The present invention relates to a method for removing carbon dioxide (CO2) from a CCh-containing stream, in particular dilute sources such as air. Methods for removing CO2 from a CCp-containing stream such as air are known in the art. As a mere example, US 10 421 913 discloses a method for the production of synthetically produced methane or other hydrocarbons, wherein carbon dioxide is adsorbed from atmosphere and subjected to an exothermic methane- or Fischer-Tropsch synthesis. Figure 1 of US 10421913 discloses a carbon dioxide recovering system in some level of detail. This carbon dioxide recovering system is preferably operated continuously, wherein recovered carbon dioxide is temporarily stored in a buffer storage.

Other example processes are described in EP0469781A2 and US4952223 which disclose methods for CO2 capture from combustion exhaust gases and other waste streams. These methods typically capture CO2 from inlet streams that have higher concentrations of CO2 and lower concentrations of oxygen, e.g., less than 10 volume %. These processes do not address the problem of capturing CO2 from very dilute sources such as air where the concentration of CO2 is much lower than combustion exhaust gases and waste streams and the oxygen content is higher than 10 volume %, typical around 21 volume %.

A problem of the above and other methods for removing CO2 from air is that the used systems do not arrange for removing entrained air from the CO2 product. A further problem is that if the CO ₂ is purified in a downstream process, the carbon dioxide is not recovered from the air to its fullest extent.

An even further problem is that buffer storage of CO ₂ product (as done in the above-mentioned US 10421913) is very expensive, if sufficient volume is required to allow intermittent operation of the CO ₂ removal step.

It is an object of the present invention to minimize one or more of the above problems.

It is a further object of the present invention to provide a simple method for removing CO ₂ from air, wherein the carbon dioxide is recovered to a fuller extent.

One or more of the above or other objects can be achieved by providing a method for removing carbon dioxide (CO ₂ ) from a C0 ₂ -containing stream, the method at least comprising the steps of: a) providing a C0 ₂ -containing stream, preferably air, wherein the C0 ₂ -containing stream (10) has a CO ₂ content in the range of from 10 to 1000 ppmv, preferably from 100 to 1000 ppmv; b) removing CO ₂ from the CC> ₂ -containing stream provided in step a) in a first CCy-removal unit, thereby obtaining a first CC> ₂ -enriched stream and a first CCy-depleted stream; c) liquefying the first CCy-enriched stream obtained in step b) in a liquefaction unit; d) removing from the liquefaction unit at least a liquefied CO ₂ stream and a gaseous stream containing at least nitrogen [N ₂ (g)], oxygen [0 ₂ (g)] and 00 ₂ (g).

It has surprisingly been found according to the present invention that CO ₂ can be removed from air in a surprisingly simple manner and can be concentrated to levels beyond 90 vol.%, even beyond 95 vol.% or even beyond 99 vol.%.

A further advantage of the present invention is that CO2 can be recovered in a non-continuous manner.

Another advantage of the present invention is that there are low CO2 losses during purification.

A further advantage of the present invention is that there is no requirement of a gaseous buffer for the intermediate storage of CO2.

In step a) of the method according to the present invention a CC>2-containing stream, preferably air, is provided. In general, it is well known that air comprises of constituents such as nitrogen, oxygen, carbon dioxide, hydrogen, helium, argon, methane, krypton. Air also comprises of water vapour. Typical concentrations of nitrogen, oxygen, argon in dry air are 78.08 vol. %,

20.95 vol.% and 0.93 vol.% respectively (Haynes, W.M., hide, D.R., & Bruno, T.J. (Eds.), Chapter 14-3, 2016, CRC Handbook of Chemistry and Physics (97th ed.), CRC Press). The concentration of carbon dioxide in air has steadily increased from around 310 ppmv in 1960 to 420 ppmv in 2022 (C. D. Keeling, S. C. Piper, R. B. Bacastow, M.

Wahlen, T. P. Whorf, M. Hermann, and H. A. Meijer, Exchanges of atmospheric CO2 and 13-C0 ₂ with the terrestrial biosphere and oceans from 1978 to 2000. I. Global aspects, SIO Reference Series, No. 01-06, Scripps Institution of Oceanography, San Diego, 88 pages, 2001). It is expected that the concentration of CO2 in air will increase unless mitigated. For all practical purposes, the present invention is applicable with any anticipated increase or decrease in the concentration of CO2 in the air. In an embodiment of the present invention, one of ordinary skill in the art will readily understand that the CC>2-containing stream is not particularly limited and may come from various sources. The CC>2-containing stream may comprise less than 10 vol.% CO2, preferably less than 5 vol.% CO2, more preferably less than 2 vol.% CO2. The remainder of the CC>2-containing stream may comprise one or more of nitrogen, oxygen, water vapour, argon.

According to a preferred embodiment of the present invention, the CCy-containing stream provided in step a) may be air, and the CCy-containing stream may have a CO2 content in the range of from 10 to 1000 ppmv, preferably from 100 to 1000 ppmv.

In step b) of the method according to the present invention, CO2 is removed from the CC>2-containing stream provided in step a) in a first CCy-removal unit, thereby obtaining a first CC>2-enriched stream and a first CO2- depleted stream.

As the person skilled in the art is familiar with such CCp-removal units, these are not discussed here in detail. Examples of such CCy-removal units are adsorption units (e.g. as described in US 10521880 B2, US 9975 087, US 9751 039 B2, the above-mentioned US 10421913, etc.). Preferably, the CCy-removal unit comprises a CO2 adsorption unit. By adjusting the operating parameters of the first CC>2-removal unit, the conditions of the first CC>2-enriched stream and the first CCy-depleted stream such as, but not limiting to, concentration of CO2 and pressure are determined.

Preferably, the first CCy-enriched stream obtained in step b) has a CO2 content (excluding water) of at least 60 vol.%, preferably at least 80 vol.%, more preferably at least 90 vol.%. Typically, the first CC>2-enriched stream obtained in step b) has a CO2 content of at most 99.5 vol.%, preferably at most 98.5 vol.%, more preferably at most 95 vol.%.

Typically, the first CCy-depleted stream obtained in step b) has a CO ₂ content of at most 200 ppmv.

According to a preferred embodiment of the method according to the present invention, the first CO ₂ - enriched stream obtained in step b) has a pressure of 0.5 to 1.5 bara, preferably from 0.9 to 1.1 bara.

In step c) of the method according to the present invention, the first CCy-enriched stream obtained in step b) is liquefied in a liquefaction unit.

The liquefaction unit is not particularly limited. As the person skilled in the art is familiar with such liquefaction units, these are not discussed here in detail. Typically, such a liquefaction unit contains a compressor, an expansion valve and a gas/liquid separation unit along with optional heat exchangers.

Other liquefaction units may also contain a compressor, a refrigeration chiller, optional heat exchangers and a gas/liquid separation unit.

In step d) of the method according to the present invention, at least a liquefied CO ₂ stream and a gaseous stream containing at least nitrogen [N ₂ (g)], oxygen [O ₂ (g)] and CO ₂ (g) are removed from the liquefaction unit. Typically, the gaseous stream containing at least N ₂ (g), 0 ₂ (g) and 00 ₂ (g) is removed from a gas/liquid separator forming part of the liquefaction unit. Typically, the gaseous stream containing at least N ₂ (g), 0 ₂ (g) and 00 ₂ (g) as removed from the liquefaction unit has a CO ₂ content of at least 40 vol.%, preferably at least 50 vol.%.

According to an especially preferred embodiment of the present invention, at least a part of the gaseous stream removed in step d) is combined with the CO ₂ - containing stream provided in step a).

In this way, the capture of CO ₂ can be further improved (instead of venting it into the atmosphere).

Further it is preferred that, alternatively or additionally, at least a part of the gaseous stream removed in step d) is separated in a second CO ₂ removal unit, thereby obtaining a second CCy-enriched stream and a second CCy-depleted stream, wherein the second CO ₂ - enriched stream is combined with the first CC> ₂ ^_ enriched stream. In general, one of skill in the art is familiar with such CO ₂ removal units, these are not discussed here in detail. By adjusting the operating parameters of the second CC-removal unit, the conditions of the second CC-enriched stream and the second CCy-depleted stream such as, but not limiting to, concentration of CO ₂ is determined. Preferably, the second CC> ₂ ^_ enriched stream has a CO ₂ content (excluding water) of at least 90 vol.%, preferably at least 95 vol.%. Typically, the second CC> ₂ ^_ depleted stream has a CO ₂ content of at most 30 vol.%.

Furthermore, it is preferred that, alternatively or additionally, at least a part of the gaseous stream removed in step d) is used as a sweep gas in the CO ₂ removal unit of step b).

The person skilled in the art will understand that the liquefied CO ₂ stream removed from the liquefaction unit can be used in various ways.

Preferably, the liquefied CO ₂ stream removed in step d) is used in a conversion process, sequestration or transport (e.g. by pipeline or ship), after optional storage and pumping. The conversion process can be selected from a broad range of processes such as RWGS (reverse water gas shift), methanation, methanol synthesis, etc. As the person skilled in the art is familiar with these conversion processes as such, these are not discussed here in detail.

Hereinafter the present invention will be further illustrated by the following non-limiting drawings.

Herein shows:

Fig. 1 schematically a flow scheme of a first embodiment of a method for removing CO2 from a CO2- containing stream according to the present invention; and

Fig. 2 schematically a flow scheme of a second embodiment of a method for removing CO2 from a CO2- containing stream according to the present invention; and

Fig. 3 schematically a flow scheme of a third embodiment of a method for removing CO2 from a CO2- containing stream according to the present invention.

For the purpose of this description, same reference numbers refer to same or similar components.

The flow scheme of Figure 1, generally referred to with reference number 1, comprises a first CC>2-removal unit 2, a CO2 liquefaction unit 3, a liquid CO2 storage unit 4, a CO2 pump 5, and a CO2 conversion unit 6.

During use of the flow scheme of Fig. 1, a CO2- containing stream 10 (preferably air) is provided.

In a first CCy-removal unit 2, e.g. in the form of a CO2 adsorption unit, CO2 is removed from the CO2- containing stream 10, thereby obtaining a first CO2- enriched stream 30 and a first CCy-depleted stream 20. In the embodiment of Fig. 1, steam stream 70 is used as a sweep gas (to desorb CO2 as adsorbed in the CO2 adsorption unit 2).

The first CCy-enriched stream 30 is then liquefied in the liquefaction unit 3. From the liquefaction unit 3 at least a liquefied CO2 stream 40 and a gaseous stream 15 are removed. The gaseous stream 15 contains at least N ₂ (g), oxygen 0 ₂ (g) and 00 ₂ (g).

The liquefied CO2 stream 40 is used in a CO2 conversion process performed in the CO2conversion unit 6, after optional storage in liquid CO2 storage unit 4 (from which it is pumped as liquid stream 50,60 by CO2 pump 5 to the CO2conversion unit 6). The CO2conversion process in the CO2conversion unit 6 along with any other reactants (shown as stream 85) as required by the conversion process) results in a products stream 90.

Instead of converting the liquefied CO2 stream 40, it can also be used for sequestration or transport (e.g. by pipeline or ship).

As shown in the embodiment of Fig. 1, at least a part of the gaseous stream 15 is combined with the CO2- containing stream 10. This surprisingly allows that a higher CO2 recovery from the CCy-containing stream 10 is achieved, without adding complexity to the system.

Fig. 2 and Fig. 3 show schematically alternative embodiments of methods for removing CO2 from a CO2- containing stream according to the present invention. It goes without saying that the embodiments of Figs 1-3 may be combined in any way.

In the embodiment of Fig. 2, a second CO2 removal unit 7 is present. At least a part of the gaseous stream 15 is separated in the second CO2 removal unit 7, thereby obtaining a second CC>2-enriched stream 25 and a second CC>2-depleted stream 80, wherein the second CC>2-enriched stream is combined with the first CC>2-enriched stream 30.

In the embodiment of Fig. 3, at least a part of the gaseous stream 15 is used as a sweep gas in the CO2 removal unit 2.

Examples Example 1

The flow scheme of Fig. 1 was used for illustrating an exemplary method according to the present invention. The compositions and conditions of the streams in the various flow lines are provided in Table 1 below.

The values in Table 1 were calculated using a model generated with commercially available UniSim software, whilst using standard thermodynamic fluid packages with settings such that CO2 removal processes and CO2 liquefaction processes are simulated.

The obtained CO2 content of the first CCy-enriched stream 30 was 90 vol.% and of the liquefied CO2 stream 40 was 99.7 vol.%.

The total CO2 recovery was defined as the ratio of moles of CO2 in the liquefied CO2 stream 40 to the moles of CO2 in the CC>2-containing stream 10; a total CO2 recovery of 58% was obtained.

Table 1

Example 2 (comparative)

For comparison with Fig. 1, and using the same UniSim software, the method of Fig. 1 but without recycling stream 15 to stream 10 was simulated. The compositions and conditions of the streams in the various flow lines are provided in Table 2 below. The total CO2 recovery in this case was 42%.

Table 2

As can be seen from Tables 1 and 2, the exemplary method of Fig. 1 (with recycle) has a higher CO2 recovery (58%) than the one without recycle (viz. 42%). Comparison of flow rates of stream 10 in Tables 1 and 2 indicate that recycling the gaseous stream 15 (containing at least N2(g), 02(g) and CO2(g)) as removed from the liquefaction unit 3 to the feed stream 10 reduces the total flow of the C0 ₂ -containing stream 10, thereby increasing efficiency of the present invention.

Discussion As can be seen from the above Figures and Examples, the method according to the present invention allows for a surprisingly simple and effective way of increasing CO2 recovery and purity from a CCy-containing stream, without adding complexity to the system. According to the present invention, CO2 concentrations of at least 90 vol.% can be achieved, and even as high as above 99 vol.%.

The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention.