A Process for Producing Ethylene

The present invention relates to a process for producing ethylene, in particular from a CO-containing stream that has been obtained from COcaptured 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 WO2021/009627A1.

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

As a mere example, the article by J. Sisler et al., “---H” in ACS Energy Lett. 2021, 6, 997-1002, discloses ‘single step’ or ‘direct’ conversion of COto CH(ethylene) using an electrolyzer.

A problem of the process as described in this article is that it requires the separation of CO, ethylene and H. 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 COcaptured 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:

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 Hcan 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 CO-containing stream, preferably COcaptured from the atmosphere. As a mere examples, the CO-containing gas stream may have been obtained by thermochemical or electrochemical COconversion 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 (N), methane (CH), carbon dioxide (CO), water (HO) and hydrogen (H) may be present in the CO-containing stream. As an example, if present, His 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 (O) and sulfur compounds such as HS and SO.

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, 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 ethylene-containing 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 Has 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 H) 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 Has 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 CO, 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.