OPTIMISED LIQUID OUTFLOW FROM MEMBRANE ELECTROLYSERS

Abstract

A method comprising operating an electrolysis apparatus comprising a plurality of electrolyzers is provided. 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. 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, a gas discharge and a liquid discharge. The liquid drains from the anode spaces and/or the cathode spaces of the electrolyzers are effected, per electrolyzer, via a pipeline siphon into the pipeline system of the liquid discharge. An electrolysis apparatus and a method comprising decoupling an operating pressure on a liquid drain side of an electrolyzer are also provided.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application, filed under 35 U.S.C. § 371, of International Application No. PCT/EP2022/069970, which was filed on Jul. 17, 2022, and which claims priority to European Patent Application No. 21186388.1, which was filed on Jul. 19, 2021. The contents of each are hereby incorporated by reference into this specification.

FIELD

The invention relates to the provision of electrolysis apparatuses comprising membrane electrolyzers with optimized liquid drain, and to a method for operating these electrolysis apparatuses.

BACKGROUND

Electrolysis processes for the production of commodity chemicals must be developed for production on a large industrial scale (several 1000 t/a). In order to produce industrial-scale amounts of product by electrolysis processes, large-area electrolysis cells and electrolyzers with a large number of electrolysis cells are needed.

As is known from chloralkali electrolysis, electrolysis cells with an electrode area of more than 2 m² per electrolysis cell are usually used. The electrolysis cells are combined in groups of up to 100 in an electrolysis frame. Multiple frames 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 sodium hydroxide solution or hydrogen.

When conducting some electrolysis processes, at least one of the electrode half-reactions liberates a gaseous product, such as for example oxygen and hydrogen in water electrolysis, or chlorine and possibly hydrogen in chloralkali electrolysis. This formation of gas often results in a pressure difference between the operating pressure of the electrolyzer and the operating pressure of the liquid outlet from the electrolyzer. For example, in the conventional chloralkali 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 solution is given here by way of example:

2 NaCl+2 H₂O→Cl₂+2 NaOH+H₂

The pressure difference described above is also observed when operating electrolyzers with gas diffusion electrodes, in which the operating pressure of the electrolyzer is influenced at least by the reactant gas introduced or the residual gas thereof removed (e.g. oxygen when operating an oxygen-depolarized cathode or e.g. carbon dioxide when operating a CO₂ electrolysis with gas diffusion electrode), possibly in combination with the product gas formed during the electrolysis.

Irrespective of the type of electrode chosen, a plurality of electrolysis apparatuses (electrolyzers) are typically operated in parallel in corresponding electrolysis plants. As described for example in DE19641125, the electrolyzers each in turn comprise a plurality of individual electrolysis cells connected hydraulically in parallel, through which electric current flows in an electrical series circuit (“bipolar electrolyzers”) or else in an electrical parallel circuit (“unipolar electrolyzers”).

The supply of the electrolyzers with the operating media (e.g. brine for the anode side, lye for the cathode side) and the discharge 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 take place via operation pipeline systems that connect the electrolyzers to the appropriate processing plants and to which the electrolyzers are connected in parallel. Typical arrangements of electrolyzers and pipeline systems in an electrolysis cell hall can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, chapter “Chlorine”.

The electrolyzers are typically operated at elevated temperature and elevated pressure, typical values being approx. 40-90° C. and an operating pressure of approximately atmospheric pressure to 200-500 mbar positive pressure. An operating positive pressure in the electrolysis cell offers the advantage that the downstream processing steps of the gaseous products, such as for example chlorine and hydrogen (condensation of moisture), especially on the anolyte side of the electrolyzers, proceed more simply and the downstream compression has better starting conditions and any intermediate compressors that may be required can be omitted. However, the additional measures described below are necessary in order to pass between an unpressurized inoperative state, via start-up/shutdown operation, and normal operation at elevated pressure/temperature.

The operating media are usually fed to the electrolysis cells from below: the products leave the electrolysis cells in the overflow. As a result, the electrolysis cells are always filled with liquid during startup/shutdown, or with a liquid/gas mixture during normal operation due to the gases produced.

Typical designs of membrane electrolysis cells and electrolyzers and typical operating data are described, for example, in the Handbook of Chlor-Alkali Technology, Chapter 5 “Chlor-Alkali Technologies”.

For startup, the electrolyzers must be heated up to the operating temperature and pressurized to the operating pressure starting from the ambient conditions (atmospheric pressure, room temperature), and for shutdown they must be correspondingly cooled down and decompressed. Exemplary procedures for start-up and shutdown are described, for example, in the Handbook of Chlor-Alkali Technology, Chapter 13 “Plant Commissioning and Operation”.

A widespread technical solution for these start-up processes is to connect the electrolyzers to start-up circuits via separate start-up pipeline systems, so that individual electrolyzers can be brought into or taken out of operation independently of the others that are in operation.

A typical start-up process proceeds as follows:

An electrolyzer is filled with the operating media via the separate start-up circuits. Thereafter, the operating media are circulated and heated until the target temperature for operation has been reached. The 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 operation pipeline systems has been opened and the circulation via these operation pipeline systems has been restarted, the electric current (hereinafter referred to as electrolysis current) can be switched on and the electrolyzer can thus be brought into operation.

A typical shutdown process proceeds analogously to the start-up, but in inverse fashion:

After the electrolysis current has been switched off, the circulation via the operation pipeline systems is stopped and the electrolyzer is disconnected from these operation pipeline systems. Anode and cathode spaces are decompressed. It should be noted that the differential pressure between the anode and cathode spaces remains within the predefined operating parameters.

The circulation is restarted via the start-up pipeline systems and the electrolyzer is cooled down.

Depending on the specifics of the technology used (e.g. type of electrode coating), a low polarization current (of the order of magnitude of a few tens of amps) is applied via an auxiliary rectifier during start-up beginning from a certain temperature, this current protecting the electrode coating from damage: when shutting down, it is correspondingly switched off again on falling below a certain temperature and as soon as the anode space has been flushed free of chlorine. The polarization rectifier can remain switched on during the brief interruption of the circulation when switching over from the start-up circuits to the operation circuits during start-up, and vice versa during shutdown. Since the electrolysis cells of conventional design are filled with liquid during these processes, no damage can be caused by the polarization current.

The parameters mentioned above with respect to the general mode of operation of the electrolyzers and with respect to the structure of the electrolysis apparatus can be applied analogously to water electrolysis. Industrial plants for alkaline water electrolysis as well as for polymer electrolyte-based electrolysis, what is known as PEM electrolysis, are known and commercially available. The principles of water electrolysis are described for example in chapter 6.3.4 in Volkmar M. Schmidt in “Elektrochemische Verfahrenstechnik” (2003 Wiley-VCH-Verlag: ISBN 3-527-29958-0).

In a new development, for example chloralkali electrolysis, by means of an additional catalyst arranged on the electrode-side current distributor of the electrolysis cells, usually referred to as a gas diffusion electrode (GDE). When using oxygen as reactant gas, the gas diffusion electrode is also referred to as an oxygen-depolarized cathode (ODC). This is for example used in chloralkali electrolysis, whereby in a modified cathode reaction with the addition of oxygen, lye (OH⁻) is produced instead of hydrogen (H₂). This modified cathode reaction is associated with a lower electrolysis voltage and corresponding energy savings. The following reaction equation results for the example of the production of sodium hydroxide lye:

4 NaCl+O₂+2 H₂O→2 Cl₂+4 NaOH

In addition to chloralkali electrolysis, mention may for example be made, as applications of a gas diffusion electrode, of the electrolysis of CO₂/CO described in the laid-open specification DE 102020207186 A, the generation of CO from CO₂ with the intermediate step of the electrolytic generation of formic acid, described in DE 102020207186 A, or the electrolysis cell with gas diffusion electrode for CO₂ reduction, described in DE 102020207186 A.

The use of a gas diffusion electrode is described below by way of example for chloralkali electrolysis. In the example, an oxygen-depolarized cathode is used as a gas diffusion electrode. The supply of oxygen to the electrolyzers that is required to maintain the oxygen depolarization reaction can be effected by simple flow through the electrolysis cells, as described for example in DE 102013011298 A, or can include an additional recycling step, as provided for in DE 10149779 A.

In any case, however, for the integration of this technology into the existing electrolyzer technology, there is the additional task of resolving the three-phase reaction between the operating liquid, catalyst and oxygen gas.

In a currently preferred industrial embodiment, this is done within the context of a chloralkali electrolysis, such as for example described in laid-open specification EP 2746429 A, in such a way that the alkali metal hydroxide lye trickles down as a liquid film in front of the catalyst layer and drains out of the electrolysis cell at the bottom, while the oxygen gas is conveyed to the catalyst layer from the rear side.

As a result, the volume of liquid on the cathode side of the electrolysis cells is very small compared to conventional chloralkali membrane electrolysis and, since the liquid, in contrast to conventional electrolysis, no longer drains via an overflow but directly through an outlet at the bottom of the electrolysis cell, the liquid content drains out of the cells within a very short time if the liquid circulation is interrupted.

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

In newly designed chloralkali electrolysis plants that operate with oxygen-depolarized cathode technology, the electrolyzers are connected in parallel to operation and start-up pipeline systems, analogously to conventional electrolysis technology.

In contrast to conventional electrolysis technology, however, the interruption of the (in this case cathode-side) liquid circulation when connecting individual electrolyzers to the operation pipeline system or disconnecting individual electrolyzers from the operation pipeline system should be avoided. The interruption of the liquid circulation and the resulting switching off of the polarization current would result in damage to the oxygen-depolarized cathode.

With the customary installations to date (separation of the systems via manual and process control-actuated valves), this task could only be achieved with great effort, if at all, by automating various previously manually operated valves (some with large nominal diameters of up to DN 400) via variable-speed drives, which valves would have to be provided with a control system which, without interrupting the liquid circulation, switches the cathode side of the electrolyzer from the start-up pipeline system to the operation pipeline system and at the same time raises the pressure to the operating pressure. The inverse correspondingly applies when switching off.

SUMMARY

It has now been found that the switchover 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 operation pipeline system no longer takes place via valves, but via a liquid-filled siphon, and the electrode-side, in particular the gas-diffusion-electrode-side, gas pressure control is no longer performed centrally for all electrolyzers together, but for each electrolyzer individually.

One subject of the invention is therefore a 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, a gas discharge and a liquid discharge, characterized in that the operating pressure of at least one liquid discharge 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 into the pipeline system of the liquid discharge, as a result of which on the liquid drain side the operating pressure of the electrolyzers is decoupled by means of each pipeline siphon from the lower operating pressure of the adjoining pipeline system of the liquid discharge, and
  • b. each gas outlet of the electrolyzers decoupled by means of pipeline siphon is effected individually for each electrolyzer via an individual control valve per electrolyzer into the common gas discharge.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a prior art electrolysis apparatus.

FIG. 2a illustrates an example of an electrolysis apparatus according to the invention, which contains a number of n membrane electrolyzers shown as “electrolyzer 1” to “electrolyzer 2 . . . n”, each of which is equipped with a standard electrode (not shown) on the anode side and on the cathode side, i.e. no gas diffusion electrode.

FIG. 2b shows an example of an electrolysis apparatus within the meaning of the invention, which contains a number of n membrane electrolyzers shown as “electrolyzer 1” to “electrolyzer 2 . . . n” with gas diffusion electrode (not shown) connected on the cathode side.

FIG. 3 shows an example of the installation of a pipeline siphon in an electrolysis apparatus according to the invention.

DETAILED DESCRIPTION

The electrolyzers of said electrolysis apparatus can be operated with conventional electrodes. It is important that during operation the operating pressure of at least one liquid discharge is set lower than the operating pressure of the electrolyzers. It has been found to be suitable with preference according to the invention if the electrolysis apparatus operated by the method is operated on the anode side and/or cathode side with gas diffusion electrodes and a gas feed provided for this purpose. For this purpose, 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, via a gas-diffusion-electrode-side liquid feed as liquid feed, a gas-diffusion-electrode-side residual gas discharge as gas discharge, and a gas-diffusion-electrode-side liquid discharge as liquid discharge.

In a preferred embodiment of the method, the electrolyzers are selected from alkali metal chloride membrane electrolyzers with oxygen-depolarized cathode as gas diffusion electrode.

A suitable alkali metal chloride that can be used 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.

As a result of the pipeline siphon, the electrode-side, preferably the gas-diffusion-electrode-side, liquid circulation can continue to be operated continuously when an electrolyzer is being started up, while the electrode-side, preferably the gas-diffusion-electrode-side, gas pressure is adjusted to the operating value via the pressure control means.

The liquid level in the leg of the pipeline siphon facing the electrolyte drain of the electrolyzer adapts itself automatically 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.

In one embodiment of the method according to the invention, it has proven 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 positive pressure, preferably in a range from 100 to 500 mbar positive pressure.

According to the invention, the reference pressure for the specification of a positive pressure is atmospheric pressure, unless explicitly defined otherwise.

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

It is preferable according to the invention when

• • (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 caused via the gas outlet according to step b. and the liquid via the liquid drain according to step a. It has proven to be suitable with preference that, when using a gas diffusion electrode, said drain manifold of the electrolyzer is operated on the gas diffusion electrode side.

In a preferred embodiment of the gas-liquid separation of the method, gas and liquid are separated by their density difference in a pipeline which adjoins the drain manifold and is preferably guided vertically with a tolerance of ±15°, and are conducted off separately. The gas flows upwards in the direction of the pressure control means assigned to each electrolyzer; the liquid drains down into the pipeline siphon.

In a further embodiment of the method according to the invention, it is advantageous when the operating pressure on the drain 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 positive pressure.

According to the invention, the operating pressure on the drain side of the pipeline siphon means the gas pressure in the gas space of the plant components following the siphon on the drain side.

According to the invention, the operating pressure on the inlet side of the pipeline siphon means the gas pressure in the drain 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.

Particularly preferably suitable 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 positive pressure, preferably in a range from 100 to 500 mbar positive pressure, and
  • (ii) the operating pressure on the drain 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 positive pressure.

Complex control means for automating a switchover from the start-up pipeline system to the operation pipeline system are avoided by the invention. A separate pipeline system for liquid and gas discharge when starting up is no longer required. The remaining necessary valves can be dimensioned smaller because only the liquid flow at the pipeline siphon and no longer the biphasic 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 in the case of a manual switchover at the drain header are eliminated. According to the invention, the liquid discharge is effected according to step a. of the method according to the invention in every operating mode of the electrolysis apparatus, in particular when starting up, shutting down and during operation of the electrolysis apparatus.

The possibly required switchover of the electrolyte supply to the electrolyzer from the start-up pipeline system to the operation pipeline system is not affected by the change on the drain side: since only liquid streams need to be switched over, the switchover can be effected manually or automatically without interruption.

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 within the meaning of DE10149779 or the gas is supplied in simple throughflow as described for example in DE102013011298, since a gas recycling would be effected within the limits defined by gas feed and discharging pressure control.

A further subject of the invention is an 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, a gas discharge and a liquid discharge, wherein

• • 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, 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 via a pipeline siphon, and
  • b. the gas outlet of all electrolyzers that are equipped with the aforementioned pipeline siphon is in fluid connection with the corresponding common gas discharge via an individual control valve per electrolyzer.

A “fluid connection” is understood by those skilled in the art to mean a connection between at least two plant components, through which a substance, which can be in any state of matter, can be transported as a material flow from one plant component (e.g. pipeline siphon) to another plant component (e.g. residual gas discharge), for example a pipeline.

In a preferred embodiment, the electrolysis apparatus has anode-side and/or cathode-side, particularly preferably cathode-side, gas diffusion electrodes, where at least one gas-diffusion-electrode-side drain manifold is present per electrolyzer, is in fluid connection with the gas space and the gas-diffusion-electrode-side liquid, and branches into at least one gas-diffusion-electrode-side gas outlet with control valve and at least one gas-diffusion-electrode-side liquid drain with pipeline siphon. This branching can very particularly preferably be implemented by a pipeline that is guided vertically with a tolerance of ±15°.

In a further embodiment of the method according to the invention, it is considered advantageous when the electrolyzers of the electrolysis apparatus each have an apparatus for pressure control which controls the operating pressure on the inlet side of the pipeline siphon via the individual control valve of the, preferably gas-diffusion-electrode-side, gas outlet in such a way that a positive pressure is present, preferably such that the operating pressure on the drain 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 positive pressure.

The required dimensioning of the pipeline siphon can be readily determined by those skilled in the art for the use according to the invention. The height of the pipeline siphon results from the maximum pressure difference between the drain side of the pipeline siphon, where the pressure is preferably between 0 mbar and 100 mbar positive pressure, and the inlet side of the pipeline siphon, where the operating pressure is preferably between 0 mbar and 1 bar positive pressure, particularly preferably is between 0 mbar and 500 mbar positive pressure, and also from the minimum density of the circulation liquid discharged on the electrode side via the pipeline siphon. The diameter of the siphon is characterized in that the pressure losses arising in the siphon can be disregarded.

A further subject of the invention is the use of a pipeline siphon at the electrode-side, preferably at the gas-diffusion-electrode-side, liquid drain of membrane electrolyzers of an electrolysis apparatus containing a plurality of electrolyzers in the form of membrane electrolyzers (preferably with gas diffusion electrode, in particular with oxygen-depolarized cathode). 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, a gas discharge and a liquid discharge, for decoupling, on the liquid drain side, the operating pressure of the electrolyzers from the operating pressure of the adjoining pipeline system.

It is preferable for the gas space of the pipeline siphon to be in fluid connection, which is controllable by means of control valve, with the gas discharge of the electrolysis apparatus.

If the use involves an electrolysis apparatus containing a plurality of electrolyzers in the form of membrane electrolyzers with gas diffusion electrode, in particular with oxygen-depolarized cathode, the electrolyzers at least on the anode side have at least one liquid drain and in each case at least one gas outlet, and separately therefrom on the cathode side have at least one liquid drain and in each case at least one gas outlet, and on the gas diffusion electrode side have a gas inlet, wherein 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 said gas feed, via a liquid feed, via a gas discharge and via a liquid discharge.

In the use according to the invention, it is preferable for the operating pressure of at least one liquid discharge to be lower than the operating pressure of the electrolyzers.

An example of a prior art electrolysis apparatus is shown in FIG. 1. For the purposes of clarification and without restricting the invention thereto, a possible electrolysis apparatus according to the invention is illustrated as an example in each of FIG. 2a and FIG. 2b. FIG. 2a shows an electrolysis apparatus which is equipped on the cathode side in each case with conventional electrodes (not shown) and does not require a gas feed for the cathodic half-cell reaction. FIG. 2b shows an electrolysis apparatus which is equipped on the cathode side with gas diffusion electrodes (not shown) which require a gas feed for the cathodic half-cell reaction that proceeds there. An example of the installation of the pipeline siphon in an electrolysis apparatus according to the invention is depicted in FIG. 3. For simplification, only the cathode-side liquid and gas feed and discharge have been shown by way of example. The connections on the anode side would be analogous. The following reference signs have been used in the figures:

• • 1.1 Gas feed for the gas diffusion electrode, for example oxygen-containing gas for the oxygen-depolarized cathode,
  • 1.11 Valve for gas feed
  • 1.12 Valve for liquid feed in normal operation
  • 1.13 Valve for start-up/shutdown systems
  • 1.14 Valve for drain-side pipeline system of the product discharge in normal operation
  • 1.15 Valve for drain-side pipeline system of the product discharge in 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 feed
  • 1.22 Valve for gas feed
  • 1.3 Operating medium inflow for the cathode side during the start-up and shutdown process, for example diluted sodium hydroxide solution
  • 1.4 Outflow of the residual gas of the gas diffusion electrode reaction in normal operation
  • 1.5 Outflow of the product-containing liquid from the electrolyzer, e.g. sodium hydroxide solution in normal operation
  • 1.6 Outflow of the residual gas of 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 means for residual gas (offgas)
  • 2.1 Gas feed for the gas diffusion electrode, for example oxygen-containing gas for the oxygen-depolarized cathode,
  • 2.11 Valve for gas feed
  • 2.12 Valve for operating medium inflow during normal operation
  • 2.13 Valve for operating medium inflow during start-up/shutdown
  • 2.14 Pipeline siphon
  • 2.15 Shut-off valve, e.g. to be used for maintenance work on the apparatus
  • 2.16 Control valve in the offgas system of the residual gas discharge of an electrolyzer
  • 2.2 Operating medium inflow for the cathode side during normal operation, for example diluted sodium hydroxide solution
  • 2.21 Gas jet pump arranged in the gas feed
  • 2.22 Control valve for gas feed
  • 2.3 Operating medium inflow for the cathode side during start-up/shutdown, for example for diluted sodium hydroxide solution
  • 2.4 Gas discharge of the product gas and/or the residual gas for normal operation and start-ups/shutdowns
  • 2.5 Liquid discharge of the liquid from the electrolyzer (e.g. concentrated sodium hydroxide solution) and associated pipeline system
  • 3.1 Drain manifold of the electrolyzer for joint discharge of gas, e.g. residual gas, and liquid
  • 3.2 Liquid drain of the electrolyzer in the form of an outflow conduit for the liquid after separation from the gas, e.g. residual gas, has been performed by gravity
  • 3.3 Gas outlet of the electrolyzer in the form of an outflow conduit for the gas after separation from the liquid has been performed by gravity
  • 3.4 Liquid discharge of the liquid from the electrolyzer (e.g. concentrated sodium hydroxide solution) and associated pipeline system
  • 3.5 Liquid level in the pipeline siphon during start-up/shutdown operation (same pressure in the electrolyzer and in the drain-side pipeline system)
  • 3.6 Different liquid levels in the pipeline siphon during normal operation (higher pressure in the electrolyzer compared to the drain-side pipeline system)
  • 3.7 Venting
  • 3.8 Pipeline siphon

FIG. 2a illustrates an example of an electrolysis apparatus according to the invention, which contains a number of n membrane electrolyzers shown as “electrolyzer 1” to “electrolyzer 2 . . . n”, each of which is equipped with a standard electrode (not shown) on the anode side and on the cathode side, i.e. no gas diffusion electrode. For simplification, only the gas and liquid connections on the cathode side are shown in FIG. 2a. Here the electrolyzers are further connected to one another at least via a liquid feed 2.2, a gas discharge 2.4 and a liquid discharge 2.5. The liquid drain is effected on the cathode side, per electrolyzer, via a pipeline siphon 2.14 for the liquid-drain-side decoupling of the operating pressure of the electrolyzers from the operating pressure of the adjoining pipeline system of the liquid discharge 2.5. The gas outlet of all electrolyzers equipped with aforementioned pipeline siphon 2.14 is further effected into the common gas discharge 2.4 via an individual control valve 2.16 present for each individual electrolyzer. On the anode side, the electrolyzers are likewise connected to one another at least via a liquid feed, a gas discharge and a liquid discharge (not shown).

FIG. 2b shows an example of an electrolysis apparatus within the meaning of the invention, which contains a number of n membrane electrolyzers shown as “electrolyzer 1” to “electrolyzer 2 . . . n” with gas diffusion electrode (not shown) connected on the cathode side, the electrolyzers being connected to one another at least via the gas-diffusion-electrode-side gas feed 2.1, a gas-diffusion-electrode-side liquid feed 2.2, a gas-diffusion-electrode-side residual gas discharge 2.4, and a gas-diffusion-electrode-side liquid discharge 2.5. Only two electrolyzers have been shown for simplification. For further simplification, only the gas and liquid connections on the gas diffusion electrode side (i.e. the cathode side) are shown in FIG. 2b. The gas-diffusion-electrode-side liquid drain of an electrolyzer is effected via a pipeline siphon 2.14 for the liquid-drain-side decoupling of the operating pressure of the electrolyzers from the operating pressure of the adjoining pipeline system 2.5. The gas-diffusion-electrode-side gas outlet of all electrolyzers equipped with aforementioned pipeline siphon 2.14 is further effected into the common gas-diffusion-electrode-side residual gas discharge 2.4 via an individual control valve 2.16 present for each individual electrolyzer. On the anode side, the electrolyzers are likewise connected to one another at least via a liquid feed, a gas discharge and a liquid discharge (not shown).

In each of FIGS. 2a and 2b, as shown in FIG. 3, on the drain side firstly gas (product gas or residual gas) and liquid are guided in the horizontally running drain manifold 3.1 of the electrolyzer together in the direction of the further-continuing pipeline systems. Gas and liquid then separate because of their density difference in an adjoining branch with a virtually vertical pipeline run (preferably ±15°); gas flows through the outlet of residual gas 3.3 upwards in the direction of the pressure control means 2.16 assigned to each electrolyzer. The liquid is discharged downwards into the outlet for the liquid 3.2.

EXAMPLES

Example 1 (cf. FIG. 1)

Sodium Chloride Electrolysis with ODC, Embodiment Corresponding to the Prior Art in Analogy to Conventional Chloralkali Electrolysis Without ODC

A plurality of electrolyzers (electrolyzer 1, electrolyzer 2 . . . n), each with an oxygen-depolarized cathode connected on the cathode side as gas diffusion electrode, were operated in parallel. Only two electrolyzers are shown in FIG. 1 for simplification. In the production site used, up to 10 or more electrolyzers were operated in parallel. For further simplification, only the gas and liquid connections on the cathode side are shown in FIG. 1.

The raw materials oxygen (1.1) and diluted sodium hydroxide solution (1.2, 1.3) were distributed from the upstream units to the electrolyzers via pipeline 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.

The products of the electrolysis process, the residual gas of 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 pipeline systems analogously to the product feed and conducted off into the downstream units. Due to the different pressure levels in normal operation and during start-up/shutdown, separate pipeline systems were required here for normal operation (1.4, 1.5) and start-up/shutdown (1.6, 1.7).

In normal operation, the operating pressure was generally controlled via a central pressure control means for the offgas (1.8). The pipeline system for the liquid discharge during normal operation was under the same operating pressure as the electrolyzers. The start-up/shutdown operation was generally effected in unpressurized fashion at atmospheric pressure.

The amounts fed to the electrolyzer and the respective path were adjusted/controlled via valves (1.11, 1.12, 1.13) in the inlet to the electrolyzer.

The path for the discharge of the products was likewise adjusted via valves (1.14, 1.15) arranged on the electrolyzer. Since gas and liquid were initially discharged through the same conduit in the drain of the electrolyzers, gas and liquid were separated downstream of the drain-side valves (1.14, 1.15) by pipelines correspondingly leading away upwards and downwards.

There were two alternative modes of operation for the gas-side operation: The simple throughflow of oxygen and subsequent discharge of the residual gas is shown on “electrolyzer 1” in FIG. 1. The recycling of oxygen-rich residual gas to the feed side via a gas jet pump (1.21) arranged in the gas feed as described in DE10149779, possibly with additional control valve (1.22), is shown on “electrolyzer 2 . . . n” in FIG. 1.

An electrolyzer was switched over from start-up to normal operation in analogy to conventional chloralkali electrolysis, by first stopping the liquid and gas circulation 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 the gas feed 1.11 or an additional auxiliary gas supply. Thereafter, the liquid and gas circulation on the operating systems (1.11, 1.12, 1.14) could be started again. Shutdown was effected analogously in reverse order.

As described above, this mode of operation harbors the risk of damage to the oxygen-depolarized cathode. In the case of manual switching, there is the risk of operating errors: the alternative automation with mechanically driven valves would be expensive since it would be required separately for each electrolyzer.

Example 2 (cf. FIG. 2b)

Sodium Chloride Electrolysis with ODC, Embodiment According to the Invention with Drain Siphon

Analogously to the variant described above in FIG. 2b the raw materials oxygen (2.1) and diluted sodium hydroxide solution (2.2, 2.3) were distributed from the upstream units to the electrolyzers via pipeline 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 differs from normal operation. For simplification, only the gas and liquid connections on the cathode side are shown in turn in FIG. 2b.

The products of the electrolysis process, the residual gas of the oxygen-depolarized cathode reaction (2.4) and the sodium hydroxide solution concentrated in the electrolyzer (2.5) were collected in pipeline systems analogously to the product feed and conducted off into the downstream units.

Due to the solution according to the invention for the pressure separation at the electrolyzer, there is no requirement for separate drain pipeline systems for the start-up/shutdown operation as in the previous example (cf. FIG. 1: 1.6, 1.7). The pressure on the gas side was controlled via control valves (2.16) in the conduit to the offgas system, which were now assigned to each electrolyzer. On the liquid side, regardless of the respective operating pressure, the concentrated sodium hydroxide solution was able to drain freely via the siphon (2.14) into the downstream pipeline system (2.5), which was to be operated at ambient pressure. With the valve (2.15), the electrolyzer could still be disconnected from the pipeline system for maintenance work.

Analogously to the variant described above in FIG. 1, there are currently two alternative modes of operation for the gas-side operation: The simple throughflow of oxygen and subsequent discharge of the residual gas is shown on electrolyzer 1. The recycling of oxygen-rich residual gas to the feed side via a gas jet pump (2.21) arranged in the gas feed as described in DE10149779, possibly with additional control valve (2.22), is shown on “electrolyzer 2 . . . n”. Within the meaning of the invention described here, both alternatives are equally usable.

As a result of the embodiment described, it was no longer necessary on the drain side of the electrolyzer for valves to be switched over between start-up/shutdown operation and normal operation. An interruption in the liquid feed and an associated switching-off of the polarization rectifier were thus avoided. The potential for operating errors was reduced and the start-up process was simplified. Furthermore, the drain-side pipeline systems for the start-up and shutdown processes can be omitted. The switchover that was still required on the inlet side was not critical and could be effected in a continuous transition since the inlet side does not have any significant influence on the operating pressure.

Example 3 (cf. FIG. 3)

Drain Side of an Electrolyzer in the Embodiment According to the Invention with Siphon

In the drain manifold (3.1) of the electrolyzer, gas and liquid initially flowed together in the direction of the further-continuing pipeline systems. Gas and liquid were then separated by their density difference in an adjoining vertical pipeline: gas flowed upwards (3.3) in the direction of the pressure control means assigned to each electrolyzer (drawing FIG. 2a and FIG. 2b, 2.16); the liquid drained down (3.2).

As a result of the embodiment according to the invention of the discharging liquid conduit as siphon, the liquid could always drain freely in the direction of the discharging pipeline system (3.4), regardless of the operating pressure that was set in each case. During unpressurized start-up/shutdown operation, the liquid level in the inlet side of the siphon was established at the same level (3.5) as on the drain side. During normal operation with positive operating gauge pressure, the liquid level in the inlet side of the siphon was lower according to the ratio of operating pressure to liquid density (3.6). Intermediate states could arise freely when the operating pressure was being raised from start-up operation to normal operation or vice versa when shutting down.

In order to enable the gas pressure to be controlled smoothly, the height of the siphon had to be selected in such a way that no gas could break through the lower end, even at the maximum possible operating pressure.

The further-continuing pipeline system (3.4) was dimensioned such that the liquid can drain freely. Positive pressure was avoided, as was negative pressure, which could arise, for example, from siphon effects. It is advantageous to configure the discharging conduit as a gravity conduit or an additional vent (3.7) that avoids negative pressure.

Claims

1. A method comprising operating an electrolysis apparatus, the 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, a gas discharge and a liquid discharge, wherein the operating pressure of at least one liquid discharge is set lower than the operating pressure of the electrolyzers, and wherein:

(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 into a pipeline system of the liquid discharge, as a result of which on the liquid drain side the operating pressure of the electrolyzers is decoupled by means of each pipeline siphon 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 is effected individually for each electrolyzer via an individual control valve per electrolyzer into a common gas discharge.

2. The method as claimed in claim 1, wherein the electrolysis apparatus operated contains a plurality of electrolyzers selected from membrane electrolyzers with gas diffusion electrode, these electrolyzers being connected to one another at least via a gas-diffusion-electrode-side gas feed, via a gas-diffusion-electrode-side liquid feed as liquid feed, a gas-diffusion-electrode-side residual gas discharge as gas discharge, and a gas-diffusion-electrode-side liquid discharge as liquid discharge.

3. The method as claimed in claim 1, wherein the membrane electrolyzers are selected from alkali metal chloride membrane electrolyzers.

4. The method as claimed in claim 3, wherein the alkali metal chloride used is selected from lithium chloride, sodium chloride, potassium chloride, or mixtures thereof.

5. The method as claimed in claim 2, wherein the electrolyzers are selected from membrane electrolyzers with oxygen-depolarized cathode as gas diffusion electrode, the gas-diffusion-electrode-side gas feed of which is connected to a source for an oxygen gas-containing gas stream.

6. The method as claimed in claim 2, wherein the electrolyzers are selected from membrane electrolyzers with gas diffusion electrode, the gas-diffusion-electrode-side gas feed of which is connected to a source for a carbon dioxide-containing gas stream.

7. The method as claimed in claim 1, wherein the operating pressure of the electrolyzers is between atmospheric pressure and 1 bar positive pressure.

8. The method as claimed in claim 1, wherein the operating pressure on the drain side of the pipeline siphon is lower than the operating pressure on the inlet side.

9. The method as claimed in claim 1, wherein of each individual electrolyzer are first guided out 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 caused via the gas outlet according to step (b) and the liquid via the liquid drain according to step (a).

(i) a gas, selected from product gas, residual gas, or a mixture of product gas and residual gas, and

(ii) a liquid

10. The method as claimed in claim 1, wherein the liquid discharge is effected according to step (a) in every operating mode of the electrolysis apparatus.

11. An electrolysis apparatus, 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, a gas discharge and a liquid discharge, wherein:

(a) for decoupling, on the liquid drain side, the operating pressure of the electrolyzers from the operating pressure of a pipeline system of at least one of the liquid discharges, 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 via a pipeline siphon, and

(b) the gas outlet of all electrolyzers that are equipped with the pipeline siphon is in fluid connection with a corresponding common gas discharge via an individual control valve per electrolyzer.

12. The electrolysis apparatus as claimed in claim 11, wherein the electrolyzers contain gas diffusion electrodes, the electrolyzers being additionally connected to one another at least via a gas feed on the gas diffusion electrode side.

13. A method comprising decoupling an operating pressure on a liquid drain side of an electrolyzer of an electrolysis apparatus from an operating pressure of an adjoining pipeline system of a liquid discharge with a pipeline siphon at a liquid drain of the electrolyzer, the electrolysis apparatus containing a plurality of membrane electrolyzers, the electrolyzers being connected to one another at least via a liquid feed, a gas discharge and the liquid discharge.

14. The method as claimed in claim 2, wherein the gas diffusion electrode comprises an oxygen-depolarized cathode.

15. The method as claimed in claim 6, wherein the gas-diffusion-electrode-side gas feed is connected to a gas stream of carbon dioxide.

16. The method as claimed in claim 7, wherein the operating pressure of the electrolyzers is in a range of from 100 to 500 mbar positive pressure.

17. The method as claimed in claim 8, wherein the operating pressure on the drain side of the pipeline siphon is between atmospheric pressure and 100 mbar positive pressure.

18. The method as claimed in claim 1, wherein the liquid discharge is effected according to step (a) when starting up, shutting down and during operation of the electrolysis apparatus.

19. A method comprising producing chlorine with the electrolysis apparatus as claimed in claim 11.

20. The electrolysis apparatus as claimed in claim 12, wherein the electrolyzers contain gas diffusion electrodes on the cathode side.