ELECTROLYSER SYSTEM OF WATER ELECTROLYSIS AND PROCESS THEREFOR

Abstract

An electrolyser system having an electrolysis stack and a direct current source, in order to generate oxygen and hydrogen as electrolysis gas by electrolysis of a water containing electrolysis medium. The electrolysis stack includes an anode section configured to generate oxygen and a cathode section configured to generate oxygen. Furthermore, the electrolyser system has an anode gas separator configured to separate oxygen from the electrolysis medium and a cathode gas separator configured to separate hydrogen from the electrolysis medium, wherein at least one of the gas separators includes a gas separating section and a gas cooling section, wherein the gas cooling section has a water inlet connected with a water supply, in order to supply cooling water to the gas cooling section of the gas separator, for the direct cooling of the electrolysis gas separated in the gas separating section of the gas separator within the gas cooling section.

Claims

1. An electrolyser system for water electrolysis, comprising an electrolysis stack and a direct current source, in order to generate oxygen and hydrogen as electrolysis gas by electrolysis of a water containing electrolysis medium, the electrolysis stack comprising an anode section configured to generate oxygen and a cathode section configured to generate hydrogen; an anode gas separator configured to separate oxygen from the electrolysis medium and a cathode gas separator configured to separate hydrogen from the electrolysis medium; wherein, at least one of the gas separators comprises a gas separating section and a gas cooling section, wherein the gas cooling section comprises a water inlet connected with a water supply, in order to supply cooling water to the gas cooling section of the gas separator, for the direct cooling of the electrolysis gas separated in the gas separating section of the gas separator within the gas cooling section.

2. The electrolyser system according to claim 1, wherein the cooling water supplied to the gas cooling section is used at least in part in order to balance out the water consumption resulting from the electrolysis.

3. The electrolyser system according to claim 1, wherein the gas separating section and the gas cooling section are arranged within one common housing.

4. The electrolyser system according to claim 1, wherein the water inlet of the gas cooling section is arranged in order to conduct the electrolysis gas to be cooled and the cooling water supplied to the gas cooling section in counter-flow.

5. The electrolyser system according to claim 1, wherein the gas cooling section of the gas separator is arranged vertically with respect to the perpendicular mediated by gravity, so that a stream of the cooling water can be conducted from top to bottom within the gas cooling section of the gas separator and a stream of the electrolysis gas can be conducted from bottom to top within the gas cooling section of the gas separator.

6. The electrolyser system according to claim 1, wherein the electrolyser system is operated at elevated pressure of 10 bar or more, and wherein the water supplied to the gas cooling section of the gas separator is used to fully balance out the water consumption resulting from the electrolysis.

7. The electrolyser system according to claim 1, wherein the electrolyser system is operated at elevated pressure of 10 bar or more, and wherein solely an amount of water required to fully balance out the water consumption resulting from electrolysis is supplied as the cooling water to the gas cooling section of the gas separator.

8. The electrolyser system according to claim 6, wherein the electrolyser system comprises an alkaline electrolyser, and the electrolyser system is operated at a pressure of 18 bar or more.

9. The electrolyser system according to claim 6, wherein the electrolyser system comprises a proton exchange membrane electrolyser, and the electrolyser system is operated at a pressure of 23 bar or more.

10. The electrolyser system according to claim 1, wherein the electrolyser system does not comprise a separator vessel to separate condensed water from cooled electrolysis gases.

11. The electrolyser system according to claim 1, wherein the water inlet comprises a nozzle to introduce the cooling water as a spray into the gas cooling section of the gas separator.

12. The electrolyser system according to claim 1, wherein a chiller is arranged between the water supply and the water inlet to pre-cool the cooling water supplied to the gas cooling section.

13. A process for performing electrolysis of a water containing electrolysis medium to generate oxygen and hydrogen as electrolysis gas, comprising: supplying a direct current to an electrolysis stack; subjecting the water containing electrolysis medium to electrolysis in the electrolysis stack, wherein the electrolysis stack comprises an anode section and a cathode section, wherein oxygen is generated in the anode section and hydrogen is generated in the cathode section; separating the generated oxygen from the electrolysis medium in an anode gas separator and separating the generated hydrogen from the electrolysis medium in a cathode gas separator; introducing cooling water from a water supply into at least one of the gas separators, for the direct cooling of the electrolysis gas separated in a gas separating section of the gas separator by the cooling water within a gas cooling section of the gas separator.

14. The process according to claim 13, wherein the electrolysis gas is cooled by the cooling water to 10° C. above ambient temperature or less.

15. The process according to claim 13, wherein the cooling water is heated to 10° C. below the operating temperature of the electrolyser system or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The invention will now be detailed by way of exemplary embodiments and examples with reference to the attached drawings. Unless otherwise stated, the drawings are not to scale. In the figures and the accompanying description, equivalent elements are each provided with the same reference marks.

In the drawings

[0063] FIG. 1 depicts a simplified flow diagram of an electrolyser system 10 according to one embodiment of the invention,

[0064] FIG. 2 depicts a simplified flow diagram of an electrolyser system 11 according to the state of the art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0065] FIG. 1 depicts one embodiment of an electrolyser system 10 according to the invention. The electrolyser system 10 comprises an electrolyser stack 13, the electrolyser stack comprising an anode section 14 for the generation of oxygen and a cathode section 15 for the generation of hydrogen. The electrolyser stack is supplied with direct current 12 from a rectifier (not shown). The electrolyser system 10 further comprises an anode gas separator 20 for the physical separation of gaseous oxygen from the liquid electrolyte, i.e. the anolyte. The electrolyser system 10 further comprises a cathode separator 21 for the physical separation of gaseous hydrogen from the liquid electrolyte, i.e. the catholyte. The anolyte circulates between the anode separator 20 and the anode section 14 of the electrolysis stack 13 via conduits 40, 44, 43 and 42. The catholyte circulates between the cathode separator 21 and the cathode section 15 of the electrolysis stack 13 via the conduits 41, 44, 43 and 42. The electrolyser stack 13 comprises a plurality of anodes and cathodes within the respective anode and cathode section, whereby the cathode section 15 and the anode section 14 are presented in a simplified form for the sake of clarity. At the surfaces of the anodes and cathodes, gaseous hydrogen and oxygen is generated and subsequently accumulates in the catholyte and the anolyte respectively. The anode section 14 and the cathode section 15 are physically separated by a membrane (not shown) so that no mixing of the anolyte and catholyte within the electrolysis stack occurs. The membrane enables the transfer of ions between the anode section 14 and the cathode section 15, e.g. hydroxyl ions in the case of alkaline electrolysis or protons in the case of proton exchange membrane electrolysis. In case of alkaline electrolysis, the circulating electrolyte is an aqueous KOH solution. In case of PEM electrolysis, the circulating electrolyte is water.

[0066] The gas separators 20 and 21 each comprise a gas separating section 20a (anode gas separator) and 21a (cathode gas separator). As the analyte and catholyte solution carrying the electrolysis gas heat up in the electrolysis stack 13 through partial conversion of electrical energy into thermal energy, the electrolysis gas separated in the gas separators 20 and 21 has to be cooled to a desired temperature, for example to 15° C. according to the example of FIG. 1. Thus, cooling water from a water supply 24 is introduced into a gas cooling section 20b (anode gas separator) respectively gas cooling section 21b (cathode gas separator) of the gas separators 20 and 21. The gas cooling sections 20b and 21b both comprise a gas inlet for the electrolysis gas withdrawn from the gas separating sections 20a, 21a (not shown). The cooling water is withdrawn from the water supply 24 via conduits 46 and 45 respectively and supplied to the gas separators 20, 21 via pumps 25a and 25b respectively. As the electrolysis gas has to be cooled below the prevailing ambient temperature (30° C.), the cooling water supplied via lines 46 and 45 is pre-cooled by chillers 26a and 26b respectively, The gas separators 20 and 21 each comprise a cooling water inlet (not shown) at the top of the gas separators 20 and 21, through which the pre-cooled cooling water of the water supply 24 is introduced into the gas cooling sections 20b and 21b of the gas separators. The gas separators also each comprise a nozzle 22 (anodes gas separator) and 23 (cathode gas separator) so that the cooling water introduced into the gas cooling section is distributed as a spray. The gas cooling sections 20b and 21b are arranged vertically, so that the cooling water is conducted from top to bottom of the gas cooling sections 20b, 21b, in counter-flow to the electrolysis gas to be cooled, which flows from bottom to top within the gas cooling sections 20b and 21b.

[0067] Within the gas cooling sections 20b and 21b, heat from the electrolysis gas is directly transferred to the water spray of the cooling water, so that the electrolysis gas (hydrogen and oxygen) is cooled. Cooled oxygen is withdrawn from anode gas separator 20 via conduit 47. Cooled hydrogen is withdrawn from cathode gas separator 21 via conduit 48. Water vapour contained in the electrolysis gas to be cooled is condensed by the cooling water, so that water separated from electrolysis gas is recycled back to the gas separating sections 20a, 21a of the gas separators and finally fed back to the electrolyte and the electrolysis stack via conduits 44, 43 and 42. The vertically arranged gas cooling sections 20b, 21b of the gas separators 20, 21 function according to the principle of gas scrubbing. So for the case of alkaline electrolysis, any caustic KOH present in the water vapour of the electrolysis gas is scrubbed out by the cooling water and returned to the electrolyte solution. The cooled and dried product gases oxygen and hydrogen are finally withdrawn from the gas separators 20 and 21 via conduits 47 and 48 respectively.

[0068] The electrolyser system according to the flow diagram of FIG. 1 is operated at elevated pressure, e.g, at 20 bar (alkaline electrolysis) or 25 bar (PEM electrolysis), Hence, the fraction of water vapour present in the electrolysis gases is relatively low compared to a system operating at low pressure, e.g, a pressure of 2-5 bar. For such a scenario, the water required to balance out the amount of water consumed by the electrolysis is sufficient to cool the electrolysis gases to the desired temperature. Hence, no additional cooling device for cooling of the electrolysis gas is required. In case the electrolysis gas is cooled only to ambient temperature, also the chillers 26a and 26b can be omitted. Furthermore, as the cooling water supplied to the anode gas separator 20 and the cathode separator 21 is at the same time used to balance out the water amount consumed by the electrolysis, no further water supply is required for this purpose. Hence, the electrolysis system according to FIG. 1 can be built much more simply than known systems.

[0069] To balance the liquid level between the anode gas separator 20 and the cathode gas separator 21, both gas separators 20, 21 are connected via a hydraulic link 37.

[0070] The oxygen depleted anolyte is withdrawn from the anode gas separator 20 via conduit 44. The hydrogen depleted catholyte is withdrawn from the cathode gas separator 21 via conduit 44. The electrolysis gas depleted electrolytes are merged in conduit 43 and introduced into a mixing device 18 via pump 19, so that any concentration difference between the anolyte and catholyte is balanced. The electrolyte is subsequently cooled in electrolyte cooler 16, which is supplied with cooling water via conduit 17a, which in turn is withdrawn from electrolyte cooler 16 via conduit 17b. The cooled electrolyte is subsequently split into two fractions and introduced into the anode section 14 and cathode section 15 of the electrolysis stack 13 via conduit 42.

[0071] FIG. 2 depicts an electrolyser system according to the state of the art. Here, the water required to balance out the water amount consumed by the electrolysis is supplied by a water supply 32 via conduit 55 and pump 35, Thus, the cooling potential of the water of water supply 32 is not used. Instead, the electrolysis gas withdrawn from the anode gas separator 30 via conduit 49 (oxygen) and from the cathode gas separator 31 via conduit 50 (hydrogen) is fed to dedicated heat exchangers 33a (anolyte) and 33b (catholyte) via conduits 51 and 52. Heat exchangers 33a and 33b each require a dedicated cooling water supply, the cooling water supplied via conduits 36a and 37a and withdrawn from the heat exchangers 33a, 33b vial conduits 36b and 37b. In the heat exchangers 33a, 33b, water vapour contained in the electrolysis gas is condensed. The cooled electrolysis gas with condensed water is fed to the separators 34a (oxygen) and 34b (hydrogen) via conduits 51 and 52. In the separators 34a, 34b, condensed water from electrolysis gas is separated and drained via conduits 53 (condensed water vapour from oxygen) and 54 (condensed water vapour from hydrogen). The cooled and dried electrolysis gas is withdrawn via conduits 47 (oxygen) and 48 (hydrogen) from the separators 34a, 34b.

[0072] The water of condensed water vapour withdrawn via conduits 53 and 53 is saturated with hydrogen and oxygen. Furthermore, in the case of alkaline electrolysis, it may contain residual caustic lye, e.g. KOH. Hence, this water has to subjected to a suitable waste water treatment.

[0073] The following numerical example represents an exemplary case in that the amount of water required to balance out the water consumed by electrolysis has a sufficiently high cooling potential. In that case, the electrolyser system is operated at elevated pressure. For such a case, no further cooling equipment for the cooling of the electrolysis gas, apart from a cooling section 20b, 21b of a gas separator 20, 21 is required.

[0074] The following operating conditions are typical operating conditions of a high pressure alkaline water electrolysis. [0075] 68 Nm.sup.3/h of hydrogen and 34 Nm.sup.3/h of oxygen are produced; [0076] Aqueous KOH solution with 30 wt.-% KOH is used as the electrolyte; [0077] The product gases hydrogen and oxygen are produced at 90° C. at 30 bar; [0078] Temperature of the water of the deionised water supply 24, fed via conduits 45, 46, is 30° C. (ambient temperature); [0079] The desired temperature of the electrolysis gas (hydrogen and oxygen) is 40° C.

[0080] The electrolysis gas separated from the KOH solution in the gas separating sections 20a, 21a enter the gas cooling sections 20b and 21b, At the top of the gas cooling sections 20b, 21b, deionised water required to offset the consumption of water in the electrolysis stack 13, is sprayed by nozzles (distributors) 22, 23. Based on the temperature requirements for the oxygen and hydrogen gas streams, the total amount of deionised water is split between the gas cooling sections 20b, 21b.

[0081] Water droplets fall from the top to the bottom of the gas cooling sections 20b, 21b, thereby directly contacting the electrolysis gas stream flowing from the bottom to the top. The design of the gas cooling sections 20b, 21b addresses the potential of the carry-over of droplets by the up-flowing electrolysis gas by employing a sufficiently large enough diameter of the gas cooling sections 20b, 21b to slow down the velocity of the electrolysis gas.

[0082] Assuming water saturated electrolysis gas streams, there should be no evaporation of water droplets from the deionised water supplied to the gas cooling sections, ensuring sufficient feed of deionised water to the electrolyser system without losses of deionised water.

[0083] The considered design temperature for the deionised water at the gas outlet of the gas cooling section 20b, 21b is 80° C., thereby maintaining a temperature difference of 10° C. between the electrolysis gas to be cooled and the supplied deionised water. In other words, for the exemplary case, the cooling water should enter the top of the gas cooling section at 30° C., and the electrolysis gas leaves the top of the gas cooling section at 40° C. At the bottom of the gas cooling section, the cooling water leaves the gas cooling section at 80° C., and the electrolysis gas enters the bottom of the gas cooling section at 90° C.

[0084] As the temperature of the electrolysis gas decreases, so does the saturation vapour pressure of the water, and therefore the water in the electrolysis gas phase starts to condense. Extra condensation heat needs to be removed by the supplied deionised water. According to the aforementioned operating conditions, the saturation water vapour pressure above the KOH solution at 90° C. is 0.44 bar (gas inlet from gas separating section 20a, 21a to gas cooling section 20b, 21b) and 0.07 bar for the water saturation pressure above water at 40° C. (gas outlet of the gas cooling section 20b, 21b to conduit 47, 48). With this information, the amount of condensed water from the gas streams can be calculated. Those amounts are 0.70 kg/h condensed water in the hydrogen stream and 0.35 kg/h condensed water in the oxygen stream respectively.

[0085] For high pressure system, such as for the exemplary system which operates at 30 bar, the fraction of water in the electrolysis gas is comparatively low (e.g. at 1 bar y(H.sub.2O)=44.0%; at 30 bar y(H.sub.2O)=1.5%). The lowest operating pressure for which the cooling capacity of the deionised water is high enough in case only the water amount to compensate for water consumption by electrolysis is used as the cooling water is about 20 bar for alkaline electrolysis and about 25 bar for PEM electrolysis. However, those numbers are for orientation only and may deviate, depending on further parameters of the electrolysis system. The pressure difference between alkaline and PEM electrolysis is due to the fact that water has a lower saturation pressure in a concentrated KOH solution compared to its pure composition, as used in PEM electrolysis.

[0086] The following energy balance calculation shows that the amount of water required to balance out the water consumed by electrolysis is sufficient to cool the hydrogen and oxygen gas streams in view of the parameters as mentioned before.

[0087] Total mass flow of deionised water feed stream required to balance out water consumption =54 kg/h;

[0088] Isobaric heat capacity of water (30-80)° C. =4.2 kJ/kg/° C.;

[0089] Cooling energy available=54 kg/h*4.2 kJ/kg/° C.*(80-30)° C.=3.2 kW;

[0090] Mass flow of hydrogen stream (68 Nm3/h)=6 kg/h;

[0091] Mass flow of oxygen stream (34 Nm3/h)=48 kg/h;

[0092] Isobaric heat capacity of hydrogen (90-40)° C.=14.4 kJ/kg/° C.;

[0093] Isobaric heat capacity of oxygen (90-40)° C.=0.9 kJ/kg/° C.;

[0094] Amount of water condensing from hydrogen stream ((90-40) ° C. @30 bar)=0.70 kg/h;

[0095] Amount of water condensing from oxygen stream ((90-40) ° C. @30 bar)=0.35 kg/h;

[0096] Evaporation heat of water (90-40) ° C.=2358 kJ/kg;

[0097] Cooling water duty in catholyte (hydrogen) cooling section=6 kg/h*14.4 kJ/kg/° C.*(90-40) ° C. +0.70 kg/h*2358 kJ/kg=1.7 kW;

[0098] Cooling water duty in anolyte (oxygen) cooling section=48 kg/h*0.9 kJ/kg/° C. *(90-40) ° C. +0.35 kg/h*2358 kJ/kg=0.8 kW

[0099] It follows that the cooling duty available in the supplied deionised water—which is 3.2 kW is greater than the total required cooling duty (hydrogen and oxygen) of 2.5 kW.

LIST OF REFERENCE SIGNS

[0100] 10 electrolyser system (invention) [0101] 11 electrolyser system (state of the art) [0102] 12 direct current [0103] 13 electrolysis stack [0104] 14 anode section [0105] 15 cathode section [0106] 16 electrolyte cooler [0107] 17a, 17b cooling water [0108] 18 mixing device [0109] 19 pump [0110] 20 anode gas separator (invention) [0111] 20a gas separation section of anode gas separator [0112] 20b gas cooling section of anode gas separator [0113] 21 cathode gas separator (invention) [0114] 21a gas separating section of cathode gas separator [0115] 21b gas cooling section of cathode gas separator [0116] 22, 23 nozzle [0117] 24 water supply (invention) [0118] 25a, 25b pump [0119] 26a, 26b chiller [0120] 30 anode gas separator (state of the art) [0121] 31 cathode gas separator (state of the art) [0122] 32 water supply (state of the art) [0123] 33a, 33b heat exchanger [0124] 34a, 34b separator [0125] 35 pump [0126] 36a, 36b cooling water [0127] 37 hydraulic link [0128] 40-55 conduit

[0129] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.