The invention relates to a method for changing the mode of operation of a device having an electrolyser, a compressor and a membrane separation device between normal and standby mode, wherein during normal operation of the device an electrolysis feed comprising carbon dioxide is converted in the electrolyzer into an electrolysis product containing carbon dioxide and carbon monoxide. at least part of which is routed via the compressor and fed to the membrane separation device at elevated pressure in order to obtain a retentate which is depleted in carbon monoxide and carbon dioxide compared to the electrolysis product.

Furthermore, the invention relates to a device that can be operated according to the method according to the invention.

Under a retentate, the expert understands those components of a

Gas mixture, which are retained by a membrane used to separate the gas mixture. The membrane separation device used in the context of the present invention is designed with at least one membrane

Allows carbon dioxide to pass preferentially and retains carbon monoxide. As a result, a gas or gas mixture is obtained as retentate, which is depleted in carbon dioxide compared to the electrolysis product used.

Correspondingly, a permeate is the components of the gas mixture to be separated that are not retained by the membrane used for the separation. The permeate considered within the scope of the present invention is on and on in carbon dioxide compared to the electrolysis product

carbon monoxide depleted.

Depending on the gas or gas mixture that can be drawn off from it, one side of a membrane that can be used to separate a gas mixture is referred to as

Membrane separation device as retentate or permeate side.

Devices of the generic type are used to generate carbon monoxide or synthesis gas, with carbon dioxide alone or together with water being electrochemically converted in the electrolyzer to form an electrolysis product which, in addition to carbon monoxide or carbon monoxide and hydrogen, also contains unreacted carbon dioxide

membrane separation device must be separated in order to remove carbon monoxide or

to obtain synthesis gas. The membrane separation device has at least one membrane which is selectively permeable to carbon dioxide and via which a CO2 partial pressure difference is generated. The selectivity of the membrane used results from the different diffusion rates of the components of the gas mixture to be separated. Corresponding polymer membranes are currently used commercially.

The principles of the reactions taking place in the electrolyser are described below using the example of the co-electrolysis of water and carbon dioxide. Instead of a co-electrolysis of water and carbon dioxide, however, pure carbon dioxide electrolysis can also be used in particular within the scope of the present invention. It goes without saying that here those relating to water electrolysis

Reaction equations do not apply or corresponding reactions do not take place. However, a separate explanation is omitted for the sake of clarity.

Depending on the electrolyte used and the catalyst used, there are different configurations of co-electrolysis, which differ in particular in terms of the operating temperature and the electrochemical reactions taking place at the electrodes of the electrolyzer.

To carry out the so-called low-temperature co-electrolysis, a

Electrolyzer used with a proton exchange membrane. In this case, the following cathode reactions take place:

C02 + 2 e- + 2 H+ -> CO + H20 (1 ) 2 e- + 2 H+ -> H2 (2)

According to the equation

H20 -> 1/2 02 + 2 H+ - 2e (3)

water is decomposed at the anode.

In variants of corresponding methods, other positive charge carriers such as the ions of an electrolyte salt can be formed at the anode instead of protons, transported via a correspondingly designed membrane and converted at the cathode. An example of an electrolyte salt is potassium hydroxide. In this case, the positive charge carriers are potassium ions. Further variants include, for example, the use of anion exchange membranes. In all variants, however, the charge carriers are not transported in the form of oxygen ions, as in the solid oxide electrolysis cells explained below, but in the form of the charge carriers explained. For details, see, for example, Delacourt et al.

(2008) J. Electrochem. society 155(1), B42-B49, DOI: 10.1149/1.2801871 , referenced.

The protons or other corresponding charge carriers are selectively transferred from the anode to the cathode side via a membrane. Depending on the selected catalyst, the respective formation reactions then compete at the cathode, so that synthesis gases with different hydrogen/carbon mo result in noxide ratios. Depending on the design of the catalyst used, other valuable products can also be formed in the low-temperature co-electrolysis.

In high-temperature co-electrolysis, which is carried out using solid oxide electrolytic cells, the following cathode reactions are observed or postulated:

C02 + 2 e- -> CO + O2- (4) H20 + 2 e- -> H2 + O2- (5)

Furthermore, the following reaction takes place at the anode:

2 O2- -> 02 + 4 e (6)

In this case, the oxygen ions are conducted essentially selectively via a ceramic membrane from the cathode to the anode.

It has not been fully clarified whether the reaction according to reaction equation 4 proceeds as shown. It is possible that only hydrogen is formed electrochemically, while carbon monoxide is formed by the reverse water-gas shift reaction in the presence of carbon dioxide:

C02 + H2 H20 + CO (7)

As a rule, the gas mixture obtained in high-temperature co-electrolysis is in water-gas shift equilibrium (or is close to it). However, the specific way in which the carbon monoxide is formed has no influence on the present invention.

As a rule, there is no complete conversion of carbon dioxide and water in either high- or low-temperature co-electrolysis, which is why the electrolysis product drawn off at the cathode contains carbon dioxide.

Due to the comparatively low investment costs, the electrolysis processes described with downstream membrane-based carbon dioxide separation can be used to advantage in particular when small or medium amounts of carbon monoxide or synthesis gas are to be produced on site for a consumer. However, such applications often place high demands on the flexibility of the system, because either the product quantities that can be delivered fluctuate greatly over time, such as when the consumer is operated in a batch process, or if the price advantages on the fluctuating electricity market are to be used optimally. Low-temperature electrolysers are particularly suitable for flexible use because their mode of operation can be switched between normal and standby mode very quickly. Since the differential pressure across the

However, membranes have to be adjusted much more slowly to avoid damage, the well-known concepts for electrolytic carbon monoxide or However, synthesis gas production is characterized by the long shutdown and start-up times of the membrane separation devices used for the carbon dioxide separations, which severely limit the flexibility of the overall process. If no carbon monoxide or synthesis gas can be delivered to the consumer in the short term,

Therefore, according to the state of the art, normal operation is maintained and the quantity of product that cannot be delivered is discarded with economic losses.

The object of the present invention is therefore to provide a method and a device of the type described at the outset which are suitable for producing the amount of carbon dioxide-depleted retentate more economically than in the prior art with great flexibility.

In terms of the method, the stated object is achieved according to the invention in that, in order to switch from normal to standby operation, the electrolyzer

is fluidically completely isolated from the membrane separator and then shut down, the pressure conditions in the

Membrane separation device are largely maintained.

That the pressure conditions in the membrane separator largely

are maintained, is to be understood in such a way that the differential pressure across each membrane of the membrane separation device changes only slowly with the same sign and preferably deviates by no more than 30% and particularly preferably no more than 15% from the mean value which the differential pressure during the

owns normal operation. A differential pressure change is considered to be slow if, based on the mean value that the differential pressure has during normal operation, it occurs at a rate of less than 30% and preferably less than 15%/min. he follows. Logically, the pressure conditions in the

Membrane separation device not only when changing from normal to standby mode, but also largely during the standby mode itself

maintain.

The electrolyser is completely fluidically isolated from the

Membrane separator by shutting off all lines that the

Connect the electrolyzer directly or via one or more other parts of the device to the membrane separation device. Since the membrane separation device can then no longer be supplied with fresh electrolysis product, the fluidic isolation would lead to a change in the pressure conditions in the membrane separation device. The membrane device is therefore preferably connected to the compressor at the same time as it is completely fluidically insulated from the electrolyzer with the compressor to form a closed system to the outside, in which the suction side of the compressor is connected to the permeate side of the membrane separation device via a first line. In order to largely maintain the pressure conditions in the membrane separation device, the pressure side of the

Compressor or the retentate side of the membrane separator are connected to the suction side of the compressor via a second line in which a with a

Pressure regulator coupled control valve is arranged.

patent claims

1. A method for changing the mode of operation of an electrolyzer (E), a compressor (V) and a membrane separator (T) having device (B) between normal and standby operation, wherein in normal operation the

Device (B) a carbon dioxide comprehensive electrolysis insert (3) in

Electrolyser (E) in a carbon dioxide and carbon monoxide containing

Electrolysis product (4) is converted, at least a part (5) of which is conducted via the compressor (V) and fed to the membrane separation device (T) at increased pressure in order to produce an increase in carbon monoxide and carbon dioxide compared to the electrolysis product (4). to obtain depleted retentate (8), characterized in that to switch from normal to standby operation of the electrolyzer (E) fluidically from the membrane separation device (T) is completely isolated and then shut down, wherein the

Pressure conditions in the membrane separation device (T) largely

be maintained.

2. The method according to claim 1, characterized in that to maintain the pressure conditions in the membrane separation device (T), the compressor (V) is connected to the membrane separation device (T) to form a fluidically closed system to the outside, in which the suction side of the compressor (V ) via a first line (10) to the permeate side of

membrane separation device (T) and via a second line (11) to the pressure side of the compressor (V) or the retentate side of the membrane separation device (T), the differential pressure between the retentate side and the permeate side being controlled via a control valve (e) arranged in the second line is controlled.

3. The method according to any one of claims 1 or 2, characterized in that to switch from standby to normal operation of the electrolyzer (E) booted and then its fluidic isolation from the membrane separator (T) while largely maintaining the

Pressure conditions in the membrane separator (T) is completely eliminated.

4. The method according to claim 2, characterized in that to change from standby to normal operation, the flow to the outside

closed, the compressor (V) and the membrane separation device (T) comprehensive system is connected to the already booted electrolyzer (E), at the same time the path (9) for the retentate downstream of the

Membrane separation device (T) is opened and the direct connections of the suction side of the compressor (V) with and the permeate side of the

Membrane separator (T) and the pressure side of the compressor (V) or the retentate side of the membrane separator (T) are interrupted.

5. Device (B) with a compressor (V), a membrane separator (T) and an electrolyzer (E) with which the device (B) enters normal operation

Carbon dioxide comprehensive electrolysis use (3) in a carbon dioxide and

Carbon monoxide-containing electrolysis product (4) can be implemented, of which at least part (5) can be fed via the compressor (V) and at elevated pressure to the membrane separation device (T) in order to achieve a

Electrolysis product to obtain a retentate depleted in carbon monoxide and carbon dioxide, characterized in that the device (B) has an insulating device with at least one valve (a) with which, when changing from normal to standby operation, the electrolyzer (E ) can be completely isolated in terms of flow from the membrane separation device (T) while largely maintaining the pressure conditions in the membrane separation device.

6. Device according to Claim 5, characterized in that the isolating device has a plurality of valves (a, b, c) and a first (10) and a second line (11, 12) for connecting the membrane separating device (T) to the compressor (V ) to an outwardly closed system, in which the suction side of the compressor (V) via the first line (10) with the permeate side of the

Membrane separator (T) and via the second line (1 1, 12) with the

Pressure side of the compressor (V) or the retentate side of the

Membrane separation device (T) is connected, with a control element (e, f) being arranged in the second line (11, 12), via which the differential pressure between the retentate and permeate side of the membrane separation device (T) can be controlled when the mode of operation changes .

7. Device according to one of claims 5 or 6, characterized in that it is arranged upstream of the electrolyser (E) and is fluidically connected to the permeate side of the membrane separation device (T).

Mixing device (A), in which a carbon dioxide-containing insert (1) can be mixed with at least a part of the permeate occurring in the membrane separation device (T) for the electrolysis insert (3), the permeate side of the membrane separation device (T) and the mixing device ( A) existing flow technical connection includes a valve (c) belonging to the isolation device, which is open during normal operation of the device and closed during standby operation.

8. Device according to one of claims 5 to 8, characterized in that the electrolyser (E) is a high- or a low-temperature electrolyser which is designed to electrochemically convert carbon dioxide alone or together with water to hydrogen and/or or convert carbon monoxide.