Water electrolysis plant comprising one or more degassers in series with a static mixer in the supply pipe

Abstract

The present invention relates to the technical field of electrolysis and very particularly to a degassing device for 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, 101) which comprises a degassing chamber (14, 114) provided with an opening for a gas-liquid mixture supply pipe (11, 111), with an opening for a a liquid discharge pipe (12, 112) arranged below the level of the gas-liquid interface (15, 115) of the degassing chamber (14, 114), and with an opening for a gas discharge pipe (13, 113) arranged above the level of the gas-liquid interface (15, 115) of the degassing chamber (14, 114). Furthermore, a static mixer (21, 121) is inserted into the gas-liquid mixture supply pipe (11, 111).

Description

WATER ELECTROLYSIS PLANT COMPRISING IN SERIES ONE OR MORE DEGAZERS WITH A STATIC MIXER IN THE SUPPLY PIPELINE

[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 (H2) and dioxygen (O2) by electrolysis of water. According to one 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.

[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 a very high energy content (285 kJ / mol).

[0005] There are several known ways to produce gaseous dihydrogen; the most advantageous is to electrolyze the water molecule because it is a high-yield reaction that does not directly produce CO2, unlike the widely used processes of reforming methane, coal and hydrocarbons.

[0006] There are three main types of electrolyzers for water electrolysis: - alkaline electrolysers (AWE), which are characterised by the use of a liquid electrolyte which allows the transfer of hydroxyl ions (OH') from the cathode to the anode, - high-temperature electrolysers, whose electrolyte is a ceramic; and - membrane electrolysers (PEM), whose electrolyte 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 brevity, 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, H2, at the cathode, and dioxygen, O2, at the anode. A diaphragm generally 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 outputs of the two gas-liquid separators are then mixed before feeding back into the electrolyzer stack. In both streams, at the outlet of the electrolyzer stack, the liquid phase (lye) is loaded with gas bubbles. At the outlet of the gas-liquid separator, only a few gas bubbles remain 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 the gas is separated from the electrolyte, the more gas is produced, which contributes to the good electrochemical efficiency of the process. Secondly, the H2 / O2 mixture is highly explosive.If the separation is not carried out correctly, a significant amount of gas, commonly called "residual gas", is carried away at the liquid outlet of the gas-liquid separator. During the next circulation in the electrolyser stack (the electrolyte rotates in a closed loop) some 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 electrolyzer stack for the dihydrogen-lye mixtures on the one hand and dioxygen-lye 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 separators. Document WO-A1-2020 / 254211 is another example.

[0010] The principle of gas bubble extraction 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 remain in the degassing chamber for a sufficient period of time to allow all the gas bubbles to be extracted from the lye. In the following 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 streamlines.Gas bubbles still present in the liquid phase can be quantitatively significant, i.e. not all gas bubbles manage to. be extracted from the liquid phase for evacuation via an ad hoc evacuation pipe on the top or side wall of the degassing chamber. This poses several problems. As already indicated above, the efficiency of the electrolyzer stack suffers from this loss of gas. Furthermore, in conventional electrolysis installations, the two fractions of lye evacuated from the gas-liquid separator are combined and mixed in an intermediate tank before being reinjected into the electrolyzer stack in a closed loop. Due to the incomplete separation, a potentially significant quantity of residual hydrogen and oxygen may be reinjected into the electrolyzer stack so that the residual oxygen ends up on the cathode side while the hydrogen goes towards the anode.As already mentioned, it is well known that the hydrogen / oxygen 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 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 creates new problems related 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 water electrolysis installation that allows for virtually complete degassing of the discharges (dihydrogen / lye and dioxygen / lye from the electrolytic cells). Ideally, such a water electrolysis installation should be able to provide the expected result when the system is used at full load (high discharge flow rate) or at reduced load (low discharge 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] Mixers intended to promote the intimate mixing of two constituents are already known in the state of the art (see for example document WO-A1-2013 / 126346).

[0015] Statement of the invention

[0016] According to the invention, this problem is solved with an installation as defined in claim 1. A static mixer is a device for continuously mixing fluids or fluids and materials in another phase (solid or gas). This device allows elements to be mixed but it can also be used with gases or to mix a gas and a liquid. It is particularly surprising that such a mixing device can be used to obtain the opposite effect, namely, to separate the gas from the liquid rather than to separate them. mixing. The inventors observed that, under the effect of the static mixer, the gas-lye mixture passing through the static mixer is subject to very strong turbulence. In this case, this turbulence must not break the gas bubbles already formed and thus create the opposite effect. This is why, with the proposed solution, only small gas bubbles remain in the static mixer. In turn, this turbulence causes the gas bubbles contained in the mixture to coalesce, which promotes the formation of larger gas bubbles that can subsequently be extracted more easily from the liquid in the gravity gas-liquid separator downstream of the static mixer.The gas separation mechanism follows the same pattern as that developed in a gravity gas-liquid separator, namely that i) the gas bubbles rise to the gas-liquid interface due to gravity as well as the liquid-gas density ratio and end their course by being evacuated through the gas evacuation pipe dedicated for this purpose and located in the upper part of the degassing chamber and ii) the liquid phase flows through the liquid evacuation pipe located in the lower part of the degassing chamber.

[0017] There are many static mixers that can be used according to the invention. Document US 3286992 describes the operating principle of the static mixer. The geometries of such mixers that can be suitable are numerous: helical, twisted, perforated, spiked or spiked ribbons, wire windings, fins, swirl generators, also called swirlers, etc. Advantageously, the static mixer is constituted by a helical insert. According to another advantageous variant of the invention, the static mixer is constituted by a swirler. These two types of static mixers make it possible to convert part of the translational energy of the flow into rotational energy and allow a strong coalescence of the gas bubbles which can then be extracted more easily from the liquid.

[0018] The invention therefore relates to a water electrolysis installation which contains one or more electrolyzer stack(s) and a degassing device as described above, the latter being defined by the coupling between a static mixer and a separator (gravity or cyclonic) downstream of the static mixer. The degassing device is supplied by the supply pipe which is connected to a pipe for discharging the liquid phase of at least one electrolyzer stack. As explained above, such a device allows efficient and rapid separation of the gas-liquid mixture to the point that the size of the degassing device can be reduced.

[0019] According to an advantageous variant of the invention, the installation comprises an additional degassing device upstream or downstream of the degassing device defined above. A first degassing device is supplied by a supply pipe which is connected to an evacuation pipe of at least one electrolyser stack. There, only the large gas bubbles are evacuated and their rupture in the mixer is avoided. static. The first gas-liquid separator is then small so that only gas bubbles larger than 300 pm (ideally 200 or 100 pm) are separated. The liquid discharge pipe of the first degassing device is connected to the supply pipe of a second degassing device. The two degassing devices are therefore arranged in series. A static mixer is arranged in the supply pipe of the first degassing device, in the supply pipe of the second degassing device or in both supply pipes.

[0020] The two degassing devices mounted in series can be aligned respectively along a horizontal or vertical main axis A in all possible combinations (horizontal-horizontal, horizontal-vertical, vertical-horizontal or vertical-vertical).

[0021] It is also possible to plan to add one or more additional degassing devices.

[0022] The preferred and simplest solution is that in which the gas / liquid flow arrives in the first gas-liquid separator in which the largest gas bubbles and the largest quantity of gas are extracted before coalescence is carried out in the pipe connecting this gas-liquid pre-separator to a second gas-liquid separator (pipe equipped with a static mixer). The static mixer can then generate very strong agitation without risking breaking large gas bubbles already formed. The two gas outlets can be connected and treated together. In this case, due to the communicating vessels, the two levels of the gas-liquid separators must be identical.

[0023] Brief description of the figures

[0024] The invention will now be described by means of figures which have no other purpose than to illustrate the present invention. These figures schematically represent:

[0025] Fig. 1 a degassing device for a water electrolysis installation according to the prior art arranged horizontally;

[0026] Fig. 2 a degassing device for a water electrolysis installation according to the prior art arranged vertically;

[0027] Fig. 3 a degassing device for a water electrolysis installation according to the invention arranged horizontally;

[0028] Fig. 4 a degassing device for a water electrolysis installation according to the invention arranged vertically;

[0029] Fig. 5 and 6 two partial representations of an electrolysis installation with two degassing devices in series (preferred and simplest solution in terms of efficiency).

[0030] Figs. 1 to 4 show different degassing devices 1 for a water electrolysis installation. All contain a degassing chamber 14. For example gas-water or gas-lye. The gas can be dihydrogen or dioxygen. The degassing chamber 14 is supplied by a supply pipe 11 of the gas-liquid mixture. from the electrolyser stack. In some cases (not shown in the figures), the degassing chamber 14 may also be supplied with a gas-liquid mixture 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 comprises 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 may be located, for example, in the upper wall or in the or a side wall of the degassing chamber 14.Similarly, the liquid discharge pipe 12 is always arranged below the gas-liquid interface 15. It may be located, for example, in the lower wall (bottom wall) or in the side wall(s) of the degassing chamber 14. The precise location of the discharge pipe 12 or 13 is not critical, but care must be taken to ensure that no vortex capable of entraining gas can form there. A vortex-breaking device is then a plus in order to prevent this phenomenon. 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 A (Figs.2 and 4) depending on construction requirements for example.

[0031] The degassing devices shown in Figs. 3 and 4 have a static mixer 21 in their supply line.

[0032] Figs. 5 and 6 show the assembly of two degassing devices connected in series. The first of the two degassing devices 1 contains a degassing chamber 14. For example, gas-water or gas-lye. The gas can 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. The degassing chamber 14 also includes a gas discharge pipe 13 and a liquid discharge pipe 12. The assemblies of Figs. 5 and 6 also contain a second degassing device 101 connected in series with the first degassing device 1. The second degassing device 101 contains a degassing chamber 114 supplied by a supply pipe 111 with the gas-liquid mixture coming from the liquid discharge pipe 12 of the first degassing device 1.The degassing chamber 114 also comprises a gas discharge pipe 113 and a liquid discharge pipe 112. It can be seen in Figs. 5 and 6 that the gas-liquid mixture supply pipe 111 of the second degassing device 101 contains a static mixer 121.

[0033] In Fig. 6, it can be seen that the gas-liquid mixture supply line of the first degassing device 1 also contains a static mixer 21. The two gas outlets can be connected for common treatment. In this case, care should be taken to adjust the two free surfaces due to the double communication (gas and liquid).

[0034] List of drawing references: I Gas-liquid separator II Gas-liquid mixture supply line 12 Liquid discharge pipe 13 Gas discharge pipe 14 Gas-liquid separation chamber 15 Gas-liquid interface 21 Static mixer 101 Gas-liquid separator III Gas-liquid mixture supply pipeline 112 Liquid discharge pipe 113 Gas discharge pipe 114 Gas-liquid separation chamber 115 Gas-liquid interface 121 Static mixer A Main axis of the separator

Claims

Claims 1. Installation for electrolysis of water or any other constituent containing one or more electrolyzer stack(s) and a degassing device (1, 101), in which the degassing device (1, 101) comprises a degassing chamber (14, 114) provided - an opening for a supply pipe (11, 111) for gas-liquid mixture; - an opening for a liquid discharge pipe (12, 112) arranged below the level of the gas-liquid interface (15, 115) of the degassing chamber (14, 114); - an opening for a gas discharge pipe (13, 113) arranged above the level of the gas-liquid interface (15, 115) of the degassing chamber (14, 114), a static mixer (21, 121) being inserted into the gas-liquid mixture supply pipe (11, 111) and in which the degassing device (1, 101) is supplied by a supply pipe (11, 111) which is connected to a gas-electrolyte mixture discharge pipe of at least one electrolyzer stack.

2. Water electrolysis installation according to the preceding claim, in which the static mixer (21, 121) is constituted by a helical insert.

3. Water electrolysis installation according to claim 1, in which the static mixer (21, 121) is constituted by a swirler.

4. Water electrolysis installation containing one or more electrolyzer stack(s) according to any one of the preceding claims and comprising several degassing devices (1, 101) mounted in series.

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