Patent Application
20240335787
The present invention generally relates to a method for selectively separating CO2 gas from a mixture of gases containing N2, O2 as well as other gases in trace amounts, 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.
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. To meet this challenge, 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. However, the technical solutions that have emerged so far are far from satisfactory. A prominent example is the amine process where CO2 is separated from flue gases, followed by compression, long distance transport and sequestration in cavities deep underground (see, e.g.: https://ccsnorway.com/capture-studies/). These processes are very energy consuming and expensive, carrying their own significant carbon footprints.
There is thus a pressing need to develop low cost, high capacity solutions that are capable of extracting CO2 from relevant gas mixtures and delivering it in a form that facilitates long term sequestration or makes it suited as a feedstock in industrial processes.
U.S. Pat. No. 4,576,615 A “Carbon dioxide hydrocarbons separation process” discloses separation of carbon dioxide from light hydrocarbons such as methane and ethane by absorbing CO2 gas with water at a pressure of at least 500 psia thus avoiding the CO2-C2H6 azeotrope problem. The CO2 is recovered from the absorbing water by pressure reduction and flashing and/or by heating and flashing.
U.S. Pat. No. 4,566,278 A “Methane-carbon dioxide scrubbing method and system” discloses a thermodynamic method and system for up-grading the quality of digester methane gas by removing substantially all of the non-combustible carbon dioxide gas from the digester gas in a scrubbing system operated by the waste heat of an internal combustion engine utilizing the up-graded methane gas as its fuel source and driving a generator to produce electric power.
Kohl A. L. et al., “Gas Purification, passage”, Jan. 1, 1997, Gas Purification, Gulf Publishing Company, Houston Texas, pages 427 and 438, presents a simple water-wash process for carbon dioxide removal.
A first aspect of the invention is a method for extracting CO2 from a gas mixture comprising CO2, where the method comprises dispensing the gas mixture in water in a first water volume, dissolving CO2 in the first water volume resulting in water enriched in dissolved CO2, and passing the water enriched in dissolved CO2 through a turbine into a recipient causing the CO2 enriched water experience a pressure drop, thereby eliciting flashing of the dissolved CO2 into gas phase and leaving degassed water at the lower part of the recipient.
Optionally, the method comprises venting the gas volume of the recipient allowing the flashed CO2 gas exiting the gas volume.
Optionally, the dispensing of the gas mixture is performed at a depth in the first water volume by bubbling in a liquid counterflow configuration.
Optionally, the dispensing of the gas mixture comprises mixing of the gas mixture and water in a mixing unit containing a part of the first water volume where optionally the mixing unit is employing one or more of the following: Counterflow bubbling columns, gas and water-fed Venturi bubble generators, water agitation in gas atmosphere (spray, mist, trompe), and gas diffusion through permeable membrane.
Optionally, the method comprises pumping degassed water out of the recipient through a second water volume, leading the gas mixture through a tube via a heat exchanger arranged in the second water volume where temperature of the gas mixture is higher than temperature of water in the second water volume thereby contributing to lifting of degassed water, and thereafter dispensing the gas mixture transported in the tube in the first water volume.
Optionally, the method comprises feeding water emanating from the second water volume into the first water volume thus forming a recirculating loop, and further optionally, the method comprises cooling the water emanating from the second water volume before feeding into the first water volume.
Optionally, the method comprises balancing bubble rising speed against descent speed of water in the first water volume to achieve optimal separation of CO2 from other gas mixture constituents.
Optionally, the gas mixture is a flue gas.
Another aspect of the invention is a system for extracting CO2 from a gas mixture comprising CO2, where the system comprises means for dispensing the gas mixture in water in a first water volume causing CO2 to be dissolved resulting in water enriched in dissolved CO2, and a turbine arranged in fluidal connection with the first water volume allowing for passing of the water enriched in dissolved CO2 into a recipient, causing the CO2 enriched water to experience a pressure drop, thereby eliciting flashing of the dissolved CO2 into gas phase and leaving degassed water at the lower part of the recipient.
Optionally, the system comprises a venting tube arranged for venting the gas volume of the recipient allowing the flashed CO2 gas exiting the gas volume.
Optionally, the recipient is arranged for collecting the degassed water in a lower part thereof.
Optionally, the means for dispensing of the gas mixture is arranged at a depth in the first water volume by bubbling in a liquid counterflow configuration.
Optionally, the means for dispensing of the gas mixture comprises a mixing unit arranged for containing a part of the first water volume and arranged for mixing of the gas mixture and water, where optionally the mixing unit is arranged for employing one or more of the following: Counterflow bubbling columns, gas and water-fed Venturi bubble generators, water agitation in gas atmosphere (spray, mist, trompe), and gas diffusion through permeable membrane.
Optionally, the system comprises a pump arranged for pumping degassed water out of the recipient through a second water volume, a tube arranged for leading the gas mixture, a heat exchanger arranged in the second water column and connected to the tube for leading the gas mixture therethrough, where temperature of the gas mixture is higher than temperature of water in the second water volume, thereby contributing to lifting degassed water, and thereafter to the means for dispensing the gas mixture in the first water volume.
Optionally, the system comprises means for feeding water emanating from the second water volume into the first water volume thus forming a recirculating loop.
Optionally, the system comprises means for cooling the water emanating from the second water volume before feeding into the first water volume.
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.
Embodiments of the present invention will now be described, by way of example only, with reference to the following figures, wherein:
The following reference numbers refer to the drawings:
The present invention exploits differences in the solubility of gases in water to obtain separation of gases in a gas mixture. This is achieved in a high throughput process where the gas mixture is brought into contact with water across a large contact area, causing the gas species with the highest solubility to dominate gas transfer into the water phase. The remaining gas species with lower solubility are dissolved to a smaller degree and are subsequently separated out by mechanical means. For concreteness, it shall be assumed in the following that the gas mixture is flue gas from a combustion process. Three species are of particular interest, namely N2, O2 and CO2, whose relative concentrations may be typically 70-80%, 2.5-4% and 1-25%, respectively. Generally, it is desired to separate the CO2 from the other gaseous species. The solubilities in water for these gases are very different, which is exploited in the present invention: Under equilibrium conditions and at 10° C. and 1 bar partial pressure, the solubilities are respectively 0.019 g. N2 gas per kg water, 0.057 g. O2 gas per kg water, and 2.5 g. CO2 gas per kg water. Thus, the solubility ratio is 44:1 between CO2 and O2 and 132:1 between CO2 and N2. At saturation, i.e. at long bubbling times, the water in the example shown in
c
a
=H
cp
p
a Eq.1
Here, Hcp is the Henry solubility constant, ca is the concentration of a species in the aqueous phase under equilibrium conditions and pa is the partial pressure of that species in the gas phase.
When CO2 dissolves in water, the carbon enters a chain of interacting processes:
CO2(gas)↔CO2(liquid) Eq.2
CO2(liquid)+H2O↔H2CO3 Eq.3
H2CO3+H2O↔H3O++HCO3− Eq.4
HCO3+H2O↔H3O++CO32− Eq.5
Here, CO2 (liquid) is carbon dioxide in solvated form in the water, H2CO3 is carbonic acid, HCO3 is bicarbonate and CO32− is carbonate. The relative concentrations of these species depend on the pH, with the dominant species at equilibrium and near-neutral pH being bicarbonate. The sum of these species is often referred to as dissolved inorganic carbon: DIC. The amount of DIC that can be absorbed in water depends on several factors, including the concentration and types of ionic species, as well as the temperature and CO2 partial pressure. Thus, at 10 C and 1 bar CO2 partial pressure, about 2.5 kg. of CO2 can be accommodated in 1 m3 of water.
Some basic principles of the invention shall now be described with reference to
The flue gas (1) is introduced via a tube (2) and brought into contact with water (3) in a first water-filled column (4) by a dispersion device (5). The dispersion device releases a cloud of bubbles (6) that exhibit a large contact area between the gases in the bubbles and the surrounding water. The bubbles tend to float upwards but encounter a downward flow of water inside the column (4). The gas transport 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 (7). Thus, a high degree of gas separation can be achieved by collecting the high solubility gas that has been dissolved in the water on the one hand and allowing the low solubility gas species to escape from the surface (7) on the other hand. Water (8) is continuously added at the top of the column and is enriched in dissolved CO2 as it descends. Ultimately the CO2 enriched water (9) passes through a turbine (10) which generates electrical power W:
Here V is the volumetric flow rate of water, p the average density of water in the column and h1 the water column height. The water that passes through the turbine experiences a sudden pressure drop ΔP:
as it exits into the volume (11) below, which communicates with the ambient atmosphere via the venting tube (12). This elicits flashing of the dissolved CO2 into the gas phase and the flashed gas (18) exits through the venting tube (12). The degassed water (13) flows down into a recipient (14).
As can be recognized at this stage, CO2 has been separated from the other flue gas components and brought into a water phase, energy has been generated in the turbine, water with the dissolved CO2 has been degassed and the separated CO2 has been brought out of the system in a separate venting tube (12). Experiments and analysis have shown that this procedure can deliver a separation efficiency exceeding 97% even with short columns (4) (less than 2 meters).
Further, the preferred embodiment shown in
An important feature of the system shown in
The preferred embodiment illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 20210864 | Jul 2021 | NO | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/NO2022/050158 | 6/30/2022 | WO |
