WO2017005594A1

WO2017005594A1 - Electrolytic system and reduction method for electrochemical carbon dioxide utilization, alkali carbonate preparation and alkali hydrogen carbonate preparation

Info

Publication number: WO2017005594A1
Application number: PCT/EP2016/065277
Authority: WO
Inventors: Günter Schmid; Maximilian Fleischer; Philippe Jeanty; Ralf Krause; Erhard Magori; Anna Maltenberger; Sebastian Neubauer; Christian Reller; Bernhard Schmid; Elena Volkova; Kerstin Wiesner
Original assignee: Siemens Aktiengesellschaft
Priority date: 2015-07-03
Filing date: 2016-06-30
Publication date: 2017-01-12
Also published as: EP3292232B1; DE102015212504A1; PL3292232T3; EP3292232A1; ES2897748T3; DK3292232T3; CN107735512B; US20180195184A1; CN107735512A

Abstract

Described is an electrolytic system comprising an electrolytic cell, a separation of the anolyte side and the catholyte side, and a separation and withdrawal device for an alkali carbonate and/or an alkali hydrogen carbonate. The disclosed reduction method allows for the continuous carbon dioxide utilization and simultaneous preparation of multiple valuable chemical products.

Description

description

Electrolysis system and reduction process for electrochemical carbon dioxide utilization, alkali carbonate and

AIkalihydrogencarbonaterzeugung

The present invention relates to a reduction method and an electrolysis system for electrochemical carbon dioxide utilization. Carbon dioxide is introduced into an electrolyte cell and reduced at a cathode.

State of the art

Currently about 80% of global energy demand is met by the burning of fossil fuels whose combustion processes worldwide emission of about 34,000 tons of carbon dioxide Millio ^¬ nen into the atmosphere per year ver ^¬ ursacht. As a result of this release into the atmosphere, most of the carbon dioxide is disposed of, which, for example, can amount to up to 50,000 tons per day for a lignite-fired power plant. Carbon dioxide is one of the so-called greenhouse gases ^¬ whose negative impact on the atmosphere and climate are discussed. Since carbon dioxide is thermodynamically very low, it can be reduced reindeer difficult to wiederverwertba- products what the actual re ^¬ recycling of carbon dioxide so far rela ^¬ hung, in theory, has left in the academic world.

Natural carbon dioxide degradation occurs, for example, through photosynthesis. Here, in a spatially into many partial steps up glie ^¬ derten time and at the molecular level process carbon dioxide are reacted to form carbohydrates. This process is not so easy on a large scale

adaptable. A copy of the natural photosynthesis process with large-scale photocatalysis is not yet sufficiently efficient. An alternative is the electrochemical reduction of carbon dioxide. Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively recent field of development. Only for a few years has there been an effort to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide. Research in the laboratory scale have shown that are to be used for the electrolysis of carbon dioxide preferably metals as catalysts Kata ^¬. From the publication

Electrochemical CO ₂ reduction on metal electrodes from Y.

Hori, published in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189, Faraday efficiencies can be seen on different metal cathodes, see Table 1.

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

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

Hg 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 produced by carbon dioxide reduction on various metal electrodes. The values given apply to a 0.1 M potassium bicarbonate solution as electrolyte and current densities below 10 mA / cm ² . On a silver cathode, for example, predominantly carbon monoxide and only a little hydrogen would be produced. THE REACTION ^¬ nen at the anode and cathode can be represented by the following reaction sliding ^¬ cations: Cathode: 2 C0 ₂ + 4 e ^"+ 4 H ⁺ - 2 CO + 2 H ₂ 0

Anode: 2 H ₂ 0 -> ■ 0 ₂ + 4 H ⁺ + 4 e ^" As can also be seen from Table 1, a large number of hydrocarbons are formed as reaction products, for example, on a copper cathode. Of particular economic interest is, for example, the electrochemical production of methane or ethylene, ethanol or monoethylene glycol. It is energetically higher quality products than carbon dioxide ^¬.

Ethylene: 2C0 ₂ + 12e ^" + 8H ₂ 0 C ₂ H ₄ + 120H ^" Methane: C0 ₂ + 8e ^" + 4H ₂ 0 - CH ₄ + 40H ^"

Ethanol: 2C0 ₂ + 12e ^" + 9H ₂ 0 C ₂ H ₅ OH + 120H ^"

Monoethylene glycol: 2C0 ₂ + lOe ^" + 8H ₂ 0 HOC ₂ H ₄ OH + 10OH ^"

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

2 Cl ^" -> ■ Cl ₂ + 2 e ^"

In the electrochemical reaction of fuel Kohlenstoffdio- hydroxide in a energetically higher quality product, the raised stabili ^¬ hung the economics of interest as well as a verb ^¬ provement in terms of the continuous operability of the electrolysis systems.

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

These objects underlying the present invention are achieved by an electrolysis system according to the patent claim 1 and by a reduction method according to the patent claim 9. Advantageous embodiments of the invention are Ge ^¬ subject of the dependent claims. Description of the invention

The electrolyzer system of the invention for carbon dioxide recycling comprises at least one electrolyzer with an anode in an anode compartment and a cathode in a cathode compartment, said cathode compartment comprising at least one access for carbon dioxide and wherein the cathode compartment is ^¬ staltet that, OBTAINED carbon dioxide in contact with the cathode bring to. Further, the cathode compartment comprises a catholyte or is configured to receive a catholyte, wherein the catholyte may be communicated with the cathode compartment via the same access as the carbon dioxide or via a separate second access. In addition, at least the anode compartment, in the operation of the cell anode and cathode compartment alkali cations.

A catholyte is an electrolyte which is directly influenced by the cathode during the electrolysis. Accordingly, anolyte is also referred to below when an electrolyte is referred to, which is in direct influence of the anode in an electrolysis. With alkali metal cations, positively charged ions are referred to, the at least one element of the ers ^¬ th main group of the periodic system.

The anode chamber of the electrolyser comprises at least one To ^¬ gear for an anolyte and comprises an anolyte or at least is adapted to receive an anolyte via this access, whereby this stand-anolyte comprises chlorine anion.

Typically, in the electrolysis system, the anode compartment and the cathode compartment are separated by a membrane. The membrane is at least one mechanical separating layer, for example a diaphragm, which separates at least the resulting in Ano ^¬ denraum and cathode compartment electrolysis products apart. One could then also speak of separator membrane or separating layer. Since it is gaseous substances, in particular for the electrolysis ^¬ products, will be ^¬ vorzugt a membrane with a high bubble point of 10 mbar or larger used. The so-called bubble point is a defining variable for the membrane used, which describes ^¬ from which pressure difference ΔΡ between the two sides of the membrane would use a gas flow through the membrane. The membrane may also be a proton- or cation-conducting or permeable membrane. While molecules, liquids or gases are separated, a proton or cation flow is ensured from the anode compartment to the cathode compartment. Preferably, a membrane is used which has sulfonated polytetrafluoroethylene, eg Nafion.

The electrolysis system further comprises at least one separation basin for the crystallization of an alkali metal bicarbonate and / or alkali metal carbonate from the catholyte. In particular ^¬ sondere has this separation basin on a product outlet. Depending on the product, whether an alkali metal bicarbonate and / or alkali metal carbonate is to be removed from the catholyte, and depending on the alkali metal, a second separating basin can also be provided for a most advantageous crystallization process. This is then typically in

Katholytkreislaufrichtung arranged after the first Abscheidebecken. In ^¬ play as produced carbon monoxide, ethylene, methane, ethanol or monoethylene glycol: Depending on the cathode material, different products are formed in the reduction of carbon dioxide. In all these cases, hydroxide ions are also formed, which are neutralized by excess carbon dioxide to bicarbonate. The source of alkaline cations lies in the anode compartment. Through the membrane, a cation current compensates for the electrical current caused by the applied voltage. For example, the alkali cations and the chloride anions are metered in the form of a chloride salt in the anolyte. While the

Chloride anions are oxidized at the anode to chlorine and chlorine gas exit as the anolyte, the Alkalika ^¬ functions migrate through the membrane into the catholyte where they in The cathode compartment react with the carbonate or hydrogen carbonate formed there to form an alkali metal carbonate or alkali metal bicarbonate and, in particular, leave the catholyte circuit via the separate product outlet of the separating basin.

The electrolysis system according to the invention has the advantage, in addition to chlorine, at least one alkali carbonate and / or alkali bicarbonate to produce as a chemical valuable material. 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, solubility is crucial. The schwerlöslichere carbonate or bicarbonate kristal ^¬ lisiert out. When sodium and potassium, it is the hydrogen carbonate ^¬ nat, the poorly soluble than the carbonate and then NEM following step must be calcined in egg. The combustion of sodium in carbon dioxide is an example in which Koh ^¬ lenstoffmonoxid directly and sodium carbonate Na ₂ Cue ₃ is generated. In addition, the electrolysis system described serves the

Kohlenstoffdioxidverwertung and it can also so typically even at least one third recyclables such example ^¬, carbon monoxide, ethylene, methane, ethanol or

Monoethylene glycol are provided. The very advantageous utilization of the compensating current by the cations thus creates an electrolysis system which enables continuous hydrogen carbonate production.

As already described, is followed by the actual Kathodenreak- tion in which the carbon dioxide is reduced, a Fol gereaktion ^¬, namely the neutralization of the hydroxide ions (OH-). This can be neutralized, in particular by excess carbon dioxide to ^¬ bicarbonate (HC0 ₃ ^~). On the one hand, this has the effect of buffering the pH in the cathode compartment in a pH range of 6 to 8.

In addition, it has the effect of increasing the electrolyte concentration continuously. However, if the catholyte is guided into a catholyte cycle, ie into the cathode space pumped and derived again, the catholyte can be removed from the resulting in the cathode compartment hydrogen carbonate. For this purpose, in each case at least one pump is arranged in particular in the catholyte circuit, for example, but also in the anolyte circuit, which ensures an electrolyte circulation.

Subsequent to the neutralization reaction of the hydroxide ions ^(OH-) with excess carbon dioxide to

Hydrogencarbonationen (HC0 ₃ ^~ ) react with these preferred

Alkali cations continue to alkali bicarbonates. The alkali metal cations present in the cathode space, are taken from the anode compartment, in which they were initially introduced, in particular as Alkalichlo ^¬ chloride as Oxidationsedukt or in the form of another Alkalisal- zes, for example, to increase the conductivity. The alkalization is preferably carried out in the anode compartment as alkali metal chloride. The membrane between the anode and cathode space is chosen in particular so that the Kat ^¬ ion current is ensured from the anode space to the cathode in the electric field of the electrolyzer. The temperature and pH dependence of the solubility of alkali hydro ^¬ gencarbonaten now leads that different processes are performed to crystallize or for removal from the catholyte:

On the one hand, the temperature dependence of the solubility of the desired as electrolysis product alkali metal bicarbonates can be used. For this purpose, the separating basin preferably comprises a cooling device, by means of which the catholyte is cooled by several degrees Kelvin, in contrast to the temperature range which prevails in the electrolyzer. Preferably, the ^adjusted temperature difference is from the separating basin to

Electrolyzer at least 15 K, in particular at least 20 K. Depending on the electrolyte concentration in the catholyte and depending on the, with which Alkalikationen the hydrogen carbonate is formed, and a temperature difference between 30 K and 50 K may be particularly suitable. The temperature difference between The electrolyzer and separating basin can be in a temperature range between 5 K and 70 K.

The lowering of the temperature in the separating basin has the additional advantage for the entire system that a cooling takes place in the catholyte circulation before the return of the catholyte into the cathode space. Thus, an excessive systembe ^¬ related heat development, avoided just in the electrolyzer. But an optionally provided for this purpose cooling device can be saved.

Also in the deposition method of crystallization of the alkali metal bicarbonate by cooling the catholyte pH buffer are preferably used, for example, in a buffer reservoir to the separating basin and / or the

Katholytkreislauf and / or the cathode compartment are made available to the catholyte volume according to puf ^¬ fern. The pH of the catholyte can also be considered as such for the

Control of the deposition of the alkali metal bicarbonate can be used from the electrolyte. In particular, the pH in the cathode compartment is first increased to a higher value, e.g. kept 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

Separation tank is then lowered the pH, preferably to a value of 6 or less, which leads to the formation and crystallization of the alkali metal bicarbonate. The pH reduction is typically done by blowing carbon dioxide into the capture tank.

Depending on with which alkali metal cation reacts hydrogen, and depending on the pH in cathode chamber, initially an alkali metal bicarbonate or an alkali carbonate may be ge ^¬ forms. In particular, the two described procedures for withdrawing the desired product from the catholyte can also be combined. In some cases, For example, in the formation of sodium bicarbonate NaHCC> 3, for example, the sodium carbonate Na ₂ CC> 3 can be obtained in retrospect from the crystallized sodium bicarbonate NaHCC> 3 by heating. Then, even preferably produced initially bicarbonate, separated, and it weiterver ^¬ ^¬ After working in the transition of the desired proportion to carbonate.

The pH-dependence of the bicarbonate or carbonate ions is e.g. shown in Figure 6 in a Hägg diagram for a sodium carbonate solution.

In the electrolysis system, a buffer reservoir is preferably also provided in the anolyte circulation, which buffer can also be used, in particular, for introducing or delivering alkali metal chloride into the electrolyte in order to maintain the salt content in the anolyte.

In a preferred embodiment of the electrolytic system, the catholyte at least one solvent on, insbesonde re ^¬ water. Typically, aqueous electrolytes and, accordingly, water-soluble conductive salts are used. The electrolyte content can be increased by the addition of other carbonates, bicarbonates, but also sulfates or other conductive salts, to increase the conductivity of the electrolyte in the

To increase catholyte as well as in the anolyte, which leads to an increase in the StoffUmsatzes in the overall system. Depending on which and in what quantities additional conductive salts are contained in the catholyte cycle, the Auskristallisa- tion process must be adjusted accordingly in order to extract the desired product in the purest possible form. Conducting salts used are therefore typically selected so that their solubility differs significantly from that of the alkali metal bicarbonate or of the alkali metal carbonate.

Typically, the electrolytic system on the anolyte side has a gas separation device which is designed to carry out the chlorine gas separation from the anolyte. A gas separation device can also be provided in the catholyte circuit, for example if it is aligned with the carbon monoxide gas generation by using a cathode containing silver. In the anolyte and in the catholyte circuit, additional devices for inlets or outlets from the system or additional buffer reservoirs can be provided.

The type and quality of the membrane used in the electrolyte ultimately makes a significant contribution to how pure the crystallized product is. Is used as a single membrane ^¬ Lich a separator, for example, chloride anions also diffuse into the cathode space, even against the electric field in the electrolyzer, so that in addition to hydrogen, where appropriate, chlorides arise. DA forth is described in the electrolysis system is preferably used a cation-conductive membrane which may eventually pass out almost ^¬ cations. A pure

anion-membrane is therefore not of pre ^¬ part.

The described reduction process for carbon dioxide utilization by means of an electrolysis system, as described above, comprises the following steps: A catholyte and carbon dioxide is introduced into a cathode space and brought into contact therewith with a cathode. Inside the cathode compartment, this catholyte has alkali cations which migrate through the membrane which separates the anode and cathode compartments. At least part of the Katholytvolumens is introduced into a separation basin, where bicarbonate is an alkali and / or alkaline carbonate auskristalli ^¬ Siert. In addition, an anolyte, which has chloride anions, introduced into an anode compartment and there brought into contact with an anode, at the anode, the

Chloride anions oxidized to chlorine and this separated as chlorine gas via a gas separation device from the anolyte. Typically, this reduction method is performed such that anolyte and catholyte is performed in each case from each other in a ge ^¬ separated circuit, ie two pumps in Provided electrolysis, which cause at least at one point in the circulation transport of the catholyte through the Katho ^¬ denraum and transport of the anolyte through the anode compartment. The circuits are separated from each other by the membrane in the electrolyte, which ideally allows only cation transport from the anode compartment into the cathode compartment. In particular, the in the cathode chamber Need Beer ^¬ saturated alkali cations are obtained from the anode chamber. For this purpose, the anolyte preferably has an alkali metal chloride, this can accordingly likewise be used as electrolyte salt but also as electrolysis product. Alternatively, the alkali metal chloride in the anolyte as Elektrolyseedukt and an additional conductive salt, for example, a sulfate, a phosphate et cetera, 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 the selectivity of the electrode, particularly the cathode, posi tive ^¬ influence. Typically generated in the reduction process in the re ^¬ production of carbon dioxide at the cathode, carbon monoxide, ethylene, methane, ethanol and / or monoethylene glycol. For this purpose, a corresponding cathode is used as a catalyst of these reactions. The cathode preferably has copper for this purpose. The great advantage of this Reduktionsverfah ^¬ proceedings that besides the chemical Kohlenstoffdioxidverwertung recyclable materials can be produced in addition.

In one embodiment of the reduction process, the hydroxide ions formed in the reduction of carbon dioxide are converted to hydrogen carbonate ions with excess carbon dioxide. The Hydrogencarbonaterzeugung directly in the cathode compartment has the advantage that they can react directly with existing in the cathode compartment alkali cations to another interesting valuable material, as he would otherwise have to be produced in separate manufacturing processes. To remove this value material to the system, a portion of the Katholytvolumens is in particular at least introduced into a deposition tank and ^¬, preferably where it is cooled by at least 15 K is at least 20 K. Here the Temperature- is exploited dependence of Carbonatlöslichkeit to extract the value ^¬ material from the catholyte. The temperature ^¬ difference of separation tank to electrolyzer may also be more than 30 K, in particular more than 50 K, depending on the present to be extracted alkali metal bicarbonate and also depending on which other salts are present in the cycle. The temperature difference between electrolyzer and 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 of the pH value is exploited. This method can be combined with the temperature-dependent method. In one embodiment of the reduction process to at least part of the Katholytvolumens is introduced into a deposition tank and there ^¬ whose pH is less lowered in particular with ^¬ means of injecting carbon dioxide of more than 8 to a pH of 6 or more. Just buffering the pH to a value above 8 in the cathode compartment has the advantage of preventing precipitation of the alkali metal hydrogencarbonate in the cathode ^compartment itself.

The reduction process can be carried out so that the precipitated alkali metal bicarbonate is converted by heating to alkali carbonate. This can be done directly after the crystallization of the bicarbonate in the

Separation tanks are made or carried separately from the described electrolysis system.

As an alternative to the temperature method of crystallization or for temperature-assisted crystallization or in combination with this, the process can be run in such a way that the pH in the cathode chamber in the upper limit of the reaction to 8 or higher is maintained, so that the equilibrium is shifted to favor nearest ^¬ of sodium carbonate: 2 NaHC0 ₃ - Na ₂ C0 ₃ + H ₂ 0 + C0. ₂

To do this, the carbon dioxide feed into the system must be very well controlled to get into and maintain this basic regime. In the separating basin, the pH would then be lowered for optimal separation of the sodium bicarbonate by blowing carbon dioxide, and thus shifting the equilibrium reaction again in favor of sodium hydrogen carbonate. The process is not be limited to ^¬ sodium bicarbonate. For example, potassium bicarbonate can also be prepared in this process. Analogous to the described deposition process for sodium bicarbonate and the potassium bicarbonate from a pure Kaliumhydrogen- carbonate electrolyte by lowering the temperature in

Separation tanks are crystallized. At 20 ° C, the solubility of potassium bicarbonate is 337 g / 1, at 60 ° C 600 g / 1. Something different must be done if an additional conductive salt, eg potassium sulfate (K ₂ S0 ₄ ) is to be used. This has a lower solubility of 111.1 g / 1 at 20 ° C and 250 g / 1 at 100 ° C, which means that in the mixed electrolyte always first the potassium sulfate would precipitate. In order to recover the potassium hydrogen carbonate (KHCO ₃₎ from an electrolyte, which holds both potassium sulfate and potassium ent ^¬, must be carried out as follows: In the separating tank AB preferably crystallized potassium sulfate K ₂ S0 ₄ from which the terminal, that is in circulation direction after Separation tank AB, the electrolyte can be recycled. The volume of electrolyte, the potassium sulfate K ₂ S0 ₄ has been removed, is then, preferably in egg ^¬ nem further separating tank, concentrated, ie the Kaliumhydrogencarbonatlösung is removed by cooling, for example, the water to obtain the crystalline material.

In principle, this method is also applicable to other cations or mixtures of cations. The migration of the cations, the catholyte concentrates on so far that the hardest most soluble salt or double ^¬ salt precipitates. It is important that the process of concentration and deposition does not take place in the cathode compartment, that is, not in the electrolysis cell itself, but instead the catholyte is integrated into an electrolysis system

Separation tank is transported. For another additional physical or chemical difference between the electrolysis cell and the separation tank, e.g. Via a temperature, pH or pressure gradient, the separation in the separating basin is achieved or promoted. A suitable pressure difference between electrolysis cell and

Separation tank can be up to 100 bar. Preferably, a pressure difference between 2 bar and 20 bar would be selected. An increased pressure in the separating basin would be the

Promote bicarbonate formation.

On the anode side, alternative Anodenre ^¬ actions are conceivable in principle, but the coupling with the chlorine production is the most economically sound because the chlorine market is about 75 million tons per year. Today's Pro ^¬ production of sodium bicarbonate (NaHCO) is about 50 million tons a year, which have hitherto been made over the energetically unfavorable Solvay process.

With the described electrolytic system and reduction method, it is possible to produce electrochemically continuously and simultaneously three valuable materials: At the cathode, a recyclable material is obtained from the carbon dioxide-reduction monoxide as carbon, ethylene, methane, ethanol or monoethylene glycol ge ^¬ gained as a result product of this reduction Ka arising in the space ^¬ Thode sodium hydrogen carbonate and / or sodium as a coproduct and on the anode side, chlorine is produced. Examples and embodiments of the present invention will be further described by way of example with reference to Figures 1 to 6 of the accompanying drawings:

FIG. 1 shows a schematic illustration of an electrolysis system with carbon dioxide reservoir and separating basin, FIG. 2 shows a schematic illustration of an electrolysis system with gas diffusion electrode,

FIG. 3 shows a schematic representation of a PEM structure of an electrolysis cell,

FIG. 4 shows a schematic representation of a PEM

Half-cell coupled to a gas diffusion electric ^¬ de, Figure 5 shows a schematic illustration of a PEM

Half cell coupled with a back-flowed Katho ^¬ de and

FIG. 6 shows a Hägg diagram.

In Figures 1 and 2 Examples of electrolysis systems are shown Carbon Reduction in schematic representation, which can be read as flowcharts for ^¬ be signed reduction process alike. The left side shows the anolyte circuit AK, while the right side shows the catholyte circuit KK. These two circuits AK, KK are connected via the electrolyzer El, E2, whose anode compartment AR and cathode compartment KR are connected to one another via a membrane M or are separated from one another via these. The membrane M a kationenlei ^¬ tend membrane M is preferably employed. In the anode space AR, an anode A, in the cathode space KR, a cathode K is arranged, which are electrically connected via a voltage source U. Both Circuits AK, KK preferably each have a pump PI, P2, which pump the electrolytes through the electrolyzer. In addition, devices N1, N2, N3 may be present in both circuits AK, KK at different points in the flow direction, which may be additional inflows or outflows or as buffer reservoirs. In the anolyte circuit AK a gas separating G2 is ^¬ least provided with a product outlet PA2 over which the product chlorine gas Cl ₂ can be removed. In the catholyte KK a gas separating Gl is provided with product outlet PA1 least the same inlet, of which for example the electrolysis ^¬ product carbon monoxide CO, for example, water ^¬ H ₂ material can be removed. But also other electrolysis products, such as ethylene, methane, ethanol, monoethylene glycol can over this or example meadow over another

Product outlet are taken from the system. The electrolyzer El, E2 has, for example, a gas diffusion electrode GDE for the carbon dioxide inlet. In the case of the electrolyzer El shown in Figure 1, a two-chamber construction is selected and the carbon dioxide C0 ₂ is introduced into the electrolyte via a reservoir C0 ₂ -R and in the direction of circulation in front of the cathode space KR. In both cases shown, the catholyte circulation KK has a separation basin AB, which can be integrated directly into the circulation or through which only a part of the catholyte volume is guided. For this purpose, a branch of the circuit can as KK shows ^¬ ge in Figures 1 and 2, may be provided. The separating tank AB or more series-connected separating tank, may be connected for example with a cooling device or with a buffer reservoir PR, so that the crystallization of bicarbonate through a ^¬ provide a temperature difference, pressure difference, or pH difference to the electrolyzer El, E2 is favored. Furthermore, the separating basin AB has a product outlet PA3. A plurality of separation basins connected in series would each have a product outlet. In the figure 1 and 2 so electrolysis systems are shown, as they can be used for an embodiment of the invention. Care is taken in this setup that separate anolyte AK and KKK catholyte circuits are present. The electrolytes used are then pumped continuously through the electrolytic cell El, E2, ie through the anode space AR and through the cathode space KR. For this purpose, in the structure in each of the two circuits AK, KK in each case a pump PI, P2 pre ^¬ see. The structure can comprise materials made of plastic, plastic-coated metal or glass. As Vorratsge ^¬ vessels glass flask can be used, the cell itself is, for example, from PTFE, the tubes made of neoprene.

The electrolyzer El, E2, as it is installed in the electrolysis systems shown, may also have a different structure, as shown for example in Figures 3 to 5. An alternative electrolysis cell is the after

Polymer electrolyte membrane construction (PEM structure). In this case, at least one electrode is located directly on the polymer electrolyte membrane ^¬ PEM. Accordingly, the electrolysis ^¬ cell can be configured as a PEM half-cell, as shown in Figures 4 and 5, in which the anode side is designed as a PEM half-cell, ie the anode A is arranged in direct contact with the membrane PEM and the anode space AR is arranged on the side facing away from the membrane of the anode A.

In the cases shown in FIGS. 4 and 5, the cathode K is porous and at least partially gas-permeable and / or electrolyte-permeable. In FIG. 4, the anode PEM half cell with a gas diffusion electrode GDE is shown in FIG

Introducing the carbon dioxide CO ₂ into the cathode space KR combined. In addition, in Figure 5, a back-flow cathode K is shown, the cathode space KR is connected via the cathode K with a gas reservoir. The gas reservoir in turn has in this case at least one gas inlet GE and, where appropriate, ^¬ outlet GA. One such embodiment is previously incorporated sets ^¬ for example, as an oxygen-consuming electrode, for example in the production of caustic soda. Then would the cathode K flows behind with oxygen. The oxygen-consuming cathode can be used, for example, to avoid the formation of hydrogen H ₂ in the cathode space KR in favor of a reaction towards water H ₂ 0. The water-forming energy reduces the necessary system voltage U and thus causes a lower energy consumption of the electrolysis system. Since the cathode K of an oxygen-consuming electrode composed of silver before ^¬ namely, it may also catalyze the oxide reduction Kohlenstoffdi-. If no oxygen to Verfü- asked supply, the oxygen-consuming reaction can not take place ^¬. Instead, the carbon dioxide reduction to carbon monoxide CO takes place with some hydrogen formation. If, for example, sodium is chosen as the alkali metal, the following reactions take place in the cathode space KR when using a copper-containing cathode K:

Ethylene: 12 NaCl + 14 C0 ₂ + 8 H ₂ O -> ■ C ₂ H ₄ + 12 NaHCO ₃ + 6 Cl ₂

Methane: 8 NaCl + 9 C0 ₂ + 4 H ₂ 0 -> ■ CH ₄ + 8 NaHC0 ₃ + 4 Cl ₂

Ethanol: 12 NaCl + 14 C0 ₂ + 9 H ₂ O -> C ₂ H ₅ OH + 12 NaHCO ₃ + 6 Cl ₂ monoethylene glycol

10 NaCl + 12 C0 ₂ + 8 H ₂ O -> ■ HOC ₂ H ₄ OH + 10 NaHCO ₃ + 5 Cl ₂ .

For a silver-containing cathode K, the following reaction would take place at the cathode K:

Carbon monoxide:

2 NaCl + 3 C0 ₂ + H ₂ 0 -> ■ CO 2 + NaHC0 ₃ + Cl _2.

These equations describe the summation process in the electrolysis cell. The chlorine gas Cl ₂ is formed, as described, by oxidation of the chloride anions Cl ^~ at the anode A, the other electrolysis products formed at the cathode K or by subsequent reactions in the cathode space KR. The example of sodium is particularly suitable since the sodium bicarbonate can be very easily separated from the electrolyte. In addition, sodium bicarbonate and sodium carbonate are important, frequently needed chemical recyclables. Worldwide annual sodium carbonate production is around 50 million tonnes, 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, it can be seen.

The solubility of sodium bicarbonate NaHC0 ₃ in water H ₂ 0 is comparatively low and also shows a strong temperature dependence, s. Table 2.

MolekuKHCO3 K2SO4 K3PO4 KI KBr KCl NaHC0 ₃ Na ₂ S0 ₄ larformel

Molarmas100.1 174.3 212.3 166.0 119.0 74.6 84.01 142.04 se in

g / mol

Solubility in H ₂ O at 20 ° C:

In g / 1 337 111 900 1400 678 344 96 170

In mol / 1 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

Table 2 lists further salts, potassium hydrogen carbonate KHCO ₃ , potassium sulfate K ₂ S0 ₄ , potassium phosphate K ₃ PO ₄ , potassium iodide KI, potassium bromide KBr, potassium chloride KCl, sodium hydrogencarbonate NaHCC> 3, sodium sulfate Na ₂ SO ₄ , which are preferably used can be. But other sulfates, Phospha ^¬ te, iodides or bromides can be used to increase the conductivity in the electrolyte. By continuously supplying the carbon dioxide or carbonates Hydrogencarbona- must te not be supplied, but are formed in the operation in the Ka ^¬ Thode space KR. The solubility of sodium bicarbonate NaHCC> 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. In contrast, sodium carbonate NaCC> 3 dissolves comparatively well, with a solubility of 217 g / l at 20 ° C. In continuous electrolysis, therefore, the sodium bicarbonate NaHCC> 3 tends to crystallize in the

Electrolysis cell El, E2. This can have an increased tempera ^¬ ture as already produced by the operation of the system and also be counteracted by an appropriate pH Value buffering. The sodium bicarbonate NaHCC> 3 should first crystallize out of the electrolyte in separating tank AB. By pumping the electrolyte into a circuit KK, the sodium bicarbonate NaHCC> 3 formed in the cathode space KR is led out of it and the

KK may catholyte by a separating tank AB reciprocated by extending or there is a branch of a Teilvolu ^¬ mens of the catholyte in a separating tank AB, in which, for example by cooling the electrolyte the Natriumhydrogencar- carbonate NaHCO can be crystallized> 3 and thus recovered. Since the electrolysis cells E1, E2 in operation by process losses anyway warm up, it can come to the effective crystallization to temperature differences of up to 70 K between the cathode space KR and Ab Abscheidebecken. Preference is given to working in a range between 30 K and 50 K Tempe ^¬ temperature difference. In particular with a temperature difference of at least 15 K or even at least 20 K.

Does the catholyte further additions to the conductivity ^¬ increase, thus increasing energy efficiency, thus minimizing additional electrolyte salt in the ohmic losses

Electrolytes, this must be considered> 3 in the crystallization of the sodium umhydrogencarbonats NaHCO to obtain a mög ^¬ lichst pure product. Preferably, a hydro ^¬ gensulfat HS0 ₄ ^~ or sulfate S0 ₄ ^{2 ~ used} as a conductive additive. This may, for example, be sodium sulfate Na ₂ S0 ₄ or sodium hydrogensulfate NaHS0 ₄ . The solubility of sodium hydrosulfide ^¬ hydrogen sulfate NaHS0 ₄ is 1080 g / 1 at 20 ° C and that of Natri ^¬ sulfate Na ₂ S0 ₄ is 170 g / 1 at 20 ° C, s. Table 2. With this large difference in solubility to sodium hydrogen carbonate NaHCC> 3, it is ensured that sodium bicarbonate preferentially crystallizes out NHCO ₃ in the separation tank. The ^¬ se variant of the reduction process has the immense pre ^¬ in part, that this is basically the for

Sodium hydrogen carbonate production can replace previously used standard Solvay process. The Solvay process for producing sodium bicarbonate has one major drawback, namely that it consumes very large amounts of water. In addition, per kilogram of soda, ie sodium carbonate Na ₂ CC> 3, about one kilogram of unusable Calcium chloride CaCl ₂ produced, which is usually discharged into the sewage and thus into rivers and seas. In a Jah ^¬ resherstellung of 50 million tons of sodium carbonate Na2C03 this is so in about 50 million tons of calcium chloride CaC12.

The natural sources of soda Na ₂ CC> 3 available besides the Solvay process are far from sufficient. Sodium hydrogen carbonate aHC0 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 extraction. A mining of particularly rich Nahcolith horizons is operated in the state of Colorado. However, the annual production in 2007 was just 93,440 tonnes. It also comes in, for example

Soda lakes in Egypt, in Turkey in Lake Van, in East Africa, eg. In Lake Natron and other lakes in the East African Trench, in Mexico, California (USA), and as Trona

(Na (HC0 ₃₎ · Na ₂ C0 ₃ · 2H ₂ 0) in Wyoming (USA), Mexico, East Africa and in the southern Saharan desert.

FIG. 6 shows an example of a Hägg diagram of a 0.05 molar solution of carbon dioxide CO ₂ in order to illustrate the dependence on the concentration and pH parameters. Be in a medium pH range

Carbon dioxide CO ₂ and its salts next to each other.

While in the preferred strongly basic carbon dioxide CO2 is preferably present as carbonate CO32-, middle pH value range as bicarbonate HC03 ^~, occurs at low pH values in the acidic medium for the expulsion of

Hydrogen carbonate ions from the solution in the form of carbon dioxide CO2.

Claims

claims

1. An electrolysis system for carbon dioxide utilization, comprising an electrolyzer (E1-E5) with an anode (A) in an anode compartment (AR), a cathode (K) in a cathode compartment (KR), wherein the cathode compartment (KR) at least one access for carbon dioxide (C0 ₂ ) and configured to bring the incoming carbon dioxide (C0 ₂ ) in contact with the Katho ^¬ de (K), wherein the cathode space (KR) a

Catholyte which may be accessed through the same access or a separate cathode compartment (KR) and which has alkali cations, and wherein the anode compartment (AR) has at least one access for an anolyte and includes or is able to access an anolyte wherein the anolyte comprises chlorine anions.

2. electrolysis system according to claim 1 with a

Abscheidebecken (AB), wherein the Abscheidebecken (AB) for the crystallization of an alkali metal bicarbonate and / or Alkali carbonate from the catholyte is configured and has a product outlet (PA3).

3. electrolysis system according to claim 1 or 2, wherein the Abscheidebecken (AB) comprises a cooling device.

4. electrolysis system according to one of the preceding claims with at least one reservoir (PR), which is configured and in such a way with the cathode space (KR) and / or the

Abscheidebecken (AB) is arranged so that it serves to buffer the catholyte.

5. electrolysis according to one of the preceding claims, wherein the catholyte comprises at least one solvent, in particular ^¬ special water.

6. electrolysis system according to any one of the preceding claims, wherein the anolyte has at least one water-soluble alkali metal salt.

7. electrolysis system according to any one of the preceding claims, wherein the anode space (AR) is connected to a gas separation means for chlorine gas separation from the anolyte.

8. electrolysis system according to any one of the preceding claims, wherein the anode space (AR) and cathode space (KR) by a cation-conducting membrane (M) are separated from each other.

9. Reduction method for carbon dioxide utilization by means of an electrolysis system according to one of the preceding claims,

in which a catholyte and carbon dioxide (C02) into a Ka ^¬ Thode space (KR) were introduced and in contact with a cathode (K) accommodated

in which carbon dioxide (CO 2) is reduced at the cathode (K),

wherein an anolyte having chloride anions (C1-) is introduced into an anode space (AR) and brought into contact with an anode (A),

in which at the anode (A) chloride anions (C1-) to chlorine (C12) are oxidized and this is separated as chlorine gas via a Gasabtren ^¬ tion device from the anolyte,

in which the anolyte has alkaline cations contained in the

Migrate catholyte and

wherein at least a portion of the catholyte volume in a

Separation basin is initiated and there crystallized an alkali hydrogen carbonate and / or alkali carbonate.

10. The reduction process of claim 9, wherein at the cathode (K) carbon dioxide (C0 ₂ ) to carbon monoxide (CO), ethylene (C ₂ H ₄ ), methane (CH ₄ ), ethanol (C ₂ H ₅ OH) and / or

Monoethylene glycol (OHC ₂ H ₄ OH) is reduced.

11. reduction method according to claim 10, wherein the hydroxide ions formed during the Carbon Reduction (OH ^~) with excess existing carbon dioxide (C0 ₂₎ to bicarbonate ions (HC0 ^~ ₃₎ converted.

12. A reduction method according to any one of claims 9 to wherein at least a portion of the catholyte volume in a

Abscheidebecken is initiated and there at least

Kelvin, preferably at least 20 Kelvin is cooled.

13. reduction process according to any one of claims 9 to 12, wherein at least a portion of the catholyte volume in a

Separation Basin is introduced and where its pH is lowered by means of blowing carbon dioxide (C02) of over 8 to a pH of 6 or less.

14. A reduction method according to any one of claims 9 to 13, wherein at least a portion of the catholyte volume in a

Separation basin is initiated and there an alkali hydrogen carbonate is crystallized, which is subsequently converted by heating in an alkali carbonate.