Method for generating synthesis gas for use in hydroformylation reactions

Abstract

A method for the generation of a gas mixture including carbon monoxide, carbon dioxide and hydrogen for use in hydroformylation plants, including the steps of evaporating water to steam; feeding the steam 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 steam to hydrogen; utilizing the effluent SOEC gas including H.sub.2 together with CO.sub.2 from an external source as feed for a RWGS reactor in which the RWGS reaction takes place, converting some of the CO.sub.2 and H.sub.2 to CO and H.sub.2O; removing some of or all the remaining steam from the raw product gas stream; using said gas mixture comprising CO, CO.sub.2 and H.sub.2 for liquid phase hydroformylation utilizing carbon monoxide and hydrogen as reactants, while recycling CO.sub.2 to the RWGS reactor.

Claims

1. A method for the generation of a gas mixture comprising carbon monoxide, carbon dioxide and hydrogen for use in hydroformylation plants, comprising the steps of: evaporating water to steam, feeding the steam 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 SOEC or SOEC stack to effect a partial conversion of steam to hydrogen forming a raw product gas, feeding CO.sub.2 from an external source, utilizing the raw product gas comprising H.sub.2 together with the CO.sub.2 as feed for a reverse water gas shift (RWGS) reactor in which the RWGS reaction takes place, converting some of the CO.sub.2 and H.sub.2 to CO and H.sub.2O forming a RWGS product gas, wherein the raw product gas is transferred directly to the RWGS reactor without cooling the raw product gas, removing some of or all the remaining steam from the RWGS product gas stream by cooling the RWGS product gas stream allowing for condensation of at least part of the steam as liquid water and separating a remaining product gas from the liquid water, the remaining product gas comprising CO, CO.sub.2 and H.sub.2, and using the remaining product gas for a liquid phase hydroformylation process utilizing carbon monoxide and hydrogen as reactants.

2. The method according to claim 1, wherein the temperature, at which H.sub.2 is produced by electrolysis of H.sub.2O in the SOEC or SOEC stack, is around 700 C.

3. The method according to claim 1, comprising removing essentially all the remaining steam from the RWGS product gas stream by cooling the RWGS product gas stream allowing for condensation of the steam as liquid water and separating the remaining product gas from the liquid water.

4. The method according to claim 1, further comprising recycling CO.sub.2 from the remaining product gas to the RWGS reactor.

5. The method according to claim 1, further comprising utilizing the CO.sub.2 to form a CO.sub.2-expanded liquid (CXL) reaction medium for the hydroformylation process.

Description

DETAILED DESCRIPTION

(1) 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. The SOEC technology is an advantageous alternative to low-temperature electrolysis technologies because of its high efficiency. The turn-on/turn-off of the apparatus is very swift, which is a further advantage.

(2) In practice it will usually be desirable to operate the SOEC stack at less than full conversion, and therefore the product gas from the SOEC or SOEC stack will contain H.sub.2 and H.sub.2O.

(3) In one embodiment of the invention, the raw product gas from the SOEC or SOEC stack is cooled, whereby most of the steam will condense, so that it can 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, now containing H.sub.2 as the main component, is then transferred to the RWGS reactor which is co-fed with CO.sub.2. This embodiment has the advantage of pushing the equilibrium in the RWGS reaction in the direction of formation of CO and H.sub.2O.

(4) In another embodiment of the invention, the raw product gas from the SOEC or the SOEC stack is not cooled, but rather transferred directly to the RWGS reactor which is co-fed with CO.sub.2. This embodiment has the advantage that the preferred operation temperatures of the SOEC or SOEC stack and the RWGS reactor are close lying; e.g. 700 C.

(5) After the RWGS reactor, the syngas will contain H.sub.2, CO, H.sub.2O and CO.sub.2. By cooling the gas, most of the H.sub.2O can be brought to condense and thus easily be separated from the gas. Further drying of the syngas may be carried out by using e.g. a drying column.

(6) 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, but such a unit is 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. Pressurizing this liquid with CO.sub.2 leads to the so-called CO.sub.2-expanded liquid (CXL). 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 alleviates mass transfer limitations in the hydroformylation reaction and increases 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 (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.

(7) 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.

(8) 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.

(9) So it is the intention of the present invention to provide a syngas-generating apparatus based on a combination of solid oxide electrolysis cells and an RWGS reactor, which can generate syngas for hydroformylation plants. The raw materials for generating the syngas will be CO.sub.2 and H.sub.2O.

(10) 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, H.sub.2 is produced from H.sub.2O, 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, steam or carbon dioxide to flush the oxygen side.

(11) More specifically, the principle of producing H.sub.2 by using a solid oxide electrolysis cell system consists in leading H.sub.2O to the fuel side of an SOEC with an applied current to convert H.sub.2O to H.sub.2 and transport the oxygen surplus to the oxygen side of the SOEC. Air, nitrogen, steam 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 H.sub.2 and H.sub.2O, whichoptionally after removal of water, e.g. by condensationcan be combined with CO.sub.2 in the RWGS reaction.

(12) If H.sub.2O is fed into an SOEC stack, the output will be a mixture of H.sub.2O and H.sub.2. Steam will be electrochemically converted into gaseous hydrogen according to the following reaction:
H.sub.2O (cathode).fwdarw.H.sub.2 (cathode)+O.sub.2 (anode)(1)

(13) The reverse water gas shift (RWGS) reaction takes place in the RWGS reactor which is fed with H.sub.2 (and optionally H.sub.2O) from the SOEC stack and CO.sub.2:
H.sub.2+CO.sub.2.Math.H.sub.2O+CO(2)

(14) When 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:

(15) $\begin{array}{cc}{X}_{H_{2}O}=\frac{p_{H_2}}{p_{H_2}+p_{H_{2}O}}=\frac{i.Math.V_m.Math.n_{\mathrm{cells}}}{z.Math.f_{H_{2}O}.Math.F}& \left(3\right)\end{array}$

(16) 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.

(17) The equilibrium constant for the RWGS reaction, K.sub.RWGS, is given by:

(18) $\begin{array}{cc}{K}_{RWGS}=\frac{p_{\mathrm{CO}}.Math.p_{H_{2}O}}{p_{{\mathrm{CO}}_2}.Math.p_{H_2}}=\exp \left(-\frac{G}{RT}\right)& \left(4\right)\end{array}$

(19) where G is the Gibbs free energy of the reaction at the operating temperature, R is the universal gas constant, and Tis absolute temperature.

(20) 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.

(21) Thus, the present invention relates to a method for the generation of a gas mixture comprising carbon monoxide, carbon dioxide and hydrogen for use in hydroformylation plants, comprising the steps of: evaporating water to steam, feeding the steam to a solid oxide electrolysis cell (SOEC) or an SOEC stack

(22) 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 steam to hydrogen, optionally removing some of or all the remaining steam 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, utilizing the effluent SOEC gas comprising H.sub.2 together with CO.sub.2 from an external source as feed for a RWGS reactor in which the RWGS reaction takes place, converting some of the CO.sub.2 and H.sub.2 to CO and H.sub.2O, removing some of or all the remaining steam 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 using said gas mixture containing CO, CO.sub.2 and H.sub.2 for liquid phase hydroformylation utilizing carbon monoxide and hydrogen as reactants, while recycling CO.sub.2 to the RWGS reactor.

(23) Preferably the temperature, at which H.sub.2 is produced by electrolysis of H.sub.2O in the SOEC or SOEC stack, is around 700 C.

(24) 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 gases.

(25) 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.

(26) Yet another advantage of the present invention is that if it is desired to switch between a low module syngas and pure Hz, this can be done using the same apparatus by simply bypassing the RWGS reactor when pure hydrogen is needed.

(27) 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. These advantages embrace higher reaction rates, improved selectivity (n/iso ratio) at mild conditions (lower temperature and lower pressure) compared to hydroformylation in a neat liquid media.

(28) 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 increased 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.

(29) The invention is illustrated further in the examples which follow.

Example 1

(30) H.sub.2O Electrolysis

(31) An SOEC stack consisting of 75 cells is operated at an average temperature of 700 C. with pure steam 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 (3), the conversion of H.sub.2O under such conditions is 26%, i.e. the gas exiting the cathode side of the stack is 26% H.sub.2, 74% H.sub.2O.

Example 2

(32) H.sub.2O Electrolysis Combined with RWGS

(33) An SOEC stack consisting of 75 cells is being operated at an average temperature of 700 C. with steam fed to the cathode with a total flow rate of 100 NI/min, while applying an electrolysis current of 50 A. In the stack, steam is electrochemically converted into H.sub.2 according to reaction (1) at a conversion of 52%. This effluent gas is fed directly from the SOEC to the RWGS reactor together with 100 NI/min CO.sub.2. The overall H.sub.2O/CO.sub.2 feed ratio is thus 50:50. The gas feeding the RWGS reactor will have the following composition: 0% CO, 50% CO.sub.2, 26% H.sub.2 and 24% H.sub.2O. Due to the RWGS reaction, some of the hydrogen will be used to generate CO. The RWGS reactor is operated isothermally at 700 C. Therefore, the gas exiting the RWGS reactor will 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 3

(34) H.sub.2O Electrolysis Combined with RWGS

(35) This example is carried out as Example 2 except that the overall H.sub.2O/CO.sub.2 feed ratio is 41:59. The gas exiting the RWGS reactor will 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 approximately 1:1.

Example 4

(36) H.sub.2O Electrolysis Combined with RWGS

(37) This example is carried out as Example 2 except that the effluent cathode gas from the SOEC stack is cooled, whereby steam condenses as liquid water which is taken out in a separator. The gas feeding the RWGS reactor will therefore have the following approximate composition: 0% CO, 50% CO.sub.2, 50% H.sub.2 and 0% H.sub.2O. Due to the RWGS reaction, some of the hydrogen will be used to generate CO. The RWGS reactor is operated isothermally at 700 C. Therefore, the gas exiting the RWGS reactor will have the following approximate composition: 22% CO, 28% CO.sub.2, 28% H.sub.2 and 22% H.sub.2O. The ratio of CO:H.sub.2 in the product gas is thus 1:1.27.