METHOD AND SYSTEM FOR SEPARATING CO2 BASED ON CHEMICAL ABSORPTION

Abstract

The present invention relates to a method and system for separating CO.sub.2 based on regenerative chemical absorption, including an absorber where the CO.sub.2 remains retained in an absorbent liquid, and including a regenerator where the CO.sub.2 is released, obtaining a regenerated absorbent that is re-used in the absorption unit. The invention proposes a configuration of the entire capturing process which allows an efficient operation that significantly reduces the energy requirements mainly associated with the regeneration of the absorbent, as well as a lesser thermal degradation of same.

Claims

1. A method for separating CO.sub.2 from a gas stream based on chemical absorption, comprising the following steps: a) absorbing the CO.sub.2 coming from the gas stream to be treated at a temperature of between 40 C. and 60 C. and a pressure in a range of between 1 and 1.5 bar, by means of putting said stream in contact in an absorber with an absorbent solution in which the CO.sub.2 will be retained; b) recirculating into the absorber up to 75% of the stream comprising the CO.sub.2-rich absorbent solution coming from step a); c) desorbing CO.sub.2 in a regenerator from the stream comprising the CO.sub.2-rich absorbent solution coming from step a) not recirculated to step b) at a temperature of between 80 C. and 120 C. a pressure of between 1.5 and 5 bar and a steam stripping flow rate of between 10 and 90% by volume with respect to the desorbed CO.sub.2 flow rate, where said stream is split into at least two streams by means of a set of heat exchangers, prior to the inlet of the regenerator; d) recovering the absorbent solution resulting from step c) from the absorber of step a).

2. The method of claim 1, wherein the CO.sub.2 of the gas stream to be treated in step a) is transferred to the liquid phase where it is dissolved and chemically bonds to the absorbent.

3. The method of claim 1, wherein the recirculated flow rate of step b) reaches between 25% and 75% of the total of the CO.sub.2-rich solution coming from step a).

4. The method of claim 1, wherein the recirculation of the stream coming from step b) takes place in the lower bed of the absorber of step a).

5. The method of claim 1, wherein the streams coming from step c) are introduced in areas located at different heights of the regenerator of step d).

6. A system for carrying out a method for separating CO.sub.2 from a gas stream based on chemical absorption, comprising the following steps: a) absorbing the CO.sub.2 coming from the gas stream to be treated at a temperature of between 40 C. and 60 C. and a pressure in a range of between 1 and 1.5 bar, by means of putting said stream in contact in an absorber with an absorbent solution in which the CO.sub.2 will be retained; b) recirculating into the absorber up to 75% of the stream comprising the CO.sub.2-rich absorbent solution coming from step a); c) desorbing CO.sub.2 in a regenerator from the stream comprising the CO.sub.2-rich absorbent solution coming from step a) not recirculated to step b) at a temperature of between 80 C. and 120 C. a pressure of between 1.5 and 5 bar and a steam stripping flow rate of between 10 and 90% by volume with respect to the desorbed CO.sub.2 flow rate, where said stream is split into at least two streams by means of a set of heat exchangers, prior to the inlet of the regenerator; d) recovering the absorbent solution resulting from step c) from the absorber of step a), wherein the system comprises: an absorber comprising a packed column and a lower bed, which has: an inlet receiving the gas stream to be treated which will come into contact in the absorber with an absorbent liquid which is used for retaining the CO.sub.2 from the gas to be treated, an outlet for a stream of CO.sub.2-rich absorbent solution, an inlet for an inlet stream of regenerated absorbent solution, an inlet for a stream of recirculated CO.sub.2-rich absorbent solution, and an outlet through which the clean gas free of CO.sub.2 is discharged, a regenerator which receives a main inlet stream into CO.sub.2-rich absorbent regenerator, from which there departs an outlet stream of poor regenerated absorbent, and an outlet stream mainly made up of CO.sub.2 and water vapor, and comprising a boiler which generates the energy necessary for regenerating the absorbent, a set of heat exchangers located between the absorber and the regenerator which receives the CO.sub.2-rich absorbent solution, as well as a poor regenerated absorbent solution coming from the regenerator, and from which there exits an inlet stream of regenerated absorbent solution directed to the absorber, and from which there exits a stream of CO.sub.2-rich absorbent which is directed to the regenerator, characterized in that the absorber additionally comprises an inlet for a recirculated CO.sub.2-rich absorbent solution recirculation line, which is conducted back to the lower bed of the absorber for the purpose of increasing the load thereof by means of a first heat exchanger which lowers its temperature.

7. The system of claim 6, wherein within the set of heat exchangers the stream of CO.sub.2-rich absorbent solution is split into a primary stream and a secondary stream, wherein both are directed to the upper part and to the intermediate bed of the regenerator, respectively.

8. The system of claim 7, wherein the distribution of the stream of CO.sub.2-rich absorbent solution between the primary stream and the secondary stream is established in the range of between 0.25 and 0.75.

9. The system of claim 7, further comprising a second indirect contact exchanger in which the primary stream is preheated using the outlet stream from the regenerator, giving rise to the main inlet stream into the regenerator.

10. The system of claim 7, wherein the secondary stream is split into an additional stream in order to be fed in at different heights of the regenerator.

11. The system of claim 7, wherein the set of heat exchangers comprises internal heat exchangers which heat the primary stream, and the secondary stream is obtained from inner streams which are removed at the outlet of each of the internal exchangers, and in that the stream of poor regenerated absorbent solution can in turn be split into different substreams which enter each of the internal heat exchangers.

12. The method of claim 2, wherein the recirculated flow rate of step b) reaches between 25% and 75% of the total of the CO.sub.2-rich solution coming from step a).

13. The method of claim 2, wherein the recirculation of the stream coming from step b) takes place in the lower bed of the absorber of step a).

14. The method of claim 3, wherein the recirculation of the stream coming from step b) takes place in the lower bed of the absorber of step a).

15. The method of claim 2, wherein the streams coming from step c) are introduced in areas located at different heights of the regenerator of step d).

16. The method of claim 3, wherein the streams coming from step c) are introduced in areas located at different heights of the regenerator of step d).

17. The method of claim 4, wherein the streams coming from step c) are introduced in areas located at different heights of the regenerator of step d).

Description

DESCRIPTION OF THE DRAWINGS

[0023] To complement the description that is being made and for the purpose of helping to better understand the features of the invention, according to a preferred practical embodiment thereof, a set of drawings is attached as an integral part of said description, wherein the following has been depicted with an illustrative and non-limiting character:

[0024] FIG. 1.Shows a diagram of the CO.sub.2 absorption-desorption system of the invention.

[0025] FIG. 2.Shows a detail of the set of heat exchangers.

[0026] FIG. 3. Shows a graph depicting the enthalpy of CO.sub.2 solubility depending on the load of the absorbent expressed in moles of CO.sub.2 per mole of absorbent (generic absorbent). The cyclic operating capacity for a conventional configuration and a configuration according to the system of the invention are indicated in a generic manner.

PREFERRED EMBODIMENT OF THE INVENTION

[0027] A preferred embodiment of the system object of this invention is described below.

[0028] Specifically, the absorption-desorption system of CO.sub.2 including the elements described below has been depicted in FIG. 1: [0029] 1.Gas stream to be treated [0030] 2.Absorber [0031] 3.Clean gas [0032] 4.CO.sub.2-rich outlet absorbent solution [0033] 5.First impeller pump [0034] 6.CO.sub.2-rich absorbent solution [0035] 7.Recirculated CO.sub.2-rich absorbent solution [0036] 8.A.First heat exchanger [0037] 8.B.Second heat exchanger [0038] 9.Set of exchangers [0039] 10.Primary stream [0040] 11.Heat exchanger for harnessing energy from the gas stream at the outlet of the regenerator [0041] 12.Main inlet stream into the CO.sub.2-rich absorbent solution regenerator [0042] 13.Secondary stream [0043] 14.Alternative inlet streams into the CO.sub.2-rich absorbent solution regenerator [0044] 15.Regenerator [0045] 16.Gas outlet stream from the regenerator [0046] 17.Condensate separator [0047] 18.Gas stream having a high concentration of CO.sub.2 [0048] 19.Condensate stream [0049] 20.Drum [0050] 21.Stripped regenerated absorbent solution [0051] 22.Second impeller pump [0052] 23.Inlet stream of regenerated absorbent solution

[0053] As can be observed in FIG. 1, the system incorporates an absorber (2) comprising a packing column which may be both structured and non-structured, and a lower bed, which receives the gas stream to be treated (1) which will come into contact in the absorber (2) with an absorbent liquid which is used for retaining CO.sub.2 of the gas to be treated (1). The absorber (2) incorporates a CO.sub.2 (4)-rich outlet absorbent solution, an inlet for the inlet stream of regenerated absorbent solution (23), a stream of recirculated CO.sub.2-rich absorbent solution (7) and an outlet through which the clean gas (3) free of CO.sub.2 is discharged.

[0054] The inlet stream of regenerated absorbent solution (23) coming from the regenerator (15) is at a temperature which has been adjusted to values close to that of the gas stream to be treated (1) by means of using a second heat exchanger (8B).

[0055] On the other hand, the absorber (2) incorporates an inlet for recirculated CO.sub.2-rich absorbent solution recirculation line (7), which is conducted back to the lower bed of the absorber (2) for the purpose of increasing the load thereof by means of a first heat exchanger (8A) which lowers its temperature.

[0056] In a preferred embodiment, the design of the absorber (2) requires an increase in section in the lower bed with respect to the rest of the column, as shown in FIG. 1.

[0057] It can also be seen in FIG. 1 that the CO.sub.2-rich outlet absorbent solution (4) is removed from the absorber (2) at the lower part thereof and impelled by means of a first impeller pump (5) which impels the CO.sub.2-rich outlet absorbent solution (4) to then be separated into the recirculated CO.sub.2-rich absorbent solution (7) and into a CO.sub.2-rich absorbent solution (6), which is previously introduced in the set of heat exchangers (9).

[0058] The set of heat exchangers (9) receives the mentioned CO.sub.2-rich absorbent solution (6), where the temperature of this stream is adjusted in an optimized manner before being split and directed to the regenerator (15), and it receives a stripped regenerated absorbent solution (21) coming from the regenerator (15), and the inlet stream of regenerated absorbent solution (23) exits the set of heat exchangers (9), directed to the absorber (2), and a primary stream (10) and a secondary stream (13) also exit as a consequence of the mentioned splitting of the CO.sub.2-rich absorbent solution (6).

[0059] The set of heat exchangers (9) comprising the following elements can be seen in FIG. 2: [0060] 6.CO.sub.2-rich absorbent solution [0061] 9.Set of exchangers [0062] 10.Primary stream [0063] 13.Secondary stream [0064] 13.A.Alternative for removing from the first exchanger the secondary inlet stream into the CO.sub.2-rich absorbent solution regenerator [0065] 13.B.Alternative for removing from the second exchanger the secondary inlet stream into the CO.sub.2-rich absorbent solution regenerator [0066] 13.C.Alternative for removing from successive exchangers the secondary inlet stream into the CO.sub.2-rich absorbent solution regenerator [0067] 21.Stripped regenerated absorbent solution [0068] 21.A.Alternative for feeding regenerated absorbent solution to successive exchangers in the set of exchangers [0069] 21.B.Alternative for feeding regenerated absorbent solution to the second exchanger in the set of exchangers [0070] 21.C.Alternative for feeding regenerated absorbent solution to the first exchanger in the set of exchangers [0071] 23.Regenerated solution inlet stream into the absorber [0072] 24.First internal exchanger of the set of exchangers [0073] 25.Second internal exchanger of the set of exchangers [0074] 26.Successive exchangers of the set of exchangers

[0075] The set of exchangers (9) depicted in FIG. 2 comprises a series of N internal heat exchangers (24, 25, 26), preferably between 2 to 4 heat exchangers, where the CO.sub.2-rich absorbent solution (6) is heated at different levels by means of the use of the stripped regenerated absorbent solution (21) coming from the bottom of the regenerator (15). The stream of -rich absorbent solution CO.sub.2 (6) is split into two main streams. The primary stream (10) is heated by means of the use of all the internal heat exchangers (24, 25, 26), whereas the secondary stream (13) can be removed at the outlet of each of the internal exchangers, giving rise to inner streams (13A, 13B, 13C). The stream of stripped regenerated absorbent solution (21) can in turn be split into different substreams, referred to as (21A, 21B, 21C), to achieve a more precise adjustment of the thermal level of the primary stream of the rich solution (10) and, therefore, of the temperature profile of the regenerator (15).

[0076] The distribution of the CO.sub.2-rich absorbent solution (6) between the primary stream (10) and the secondary stream (11) is preferably established in the range of between 0.25 and 0.75. The primary stream (10) is then preheated in an indirect contact second exchanger (11) indirect contact using the outlet stream from the regenerator (16), at a temperature greater than 100 C., giving rise to a main inlet stream into the regenerator (12).

[0077] The regenerator (15) receives the stream from the absorber (2) at different heights and temperatures, such that the degree of regeneration of the absorbent is adjusted in an optimal manner.

[0078] The main inlet stream into the regenerator (12) is introduced in the upper part of the regenerator (15). On the other hand, the secondary stream (13) is introduced at a temperature less than the temperature set for the primary stream (10) in an intermediate bed of the regenerator (15), Achieving a temperature profile which optimizes the energy requirements of the entire capturing process. The secondary stream (13) can in turn be split into another additional stream (14) in order to be fed in at different heights of the regenerator (15).

[0079] This configuration allows obtaining a partial regeneration of the absorbent, shifting the cyclic capacity thereof into areas with a lower energy requirement of CO.sub.2 desorption. The energy necessary for the regeneration of the absorbent to occur is provided to the regenerator (15) by means of a drum (20) preferably using vapor as the working fluid.

[0080] On the other hand, the outlet stream (16) at the upper part of the regenerator, which stream is primarily made up of CO.sub.2 and water vapor, is introduced in a separator (17), where the stream having a high concentration of CO.sub.2 saturated in water (18) and a condensate stream (19) are obtained, which is subsequently recirculated to the regenerator (15).

[0081] Lastly, the stripped regenerated absorbent solution (21) is removed from the lower part of the regenerator (15) and impelled by means of a second pump (22) to the set of exchangers (9) prior to being reincorporated into the absorption system (23).

[0082] The regenerator (15) preferably works in a pressure range comprised between 1.5 and 5 bar, and at a maximum temperature less than 120 C., more preferably, in a temperature range comprised between 100 C. y 120 C., such that lesser degradation of the absorbent is assured.

[0083] The invention is illustrated below by means of tests performed by the inventors, which clearly shows the specificity and effectiveness of the method of the invention for capturing CO.sub.2.

[0084] Particularly, a process for separating CO.sub.2 from a synthetic gas stream has been performed in a laboratory-scale unit based on two operative configurations which correspond on one hand to a conventional configuration and on the other to a configuration according to the system of the invention.

[0085] In this sense, A synthetic gas flow rate of 7 L/min, with a composition of 60% v/v CO.sub.2, saturated with water vapor and completed with N.sub.2 has been used. Monoethanolamine in aqueous solution at 30% w/w has been used as absorbent, as it is a reference absorbent. The amount total of absorbent used in the system is 2 L. The absorption of CO.sub.2 is performed at a pressure of 1 atm and at a temperature of 50 C. in a column having 3 cm in diameter and 2 m in height, using as an absorption bed 6 mm ceramic Raschig rings. The regeneration of the absorbent is performed at a pressure of 2 bar in a column having 3 cm in diameter and 1 m in height using 6 mm stainless steel 316L Raschig rings.

[0086] The conventional configuration consisted of having a recirculation rate in the absorber of 0 (7), a single internal heat exchanger (24) makes up the set of exchangers (9) and the infeeding of the regenerator (15) is performed by means of using a single primary stream (10) introduced at the upper part of the regenerator (15). The absorbent flow rate was set at 7.01 kg/h, which corresponds with an L/G ratio equal to 12, with the inlet temperature into the absorber being 49 C.

[0087] The configuration of the invention uses a partial recirculation of the stream of recirculated CO.sub.2-rich absorbent solution (7), a set of exchangers (9) made up of internal heat exchangers (24, 25), and the inlet stream has been distributed to the regenerator in two streams: a primary stream (10) in the upper part of the regenerator (15) and a secondary stream (13) in the intermediate area of the regenerator (15). This secondary stream (13) was removed at the outlet of the first internal heat exchanger (24) of the set of exchangers (9). The absorbent flow rate was set at 8.18 kg/h, which corresponds with an L/G ratio equal to 14, with the inlet temperature of the gas into the absorber being 47 C.

[0088] The most relevant operating conditions and the results obtained are summarized in Table 1. The operation by means of the method of the invention allowed increasing the cyclic capacity of the absorbent and the CO.sub.2 separation yield during the separation operation as a result of a higher load of the rich absorbent in the absorption step. This increase in load is primarily due to the recirculation of part of the recirculated CO.sub.2-rich absorbent solution (7). Stratifying the feed into the regenerator (15) caused a decrease in the temperature profile in the regenerator (15) and, therefore, a stripped solution with a higher load of CO.sub.2. This shift in the cyclic operating capacity of the absorbent allowed the use of the new configuration to achieve an 11% reduction of the specific consumption of energy associated with the regeneration of the absorbent, producing a net benefit with respect to the conventional configuration of processes of this type. Furthermore, the lower thermal level obtained in the regenerator favors a reduction of the degradation of the absorbent associated with thermal mechanisms.

TABLE-US-00001 TABLE 1 Configuration Conven- Inven- Units tional tion Operating parameters Absorber L/G ratio kg/kg 12 14 Recirculation rate % 20 Regenerator Temperature at bottom C. 120 118 Primary Flow rate kg/h 7.01 5.73 feed Temperature C. 112 108 Secondary Flow rate kg/h 2.45 feed Temperature C. 100 Distribution in set (21C/21B) 100/0 80/20 Results Stripped absorbent load mol 0.15 0.19 CO.sub.2/mol absorbent Rich absorbent load mol 0.34 0.41 CO.sub.2/mol absorbent Cyclic capacity mol 0.19 0.22 CO.sub.2/mol absorbent CO.sub.2 capture yield % 96 98 Specific consumption of captured CO.sub.2 GJ/t CO.sub.2 4.55 4.05