NL2036130B1 - Process for removing co2 from gas - Google Patents

Abstract

The invention relates to a process for removing COZ from a 002 containing gas, the process comprising: i) introducing the 002 containing gas into an absorber in which the 002 containing gas is brought into counter current contact with an absorbent to produce a 002 depleted gas and a 002 loaded absorbent, ii) withdrawing the 002 loaded absorbent of step i) from the absorber, iii) heating the 002 loaded absorbent of step ii) to obtain a heated COZ loaded absorbent, iv) introducing the heated COZ loaded absorbent of step iii) into a desorber in which the heated COZ loaded absorbent is brought into counter current contact with a stripping gas to obtain a 002 rich gas and a regenerated absorbent, v) withdrawing the regenerated absorbent from the desorber for introduction to the absorber, wherein step iv) further comprises withdrawing the COZ rich steam from a top part of the desorber and separating COZ absorbent from the 002 rich steam and returning the 002 absorbent to the desorber, wherein part of the 002 loaded absorbent of step ii) is fed to the top part of the desorber without being heated in step iii).

Description

PROCESS FOR REMOVING CO; FROM GAS

The invention relates to a process for removing CO2 from a gas.

Many gaseous streams, such as biogas, natural gas and flue gas, require the removal of CO: for upgrading the gas or to mitigate climate change.

US8470079 discloses a process for removing CO; from a gas mixture by contacting the gas mixture with at least one CO2 absorbing agent in aqueous solution or suspension, partially removing the at least one CO2 absorbing agent and inducing a phase separation into an aqueous phase and a nonaqueous phase, wherein the at least one CO2 absorbing agent, after the phase separation, resides at least predominantly in the nonaqueous phase; and predominantly transferring the CO: to the aqueous phase.

US8858906 discloses a process for removing COz from a gas by contacting the gas with an absorbing liquid to obtain an absorbing liquid enriched in CO; and a purified gas; heating the absorbing liquid enriched CO. to obtain a heated absorbing liquid enriched in CO:; and (c) contacting the heated absorbing liquid enriched in CO, with a stripping gas at an elevated temperature in a regenerator to obtain a regenerated absorbing liquid and a hot gas stream enriched in CO:; wherein 30% of the absorbing liquid enriched in COzis heated through external heat exchange with the hot gas stream enriched in CO:.

There is a demand for an improved process for removing CO2 from a CO2 containing gas.

The invention relates to a process for removing CO2 from a CO2 containing gas, the process comprising: 0) introducing the CO2 containing gas into an absorber in which the CO2 containing gas is brought into counter current contact with an absorbent to produce a

CO2 depleted gas and a CO2 loaded absorbent, ii) withdrawing the CO2 loaded absorbent of step i) from the absorber,

iii) heating the CO2 loaded absorbent of step ii) to obtain a heated CO2 loaded absorbent, iv) introducing the heated CO2 loaded absorbent of step iii) into a desorber in which the heated CO2 loaded absorbent is brought into counter current contact with a stripping gas to obtain a CO2 rich gas and a regenerated absorbent,

Vv) withdrawing the regenerated absorbent from the desorber for introduction to the absorber.

The absorbent used in the present invention is a liquid absorbent comprising at least one CO2 absorbing agent. Preferably, the absorbent changes between monophasic {homogeneous without phase separation) and biphasic (separated into a phase rich in the at least one CO2 absorbing agent (herein sometimes referred as “amine-rich phase”) and a phase poor in the at least one CO2 absorbing agent (herein sometimes referred as “amine-poor phase”)) depending on the CO2 loading and the temperature.

The biphasic absorbent may take the form of an emulsion of an amine-poor phase in an amine-rich phase or an emulsion of an amine-rich phase in an amine-poor phase.

The liquid absorbent introduced to the absorber may be biphasic. As the liquid absorbent flows down the absorber and the CO2 loading is increased, the liquid absorbent can gradually become monophasic. This liquid absorbent loaded with CO2 is withdrawn from the absorber and is heated. Upon heating the loaded CO2 is liberated and can leave the absorbent as a gas, which also leads to a phase separation and the absorbent becomes biphasic having an amine-poor phase and an amine-rich phase. A regenerated biphasic liquid absorbent is thus obtained having a CO2 lean amine-poor phase and a CO2 lean amine-rich phase. This regenerated liquid absorbent is then reintroduced to the absorber optionally after being cooled.

In such process, the separation of CO2 from the CO2 loaded absorbent in the desorber is easy and fast since CO2 is separated from the absorbent by phase separation.

It is an objective of the present invention to improve the process by at least one of the following: improve the efficiency of CO2 absorption, reduce degradation of the absorbent and decrease the amount of absorbent lost during absorbent regeneration.

In a first aspect, step i) comprises bringing the CO2 containing gas into counter current contact with the absorbent in an absorber packing comprising a first absorber packing section and a second absorber packing section located at a lower vertical position than the first absorber packing section and which is more hydrophilic than the first absorber packing section.

To improve CO2 absorption kinetics, it is beneficial to have the amine-rich phase as the continuous phase and the amine-poor phase as the dispersed phase. According to the first aspect, the packing material is selected such that the packing material is always wetted by the liquid. The type of absorber packing material can promote the preferential wetting of the packing material with the continuous amine-rich phase. The wettabilitiy of the packing material with the amine rich phase can be promoted by packing materials which are more hydrophobic of nature, such as polymeric materials (e.g. polypropylene, PVC). Upon CO2 loading the biphasic solvent becomes a monophasic solvent. The optimisation of the packing wetting behavior of a monophasic solvent requires a different packing material than in the case of a biphasic solvent.

In a second aspect, step ii) comprises degassing the CO2 loaded absorbent at a bottom part of the absorber to remove entrained bubbles of the CO2 containing gas and withdrawing the degassed CO2 loaded absorbent from the bottom part of the absorber.

The CO2 loaded absorbent comprises not only CO2 but also dissolved oxygen. The

CO2 loaded absorbent further comprises entrained bubbles of the CO2 containing gas which also comprises oxygen. Oxygen in the CO2 loaded absorbent causes oxidative degeneration of the absorbent particularly at elevated temperatures. It is therefore beneficial to decrease the amount of the dissolved oxygen and the entrained bubbles in the CO2 loaded absorbent before oxidative degeneration takes place.

The degassing of the CO2 loaded absorbent before it is withdrawn from the absorber reduces the amount of entrained bubbles. This reduction of the entrained bubbles leads to reduction of the amount of oxygen in the absorbent which in turn reduces oxidative degradation of the absorbent.

In a third aspect, step iii) is performed by a heat exchanger system comprising an optional first pump, a first heat exchanger, a back pressure control system, an optional second pump and a second heat exchanger connected in series in this order, wherein the back pressure control system reduces pressure of the CO2 loaded absorbent to induce flashing and the remainder is passed through the second pump and the second heat exchanger.

The back pressure control system reduces the pressure to induce flashing such that the dissolved oxygen is flashed out from the CO2 loaded absorbent together with some amount of CO2. This removal of the dissolved oxygen leads to reduction of the amount of oxygen in the absorbent which in turn reduces the oxidative degradation of the absorbent.

In a fourth aspect, step iv) further comprises withdrawing the CO2 rich gas from a top part of the desorber and separating CO2 absorbent from the CO2 rich gas and returning the CO2 absorbent to the top part of the desorber, wherein part of the CO2 loaded absorbent of step ii) is introduced to the top part of the desorber without being heated in step iii).

The CO2 rich gas withdrawn from the top part of the desorber comprises not only CO2 but also CO2 absorbent. By cooling the CO2 rich gas, the CO2 absorbent can be condensed and separated out from the CO2 rich gas before the CO2 rich gas leaves the system as CO2 product stream to be used for suitable purposes. However, some amounts of CO2 absorbent will remain in the CO2 rich gas, leading to a loss of CO2 absorbent. The cooling and condensation also require a large amount of energy. The fourth aspect of the invention solves this problem.

Introducing a relatively cool CO2 loaded absorbent of step ii) to the top part of the desorber decreases the temperature of the CO2 rich gas to be withdrawn from the top part of the desorber. The decreased temperature of the CO2 rich gas withdrawn from the top part of the desorber reduces the amount of the CO2 absorbent in the CO2 rich gas. This reduces the amount of the CO2 absorbent in the CO2 product stream obtained and in turn reduces the amount of CO2 absorbent lost during absorbent regeneration. Further, the decreased temperature reduces the energy required for cooling and condensing the absorbent.

Further, the decreased temperature also reduces the risk of the formation of a solid 5 reaction product which can be precipitated on unwanted surfaces or even carried as entrained solids with the CO2 product stream.

In a fifth aspect, the desorber comprises a bottom part comprising a desorber sump, a chimney tray above the desorber sump and a pipe extending down from the chimney tray to the desorber sump and then to a reboiler, wherein step iv) further comprises collecting the regenerated absorbent by the chimney tray and transporting it to the reboiler by the pipe and reboiling the absorbent by the reboiler and introducing the reboiled absorbent to the bottom part via an inlet located between the chimney tray and the desorber sump and step v) comprises withdrawing the regenerated absorbent from the desorber sump for introduction to the absorber.

The reboileris fed by the regenerated absorbent collected by the chimney tray instead of being fed from the desorber sump. Therefore the reboiler does not reboil the regenerated absorbent in the desorber sump which contains regenerated absorbent that has already been reboiled. Accordingly, efficient use of energy by the reboiler is achieved.

The regenerated absorbent collected by the chimney tray is heated by the hot vapor from the desarber sump. The regenerated absorbent is further heated indirectly in the pipe by the desorber sump before reaching the reboiler. Accordingly, the energy required by the reboiler is reduced by the use of the chimney tray and the pipe.

Step i)

The process according to the invention comprises the step of introducing the CO2 containing gas into an absorber in which the CO2 containing gas is brought into counter current contact with an absorbent to produce a CO2 depleted gas and a CO2 loaded absorbent.

Preferably, the CO2 loaded absorbent produced in step i} is monophasic.

The CO2 depleted gas can exit the absorber to be suitably used.

The process is suitable for any gas comprising CO:. For example, the gases to be treated may be biogas, natural gas, synthesis gas, obtained for instance by (catalytic) partial oxidation and/or by steam methane reforming of hydrocarbons, e.g. methane, natural or associated gas, naphtha, diesel and liquid residual fractions, gases originating from coal gasification, coke oven gases, refinery gases, hydrogen and hydrogen containing gases, and flue gases.

Suitably, the gas comprises in the range of from 0.25 to 70% (v/v) of CO, preferably from 1 to 45% (v/v).

In the event that the gas is a flue gas, the amount of CO. will generally be lower, suitably from 0.25 to 20 vol% and the gas will usually also comprise oxygen, preferably in the range of from 0.25 to 20 vol%, more preferably from 0.5 to 15 vol%, still more preferably from 1 to 10 vol%.

The absorbent comprises at least one CO2 absorbing agent. The at least one CO2 absorbing agent comprises an amine.

Preferably, the absorbent used in step i) is an aqueous solution of the at least one

CO2 absorbing agent having a concentration of 21 M and =5 M.

The at least one CO2 absorbing agent may be selected from those described e.g. in

US8470079 and Jiafei Zhang, “Study on CO2 Capture Using Thermomorphic Biphasic

Solvents with Energy-Efficient Regeneration”, Dortmund 2013, available at: https: fd-ni info/ 110 1476888/34

As described in US8470079, following can be mentioned regarding preferred CO2 absorbing agents used in the present invention.

Preferably, the at least one CO2 absorbing agent has a boiling point of 2100° C, more preferably 110° C, more preferably 2120° C. Such a boiling point ensures good phase separation. Preferably, the at least one CO2 absorbing agent has a boiling point of 2100° C and =180 °C, more preferably 110° C and =170 °C, more preferably =120° C and =160 °C.

Preferably, the at least one CO2 absorbing agent has a pKa of 27.5 and =11, more preferably 28 and =10.5, more preferably 29 and =10. Such a pKa leads to a particularly high binding of the CO2 to the absorbent.

Preferably, the at least one CO2 absorbing agent has a density of 20.7 and =1.1 g/ml at 25° C. Such a density ensures good phase separation.

Preferably, the at least one CO2 absorbing agent has, at the absorption temperature (=the temperature at which step i) is carried out), a solubility in water of 20.1 M and =5 M, preferably 20.2 M and =4.5 M, more preferably 20.1 M and =4 M. Such solubility properties allow a highly concentrated aqueous solution of the absorbent to be used.

Preferably, the at least one CO2 absorbing agent has, at the regeneration temperature (=the temperature which is set for inducing the phase separation), a solubility in water of 20.001 M and =0.3 M, preferably 20.01 M and =0.1 M. Such solubility properties allow virtually no CO2 absorbing agent to be transferred to the amine-poor phase or remains there.

As described above, the at least one CO2 absorbing agent can be defined by various preferred properties. When the at least one CO2 absorbing agent is a mixture of CO2 absorbing agents, it is understood that the mixture has such property. Thus, for example, the at least one CO2 absorbing agent having a boiling point of 100° C means the mixture of the CO2 absorbing agent has a boiling point of 2100° C.

Preferably, the at least one CO2 absorbing agent comprises at least one primary amine and/or at least one secondary amine and/or at least one tertiary amine.

In some embodiments, the at least one CO2 absorbing agent comprises at least one primary amine and at least one secondary amine.

In some embodiments, the at least one CO2 absorbing agent comprises at least one primary amine and at least one tertiary amine.

In some embodiments, the at least one CO2 absorbing agent comprises at least one secondary amine and at least one tertiary amine.

In some embodiments, the at least one CO2 absorbing agent comprises at least one primary amine, at least one secondary amine and at least one tertiary amine.

The primary amine may be selected from the group consisting of pentylamine, hexylamine, heptylamine, octylamine, cyclohexylamine, 2-methylcyclohexylamine, 2- methyl-butylamine, 2-aminopentane, 2-aminoheptane, 2-amino-hexane, 2- aminooctane, 2-aminononane, 3-methoxypropyl-amine, 2-methyl-1,5- diaminopentane, geranylamine, 2-ethyl-1-hexylamine, 6-methyl-2-heptylamine, cyclooctylamine, aniline, N-phenylethylenediamine, 2-phenylethylamine, N,N- dimethyl-4-cyclooctene-1-amines, and mixtures thereof.

The secondary amine may be selected from the group consisting of dipropylamine, N- ethylbutylamine, dibutylamine, diisopropylamine, dicyclohexylamine, bis(2- ethylhexylyamine, bis(alpha-methylbenzyl)amine, bis(1,3-dimethylbutyl)amine, diallylamine, bis[(s)-1-phenylethyl]lamine, di-sec-butylamine, 2,2,6,6- tetramethylpiperidine, N-methylcyclohexylamine, benzyl-tertbutylamine, bis(2- ethylhexyl)amine, 4-tert-butylcyclohexylamine and mixtures thereof.

The tertiary amine may be selected from the group consisting of triethylamines, 2- (diethylamino)ethanol, tripropyl-amine, tributylamine, N,N-dimethylcyclohexylamine, dimethyloctylamine, dimethyl-(1-methylheptyl)amine, dimethylallylamine, N- ethyldiisopropylamine, tris(2-ethylhexyl)amine, bis(2- cyclohexyloxyethylymethylamines, bis(2-(2,4-diethyloctyloxy)ethyl)methylamines, (2- (2-dimethylaminoethoxy)ethyl)dimethylamines, N-isopropylethylenediamine, N- methylenediamine, N,N-dimethylethylenediamine, N,N-dibutyltrimethylenediamine, tris[2-(isopropylamino)ethyllamine, tris[2-(methylamino)-ethylJamine and mixtures thereof.

Preferably, step i) comprises bringing the CO2 containing gas into counter current contact with the CO2 absorbent in an absorber packing comprising a first absorber packing section and a second absorber packing section located at a lower vertical position than the first absorber packing section and which is more hydrophilic than the first absorber packing section.

The hydrophilicity is indicated by the water contact angle of the material. The water contact angle can be measured at 25 °C. The water contact angle generally ranges between 0° to smaller than 90° (hydrophilic) or 90° to 150° (hydrophobic).

Preferably, the second absorber packing section has a water contact angle which is at least 1° larger than a water contact angle of the first absorber packing section. More preferably, the second absorber packing section has a water contact angle which is at least 5°, at least 10°, at least 15°, at least 20°, at least 25°, at least 30°, at least 35°, at least 40° or at least 45°, larger than a water contact angle of the first absorber packing section.

Suitably, the first absorber packing section comprises a material selected from metals, ceramics and polymers and the second absorber packing section comprises a material selected from metals such as steel, ceramics, glass, carbon and polymers such as polyethylene, polypropylene, polyvinyl chloride, polyvinylidene fluoride, perfluoroalkoxy alkane and ethylene chlorotrifluoroethylene .

Step ii)

The process according to the invention comprises withdrawing the CO2 loaded absorbent from a bottom part of the absorber.

Preferably, step ii) comprises degassing the CO2 loaded absorbent at a bottom part of the absorber to remove entrained bubbles of the CO2 containing gas and withdrawing the degassed CO2 loaded absorbent from the bottom part of the absorber.

In some preferred embodiments, the CO2 loaded absorbent is collected in an absorber sump at the bottom part of the absorber, wherein the absorber comprises a demister above the absorber sump for separating gas phase and liquid phase.

In other preferred embodiments, the CO2 loaded absorbent is collected in an absorber sump at the bottom part of the absorber, wherein the absorber sump comprises a mixing means for degassing the CO2 loaded absorbent by centrifugal force.

Step iii)

The process according to the invention comprises heating the CO2 loaded absorbent of step ij} to obtain a heated CO2 loaded absorbent.

Preferably, in step iii), the heated CO2 loaded absorbent has a temperature of 80 to 110 °C.

Preferably, step iii) is performed by a heat exchanger system comprising an optional first pump, a first heat exchanger, a back pressure control system, an optional second pump and a second heat exchanger connected in series in this order, wherein the back pressure control system reduces pressure of the CO2 loaded absorbent to induce flashing and the remainder is passed through the second pump and the second heat exchanger.

Preferably, the heat exchanger system comprises the first pump and/or the second pump, more preferably both the first pump and the second pump. In some embodiments, the heat exchanger system does not comprise the first pump and does not comprise the second pump.

Preferably, step iii) comprises heating the CO2 loaded absorbent of step ii) by the regenerated liquid absorbent from the desorber. The regenerated liquid absorbent from the desorber may heat the second heat exchanger and then the first heat exchanger.

In the first heat exchanger, the temperature of the CO2 loaded absorbent may be increased to e.g. 50 to 80 °C.

In the second heat exchanger, the temperature of the CO2 loaded absorbent may be increased to e.g. 80 to 110 °C.

The gas flashed out may be introduced to the bottom part of the absorber.

Step iv)

The process according to the invention comprises introducing the heated CO2 loaded absorbent of step iii) into a desorber in which the heated CO2 loaded absorbent is brought into counter current contact with a stripping gas to obtain a CO2 rich gas and a regenerated absorbent.

Preferably, step iv) further comprises withdrawing the CO2 rich gas from a top part of the desorber and separating CO2 absorbent from the CO2 rich gas and returning the

CO2 absorbent to the top part of the desorber.

The CO2 rich gas from which CO2 absorbent has been separated can be withdrawn as a CO2 product stream to be suitably used.

Preferably, the separation of CO2 absorbent from the CO2 rich gas is performed by condensing the CO2 absorbent, e.g. at a temperature of 35 to 50 °C, for example 40 °C.

Preferably, part of the CO2 loaded absorbent of step ii) is introduced to the top part of the desorber without being heated in step iii).

In this case, the absorbent is introduced at three inlets positioned at different vertical positions and have different temperatures and different concentrations of the CO2 absorbing agent. The CO2 rich gas withdrawn from the top part of the desorber has a temperature and CO2 absorbing agent concentration influenced by these different absorbents introduced to the top part of the desorber.

Preferably, the CO2 rich gas withdrawn from the top part of the desorber has a temperature of at most 70°C.

Preferably, the CO2 rich gas withdrawn from the top part of the desorber comprises the CO2 absorbing agent at a concentration of at most 5 vol%, preferably at most 2 vol%, more preferably at most 1 vol%, with respect to said CO2 rich gas.

The CO2 loaded absorbent of step iii) (heated) is introduced at the inlet of the lowest vertical position. Among the three absorbents, this absorbent has the highest temperature and may have a temperature of e.g. 80 to 110°C. This absorbent has the highest concentration of the CO2 absorbing agent.

The CO2 loaded absorbent of step ii) which has not been heated in step iii) is introduced at the inlet of the vertical position higher than the vertical position of the inlet for the CO2 loaded absorbent of step iii). This absorbent has a temperature lower than the CO2 loaded absorbent of step iii) and may have a temperature of e.g. 50 to 85 °C, for example 55 °C. This absorbent has a concentration of the CO2 absorbing agent lower than the CO2 loaded absorbent of step iii).

The CO2 absorbent separated from the CO2 rich gas is introduced at the inlet of the highest vertical position. Among the three absorbents, this absorbent has the lowest temperature and may have a temperature of e.g. 35 to 50 °C, for example 40 °C. This absorbent has the lowest concentration of the CO2 absorbing agent.

The amount of the CO2 loaded absorbent introduced to the top part of the desorber without being heated in step iii) with respect to the CO2 loaded absorbent of step ii) may e.g. be 5 to 20 vol%, preferably 7 to 15 vol%.

Preferably, in step iv), the heated CO2 loaded absorbent of step iii) is brought into counter current contact with the stripping gas in an absorber packing comprising a higher absorber packing section and a lower absorber packing section located at a lower vertical position than the higher absorber packing section and which is less hydrophilic than the higher absorber packing section.

Step v)

The process according to the invention comprises withdrawing the regenerated liquid absorbent from the desorber for introduction to the absorber.

Preferably, the regenerated liquid absorbent is biphasic.

Preferably, the desorber comprises a bottom part comprising a desorber sump, a chimney tray above the desorber sump and a pipe extending down from the chimney tray to the desorber sump and then to a reboiler, wherein step iv) further comprises collecting the regenerated liquid absorbent by the chimney tray and transporting it to the reboiler by the pipe, reboiling the absorbent by the reboiler and introducing the reboiled absorbent to the bottom part via an inlet located between the chimney tray and the desorber sump and step v) comprises withdrawing the regenerated absorbent from the desorber sump for introduction to the absorber.

Preferably, the desorber sump is provided with a pump around which pumps the regenerated absorbent from the desorber sump back to the desorber sump.

It is noted that the invention relates to the subject-matter defined in the independent claims alone or in combination with any possible combinations of features described herein, preferred in particular are those combinations of features that are present in the claims. It will therefore be appreciated that all combinations of features relating to the composition according to the invention; all combinations of features relating to the process according to the invention and all combinations of features relating to the composition according to the invention and features relating to the process according to the invention are described herein.

It is further noted that the term ‘comprising’ does not exclude the presence of other elements. However, it is also to be understood that a description on a product/composition comprising certain components also discloses a product/composition consisting of these components. The product/composition consisting of these components may be advantageous in that it offers a simpler, more economical process for the preparation of the product/composition. Similarly, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps. The process consisting of these steps may be advantageous in that it offers a simpler, more economical process.

When values are mentioned for a lower limit and an upper limit for a parameter, ranges made by the combinations of the values of the lower limit and the values of the upper limit are also understood to be disclosed.

The invention is now elucidated referring to figures, without however being limited thereto.

Figure 1 is a diagrammatic representation of a plant suitable for carrying out an embodiment of the process of the invention and

Figure 2 is a diagrammatic representation of an example of a heat exchanger system used in the plant of Figure 1.

According to Figure 1, a suitably pretreated CO2 containing gas (e.g. flue gas) is fed to a lower part of an absorber 2 via a line 1 and brought into contact in countercurrent flow with a regenerated absorbent which is fed to a higher part of the absorber 2 via an absorbent line 3.

The absorber 2 comprises a first absorber packing section 13 located below the inlet of the absorbent line 3 and a second absorber packing section 14 located below the first absorber packing section 13 and above the inlet of the line 1. The absorbent removes CO2 from the CO2 containing gas in the first absorber packing 13 and the second absorber packing 14. A gas with low CO2 loading is obtained and exits the absorber 2 via an offgas line 4 at the top of the absorber 2.

The CO2 loading of the absorbent increases as it flows down the first and second absorber packing sections 13 and 14. The absorbent may be biphasic when it is introduced to the absorber and may gradually become monophasic as it flows down the first and second absorber packing sections 13 and 14. The first absorber packing section 13 has a lower hydrophilicity than the second absorber packing section 14.

The higher hydrophilicity in the lower absorber packing section than in the higher absorber packing section increases the efficiency of CO2 absorption by the absorbent as it undergoes the change from biphasic to monophasic.

It will be appreciated that the number of the absorber packing sections is not limited to 2 and can e.g. be 3, 4, 5, 6, 10 or 100. Preferably, the hydrophilicity of an absorber packing section is equal to or lower than the hydrophilicity of an absorber packing section located at a lower vertical position.

Upon the absorbent contacting the CO2 containing gas, a CO2 loaded absorbent is obtained. The CO2 loaded absorbent comprises not only CO2 but also dissolved oxygen. The CO2 loaded absorbent further comprises entrained bubbles of the CO2 containing gas which also comprises oxygen. These entrained bubbles have a very high interfacial area and can rapidly exchange oxygen with the liquid absorbent.

Oxygen in the CO2 loaded absorbent causes oxidative degeneration of the absorbent particularly at elevated temperatures. It is therefore beneficial to decrease the amount of the dissolved oxygen and the entrained bubbles in the CO2 loaded absorption medium before oxidative degeneration takes place.

The CO2 loaded absorbent in a liquid phase is collected in a sump 27 at the bottom of the absorber 2. The absorber 2 comprises a demister 26 on top of the liquid present in the absorber sump. This demister 26 separates the gas from the liquid phase and leads to reduction of the entrained bubbles of the CO2 containing gas in the CO2 loaded absorbent in the sump 27. This reduction of the bubbles leads to reduction of the amount of oxygen in the absorbent and in turn reduces the oxidative degradation of the absorbent. Instead of a demister 26, a mixing means may be provided in the absorber sump for removing the entrained bubbles by centrifugal forces.

The CO2 loaded absorbent is withdrawn from the sump 27 by a line 5. Part of the CO2 loaded absorbent is passed through a solvent-solvent heat exchanger system 11 via aline 15 and is heated in the solvent-solvent heat exchanger system 11 with the heat of the regenerated absorbent exiting from the bottom of the desorber 7. As described further later, part of the CO2 loaded absorbent from the absorber 2 is fed to a top part 17 of a desorber 7 via an absorbent line 16.

As illustrated in Figure 2, the solvent-solvent heat exchanger system 11 consists of a first pump 11A, a first heat exchanger 11B, a back pressure control system 11C, a second pump 11D and a second heat exchanger 11E. The first pump 11A increases the pressure of the absorbent to prevent flashing from occurring in the following first heat exchanger 11B in which the temperature is increased to an intermediate temperature of e.g. 50 to 80 °C. The following back pressure control system 11C reduces the pressure to induce flashing such that the dissolved oxygen is flashed out from the absorbent together with CO2. This removal of the dissolved oxygen leads to reduction of the amount of oxygen in the absorbent and in turn reduces the oxidative degradation of the absorbent.

The duration of the presence of the dissolved oxygen in the absorbent should be as short as possible for preventing oxidative degradation. Accordingly, it is preferred that the residence time in the first heat exchanger 11B is selected to be short and it is followed by a flash. Further, the duration in which the absorbent stays in the sump 27 is preferably short.

The gas flashed out containing oxygen and CO2 may be transported to the lower part of the absorber 1. This is advantageous for environmental point of view. Alternatively, this flashed out gas may be released in the air.

The remaining CO2 loaded absorbent is transported to the second pump 11D. The second pump 11D increases the pressure to prevent flashing from occurring in the following second heat exchanger 11E in which the temperature is increased to a high temperature suitable to be fed to the desorber 7, e.g. 80 to 110 °C.

The heated CO2 loaded absorbent from the solvent-solvent heat exchanger system 11 is fed to the desorber 7 at a location above a second desorber packing section 21.

The absorbent flows down countercurrent to a stripping gas that streams upwards through the second desorber packing section 21 and CO2 is released from the absorbent to obtain a regenerated absorbent.

It is noted that in place of one desorber packing section (second desorber packing section 21), a plurality of desorber packing sections having different hydrophilicities can also be present. Preferably, the hydrophilicity of the second desorber packing section is higher at a higher vertical position. This change in the hydrophilicity depending on the vertical position increases the CO2 desorption from the absorbent.

The stream flowing upwards from the second desorber packing 21 in the desorber 7 is washed by counter current flow of the absorbent in a first desorber packing section 20 and arrives at the top part of the desorber 7. As mentioned earlier, part of the CO2 loaded absorbent from the absorber 2 is also fed to the top part 17 of via the absorbent line 16. The amount of the CO2 loaded absorbent fed to the top part 17 may e.g. be 10 vol% of the CO2 loaded absorbent from the absorber 2. Since the absorbent fed via the absorbent line 16 has bypassed the solvent-solvent heat exchanger system 11, its temperature is lower and results in a decrease in the temperature of the top part 17. This in turn results in a decrease in the amount of the absorbent in the vapor phase in the top part 17.

Gas comprising CO2 and the absorbent is withdrawn from the top part 17 of the desorber 7 and cooled e.g. to a temperature of 40 °C and compressed by a condenser 19 so that the absorbent is separated from CO2. CO2 is withdrawn from the plant through a CO2 line (CO2 product stream). The condensed absorbent from the condenser 18 returns to the top part 17 of the desorber 7.

Thus, the absorbent is introduced at three inlets positioned at different vertical positions. The absorbents have different temperatures and different concentrations of the CO2 absorbing agent.

The CO2 loaded absorbent introduced via line 15 (heated by the heat exchanger system 11} is introduced at the inlet of the lowest vertical position among the three inlets. This absorbent has the highest temperature and has a temperature of e.g. 110 °C. This absorbent has the highest concentration of the CO2 absorbing agent.

The CO2 loaded absorbent introduced via line 16 (bypassed the heat exchanger system 11) is introduced at the inlet of the vertical position higher than the vertical position of the inlet of line 15. This absorbent has a temperature lower than the CO2 loaded absorbent of step iii) and has a temperature of e.g. 55 °C. This absorbent has a concentration of the CO2 absorbing agent lower than the CO2 loaded absorbent introduced via line 15.

The CO2 absorbent introduced via line 19 (separated from the CO2 rich gas) is introduced at the inlet of the highest vertical position. This absorbent has the lowest temperature and has a temperature of e.g. 40 °C. This absorbent has the lowest concentration of the CO2 absorbing agent.

The introduction of the cool CO2 loaded absorbent via line 16 results in a decreased temperature of the top par 17. This reduces the amount of the COZ absorbent in the

CO2rich gas. Further, the decreased temperature reduces the energy required by the condenser 19 for cooling and condensing the absorbent. Further, the decreased temperature reduces the reaction of the absorber with CO2 upon condensation, which increases the CO2 product stream. This also reduces the risk of the formation of a solid reaction product which can be precipitated on unwanted surfaces or even carried as entrained solids with the CO2 product stream.

The regenerated absorbent from the second desorber packing 21 may be biphasic having a phase rich in water and a phase rich in amine. When such absorbent is used, it is of importance to prevent building up of one or the other phase in the desorber 7.

Otherwise, this will lead to non-controlled solvent composition towards the absorber and therefore a lower CO2 removal rate. It is desirable that an emulsion of the two phases is formed in the desorber 7 and fed to the absorber 2.

The regenerated absorbent from the second desorber packing 21comes into contact with a chimney tray 22. Part of the regenerated absorbent is collected by the chimney tray 22 and the remainder passes through the chimney tray 22 to be collected at a desorber sump 25 at the bottom of the desorber 7.

The regenerated absorbent collected by the chimney tray 22 is passed to a reboiler 23 via a pipe 29 extending down from the chimney tray 22 to the desorber sump 25 and then to the reboiler 23. The regenerated absorbent is vaporized by the reboiler 23 and the vapor is fed back to the desorber 7 at a location between the chimney tray 22 and the desorber sump 25.

The reboiler 23 is fed by the regenerated liquid absorbent collected by the chimney tray instead of the desorber sump. Therefore the reboiler does not reboil the regenerated liquid absorbent in the desorber sump which has already been reboiled.

Accordingly, efficient use of energy by the reboiler is achieved.

The regenerated liquid absorbent collected by the chimney tray is heated by the hot vapor from the desorber sump and may have a temperature of e.g. 115 °C. This is heated indirectly in the pipe by the desorber sump which may have a temperature of e.g. 120 °C and may reach a temperature of e.g. 116 °C before reaching the reboiler 23. The vapor returned to the inlet between the chimney tray 22 and the desorber sump 25 may have a temperature of e.g. 120 °C.

The desorber sump 25 is provided with a pump around which pumps the regenerated absorbent from the desorber sump 25 back to the desorber sump 25. This mixes the regenerated absorbent well in order to maintain the composition of the absorbent.

The regenerated absorbent is subsequently fed to the solvent-solvent heat exchanger system 11, in which the regenerated absorbent heats the CO2 loaded absorbent and is itself cooled in the process, and after being further cooled by a heat exchanger 10 is fed back to the absorber 2 via line 3.

Claims (15)

CONCLUSIONS 1. A process for removing CO2 from a CO2 containing gas, the process comprising: i) introducing the CO2 containing gas into an absorber wherein the CO2 containing gas is brought into countercurrent contact with an absorbent to produce a CO2 depleted gas and an absorbent incorporating CO2, ii) withdrawing the CO2 incorporated absorbent from step i) from the absorber, iii) heating the CO2 incorporated absorbent from step ii) to produce a heated CO2 incorporated absorbent, iv) introducing the heated CO2 incorporated absorbent from step iii) into a desorber wherein the heated CO2 incorporated absorbent is brought into countercurrent contact with a stripping gas to produce a CO2 rich gas and a regenerated absorbent, vv) withdrawing the regenerated absorbent from the desorber for introduction into the absorber, wherein step iv) further comprises withdrawing of the CO2 rich stream from an upper part of the desorber and separating CO2 absorbent from the CO2 rich steam and returning the CO2 absorbent to the desorber, wherein a portion of the CO2 incorporated absorbent from step ij) is fed to the upper part of the desorber without being heated in step iii). 2. The process of claim 1, wherein the CO2 rich gas withdrawn from the upper portion of the desorber has a temperature of at least 70°C. 3. Process according to any one of the preceding claims, wherein the CO2 rich gas withdrawn from the upper part of the desorber comprises the CO2 absorbing agent at a concentration of at least 5 vol%, preferably at least 2 vol%, more preferably at least 1 vol%, relative to said CO2 rich gas. Process according to any one of the preceding claims, wherein the amount of the CO2-containing absorbent introduced into the upper part of the desorptor without being heated in step iii) relative to the CO2-containing absorbent of step ii) is from 5 to 20 vol.%, preferably from 7 to 15 vol.%. 5. A process according to any preceding claim, wherein step i) comprises bringing the CO2 containing gas into countercurrent contact with the liquid absorbent in a packing of the absorber comprising a first packing section of the absorber and a second packing section of the absorber which is located at a lower vertical position than the first packing section of the absorber and which is more hydrophilic than the first packing section of the absorber. 6. The process of claim 5, wherein the second packing section of the absorber has a water contact angle at least 1° greater than a water contact angle of the first packing section of the absorber 7. A process according to any preceding claim, wherein step iv) comprises bringing the heated liquid CO2 absorbent containing CO2 from step iii) into countercurrent contact with the stripping gas in a packing of the desorber comprising an upper packing section of the absorber and a lower packing section of the absorber which is at a lower vertical position than the upper packing section of the absorber and which is less hydrophilic than the upper packing section of the absorber. 8. A process according to any preceding claim, wherein step ii) comprises degassing the CO2-containing absorbent in a lower portion of the absorber to remove entrapped bubbles of the CO2-containing gas and withdrawing the degassed CO2-containing absorbent from the lower portion of the absorber. 9. Process according to claim 8, wherein the absorbent containing CO2 is collected in an absorbent collection basin in the lower part of the absorber, the absorber comprising above the absorbent collection basin a de-evaporation device for separating gas phase and liquid phase 10. The process of claim 8, wherein the CO2-containing absorbent is collected in an absorbent collection basin in the lower part of the absorber, the absorbent collection basin comprising a mixing means for degassing the CO2-containing absorbent by means of centrifugal force. 11. A process according to any preceding claim, wherein step iii) is carried out by a heat exchanger system comprising an optional first pump, a first heat exchanger, a back-pressure control system, an optional second pump and a second heat exchanger, connected in series in that order, the back-pressure control system reducing pressure of the absorbent containing CO2 to induce flaring and the remaining portion being passed through the second pump and the second heat exchanger. 12. The process of claim 11, wherein step iii) comprises heating the CO2-containing absorbent from step ii) by means of the regenerated liquid absorbent from the desorber. 13. A process according to any preceding claim, wherein the desorber comprises a lower portion comprising a desorbent collection basin, an apertured plate above the desorbent basin and a pipe extending downwardly from the apertured plate to the desorbent basin and thence to a degassing device, wherein step iv) further comprises collecting the regenerated absorbent by means of the apertured plate and conveying it through the pipe to the means for heating it to a boiling point and reheating the absorbent by means of the means for reheating it to a boiling point and introducing the reheated absorbent into the lower portion through an inlet located between the apertured plate and the desorbent basin, and step v) comprises withdrawing the regenerated absorbent from the desorbent basin for introduction into the absorber. 14. The process of claim 13, wherein the desorbent basin is provided with a recirculation device that pumps the regenerated absorbent from the desorbent basin back to the desorbent basin. A process according to any preceding claim, wherein the absorbent changes between monophasic and biphasic depending on the level of CO2 incorporated and the temperature, preferably wherein the CO2 incorporated absorbent produced in step i) is monophasic and the regenerated absorbent withdrawn from the desorber in step v) is biphasic.