EP4269658A1 - Electrolysis system and method for operating an electrolysis system with optimised water management - Google Patents

Abstract

The invention relates to a method for operating an electrolysis system (200), in which ion components are reduced in water as the feed medium (a) by means of an ion reduction unit (120), the feed medium (b) then being transferred to an electrolysis unit (130) with a proton exchange membrane (134) is supplied, in which the feed medium is converted into hydrogen and oxygen, the feed medium (d) being circulated on an oxygen side of the electrolysis unit (130), and a part (n, o) of the feed medium being removed. The invention also relates to a corresponding system (200).

Description

The invention relates to a method for operating an electrolysis system, in particular water management with regard to the ion concentration, as well as such an electrolysis system, which is used, for example, to obtain hydrogen from water.

State of the art

To obtain hydrogen, so-called electrolysis can be used, in which, for example, water is split into oxygen and hydrogen using electrical energy. In this context, one also speaks of water electrolysis, which is known, for example, as so-called proton exchange membrane electrolysis (PEM electrolysis, " Proton Exchange Membrane" electrolysis) can be carried out, the basics of which can be found, for example, in "Bessarabov et al: PEM electrolysis for hydrogen production. CRC Press " are known.

The electrolysis of water can produce ions that increase the electrical conductivity of the water and thus the risk of short circuits and reduce the lifespan of the electrolysis cells. The present invention therefore has the task of providing improved options for operating an electrolysis system, particularly with regard to water management.

Disclosure of the invention

This task is solved by a method for operating an electrolysis system and an electrolysis system with the features of the independent patent claims. Refinements are the subject of the dependent patent claims and the following description.

Advantages of the invention

Electrolysis systems are typically used to produce or obtain hydrogen using electrolysis. For electrolysis, water, in particular demineralized water, is supplied as the feed medium to an electrolysis unit with a proton exchange membrane (PEM electrolysis), in which the feed medium, i.e. the water, is converted (split) into hydrogen and oxygen (in addition to Hydrogen is therefore always obtained or produced oxygen at the same time).

It has been shown that the water used in PEM electrolysis has a corrosive effect and, for example, dissolves components such as iron, chromium, nickel, cobalt or molybdenum from stainless steel components. On a platinum catalyst, as is often used in the cathodes of PEM electrolysis units, protons and oxygen can react to form hydrogen peroxide:

O ₂ + 2H ^- + 2e ^- → H ₂ O ₂

The resulting hydrogen peroxide forms highly reactive radicals (HO ^- ). This reaction is strongly catalyzed by iron ions (Fe ²⁺ ) and is known as the so-called Fenton reaction:

H ₂ O ₂ + Fe ²⁺ → Fe ³⁺ HO ^. + HO ^-

HO ^- radicals increase the decomposition reactions on the fluoridated perfluorosulfonic acid membranes, which are preferably used as cation-conducting solid electrolytes, which leads to the release of fluoride into the water used in the electrolysis unit, which is usually circulated in a circuit on the anode side, but also via the membrane reaches the cathode side.

The release of cations and anions into the demineralized water increases the electrical conductivity of the water and thus the risk of a short circuit in the electrolysis unit or the actual electrolysis cell. In addition, organic carbon molecules from polymeric materials (e.g. water pipes) or additives (e.g. plasticizers, dyes, stabilizers) can get into the demineralized water. Oxidation reactions with hydrogen peroxide or OH ^- radicals can break down large molecules into smaller, ionic molecules, which also increase the electrical conductivity of the water.

In order to avoid the risk of short circuits, the electrical conductivity of the water should be limited, for example to values below 10 µS/cm, preferably below 3 µS/cm, more preferably below 1.5 µS/cm.

A continuous concentration of ions can be counteracted, for example, by using a so-called mixed-bed polisher ion exchange unit (MBP). For this purpose, ion components in the water supplied to the electrolysis unit as a feed medium and in the water circulated in the circuit are reduced by means of such an ion reduction unit.

As it turns out, the release rate of ions such as fluoride is typically relatively high in the first hundred or several hundred hours after starting up a PEM electrolysis, then decreases and ultimately remains at a low level. The maximum fluoride concentration in the circulated water or the feed medium must be a maximum of 50 or if necessary 100, possibly even up to 250 ppb in order to achieve an electrical conductivity of less than 1.5 µS/cm.

Further explanations of the release of fluoride and its measurement can be found, for example, in " P. Marocco et al.: Online measurements of fluoride ions in proton exchange membrane water electrolysis through ion chromatography. Journal of power sources, 2021. https://doi.org/10.1016/j.jpowsour.2020.229179 ". More detailed explanations of the effect of erosion of the membrane by fluoride can be found, for example, " SA Grigoriev et al: Failure of PEM water electrolysis cells: Case study involving anode dissolution and membrane thinning. Int Journal of Hydrogen Energy (2014). http://dx.doi.org/10.1016/j.ijhydene.2014.05.043 ". Further explanations can be found in " S. Siracusano et al: Degradation issues of PEM electrolysis MEAs. Renewable Energy (2018). https://doi.org/10.1016/j.renene.2018.02.024 " and " Frensch et al: Impact of iron and hydrogen peroxide on membrane degradation for polymer electrolyte membrane water electrolysis: Computational and experimental investigation on fluoride emission. Journal of Power Sources, 2019. https://doi.org/10.1016/j.jpowsour.2019.02.076 ".

In large electrolysis systems, in particular for PEM electrolysis, the concentration of ions such as fluoride and thus the electrical conductivity can be controlled, i.e. limited to the values mentioned, by using a certain proportion, for example up to 6%, of the circulating medium or water is branched off and cleaned using an ion reduction unit such as an MBP.

The ion reduction unit must therefore be designed to clean of ions not only the feed medium supplied from outside (water), which is converted into hydrogen and oxygen during electrolysis, but also the portion diverted from the circulated feed medium and, if necessary, that through the membrane Water that reached the cathode side of the electrolysis unit was collected there and returned to the circuit. Since the amount of water circulated in the circuit per unit of time is usually very high, the ion reduction unit must also be very large. The proportion of up to 6% of the circulating feed medium is usually many times higher than the amount of water supplied as feed medium. In particular, the water occurring on the cathode side of the electrolysis unit has been shown to contribute particularly strongly to increasing the concentration of ions. This ultimately means that the ion reduction unit (e.g. an MBP) has to be very large or very powerful, which is associated with high costs.

Against this background, it is now proposed that part of the feed medium is removed from the circuit, in particular a so-called blown-down is carried out, ie this part of the feed medium is removed for desalination or for the purpose of desalination. This part of the feed medium is in particular removed from the water circuit of the electrolysis unit. This leads to a significantly smaller amount of feed medium or water that has to be passed through the ion reduction unit (e.g. MBP), so that it can be designed to be significantly smaller. The discharged water can be used in various ways and/or ion contents can be reduced in other ways so that the water can be returned to the circuit. If the water that is removed is not returned, more fresh feed medium can be added.

Preferably, at least part of the discharged feed medium is removed from the circulated feed medium. The amount of circulating feed medium is thereby reduced, so that less feed medium has to be fed into the ion reduction unit, which can therefore be made smaller.

Advantageously, the amount of feed medium to be freshly supplied per unit of time is greater than the amount of feed medium converted into hydrogen and oxygen per unit of time (possibly plus other losses that otherwise have to be compensated for). Excess, i.e. supplied but not converted feed medium, in particular as part of the circulated feed medium, can then be removed; This part then no longer has to be returned. The excess feed medium can, for example, be at least partially discharged as wastewater. Likewise, the excess feed medium can at least partially be used (or reused) externally, for example as cooling water. Due to its quality, the discharged feed medium can easily be used elsewhere as process water.

It is particularly useful to determine the amount of feed medium to be removed per unit of time depending on the conductivity of the feed medium. For this purpose, for example, the ion concentration can be determined at a suitable location in the electrolysis unit (e.g. together or separately for several types of ions) and/or the conductivity can be measured directly. Depending on the level of the current conductivity, more or less (or even no) water can be removed. Accordingly, more or less feed medium is then supplied fresh.

Advantageously, ion components of the removed part of the circulated feed medium are at least partially reduced by means of a second ion reduction unit, in particular selectively (i.e. only certain types of ions such as fluoride). The part of the discharged feed medium treated in this way is then at least partially fed to the first ion reduction unit. The first ion reduction unit itself can therefore be smaller in size since there is a second, possibly different, ion reduction unit.

As mentioned, a significant amount of water can accumulate on the hydrogen or cathode side of the electrolysis unit. This water (collected water) is collected and preferably supplied as a feed medium at least partially directly, ie without treatment in an ion reduction unit, to the oxygen or anode side of the electrolysis unit.

The collection water is expediently freed of hydrogen before it is at least partially fed to the oxygen side of the electrolysis unit. This can be done, for example, by stripping with nitrogen. This not only reduces the proportion of hydrogen in the water, but also improves explosion protection.

Advantageously, the collection water is comprised of the part of the circulated feed medium that is discharged. Before it is at least partially fed to the oxygen side of the electrolysis unit, and in particular after it has been freed of hydrogen, ion components can be reduced therein by means of a third ion reduction unit, in particular selectively (i.e. only certain types of ions such as fluoride). For example, an anion exchanger can be used; For example, selective fluoride removal is possible (this can be done both regeneratively and non-regeneratively).

It is also expedient if the part of the circulated feed medium that is removed is fed to the third ion reduction unit, for example as so-called backwash water.

It is particularly preferred if the third ion reduction unit is only used in at least one predetermined phase of the electrolysis, in particular the start-up or commissioning (of the system). This makes it possible to take into account the effect that the ion concentration when the system is started up is typically higher than in normal operation, during which the third ion reduction unit is no longer required. Likewise, the third ion reduction unit can also be switched on or off in other operating phases (correspondingly, the water flows may not have to be routed over it). If the third ion reduction unit is not used, the feed medium taken as collection water, in particular after it has been freed of hydrogen, can be fed directly, at least in part, to the oxygen side of the electrolysis unit.

The invention also relates to an electrolysis system for producing hydrogen from water, which is set up to carry out a method described above.

The invention is explained in more detail below with reference to the accompanying drawings, which show systems according to preferred embodiments of the present invention.

Short description of the drawing

Figure 1

shows schematically a system for producing hydrogen using electrolysis according to the state of the art.

Figure 2

shows schematically a system for carrying out a method according to the invention in a preferred embodiment.

Figure 3

shows schematically a system for carrying out a method according to the invention in a further preferred embodiment.

Figure 4

shows schematically a system for carrying out a method according to the invention in a further preferred embodiment.

Figure 5

shows schematically a system for carrying out a method according to the invention in a further preferred embodiment.

Detailed description of the drawing

In Figure 1 A system or electrolysis system 100 is shown schematically, as is known from the prior art, on which the background of the invention will be explained in more detail.

The system 100 has a tank or storage tank 110 for water to be supplied as an insertion medium. New water (so-called make-up water) can be supplied externally. From the tank 110, the water can then be fed as stream b to an ion reduction unit 120, in which the ion content of the water is reduced. The ion reduction unit can in particular be a so-called mixed bed polisher ion exchange unit (MBP). However, an adsorber or a so-called RO-CDI unit (RO stands for "Reverse Osmosis", CDI stands for "Capacitive Deionization") also comes into consideration.

Furthermore, the system 100 has an electrolysis unit 130, which in turn has an oxygen-water separator 131 (on the oxygen side), an actual electrolysis cell 133 with a proton exchange membrane 134, and a hydrogen-water separator 132 (on the hydrogen side). Water or feed medium, see stream c, is removed from the ion reduction unit 120 and fed to the electrolysis unit 130 via the oxygen-water separator 131. From there, the water, cf. stream d, is supplied to the oxygen side of the electrolysis cell 133 by means of a pump 135.

By applying electrical voltage or current to the electrolysis cell 133, the water is split into oxygen and hydrogen. Together with water, oxygen e collects in the oxygen-water separator 131, where oxygen g is separated and removed. The hydrogen f also collects together with water in the hydrogen-water separator 132, where hydrogen h is separated and discharged. Depending on the desired use, both the hydrogen h and the oxygen g can be stored or used directly.

The water (collective water) in the hydrogen-water separator 132 is returned via line k to the tank 110, from where it is fed to the ion reduction unit 120 to reduce its ion content. The amount of water per unit of time (volume flow) is therefore greater for stream b than for stream a.

Water circulates via lines d and e and the oxygen-water separator 131, ie the feed medium is circulated. This serves the proper operation of the electrolysis cell 133. In addition to the newly or freshly supplied water a and the collection water k, part i of the circulated feed medium is also fed into the ion reduction unit 120 with the help of the pump 22 to reduce the ion content.

The amount per time or the volume flow of water that the ion reduction unit 120 has to cope with is therefore the sum of the amounts per time or the volume flows of the streams a, k and i, which together correspond to the current c.

An exemplary volume flow for stream a (make-up water) is 4.2 m ³ /h, that for stream d (and thus the circulated water) is 1590 m ³ /h, and that for stream k is 23, 2 ^m3 /h. In order to sufficiently reduce the overall ion content, for example 6% of stream d can be branched off as stream i, which corresponds to a volume flow of 94.4 m ³ /h. The ion reduction unit 120 must therefore handle a volume flow of 122.8 m ³ /h.

Possibilities of how the ion reduction unit 120 can be made smaller will be described below. In order to be able to make a comparison, the exemplary volume flows mentioned here should also be used again.

In Figure 2 a system 200 for carrying out a method according to the invention is shown schematically in a preferred embodiment. Annex 200 largely corresponds to Annex 100 Figure 1 , so that only other or additional components as well as other or additional streams of water or feed medium or anything else should be explicitly explained. By the way, be aware of the Figure 1 and the associated description.

In the system 200, no part of the circulated feed medium d is diverted and fed to the ion reduction unit 120. The collection water k from the hydrogen-water separator 132 is not fed to the tank 110, but is first treated in a stripper unit or a stripper 140 with a stripping gas I - such as nitrogen - in order to remove any hydrogen contained. The feed medium or water purified in this way is then fed to the oxygen-water separator 131 without further treatment.

Furthermore, part of the circulated feed medium d is branched off via lines n and o as a so-called blow-down (for desalination). For example, the stream n can be wastewater, while the stream o can be water that is used for other purposes, for example for cooling purposes, in particular outside the system 200. Depending on the circumstances, only the current n or only the current o or both can be used in appropriate proportions.

Furthermore, it is provided that the amount per unit of time (or volume flow) that is discharged from the circulated feed medium d as streams n and/or o depends on the electrical conductivity of the circulated feed medium d. For this purpose, a measuring and control device 142 is provided, by means of which, for example, the electrical conductivity in the stream d is measured and a valve 144 is controlled in order to regulate the volume flow of water discharged depending on the electrical conductivity.

Depending on how much water is removed, new water must be added accordingly, ie the volume flow of stream a is then higher than in the example Figure 1 or system 100. If, for example, 3.6 m ³ /h of water is removed, 7.8 m ³ /h must be added accordingly; this is also the amount per unit of time that the ion reduction unit 120 has to cope with. This value is significantly lower than in the example Figure 1 or system 100. This achieves, for example, a fluoride concentration in the system of ≤ 100 ppb.

If, for example, 7.2 m ³ /h of water is removed, 11.4 m ³ /h must be added accordingly; this is also the amount per unit of time that the ion reduction unit 120 has to cope with. This value is still significantly lower than in the example Figure 1 or system 100. This achieves, for example, a fluoride concentration in the system of ≤ 50 ppb (less or equal to 50 ppb).

In Figure 3 a system 300 for carrying out a method according to the invention is shown schematically in a further preferred embodiment. Appendix 300 largely corresponds to Appendix 100 Figure 1 , so that only other or additional components as well as other or additional streams of water or feed medium or anything else should be explicitly explained. By the way, be aware of the Figure 1 and the associated description.

In the system 300, no part of the circulated feed medium d is diverted and fed to the ion reduction unit 120. The collection water k from the hydrogen-water separator 132 is also not supplied to the tank 110, but is first treated in a stripper unit or a stripper 140 with a stripping gas I - such as nitrogen - in order to remove any hydrogen contained. The feed medium or water purified in this way is then fed to the oxygen-water separator 131 without further treatment.

Furthermore, part of the circulated feed medium d is fed via lines p and q to a second ion reduction unit 150 in order to remove ions, i.e. to reduce ion components; This can be done particularly selectively, for example for the particularly relevant fluoride. In addition, for example, NaOH r can be supplied from a storage unit 152, which serves to regenerate the anion exchange material. If an MBP is also used here (anion and cation exchange), HCl can also be used as a regeneration chemical. Any waste water s can be discharged from the second ion reduction unit 150 and used for other purposes, for example, while the water t with a reduced ion content is fed to the ion reduction unit 120.

Furthermore, it is provided that the amount per unit of time (volume flow) that is discharged from the circulated feed medium d as current p depends on the electrical conductivity of the circulated feed medium d. For this purpose, a measuring and control device 142 is provided, by means of which, for example, the electrical conductivity and/or the (degassed) cation conductivity in the stream d is measured and a valve 144 is controlled in order to regulate the volume flow of water discharged depending on the electrical conductivity. Depending on the situation, only the current p, only the current q or both can be dissipated.

Depending on how much water is discharged into the second ion reduction unit 150, the ion reduction unit 120 must handle a correspondingly larger volume flow. For example, if 3.64 m ³ /h of water is removed, 7.84 m ³ /h must be handled in the ion reduction unit 120. This value is significantly lower than in the example Figure 1 or system 100. This achieves, for example, a fluoride concentration in the system of ≤ 100 ppb.

For example, if 7.35 m ³ /h of water is removed, 11.55 m ³ /h must be handled in the ion reduction unit 120. This value is still significantly lower than in the example Figure 1 or system 100. This achieves, for example, a fluoride concentration in the system of ≤ 50 ppb.

In Figure 4 A system 400 for carrying out a method according to the invention is shown schematically in a further preferred embodiment. Annex 400 largely corresponds to Annex 100 Figure 1 , so that only other or additional components as well as other or additional streams of water or feed medium or anything else should be explicitly explained. By the way, be aware of the Figure 1 and the associated description.

In the system 400, part of the circulated feed medium d is branched off and fed to the ion reduction unit 120 via line i, but this part (in the sense of a volume flow) can be lower than in the system 100 according to Figure 1 . The collected water k from the hydrogen-water separator 132 is not fed to the tank 110, but is first treated in a stripper unit or a stripper 140 with a stripping gas I - such as nitrogen - in order to remove any hydrogen contained. The feed medium or water purified in this way is then fed to a third ion reduction unit 160. In addition, part q of the circulated feed medium d is also passed into the third ion reduction unit 160 for treatment.

In the ion reduction unit 160, ions are removed from the water streams m and q, i.e. ion components are reduced; This can be done particularly selectively, for example for the particularly relevant fluoride. In addition, for example, NaOH r can be supplied from a storage 162, and possibly also HCl as described for the system 300. Any wastewater s can be discharged from the ion reduction unit 160 and used for other purposes, for example, while the water v is fed directly to the oxygen-water separator 131.

If, for example, 31.8 m ³ /h of water is diverted from the circulated feed medium d, see stream i, 36 m ³ /h must be handled in the third ion reduction unit 120. This value is significantly lower than in the example Figure 1 or system 100. Water that is discharged into the ion reduction unit 160, for example 23.2 m ³ /h, does not have to be handled by the ion reduction unit 120 here.

It should be mentioned that the second ion reduction unit 150 of the system 300 and the third ion reduction unit 160 of the system 400 can be functionally identical.

In Figure 5 A system 500 for carrying out a method according to the invention is shown schematically in a further preferred embodiment. Annex 500 largely corresponds to Annex 100 Figure 1 , so that only other or additional components as well as other or additional streams of water or feed medium or anything else should be explicitly explained. By the way, be aware of the Figure 1 and the associated description.

In the system 500, part of the circulated feed medium d is branched off and fed to the ion reduction unit 120 via line i, but this part (in the sense of a volume flow) can be lower than in the system 100 according to Figure 1 . The collected water, see stream k, from the hydrogen-water separator 132 is not fed to the tank 110, but is first treated in a stripper unit or a stripper 140 with a stripping gas I - such as nitrogen - in order to contain to remove hydrogen. The feed medium or water purified in this way is then fed to a fourth ion reduction unit 170.

In the fourth ion reduction unit 170 m ions are removed from the water from the stream, i.e. ion components are reduced; This can be done particularly selectively, for example for the particularly relevant fluoride. The water v with reduced ion content is then fed directly to the oxygen-water separator 131.

If, for example, 31.8 m ³ /h of water is diverted from the circulated feed medium d via line i, 36 m ³ /h must be handled in the ion reduction unit 120. This value is significantly lower than in the example Figure 1 or system 100. Water that is discharged into the fourth ion reduction unit 170, for example 23.2 m ³ /h, does not have to be handled by the ion reduction unit 120 here.

The functional principle of the system 500 basically corresponds to that of the system 400, at least with regard to the water k or m collected on the cathode side. A special feature of the system 500, however, is that the fourth ion reduction unit 170 is mobile and, for example, only for Commissioning of the system 500 is used. After a hundred or a few hundred hours of operation, it is no longer needed. As mentioned, the ion content in the water is no longer that high anyway. This principle of only temporarily using a further ion reduction unit 170 for the primary ion reduction unit 120 can also be transferred to the other systems 300 and 400, where it concerns the ion reduction units 150 and 160.

Claims (16)

Method for operating an electrolysis system (200, 300, 400, 500), in which ion components are reduced in water as the feed medium (a) by means of an ion reduction unit (120), the feed medium (b) treated in this way being transferred to an electrolysis unit (130) with a proton -Exchange membrane (134) is supplied, in which the feed medium is converted into hydrogen and oxygen, wherein the feed medium (d) is circulated on the oxygen side of the electrolysis unit (130), and whereby a part (n, o, p, q, k) of the feed medium is removed for the purpose of desalination. The method of claim 1, wherein at least a portion of the feed that is discharged is discharged from the recirculated feed (d). Method according to claim 1 or 2, wherein the amount of the feed medium removed per unit of time is determined as a function of a conductivity of the feed medium. Method according to one of the preceding claims, wherein an amount of feed medium (a) supplied per unit of time is greater than an amount of feed medium converted into hydrogen and oxygen per unit of time, and
wherein excess feed medium, in particular as the part of the circulated feed medium, is removed. Method according to claim 4, wherein the removed part of the feed medium is at least partially removed as wastewater (o). Method according to claim 4 or 5, wherein the removed part of the feed medium is at least partially used externally as cooling water (o). Method according to one of the preceding claims, wherein ion components of the removed part of the circulated feed medium are at least partially reduced by means of a second ion reduction unit (150). are, in particular selectively, and wherein the removed part (u) is then at least partially fed to the ion reduction unit (120). Method according to one of the preceding claims, wherein collecting water (m) on the hydrogen side of the electrolysis unit (130) is at least partially fed to the oxygen side of the electrolysis unit (130) as a feed medium. Method according to claim 8, wherein the feed medium taken as collection water (m) is freed from hydrogen before it is at least partially fed to the oxygen side of the electrolysis unit (130). A method according to claim 8 or 9, wherein the feed medium taken as collection water (m) is included in the portion of the feed medium that is discharged, and wherein therein before it is at least partially supplied to the oxygen side of the electrolysis unit (130), and in particular after it has been freed of hydrogen, ion components are reduced, in particular selectively, by means of a further ion reduction unit (160, 170). The method of claim 10, wherein the portion of the circulated feed medium that is discharged is fed to the further ion reduction unit (160). Method according to claim 10, wherein the further ion reduction unit (170) is only used in a predetermined phase of the electrolysis, in particular the start-up or commissioning. Method according to claim 12, wherein, if the further ion reduction unit (170) is not used, the feed medium (k) taken as collection water, in particular after it has been freed of hydrogen, is fed directly at least partially to the oxygen side of the electrolysis unit (130). . Method according to one of the preceding claims, wherein part (i) of the circulated feed medium is diverted and fed to the ion reduction unit (120). Method according to one of the preceding claims, wherein a mixed-bed polisher ion exchange unit, an adsorbent, or an RO-CDI unit is used as the cleaning unit (120). Electrolysis system (200, 300, 400, 500), which is set up to carry out a method according to one of the preceding claims.

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