The present invention relates to a process for producing ethylene, in particular from a CO-containing stream that has been obtained from CO2 captured from the atmosphere, flue gas or the like.

Ethylene is an important raw material for multiple end products like ethylene oxide, ethylene glycol, polymers, rubbers, plastics, etc. Processes for producing ethylene are known in the art.

To date, ethylene has been predominantly produced via steam cracking of hydrocarbons derived from crude oil or via conversion of natural gas.

An example of selective production of ethylene from methane has been disclosed in W02021/009627A1.

However, the increasing availability of low-cost renewable electricity and the desire to decrease carbon emissions through CO2 capture presents an opportunity to produce carbon-based feedstocks and fuels via the electrochemical reduction of carbon dioxide (CO2) to chemical feedstocks. As a result, there has been an increasing amount of research into identifying pathways of electrochemical CO2 reduction to ethylene.

As a mere example, the article by J. Sisler et al., "Ethylene Electrosynthesis: A comparative techno-economic analysis of alkaline vs membrane electrode vs CO2-CO-C2H4 tandems" in ACS Energy Lett. 2021, 6, 997-1002, discloses 'single step' or 'direct' conversion of CO2 to C2H4 (ethylene) using an electrolyzer . A problem of the process as described in this article is that it requires the separation of CO, ethylene and H2. This separation step can be cumbersome or at least expensive as it typically requires cryogenic distillation.

It is an object of the present invention to solve, minimize or at least reduce one or more of the above problems .

It is a further object of the present invention to provide an alternative process for producing ethylene, in particular from a CO-containing stream which CO- containing stream has been obtained from CO2 captured from the atmosphere, flue gas, or the like.

One or more of the above or other objects may be achieved according to the present invention by providing a process for producing ethylene, the process at least comprising the steps of:

(a) providing a CO-containing stream;

(b) converting the CO-containing stream provided in step (a) in an electrolyzer thereby producing an ethylene-containing vapour stream and an ethanol- containing liquid stream;

(c) subjecting at least a part of the ethylenecontaining vapour stream obtained in step (b) to hydration thereby obtaining a first ethanol-enriched stream;

(d) separating the first ethanol-enriched stream obtained in step (c) thereby obtaining a second ethanol- enriched stream and a water-enriched stream; and

(e) subjecting the second ethanol-enriched stream to dehydration thereby obtaining ethylene.

It has surprisingly been found according to the present invention that by the 'indirect' conversion of carbon monoxide (CO) to ethylene (via the intermediate conversion into ethanol) the separation (via cryogenic distillation) of CO, ethylene and H2 can be avoided.

A further advantage of the process according to the present invention is that ethanol can be easily stored in for example low-cost tanks. This can be of importance in case the electrolyzer is driven by renewable power such as wind, solar and other forms of renewable power that has intermittency issues. By using low-cost storage facilities for the ethanol, the use of relatively expensive batteries to ensure a (n otherwise needed) continuous operation of the electrolyzer can be avoided; this, as in case the electrolyzer is not working because of intermittency issues, the stored ethanol can be used in downstream processes (which often operate continuously) .

In step (a), a CO-containing gas stream is provided. The CO-containing gas stream is not limited in any way (in terms of composition, temperature, pressure, etc.), as long as it contains CO. The CO-containing gas stream may have various origins.

Preferably, the CO-containing stream has been obtained from a C02-containing stream, preferably CO2 captured from the atmosphere. As a mere examples, the CO-containing gas stream may have been obtained by thermochemical or electrochemical CO2 conversion routes; methane conversion routes; gasification of biomass or waste; etc.

Preferably, the CO-containing stream provided in step (a) comprises at least 25 mol.% CO, preferably at least 50 mol.% CO.

Some other compounds such as nitrogen (N2), methane (CH4), carbon dioxide (CO2), water (H2O) and hydrogen (H2) may be present in the CO-containing stream. As an example, if present, H2 is present in the CO-containing stream in an amount of at most 75 mol.%, preferably at most 50 mol.%. Preferably, the CO-containing stream does not contain oxygen (O2) and sulfur compounds such as H2S and SO _X .

Typically, the CO-containing stream as provided in step (a) has a temperature in the range of from 0 to 90°C, preferably from 15 to 80°C, more preferably below 65°C. Further, the CO-containing gas stream as provided in step (a) typically has a pressure in the range of from 0.5 to 30.0 bara, preferably below 10.0 bara and preferably around 1.0 bara. If appropriate, the CO- containing stream may have been pre-processed to obtain the desired composition and conditions.

In step (b), the CO-containing stream provided in step (a) is converted in an electrolyzer thereby producing an ethylene-containing vapour stream and an ethanol-containing liquid stream.

As the person skilled in the art is familiar with electrolyzers, this will not be discussed here in full detail. In general, an electrolyzer uses electricity to drive an otherwise non-spontaneous chemical reaction. The Electrolyzer will typically comprise an anode and a cathode separated by a membrane, and electrolyte. For more information on electrolyzers, reference is made to the article by B. Endrodi et al., Continuous-flow electroreduction of carbon dioxide, Progress in Energy and Combustion Science, Volume 62, September 2017, pages 133-154 .

According to a preferred embodiment of the present invention, the electrolyzer is driven by renewable power. Renewable power can be either intermittent or continuous. The person skilled in the art will readily understand that the components of the electrolyzer may be constructed from a wide range of materials.

The cathode as used in the electrolyzer is not particularly limited. Generally, the cathode of the electrolyzer is selected from copper, silver, gold, platinum, tin, lead, palladium, aluminium, zinc, titania, carbon black, carbon nanotubes, graphene (with or without nitrogen, sulphur, phosphorus doping) or combinations thereof. Preferably, the electrolyzer in step (b) has a cathode that comprises copper (Cu) or a copper-based alloy.

Also, the anode as used in the electrolyzer is not particularly limited. Preferably, the electrolyzer in step (b) has an anode that comprises a metal selected from the group consisting of nickel (Ni), iron (Fe), iridium (Ir), cobalt (Co), manganese (Mn), ruthenium (Ru) or combinations thereof. The anode may comprise an oxide of the above metals.

The electrolyte is also not particularly limited. Typically, the electrolyte is an aqueous electrolyte containing a compound selected from the group consisting of carbonates, bicarbonates, hydroxides, halides of Na+, K+, Rb+, Cs+, NH ₄ ⁺ , deionized water, preferably KOH.

Although the temperature at which the electrolyzer is operated is not limited, preferably the electrolyzer in step (b) is operated at a temperature of from 20 to 100°C, preferably from 40 to 90°C. Further, the electrolyzer is preferably operated at a pressure of 0.5- 30 bara, preferably 1.0-10 bara.

As mentioned above, in step (b) an ethylenecontaining vapour stream and an ethanol-containing liquid stream are produced. Preferably, the ethylene-containing vapour stream obtained in step (b) comprises at least 0.5 mol.% ethanol, preferably at least 1.0 mol.% ethanol. Preferably, the ethylene-containing vapour stream obtained in step (b) comprises at least 20 mol.% ethylene .

Further, the ethanol-containing liquid stream obtained in step (b) comprises at most 5.0 mol.% ethylene, preferably at most 1.0 mol.%.

The ethanol-containing liquid stream produced in step

(b) comprises at least 0.2 mol.% ethanol, preferably at least 1.0 mol.% ethanol. Typically, the ethanol- containing liquid stream contains some electrolyte, acetate and propanol as well.

According to an especially preferred embodiment of the process according to the present invention, the ethylene-containing vapour stream produced in step (b) is, before subjecting to hydration in step (c), separated thereby obtaining an ethylene-enriched gas stream and a third ethanol-enriched stream, and wherein the ethylene- enriched gas stream is subjected to the hydration in step

(c). Typically, the third ethanol-enriched stream is liquid.

Although not limited thereto, the ethylene-containing vapour stream is typically separated in a gas/liquid separation vessel, after first being flashed.

The ethylene-enriched gas stream typically comprises at least 20 mol.% ethylene, preferably at least 30 mol.%. The ethylene-enriched gas stream typically contains some CO and H2 as well.

The third ethanol-enriched liquid stream is typically an aqueous stream and typically comprises at least 10 mol.% ethanol, preferably at least 20 mol.%. In step (c), at least a part, and preferably all, of the ethylene-containing vapour stream obtained in step (b) is subjected to hydration thereby obtaining a first ethanol-enriched stream.

As the person skilled in the art is familiar with hydration, this is not discussed here in detail. In addition to the first ethanol-enriched stream, typically also a residual CO-containing stream is obtained. This residual CO-containing stream (typically also containing some H2) can be recycled.

The first ethanol-enriched stream typically comprises at least 20 mol.% ethanol, preferably at least 30 mol.% ethanol. It is preferred that the first ethanol-enriched stream (and preferably also the third ethanol-enriched stream) is temporarily stored before separating in step (d).

The temporary storing can for example take place in a storage tank (hereinafter referred to with a 'first buffer tank'). An important advantage of the use of such a storage tank (which can be low-cost) is that in case the electrolyzer is driven by renewable power such as wind, solar and other forms of renewable power intermittency issues can be accommodated without the use of expensive batteries. This, as in case the electrolyzer is not working because of intermittency issues, the stored ethanol can be used in downstream processes (which often operate continuously). A further advantage of the use of a storage tank, is that it can be easily scaled up .

According to an especially preferred embodiment of the process according to the present invention, the third ethanol-enriched stream and the first ethanol-enriched stream formed in step (c) are combined thereby obtaining a combined ethanol stream, and wherein the combined ethanol stream is used in the separation of step (d). Again, it is preferred that the combined ethanol stream is temporarily stored before separating in step (d).

The combined ethanol stream typically comprises at least 20 mol.% ethanol, preferably at least 30 mol.%.

In step (d), the first ethanol-enriched stream obtained in step (c) - or the combined ethanol stream - is separated thereby obtaining a second ethanol-enriched stream and a water-enriched stream.

Although the separation in step (d) may be performed in many ways, the separation is typically done by distillation. As the person skilled in the art is familiar with such distillation, this is not further discussed here in detail.

The second ethanol-enriched stream, which may be a vapour or liquid, typically comprises at least 85 mol.% ethanol, preferably at least 90 mol.%. The second ethanol-enriched stream typically contains some water as well.

The water-enriched stream typically comprises at least 90 mol.% water and may contain some ethanol, acetate/acetic acid and propanol as well.

In step (e), the second ethanol-enriched stream is subjected to dehydration thereby obtaining ethylene. As the person skilled in the art is familiar with this dehydration step, this is not further discussed here in detail. If desired, the obtained ethylene may be further purified; typically, some water as generated during the dehydration step will be removed.

According to a particularly preferred embodiment according to the present invention, the ethanol- containing liquid stream obtained in step (b) is separated thereby obtaining a fourth ethanol-enriched stream and an oxygen-enriched gas stream.

Although not limited thereto, the ethanol-containing liquid stream is typically separated in a gas/liquid separation vessel, after first being flashed.

The fourth ethanol-enriched stream is typically liquid and typically comprises at least 0.2 mol.% ethanol, preferably at least 2.0 mol.%. The fourth ethanol-enriched stream typically contains some CO and H2 as well.

The oxygen-enriched gas stream typically comprises at least 90 mol.% oxygen, preferably at least 95 mol.%. The remainder is typically water, with some trace amounts of CO2, ethanol and propanol.

Preferably, the fourth ethanol-enriched stream is separated thereby obtaining a fifth ethanol-enriched stream and a propanol-enriched stream.

Typically, the separation of the fourth ethanol- enriched stream is via distillation in an alcohol separation unit.

The fifth ethanol-enriched stream typically comprises at least 70 mol.% ethanol, preferably at least 80 mol.%. The fifth ethanol-enriched stream typically contains some water as well.

The propanol-enriched stream typically comprises at least 0.1 mol.% propanol, preferably at least 0.2 mol.%. The propanol-enriched stream will typically obtain some electrolyte, acetate and water as well.

According to an especially preferred embodiment according to the present invention, the fourth ethanol- enriched stream is temporarily stored before being separated (as discussed hereinafter) to obtain the fifth ethanol-enriched stream. The temporary storing can for example take place in a storage tank (hereinafter referred to with a 'second buffer tank'). As mentioned earlier, an important advantage of the use of such a storage tank (which can be low-cost) is that in case the electrolyzer is driven by renewable power such as wind, solar and other forms of renewable power intermittency issues can be accommodated without the use of expensive batteries.

Further it is preferred that the fifth ethanol- enriched stream is combined with the combined ethanol stream.

According to a particularly preferred embodiment of the process according to the present invention, the propanol-enriched stream is separated thereby obtaining a purified propanol stream and an electrolyte-enriched stream; wherein at least a part of the electrolyte- enriched stream is separated in a further electrolyte- enriched stream and an acetic acid-enriched stream; wherein the acetic acid-enriched stream is hydrogenated thereby obtaining a further ethanol stream; and wherein the further ethanol stream is subjected to dehydration in step (e).

In another aspect, the present invention provides an apparatus suitable for performing the process for producing ethylene according to the present invention, the apparatus at least comprising:

- an electrolyzer for converting a CO-containing stream, thereby producing an ethylene-containing vapour stream and an ethanol-containing liquid stream;

- a hydration unit for hydrating at least a part of the ethylene-containing vapour stream, thereby obtaining a first ethanol-enriched stream; - an alcohol separation unit for separating the first ethanol-enriched stream, thereby obtaining a second ethanol-enriched stream and a water-enriched stream; and

- a dehydration unit for dehydrating the second ethanol-enriched stream thereby obtaining ethylene.

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

Fig. 1 schematically a flow scheme of the process for producing ethylene 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, shows an electrolyzer 2, a first g/1 vessel 3, a hydration process unit 4, an ethanol separation (distillation) unit 5, a dehydration unit 6, a first liquid storage tank 7, a mol-sieve unit 8, a second g/1 vessel 9, an alcohol separation (distillation) unit 11, a second liquid storage tank 12, a propanol separation (distillation) unit 13, an acetate/acetic acid separation unit 14 and a hydrogenation unit 15.

During use, a CO-containing stream 10 is provided. The CO-containing stream 10 has preferably been obtained from a C02-containing stream that has been captured from the atmosphere. The CO-containing stream 10 is fed, together with an electrolyte 20 to the electrolyzer 2. In the electrolyzer 2, the CO-containing stream 10 is converted thereby producing an ethylene-containing vapour stream 30 and an ethanol-containing liquid stream 40.

The ethylene-containing vapour stream 30 is subsequently separated - after being flashed in a flash separator (not shown) - in the first g/1 vessel 3, thereby obtaining an ethylene-enriched gas stream 50 and a liquid, third ethanol-enriched stream 60.

At least a part, but typically all, of the ethylene- enriched gas stream 50 is subjected in the hydration process unit 4 to hydration (enabled by process water stream 70) thereby obtaining a liquid, first ethanol- enriched stream 90 and a residual CO-containing stream 80. This residual CO-containing stream 80 (typically also containing some H2) can be recycled to or combined with the output of e.g. a CO-producing unit (not shown) to obtain the CO-containing stream 10.

The third ethanol-enriched stream 60 and the first ethanol-enriched stream 90 are combined thereby obtaining a combined ethanol stream 100. In the embodiment of Fig. 1 the combining takes place in the first storage tank 7 where the combined stream is temporarily stored, but this may also happen before entering the first storage tank 7. In the absence of the first storage tank 7, the combination may take place somewhere before or in the ethanol separation (distillation) unit 5.

In the ethanol separation (distillation) unit 5 the combined ethanol stream 100 is separated thereby obtaining a second ethanol-enriched stream 110 and a water-enriched stream 120.

The second ethanol-enriched stream 110 is then subjected to dehydration in dehydration unit 6, usually in the presence of additional water, thereby obtaining ethylene stream 130 and a further water-enriched stream 135. As can be seen from Fig. 1, the ethylene stream 130, may be further purified in e.g. a mol-sieve unit 8 to obtain a purified ethylene stream 140 and a water stream 150. As shown in the embodiment of Fig. 1, and as particularly preferred according to the present invention, the ethanol-containing liquid stream 40 is separated - after being flashed in a flash separator (not shown) - in the second g/1 vessel 9, thereby obtaining a liquid, fourth ethanol-enriched stream 160 and an oxygen- enriched gas stream 170. The fourth ethanol-enriched stream 160 is temporarily stored in second storage tank 12 and subsequently separated in the alcohol separation (distillation) unit 11 thereby obtaining a fifth ethanol- enriched stream 190 and a propanol-enriched stream 180. The fifth ethanol-enriched stream 190 is combined with the combined ethanol stream 100. In the embodiment of Fig. 1, this combination takes places in the ethanol separation (distillation) unit 5, but this can also be done upstream of the ethanol separation (distillation) unit 5.

As shown in the embodiment of Fig. 1, the propanol- enriched stream 180 (which also contains electrolyte, acetic acid and acetate) is separated in propanol separation (distillation) unit 13 thereby obtaining a purified propanol stream 200 and an electrolyte-enriched stream 210 which may be recycled to the electrolyzer 2 (not shown).

Also shown in Fig. 1 is an optional acetate/acetic acid separation unit 14 for separating (at least a part of) the electrolyte-enriched stream 210 into a further electrolyte-enriched stream 220 (which may be recycled to the electrolyzer 2) and an acetic acid enriched stream 230. The acetic acid enriched stream 230 may be treated in the hydrogenation unit 15 to obtain a further ethanol stream 240 which may be fed to the ethanol separation

(distillation) unit 5. Example

The flow scheme of Fig. 1 (without the acetate/acetic acid separation unit 14 and hydrogenation unit 15) was used for illustrating the production of ethylene from a CO-containing stream in a non-limiting manner. The compositions and conditions of the fluid (i.e. gas and liquid) streams in the various flow lines are provided in Table 1 below (V means vapour, whilst L means liquid).

The values in Table 1 were calculated using a model generated with commercially available Aspen Plus software, whilst using standard thermodynamic packages with setting such that CO2 conversion, alcohol distillation, gas/liquid separation, etc. are simulated. For the electrolyzer block, experimental data available from D.S. Ripatti et al., Carbon Monoxide Gas Diffusion Electrolysis that Produces Concentrated C2 Products with High Single-Pass Conversion, Joule, Volume 3, Issue 1, pages 240-256, 2019 (DOI: https://doi.org/10.1016/j.joule.2018.10.007) was used to supplement the Aspen model.

As electrolyte, an aqueous (1.98 mol.%) KOH solution was used.

Discussion

As can be seen from Table 1, the process according to the present invention allows for an effective way of producing ethylene from a CO-containing stream.

An important advantage of the present invention is that by the 'indirect' conversion of carbon monoxide (CO) to ethylene (via the intermediate conversion into ethanol) the separation (via cryogenic distillation) of CO, ethylene and H2 can be avoided.

A further important advantage of the process according to the present invention is that ethanol can be easily stored in for example low-cost tanks. This can be of importance in case the electrolyzer is driven by renewable power such as wind, solar and other forms of renewable power that has intermittency issues.

The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention. Further, the person skilled in the art will readily understand that, while the present invention in some instances may have been illustrated making reference to a specific combination of features and measures, many of those features and measures are functionally independent from other features and measures given in the respective embodiment (s) such that they can be equally or similarly applied independently in other embodiments.