ES2897748T3 - Reduction procedure for the use of electrochemical carbon dioxide, generation of alkaline carbonate and alkaline hydrogen carbonate - Google Patents

Abstract

Reduction procedure for the use of carbon dioxide by means of an electrolysis system, in which a catholyte comprising water and hydroxide ions and carbon dioxide (CO2) is introduced into a cathode space (KR) and is brought to contact with a cathode (K), in which carbon dioxide (CO2) is reduced at the cathode (K), in which an anolyte, which has chloride anions (Cl-), is introduced into an anode space (AR ) and is brought into contact with an anode (A), in which chloride anions (Cl-) are oxidized at the anode (A) to give chlorine (Cl2) and this is separated as chlorine gas from the anolyte through a device of gas separation, in which the anolyte has alkaline cations, which migrate through a cation-conducting membrane towards the catholytes, where the carbon dioxide in excess in the cathode space reacts with the hydroxide ions to give carbonate and/or hydrogen carbonate, which react with the alkaline cations that have migrated through the membrane to ia the catholyte to give alkali hydrogen carbonate and/or alkali carbonate and in which at least a part of the volume of catholyte is introduced into the deposition tank and there crystallizes the alkali hydrogen carbonate and/or alkali carbonate, where - the cathode (K ) is a silver-containing cathode (K) and forms from carbon dioxide and catholyte predominantly carbon monoxide, hydroxide ions and only a little hydrogen as products; or wherein - the cathode (K) is a copper-containing cathode (K) and forms from carbon dioxide and the catholyte ethylene, methane, ethanol and/or monoethylene glycol and hydroxide ions as products.

Description

DESCRIPTION

Reduction procedure for the use of electrochemical carbon dioxide, generation of alkaline carbonate and alkaline hydrogen carbonate

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

State of the art

Currently it covers approx. 80% of the world's energy demand through the combustion of fossil fuels, the combustion process of which causes a global emission of approximately 34 billion tons of carbon dioxide into the atmosphere per year. This release into the atmosphere removes most of the carbon dioxide, which can amount, for example, in a lignite-fired power plant to up to 50,000 tons per day. Carbon dioxide belongs to the so-called greenhouse gases, whose negative repercussions on the atmosphere and climate are discussed. Since carbon dioxide is thermodynamically very low, it can be reduced only with difficulty to reusable products, which has left the true use of carbon dioxide until now in theory or in academia.

Natural carbon dioxide degradation takes place, for example, by photosynthesis. In this case, carbon dioxide is converted into carbohydrates in a spatially and temporally classified process in many sub-stages. This process cannot thus easily be adapted on a technical scale. A copy of the natural photosynthesis process with photocatalysis on a technical scale is so far not efficient enough.

An alternative is represented by the electrochemical reduction of carbon dioxide. Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively young field of development. Only since a few years have there been efforts to develop an electrochemical system, which can reduce an acceptable amount of carbon dioxide. Investigations on a laboratory scale have shown that metals can preferably be used as catalysts for the electrolysis of carbon dioxide.

US 2012/228147 A1 discloses processes and systems for the electrochemical preparation of formic acid from carbon dioxide. The cathode is selected from the group consisting of indium, lead, tin, cadmium, and bismuth. US 3959094 A discloses a method and system for the synthesis of methanol from CO2 in air using electrical energy. CO2 is absorbed by a KOH solution to form K2CO3, which is electrolyzed to generate methanol, a liquid hydrocarbon fuel. US 2008/223727 A1 discloses electrochemical processes for the reduction of carbon dioxide, eg the transformation of carbon dioxide into formate salts or formic acid.

From the publication Electrochemical CO2 reduction on metal electrodes by Y. Hori, published in: C. Vayenas, et al. (eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, p. 89-189, the Faraday efficiencies on different metal cathodes can be derived, see Table 1.

If carbon dioxide is reduced, for example on silver, gold or zinc cathodes, almost exclusively carbon monoxide is produced.

Table 1:

The table shows the Faraday efficiencies [%] of products that are produced in the reduction of carbon dioxide on different metal electrodes. The indicated values apply for a 0.1 M potassium hydrogen carbonate solution as electrolyte and current densities below 10 mA/cm2

At a silver cathode, for example, predominantly carbon monoxide and only a little hydrogen would be produced. The reactions at the anode and cathode can be represented by the following reaction equations:

Cathode: 2 CO2 4 e- 4 H+ ^ 2 CO 2 H2O

Anode: 2 H2O ^ O2 4 H+ 4 e-

As can also be seen from Table 1, a plurality of hydrocarbons are produced as reaction products, for example, on a copper cathode. Of particular interest from an economic point of view, for example, is the electrochemical generation of methane or ethylene, ethanol or monoethylene glycol. In this respect, they are energetically higher value products than carbon dioxide.

Ethylene: 2CO2 12e- 8H2O ^ C2H4 12OH-Methane: CO2 8e- 4H2O ^ CH4 4OH-Ethanol: 2CO2 12e- 9H2O ^ C2H5OH 12OH-Monoethylene glycol: 2CO2 10e- 8H2O ^ HOC2H4OH 10OH-

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

2 Cl- ^ Cl2 2 e-

In the electrochemical conversion of carbon dioxide matter into an energetically higher value product, increased cost-effectiveness as well as an improvement in the continuous operability of electrolysis systems are of interest.

Consequently, it is stated as technically necessary to propose an improved solution for the use of electrochemical carbon dioxide, which avoids the drawbacks known by the state of the art. In particular, the solution to be proposed must allow a continuous conversion of carbon dioxide. The object of the invention is to indicate 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 claim 1. Advantageous embodiments of the invention are the subject of the claims.

Description of the invention

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

An electrolysis system not according to the invention for the utilization of carbon dioxide comprises at least one electrolyser with an anode in an anode space and a cathode in a cathode space, where the cathode space has at least one inlet for carbon dioxide and wherein the cathode space is configured to bring entering carbon dioxide into contact with the cathode. Furthermore, the cathode space comprises a catholyte or is configured to accommodate a catholyte, wherein the catholyte may enter the cathode space through the same inlet as the carbon dioxide or through a second separate inlet. In addition, at least the anode space, in cell operation the anode and cathode space, has alkali metal cations.

Catholyte refers to an electrolyte which is under direct influence of the cathode during electrolysis. Correspondingly, anolyte is also referred to below when an electrolyte is designated which is directly influenced by the anode during electrolysis. Alkali metal cations are designated positively charged ions which have at least one element from the first main group of the periodic system.

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

Normally, in the electrolysis system not according to the invention, the anode space and the cathode space are separated from each other by a membrane. The membrane is here at least one mechanically separating layer, for example a diaphragm, which separates at least the electrolysis products occurring in the anode space and cathode space from one another. One could then also speak of a separating membrane or separating layer. Since the electrolysis products are, in particular, gaseous substances, a membrane with a high bubble point of 10 mbar or higher is preferably used. The so-called bubble point is, in this connection, a determining quantity for the membrane used, which describes as of which pressure difference AP between the two sides of the membrane a gas flow through the membrane would be used. The membrane can also be a membrane that is either conductive or permeable to protons or cations. While the molecules, liquids or gases are separated, a flow of protons or cations from the anode space to the cathode space is ensured. Preferably, a membrane having sulfonated polytetrafluoroethylene, eg Nafion, is used.

The electrolysis system further comprises at least one deposition tank for the crystallization of an alkaline hydrogen carbonate and/or alkaline carbonate from the catholyte. In particular, this deposition tank has a product outlet. Depending on the product, whether an alkali metal hydrogen carbonate and/or alkali metal carbonate is to be removed from the catholyte, and depending on the alkali metal, a second deposition tank can also be provided for a crystallization process if possible advantageously. This is then arranged, normally in the catholyte flow direction after the first deposition tank.

Depending on the cathode material used, different products are produced during the reduction of carbon dioxide: for example, carbon monoxide, ethylene, methane, ethanol or monoethylene glycol are produced. In all these cases, hydroxide ions are also produced, which are neutralized by excess carbon dioxide to form hydrogen carbonate. The source of the alkali cations is in the anode space. Across the membrane, a flux of cations offsets the electrical current caused by the applied voltage. For example, alkali metal cations and chloride anions are metered into the anolyte in the form of a chloride salt. While the chloride anions are oxidized at the anode to chlorine and leave the anolyte circuit as chlorine gas, the alkali cations migrate across the membrane to the cathode circuit, where they react in the cathode space with the carbonate or hydrogen carbonate thus produced. to give an alkali metal carbonate or alkali hydrogen carbonate and, in particular, leave the catholyte circuit through the separate product outlet of the deposition tank.

The electrolysis system not according to the invention has the advantage of generating, in addition to chlorine, additionally at least one alkali metal carbonate and/or alkali metal hydrogen carbonate as chemically usable material. Whether alkali metal carbonate or alkali hydrogen carbonate is produced depends, for example, on the alkali metal and the utilization method. In aqueous solution, for example, the solubility is decisive. The less soluble carbonate or hydrogen carbonate crystallizes. In the case of sodium and potassium, it is the hydrogen carbonate that is less soluble than the carbonate and must therefore be calcined in a subsequent stage. The combustion of sodium to carbon dioxide is an example, in which carbon monoxide and directly sodium carbonate Na ² CO ³ are generated.

In addition, the described electrolysis system, not according to the invention, is used for the utilization of carbon dioxide and can normally also provide at least one third utilizable material, such as, for example, carbon monoxide, ethylene, methane, ethanol. or monoethylene glycol. Through the very advantageous utilization of the composition stream by means of the cations, an electrolysis system not according to the invention is thereby created, which allows a continuous production of hydrogen carbonate.

As already described, the true cathode reaction, in which carbon dioxide is reduced, is followed by a secondary reaction, namely the neutralization of hydroxide ions (OH-). These are neutralized, in particular, by excess carbon dioxide to give hydrogen carbonate (HCO3"). On the one hand, this has the effect that the pH value in the cathode space is thereby damped over a value range of pH 6 to 8. In addition, this has the action that the electrolyte concentration increases continuously.However, if the catholyte is led into a catholyte circuit, that is, it is pumped into the cathode space and from this is also discharged again, the hydrogen carbonate produced in the cathode space can be removed from the catholyte, for this it is arranged in particular in the catholyte circuit, for example, however, also in the anolyte circuit, in each case , at least one pump providing electrolyte circulation.

Following the neutralization reaction of the hydroxide ions (OH-) with carbon dioxide in excess to give hydrogen carbonate ions (HCO3"), these react preferentially with alkali metal cations subsequently to give alkali metal hydrogen carbonates. The alkali metal cations, which are found in the cathode space, come from the anode space, into which they were initially introduced, in particular as alkali metal chloride, oxidation starting product or in the form of another alkali metal salt, for example to increase the conductivity. In the anode space, the alkali metal cation is preferably followed as alkali metal chloride.The membrane between the anode and cathode space is selected in particular in such a way that the flow of cations from the anode space to the cathode space is ensured. cathode in the electric field of the electrolyser The dependence on temperature and also on the pH value of the solubility of alkali metal hydrogen carbonates leads to aho for different processes to be carried out for the recrystallization or for the extraction of the catholyte:

On the one hand, the temperature dependence of the solubility of the desired alkali metal hydrogen carbonates can be used as electrolysis product. For them, the deposition tank preferably comprises a cooling device, by means of which the catholyte is cooled by several degrees Kelvin in contrast to the temperature range prevailing in the electrolyser. Preferably, the adjusted temperature difference from the deposition tank to the electrolysate is at least 15 K, in particular at least 20 K. Depending on the concentration of electrolyte in the catholyte and depending on which alkaline cations it has been formed with hydrogen carbonate, a temperature difference between 30 K and 50 K may also be particularly suitable. The temperature difference between the electrolyser and the deposition tank can be in a temperature range between 5 K and 70 K.

The lowering of the temperature in the deposition tank has the additional advantage for the entire system that cooling in the catholyte circuit takes place before the catholyte is returned to the cathode space. A too high system-caused heat development, precisely in the electrolyser, is thus avoided. However, a cooling device optionally provided extra for this purpose can be dispensed with.

Also in the deposition method of alkali hydrogen carbonate crystallization by cooling the catholyte, preferably pH-value buffers are used, which are supplied, for example, in a buffer container to the deposition tank and/or to the catholyte circuit. and/or to the cathode space, to correspondingly buffer the catholyte volume.

The pH value of the catholyte can also be consulted as such for the control of the deposition process of the alkaline hydrogen carbonate from the electrolyte. For this purpose, in particular, the pH value in the cathode space is initially kept at a higher value, for example at 8 or higher. This can shift the equilibrium in favor of the alkali carbonate as opposed to the alkali hydrogen carbonate. For crystallization in the deposition tank, the pH value is then lowered, preferably to a value of 6 or less, which leads to the formation and crystallization of alkali metal hydrogen carbonate. The reduction of the pH value is normally carried out by blowing carbon dioxide into the deposition tank.

Depending on which alkali metal cation the hydrogen carbonate reacts with and depending on the pH value in the cathode space, an alkali metal hydrogen carbonate or an alkali metal carbonate may be formed first. In particular, the two procedures described for extracting the desired product from the catholyte can also be combined. In some cases, for example with the formation of sodium hydrogen carbonate NaHCO ³ , for example sodium carbonate Na ² CO ³ can also be obtained subsequently from crystallized sodium hydrogen carbonate NaHCO ³ by heating. In this case hydrogen carbonate is even preferably generated first, deposited and subsequently further processed in the desired amount of hydrogen carbonate to give carbonate.

The pH value dependence of hydrogen carbonate or carbonate ions is shown, for example, in FIG. 6 in a Hagg 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 for the introduction or subsequent delivery of alkali metal chloride to the electrolyte in order to maintain the salt content in the anolyte.

In a preferred embodiment of the electrolyte system not according to the invention, the catholyte has at least one solvent, in particular water. Normally, one works with aqueous electrolytes and correspondingly water-soluble conductive salts. The conductive salt content can be increased by adding other carbonates, hydrogen carbonates but also sulfates or other conductive salts, in order to increase the conductivity of the electrolyte in the catholyte circuit as well as in the anolyte circuit, which leads throughout the system to an increased conversion of substance. Depending on what and in what amounts the additional conductive salts are contained in the catholyte circuit, the crystallization process must be adapted accordingly in order to extract the desired product in the purest possible form. The conductive salts used are therefore normally selected so that their solubility differs significantly from that of alkali metal hydrogen carbonate or alkali metal carbonate.

Normally, the electrolyte system not according to the invention has a gas separation device on the anolyte side, which is equipped to carry out the separation of chlorine gas from the anolyte. A gas separation device can also be provided in the catholyte circuit, for example when this is intended for the generation of carbon monoxide gas by using a silver-containing cathode. In the anolyte circuit as well as in the catholyte circuit, additional devices for inlet and outlet of the system or additional buffer reservoirs can be provided.

The type and quality of the membrane used in the electrolyser ultimately contributes in an essential way to how pure the crystallized product is. If only a separator is used as membrane, for example, chloride anions can also diffuse into the cathode space, even against the electric field in the electrolyser, so that, in addition to hydrogen carbonate, chlorides may also be produced. . In the described electrolysis system not according to the invention, therefore, preferably a cation-conducting membrane is used, through which almost exclusively cations can pass. A purely anion-conducting membrane is correspondingly not advantageous.

The reduction process according to the invention, described for the use of carbon dioxide by means of an electrolysis system, as described above, has been defined in claim 1 and comprises the following steps: a catholyte comprising water and hydroxide and carbon dioxide ions are introduced into a cathode space and there brought into contact with a cathode. Inside the cathode space, this catholyte has alkali metal cations which migrate through the cation-conducting membrane, which separates the anode and cathode space. Excess carbon dioxide reacts in the cathode space with hydroxide ions to give carbonate and/or hydrogen carbonate, which react with alkali cations that have migrated through the membrane to the catholyte to give alkali hydrogen carbonate and/or alkali carbonate. At least part of the catholyte volume is introduced into a deposition tank and an alkali metal hydrogencarbonate and/or an alkali metal carbonate crystallizes there. In addition, an anolyte containing chloride anions is introduced into an anode space and brought into contact with an anode there, the chloride anions are oxidized to chlorine at the anode, and this is separated from the anolyte as chlorine gas via a gas separation device. Normally, this reduction process is carried out in such a way that the anolyte and the catholyte are in each case conducted in a separate circuit from one another, i.e. two pumps are provided in the electrolyte system which cause at least one point in the circuit a transport of catholyte through the cathode space and a transport of anolyte through the anode space. The circuits are separated from one another by the membrane in the electrolyser, which ideally exclusively permits a transport of cations from the anode space to the cathode space. In particular, the necessary alkali metal cations in the cathode space are obtained from the anode space. For this purpose, the anolyte preferably has an alkali metal chloride, which can also be used correspondingly as a conductive salt, however, also as an electrolysis starting product. Alternatively, the alkali metal chloride in the anolyte can be used as electrolysis starting product and an additional conductive salt, eg a sulfate, a phosphate, etc., preferably an alkali metal sulfate. Alternatively, ammonium salts or their homologues can also be used. Imidazolium salts or other ionic liquids can positively influence the selectivity of the electrode, especially the cathode.

Carbon monoxide, ethylene, methane, ethanol and/or monoethylene glycol are normally generated in the reduction process during the reduction of carbon dioxide at the cathode. For this, a corresponding cathode is used as a catalyst for these reactions. The cathode is a silver-containing cathode and forms from carbon dioxide and catholyte predominantly carbon monoxide, hydroxide ions and only a little hydrogen as products. Alternatively, the cathode is a copper-containing cathode and forms from carbon dioxide and the catholyte ethylene, methane, ethanol and/or monoethylene glycol and hydroxide ions as products. The great advantage of this reduction process is that, in addition to the utilization of carbon dioxide, usable chemical materials can also be generated.

In one embodiment of the reduction process, the hydroxide ions produced during the reduction of carbon dioxide are converted with excess carbon dioxide into hydrogen carbonate ions. The generation of hydrogen carbonate directly in the cathode space has the advantage that it can subsequently react directly with alkali metal cations present in the cathode space to give another interesting usable material, such as should be produced, on the contrary, in processes of separate preparation.

In order to remove this usable material from the system, in particular at least part of the catholyte volume is introduced into a deposition tank and cooled there to at least 15 K, preferably at least 20 K. In this case, the dependence on the temperature of carbonate solubility, to extract the usable material from the catholyte circuit. The temperature difference from the deposition tank to the electrolyser can also amount to more than 30 K, in particular also to more than 50 K, depending on the existing alkaline hydrogen carbonate to be extracted and also depending on what other salts are present in it. The circuit. The temperature difference between the electrolyser and the deposition unit can be between 5 K and 70 K.

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

In one embodiment of the reduction process, at least part of the catholyte volume is introduced into a deposition tank and its pH value is lowered there, in particular by blowing in carbon dioxide from more than 8 to a pH value of 6 or less. Precisely, the buffering of the pH value to a value above 8 in the cathode space has the advantage of preventing the precipitation of the alkali hydrogen carbonate in the cathode space itself.

The reduction process can be carried out in such a way that the precipitated alkali hydrogen carbonate is converted into alkali carbonate by heating. This can be done directly following crystallization of the hydrogen carbonate in the deposition tank or it can be done separately from the described electrolysis system.

As an alternative to, or in combination with, the temperature method of crystallization or temperature-supported crystallization, the process can be conducted in such a way that the pH value in the cathode space remains at the upper limit of the reaction at 8 o higher, so that the equilibrium has shifted first in favor of sodium carbonate:

2 NaHCOa ^ Na2CO ³ + H2O CO2.

For this, the supply of carbon dioxide in the system must be controlled very well, to reach this basic regime and maintain it. In the deposition tank, the pH value for optimal sodium hydrogen carbonate deposition would then be lowered by blowing in carbon dioxide, and thus the equilibrium reactions would again shift in favor of sodium hydrogen carbonate.

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

Something different must happen when an additional conductive salt is to be used, eg potassium sulfate (K2SO4). This has a lower solubility of 111.1 g/l at 20 °C and 250 g/l at 100 °C, which means that potassium sulfate would always precipitate first in the mixed electrolyte. To obtain potassium hydrogen carbonate (KHCO3) from an electrolyte containing both potassium sulfate and potassium hydrogen carbonate, proceed as follows: In deposition tank AB preferably potassium sulfate K2SO4 crystallizes, which then, i.e. in the flow direction to the deposition tank AB, it can be fed back into the electrolyte. The electrolyte volume, from which the potassium sulfate K2SO4 has already been removed, is then concentrated, preferably in another deposition tank, i.e. the water is removed from the potassium hydrogen carbonate solution, for example by cooling, to obtain the crystalline material.

In principle, this process can also be used for other cations or cation mixtures. The migration of the cations in this case concentrates the catholyte until the most sparingly soluble salt or double salt is deposited. In this regard, it is important that the concentration and deposition process is not carried out in the cathode space, that is, it is not carried out in the electrolysis cell itself, but rather that the catholyte is transported to a deposition tank integrated in the cell. the electrolysis system. Through another additional physical or chemical difference between the electrolysis cell and the deposition tank, that is, for example, through a gradient of temperature, pH value or pressure, the deposition in the tank is achieved or favored. deposition tank. A suitable pressure difference between the electrolysis cell and the deposition tank can be up to 100 bar. Preferably, a pressure difference between 2 bar and 20 bar would be selected. A high pressure in the deposition tank would favor the formation of hydrogen carbonate.

Alternative anode reactions are basically also conceivable on the anode side, however, it is the coupling with chlorine production that is the most economically significant, since the chlorine market is approx. 75 million tons per year. The current production of sodium hydrogen carbonate (NaHCO ³ ) is approx. 50 million tons per year, which are prepared so far through the energetically unfavorable Solvay process.

With the electrolysis system described and the reduction process, it is possible to generate three usable materials in a continuous and simultaneous electrochemical manner: at the cathode, a usable material such as carbon monoxide, ethylene, methane is obtained from the reduction of carbon dioxide. , ethanol or monoethylene glycol, as a by-product of this reduction which occurs in the cathode space sodium hydrogen carbonate and/or sodium carbonate as a coupling product and chlorine is produced on the anode side.

Examples and embodiments of the present invention are further described by way of example with reference to Figures 1 to 6 of the accompanying drawing:

FIG. 1 shows in schematic representation an electrolysis system not according to the invention with carbon dioxide reservoir and deposition tank,

Figure 2 shows in schematic representation an electrolysis system not according to the invention with 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 coupled PEM half-cell with a back-flow cathode and

Figure 6 shows a Hagg diagram.

Examples of electrolysis systems not according to the invention for the reduction of carbon dioxide are shown in schematic representation in FIGS. 1 and 2, which can also be read as a flow chart for the described reduction process. On the left side, in each case, the anolyte circuit AK is shown, on the right side the catholyte circuit KK. These two circuits AK, KK are connected via the electrolyser E1, E2, whose anode space AR and cathode space KR are connected to one another via a membrane M or are separated from one another via the latter. Preferably, a cation-conducting membrane M is used as the membrane M. In the space Arranged in the anode AR is an anode A, and in the cathode space KR a cathode K, which are electrically connected via a voltage source U. Both circuits AK, KK preferably each have a pump P1, P2, that pump the electrolytes through the electrolyser. Additionally, devices N1, N2, N3 may be present in the two circuits AK, KK at different places in the flow direction, which may be additional inlets or outlets or as reservoirs for buffer. Provided in the anolyte circuit AK is at least one gas separation device G2 with a product outlet PA2, via which the product chlorine gas Cl2 can be withdrawn. In the catholyte circuit KK, at least one gas separation device G1 with product outlet PA1 is also provided, via which, for example, the electrolysis product carbon monoxide CO, for example also hydrogen H2, can be removed. . However, also other electrolysis products, such as ethylene, methane, ethanol, monoethylene glycol can be removed from the system via these or, for example, via another product outlet. The electrolyser E1, E2 has, for example, a gas diffusion electrode GDE for the input of carbon dioxide.

In the case of the electrolyser E1 shown in Fig. 1, a two-chamber structure is selected and the carbon dioxide CO2 is introduced through a reservoir CO2-R and in the flow direction in front of the cathode space KR into the electrolytes. In the two cases shown, the catholyte circuit KK has a deposition tank AB, which can be directly connected to the circuit or through which only part of the catholyte volume is conducted. For this purpose, as shown in FIGS. 1 and 2, a branch of the circuit KK can be provided. The deposition tank AB or several deposition tanks connected one behind the other can be connected, for example, to a cooling device or to a buffer tank PR, so that the crystallization of the hydrogen carbonate is promoted by setting a difference temperature difference, pressure difference or pH value difference with respect to the electrolyser E1, E2. In addition, the AB deposition tank has a PA3 product outlet. Several deposition tanks connected one after the other would present, in each case, a product outlet.

In Fig. 1 and 2 electrolysis systems have therefore been shown, such as can be used for an embodiment of the reduction process according to the invention. In this structure, care is taken that separate anolyte circuit AK and catholyte circuit KK are present. The used electrolytes are then continuously pumped through the electrolysis cell E1, E2, ie through the anode space AR and through the cathode space KR. For this, a pump P1, P2 is provided in the structure in each of the two circuits AK, KK. The frame may feature plastic, plastic-coated metal, or glass materials. Glass flasks can be used as storage vessels, the cell itself is eg PTFE, the hoses are neoprene.

The electrolyser E1, E2, as it is constructed in the electrolysis systems shown, can also have another structure, as it is shown for example in figures 3 to 5. An alternative electrolysis cell is one according to the membrane structure of polymer electrolyte (PEM structure). In this case, at least one electrode is in direct contact with the PEM polymer electrolyte membrane. Correspondingly, the electrolysis cell can be configured as a PEM half-cell, as shown in FIGS. 4 and 5, in which the anode side is configured as a PEM half-cell, i.e. the anode A is arranged in direct contact with the PEM membrane and the anode space AR is disposed on the side of the anode A opposite the membrane.

In the cases as shown in FIGS. 4 and 5, the cathode K is made porous and at least partially gas-permeable and/or electrolyte-permeable. In FIG. 4, the anode PEM half-cell is combined with a gas diffusion electrode GDE for the introduction of carbon dioxide CO 2 into the cathode space KR. Furthermore, a backflow cathode K is shown in FIG. 5, the cathode space KR of which is connected via the cathode K with a gas reservoir. In this case, the gas reservoir has for its part at least one gas inlet GE and, if appropriate, a gas outlet GA. Such an embodiment has hitherto been used, for example, as an oxygen consumption electrode, for example in the preparation of sodium hydroxide solution. Oxygen would then flow behind the cathode K. The oxygen consuming cathode can be used, for example, to prevent the formation of hydrogen H2 in the cathode space KR in favor of a reaction to give water H2O. In this respect, the energy of water formation reduces the necessary system voltage U and consequently causes a lower energy consumption of the electrolysis system. Since the cathode K of an oxygen consuming electrode preferably consists of silver, it can also catalyze the reduction of carbon dioxide. If oxygen is not supplied, the oxygen consumption reaction cannot proceed. Instead, the reduction of carbon dioxide to carbon monoxide CO takes place with some formation of hydrogen.

If, for example, sodium is selected as the alkali metal, the following reactions take place in the cathode space KR with the use of a copper-containing cathode K:

ethylene: 12 NaCl 14 CO2 8 H2O ^ C2H4 12 NaHCOa 6 Cl2

methane: 8 NaCl 9 CO2 4 H2O ^ CH4 8 NaHCOa 4 Cl2

ethanol: 12 NaCl 14 CO2 9 H2O ^ C2H5OH 12 NaHCOa 6 Cl2

monoethylene glycol:

10 NaCl 12 CO2 8 H2O ^ HOC2H4OH 10 NaHCOa 5 Cl2.

With a silver-containing K cathode the following reaction would proceed at the K cathode:

carbon monoxide:

2 NaCl 3 CO2 H2O ^ CO 2 NaHCOa Cl ² .

These equations describe the total process in the electrolysis cell. Chlorine gas CI2 is produced, as described, by oxidation of chloride anions Cl- at anode A, which produce further electrolysis products at cathode K, or by secondary reactions in the cathode space KR.

The sodium example is particularly suitable, since sodium hydrogen carbonate can be separated very well from the electrolyte. Furthermore, in the case of sodium bicarbonate and sodium carbonate, these are often necessary, important, usable chemical materials. The annual world production of soda ash is approximately 50 million tons, as can be extracted, for example, from 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.

The solubility of sodium hydrogen carbonate NaHCO ³ in water H2O is comparatively low and also shows a strong temperature dependence, see table 2.

Table 2

Other salts have been mentioned in table 2, potassium hydrogen carbonate KHCO3, potassium sulfate K2SO4, potassium phosphate K3PO4, potassium iodide KI, potassium bromide KBr, potassium chloride KCl, sodium hydrogen carbonate NaHCO ³ , sodium sulfate Na ² SO ⁴ , which can be used preferentially. However, other sulfates, phosphates, iodides or bromides can also be used to increase the conductivity in the electrolyte. By continuous feeding of carbon dioxide, carbonates or hydrogen carbonates do not have to be fed in, but are formed in operation in the cathode space KR.

The solubility of sodium bicarbonate 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. On the contrary, sodium carbonate NaCO ³ dissolves comparatively well, whose solubility is 217 g/l at 20 °C. In the case of continuous electrolysis, therefore, the sodium bicarbonate NaHCO ³ tends to crystallize in the electrolysis cell E1, E2. This can be counteracted by an increased temperature, as it is produced in any case by the operation of the system and also by a corresponding damping of the pH value. Sodium hydrogen carbonate NaHCO ³ must crystallize only in deposition tank AB from the electrolyte. By pumping the electrolyte through a KK circuit, the sodium bicarbonate NaHCO ³ formed in the cathode space KR is led out of the cathode space KR and the catholyte circuit KK can run through a deposition tank AB or a bypass from a partial volume of the catholyte to a deposition tank AB, in which, for example, by cooling the electrolyte sodium hydrogen carbonate NaHCO ³ crystallizes and can thus be obtained. Since the electrolysis cells E1, E2 heat up strongly during operation due to process losses, effective crystallization can take place with temperature differences of up to 70 K between the cathode space KR and the deposition tank AB. Preferably, work is carried out in a temperature difference range between 30 K and 50 K. In particular, with a temperature difference of at least 15 K or even at least 20 K.

If the catholyte contains further additives for the increase in conductivity and thus the increase in energy efficiency, thus an additional conductive salt minimizes the ohmic losses in the electrolyte, this must be considered in the crystallization of sodium hydrogen carbonate NaHCO ³ , to obtain a product that is as pure as possible. Preferably, a hydrogen sulfate HSO4- or sulfate SO42- is used as 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. With this large difference in solubility compared to Sodium hydrogen carbonate NaHCO ³ is guaranteed to crystallize in the deposition tank, preferentially, sodium hydrogen carbonate NHCO 3 . This variant of the reduction process has the immense advantage that it can essentially replace the Solvay process used in a standardized manner hitherto for the preparation of sodium bicarbonate. The Solvay procedure for preparation of sodium hydrogencarbonate namely has a great disadvantage, namely that it consumes large amounts of water. In addition, approximately one kilogram of calcium chloride CaCh is generated per kilogram of soda, that is, sodium carbonate Na ² CO ³ , which cannot be used, which, in most cases, is discharged into the wastewater and thus in rivers and seas. With an annual production of 50 million tons of sodium carbonate Na ² CO ³ this amounts to approximately 50 million tons of calcium chloride CaCl2.

Available natural sources, other than the Solvay process, for soda Na ² CO ³ are far from sufficient. Sodium hydrogen carbonate NaHCO ³ exists as a natural mineral nahcolite in the United States of America. It appears in most cases finely divided in oil shale and can then be obtained as a by-product of oil extraction. A particularly rich nahcolite horizon mine is exploited in the state of Colorado. The annual extraction was, however, in 2007 only 93,440 tons. In addition, it appears, for example, in calcium carbonate lakes in Egypt, in Turkey in Lake Van, in East Africa, for example, in Lake Natron and other East African rift lakes, 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 south of the Sahara.

An example of a Hagg diagram of a 0.05 molar carbon dioxide CO 2 solution is shown in FIG. 6 to illustrate the dependence of the concentration and pH value parameters. In an average pH value range, carbon dioxide CO2 and its salts are found next to each other. While strongly basic CO2 is present in strongly basic carbon dioxide as carbonate CO32-, in the average pH range, preferably as hydrogen carbonate HCO3-, at lower pH values in the acidic medium, hydrogen carbonate ions are expelled from solution as carbon dioxide CO2.

Claims (4)

1. Reduction procedure for the use of carbon dioxide by means of an electrolysis system, in which a catholyte comprising water and hydroxide ions and carbon dioxide (CO2) is introduced into a cathode space (KR) and is brings into contact with a cathode (K), in which carbon dioxide (CO2) is reduced at the cathode (K), in which an anolyte containing chloride anions (Cl-) is introduced into an anode space (AR) and is brought into contact with an anode (A), in which chloride anions (Cl-) are oxidized at the anode (A) to chlorine (Ch) and this is separated as chlorine gas from the anolyte through a gas separation device, in which the anolyte has alkaline cations, which migrate through a cation-conducting membrane towards the catholytes, where the excess carbon dioxide in the cathode space reacts with the hydroxide ions to form carbonate and/or hydrogen carbonate, which react with the alkali cations that have migrated through the membrane towards the catholyte to give alkali hydrogen carbonate and/or alkali carbonate and in which at least a part of the catholyte volume is introduced into the deposition tank and the alkali hydrogencarbonate and/or alkali carbonate crystallizes there, where - cathode (K) is a silver-containing cathode (K) and forms from carbon dioxide and catholyte predominantly carbon monoxide, hydroxide ions and only a little hydrogen as products; or where - cathode (K) is a cathode (K) which contains copper and forms from carbon dioxide and the catholyte ethylene, methane, ethanol and/or monoethylene glycol and hydroxide ions as products. Reduction method according to Claim 1, in which the catholyte volume introduced therein is cooled to at least 15 Kelvin, preferably at least 20 Kelvin, during the introduction of at least part of the catholyte volume into the deposition tank. Reduction method according to claim 1 or claim 2, wherein during the introduction of the at least part of the volume of catholyte in the deposition tank the pH value of the volume of catholyte introduced is reduced by means of introduction by blowing carbon dioxide (CO2) from more than 8 to a pH value of 6 or less. 4. Reduction process according to one of claims 1 to 3, in which during the introduction of at least a part of the volume of catholyte in the deposition tank an alkaline hydrogen carbonate is crystallized from the volume of catholyte introduced, which is converted to followed by heating in an alkali carbonate.