Temperature-swing adsoroption process

Abstract

A temperature swing adsorption process for removing a target component from a gaseous mixture containing water and at least one side component, said process comprising: (a) at least one adsorption step, providing a target component-loaded adsorbent and at least one waste stream depleted of the target component; (b) a desorption step, comprising heating of the loaded adsorbent to a desorption temperature and providing a first output stream containing the desorbed target component; (c) a conditioning step; (d) at least one target component-releasing releasing step bringing the solid adsorbent to a temperature lower than said desorption temperature and providing at least one second output stream containing an amount of the target component and containing water; (e) separating water from said second output stream(s) and (f) subjecting the so obtained water-depleted stream(s) to said adsorption step or to at least one of said adsorption steps.

Claims

1. A temperature swing adsorption process for removing a target component from a gaseous mixture containing water and at least one side component besides the target component, said temperature swing process comprising: (a) at least one adsorption step, comprising adsorption of target component over a solid adsorbent, providing a target component-loaded adsorbent and at least one waste stream depleted of the target component; (b) a desorption step, comprising heating of said target component-loaded adsorbent to a desorption temperature and desorption of an amount of target component, providing an at least partially regenerated adsorbent and a first output stream containing the desorbed target component; (c) a conditioning step, comprising cooling of said at least partially regenerated adsorbent to a conditioning temperature; (d) at least one target component-releasing step bringing the solid adsorbent to a temperature lower than said desorption temperature and providing at least one second output stream containing an amount of the target component and containing water; (e) separating water from said second output stream(s), producing at least one water-depleted stream; and (f) subjecting said water-depleted stream(s) to said adsorption step or to at least one of said adsorption steps; wherein said at least one target component-releasing step is performed before or after said desorption step (b); and wherein said step (e) comprises cooling of said second output stream(s) to condense at least a portion of the water contained therein and also comprises separation of the condensed water, obtaining said water-depleted stream(s).

2. The temperature swing process of claim 1, wherein said target component-releasing step is performed before the desorption step (b) and comprises heating of the target component-loaded adsorbent up to a temperature which is lower than said desorption temperature.

3. The temperature swing process of claim 1, wherein said target component-releasing step is performed after the desorption step (b) and comprises cooling of said at least partially regenerated adsorbent to a temperature which is preferably higher than said conditioning temperature, said cooling taking place with the aid of at least a portion of said waste stream or at least one of said waste streams, which is cooled prior to be subjected to said target component-releasing step(s).

4. The temperature swing process of claim 1, wherein said target component-releasing step includes more than one of said target component-releasing step, wherein one of them is performed before said desorption step (b) and another one is performed thereafter with the aid of at least a portion of said waste stream or at least one of said waste streams, which is optionally cooled prior to be subjected thereto.

5. The temperature swing process of claim 1, wherein said temperature swing process is carried out in a plurality of reactors containing an adsorbent and each reactor of the plurality of reactors performing said steps (a) to (f).

6. The temperature swing process of claim 5, wherein said water-depleted stream or at least one of said water-depleted streams provided by one reactor is subjected to at least one other reactor of the plurality while performing said adsorption step (a) or one of said adsorption steps.

7. The temperature swing process of claim 6, wherein said water-depleted stream or at least one of said water-depleted streams is subjected with or without an intermediate storage in a suitable tank to said at least one other reactor performing said adsorption step (a) or one of said adsorption steps.

8. The temperature swing process of claim 6, wherein said target component-releasing step or at least one of said target component-releasing steps being performed after the desorption step (b) and comprising cooling of said at least partially regenerated adsorbent to a temperature which is higher than said conditioning temperature with the aid of at least a portion of said waste stream or at least one of said waste streams which is provided by at least one other reactor of said plurality.

9. The temperature swing process of claim 8, wherein said at least a portion of waste stream is exchanged with or without an intermediate storage in a tank from said at least one other reactor to the reactor undergoing said target component-releasing step (d).

10. The temperature swing process of claim 8, wherein the waste stream or at least one of the waste streams subjected to said target component-releasing step and the water-depleted stream or at least one of the water-depleted streams subjected to said adsorption step or to at least one of said adsorption steps are provided by two different reactors.

11. The temperature swing process of claim 10, wherein: a first reactor performs said target component-releasing step before the desorption step (b) providing the second output stream, which is subjected to said step (e) producing said water-depleted stream; a second reactor performs said at least one adsorption step providing said at least one waste stream; and at least a portion of said water-depleted stream is subjected to said second reactor performing the adsorption step, and at least a portion of said waste stream is used for the target component-releasing step (d) of said first reactor, thus forming a closed loop between said first and second reactor.

12. The temperature swing process of claim 11, wherein each reactor of said plurality performs a first adsorption step and a second adsorption step, said second adsorption step being carried out after said first adsorption step and before said desorption step (b); said first adsorption step comprising contacting an input stream of said gaseous mixture with a solid adsorbent and adsorption of target component from said input stream, providing a target component-loaded adsorbent and a first waste stream depleted of the target component; said second adsorption step comprising contacting said loaded adsorbent with the water-depleted stream or at least one of the water-depleted streams provided by at least one other reactor of said plurality of reactors while performing said step (e), wherein an amount of the target component contained in said water-depleted stream is adsorbed and a second waste stream depleted of the target component is produced.

13. The temperature swing process of claim 12, wherein said target component-releasing step performed after the desorption step (b) is carried out with the aid of at least a portion of the second waste stream provided by at least one other reactor of said plurality of reactors while performing said second adsorption step.

14. The temperature swing process of claim 13, wherein said conditioning step (c) is carried out by at least a portion of the first waste stream provided by said at least one other reactor of said plurality of reactors while performing said first adsorption step (a), said at least a portion of the first waste stream being cooled prior to subjection to said conditioning step (c).

15. The temperature swing process of claim 14, wherein each reactor of the plurality of reactors additionally performs one target component-releasing step before the desorption step (b), and provides a first water-depleted stream resulting from the target component-releasing step performed before the step (b) and a second water-depleted stream resulting from the target component-releasing step performed after the step (b), said first stream being supplied to a reactor performing said first adsorption step and said second stream being supplied to a reactor performing said second adsorption step.

16. The temperature swing process of claim 1, wherein the desorption temperature is not greater than 250° C.

17. The temperature swing process of claim 16, wherein the desorption temperature is not greater than 200° C.

18. The temperature swing process of claim 17, wherein the desorption temperature is not greater than 170° C.

19. The temperature swing process of claim 1, wherein the conditioning temperature is not greater than 60° C.

20. The temperature swing process of claim 1, wherein said target component includes carbon dioxide.

21. The temperature swing process of claim 1 wherein said gaseous mixture includes a flue gas.

22. The temperature swing process of claim 21 wherein said flue gas includes a flue gas of any of: an ammonia plant, a methanol plant, a urea plant, or a fossil fuel fired power plant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1 to 5 are block diagrams of temperature swing adsorption processes for removing the carbon dioxide from a flue gas, according to various embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiment of FIG. 1

(2) Referring to FIG. 1, the process of the invention is carried out in a plurality of reactors, for example including reactors 101, 102, 103. Each reactor 101-103 contains a fixed bed of an adsorbent for a target component, for example zeolite 13X for adsorption of CO.sub.2.

(3) Each reactor performs a number of steps, namely: a first adsorption step (a), a second adsorption step (a1), a desorption step (b), a purge step (b1), a conditioning step (c) and a condensation step. The reactors are interconnected and, during some of said process steps, a reactor may exchange one or more stream(s) with one or more other reactor(s). In FIG. 1, the blocks (a), (a1), (b), (b1), (c) denote the reactors 101, 102, 103 while performing said process steps.

(4) During the first adsorption step (a), a gas to be treated, for example a flue gas, is admitted to the reactor and the target component is adsorbed, resulting in a waste stream and partially loading the adsorbent with the target component. During the second adsorption step (a1), the adsorbent is contacted with a stream rich of the target component which is obtained by subjecting to condensation the effluent of another reactor performing the purge step (b1). As a consequence, a further amount of the target component is adsorbed and the one or more side components are expelled, thus generating another waste stream. During the desorption step (b), the adsorbent is heated by direct or indirect heat exchange, resulting in desorption of the target component and partial regeneration of the adsorbent. During the purge step (b1), the partially regenerated adsorbent is contacted by at least a portion of a waste stream (mainly containing the one or more side components) taken from another reactor while performing the first adsorption step (a). Step (c) is the conditioning step which brings the adsorbent back to the adsorption temperature in order to start again the cycle.

(5) Said steps and said interactions between the reactors will be described with a greater detail with reference to the working cycle of reactor 101.

(6) First Adsorption Step (a)

(7) A flue gas 111 coming from a combustion process and containing predominantly carbon dioxide (CO.sub.2), nitrogen (N.sub.2) and water (H.sub.2O) is supplied to the reactor 101, where CO.sub.2 and some water are adsorbed over the zeolite bed of the reactor, having a greater affinity with said adsorbent compared to nitrogen.

(8) As a result, step (a) provides a CO.sub.2-loaded adsorbent and a first waste stream 112 containing predominantly N.sub.2. A portion 113 of said waste stream 112 can be used in another reactor (for example in reactor 103) while performing the purge step (b1) taking place before the desorption step (b), as will be explained below. The remaining portion 114 of the waste stream 112 is exported and can be vented or used for a further scope if appropriate. For example in an ammonia plant, said stream 114, which is rich in nitrogen, can be used for the synthesis of ammonia.

(9) Preferably, the first adsorption step (a) takes place at ambient temperature, for example at a temperature in the range 15 to 30° C. Preferably said step (a) is carried out upflow, which means that the flue gas 111 is supplied from the bottom of the reactor 101 and the waste stream 112 leaves the reactor 101 from the top.

(10) Second Adsorption Step (a1)

(11) The reactor 101 receives a gaseous CO.sub.2-rich stream 128. Said stream 128 is obtained by subjecting the output stream 127 of another reactor (for example reactor 102) while performing the purge step (b1) to a condensation step in a dedicated condenser 104 for removal of water 129. Therefore, said CO.sub.2-rich stream 128 is deprived of water also and is referred to as water-depleted stream.

(12) Said water-depleted stream 128 is fed to the bottom of the reactor 101, meaning that step (a1) is carried out in the same flow direction as step (a).

(13) Said water-depleted stream 128 is optionally heated in an external heat exchanger 10 prior to admission to said reactor 101. For example said stream 126 is optionally heated to a temperature of 343 K (70° C.).

(14) During said step (a1), some of the carbon dioxide contained in the CO.sub.2-rich stream 128 is adsorbed over the adsorbent bed, which is already partially loaded with CO.sub.2 as a consequence of the first adsorption step (a); a second waste stream 115 mainly containing N.sub.2 is obtained, which is exported and can be vented or used in the process, similarly to the above mentioned stream 112.

(15) In some embodiments, the second adsorption step (a1) of reactor 101 and the purge step (b1) of the reactor 102 are synchronized, which means that the water-depleted stream 128 leaving the condenser 104 at the outlet of the reactor 102 passes into the reactor 101 without an intermediate storage. In other embodiments, said water-depleted stream 128 is stored in a suitable tank (not shown) and subsequently introduced into the reactor 101 for the above described step (a1). The latter embodiment with intermediate storage may provide a greater flexibility since the duration of steps (a1) and (b1) of the two reactors may be different.

(16) Desorption Step (b)

(17) The CO.sub.2-loaded adsorbent is heated, for example to 420 K (147° C.); as a consequence, CO.sub.2 is desorbed producing a current 116 of CO.sub.2 of a high purity and the adsorbent of the reactor 101 is partially regenerated.

(18) The desorption step (b) can be performed either by means of indirect heat exchange or direct heat exchange.

(19) In case of indirect heat exchange, preferably one of the reactor ends is kept open while the other is kept closed, meaning that it is a semi-open heating step.

(20) In case of direct heat exchange, a hot regeneration medium is supplied to the reactor for direct contact with the adsorbent. Preferably, both ends of the reactor 101 are kept open and said regeneration medium flows opposite with respect to steps (a) and (a1), namely from the top to the bottom. Preferably said regeneration medium is made predominantly of CO.sub.2 (i.e. of the target component).

(21) Purge Step (b1)

(22) The adsorbent in the reactor 101 is purged with a portion 133 of the waste stream 132 resulting from the first adsorption step (a) of another reactor (for example of reactor 103). Said waste stream 132 is similar in composition to the previously described stream 112 obtained from the reactor 101.

(23) Said portion 133 is preferably fed to the reactor 101 from the top, meaning that step (b1) is carried out in the opposite flow direction with respect to steps (a) and (a1).

(24) Said waste stream 133 is optionally cooled in an external heat exchanger 20 prior to admission into the reactor 101. For example the waste stream 133 is cooled to a temperature of 283 K (10° C.).

(25) During said step (b1), the waste stream 133 “cleans” the adsorbent by displacing at least a portion of the non-adsorbed CO2 from the reactor (and optionally desorbing a further portion of CO2), thus forming a CO.sub.2-rich stream 117, so that the recovery is increased. Said CO.sub.2-rich stream 117 can be used in the second adsorption step (a1) of another reactor, in the same manner as the CO.sub.2-rich stream 127 previously described.

(26) In some embodiments, the first adsorption step (a) of reactor 103, the condensation step and the purge step (b1) of reactor 101 are synchronized, so that the waste stream 133 leaving the reactor 103 passes into the reactor 101 without an intermediate storage. In other embodiments, a storage tank for said stream 133 is provided.

(27) Conditioning Step (c)

(28) The adsorbent is cooled down to the adsorption temperature in order to restart the cycle. Said conditioning step (c) can be carried out either at constant pressure, where one end of the reactor 101 is kept open and the other end is kept closed, or under slightly vacuum conditions, where both ends of the reactor 101 are closed.

(29) The other reactors, such as reactors 102 and 103, perform the same steps.

Embodiment of FIG. 2

(30) Referring to FIG. 2, the process of the invention is carried out in a plurality of reactors, for example including reactors 201, 202, 203. Each reactor 201-203 contains a fixed bed of an adsorbent for a target component, for example zeolite 13X for adsorption of CO2.

(31) Each reactor performs a number of steps, namely: an adsorption step (a), a first desorption step (a2), a second and main desorption step (b), a conditioning step (c) and a condensation step. In FIG. 2, the blocks (a), (a2), (b), (c) denote the reactors 201, 202, 203 while performing said process steps.

(32) During the adsorption step (a), a gas to be treated, for example a flue gas, is admitted to the reactor and the target component is preferentially adsorbed, resulting in a waste stream and loading the adsorbent with the target component. During the first desorption step (a2), the adsorbent is slightly heated in order to remove the one or more side components from the adsorbent, which also results in desorption of some of the target component. During the second (main) desorption step (b), the adsorbent is heated by direct or indirect heat exchange, resulting in desorption of the target component and regeneration of the adsorbent. During the conditioning step (c), the temperature of the adsorbent is lowered in order to start again the cycle with step (a).

(33) The above steps are now elucidated with reference to the reactor 201 and to a preferred embodiment.

(34) Adsorption Step (a)

(35) A combustion flue gas 211 predominantly containing carbon dioxide (CO2), nitrogen (N2) and water (H2O) is mixed with a gaseous product 232 predominantly containing N2 and a small amount of CO2, and the resulting mixture 240 is supplied to the reactor 201. Said gaseous product 232 is obtained by subjecting the output stream 230 of another reactor of the plurality (for example reactor 203) performing said first desorption step (a2) to a condensation step in a dedicated condenser 204 for removal of water 233. Said stream 238 is also referred to as water-depleted stream.

(36) During the adsorption step (a), CO2 and some water are adsorbed over the zeolite bed of the reactor 201 providing a CO2-loaded adsorbent, and a CO2-depleted effluent 212 predominantly containing N2 is exported which can be vented or used for a further scope if appropriate. For example, in an ammonia plant, said stream 212, which is rich in nitrogen, can be used for the synthesis of ammonia. A minor portion of N2 is also adsorbed over the zeolite bed, such portion being much smaller than the adsorbed CO2.

(37) In some embodiments, the adsorption step (a) of the reactor 201, the condensation step and the first desorption step (a2) of the reactor 203 are synchronized, which means that the gaseous product 232 from the condenser 204 passes into the reactor 201 without an intermediate storage. In other embodiments, said gaseous product 232 is stored in a suitable tank outside the reactor 203 and subsequently introduced into the reactor 201 undergoing step (a).

(38) First Desorption Step (a2)

(39) The CO2-loaded adsorbent contained in the reactor 201 is heated to a selected temperature lower than the temperature of the subsequent main desorption step (b). For example, the temperature reached by the adsorbent during said first desorption step (a2) is comprised between 360 and 380 K (87-107° C.).

(40) During said step (a2), some nitrogen, water and a small amount of CO2 are desorbed providing a gaseous product 220. During said step (a2), the pressure is kept constant and only the bottom end of the reactor is kept open.

(41) The so obtained gaseous product 220 is subsequently subjected to a condensation step in a dedicated condenser 205 providing a water-depleted stream 222 and water 223. Said stream 222 is then mixed with the flue gas feed of the reactor 202, in the same manner as the gaseous product 232 previously described, in order to recover the CO2 contained therein. For example, said gaseous product 222 is mixed with a flue gas 221 admitted to a second reactor 202, to form a mixture 250.

(42) In some embodiments, the gaseous product 222 can be subjected to adsorption step (a) in the same reactor 201. In a such a case, said gaseous product 222 is stored in a suitable tank (not shown) before being recycled to the reactor.

(43) Second (Main) Desorption Step (b) and Conditioning Step (c)

(44) The adsorbent still loaded with CO2 is heated, for example to 420 K (147° C.); as a consequence, the CO2 is desorbed producing a current 216 of CO2 of a high purity and the adsorbent of the reactor 201 is regenerated.

(45) The regenerated adsorbent is subsequently cooled down to the adsorption temperature, for example to the ambient temperature in order to restart the cycle.

Embodiment of FIG. 3

(46) Referring to FIG. 3, the process of the invention is carried out in a plurality of reactors, for example including reactors 301, 302, 303. Each reactor 301-303 contains a fixed bed of an adsorbent for a target component, for example zeolite 13X for adsorption of CO.sub.2.

(47) Each reactor performs a sequence of steps which is the same sequence as the first embodiment, with the addition of a further desorption step (a2) which is the same as the second embodiment. Said further desorption step (a2) is carried out after the second adsorption step (a1) and before the desorption step (b). For the sake of simplicity, said further desorption step (a2) and said desorption step (b) will be also referred to as first desorption step and second (main) desorption step (b), respectively.

(48) Since these steps are in common to the first and second embodiments, they are not described in detail for the sake of brevity.

(49) Combining steps (a1) and (b1) with a further desorption step (a2) gives rise to a synergy, allowing to obtain high recovery and high purity of step (a2) and low energy consumption of steps (a1) and (b1).

(50) Referring to a reactor 301, a gas mixture 311 containing predominantly carbon dioxide (CO.sub.2), nitrogen (N.sub.2) and water (H.sub.2O) is mixed with a gaseous product 322 predominantly containing N.sub.2 and a small amount of CO.sub.2 and also containing residual water, to provide a gaseous input stream 340. Said gaseous product 322 is obtained from a condensation step carried out on the effluent 320 of the first desorption step (a2). Said condensation step takes place in a condenser 305 and also separates water 323. Accordingly said gaseous product 322 contains less water than the effluent 320 and is also referred to as water-depleted stream.

(51) Said input stream 340 is supplied to the reactor 301 for the adsorption step (a) wherein a waste stream 312 is produced and the adsorbent is loaded with CO.sub.2 and some water. A portion 313 of the waste stream can be used in the purge step (b1) of another reactor and the remaining portion 314 is exported or vented.

(52) Then, the reactor 301 undergoes the second adsorption step (a1) with the help of a water-depleted stream 328, which is obtained by subjecting the output stream 327 of another reactor of the plurality (for example reactor 302) performing the purge step (b1) to a condensation step in a dedicated condenser 304 for removal of water 329. Said water-depleted stream is optionally subjected to heating in an exchanger 10′ before being supplied to the reactor 301.

(53) Then, the reactor 301 undergoes the first desorption step (a2), during which the CO.sub.2-loaded adsorbent contained in the reactor 301 is further heated. The temperature reached by the adsorbent during said step (a2) is lower than the temperature reached during the subsequent main desorption step (b). For example, the adsorbent is heated to a temperature ranging between 360 and 380 K (i.e. between 87 and 107° C.) during said step (a2).

(54) During said step (a2), the nitrogen, the water and a small amount of CO.sub.2 are desorbed providing the gaseous product 320. During said step (a2), only the bottom end of the reactor is kept open.

(55) Said gaseous product 320 is subjected to condensation in the condenser 305 providing the aforementioned water-depleted stream 322.

(56) In some embodiments, said water-depleted stream 322 is stored in a tank 30 and subsequently mixed with the flue gas 311 to provide the gaseous stream 340 feeding the reactor 301 undergoing the first adsorption step (a), in order to recover the CO.sub.2 contained therein. In other embodiments (not shown), said water-depleted stream 322 is mixed with the flue gas feed of another reactor, for example of reactor 302 or 303.

(57) After the first desorption step (a2), the reactor 201 undergoes the sequence of the second (main) desorption (b), purge (b1) and conditioning (c), which are equivalent to the same steps of the first embodiment. In particular, the purge step (b1) is carried out with the help of a waste stream 333 taken from another reactor, e.g. from reactor 303, optionally with intermediate cooling in a heat exchanger 20′. The main desorption step (b) releases a CO.sub.2 stream 316.

(58) The other reactors, such as reactors 302 and 303, perform the same steps.

Embodiment of FIG. 4

(59) Referring to FIG. 4, the process of the invention is carried out in a plurality of reactors, for example including reactors 401, 402, 403. Each reactor 401-403 contains a fixed bed of an adsorbent for a target component, for example zeolite 13X for adsorption of CO.sub.2.

(60) Each reactor performs a number of steps, which is the same sequence as the first embodiment, with the difference that reactors undergoing the purge step (b1) are supplied with the effluent waste stream of at least another reactor performing the second adsorption step (a1), the latter being fed with the water-depleted stream, thus forming a closed loop.

(61) Since these steps are in common to the first embodiments, they are not described in detail for the sake of brevity.

(62) During the first adsorption step (a), a wet flue gas 411 predominantly containing CO2, N2 and water is admitted to the reactor 401, wherein CO2 and some water are adsorbed, resulting in a first waste stream 412 and partially loading the adsorbent with CO2.

(63) During the second adsorption step (a1), the adsorbent is contacted with a CO2-rich stream 428. Said stream 428 is obtained by subjecting the output stream 427 of another reactor of the plurality (for example reactor 402) performing the purge step (b1) to a condensation step in a dedicated condenser 404 for removal of water 429, and optionally by subjecting the water-depleted stream 428 to a heat exchanger 10″. As a consequence, a further amount of CO2 is adsorbed and N2 is expelled, thus generating a second waste stream 415. Said second waste stream is recycled to said another reactor 402 while performing said step (b1), thus forming a closed loop between reactors 401 and 402.

(64) During the desorption step (b), the adsorbent is heated by direct or indirect heat exchange, resulting in desorption of CO2 as stream 416 and regeneration of the adsorbent.

(65) The purge step (b1) is made with the help of the second waste stream 435 taken from step (a1) of another reactor (for example of the reactor 403). The effluent of said step (b1) is a CO2-rich stream 417, which is supplied to a condenser 406 for removal of water 419 and the resulting water-depleted stream 418 is recycled to step (a1) of said another reactor via optional passage through heat exchanger 20″, thus forming a closed loop between reactors 401 and 403.

(66) The conditioning step (c) is made with the help of at least a portion 433 of the first waste stream 432 (mainly containing N2) taken from the adsorption step (a) of another reactor (for example from the reactor 403) and optionally passing through a heat exchanger 50. Said step (c) brings the adsorbent back to the adsorption temperature in order to start again the cycle with step (a).

Embodiment of FIG. 5

(67) Referring to FIG. 5, the process of the invention is carried out in a plurality of reactors, for example including reactors 501, 502, 503. Each reactor 501-503 contains a fixed bed of an adsorbent for a target component, for example zeolite 13X for adsorption of CO.sub.2.

(68) Each reactor performs a sequence of steps which is the same sequence as the third embodiment, with the difference that reactors undergoing the purge step (b1) are supplied with the effluent waste stream of at least another reactor performing the second adsorption step (a1) and the latter is fed with the water-depleted stream as in the fourth embodiment, thus forming a closed loop between two reactors of the plurality.

(69) Referring to a reactor 501, a gas mixture 511 containing predominantly CO.sub.2, N.sub.2 and water is mixed with a gaseous product 522 predominantly containing N.sub.2 and a small amount of CO.sub.2 and also containing residual water, to provide a gaseous input stream 540. Said gaseous product 522 is obtained from a condensation step carried out on the effluent 520 of the first desorption step (a2), which also separates condensed water 523. Said condensation step takes place in the condenser 505 and said gaseous product 522 is also referred to as water-depleted stream.

(70) Said input stream 540 is supplied to the reactor 501 for the adsorption step (a) wherein a waste stream 512 is produced and the adsorbent is loaded with CO.sub.2.

(71) Then, the reactor 501 undergoes the second adsorption step (a1) and the adsorbent is contacted with a CO2-rich stream 528. Said stream 528 is obtained by subjecting the output stream 527 of another reactor of the plurality (for example reactor 302) performing said purge step (b1) to a condensation step in a dedicated condenser 504 for removal of water 529, optionally with intermediate heating in the exchanger 10′″. As a consequence, a further amount of CO2 is adsorbed and N2 is expelled, thus generating a second waste stream 515. Said second waste stream 515 is recycled to said another reactor 502 while performing said step (b1), thus forming a closed loop between reactors 501 and 502.

(72) Then, the reactor 501 undergoes the first desorption step (a2), during which the CO.sub.2-loaded adsorbent contained in the reactor 501 is further heated. During said step (a2), the nitrogen, the water and a small amount of CO.sub.2 are desorbed providing the gaseous product 520. Said gaseous product 520 is subjected to condensation in the condenser 505 providing the aforementioned water-depleted stream 522.

(73) In some embodiments, said water-depleted stream 522 is stored in a tank 30 and subsequently mixed with the flue gas 511 to provide the gaseous stream 540 feeding the reactor 501 undergoing the adsorption step (a), in order to recover the CO.sub.2 contained therein. In other embodiments (not shown), said water-depleted stream 522 is mixed with the flue gas feed of another reactor, for example of reactor 502 or 503 (not shown).

(74) After the first desorption step (a2), the reactor 201 undergoes the sequence of the second (main) desorption (b), purge (b1) and conditioning (c), which are equivalent to the same steps of the forth embodiment.

(75) In particular, the purge step (b1) is made with the help of the second waste stream 535 taken from step (a1) of another reactor (for example of the reactor 503). The effluent of said step (b1) is a CO2-rich stream 517, which is supplied to a condenser 506 for removal of water 519 and the resulting water-depleted stream 518 is recycled to step (a1) of said another reactor via optional passage through heat exchanger 20′″.

(76) The conditioning step (c) is made with the help of at least a portion 533 of the first waste stream 532 (mainly containing N2) taken from the adsorption step (a) of another reactor (for example from the reactor 503) and optionally passing through a heat exchanger 50′. Said step (c) brings the adsorbent back to the adsorption temperature in order to start again the cycle with step (a).