OPTIMIZED LIQUID DRAINAGE FROM MEMBRANE ELECTROLYSER

Description

[0001] The invention relates to the provision of electrolysis devices with membrane electrolyzers with optimized liquid drainage, as well as a method for operating these electrolysis devices.

[0002] Electrolysis processes for the production of basic chemicals must be developed for large-scale industrial production (several thousand tons per year). To produce large-scale quantities of product using electrolysis processes, large-area electrolysis cells and electrolyzers with a large number of electrolysis cells are necessary.

[0003] Typically, electrolysis cells with an electrode area of more than 2m² per electrolysis cell are used, as is the case with chlor-alkali electrolysis. The electrolysis cells are grouped together in groups of up to 100 in an electrolysis rack. Several racks then form an electrolyzer. The capacity of an industrial electrolyzer, for example, for chlorine production, is currently up to 30,000 t / a of chlorine and the respective equivalents of caustic soda or hydrogen.

[0004] During some electrolysis processes, at least one of the electrode half-reactions releases a gaseous product, such as oxygen and hydrogen in water electrolysis, or chlorine and possibly hydrogen in chlor-alkali electrolysis. This gas formation often creates a pressure difference between the operating pressure of the electrolyzer and the operating pressure of the liquid discharge from the electrolyzer. For example, in conventional chlor-alkali membrane electrolysis, the products chlorine, aqueous alkali metal hydroxide solution (lye), and hydrogen are produced by electrolysis of an aqueous alkali metal salt solution. The reaction equation for the production of sodium hydroxide lye is exemplified here: 2 NaCl + 2 H 2 O → Cl 2 + 2 NaOH + H 2

[0005] The pressure difference described above is also observed during the operation of electrolyzers with gas diffusion electrodes, in which the operating pressure of the electrolyzer is influenced at least by the reactant gas introduced or its discharged residual gas (e.g. oxygen when operating an oxygen-consuming cathode or e.g. carbon dioxide when operating a CO 2 electrolysis with gas diffusion electrode), possibly in combination with the product gas formed during the electrolysis.

[0006] Regardless of the electrode type chosen, several electrolysis devices (electrolyzers) are typically operated in parallel in corresponding electrolysis plants. As described, for example, in DE19641125, the electrolyzers each comprise several individual electrolysis cells connected hydraulically in parallel, through which electric current flows in an electrical series circuit ("bipolar electrolyzers") or in an electrical parallel circuit ("monopolar electrolyzers").

[0007] The supply of the electrolyzers with the operating media (e.g., brine for the anode side, lye for the cathode side) and the removal of the products (e.g., chlorine gas and depleted brine ("anolyte" from the anode side, and hydrogen and enriched lye "catholyte" from the cathode side) generally takes place via operating piping systems that connect the electrolyzers to the corresponding treatment plants and to which the electrolyzers are connected in parallel. Typical arrangements of electrolyzers and piping systems in an electrolysis cell room can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, chapter "Chlorines."

[0008] EP3489389 describes an electrolysis device comprising at least two electrolysis cells and at least two first and two second gas separation devices, wherein the gas separation devices are connected via siphon-like lines suitable for conducting the water.

[0009] Electrolyzers are usually operated at elevated temperature and pressure; typical values are approximately 40–90°C and an operating pressure of approximately atmospheric pressure to 200–500 mbar overpressure. Operating overpressure in the electrolysis cell offers the advantage that the subsequent processing steps of the gaseous products, such as chlorine and hydrogen (condensation of moisture), are easier, particularly on the anolyte side of the electrolyzers. The subsequent compression has better starting conditions and any necessary intermediate compressors can be eliminated. However, the additional measures described below are necessary to transition from a pressureless standstill via start-up and shutdown operations to normal operation at elevated pressure / temperature.

[0010] The operating media are usually fed into the electrolysis cells from below; the products leave the electrolysis cells via the overflow. This ensures that the electrolysis cells are always filled with liquid during startup and shutdown, or with a liquid / gas mixture during normal operation due to the gases produced.

[0011] Typical designs of membrane electrolysis cells and electrolyzers and common operating data are described, for example, in the Handbook of Chlor-Alkali Technology, Chapter 5 "Chlor-Alkali Technologies".

[0012] For commissioning, the electrolyzers must be heated from ambient conditions (atmospheric pressure, room temperature) to operating temperature and pressurized to operating pressure. For shutdown, they must be cooled down and depressurized accordingly. Example procedures for startup and shutdown are described, for example, in the Handbook of Chlor-Alkali Technology, Chapter 13, "Plant Commissioning and Operation."

[0013] A common technical solution for these commissioning processes is to connect the electrolyzers to start-up circuits via separate start-up piping systems so that individual electrolyzers can be started up or shut down independently of the others in operation.

[0014] A typical commissioning process proceeds as follows: An electrolyzer is filled with the operating media via separate start-up circuits. The operating media are then circulated and heated until the target operating temperature is reached. Circulation via the start-up circuit is then stopped. The anode and cathode sides of the electrolyzer are pressurized to the operating pressures (e.g., by adding nitrogen). After the connection to the operating piping systems has been opened and circulation via these operating piping systems has been restarted, the electrical current (hereinafter referred to as the electrolysis current) can be switched on, thus putting the electrolyzer into operation.

[0015] A typical decommissioning process proceeds in reverse analogy to commissioning: After the electrolysis current has been shut down, circulation via the operating piping systems is stopped, and the electrolyzer is disconnected from these piping systems. The anode and cathode chambers are depressurized. It is important to ensure that the differential pressure between the anode and cathode chambers remains within the specified operating parameters. Circulation is restarted via the start-up piping systems, and the electrolyzer is cooled down.

[0016] Depending on the details of the technology used (e.g. type of electrode coating), a low polarization current (order of magnitude: several 10 A) is applied via an auxiliary rectifier above a certain temperature during commissioning to protect the electrode coating from damage. During shutdown, the polarization current is switched off again when the temperature falls below a certain level and as soon as the anode chamber has been rinsed free of chlorine. During the brief interruption of circulation when switching from the start-up to the operating circuits during commissioning, or vice versa during shutdown, the polarization rectifier can remain switched on. Since conventional electrolysis cells are filled with liquid during these processes, the polarization current cannot cause any damage.

[0017] Analogously, the previously mentioned parameters for the general operation of electrolyzers and the design of the electrolysis device can be applied to water electrolysis. Technical systems for alkaline water electrolysis as well as for polymer electrolyte-based electrolysis, so-called PEM electrolysis, are well-known and commercially available. The principles of water electrolysis are described as examples in Chapter 6.3.4 of Volkmar M. Schmidt's "Elektrochemische Verfahrenstechnik" (2003, Wiley-VCH-Verlag; ISBN 3-527-29958-0).

[0018] In a new development, e.g., chlor-alkali electrolysis, an additional catalyst, commonly referred to as a gas diffusion electrode (GDE), is arranged on the electrode-side current distributor of the electrolysis cells. When oxygen is used as the reactant gas, the gas diffusion electrode is also referred to as an oxygen-consuming cathode (ODC) or oxygen depolarized cathode (ODC). This is used, for example, in chlor-alkali electrolysis, whereby a modified cathode reaction with the addition of oxygen produces lye (OH -< ) instead of hydrogen (H 2 ). This modified cathode reaction is associated with a lower electrolysis voltage and a corresponding energy saving. For the example of the production of sodium hydroxide lye, the following reaction equation results: 4 NaCl + O 2 + 2 H 2 O → 2 Cl 2 + 4 NaOH

[0019] Examples of applications of a gas diffusion electrode include, in addition to chlor-alkali electrolysis, the electrolysis of CO 2 / CO described in the published patent application DE 102020207186 A, the production of CO from CO 2 with the intermediate step of the electrolytic production of formic acid described in DE 102020207186 A, or the electrolysis cell with gas diffusion electrode for CO 2 reduction described in DE 102020207186 A.

[0020] The use of a gas diffusion electrode is described below using chlor-alkali electrolysis as an example. An oxygen-consuming cathode is used as the gas diffusion electrode. The oxygen supply to the electrolyzers, necessary to maintain the oxygen-consuming reaction, can be achieved by simply flowing through the electrolysis cells, as described in DE 102013011298 A, for example, or by including an additional recycling step, as provided for in DE 10149779 A.

[0021] In any case, however, for the integration of this technology into the existing electrolyzer technology, the three-phase reaction between operating fluid, catalyst and oxygen gas must be solved as an additional task.

[0022] In a currently preferred technical embodiment, this takes place within the framework of a chlor-alkali electrolysis, as described, for example, in the published patent application EP 2746429 A, in such a way that the alkali hydroxide solution trickles down as a liquid film in front of the catalyst layer and runs out of the bottom of the electrolysis cell, while the oxygen gas is supplied from the back of the catalyst layer.

[0023] This means that the liquid volume on the cathode side of the electrolysis cells is very small compared to conventional chlor-alkali membrane electrolysis and, since the liquid no longer drains via an overflow but directly through an outlet at the bottom of the electrolysis cell, unlike in conventional electrolysis, the liquid content drains out of the cells within a very short time when the liquid circulation is interrupted.

[0024] A polarization current applied to protect the electrode coating and the catalyst of the oxygen-consuming cathode during commissioning / shutdown would have to be switched off immediately in the event of an interruption in the liquid circulation in order to avoid, for example, short circuits in an electrolysis cell that has run dry on the cathode side.

[0025] In newly designed chlor-alkali electrolysis plants that use oxygen-depleting cathode technology, the electrolyzers are connected in parallel to operating and start-up piping systems, similar to conventional electrolysis technology.

[0026] In contrast to conventional electrolysis technology, however, interruption of the liquid circulation (here on the cathode side) must be avoided when connecting or disconnecting individual electrolyzers from the operating piping system. Interruption of the liquid circulation and the resulting shutdown of the polarization current would lead to damage to the oxygen-depleting cathode.

[0027] This task could only be achieved with the current installations (separation of the systems via manual and process control-controlled valves), if at all, with great effort. Various previously manually operated valves (some with large nominal diameters up to DN 400) would have to be automated via control actuators and equipped with a control system that switches the cathode side of the electrolyzer from the start-up to the operating piping system without interrupting the fluid circulation and, in parallel, raises the pressure to the operating pressure. The process is reversed during shutdown.

[0028] It has now been found that the switching process for the electrode side, in particular for the gas diffusion electrode side, of the electrolyzers can be significantly simplified if the liquid-side decoupling of the electrolyzers from the operating piping system is no longer carried out via fittings, but via a liquid-filled siphon and the electrode-side, in particular the gas diffusion electrode side, gas pressure control is no longer carried out centrally for all electrolyzers together, but for each electrolyzer individually.

[0029] The invention therefore relates to a method for operating an electrolysis device with a plurality of electrolyzers selected from membrane electrolyzers, wherein at least each electrolyzer has at least one liquid outlet and at least one gas outlet on the anode side, and separately therefrom at least one liquid outlet and at least one gas outlet on the cathode side, and the anode spaces of these electrolyzers are connected to one another and separately therefrom the cathode spaces of these electrolyzers are connected to one another at least via a liquid inlet, a gas outlet and a liquid outlet, characterized in that the operating pressure of at least one liquid outlet is set lower than the operating pressure of the electrolyzers and a. the liquid discharges from the anode compartments or the cathode compartments or from both of these compartments of the electrolyzers take place via a pipeline siphon into the liquid discharge piping system for each electrolyzer, whereby the operating pressure of the electrolyzers is decoupled from the lower operating pressure of the subsequent liquid discharge piping system with each pipeline siphon on the liquid discharge side, and b. each gas discharge from the electrolyzers decoupled by a pipeline siphon takes place individually for each electrolyzer via an individual control valve per electrolyzer into the common gas discharge.

[0030] The electrolyzers of the electrolysis device in question can be operated with conventional electrodes. It is important that the operating pressure of at least one liquid discharge is set lower than the operating pressure of the electrolyzers during operation. It has proven particularly suitable according to the invention if the electrolysis device operated by the method is operated with gas diffusion electrodes on the anode side and / or the cathode side and a gas supply provided for this purpose.For this purpose, the operated electrolysis device contains a plurality of electrolyzers selected from membrane electrolyzers with a gas diffusion electrode, in particular with an oxygen-consuming cathode, wherein these electrolyzers are connected to one another at least via a gas supply on the gas diffusion electrode side, a liquid supply on the gas diffusion electrode side as a liquid supply, a residual gas discharge on the gas diffusion electrode side as a gas discharge and a liquid discharge on the gas diffusion electrode side as a liquid discharge.

[0031] In a preferred embodiment of the process, the electrolyzers are selected from alkali metal chloride membrane electrolyzers with an oxygen-consuming cathode as the gas diffusion electrode. A suitable alkali metal chloride for this embodiment is, for example, at least one alkali metal chloride selected from lithium chloride, sodium chloride, and potassium chloride, with sodium chloride being preferred.

[0032] The pipeline siphon allows the liquid circulation on the electrode side, preferably the gas diffusion electrode side, to continue to operate continuously during commissioning of an electrolyzer, while the gas pressure on the electrode side, preferably the gas diffusion electrode side, is adjusted to the operating value via the pressure control.

[0033] The liquid level in the leg of the pipeline siphon facing the electrolyte drain of the electrolyzer automatically adapts to the changed operating pressure of the electrolyzer when the drain side of the siphon drains into a pipeline system with a lower operating pressure.

[0034] Within the scope of one embodiment of the method according to the invention, it has been found to be advantageous if the operating pressure on the electrode side, preferably the gas diffusion electrode side, of the electrolyzers is between atmospheric pressure and 1 bar overpressure, preferably in a range of 100 to 500 mbar overpressure.

[0035] Unless explicitly defined otherwise, the reference pressure for specifying an overpressure is atmospheric pressure.

[0036] According to the invention, the operating pressure on the electrode side, preferably the gas diffusion electrode side, is the gas pressure in the electrode-side, preferably the gas diffusion electrode-side, gas space of the electrolysis cells.

[0037] It is preferred according to the invention if (i) the gas selected from product gas, residual gas, mixture of product gas and residual gas and (ii) the liquid First, they are led out of the electrolyzer together as a mixture in a discharge manifold of each individual electrolyzer, and then this mixture is subjected to a gas-liquid separation, wherein, after separation, the gas is discharged via the gas discharge line according to step b. and the liquid via the liquid discharge line according to step a. It has proven particularly suitable that, when using a gas diffusion electrode, said discharge manifold of the electrolyzer is operated on the gas diffusion electrode side.

[0038] In a preferred embodiment of the gas-liquid separation process, the gas and liquid are separated by their density difference in a pipeline connected to the discharge manifold, which is preferably vertical with a tolerance of ±15°, and then discharged separately. The gas flows upward toward the pressure regulator assigned to each electrolyzer; the liquid flows downward into the pipeline siphon.

[0039] Within the scope of a further embodiment of the method according to the invention, it is advantageous if the operating pressure on the outlet side of the pipeline siphon is lower than the operating pressure on the inlet side of this pipeline siphon, preferably between atmospheric pressure and 100 mbar overpressure.

[0040] According to the invention, the operating pressure on the outlet side of the pipeline siphon is the gas pressure in the gas space of the system components downstream of the siphon.

[0041] According to the invention, the operating pressure on the inlet side of the pipeline siphon is the gas pressure in the discharge manifold of the electrolyzer, which is connected on the one hand to the gas space of the electrolysis cells and on the other hand to the inlet of the siphon.

[0042] Particularly preferred is an embodiment of the method according to the invention in which (i) the operating pressure on the electrode side, preferably the gas diffusion electrode side, of the electrolyzers is between atmospheric pressure and 1 bar overpressure, preferably in a range of 100 to 500 mbar overpressure, and (ii) the operating pressure on the outlet side of the pipeline siphon is lower than the operating pressure on the inlet side of this pipeline siphon, preferably between atmospheric pressure and 100 mbar overpressure

[0043] Complex controls for automating switching from the start-up to the operating piping system are avoided by the invention. A separate piping system for liquid and gas removal during start-up is no longer required. The remaining necessary fittings can be dimensioned smaller because only the liquid flow at the pipeline siphon and no longer the two-phase flow of liquid and gas at the drain header of the electrolyzer needs to be adjusted / shut off. Possible operating errors and consequential damage to the electrolyzer during manual switching at the drain header are eliminated. According to the invention, the liquid is preferably removed in every operating mode of the electrolysis device according to step a. of the method according to the invention, in particular during start-up, shut-down and operation of the electrolysis device.

[0044] Any necessary switching of the electrolyte supply to the electrolyzer from the start-up to the operating piping system is not affected by the change on the outlet side; since only liquid flows need to be switched, the switching can be done manually or automatically without interruption.

[0045] Furthermore, it is not relevant for the change whether, in the embodiment of the electrolyzers operated with gas diffusion electrodes, these electrolyzers are equipped with gas recycling on the gas diffusion electrode side as defined in DE10149779 or whether the gas supply takes place in a simple flow-through manner as described, for example, in DE102013011298, since gas recycling would take place within the limits specified by the gas supply and discharge pressure control.

[0046] A further subject of the invention is an electrolysis device, in particular for the production of chlorine, comprising a plurality of electrolyzers selected from membrane electrolyzers, wherein at least each electrolyzer has at least one liquid outlet and at least one gas outlet on the anode side, and separately therefrom at least one liquid outlet and at least one gas outlet on the cathode side, and the anode spaces of these electrolyzers are connected to one another, and separately the cathode spaces of these electrolyzers are connected to one another at least via a liquid supply, a gas outlet and a liquid outlet, wherein a. in order to decouple the operating pressure of the electrolyzers from the operating pressure of the piping system of at least one of the liquid outlets on the liquid outlet side, the liquid outlets from the anode compartments or the cathode compartments or from both of these compartments of the electrolyzers are in fluid communication with the piping system of the liquid outlet via a pipeline siphon for each electrolyzer, and b. the gas outlet of all electrolyzers equipped with the aforementioned pipeline siphon is in fluid communication with the corresponding common gas outlet via an individual control valve per electrolyzer.

[0047] A "fluid connection" is understood by the person skilled in the art to mean a connection between at least two parts of the plant through which a substance, which can be present in any state of aggregation, can be transported as a material flow from one part of the plant (e.g. pipeline siphon) to another part of the plant (e.g. residual gas discharge), for example a pipeline.

[0048] In a preferred embodiment, the electrolysis device comprises gas diffusion electrodes on the anode side and / or the cathode side, particularly preferably on the cathode side. At least one gas diffusion electrode-side drain manifold is provided per electrolyzer, which is in fluid communication with the gas space and the liquid on the gas diffusion electrode side, and which branches into at least one gas diffusion electrode-side gas discharge line with a control valve and at least one gas diffusion electrode-side liquid drain with a pipeline siphon. This branching can be realized most preferably by a pipeline that is vertically routed with a tolerance of ±15°.

[0049] Within the scope of a further embodiment of the method according to the invention, it is considered advantageous if the electrolyzers of the electrolysis device each have a device for pressure control, which regulates the operating pressure on the inlet side of the pipeline siphon via the individual control valve of the gas discharge, preferably on the gas diffusion electrode side, in such a way that an overpressure is present, preferably so that the operating pressure on the outlet side of the pipeline siphon is lower than the operating pressure on the inlet side of the pipeline siphon, preferably between atmospheric pressure and 100 mbar overpressure.

[0050] The required dimensions of the pipeline siphon can be easily determined by a specialist. The height of the pipeline siphon is determined by the maximum pressure difference between the outlet side of the pipeline siphon, where the pressure is preferably between 0 mbar and 100 mbar overpressure, and the inlet side of the pipeline siphon, where the operating pressure is preferably between 0 mbar and 1 bar overpressure, particularly preferably between 0 mbar and 500 mbar overpressure, as well as the minimum density of the circulating fluid discharged from the electrode side via the pipeline siphon. The diameter of the siphon is characterized by the fact that the resulting pressure losses in the siphon can be neglected.

[0051] Also disclosed, but not claimed, is the use of a pipeline siphon on the electrode-side, preferably on the gas diffusion electrode-side, liquid outlet of membrane electrolyzers of an electrolysis device comprising a plurality of electrolyzers in the form of membrane electrolyzers (preferably with a gas diffusion electrode, in particular with an oxygen-consuming cathode), wherein at least each electrolyzer has at least one liquid outlet and at least one gas outlet on the anode side, and separately therefrom at least one liquid outlet and at least one gas outlet on the cathode side, and the anode spaces of these electrolyzers are connected to one another and separately therefrom the cathode spaces of these electrolyzers are connected to one another at least via a liquid inlet, a gas outlet and a liquid outlet,for decoupling the operating pressure of the electrolyzers from the operating pressure of the connected piping system on the liquid outlet side.

[0052] It is preferred if the gas space of the pipeline siphon is in controllable fluid communication with the gas outlet of the electrolysis device via a control valve. If the electrolysis device used comprises a plurality of electrolyzers in the form of membrane electrolyzers with a gas diffusion electrode, in particular with an oxygen-consuming cathode, the electrolyzers have at least one liquid outlet and at least one gas outlet at least on the anode side, and separately from this, at least one liquid outlet and at least one gas outlet on the cathode side, as well as a gas inlet on the gas diffusion electrode side, wherein the anode spaces of these electrolyzers are connected to one another, and separately from this, the cathode spaces of these electrolyzers are connected to one another, each at least via said gas inlet, a liquid inlet, a gas outlet, and a liquid outlet.

[0053] In use, it is preferred if the operating pressure of at least one liquid discharge is lower than the operating pressure of the electrolyzers.

[0054] An example of a prior art electrolysis device is shown in Figure Fig.1 For clarification and without limiting the invention thereto, Figure Fig.2a and Fig.2b each illustrates a possible electrolysis device according to the invention as an example. In Fig. 2a It is an electrolysis device that is equipped with conventional electrodes (not shown) on the cathode side and does not require a gas supply for the cathodic half-cell reaction. Fig. 2bAn electrolysis device is shown which is equipped with gas diffusion electrodes (not shown) on the cathode side, which require a gas supply for the cathodic half-cell reaction taking place there. An example of the installation of the pipeline siphon in an electrolysis device according to the invention is shown in Fig.3 For simplicity, only the cathode-side liquid and gas supply and discharge are shown as examples. The anode-side connections would be implemented analogously. The following reference symbols are used in the figures: 1.1 Gas supply for the gas diffusion electrode, for example oxygen-containing gas for the oxygen-consuming cathode, 1.11 Valve for the gas supply 1.12 Valve for the liquid supply during normal operation 1.13 Valve for the start-up / shutdown systems 1.14 Valve for the downstream piping system for the product removal during normal operation 1.15 Valve for the downstream piping system for the product removal during start-up / shutdown operation 1.2 Operating medium inflow for the cathode side during normal operation, for example diluted sodium hydroxide solution 1.21 Gas jet pump arranged in the gas supply 1.22 Gas supply valve 1.3 Operating medium inflow for the cathode side during the start-up and shut-down process, for example diluted sodium hydroxide solution 1.4 Outlet of the residual gas from the gas diffusion electrode reaction during normal operation 1.5 Outlet of the product-containing liquid from the electrolyzer, e.g. caustic soda in normal operation 1.6 Outflow of the residual gas from the gas diffusion electrode reaction during the start-up / shutdown process 1.7 Outflow of the product-containing liquid from the electrolyzer, e.g. sodium hydroxide solution during the start-up / shutdown process 1.8 Pressure control for residual gas (exhaust gas) 2.1 Gas supply for the gas diffusion electrode, e.g. oxygen-containing gas for the oxygen-consuming cathode, 2.11 Gas supply valve 2.12 Valve for the operating medium inflow during normal operation 2.13 Valve for the operating medium inflow during start-up / shutdown 2.14 Pipeline siphon 2.15 Shut-off valve, e.g. for use for maintenance work on the device 2.16 Control valve in the exhaust system for the residual gas removal of an electrolyzer 2.2 Operating medium inflow for the cathode side during normal operation, e.g. diluted sodium hydroxide solution 2.21 Gas jet pump arranged in the gas supply 2.22 Gas supply control valve 2.3 Operating medium inflow for the cathode side during start-up / shutdown, e.g. for diluted caustic soda 2.4 Gas discharge of the product gas and / or the residual gas for normal operation and shut-down and start-up 2.5 Liquid discharge of the liquid from the electrolyzer (e.g. strengthened caustic soda) and associated piping system 3.1 Drain manifold of the electrolyzer for the joint discharge of gas, e.g. residual gas, and liquid 3.2 Liquid discharge of the electrolyzer in the form of a discharge line for the liquid after separation from the gas by gravity, e.g. residual gas 3.3 Gas discharge of the electrolyzer in the form of a discharge line for the gas after separation from the liquid by gravity 3.4 Liquid discharge of the liquid from the electrolyzer (e.g. strengthened caustic soda) and associated piping system 3.5 Liquid level in the pipe siphon during start-up / shutdown operation (same pressure in the electrolyzer and in the drain-side piping system) 3.6 Different liquid levels in the pipe siphon during normal operation (higher pressure in the electrolyzer compared to the drain-side piping system) 3.7 Ventilation 3.8 Pipe siphon .

[0055] Fig. 2a illustrates an example of an electrolysis device according to the invention, which contains a number of n membrane electrolyzers, represented as "Electrolyzer 1" to "Electrolyzer 2...n", each of which is equipped on the anode side and cathode side with a normal electrode (not shown), ie, no gas diffusion electrode. For simplification, Fig.2aOnly the gas and liquid connections on the cathode side are shown. Here, the electrolyzers are further connected to each other via at least one liquid supply 2.2, one gas discharge 2.4, and one liquid discharge 2.5. The liquid discharge on the cathode side of each electrolyzer takes place via a pipe siphon 2.14 for decoupling the operating pressure of the electrolyzers on the liquid discharge side from the operating pressure of the subsequent piping system of the liquid discharge 2.5. Furthermore, the gas discharge from all electrolyzers equipped with the aforementioned pipe siphon 2.14 into the common gas discharge 2.4 takes place via an individual control valve 2.16 for each electrolyzer. On the anode side, the electrolyzers are also connected to each other via at least one liquid supply, one gas discharge, and one liquid discharge (not shown).

[0056] Fig. 2bshows an example of an electrolysis device according to the invention, which contains a number of n membrane electrolyzers, represented as "Electrolyzer 1" to "Electrolyzer 2...n" with a gas diffusion electrode (not shown) connected to the cathode side, wherein the electrolyzers are connected to one another at least via the gas diffusion electrode-side gas supply 2.1, a gas diffusion electrode-side liquid supply 2.2, a gas diffusion electrode-side residual gas discharge 2.4, and a gas diffusion electrode-side liquid discharge 2.5. For simplification, only two electrolyzers are shown. For further simplification, Fig.2bOnly the gas and liquid connections of the gas diffusion electrode side (i.e., the cathode side) are shown. The liquid outlet of an electrolyzer on the gas diffusion electrode side is via a pipeline siphon 2.14 for decoupling the operating pressure of the electrolyzers from the operating pressure of the connected piping system 2.5 on the liquid outlet side. Furthermore, the gas discharge on the gas diffusion electrode side of all electrolyzers equipped with the aforementioned pipeline siphon 2.14 into the common gas diffusion electrode-side residual gas discharge 2.4 is via an individual control valve 2.16 for each electrolyzer. On the anode side, the electrolyzers are also connected to one another via at least one liquid supply, gas discharge, and liquid discharge (not shown).

[0057] In each case Fig.2a and 2b drain side as in Fig.3As shown, gas (product gas or residual gas) and liquid are initially conducted together in the electrolyzer's horizontally arranged discharge manifold 3.1 toward the downstream piping systems. Gas and liquid then separate due to their density difference in a subsequent branch with a nearly vertical piping path (preferably ±15°); gas flows upward through the residual gas discharge 3.3 toward the pressure control 2.16 assigned to each electrolyzer. The liquid is discharged downward into the liquid discharge 3.2. Examples Example 1 (see Fig. 1): Sodium chloride electrolysis with ODC, state-of-the-art design analogous to conventional chlor-alkali electrolysis without ODC

[0058] Several electrolyzers (Electrolyzer 1, Electrolyzer 2...n), each with an oxygen-consuming cathode as a gas diffusion electrode on the cathode side, were operated in parallel. For simplification, Fig.1Only two electrolyzers are shown. In the production situation used, up to 10 or more electrolyzers were operated in parallel. For further simplification, Fig.1 only the gas and liquid connections on the cathode side are shown.

[0059] The raw materials oxygen (1.1) and diluted caustic soda (1.2, 1.3) were distributed from the upstream plants to the electrolyzers via piping systems. On the liquid side, there were separate systems for normal operation (1.2) and start-up / shutdown (1.3), since the start-up / shutdown processes generally follow a pressure / temperature profile that differs from normal operation.

[0060] The products of the electrolysis process, the residual gas from the oxygen-depolarized cathode reaction (1.4, 1.6), and the sodium hydroxide solution concentrated in the electrolyzer (1.5, 1.7) were collected in piping systems similar to the product feed and discharged to the downstream plants. Due to the different pressure levels during normal operation and startup / shutdown, separate piping systems were required for normal operation (1.4, 1.5) and startup / shutdown (1.6, 1.7).

[0061] During normal operation, the operating pressure was generally controlled via a central pressure control for the exhaust gas (1.8). The piping system for liquid removal during normal operation was at the same operating pressure as the electrolyzers. Startup and shutdown operations were generally carried out without pressure at atmospheric pressure.

[0062] The feed quantities to the electrolyzer and the respective path were adjusted / regulated via valves (1.11, 1.12, 1.13) in the feed line to the electrolyzer.

[0063] The product discharge path was also controlled by valves (1.14, 1.15) located on the electrolyzer. Since gas and liquid were initially discharged through the same line in the electrolyzer outlet, gas and liquid were separated after the outlet-side valves (1.14, 1.15) by pipes leading upwards and downwards.

[0064] For the gas-side operation, there were two alternative operating modes: At "Electrolyser 1" of the Fig.1 The simple flow of oxygen and subsequent removal of the residual gas are shown. At "Electrolyzer 2...n" of the Fig.1shown the recycling of oxygen-rich residual gas to the supply side via a gas jet pump (1.21) arranged in the gas supply as described in DE10149779, with, if necessary, an additional control valve (1.22).

[0065] The transition of an electrolyzer from start-up to normal operation was carried out analogously to conventional chlor-alkali electrolysis. First, the liquid and gas circulation was stopped by closing the valves to the start-up / shutdown systems (1.11, 1.13, 1.15). The pressure was then raised to the operating pressure, for example, via gas supply 1.11 or an additional auxiliary gas feed. After that, the liquid and gas circulation could be restarted at the operating systems (1.11, 1.12, 1.14). The shutdown was carried out analogously in reverse order.

[0066] As described above, this mode of operation carries the risk of damage to the oxygen-consuming cathode. Manual changeover carries the risk of operator error; the alternative automation with mechanically driven valves would be costly, as it would be required separately for each electrolyzer. Example 2 (see Fig.2b): Sodium chloride electrolysis with SVK, inventive design with drain siphon

[0067] Analogous to the previously described variant in Fig. 2b The raw materials oxygen (2.1) and diluted sodium hydroxide (2.2, 2.3) were distributed from the upstream plants to the electrolyzers via piping systems. On the liquid side, there were separate systems for normal operation (2.2) and start-up / shutdown (2.3), since the start-up / shutdown processes generally followed a pressure / temperature profile that differed from normal operation. For simplification, Fig.2b Again, only the gas and liquid connections on the cathode side are shown.

[0068] The products of the electrolysis process, the residual gas from the oxygen depletion cathode reaction (2.4) and the sodium hydroxide solution (2.5) concentrated in the electrolyzer were collected in pipeline systems analogous to the product feed and discharged to the downstream plants.

[0069] Due to the inventive solution for pressure separation at the electrolyzer, no separate drain piping systems are required for start-up / shutdown operation as in the previous example (cf. Fig.1 : 1.6, 1.7) was required. The gas-side pressure control was now carried out via control valves (2.16) assigned to each electrolyzer in the line to the exhaust system. On the liquid side, the strengthened caustic soda could drain freely via the siphon (2.14) into the downstream, pressureless piping system (2.5), regardless of the current operating pressure. The valve (2.15) allowed the electrolyzer to be separated from the piping system for maintenance work.

[0070] Analogous to the previously described variant in Fig.1 There are currently two alternative operating modes for gas-side operation: Electrolyzer 1 depicts the simple flow of oxygen followed by the removal of the residual gas. Electrolyzer 2...n depicts the recycling of oxygen-rich residual gas to the supply side via a gas jet pump (2.21) arranged in the gas supply, as described in DE10149779, with an additional control valve (2.22) if necessary. Both alternatives are equally applicable within the meaning of the invention described here.

[0071] Thanks to the described design, valves on the outlet side of the electrolyzer no longer needed to be switched between startup / shutdown and normal operation. This avoided interruptions in the liquid supply and the associated shutdown of the polarization rectifier. The potential for operating errors was reduced and the commissioning process was simplified. Furthermore, the outlet-side piping systems for startup / shutdown processes were eliminated. The switchover on the inlet side, which was still required, was uncritical and could be carried out seamlessly, as the inlet side does not significantly influence the operating pressure. Example 3 (see Fig.3): Outlet side of an electrolyzer in the inventive design with siphon

[0072] In the electrolyzer's discharge manifold (3.1), gas and liquid initially flowed together toward the downstream piping systems. Gas and liquid were then separated by their density difference in a subsequent vertical pipe; gas flowed upwards (3.3) toward the pressure regulator assigned to each electrolyzer (drawing Fig.2a & Fig.2b , 2.16); the liquid drained downwards (3.2).

[0073] The inventive design of the discharge liquid line as a siphon allowed the liquid to always drain freely toward the discharge piping system (3.4), regardless of the set operating pressure. During pressureless start-up / shutdown operation, the liquid level on the inlet side of the siphon was at the same height (3.5) as on the outlet side. During normal operation with positive operating pressure, the liquid level on the inlet side of the siphon was lower (3.6) in accordance with the ratio of operating pressure to liquid density. Intermediate states could freely occur when the operating pressure was increased from start-up operation to normal operation, or vice versa during shut-down.

[0074] In order to enable a smooth regulation of the gas pressure, the height of the siphon had to be selected so that even at the maximum possible operating pressure no gas can penetrate through the lower end.

[0075] The downstream piping system (3.4) was dimensioned to allow the fluid to drain freely. Overpressure was avoided, as was negative pressure, which could arise, for example, from siphoning. It is advantageous to design the downstream line as a gravity line or to incorporate additional ventilation (3.7) to prevent negative pressure.

Claims

1. Method for operating an electrolysis apparatus comprising a plurality of electrolyzers selected from membrane electrolyzers, wherein at least each electrolyzer on the anode side has at least one liquid drain and in each case at least one gas outlet, and separately therefrom on the cathode side has at least one liquid drain and in each case at least one gas outlet, and the anode spaces of these electrolyzers are connected to one another and separately therefrom the cathode spaces of these electrolyzers are connected to one another, in each case at least via a liquid feed (2.2), a gas discharge (2.4) and a liquid discharge (2.5), characterized in that the operating pressure of at least one liquid discharge (2.5) is set lower than the operating pressure of the electrolyzers and a. the liquid drains from the anode spaces or the cathode spaces or from both of these spaces of the electrolyzers are effected, per electrolyzer, via a pipeline siphon (2.14) into the pipeline system of the liquid discharge (2.5), as a result of which on the liquid drain side the operating pressure of the electrolyzers is decoupled by means of each pipeline siphon (2.14) from the lower operating pressure of the adjoining pipeline system of the liquid discharge (2.5), and b. each gas outlet of the electrolyzers decoupled by means of pipeline siphon (2.14) is effected individually for each electrolyzer via an individual control valve (2.16) per electrolyzer into the common gas discharge (2.4).

2. Method according to Claim 1, characterized in that the electrolysis apparatus operated contains a plurality of electrolyzers selected from membrane electrolyzers with gas diffusion electrode, in particular with oxygen-depolarized cathode, these electrolyzers being connected to one another at least via a gas-diffusion-electrode-side gas feed (2.1), via a gas-diffusion-electrode-side liquid feed as liquid feed (2.2), a gas-diffusion-electrode-side residual gas discharge as gas discharge (2.4), and a gas-diffusion-electrode-side liquid discharge as liquid discharge (2.5).

3. Method according to Claim 1 or 2, characterized in that the membrane electrolyzers are selected from alkali metal chloride membrane electrolyzers.

4. Method according to Claim 3, characterized in that the alkali metal chloride used is selected from lithium chloride, sodium chloride, potassium chloride, or mixtures thereof.

5. Method according to any of Claims 2 to 4, characterized in that the electrolyzers are selected from membrane electrolyzers with oxygen-depolarized cathode as gas diffusion electrode, the gas-diffusion-electrode-side gas feed (2.1) of which is connected to a source for an oxygen gas-containing gas stream.

6. Method according to Claim 2, characterized in that the electrolyzers are selected from membrane electrolyzers with gas diffusion electrode, the gas-diffusion-electrode-side gas feed (2.1) of which is connected to a source for a carbon dioxide-containing gas stream, in particular for a gas stream of carbon dioxide.

7. Method according to any of Claims 1 to 6, characterized in that the operating pressure of the electrolyzers is between atmospheric pressure and 1 bar positive pressure, preferably in a range from 100 to 500 mbar positive pressure.

8. Method according to any of Claims 1 to 5, characterized in that the operating pressure on the drain side of the pipeline siphon is lower than the operating pressure on the inlet side, preferably between atmospheric pressure and 100 mbar positive pressure.

9. Method according to any of the preceding claims, characterized in that (i) the gas, selected from product gas, residual gas, mixture of product gas and residual gas, and (ii) the liquid are first guided out of the electrolyzer together as a mixture in a drain manifold of each individual electrolyzer and this mixture is then subjected to a gas-liquid separation, where after separation has been effected the gas is guided via the gas outlet according to step b. and the liquid via the liquid drain according to step a.

10. Method according to any of the preceding claims, characterized in that the liquid discharge (2.5) is effected according to step a. in every operating mode of the electrolysis apparatus, in particular when starting up, shutting down and during operation of the electrolysis apparatus.

11. Electrolysis apparatus, in particular for the production of chlorine, containing a plurality of electrolyzers selected from membrane electrolyzers, wherein at least each electrolyzer on the anode side has at least one liquid drain and at least one gas outlet, and separately therefrom on the cathode side has at least one liquid drain and at least one gas outlet, and the anode spaces of these electrolyzers are connected to one another and separately the cathode spaces of these electrolyzers are connected to one another, in each case at least via a liquid feed (2.2), a gas discharge (2.4) and a liquid discharge (2.5), characterized in that a. for decoupling, on the liquid drain side, the operating pressure of the electrolyzers from the operating pressure of the pipeline system of at least one of the liquid discharges (2.5), the liquid drains from the anode spaces or the cathode spaces or from both of these spaces of the electrolyzers, per electrolyzer, are in fluid connection with the pipeline system of the liquid discharge (2.5) via a pipeline siphon (2.14), and b. the gas outlet of all electrolyzers that are equipped with the aforementioned pipeline siphon (2.14) is in fluid connection with the corresponding common gas discharge (2.4) via an individual control valve (2.16) per electrolyzer.

12. Electrolysis apparatus according to Claim 11, in particular for the production of chlorine, characterized in that the electrolyzers contain gas diffusion electrodes, preferably on the cathode side, the electrolyzers being additionally connected to one another at least via a gas feed (2.1) on the gas diffusion electrode side.