Arrangement for the Electrolysis of Carbon Dioxide

Abstract

Various embodiments may include a system for carbon dioxide electrolysis comprising: an anode; a cathode including a gas diffusion electrode adjoined by a gas space and a cathode space; an electrolyte circuit; a gas supply delivering carbon dioxide-containing gas to the gas space, wherein the gas space includes an outlet for an electrolyte, carbon dioxide, and product gases resulting from the electrolysis; and a throttle connecting the outlet to the electrolyte circuit, wherein the throttle creates a predefined pressure differential between the gas space and the cathode space when a mixture of product gases and liquid electrolyte flows through the throttle.

Claims

1. A system for carbon dioxide electrolysis, the system comprising: an electrolysis cell having an anode and a cathode both connected to a voltage supply, wherein the cathode comprises a gas diffusion electrode adjoined on a first side by a gas space and on a second side by a cathode space; an electrolyte circuit adjoining the electrolysis cell; a gas supply delivering carbon dioxide-containing gas to the gas space; wherein the gas space includes an outlet for an electrolyte, carbon dioxide, and product gases resulting from the electrolysis; and a throttle connecting the outlet to the electrolyte circuit, wherein the throttle creates a predefined pressure differential between the gas space and the cathode space when a mixture of product gases and liquid electrolyte flows through the throttle.

2. A system as claimed in claim 1, wherein the throttle comprises a pipe arranged at an angle of between 0 and 80 to the vertical.

3. A system as claimed in claim 2, wherein the angle of the pipe is essentially vertical.

4. A system as claimed in claim 2, wherein the angle of the pipe is adjustable by rotation.

5. A system as claimed in claim 2, wherein an internal diameter of the pipe is at least twice a diameter of a remainder of the connection between the gas space and the electrolyte circuit.

6. A system as claimed in claim 1, further comprising a return connection joining the outlet to the gas supply.

7. A system as claimed in claim 6, further comprising a pump for circulation of carbon dioxide and product gas in the gas space and the return connection.

8. A system as claimed in claim 7, wherein the pump is disposed in the return connection.

9. A system as claimed in claim 7, wherein the pump is disposed in the gas space.

10. A system as claimed in claim 1, wherein the pressure differential between the gas space and the cathode space is kept between 10 and 100 hPa.

11. A system as claimed in claim 1, wherein the outlet in the gas space is disposed at the bottom end.

12. A system as claimed in claim 1, wherein the outlet is connected to an overflow vessel.

13. A system as claimed in claim 1, further comprising turbulence promoters disposed in the gas space.

14. A system as claimed in claim 10, wherein the turbulence promoters define an air gap of at least 0.1 mm between the turbulence promoters and a surface of the cathode.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0027] A working example, one which is by no means limiting, is now elucidated in detail with reference to the drawing. The features are shown here in schematic form.

DETAILED DESCRIPTION

[0028] In some embodiments, an arrangement for carbon dioxide electrolysis comprises an electrolysis cell having an anode and a cathode, where anode and cathode are connected to a voltage supply, where the cathode takes the form of a gas diffusion electrode adjoined on a first side by a gas space and on a second side by a cathode space, an electrolyte circuit that adjoins the electrolysis cell and a gas supply for supplying carbon dioxide-containing gas to the gas space. In addition, the gas space has an outlet for electrolyte, carbon dioxide and product gases from the electrolysis and the outlet is connected via a throttle to the electrolyte circuit, where the throttle is configured so as to bring about a definable pressure differential between gas space and cathode space when a mixture of product gases and liquid electrolyte flows through it.

[0029] What is thus created is a carbon dioxide electrolysis plant that works in flow-by mode. The carbon dioxide is not forced here through the cathode, i.e. the gas diffusion electrode, to the catholyte side (flow-through) but guided past it in the gas space. The pressure differential between cathode space and gas space is low in flow-by operation. In order on the one hand to allow sufficient electrolyte to flow through the cathode to prevent salting-up and on the other hand also to prevent the formation of a liquid film on the gas space side of the cathode, a pressure differential is generated and maintained by means of the throttle. In some embodiments, the following features can additionally be provided for the arrangement: [0030] The throttle may comprise a pipe arranged at an angle of between 0 and 80 to the vertical. In some embodiments, the throttle comprises a vertical pipe. The pipe may have a length of between 60 cm and 140 cm, especially between 90 cm and 110 cm. [0031] The pipe may be in a rotatable arrangement. This allows alteration of the absolute height that the pipe bridges. As a result, in turn, the pressure differential brought about by the pipe is altered. It is thus possible to establish a desired pressure differential between gas space and cathode space by a rotation of the pipe. The maximum pressure differential exists when the pipe is vertical. If the pipe is rotated to the horizontal, the pressure differential is close to zero. [0032] The pipe has an internal diameter corresponding to at least twice the internal diameter of the rest of the connection between gas space and electrolyte circuit. In some embodiments, the internal diameter is five times the internal diameter of the rest of the connection. The internal diameter may be less than ten times the internal diameter of the rest of the connection. In the pipe, it is the length that ensures the magnitude of the hydrostatic pressure, but it is the cross-sectional extent that actually enables the liquid to persist in this pipe region as well. It is assumed here that the further pipe connections, i.e. more particularly another part of the connection between gas space and electrolyte circuit, especially between overflow vessel and electrolyte reservoir, may be implemented with a minimum cross section in order to bring about rapid flow. The greater cross section of the pipe breaks up the plug flow (liquid plug in the gas stream), and the gas bubbles are freed. [0033] The outlet may be disposed in the gas space at the bottom end. This means that the electrolyte that enters the gas space from the cathode space and runs off at the cathode to the bottom of the gas space can be conducted out of the gas space without difficulty. [0034] The outlet may be connected to the gas supply via a return connection. [0035] A pump apparatus for circulation of carbon dioxide and product gas may be present in the circuit formed from the gas space and the return connection. [0036] The outlet may be connected to an overflow vessel. The outlet and any connected pipe conduct electrolyte and carbon dioxide and product gases. For the further work of the electrolysis cell, gases and electrolyte may be divided, which is accomplished by introduction into the overflow vessel. The electrolyte collects at the base of the overflow vessel, and the carbon dioxide and any product gases in the region above the electrolyte. Appropriately, the return connection to the gas feed is connected in the upper region of the overflow vessel, such that the carbon dioxide can be recycled without electrolyte. The electrolyte is guided to the overflow vessel preferably in a gravity-driven manner. [0037] The overflow vessel may be constructed separately from the gas space and may be connected, for example, via a pipe connection. The overflow vessel may also be integrated into the gas space. [0038] The overflow vessel may be connected to the electrolyte circuit via a throttle, where the throttle is configured to bring about a definable pressure differential between gas space and cathode space. The pressure differential is not to be dependent on whether gas, electrolyte or a mixture thereof is passing through the throttle. This keeps the pressure differential within a predetermined range. As a result, a constant flow of electrolyte through the gas diffusion electrode into the gas space is maintained, which prevents salting-up, but on the other hand limits the flow of the electrolyte in order to prevent coverage of the gas diffusion electrode with a liquid film that would reduce the efficiency of the electrolysis. The throttle may be disposed, for example, at a moderate height in the overflow vessel. As soon as the liquid level in the overflow vessel reaches this moderate height, the electrolyte is transported away through the throttle. The liquid level in the overflow vessel is thus kept constant at the moderate height. [0039] A first pressure sensor may be present in the gas space. This gives a pressure signal, for example, to a control device for actuation of the shut-off device. A second pressure sensor may be disposed within the cathode space. This can likewise give a pressure signal to the control device. The two pressure signals can be used by the control device to determine the pressure differential. [0040] In some embodiments, a pressure differential sensor for gas space and cathode space may be present. This directly gives a signal for the pressure differential to a control device. [0041] The pressure differential between gas space and cathode space may be kept between 10 and 100 hPa. This slight pressure increase on the gas side still permits sufficiently good passage of the electrolyte through the gas diffusion electrode, i.e. washes the salts away efficiently, and simultaneously moves the three-phase boundary somewhat into the gas diffusion electrode. What is thus used is a modified flow-by operation in which the reactant gas is forced slightly into the gas diffusion electrode. This increases the yield of product gas, for example carbon monoxide. [0042] The gas phase may comprise turbulence promoters. The electrolysis takes place in flow-by operation, meaning that the carbon dioxide is guided past the gas diffusion electrode and is not forced through it. Without additional internals, a laminar flow thus develops, in which the gas velocity is very low at the surface of the gas diffusion electrode. The gas space is therefore remodeled such that the incoming gas is vortexed and hence the flow film at the surface of the cathode breaks. This results in better penetration of the carbon dioxide into the gas diffusion electrode and hence in a better yield of product gas, for example CO. Turbulence promoters may include, for example: flow channel, baffle, reduction in the cross section. [0043] The turbulence promoters may be configured such that an air gap of between 0.1 mm and 5 mm remains between them and the surface of the cathode. What this achieves is that electrolyte passing through the gas diffusion electrode does not wet and is not retained at the turbulence promoters. This in turn would lead to a reduced flow of carbon dioxide and would significantly damage the efficiency of the electrolysis overall. However, the air gap creates a separation of the turbulence promoters from the surface of the gas diffusion electrode, such that the electrolyte can run off and collect at the bottom end in the gas space. However, there are preferably supporting connections at multiple points between the turbulence promoters and the gas diffusion electrode, which means that the gas diffusion electrode experiences mechanical reinforcement. [0044] The turbulence promoters may have efflux channels, by means of which the electrolyte is guided to the edge of the gas space. [0045] In some embodiments, the volume flow rate of the pump is distinctly greater than the feed gas volume flow rate, i.e. the volume flow rate of new carbon dioxide. This means that there is firstly higher flow through the gas space, which in turn results in more turbulent flow; secondly, this improves the conversion of the carbon dioxide. Furthermore, the overflow from the gas space is transported away better owing to the higher gas flow rate. [0046] The pump apparatus may be disposed within the gas space. For example, the pump apparatus may be disposed at the inlet to the gas space into which the gas feed opens or in the region of the outlet. The pump apparatus may, for example, be a membrane pump which is advantageously chemical-resistant. Other pump types are also useful, such as gear pumps, piston pumps, stroke pumps or peristaltic pumps. The volume flow rate of the pump apparatus may, for example, be 2 L/min to 5 L/min. It should be at least ten times the volume flow rate of the incoming carbon dioxide. [0047] The pump apparatus may alternatively be disposed in the return connection. In other words, the pump apparatus is disposed outside the gas space.

[0048] The construction of an electrolysis cell 11 shown in schematic form in the FIGURE is typically suitable for undertaking a carbon dioxide electrolysis. This embodiment of the electrolysis cell 11 comprises at least one anode 13 with an adjoining anode space 12, and a cathode 15 and an adjoining cathode space 14. Anode space 12 and cathode space 14 are separated from one another by a membrane 21. The membrane 21 is typically manufactured from a PTFE-based material. According to the electrolyte solution used, a construction without a membrane 21 is also conceivable, in which case pH balancing then goes beyond that by the membrane 21.

[0049] 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 may apply a protection voltage or an operating voltage to the electrodes 13, 15, i.e. the anode 13 and the cathode 15. The anode space 12 of the electrolysis cell 11 shown is equipped with an electrolyte inlet. The anode space 12 depicted likewise comprises an outlet for electrolyte and, for example, oxygen O.sub.2 or another gaseous by-product which is formed in the carbon dioxide electrolysis at the anode 13. The cathode space 14 in each case likewise has at least one product and electrolyte outlet. The overall electrolysis product may be composed of a multitude of electrolysis products.

[0050] The electrolysis cell 11 is also executed in a three-chamber construction in which the carbon dioxide CO.sub.2 is introduced into the cathode space 14 via the cathode 15 executed as a gas diffusion electrode. Gas diffusion electrodes enable mutual contacting of a solid catalyst, a liquid electrolyte and a gaseous electrolysis reactant. For this purpose, for example, the catalyst may be executed in porous form and assume the electrode function, or a porous electrode assumes the catalyst function. The pore system of the electrode is configured here such that the liquid phase and the gaseous phase can penetrate equally into the pore system and may be present simultaneously therein, i.e. at the electrically accessible surface thereof. One example of a gas diffusion electrode is an oxygen-depolarized electrode which is used in chloralkali electrolysis.

[0051] For configuration as a gas diffusion electrode, the cathode 15 in this example comprises a metal mesh to which a mixture of PTFE, activated carbon and a catalyst has been applied. For introduction of the carbon dioxide CO2 into the catholyte circuit, the electrolysis cell 11 comprises a carbon dioxide inlet 24 into the gas space 16. In the gas space 16, the carbon dioxide reaches the cathode 15, where it can penetrate into the porous structure of the cathode 15 and hence be reacted.

[0052] In addition, the arrangement 10 comprises an electrolyte circuit 20, by means of which the anode space 12 and the cathode space 14 are supplied with a liquid electrolyte, for example K2SO4, KHCO3, KOH, Cs2SO4, and the electrolyte is recycled into a reservoir 19. The electrolyte is circulated in the electrolyte circuit 20 by means of an electrolyte pump 18.

[0053] In the present example, the gas space 16 comprises an outlet 25 disposed in the base region. The outlet 25 is configured as an opening with sufficient cross section, so that both electrolyte that passes through the cathode 15 and carbon dioxide and product gases can get through the outlet into the connecting pipe. The outlet 25 leads to an overflow vessel 26. The liquid electrolyte is collected and accumulates in the overflow vessel 26. Carbon dioxide and product gases that come from the gas space 16 are separated from the electrolyte and accumulate above it.

[0054] From a point toward the top of the overflow vessel 26, a further pipe 28 leads to a pump 27, in this working example a membrane pump, and further to the gas supply 17. The pump 27 may also be a piston pump, stroke pump, extruder pump or gear pump. Part of the gas supply 17, the gas space 16, the pipe 18 and the overflow vessel 26 together with the connection thereof to the outlet 25 thus form a circuit. By means of the pump 27, the carbon dioxide and product gases present are guided from the overflow vessel 26 back into the gas supply and hence the gas is partly circulated. The volume flow rate of the pump 27 is much higher than the volume flow rate of new carbon dioxide. Reactant gas that has not been consumed is thus advantageously guided once again past the cathode 15 and once or more than once has the opportunity to be reduced. Product gases are likewise partly circulated here. The repeated passage of the carbon dioxide past the cathode 15 increases the efficiency of the conversion.

[0055] From the overflow vessel 26, there is a further connection that leads back to the electrolyte circuit 20. This connection begins with an outlet 29 disposed at a side wall of the overflow vessel 26, in some embodiments close to the base, but not at the base. The outlet 29 is connected to a throttle 30 in the form of a vertical pipe section having a length of 90 cm, for example. The diameter of this pipe section is much greater than that of the inlets to the throttle 30. The inlet has, for example, an internal diameter of 4 mm; the pipe section has an internal diameter of 20 mm. The throttle 30 is connected on the outlet side, i.e. at the upper end of the pipe section, to the electrolyte circuit 20.

[0056] In continuous operation, the throttle 30 establishes and maintains a pressure differential between the electrolyte circuit 20 connected at the top end and hence also the cathode space 14 on the one hand and the overflow vessel 26 and the gas space 16 on the other hand. This pressure differential is between 10 and 100 hPa (mbar), meaning that the gas space 16 remains at only slightly elevated pressure relative to the cathode space 14. In some embodiments, the throttle 30 establishes the pressure differential irrespective of whether a liquid or gaseous medium is currently flowing through, or a mixture thereof. In the pipe section of the throttle 30 filled with electrolyte, the pressure differential is established depending on the height of the pipe section, owing to the hydrostatic pressure. If the pipe section is mounted in a rotatable manner, the pressure differential of the throttle 30 can be lowered in an infinitely variable manner, down to virtually zero in a horizontal position.

[0057] When starting up the electrolysis, in spite of the slightly elevated pressure on the gas side, i.e. in the gas space 16, the electrical voltage applied to the cathode 15 results in pumping of electrolyte out of the catholyte space 14 through the gas diffusion electrode, i.e. the cathode 15, in the direction of gas space 16. Droplets form on the side of the gas space 16 at the surface of the cathode 15, which coalesce and collect in shape in the lower region of the cathode 15.

[0058] As a result, the backup of electrolyte causes a pressure rise in the gas space 16. However, this pressure rise is compensated for again by the throttle 30 in that electrolyte and/or gas is recycled from the overflow vessel 26 back into the electrolyte circuit 20. The pressure differential between the two sides of the cathode 15 thus remains within the desired range between 10 and 100 hPa.

[0059] The OH.sup. ions that pass through the cathode 15 do cause salt formation together with the carbon dioxide and the alkali metal cations from the electrolyte, but the pressure differential at the cathode 15 is so small that sufficient liquid is flushed through the cathode 15 and brings the salt formed into solution, permanently washes it away and transports it out of the gas space 16 into the overflow vessel 26. A further pressure rise that would lead to crystallization of the salt formed is prevented by the throttle 30.