EP3292232B1 - Reduction method for electrochemical carbon dioxide utilization, alkali carbonate preparation and alkali hydrogen carbonate preparation - Google Patents

Description

The present invention relates to a reduction process for the electrochemical utilization of carbon dioxide. Carbon dioxide is fed into an electrolysis cell and reduced at a cathode.

State of the art

Currently, around 80% of the world's energy needs are covered by the combustion of fossil fuels, the combustion processes of which cause global emissions of around 34,000 million tons of carbon dioxide into the atmosphere per year. As a result of this release into the atmosphere, most of the carbon dioxide is disposed of, which can be up to 50,000 tons per day in a lignite power station, for example. Carbon dioxide is one of the so-called greenhouse gases, the negative effects of which on the atmosphere and the climate are being discussed. Since carbon dioxide is thermodynamically very low, it can only be reduced to recyclable products with difficulty, which has so far left the actual recycling of carbon dioxide in theory or in the academic world.

A natural breakdown of carbon dioxide takes place, for example, through photosynthesis. In this process, carbon dioxide is converted into carbohydrates in a process that is broken down into many sub-steps in terms of time and space. This process cannot easily be adapted on a large scale. A copy of the natural photosynthesis process with large-scale photocatalysis has so far not been sufficiently efficient.

An alternative is the electrochemical reduction of carbon dioxide. Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively young field of development. Efforts have only been made for a few years to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide. Research on a laboratory scale has shown that metals are the preferred catalysts for the electrolysis of carbon dioxide.

US 2012/228147 A1 discloses methods and systems for the electrochemical production of formic acid from carbon dioxide. The cathode is selected from the group consisting of indium, lead, tin, cadmium and bismuth. U.S. 3,959,094 A discloses a method and system for synthesizing methanol from the CO ₂ in air using electrical energy. The CO ₂ is absorbed into a solution of KOH _{to form K 2} CO ₃ , which is electrolyzed to produce methanol, a liquid hydrocarbon fuel. US 2008/223727 A1 discloses electrochemical processes for the reduction of carbon dioxide, for example the conversion of carbon dioxide into formate salts or formic acid.

From the publication Electrochemical CO ₂ reduction on metal electrodes by Y. Hori, published in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189 , Faraday efficiencies can be found on different metal cathodes, see Table 1.

If carbon dioxide is reduced on silver, gold or zinc cathodes, for example, carbon monoxide is formed almost exclusively. electrode CH ₄ C ₂ H ₄ C ₂ H ₅ OH C ₃ H ₇ OH CO HCOO H ₂ Total Cu 33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0 102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 Ed 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6 100.1 CD 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.0 0.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.0 0.0 0.0 0.0 0.0 99.7 99.7

Table 1:

The table shows Faraday efficiencies [%] of products that are produced during the reduction of carbon dioxide on various metal electrodes. The specified values apply to a 0.1 M potassium hydrogen carbonate solution as the electrolyte and current densities below 10 mA / cm ² .

For example, a silver cathode would produce predominantly carbon monoxide and only a little hydrogen. The reactions at the anode and cathode can be represented with the following reaction equations: Cathode: 2 CO ₂ + 4 e ^- + 4 H ⁺ → 2 CO + 2 H ₂ O Anode: 2 H ₂ O → O ₂ + 4 H ⁺ + 4 e ^-

As can also be seen in Table 1, a large number of hydrocarbons are produced as reaction products on a copper cathode. The electrochemical production of methane or ethylene, ethanol or monoethylene glycol, for example, is of particular economic interest. In terms of energy, these are products of higher value than carbon dioxide. Ethylene: 2CO ₂ + 12e ^- + 8H ₂ O → C ₂ H ₄ + 12OH ^- Methane: CO ₂ + 8e ^- + 4H ₂ O → CH ₄ + 4OH ^- Ethanol: 2CO ₂ + 12e ^- + 9H ₂ O → C ₂ H ₅ OH + 12OH ^- Monoethylene glycol: 2CO ₂ + 10e ^- + 8H ₂ O → HOC ₂ H ₄ OH + 10OH ^-

With an electrolyte containing chloride, the following reaction can take place at the anode:

2 Cl ^- → Cl ₂ + 2 e ^-

In the electrochemical conversion of carbon dioxide into an energetically higher-value product, the increase in economic efficiency and an improvement in terms of the continuous operability of the electrolysis systems are of interest.

Consequently, it is technically necessary to propose an improved solution for electrochemical carbon dioxide utilization which avoids the disadvantages known from the prior art. In particular, the solution to be proposed should enable continuous conversion of carbon dioxide. It is the object of the invention to provide an improved reduction process for the utilization of carbon dioxide.

These objects on which the present invention is based are achieved by a reduction method according to patent claim 1. Advantageous refinements of the invention are the subject matter of the subclaims.

Description of the invention

The reduction process according to the invention for utilizing carbon dioxide by means of an electrolysis system is defined in claim 1.

An electrolysis system not according to the invention for carbon dioxide utilization comprises at least one electrolyzer with an anode in an anode compartment and a cathode in a cathode compartment, the cathode compartment having at least one access for carbon dioxide and the cathode compartment being designed to bring the incoming carbon dioxide into contact with the cathode . Furthermore, the cathode compartment comprises a catholyte or is designed to be able to accommodate a catholyte, wherein the catholyte can access the cathode compartment via the same access as the carbon dioxide or via a separate second access. In addition, at least the anode compartment, when the cell is in operation, anode compartment and cathode compartment have alkaline cations.

A catholyte is an electrolyte that is directly influenced by the cathode during electrolysis. Correspondingly, anolyte is also used in the following when an electrolyte is referred to that is directly influenced by the anode during electrolysis. Alkaline cations refer to positively charged ions which have at least one element of the first main group of the periodic table.

The anode compartment of the electrolyser has at least one access for an anolyte and comprises an anolyte or is at least designed to receive an anolyte via this access, this anolyte having chlorine anions.

Typically, in the electrolysis system not according to the invention, the anode compartment and the cathode compartment are separated from one another by a membrane. The membrane is at least one mechanically separating layer, for example a diaphragm, which separates at least the electrolysis products produced in the anode compartment and cathode compartment from one another. Then you could too speak of a separator membrane or separating layer. Since the electrolysis products are in particular gaseous substances, a membrane with a high bubble point of 10 mbar or greater is preferably used. The so-called bubble point is a defining variable for the membrane used, which describes the pressure difference ΔP between the two sides of the membrane from which a gas flow through the membrane would start. The membrane can also be a proton- or cation-conducting or permeable membrane. While molecules, liquids or gases are separated, a flow of protons or cations is guaranteed from the anode compartment to the cathode compartment. A membrane is preferably used which has sulfonated polytetrafluoroethylene, for example Nafion.

The electrolysis system further comprises at least one separation basin for crystallizing an alkali hydrogen carbonate and / or alkali carbonate from the catholyte. In particular, this separation basin has a product outlet. Depending on the product, whether an alkali hydrogen carbonate and / or alkali carbonate is to be removed from the catholyte, and depending on the alkali metal, a second separation basin can also be provided for the most advantageous possible crystallization process. This is then typically arranged in the catholyte circulation direction after the first separation basin.

Depending on the cathode material used, the reduction of carbon dioxide produces different products: For example, carbon monoxide, ethylene, methane, ethanol or monoethylene glycol are produced. In all these cases, hydroxide ions are also formed, which are neutralized to hydrogen carbonate by excess carbon dioxide. The source of the alkaline cations lies in the anode compartment. A cation current through the membrane compensates for the electrical current caused by the applied voltage. For example, the alkali cations and the chloride anions are metered into the anolyte in the form of a chloride salt. While the chloride anions are oxidized to chlorine at the anode and as chlorine gas Leaving the anolyte circuit, the alkali cations migrate through the membrane into the catholyte circuit, where they react in the cathode compartment with the carbonate or hydrogen carbonate formed there to form an alkali metal carbonate or alkali hydrogen carbonate and in particular leave the catholyte circuit via the separate product outlet of the separation basin.

The electrolysis system not according to the invention has the advantage that, in addition to chlorine, at least one alkali metal carbonate and / or alkali metal hydrogen carbonate is generated as a valuable chemical substance. Whether alkali carbonate or alkali hydrogen carbonate is produced depends, for example, on the alkali metal and the recycling process. In aqueous solution, for example, the solubility is crucial. The less soluble carbonate or hydrogen carbonate crystallizes out. In the case of sodium and potassium, it is the hydrogen carbonate that is more sparingly soluble than the carbonate and then has to be calcined in a subsequent step. The combustion of sodium in carbon dioxide is an example that produces _{carbon monoxide and directly sodium carbonate Na 2} CO _3.

In addition, the electrolysis system described, not according to the invention, serves to utilize carbon dioxide and it can typically also be used to provide at least one third valuable substance, such as carbon monoxide, ethylene, methane, ethanol or monoethylene glycol. Via the very advantageous utilization of the compensating current by the cations, an electrolysis system not according to the invention is thus created which enables continuous hydrogen carbonate production.

As already described, the actual cathode reaction, in which the carbon dioxide is reduced, is followed by a subsequent reaction, namely the neutralization of the hydroxide ions (OH-). These are neutralized in particular by excess carbon dioxide to form hydrogen carbonate (HCO ₃ ^- ). On the one hand, this has the effect that the pH value in the cathode compartment is buffered in a pH value range of 6 to 8 will. It also has the effect that the electrolyte concentration increases continuously. If, however, the catholyte is fed into a catholyte circuit, that is to say is pumped into the cathode compartment and also discharged from it again, the hydrogen carbonate formed in the cathode compartment can be removed from the catholyte. For this purpose, in particular in the catholyte circuit, but for example also in the anolyte circuit, there is in each case at least one pump which provides for an electrolyte circuit.

Following the neutralization reaction of the hydroxide ions (OH ^- ) with excess carbon dioxide to form hydrogen carbonate ions (HCO ₃ ^- ), these preferably react further with alkali metal cations to form alkali metal hydrogen carbonates. The alkali cations that are present in the cathode compartment originate from the anode compartment, into which they were initially introduced, in particular as alkali metal chloride as an oxidation reactant or in the form of another alkali metal salt, for example to increase conductivity. The alkali ions are preferably fed into the anode compartment as alkali chloride. The membrane between the anode and cathode compartments is in particular chosen so that the cation current from the anode compartment to the cathode is ensured in the electric field of the electrolyzer. The temperature and pH value dependency of the solubility of alkali hydrogen carbonates now leads to different processes for crystallization or removal from the catholyte being carried out:

On the one hand, the temperature dependency of the solubility of the alkali hydrogen carbonates desired as electrolysis product can be used. For this purpose, the separation basin preferably comprises a cooling device, by means of which the catholyte is cooled by several degrees Kelvin, in contrast to the temperature range that prevails in the electrolyzer. The set temperature difference between the separation basin and the electrolyzer is preferably at least 15 K, in particular at least 20 K. Depending on the electrolyte concentration in the catholyte and depending on the alkali cations with which the hydrogen carbonate is formed is, a temperature difference between 30 K and 50 K can be particularly suitable. The temperature difference between the electrolyzer and the separation basin can be in a temperature range between 5 K and 70 K.

The lowering of the temperature in the separation basin has the additional advantage for the overall system that cooling takes place in the catholyte circuit before the catholyte is returned to the cathode compartment. This avoids excessive system-related heat development, especially in the electrolyser. However, a cooling device which may be provided especially for this purpose can be saved.

Also with the separation method of crystallizing out the alkali hydrogen carbonate by cooling the catholyte, pH buffers are preferably used, which are made available, for example, in a buffer reservoir to the separation basin and / or the catholyte circuit and / or the cathode compartment in order to buffer the catholyte volume accordingly.

The pH of the catholyte can also be used as such to control the process of separating the alkali hydrogen carbonate from the electrolyte. For this purpose, the pH value in the cathode compartment in particular is initially kept at a higher value, e.g. 8 or higher. This can shift the equilibrium in favor of the alkali carbonate in contrast to the alkali hydrogen carbonate. For crystallization in the separation basin, the pH is then lowered, preferably to a value of 6 or less, which leads to the formation and crystallization of the alkali hydrogen carbonate. The lowering of the pH value is typically done by blowing carbon dioxide into the separation basin.

Depending on the alkaline cation with which the hydrogen carbonate reacts, and depending on the pH value in the cathode compartment, an alkali hydrogen carbonate or an alkali carbonate can initially be formed. In particular, the two procedures described can be used to remove the desired product from the catholyte can also be combined. In some cases, for example when sodium hydrogen carbonate NaHCO _{3 is formed} , the sodium carbonate Na ₂ CO ₃ , for example, can also be obtained afterwards from the crystallized sodium hydrogen carbonate NaHCO ₃ by heating. Then hydrogen carbonate is even preferably first produced, deposited, and then the desired proportion of it is further processed into carbonate.

The pH dependence of the hydrogen carbonate or carbonate ions is, for example, in Figure 6 shown in a Hägg diagram for a sodium carbonate solution.

In the electrolysis system not according to the invention, a buffer reservoir is preferably also provided in the anolyte circuit, which can in particular also serve to introduce or replenish alkali chloride in the electrolyte in order to maintain the salt content in the anolyte.

In a preferred embodiment of the electrolysis system not according to the invention, the catholyte has at least one solvent, in particular water. Typically, aqueous electrolytes and correspondingly water-soluble conductive salts are used. The conductive salt content can be increased by adding further carbonates, hydrogen carbonates, but also sulfates or other conductive salts in order to increase the conductivity of the electrolyte in the catholyte as well as in the anolyte circuit, which leads to an increase in the metabolism in the overall system. Depending on which and in which quantities additional conductive salts are contained in the catholyte cycle, the crystallization process must be adapted accordingly in order to extract the desired product in as pure a form as possible. The conductive salts used are therefore typically chosen so that their solubility differs significantly from that of the alkali hydrogen carbonate or the alkali carbonate.

Typically, the electrolysis system not according to the invention has a gas separation device on the anolyte side, which is designed to separate the chlorine gas from the anolyte. A gas separation device can also be provided in the catholyte circuit, e.g. if it is geared towards the generation of carbon monoxide gas by using a cathode containing silver. Additional devices for inlets or outlets from the system or additional buffer reservoirs can be provided in the anolyte and catholyte circuits.

The type and quality of the membrane used in the electrolyser ultimately plays a major role in how pure the product that has crystallized out is. If only a separator is used as the membrane, chloride anions, for example, can also diffuse into the cathode space, even against the electric field in the electrolyzer, so that chlorides may also be formed in addition to hydrogen carbonate. Therefore, in the electrolysis system described, which is not according to the invention, a cation-conducting membrane is preferably used, which can almost exclusively pass cations. A purely anion-conducting membrane is accordingly not advantageous.

The described, inventive reduction process for carbon dioxide utilization by means of an electrolysis system, as described above, is defined in claim 1 and comprises the following steps: A catholyte comprising water and hydroxide ions and carbon dioxide are introduced into a cathode compartment and brought into contact there with a cathode. In the interior of the cathode compartment, this catholyte has alkaline cations which migrate through the cation-conducting membrane that separates the anode and cathode compartments. Excess carbon dioxide reacts in the cathode space with the hydroxide ions to form carbonate and / or hydrogen carbonate, which react with the alkali cations that have migrated through the membrane into the catholyte to form alkali hydrogen carbonate and / or alkali carbonate. At least part of the catholyte volume is introduced into a separation basin and an alkali hydrogen carbonate and / or an alkali carbonate is crystallized there. In addition, an anolyte, which has chloride anions, is introduced into an anode space and brought into contact there with an anode, at the anode the chloride anions are oxidized to chlorine and this is separated from the anolyte as chlorine gas via a gas separation device. Typically, this reduction process is carried out in such a way that the anolyte and catholyte are each conducted in a separate circuit, i.e. two pumps are provided in the electrolysis system, which at least at one point in the circuit transport the catholyte through the cathode compartment and transport the anolyte through the anode compartment effect. The circuits are separated from one another by the membrane in the electrolyser, which ideally only allows the transport of cations from the anode compartment into the cathode compartment. In particular, the alkali cations required in the cathode compartment are obtained from the anode compartment. For this purpose, the anolyte preferably has an alkali chloride, which can accordingly be used as a conductive salt but also as an electrolysis product. Alternatively, the alkali metal chloride in the anolyte can be used as the electrolysis product and an additional conductive salt, for example a sulfate, a phosphate, etc., preferably an alkali metal sulfate, can be used. Alternatively, ammonium salts or their homologues can also be used. Imidazolium salts or other ionic liquids can have a positive effect on the selectivity of the electrode, especially the cathode.

Typically, carbon monoxide, ethylene, methane, ethanol and / or monoethylene glycol is produced in the reduction process during the reduction of the carbon dioxide at the cathode. For this purpose, a suitable cathode is used as a catalyst for these reactions. The cathode is a silver-containing cathode and mainly forms carbon monoxide, hydroxide ions and only a little hydrogen as products from the carbon dioxide and the catholyte. Alternatively, the cathode is a copper-containing cathode and forms ethylene, methane, ethanol and / or from the carbon dioxide and the catholyte Monoethylene glycol and hydroxide ions as products. The great advantage of this reduction process is that, in addition to the utilization of carbon dioxide, chemical valuable substances can also be generated.

In one embodiment of the reduction process, the hydroxide ions formed during the carbon dioxide reduction are converted to hydrogen carbonate ions with excess carbon dioxide. The hydrogen carbonate production directly in the cathode compartment has the advantage that it can react further directly with the alkali cations present in the cathode compartment to form another interesting valuable material that would otherwise have to be produced in separate manufacturing processes.

In order to remove this valuable material from the system, in particular at least part of the catholyte volume is introduced into a separation basin and cooled there by at least 15 K, preferably by at least 20 K. The temperature dependence of the carbonate solubility is used here to remove the valuable substance from the catholyte cycle. The temperature difference between the separation basin and the electrolyser can also be more than 30 K, in particular more than 50 K, depending on the alkali hydrogen carbonate to be extracted and also depending on which other salts are present in the circuit. The temperature difference between the electrolyzer and the separation unit can be between 5 K and 70 K.

In an alternative variant for the extraction of the hydrogen carbonate product from the catholyte volume, the dependence of the solubility on the pH value is used. This method can be combined with the temperature-dependent method.

In one embodiment of the reduction process, at least part of the catholyte volume is introduced into a separation basin and its pH value there, in particular by blowing in carbon dioxide from above 8 to a pH value lowered by 6 or less. Buffering the pH value to a value above 8 in the cathode compartment has the advantage of preventing the alkali hydrogen carbonate from precipitating in the cathode compartment itself.

The reduction process can be carried out so that the precipitated alkali hydrogen carbonate is converted to alkali carbonate by heating. This can be done directly after the hydrogen carbonate has crystallized out in the separation basin or it can be done separately from the electrolysis system described.

As an alternative to the temperature method of crystallization or temperature-assisted crystallization, or in combination with this, the process can be run in such a way that the pH value in the cathode compartment is kept at the upper limit of the reaction by 8 or higher, so that the equilibrium is initially shifted in favor of sodium carbonate is:

2 NaHCO ₃ → Na ₂ CO ₃ + H ₂ O + CO ₂ .

To do this, the supply of carbon dioxide into the system must be very carefully controlled in order to get into and maintain this basic regime. In the separation basin, the pH value would then be lowered for optimal separation of the sodium hydrogen carbonate by blowing in carbon dioxide and thus the equilibrium reaction would be shifted again in favor of sodium hydrogen carbonate.

However, the process is not limited to sodium hydrogen carbonate. For example, potassium hydrogen carbonate can also be produced in this process. Analogously to the described separation process for sodium hydrogen carbonate, the potassium hydrogen carbonate can also be crystallized from a pure potassium hydrogen carbonate electrolyte by lowering the temperature in the separation basin. At 20 ° C the solubility of potassium hydrogen carbonate is 337 g / l, at 60 ° C 600 g / l.

The procedure is somewhat different if an additional conductive salt, eg potassium sulfate (K ₂ SO ₄ ) is to be used. This has a lower solubility of 111.1 g / l at 20 ° C and 250 g / l at 100 ° C, which means that the potassium sulfate would always precipitate first in the mixed electrolyte. In order to obtain the potassium hydrogen carbonate (KHCO ₃ ) from an electrolyte that contains both potassium sulphate and potassium hydrogen carbonate, the following procedure must be followed: In the separation tank AB, potassium sulphate K ₂ SO ₄ preferably crystallizes out, which is then, in the direction of the circulation, after the separation tank AB , can be fed back into the electrolyte. The electrolyte volume from which the potassium sulfate K ₂ SO _{4 has} already been removed is then concentrated, preferably in a further separation basin, ie the water is removed from the potassium hydrogen carbonate solution, for example by cooling, in order to obtain the crystalline material.

In principle, this method can also be used for other cations or mixtures of cations. Due to the migration of the cations, the catholyte concentrates until the most difficult to dissolve salt or double salt separates. It is important that the process of concentration and separation does not take place in the cathode compartment, i.e. not in the electrolysis cell itself, but rather that the catholyte is transported to a separation basin integrated in the electrolysis system. A further additional physical or chemical difference between the electrolysis cell and the separation basin, for example a temperature, pH value or pressure gradient, enables or facilitates the separation in the separation basin. A suitable pressure difference between the electrolysis cell and the separation basin can be up to 100 bar. A pressure difference between 2 bar and 20 bar would preferably be selected. An increased pressure in the separation basin would favor the formation of hydrogen carbonate.

In principle, alternative anode reactions are also conceivable on the anode side, but the coupling with chlorine production makes the most economic sense, since the chlorine market is around 75 million tons per year. Today's production of sodium hydrogen carbonate (NaHCO ₃ ) is around 50 million tons per year, which has so far been produced using the energetically unfavorable Solvay process.

With the described electrolysis system and reduction process it is possible to electrochemically continuously and simultaneously generate three valuable substances: At the cathode, a valuable substance such as carbon monoxide, ethylene, methane, ethanol or monoethylene glycol is obtained from the carbon dioxide reduction; and / or sodium carbonate as a by-product and chlorine is produced on the anode side.

Figur 1

zeigt in schematischer Darstellung ein nichterfindungsgemäßes Elektrolysesystem mit Kohlenstoffdioxid-Reservoir und Abscheidebecken,

Figur 2

zeigt in schematischer Darstellung ein nichterfindungsgemäßes Elektrolysesystem mit Gasdiffusionselektrode,

Figur 3

zeigt in schematischer Darstellung einen PEM-Aufbau einer Elektrolysezelle,

Figur 4

zeigt in schematischer Darstellung eine PEM-Halbzelle gekoppelt mit einer Gasdiffusionselektrode,

Figur 5

zeigt in schematischer Darstellung eine PEM-Halbzelle gekoppelt mit einer hinterströmten Kathode und

Figur 6

zeigt ein Hägg-Diagramm.

Examples and embodiments of the present invention will be further exemplified with reference to FIG Figures 1 to 6 described in the attached drawing:

Figure 1

shows a schematic representation of an electrolysis system not in accordance with the invention with a carbon dioxide reservoir and separation basin,

Figure 2

shows a schematic representation of an electrolysis system not according to the invention with a gas diffusion electrode,

Figure 3

shows a schematic representation of a PEM structure of an electrolysis cell,

Figure 4

shows a schematic representation of a PEM half-cell coupled with a gas diffusion electrode,

Figure 5

shows a schematic representation of a PEM half-cell coupled with a cathode flowing behind and

Figure 6

shows a Hägg diagram.

In the Figures 1 and 2 Examples of electrolysis systems not according to the invention for carbon dioxide reduction are shown in a schematic representation, which can also be read as flow charts for the reduction process described. The anolyte circuit AK is shown on the left-hand side and the catholyte circuit KK is shown on the right-hand side. These two circuits AK, KK are connected via the electrolyzer E1, E2, the anode compartment AR and cathode compartment KR of which are connected to one another via a membrane M or are separated from one another via these. A cation-conducting membrane M is preferably used as the membrane M. An anode A is arranged in the anode space AR and a cathode K is arranged in the cathode space KR, which are electrically connected via a voltage source U. Both circuits AK, KK preferably each have a pump P1, P2 which pump the electrolytes through the electrolyzer. In addition, devices N1, N2, N3 can be present in both circuits AK, KK at different points in the direction of flow, which can be additional inflows or outflows or as buffer reservoirs. At least one gas separation device G2 with a product outlet PA2, via which the product chlorine gas Cl ₂ can be removed, is provided in the anolyte circuit AK. At least one gas separation device G1 with a product outlet PA1 is also provided in the catholyte circuit KK, via which, for example, the electrolysis product carbon monoxide CO, for example also hydrogen H ₂ , can be withdrawn. However, other electrolysis products such as ethylene, methane, ethanol, monoethylene glycol can also be taken from the system via this or, for example, via a further product outlet. The electrolyzer E1, E2 has, for example, a gas diffusion electrode GDE for the carbon dioxide inlet.

In the case of the in Figure 1 The electrolyser E1 shown is a two-chamber structure and the carbon dioxide CO ₂ is introduced into the electrolyte via a reservoir CO ₂ -R and in the direction of the circulation in front of the cathode chamber KR. In both cases shown, the catholyte circuit KK has a separation basin AB, which can be integrated directly into the circuit or through which only part of the catholyte volume is passed. As in the Figures 1 and 2 shown, a branch of the circuit KK may be provided. The separation basin AB or several separation basins connected in series can be connected, for example, to a cooling device or to a buffer reservoir PR, so that the crystallization of the hydrogen carbonate is promoted by setting a temperature difference, pressure difference or pH value difference to the electrolyzer E1, E2. Furthermore, the separation basin AB has a product outlet PA3. Several separation basins connected in series would each have a product outlet.

In the Figure 1 and 2 That is, electrolysis systems are shown as they can be used for carrying out the reduction process according to the invention. In this setup, care is taken to ensure that separate anolyte AK and catholyte circles KK are available. The electrolytes used are then pumped continuously through the electrolysis cell E1, E2, ie through the anode space AR and through the cathode space KR. For this purpose, a pump P1, P2 is provided in each of the two circuits AK, KK in the structure. The structure can have materials made of plastic, plastic-coated metal or glass. Glass flasks can be used as storage vessels, the cell itself is made of PTFE, for example, and the tubes are made of neoprene.

The electrolyzer E1, E2, as it is installed in the electrolysis systems shown, can also have a different structure, as it is, for example, in FIG Figures 3 to 5 is shown. An alternative electrolysis cell is the one based on the polymer electrolyte membrane structure (PEM structure). In this case lies at least one electrode directly on the polymer electrolyte membrane PEM. Correspondingly, the electrolysis cell can be designed as a PEM half-cell, as in FIG Figures 4 and 5 in which the anode side is designed as a PEM half-cell, that is, the anode A is arranged in direct contact with the membrane PEM and the anode space AR is arranged on the side of the anode A facing away from the membrane.

In the cases as in Figure 4 and 5 As shown, the cathode K is designed to be porous and at least partially gas-permeable and / or electrolyte-permeable. In the Figure 4 the anode PEM half-cell is combined with a gas diffusion electrode GDE for introducing the carbon dioxide CO ₂ into the cathode space KR. In addition, the Figure 5 a cathode K flowing behind is shown, the cathode space KR of which is connected to a gas reservoir via the cathode K. The gas reservoir for its part has at least one gas inlet GE and possibly gas outlet GA. Such an embodiment has hitherto been used, for example, as an oxygen-consuming electrode, for example in the production of sodium hydroxide solution. Then the cathode K would flow behind with oxygen. The oxygen-consuming cathode can be used, for example, _{to avoid the formation of hydrogen H 2} in the cathode chamber KR in favor of a reaction to water H ₂ O. The water formation energy lowers the necessary system voltage U and thus results in a lower energy consumption of the electrolysis system. Since the cathode K of an oxygen-consuming electrode consists primarily of silver, it can also catalyze the carbon dioxide reduction. If no oxygen is made available, the oxygen consumption reaction cannot take place. Instead, the carbon dioxide reduction to carbon monoxide CO takes place with a certain amount of hydrogen formation.

• Ethylen: 12 NaCl + 14 CO₂ + 8 H₂O → C₂H₄ + 12 NaHCO₃ + 6 Cl₂
• Methan: 8 NaCl + 9 CO₂ + 4 H₂O → CH₄ + 8 NaHCO₃ + 4 Cl₂
• Ethanol: 12 NaCl + 14 CO₂ + 9 H₂O → C₂H₅OH + 12 NaHCO₃ + 6 Cl₂
• Monoethylenglykol:
  10 NaCl + 12 CO₂ + 8 H₂O → HOC₂H₄OH + 10 NaHCO₃ + 5 Cl₂.

If, for example, sodium is selected as the alkali metal, the following reactions take place in the cathode compartment KR when a copper-containing cathode K is used:

• Ethylene: 12 NaCl + 14 CO ₂ + 8 H ₂ O → C ₂ H ₄ + 12 NaHCO ₃ + 6 Cl ₂
• Methane: 8 NaCl + 9 CO ₂ + 4 H ₂ O → CH ₄ + 8 NaHCO ₃ + 4 Cl ₂
• Ethanol: 12 NaCl + 14 CO ₂ + 9 H ₂ O → C ₂ H ₅ OH + 12 NaHCO ₃ + 6 Cl ₂
• Monoethylene glycol:
  10 NaCl + 12 CO ₂ + 8 H ₂ O → HOC ₂ H ₄ OH + 10 NaHCO ₃ + 5 Cl ₂ .

In the case of a silver-containing cathode K, the following reaction would take place at the cathode K:
Carbon monoxide:
2 NaCl + 3 CO ₂ + H ₂ O → CO + 2 NaHCO ₃ + Cl ₂ .

These equations describe the total process in the electrolysis cell. The chlorine gas Cl ₂ arises, as described, through oxidation of the chloride anions Cl ^- at the anode A, the other electrolysis products arise at the cathode K or through subsequent reactions in the cathode space KR.

The sodium example is particularly suitable because the sodium hydrogen carbonate can be separated out very easily from the electrolyte. In addition, sodium hydrogen carbonate and sodium carbonate are important, frequently required chemical substances. Worldwide annual sodium carbonate production is around 50 million tons, such as the Roskill market report " Soda Ash: Market Outlook to 2018 ", available from Roskill Information Services Ltd, email: info@roskill.co.uk, www.roskill.co.uk/soda-ash , can be found.

The solubility of sodium hydrogen carbonate NaHCO ₃ in water H ₂ O is comparatively low and also shows a strong temperature dependence, see Table 2. Table 2 Molecular formula KHCO ₃ K ₂ SO ₄ K ₃ PO ₄ AI KBr KCl NaHCO ₃ Na ₂ SO ₄ Molar mass in g / mol 100.1 174.3 212.3 166.0 119.0 74.6 84.01 142.04 Solubility in H ₂ O at 20 ° C: In g / l 337 111 900 1400 678 344 96 170 In mol / l 3.37 0.64 4.24 8.43 5.70 4.61 1.19 1.14 Conductivities σ in mS / cm: At 0.05M 4.8 9.9 17.3 7.2 7.7 7.4 5.8 14.8 At 0.1M 9.1 19.2 30.1 14.0 14.3 13.8 28.1 51.6 At 0.5M 38.9 (69.9) 108 65.2 67.5 62.8

Table 2 shows other salts, potassium hydrogen carbonate KHCO ₃ , potassium sulfate K ₂ SO ₄ , potassium phosphate K ₃ PO ₄ , potassium iodide KI, potassium bromide KBr, potassium chloride KCl, sodium hydrogen carbonate NaHCO ₃ , sodium sulfate Na ₂ SO ₄ , which can be used with preference. However, other sulfates, phosphates, iodides or bromides can also be used to increase the conductivity in the electrolyte. Due to the constant supply of carbon dioxide, carbonates or hydrogen carbonates do not have to be supplied, but are formed in the cathode space KR during operation.

The solubility of sodium hydrogen carbonate NaHCO ₃ in water is 69 g / l at 0 ° C, 96 g / l at 20 ° C, 165 g / l at 60 ° C and 236 g / l at 100 ° C. Sodium carbonate NaCO ₃ , on the other hand, dissolves comparatively well, its solubility is 217 g / l at 20 ° C. With ongoing electrolysis, the sodium hydrogen carbonate NaHCO ₃ tends to crystallize out in the electrolysis cell E1, E2. This can be counteracted by means of an increased temperature, which is already created by the operation of the system, and also by means of appropriate pH value buffering. The sodium hydrogen carbonate NaHCO ₃ should only crystallize out of the electrolyte in the separation basin AB. By pumping the electrolyte into a circuit KK, the sodium hydrogen carbonate NaHCO ₃ formed in the cathode chamber KR is led out of this and the catholyte circuit KK can run through a separation basin AB or a partial volume of the catholyte is branched off into a separation basin AB, in which, for example, the cooling of the electrolyte, the sodium hydrogen carbonate NaHCO ₃ crystallizes out and can thus be obtained. Since the electrolysis cells E1, E2 heat up considerably during operation due to process losses, effective crystallization to temperature differences of up to 70 K between the cathode space KR and the separation basin AB can occur. A temperature difference between 30 K and 50 K is preferred. In particular with a temperature difference of at least 15 K or even at least 20 K.

If the catholyte contains further additives to increase conductivity and thus increase energy efficiency, i.e. if an additional conductive salt minimizes the ohmic losses in the electrolyte, this must be _{taken into account when the sodium hydrogen carbonate NaHCO 3} crystallizes out in order to obtain the purest possible product. A hydrogen sulfate HSO ₄ ^- or sulfate SO ₄ ^2- is preferably used as the conductive additive. This can be, for example, sodium sulfate Na ₂ SO ₄ or sodium hydrogen sulfate NaHSO ₄ . The solubility of sodium hydrogen sulfate NaHSO ₄ is 1080 g / l at 20 ° C and that of sodium sulfate Na ₂ SO ₄ is 170 g / l at 20 ° C, see Table 2. This large difference in solubility compared to sodium hydrogen carbonate NaHCO ₃ ensures that Sodium hydrogen carbonate NHCO ₃ preferably crystallizes out in the separation basin. This variant of the reduction process has the immense advantage that it can basically replace the Solvay process used as standard for the production of sodium hydrogen carbonate. The Solvay process for producing sodium hydrogen carbonate has a major disadvantage, namely that it consumes very large amounts of water. In addition, for every kilogram of soda, i.e. sodium carbonate Na ₂ CO ₃ , roughly one kilogram of unusable calcium chloride is also produced CaCl _{2 is} generated, which is mostly released into wastewater and thus into rivers and seas. With an annual production of 50 million tons of sodium carbonate Na ₂ CO ₃ , this is also around 50 million tons of calcium chloride CaCl2.

_{The natural sources of soda Na 2} CO ₃ available in addition to the Solvay process are nowhere near sufficient. Sodium hydrogen carbonate NaHCO ₃ occurs as a natural mineral Nahcolith in the United States of America. It usually occurs finely distributed in oil shale and can then be obtained as a by-product of oil production. Particularly rich Nahcolith horizons are being mined in the state of Colorado. However, the annual production in 2007 was only 93,440 tons. It also occurs, for example, in soda lakes in Egypt, in Turkey in Lake Van, in East Africa, e.g. B. in Lake Natron and other lakes of the East African Rift, in Mexico, in California (USA), and as Trona (Na (HCO ₃ ) · Na ₂ CO ₃ · 2H ₂ O) in Wyoming (USA), Mexico, East Africa and in the southern Sahara.

In the Figure 6 an example of a Hägg diagram of a 0.05 molar solution of carbon dioxide CO _{2 is} shown to illustrate the dependence on the concentration and pH value parameters. In a medium pH range, carbon dioxide CO ₂ and its salts are present next to one another. While in the strongly basic carbon dioxide CO _{2 is} preferably present as carbonate CO ₃ ^2- , in the medium pH range preferably as hydrogen carbonate HCO ₃ ^- , the hydrogen carbonate ions are expelled from the solution in the form of at low pH values in an acidic environment Carbon dioxide CO ₂ .

Claims (4)

1. Reduction process for carbon dioxide utilization by means of an electrolysis system,
   in which a catholyte comprising water and hydroxide ions, and carbon dioxide (CO₂) are introduced into a cathode space (KR) and brought into contact with a cathode (K),
   in which carbon dioxide (CO₂) is reduced at the cathode (K),
   in which an anolyte including chloride anions (Cl^-) is introduced into an anode space (AR) and brought into contact with an anode (A),
   in which chloride anions (Cl^-) are oxidized at the anode (A) to chlorine (Cl₂) and the latter is separated from the anolyte as chlorine gas by means of a gas separation unit,
   in which the anolyte includes alkali metal cations that migrate into the catholyte through a cation-conducting membrane,
   wherein excess carbon dioxide reacts in the cathode space with the hydroxide ions to give carbonate and/or hydrogencarbonate, which react with the alkali metal cations that have migrated into the catholyte through the membrane to give alkali metal hydrogencarbonate and/or alkali metal carbonate, and
   in which at least a portion of the catholyte volume is introduced into a deposition tank, where the alkali metal hydrogencarbonate and/or alkali metal carbonate crystallizes out,
   wherein

   - the cathode (K) is a silver-containing cathode (K), and the carbon dioxide and the catholyte form predominantly carbon monoxide, hydroxide ions and only a little hydrogen as products; or wherein

   - the cathode (K) is a copper-containing cathode (K), and the carbon dioxide and the catholyte form ethylene, methane, ethanol and/or monoethylene glycol and hydroxide ions as products.
2. Reduction process according to Claim 1, in which, on introduction of at least a portion of the catholyte volume into the deposition tank, the catholyte volume introduced is cooled down therein by at least 15 kelvin, preferably at least 20 kelvin.
3. Reduction process according to Claim 1 or Claim 2, in which, on introduction of at least a portion of the catholyte volume into the deposition tank, the pH of the catholyte volume introduced is lowered from above 8 to a pH of 6 or less by blowing in carbon dioxide (CO₂).
4. Reduction process according to any of Claims 1 to 3, in which, on introduction of at least a portion of the catholyte volume into the deposition tank, an alkali metal hydrogencarbonate is crystallized out of the catholyte volume introduced and is subsequently converted to an alkali metal carbonate by heating.

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