Temperature-swing adsorption process

Abstract

A temperature swing adsorption (TSA) process for removing a target component from a gaseous mixture, where the process is carried out in a plurality of reactors. Each reactor performs the following steps: an adsorption step wherein an input stream of said gaseous mixture is contacted with a solid adsorbent selective for said target component, producing a first waste stream depleted of the target component; a heating step for regeneration of the loaded adsorbent providing a first output stream containing the target component; and a cooling step of the regenerated adsorbent.

Claims

1. A temperature swing adsorption process for removing a target component from a gaseous mixture containing at least one side component besides the target component, said process being carried out in a plurality of reactors, wherein each reactor performs the following steps: (a) an 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; (b) heating of said loaded adsorbent and desorption of a first amount of target component, providing a partially regenerated adsorbent and a first output stream containing the desorbed target component; and (c) cooling of said at least partially regenerated adsorbent, the process of each reactor being characterized by: a pre-cooling step before said cooling step, wherein said partially regenerated adsorbent is contacted with a waste stream depleted of the target component which is provided by at least one other reactor of said plurality of reactors while performing the adsorption step, wherein a second amount of target component is desorbed providing a second output stream containing the target component; and a pre-heating step after said adsorption step and before said heating step, wherein said loaded adsorbent is contacted with a rinse stream containing the target component which is provided by at least one other reactor of said plurality of reactors while performing the pre-cooling step, wherein an amount of target component contained in said rinse stream is adsorbed and a second waste stream depleted of the target component is produced, wherein a first reactor provides said waste stream for the pre-cooling step, and a second reactor provides said rinse stream for the pre-heating step.

2. The process according to claim 1, wherein said waste stream for the pre-cooling step is cooled prior to said pre-cooling step.

3. The process according to claim 1, wherein said rinse stream is heated prior to said pre-heating step.

4. The process according to claim 1, wherein the heating step comprises direct heat exchange with a heating medium in contact with the adsorbent.

5. The process according to claim 1, wherein the cooling step comprises direct heat exchange with a cooling medium in contact with the adsorbent.

6. The process according to claim 1, wherein the heating step and/or the cooling step comprises indirect heat exchange.

7. The process according to claim 6, wherein said heating step partially takes places during the pre-heating, and said cooling step partially takes places during the pre-cooling step.

8. The process according to claim 1, wherein said rinse stream is exchanged without an intermediate storage from said at least one other reactor undergoing the pre-cooling step to said reactor undergoing the adsorption step.

9. The process according to claim 1, wherein said rinse stream is exchanged with an intermediate storage in a suitable tank from said at least one other reactor undergoing the pre-cooling step to said reactor undergoing the adsorption step.

10. The process according to claim 1, each reactor of said plurality performing a preliminary heating step after said pre-heating step and before said heating step, wherein during said preliminary heating step a gaseous product containing said at least one side component is released from the adsorbent and is recycled to a reactor undergoing the adsorption step.

11. The process according to claim 10, wherein at least one of the following conditions applies: the time duration of the preliminary heating step is from 3 to 10 times the time duration of the pre-heating step; the time duration of the heating step is from 15 to 70 times the time duration of the pre-heating step; or the time duration of the cooling step is from 10 to 50 times the time duration of the pre-cooling step.

12. The process according to claim 10, wherein said preliminary heating step is carried out at a temperature lower than the temperature of the subsequent heating step.

13. The process according to claim 1, wherein said solid adsorbent is suitable to selectively adsorb said target component over said at least one side component and over water.

14. The process according to claim 1, wherein said solid adsorbent comprises at least a first layer of a first material suitable for selectively adsorbing water and a second layer of a second material suitable for selectively adsorbing the target component, and the adsorption step comprising a first step of adsorption of water contained in the gaseous mixture, through said first layer, and a second step of adsorption of the target component, through said second layer.

15. The process according to claim 1, wherein the temperature of said heating step is not greater than 250 C.

16. The process according to claim 1, wherein the target component is carbon dioxide.

17. Use of the process according to claim 1 for treating a flue gas of an ammonia or methanol or urea plant.

18. A plant for treating a gaseous mixture and removing a target component from said gaseous mixture with a process according to claim 1, the plant comprising a plurality of reactors, each reactor containing an adsorbent bed for selectively adsorbing said target component, wherein: each reactor operates by a sequence of steps comprising: adsorption of the target component in the adsorbent bed, pre-heating and subsequent heating of loaded adsorbent for desorption of the target component, pre-cooling and subsequent cooling of the so obtained regenerated adsorbent, and wherein the reactors are interconnected so that each reactor: during the pre-cooling step, receives a waste stream depleted of the target component which is provided by at least one other reactor of said plurality of reactors while said at least one other reactor is performing the adsorption step; and during the pre-heating step, receives a rinse stream containing the target component which is provided by at least one other reactor of said plurality of reactors while said at least one other reactor is performing the pre-cooling step.

19. The process according to claim 15, wherein the temperature of said heating step is not greater than 200 C.

20. The process according to claim 15, wherein the temperature of said heating step is not greater than 170 C.

21. The process according to claim 4, wherein said heating medium being a stream containing predominantly the target component.

22. The process according to claim 5, wherein said cooling medium being a target component depleted-waste stream.

23. The process according to claim 11, wherein the time duration of the preliminary heating step is six times the duration of said pre-heating step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram of a temperature swing adsorption process for removing the carbon dioxide from a flue gas, according to a first embodiment of the invention.

(2) FIG. 2 is a block diagram of a temperature swing adsorption process, according to a second embodiment of the invention.

(3) FIG. 3 is a purity vs recovery curve of a TSA process according to the prior art.

(4) FIG. 4 is a productivity vs energy consumption curve of a TSA process according to the prior art.

(5) FIG. 5 shows a purity vs recovery curve of a TSA process according to the embodiment of FIG. 1, in comparison with a curve of the prior art.

(6) FIG. 6 shows the energy consumption and productivity of one operating point of the TSA process according to the embodiment of FIG. 1, in comparison with the curve of the energy consumption and productivity of the prior art.

(7) FIG. 7 shows a purity vs recovery curve of a TSA process according to the embodiment of FIG. 2, in comparison with a curve of the prior art.

(8) FIG. 8 shows a productivity vs energy consumption curve of a TSA process according to the embodiment of FIG. 2, in comparison with a curve of the prior art.

(9) FIG. 9 shows the outlet temperature and composition profiles over time of one reactor undergoing all cycle steps of a TSA process according to the embodiment of FIG. 2.

(10) FIG. 10 shows the outlet temperature and composition profiles over time of one reactor undergoing all cycle steps of a TSA process according to the prior art.

(11) FIG. 11 shows a productivity vs energy consumption curve (curve 1) of a TSA process according to the embodiment of FIG. 2 comprising the sequence of steps (a), (a1), (a2), (b), (b1), (c), in comparison with the curve (curve 2) of a process comprising the sequence of steps (a), (a2), (b), (c).

(12) FIG. 12 shows a schedule for the operation of the process according to the second embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First Embodiment

(13) 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.

(14) Each reactor performs a number of steps, namely: an adsorption step (a), a pre-heating step (a1), a main heating step (b), a preliminary cooling step (b1) and a cooling step (c). 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.

(15) During 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.

(16) During the pre-heating step (a1), the adsorbent is slightly heated by direct contact with a stream rich of the target component which comes from the preliminary cooling step (b1) of another reactor. As a consequence, a further amount of the target component can be adsorbed and the one or more side components are expelled, thus generating another waste stream. During the main heating step (b), the adsorbent is heated by direct or indirect heat exchange, resulting in desorption of the target component and regeneration of the adsorbent. Step (b1) is a preliminary cooling which is made with the help of at least a portion of a waste stream (mainly containing the one or more side components) taken from the adsorption step (a) of another reactor. Step (c) is the main cooling which brings the adsorbent back to the adsorption temperature in order to start again the cycle with step (a).

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

(18) Adsorption Step (a)

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

(20) As a result, step (a) provides a CO.sub.2-loaded adsorbent and a CO.sub.2-depleted effluent 112, containing predominantly N.sub.2. A portion 117 of said effluent 112 can be used for the pre-cooling step (b1) of another reactor (for example of reactor 103), as will be explained below. The remaining portion 118 of the effluent 112 is exported and can be vented or used for a further scope if appropriate. For example in an ammonia plant, said stream 118, which is rich in nitrogen, can be used for the synthesis of ammonia.

(21) Preferably, the 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, being N.sub.2 lighter than CO.sub.2.

(22) Pre-Heating Step (a1)

(23) The reactor 101 receives a gaseous CO.sub.2-rich rinse stream 126 produced by another reactor of the plurality, for example by the reactor 102, while performing the pre-cooling step (b1). Said rinse stream 126 is fed to the bottom of the reactor 101, meaning that step (a1) is carried out in the same upflow direction as step (a).

(24) The rinse stream 126 is optionally heated in an external heat exchanger 20 prior to admission to said reactor 101. For example the rinse stream 126 is heated to a temperature of 343 K (70 C.).

(25) During said pre-heating step (a1), some of the carbon dioxide contained in the rinse stream 126 is adsorbed over the adsorbent bed, which is already partially loaded with CO.sub.2 as a consequence of the previous adsorption step (a); a second waste stream 113 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 118.

(26) In some embodiments, the pre-heating step (a1) of reactor 101 and the pre-cooling step (b1) of the reactor 102 are synchronized, which means that the rinse stream 126 leaving the reactor 102 passes into the reactor 101 without an intermediate storage. In other embodiments, said CO.sub.2-rich gas 126, produced by the pre-cooling step (b1) of reactor 102, is stored in a suitable tank (not shown) outside the source reactor 102 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.

(27) Main Heating Step (b)

(28) 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 115 of CO.sub.2 of a high purity and the adsorbent of the reactor 101 is regenerated.

(29) The main heating step (b) can be performed either by means of indirect heat exchange or direct heat exchange.

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

(31) 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).

(32) Preliminary Cooling Step (b1)

(33) The adsorbent in the reactor 101 is purged with a pre-cooling waste stream 137 which results from the main adsorption step (a) of another reactor, for example of reactor 103. Said stream 137 is similar in composition to the previously described stream 117 obtained in the reactor 101 itself.

(34) Said waste stream 137 is 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).

(35) Said waste stream 137 is optionally cooled in an external heat exchanger 30 prior to admission into the reactor 101. For example the waste stream 137 is cooled to a temperature of 283 K (10 C.).

(36) During said pre-cooling step (b1), the pre-cooling stream 137 cleans the adsorbent by displacing a CO.sub.2-rich stream 116, so that more CO.sub.2 can be adsorbed during the adsorption step (a) and the recovery is increased. Said CO.sub.2-rich stream 116 can be used for the pre-heating step (a1) of another reactor, in the same manner as the CO.sub.2-rich stream 126 previously described.

(37) In some embodiments, the adsorption step (a) of reactor 103 and the preliminary cooling step (b1) of reactor 101 are synchronized, so that the waste stream 137 leaving the reactor 103 passes into the reactor 101 without an intermediate storage. In other embodiments, a storage tank for said stream 137 is provided.

(38) Cooling Step (c)

(39) The adsorbent is cooled down to the adsorption temperature in order to restart the cycle. Said cooling 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.

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

Second Embodiment

(41) 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 CO.sub.2.

(42) Each reactor performs a sequence of steps which is the same sequence as the first embodiment, with the addition of a preliminary heating step (a2), after the pre-heating step (a1) and before the main heating step (b). The steps common to the first embodiment are not described in detail for the sake of brevity.

(43) Combining steps (a1) and (b1) with a further pre-heating step (a2) gives rise to a synergy, which allows obtain the high recovery and purity of step (a2) and the low energy consumption of steps (a1) and (b1).

(44) Referring to a reactor 201, a gas mixture 211 containing predominantly carbon dioxide (CO.sub.2) and nitrogen (N.sub.2) is mixed with a gaseous product 219 predominantly containing N.sub.2 and a small amount of CO.sub.2, obtained from said preliminary heating step (a2), to provide a gaseous input stream 214.

(45) Said input stream 214 is supplied to the reactor 201 for the adsorption step (a) wherein a waste stream 212 is produced and the adsorbent is loaded with CO.sub.2. A portion 217 of the waste stream can be used for pre-cooling of another reactor and the remaining portion 218 is exported or vented.

(46) Then, the reactor 201 undergoes the pre-heating step (a1) with the help of a rinse stream 226 from the reactor 202 undergoing the preliminary cooling step (b1), optionally with intermediate heating in the exchanger 20.

(47) Then, the reactor 201 undergoes the preliminary heating step (a2), during which the CO.sub.2-loaded adsorbent contained in the reactor 201 is further heated. The temperature reached by the adsorbent during said preliminary heating step (a2) is lower than the temperature reached during the subsequent main heating 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 preliminary heating step (a2).

(48) During said step (a2), the nitrogen and a small amount of CO.sub.2 are desorbed providing the gaseous product 219. During said step (a2), the pressure is kept constant and only the bottom end of the reactor is kept open.

(49) In some embodiments, the so obtained gaseous product 219 is stored in a tank 40 and subsequently mixed with the flue gas 211 to provide the gaseous stream 214 feeding the adsorption step (a), in order to recover the CO.sub.2 contained therein. In other embodiments (not shown), said gaseous product 219 is mixed with the flue gas feed of another reactor, for example of reactor 202 or 203.

(50) After the preliminary heating step (a2), the reactor 201 undergoes the sequence of main heating (b), pre-cooling (b1) and cooling (c), which are equivalent to the same steps of the first embodiment. In particular, the pre-cooling step (b1) is carried out with the help of a pre-cooling waste stream 237 taken from another reactor, e.g. from reactor 203, optionally with intermediate cooling in a heat exchanger 30. The main heating (b) releases a stream 215 of the target component, in this case of CO.sub.2.

(51) The other reactors, such as reactors 202 and 203, perform the same steps.

COMPARATIVE EXAMPLES

Example 1: Prior Art

(52) A combustion flue gas containing carbon dioxide, nitrogen, argon and saturated with water is subjected to a dehydration process to remove water. The energy consumption of the dehydration process is 8 MJ/(kg H.sub.2O removed).

(53) A flue gas with the following dry molar composition:
CO.sub.2=0.12,N.sub.2=0.85,Ar+O.sub.2=0.03
is obtained and subjected to adsorption (a) at a temperature of 300 K (27 C.) and 1.3 bar abs in a reactor over a commercial zeolite 13X as adsorbent, in order to separate the carbon dioxide (i.e. the target component) from nitrogen, argon and oxygen (i.e. the side components).

(54) Said process provides an adsorbent partially loaded with CO.sub.2 and a waste stream mainly containing nitrogen. The partially loaded adsorbent is subjected to a heating step (b), wherein it is heated to 420 K (147 C.) and a CO.sub.2-rich stream is collected as target product. At the end of the heating step, the adsorbent is subjected to a cooling step (c), wherein it is cooled down to the adsorption temperature of 300K (27 C.). Said steps of heating and cooling occur by indirect heat exchange with external fluid streams.

(55) By varying the time duration of the above three steps of adsorption (a), heating (b) and cooling (c), the curves of FIGS. 3 and 4 have been identified with computer simulation.

(56) The curve of FIG. 3 delimits the maximum feasible CO.sub.2 purity and CO.sub.2 recovery in a two dimensional plot of purity vs recovery. As shown in FIG. 3, the maximum feasible purity is 88% and is achievable only for a recovery of less than 30%. On the other hand, the maximum feasible recovery is 90% and is achievable only with a purity of less than 80%.

(57) The curve of FIG. 4 delimits the energy requirement and the productivity of said process in a two dimensional plot of productivity vs specific energy consumption. The productivity is indicated as a number between 0 and 100 of a reference value and only the data points with purity higher than 85% and recovery of 50% have been considered to determine said curve.

(58) As shown in FIG. 4, the thermal energy required for the process is in the range 6.5 to 7.5 MJ/kgCO.sub.2 and the productivity is in the range 30% to 60% of the reference value.

(59) The productivity is negatively impacted by the time duration of the heating and cooling steps; in particular, any time that is not used to adsorb CO.sub.2 from the feed or desorb it into the product is wasted. Moreover, the thermal inertia of the equipment (e.g. the adsorbent, the adsorber pressure vessel, the heat transfer elements) affect negatively the time duration of the process.

Example 2: First Embodiment of the Invention

(60) A combustion flue gas with the same composition of the gas of the previous example is subjected to dehydration and subsequently to adsorption (a) inside a first reactor.

(61) The adsorption provides an adsorbent partially loaded with CO.sub.2 and a waste stream mainly containing nitrogen. During the subsequent pre-heating step (a1), the loaded adsorbent is contacted with a CO.sub.2-rich rinse stream provided by a second reactor undergoing a pre-cooling step (b1) and heated to 343 K (70 C.) in an external heat exchanger. The CO.sub.2 contained in said rinse stream is adsorbed and a further waste stream containing nitrogen is released.

(62) The resulting loaded adsorbent is subjected to the main heating step (b), wherein it is heated to 420 K (147 C.) and a CO.sub.2-rich stream is collected as target product. At the end of the heating step, the adsorbent undergoes the pre-cooling (b1), during which it is purged with a portion of a waste stream containing nitrogen provided by a third reactor undergoing the adsorption step (a) and cooled down to 283 K (10 C.) in an external cooler. The adsorbent is finally cooled down to the adsorption temperature of 300 K (27 C.) by indirect cooling (c).

(63) FIG. 5 shows the curves delimiting the feasible CO.sub.2 purity and CO.sub.2 recovery for the process of the invention and the process of the prior art.

(64) The new process largely outperforms the process of the prior art, according to which more than 90% of the CO.sub.2 can be recovered with a purity higher than 90%. Moreover, the much improved separation performance does not entail either a higher energy requirement or lower productivity, as evident from FIG. 6.

(65) FIG. 6 shows a point delimiting the energy requirement and the productivity of the process of the invention, compared with a corresponding curve of the prior art. Such point has been determined by considering the data points with purity of 90% and recovery of 50%.

(66) Despite the higher purity and recovery achieved compared to the prior art, the new TSA process has a much lower energy consumption (i.e. 70% of the prior art) and a much higher productivity (i.e. 70% instead of 30-60%).

(67) Hence, the process of the invention has higher recovery and purity, lower consumption, higher productivity and lower capital cost.

Example 3: Second Embodiment of the Invention

(68) As for example 2, a dried flue gas is subjected to an adsorption step (a) inside a first reactor.

(69) The adsorption provides an adsorbent partially loaded with CO.sub.2 and a waste stream mainly containing nitrogen. During the subsequent pre-heating step (a1), the loaded adsorbent is contacted with a CO.sub.2-rich rinse stream provided by a second reactor undergoing a pre-cooling step (b1) and heated to 343 K (70 C.) in an external cooler. The CO.sub.2 contained in said rinse stream is adsorbed and a further waste stream containing nitrogen is released.

(70) At the end of this step, the adsorbent undergoes another pre-heating step (a2), during which it is heated to a temperature of 360 K to 380 K (87-107 C.), providing a gaseous product manly containing nitrogen and some carbon dioxide. Said gaseous product is collected in a tank and subsequently mixed with the flue gas feed.

(71) The resulting adsorbent is subjected to the main heating step (b), wherein it is heated to 420 K (147 C.) and a CO.sub.2-rich stream is collected as target product. At the end of the heating step, the adsorbent undergoes the pre-cooling (b1), during which it is purged with a portion of a waste stream containing nitrogen provided by a third reactor undergoing the adsorption step (a) and cooled down to 283 K (10 C.) in an external cooler. The adsorbent is finally cooled down to the adsorption temperature by indirect cooling (c).

(72) FIG. 7 shows the curves delimiting the feasible purity and recovery for said process of the invention and the process of the prior art.

(73) The new process largely outperforms the process of the prior art, according to which more than 95% of the CO.sub.2 can be recovered at a purity higher than 95%. Moreover, the much improved separation performance does not entail either a higher energy requirement or lower productivity, as evident from FIG. 8.

(74) As shown in FIG. 8, the second new TSA process has much lower energy consumption and similar productivity than the prior art even at very much higher separation specifications. The curve calculated for the new process has an energy requirement in the range of 5-6 MJ/kg of CO2 and a productivity in the range of 30-60%.

(75) FIGS. 9 and 10 show the outlet temperature and composition profiles of one reactor undergoing all cycle steps of the process according to the second embodiment of the invention (i.e. example 3, FIG. 2) and the process according to the prior art (i.e. example 1) under exemplary operating conditions, respectively.

(76) Three simulations (Runs 1-3) have been carried out for the process according to the invention using different times for the steps a, b, c and one simulation (Run 4) has been carried out for the process according to the prior art. The applied times are reported in Table 1.

(77) In FIG. 9 the outlet composition and temperature profiles are represented with solid line (Run 1), dashed line (Run 2) and dash-dotted (Run 3). A scaled time axis is used so as, though the duration of the same step in the different runs is different, the same interval along the horizontal coordinate is used for all the cases.

(78) The resulting performances of the above processes are also reported in Table 1, with respect to the following parameters: CO.sub.2 purity (), CO.sub.2 recovery (r.sub.TSA) and energy consumption (e.sub.TSA).

(79) The CO.sub.2 purity is calculated as the average composition of the CO.sub.2 product:

(80) $=\frac{n_{{\mathrm{CO}}_2}^P}{n_{{\mathrm{CO}}_2}^P+n_{N_2}^P}$
where n.sub.CO2.sup.P is the amount of CO.sub.2 produced from one reactor during one cycle and collected in the target product, and n.sub.N2.sup.P is the amount of N.sub.2 collected in the same target product.

(81) The CO.sub.2 recovery r.sub.TSA is the fraction of CO.sub.2 recovered in the target product:

(82) $r_{\mathrm{TSA}}=\frac{n_{{\mathrm{CO}}_2}^P}{n_{{\mathrm{CO}}_2}^F}$
where n.sub.CO2.sup.F is the amount of CO.sub.2 fed to the process during one cycle and is the fraction of flue gas processed in the TSA.

(83) The specific thermal energy consumption e.sub.TSA of the TSA unit is determined by integrating the heat flow into one reactor (Q.sub.in) during one cycle:

(84) $e_{\mathrm{TSA}}=\frac{1}{n_{{\mathrm{CO}}_2}^P}{}_0^{t_{\mathrm{cycle}}}\max \left(0,{\stackrel{.}{Q}}_{\mathrm{in}}\right)\mathrm{dt}$

(85) TABLE-US-00001 TABLE 1 Second Embodiment Prior Art Run 1 Run 2 Run 3 Run 4 t.sub.step a2 [s] 150 150 150 t.sub.step b [s] 1200 1200 800 2100 t.sub.step b1 [s] 25 25 25 t.sub.step c [s] 850 850 600 1450 t.sub.step a [s] 300 400 250 300 t.sub.step a1 [s] 25 25 25 [] 0.968 0.974 0.962 0.833 r.sub.TSA [] 0.901 0.770 0.875 0.802 e.sub.TSA [MJ/kgCO.sub.2] 4.39 4.07 5.00 5.73

(86) The simulations of the process according to the invention have been carried out for the optimal time duration of steps a, a1, a2, b, b1 and c based on the following reasons.

(87) The absolute values of the optimal time duration of each step may vary according to the geometry of the reactor, in particular according to the ratio of heat exchange surface over volume of the adsorbent. However, the applicant has found that the following relative values are substantially constant even for different geometries: time duration of step (a2) over time duration of step (b); time duration of step (c) over time duration of step (b); time duration of step (a1) over time duration of step (a); time duration of step (b1) over time duration of step (a).

(88) Although longer times of the pre-cooling step (b1) are beneficial to the recovery since more CO.sub.2 is desorbed prior to the adsorption step (a), they are not beneficial to the CO.sub.2 purity. In fact, an increase in purity is only achieved if the average CO.sub.2 content of the rinse stream is larger than that of the feed and this is the case for short times of pre-cooling (b1); on the other hand, in the case of long pre-cooling times, the nitrogen contained in the waste stream leaves the reactor diluting the rinse stream.

(89) Moreover, the heat effects arising during said steps of pre-cooling (b1) and pre-heating (a1) are beneficial to both energy consumption and productivity. In fact, the desorption of CO.sub.2 happening during the pre-cooling step (b1) cools down the bed, thus requiring a shorter cooling step (c), while the heat released during the pre-heating step (a1) due to CO.sub.2 adsorption contributes to heat the bed before the actual heating step (b). This is indeed observed in the temperature profiles shown in FIG. 9.

(90) As shown in table 1, the CO.sub.2 purity () obtained with the process of the invention is much greater than the purity obtained with the process of the prior art, namely 0.968 (Runs 1), 0.974 (Run 2) and 0.962 (Run 3) against 0.833 (Run 4). An increased purity is obtained in the process of the invention thanks to the presence of the pre-heating step (a1), during which the loaded adsorbent is enriched with the CO.sub.2 contained in the rinse stream and a further N.sub.2-containing stream is released. As a result of this, the overall recovery of CO.sub.2 is also increased.

(91) Moreover, the CO.sub.2 purity of the process of the invention is able to approach 100% because during the preliminary heating step (a2) the resulting N2-containing product is not collected in the target product but submitted to a further adsorption step (a).

(92) The applicant has surprisingly found that the implementation of the preliminary heating step (a2) after said pre-heating step (a1) and before said heating step (b) has a significantly beneficial effect on the purity of the CO.sub.2 product without compromising the productivity and energy consumption. This effect was completely unexpected.

(93) Indeed, a skilled person desirous to recover more than 95% of CO.sub.2 with a purity higher than 95%, while maintaining low energy consumption and achieving high productivity, would have never considered to implement the preliminary heating step (a2) in a process according to FIG. 1, thus obtaining a process according to FIG. 2. This is justified by the following considerations.

(94) A process comprising the sequence of steps (a), (a1), (b), (b1), (c) as in FIG. 1 allows obtaining high CO.sub.2 purity, which is however not unitary especially for high CO.sub.2 recovery (as evident from FIG. 5). On the other hand, a process comprising the sequence of steps (a), (a2), (b), (c) would be characterized by high energy consumptions and low productivity, because the step (a2) of preliminary heating consumes energy and time and does not produce CO.sub.2.

(95) As a consequence, a skilled person would have not contemplated to introduce the step (a2) of preliminary heating in the process of FIG. 1, providing the process of FIG. 2, in order to further increase the CO.sub.2 purity especially at high CO.sub.2 recovery, while achieving low energy consumption and high productivity.

(96) As clearly visible from FIG. 11, the process according to FIG. 2 has much lower energy consumption and much higher productivity than a process comprising a preliminary heating step (a2), but not pre-heating (a1) and pre-cooling (b1) steps. The curves shown in FIG. 11 have been determined by considering the data points with purity equal to or higher than 96% and recovery equal to or higher than 90%.

(97) The purity is also related to the time of the adsorption step (a). As it increases, the transition front through which CO.sub.2 is adsorbed approaches the end of the reactor; the purity will increase because the reactor is increasingly loaded with CO.sub.2 until the front eventually breaks through, at which point the recovery decreases rapidly. The increase in CO.sub.2 loading within the reactor at the end of the adsorption step will also result in a lower specific energy consumption because the provided heat is more efficiently used (a larger fraction is consumed for the actual desorption of CO.sub.2).

(98) The above effect can be seen when comparing the outlet composition profile according to Runs 1 and 2 (FIG. 9), where the CO.sub.2 front clearly breaks through earlier the reactor to a greater extent in the case of Run 2, whose adsorption step is longer.

(99) However, the lower CO.sub.2 recovery achieved in Run 2 is compensated by a higher purity and much lower energy consumption.

(100) The increased CO.sub.2 recovery achieved in Runs 1 and 3 is obtained by purging the reactor with the N.sub.2-containing waste stream during the pre-cooling step (b1), wherein a further amount of CO.sub.2 is desorbed meaning that less CO.sub.2 is lost during the adsorption step (a).

(101) FIG. 12 shows a schedule for the operation of the process according to the second embodiment of the invention with the operating conditions corresponding to those of Run 1 (Table 1).

(102) In order to ensure a continuous production and synchronous pre-heating (a1) and pre-cooling (b1) steps, an idle time of 150 seconds, splitted in two intervals of 130 seconds (before pre-cooling) and 20 seconds (before pre-heating), and 9 reactors are required, which are operated shifted in time by 300 s.

(103) It is worth noting that there is a minimum number of reactors required to ensure given scheduling constraints, and that in general increasing the number of reactors decreases the idle times and vice versa.