Method and system for temperature-controlled co2 capture and utilization

Abstract

A method for capturing CO2 from a gas mixture by dissolution in water is disclosed. The method comprises injecting the gas mixture as bubbles into a water volume, allowing CO2 enriched water to exit out of the volume, and performing a depressurizing process of the CO2 enriched water. The method saves energy by thermal contact with heat sources or cold sinks available at site. A corresponding system is also disclosed.

Description

[0001] TITLE: Method and system for temperature-controlled CO2 capture and utilization

[0002] Field of the invention

[0003] The present invention generally relates to a method for temperature-controlled selectively separating CO2 gas from a mixture of gases, and systems for carrying out the method. It also includes methods and systems for recovering the CO2 that has been separated from the other gases.

[0004] Background of the invention

[0005] Emissions of CO2 into the atmosphere from human activities are generally acknowledged as a major unsolved problem both locally where concentrations of CO2 may become high and cause acute damage to flora and fauna, and on a global scale where the background concentration of CO2 in the air is steadily rising, causing global warming and destruction of marine habitats. Furthermore, separating CO2 gas from a mixture of gases is of extreme importance in many industries, e.g. the transport and use of oil and natural gas and reforming processes involving synthesis of hydrogen from gas or oil.

[0006] To meet these challenges, great efforts have been spent worldwide to capture and dispose of CO2, in particular from high volume point emitters such as fossil fueled power plants, cement factories and garbage incinerators. Several methods are under development for capture of CO2 from flue or synthesis gases, employing chemical adsorption, membrane systems and solid adsorbents. These systems are environmentally challenging (chemical adsorption) and / or energy or process demanding. A prominent example is the amine process where CO2 is separated from flue gases (see, e.g.: https . studies / ) which is very energy consuming, expensive, and may employ toxic chemicals. There is thus a pressing need to develop new clean, low cost, high capacity, and energy efficient solutions that are capable of extracting CO2 from relevant gas mixture in an environmentally and energy friendly manner, including optimum use of any excess energy (hot or cold) available at site.

[0007] Summary of the invention

[0008] A first aspect of the invention is a method for capturing CO2 from a gas mixture comprising CO2 and one or more other gases by a system for capturing CO2, where the CO2 gas has higher solubility in the water than the one or more other gases, where the method comprises the following steps:

[0009] - injecting the gas mixture as bubbles into a water volume with a surface in an absorber unit, allowing the gas mixture in the bubbles to interact with the water, thereby causing CO2 gas to dissolve in the water to a higher degree than the one or more other gases, further allowing the bubbles enriched in gas not dissolved to ascend to and exit from the surface of the water volume, resulting in the water becoming enriched in CO2, and further allowing the CO2 enriched water to exit out of the volume;

[0010] - performing a first step depressurizing of the CO2 enriched water in a first step depressurizing unit to a first pressure level thereby degassing the one or more other gases from the CO2 enriched water to a higher degree than CO2 giving first step depressurized water;

[0011] - performing a second step depressurizing of the first step depressurized water in a second step depressurizing unit, where depressurizing is to a lower pressure level than that in the first step depressurizing, thereby degassing CO2 from the first step depressurized water giving second step depressurized water depleted in CO2; and

[0012] - performing temperature control by: i) cooling content (fluid) in the absorber unit and the first step depressurizing unit by a water phase in which said units are at least partially submerged.

[0013] Optionally, the performing temperature control comprises: ii) heating water phase from the first step depressurizing before entering the second step depressurizing thereby making the second step depressurizing more efficient, where the heating utilizes one or more of i) excess heat from an external industrial process and ii) heat from an internal heat exchanger comprised by the system.

[0014] Optionally, the performing temperature control further comprises cooling content in the second step depressurizing unit by a water phase in which said second step depressurizing unit is at least partially submerged.

[0015] Optionally, the performing temperature control further comprises one or more of the following steps: iii) cooling gas and vapor from the second step depressurizing for condensation before performing separation; iv) cooling second step depressurized water before returning it to the absorber unit; v) cooling water before disposal by mixing with water from the water phase.

[0016] Optionally, depressurizing is by one or more of the following: A throttling process, flashing over a valve / choke, or pumping by a vacuuming device.

[0017] Optionally, the method comprises collecting the CO2 degassed from the water in the second step depressurizing.

[0018] Optionally, the method comprises, prior to the injecting, at least one of i) compressing the gas mixture preferably to a minimum pressure of 2 to 10 bara and ii) cooling the gas mixture preferably to 5-30 degrees C.

[0019] Optionally, the water in the water volume is counter current to the bubbles, with a water descent rate preferably lower than ascent rate of the bubbles.

[0020] Optionally, the method comprises obtaining pressure of the liquid water phase in the water volume by at least one of the following: A static head hi in the water volume and: Gas pressure acting on the surface of the water volume. Optionally, the method comprises using gas under pressure from pipe system in compressor for compression of incoming gas.

[0021] Optionally, the heating water phase from the first step depressurizing before entering the second step depressurizing comprises using heat from cooling water used to cool the compressed incoming gas.

[0022] Optionally, gas degassed from the water in the first step depressurizing is reinjected in the gas mixture prior to the injecting and, if applicable, the compressing as defined above.

[0023] Optionally, the method comprises cooling gas and vapour degassed from the water in the second step depressurizing giving condensed water and gas enriched in CO2, and further optionally, reinjecting the condensed water into the second step depressurized water prior to the second step depressurizing.

[0024] Optionally, the method comprises releasing the gas enriched in CO2 to any further treatment.

[0025] Optionally, the method comprises injecting the second step depressurized water depleted in CO2 into the water volume, and further optionally, injecting water into the water volume.

[0026] Optionally, the method comprises diverting at least parts of the CO2 enriched water from the absorber unit, thereby producing carbonated water.

[0027] A second aspect of the invention is a system for capturing CO2 from a gas mixture comprising CO2 and one or more other gases by dissolution in a liquid water phase, where the CO2 gas has higher solubility in the water than the one or more other gases, where the system is arranged for performing the method described above. A further aspect of the invention is a system for capturing CO2 from a gas mixture comprising CO2 and one or more other gases by dissolution in a liquid water phase, where the CO2 gas has higher solubility in the water than the one or more other gases, where the system comprises:

[0028] - an absorber unit, and means for injecting the gas mixture as bubbles into a water volume with a surface in the absorber unit, allowing the gas mixture in the bubbles to interact with the water, thereby causing CO2 gas to dissolve in the water to a higher degree than the one or more other gases, further allowing the bubbles enriched in gas not dissolved to ascend to and exit from the surface of the water volume, resulting in the water becoming enriched in CO2, and further allowing the CO2 enriched water to exit out of the volume;

[0029] - a first step depressurizing unit arranged for performing a first step depressurizing of the CO2 enriched water to a first pressure level thereby degassing the one or more other gases from the CO2 enriched water to a higher degree than CO2 giving first step depressurized water;

[0030] - a second step depressurizing unit arranged for performing a second step depressurizing of the first step enriched water where depressurizing is to a lower pressure level than that in the first step depressurizing, thereby degassing CO2 from the first step depressurized water giving second step depressurized water depleted in CO2; and

[0031] - means for performing temperature control comprising: i) means for cooling content in the absorber unit and the first step depressurizing unit by a water phase in which said units are at least partly submerged.

[0032] Optionally, the system comprises means for heating water phase from the first step depressurizing before entering the second step depressurizing thereby making the second step depressurizing more efficient, where the heating utilizes one or more of i) excess heat from an external industrial process and ii) heat from an internal heat exchanger comprised by the system.

[0033] Optionally, the means for performing temperature control further comprises the second step depressurizing unit being at least partly submerged in a water phase by which the content of said unit is cooled.

[0034] Optionally, the means for performing temperature control further comprises means for one or more of the following steps: iii) cooling gas and vapor from the second step depressurizing for condensation before performing separation; iv) cooling second step depressurized water before returning it to the absorber unit; v) cooling water before disposal by mixing with water from the water phase.

[0035] Optionally, the means for second step depressurizing is or comprises a membrane unit suitable for gas / water separation.

[0036] Optionally, the water phase is inside a volume optionally formed as one of i) a shaft in the ground, ii) a tank onshore, iii) a dry dock and iv) a barge placed in a natural water volume, where further optionally, the volume is formed according to iv) above, where the system comprises a floating device which at least parts of the system is mechanically connected to. Further optionally, the volume is formed according to iv), and with an open bottom.

[0037] Description of the figures

[0038] The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of exemplary embodiments of the invention given with reference to the accompanying drawings.

[0039] Embodiments of the present invention will now be described, by way of example only, with reference to the following figures, wherein:

[0040] Figure 1 shows a first preferred embodiment of the present invention.

[0041] Figure 2 shows a second preferred embodiment of the present invention List of reference numbers in the figures

[0042] The following reference numbers refer to the drawings:

[0043] No. Designation

[0044] 1. Flue gas entry

[0045] 2. Compressor

[0046] 3. Tank / Absorber unit

[0047] 4. Mixer

[0048] 5. Pipe system

[0049] 6. First depressurizing / flashing unit

[0050] 7. Chimney

[0051] 8. Pipe system

[0052] 9. Pipe system

[0053] 10. Pipe system

[0054] 11. Pipe system

[0055] 12. Second depressurizing / flashing unit

[0056] 13. Pipe system

[0057] 14. Pipe system

[0058] 15. Heat exchanger

[0059] 16. Pipe system

[0060] 17. Liquid pump

[0061] 18. Valve or choke

[0062] 19. Pipe system

[0063] 20. Liquid pump

[0064] 21. Vacuum pump / compressor

[0065] 22. Gas / liquid separator

[0066] 23. Pipe system

[0067] 24. Pipe system

[0068] 25. Pipe system

[0069] 26. Liquid pump

[0070] 27. Pipe system 28. Pipe system

[0071] 29. Pipe system

[0072] 30. Liquid pump

[0073] 31. Pipe system

[0074] 32. Pipe system

[0075] 33. Pipe system

[0076] 34. Compressor

[0077] 35. Water phase

[0078] 36. Wall

[0079] 37. Barge (optional)

[0080] 38. Construction on top of 36

[0081] 39. Floating device

[0082] 40. Water level

[0083] 41. Bottom

[0084] 42. Pipe / tunnel

[0085] 43. Pipe / tunnel

[0086] 44. Pipe / tunnel

[0087] 45. Propel

[0088] 46. Coating

[0089] 47. Valve or choke

[0090] 48. Pipe system

[0091] 49. Pipe system

[0092] 50. Pipe system

[0093] 51. Heat exchanger

[0094] 52. Pump

[0095] 53. Pipe system

[0096] 54. Pipe system

[0097] 55. Heat exchanger

[0098] 56. Heat exchanger

[0099] 57. Pipe system

[0100] 58. Insulation

[0101] 59. Pipe system hi Static head h2 Static head h3 Static head

[0102] Description of preferred embodiments of the invention

[0103] The present invention may be used for capture of CO2 from an input gas containing CO2 in a mixture with other gases, e.g. flue or synthetic gas. The input gas may be of any composition and state, but preferentially with a temperature between 0 and 30 °C. Flue gas may also preferentially be free of sour gasses as e.g., NOx, SO2, HCI and HF, and environmentally unfriendly components as e.g. Hg. This may be obtained by any suitable method for pre-treatment of flue gas if needed.

[0104] The present invention preferentially requires temperature control (hot / cold) for process units. Possible use of available temperature sources (hot / cold) at site are therefore illustrated in Figure 1 and Figure 2. Temperature sources should preferentially be available cold water (e.g. from sea, river, or lake) or excess warm water from industrial processes (including present invention). Use of any cold water should preferentially be used at minimum energy requirements, e.g. by using present water pressure (from e.g. upstream river or lake), or by placing process units below e.g. sea level as illustrated in Figure 1.

[0105] The basic principles of the present invention can be understood by inspection of Figure 1 : Flue gas (1) is compressed by one (2) or more (34) compressors before being injected into a liquid water phase in a tank (3) as compressed gas by a mixer (4). The gas phase is then contacted with the water phase in the tank (3) where CO2 is dissolved in the water phase together with other flue gas constituents, e.g. nitrogen and some oxygen. The amount of gas dissolved in the water phase will mainly depend on pressure, temperature, and pH. The injected gas forms bubbles that float upwards in the column against downward flowing water. The constituent gases in the flue gas interact with the water: The gas transported out of the bubbles and into the water will differ between the gas species, reflecting differences in diffusivity and solubility in the water surrounding the bubble. This results in segregation of gas species where CO2 which has the highest diffusivity and solubility is more easily transported into the water outside the bubble, while a larger proportion of the other gas species (e.g. N2) remain inside the bubble and are transported out of the water volume when the bubble floats to the surface. Thus, a high degree of gas separation can be achieved by collecting the water with dissolved high solubility gas on the one hand and allowing the bubbles with the low solubility gas species to escape from the surface on the other hand. Excess gas is depleted from the tank (3) via a pipe system (5). The water phase in the tank (3) is transported via the pipe system (16) and partly degassed in the first depressurizing / flashing unit (6) where some of the CO2 and other gases is released to a pipe system (9). Gas stream in pipe system (9) may be depleted via a pipe system (10) to a chimney (7) or injected via a pipe system (11) back to the flue gas entry (1). Water phase from the first depressurizing / flashing unit (6) is then degassed in the second depressurizing / flashing unit (12), where a CO2 rich gas is obtained and transported via a pipe system (13). Water phase from the second depressurizing / flashing unit (12) is depleted in separator (22) and re-injected back to the tank (12). The main system, tanks (3), (6), and optional (12) and (22), may be temperature controlled by being submerged in a water phase (35).

[0106] The second depressurizing / flashing unit (12) may be a tank with any kind of interior or may be on or more membrane units giving a degassed water phase to stream (25) and a gas and water vapor phase to (13).

[0107] A preferred embodiment of the invention shall now be described in detail with reference to Figure 1 :

[0108] Flue gas is entering the system at the flue gas entry in pipe (1) and may be at pressure above atmospheric or if needed compressed to a given pressure e.g. between 2 and 10 bara in the pipe system (14) by one or more compressors (2) and (34). Compressor (2) may be of any suitable type powered by any suitable means (e.g. electric, steam, high pressure gas), while compressor (34) is optional and preferentially powered by compressed gas from tank (3) through pipe (5). Pipe systems (1), (5) and (14) may be of any pipe diameter, pressure rate and material and may contain any valve, fan, cooler, heat exchanger, or any other item suitable for the process. Flue gas at (1) may be compressed to any suitable pressure for the invention, but preferably to a pressure between 2 and 10 bara. Before entering the tank (3), flue gas in pipe system (14) may be cooled to a temperature suited for the process (e.g. 2 to 30 °C for a flue gas) in a cooler (15). Cooler (15) may be any kind of cooler or heat exchanger and may be an integrated part of compressors (2) and (34).

[0109] Compressed flue gas in (14) is injected into a liquid water phase in the tank (3) by any suitable means at mixer (4). Mixer (4) may be of any kind, distributing gas from (14) as small bubbles into the water phase in the tank (3). At the injection point (4), the compressed flue gas (14) will be at a higher pressure than the water phase in the tank (3). Pressure of water phase in the tank (3) at injection point (4) may be obtained by a static head hi and by pump (26). The water phase in tank (3) may be from fresh or salt water. If any conditioning of the water phase is needed, any environmentally friendly inhibitor or conditioning chemical may be added to the water phase, e.g. for inhibition of any scale formation from e.g. salt water. The tank (3) may be a vertical pipe or tank of any suitable pressure range, diameter, or height, made from any suitable material. Flue gas from mixing point (4) will rise in the water phase in the tank (3) due to buoyancy, leaving the tank (3) into pipe system (5). Pressure in pipe (5) may be atmospheric or at a pressure given by the pressure in pipe (14) minus pressure loss due to the static head, hi , in tank (3). Water phase in the tank (3) is counter current to the gas phase from the pipe system (14). The fluid flow rate of water should preferentially be lower than gas flow ascending rate in the tank (3). Inside, tank (3) may be open or filled with any suitable means (e.g. mesh, any gas / water mixer, or any suitable gas / water contactor for optimum contact area between water and flue gas). In the tank (3) CO2 from flue gas (1) will be dissolved in the water phase. Other gases in the flue gas (1) as e.g. nitrogen and some oxygen will also to some lesser degree be dissolved in the water phase. The amount of CO2 dissolved in the water phase in the tank (3) is mainly given by pressure, temperature, and pH in the water phase, and the ratio between water and gas in the tank (3). Given correct conditions in the tank (3), up to nearly 100% of CO2 in the flue gas (1) may be dissolved in the water phase in the tank (3).

[0110] Flue gas in the tank (3) not dissolved in the water phase in the tank (3), usually at a pressure above atmospheric is entering pipe system (5). Gas at high pressure from pipe (5) may then be used in compressor (34) for compression of incoming gas (11) and (1). Gas released from compressor (34) may go into pipe system (48) which may be, but not restricted to, a chimney (7) of any kind for releasing the gas to the atmosphere, or to a pipe system (8) for any after treatment or use of the gas. Pipe systems (48) and (8) and chimney (7) may be of any pipe diameter, pressure rate and material and may contain any valve, choke, or any other item suitable for the process.

[0111] Water enriched in CO2 from the tank (3) goes through pipe system (16) to be depressurized / flashed in unit (6). Pipe system (16) may include a choke / valve (18) and or a pump (17). Choke / Valve (18) and pump (17) may be of any kind, suitable for the purpose. Pressure difference across choke / valve (18) may be zero or any higher pressure suitable for the process.

[0112] The first depressurizing / flashing unit (6) may be a vertical pipe or tank of any suitable pressure range, diameter, or height, made from any suitable material. If unit (6) includes a static head h2, some of the gas (mainly nitrogen and oxygen) dissolved in the water phase in (3) may start to form bubbles in the water phase in (6) due to decreased pressure when rising in unit (6). Gas phase formed in unit (6) will leave in pipe system (9) through valve / choke (47). Valve / choke (47) may be any valve or choke suitable for the purpose. Pressure difference across the valve (47) may be zero or any higher pressure suitable for the process. After valve / choke (47) gas stream in (9) may be depleted via pipe system (10) to the chimney (7) or injected via a pipe system (11) back to the flue gas entry (1) before compressor (34) in order to recapture CO2 from the gas phase released in depressurizing / flashing unit (6). Pipe systems (9), (10) and (11) may be of any pipe diameter, pressure rate and material and may contain any valve, choke, or any other item suitable for the process. The first depressurizing / flashing unit (6) may include any interior for an efficient gas / water separation. Pressure inside the first depressurizing / flashing unit (6) may be atmospheric or any higher pressure suitable for the process.

[0113] Water phase from the first depressurizing / flashing unit (6), carrying dissolved CO2, goes through pipe systems (19), (31) and (32) to vacuum / reduced pressure chamber (12). Pipe system (19) may include a pump (20) of any kind suitable for the process. (20) may also by a valve or choke of any kind. Pipe systems (19), (31) and (32) may be of any pipe diameter, pressure rate and material and may contain any valve, cooler, heat exchanger, or any other item suitable for the process.

[0114] The second depressurizing unit (12) may be any tank or pipe system suitable for the purpose and may include any interior for an efficient gas / water separation. In (12) vacuum / reduced pressure will de-gas the water phase, i.e. draw out gas dissolved in the water phase (mainly CO2) together with some water vapor into the gas phase, by a vacuum / reducing pressure pump, compressor or by any other suitable means (21), and pipe system (13). Pipe system (13), before and after (21) may be of any pipe diameter, pressure rate and material and may contain any valve, heat exchanger, or other item suitable for the process.

[0115] The second depressurization unit (12) may optionally be or comprise a membrane unit suitable for the purpose for an efficient gas / water separation. In (12) vacuum / reduced pressure will de-gas the water phase, i.e. draw out gas dissolved in the water phase (mainly CO2) through a membrane together with some water vapor into the gas phase, by a vacuum / reducing pressure pump, compressor or by any other suitable means (21), and pipe system (13). Gas and vapor from pipe system (13) may be pressurized and / or cooled in (22), which may be any tank or pipe system suitable for the purpose. Condensed water from (22) may be reinjected into the second depressurizing / flashing unit (12) by pipe system (23) and pump / choke (30). Pump / choke (30) maybe of any kind suitable for the process and pipe system (32). Pipe system (23) may be of any pipe diameter, pressure rate and material and may contain any valve, heat exchanger, cooler and / or any other item suitable for the process.

[0116] CO2 rich gas from (22) is released through pipe system (24) for any further treatment (e.g. before being purified and transported to a permanent storage) or any other use.

[0117] Water from (12), depleted of most dissolved gases, goes through pipe systems (25), liquid pump (26) and pipe system (29), to the top of the tank (3). The liquid pump (26) is a pump of any suitable kind. Pipe systems (25) and (29) may be of any pipe diameter, pressure rate and material and may contain any valve, cooler, heat exchanger, or any other item suitable for the process. Some water may have to bleed out of the process via the pipe system (27) if any unwanted components are increased in the water phase from flue gas (1). Fresh water from any suitable source will then be added to the process via pipe system (28). If needed, water from (27) will be cleaned by any means before further disposal. In some applications water phase may be used "once through" in the process. All water will then be injected at (28) and depleted at (27). Water at (28) may be fresh or salt water and at a temperature suitable for the process. If carbonated water is wanted from the system, this may be depleted at (33) or at (49).

[0118] The process for CO2 capture given in the present invention will adsorb heat to the water phase in tank (3) due to absorption heat of gases and from heat generated in compressors and pumps used in the system given. The CO2 capture process optimum operates at temperatures between 2 and 20 °C, preferentially between 2 and 15 °C. These temperatures may be obtained by tanks (3) and (6), optional also tanks (12) and (22) together with any wanted infrastructure (e.g. pipes, valves, and pumps) being submerged in a water phase (35). This may be obtained by placing the system in a water filled shaft in the ground, in a tank onshore, in a water filled dry dock, from a barge in sea / lake / river or by a pipe in pipe cooling system.

[0119] The principle of placing the CO2 capture system in a water phase is illustrated in Figure 1. The water phase (35) is inside a closed volume (36) and (41) which may be formed by a shaft in the ground, a tank onshore, a dry dock or a barge (37) placed in the sea or a river or lake. (36) is the side of the water phase (35), while (41) is the bottom of the water phase (35). (37) and (41) may be insulated for control of the temperature in water phase (35). If the water phase (35) is placed on a barge (37), the bottom (41) may be open to sea / river or lake. Water for cooling the process may be supplied through (42) and or (43), which may be, but not restricted to, a pipe or tunnel connecting the water phase (35) to a cold water supply. This may e.g. be to a deep water intake in the sea or a lake, suppling cold water throughout the year. Cold water coming in through (42) or (43) may be evenly distributed in the water phase (35) by e.g. one or more propels (45), or by any other means suited for the purpose. Warm water from the system is taken out in top of (36) through (44), which may be a pipe or tunnel or any other means suited for the purpose. The cold water to water phase (35) should preferentially be from a source with higher pressure than the static hight, h3, in (35), e.g. from a lake or river, or from the sea with a surface level equal to surface level (40) in water phase (35). In this way circulation of cold water from the sea will only require energy for movement of water through (42) and (43) to output in (44) without lifting the natural static head of the sea water.

[0120] If the water phase (35) is connected to the sea, or any other water source with varying water level (40) (e.g. ebb and flow), tanks (3) and (6), optional tanks (12) and (22), together with any wanted infrastructure (e.g. pipes, valves, and pumps) may be placed on any suitable floating device (39) in water phase (35). The top of water phase (35) may be covered by a any suitable construction (38), which may contain any items suitable for the CO2 capture system given in the invention as e.g., but not restricted to, compressor, pipes, and valves. Tanks (3), (6), (12), and (22) and any other surfaces exposed to water phase (35) may be covered by any suitable means (46) for protection of biogenic fouling or any other fouling or corrosion that may appear over time. If water phase (35) is inside a closed volume (36) and (41), the volume (35) may be drained for water for maintenance and repairs when needed. For industrial plants, the CO2 capture process given in the invention may be divided into parallel units (complete or partial), allowing one or more units not to be operative at any time for e.g. maintenance.

[0121] A second preferred embodiment of the invention is given in Figure 2 as the pipe systems in the present invention may be of any pipe diameter, pressure rate and material and may contain any valve, choke, heat exchanger or any other item suitable for the process. Similar numbers in Figure 1 and Figure 2 are equivalent process units and pipe systems. In this embodiment, excess heat (50), preferably hot water, from e.g. industrial processes or (59) from heat exchanger (15) of any temperature above e.g. 30 °C may be used to improve the performance of present invention by heating (51) the water phase (19) from first depressurized / flash unit (6) before entering second depressurization / vacuum unit (12). Heater (51) may be any kind of heater. When entering depressurization / vacuum unit (12), the water phase from tank (6) will have a higher temperature than in tank (6), making de-gassing in (12) more energy efficient. Unit (12) may be insulated (58) and be at any place suited for the process. Unit (12) may optionally be or comprise a membrane system.

[0122] Gas and vapor from pipe system (13) will be cooled in heat exchanger (55) in order to condensate water vapor in (13) before entering separator (22). Condensed water from (22) is returned to the second depressurizing / flashing unit (12) by pipe system (23) and pump / choke (30). Degassed water from depressurization / vacuum unit (12) is cooled in heat exchanger (56) before returned to tank (3). Heat exchangers (55) and (56) may be any kind of heat exchangers. Cooling water (53) and (54) for heat exchangers (55) and (56) may be from water phase (35) by pipe system (57) and pump (52). Pump (52) may be any kind of pump. After heat exchangers (55) and (56) warm water in pipeline systems (53) and (54) is mixed with warm water in pipeline system (50). Warm water from pipeline system (50) is then mixed with output water (44) from water phase (35). Temperature in (44) after mixing with warm water from (50) should have a temperature acceptable for disposal to e.g. sea (e.g. less than 30 °C). Pipe systems (50), (53), (54) and (57) may be of any pipe diameter, pressure rate and material and may contain any valve, heat exchanger, cooler and / or any other item suitable for the process.

Claims

Claims1. Method for capturing CO2 from a gas mixture comprising CO2 and one or more other gases by a system for capturing CO2, where the CO2 gas has higher solubility in the water than the one or more other gases, where the method comprises the following steps:- injecting the gas mixture as bubbles into a water volume with a surface in an absorber unit (3), allowing the gas mixture in the bubbles to interact with the water, thereby causing CO2 gas to dissolve in the water to a higher degree than the one or more other gases, further allowing the bubbles enriched in gas not dissolved to ascend to and exit from the surface of the water volume, resulting in the water becoming enriched in CO2, and further allowing the CO2 enriched water to exit out of the water volume;- performing a first step depressurizing of the CO2 enriched water in a first step depressurizing unit (6) to a first pressure level thereby degassing the one or more other gases from the CO2 enriched water to a higher degree than CO2 giving first step depressurized water;- performing a second step depressurizing of the first step depressurized water in a second step depressurizing unit (12), where depressurizing is to a lower pressure level than that in the first step depressurizing, thereby degassing CO2 from the first step depressurized water giving second step depressurized water depleted in CO2; and- performing temperature control by: i) cooling content (fluid) in the absorber unit (3) and the first step depressurizing unit (6) by a water phase (35) in which said units are at least partially submerged.

2. Method according to claim 1 , where the perform temperature control comprises: ii) heating water phase from the first step depressurizing before entering the second step depressurizing thereby making the second step depressurizing more efficient, where the heating utilizes one or more of i) excess heat from an external industrial process and ii) heat from an internal heat exchanger comprised by the system.

3. Method according to one of the claims above, where the performing temperature control further comprises cooling content in the second step depressurizing unit (12) by water a water phase (35) in which said second step depressurizing unit (12) is at least partially submerged.

4. Method according to one of the claims above, where the performing temperature control further comprises one or more of the following steps: iii) cooling gas and vapor from the second step depressurizing for condensation before performing separation; iv) cooling second step depressurized water before returning it to the absorber unit (3); v) cooling water before disposal by mixing with water from the water phase (35).

5. Method according to one of the claims above, where depressurizing is by one or more of the following: A throttling process, flashing over a valve / choke, or pumping by a vacuuming device.

6. Method according to one of the claims above, further comprising collecting the CO2 degassed from the water in the second step depressurizing.

7. Method according to one of the claims above, comprising, prior to the injecting, at least one of i) compressing the gas mixture preferably to a minimum pressure of 2 to 10 bara and ii) cooling the gas mixture preferably to 5-30 degrees C.

8. Method according to one of the claims above, where the water in the water volume is counter current to the bubbles, with a water descent rate preferably lower than ascent rate of the bubbles.

9. Method according to one of the claims above, comprising obtaining pressure of the liquid water phase in the water volume by at least one of the following: A static head hi in the water volume and: Gas pressure acting on the surface of the water volume.

10. Method according to one of the claims above, comprising using gas under pressure from pipe system (5) in compressor (34) for compression of incoming gas (1) and (11).11 . Method according to claim 10, where the heating water phase from the first step depressurizing before entering the second step depressurizing comprises using heat from cooling water (59) used to cool the compressed incoming gas (1) and (11).

12. Method according to one of the claims above, where gas degassed from the water in the first step depressurizing is reinjected in the gas mixture prior to the injecting and, if applicable, the compressing according to claim 7.

13. Method according to one of the claims above, further comprising cooling gas and vapour degassed from the water in the second step depressurizing giving condensed water and gas enriched in CO2.

14. Method according to claim 13, comprising reinjecting the condensed water into the second step depressurized water prior to the second step depressurizing.

15. Method according to claim 13 or 14, further comprising releasing the gas enriched in CO2 to any further treatment.

16. Method according to one of the claims above, comprising injecting the second step depressurized water depleted in CO2 into the water volume.

17. Method according to one of the claims above, injecting water into the water volume.

18. Method according to one of the claims above, comprising diverting at least parts of the CO2 enriched water from the absorber unit (3) thereby producing carbonated water.

19. System for capturing CO2 from a gas mixture comprising CO2 and one or more other gases by dissolution in a liquid water phase, where the CO2 gas has higher solubility in the water than the one or more other gases, where the system is arranged for performing the method according to one of the claims above.

20. System for capturing CO2 from a gas mixture comprising CO2 and one or more other gases by dissolution in a liquid water phase, where the CO2 gas has higher solubility in the water than the one or more other gases, where the system comprises:- an absorber unit (3), and means for injecting the gas mixture as bubbles into a water volume with a surface in the absorber unit (3), allowing the gas mixture in the bubbles to interact with the water, thereby causing CO2 gas to dissolve in the water to a higher degree than the one or more other gases, further allowing the bubbles enriched in gas not dissolved to ascend to and exit from the surface of the water volume, resulting in the water becoming enriched in CO2, and further allowing the CO2 enriched water to exit out of the volume;- a first step depressurizing unit (6) arranged for performing a first step depressurizing of the CO2 enriched water to a first pressure level thereby degassing the one or more other gases from the CO2 enriched water to a higher degree than CO2 giving first step depressurized water;- a second step depressurizing unit (12) arranged for performing a second step depressurizing of the first step enriched water where depressurizing is to a lower pressure level than that in the first step depressurizing, thereby degassing CO2 from the first step depressurized water giving second step depressurized water depleted in CO2; and- means for performing temperature control comprising: i) means for cooling content in the absorber unit (3) and the first step depressurizing unit (6) by a water phase (35) in which said units are at least partly submerged.

21. System according to claim 20, comprising means for heating water phase from the first step depressurizing before entering the second step depressurizing therebymaking the second step depressurizing more efficient, where the heating utilizes one or more of i) excess heat from an external industrial process and ii) heat from an internal heat exchanger comprised by the system.

22. System according to one of the claims 20-21 , where the means for performing temperature control further comprises the second step depressurizing unit (12) being at least partly submerged in a water phase (35) by which the content of said unit is cooled.

23. System according to one of the claims 20-22, where the means for performing temperature control further comprises means for one or more of the following steps: iii) cooling gas and vapor from the second step depressurizing for condensation before performing separation; iv) cooling second step depressurized water before returning it to the absorber unit (3); v) cooling water before disposal by mixing with water from the water phase (35).

24. System according to one of the claims 20-23, where the means for second step depressurizing is or comprises a membrane unit suitable for gas / water separation.

25. System according to one of the claims 20-24, where the water phase (35) is inside a volume (36, 41) optionally formed as one of i) a shaft in the ground, ii) a tank onshore, iii) a dry dock and iv) a barge (37) placed in a natural water volume.

26. System according to claim 25, where the volume (36, 41) is formed according to iv), where the system comprises a floating device (39) which at least parts of the system is mechanically connected to.

27. System according to claim 25 or 26, where the volume (36, 41) is formed according to iv), and with an open bottom.

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