EP3445893A1

EP3445893A1 - Arrangement for the electrolysis of carbon dioxide

Info

Publication number: EP3445893A1
Application number: EP17724054.6A
Authority: EP
Inventors: Philippe Jeanty; Erhard Magori; Christian Scherer; Angelika Tawil; Kerstin WIESNER-FLEISCHNER; Oliver von Sicard
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2016-06-30
Filing date: 2017-05-18
Publication date: 2019-02-27
Anticipated expiration: 2037-05-18
Also published as: ES2795698T3; DK3445893T3; CN109415821A; US20190233957A1; AU2017288319B2; PL3445893T3; BR112018075707A2; DE102016211824A1; AU2017288319A1; EP3445893B1; WO2018001636A1; CL2018003721A1

Abstract

The invention relates to an arrangement for the electrolysis of carbon dioxide, comprising an electrolytic cell with an anode and a cathode, the anode and cathode being connected to a voltage supply, the cathode being a gas diffusion electrode and a gas chamber being connected to a first side of the cathode and a cathode chamber being connected to a second side of the cathode, also comprising an electrolytic circuit which is connected to the electrolytic cell, and a gas supply for supplying carbon dioxide-containing gas into the gas chamber, characterised in that the gas chamber comprises an outlet for the electrolyte, carbon dioxide and product gases of the electrolysis, the outlet being connected to the electrolytic circuit by a throttle which is designed to produce a definable pressure difference between the gas chamber and cathode chamber when a mixture of product gases and liquid electrolytes flow through.

Description

description

The invention relates to an arrangement for the carbon dioxide electrolysis according to the preamble of claim 1.

The burning of fossil fuels currently covers about 80% of global energy needs. Through these combustion processes approximately 34 billion tons of carbon dioxide (C02) were emit ^¬ advantage into the atmosphere in 2011 worldwide. This release is the easiest way to dispose of even large amounts of C02 (large lignite-fired power plants over 50,000 tons per day).

The discussion about the negative impact of the greenhouse gas C02 on the climate has meant that we think about a How To ^¬ derverwertung C02. C02 is a strongly bound molecule and therefore it is difficult to reduce it back to useful products.

In nature, the C02 process of photosynthesis to carbohydrate ^¬ th. This complex process is very difficult to reproduce on an industrial scale. A currently technically feasible way is the electrochemical reduction of C02 dar. The carbon dioxide is converted with the supply of electrical energy in a higher energy product such as CO, CH4, C2H4 or C 1 -C 4 alcohols. The electrical ^¬ cal energy in turn is preferably derived from renewable energy sources such as wind power or photovoltaics.

For the electrolysis of C02 usually metals are used as catalysts Kata ^¬. The type of metal influences the products of electrolysis. Thus, C02 is, for example, of Ag, Au, Zn, and with limitations of Pd, Ga, almost from ^¬ finally reduced to CO, while copper is a variety of hydrocarbons as reducing products to be ^¬ obachten. In addition to pure metals, metal alloys are also gen as well as mixtures of metal and metal oxide which is catalytically effective co-, of interest because they may increase the Selekti ^¬ tivity of a specific hydrocarbon. When C02-electrolysis, a gas diffusion electrode (GDE) are used as the cathode, similar to the chlor-alkali electrolysis to Zvi ^¬ rule the liquid electrolyte, the gaseous C02 and the solid silver particles to produce a three-phase boundary. In this case, an elec- rolysezelle is, as known also from the fuel cell technology used with two electrolyte compartments, the Elect ^¬ rolytkammern are separated by an ion exchange membrane.

The working electrode is a porous gas diffusion electrode. It comprises a metal net onto which a mixture of PTFE,

Activated carbon, a catalyst and other components is applied. It includes a pore system into which the

Reactants invade and react at the three-phase interfaces.

The counter electrode is a sheet applied with platinum or an iridium mixed oxide. The GDE is in contact with the electrolyte on one side. On the other hand, it is supplied with C02, which is forced through the GDE with overpressure (so-called convective mode of operation). The GDE can while holding various metals and metal compounds ent ^¬ that have a catalytic effect on the process. The functioning of a GDE is known, for example, from EP 297377 A2, EP 2444526 A2 and EP 2410079 A2.

Unlike the chlor-alkali electrolysis and fuel cell technology ^¬ material, the resulting product is gaseous and not liquid at the Koh ^¬ dioxide electrolysis. Furthermore, the CO 2 used forms salts with the alkali metal or alkaline earth metal hydroxide formed from the electrolyte. Beispielswei ^¬ se is formed with the use of potassium salts as the electrolyte KOH and there are the salts of KHC03 and K2C03. by virtue of The operating conditions lead to a crystallization of the salts in and on the GDE from the gas side.

The electrochemical conversion of CO 2 to silver electrodes takes place according to the following equation:

Cathode: C02 + 2e- + H20 -> CO + 20H- with the backreaction

Anode: 6H20 -> 02 + 4e- + 4H30 +

Due to the electrochemical conditions, the charge balance of the chemical equations is not uniform with H30 + or OH-. Despite acidic electrolyte, the GDE has local alkaline pH values. For operating a see alkaline fuel cell technology, the introduced oxygen must be C02-free, because otherwise KHCO would form K2C03 as fol ^¬ constricting equations /:

C02 + KOH -> KHCO3

C02 + 2KOH -> K2C03 + H20

The same process can now also be observed in CO 2 electrolysis, with the difference that the gas fed in can not be CO 2 -free. As a result, after a finite period of time (depending on the current density), salt in and on the GDE crystallizes from the gas side and clogs the pores of the GDE. The gas pressure rises, the GDE is heavily loaded and tears from a certain pressure. In addition, the potassium ions required for the process are removed from the process and the gas space is gradually filled with salt. An analogous process is observed with ande ren ^¬ alkali / alkaline earth metals, such as cesium.

A stable long-term operation of the gas diffusion electrode in the range of more than 1000 h is not possible in the C02 electrolysis, since the resulting salt clogs the pores of the GDE and thus this gas-impermeable. It is an object of the present invention to provide an improved arrangement for the carbon dioxide electrolysis, with a stable long-term operation while avoiding the aforementioned disadvantages is made possible.

This object is achieved by an arrangement having the features of claim 1. The subclaims relate to advantageous embodiments of the arrangement. The arrangement according to the invention for the carbon dioxide electrolysis comprises an electrolytic cell having an anode and a cathode, said anode and cathode are connected to a voltage ^¬ supply, wherein the cathode is designed as Gasdiffusi ^¬ onselektrode to which on a first side of a gas space and a second side of a cathode chamber connects, a subsequent to the electrolysis cell Elect ^¬ rolyt circuit and a gas supply for supplying carbon-lendioxidhaltigem gas into the gas space. Furthermore, the gas space has an outlet for electrolyte,

Carbon dioxide and product gases of the electrolysis and off ^¬ lass is ver ^¬ connected via a throttle with the electrolyte circuit, wherein the throttle is designed to cause a definable pressure difference between the gas space and the cathode space at a flow of a mixture of product gases and liquid electrolyte.

Thus, a carbon dioxide electrolysis plant is created, which operates in the "flow-by" mode.The carbon dioxide is not through the cathode, so the gas diffusion electrode, on the catholyte side through-pressed ("flow-through"), but at this in Gas room bypassed. The pressure difference between the cathode space and gas space is low during flow-by operation. But on the one hand to allow enough electrolyte to flow through the cathode to prevent salinization and on the other hand to avoid the formation of a liquid film on the gas space side of the cathode, a pressure difference is generated and maintained by means of the throttle. Advantageous embodiments of the device according to the invention will become apparent from the dependent of claim 1 claims. In this case, the embodiment can be combined according to claim 1 with the features of one of the subclaims or preferably also with those of several subclaims. Accordingly, the following features can additionally be provided for the arrangement: The throttle can be inserted at an angle of between 0 ° and

80 ° to the vertical arranged pipe. In one embodiment, the throttle comprises a vertical pipe. The tube preferably has a length of between 60 cm and 140 cm, in particular between 90 cm and 110 cm.

- The tube can be arranged rotatably. This allows you to change the absolute height that bridges the pipe. Since ^¬ by turn, caused the pipe pressure difference is changed. Thus, therefore, a desired pressure difference between gas space and cathode space can be achieved by a rotation of the

Adjust tube. The maximum pressure difference exists when the pipe is vertical. The pipe rotates ^¬ ge into the horizontal, is the pressure difference close to zero. - The tube has an inner diameter which corresponds to at least twice the inner diameter of the other connection between the gas space and the electrolyte circuit. Insbesonde ^¬ re is the inner diameter of five times the inner diameter of the other compound. The inner diameter is preferably less than ten times the inner diameter of the other compound. In the tube, the length provides for the amount of hydrostatic pressure, the

Extending the cross-section first allows the liquid to hold in this tube area. It is assumed that the other pipe connections, ie in particular a different part of the connection between the gas space and the electrolyte circulation, especially between the overflow tank and the electrolyte reservoir, with as small a cross-section as possible. is realized cut to cast a rapid flow to be ^¬. Due to the larger cross-section of the tube, the plug flow (liquid plug in the gas stream) is torn open and the gas bubbles are released.

- The outlet is preferably in the gas space on the bottom angeord ^¬ net. As a result, the electrolyte, which passes from the cathode chamber into the gas space and runs off at the cathode to the bottom of the gas space, can easily be led out of the gas space.

- The outlet can be connected via a return connection to the gas supply.

There may be a pumping device for the circulation of carbon dioxide and product gas in the circuit formed by the gas space and the return connection.

- The outlet is conveniently connected to an overflow tank. The outlet and a possibly subsequent pipe carry electrolyte and carbon dioxide and product gases. For the further work of the electrolytic cell gases and electrolyte must be shared ^¬ , which happens by the introduction into the overflow tank. At the bottom of the overflow tank, the electrolyte collects and in the area above the electrolyte, the carbon dioxide and, if necessary, product gases. Appropriately includes the Rückver ^¬ bond to the gas supply in the upper region of the overflow container, so that the carbon dioxide can be recycled without electrolyte. The leadership of electrolyte to the overflow tank is preferably gravity driven.

- The overflow tank can be constructed separately from the gas space and connected for example via a pipe connection. The overflow tank can also be integrated in the gas space.

The overflow container can be connected to the electrolyte circuit via a throttle, wherein the throttle is configured, a definable pressure difference between the gas space and the effecting method space. The pressure difference should not be dependent on whether gas, electrolyte or a mixture thereof passes through the throttle. As a result, the pressure difference is kept within a predetermined range. Thereby a continuous flow of electrolyte through the gas diffusion electric ^¬ en is maintained in the gas space, which prevents salinization, on the other hand be ^¬ adjoins the flow of the electrolyte to prevent the cover of the gas diffusion electrode with a liquid film which rolyse the efficiency of the elec- would reduce. The throttle may be arranged, for example, at a medium height in the overflow container. As soon as the liquid level reaches this mitt ^¬ sized height in the overflow container the electrolyte is transported through the choke from ^¬. The liquid level in the overflow tank is thus kept constant at the middle level.

- A first pressure sensor may be present in the gas space. This is a pressure signal, for example, to a control ^¬ device for controlling the shut-off device. A second pressure sensor can be arranged in the cathode compartment. This can also give a pressure signal to the controller. From the two pressure signals, the controller can determine the pressure difference. - Alternatively, a differential pressure sensor for gas space and cathode space may be present. This directly gives a signal for the pressure difference to a control device.

- The pressure difference between the gas space and the cathode space is preferably maintained between 10 and 100 hPa. This light one

Increasing the pressure on the gas side still allows a sufficiently good passage of the electrolyte through the gas diffusion electrode, so it washes the salts off well, and at the same time shifts the three-phase boundary slightly into the gas diffusion electrode. There is thus a modified flow-by mode ver ^¬ applies where the feed gas is pressed lightly into the Gasdiffusionselekt ^¬ rode. Thus, the yield of Pro ^¬ duktgas, such as carbon monoxide increases. - The gas space may include turbulence promoters. The electrolysis takes place in flow-by mode, ie the carbon dioxide is conducted past the gas diffusion electrode and not pressed through it. Without additional equipment bil ^¬ det thus a laminar flow from where on the surface of the gas diffusion electrode, the gas velocity is very low. The gas space is therefore advantageously so umge ^¬ staltet that the inflowing gas is swirled and thus the flow film breaks off at the surface of the cathode. Since ^¬ through it comes to a better penetration of the carbon dioxide in the gas diffusion electrode and thus to a better yield of product gas, for example CO. Turbulence ^promoters may include, for example: flow channel, flow breaker, reduction of the cross section.

The turbulence promoters may be designed so that an air gap of between 0.1 mm and 5 mm remains between them and the surface of the cathode. This advantageously ensures that the turbulence promoters are not wetted by the electrolyte passing through the gas diffusion electrode and held there. This in turn would lead to a reduced flow of carbon dioxide and severely damage the overall efficiency of the ^electrolysis . However, the air gap creates a distance of the turbulence promoters from the surface of the gas diffusion electrode, so that the electrolyte can drain and collect on the bottom side in the gas space. Preferably be ^¬ but are supporting connections at several points between the turbulence promoter and the gas diffusion electrode, whereby the gas diffusion electrode undergoes a mechanical strengthening.

- The turbulence promoters may have flow channels, by means of which the electrolyte is guided to the edge of the gas space.

- Preferably, the volume flow of the pump is significantly greater than the feed gas volume flow, ie the volume flow of new Carbon dioxide. This results in a higher flow through the gas space, which in turn has a turbulent flow result, on the other hand, the conversion of carbon dioxide is improved. Furthermore, a Besse ^¬ rer removal of the overflow from the headspace is due to the higher gas flow rate.

- The pump device may be arranged in the gas space. For example, the pumping device can be arranged at the entrance to the gas space into which the gas feed opens or in the region of the outlet. The pump device may be, for example, a diaphragm pump, which is advantageously resistant to chemicals. Other pump types are also possible, such as gear, piston, stroke or peristaltic pumps. The volume flow of the pumping device can be, for example, 2 l / min to 5 l / min. He should be at least the Zehnfa ^¬ che the flow rate of the incoming carbon dioxide.

The pumping device may alternatively be arranged in the return connection. In other words, the pumping device is arranged outside the gas space.

A preferred, but by no means limitative exporting ^¬ approximately example of the invention will now be further explained with reference to the figure of the drawing. The characteristics are shown specific ^¬ matically.

The construction of an electrolysis cell 11, shown schematically in Figure 1 is typically adapted to carry out a carbon dioxide ^¬ electrolysis. The off ^¬ guide shape of the electrolytic cell 11 comprises at least one anode 13 with adjoining anode compartment 12 and a cathode 15 and ei ^¬ NEN adjacent the cathode compartment 14 and anode compartment 12 cathode chamber 14 are separated by a membrane 21st The membrane 21 is typically made of a PTFE-based material. Depending on the electrolyte solution used, a construction without membrane 21 is also conceivable in which a pH compensation then exceeds that of the membrane 21. Anode 13 and cathode 15 are electrically connected to a voltage ^¬ supply 22, which is controlled by the control unit 23. The control unit 23 can apply a protective voltage or an operating voltage to the electrodes 13, 15, that is to say the anode 13 and the cathode 15. The anode compartment 12 of the electrolysis cell 11 shown is equipped with an electrolyte inlet. Likewise, the illustrated Ano ^¬ denraum 12 includes an outlet for electrolyte and, for example, oxygen O ₂ or other gaseous by-product, which is gebil ^¬ det in the carbon dioxide electrolysis at the anode 13. The cathode compartment 14 likewise has at least one product and electrolyte outlet in each case. In this case, the total electrolysis product can be composed of a large number of electrolysis products.

The electrolytic cell 11 is also designed in a three-chamber structure, in which the carbon dioxide CO _{2 is} flowed via the designed as a gas diffusion electrode cathode 15 in the Katho- denraum 14. Gas diffusion electrodes make it possible a solid catalyst, a liquid electrolyte and a gaseous Elektrolyseedukt in contact to bring MITEI ^¬ Nander. For this purpose, for example, the catalyst can be made porous and take over the electrode function, or a porous electrode takes over the catalyst function. The pore system of the electrode is designed so that the liquid and the gaseous phase can equally penetrate into the pore system and can simultaneously present therein or at its electrically accessible surface. An example of a gas diffusion electrode is an oxygen-consuming electrode used in chloralkali electrolysis.

For the embodiment as a gas diffusion electrode comprising the Ka Thode 15 in this example, a metal mesh on which a Mi ^¬ research made of PTFE, activated carbon and a catalyst is applied. For the introduction of the carbon dioxide C02 in the

Catholyte cycle, the electrolytic cell 11 comprises a The carbon dioxide reaches in the gas space 16, the cathode 15 and there can penetrate into the porous structure of the cathode 15 and thus come to the reaction.

Furthermore, the arrangement 10 comprises an electrolyte circuit 20, via which the anode space 12 and the cathode space 14 are supplied with a liquid electrolyte, for example K 2 SO 4, KHCO 3, KOH, Cs 2 SO 4, and the electrolyte is returned to a reservoir 19. The circulation of the electrolyte in the electrolyte ^¬ rolytkreislauf 20 is carried out by an electrolyte pump 18th

The gas space 16 in the present example comprises an outlet 25, which is arranged in the bottom area. The outlet 25 is designed as an opening with a sufficient cross section, so that both electrolyte, which passes through the cathode 15, as well as carbon dioxide and product gases can pass through the outlet in the ^¬ bound pipe. The outlet 25 leads to an overflow vessel 26. In the overflow vessel 26, the liquid

Electrolyte collected and collected. Carbon dioxide and

Product gases coming from the gas chamber 16 are separated from Elect ^¬ rolyten and accumulate them above.

From an upper point of the overflow vessel 26, another pipe 28 leads to a pump 27, in this embodiment a diaphragm pump, and further to the gas supply 17. The pump 27 may also be a piston, lift, extruder or gear pump. A portion of the gas supply 17, the gas space 16, the tube 18 and the overflow vessel 26 together with its connection to the outlet 25 thus together form a circuit. By means of the pump 27, the carbon dioxide and pre ^¬ existing product gases are guided by the overflow vessel 26 back into the Gaszu ^¬ management and thus partially guided the gas in a circle. The volume flow of the pump 27 is significantly higher than the volume flow of new carbon dioxide. Feed gas which is not consumed is thereby guided past advantageously again at the cathode 15, and has to be reduced once more or several times re ^¬ the opportunity. product gases are partly also led in a circle. By passing the carbon dioxide past the cathode 15 several times, the efficiency of the conversion is increased. From the overflow vessel 26 there is a further connection, which leads back to the electrolyte circuit 20. This compound begins with an outlet 29 which is disposed on a side wall of the Überlaufge ^¬ fäßes 26, preferably near the bottom, but not in the ground. The outlet 29 is connected to a throttle 30, which is designed as a vertical piece of pipe with a length of example ^¬ 90 cm. The diameter of the pipe section is significantly greater than that of the leads to the throttle 30. The supply line has for example an inner ^¬ diameter of 4mm, the pipe section has an inner diameter of 20mm. The throttle 30 is the output side, ie at the top

End of the pipe section connected to the electrolyte circuit 20.

During operation, a Druckdif ^¬ ference between the upper side is connected electrolyte circulation 20 and therefore also to the cathode chamber 14 on the one hand and the overflow vessel 26 and the gas chamber 16 on the other hand, made her ^¬ and held by the throttle 30th This pressure difference is between 10 and 100 hPa (mbar), ie the gas space 16 remains at ei ^¬ nem only slight overpressure relative to the cathode compartment 14. It is important that the throttle 30, the pressure difference un ^¬ depending manufactures depending on whether just a liquid or Gas ^¬ shaped medium flows through or a mixture thereof. In the pipe section of the throttle 30, which is filled with electrolyte, depending on the height of the pipe section due to the hydrostatic pressure of the differential pressure. If the tube piece rotatably mounted, so the differential pressure of the reactor can be continuously lowered 30 to almost zero in horizontal ^¬ right position. When starting the electrolysis, despite the slight overpressure on the gas side, ie in the gas space 16 due to the an ^¬ lying electrical voltage at the cathode 15 electrolyte from the catholyte compartment 14 through the gas diffusion electrode, al Thus, the cathode 15 is "pumped" in the direction of the gas space 16. es ent ^¬ are on the side of the gas chamber 16 drops on the Oberflä ^¬ surface of the cathode 15, which coalesce and collect in the form of the bottom of the cathode 15th

The accumulating electrolyte thereby causes a

Pressure rise in the gas space 16. However, this pressure increase is compensated by the throttle 30 again by electrolyte and / or gas from the overflow vessel 26 is lytkreislauf 20 back into the elec-. Thus, the remains

Pressure difference between the two sides of the cathode 15 in the desired range between 10 and 100 hPa.

Although passing through the cathode 15 OH ^~ ions cause together with the carbon dioxide and alkali cations from the electrolyte salt formation, however, the differential pressure at the cathode 15 is so low that sufficient liquid ^{¬ is} flushed through the cathode 15 and the formed Bring salt into solution, permanently washed off and transported away from the gas space 16 into the overflow vessel 26. Another ^¬ pressure increase, which would lead to crystallization of the salt formed, is prevented by the throttle 30th

Claims

claims

1. arrangement (10) for the carbon dioxide electrolysis, comprising

- An electrolytic cell (11) having an anode (13) and a cathode (15), said anode (13) and cathode (15) having a

Voltage supply (22) are connected, wherein the cathode (15) is designed as a gas diffusion electrode to which on ei ^¬ ner first side, a gas space (16) and on a second side of a cathode space (14),

- a subsequent to the electrolysis cell (11) Electric ^¬ lytkreislauf (20)

a gas feed (17) for feeding carbon dioxide-containing gas into the gas space (16),

characterized in that

- the gas space (16) has an outlet (25) for electrolyte having carbon dioxide ^¬ and product gases of the electrolysis,

- The outlet (25) via a throttle (30) to the electrolyte ^¬ circuit (20) is connected, wherein the throttle (30) is designed ^¬ staltet, a definable pressure difference between the gas space (16) and the cathode space (14) at flow of to effect a mixture of product gases and liquid electrolyte.

2. Arrangement (10) according to claim 1, wherein the throttle (30) comprises an arranged at an angle of between 0 ° and 80 ° to the vertical pipe.

3. Arrangement (10) according to claim 2, wherein the tube is arranged substantially vertically.

4. Arrangement (10) according to claim 2, wherein the tube is rotatably arranged.

5. Arrangement (10) according to any one of the preceding claims 2 to 4, wherein the inner diameter of the tube is at least twice, in particular at least five times the other connection between the gas space (16) and the electrolyte circuit (20).

An assembly (10) according to any one of the preceding claims, wherein the outlet (25) is connected to the gas supply via a return connection.

7. Arrangement (10) according to claim 6 with a pumping device

(27) for circulating carbon dioxide and product gas in the circuit consisting of the gas space (16) and the return compound

(28) is formed.

The assembly (10) of claim 7, wherein the pumping device (27) is disposed in the return connection.

9. Arrangement (10) according to claim 7, wherein the pumping device (27) in the gas space (16) is arranged.

10. Arrangement (10) according to one of the preceding claims, configured such that the pressure difference between the gas space (16) and the cathode space (14) is maintained between 10 and 100 hPa.

11. The arrangement (10) according to any one of the preceding claims, wherein the outlet (25) in the gas space (16) is the bottom side angeord ^¬ net.

12. An assembly (10) according to any one of the preceding claims, wherein the outlet (25) is connected to an overflow container (26).

13. Arrangement (10) according to one of the preceding claims, wherein the gas space (16) comprises turbulence promoters.

14. Arrangement (10) according to claim 10, wherein the turbulence promoters are designed so that an air gap of at least 0.1 mm remains between them and the surface of the cathode (15).