Acidic gas recovery apparatus and acidic gas recovery method

The present embodiments provide an acidic gas recovery apparatus or method capable of efficiently reducing emission of amine to the environment. In the embodiment, an acidic gas recovery apparatus 10A comprises: an acidic gas-absorber 11a; a regeneration tower 12; a gas-cleaner 11b; a cleaning liquid drawing-out line L23; an absorbing liquid drawing-out line L21; and an acidic component-remover 13A. The cleaning water 27c from the cleaning liquid drawing-out line L23 and a purified lean solution 23C from the absorbing liquid drawing-out line L21 are supplied to the acidic component-remover 13A. The acidic component-remover 13A comprises a cathode 53, an anode 54, an absorbing liquid-purification compartment 57 for removing acidic components from the objective lean solution 23B, and cleaning liquid compartments 58-1 and 58-2. The cleaning water 27 is supplied to the cleaning liquid compartments.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2016-170879, filed on Sep. 1, 2016, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments of the present invention relate to an acidic gas recovery apparatus and an acidic gas recovery method.

BACKGROUND

Large fossil-fuel consumers, such as thermal power plants and ironworks, burn fossil fuels in boilers to generate combustion discharge gases. Those gases as well as natural gases and coal gasification gases (gasification gases) contain acidic gas components, such as, carbon dioxide (CO2), SOx, NOxand H2S. In order not to emit those acidic gas components in the discharge gases into the atmosphere, methods for recovering the acidic gas components have been vigorously studied and developed. In those methods, discharge gases containing the acidic gas components are introduced into an absorption tower and brought into gas-liquid contact with an absorbing liquid containing an amino group-containing compound (amine-based compound) so that the absorbing liquid may absorb the acidic gas components to remove them from the treated gases.

For example, there is a known carbon dioxide recovery apparatus comprising: an absorption tower in which a discharge gas is brought into contact with an absorbing liquid containing an amino group-containing compound so that the absorbing liquid can absorb the acidic gas components such as carbon dioxide; and a regeneration tower in which the absorbing liquid loaded with the absorbed acidic gas components is heated to release the acidic gas components and thereby to be regenerated. The regenerated absorbing liquid is then supplied again to the absorption tower and reused there. In the apparatus, the absorbing liquid is thus circularly used in the system including the absorption and regeneration towers.

However, the above carbon dioxide recovery apparatus has a problem in that the discharge gas from which carbon dioxide is absorbed and removed by the amine-based absorbing liquid in the absorption tower, namely, the CO2-removed discharge gas (decarbonated combustion discharge gas) is finally emitted together with the amine from the absorption tower into the atmosphere. Accordingly, since thermal power plants and the like exhaust a large amount of decarbonated combustion discharge gas, they may also release a large amount of amine at the same time. It is hence necessary to effectively reduce the amine emitted together with the decarbonated discharge gas into the atmosphere. In view of that, there is, for example, a known cleaning apparatus in which the decarbonated discharge gas is brought into contact with a cleaning liquid so as to recover the amine accompanying the discharge gas.

Further, when carbon dioxide in the discharge gas is absorbed in the recovery apparatus, not only SOxand NOxbut also other inorganic acids, such as, carbonyl sulfide, hydrogen cyanate, thiocyanic acid and thiosulfuric acid, may react with the amino group-containing compound contained in the absorbing liquid to form degradation products referred to as “heat stable amine salts: HSAS”. Further, when the absorbing liquid is heated to be regenerated, the amino group-containing compound may be decomposed thermally or by reaction with oxygen in the discharge gas and, as a result of that, heat stable amine salts are also formed.

The heat stable amine salts are not thermally decomposed in the heating procedure for regenerating the absorbing liquid in the regeneration tower, and hence are not removed from but accumulated in the absorbing liquid. The heat stable amine salts thus accumulated not only impair the absorbing liquid to lower the efficiency of acidic gas absorption but also cause corrosion of the apparatus. It is hence desired to remove the heat stable amine salts from the absorbing liquid.

As a method for removing the heat stable amine salts from the absorbing liquid, it is known to adopt an electrodialyser of three-compartment structure, for example. The electrodialyser comprises a bipolar membrane in combination with plural ion-exchange membranes, and has three compartments, namely, an amine-purification compartment, an amine-recovery compartment and an acid-recovery compartment. Those compartments are provided between the electrodes facing each other, and aligned in the above order from the cathode side to the anode side. In the electrodialyser, the absorbing liquid undergoes electrodialysis so that the heat stable amine salts migrate from the absorbing liquid to a concentrate to be removed.

DETAILED DESCRIPTION

The acidic gas recovery apparatus according the embodiment comprises:

an acidic gas-absorber configured to absorb at least a part of acidic gases contained in an objective gas into an absorbing liquid and to discharge said objective gas as an acid gas-removed gas;

a regenerator configured to be provided with said absorbing liquid from said acidic gas-absorber and to release said acidic gases absorbed in said absorbing liquid;

a gas-cleaner configured to clean said acid gas-removed gas discharged from said acidic gas-absorber with a cleaning liquid;

a cleaning liquid drawing-out line configured to draw out said cleaning liquid;

an absorbing liquid drawing-out line configured to draw a part of the absorbing liquid supplied to said acidic gas-absorber; and

an acidic component-remover configure to be supplied with said cleaning liquid and said absorbing liquid via said cleaning liquid drawing-out line and said absorbing liquid drawing-out line, respectively; wherein

said acidic component-remover comprises a cathode, an anode, an absorbing liquid-purification compartment configured to remove acidic components of said absorbing liquid, and a cleaning liquid compartment configured to be supplied with said cleaning liquid.

Further, the acidic gas recovery method according to the embodiment comprises:

an absorption step absorbing at least a part of acidic gases contained in an objective gas into an absorbing liquid and discharging said objective gas as an acid gas-removed gas;

a gas-cleaning step cleaning said acid gas-removed gas with a cleaning liquid; and

an acidic component-removal step removing acidic components from said absorbing liquid in an acidic component-remover, the acidic component-remover comprising an anode, a cathode, an absorbing liquid-purification compartment provided between said anode and said cathode and a cleaning liquid compartment provided between said anode and said cathode, wherein said absorbing liquid-purification compartment and said cleaning liquid compartment are separated with at least one membrane,

wherein the acidic component-removal step comprises:supplying at least a part of said absorbing liquid to be used in said absorption step to said absorbing liquid-purification compartment in said acidic component-remover;supplying at least a part of said cleaning liquid to said cleaning liquid compartment in said acidic component-remover; andapplying voltage between said anode and said cathode.

Embodiments will now be explained with reference to the accompanying drawings. In the following description, the present embodiments will be explained provided that the acidic gas is carbon dioxide (CO2).

First Embodiment

FIG. 1schematically shows a structure of the acidic gas recovery apparatus according a first embodiment. As shown inFIG. 1, the acidic gas recovery apparatus10A comprises an absorption tower11, a regeneration tower (regenerator)12, and an acidic component-remover13A. In the acidic gas recovery apparatus10A, an absorbing liquid for absorbing CO2in a CO2-containing discharge gas (objective gas)21is circulated between the absorption tower11and the regeneration tower12(hereinafter, this circulation area is often referred to as “inner system”). The absorbing liquid loaded with the CO2absorbed from the discharge gas21(hereinafter, this absorbing liquid is often referred to as “rich solution22”) is supplied from the absorption tower11to the regeneration tower12. In the regeneration tower12, the CO2is partly or almost completely removed from the rich solution22to regenerate the absorbing liquid (hereinafter, this regenerated absorbing liquid is often referred to as “lean solution23A”). The lean solution23A is then supplied to the absorption tower11from the regeneration tower12.

The absorbing liquid is an aqueous amine-based solution containing water and an amine-based compound (amino group-containing compound). Examples of the amine-based compound include: primary amines, such as, monoethanolamine and 2-amino-2-methyl-1-propanol; secondary amines, such as, diethanolamine and 2-methylaminoethanol; tertiary amines, such as. triethanolamine and N-methyldiethanolamine; polyethylene-polyamines, such as, ethylenediamine, triethylenediamine and diethylenetriamine; cyclic amines, such as, piperazines, piperidines and pyrrolidines; polyamines, such as, xylylenediamine; and amino acids, such as, methylaminocarboxylic acid. Those may be adopted singly or in combination of two or more. The absorbing liquid normally contains the amine-based compound normally in an amount of 10 to 70 wt %, and may further contain other compounds, such as, a reaction accelerator, a nitrogen-containing compound for improving absorption of acidic gases such as CO2, an anticorrosive agent for preventing corrosion of the plant facilities, an antifoaming agent for preventing foaming, an oxidation inhibitor for preventing deterioration of the absorbing liquid, and a pH adjuster. Those compounds may be added in such amounts that they do not impair the function of the absorbing liquid.

As described later in detail, the lean solution23A in the present embodiment is an absorbing liquid regenerated by removing CO2partly or almost completely in the regeneration tower12. On the way to the absorption tower11, the lean solution23A is partly drawn out to be an objective lean solution23B. The objective lean solution23B is then fed to the acidic component-remover13A from the inner system. In the acidic component-remover13A, acidic components of heat stable amine salts are removed from the objective lean solution23B to produce a purified lean solution23C. The purified lean solution23C is returned to the inner system, and mixed with the lean solution23A to be a mixed lean solution23D, which is then supplied to the absorption tower11. In the present specification, the “absorbing liquid” inclusively means the lean solution23A, the objective lean solution23B, the purified lean solution23C and the mixed lean solution23D.

The discharge gas21is a CO2-containing exhaust gas, such as, a combustion discharge gas exhausted from boilers, gas turbines and the like in thermal power plants or the like, or a process discharge gas generated from ironworks. The discharge gas21is pressurized with a ventilator or the like, cooled in a cooling tower, and then supplied through a flue to the absorption tower11from an intake provided on the side wall near the tower bottom (foot of the tower).

The absorption tower11comprises a CO2-absorption unit (acidic gas-absorber)11aand a gas-cleaner11b. In the CO2-absorption unit, the mixed lean solution23D absorbs CO2in the discharge gas21to produce an acid gas-removed gas (CO2-removed discharge gas)26. In the gas-cleaner11b, the acid gas-removed gas26produced in the CO2-absorption unit11ais then washed with a cleaning water27aserving as the cleaning liquid to recover amine accompanying the CO2-removed discharge gas26a.

The CO2-absorption unit11ais filled with a packing for enhancing the efficiency of gas-liquid contact, and is equipped with a spray nozzle on the top. From the spray nozzle, the mixed lean solution23D supplied to the absorption tower11is scatteringly showered down to the CO2-absorption unit11a. On the other hand, the discharge gas21fed into the tower flows from the bottom area to the tower top (upper area). In the CO2-absorption unit11a, the discharge gas21moving upward in the tower is thus brought into counterflow contact with the mixed lean solution23D. As a result, the following reactions (1) and (2) proceed to form a heat-decomposable salt (RNH2CO3) and a heat stable amine salt (RNHX) if the absorbing liquid contains a tertiary amine, so that CO2in the discharge gas21is absorbed in the mixed lean solution23D and thereby removed from the discharge gas21. Meanwhile, the mixed lean solution23D thus absorbs CO2in the discharge gas21and converts to the rich solution22, which is stored in the bottom area. The rich solution22contains the heat-decomposable salt and the heat stable amine salt. Heat stable amine salts are also formed from other acidic substances absorbed in the solution. Examples of those acidic substances include: organic acids formed by reactions with oxygen contained in the discharge gas21, and inorganic acids contained in the discharge gas21, such as, SOx, NOx, carbonyl sulfide, hydrogen cyanate, thiocyanate and thiosulfate. The heat stable amine salts are accumulated in the rich solution22.
RN+CO2+H2O→RNH2CO3(1)
RN+HX→RNHX  (2)

The CO2-removed discharge gas26adischarged from the CO2-absorption unit11amoves upward in the absorption tower11and comes into the gas-cleaner11b.

In the gas-cleaner11b, the CO2-removed discharge gas26ais washed with the cleaning water27ato recover amine accompanying the CO2-removed discharge gas26a. In the present embodiment, the gas-cleaner11bcomprises a water-cleaner28where the CO2-removed discharge gas26ais washed with the cleaning water27a. The water-cleaner28is installed in the absorption tower11, and is provided on the downstream side of the CO2-absorption unit11aalong the flow of the CO2-removed discharge gas26a. Accordingly, the water-cleaner28is positioned above the CO2-absorption unit11a. The water-cleaner28is equipped with a spray nozzle on the top. From the spray nozzle, the cleaning water27asupplied to the absorption tower11is scatteringly showered down to the water-cleaner28. In the water-cleaner28, the CO2-removed discharge gas26ais washed with the cleaning water27ato remove amine accompanying the CO2-removed discharge gas26a. Although included in the absorption tower11inFIG. 1, the water-cleaner28may be installed outside of the absorption tower11to be a gas-cleaning tower independent from the absorption tower11.

The cleaning water27ais, for example, stored in a cleaning water-tank (not shown) installed in the lower area of the water-cleaner28. The cleaning water-tank is connected to a cleaning water-circulation line L11equipped with a circulation pump31. The cleaning water27ais pressurized by the circulation pump31and returned to the tower from the upper area of the water-cleaner28.

The more acidic the cleaning water27ais, the higher cleaning efficiency it has. Accordingly, for example, at the time of starting operation of the acidic gas recovery apparatus, pure water, aqueous sulfuric acid or the like are preferably adopted. However, according as the apparatus is kept operated, the amine-based solution used as the absorbing liquid is accumulated in the cleaning water27aand hence the cleaning water27atends to become alkaline.

The cleaning water27acontains amine absorbed from the CO2-removed discharge gas26a, and the amine concentration therein continues to increase while the cleaning water27akeeps circulating through between the water-cleaner28and the cleaning water-circulation line L11. Accordingly, the cleaning water27abecomes impaired in the performance of recovering amine. In the present embodiment, the cleaning water27ais partly drawn out and amine contained therein is removed in the acidic component-remover13A. For compensating the drawn-out cleaning water27a, a fresh cleaning water27ein the same amount as the drawn-out cleaning water may be supplied to the cleaning water-circulation line L11. It is also possible to partly drain out the cleaning water27acirculating through between the water-cleaner28and the cleaning water-circulation line L11.

After purified in the gas-cleaner11b, the CO2-removed discharge gas26ais discharged to the outside as a purified gas32from the top of the absorption tower11.

Meanwhile, the rich solution22stored in the bottom area of the absorption tower11is discharged from the bottom, and led to a rich solution-supply line L12equipped with a pump (not shown). The rich solution22pressurized by the pump is sent to a heat exchanger33, then undergoes heat exchange with the lean solution23A regenerated in the regeneration tower12, and subsequently is fed to the regeneration tower12. In course of the heat exchange between the rich solution22and the lean solution23A in the heat exchanger33, the lean solution23A serves as a heat source to heat the rich solution22and, in contrast, the rich solution22serves as a cooling source to cool the lean solution23A. The heat exchanger may be a known one, such as, a plate heat exchanger or a shell and tube heat exchanger.

In the regeneration tower12, CO2is released and separated from the rich solution22so that the rich solution22may be regenerated as the lean solution23A. The rich solution22is supplied to the regeneration tower12, and then heated with the lean solution23A and steam35supplied to the regeneration tower12. The lean solution23A and the steam35are generated by heat exchange between the lean solution23A and saturated steam in a reboiler36. From the rich solution22thus heated with the steam, CO2contained therein is eliminated. In this way, CO2in the rich solution22is partly or almost completely removed and thereby the rich solution22is converted into the lean solution23A.

The lean solution23A stored in the regeneration tower12is partly discharged therefrom and heated in the reboiler36, and thereafter returned to the regeneration tower12. When heated in the reboiler36, the lean solution23A generates steam and releases CO2residually remaining therein. The generated steam and CO2gas are returned to the regeneration tower12to heat the rich solution22supplied to the regeneration tower12. The CO2gas is then exhausted out of the regeneration tower12. The lean solution23A discharged from the regeneration tower12is pressurized by a lean solution-pump (not shown) and fed to the absorption tower11via the heat exchanger33.

From the upper area of the regeneration tower12, the CO2gas is exhausted together with the steam simultaneously generated from the lean solution23A. The mixed gas41comprising the CO2gas and the steam is cooled with cooling water in a cooler42, and the steam condenses to water. The mixed fluid44containing the condensed water and the CO2gas is supplied to a gas-liquid separator45, where the CO2gas46and water47are separated. While the CO2gas46is exhausted, the water47is drained out from the bottom area of the separator45and sent to the upper area of the regeneration tower12.

The lean solution23A stored in the regeneration tower12is drained out from the bottom area of the regeneration tower12into a lean solution-evacuation line L13, and then led to the heat exchanger33. In the heat exchanger33, the lean solution23A is cooled by heat exchange with the rich solution22. Subsequently, the lean solution23A is pressurized by a pump (not shown), cooled with cooling water in a cooler48, and then supplied to the absorption tower11.

The lean solution-evacuation line L13diverges into an absorbing liquid drawing-out line L21connecting to the acidic component-remover13A. Through the absorbing liquid drawing-out line L21, the lean solution23A sent to the absorption tower11is partly drawn out and supplied to the acidic component-remover13A as the objective lean solution23B. The acidic component-remover13A and the lean solution-evacuation line L13are connected by a purified absorption liquid-supply line L22, through which the purified lean solution23C discharged from the acidic component-remover13A is transferred to the lean solution-evacuation line L13. The lean solution-evacuation line L13diverges into the absorbing liquid drawing-out line L21at a position between the cooler48and the absorption tower11, but the diverging point may be provided on the upstream side of the cooler48along the flow of the absorption liquid.

The cleaning water-circulation line L11diverges into a cleaning liquid drawing-out line L23connecting to the acidic component-remover13A. Through the cleaning liquid drawing-out line L23, the cleaning water27asent to the absorption tower11is partly drawn out and supplied to the acidic component-remover13A as the cleaning water27b. The cleaning water-circulation line L11and the evacuation side of the acidic component-remover13A are connected by a cleaning liquid-evacuation line L24, through which the cleaning water27cdischarged from the acidic component-remover13A is transferred to the cleaning water-circulation line L11. The cleaning waters27aand27care mixed and supplied to the water-cleaner28as the cleaning water27d.

The acidic component-remover13A is connected to a concentrate tank51via a concentrate-circulation line L25, through which a concentrate52ais supplied from the concentrate tank51to the acidic component-remover13A and also through which a concentrate52bis discharged from the acidic component-remover13A and transferred to the concentrate tank51.

In the acidic component-remover13A, the cleaning water27bintroduced from the cleaning liquid drawing-out line L23is treated to remove amine therefrom and also the objective lean solution23B introduced from the absorbing liquid drawing-out line L21is treated to remove acidic components of heat stable amine salts accumulated therein.FIG. 2shows a structure of the acidic component-remover13A. As shown inFIG. 2, the acidic component-remover13A comprises a cathode53, an anode54, cation-exchange membranes55C-1and55C-2, and anion-exchange membranes56A-1and56A-12. The acidic component-remover13A is divided in such three areas as are an absorbing liquid-purification compartment57, cleaning liquid compartments58-1and58-2, and concentrate compartments59-1and59-2. Those compartments are separated with the cation-exchange membranes55C-1and55C-2and the anion-exchange membranes56A-1and56A-2. In the acidic component-remover13A, the membranes are provided between the cathode53and the anode54, and are aligned in the order of the cation-exchange membrane55C-1, the anion-exchange membranes56A-1and56A-2, and the cation-exchange membrane55C-2from the cathode53side to the anode54side. Accordingly, in the present embodiment, the acidic component-remover13A comprises five compartments separated with the ion-exchange membranes, and the compartments are aligned in the order of the concentrate compartment59-1, the cleaning liquid compartment58-1, the absorbing liquid-purification compartment57, the concentrate compartment59-2and the cleaning liquid compartment58-2from the cathode53side to the anode54side. Specifically, the five compartments, namely, the concentrate compartment59-1, the cleaning liquid compartment58-1, the absorbing liquid-purification compartment57, the concentrate compartment59-2and the cleaning liquid compartment58-2are provided between the cathode53and the anode54, and are separated with the cation-exchange membranes55C-1and55C-2and the anion-exchange membranes56A-1and56A-2. Accordingly, the insides of the compartments can be individually impressed with voltage via the cation-exchange membranes55C-1and55C-2and the anion-exchange membranes56A-1and56A-12.

The cation-exchange membranes55C-1and55C-2are cation exchange group-containing polymer films through which cations can permeate but anions cannot. For example, polymer films having one or more of sulfonic acid groups, carboxylic acid groups, phosphonic acid groups, sulfate groups and phosphate groups are usable as the cation-exchange membranes55C-1and55C-2. Examples of the usable films include known cation-exchange membranes, such as, NEOSEPTA CMX, NEOSEPTA CMB ([trademark], manufactured by ASTOM Corporation), SELEMION CMV, SELEMION CMD, SELEMION CSO, and SELEMION CMF ([trademark], manufactured by ASAHI GLASS CO., LTD.).

The anion-exchange membranes56A-1and56A-2are anion exchange group-containing polymer films through which anions can permeate but cations cannot. For example, polymer films having strongly basic quaternary ammonium groups and weakly basic functional groups such as primary, secondary and tertiary amino groups are usable as the anion-exchange membranes56A-1and56A-2. Examples of the usable films include known anion-exchange membranes, such as, NEOSEPTA AMX, NEOSEPTA AHA ([trademark], manufactured by ASTOM Corporation), SELEMION AMV, SELEMION AMT, SELEMION DSV, SELEMION ASV and SELEMION AHO ([trademark], manufactured by ASAHI GLASS CO., LTD.).

The absorbing liquid-purification compartment57is provided between the anion-exchange membranes56A-1and56A-2, is supplied with the objective lean solution23B, and is positioned on the anode54side of the cleaning liquid compartment58-1via the anion-exchange membrane56A-1and on the cathode53side of the concentrate compartment59-2via the anion-exchange membrane56A-2.

The cleaning liquid compartment58-1is provided between the cation-exchange membrane55C-1and the anion-exchange membrane56A-1, and the cleaning liquid compartment58-2is provided between the cation-exchange membrane55C-2and the anode54. Those cleaning liquid compartments58-1,58-2are supplied with the cleaning water27bserving as the cleaning liquid.

The concentrate compartment59-1is provided between the cation-exchange membrane55C-1and the cathode53, and the concentrate compartment59-2is provided between the anion-exchange membrane56A-2and the cation-exchange membrane55C-2. The concentrate52ais supplied to the concentrate compartment59-2. The concentrate compartments59-1and59-2are positioned on the cathode53side of the cleaning liquid compartments58-1and58-2via the cation-exchange membranes55C-1and55C-2, respectively. The cathode53and the anode54may be immersed in an electrode liquid.

The absorbing liquid-purification compartment57is connected to the absorbing liquid drawing-out line L21through which the objective lean solution23B is introduced, and also is connected to the purified absorption liquid-supply line L22through which the purified lean solution23C is drained out.

The cleaning liquid compartments58-1and58-2are individually connected to the cleaning liquid drawing-out line L23through which the cleaning water27bis supplied. The cleaning water-circulation line L11is connected to the cleaning liquid-evacuation line L24, through which the cleaning water27cis supplied from the cleaning liquid compartments58-1and58-2to the cleaning water-circulation line L11. The cleaning water27cis mixed with the cleaning water27ato be the cleaning water27d, which is then supplied to the gas-cleaner11b.

The concentrate tank51and the cleaning liquid compartments58-1and58-2are connected by the concentrate-circulation line L25, through which the concentrate52ais supplied to the cleaning liquid compartments58-1and58-2and also through which the concentrate52bis supplied to the concentrate tank51.

The lean solution23A is partly drawn out as the objective lean solution23B from the lean solution-evacuation line L13to the absorbing liquid drawing-out line L21, through which the objective lean solution23B is supplied to the absorbing liquid-purification compartment57in the acidic component-remover13A. Meanwhile, the cleaning water27acirculating in the gas-cleaner11bis partly supplied as the cleaning water27bto the cleaning liquid compartments58-1and58-2. The concentrate52ain the concentrate tank51is supplied to the concentrate compartments59-1and59-2. As described above, heat stable amine salts contained in the rich solution22are accumulated in the objective lean solution23B. If containing the accumulated heat stable amine salts, the absorbing liquid shows a low pH (hydrogen ion exponent) value.

When voltage is applied between the cathode53and the anode54, a heat stable amine salt (RNHX) in the objective lean solution23B is decomposed into an amine cation (RNH+) and an acidic anion (X−) as shown in the following formula (3). The acidic anion (X−) is attracted to the anode54side, and accordingly migrates from the absorbing liquid-purification compartment57to the concentrate compartment59-2through the anion-exchange membrane56A-2. In this way, the acidic component (X) of the heat stable amine salt is removed from the objective lean solution23B. On the other hand, the amine cation (RNH+) is attracted to the cathode53side, but cannot permeate the anion-exchange membrane56A-1and hence remains in the absorbing liquid-purification compartment57. The acidic component (X) of the heat stable amine salt in the objective lean solution23B is thus removed in the absorbing liquid-purification compartment57, to recover the purified lean solution23C.
RNHX→RNH++X−(3)

Meanwhile, the cleaning waters27a,27bsupplied to the cleaning liquid compartments58-1and58-2contain amine recovered from the CO2-removed discharge gas26aand hence are alkaline. Accordingly, hydroxyl ions (OH−) therein are transferred from the cleaning liquid compartment58-1to the absorbing liquid-purification compartment57through the anion-exchange membranes56A-1,56A-2, and compensate the acidic component anions (X−) removed from the absorbing liquid-purification compartment57. On the other hand, since being in the form of cation (RNH+), the amine is attracted to the cathode53side. Accordingly, the amine permeates the cation-exchange membranes55C-1and55C-2and migrates from the cleaning liquid compartments58-1and58-2to the concentrate compartments59-1and59-2, respectively. In this way, the amine is removed from the cleaning water27b. The amine and the acidic component are thus concentrated in the concentrate52a, and thereby the concentrate52ais converted into the concentrate52b, which is then drained out to the concentrate-circulation line L25. The amine in the cleaning water27bis thus removed in the cleaning liquid compartments58-1and58-2, to recover the amine-recovering ability of the cleaning water27b.

After the acidic components of the heat stable amine salts are removed, the purified lean solution23C is returned to the lean solution23A through the purified absorption liquid-supply line L22. Meanwhile, after the amine-recovering ability is recovered in the cleaning liquid compartments58-1and58-2, the cleaning water27bis drained out from the cleaning liquid-evacuation line L24. The concentrate52bis returned to the concentrate tank51from the concentrate compartments59-1and59-2through the concentrate-circulation line L25. Purified absorption liquid-supply line L22may be connected to a location other than lean solution23A, such as, for example, rich solution-supply line L12.

The apparatus may be provided with a first storage tank between the absorbing liquid drawing-out line L21and the absorbing liquid-purification compartment57. The lean solution23B is stored temporally in the first storage tank, and then supplied to the absorbing liquid-purification compartment57. Further, the purified absorption liquid-supply line L22and the first storage tank may be connected by a line through which the purified lean solution23C is returned to the first storage tank. Furthermore, the first storage tank and the lean solution-evacuation line L13may be connected by a line through which the lean solution23B stored in the first storage tank is partly or fully supplied continuously or intermittently to the lean solution-evacuation line L13so as to be mixed with the lean solution23A.

The apparatus may be also provided with a second storage tank between the cleaning liquid drawing-out line L23and the cleaning liquid compartments58-1and58-2. The cleaning water27bis stored temporally in the second storage tank, and then supplied to the cleaning liquid compartments58-1and58-2. Further, the cleaning liquid-evacuation line L24and the second storage tank may be connected by a line through which the cleaning water27cdrained out from the cleaning liquid compartments58-1and58-2is returned to the second storage tank. Furthermore, the second storage tank and the cleaning water-circulation line L11may be connected by a line through which the cleaning water27bstored in the second storage tank is partly or fully supplied continuously or intermittently to the cleaning water-circulation line L11so as to be mixed with the cleaning water27a.

The objective lean solution23B and the cleaning water27bare supplied to the absorbing liquid-purification compartment57and to the cleaning liquid compartment58-1or58-2, respectively. The objective lean solution23B and the cleaning water27bmay be only once or plural times made to pass through the absorbing liquid-purification compartment57and through the cleaning liquid compartment58-1or58-2, respectively.

In the present embodiment, only the cleaning water27bis supplied as the cleaning liquid to the cleaning liquid compartments58-1and58-2. However, the apparatus is not limited to that embodiment. Specifically, pure water and aqueous solutions may be introduced from the outside and supplied together with the cleaning water27bto the cleaning liquid compartments58-1and58-2as the cleaning liquid. The aqueous solutions must be capable of dissolving acids and amines, and needs to provide electroconductivity for enhancing the current efficiency. In view of that, it is preferred to adopt aqueous solutions containing acids, alkalis or salts dissolved therein.

The cation-exchange membranes55C-1,55C-2and the anion-exchange membranes56A-1,56A-2, which are used in the acidic component-remover13A, are liable to deteriorate at a high temperature. Accordingly, the temperature of the cleaning water27bis preferably the same as or lower than that of the objective lean solution23B, and is preferably 40° C. or below.

As shown inFIG. 1, the purified lean solution23C discharged from the acidic component-remover13A is introduced to the lean solution-evacuation line L13, mixed with the lean solution23A, and then supplied to the absorption tower11as the mixed lean solution23D.

According to the present embodiment, amine contained in the cleaning water27bcan be thus removed in the cleaning liquid compartments58-1and58-2and thereby the acidic component-remover13A can fulfill the function of reducing the amine concentration in the cleaning water27b. Hence, it can be realized to lower the pH value of the cleaning water27cdrained out from the acidic component-remover13A. In this way, the cleaning water27cis introduced through the cleaning liquid-evacuation line L24into the cleaning water-circulation line L11and mixed with the cleaning water27ato be the cleaning water27d, so that the pH value of the cleaning water27dcan be reduced enough to recover the amine-recovering ability of the cleaning water27a. As a result, the amine-cleaning efficiency can be improved in the gas-cleaner11b.

Also, according to the present embodiment, it can be realized in the absorbing liquid-purification compartment57of the acidic component-remover13A to remove acidic components of heat stable amine salts contained in the objective lean solution23B. While the lean solution23A serving as the absorption liquid is kept circulating in the inner system, heat stable amine salts and the like are accumulated in the lean solution23A. However, the acidic components of heat stable amine salts are thus removed from the objective lean solution23B, which is then mixed again with the lean solution23A and reused. In this way, it can be realized to remove acidic components of heat stable amine salts accumulated in the lean solution23A. As a result, the CO2-absorption efficiency can be improved in the CO2-absorption unit11a.

According to the prior art, both the gas-cleaner and the acidic component-remover individually need to be supplied with water or chemicals (e.g., aqueous solutions of sulfuric acid and the like). Further, it is also necessary to treat the effluent water or chemicals used in the gas-cleaner and the acidic component-remover. In contrast, according to the present embodiment, the cleaning water27asupplied to the gas-cleaner11bis partly introduced into the acidic component-remover13A, so as to disuse or reduce the water or chemicals employed in the acidic component-remover13A for removing heat stable amine salts from the objective lean solution23B. This also can suppress expansion of the effluent treatment facility.

According to the present embodiment, it is also possible to remove heat stable amine salts and the like accumulated in the objective lean solution23B without adding chemicals such as hydroxides of alkali metals (e.g., sodium) in the acidic component-remover13A. If alkali metal hydroxides are added to the objective lean solution23B, impurities such as salts other than the heat stable amine salts may be formed to increase salts in the absorption liquid and consequently the apparatus may corrode and/or the impurities may precipitate. In contrast, since it is unnecessary in the present embodiment to incorporate alkali metal hydroxides into the objective lean solution23, it can be realized to dispense with a work for removing the above impurities. It is hence easy to remove heat stable amine salts and the like accumulated in the objective lean solution23B.

As described above, since the acidic gas recovery apparatus10A comprises the acidic component-remover13A, it is possible to recover the amine-recovering ability of the cleaning water27bused in the gas-cleaner11band thereby to keep the amine-recovering efficiency of the gas-cleaner11band further it is also possible to stabilize the performance of absorbing CO2from the discharge gas21in the absorption tower11. Furthermore, since the cleaning water27asupplied to the gas-cleaner11bis partly used in the acidic component-remover13A, it is possible to reduce water or chemicals used there. Accordingly, the effluent treatment facility can be kept from expanding and hence the cost for the apparatus can be prevented from increasing. Still further, since the mixed lean solution23D containing heat stable amine salts in a low concentration is circulated in the inner system between the absorption tower11and the regeneration tower12, the acidic gas recovery apparatus10A is prevented from undergoing internal corrosion or damage and hence can be stably operated. The acidic gas recovery apparatus10A thus makes it possible to efficiently reduce the emission of amine to the environment.

In the present embodiment, the cleaning water27cdischarged from the acidic component-remover13A is transferred through the cleaning liquid-evacuation line L24to the gas-cleaner11band reused there. However, the amine concentration in the cleaning water27cis lower than that in the cleaning water27b, and hence the cleaning water27cmay be directly drained out to the outside, for example, as shown inFIG. 3.

Also in the present embodiment, the cleaning water27ais circulated in the cleaning water-circulation line L11while the cleaning water27bis partly supplied to the cleaning liquid drawing-out line L23. However, the cleaning water27acontains amine. In view of that, as shown inFIG. 4, the cleaning water-circulation line L11and the lean solution-evacuation line L13may be connected by a cleaning water-intake line L26, through which the cleaning water27acirculating in the cleaning water-circulation line L11is mixed with the mixed lean solution23D so that amine in the cleaning water27acan be utilized to absorb CO2in the discharge gas21. Furthermore, cleaning water27acirculating in the cleaning water-circulation line L11may be supplied to CO2-absorption unit11a.

Second Embodiment

The acidic gas recovery apparatus according to a second embodiment will be described with reference to the drawings. The same number or sign will be applied to a member having the same function as that in the embodiment described above, and the detailed description thereof will be omitted. The present embodiment is the same as the first embodiment except for the structure of the acidic component-remover, and therefore only the structure of the acidic component-remover will be explained with reference to the drawings. In the acidic gas recovery apparatus of the present embodiment, the acidic component-remover comprises bipolar membranes in place of the cation-exchange membranes.

FIG. 5schematically shows the structure of the acidic component-remover. As shown inFIG. 5, the acidic component-remover13B comprises a cathode53, an anode54, bipolar membranes61BP-1and61BP-2, and an anion-exchange membranes56A. The acidic component-remover13B is divided in such two areas as are absorbing liquid-purification compartments57-1,57-2and cleaning liquid compartments58-1,58-2. Those compartments are separated with the anion-exchange membranes56A and the bipolar membranes61BP-1and61BP-2. The membranes are provided between the cathode53and the anode54, and are aligned in the order of the bipolar membrane61BP-1, the anion-exchange membrane56A, and the bipolar membrane61BP-2from the cathode53side to the anode54side. Accordingly, in the present embodiment, the acidic component-remover13B comprises four compartments separated with the bipolar membranes61BP-1,61BP-2and the anion-exchange membrane56A, and the compartments are aligned in the order of the cleaning liquid compartment58-1, the absorbing liquid-purification compartment57-1, the cleaning liquid compartment58-2, and the absorbing liquid-purification compartment57-2from the cathode53side to the anode54side. Specifically, the four compartments, namely, the cleaning liquid compartment58-1, the absorbing liquid-purification compartment57-1, the cleaning liquid compartment58-2and the absorbing liquid-purification compartment57-2are so provided between the cathode53and the anode54that the inside of each compartment can be individually impressed with voltage via at least one membrane. In the present embodiment, the acidic component-remover13B comprises two pairs of the absorbing liquid-purification compartments and cleaning liquid compartments. However, it may comprise one pair of them or plural pairs of them.

The bipolar membranes61BP-1and61BP-2are composite films in which anion-exchange membranes and cation-exchange membranes are laminated. In the presence of water, the bipolar membranes are provided so that the anion-exchange membranes and the cation-exchange membranes may be on the anode side and on the cathode side, respectively. Examples of the anion-exchange membranes and the cation-exchange membranes are the same as those described in the first embodiment.

The absorbing liquid-purification compartments57-1and57-2are supplied with the objective lean solution23B.

The absorbing liquid-purification compartment57-1is provided between the bipolar membrane61BP-1and the anion-exchange membrane56A, and is positioned on the anode54side of the cleaning liquid compartment58-1via the bipolar membrane61BP-1. Further, the absorbing liquid-purification compartment57-1is sandwiched between the cleaning liquid compartments58-1and58-2. The anion-exchange membrane56A is positioned on the anode54side of the absorbing liquid-purification compartment57-1.

The absorbing liquid-purification compartment57-2is provided between the bipolar membrane61BP-2and the inner wall, and is positioned on the anode54side of the cleaning liquid compartment58-2via the bipolar membrane61BP-2.

The absorbing liquid-purification compartments57-1and57-2are supplied with the objective lean solution23B while the cleaning liquid compartments58-1and58-2are supplied with the cleaning water27b.

When voltage is applied between the electrodes, the anionic acidic component (X−) of heat stable amine salt in the objective lean solution23B is attracted to the anode54side in the absorbing liquid-purification compartments57-1and57-2. The acidic component (X−) of heat stable amine salt in the absorbing liquid-purification compartment57-1then migrates from the absorbing liquid-purification compartment57-1to the cleaning liquid compartment58-2though the anion-exchange membrane56A. As a result, the acidic component (X−) of the heat stable amine salt is removed from the objective lean solution23B.

Meanwhile, when voltage is applied between the electrodes, water in the bipolar membranes61BP-1and61BP-2undergoes electrolysis and the resultant hydrogen ions and hydroxyl ions migrate to the cation-exchange membrane side (cathode53side) and to the anion-exchange membrane side (anode54side), respectively, in the bipolar membranes61BP-1and61BP-2. Consequently, the hydroxyl ions move from the bipolar membranes61BP-1and61BP-2to the absorbing liquid-purification compartments57-1and57-2while the hydrogen ions move from the bipolar membranes61BP-1and61BP-2to the cleaning liquid compartments58-1and58-2. The hydroxyl ions can be thus transferred to the absorbing liquid-purification compartments57-1and57-2. Also, since the hydrogen ions are transferred to the cleaning water27b, the pH value of the cleaning water27bcan be lowered.

The purified lean solution23C is drained out from the acidic component-remover13B, and then returned to the lean solution23A (see,FIG. 1) through the purified absorption liquid-supply line L22(see,FIG. 1). Meanwhile, the cleaning water27cis drained out from the acidic component-remover13B, transferred through the cleaning liquid-evacuation line L24(see,FIG. 1) and mixed with the cleaning water27a(see,FIG. 1) flowing in the cleaning water-circulation line L11(see,FIG. 1). The drained cleaning water27c(see,FIG. 1) contains accumulated acidic components of heat stable amine salts.

According to the present embodiment, the acidic component (X−) of heat stable amine salt contained in the objective lean solution23B is transferred to the cleaning water27bin the cleaning liquid compartments58-1and58-2and thereby the acidic component-remover13B can fulfill the function of lowering the pH value of the cleaning water27b. The cleaning water27bis suppled through the cleaning liquid-evacuation line L24to the cleaning water-circulation line L11, and mixed with the cleaning water27a. In this way, the pH value of the cleaning water27ccan be lowered enough to recover the amine-recovering ability of the cleaning water27c. As a result, the amine-cleaning efficiency can be improved in the gas-cleaner11b.

Also, according to the present embodiment, acidic components of heat stable amine salts can be removed from the objective lean solution23B in the absorbing liquid-purification compartments57-1and57-2of the acidic component-remover13B. After the acidic components of heat stable amine salts are thus removed from the objective lean solution23B to lower the concentration thereof in the acidic component-remover13B, the purified lean solution23C is supplied through the purified absorption liquid-supply line L22to the lean solution-evacuation line L13and mixed with the lean solution23A. The purified lean solution23C is thus mixed again with the lean solution23A and reused, and thereby it can be realized to reduce the concentration of acidic components of heat stable amine salts in the mixed lean solution23D and hence to recover the CO2-absorption performance of the mixed lean solution23D.

In the present embodiment, if pure water is adopted as the liquid supplied to the cleaning liquid compartments58-1and58-2, the supplied liquid has such low electroconductivity as to lower the current efficiency of electrodialysis carried out in the acidic component-remover13B. Accordingly, it is known to add acids, alkalis or salts for providing electroconductivity. Actually, however, the objective lean solution23B in the absorbing liquid-purification compartments57-1and57-2contains amine, acidic components of heat stable amine salts, and hydroxyl ions and also the cleaning water27bin the cleaning liquid compartments58-1and58-2contains amine and hydroxyl ions, and hence they necessarily have electroconductivity. It is therefore unnecessary to incorporate the additives, such as acids, alkalis or salts, for providing electroconductivity to the objective lean solution23B and the cleaning water27bwhen they are subjected to electrodialysis in the acidic component-remover13B. The acidic components of heat stable amine salts can be thus removed from the objective lean solution23B without adding the additives or by adding a reduced amount of the additives.

Further in the present embodiment, the cleaning water27aused in the gas-cleaner11bis partly supplied to the acidic component-remover13B, so as to disuse or reduce water or chemicals mixed with the cleaning water27ain the acidic component-remover13B for removing heat stable amine salts from the objective lean solution23B. This also can suppress expansion of the effluent treatment facility for the water and chemicals.

According to the present embodiment, it also becomes possible in the acidic component-remover13B to remove acidic components of heat stable amine salts from the objective lean solution23B without adding therein chemicals such as hydroxides of alkali metals, such as, sodium. Accordingly, there is no fear of forming impurities, such as, salts other than the heat stable amine salts, when alkali metal hydroxides are added to the objective lean solution23B. The present embodiment therefore dispenses with a work for removing the above impurities, and hence it is easy to remove heat stable amine salts and the like accumulated in the objective lean solution23B.

As described above, since the acidic gas recovery apparatus comprises the acidic component-remover13B, it is possible to recover the amine-recovering ability of the cleaning water27aused in the gas-cleaner11band thereby to keep the amine-cleaning efficiency of the gas-cleaner11band further it is also possible to stabilize the performance of absorbing CO2from the discharge gas21in the absorption tower11. In addition, since the objective lean solution23B and the cleaning water27bnecessarily have electroconductivity in the absorbing liquid-purification compartments57-1,57-2and in the cleaning liquid compartments58-1,58-2, respectively, they can be subjected to electrodialysis in the acidic component-remover13B without addition of the additives. Further, since the cleaning water27aused in the gas-cleaner11bis partly employed in the acidic component-remover13B, it is possible to reduce the consumed amount of water or chemicals and to suppress expansion of the effluent treatment facility. As a result, the cost for the apparatus can be prevented from increasing. Furthermore, the cleaning water27cdischarged from the acidic component-remover13B is reused, so that it can be realized to reduce the disposal amount of the cleaning water27band to prevent amine contained in the cleaning water27bfrom leaking out to the outside. Still furthermore, since the mixed lean solution23D containing heat stable amine salts in a low concentration is circulated in the inner system between the absorption tower11and the regeneration tower12, the acidic gas recovery apparatus10A is prevented from undergoing internal corrosion or damage and hence can be stably operated.

Third Embodiment

The acidic gas recovery apparatus according to a third embodiment will be described with reference to the drawings. The same number or sign will be applied to a member having the same function as that in the embodiment described above, and the detailed description thereof will be omitted.

FIG. 6schematically shows a structure of the acidic gas recovery apparatus according the third embodiment. As shown inFIG. 6, in the acidic gas recovery apparatus10B, the gas-cleaner11bcomprises an acid-cleaner62in addition to the water-cleaner28. The cleaning liquid-evacuation line L24is connected to a cleaning acid-circulation line L31through which a cleaning acid63ais supplied to the acid-cleaner62. Here, the “cleaning acid” means a second cleaning liquid having a pH value lower than the cleaning water27a, which is the first cleaning liquid used in the water-cleaner28. The cleaning acid is, for example, an acidic aqueous solution having a pH value of 7 or less. Examples of the acidic aqueous solution include aqueous solutions of sulfuric acid, acetic acid and nitric acid.

In the present embodiment, the acidic component-remover13A is employed to remove the acidic components. For supplying the cleaning water27aused in the water-cleaner28, it is preferred to adopt the acidic component-remover13A shown inFIG. 3. On the other hand, for supplying the cleaning acid63aused in the acid-cleaner62, it is preferred to adopt the acidic component-remover13B shown inFIG. 5. In the present embodiment, the cleaning water27ais used as the cleaning liquid and hence the acidic component-remover13A is employed. However, the acidic component-remover13B can be employed as well as the acidic component-remover13A in the present embodiment.

In the acid-cleaner62, the CO2-removed discharge gas26bdischarged from the water-cleaner28is washed with the cleaning acid63having a lower pH value than the cleaning water27a. The acid-cleaner62is provided above the water-cleaner28, namely, on the downstream side of the water-cleaner28along the flow of the CO2-removed discharge gas26bin the absorption tower11. The acid-cleaner62is equipped with a spray nozzle on the top. From the spray nozzle, the cleaning acid63bsupplied to the absorption tower11is scatteringly showered down to the acid-cleaner62. In the acid-cleaner62, the CO2-removed discharge gas26bis washed with the cleaning acid63bto remove amine remaining in the CO2-removed discharge gas26b. It thus becomes possible to recover amine and the like left unrecovered in the water-cleaner28and thereby to enhance the recovering yield.

The cleaning acid63is, for example, stored in a cleaning acid-tank (not shown) provided in the lower area of the acid-cleaner62. The cleaning acid-tank is connected to a cleaning acid-circulation line L31. The cleaning acid63ais pressurized by a circulation pump64installed in the cleaning acid-circulation line L31, and returned into the tower from the upper area of the acid-cleaner62.

The pH value of the cleaning acid63acan be shifted to the acidic side (the pH value of the cleaning acid63acan be lowered). For the purpose of that, acidic solutions may be introduced from the outside to the cleaning acid-circulation line L31and/or the mine concentration in the cleaning acid63amay be made lower than that in the cleaning water27a.

The cleaning liquid-evacuation line L24is connected to the cleaning acid-circulation line L31, so that the cleaning water27cdrained out from the acidic component-remover13A is mixed with the cleaning acid63a. In the present embodiment, the cleaning water27ais used as the cleaning liquid, and the cleaning water27cdrained out from the acidic component-remover13A is mixed with the cleaning acid63aand then supplied as the cleaning acid63bto the acid-cleaner62. The cleaning water27cused as the cleaning liquid is thus utilized as the cleaning acid63btogether with the cleaning acid63aso that amine remaining in the CO2-removed discharge gas26bcan be recovered in the acid-cleaner62.

As described above, also in the acidic gas recovery apparatus10B, the cleaning water27cused as the cleaning liquid can be effectively used together with the cleaning acid63afor recovering amine remaining in the CO2-removed discharge gas26bin the acid-cleaner62.

Although included in the absorption tower11in the present embodiment, the water-cleaner28and the acid-cleaner62may be installed outside of the absorption tower11to be a gas-cleaning tower independent from the absorption tower11.

Fourth Embodiment

The acidic gas recovery apparatus according to a fourth embodiment will be described with reference to the drawings. The same number or sign will be applied to a member having the same function as that in the embodiment described above, and the detailed description thereof will be omitted.

FIG. 7schematically shows a structure of the acidic gas recovery apparatus according the fourth embodiment. As shown inFIG. 7, the acidic gas recovery apparatus10C is the same as the acidic gas recovery apparatus10B shown inFIG. 6according the third embodiment except that the line into which the cleaning liquid drawing-out line L23diverges is changed from the cleaning water-circulation line L11to the cleaning acid-circulation line L31and that the cleaning liquid supplied to the acidic component-remover13A is changed from the cleaning water27ato the cleaning acid63a. In the present embodiment, since the cleaning acid63ais used as the cleaning liquid in the acid-cleaner62, the acidic component-remover13B shown inFIG. 5is employed to remove the acidic components.

The cleaning liquid drawing-out line L23is connected to the cleaning acid-circulation line L31, so that the cleaning acid63adischarged from the acid-cleaner62is partly drawn out as the cleaning acid63cthrough the cleaning liquid drawing-out line L23and supplied to the acidic component-remover13B. The cleaning acid63ddrained out from the acidic component-remover13B is mixed with the cleaning acid63aflowing to the acid-cleaner62, and then supplied as the cleaning acid63dto the acid-cleaner62. The cleaning acid63cused as the cleaning liquid is thus utilized together with the cleaning acid63aso that amine remaining in the CO2-removed discharge gas26bcan be recovered in the acid-cleaner62.

As described above, also in the acidic gas recovery apparatus10C, the cleaning acid63dused in the acidic component-remover13B can be effectively used together with the cleaning acid63ain the acid-cleaner62for recovering amine remaining in the CO2-removed discharge gas26b.

In the present embodiment, only the cleaning acid63cis supplied as the cleaning liquid to the cleaning liquid compartments58-1and58-2. However, the apparatus is not limited to that embodiment. Specifically, acid-replenishing solutions may be introduced from the outside and supplied together with the cleaning acid63cto the cleaning liquid compartments58-1and58-2as the cleaning liquid. The acid-replenishing solutions must be aqueous solutions capable of dissolving acids, and needs to provide electroconductivity for enhancing the current efficiency. In view of that, it is preferred to adopt aqueous solutions containing acids, alkalis or salts dissolved therein.

Also in the present embodiment, the cleaning liquid-evacuation line L24is connected to the cleaning acid-circulation line L31so that the cleaning acid63ddischarged from the acidic component-remover13B can be reused in the acid-cleaner62d. However, the amine concentration in the cleaning acid63dis lower than that in the cleaning acid63c, and hence the cleaning acid63dmay be directly drained out from the cleaning acid-circulation line L31to the outside, for example, as shown inFIG. 8.

Fifth Embodiment

The acidic gas recovery apparatus according to a fifth embodiment will be described with reference to the drawings. The same number or sign will be applied to a member having the same function as that in the embodiment described above, and the detailed description thereof will be omitted.

FIG. 9schematically shows a structure of the acidic gas recovery apparatus according the fifth embodiment. As shown inFIG. 9, the acidic gas recovery apparatus10D is the same as the acidic gas recovery apparatus10C shown inFIG. 7according the fourth embodiment except for additionally comprising a pH-meter65for measuring the pH value of the cleaning acid63aused in the acid-cleaner62, a cleaning acid-supply66for supplying a fresh cleaning acid63eto the cleaning acid-circulation line L31, a controller67A, and a control valve V11.

The pH-meter65is installed in the cleaning acid-circulation line L31so as to measure the pH value of the cleaning acid63aflowing through the cleaning acid-circulation line L31. The pH-meter65is. for example, a pH meter.

The controller67A is connected to the pH-meter65, the control valve V11and other members constituting the CO2-recovery apparatus10, and has a function of controlling the aperture of the control valve V11in accordance with the measured result given by the pH-meter65. The controller67A comprises, for example, a memory bank for storing a control program and various memory information, and a calculating means working according to the control program. In the controller67A, the memory bank is beforehand stored with a map or the like of the relation between the pH value of the cleaning acid63aand the efficiency of recovering amine from the CO2-removed discharge gas26b.

When the measured result is sent from the pH-meter65to the controller67A, the controller67A controls the aperture of the control valve V11in accordance with the pH value of the cleaning acid63ameasured in the pH-meter65so that the cleaning acid63ein an adequate amount can flow into the cleaning acid-circulation line L31from the cleaning acid-supply66. If the cleaning acid63eis newly suppled to the cleaning acid-circulation line L31, the cleaning acid63aflowing in the cleaning acid-circulation line L31is preferably drained out to the outside in the amount of the newly supplied cleaning acid63e.

According to the present embodiment, the cleaning acid63ddischarged from the acidic component-remover13A is supplied to the cleaning acid-circulation line L31and used in the acid-cleaner62and also the cleaning acid63eis newly suppled to the cleaning acid-circulation line L31in accordance with the pH value of the cleaning acid63a, and thereby the amine-recovering ability of the cleaning acid63ecan be stably recovered and accordingly the amine-recovering performance in the acid-cleaner62can be stably maintained or improved.

In the present embodiment explained above, the cleaning acid63eis newly suppled to the cleaning acid-circulation line L31. However, the pH-meter65may be installed in the cleaning water-circulation line L11so as to measure the pH value of the cleaning acid27aflowing through the cleaning water-circulation line L11, and the cleaning water may be newly supplied to the cleaning water-circulation line L11in accordance with the measured pH value. In that case, while the pH value of the cleaning water27aflowing through the cleaning water-circulation line L11and that of the cleaning acid63aflowing through the cleaning acid-circulation line L31are individually kept constant, the pH value of the cleaning acid63ais kept lower than that of the cleaning water27aso as to stabilize the performance of recovering amine from the CO2-removed discharge gas26b.

Sixth Embodiment

The acidic gas recovery apparatus according to a sixth embodiment will be described with reference to the drawings. The same number or sign will be applied to a member having the same function as that in the embodiment described above, and the detailed description thereof will be omitted.

FIG. 10schematically shows a structure of the acidic gas recovery apparatus according the sixth embodiment. As shown inFIG. 10, the acidic gas recovery apparatus10E is the same as the acidic gas recovery apparatus10B shown inFIG. 6according the third embodiment except for additionally comprising a pH-meter65-1for measuring the pH value of the cleaning liquid27cdischarged from the acidic component-remover13A, another pH-meter65-2for measuring the pH value of the cleaning acid63aused in the acid-cleaner62, a controller67B, control valves V12and V13, and a circulation line L32.

The pH-meter65-1is installed in the cleaning liquid-evacuation line L24to measure the pH value of the cleaning liquid27cdischarged from the acidic component-remover13A. The pH-meter65-2is installed in the cleaning acid-circulation line L31to measure the pH value of the cleaning acid63aflowing through the cleaning acid-circulation line L31. The pH-meters65-1and65-2are. for example, pH meters.

The controller67B is connected to the pH-meters65-1,65-2, the control valves V12, V13and other members constituting the CO2-recovery apparatus10, and has a function of controlling the flow passing through the control valve V12in accordance with the measured results given by the pH-meters65-1and65-2. Like the controller67A, the controller67B can comprise a memory bank, a calculating means and the like. In the controller67B, the memory bank is beforehand stored with a map or the like of the relation between the pH value of the cleaning acid63aand the efficiency of recovering amine from the CO2-removed discharge gas26b.

The valves V12and V13are installed in the cleaning liquid-evacuation line L24and in the cleaning acid-circulation line L31, respectively. The aperture of the valve V12or V13is so controlled that an adequate amount of the cleaning water27cdischarged from the acidic component-remover13A can flow into the acid-cleaner62or into the acidic component-remover13A, respectively.

The circulation line L32serves as a line through which the cleaning water27cdischarged from the acidic component-remover13A circulate from the cleaning liquid-evacuation line L24to the absorbing liquid drawing-out line L21.

When the measured results are sent from the pH-meters65-1and65-2to the controller67B, the controller67B controls the apertures of the control valves V12and V13in accordance with the pH value of the cleaning water27cmeasured in the pH-meter65-1and with that of the cleaning acid63ameasured in the pH-meter65-2so that the cleaning water27cin an adequate amount can flow into the acid-cleaner62or into the acidic component-remover13A. The cleaning water27cmay be supplied to only one of or both of the acid-cleaner62and the acidic component-remover13A.

Specifically, in the present embodiment, when receiving the measured result given by the pH-meter65-2and judging therefrom that the pH value of the cleaning acid63ais lower than a predetermined value (e.g., 7.0), the controller67B recognizes that the performance of recovering amine from the CO2-removed discharge gas26bis maintained or not impaired and controls the flow passing through the control valve V12so that the cleaning water27cmay be supplied to the circulation line L32. The cleaning water27cis thus mixed with the cleaning water27band transferred to the acidic component-remover13A.

On the other hand, when receiving the measured result given by the pH-meter65-2and judging therefrom that the pH value of the cleaning acid63ais higher than a predetermined value (e.g., 7.0), the controller67B recognizes that the performance of recovering amine from the CO2-removed discharge gas26bis impaired and controls the control valves V12and V13so that the cleaning water27cmay be supplied to the cleaning acid-circulation line L31. The cleaning water27cis thus mixed with the cleaning acid63ato lower the pH value of the cleaning acid63band thereby to recover the performance of recovering amine from the CO2-removed discharge gas26b.

In this way, according to the present embodiment, only at the time of need in accordance with the pH value of the cleaning acid63aused in the acid-cleaner62, the cleaning water27cdischarged from the acidic component-remover13A is mixed with the cleaning acid63aso as to lower the pH value of the cleaning acid63band thereby to recover the performance of recovering amine from the CO2-removed discharge gas26b. It can be therefore realized to stably keep or improve the performance of amine-recovering in the acid-cleaner62.

Seventh Embodiment

The acidic gas recovery apparatus according to a seventh embodiment will be described with reference to the drawings. The same number or sign will be applied to a member having the same function as that in the embodiment described above, and the detailed description thereof will be omitted.

FIG. 11schematically shows a structure of the acidic gas recovery apparatus according the seventh embodiment. As shown inFIG. 11, the acidic gas recovery apparatus10F is the same as the acidic gas recovery apparatus10C shown inFIG. 7according the fourth embodiment except for additionally comprising a pH-meter65for measuring the pH value of the cleaning acid63aused in the acid-cleaner62, a controller67C, a three-way valve V21, and a circulation line L32.

The controller67C is connected to the pH-meter65, the three-way valve V21and other members constituting the CO2-recovery apparatus10, and has a function of controlling the flow passing through the three-way valve V21in accordance with the measured result given by the pH-meter65. Like the controller67A, the controller67C can comprise a memory bank, a calculating means and the like. In the controller67C, the memory bank is beforehand stored with a map or the like of the relation between the pH value of the cleaning acid63aand the efficiency of recovering amine from the CO2-removed discharge gas26b.

The three-way valve V21is installed in the cleaning liquid-evacuation line L24to control the path through which the cleaning water27cdischarged from the acidic component-remover13A is supplied to the acid-cleaner62or to the acidic component-remover13A.

When the measured result is sent from the pH-meter65to the controller67C, the controller67C controls the flow passing through the three-way valve V21in accordance with the pH value of the cleaning acid63ameasured in the pH-meter65so that the cleaning water27cin an adequate amount can flow into the acid-cleaner62or into the acidic component-remover13A.

Specifically, in the present embodiment, when receiving the measured result given by the pH-meter65and judging therefrom that the pH value of the cleaning acid63ais lower than a predetermined value (e.g., 7.0), the controller67C recognizes that the performance of recovering amine from the CO2-removed discharge gas26bis maintained or not impaired and controls the flow passing through the three-way valve V21so that the cleaning water27cmay be supplied to the circulation line L32. The cleaning acid63dis thus mixed with the cleaning acid63cand transferred to the acidic component-remover13A. The cleaning acid63dis circulated between the circulation line L32and the acidic component-remover13A, and thereby the pH value of the cleaning acid63dcan be lowered.

On the other hand, when receiving the measured result given by the pH-meter65and judging therefrom that the pH value of the cleaning acid63ais higher than a predetermined value (e.g., 7.0), the controller67C recognizes that the performance of recovering amine from the CO2-removed discharge gas26bis impaired and controls the flow passing through the three-way valve V21so that the cleaning acid63dmay be supplied to the cleaning acid-circulation line L31. The cleaning acid63dis thus mixed with the cleaning acid63ato lower the pH value of the cleaning acid63band thereby to recover the performance of recovering amine from the CO2-removed discharge gas26b. The cleaning acid63dis circulated between the acidic component-remover13A and the circulation line L32to further lower the pH value of the cleaning acid63d. The cleaning acid63dhaving a thus lowered pH value may be supplied to the acid-cleaner62so as to reduce the amount of chemicals added for the purpose of shifting the pH value of the cleaning acid63ato the acidic side.

Thus, also in the present embodiment, only at the time of need in accordance with the pH value of the cleaning acid63aused in the acid-cleaner62, the cleaning acid63ddischarged from the acidic component-remover13A is mixed with the cleaning acid63aso as to lower the pH value of the cleaning acid63band thereby to recover the performance of recovering amine from the CO2-removed discharge gas26b. It can be therefore realized to stably keep or improve the performance of amine-recovering in the acid-cleaner62.

In the present embodiment explained above, the supplying flow of the cleaning acid63ddischarged from the acidic component-remover13A is controlled in accordance with the measured pH value of the cleaning acid63aflowing through the cleaning acid-circulation line L31. However, the pH-meter65may be also installed in the cleaning water-circulation line L11so that the supplying flow of the cleaning acid63ddischarged from the acidic component-remover13A can be controlled also in consideration of the measured pH value of the cleaning water27aflowing through the cleaning water-circulation line L11. Even if, for the purpose of lowering the pH value of the cleaning acid63b, the cleaning acid63ddischarged from the acidic component-remover13A is supplied to the cleaning acid63ain accordance with not only the pH value of the cleaning acid63abut also that of the cleaning water27aused in the water-cleaner28, it can be realized to stably keep or improve the performance of recovering amine from the CO2-removed discharge gas26bin the acid-cleaner62.

The embodiments described above are explained provided that the discharge gas21contains CO2as the objective gas. However, the present embodiments can be also applied in the same manner even if the discharge gas21contains not only CO2but also other acidic gas components, such as, SOx, NOx, H2S, COS, CS2, NH3and HCN. Further, the present embodiments can be still also applied in the same manner even if the discharge gas21does not contain CO2but contains other acidic gas components.