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.
Priority Claims (1)
Number Date Country Kind
2020-066881 Apr 2020 JP national
PCT Information
Filing Document Filing Date Country Kind
PCT/JP2021/000852 1/13/2021 WO