BE1031541B1 - ELECTROLYSIS UNIT WITH INCREASED PRODUCTIVITY AND AVAILABILITY - Google Patents

Abstract

The present invention relates to an electrolyser (10) comprising: - a stack (12) immersed in an electrolytic solution - a path of first flow (Fa) of electrolytic solution, comprising a first gas-liquid separator (16a) adapted to isolate a first electrolysis product from the first flow (Fa), a first pipe (14a) connecting the solution to an inlet of the first separator, and a first conduit (18a) which connects an outlet of the first separator to the electrolytic solution; - a path of second flow (Fb) of electrolytic solution, comprising a second gas-liquid separator (16b) adapted to isolate a second electrolysis product from the second flow (Fb), a second pipe (14b) connecting the electrolytic solution to the second separator, and a second conduit (18a) which connects an outlet of the second separator to the electrolytic solution; - a preheating unit (20) for heating at least one of the first and second flows (Fa, Fb).

Description

2023/5464 ,Ç

BE2023/5464

DESCRIPTION

Title of the invention: ELECTROLYSIS UNIT WITH PRODUCTIVITY AND

INCREASED AVAILABILITY

TECHNICAL FIELD OF THE INVENTION

The present invention relates to electrolysis technology, in particular for the production of dihydrogen (Hz) and dioxygen (O2). It relates more particularly to an alkaline electrolysis unit with increased instantaneous efficiency.

TECHNICAL BACKGROUND

An electrolysis unit, called an electrolyser, typically comprises a stack formed of a multitude of stacked electrolytic cells. Each cell comprises two electrodes, including a cathode and an anode, immersed in an electrolytic solution.

Electrolysis consists of imposing the passage of an electric current between these two electrodes to cause a break in bonds and the generation of new products.

The electrolytic solution is a solution serving as a seat for the reaction. In the case of so-called alkaline electrolysis for the production of dihydrogen (Hz) and dioxygen (O2), this solution is in the form of an aqueous solution comprising pure water, i.e. demineralized, and a water-soluble alkaline additive, called electrolyte. The electrolyte generally corresponds to potassium hydroxide (KOH) or sodium hydroxide (NaOH).

Since demineralized water is a poor conductor of electricity, the use of this electrolyte makes it possible to achieve an admissible conductance by means of the formation of ions which pass from one electrode to the other to ensure the passage of current.

As this electric current passes, water molecules (H2O) dissociate into hydroxide ions (HO7) and hydrogen ions (H*). The hydrogen cations (H*) accept electrons at the cathode in a redox reaction to form dihydrogen (Hz), while oxidation of the hydroxide anions (HO7) occurs at the anode to form dioxygen (O2).

At the outlet, two mixtures are recovered, including a mixture formed of dioxygen (O2) and electrolytic solution at the anode, and a mixture formed of dihydrogen (Hz) and electrolytic solution at the cathode. In a known manner, these mixtures are each routed to a gas-liquid separator to isolate the dioxygen (O2) and the dihydrogen (Hz).

The residual electrolytic solution of the two mixtures, called “lye”, is then reinjected into the electrolytic solution in which the two electrodes are bathed, in the form of a closed circuit.

Since temperature is linked to the thermal agitation of molecules and ions in solution, it is understood that a rise in temperature of the electrolytic solution increases the mobility of the ions and consequently the intensity of the electric current flowing through it. The electrolysis reaction is thus favored.

When starting the electrolyser at room temperature, the conductance of the electrolytic solution is low, which leads to: - a significantly reduced power of the stack compared to its nominal speed; and - a loss by leakage of dihydrogen (Hz), called HTO from the English "amount of hydrogen in oxygen", which is unexpectedly found in the mixture intended for the gas-liquid separator adapted to isolate the dioxygen (O2), and corollary; - a loss by leakage of dioxygen (O2), called OTH from the English "amount of oxygen in hydrogen", found in the mixture intended for the separator contrary adapted to isolate the dihydrogen (H2).

In practice, the electrolytic solution heats spontaneously by the effect

Joule effect when the current passes. However, this Joule effect is significantly limited at start-up because the current flowing through is limited, which makes the heating time particularly long.

The aim of the invention is to propose a solution for limiting the heating time in order to overcome the aforementioned production failures.

STATEMENT OF THE INVENTION

For this purpose, the invention relates to an electrolyser comprising: - a stack traversed from upstream to downstream by an electrolytic solution and comprising a stack of electrolytic cells immersed in this electrolytic solution; - a first electrolytic solution flow routing loop, comprising a first gas-liquid separator adapted to isolate a first electrolysis product from the first flow, a first electrolytic solution sampling pipe downstream of the stack which is connected to an inlet of the first gas-liquid separator, and a first conduit which connects an outlet of the first gas-liquid separator directly or indirectly to the electrolytic solution upstream of the stack; - a second electrolytic solution flow routing loop, comprising a second gas-liquid separator adapted to isolate a second electrolysis product from the second flow, a second electrolytic solution sampling pipe downstream of the stack which is connected to an inlet of the second gas-liquid separator, and a second conduit which connects an outlet of the second gas-liquid separator directly or indirectly to the electrolytic solution upstream of the stack; - a preheating unit for heating at least one of the first and second streams.

The invention also relates to an electrolyser thus defined, in which: - the first conduit and the second conduit are connected to a mixing chamber, and - the preheating unit is arranged to heat the mixture of the first and second flows at or at the outlet of the mixing chamber.

The invention also relates to an electrolyser thus defined, in which:

- the first conduit and the second conduit are insulated from each other; and - the preheating unit comprises a first preheating module arranged to heat the first flow and a second preheating module arranged to heat the second flow.

The invention also relates to an electrolyser thus defined, in which: - the first preheating module is arranged to heat the first flow at the first conduit; and - the second preheating module is arranged to heat the second flow at the second conduit.

The invention also relates to an electrolyser thus defined, in which the preheating unit comprises at least one heat exchanger.

The invention also relates to an electrolyser thus defined, in which the preheating unit comprises at least one heating resistor.

The invention also relates to a method for synthesizing a product by electrolysis using an electrolyser thus defined, comprising a step of heating at least one of the first stream and the second stream by the preheating unit when the electrolytic solution temperature is below a target temperature.

The invention also relates to a process thus defined for the synthesis of dihydrogen (Hz) and dioxygen (O2) by electrolysis of an electrolytic solution comprising demineralized water and potassium hydroxide (KOH) or sodium hydroxide (NaOH), in which the temperature oscillates around a "target" temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will become apparent when reading the detailed description which follows, for the understanding of which reference will be made to the appended drawings in which: — [Fig.1] is a schematic diagram of a first embodiment of the electrolyser according to the invention.

— [Fig.2] is a schematic diagram of a second embodiment of the electrolyser according to the invention.

DETAILED DESCRIPTION OF THE INVENTION 5 With reference to Figures 1 and 2, an electrolysis unit 10, or electrolyzer, according to the invention comprises a stack 12 crossed from upstream AM to downstream AV by an electrolytic solution. This stack 12, forming the core of the electrolyzer 10, comprises a multitude of stacked electrolytic cells each comprising two electrodes immersed in the electrolytic solution. The electrodes of each cell of the stack 12, including a cathode and an anode, are separated from each other by a membrane, or separator.

In the following, the electrolyser 10 according to the invention will be described in the context of an application of so-called alkaline electrolysis for the production of dihydrogen (Hz) and dioxygen (O2). The electrolytic solution corresponds to an aqueous alkaline solution comprising demineralised water (H20) as well as potassium hydroxide (KOH) in a predefined concentration.

The electrolyser 10 comprises a first and a second pipe 14a, 14b provided to collect downstream AV of the stack 12 the electrolysis reaction products and convey them respectively to a first and a second gas-liquid separator 16a, 16b.

The first pipe 14a conveys to the first gas-liquid separator 16a a first flow Fa of electrolytic solution enriched in dihydrogen (Hz) after reaction at the electrolytic cells. In more detail, this dihydrogen (Hz) comes in cascade from the dissociation of water (H2O) into hydrogen cations (H*) when an electric current passes, followed by an oxidation-reduction reaction of these ions with the cathode. At the outlet of the first gas-liquid separator 16a, the dihydrogen (Hz) is isolated while the residual electrolytic solution, called lye, of the first flow Fa is conveyed along a pipe 18a by pumping to reintegrate the flow of electrolytic solution upstream AM of the stack 12.

The second pipe 14b conveys to the second gas-liquid separator 16b a second flow of electrolytic solution enriched in dioxygen (O2) after reaction at the electrolytic cells. The dioxygen (O2) is the product of the dissociation of water into hydroxide ions (HOT) and of an oxidation of these ions at the anode. In the same way as for the first gas-liquid separator 16a, the second gas-liquid separator 16b makes it possible to isolate the dioxygen (O2) while the lye of the second flow Fb is guided by pumping along a pipe 18b to reintegrate the flow of electrolytic solution upstream AM of the stack 12.

As understood, after isolation of the products, the flows Fa and Fb continuously reform the electrolytic solution before its passage to the level of the electrolytic cells of stack 12 where the reaction takes place, in the form of routing loops.

The chemical yield of this reaction to form dihydrogen (Hz) and dioxygen (O2), namely its efficiency, is a function of the passage of the current which allows the dissociation of the modules (H20) into hydroxide ions (HOT) and hydrogen (H*). This passage of the current depends directly on the temperature: the temperature being linked to the thermal agitation of the molecules and ions in solution, it follows that the hotter it is, the more the mobility of the ions increases and therefore promotes their migration between the cathode and the anode.

The intensity of the electric current flowing through the electrolytic solution therefore increases as the temperature rises.

In this regard, the idea behind the invention lies in the implementation of measures aimed at limiting the heating time of the electrolytic solution to reach its “target” temperature when starting the electrolyser 10.

In this regard, the electrolyser 10 according to the invention comprises a preheating unit 20 which makes it possible to limit the start-up time required to reach the “target” temperature.

Figure 1 illustrates a first variant arrangement of the electrolyser 10 in which the lye from the first flow Fa and the lye from the second flow Fb are conveyed into a mixing chamber 19 via the conduits 18a and 18b. In such a case, the mixing chamber 19 is judiciously heated by the preheating unit 20 so as to reinject the mixture of the two flows at a higher temperature into the flow of electrolytic solution passing through the stack 12. This results in an overall rise in temperature of the electrolytic solution.

Note that the preheating unit 20 is not limited to this particular arrangement. It can be arranged along a pipe, denoted 22, which carries the mixture to the stack 12 at the outlet of this mixing chamber 19.

Figure 2 illustrates a second variant arrangement of the electrolyser 10 in which the conduits 18a and 18b each convey the lye in isolation to reform the electrolytic solution passing through the stack 12. It is retained according to this architecture that the preheating unit 20 comprises: - a first preheating module, denoted 20a, which is arranged to heat the lye at the conduit 18a at the outlet of the first gas-liquid separator 16a; and - a second preheating module 20b which is arranged to heat the lye at the conduit 18b at the outlet of the second gas-liquid separator 16b.

The injection of the preheated lye thus leads to an overall increase in the temperature of the electrolytic solution. It is therefore possible to quickly and efficiently reach the “target” temperature, i.e. the optimum temperature, for operating the stack 12. The “target” temperature is set in a non-limiting manner at approximately 70°C for this application of synthesizing dihydrogen (Hz) and dioxygen (O2) on the basis of an electrolytic solution comprising demineralized water and potassium hydroxide (KOH).

The heat supply according to the invention therefore allows the electrolyser 10 to reach the appropriate temperature as quickly as possible to operate at its nominal speed, namely to operate at its full potential.

In practice, it has been observed that the preheating unit 20 makes it possible to heat the electrolytic solution to its “target” temperature in 30 minutes from the start-up at room temperature of the electrolyser 10 in question. For the same electrolyser 10, but without a preheating unit 20 in accordance with the invention, reaching the “target” temperature would take significantly longer, of the order of five to six hours. This particularity is explained by the fact that in the absence of the preheating unit 20 according to the invention, the solution would rise in temperature solely by the Joule effect, when the current passes. Since the Joule effect depends on the conductance of the electrolytic solution, it is understood that this rise in temperature solely on this basis is particularly limited and long when the electrolyser 10 is started up, as the mobility of the ions is low at room temperature.

In the examples of figures 1 and 2, the preheating unit 20 is advantageously located upstream AM of the stack 12, and more particularly arranged to heat the flows Fa and Fb between the gas-liquid separators 16a, 16b and the reinjection point into the electrolytic solution flowing through the stack 12. This arrangement is not limiting within the scope of the invention but remains particularly recommended.

Indeed, on the one hand, if the preheating unit 20 were arranged at the level of the stack 12, a gas-liquid mixture could appear at the level of the electrolytic cells, which would penalize the reaction yield.

On the other hand, an arrangement between the stack 12 and the gas-liquid separators 16a and 16b would lead to heating the dihydrogen (Hz) and the dioxygen (O2) still present in the flows Fa and Fb, which is counterproductive given that these elements are: - extracted from the electrolytic solution, which would amount to using part of the calories so as not to heat the electrolytic solution; and - commonly cooled after their capture, which would therefore subsequently require consuming more energy for their cooling.

The preheating unit 20 can advantageously be in the form of one or more heat exchangers, for example of the type

ACOC advantageously comprising fins bathed in the flows Fa and Fb considered. These fins can protrude from the delimiting wall of the conduits 18a and 18b in the case of figure 1 or either from the mixing chamber 19 or from the pipe 22 in the example of figure 2.

It should be noted that other devices may be retained as preheating unit 20 without departing from the scope of the invention. For example, the unit may comprise one or more heating resistors or other types of heat input (heat pump, plate exchanger, etc.).

The invention has been explained as relating to the integration of a preheating unit 20 making it possible to limit the so-called priming phase of the electrolyser 10 during which it does not operate at full potential. It should be noted, however, that the preheating unit 20 finds its application throughout the operating cycle, by allowing continuous control of the temperature: as soon as the temperature of the electrolytic solution drops unexpectedly, it is possible to reheat it to return to the optimum operating temperature.

The electrolyser 10 according to the invention also makes it possible to increase the quality and quantity of dihydrogen (Hz) and dioxygen (O2) produced, since the stack 12 works more quickly at high speed. The quality and quantity of dihydrogen (Hz) and dioxygen (O2) being higher, the energy used to power the preheating unit 20 is amortized.

The electrolyser 10 has been described for an alkaline electrolysis application involving an electrolytic solution composed of potassium hydroxide (KOH). It should be noted that the invention is not limited to this particular composition. It finds application for any type of alkaline electrolyte, for example sodium hydroxide (NaOH).

Claims (8)

1. Electrolyser (10) comprising: - a stack (12) crossed from upstream (AM) to downstream (AV) by an electrolytic solution and comprising a stack of electrolytic cells immersed in this electrolytic solution, - a first flow routing loop (Fa) of electrolytic solution comprising a first gas-liquid separator (16a) adapted to isolate a first electrolysis product from the first flow (Fa), a first pipe (14a) for sampling electrolytic solution downstream (AV) of the stack (12) which is connected to an inlet of the first gas-liquid separator (16a), and a first conduit (18a) which connects an outlet of the first gas-liquid separator (16a) directly or indirectly to the electrolytic solution upstream (AM) of the stack (12); - a second flow path loop (Fb) of electrolytic solution, comprising a second gas-liquid separator (16b) adapted to isolate a second electrolysis product from the second flow (Fb), a second electrolytic solution sampling pipe (14b) downstream (AV) of the stack (12) which is connected to an inlet of the second gas-liquid separator (16b), and a second conduit (18b) which connects an outlet of the second gas-liquid separator (16b) directly or indirectly to the electrolytic solution upstream (AM) of the stack (12); the electrolyser (10) further comprising a preheating unit (20) for heating at least one of the first and second flows (Fa, Fb). 2. Electrolyser (10) according to claim 1, in which: - the first conduit (18a) and the second conduit (18b) are connected to a mixing chamber (19) at the outlet of the gas-liquid separators (16a, 16b), and - the preheating unit (20) is arranged to heat the mixture of the first and second flows (Fa, Fb) at or at the outlet of the mixing chamber (19). 3. Electrolyzer (10) according to claim 1, wherein: - the first conduit (18a) and the second conduit (18b) are isolated from each other; and - the preheating unit (20) comprises a first preheating module (20a) arranged to heat the first flow (Fa) and a second preheating module (20b) arranged to heat the second flow (Fb). 4. Electrolyser (10) according to claim 3, wherein: - the first preheating module (20a) is arranged to heat the first flow (Fa) at the first conduit (18a); and - the second preheating module (20b) is arranged to heat the second flow (Fb) at the second conduit (18b). 5. Electrolyser (10) according to any one of the preceding claims, in which the preheating unit (20) comprises at least one heat exchanger. 6. Electrolyser (10) according to any one of claims 1 to 4, in which the preheating unit (20) comprises at least one heating resistor or any other heating mechanism. 7. A method of synthesizing a product by electrolysis using an electrolyser (10) according to any one of the preceding claims, comprising a step of heating at least one of the first flow (Fa) and the second flow (Fb) by the preheating unit (20) when the electrolytic solution temperature is below a “target” temperature. 8. A method according to claim 7 for the synthesis of dihydrogen (Hz) and dioxygen (O2) by electrolysis of an electrolytic solution comprising demineralized water (H2O) and potassium hydroxide (KOH) or sodium hydroxide (NaOH).