BE1031653B1 - Degassing device for an electrolysis plant and electrolysis plant - Google Patents

Abstract

The present invention relates to the technical field of electrolysis and more particularly to an electrolysis plant for the production of dihydrogen (H2) and dioxygen (O2) by electrolysis of water. According to the present invention, the plant comprises a degassing device (1) which comprises a degassing chamber (14) provided with an opening for a gas-liquid mixture supply pipe (11), an opening for a liquid discharge pipe (12) arranged below the level of the gas-liquid interface (15) of the degassing chamber (14) and an opening for a gas discharge pipe (13) arranged above the level of the gas-liquid interface (15) of the degassing chamber (14). Furthermore, in the device of the invention, the degassing chamber (14) comprises a cyclonic gas-liquid separator (2) supplied with gas-liquid mixture by the gas-liquid mixture supply pipe (11) of the degassing chamber (14).

Description

1 BE2023/5434

Degassing device for an electrolysis plant and electrolysis plant.

[0001] Description

[0002] The present invention relates to the technical field of electrolysis and more particularly to an electrolysis installation for the production of dihydrogen (H>) and dioxygen (07) by electrolysis of water. According to a first of its aspects, the invention relates to a degassing device which can be used in an installation for the production of dihydrogen and dioxygen by electrolysis of water. Another aspect of the invention relates to an installation for the production of dihydrogen and dioxygen by electrolysis of water containing such a degassing device.

[0003] Indication of prior art

[0004] The need to reduce greenhouse gas production and use renewable energy is now well known. Dihydrogen is an alternative to hydrocarbons because it is an easily storable energy vector, unlike electricity, and its oxidation releases very significant energy (285 kJ/mole).

[0005] Several ways of producing gaseous dihydrogen are known; the most advantageous is to electrolyze the water molecule because it is a high-yield reaction that does not directly produce CO) unlike the massively used processes of reforming methane, coal and hydrocarbons.

[0006] Three main types of electrolysers are known for the electrolysis of water: - alkaline electrolysers (AWE), which are characterised by the use of a liquid electrolyte which allows the transfer of hydroxyl ions (OH7) from the cathode to the anode, - high-temperature electrolysers, the electrolyte of which is a ceramic; and - membrane electrolysers (PEM), the electrolyte of which is a proton-conducting ion exchange membrane.

[0007] In all three cases, the system must be supplied with water of very high purity (supplying, in the case of alkaline electrolysers, an electrolytic solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH)). In the remainder of the description, for reasons of conciseness, reference will be made to an alkaline electrolyser, but it is understood that the present invention also applies to the membrane electrolyser (for example a proton exchange membrane).

[0008] According to the well-known method of the prior art, an electrolytic solution (known by the English term lye) is brought into a set of electrolytic cells (known as an electrolyzer stack) through a specific inlet. The electrolytic solution passes through the electrolyzer stack. The water is decomposed into gaseous molecules of dihydrogen,

H>, at the cathode, and dioxygen, O7, at the anode. A diaphragm usually separates the anode from the cathode so that, under normal conditions, dihydrogen and dioxygen do not mix. The installation includes an outlet for dihydrogen and electrolyte flowing on the cathode side (catholyte) and an outlet for dioxygen and electrolyte flowing on the anode side (anolyte). In other words, these are two separate flows so that there is a gas-liquid separator dedicated to the separation of dihydrogen from the catholyte, and a gas-liquid separator for the separation of dioxygen from the anolyte. The liquid outlets of the two gas-liquid separators are then mixed before feeding the electrolyzer stack again. In both flows, at the outlet of the electrolyzer stack, the liquid phase (lye) is loaded with gas bubbles. At the outlet of the gas-liquid separator, there are only a few gas bubbles left in the lye evacuated through the lower orifice of the gas-liquid separator dedicated to the liquid phase, while the majority of the gas phase is extracted from the gas-liquid separator through the upper orifice of the gas-liquid separator. For various reasons, it is important to separate the gas from the lye. First of all, the more gas is separated from the electrolyte, the more gas is produced, which contributes to the good electrochemical efficiency of the process. Then, the H/02 mixture is highly explosive. If the separation is not carried out correctly, a significant quantity of gas, commonly called "residual gas", is entrained at the liquid outlet of the gas-liquid separator. During the next circulation in the electrolyser stack (the electrolyte rotates in a closed loop), part of this gas passes into the other compartment and therefore to the wrong side.

[0009] These gas-liquid separators, well known in the art, comprise a degassing chamber provided with an opening for a gas-liquid mixture supply pipe, an opening for a liquid discharge pipe arranged below the level of the gas-liquid interface of the degassing chamber and an opening for a gas discharge pipe arranged above the level of the gas-liquid interface of the degassing chamber. Each of the outlets of the electrolyser stack for the dihydrogen-lye mixtures on the one hand and the dioxygen-lye mixtures on the other hand is connected to such a gas-liquid separator. Figs. 1 and 2 schematically represent known gas-liquid separators aligned respectively along a horizontal or vertical main axis A. Document FR-A1-2949479 for example describes such gas-liquid separators.

[0010] The principle of extraction of gas bubbles from the liquid phase is based on Archimedes' principle. The efficiency of the separation depends mainly on gravity and the difference in density between the liquid and gas phases, but also on viscosity (friction of the gas bubbles in the liquid part). The gas-liquid mixture must therefore reside long enough in the degassing chamber to allow all of the gas bubbles to be extracted from the lye. In the remainder of the description, such gas-liquid separators will be referred to as gravity gas-liquid separators. These gravity gas-liquid separators are characterized by substantial dimensions. In some gas-liquid separators, equipment can be inserted to accelerate the separation (for example a honeycomb structure) or to standardize the flow and have a uniform residence time for all the flow lines. Gas bubbles still present in the liquid phase may be quantitatively significant, i.e. not all gas bubbles can be extracted from the liquid phase for evacuation through a dedicated evacuation pipe on the top or side wall of the degassing chamber. This poses several problems.

As already indicated above, the efficiency of the electrolyser stack suffers from this gas loss. Furthermore, in conventional electrolysis plants, the two lye fractions discharged from the gas-liquid separator are combined and mixed in an intermediate tank before being reinjected into the electrolyser stack in a closed loop.

Due to the incomplete separation, a potentially significant amount of residual dihydrogen and dioxygen can be reinjected into the electrolyser stack so that the residual dioxygen ends up on the cathode side while the dihydrogen goes towards the anode. As already mentioned, it is well known that the dihydrogen/dioxygen gas mixture is explosive even at a fairly low concentration and this situation is dangerous for personnel and the installation. The gases thus produced also have a very poor purity which requires an additional purification step.

[0011] One solution to this problem is to increase the size of the gas-liquid separators, which in turn generates new problems, linked to the additional cost, complexity of manufacturing and transporting these gas-liquid separators as well as the increase in the size of the production plant.

[0012] It would therefore be desirable to be able to provide a degassing device allowing practically complete degassing of the exhausts (dihydrogen/lye and dioxygen/lye from the electrolytic cells). Ideally, such a degassing device should be able to provide the expected result when the system is used at full load (high exhaust flow rate) or at reduced load (low exhaust flow rate).

[0013] It is also necessary to take into account the fact that the gas-liquid separators of electrolysers do not always produce dihydrogen and dioxygen at their nominal load (unlike similar systems for other industries or applications) and that the system must be efficient regardless of the quantity of gas to be separated. Indeed, when the volume of gas decreases, the purity of the gases deteriorates because proportionally the coalescence of the larger and easier to extract gas bubbles is not the same.

[0014] Statement of the invention

[0015] The cyclonic gas-liquid separator is well known in the prior art. Its operation relies on centrifugal forces as the driving force for the separation of the liquid and gas phases unlike gravity gas-liquid separators.

The operating principle of the cyclonic gas-liquid separator is based on swirling, in other words the rotation of the incoming gas-lye mixture. This swirl produces a centrifugal force that drives the densest phase, the lye, towards the walls of the cyclonic gas-liquid separator while the less dense phase, the gas bubbles, remains in the center of said cyclonic gas-liquid separator. It is also well known that the efficiency of the cyclonic gas-liquid separator depends heavily on the incoming flow rates. Indeed, at low incoming flow rates, the energy linked to the flow is not sufficient to effectively separate the gas bubbles. However, alkaline electrolysers aim to produce dihydrogen with a low carbon footprint. They are therefore generally linked to renewable energy production which can vary to a large extent depending on demand. As a consequence, the volume of gas-liquid mixture that must be separated is also variable.

Thus, the cyclonic gas-liquid separator alone is not sufficient to meet the needs of this application.

[0016] According to the invention, this problem has been solved with a degassing device according to claim 1. The inventor has in fact noticed that by combining a cyclonic gas-liquid separator and a gravity gas-liquid separator, it is possible to obtain a satisfactory separation of the gas and the liquid. When the cyclonic gas-liquid separator is coupled to a gravity gas-liquid separator, the latter performs an additional separation which makes it possible to separate the gas phase at the outlet of the cyclonic gas-liquid separator. At partial load, the efficiency of the cyclonic separation is lower, but the gravity separation is better due to an increase in the residence time.

[0017] In the device according to the invention, the cyclonic gas-liquid separator is arranged in the degassing chamber and is supplied by the gas-liquid mixture supply pipe of the degassing chamber.

[0018] Preferably, the cyclonic gas-liquid separator is provided with a first outlet opening for the gas emerging into the degassing chamber above the level of the gas-liquid interface of the degassing chamber; and a second outlet opening for the liquid emerging into the degassing chamber below the level of the gas-liquid interface of the degassing chamber. This configuration makes it possible to prevent liquid from being entrained into the gas phase by gas bubbles which would percolate through said liquid.

Similarly, the rejection of the liquid phase below the gas-liquid interface level makes it possible to avoid the entrainment of gas into the liquid phase.

[0019] The inventor also observed that the presence of the cyclonic gas-liquid separator within the gas-liquid separator thus allows a first separation of the gas molecules from the gas-lye mixture. Since the gas-lye mixture is already undergoing significant degassing, the residence time of the gas-liquid mixture in the device can be significantly lower than in the case of a gravity gas-liquid separator alone. In this configuration, the size of said gravity gas-liquid separator can therefore be reduced compared to a conventional installation.

[0020] When the cyclone gas-liquid separator is coupled to a gravity gas-liquid separator, the latter performs an additional separation which makes it possible to separate the gas phase at the outlet of the cyclone gas-liquid separator.

[0021] According to an advantageous variant of the invention, to further improve the efficiency of the system, a deflector can be arranged in the degassing chamber, under the liquid discharge opening of the cyclonic gas-liquid separator. Said plate thus prevents gas bubbles still possibly present in the liquid from being dispersed in the bottom of the degassing chamber. The deflector can consist of a flat, convex or concave plate.

[0022] The degassing chamber of the gas-liquid separator may have a horizontal or vertical main axis.

[0023] Ideally, the liquid-gas mixture feed opening of the degassing chamber is located opposite, along the main axis of the degassing chamber, at least one of the outlet openings of the degassing chamber. Thus the liquid or the gas respectively follow longer paths in the degassing chamber before being evacuated and the separation is more efficient.

[0024] Apart from this, it should also be noted that the precise location of the discharge openings of the degassing chamber is not critical, i.e. the design of the degassing device may have some flexibility in choosing the location of these openings. Of course, the discharge opening for the degassed liquid phase must be located below the gas-liquid interface of the degassing chamber, for example through the bottom wall or one of the side walls of the degassing chamber. Similarly, the discharge opening for the gas phase must be located above the gas-liquid interface of the degassing chamber, for example through the top wall or one of the side walls of the degassing chamber.

[0025] According to another of its aspects, the invention relates to a water electrolysis installation comprising a degassing device as defined above. The electrolysis installation may comprise an alkaline electrolyser, a membrane electrolyser or a high-temperature electrolyser. Preferably, an alkaline electrolyser is used. As explained above, the degassing device according to the invention allows the use of a degassing device of reduced size. It has been observed that according to the invention, the degassing device can provide the expected result when the installation is used at full load (high discharge flow rate) or at reduced load (low discharge flow rate).

[0026] It is possible to provide a sensor for detecting the quantity of residual gas at the outlet of the degassing chamber. The information measured by the sensor is supplied to a control module of a valve allowing, depending on the level of gas detected in the liquid, to reinject the liquid evacuated from the degassing chamber into the degassing chamber in order to refine the separation or to supply an intermediate tank with the liquid evacuated from the degassing chamber where it is mixed with the liquid phase coming from the other gas-liquid separator.

[0027] The invention will now be described by means of the figures which have no other purpose than to illustrate the present invention. These figures schematically represent.

[0028] Fig. 1 a degassing device according to the prior art arranged horizontally

[0029] Fig. 2 a degassing device according to the prior art arranged vertically

[0030] Fig. 3 a degassing device according to the invention arranged horizontally

[0031] Fig. 4 a degassing device according to the invention arranged vertically

[0032] In Figs. 1 to 4, different degassing devices 1 are shown. All contain a degassing chamber 14. For example gas-water or gas-lye. The gas may be dihydrogen or dioxygen. The degassing chamber 14 is supplied by a supply pipe 11 of the gas-liquid mixture coming from the electrolyser stack. In certain cases (not shown in the figures), the degassing chamber 14 can also be supplied with gas-liquid mixture coming from the liquid discharge pipe 12 of the degassing chamber 14 by means of a loop controlled by a valve if a sensor has detected that the quantity of gas present in the discharge of the degassing chamber 14 was greater than a predetermined value. The degassing chamber 14 also includes a gas discharge pipe, gas having been separated from the liquid phase, which can then either be discharged from the installation or combined with the same gas from the electrolyser stack. It will be noted that the gas discharge pipe 13 is always arranged above the gas-liquid interface 15. It can be located for example in the upper wall or in the or one side wall of the degassing chamber 14. Similarly, the liquid discharge pipe 12 is always arranged below the gas-liquid interface 15. It can be located for example in the lower wall (bottom wall) or in the or one side wall of the degassing chamber 14. The precise location of the discharge pipe 12 or 13 is not critical. It can be seen, however, that in all the cases shown, the discharge pipe 12 or 13 has been arranged opposite, that is to say at the furthest distance from the supply pipe 11 of the degassing chamber 14 in order to allow a longer path for the liquid in the degassing device 1. It can also be seen that the degassing device can be arranged along a horizontal axis A (Figs. 1 and 3) or vertical axis (Figs. 2 and 4) depending on the construction requirements for example.

[0033] The prior art devices of Figs. 1 and 2 are generally larger in size than the devices according to the invention of Figs. 3 and 4 because their efficiency is lower and a longer residence time in the degassing device is required.

[0034] The degassing devices 1 according to the present invention contain a cyclonic gas-liquid separator 2 whose feed opening 21 is connected to the gas-liquid mixture feed pipe 11 of the degassing chamber 14. The cyclonic gas-liquid separator 2 is provided with a first outlet opening 23 for the gas emerging into the degassing chamber 14 above the level of the gas-liquid interface 15 of the degassing chamber 14 and a second outlet opening 22 for the liquid emerging into the degassing chamber 14 below the level of the gas-liquid interface 15. A deflector 3 has been arranged under the liquid outlet so as to intercept the jet of liquid discharged from the cyclonic gas-liquid separator 2. This prevents the residual gas bubbles from being dispersed in the liquid present in the degassing chamber 14, more particularly in the bottom of the chamber. said degassing chamber 14. The deflector 3 may consist of a flat, convex or concave plate. The deflector 3 is supported by an arm (not shown) which may be connected to one of the walls (for example lower or lateral) of the degassing chamber 14 or to the cyclonic gas-liquid separator 2 or to any construction element present in the degassing chamber 14. A flat deflector is shown in Fig. 3 and a concave deflector in Fig. 4. The reverse configuration could just as well have been shown. It can be seen that in Figs. 3 and 4, the liquid discharge pipe 13 is shown in the upper wall of the degassing chamber 14. The pipe 13 could just as well have been shown in a lateral wall of the degassing chamber 14. The liquid discharge pipe 12 is shown in the bottom wall in Fig. 3 and in a lateral wall in Fig. 4. We could just as well have represented the reverse configuration.

[0035] List of references of the drawings: 1 Gas-liquid separator 11 Gas-liquid mixture supply pipe 12 Liquid discharge pipe 13 Gas discharge pipe 14 Gas-liquid separation chamber 15 Gas-liquid interface 2 Cyclonic gas-liquid separator 21 Supply opening of the cyclonic gas-liquid separator 22 Liquid discharge opening of the cyclonic gas-liquid separator 23 Gas discharge opening of the cyclonic gas-liquid separator 3 Deflector

A Main axis of the gas-liquid separator

Claims (10)

Claims 1. Degassing device (1) for an electrolysis installation comprising a degassing chamber (14) provided with - an opening for a gas-liquid mixture supply pipe (11); - an opening for a liquid discharge pipe (12) arranged below the level of the gas-liquid interface (15) of the degassing chamber (14); - an opening for a gas discharge pipe (13) arranged above the level of the gas-liquid interface (15) of the degassing chamber (14); characterized in that the degassing chamber (14) further comprises - A cyclonic gas-liquid separator (2) supplied with gas-liquid mixture by the gas-liquid mixture supply pipe (11) of the degassing chamber (14). 2. Degassing device (1) according to claim 1, wherein the cyclonic gas-liquid separator (2) has - a feed opening (21) connected to the feed pipe (11) for gas-liquid mixture in the degassing chamber (14); - a first outlet opening (23) for the gas emerging into the degassing chamber (14) above the level of the gas-liquid interface (15) of the degassing chamber (14); and - a second outlet opening (22) for the liquid emerging into the degassing chamber (14) below the level of the gas-liquid interface (15) of the degassing chamber. 14. 3. Degassing device (1) according to claim 2, in which a deflector (3) is arranged under the liquid outlet opening (22). 4. Degassing device (1) according to claim 3, in which the deflector (3) consists of a flat or concave plate. 5. Degassing device (1) according to any one of the preceding claims, in which the degassing chamber (14) has a horizontal main axis (A). 6. Degassing device (1) according to any one of claims 1 to 5, in which the degassing chamber (14) has a vertical main axis (B). 7. Degassing device (1) according to any one of claims 5 or 6, in which the supply pipe (11) for liquid-gas mixture of the degassing chamber (14) is located opposite, along the main axis (A) of the degassing chamber (14), at least one of the evacuation pipes (23, 13) of the degassing chamber (14). 8. Degassing device (1) according to claim 7, in which the supply pipe (11) for liquid-gas mixture of the degassing chamber (14) is located opposite, along the main axis (A) of the degassing chamber (14), the evacuation pipes (23, 13) of the degassing chamber (14). 9. Water electrolysis installation comprising a degassing device (1) according to any one of claims 1 to 8. 10. Electrolysis installation according to the preceding claim in which the electrolysis cell operates on the principle of alkaline electrolysis.