NL2032604B1

NL2032604B1 - Method for flushing in an electromembrane process, a device, a membrane stack, and a system to perform said method.

Info

Publication number: NL2032604B1
Application number: NL2032604A
Authority: NL
Inventors: Victor Marie Hamelers Hubertus; Kuntke Philipp; Van Der Kooi Harm; Johanna Donker Ellis; Saakes Machiel
Original assignee: Stichting Wetsus European Centre Of Excellence For Sustainable Water Tech
Priority date: 2022-07-26
Filing date: 2022-07-26
Publication date: 2024-02-05

Abstract

The invention relates to a method a method for flushing in an electromembrane process, such as electrolysis of salt water, using an electrochemical cell, a device, a membrane stack, and a system to perform said method. The method comprises the steps of: — providing the electrochemical cell, comprising: — two or more compartments, wherein one of the two or more compartments comprises a cathode and wherein another of the two or more compartments comprises an anode, and — at least one separator between the two or more compartments, — providing a flow to at least one of the two or more compartments, — applying an electrical potential difference between the anode and the cathode, and — periodically or continuously flushing of at least one of the two or more compartments with a flushing flow using a flushing system, wherein the step of flushing comprises a pH swing in the at least one of the two or more compartments

Description

METHOD FOR FLUSHING IN AN ELECTROMEMBRANE PROCESS, A DEVICE, A

MEMBRANE STACK, AND A SYSTEM TO PERFORM SAID METHOD.

The present invention relates to a method for flushing in an electtomembrane process, such as electrolysis of salt water, using an electrochemical cell. a device, a membrane stack. and a system to perform said method.

Conventional methods and/or systems for flushing in an electromembrane process, such as electrolysis of salt water, using an electrochemical cell are known to pollute during use. Said (undesired) pollution may cause fouling and/or scaling of the conventional method and/or system.

For example when performing electrolysis of a flow chlorine gas may be formed at the anode and scaling of for example magnesium and calcium derivatives at the cathode occurs.

The formation of chlorine gas at the anode is formed due to the slow kinetics of oxygen gas formation and the relatively fast kinetics of chlorine gas formation at the anode. This follows from the fact that the local pH at the anode is low due to formation of protons as a product of the water oxidation. Therefore, no scaling occurs at the anode. However, at the cathode the reduction of water to hydrogen gas leads to high local pH because hydroxyl ions are formed. Said hydroxyl ions may react with divalent magnesium (Mg**) and divalent calcium (Ca?*).

Therefore, conventional methods and/or systems are not suitable for an electtomembrane process in high throughput, as maintenance is limiting the electromembrane process.

An objective of the present invention is to provide a method for flushing in an electromembrane process, such as electrolysis of salt water, using an electrochemical cell that obviates or at least reduces one or more of the aforementioned problems and/or 1s more effective as compared to conventional methods and systems.

This objective is achieved with the method for flushing in an electromembrane process. such as electrolysis of salt water, using an electrochemical cell, comprising the steps of: — providing the electrochemical cell, comprising: — two or more compartments, wherein one of the two or more compartments comprises a cathode and wherein another of the two or more compartments comprises an anode; and — at least one separator between the two or more compartments; — providing a flow to at least one of the two or more compartments; — applying an electrical potential difference between the anode and the cathode; and — periodically or continuously flushing of at least one of the two or more compartments with a flushing flow using a flushing system, wherein the step of flushing comprises a pH swing in the at least one of the two or more compartments.

The method according to the invention may start with the step of providing an electrochemical cell. Said electrochemical cell may comprise two or more compartments, wherein one of the two or more compartments comprises a cathode and wherein another of the two or more compartments comprises an anode. Furthermore, said electrochemical cell comprises at least one separator between the two or more compartments. The at least one separator is configured between the cathode and the anode, and thus forming two or more compartments.

In other words, the two or more compartments extend at least partially between the anode and the cathode, wherein the two or more compartments are separated by the at least one separator.

Thus, the at least one separator is configured to define two or more compartments.

In a preferred embodiment, the separator may be a porous separator, an anion exchange membrane, a cation exchange membrane, a monovalent cation exchange membrane, a proton blocking membrane, or a combination thereof. Preferably, the separator may be a monovalent cation exchange membrane and/or anion exchange membrane and/or proton blocking membrane, more preferably the separator may be an anion exchange membrane and/or a proton blocking membrane. For example, the proton blocking membrane may be a proton blocking anion exchange membrane.

Providing a separator enables a water intake smart hydrogen cell (WISH cell), wherein the method according to the invention enables an efficient and effective electromembrane process, such as electrolysis of salt water. Furthermore, said separator enables to achieve different pH levels in the electrochemical cell. For example, a low pH may be achieved in the compartment at least partly delineated by the separator and being closest to the anode compared to the compartment at least partly delineated by the separator and being closest to the cathode, due to the dissociation of for example (salt) water in the flow and/or flushing flow. Therefore, in-situ prevention and/or reduction of scaling is achieved.

It is noted that in this application the electromembrane process, such as electrolysis of salt water, using an electrochemical cell may include the step of forming hydrogen. Said step of forming hydrogen may also be referred to as producing hydrogen, making hydrogen, manufacturing hydrogen. and the like. Furthermore, producing hydrogen via electrolysis includes the passing of a direct electric current through an electrolyte producing chemical reactions at the electrodes and decomposition of the materials, such as water. At high pH. such as a pH of at least 8, the reaction at the cathode results in hydrogen (gas) and hydroxide ions, at the reaction at the anode results in water and oxygen (gas). Said reactions may be referred to as:

Cathode: 4H:20 +42 => 2H; +4 OH

Anode: 4 OH 2 0, + 2H, 0 +4 ¢

Furthermore, the anode and/or cathode may be entirely or partly provided to the compartment with the anode or the compartment with the cathode respectively. Alternatively, the anode and/or cathode form a boundary of the compartment with the anode or compartment with the cathode respectively.

The step of providing the electrochemical cell may be followed by the step of providing a flow to at least one of the two or more compartments. In a preferred embodiment, said flow may act as anolyte and/or catholyte. Furthermore, said step of providing a flow to at least one of the two or more compartments may include providing a flow to two or more compartments.

It is noted that providing a flow to at least one of the two or more compartments may be the compartment with the anode or a compartment without the anode.

The step of providing a flow to at least one of the two or more compartments may be performed in a continuous manner, semi-continuous manner, and/or periodically. Furthermore, said step may include providing multiple streams of salt water to different compartments.

The step of providing a flow to at least one of the two or more compartments may be followed by the step of applying an electrical potential difference between the anode and the cathode, and the step of periodically or continuously flushing of at least one of the two or more compartments with a flushing flow using a flushing system. Preferably, the flushing of at least one of the two or more compartments with a flushing flow using a flushing system is performed periodically.

In a preferred embodiment according to the invention, the step of forming hydrogen in at least one of the two or more compartments 1s performed in the compartment comprising the cathode. Said step may be performed after the step of applying an electrical potential difference between the anode and the cathode. Preferably, the step of flushing comprises flushing the compartments which are at least partly delineated by the separator.

The step of periodically or continuously flushing of at least one of the two or more compartments with a flushing flow using a flushing system comprises a pH swing in the at least one of the two or more compartments.

Said pH swing includes an increase and/or decrease of the pH in the at least one of the two or more compartments. For example, the pH in the compartment which is at least partly delineated by the separator and being closest to the anode may be provided with a flow comprising for example a neutral pH. The pH in said compartment may decrease over time due to electrolysis of the flow, for example salt water. The flow comprising a decreased pH may than be at least partly replaced by a (fresh) flow and thus the pH in the compartment which is at least partly delineated by the separator and being closest to the anode is increased.

Periodically or (semi)-continuously flushing wherein the step of flushing comprises a pH swing in the at least one of the two or more compartments has as advantage that scaling and/or fouling of the electrochemical cell is reduced and/or prevented. Furthermore, this enables an efficient and effective electrolysis of (salt) water.

Providing a flow to one of the at least two or more compartments enables to perform an electromembrane process, such as electrolysis of salt water, using an electrochemical cell. An advantage of the method according to the invention is that efficient and effective electromembrane process, such as electrolysis of salt water, is achieved, without blocking/clogging and/or fouling and/or scaling of the electrochemical cell. As a result, the method according to the invention increases the lifespan of the electrochemical cell and thus the operation time of the method according to the invention.

In addition, providing a flow to one of the at least two or more compartments while the compartment comprising the catholyte comprises an alkaline solution prevents and/or reduces the formation of undesired/unwanted chlorine species. This can be understood as the redox potential for the oxidation of hydroxyl ions is 0.49 Volt lower compared to the redox potential for the oxidation of chloride to hypochlorite. Preferably, at the start only e.g., a pure sodium hydroxide solution is present, and no chloride ions are present in the catholyte and/or anolyte. Consequently, no chloride species can be oxidized since the redox potential for the oxidation hydroxyl is lower.

At the anode a flow, such as salt water, is passed. Due to the water dissociation at the anode and/or cathode and/or in the at least one separator between the two or more compartments, protons and hydroxyl ions are transported to the compartment comprising the cathode and compartment comprising the anode. The required water for this water dissociation process originates from the flow and/or flushing flow. This is done by using a lower water activity for the alkaline solution for the compartment with the cathode. At a current density of 1000A m™ for the water splitting, it was found that the pH of the compartment comprising the cathode may be increased till values above pH of 10. This is caused by the co-ion transport of chloride ions from the flow towards the compartment with the anode. To keep electroneutrality, hydroxyl ions are transported towards the compartment with the cathode. The pH of the catholyte is very sensitive to scaling and therefore a periodic and/or continuous flow of for example salt water is preferred to be transported through the compartment with the anode and/or compartment configured for transporting the flow. The formation of scaling at the cathode in the form of e.g., magnesium hydroxide and calcium carbonate, can therefore be prevented and/or reduced.

Furthermore, the method according to the invention requires less maintenance compared to a conventional method for flushing in an electromembrane process. Therefore, the operation costs are lower compared to said conventional methods.

Therefore, the method according to the invention is enabled to be performed offshore, without the use of fresh water. As a result, hydrogen may be produced offshore and thus expensive (copper) electricity cables and infrastructure for transporting electricity from the production site to the mainland are not necessary. It is noted that electricity which is produced offshore may be produced by windmills, tidal power station, solar panels. and the like.

A further advantage of the method according to the mvention is that a surplus of electricity can efficiently and effectively be stored as hydrogen gas. Therefore. environmental sources for producing electricity, such as windmills, solar panels, and tidal power stations, may be used efficiently and effectively, and a higher efficiency is achieved. 5 Thus, it has been found that the method according to the invention reduces and/or prevents the forming of (toxic) chlorine species such as hvpochloride, magnesium hydroxide (Mg(OH):) and/or calcium carbonate (CaCO:). Therefore, an efficient and effective method for electrolysis of a flow, such as a flow of salt water, for example sea water, using an electrochemical cell is achieved.

A further advantage of the method according to the invention is that polarity reversal is reduced and/or prevented. Therefore, a robust method is achieved which requires less maintenance compared to conventional methods.

In a preferred embodiment according to the invention, the flow and/or flushing flow comprises water, preferably salt water such as brackish water and/or sea water. It is noted that salt water comprises sodium chloride.

In addition, the flushing flow may originate from the flow, wherein the flushing flow may have a different pH compared to the flow.

In a presently preferred embodiment according to the invention, the compartment which may be at least partly delineated by the separator and being closest to the cathode may be at least partly flushed with the flushing flow provided to the compartment which is at least partly delineated by the separator and being closest to the anode.

It is noted that the flow provided to the compartment which is at least partly delineated by the separator and being closest to the anode may become the flushing flow.

The flushing flow has decreased/lower pH compared to the flow. In particular when salt water is used, said decreased/lower pH. also referred to as pH swing, enables to efficiently and effectively clean the electrochemical call and/or prevent and/or reduce scaling and/or fouling in the electrochemical cell.

It was found that providing the flow, for example a flow of salt water, to the compartment delineated at least partly by the at least one separator and being closest to the anode, and then at least partly flushing the compartment delineated at least partly by the at least one separator and being closest to the cathode with the flushing flow prevents and/or reduces the fouling and/or scaling at the anode and/or cathode. In fact, it was found that the flushing flow used in the method according to the invention may be purified such that the number of solid particles is reduced, preferably substantially removed. Further purifications of the salt water may be performed if desired.

Therefore, the method according to the invention is enabled to be performed at sea/offshore and/or remote places. Furthermore, less maintenance needs to be performed and operations times are extended.

In addition, the number of hazardous trips to a membrane device or place said method is performed is reduced.

A further advantage is that a surplus of electricity generated by a windmill and/or a solar panel and/or a tidal power station may be easily used for electrolysis to form hydrogen. As a result, the capacity of a windmill and/or a solar panel and/or a tidal power station can be optimized and less (potential) electricity must be wasted.

In a further preferred embodiment, the step of providing a flow to at least one of the at least two or more compartments comprises the step of providing a flow to the compartment at least partly delineated by the separator and being closest to the anode, and further comprising the step of providing a flow to the compartment at least partly delineated by the separator and being closest to the cathode.

The step of providing a flow to at least one of the two or more compartments and the step of periodically or (semi)-continuously flushing of at least one of the two or more compartments with a flushing flow using a flushing system may be performed simultaneously in a continuous mode, or one or the other step may be performed in interval whilst the other step is performed continuously.

In other words, one step may be performed continuously, and/or the other step may be performed semi-continuously or periodically. For example, the step of providing a flow to the compartment at least partly delineated by the separator and being closest to the anode may be performed semi- continuously or periodically, the step of providing a flow to the compartment at least partly delineated by the separator and being closest to the cathode may be performed continuously, and the step of flushing of at least one of the two or more compartments may be performed semi- continuously or periodically.

In a preferred embodiment. the step of providing a flow to the compartment at least partly delineated by the separator and being closest to the anode is performed in a semi-continuous mode or periodically, wherein the step of providing a flow to the compartment at least partly delineated by the separator and being closest to the anode is performed for at most 80% of the time, preferably at most 65% of the time, more preferably about 50% of the time, of the step of providing a flow to at least one of the two or more compartments. Preferably, the step of providing a flow to the compartment at least partly delineated by the separator and being closest to the anode and the step of periodically or continuously flushing of at least one of the two or more compartments are simultaneously performed.

Furthermore, at least 50% of the volume, preferably at least 75% of the volume, more preferably substantially about 100% of the volume of the flow provided to the compartment at least partly delineated by the separator and being closest to the anode may be provided to the compartment at least partly delineated by the separator and being closest to the cathode.

An advantage of at least partly flushing the compartment which may be at least partly delineated by the separator and being closest to the cathode with the flow, referred to flushing flow when the pH is lowered. provided to the compartment which is at least partly delineated by the separator and being closest to the anode is that a pH swing within the two or more compartments and between the two or more compartments may be established. As a result, scaling and/or fouling in the WISH cell is prevented and/or reduced compared to conventional methods.

It was found that periodically or continuously flushing of at least one of the two or more compartments with a flushing flow, comprising a low pH. of the compartment which is at last partly delineated by the separator and being closest to the cathode at an interval (in a semi- continuous manner) enables an efficient and effective electrolysis of the flow, such as a flow of salt water. The interval, also referred to of the interval of the semi-continuous manner, may for example be at periods of 30% running time and 50% down time of flushing. During the 50% running time the compartment which is at least partly delineated by the separator being closest to the anode is emptied (a flow with a low pH, such as salt water with a low pH) and filled with a flow, such as (fresh) salt water, wherein the flow, such as salt water, coming from the compartment which is at least partly delineated and being closest to the anode is flushed/circulated/provided to the compartment which is at least partly delineated and being closest to the cathode. In addition, a flow, such as (fresh) salt water, may be provided, in a continuous manner, to the compartment which is at least partly delineated by the separator and being closest to the cathode. It will be understood that other configurations or regimes for the circulation/flushing can be envisaged in accordance with the present invention. An advantage of said step is that that in-situ cleaning may be performed and scaling is prevented and/or reduced. For example, the method according to the invention prevents and/or reduces growth of bacteria, algae, formation of magnesium hydroxide, and formation of calcium carbonate.

As a result of the method according to the invention, the life-span of the membrane device is increased and the operational costs are lowered. This is especially advantageous for off-shore applications.

For example, at the time interval of 7; to £; a flow, such as (fresh) salt water, is provided to the compartment which is at least partly delineated by the separator and being closest to the anode with a flow rate of X L min. Furthermore, at the time interval of #; to #; a flushing flow, such as salt water with a low pH, is provided to the compartment which is at least partly delineated by the separator and being closest to the cathode with a flow rate of Y L min™. In addition. an outlet of the compartment which is at least partly delineated by the separator and being closest to the anode is operatively coupled with an inlet of the compartment which is at least partly delineated by the separator and being closest to the cathode. Thus, the output of the compartment which is at least partly delineated by the separator and being closest to the anode is flushed/provided (as input) to the compartment which is at least partly delineated by the separator and being closest to the cathode. Therefore, at the time interval of fa to 7: the total flow rate in the compartment which is at least partly delineated by the separator and being closest to the cathode is at least X + Y L min.

Additionally, at the time interval of #9 to #; a flow, such as (fresh) salt water, is provided to the compartment which is at least partly delineated by the separator and being closest to the cathode with a flow rate of Z L min”. Therefore, at the time interval of #, to #; the total flow rate in the compartment which is at least partly delineated by the separator and being closest to the cathode is at least X + Y + ZL min".

At the time interval of f: to £; no flow, such as (fresh) salt water, is provided to the compartment which is at least partly delineated by the separator and being closest to the anode.

Furthermore, at the time interval of #; to £2 a flow, such as (fresh) salt water, is provided to the compartment which is at least partly delineated by the separator and being closest to the cathode with a flow rate of Z L min’! Therefore, at the time interval of #; to the total flow rate in the sub- compartment closest to the compartment with the cathode is at least Z L min".

Providing a flow, such as (fresh) salt water, to the compartment which is at least partly delineated by the separator and being closest to the cathode enables an efficient and effective electrolysis of salt water.

The time interval of ¢; to #7 and #; to £> may alternate. In other words, the #; to £; may be followed by £; to 72, which than may be followed by £ to f;, etc. The alternating time interval enables a pH swing in at least one of the two or more compartments. The compartment which is at least partly delineated by the separator and being closest to the cathode may particularly benefit from said pH swing. A low pH in said compartment prevents and/or reduces fouling in said compartment.

In a further presently preferred embodiment according to the invention, the flushing flow has a pH of at most 5, preferably a pH of at most 4, more preferably a pH of at most 3. Preferably, the pH of the flushing flow may formed in-situ.

It was found that a flushing flow having a pH of at most 5, preferably a pH of at most 4, more preferably a pH of at most 3, most preferably a pH of at most 2.5 prevents and/or reduces fouling and/or scaling in said sub-compartment.

In a preferred embodiment, the pH of the flushing flow has a pH of at least 1, preferably a pH of at least 2.

For example, the flushing flow may have a pH in the range of 1 to 5, preferably in the range of I to 4, more preferably in the range of 1 to 3.

The flushing of the compartment which is at least partly delineated by the separator and being closest to the cathode with a flushing flow which originates from the compartment which is at least partly delineated by the separator and being closest to the anode enables an in-situ pH swing and use of the in-situ formed flushing flow comprising a pH of at most 5, preferably a pH of at most 4, more preferably a pH of at most 3, most preferably a pH of at most 2.5, and a pH of at least 1, preferably a pH of at least 2.

Preferably, a flow, such as (fresh) salt water, is provided to the compartment which is at least partly delineated by the separator and being closest to the anode.

The pH of the flow, such as salt water, in said compartment is decreased to a pH of at most 5, preferably a pH of at most 4, more preferably a pH of at most 3, most preferably a pH of at most 2.5. The flushing flow, such as salt water with lowered pH, with lowered pH may periodically be provided to the compartment which is at least partly delineated by the separator and being closest to the cathode. In addition, a flow. such as (fresh) salt water, may continuously be provided to the compartment which is at least partly delineated by the separator and being closest to the cathode.

An advantage of the in-situ formation of an acidic flow which may be used as flushing flow reduces the use of added anodic acid in order to achieve an efficient and effective electromembrane process.

Thus, a pH swing enables an efficient and effective method according to the invention.

It was found that in certain embodiments of the invention said pH swing in the compartment which is at least partly delineated by the separator and being closest to the anode may be from pH 8 to 2, and for the compartment which is at least partly delineated by the separator and being closest to cathode may be from pH 10 to 2. A (periodically) low pH for both sub-compartments is preferred as scaling and/or fouling is prevented and/or reduced.

In a further preferred embodiment the anode and/or cathode are one or more of a platinum electrode, platinum coated electrode, nickel electrode, nickel foam electrode, carbon electrode.

Preferably, the cathode is nickel foam electrode. Alternatively, a non-chlorine evolving anode may be used e.g. platinum coated with a thin layer of silicon dioxide or coated nickel foam electrode.

An advantage of a nickel foam electrode is that the formed hydrogen (gas) may easily be recovered from the catholyte. Furthermore, an advantage of the non-chlorine evolving anode is that the formation of chlorine gas is prevented and/or reduced.

In a further presently preferred embodiment according to the invention, the flow and/or flushing flow has a salt content in the range of 0.5 part per thousand to 50 parts per thousand, preferably in the range of 7 parts per thousand to 50 parts per thousand, more preferably in the range of 30 parts per thousand to 50 parts per thousand.

Said salt content enables an efficient and effective electrolysis and therefore production of hydrogen (gas).

In a further presently preferred embodiment according to the invention, the flow has a pH in the range of 7.2 to 9.5, preferably a pH in the range of 7.5 to 9.0, more preferably a pH in the range of 7.5 to 8.5.

The flow, such as salt water, having said pH refers to the flow provided to the at least one of the two or more compartments. Thus, the flow which has not been treated in an electromembrane process, such as electrolysis of salt water.

In a further presently preferred embodiment according to the invention, further comprises the step of obtaining hydrogen gas.

The step of obtaining hydrogen gas, for example from the flow which is present in and/or removed from the compartment which is at least partly delineated by the separator and being closest to the cathode, may be performed simultaneously or after the step of applying an electrical potential difference between the anode and the cathode and/or the step of forming hydrogen in the compartment which is at least partly delineated by the separator and being closest to the cathode.

In a further presently preferred embodiment according to the invention, wherein the flow and/or the flushing flow may be provided with a flushing rate in the range of 5 L min"! per cubic decimetre of the compartment to 50 L min’! per cubic decimetre of the compartment the flow and/or the flushing flow is provided to, preferably in the range of 10 L min™ per cubic decimetre of the compartment to 40 L min"! per cubic decimetre of the compartment the flow and/or the flushing flow is provided to, more preferably in the range of 20 L min’! per cubic decimetre of the compartment to 30 L min’! per cubic decimetre of the compartment the flow and/or the flushing flow is provided to.

It was found that providing the flow and/or the flushing flow with a flushing rate in said range, efficient and effective electrolysis of the flow and/or flushing flow was achieved. In addition, said range enabled to prevent and/or reduce fouling and/or scaling in the electrochemical cell during an electromembrane process, such as electrolysis of the flow, for example electrolysis of salt water.

In a further presently preferred embodiment according to the invention, further comprises the step of treating the flow with UV-light before the step of providing a flow to at least one of the two or more compartments.

The step of treating the salt water with UV-light before the step of providing a flow to one of the at least two compartments may be performed before the step of providing salt water to another one of the at least two or more compartments. An advantage of treating the salt water with UV- light is that the amount of micro-organism present in the salt water is reduced. As a result, the method becomes more robust, and less mamtenance has to be performed.

In a further presently preferred embodiment according to the invention, further comprises the step of filtering the flow before the step of providing a flow to one of the at least two compartments.

The step of filtering the flow, such as salt water, before the step of providing a flow to one of the at least two compartments may be performed before the step of providing a flow to another one of the at least two or more compartments. An advantage of the step of filtering is that solid particles are removed from the salt water provided to the membrane device. As a result, the membranes of the membrane device and/or the separator are less damaged by solid particles present in the provided salt water.

A further advantage is that microplastics may be filtered from the flow, such as salt water, provided to the electrochemical cell. Therefore, the flow used in the method according to the invention is also purified, and the less microplastics may harm the environment.

The invention also relates to a device for flushing in an electromembrane process, such as electrolysis of salt water, configured to perform the method according to the invention, comprising: — at least two or more compartments, wherein one of the two or more compartments comprises a cathode and wherein another compartment of the two or more compartments comprises an anode; — at least one separator between the two or more compartments; — means to provide an electrical potential difference between the cathode and the anode; and — a flushing system configured for flushing the compartments at least partly delineated by the separator. comprising periodically or continuously providing a flushing flow from the compartment at least partly delineated by the separator and being closest to the anode to the compartment which is at least partly delineated by the separator and being closest to the cathode.

The device for flushing in an electtromembrane process. such as electrolysis of salt water, configured to perform the method according to the invention provides the same effects and advantages as those described for the method for flushing in an electromembrane process according to the invention.

An advantage of the device for flushing in an electromembrane process, such as electrolysis of salt water, is that fouling and/or scaling is prevented and/or reduced. Therefore, said device for flushing in an electromembrane process, such as electrolysis of salt water, may be used under corrosive conditions such as at sea.

Furthermore, the device for flushing in an electromembrane process, such as electrolysis of salt water, according to the invention may further comprise features as described for the method according to the invention. Preferably the flushing system comprises at least one pump configured to flush at least one of the two or more compartments, and/or at least one valve, and/or at least control unit configured to control the flow and/or flushing flow, and/or conduits/connections to operatively couple the two or more compartments with the flow and/or flushing flow and/or the compartments.

In a preferred embodiment, the device for flushing in an electromembrane process, such as electrolysis of salt water, according to the invention further comprises at least two membranes, wherein said two membranes are bipolar membranes, and a separator, wherein said separator may be a monovalent cation exchange membrane and/or anion exchange membrane and/or proton blocking membrane, more preferably the separator may be an anion exchange membrane and/or a proton blocking membrane. For example, the proton blocking membrane may be a proton blocking anion exchange membrane.

Furthermore, the device for flushing in an electromembrane process. such as electrolysis of salt water, may further comprise controlling and/or communicating means. Said controlling and/or communicating means enable to monitor the device for electrolysis remotely. Therefore, the maintenance of the device for electrolysis according to the invention may be optimised.

It is noted that the controlling means may include sensors and the like.

In a presently preferred embodiment according to the invention, the flushing system further comprises a pH sensor which may be configured to determine the pH of the flushing flow.

An advantage of the pH sensor is that the pH swing may be monitored and timely maintenance may be performed.

The invention also relates to a membrane stack for flushing in an ¢lectromembrane process. such as electrolysis of salt water, the stack comprising a number of cells which are configured to perform the method according to the invention.

The membrane stack for flushing in an electromembrane process, such as electrolysis of salt water, provides the same effects and advantages as those described for the method and device for flushing m an electromembrane process according to the invention.

The membrane stack according to the invention comprises a number of cells, wherein said cells are separated via a bipolar membrane. In addition, the cells may also be referred to as a membrane device.

The invention also relates to a system for flushing in an electromembrane process, such as electrolysis of salt water, the system comprising: — adevice according to the invention; — a flow supply that is operatively connected to at least one of the two or more compartments and that is configured to supply the flow to said compartment; and — an outlet that is configured to discharge hydrogen from the device.

The system for flushing in an electromembrane process, such as electrolysis of salt water, provides the same effects and advantages as those described for the method, device, and membrane stack for flushing in an electromembrane process according to the invention.

Preferably, the system comprises an electrical connection for providing a voltage and/or current to the system.

In a preferred embodiment, the compartment with the anode and/or compartment with the cathode at least comprises partly the anode and cathode respectively.

In a presently preferred embodiment according to the invention, the means to provide an electrical potential difference is a windmill and/or a tidal power station.

The means to provide an electrical potential difference via the electrical connection may be a windmill and/or tidal power station. This is especially advantage in offshore applications.

Further advantages. features and details of the invention are elucidated on the basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, in which: — Figure 1 shows a schematic overview of the method according to the invention; — Figure 2 shows a schematic overview of the electrochemical cell according to the invention comprising one separator; — Figure 3 shows a schematic overview of the electrochemical cell according to the invention comprising two bipolar membranes and a separator (WISH cell); — Figure 4 shows a schematic overview of a pump regime; — Figure 5 shows the experimental results of the cell voltage of the method according to the invention having a pH swing: — Figure 6 shows a pH swing of the experimental results of Figure 15; — Figure 7 shows the pH of the salt water during electrolysis at current density of 1000 A min WISH cell; — Figure 8 shows a segment of Figure 5; — Figure 9 shows the cell voltage during electrolysis at current density of 1000 A m™ in

WISH cell; and — Figure 10 temperature removed from the WISH cell during experiment.

Method 10 (Figure 1) for flushing in an electromembrane process, such as electrolysis of salt water, follows a sequence of steps.

In the illustrated embodiment method 10 may start with step 12 of providing the electrochemical cell.

The electrochemical cell provided in step 12 comprises two or more compartments, wherein one of the two or more compartments comprises a cathode and wherein another of the two or more compartments comprises an anode, and at least one separator between the two or more compartments.

Step 12 of providing the electrochemical cell may be followed by step 14 of providing a flow to at least one of the two or more compartments. Said step may, preferably, comprise step 16 of providing a flow to the compartment at least partly delineated by the separator and being closest to the anode, and step 18 of providing a flow to the compartment at least partly delineated by the separator and being closest to the cathode.

Alternatively, step 12 of providing the electrochemical cell may be followed by step 20 treating the flow with UV-light before the step of providing a flow to one of the at least two compartments, and/or step 22 of filtering the flow before the step of providing a flow to one of the at least two compartments.

Step 14 may be followed by step 24 of applying an electrical potential difference between the anode and the cathode in a presently preferred embodiment of the invention. Applying an electrical potential difference between the anode and the cathode may enable to form hydrogen.

Therefore, step 24 may be followed by step 26 of forming hydrogen in at least one of the two or more compartments. Step 24 and/or step 26 may be followed by step 28 of periodically or continuously flushing of at least one of the two or more compartments with a flushing flow using a flushing system.

Step 28 is followed by step 30 of obtaining hydrogen gas.

Electrochemical cell 32 (Figure 2) comprises two compartments 34 and 36, wherein compartment 34 comprises cathode 38. and wherein compartment 36 comprises anode 40.

Electrochemical cell 32 is one of the embodiments that can be used in method 10. Furthermore, electrochemical cell 32 comprises separator 42, and means for applying an electrical potential 44.

Separator 42 is assembled between compartments 34 and 36 such that compartments 34 and 36 are (partly) delineated by separator 42.

Compartment 36 further comprises inlet 46 and outlet 48, wherein inlet 46 is configured for providing a flow to compartment 36 and outlet 48 is configured for removing flushing flow from compartment 36. Compartment 36 is provided with flow 50.

In use, electrochemical cell 32 enables to split water at cathode 38 into hydrogen and hydroxide ions. Said hydroxide ions may diffuse to compartment 36 where it may react with H to form water. In addition, chloride ions (which are present in salt water) may form chlorine at anode 40, as the formation of chlorine is energetically more favourable compared to the formation of oxygen (02).

Furthermore, compartment 34 may comprise inlet 52 which is configured for providing a flow, and outlet 54 which is configured to remove the flow. In addition, compartment 34 may comprise flow 56.

Furthermore, electrochemical cell 32 may further comprise flushing system 58, wherein flushing system 58 is configured for flushing compartment 34 with flushing flow 60.

Electrochemical cell 32 further comprises controlling and/or communicating means 62 such that electrochemical cell 32 (in use) may be monitored via said means.

Electrochemical cell 64 (Figure 3) comprises three compartments 34, 36, and 66, wherein compartment 34 comprises cathode 38, and wherein compartment 36 comprises anode 40.

Furthermore, electrochemical cell 64 comprises bipolar membranes 68 and 70, means for applying an electrical potential 44, and means 72 for applying a current density to bipolar membrane 68 and/or 70. Bipolar membrane 68 is assembled between compartments 34 and 66 such that compartments 34 and 66 are (partly) delineated by bipolar membrane 68. Furthermore, bipolar membrane 68 is assembled between compartments 36 and 66 such that compartments 36 and 66 are (partly) delineated by bipolar membrane 70.

Compartment 66 is separated by separator 74 into sub-compartments 76 and 78. Separator 74 is configured to transport (monovalent) cations and/or anions from sub-compartment 78 to sub- compartment 76 or vice versa. Sub-compartment 76 further comprises inlet 80 and outlet 82, wherein inlet 80 is configured for providing a flow via pump 84 to sub-compartment 76. In addition, outlet 82 is configured for removing acidified flow, also referred to as a flow with a lowered pH, from sub-compartment 76 and providing the acidified flow to sub-compartment 78 via pump 86. Preferably, removing the flow, such as salt water, via outlet 82 and providing the acidified flow, such as acidified salt water, to sub-compartment 78 is performed by intervals.

It is noted that sub-compartment 76 may also be referred to as the compartment which is at least partly delineated by separator 74 and being closest to anode 40, and that sub-compartment 78 may also be referred to as the compartment which is at least partly delineated by separator 74 and being closest to cathode 38.

Furthermore, sub-compartment 78 further comprises inlet 88 and outlet 90, wherein inlet 88 is configured for providing flow. such as salt water. via pump 92 to sub-compartment 78 and outlet 90 is configured for removing the flow, such as salt water, from sub-compartment 78. Preferably, the flow, such as salt water, provided to sub-compartment 78 via inlet 88 is in a continuous manner. Compartment 34 is provided with catholyte 94, and compartment 36 is provided with anolyte 96. Preferably, anolyte 96 is a solution of 1 M sodium hydroxide and 0.5 M sodium chloride.

In use, electrochemical cell 64 enables to split water at cathode 38 into hydrogen and hydroxide ions. The hydroxide ions may react with the H* provided by splitting water in the bipolar membrane. Due to the energetically favourable reactions between hydroxide and H, scaling and/or fouling is prevented.

Furthermore, electrochemical cell 64 (in use) may be monitored via controlling and/or communicating means 62.

Pump regime 98 (Figure 4) shows a pump regime for pump 86 and pump 92. Pump 86 pumps at an interval, wherein the interval ¢ to #; refers to a switched-off position and #; to £ refers toa switched-on position of said pump. Furthermore, pump 92 pumps continuously.

It is noted that other pump regimes are also feasible.

In a further experiment, the embodiment disclosed in Figure 3 (WISH cell) was used. Said embodiment enables a pH swing. The bipolar membranes were provided with a current density of 1000 A m™, and a nickel foam anode 1.4 mm (350 g m™) and a nickel foam cathode 1.4 mm (350 g m™) were used. Furthermore, anolyte was 1 M sodium hydroxide, the catholyte was 1 M sodium hydroxide. The cell voltage of the experiment is shown in Figure 5, wherein Cell voltage (V) is shown on the y-axis and time (s) on the x-axis.

The pump providing salt water to the sub-compartment closest to the compartment with the anode was switched-on for 30 seconds and switched-off for 30 seconds which than alternates. Said pump provided salt water with 600 mL min" to said sub-compartment. The pump providing (fresh) salt water in a continuous manner to the sub-compartment closest to the compartment with the cathode provided (fresh) salt water with 600 mL min’! to said sub-compartment.

The acidified salt water provided a pH swing in the sub-compartment closest to the compartment with the cathode. Said pH swing is shown in Figure 6, wherein the y-axis shows the pH and the x-axis shows the time (s).

In a further experiment the WISH cell (also disclosed in Figure 3), the pH of the salt water in the sub-compartment of the compartment closest to the cathode was further explored. The experiment was performed using an anolyte of 1 M sodium hydroxide, a catholyte of 1 M sodium hydroxide, salt water comprising 30 g L™ atlantic sea salt, a 1.4 mm nickel foam 350 g m™ anode and cathode, 40 micron anion exchange membrane, run time of 5 hours, a current density of 1000

A m? provided to the bipolar membranes and the salt water had a temperature of 22.2 °C.

Furthermore, the geometric area of the electrochemical cell 64 was 100 cm}.

The pump providing salt water to the sub-compartment closest to the compartment with the anode was switched-on for 30 seconds and switched-off for 30 seconds which than alternates. Said pump provided salt water with 600 mL min’! to said sub-compartment. The pump providing (fresh) salt water in a continuous manner to the sub-compartment closest to the compartment with the cathode provided (fresh) salt water with 600 mL min™ to said sub-compartment.

Figure 7 shows the pH swing in the sub-compartment closest to the compartment with the cathode. Figure 8 is a segment thereof. The y-axis shows the pH of the salt water in the sub- compartment closest to the compartment with the cathode and the x-axis shows the time (s).

Furthermore, Figure 9 shows the cell voltage (V) being in the range of 6 V to 6.1 V, preferably in the range of 6.042 V to 6.049 V.

It also becomes clear that the temperature of the salt water slightly increases over time (Figure 10). Figure 10 shows said increase of the temperature, wherein the y-axis shows the temperature (°C) and the x-axis shows the time (s). Said increase can be explained because a tank of about 120 Litres was used to provide the salt water to the WISH cell. The temperature increase will be less of an issue, or no issue when the method according to the invention is applied off- shore.

From these experiments it becomes clear that an efficient, effective, and stable method and device for electrolysis of salt water is achieved.

The present invention is by no means limited to the above described preferred embodiments and/or experiments thercof. The rights sought are defined by the following claims within the scope of which many modifications can be envisaged.

CLAUSES

1. Method for flushing in an electromembrane process, such as electrolysis of salt water, using an electrochemical cell, comprising the steps of: — providing the electrochemical cell, comprising: — two or more compartments, wherein one of the two or more compartments comprises a cathode and wherein another of the two or more compartments comprises an anode; and — at least one separator between the two or more compartments; — providing a flow to at least one of the two or more compartments; — applying an electrical potential difference between the anode and the cathode; and — periodically or continuously flushing of at least one of the two or more compartments with a flushing flow using a flushing system, wherein the step of flushing comprises a pH swing in the at least one of the two or more compartments. 2. Method according to clause 1, wherein the compartment which is at least partly delineated by the separator and being closest to the cathode is at least partly flushed with the flushing flow provided to the compartment which is at least partly delineated by the separator and being closest to the anode. 3. Method according to any one of the preceding clauses, wherein the flushing flow has a pH of at most 5, preferably a pH of at most 4, more preferably a pH of at most 3. 4. Method according to clause 3, wherein the pH of the flushing flow is formed in-situ. 5. Method according to any one of the preceding clauses, wherein the flow and/or flushing flow has a salt content in the range of 0.5 part per thousand to 30 parts per thousand, preferably in the range of 7 parts per thousand to 50 parts per thousand, more preferably in the range of 30 parts per thousand to 50 parts per thousand. 6. Method according to any one of the preceding clauses, wherein the flow has a pH in the range of 7.2 to 9.5, preferably a pH in the range of 7.5 to 9.0, more preferably a pH in the range of 7.510 8.5.

7. Method according to any one of the preceding clauses, further comprising the step of obtaining hydrogen gas.

8. Method according to any one of the preceding clauses, wherein the flow and/or the flushing flow is provided with a flushing rate in the range of 5 L min’! per cubic decimetre of the compartment to 50 L min’! per cubic decimetre of the compartment the flow and/or the flushing flow is provided to, preferably in the range of 10 L min"! per cubic decimetre of the compartment to 40 L min"! per cubic decimetre of the compartment the flow and/or the flushing flow is provided to, more preferably in the range of 20 L min’! per cubic decimetre of the compartment to 30 L min’

tper cubic decimetre of the compartment the flow and/or the flushing flow is provided to.

9. Method according to any one of the preceding clauses, further comprising the step of treating the flow with UV-light before the step of providing a flow to at least one of the two or more compartments.

10. Method according to any one of the preceding clauses, further comprising the step of filtering the flow before the step of providing a flow to one of the at least two compartments.

11. Device for flushing in an ¢lectromembrane process, such as electrolysis of salt water,

configured to perform the method according to any one of the preceding clauses, comprising:

— at least two or more compartments, wherein one of the two or more compartments comprises a cathode and wherein another compartment of the two or more compartments comprises an anode;

— at least one separator between the two or more compartments;

— means to provide an electrical potential difference between the cathode and the anode; and

— aflushing system configured for flushing the compartments at least partly delineated by the separator, comprising periodically or continuously providing a flushing flow from the compartment at least partly delineated by the separator and being closest to the anode to the compartment which is at least partly delineated by the separator and being closest to the cathode.

12. Device according to clause 13, wherein the flushing system further comprises a pH sensor which is configured to determine the pH of the flushing flow.

13. Membrane stack for flushing in an electromembrane process, such as electrolysis of salt water, the stack comprising a number of cells which are configured to perform the method according to any one of the clauses 1 to 10. 14. System for flushing in an electromembrane process, such as electrolysis of salt water, comprising: —- adevice according to clause 12 or 13; —- a flow supply that is operatively connected to at least one of the two or more compartments and that is configured to supply the flow to said compartment; and — an outlet that is configured to discharge hydrogen from the device. 15. System according to clause 14, wherein the means to provide an electrical potential difference is a windmill and/or a tidal power station.

Claims

CONCLUSIONS

1. Method for rinsing in an electromembrane process, such as salt water electrolysis, using an electrochemical cell, comprising the steps of: - providing the electrochemical cell comprising: - two or more compartments, one of the two or more compartments comprises a cathode and another of the two or more compartments comprises an anode; and — at least one separator between the two or more compartments: — providing power to at least one of the two or more compartments; - applying an electrical potential difference between the anode and the cathode; and - periodically or continuously flushing at least one of the two or more compartments with a flushing flow using a flushing system, wherein the flushing step includes a pH fluctuation in at least one of the two or more compartments.

2. Method according to claim 1, wherein the compartment that is at least partially delimited by the separator and closest to the cathode is at least partially flushed with the flushing current provided to the compartment that is at least partially demarcated by the separator and is closest to the anode.

Method according to any one of the preceding claims, wherein the rinse flow has a pH of at most 5, preferably a pH of at most 4, more preferably a pH of at most 3.

Method according to claim 3, wherein the pH of the rinse stream is formed in-situ.

5. Method according to any one of the preceding claims, wherein the stream and/or the rinse stream has a salt content in the range of 0.5 parts per thousand to 50 parts per thousand, preferably in the range of 7 parts per thousand to and at 50 parts per thousand, more preferably in the range of 30 parts per thousand to 50 parts per thousand.

Method according to any one of the preceding claims, wherein the stream has a pH in the range of 7.2 to 9.5, preferably a pH in the range of 7.5 to 9.0, more preferably a pH in the range of 7.5 to 8.5.

7. Method according to any one of the preceding claims, further comprising the step of obtaining hydrogen gas.

Method according to any one of the preceding claims, wherein the current and/or the flushing current is provided with a refresh rate in the range of 5 L min. per cubic decimeter of the compartment up to and including 50 L min'! the flow and/or flushing flow is provided per cubic decimeter of the compartment, preferably in the range of 10 L min'! per cubic decimeter of the compartment up to and including 40 L min'! per cubic decimeter of the compartment the flow and/or the flushing flow is provided, in the range of 20 L min'! per cubic decimeter of the compartment up to and including 30 L min'! per cubic decimeter of the compartment the current and/or the flushing current is provided.

9. Method according to any one of the preceding claims, further comprising the step of treating the flow with UV light prior to the step of providing a flow to at least one of the two or more compartments.

A method according to any one of the preceding claims, further comprising the step of filtering the flow prior to the step of providing a flow to one of the two or more compartments.

11. Device for rinsing in an electromembrane process, such as electrolysis of salt water, designed for carrying out the method according to any one of the preceding claims. comprising: - at least two or more compartments, wherein one of the two or more compartments comprises a cathode and wherein another compartment of the two or more compartments comprises an anode; — at least one membrane between the two or more compartments; - means for applying an electrical potential difference between the cathode and the anode; and — a flushing system designed for flushing the compartments that are at least partially demarcated by the separator, including periodically or continuously providing a flushing flow of the compartment that is at least partially demarcated by the separator and closest to the anode is located on the compartment that is at least partially delimited by the separator and is closest to the cathode.

12. Device according to claim 13, wherein the flushing system further comprises a pH sensor that is designed to determine the pH of the flushing flow.

13. Membrane stack for rinsing in an electromembrane process, such as electrolysis of salt water, wherein the stack comprises a number of cells designed to carry out the method according to any one of claims 1 to 10.

14. System for rinsing in an electromembrane process. such as electrolysis of salt water, comprising: - a device according to claim 12 or 13; - a power supply that is operatively connected to at least one of the two or more compartments and that is designed to supply power to the compartment; and — an outlet designed to let hydrogen out of the device.

15. System according to claim 14, wherein the means for providing an electric potential is a windmill and/or a tidal power station.