A is provided with a first absorbent circulation line L₁₁ that is arranged to discharge a whole amount of the semi-rich solution 13B stored in the solution reservoir 143A through a discharge position A of the CO₂ absorber 14 and introduce the discharged solution into an introduction position B located at the upper portion of the first absorption stage 141A that is the same stage as that for the discharge.

The first absorbent circulation line L₁₁ is provided with a semi-rich solution pump 25 to circulate the semi-rich solution 13B into the upper portion of the first absorption stage 141A. The first absorbent circulation line L₁₁ is provided with a circulation flow control valve V₁ that controls a circulation flow rate of the circulated semi-rich solution 13B.

A front end of a lean solution supply line L₂ is connected to the first absorbent circulation line L₁₁ at a connection position H, such that the lean solution 13A regenerated in the absorbent regenerator 15 is mixed with the circulated semi-rich solution 13B (circulation solution 13B) to form a mixture (13A/13B). The mixture is introduced into an upper stage liquid distributor 140A. The front end of the lean solution supply line L₂ may be connected directly to the upper stage liquid distributor 140A at the upper portion of the first absorption stage 141A without connected to the first absorbent circulation line L₁₁ such that the lean solution 13A and the circulated semi-rich solution 13B (circulation solution 13B) are individually introduced into the upper stage liquid distributor 140A.

A second absorbent circulation line L₁₂ is provided to be connected at a liquid reservoir 144 located at a bottom of the CO₂ absorber to discharge a whole amount of the rich solution 13C stored in the liquid reservoir 144 through a discharge position C of the CO₂ absorber 14 and introduce the discharged solution into an introduction position D positioned at an upper part of the second absorption stage 141B that is the same stage as the stage for the discharge.

A part of the rich solution 13C is separated as a circulation solution 13C-1 at a discharge position E by the second absorbent circulation line L₁₂. The second absorbent circulation line L₁₂ is connected at the discharge position E to a base end of the rich solution supply line L₁. The rich solution 13C discharged is supplied through the rich solution supply line L₁ to the absorbent regenerator 15. The rich solution supply line L₁ is provided with a rich solution flow control valve V₂ to control the amount of the rich solution 13C supplied to the absorbent regenerator 15. The second absorbent circulation line L₁₂ is provided with a circulation flow control valve V₃ to control the circulation flow rate of the circulated circulation solution 13C-1.

An absorbent discharge line L₂₁ is provided to discharge a part of the semi-rich solution 13B (circulation solution 13B) circulated from the first absorbent circulation line L₁₁ as a discharged solution 13B-1 at a discharge position F and supply the discharged solution 13B-1 to the second absorption stage 141B that is the adjacent stage immediately below the stage for the discharge.

In the present embodiment, the front end of the absorbent discharge line L₂₁ is connected at a connection position G to the second absorbent circulation line L₁₂, such that the discharged solution 13B-1 is mixed with the circulation solution 13C-1 to form the mixture (13B-1/13C-1). The mixture is supplied through a single line to the introduction position D located at the upper portion of the second absorption stage 141B. The front end of the absorbent discharge line L₂₁ may be connected directly to the lower stage liquid distributor 140B at the upper portion of the second absorption stage 141B without connected to the second absorbent circulation line L₁₂ such that the discharged solution 13B-1 and the circulation solution 13C-1 are individually introduced into the lower stage liquid distributor 140B.

In addition, a discharge flow control valve V₄ is provided in the absorbent discharge line L₂₁ to control the discharge flow rate of the discharged solution 13B-1.

In the first absorption stage 141A, the lean solution 13A regenerated in the absorbent regenerator 15 and the circulated semi-rich solution 13B (circulation liquid 13B) are supplied as the CO₂ absorbents. The supplied lean solution 13A and the circulated semi-rich solution 13B (circulation liquid 13B) are distributed from the upper stage liquid distributor 140A that is located at the upper portion of the first absorption stage 141A, and then flowed down within a packed bed. Here, the semi-rich solution 13B as the circulation solution has a higher CO₂ concentration than the lean solution 13A, but is still capable of absorbing CO₂. Thus, the semi-rich solution 13B can be reused in the first absorption stage 141A to absorb CO₂ from the introduced gas 11B.

To the second absorption stage 141B, the circulation solution 13C-1 is supplied together with the discharged solution 13B-1 of the semi-rich solution 13B containing CO₂ absorbed partially from the introduced gas 11B in the first absorption stage 141A. The supplied discharged solution 13B-1 and the circulation solution 13C-1 are distributed from the lower stage liquid distributor 140B at the upper portions of the second absorption stage 141B and flowed down within the packed bed. Here, the rich solution 13C as the circulation solution has a higher CO₂ concentration than the semi-rich solution 13B, but is still capable of absorbing CO₂. Thus, the rich solution 13C can be reused in the second absorption stage 141B to absorb CO₂ from the introduced gas 11A.

In the first absorption stage 141A of the CO₂ absorber 14, the introduced gas 11B containing CO₂ is brought into countercurrent contact with the mixture of the lean solution 13A made of the amine compound-based CO₂ absorbent and the circulated semi-rich solution 13B (circulation solution 13B) that are introduced into the tower. As a result, the gas introduced into the tower becomes an outlet gas 11C by removing CO₂ in the gas while the lean solution 13A becomes the semi-rich solution 13B.

In the second absorption stage 141B, the semi-rich solution 13B (discharged solution 13B-1 and the circulation solution 13C-1) partially absorbing CO₂ is brought into countercurrent contact with the introduced gas 11A that contains CO₂ and is introduced from a bottom side of the tower, allowing CO₂ to be absorbed into the semi-rich solution 13B from the introduced gas 11A by a chemical reaction. As a result, the introduced gas 11A becomes the introduced gas 11B with a reduced CO₂ concentration by removing CO₂ in the gas while the semi-rich solution 13B becomes the rich solution 13C with higher amount of absorbed CO₂. This arrangement allows the introduced gases 11A and 11B containing CO₂ to be flowed through the first absorption stage 141A and the second absorption stage 141B, removing CO₂ to provide a decarbonated outlet gas 11C.

A mist eliminator 145 is installed inside a tower top 14a to capture mist contained in the decarbonated outlet gas 11C and then discharge the gas outwardly from the tower top 14a of the CO₂ absorber 14.

The absorbent regenerator 15 is provided at its interior with a packed bed 151 to release CO₂ with aid of water vapor from the rich solution 13C containing absorbed CO₂. The absorbent regenerator 15 is provided at the vicinity of a tower bottom 15b with a circulation line L₄ that circulates therethrough a part of the lean solution 13A flowed down to the tower bottom 15b. The circulation line L₄ is provided with a reboiler 31 that generates water vapor by indirect heating of the lean solution 13A with use of saturated steam S, and a control valve 32 that controls the amount of saturated steam S supplied to the reboiler 31. The saturated steam S becomes steam condensate W₄ after the heating.

The absorbent regenerator 15 is provided at its top 15a with a gas discharge line Ls to discharge CO₂ gas 41 together with water vapor. The gas discharge line Ls is provided with a condenser 42 that condenses the water vapor discharged together with the CO₂ gas 41 into water, and a separation drum 43 to separate a CO₂ gas 44 from condensed water W₅. The CO₂ gas 44 separated from the condensed water W₅ is discharged outwardly from an upper portion of the separation drum 43. A condensed water line L₆ is provided between the bottom part of the separation drum 43 and the top 15a of the absorbent regenerator 15 to supply the condensed water W₅ separated in the separation drum 43 to the top 15a of the absorbent regenerator 15. The condensed water line L₆ is provided with a condensed water circulation pump 45 to supply the condensed water W₅ separated in the separation drum 43 to the top 15a of the absorbent regenerator 15.

The lean solution supply line L₂ is provided between the bottom 15b of the absorbent regenerator 15 and the top 14a of the CO₂ absorber 14 to supply the lean solution 13A of the CO₂ absorbent from the bottom 15b of the absorbent regenerator 15 to the upper side of the CO₂ absorption section 141. The lean solution supply line L₂ is provided with: a rich/lean solution heat exchanger 52 that heats the rich solution 13C containing the absorbed CO₂ with use of the lean solution 13A that is obtained by heating with water vapor in the absorbent regenerator 15 for the removal of CO₂; a lean solution pump 54 that supplies the lean solution 13A from the bottom 15b of the absorbent regenerator 15 to the upper portion of the CO₂ absorption section 141; and a cooling part 55 that cools the lean solution 13A of the CO₂ absorbent down to a predetermined temperature.

In the present embodiment, this cooling part 55 is configured to cool the lean solution 13A down to the same temperature as or a temperature close to a gas temperature (room temperature: e.g., 25° C.) of the introduced gas 11A introduced into the CO₂ absorber 14.

Next, the overall operation of the CO₂ recovery unit 10A according to the present embodiment is described. For example, the introduced gas (atmospheric gas) 11A containing a subtle amount of CO₂ is introduced into the interior of the CO₂ absorber 14 through the bottom 14b of the CO₂ absorber 14 and flows upward inside the tower. The gas introduced into the CO₂ absorber 14 is brought into countercurrent contact with the CO₂ absorbent containing the amine compound such as alkanolamine in the first absorption stage 141A and second absorption stage 141B of the CO₂ absorption section 141, such that CO₂ is absorbed from the gas into the CO₂ absorbent and then removed for the purpose of providing the decarbonated outlet gas 11C.

The decarbonated outlet gas 11C is discharged outwardly from the top 14a of the CO₂ absorber 14.

After the absorption of CO₂ in the CO₂ absorber 14, the rich solution 13C of the CO₂ absorbent is flowed in the rich solution supply line L₁ to pass through the rich/lean solution heat exchanger 52 for heat exchange with the lean solution 13A and then supplied to the top side of the absorbent regenerator 15 with the aid of the rich solution pump 51.

The rich solution 13C of the CO₂ absorbent supplied to the absorbent regenerator 15 is allowed to release CO₂ by the heat of the water vapor supplied from the reboiler 31 while flowing downward inside the packed bed 151 of the absorbent regenerator 15, so as to become the lean solution 13A. In the absorbent regenerator 15, the solution is circulated from the liquid reservoirs 151B of the chimney tray 151A into the circulation line L₄ while heated by saturated steam S in the reboiler 31 to generate water vapor inside the absorbent regenerator 15. The lean solution 13A is obtained as a result of releasing CO₂ from the rich solution 13C with the aid of the heat of this generated water vapor. After the heating, the saturated steam S becomes steam condensate W₄. The CO₂ gas 41 released together with the water vapor from the CO₂ absorbent is cooled down for the condensation into water with use of the condenser 42, separated from the condensed water W₅, and then discharged as the CO₂ gas 44 outwardly through the upper portion of the separation drum 43. The condensed water W₅ separated is supplied to the absorbent regenerator 15.

After discharged from the bottom 15b of the absorbent regenerator 15, the lean solution 13A is flowed in the lean solution supply line L₂ to pass through the rich/lean solution heat exchanger 52 for heat exchange with the rich solution 13C and then supplied to the upper side of the CO₂ absorption section 141 of the CO₂ absorber 14 with the aid of the lean solution pump 54.

After supplied to the upper side of the CO₂ absorption section 141, the lean solution 13A absorbs CO₂ from the introduced gas 11B in the first absorption stage 141A to become the semi-rich solution 13B, which is discharged at the discharge position A at the lower portion of the first absorption stage 141A and flowed into the first absorbent circulation line L₁₁. The discharged semi-rich solution 13B is supplied with the aid of the semi-rich solution pump 25 together with the lean solution 13A to the introduction position B positioned at the upper side of the first absorption stage 141A.

In the second absorption stage 141B, CO₂ is absorbed from the introduced gas 11B to form the rich solution 13C. The rich solution 13C is discharged at the discharge position C at the lower part of the second absorption stage 141B and flowed into the second absorbent circulation line L₁₂. The discharged rich solution 13C is separated at the discharge position E, as the circulation solution 13C-1. Then, the circulation solution 13C-1 is supplied with the aid of the rich solution pump 51 to the introduction position D at the upper portion of the second absorption stage 141B.

A part of the semi-rich solution 13B is separated at the discharge position F of the first absorbent circulation line L₁₁ by the absorbent discharge line L₂₁. The discharged solution 13B-1 is mixed with the circulation solution 13C-1 at the connection position G of the second absorbent circulation line L₁₂. This discharged solution 13B-1 and the circulation solution 13C-1 are mixed with each other (13B-1/13C-1) and then supplied through a single line to the introduction position D at the upper portion of the second absorption stage 141B.

After separated from the circulation solution 13C-1 at the discharge position E, the rich solution 13C is supplied to the absorbent regenerator 15 through the rich solution supply line L₁.

Therefore, the configuration according to the present embodiment includes: the first absorbent circulation line L₁₁ for supplying the semi-rich solution 13B discharged at the lower side of the first absorption stage 141A in the CO₂ absorber 14 to the upper side of the same stage 141A as the stage for the discharge; the second absorbent circulation line L₁₂ for supplying the rich solution 13C discharged at the lower side of the second absorption stage 141B of the CO₂ absorber 14 to the upper side of the second absorption stage 141B that is the same stage as the stage for the discharge; the absorbent discharge line L₂₁ for separating the circulated semi-rich solution 13B (circulation solution 13B) discharged at the lower side of the first absorption stage 141A and then mixing the separated solution with the lean solution 13C-1 that is discharged at the lower side of the second absorption stage 141B that is the adjacent stage immediately below the stage for the discharge; the lean solution supply line L₂ for supplying the lean solution from the absorbent regenerator 15 to the upper portion of the first absorption stage 141A that is the highest stage of the CO₂ absorber 14 and then mixing the supplied solution with the semi-rich solution 13B discharged at the lower side of the first absorption stage 141A; and the rich solution supply line L₁ for flowing the rich solution 13C from the bottom part of the CO₂ absorber 14 to the absorbent regenerator 15.

Thus, according to the first embodiment, after absorbing CO₂, the semi-rich solution 13B and the rich solution 13C that contain absorbed CO₂ can be discharged and flowed outwardly through the lower sides of the first absorption stage 141A and the second absorption stage 14B, respectively, and supplied as the circulation solutions to the upper portions at the corresponding stages above the absorbent-discharged portions. This allows the circulated circulation solutions to be reused as the absorbents in the first absorption stage 141A and the second absorption stage 14B to recover CO₂ in the gas. As a result, with increase of the CO₂ concentration in the CO₂ absorbent, it is possible to improve the CO₂ recovery efficiency in the CO₂ absorber 14 and thereby increase the CO₂ recovery amount per unit volume of the CO₂ absorbent. Furthermore, the CO₂ concentration in the rich solution 13C can be increased at the bottom 14b of the CO₂ absorber 14, thereby allowing the rich solution 13C having an increased CO₂ concentration to be supplied to the absorbent regenerator 15.

Here, when the two-stage configuration for the absorption section, the first absorption stage 141A and the second absorption stage 141B, as in the present embodiment, the solution discharged from the circulation solution 13C-1 is the rich solution 13C, while the rich solution supply line L₁ functions as an absorbent discharge line to supply the discharged rich solution 13C to the absorbent regenerator 15. In the case of three-stage configuration for the absorption section, the first absorption stage 141A, the second absorption stage 141B, and a third absorption stage, a solution discharged from the circulation solution 13C-1, which is supplied from the second absorption stage 141B, is supplied as the discharged solution to the third absorption stage immediately below the discharged stage of the second absorption stage.

As a result, the semi-rich solution 13B and the rich solution 13C are reused twice in the CO₂ absorber 14, enabling it to reduce the flow rate of the circulated CO₂ absorbent in the system circulating between the CO₂ absorber 14 and the absorbent regenerator 15. This can reduce the flow rate of the rich solution 13C supplied to the absorbent regenerator 15, reducing the steam supply amount of the reboiler 31 used in the absorbent regenerator 15 for the regeneration of the CO₂ absorbent, and thereby reducing heat energy consumption to improve energy efficiency.

In the present embodiment, even in the case of recovering CO₂ from a gas containing a very subtle amount of CO₂ (e.g., air), the CO₂ concentration can be increased in the rich solution 13C supplied to the absorbent regenerator 15 so as to reduce the steam amount required in the absorbent regenerator 15, thereby reducing the thermal energy as well as achieving further energy saving in the process of regenerating the CO₂ absorbent.

Next, the separation ratios of the circulation solution to the discharged solution in the CO₂ recovery unit 10A according to the present embodiment, is described.

In the present embodiment, regarding the flow rate (f1) of the semi-rich solution 13B (circulation solution 13B) circulated through the first absorbent circulation line L₁₁ from the lower portion of the first absorption stage 141A and the flow rate (f2) of the discharged solution 13B-1 discharged via the absorbent discharge line L₂₁, the separation ratio of the flow rates (referred to as “separation flow ratio”, hereinafter) is preferably 1:1 to 60:1, preferably 10:1 to 60:1, more preferably 30:1 to 60:1.

Regarding the flow rate (f3) of the circulation solution 13C-1 flowed into the second absorbent circulation line L₁₂ and the flow rate (f4) of the rich solution 13C flowed into the rich solution supply line L₁ from the bottom of the CO₂ absorber 14, the flow separation ratio of the flow rates is preferably 1:1 to 60:1, preferably 10:1 to 60:1, more preferably 30:1 to 60:1.

Hereinafter, preferred test examples exhibiting effects of the separation flow ratio according to the present embodiment are described. The present invention is not limited to these examples. FIGS. 4 to 9 in these test examples illustrate examples for gas having a CO₂ concentration of 1% or less.

FIG. 4 a graph representing CO₂ concentration ratios in the rich solution in test example 1. Herein, FIG. 4 represents a performance improvement ratio calculated based on standardized CO₂ concentration (1) without the discharge at each stage. In FIG. 4, the horizontal axis represents the separation flow ratio of the flow rate (f1) of the semi-rich solution 13B (circulation solution 13B) circulated through the first absorbent circulation line L₁₁ to the flow rate (f2) of the discharged solution 13B-1 supplied to the absorbent discharge line L₂₁, in the range of 1:1 to 60:1 (the same applies to FIGS. 5 and 6). The same applies to the separation flow ratio of the flow rate (f1) of the circulation solution 13C-1 circulated through the second absorbent circulation line L₁₂ to the flow rate (f2) of the rich solution 13C supplied to the rich solution supply line L₁, in the range of 1:1 to 60:1.

FIG. 5 is a graph representing CO₂ recovery amount ratios in test example 1. Herein, FIG. 5 represents a performance improvement ratio calculated based on standardized CO₂ recovery amount (1) without the discharge at each stage. FIG. 6 is a graph representing reduction percentages (%) of reboiler heat duty in the absorbent regenerator in test example 1. FIG. 6 represents the reduction percentages (%) calculated based on standardized reboiler heat duty reduction percentages (0) without the discharge.

As illustrated in FIGS. 4 to 6, in test example 1, the separation flow ratio in the range of 1:1 to 60:1 allows for improvement in the separation effects on the CO₂ concentration ratio in the rich solution, the CO₂ recovery amount ratio, and the reboiler heat duty reduction percentage (%). In particular, the separation flow ratio in the range of 1:1 to 40:1 allows for improvement in the increase ratio of the separation effects on the CO₂ concentration ratio in the rich solution, the CO₂ recovery amount ratio, and the reboiler heat duty reduction percentage (%).

Here, the CO₂ absorption rate of the absorbent is represented as the product of the volumetric mass transfer coefficient (the product of the mass transfer coefficient and the gas-liquid contact area) and the driving force (the difference between the CO₂ partial pressure and the CO₂ equilibrium pressure). In the case of returning the absorbent of the semi-rich solution 13B (circulation solution 13B) circulating from the rich solution side at the bottom side of the CO₂ absorber 14 to the top of the CO₂ absorber 14, as disclosed in the prior art (Patent Literature 1), the concentration of acid gas (CO₂ ) may be increased at the outlet side due to decrease in the driving force when the CO₂ content is high in the gas (such as boiler flue gas with a CO₂ concentration of more than 10% and 15% or less).

In contrast, when CO₂ is removed from the gas with a low CO₂ concentration of 1% such as atmospheric air as in this test example, even if the absorbent is circulated from the rich solution side on the bottom side of the CO₂ absorber to the lean solution side on the upper side of the CO₂ absorber including the top thereof, it is possible to improve the volumetric mass transfer coefficient to such a great extent as to outweigh the decrease of the driving force, and thereby improve the CO₂ absorption rate. Thus, the improvement in CO₂ absorption rate in the present invention is particularly noticeable under conditions with the relatively low CO₂ concentration in the target gas.

Therefore, examples of the gas with low CO₂ concentration applicable to the present invention include a gas at nearly atmospheric pressure containing a subtle amount (with a CO₂ concentration of 1% or less), preferably a gas at nearly atmospheric pressure with a CO₂ concentration of 10% or less, more preferably a gas at nearly atmospheric pressure with a CO₂ concentration of 5% or less. Examples of the gas with a CO₂ concentration of 10% or less include flue gases from boilers, gas turbines, combustion furnaces, heating furnaces, incinerators, internal combustion engines, and the like, for example, as well as atmospheric air and air in closed and substantially closed spaces.

In the present invention, in the case of targeting air, or air in the closed or substantially closed space, the lean absorbent temperature is desired to be same as or as close as possible to the target gas temperature.

When the introduced gas 11 in the CO₂ absorber 14 is atmospheric gas or air with unsaturated water vapor, the water content of the absorbent is evaporated and then exhausted as moisture saturated gas, thereby requiring water to be supplied to the absorbent. The higher the lean solution temperature, the higher amount of water supplied to the absorbent, due to increase in the temperature of the outlet gas 11C of the CO₂ absorber 14 and increased amount of exhausted water content resulting from increase of saturated water content. For this reason, as illustrated in FIG. 1, the lean solution supply line L₂ is provided with a supply water introduction line L₉ to introduce supplied water 70 to dilute the absorbent in the present embodiment, preventing the amine concentration from increased in the absorbent.

The amount of absorbed CO₂ is small when the introduced gas 11A in the CO₂ absorber 14 is atmospheric gas or air with unsaturated water vapor. Thus, the amount of heat absorption resulting from the evaporative latent heat of water content is greater than the amount of heat generated by the reaction when CO₂ is absorbed from the flue gas exhausted from the boiler as in the conventional system. Therefore, even when the lean solution 13A with the same temperature as the introduced gas 11A is used, for example, the outlet gas temperature (T₂) of the outlet gas 11C is lower by approximately 3° C. than that (T₁) of the introduced gas 11A under the condition that the gas temperature (T₁) of the introduced gas 11A in test example 1 according to first embodiment is 25° C.

Thus, under the condition that CO₂ is removed from a gas containing a subtle amount of CO₂ (a CO₂ concentration of 1% or less), the temperature of the circulation solution in the absorption section is lower than that of the lean solution 13A introduced into the CO₂ absorber 14, without a cooler installed in the first absorbent circulation line L₁₁ and the second absorbent circulation line L₁₂. As a result, the reaction heat of the amine solution is not generated when CO₂ is absorbed inside the absorber as in the prior arts, thereby not requiring the cooler in the first absorbent circulation line L₁₁ and the second absorbent circulation line L₁₂ in which the semi-rich solution 13B and the rich solution 13C are circulated.

FIGS. 7 to 9 illustrate performances with comparison to the prior art (Patent Literature 2: International Publication No. 09/104744) in which only the second absorbent circulation line L₁₂ is used to circulate the absorbent, without the first absorbent circulation line L₁₁ installed as in the present invention, (with circulation at the lower stage without circulation at the upper stage) (Comparative Example 2).

FIG. 7 is a graph representing CO₂ concentration ratios in the rich solution in test example 2. Herein, FIG. 7 represents a performance improvement ratio calculated based on standardized CO₂ concentration (1) with circulation at the lower stage without circulation at the upper stage. In FIG. 7, the horizontal axis represents the separation flow ratio of the flow rate (f1) of the semi-rich solution 13B (circulation solution 13B) circulated through the first absorbent circulation line L₁₁ to the flow rate (f2) of the discharged solution 13B-1 supplied to the absorbent discharge line L₂₁, in the range of 1:1 to 60:1 (the same applies to FIGS. 9 and 10). The same applies to the separation flow ratio of the flow rate (f1) of the circulation solution 13C-1 circulated through the second absorbent circulation line L₁₂ to the flow rate (f2) of the rich solution 13C supplied to the rich solution supply line L₁, in the range of 1:1 to 60:1.

FIG. 8 is a graph representing CO₂ recovery amount ratios in test example 2. Herein, FIG. 8 represents the performance improvement ratio calculated based on standardized CO₂ recovery amount (1) with circulation at the lower stage without circulation at the upper stage.

FIG. 9 is a graph representing reduction percentages (%) of reboiler heat duty in the absorbent regenerator in test example 2. FIG. 9 represents the reduction percentages (%) calculated based on standardized reboiler heat duty reduction percentages (0) with circulation at the lower stage without circulation at the upper stage.

As illustrated in FIGS. 7 to 9, in test example 2, the separation ratio of the flow rates in the range of 1:1 to 60:1 allows for improvement in the separation effects on the CO₂ concentration ratio in the rich solution, the CO₂ recovery amount ratio, and the reboiler heat duty reduction percentage (%). In particular, the separation ratio of the flow rates in the range of 1:1 to 30:1 allows for improvement in the increase ratio of the separation effects on the CO₂ concentration ratio in the rich solution, the CO₂ recovery amount ratio, and the reboiler heat duty reduction percentage (%).

Although explanations are given for the CO₂ recovery unit 10A according to the present embodiment in which two absorption stages (the first absorption stage 141A and the second absorption stage 141B) are provided in the CO₂ absorber 14, the present invention is not limited to this. Two or more absorption stages are provided (another absorption stage is provided between the first absorption stage 141A and the second absorption stage 141B) in the CO₂ absorber 14 such that the circulation and discharge are performed at the predetermined separation ratio of the flow rates to circulate the absorbent.

Second Embodiment

FIG. 2 is a schematic view representing configuration of a CO₂ recovery unit according to a second embodiment. Descriptions are given for new configuration in the second embodiment added to the CO₂ recovery unit 10A according to the first embodiment illustrated in FIG. 1. The same components as in the configuration according to the first embodiment are not described.

A CO₂ recovery unit 10B according to the second embodiment is provided with a wash section 142 that is added inside the top part 14a of the first absorption stage 141A of the CO₂ absorber 14 on a downstream side of the gas flow in the CO₂ recovery unit 10A according to the first embodiment.

The wash section 142 is provided at its bottom with a liquid reservoir 144A that stores therein washing water W₂ to wash the decarbonated outlet gas 11C. A circulation line L₃ is provided between the liquid reservoir 144A and the top of the wash section 142 to supply wash water W₂ containing the CO₂ absorbent collected in the liquid reservoir 144A for circulation from the top part 14a side of the wash section 142. The circulation line L₃ is equipped with a heat exchanger 21 that cools the wash water W₂ and a circulation pump 22 that circulates the wash water W₂ containing the CO₂ absorbent collected in the liquid reservoir 144A through the heat exchanger 21 in the circulation line L₃.

In the present embodiment, CO₂ is recovered in the gas with low CO₂ concentration, thereby the CO₂ absorption does not involve the exothermic reaction of the amine-based absorbent, as described above. However, the water wash section 142 illustrated in FIG. 2 is preferably provided to minimize loss of the amine-based absorbent in the outlet gas 11C discharged outwardly, from the perspective of achieving near zero emission.

Third Embodiment

FIG. 3 is a schematic view representing configuration of a CO₂ recovery unit according to a third embodiment. Descriptions are given for new configuration in the third embodiment added to the CO₂ recovery unit 10B according to the second embodiment illustrated in FIG. 2. The same components as in the configuration according to the first embodiment are not described.

A CO₂ recovery unit 10C according to the third embodiment is provided with a gas quencher 12, which is added on an upstream side of the CO₂ absorber 14 in the CO₂ recovery unit 10B according to the second embodiment, to cool the introduced gas 11A down to a predetermined temperature. The gas quencher 12 has a quencher section 121 to cool the introduced gas 11A. A water circulation line L₇ is provided to circulate the cooling water W₁ between the bottom 12b side of the gas quencher 12 and the top 12a side of the quencher section 121. The water circulation line L₇ is equipped with a heat exchanger 122 that cools the cooling water W₁ and a circulation pump 123 that circulates the cooling water W₁ in the water circulation line L₇.

In the quencher section 121, the introduced gas 11A is brought into countercurrent contact with the cooling water W₁ so as to be cooled down to a predetermined temperature. The heat exchanger 122 cools the cooling water W₁ heated by heat exchange with the introduced gas 11A. The circulation pump 123 supplies the cooling water W₁ flowing down to the bottom 12b of the gas quencher 12 via the heat exchanger 122 to the top 12a of the quencher section 121. After cooled, the introduced gas 11A is introduced into the CO₂ absorber 14 from the vicinity of the bottom of the CO₂ absorber 14 via the introduction line L₈.

In the third embodiment, the cooling part 55, which is installed in the lean solution supply line L₂ in the preceding configuration, is installed in the first absorbent circulation line L₁₁. This allows for cooling of the mixed solution (13A/13B) of the semi-rich solution 13B (circulation solution 13B) circulating from the first absorbent circulation line L₁₁ and the lean solution 13A flowed from the lean solution supply line L₂.

The present embodiment differs from the first embodiment in that it is preferable to aim for gases (e.g., turbine flue gas and various flue gases) containing 3% or more and 10% or less of CO₂ as the introduced gas 11A. For the gases containing 3% or more of CO₂, the liquid temperature of the lean solution 13A is expected to be a lean absorbent temperature at a temperature level of the water vapor-saturated gas flowed at the outlet of the gas quencher 12.

For example, under the condition that the gas temperature of the introduced gas 11A in the present embodiment is 40° C., when the absorbent is not circulated at each stage as in the prior art, the gas temperature of the outlet gas 11C discharged from the top 14a is approximately 10° C. higher than the gas temperature of the introduced gas. Therefore, under such a condition, it is more advantageous to install the cooling part 55 in the first absorbent circulation line L₁₁ to prevent the temperature of the absorbent from rising, for increasing the CO₂ concentration in the absorbent.

Hereinafter, preferred test examples exhibiting effects of the separation flow ratio according to the present embodiment are described. The present invention is not limited to these examples. FIGS. 10 to 15 of the test examples illustrate examples aimed for gases with a CO₂ concentration of 3% or more.

FIG. 10 a graph representing CO₂ concentration ratios in the rich solution in test example 3. Herein, FIG. 10 represents a performance improvement ratio calculated based on standardized CO₂ concentration (1) without the discharge at each stage. In FIG. 10, the horizontal axis represents the separation flow ratio of the flow rate (f1) of the semi-rich solution 13B (circulation solution 13B) circulated through the first absorbent circulation line L₁₁ to the flow rate (f2) of the discharged solution 13B-1 supplied to the absorbent discharge line L₂₁, in the range of 1:1 to 60:1 (the same applies to FIGS. 11 and 12). The same applies to the separation flow ratio of the flow rate (f1) of the circulation solution 13C-1 circulated through the second absorbent circulation line L₁₂ to the flow rate (f2) of the rich solution 13C supplied to the rich solution supply line L₁, in the range of 1:1 to 60:1.

FIG. 11 is a graph representing CO₂ recovery amount ratios in test example 3. Herein, FIG. 11 represents a performance improvement ratio calculated based on standardized CO₂ recovery amount (1) without the discharge at each stage. FIG. 12 is a graph representing reduction percentages (%) of reboiler heat duty in the absorbent regenerator in test example 3. FIG. 13 represents the reduction percentages (%) calculated based on standardized reboiler heat duty reduction percentages (0) without the discharge.

As illustrated in FIGS. 10 to 12, in test example 3, the separation flow ratio in the range of 1:1 to 60:1 allows for increase of the separation effects on the CO₂ concentration ratio in the rich solution, the CO₂ recovery amount ratio, and the reboiler heat duty reduction percentage (%). In particular, the separation flow ratio in the range of 1:1 to 20:1 allows for improvement in the increase ratio of the separation effects on the CO₂ concentration ratio in the rich solution, the CO₂ recovery amount ratio, and the reboiler heat duty reduction percentage (%).

FIGS. 13 to 15 illustrate performances with comparison to the prior art (Patent Literature 2: International Publication No. 09/104744) in which only the second absorbent circulation line L₁₂ is used to circulate the absorbent, without the first absorbent circulation line L₁₁ installed as in the present invention, (with circulation at the lower stage without circulation at the upper stage) (Comparative Example 2).

FIG. 13 is a graph representing CO₂ concentration ratios in the rich solution in test example 4. Herein, FIG. 13 represents a performance improvement ratio calculated based on standardized CO₂ concentration (1) with circulation at the lower stage without circulation at the upper stage. In FIG. 7, the horizontal axis represents the separation flow ratio of the flow rate (f1) of the semi-rich solution 13B (circulation solution 13B) circulated through the first absorbent circulation line L₁₁ to the flow rate (f2) of the discharged solution 13B-1 supplied to the absorbent discharge line L₂₁, in the range of 1:1 to 60:1 (the same applies to FIGS. 14 and 15). The same applies to the separation flow ratio of the flow rate (f1) of the circulation solution 13C-1 circulated through the second absorbent circulation line L₁₂ to the flow rate (f2) of the rich solution 13C supplied to the rich solution supply line L₁, in the range of 1:1 to 60:1.

FIG. 14 is a graph representing CO₂ recovery amount ratios in test example 4. Herein, FIG. 14 represents the performance improvement ratio calculated based on standardized CO₂ recovery amount (1) with circulation at the lower stage without circulation at the upper stage. FIG. 15 is a graph representing reduction percentages (%) of reboiler heat duty in the absorbent regenerator in test example 4. FIG. 15 represents the reduction percentages (%) calculated based on standardized the reboiler heat duty reduction percentages (0) with circulation at the lower stage without circulation at the upper stage.

As illustrated in FIGS. 13 to 15, in test example 4, the separation ratio of the flow rates in the range of 1:1 to 60:1 allows for improvement of the separation effects on the CO₂ concentration ratio in the rich solution, the CO₂ recovery amount ratio, and the reboiler heat duty reduction percentage (%). In particular, the separation ratio of the flow rates in the range of 1:1 to 40:1 allows for improvement in the increase ratio of the separation effects on the CO₂ concentration ratio in the rich solution, the CO₂ recovery amount ratio, and the reboiler heat duty reduction percentage (%).

The CO₂ recovery unit and the CO₂ recovery method described in the embodiments are understood, for example, as follows.

The CO₂ recovery unit 10A, 10B, 10C according to a first aspect includes: the CO₂ absorber 14 configured to bring the introduced gas 11A having a low CO₂ concentration into countercurrent contact with the CO₂ absorbent and remove CO₂; the absorbent regenerator 15 configured for heat exchange between the rich solution 13C containing absorbed CO₂ and the vapor flowed from the reboiler 31 to regenerate the CO₂ absorbent; the rich solution supply line L₁ configured to discharge the rich solution 13C containing absorbed CO₂ from the bottom 14b of the CO₂ absorber 14 and supply the rich solution to the top side of the absorbent regenerator 15; and the lean solution supply line L₂ configured to discharge the lean solution 13A, which is obtained after releasing CO₂, from the bottom 15b of the absorbent regenerator 15, and supply it to the top side of the CO₂ absorber 14, in which the CO₂ absorber 14 has at least two or more CO₂ absorption stages 141A, 141B. The CO₂ recovery unit 10A further includes: the absorbent circulation lines L₁₁, L₁₂ configured to discharge the CO₂ absorbent from the lower sides of the CO₂ absorption stages 141A, 141B and supply the CO₂ absorbent as the circulating semi-rich solution 13B (circulation solution 13B) and the circulating rich solution 13C (circulation solution 13C-1) to the upper sides of the corresponding CO₂ absorption stages 141A, 141B that are used as the discharge stages; and the absorbent discharge lines L₂₁, L₁ configured to discharge parts of the semi-rich solution 13B (circulation solution 13B) circulation solution 13C-1 from the absorbent circulation lines L₁₁, L₁₂ and supply the solution as the discharged solutions 13B-1, 13C to adjacent stage immediately below the discharge stages.

This configuration enables it to discharge the CO₂ absorbent containing the absorbed CO₂ from the lower portion of each of the CO₂ absorption stages, supply it to the upper portion of the corresponding stage at which the CO₂ absorbent is discharged, and reuse the solution as the CO₂ absorbent for the purpose of recovering CO₂ in the gas, increasing the CO₂ concentration in the CO₂ absorbent, and reducing the reboiler heat duty needed for recovering CO₂ from a variety of gases with low CO₂ contents.

The CO₂ recovery unit 10A, 10B, 10C according to a second aspect is aimed for the introduced gas 11A with a CO₂ concentration of 10% by volume or less.

This configuration enables it to reduce the reboiler heat duty needed for recovering CO₂ from the introduced gas 11A with a CO₂ concentration of 10% by volume or less.

In the CO₂ recovery unit 10A, 10B, 10C according to a third aspect, the flow ratio is set at 1:1 to 60:1 for the separation ratio of the flow rates of the circulation solution 13B, 13C-1 to the discharged solution 13B-1, 13C.

With the separation ratio of the flow rates of the circulation solution 13B, 13C-1 to the discharged solution 13B-1, 13C ranging from 1:1 to 60:1 in this configuration, it is possible to reduce the reboiler heat duty needed for recovering CO₂ from a variety of gases with low CO₂ contents.

The CO₂ recovery unit 10B, 10C according to a fourth aspect is provided with the wash section 142 at the downstream side of gas flow in the CO₂ absorption section at the highest stage.

This configuration having the wash section 142 enables it to wash the outlet gas 11C with water, and thereby minimize the loss of the amine-based absorbent in the outlet gas 11C released outwardly.

The CO₂ recovery unit 10C according to a fifth aspect has the cooling part 55 in the first absorbent circulation line L₁₁.

With the cooling part 55 in the first absorbent circulation line L₁₁, this configuration enables it to prevent the temperature of the absorbent from increasing, achieving advantageous effects on the increase in the CO₂ concentration in the absorbent.

A CO₂ recovery method according to a sixth aspect includes: a CO₂ absorption step of bringing the introduced gas 11A having low CO₂ concentration into countercurrent contact with the CO₂ absorbent to remove CO₂; an absorbent regeneration step of performing the heat exchange between the rich solution 13C containing the absorbed CO₂ and the steam flowed from the reboiler 31 to regenerate the CO₂ absorbent; and a step of discharging the rich solution 13C containing CO₂ that is absorbed at the CO₂ absorption step, supplying it to the absorbent regeneration step, discharging the lean solution 13A that is obtained by releasing CO₂ at the absorbent regeneration step, and supplying it to the CO₂ absorption step to recover CO₂ in the gas. The CO₂ absorption step includes an absorbent circulation step of, in the CO₂ absorption section with at least two or more stages 141A, 141B, discharging the CO₂ absorbent from the lower sides of the CO₂ absorption stages 141A, 141B and then supplying it as the circulation solution 13B, 13C-1 to the upper sides of the corresponding CO₂ absorption stages 141A, 141B that are used for the discharge; and an absorbent discharge step of discharging a part of the circulation solution 13B from the absorbent circulation step and supplying it as the discharged solution 13B-1, 13C to the stage immediately below the stage used for the discharge.

This configuration enables it to discharge the CO₂ absorbent containing the absorbed CO₂ from the lower portion of each of the CO₂ absorption stages, supply it to the upper portion of the corresponding stage at which the CO₂ absorbent is discharged, and reuse the solution as the CO₂ absorbent for the purpose of recovering CO₂ in the gas, increasing the CO₂ concentration in the CO₂ absorbent, and reducing the reboiler heat duty needed for recovering CO₂ from a variety of gases with low CO₂ contents.

REFERENCE SIGNS LIST

10A to 10C CO₂ recovery unit

11A, 11B Introduced gas

11C Outlet gas

13A Lean solution

13B Semi-rich solution

13B-1 Discharged solution

13C Rich solution

13C-1 Circulation solution

14 CO₂ absorber

15 Absorbent regenerator

31 Reboiler

55 Cooling part

141A First CO₂ absorption stage (first absorption stage)

141B Second CO₂ absorption stage (second absorption stage)

144 Liquid reservoir

L₁ Rich solution supply line

L₂ Lean solution supply line

L₁₁ First absorbent circulation line

L₁₂ Second absorbent circulation line

L₂₁ Absorbent discharge line

Claims

1. A CO2 recovery unit comprising: a CO2 absorber that brings a gas having a low CO2 concentration into countercurrent contact with a CO2 absorbent to remove CO2 from the gas;an absorbent regenerator that performs a heat exchange between a rich solution containing absorbed CO2 and water vapor flowed from a reboiler to regenerate the CO2 absorbent;a rich solution supply line that discharges a rich solution in which CO2 has been absorbed by the CO2 absorbent in the CO2 absorber, from a bottom part of the CO2 absorber and supplies the rich solution to an upper side of the absorbent regenerator; anda lean solution supply line that discharges a lean solution in which CO2 has been released from the rich solution in the absorbent regenerator, from a bottom part of the absorbent regenerator and supplies the lean solution as the CO2 absorbent to an upper side of the CO2 absorber,wherein the CO2 absorber includes at least two or more CO2 absorption sections including a first CO2 absorption section and a second CO2 absorption section located below the first CO2 absorption section, andthe CO2 recovery unit further comprises: a first absorbent circulation line that discharges the CO2 absorbent from the first CO2 absorption section and supplies the CO2 absorbent as a first circulation solution to an upper side of the first CO2 absorption section;a second absorbent circulation line that discharges the CO2 absorbent from the second CO2 absorption section and supplies the CO2 absorbent as a second circulation solution to an upper side of the second CO2 absorption section; andan absorbent discharge line that discharges a part of the first circulation solution from the first absorbent circulation line and supply the part of the first circulation solution as a discharged solution to the second absorbent circulation line.

2. The CO2 recovery unit according to claim 1, wherein the gas has a CO2 concentration of 10% by volume or less.

3. The CO2 recovery unit according to claim 1, wherein a separation ratio of flow rates of the first circulation solution to the discharged solution is set to 1:1 to 60:1.

4. The CO2 recovery unit according to claim 1, further comprising a water wash section on a downstream side of a gas flow in the first CO2 absorption section.

5. The CO2 recovery unit according to claim 1, wherein the first absorbent circulation line includes a cooling part.

6. A CO2 recovery method comprising: a CO2 absorption step of bringing a gas having a low CO2 concentration into countercurrent contact with a CO2 absorbent to remove CO2 from the gas;an absorbent regeneration step of performing a heat exchange between water vapor and a rich solution containing absorbed CO2 to regenerate the CO2 absorbent; anda step of discharging a rich solution in which CO2 has been absorbed at the CO2 absorption step, supplying the rich solution to the absorbent regeneration step, discharging a lean solution in which CO2 has been released from the rich solution at the absorbent regeneration step, and supplying the lean solution as the CO2 absorbent to the CO2 absorption step to recover CO2 in the gas,wherein the CO2 absorption step includes at least two or more CO2 absorption steps including a first CO2 absorption step and a second CO2 absorption step following the first CO2 absorption step,the CO2 recovery method further comprises: a first absorbent circulation step of discharging the CO2 absorbent obtained at the first CO2 absorption step and supplying the CO2 absorbent as a first circulation solution to the first CO2 absorption step; anda second absorbent circulation step of discharging the CO2 absorbent obtained at the second CO2 absorption step and supplying the CO2 absorbent as a second circulation solution to the second CO2 absorption step; andan absorbent discharge step of discharging a part of the first circulation solution obtained at the first absorbent circulation step and supplying the part of the first circulation solution as a discharged solution to the second absorbent circulation step.