The present invention relates to a method for generating synthesis gas (syngas) for use in hydroformylation plants.

Hydroformylation, also known as "oxo synthesis" or "oxo process", is an industrial process for the production of aldehydes from alkenes. More specifically, the hydro- formylation reaction is the addition of carbon monoxide

(CO) and hydrogen (¾) to an alkene. This chemical reaction entails the net addition of a formyl group (CHO) and a hy ^¬ drogen atom to a carbon-carbon double bond. The reaction yields an aldehyde with a carbon chain one unit longer than the parent alkene. If the aldehyde is the desired product, then the syngas should have a composition close to ΟΌ:¾ ₌ 1:1.

In some cases, the alcohol corresponding to the aldehyde is the desired product. When this is the case, more hydrogen is consumed to reduce the intermediate aldehyde to an alco ^¬ hol, and therefore the syngas should have a composition of approximately ΟΌ:¾ = 1:2. Sometimes it is desired to purify the intermediate aldehyde before converting it into an alcohol. Accordingly, in such case, a syngas with the composition ΟΌ:¾ ₌ 1:1 must first be used, followed by pure H ₂ . Thus, the need for low-module syngas (i.e. low hydrogen-to- carbon monoxide ratio) is characteristic for the hydro- formylation reaction. Such a syngas composition is rather costly to provide since it cannot be obtained directly from steam reforming of natural gas or naphtha. At least a steam reformed gas must undergo reverse shift, i.e. the reaction CO ₂ + ¾ -> CO + H ₂ O, to provide sufficient CO. Otherwise, a cold box for condensing CO has to be installed to separate the CO. This is also a costly solution, and there will be an excess of hydrogen for which a use purpose has to be found . Alternatively, gasification plants may provide low-module syngas, but gasification plants need to be very large to be efficient, and they are expensive, both with respect to CAPEX and to OPEX. Furthermore, coal-based gasification plants are increasingly undesired due to the substantial environmental implications and a large CO ₂ footprint.

Low-module (i.e. CO-rich) syngas for hydroformylation is therefore generally costly. Large hydroformylation plants are often placed in industrial areas and may thus obtain the necessary syngas "over the fence" from a nearby syngas producer. In many cases, however, this is not possible for medium or small size hydroformylation plants. Instead, such smaller plants will need to import the syngas, e.g. in gas cylinders, which is very expensive. Furthermore, transpor- tation and handling of such gas containers is connected with certain elements of risk since syngas (not least low- module syngas) is highly toxic and extremely flammable, and syngas may form explosive mixtures with air. Import of CO by tube trailers will face similar challenges, both in terms of costs and in terms of safety. Regarding prior art, US 8,568,581 discloses a hydroformyla ^¬ tion process using a traditional electrochemical cell, not a solid oxide electrolysis cell (SOEC) or an SOEC stack, for preparation of the synthesis gas to be used in the pro- cess. Water is introduced in a first (anode) compartment of the cell, and CO ₂ is introduced into the second (cathode) compartment of the cell followed by alkene and catalyst ad ^¬ dition to the cell, and the cathode induces liquid phase hydroformylation when an electrical potential is applied between the anode and the cathode.

In WO 2017/014635, a method for electrochemically reducing carbon dioxide is described. The method involves the con ^¬ version of CO ₂ into one or more platform molecules such as syngas, alkenes, alcohols (including diols) , aldehydes, ke ^¬ tones and carboxylic acids, and also conversion of CO ₂ into i.a. CO, hydrogen and syngas. The method does not, however, include preparation of low-module syngas for hydroformyla ^¬ tion .

US 2014/0291162 discloses a multi-step method for prepara ^¬ tion of various compounds, such as aldehydes, by electroly ^¬ sis of previously prepared CO ₂ and/or CO and steam. The method includes i.a. heat transfer from a heating means to- wards a proton-conductive electrolyser comprising a proton- conducting membrane arranged between the anode and the cathode .

Finally, US 2011/0253550 discloses a method for producing a synthetic material, where water is converted into ¾ and O ₂ using high-temperature electrolysis. Depending on how the catalytic process is carried out, the mixture of water va ^¬ pour, CO ₂ and ¾ can additionally be converted catalyti- cally into functionalized hydrocarbons, such as aldehydes. This publication is very unspecific and does not define the concept of high-temperature electrolysis, neither in terms of temperature range nor in terms of the kind(s) of equip ^¬ ment being usable for the purpose.

Now it has turned out that the above-described elements of risk in relation to syngas can effectively be counteracted by generating the syngas, which is necessary for hydro- formylation plants, in an apparatus based on solid oxide electrolysis cells (SOECs) or SOEC stacks. A solid oxide electrolysis cell is a solid oxide fuel cell (SOFC) run in reverse mode, which uses a solid oxide electrolyte to pro ^¬ duce e.g. oxygen and hydrogen gas by electrolysis of water. Importantly, it can also be used for converting CO ₂ elec- trochemically into the toxic, but for many reasons attrac ^¬ tive CO directly at the site where the CO is to be used, which is an absolute advantage. The turn-on/turn-off of the apparatus is very swift, which is a further advantage.

So it is the intention of the present invention to provide an apparatus generating syngas based on solid oxide elec- trolysis cells, which can generate syngas for hydroformyla- tion plants. The raw materials for generating the syngas will be mixtures of CO ₂ and ¾0.

A solid oxide electrolysis cell system comprises an SOEC core wherein the SOEC stack is housed together with inlets and outlets for process gases. The feed gas or "fuel gas" is led to the cathode part of the stack, from where the product gas from the electrolysis is taken out. The anode part of the stack is also called the oxygen side, because oxygen is produced on this side. In the stack, CO and ¾ are produced from a mixture of CO2 and water, which is led to the fuel side of the stack with an applied current and excess oxygen is transported to the oxygen side of the stack, optionally using air or nitrogen to flush the oxygen side. The product stream from the SOEC, containing CO and ¾ mixed with CO2 , is normally subjected to a separation process.

More specifically, the principle of producing CO and ¾ by using a solid oxide electrolysis cell system consists in leading CO2 and ¾0 to the fuel side of an SOEC with an ap- plied current to convert CO2 to CO and ¾0 to ¾ and

transport the oxygen surplus to the oxygen side of the SOEC. Air, nitrogen or carbon dioxide may be used to flush the oxygen side. Flushing the oxygen side of the SOEC has two advantages, more specifically (1) reducing the oxygen concentration and related corrosive effects and (2) provid ^¬ ing means for feeding energy into the SOEC, operating it endothermic. The product stream from the SOEC contains a mixture of CO, ¾, ¾0 and CO2 , which - after removal of wa ^¬ ter, e.g. by condensation - can be led to a separation pro- cess such as pressure swing adsorption (PSA) , temperature swing adsorption (TSA) , membrane separation, cryogenic separation or liquid scrubber technology, such as wash with N- methyl-diethanolamine (MDEA) . PSA is especially suitable for the production of high purity syngas.

The overall principle in the production of CO by electroly ^¬ sis is that CO2 (possibly including some CO) is fed to the cathode. As current is applied to the stack, CO2 is con ^¬ verted to CO to provide an output stream with a high con ^¬ centration of CO: 2 C0 ₂ (anode) -> 2 CO (cathode) + 0 ₂ (anode)

H ₂ 0 (anode) -> H ₂ (cathode) + ½ 0 ₂ (anode)

If pure CO2 is fed into the SOEC stack, the output will be CO (converted from CO2 ) and unconverted CO2. If needed, the unconverted CO2 can be removed in a CO/ CO2 separator to pro ^¬ duce high-purity CO.

If a mixture of CO2 and ¾0 is fed into the SOEC stack, the output will be a mixture of CO, CO2 , ¾0, and ¾ . In addi- tion to the electrochemical conversion reaction of CO2 to CO (1) given above, steam will be electrochemically con ^¬ verted into gaseous hydrogen according to the following reaction : H ₂ 0 (cathode) -> H ₂ (cathode) + ½ 0 ₂ (anode) (2)

Additionally, a non-electrochemical process, namely the re ^¬ verse water gas shift (RWGS) reaction takes place within the pores of the cathode:

H ₂ (cathode) + C0 ₂ (cathode) <-> <-> H ₂ 0 (cathode) + CO (cathode) (3)

In state-of-the-art SOEC stacks, where the cathode com- prises Ni metal (typically a cermet of Ni and stabilized zirconia) , the overpotential for reaction (1) is typically significantly higher than for reaction (2). Furthermore, since Ni is a good catalyst for RWGS reaction, reaction (3) occurs almost instantaneously at SOEC operating tempera ^¬ tures. In other words, the vast majority of the electroly ^¬ sis current is used for converting ¾0 into ¾ (reaction 2), and the produced ¾ rapidly reacts with CO2 (according to reaction 3) to provide a mixture of CO, CO2 , ¾0, and H ₂ . Under typical SOEC operating conditions, only a very small amount of CO is produced directly via electrochemical con ^¬ version of CO2 into CO (reaction 1) .

In case pure ¾0 is fed into the SOEC stack, the conversion ¾2o of H ₂ 0 to H ₂ is given by Faraday's law of electrolysis:

PH, i-V„ -n cells

where p _H 2 is the partial pressure of ¾ at cathode outlet, p _H 2o is the partial pressure of steam at cathode outlet, i is the electrolysis current, V _m is the molar volume of gas at standard temperature and pressure, n _Ce ii _s is the number of cells in an SOEC stack, z is the number of electrons transferred in the electrochemical reaction, i¾2o is the flow of gaseous steam into the stack (at standard tempera ^¬ ture and pressure), and F is Faraday's constant. In case pure CO2 is fed into the SOEC stack, the conversion Co2 of CO2 to CO is given by an analogous expression:

V _ PcO _i ^'~ V _m ^{' n} cells π \

« _¾ 7 — — ( )

Pco ⁺ Pco ₂ ^{z '} Jco ₂ ^' r

where p _C o is the partial pressure of CO at cathode outlet, pco2 is the partial pressure of steam at cathode outlet, i is the electrolysis current, V _m is the molar volume of gas at standard temperature and pressure, n _Ce ii _s is the number of cells in an SOEC stack, z is the number of electrons transferred in the electrochemical reaction, f o2 is the flow of gaseous steam into the stack (at standard tempera ^¬ ture and pressure), and F is Faraday's constant.

In case steam and CO2 are both fed into the SOEC stack, the gas composition exiting the stack will further be affected by the RWGS reaction (3) . The equilibrium constant for RWGS reaction, JRW _GS / is given by: where AG is the Gibbs free energy of the reaction at SOEC operating temperature, R is the universal gas constant, and T is absolute temperature.

The equilibrium constant and therefore the extent to which electrochemically produced ¾ is used to convert CO2 into

CO, is temperature-dependent. For example, at 500 °C, .K _RWGS = 0.195. At 600°C, _RWGS = 0.374. At 700°C, i¾ _WGS = 0.619.

Thus, the present invention relates to a method for the generation of syngas for use in hydroformylation plants, comprising the steps of:

- evaporating water to steam, - mixing the steam with carbon dioxide in the desired molar ratio, and - feeding the resulting gas to a solid oxide electrolysis cell (SOEC) or an SOEC stack at a sufficient temperature for the cell or cell stack to operate while supplying an electrical current to the cell or cell stack to effect the conversion of the feed gas to syngas, either fully or in part .

In the method of the invention, steam is electrochemically converted to hydrogen in an SOEC or an SOEC stack, and part of the hydrogen formed is allowed to react with carbon di ^¬ oxide to form carbon monoxide and steam via the reverse wa ^¬ ter gas shift (RWGS) reaction, thus resulting in a mixture of hydrogen, steam, carbon monoxide and carbon dioxide.

The molar ratio between steam and carbon dioxide is preferably around 1:1, more preferably around 2:3 and most pref ^¬ erably around 0.41:0.59, since this ratio, at an operation temperature of 700°C and a current of 50 A, will provide a syngas with the preferred CO:¾ ratio around 1:1 as it is explained in Example 4 below.

The temperature, at which CO is produced by electrolysis of CO ₂ in the SOEC or SOEC stack, is in the range from 650 to 800°C, preferably around 700°C.

The ratio between carbon monoxide and hydrogen in the gas mixture is in the range from 0.85:1.15 to 1.15:0.85, pref ^¬ erably from 0.90:1.10 to 1:10:0.90 and most preferably from 0.95:1.05 to 1.05:0.95, especially close to 1:1.

The product stream from the SOEC stack is subjected to a separation process in a separation unit to remove unconverted carbon dioxide from the syngas product. This separa ^¬ tion unit is preferably a pressure swing adsorption (PSA) unit comprising an adsorption step consisting of two or more adsorption columns, each containing adsorbents which have selective adsorption properties towards carbon diox ^¬ ide .

One of the great advantages of the method of the present invention is that the syngas can be generated with the use of virtually any desired CO/ H2 ratio, since this is simply a matter of adjusting the CO2 / H2O ratio of the feed gas.

Another great advantage of the invention is, as already mentioned, that the syngas can be generated "on-site", i.e. exactly where it is intended to be used, instead of having to transport the toxic and highly flammable syngas from the preparation site to the site of use. Yet another advantage of the present invention is that if it is desired to switch between a ΟΌ:¾ ₌ 1:1 syngas and pure ¾, this can be done using the same apparatus, simply by adjusting the feed from 1:1 CO2 / H2O to pure H ₂ 0. A still further advantage of the present invention is that syngas of high purity can be produced without in any way being more expensive than normal syngas, even though this desired high purity would prima facie be expected to entail increasing production costs. This is because the purity of the syngas is largely determined by the purity of the

CO2 / H2O feed, and provided that a feed consisting of food grade or beverage grade CO ₂ and ion-exchanged water is cho ^¬ sen, very pure syngas can be produced.

The invention is illustrated further in the examples which follow.

Example 1

CO2 electrolysis

An SOEC stack consisting of 75 cells is operated at an av ^¬ erage temperature of 700 °C with pure CO ₂ fed to the cathode at a flow rate of 100 Nl/min, while applying an electroly ^¬ sis current of 50 A. Based on the above equation (5) , the conversion of CO ₂ under such conditions is 26%, i.e. the gas exiting the cathode side of the stack consists of 26% CO and 74% C0 ₂ .

Example 2

H ₂ 0 electrolysis

An SOEC stack consisting of 75 cells is operated at an av ^¬ erage temperature of 700 °C with pure steam fed to the cath- ode at a flow rate of 100 Nl/min (corresponding to a liquid water flow rate of approximately 80 g/min) , while applying an electrolysis current of 50 A. Based on the above equa ^¬ tion (4), the conversion of ¾0 under such conditions is 26%, i.e. the gas exiting the cathode side of the stack consists of 26% H ₂ and 74% H ₂ 0.

Example 3 co-electrolysis

An SOEC stack consisting of 75 cells is operated at an av ^¬ erage temperature of 700 °C with a mixture of steam and CO ₂ in a molar ratio of 1:1 being fed to the cathode with a to ^¬ tal flow rate of 100 Nl/min, while applying an electrolysis current of 50 A. In the stack, steam is electrochemically converted into H ₂ according to reaction (2) . Assuming that any electrochemical conversion of CO ₂ via reaction (1) is negligible, 52% of the fed steam is electrochemically con ^¬ verted into hydrogen. Were the RWGS reaction not present, the gas exiting the stack would have the following composi ^¬ tion: 0% CO, 50% C0 ₂ , 26% H ₂ and 24% H ₂ 0. However, due to the RWGS reaction, some of the produced hydrogen will be used to generate CO. Therefore, the gas exiting the stack will actually have the following composition: 10.7% CO, 39.3% C0 ₂ , 15.3% H ₂ , and 34.7% H ₂ 0. The ratio of CO:H ₂ in the product gas is thus 1:1.43. Example 4 co-electrolysis

An SOEC stack consisting of 75 cells is operated at an av- erage temperature of 700°C with a mixture of steam and C0 ₂ being fed to the cathode in a molar ratio of 41:59 with a total flow rate of 100 Nl/min, while applying an electroly ^¬ sis current of 50 A. In the stack, steam is electrochemically converted into H ₂ according to reaction (2) . Assuming that any electrochemical conversion of C0 ₂ via reaction (1) is negligible, 64% of the fed steam is electrochemically converted into hydrogen. Were the RWGS reaction not pre ^¬ sent, the gas exiting the stack would have the following composition: 0% CO, 59% C0 ₂ , 26% H ₂ and 15% H ₂ 0. However, due to the RWGS reaction, some of the produced hydrogen will be used to generate CO. Therefore, the gas exiting the stack will actually have the following composition: 13.2% CO, 45.8% C0 ₂ , 13.0% H ₂ , and 28.0% H ₂ 0. The ratio of CO:H ₂ in the product gas is thus 1:1.01.