METHOD FOR GENERATING GAS MIXTURES COMPRISING CARBON MONOXIDE AND CARBON DIOXIDE FOR USE IN SYNTHESIS REACTIONS

Abstract

A method for the generation of a gas mixture including carbon monoxide, carbon dioxide and optionally hydrogen for use in hydroformylation plants or in carbonylation plants, including mixing an optional steam with carbon dioxide in the desired molar ratio, 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 effecting a partial conversion of carbon dioxide to carbon monoxide and optionally of steam to hydrogen, removing some or all the remaining steam from the raw product gas stream by cooling the raw product gas stream and separating the remaining product gas from a liquid, and using the gas mixture containing CO and CO.sub.2 for liquid phase synthesis reactions utilizing carbon monoxide as one of the reactants while recycling CO.sub.2 to the SOEC or SOEC stack.

Claims

1. A method for the generation of a gas mixture comprising carbon monoxide, carbon dioxide, and hydrogen for use in a liquid phase hydroformylation or a liquid phase carbonylation, comprising the steps of: mixing steam with carbon dioxide in a desired molar ratio to form a resulting gas; feeding the resulting gas to a solid oxide electrolysis cell (SOEC) or an SOEC stack at a sufficient temperature for the SOEC or SOEC stack to operate while supplying an electrical current to the SOEC or SOEC stack to effect a partial conversion of carbon dioxide to carbon monoxide and of steam to hydrogen forming a raw product gas comprising carbon monoxide and carbon dioxide; removing some of or all remaining steam from the raw product gas by cooling the raw product gas allowing for condensation of at least part of the steam as liquid water and separating a remaining product gas comprising carbon monoxide and carbon dioxide from the liquid water; and using the remaining product gas for the liquid phase hydroformylation or the liquid phase carbonylation, the liquid phase hydroformylation or the liquid phase carbonylation utilizing carbon monoxide as a reactant in the liquid phase hydroformylation or the liquid phase carbonylation, while utilizing at least some of the carbon dioxide to pressurize the liquid phase hydroformylation or the liquid phase carbonylation.

2. The method of claim 1, wherein utilizing at least some of the carbon dioxide to pressurize the liquid phase hydroformylation or the liquid phase carbonylation increases the reaction rate of the liquid hydroformylation reaction or the liquid phase carbonylation.

3. The method of claim 1, further comprising recycling at least some of the carbon dioxide remaining from after the liquid phase hydroformylation or the liquid phase carbonylation to the SOEC or SOEC stack.

4. The method of claim 1, comprising utilizing carbon monoxide and hydrogen as reactants in liquid phase hydroformylation.

5. The method of claim 1, wherein the liquid phase hydroformylation or the liquid phase carbonylation is liquid phase carbonylation.

6. The method of claim 1, comprising flushing an oxygen side of the SOEC or SOEC stack.

7. The method of claim 6, wherein the flushing is with air, nitrogen, steam, or carbon dioxide.

8. The method of claim 6, wherein the flushing reduces the oxygen concentration on the oxygen side of the SOEC or SOEC stack.

9. The method of claim 6, wherein the flushing feeds energy into the SOEC or SOEC stack allowing the SOEC or SOEC stack to operate endothermically.

10. The method of claim 1, comprising operating the SOEC or SOEC stack endothermically.

11. The method of claim 1, comprising utilizing at least some of the remaining product gas to pressurize the liquid phase hydroformylation or the liquid phase carbonylation.

12. A method for the generation of a gas mixture comprising carbon monoxide, carbon dioxide, and hydrogen for use in a liquid phase synthesis reaction, comprising the steps of: mixing steam with carbon dioxide in a desired molar ratio to form a resulting gas; feeding the resulting gas to a solid oxide electrolysis cell (SOEC) or an SOEC stack at a sufficient temperature for the SOEC or SOEC stack to operate while supplying an electrical current to the SOEC or SOEC stack to effect a partial conversion of carbon dioxide to carbon monoxide and of steam to hydrogen forming a raw product gas comprising carbon monoxide and carbon dioxide; removing some of or all remaining steam from the raw product gas by cooling the raw product gas allowing for condensation of at least part of the steam as liquid water and separating a remaining product gas comprising carbon monoxide and carbon dioxide from the liquid water; and using the remaining product gas for the liquid phase synthesis reaction, the liquid phase synthesis reaction utilizing carbon monoxide as a reactant in the liquid phase synthesis reaction, while utilizing at least some of the carbon dioxide to pressurize the liquid phase synthesis reaction.

13. The method of claim 12, further comprising recycling at least some of the carbon dioxide remaining from after the liquid phase synthesis reaction to the SOEC or SOEC stack.

14. The method of claim 12, comprising flushing an oxygen side of the SOEC or SOEC stack.

15. The method of claim 14, wherein the flushing is with air, nitrogen, steam, or carbon dioxide.

16. The method of claim 14, wherein the flushing reduces the oxygen concentration on the oxygen side of the SOEC or SOEC stack.

17. The method of claim 14, wherein the flushing feeds energy into the SOEC or SOEC stack allowing the SOEC or SOEC stack to operate endothermically.

18. A method for the generation of a gas mixture comprising carbon monoxide and carbon dioxide for use in a liquid phase hydroformylation or a liquid phase carbonylation, comprising the steps of: feeding a gas comprising carbon dioxide to a solid oxide electrolysis cell (SOEC) or an SOEC stack at a sufficient temperature for the SOEC or SOEC stack to operate while supplying an electrical current to the SOEC or SOEC stack to effect a partial conversion of carbon dioxide to carbon monoxide forming a raw product gas comprising carbon monoxide and carbon dioxide; using the raw product gas for the liquid phase hydroformylation or the liquid phase carbonylation, the liquid phase hydroformylation or the liquid phase carbonylation utilizing carbon monoxide as a reactant in the liquid phase hydroformylation or the liquid phase carbonylation, while utilizing at least some of the carbon dioxide to pressurize the liquid phase hydroformylation or the liquid phase carbonylation, which increases the reaction rate of the liquid hydroformylation reaction or the liquid phase carbonylation; and recycling at least some of the carbon dioxide remaining from after the liquid phase hydroformylation or the liquid phase carbonylation to the SOEC or SOEC stack.

Description

DETAILED DESCRIPTION

[0023] 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 hydroformylation 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 produce e.g. oxygen and hydrogen gas by electrolysis of water. Importantly, it can also be used for converting CO.sub.2 electrochemically into the toxic, but for many reasons attractive 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.

[0024] Thus, co-electrolysis of water and carbon dioxide in an SOEC stack may produce a mixture of hydrogen and carbon monoxide in the desired ratio. If hydrogen is already available from other sources, then the SOEC may be used to generate carbon monoxide. This includes the option of preparing H.sub.2 and CO in separate SOEC stacks. In practice it is usually desirable to operate the SOEC stack at less than full conversion and therefore the product gas will contain CO, CO.sub.2 and optionally H.sub.2 and H.sub.2O. By cooling the raw product gas, most of the steam (if present) will condense, and it can then be separated from the gas stream as liquid water in a separator. The product gas may be further dried, e.g. over a drying column, if desired. The product gas will then contain CO, CO.sub.2 and optionally H.sub.2 as the main components. The separation of CO.sub.2 from the reactive components CO and H.sub.2 is more complicated and costly than the separation of water from the product gas. It can be done by using a PSA (pressure swing adsorption) unit, which unfortunately is quite expensive. However, the presence of CO.sub.2 in the hydroformylation reaction actually is an advantage: The hydroformylation reaction is carried out in a liquid medium, and pressurizing this liquid with CO.sub.2 entails a CO.sub.2-expanded liquid (CXL) as defined above. It has been described in the literature (see Fang et al., Ind. Eng. Chem. Res. 46 (2007) 8687-8692 and references therein) that CXL media alleviate mass transfer limitations in the hydroformylation reaction and increase the solubility of the reactant gases in the CXL medium compared to the neat liquid medium. As a result of this, the rate of the hydroformylation reaction may be increased by up to a factor of four in CXL-media compared to neat organic solvents. Furthermore, the n/iso ratio, i.e. the ratio between linear and branched aldehydes, may be improved by using a CXL solvent compared to using the neat solvent as taught in U.S. Pat. No. 7,365,234 B2.

[0025] Therefore, the present invention offers a way to provide a syngas with the appropriate H.sub.2/CO ratio while at the same time providing the CO.sub.2 needed for obtaining a CO.sub.2-expanded liquid reaction medium for the hydroformylation process. If hydrogen is available from other sources, the present invention offers a way to provide a CO/CO.sub.2-mixture which, when mixed with hydrogen, is suitable for carrying out the hydroformylation reaction in a CXL medium.

[0026] An example of an olefin used for the hydroformylation reaction is 1-octene, but in principle any olefin may be used according to the present invention. An example of a liquid solvent for the hydroformylation reaction is acetone, but a long range of other organic solvents may be used.

[0027] Many other catalyzed liquid-phase carbonylation processes are used industrially, and the present invention can be applied to all of them.

[0028] So it is the intention of the present invention to provide an apparatus generating syngas or a mixture of carbon oxides based on solid oxide electrolysis cells, which can generate syngas for hydroformylation plants or other plants which are based on synthesis with CO in the liquid phase. The raw materials for generating the syngas will be mixtures of CO.sub.2 and optionally H.sub.2O.

[0029] 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 H.sub.2 are produced from a mixture of CO.sub.2 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, nitrogen or carbon dioxide to flush the oxygen side.

[0030] More specifically, the principle of producing CO and H.sub.2 by using a solid oxide electrolysis cell system consists in leading CO.sub.2 and H.sub.2O to the fuel side of an SOEC with an applied current to convert CO.sub.2 to CO and H.sub.2O to H.sub.2 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) providing means for feeding energy into the SOEC, operating it endothermic. The product stream from the SOEC contains a mixture of CO, H.sub.2, H.sub.2O and CO.sub.2, whichafter removal of water, e.g. by condensationcan be used directly in the hydroformylation reaction.

[0031] In one embodiment of the invention, CO and H.sub.2 are both made by electrolysis, but in separate SOECs or SOEC stacks. This has the advantage that each SOEC or SOEC stack may be optimized for its specific use.

[0032] The present invention pertains not only to the hydroformylation reaction, but in principle to all catalyzed liquid phase reactions where CO is one of the reactant chemicals.

[0033] The overall principle in the production of CO by electrolysis is that CO.sub.2 (possibly including some CO) is fed to the cathode. As current is applied to the stack, CO.sub.2 is converted to CO to provide an output stream with a high concentration of CO:

##STR00001##

[0034] If pure CO.sub.2 is fed into the SOEC stack, the output will be CO (converted from CO.sub.2) and unconverted CO.sub.2.

[0035] If a mixture of CO.sub.2 and H.sub.2O is fed into the SOEC stack, the output will be a mixture of CO, CO.sub.2, H.sub.2O and H.sub.2. In addition to the electrochemical conversion reaction of CO.sub.2 to CO (1) given above, steam will be electrochemically converted into gaseous hydrogen according to the following reaction:

##STR00002##

[0036] Additionally, a non-electrochemical process, namely the reverse water gas shift (RWGS) reaction, takes place within the pores of the cathode:

##STR00003##

[0037] In state-of-the-art SOEC stacks, where the cathode comprises Ni metal (typically a cermet of Ni and stabilized zirconia), the overpotential for reaction (1) is typically significantly higher than that for reaction (2). Furthermore, since Ni is a good catalyst for the RWGS reaction, reaction (3) occurs almost instantaneously at SOEC operating temperatures. In other words, the vast majority of the electrolysis current is used for converting H.sub.2O into H.sub.2 (reaction 2), and the produced H.sub.2 rapidly reacts with CO.sub.2 (according to reaction 3) to provide a mixture of CO, CO.sub.2, H.sub.2O and H.sub.2. Under typical SOEC operating conditions, only a very small amount of CO is produced directly via electrochemical conversion of CO.sub.2 into CO (reaction 1).

[0038] In case pure H.sub.2O is fed into the SOEC stack, the conversion X.sub.H2O of H.sub.2O to H.sub.2 is given by Faraday's law of electrolysis:

[00001]$\begin{array}{cc}{X}_{\text{?}}=\frac{p_{\text{?}}}{p_{\text{?}}+p_{\text{?}}}=\frac{i.Math.V_{\text{?}}.Math.n_{\text{?}}}{z.Math.f_{\text{?}}.Math.F}& \left(4\right)\end{array}$$\text{?}\text{indicates text missing or illegible when filed}$

where p.sub.H2 is the partial pressure of H.sub.2 at cathode outlet, p.sub.H2O is the partial pressure of steam at cathode outlet, i is the electrolysis current, V.sub.m is the molar volume of gas at standard temperature and pressure, n.sub.cells is the number of cells in an SOEC stack, z is the number of electrons transferred in the electrochemical reaction, f.sub.H2O is the flow of gaseous steam into the stack (at standard temperature and pressure), and F is Faraday's constant.

[0039] In case pure CO.sub.2 is fed into the SOEC stack, the conversion X.sub.CO2 of CO.sub.2 to CO is given by an analogous expression:

[00002]$\begin{array}{cc}{X}_{\text{?}}=\frac{p_{\text{?}}}{p_{\text{?}}+p_{\text{?}}}=\frac{i.Math.V_{\text{?}}.Math.n_{\text{?}}}{z.Math.f_{\text{?}}.Math.F}& \left(5\right)\end{array}$$\text{?}\text{indicates text missing or illegible when filed}$

where p.sub.CO is the partial pressure of CO at cathode outlet, p.sub.CO2 is the partial pressure of CO.sub.2 at cathode outlet, i is the electrolysis current, V.sub.m is the molar volume of gas at standard temperature and pressure, n.sub.cells is the number of cells in an SOEC stack, z is the number of electrons transferred in the electrochemical reaction, f.sub.CO2 is the flow of gaseous CO.sub.2 into the stack (at standard temperature and pressure), and F is Faraday's constant.

[0040] In case both steam and CO.sub.2 is 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, K.sub.RWGS, is given by:

[00003]$\begin{array}{cc}{K}_{\text{?}}=\frac{p_{\text{?}}.Math.p_{\text{?}}}{p_{\text{?}}.Math.p_{\text{?}}}=\exp \left(-\frac{G}{\mathrm{RT}}\right)& \left(6\right)\end{array}$$\text{?}\text{indicates text missing or illegible when filed}$

where G is the Gibbs free energy of the reaction at SOEC operating temperature, R is the universal gas constant, and T is the absolute temperature.

[0041] The equilibrium constant, and therefore the extent to which electrochemically produced H.sub.2 is used to convert CO.sub.2 into CO, is temperature-dependent. For example, at 500 C., K.sub.RWGS=0.195. At 600 C., K.sub.RWGS=0.374. At 700 C., K.sub.RWGS=0.619.

[0042] Thus, the present invention relates to a method for the generation of a gas mixture comprising carbon monoxide, carbon dioxide and optionally hydrogen for use in hydroformylation plants or in carbonylation plants, comprising the steps of: [0043] optionally evaporating water to steam, [0044] mixing the optional steam with carbon dioxide in the desired molar ratio, and [0045] 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 a partial conversion of carbon dioxide to carbon monoxide and optionally of steam to hydrogen, wherein [0046] optionally some of or all the remaining steam is removed from the raw product gas stream by cooling the raw product gas stream allowing for condensation of at least part of the steam as liquid water and separating the remaining product gas from the liquid, and [0047] the gas mixture containing CO and CO.sub.2 is used for liquid phase synthesis reactions, utilizing carbon monoxide as one of the reactants while recycling CO.sub.2 to the SOEC.

[0048] For use in the hydroformylation reaction, the molar ratio between steam and carbon dioxide is preferably in the interval 0-2, more preferably in the interval 0-1.5 and most preferably in the interval 0-1, since this ratio will provide a syngas with a CO:H.sub.2 ratio of 1.015:1 (see Example 4 below).

[0049] Preferably the temperature, at which CO is produced by electrolysis of CO.sub.2 in the SOEC or SOEC stack, is around 700 C.

[0050] 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/H.sub.2 ratio, since this is simply a matter of adjusting the CO.sub.2/H.sub.2O ratio of the feed gas.

[0051] 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.

[0052] Yet another advantage of the present invention is that if it is desired to switch between a CO:H.sub.2=1:1 syngas and pure H.sub.2, this can be done using the same apparatus, simply by adjusting the feed from CO.sub.2/H.sub.2O to pure H.sub.2O.

[0053] A further advantage of the present invention is that it provides a CO/H.sub.2 stream diluted in CO.sub.2, which enables the subsequent hydroformylation reaction to be carried out in a CO.sub.2-expanded liquid (CXL) reaction medium. This advantage embraces higher reaction rates, improved selectivity (n/iso ratio) at mild conditions (lower temperature and lower pressure) compared to hydroformylation in neat liquid media. Similar advantages in other carbonylation reactions are to be expected.

[0054] A still further advantage of the present invention is that syngas of high purity can be produced without being more expensive than normal syngas in any way, 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 CO.sub.2/H.sub.2O feed, and provided that a feed consisting of food grade or beverage grade CO.sub.2 and ion-exchanged water is chosen, very pure syngas can be produced.

[0055] The invention is illustrated further in the examples which follow.

Example 1

CO.SUB.2 .Electrolysis

[0056] An SOEC stack consisting of 75 cells is operated at an average temperature of 700 C. with pure CO.sub.2 being fed to the cathode at a flow rate of 100 Nl/min CO.sub.2, while applying an electrolysis current of 50 A. Based on equation (5) above, the conversion of CO.sub.2 under such conditions is 26%, i.e. the gas exiting the cathode side of the stack consists of 26% CO and 74% CO.sub.2.

Example 2

H.SUB.2.O Electrolysis

[0057] An SOEC stack consisting of 75 cells is operated at an average temperature of 700 C. with pure steam being fed to the cathode at a flow rate of 100 NI/min steam (corresponding to a liquid water flow rate of approximately 80 g/min), while applying an electrolysis current of 50 A. Based on equation (4) above, the conversion of H.sub.2O under such conditions is 26%, i.e. the gas exiting the cathode side of the stack consists of 26% H.sub.2 and 74% H.sub.2O.

Example 3

Co-Electrolysis

[0058] An SOEC stack, consisting of 75 cells, is operated at an average temperature of 700 C. with a mixture of steam and CO.sub.2 being fed to the cathode in a molar ratio of 1:1 with a total flow rate of 100 Nl/min, while applying an electrolysis current of 50 A. In the stack, steam is electrochemically converted into H.sub.2 according to reaction (2) above. Assuming that Pelectrochemical conversion of CO.sub.2 via reaction (1) is negligible, 52% of the fed steam is electrochemically converted into hydrogen. Were the RWGS reaction not present, the gas exiting the stack would have the following composition: 0% CO, 50% CO.sub.2, 26% H.sub.2 and 24% H.sub.2O. 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% CO.sub.2, 15.3% H.sub.2 and 34.7% H.sub.2O. The ratio of CO:H.sub.2 in the product gas is thus 1:1.43.

Example 4

Co-Electrolysis

[0059] An SOEC stack consisting of 75 cells is operated at an average temperature of 700 C. with a mixture of steam and CO.sub.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 electrolysis current of 50 A. In the stack, steam is electrochemically converted into H.sub.2 according to reaction (2) above. Assuming that electrochemical conversion of CO.sub.2 via reaction (1) is negligible, 64% of the fed steam is electrochemically converted into hydrogen. Were the RWGS reaction not present, the gas exiting the stack would have the following composition: 0% CO, 59% CO.sub.2, 26% H.sub.2 and 15% H.sub.2O. 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% CO.sub.2, 13.0% H.sub.2 and 28.0% H.sub.2O. The ratio of CO:H.sub.2 in the product gas is thus 1.015:1.