Method and system for removing carbon dioxide from an aqueous liquid

Abstract

A method and system for removing carbon dioxide (CO2) from an aqueous liquid, such as seawater, are described. The method provides a first membrane system in which a gas stream, enriched in CO2 compared to the atmosphere, is separated from the aqueous liquid by a first vacuum pump. A sorbent liquid, such as an amine solvent, is carried through the first vacuum pump and absorbs CO2 from the gas stream. The first vacuum pump is an optionally adapted liquid ring vacuum pump which serves as a vacuum system, as sorbent circulation pump and as an absorber. The sorbent liquid is carried through a second membrane unit in which a nearly pure CO2 gas stream is separated from the sorbent by a second vacuum pump. The sorbent liquid is recycled to the first vacuum pump. The second vacuum pump discharges nearly pure CO2, such as to an end-user or for sequestration. The method may operate thermally neutral with low energy consumption. Only shaft power to drive the (vacuum) pumps is required.

Description

[0001] METHOD AND SYSTEM FOR REMOVING CARBON DIOXIDE FROM AN AQUEOUS LIQUID

[0002] TECHNICAL FIELD

[0003] The present invention concerns a method of removing carbon dioxide (CO2) from an aqueous liquid such as seawater using membrane separator devices and chemical sorbent liquids such as mono ethanolamine (MEA). In particular, a method is provided for direct capture of CO2 from seawater with low energy consumption. The invention further relates to a system for removing CO2 from an aqueous liquid.

[0004] BACKGROUND ART

[0005] Capturing CO2 directly from ambient air (as opposed to capturing CO2 from point sources such as fossil fuel combustion facilities for instance) is known in the art. In a known method, CO2 removal is achieved by contacting ambient air with chemical media such as amines, alkaline solutions or solid sorbents, able to capture CO2 . In a subsequent process step the CO2 may then be removed from the capturing chemical medium, typically by raising the temperature. The known method produces a CO2 stream for subsequent use or for sequestration. In case of a solid sorbent, the captured CO2 chemically bound to such a solid sorbent may be sequestrated.

[0006] A drawback of CO2 capture directly from (ambient) air is that very large volumes of air must be carried through the capture process due to the low concentration of CO2 in the atmosphere (currently about 400 ppm) with an associated relatively high energy consumption. Subsequently, energy is required to release the chemically bound CO2 from the sorbent. The amount of energy required for direct capture of CO2 from air is therefore high in comparison to capture of CO2 from point sources.

[0007] The feasibility of extraction of CO2 from an aqueous liquid, in particular seawater, using membrane separator devices has also been disclosed in the prior art. In such methods, advanced membrane separator systems have been developed and tested, such as bipolar membranes. However, achieving adequate CO2 capture rates and a relatively high selectivity remain a challenge.

[0008] It is an aim of the present invention therefore to provide an improved method of removing CO2 from an aqueous liquid, in particular seawater, with a lower energy consumption than known in the art, using CO2 neutral energy sources. SUMMARY OF THE INVENTION

[0009] These and other aims are provided by the method in accordance with claim 1. The invention provides a method for removing carbon dioxide (CO2) from an aqueous liquid, such as seawater, the method comprising: a) pumping the aqueous liquid to a first membrane separator device; b) separating the aqueous liquid in the first membrane separator device into an aqueous liquid stream with a reduced concentration of CO2 and a C02-enriched gas stream which is enriched in CO2 compared to the atmosphere; c) performing the following steps in a first vacuum pump device fluidly connected to the first membrane separator device: i. extracting the CCE-enriched gas stream from the first membrane separator device, ii. intensively contacting the CCE-enriched gas with a sorbent liquid, such as an amine solvent, and absorbing the CO2 in the sorbent liquid to obtain a gas with a low concentration of CO2 and a sorbent liquid enriched with CO2, and iii. circulating the CCE-enriched sorbent liquid to a second membrane separator device; d) separating the CCE-enriched sorbent liquid in the second membrane separator device to obtain a substantially pure CO2 gas stream and a sorbent liquid with a reduced CO2 content; e) recycling the sorbent liquid with a reduced CO2 to the first vacuum pump device; f) performing the following steps in a second vacuum pump device fluidly connected to the second membrane separator device: i. extracting the substantially pure CO2 gas stream from the second membrane separator device, and ii. discharging the substantially pure CO2 gas stream, for instance to an end-user or for sequestration.

[0010] The invented method uses a chemical sorbent liquid for capturing the CO2, such as an amine solvent, preferably in a closed loop.

[0011] The method further uses a first and second membrane separator device. Such devices are known per se and any membrane separator device with a suitable CO2 / N2 selectivity and sufficiently high CO2 permeability may be used. One skilled in the art of membrane separation will also be able to select the type of membrane material for the purpose. The method also uses first and second vacuum pump devices in an innovative way by employing at least the first vacuum pump device for multiples purposes, i.e. extracting the CCh-enriched gas stream from the first membrane separator system; intensively contacting the CCf-cnrichcd gas stream with the sorbent liquid; absorbing the CO2 in the sorbent liquid, so as to obtain a gas with a low concentration of CO2 and a sorbent liquid enriched with CO2; and circulating the CCf-cnrichcd sorbent liquid to a second membrane system.

[0012] The invention therefore also relates to a system for removing carbon dioxide (CO2) from an aqueous liquid, such as seawater, the system comprising: a pump for supplying the aqueous liquid to a first membrane separator device; wherein the first membrane separator device is adapted to separate the aqueous liquid into an aqueous liquid stream with a reduced concentration of CO2 and a CCf-cnrichcd gas stream which is enriched in CO2 compared to the atmosphere; a first vacuum pump device that is fluidly connected to the first membrane separator device and adapted to: extract the CCf-cnrichcd gas stream from the first membrane separator system; intensively contact the CCf-cnrichcd gas stream with a sorbent liquid, such as an amine solvent; absorb the CO2 in the sorbent liquid, so as to obtain a gas with a low concentration of CO2 and a sorbent liquid enriched with CO2; and circulate the CCf-cnrichcd sorbent liquid to a second membrane system; wherein the second membrane system is fluidly connected to the first vacuum pump device and adapted to separate the CCf-cnrichcd sorbent liquid into a substantially pure CO2 gas stream and a sorbent liquid with a reduced CO2 content; a feed-back conduit adapted to feed the sorbent liquid with reduced CO2 content back to the first vacuum pump device; and a second vacuum pump device adapted to extract the substantially pure CO2 gas stream from the second membrane system and discharge the substantially pure CO2 gas stream, for instance to an end-user or for sequestration.

[0013] The invention provides an efficient method and system for extracting dissolved CO2 from an aqueous liquid such as seawater. It enables separation of CO2 from the atmosphere while avoiding the need to process large volumes of air. The system can be operated stand-alone and can be powered by CCf-ncutral energy such as solar- and wind-power. CO2 removed from the aqueous liquid may be used in a subsequent process such as synthesis of renewable CCf-ncutral fuel, for enhancement of crop growth, or sequestrated. CO2 sourced from an aqueous liquid such as seawater can replace CO2 that would otherwise be produced from fossil fuels.

[0014] DETAILED DISCLOSURE OF THE INVENTION

[0015] The invention provides a method of removing carbon dioxide (CO2) from an aqueous liquid, such as seawater, as disclosed in claim 1. The invented method is used for removing carbon dioxide (CO2) from an aqueous liquid, wherein the aqueous liquid is defined as a liquid containing water to a substantial extent. Non-limiting examples of such aqueous liquids include industrial water, for instance as used in chemical plants, seawater or water that originates from other waterways such as rivers, canals and the like.

[0016] As used herein, the terms ‘ substantial (ly)’, ‘ essential (ly)’, ‘consist(ing) essentially of, ‘essentially all’ and equivalents thereof have, unless noted otherwise, in relation to a composition or a process step the usual meaning that deviations in the composition or process step may occur, but only to such an extent that the essential characteristics and effects of the composition or process step are not materially affected by such deviations.

[0017] In step b) of the method, the first membrane separator device separates the aqueous liquid into an aqueous liquid stream with a reduced concentration of CO2 and a CCE-enriched gas stream which is enriched in CO2 compared to the atmosphere. The aqueous liquid stream with a reduced concentration of CO2 may be fed to a suitable recipient, wherein the recipient may be a storage tank or vessel and / or may be surrounding aqueous liquid, for instance surrounding seawater, when the aqueous liquid comprises seawater.

[0018] According to the invention, the CCf-cnrichcd gas is intensively contacted with a sorbent liquid, such as an amine solvent, and the CO2 is absorbed in the sorbent liquid to obtain a gas with a low concentration of CO2 and a sorbent liquid enriched with CO2. The chemical sorbent liquid for capturing the CO2, such as an amine solvent, may be chosen according to the circumstances. Suitable sorbent liquids include but are not limited to amine solvents comprising for instance monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA) and also aminoethoxyethanol (diglycolamine) (DGA). Other solvents and mixtures of solvents may also be used, with or without additives.

[0019] According to the invented method, the steps of extracting the C02-enriched gas stream from the first membrane separator device, intensively contacting the C02-enriched gas with a sorbent liquid, such as an amine solvent, and absorbing the CO2 in the sorbent liquid to obtain a gas with a low concentration of CO2 and a sorbent liquid enriched with CO2, and circulating the CCh-enriched sorbent liquid to a second membrane separator device are carried out in a first vacuum pump device that is fluidly connected to the first membrane separator device.

[0020] As used herein, two devices are ‘fluidly connected’ when, unless noted otherwise, the two devices are connected by a conduit through which a fluid can be transported, wherein the term ‘fluid’ may comprise a liquid, a slurry, a solution, a dispersion, and the like, as well as a gas or gas mixture.

[0021] According to an embodiment of the invention, a method is provided wherein the sorbent liquid is circulated in the first vacuum pump device by letting in sorbent liquid through an inlet and letting out sorbent liquid through an outlet of the first vacuum pump device. The inlet is provided to let in sorbent liquid into the first vacuum pump device through an inlet conduit, whereas the outlet is provided to let out sorbent liquid enriched with CO2. The inlet may be provided to let in virgin sorbent liquid, or, preferably, may be provided to let in sorbent liquid with a reduced CO2 content, such as resulting from separating the CCf-cnrichcd sorbent liquid in the second membrane separator device to obtain a substantially pure CO2 gas stream and a sorbent liquid with a reduced CO2 content.

[0022] The method in accordance with the invention further uses a first and a second membrane separator device. Such devices are known per se and any membrane separator device with a suitable CO2 / N2 selectivity and sufficiently high CO2 permeability may be used. One skilled in the art of membrane separation will be able to select the type of membrane material suitable for the purpose. Dense polymer membranes with separation based on a physical separation principle may be used for instance. These polymer membranes are commercially available. Membranes with significantly increased CO2 / N2 selectivity at the same time providing high CO2 permeability may also be used, such as those employing nano-structured and / or chemically reactive materials.

[0023] The first vacuum pump device, and optionally also the second vacuum pump device, are adapted to combine the steps ci) to ciii) and, optionally also the steps fi) to fii). In an embodiment of the invention, a method is provided wherein the steps ci) to ciii) and optionally steps fi) to fii) are performed in a liquid ring vacuum pump. A liquid ring vacuum pump comprises a rotating positive-displacement pump. It functions similar to a rotary vane pump, with the difference that the vanes are an integral part of a rotor of the liquid ring vacuum pump and the vanes drive a rotating ring of liquid to form a seal for a number of compression-chambers formed between two subsequent vanes. Liquid ring vacuum pumps may be powered by an induction motor for instance. In operation, the liquid ring vacuum pump draws in a CCf-cnrichcd gas stream originating from the first membrane separator system. The CCfi-cnrichcd gas stream is then compressed by the rotating vaned impeller (or rotor) which is provided eccentrically within a cylindrical casing of the liquid ring vacuum pump. According to the invention, the CCf-cnrichcd gas stream is contacted with the sorbent liquid provided inside the casing. By centrifugal acceleration, the sorbent liquid forms a moving cylindrical ring against the inside of the casing of the liquid ring vacuum pump. This liquid ring creates a series of seals in the spaces between the impeller vanes, which form compression chambers. The eccentricity between the impeller's axis of rotation and the (cylindrical) casing geometric axis results in a cyclic variation of the volume of the compression chambers, which decreases from an inlet through which the CCf-cnrichcd gas stream is sucked in to an outlet through which CCf-cnrichcd sorbent liquid is exited and fed to the second membrane separator device. The CCf-cnrichcd gas stream is trapped in the compression chambers formed by the impeller vanes and the sorbent liquid ring and is intensively contacted with the sorbent liquid in there. This causes the sorbent liquid to take up a large amount of CO2 from the CCf-cnrichcd gas stream. The reduction in volume caused by the impeller rotation compresses the sorbent liquid enriched with CO2, which is exited to the outlet under pressure.

[0024] The above use of a liquid ring vacuum pump in this embodiment of the invention surprisingly enables to absorb a relatively large amount of CO2 in the sorbent liquid, to obtain a gas with a low concentration of CO2 and a sorbent liquid enriched with CO2.

[0025] In an improved embodiment, a method is provided wherein gas / liquid contact area and mass transfer coefficients are enhanced by providing an impeller of the liquid vacuum pump with means for enhancing gas / liquid contact area and mass transfer coefficients, such as a packing.

[0026] The method is efficient in that it does not need high temperatures and pressures for a proper functioning. A preferred embodiment provides a method wherein at least one of the steps a) to f) are performed at ambient temperatures ranging from 0 to 50°C, more preferably from 5 to 45 °C, even more preferably from 10 to 40°C. More preferably at least two, even more preferably at least three, even more preferably at least four, even more preferably at least five, and most preferably all the steps a) to f) are performed at ambient temperatures ranging from 0 to 50°C, more preferably from 5 to 45°C, even more preferably from 10 to 40°C.

[0027] The invented method yields good results for a variety of aqueous liquids fed to the first membrane separator device. In a method according to a preferred embodiment, the CCf-cnrichcd gas obtained from the first membrane separator device in step b) contains at least 0,2 vol.-% CO2, more preferably at least 0,5 vol.-% CO2, and most preferably at least 1 vol.-% CO2.

[0028] In another embodiment of the method, at least 50 wt.% of the CO2 present in the C Ch-enriched gas stream extracted from the first membrane separator device is transferred by the sorbent liquid, circulating between the first vacuum pump device and the second membrane separator device, to the second vacuum pump device, more preferably at least 70 wt.% of the CO2, even more preferably at least 90 wt.% of the CO2.

[0029] In yet another embodiment, a method is provided wherein the substantially pure CO2 gas stream discharged by the second vacuum pump device contains at least 50 vol.-% CO2, more preferably at least 70 vol.-% CO2, and most preferably at least 90 vol.-% CO2.

[0030] In a second aspect of the invention, a system for removing carbon dioxide (CO2) from an aqueous liquid, such as seawater, is provided, wherein the system comprises: a pump for supplying the aqueous liquid to a first membrane separator device; wherein the first membrane separator device is adapted to separate the aqueous liquid into an aqueous liquid stream with a reduced concentration of CO2 and a CCh-enriched gas stream which is enriched in CO2 compared to the atmosphere; a first vacuum pump device that is fluidly connected to the first membrane separator device and adapted to: extract the CCf-cnrichcd gas stream from the first membrane separator system; intensively contact the CCf-cnrichcd gas stream with a sorbent liquid, such as an amine solvent; absorb the CO2 in the sorbent liquid, so as to obtain a gas with a low concentration of CO2 and a sorbent liquid enriched with CO2; and circulate the CCf-cnrichcd sorbent liquid to a second membrane system; wherein the second membrane system is fluidly connected to the first vacuum pump device and adapted to separate the CCf-cnrichcd sorbent liquid into a substantially pure CO2 gas stream and a sorbent liquid with a reduced CO2 content; a feed-back conduit adapted to feed the sorbent liquid with reduced CO2 content back to the first vacuum pump device; and a second vacuum pump device adapted to extract the substantially pure CO2 gas stream from the second membrane system and discharge the substantially pure CO2 gas stream, for instance to an end-user or for sequestration. A preferred embodiment provides a system wherein the first vacuum pump device comprises sorbent liquid in-and outlets to support sorbent liquid circulation.

[0031] Another preferred embodiment provides a system wherein the first and optionally also the second vacuum pump system comprises a liquid ring vacuum pump.

[0032] Yet another preferred embodiment provides a system wherein an impeller of the liquid vacuum pump comprises means for enhancing gas / liquid contact area and mass transfer coefficients, such as a packing.

[0033] The first vacuum system preferably comprises a liquid ring vacuum pump, provided with a modified impeller comprising means to enhance gas / liquid contact area and mass transfer coefficients, and provided with adapted liquid in-and outlets to support sorbent liquid circulation.

[0034] The first vacuum pump device is providing multiple functions: extracting CO2 enriched gas from the first membrane system by vacuum, providing solvent circulation through the second membrane system, and absorption of CO2 into the sorbent liquid within the pump device.

[0035] The second vacuum pump device may be any vacuum pump known in the art. Preferably, the second vacuum pump device also comprises a (second) liquid ring vacuum pump, provided with liquid in-and outlets to support liquid circulation inside the second liquid ring vacuum pump. The liquid used in the second liquid ring vacuum pump is preferably water.

[0036] It is explicitly mentioned that the embodiments disclosed in the present application may be combined in any possible combination of these embodiments, and that each separate embodiment may be the subject of a divisional application.

[0037] BRIEF DESCRIPTION OF THE FIGURES

[0038] The above description, as well as other objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred, but nonetheless illustrative embodiment, referring to the accompanying figures in which:

[0039] Fig. 1 schematically shows a representation of CO2 absorption from a sea water stream (SI) according to an embodiment of the invention;

[0040] Fig.2 schematically shows a liquid ring vacuum pump as used in an embodiment of the invention; Fig. 3 schematically shows a graph of the partial pressure of CO2 in the CCf-cnrichcd gas stream in function of the CO2 loading of the absorbent liquid MEA in chemical equilibrium.

[0041] Figure 1 illustrate a method according to an embodiment of invention. In the embodiment shown, CO2 is extracted from seawater by a first membrane unit 2. Seawater with reduced CO2 content is discharged back to the sea as stream S2. A CO2 -rich gas stream is extracted from the first membrane unit 2 by an adapted first liquid ring vacuum pump 3. Using commercially available polymeric membranes, the CO2 / N2 ratio in the gas leaving the first membrane unit 2 may be increased with a factor of 20 to 50. With advanced membrane materials, this factor may even be increased to well beyond 100 offering reduced energy consumption of the invented system. A “lean” (low CO2 content) chemical sorbent liquid, such as MEA, is circulated through the first liquid ring vacuum pump 3. CO2 is selectively absorbed by the sorbent liquid within the liquid ring vacuum pump 3. Remaining gas, mainly consisting of nitrogen and oxygen, is discharged to the atmosphere as stream S3. The adapted first liquid ring vacuum pump 3 has three functions: vacuum system, solvent circulation pump, and absorber. In a second membrane unit 4 the absorbed CO2 is recovered as stream S4 from the “rich” sorbent liquid by a vacuum pump 5. The sorbent liquid is recirculated to the first liquid ring vacuum pump 3.

[0042] A set-up of the system to recover CO2 from seawater according to an embodiment as shown in Fig. 1 comprises a pump 1 to supply seawater to the first membrane unit 2, a first liquid ring vacuum pump 3, a second membrane unit 4, and a second vacuum pump 5. Each membrane unit may comprise multiple membrane units provided in parallel, if desired. The seawater stream SI is carried by the pump 1 through the first membrane unit 2 and, after extraction of part or most of the CO2, discharged to the sea / ocean as stream S2. Part or most of the CO2 present in stream SI passes through the membrane unit due to vacuum created by the liquid ring vacuum pump 3. The gas stream extracted by the pump 3 from the membrane unit 2 may contain 0,5-2 vol.-% CO2, depending on the CO2 / N2 selectivity of the membrane.

[0043] Fig.2 schematically shows a liquid ring vacuum pump (3 in Fig. 1). The liquid ring vacuum pump 3 is adapted in that it is fitted with an adapted rotor 15 and with sorbent liquid in- and outlets (18, 19). The liquid ring vacuum pump 3 is used in an innovative way as a vacuum pump system, as a sorbent liquid circulation pump, and as an absorber.

[0044] In Fig, 2, a gas inlet 11 fluidly connects to a gas inlet aperture (or upstream suction chamber) 13 within a sorbent liquid ring 16 and into the rotor 15 of the pump. Gas with a low concentration of CO2 is discharged via aperture (or downstream compression chamber) 14 which is fluidly connected to a gas outlet 12. The pump rotor 15 is surrounded by the sorbent liquid ring 16, which forms by supplying sorbent liquid 16 to the liquid ring vacuum pump 3 and by centrifugal forces caused by rotation of the rotor 15. A suitable chemical- or physical sorbent liquid (or solvent) is used to form the sorbent liquid ring 16 in the pump 3 and absorb the CO2. The rotation of the liquid ring 16 within the vacuum pump 3 generates the driving force for solvent circulation, similar to the operating principle of a centrifugal pump. The sorbent liquid inlet 18 is combined with the gas inlet 11 of the liquid ring vacuum pump 3 and the sorbent liquid outlet 13 is located at the perimeter of the sorbent liquid ring 16 to obtain sufficient pressure drop for sorbent circulation. The pump rotor 15 may be driven by an electric motor, such as an induction motor. The rotor 15 may be fitted with a gas / liquid (G / L) contacting packing 17 provided in between the impeller vanes of the rotor 15 to increase CO2 transfer to the sorbent liquid 16. The packing 17 is preferably constructed from metal. The specific contact area preferably is at least 100 m2 / m3, the void content is preferably at least 95%, and the material and method of installation of the packing are suitably chosen and designed to withstand the typically high hydrodynamic- and centrifugal forces within the liquid ring vacuum pump 3. Notably, with a rotor rotational speed of 1750 rpm for instance, centrifugal forces at a radius of 10 cm from the axis of rotation may exceed 300 times the gravitational acceleration G.

[0045] The CCh-enriched gas stream extracted from the first membrane unit 2 is contacted with sorbent liquid inside the liquid ring vacuum pump 3. Part or most of the CO2 is transferred to the sorbent liquid. The sorbent liquid 16 may preferably be circulated through the pump 3 at an increased rate compared to conventional operation of a liquid ring vacuum pump. The enriched sorbent liquid discharged by the pump 3 is supplied to the second membrane unit 4. Gas exiting the liquid ring vacuum pump 3, mainly a mixture of nitrogen and oxygen, is discharged to the atmosphere as stream S3.

[0046] As known in the art, CO2 absorption by a sorbent liquid (or solvent) is enhanced by increasing the partial pressure of CO2 in the gas, by increasing the gas / liquid contact surface area, and by increasing the gas- and liquid side mass transfer coefficients. Within a liquid ring pump these mass transfer parameters are enhanced as the gas pressure is increased and by the high shear velocities between gas and liquid.

[0047] It is also known in the art that rotating packed beds are able to intensify the physical -chemical process of multiphase transport and reaction. They have been applied in the field of CO? emission control. The reference describes CO? capture by a NaOH solution in rotating packed beds. (Reference: Zhao, Bingtao & Su, Yaxin & Tao, Wenwen, 2014. Mass transfer performance of C02 capture in rotating packed bed: Dimensionless modeling and intelligent prediction, Applied

[0048] Energy, Elsevier, vol. 136(C), pages 132-142.)

[0049] The gas / liquid contact surface area in the liquid ring pump can be increased by installing a suitable packing 17 in substantially all compression chambers of the rotor 15 of the pump 3, only indicated for a single segment of the rotor. During each rotation of the rotor 15, the packing 17 is immersed into the sorbent liquid 16 and wetted with sorbent liquid. Subsequently, the wetted rotor blades and the packing 17 are exposed to the incoming gas. The solvent on the packing flows radially outward to the liquid ring 16 due to the centrifugal forces caused by the rotation of the rotor 15, resulting in high transfer coefficients between gas and liquid. The rapid sequence of solvent film renewal on the rotor blades and on the packing 17, according to a typical rotor speed of 1000-2000 rpm, and the high transfer coefficients due to the high differential velocities between gas and liquid, intensify transfer of CO2 from gas to solvent. The adapted liquid ring vacuum pump 3 integrates three functions in a single device in an innovative method: vacuum pump to extract gas from the membrane system, added to that circulation pump for the solvent, and CO2 absorber. The pump 3 discharges gas, mainly consisting of nitrogen and oxygen and with low CO2 content, as stream S3 to the atmosphere.

[0050] Due to the pressure increase in the liquid ring pump 3, the sorbent circulating through the pump absorbs CO2 from the gas extracted from the membrane unit 2. Fig.3 indicates that a 30 m.-% mono-ethanolamine (MEA) in water sorbent in equilibrium with 1 vol.-% CO2 in air at atmospheric total pressure can be enriched to about 0,54 molCCE / mol MEA at 20°C. Exposed to a vacuum of approx. 0, 1 bar absolute total pressure in the second membrane unit 4, CO2 can be extracted from this solvent to about 0,42 mol CCE / molMEA.

[0051] The amine solvent leaving liquid ring pump 3, enriched with CO2, is carried through the second membrane unit 4. Part or most of the CO2, from the amine solvent is removed in the membrane unit 4 due to vacuum created by the second vacuum pump 5. The lean amine is recycled to the first vacuum pump 3. Nearly pure CO2 (>90 v.-%) is discharged by the second vacuum pump 5 as stream S4. The second vacuum pump 5 may also be a liquid ring vacuum / compression pump, but any standard vacuum equipment can be used. In case a liquid ring vacuum pump is used as second vacuum pump 5, the liquid used to maintain the liquid ring in the pump 5 is water. A small makeup supply stream S5 may then be required to maintain the water inventory. In case a vacuum system other than a liquid ring pump is provided, stream S5 is not applicable. The CO2 gas leaving the system as stream S4 may directly be carried to an end-user or can be sequestrated. Shaft power, required to drive the pump 1 and the vacuum pumps 3 and 5 may be provided by electric drive systems. The energy is preferably obtained from CO2 neutral sources such as solar- or wind-power.

[0052] The system in the shown embodiments may operate at ambient temperature in the range 0- 50°C, preferably 10-40°C. In principle, the sorbent liquid circulation loop operates thermally neutral. Absorption heat release and desorption heat consumption are balanced in that heat released during absorption of CO2 in the vacuum pump 3 is carried with the enriched solvent as sensible heat. This sensible heat is consumed during desorption of CO2 in the membrane unit 4. A small cooler may, if desired, be provided into the sorbent liquid or solvent feedback loop to control the solvent temperature and / or to rectify eventual solvent heat-up due to energy dissipation in the pump 3. The driving force for absorption is (over) pressure, generated by the first liquid ring pump 3. The driving force for desorption is vacuum, generated by the second vacuum pump 5.

[0053] This is a major difference compared to CO2 removal by coupled absorber and desorber units, with suitable accessory equipment, operated at elevated pressures and temperatures. As is known to a person skilled in the art, such systems have an energy requirement in the range of 3-4 GJ per ton of recovered CO2. The method and system according to embodiments of the present invention allow to substantially limit the energy consumption to the shaft power of the pumps 3 and 5.

Claims

CLAIMS1. Method of removing carbon dioxide (CO2) from an aqueous liquid, such as seawater, the method comprising: a) pumping the aqueous liquid to a first membrane separator device; b) separating the aqueous liquid in the first membrane separator device into an aqueous liquid stream with a reduced concentration of CO2 and a CCh-enriched gas stream which is enriched in CO2 compared to the atmosphere; c) performing the following steps in a first vacuum pump device fluidly connected to the first membrane separator device: i. extracting the CCf-cnrichcd gas stream from the first membrane separator device, ii. intensively contacting the CCf-cnrichcd gas with a sorbent liquid, such as an amine solvent, and absorbing the CO2 in the sorbent liquid to obtain a gas with a low concentration of CO2 and a sorbent liquid enriched with CO2, and iii. circulating the CCf-cnrichcd sorbent liquid to a second membrane separator device; d) separating the CCf-cnrichcd sorbent liquid in the second membrane separator device to obtain a substantially pure CO2 gas stream and a sorbent liquid with a reduced CO2 content; e) recycling the sorbent liquid with a reduced CO2 to the first vacuum pump device; f) performing the following steps in a second vacuum pump device fluidly connected to the second membrane separator device: i. extracting the substantially pure CO2 gas stream from the second membrane separator device, and ii. discharging the substantially pure CO2 gas stream, for instance to an end-user or for sequestration.

2. Method as claimed in claim 1, wherein sorbent liquid is circulated in the first vacuum pump device by letting in sorbent liquid through an inlet and letting out sorbent liquid through an outlet of the first vacuum pump device.

3. Method as claimed in claim 1 or 2, wherein the steps ci) to ciii) and optionally steps fi) to fii) are performed in a liquid ring vacuum pump.

4. Method as claimed in claim 3, wherein gas / liquid contact area and mass transfer coefficients are enhanced by providing an impeller of the liquid vacuum pump with means for enhancing gas / liquid contact area and mass transfer coefficients, such as a packing.

5. Method as claimed in any one of the preceding claims, wherein at least one of the steps a) to f), and preferably all steps a) to f), are performed at ambient temperatures ranging from 0 to 50°C, more preferably from 5 to 45°C, even more preferably from 10 to 40°C.

6. Method as claimed in any one of the preceding claims, wherein the CCf-cnrichcd gas obtained from the first membrane separator device in step b) contains at least 0,2 vol.-% CO2, more preferably at least 0,5 vol.-% CO2, and most preferably at least 1 vol.-% CO2.

7. Method as claimed in any one of the preceding claims, wherein at least 50 wt.% of the CO2 present in the CCf-cnrichcd gas stream extracted from the first membrane separator device is transferred by the sorbent liquid, circulating between the first vacuum pump device and the second membrane separator device, to the second vacuum pump device, more preferably at least 70 wt.% of the CO2, even more preferably at least 90 wt.% of the CO2.

8. Method as claimed in any one of the preceding claims, wherein the substantially pure CO2 gas stream discharged by the second vacuum pump device contains at least 50 vol.-% CO2, more preferably at least 70 vol.-% CO2, and most preferably at least 90 vol.-% CO2.

9. A system for removing carbon dioxide (CO2) from an aqueous liquid, such as seawater, the system comprising: a pump for supplying the aqueous liquid to a first membrane separator device; wherein the first membrane separator device is adapted to separate the aqueous liquid into an aqueous liquid stream with a reduced concentration of CO2 and a CCf-cnrichcd gas stream which is enriched in CO2 compared to the atmosphere; a first vacuum pump device that is fluidly connected to the first membrane separator device and adapted to: extract the CCf-cnrichcd gas stream from the first membrane separator system; intensively contact the CCf-cnrichcd gas stream with a sorbent liquid, such as an amine solvent; absorb the CO2 in the sorbent liquid, so as to obtain a gas with a low concentration of CO2 and a sorbent liquid enriched with CO2; andcirculate the CCh-enriched sorbent liquid to a second membrane system; wherein the second membrane system is fluidly connected to the first vacuum pump device and adapted to separate the CCfi-cnrichcd sorbent liquid into a substantially pure CO2 gas stream and a sorbent liquid with a reduced CO2 content; a feed-back conduit adapted to feed the sorbent liquid with reduced CO2 content back to the first vacuum pump device; and a second vacuum pump device adapted to extract the substantially pure CO2 gas stream from the second membrane system and discharge the substantially pure CO2 gas stream, for instance to an end-user or for sequestration.

10. System as claimed in claim 9, wherein the first vacuum pump device comprises sorbent liquid in-and outlets to support sorbent liquid circulation.

11. System as claimed in claim 9 or 10, wherein the first and optionally also the second vacuum pump system comprises a liquid ring vacuum pump.

12. System as claimed in claim 11, wherein an impeller of the liquid vacuum pump comprises means for enhancing gas / liquid contact area and mass transfer coefficients, such as a packing.

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