OF CARBON DIOXIDE

FIELD OF THE INVENTION

The present invention relates to a process for capture of carbon dioxide and electrochemical conversion thereof while regenerating the sorbent. Said process is useful for reducing carbon dioxide in the atmosphere and provides an improved process for producing chemicals and/or fuels from carbon dioxide.

BACKGROUND OF THE INVENTION

Problems with global warming and the need to reduce the world' s carbon footprint are currently high on the political agenda. In fact, solving the global warming problem is regarded as the most important challenge facing mankind in the 21st century. The capacity of the earth system to absorb greenhouse gas emissions is already exhausted, and under the Paris climate agreement, current emissions must be fully stopped until around 2070. This decarbonization of the energy system requires an energy transition moving away from conventional fossil fuels, but also a solution to the problem to reduce the amount of produced carbon dioxide for example by

recycling it to produce other compounds.

The interest in the subject of carbon dioxide

electrochemical conversion is driven by the challenge of mitigating risk of climate change by transitioning to a low-carbon energy future, while extending the economic and social benefits of energy to everyone on the planet. To meet the increasing global energy demand, the usage of fossil fuels and thus production of carbon dioxide emissions is unavoidable in the short term. To reduce carbon dioxide emissions, solutions like carbon dioxide capture (from flue gases or air) and storage or

utilization have been widely investigated. In this context, electrochemical processes to capture and convert carbon dioxide are part of the emerging technologies gaining increased attention as they solve the dual challenges of meeting the increasing energy demand while storing renewable electricity into carbon dioxide neutral molecules, i.e. molecules for which net carbon dioxide emissions are zero.

Technologies are known in which carbon dioxide is electrochemically reduced in an electrolysis cell to obtain several types of products. By varying cell

potentials, electrocatalyst ( s ) and electrolyte type the product distribution can be tuned. For example, carbon dioxide can electrochemically be converted to carbon monoxide, ethylene, methane, etcetera. These technologies typically require a concentrated carbon dioxide gas stream to be fed to an electrolyzer. Proposed process line-ups available in the art consist of a carbon dioxide capturing unit (a scrubber, where a liquid and gas stream are in contact with each other and carbon dioxide is selectively absorbed/captured in the liquid from the gas stream) , a carbon dioxide release unit where the sorbent is regenerated (typically a thermal step delivering concentrated carbon dioxide at elevated temperature and ambient pressure) , and a concentrated carbon dioxide stream that is fed to the electrolyzer.

In the art technologies are described comprising integration of the capture and conversion of carbon dioxide. For example, it is known that carbon dioxide can be reduced when contained in a liquid sorbent,

particularly methanol, ionic liquids, and amine-type of molecules. If the products of this process are gaseous, theygenerally need to be brought at pressure for

downstream processing.

Different technologies are known in the art

comprising electrochemical conversion of gaseous carbon dioxide at higher pressure. Allegedly, mass transport and ohmic limitations are reduced in such conversions. High pressure electrochemical conversion of carbon dioxide comprises capturing carbon dioxide in a sorbent, release of carbon dioxide from the sorbent, generally using heat, compressing the carbon dioxide, converting the

pressurized carbon dioxide producing gaseous products, separating unreacted carbon dioxide from the gaseous products and recycling the carbon dioxide back into the process .

The climate change still continues and the energy consumption of the world still increases. Therefore, in general there is a need for efficient and less energy demanding processes. Thus, there is a need for more efficient and less energy demanding processes for capture and conversion of carbon dioxide to produce useful chemicals and/or fuels. The present disclosure provides a solution to said problem.

SUMMARY OF THE INVENTION

The present invention relates to an integrated process for low pressure capture of carbon dioxide and high pressure conversion thereof, thereby producing pressurized gaseous products, while at the same time regenerating the sorbent. In particular, the present disclosure relates to a process for converting carbon dioxide into carbon

containing products, comprising capturing carbon dioxide from air or flue gases and electrochemically converting the carbon dioxide, wherein

(a) the carbon dioxide is introduced into a vessel

(carbon dioxide capturing vessel) comprising a liquid sorbent thereby capturing the carbon dioxide in the liquid sorbent; followed by

(b) pressurizing the liquid sorbent comprising the captured carbon dioxide by introducing the liquid sorbent into a pump, thereby providing a pressurized carbon dioxide comprising liquid sorbent; followed by

(c) feeding the pressurized carbon dioxide comprising liquid sorbent of step (b) to a cathode compartment of an electrolyzer comprising a catalyst,

thereby producing carbon containing gaseous products.

The process of the present disclosure advantageously combines the high pressure carbon dioxide conversion step with sorbent regeneration, which is a more efficient process than carbon dioxide conversion processes known in the art. Pumping and pressurizing a liquid according to the presently disclosed process is technically less demanding in terms of energy and costs than compressing gases, which is required in processes known in the art. Further, according to the presently disclosed process, carbon dioxide is converted at pressure into products while in the liquid sorbent. The gaseous products have a low solubility in the solvent and can easily be separated from the solvent at pressure. The sorbent, with a

remainder of unreacted carbon dioxide, can be recycled back to be re-used for capturing carbon dioxide. DETAILED DESCRIPTION OF THE DISCLOSURE

According to the present disclosure, in step (a) the carbon dioxide is captured from a gaseous carbon dioxide source using a liquid sorbent.

The carbon dioxide source may be air (with a CO2 content as low as 0.05 to 0.06 weight percent), flue gas or any other off-gas from processes comprising carbon dioxide. Also pure carbon dioxide gas sources may be used .

Liquid sorbents for capturing carbon dioxide are generally known in the art. The liquid sorbent that is used in the process of this disclosure is selected from physical sorbents and chemical sorbents. Physical sorbents are known in the art and are selected from solvents in which carbon dioxide dissolves, like water, certain ketones, like acetone, and alcohols, specifically methanol, and mixtures thereof. Chemical sorbents for carbon dioxide are also known in the art per se, and are preferably selected from amines (in particular

alkanolamines ) , ionic liquids and metal hydroxides, and mixtures thereof, and aqueous solutions comprising such compounds. In particular, alkanolamines and metal hydroxides are preferred. Alkanolamines preferably are selected from monothanolamine (MEA) , diethanolamine

(DEA) , di-isopropanolamine (DIPA) and

methyldiethanolamine (MDEA) . Metal hydroxides are

preferably selected from sodium hydroxide, potassium hydroxide, calcium hydroxide and cesium hydroxide.

Preferably, the liquid sorbent used in the process of the present disclosure is a chemical sorbent.

Except from capturing carbon dioxide, the liquid sorbent may also act as co-catalyst when a sorbent is selected that decreases the activation energy of carbon dioxide by interacting with the molecule, which makes the carbon dioxide more easily reactive. Preferred examples of such sorbents are amine-type molecules and ionic liquids .

The carbon dioxide captured by the sorbent is

electrochemically reduced to a number of products in an electrolyzer (electrolysis cell) . The target products can be tuned by varying cell potentials, electrocatalyst ( s ) and electrolyte type. The electrocatalytic reduction of carbon dioxide at the cathode is coupled with an

oxidation reaction at the anode, for example with water electrolysis. Devices are known in the art for this purpose, being called H ₂ O/CO ₂ co-electrolyzers. Oxygen is evolved from water at the anode and carbon dioxide reduction occurs at the cathode. Anode and cathode may be separated by a solid, or solid and liquid electrolyte, that only allows the transport of anions/cations.

Preferably, the cathode compartment and an anode

compartment are separated by a membrane, a diaphragm, or a gas separator. Suitable membranes or diaphragms are, for example, anionic exchange membranes or diaphragms exchanging OH-, cationic exchange membranes exchanging H+ and bipolar membranes dissociating water in OH- and H+ .

Examples of liquid electrolytes used in combination with a membrane, or not, and suitable for electrolysis are metal hydroxides, carbonates and bicarbonates, acid as sulphuric acid and alike, bases as metal hydroxides and alike.

The electrolyzer cathode comprises a catalyst, which generally is a heterogenous catalyst. Preferably, the heterogenous catalyst comprising a metal. The metal is selected from one or more metals being gold, silver, copper, lead, tin, zinc, platinum, mercury, palladium, nickel, cobalt, iron, or combinations thereof, e.g.

alloys or combinations of particles. Gold, silver, copper, palladium, tin and lead, or any combinations thereof are preferred. In a process for producing carbon monoxide as the preferred product, a gold and/or silver comprising catalyst is preferably selected. In a process for producing ethylene as the preferred product, a copper comprising catalyst is preferably selected. The

heterogenous catalyst may comprise an organometallic compound or organic compound, like porphyrines and similar substances.

The electrolyzer also comprises an anode compartment in which an oxidation reaction takes place. The

electrolyzer anode comprises a catalyst which generally is a heterogenous catalyst. The heterogenous catalyst is selected from a catalyst comprising a metal selected from one or more of iridium, ruthenium, platinum, nickel, iron, oxides thereof, and combinations thereof.

Any of the catalysts of the electrolyzer can be supported or unsupported. Suitable support materials such as carbon paper or cloth, titanium and stainless steel meshes and the like are known in the art.

Preferably, step (a) of the process of the present disclosure is carried out at ambient pressure, however it is also possible to introduce a carbon dioxide containing gas stream at slightly elevated pressure. The pressure in steps (b) and (c) is higher than in step (a) , preferably from 1 MPa to 5 MPa, more preferably from 3 MPa to 5 MPa. For conversion purposes, higher pressure can be

beneficial .

Preferred products of the process of this disclosure in step (c) are gaseous carbon containing products, preferably being gaseous products selected from carbon monoxide, methane, dimethyl ether (DME) , ethylene and propylene. Even more preferred are products selected from carbon monoxide, ethylene, propylene. The preferred catalyst selected for producing these preferred products according to the process of this disclosure are selected from gold and silver comprising catalysts for carbon monoxide and a copper comprising catalyst for ethylene, propylene, dimethyl ether and methane.

The sorbent after step (c) may comprise unreacted carbon dioxide still present in the liquid. The preferred products of the process of this disclosure are gaseous and have low solubility in the sorbent at pressure.

Therefore, these products easily separate from the sorbent either in the electrolyzer or in a separate separation vessel connected to the electrolyzer. The pressurized gaseous products do not contain unreacted carbon dioxide, which is present in the liquid sorbent and that sorbent is recycled. The pressurized products are led away for further use, for example for downstream processing or storage.

After step (c) , the sorbent, partially regenerated if carbon dioxide is still present, or fully regenerated if all carbon dioxide is converted, may contain some traces of products and is recycled back into the process.

Depending on the pressure drop of the line between the electrolyzer and the carbon dioxide capturing vessel, depressurization might be necessary.

DESCRIPTION OF THE DRAWINGS

Figure 1. Depicts a line-up for use in the process according to this disclosure. Legend: 1 = carbon dioxide capturing vessel; 2 = pump for pressurizing the carbon dioxide comprising sorbent; 3 = cathode compartment of electrolyzer; 4 = anode compartment of electrolyzer; and A = carbon dioxide source; B = depleted carbon dioxide source; C = carbon dioxide comprising liquid sorbent; D = pressurized carbon dioxide comprising liquid sorbent; E = regenerated liquid sorbent, optionally comprising

unreacted carbon dioxide; F = gaseous products at

pressure; G = water; H = oxygen.

Figure 2. Depicts a line-up according to the prior art. Legend: 1 = carbon dioxide capturing vessel; 2 = carbon dioxide regenerator; 3 = gas compressor; 4 = cathode compartment of electrolyzer; 5 = anode compartment of electrolyzer; 6 = separator; and A = carbon dioxide source; B = depleted carbon dioxide source; C = carbon dioxide comprising liquid sorbent; D = regenerated liquid sorbent; E = gaseous carbon dioxide; F = pressurized gaseous carbon dioxide; G = pressurized gaseous carbon dioxide and recycled carbon dioxide; H = gaseous products and unreacted carbon dioxide; I = gaseous products and unreacted carbon dioxide (at pressure) ; J = recycled carbon dioxide; K = water; L = oxygen.

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

EXAMPLE

The process of the present invention enables the

regeneration of chemically captured carbon dioxide by direct electrochemical conversion of the absorbed carbon dioxide to gas product.

Reference is made here to Figure 1.

A gaseous stream containing carbon dioxide (A), e.g. flue gas or air, and a carbon dioxide capturing liquid sorbent (E) , being a liquid containing a capturing molecule (e.g. KOH, amines, methanol, ionic liquids, etc.) _/ are fed to a carbon dioxide capturing vessel (1), such as an

absorption column, where carbon dioxide is captured by the sorbent (E) forming a carbon dioxide comprising liquid sorbent (C) .

The depleted carbon dioxide source (B) exits the

capturing vessel and is either vented or recirculated to the upstream or downstream process. The sorbent loaded with carbon dioxide (C) is introduced into a pump (2) and pressurized to provide a pressurized carbon dioxide comprising liquid sorbent (D) (pressure: e.g. 30 bar), exiting the pump (2) . The pressurized sorbent (D) is fed to a co-electrolyzer cathode (3) comprising a catalyst (e.g. a silver-, gold- or copper- based catalyst). Water (G) is fed to cathode compartment (3) or to the anode compartment of the electrolyzer (4) . A potential is applied between the cathode and anode (divided by a membrane, diaphragm, or separator) . In the cathode compartment (3), the carbon dioxide contained in/absorbed on the liquid (D) is directly reduced to gaseous products (e.g. CO or ethylene) at pressure (F) . The gas products

(F) easily separate from the sorbent either in the cathode compartment (3) or in a distinct separation vessel, at pressure, and are sent for storage or

downstream processing. The regenerated liquid sorbent (E) , optionally comprising unreacted carbon dioxide, is recycled back to vessel (1) for re-use. In the anode compartment (4), water (G) , at pressure, is converted into oxygen (H) that is vented or integrated with the upstream or downstream processes.