ELECTROLYSIS ARRANGEMENT AND METHOD WITH ANOLYTE COOLER

Abstract

The invention relates to an electrolysis arrangement and a method for producing hydrogen and oxygen by electrolysis of an aqueous electrolysis medium, in particular a corrosive electrolysis medium. According to the invention, the electrolyte cooler to maintain the desired operating temperature of the electrolysis cell stack is arranged downstream of the electrolysis cell stack and upstream of the anolyte gas-liquid separator. By this arrangement, less corrosion resistant materials can be used in particular on the anode side of the electrolysis arrangement, since conduits and further components on the anode side of the electrolysis arrangement are exposed to lower temperatures.

Claims

1. An electrolysis arrangement, comprising: an electrolysis cell stack comprising a plurality of electrolysis cells for the electrochemical generation of hydrogen and oxygen from an electrolysis medium, wherein the electrolysis cell stack comprises an anode section for the generation of oxygen and a cathode section for the generation of hydrogen; an anolyte gas-liquid separator for the separation of oxygen gas from an oxygen loaded anolyte portion of the electrolysis medium; a catholyte gas-liquid separator for the separation of hydrogen gas from a hydrogen loaded catholyte portion of the electrolysis medium; an anolyte cooler, wherein said anolyte cooler is arranged downstream of the electrolysis cell stack and upstream of the anolyte gas-liquid separator, in order to cool the oxygen loaded anolyte portion of the electrolysis medium before entering the anolyte gas-liquid separator.

2. The electrolysis arrangement according to claim 1, wherein the electrolysis medium is an aqueous concentrated potassium hydroxide solution with a potassium hydroxide concentration of up to 30 wt.-%.

3. The electrolysis arrangement according to claim 1, wherein the anolyte cooler is arranged within a first piping system, wherein the first piping system connects an outlet of the anode section of the electrolysis cell stack and an inlet of the anolyte gas-liquid separator.

4. The electrolysis arrangement according to claim 1, wherein the electrolysis arrangement comprises a second piping system, wherein the second piping system connects an outlet of the anolyte gas-liquid separator and an outlet of the catholyte gas-liquid separator with an inlet of the electrolysis cell stack, configured to withdraw hydrogen depleted catholyte from the catholyte gas-liquid separator and to withdraw oxygen depleted anolyte from the anolyte gas-liquid separator, and to supply the hydrogen depleted catholyte and the oxygen depleted anolyte to the electrolysis cell stack.

5. The electrolysis arrangement according to claim 4, wherein the second piping system comprises a mixing device arranged downstream of the anolyte gas-liquid separator and downstream of the catholyte gas-liquid separator, configured to at least partially mix the hydrogen depleted catholyte and the oxygen depleted anolyte to obtain a mixed hydrogen and oxygen depleted electrolyte, to supply the mixed hydrogen and oxygen depleted electrolyte to the inlet of the electrolysis cell stack.

6. The electrolysis arrangement according to claim 4, wherein no cooling device is arranged within the second piping system.

7. The electrolysis arrangement according to claim 5, wherein no cooling device is arranged within the second piping system downstream of the mixing device and upstream of the inlet of the electrolysis cell stack.

8. The electrolysis arrangement according to claim 1, wherein the electrolysis arrangement comprises a third piping system, wherein the third piping system connects an outlet of the cathode section of the electrolysis cell stack and an inlet of the catholyte gas-liquid separator.

9. The electrolysis arrangement according to claim 3, wherein the part of the first piping system connecting an outlet of the anolyte cooler with the inlet of the anolyte gas-liquid separator is made of a stainless steel material.

10. The electrolysis arrangement according to claim 1, wherein a housing of the anolyte gas-liquid separator is made of a stainless steel material.

11. The electrolysis arrangement according to claim 4, wherein the second piping system as a whole is made of a stainless steel material.

12. The electrolysis arrangement according to claim 1, wherein the anolyte cooler is arranged directly at or proximately to the outlet of the anode section of the electrolysis cell stack.

13. A method for producing hydrogen and oxygen by electrolysis of an electrolysis medium, the method comprising: electrochemical splitting of water by means of an electrolysis cell stack, whereby a hydrogen loaded catholyte withdrawn from a cathode section of the electrolysis cell stack and an oxygen loaded anolyte withdrawn from an anode section of the electrolysis cell stack is obtained; supplying the hydrogen loaded catholyte to a catholyte gas-liquid separator to separate hydrogen from the hydrogen loaded catholyte, whereby hydrogen gas and a hydrogen depleted catholyte is obtained; supplying the oxygen loaded anolyte to an anolyte gas-liquid separator to separate oxygen from the oxygen loaded anolyte, whereby oxygen gas and an oxygen depleted anolyte is obtained; withdrawing the hydrogen depleted catholyte from the catholyte gas-liquid separator and recycling the hydrogen depleted catholyte to the cathode section of the electrolysis cell stack; withdrawing the oxygen depleted anolyte from the anolyte gas-liquid separator and recycling the oxygen depleted anolyte to the anode section of the electrolysis cell stack; supplying the oxygen loaded anolyte withdrawn from the anode section of the electrolysis cell stack to an anolyte cooler to obtain a cooled oxygen loaded anolyte, before supplying the oxygen loaded anolyte to the anolyte gas-liquid separator.

14. The method according to claim 13, wherein the electrolysis medium is an aqueous concentrated potassium hydroxide solution with a potassium hydroxide concentration of up to 30 wt.-%.

15. The method according to claim 13, wherein the hydrogen depleted catholyte withdrawn from the catholyte gas-liquid separator and/or the oxygen depleted anolyte withdrawn from the anolyte gas-liquid separator is/are not cooled before being recycled to the electrolysis cell stack.

16. The method according to claim 13, wherein the hydrogen depleted catholyte withdrawn from the catholyte gas-liquid separator and the oxygen depleted anolyte withdrawn from the anolyte gas-liquid separator are at least partially mixed to obtain a hydrogen and oxygen depleted mixed electrolysis medium, and the hydrogen and oxygen depleted mixed electrolysis medium is recycled to the electrolysis cell stack.

17. The method according to claim 16, wherein the hydrogen and oxygen depleted mixed electrolysis medium is not cooled before being recycled to the electrolysis cell stack.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0061] The invention will now be detailed by way of an exemplary embodiment with reference to the attached drawing. Unless otherwise stated, the drawings are not to scale. In the FIGURE and the accompanying description, equivalent elements are each provided with the same reference marks.

[0062] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein the figure depicts a simplified flow diagram of an electrolysis arrangement 1 according to one exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0063] The figure depicts one exemplary embodiment of an electrolysis arrangement 1 according to the invention. It is assumed that the electrolysis arrangement 1 performs an alkaline type electrolysis with 30 wt.-% KOH lye solution as the electrolyte. The electrolysis arrangement 1 comprises an electrolysis cell stack 34, a rectifier 2, an anolyte cooler 11, an anolyte gas-liquid separator 12 with oxygen cooler 14 and a catholyte gas-liquid separator 13 with hydrogen cooler 15 as the main components. The main components are connected via conduits of a first, second and third piping system. The first piping system comprises the conduits 31a and 31b, the second piping system comprises the conduits 32a, 32b and 32c, and the third piping system comprises conduit 33. Furthermore, the electrolysis arrangement comprises a mixing device 21 in order to mix hydrogen depleted catholyte and oxygen depleted anolyte, an electrolyte pump 20 for circulating the electrolysis medium, and a deionised (DI) water supply 19 to compensate for the amount of water consumed by the electrochemical water splitting reaction.

[0064] Direct current 3 is supplied to the electrolysis cell stack 34 by means of the rectifier 2. For the sake of simplification, only one electrolysis cell is shown in regards of electrolysis cell stack 34, but the electrolysis cell stack 34 actually contains a plurality of electrolysis cells. The electrolysis cell shown comprises an anode section 6, which consists of an anode 4 and an anode compartment 5. The electrolysis cell further comprises a cathode section 9, which consists of a cathode 7 and a cathode compartment 8. The anode section 6 and the cathode section 9 are separated by a liquid tight membrane 10 which enables the exchange of hydroxyl ions between the anode section 6 and the cathode section 9.

[0065] By supplying direct current 3 to the electrolysis cell stack 34, water of the electrolyte lye solution is split into hydrogen (cathode side) and oxygen (anode side). This results in the generation of an oxygen loaded anolyte in the anode section 6 of the electrolysis cell stack 34 and a hydrogen loaded catholyte in the cathode section 9 of the electrolysis cell stack 34.

[0066] The oxygen loaded anolyte generated in the anode section 6 is withdrawn from the electrolysis cell stack 34 via an outlet of the anode section 6 (outlet not shown) and supplied via conduit 31a to an inlet of the anolyte cooler 11 (inlet not shown). In the anolyte cooler 11, which is arranged downstream of the electrolysis cell stack 34 and upstream of the anolyte gas-liquid separator 12, the oxygen loaded anolyte is cooled to a sufficiently low temperature by cooling water 26, which is fed to the anolyte cooler 11 in countercurrent to the flow of the oxygen loaded anolyte. The heated cooling water 27 is withdrawn from the anolyte cooler 11 and is re-cooled by e.g. an air cooler (not shown).

[0067] The cooled oxygen loaded anolyte is withdrawn via an outlet of the anolyte cooler 11 (outlet not shown) and supplied via conduit 31b to an inlet of the anolyte gas-liquid separator 12 (inlet not shown). In anolyte gas-liquid separator 12, oxygen gas is separated from the oxygen-loaded anolyte, whereby an oxygen depleted anolyte and oxygen gas is obtained. The oxygen gas still contains uncondensed water. Hence, it is cooled in oxygen cooler 14 by indirect cooling. Therefore, oxygen cooler 14 is supplied with cooling water 24. The thereby condensed water is returned to the anolyte gas-liquid separator 12. Heated cooling water 25 is withdrawn from oxygen cooler 14 and re-cooled by e.g. an air cooler (not shown). Dry oxygen gas 17 is withdrawn from oxygen cooler 14 and fed to a further processing step.

[0068] As water is continuously consumed by the water electrolysis process, the electrolysis arrangement 1 comprises a DI water storage tank to compensate for the consumed water and balance the amount of water which is in the process. According to the embodiment of the figure, DI water 19 is supplied from the DI water storage tank 18 to the anolyte gas-liquid separator 12 to be introduced into the electrolysis system.

[0069] The hydrogen loaded catholyte generated in the cathode section 9 is withdrawn from the electrolysis cell stack 34 via an outlet of the cathode section 9 (outlet not shown) and supplied via conduit 33 to an inlet of the catholyte gas-liquid separator 13 (inlet not shown). No further cooling device is arranged within conduit 33. In catholyte gas-liquid separator 13, hydrogen gas is separated from the hydrogen loaded catholyte, whereby a hydrogen-depleted catholyte and hydrogen gas is obtained. The hydrogen gas still contains uncondensed water. Hence, it is cooled in hydrogen cooler 15 by indirect cooling. Therefore, hydrogen cooler 15 is supplied with cooling water 22. The thereby condensed water is returned to the catholyte gas-liquid separator 13. Heated cooling water 23 is withdrawn from hydrogen cooler 15 and re-cooled by e.g. an air cooler (not shown). Dry hydrogen gas 16 is withdrawn from hydrogen cooler 15 and fed to a further processing step.

[0070] Oxygen depleted anolyte is withdrawn from the anolyte gas-liquid separator 12 and supplied via conduit 32a to the mixing device 32b. Hydrogen depleted catholyte is withdrawn from the catholyte gas-liquid separator 13 and supplied via conduit 32b to mixing device 21. In mixing device 21, oxygen depleted anolyte and hydrogen depleted catholyte are fully mixed, whereby a mixed hydrogen and oxygen depleted electrolyte is obtained. The mixed oxygen and hydrogen depleted electrolyte is recycled via conduit 32c to the electrolysis cell stack 34, whereby it is routed to an anode inlet and a cathode inlet after splitting the volume flows in a ratio of one to one. In the electrolysis cell stack 34, the oxygen and hydrogen depleted electrolyte is processed for the renewed splitting of water by the electrolysis reaction. Conduits 32a, 32b and 32c form the second piping system. No further cooling device is arranged within this second piping system.

[0071] According to the alkaline electrolysis arrangement 1 of the figure, the electrolysis cell stack 34 is operated at a set temperature of 90° C. That is the hydrogen loaded catholyte and the oxygen loaded anolyte are withdrawn from the electrolysis cell stack having a temperature of approximately 90° C. By the anolyte cooler 11, the oxygen loaded anolyte is cooled down to a temperature of 60° C. That is, the oxygen loaded anolyte in conduit 31b and the oxygen depleted anolyte of conduit 32a have a temperature of approximately 60° C. The hydrogen loaded catholyte in conduit 33 and the hydrogen depleted catholyte in conduit 32b have a temperature of approximately 90° C., as no further cooling device is arranged within the second piping system. In the mixing device 21, hydrogen depleted catholyte having a temperature of approximately 90° C. and oxygen depleted anolyte having a temperature of approximately 60° C. are mixed, which results in a mixed hydrogen and oxygen depleted electrolyte temperature of approximately 75° C. Hence, the mixed electrolyte is recycled to the electrolysis cell stack 34 having a temperature of approximately 75° C. In the electrolysis cell stack 34, said mixed electrolyte is again heated to 90° C. as a consequence of the electrolysis reaction within the stack.

[0072] The aforementioned scenario is further discussed in detail in the following.

[0073] Operating conditions of the alkaline electrolysis arrangement are: [0074] 30 wt.-% aqueous KOH lye solution as the electrolysis medium; [0075] The outlet streams of the electrolysis cell stack 34 (conduits 31a and 33) are at 90° C.; [0076] The allowable temperature difference from inlet to outlet of the electrolysis cell stack 34 is 15° C.; [0077] The inlet mixed electrolyte stream into the electrolysis cell stack 34 is split 1:1 between the anode section 6 and cathode section 9 (not shown in the figure).

[0078] By arranging the electrolyte cooler downstream of the anode section 6 of the cell stack 34, it is possible to reduce the temperature of the stream by twice the amount of temperature increase in the cell stack 34 (dT.sub.stack). This is not possible when the electrolyte cooler is arranged in conduit 32c since, if one would cool by more than the dT.sub.stack, one would reduce the operating temperature of the electrolysis arrangement, making it less efficient. The present invention enables to reach these lower temperatures because an electrolyte stream is cooled (i.e. the anolyte stream) which is 50% of the total electrolyte inlet stream into the cell stack 34. This is shown in the energy balance below. [0079] Allowable temperature increase in the cell stack for the scenario if the figure is dTstack = 15° C. [0080] Total electrolyte flow (anode + cathode side) to maintain temperature increase in cell stack = V.sub.total [0081] Electrolyte flow into anode and cathode section (50/50 split), Vanode = V.sub.total / 2 = V.sub.cathode [0082] To maintain a steady operating temperature of the electrolysis arrangement, the heat generated in the cell stack (Q.sub.stack) has to equal the heat removed from the system (Q.sub.cooling): Q.sub.stack = Q.sub.cooling [0083] Q.sub.stack and Q.sub.cooling equal to a temperature change of the electrolyte in the stack and in the cooler (heat exchanger - HX) respectively: V.sub.total * rho.sub.KOH * cp.sub.KOH * dT.sub.stack = V.sub.anode * rho.sub.KOH * CpKOH * dTHX, wherein rho.sub.KOH is the density of the electrolysis medium and cp.sub.KOH is the heat capacity of the electrolysis medium [0084] This can be simplified, since rho.sub.KOH and cp.sub.KOH can be regarded as being constant with these small temperature differences: V.sub.total * dT.sub.stack = (V.sub.total / 2) * dT.sub.HX .fwdarw. dT.sub.HX = 2 * dT.sub.stack [0085] For our scenario, this leads to: dT.sub.HX = 30° C. .fwdarw. T.sub.outlet from .sub.HX = 60° C. [0086] Since the hydrogen stream is not cooled, it remains at 90° C., and since it has the same flowrate as the oxygen stream after their mixing upstream the pump, it results: T .sub.after .sub.mixing = 75° C. = T.sub.inlet .sub.to .sub.stack

[0087] Reducing the temperature of the oxygen loaded anolyte to 60° C. instead of 90° C., when the electrolyte cooler would be arranged in conduit 32c, less corrosion resistant materials can be used to still ensure low corrosion rates. For instance, stainless steel can be chosen as a material for conduit 31b, for the housing of anolyte gas-liquid separator 12 and for conduit 32a. In case the electrolyte cooler would be arranged within conduit 32c upstream of the electrolysis cell stack 34, a pure Nickel material would have to be used for the oxygen loaded anolyte carrying conduits and housings. Because in the case, those conduits and housings would be exposed to a temperature of 90° C.

[0088] The solubility of the oxygen gas does not change significantly with temperatures within the aforementioned temperature range (60° C. to 90° C.). For instance, in case of 30 wt.-% KOH at 30 bar, the solubility of oxygen at 90° C. is 56.5 wt.-ppm and for 60° C. it is 56.1 wt.-ppm according to the solubility model and data of Shoor et al., published in 1968 as “Salting out of nonpolar gases in aqueous potassium hydroxide solution”.

[0089] The anolyte cooler 11 has to handle a two-phase flow (anolyte and gaseous oxygen) and therefore preferably has a straight vertical arrangement preventing any possible dead-spots where oxygen gas can be trapped. The tubing or plating, depending on the heat exchanger type chosen, is made out of corrosion resistant material as it is subject to an anolyte-oxygen mixture under a high temperature. However, these components are easier to manufacture and are available in standardized pieces, making them relatively cheaper than an entire gas-liquid separator made out of the respective corrosion resistant material.

[0090] To maintain a 50/50 split at the inlet to the electrolysis cell stack 34, a pressure resistive element can be placed on the cathode section outlet to balance the pressure drop introduced by placing the electrolysis cell stack on the anode outlet stream.

List of Reference Signs

[0091] 1 electrolysis arrangement

[0092] 2 rectifier

[0093] 3 direct current

[0094] 4 anode

[0095] 5 anode compartment

[0096] 6 anode section

[0097] 7 cathode

[0098] 8 cathode compartment

[0099] 9 cathode section

[0100] 10 membrane

[0101] 11 anolyte cooler

[0102] 12 anolyte gas-liquid separator

[0103] 13 catholyte gas-liquid separator

[0104] 14 oxygen cooler

[0105] 15 hydrogen cooler

[0106] 16 hydrogen gas

[0107] 17 oxygen gas

[0108] 18 DI water storage tank

[0109] 19 DI water supply

[0110] 20 electrolyte pump

[0111] 21 mixing device

[0112] 22 cooling water hydrogen cooler (in)

[0113] 23 cooling water hydrogen cooler (out)

[0114] 24 cooling water oxygen cooler (in)

[0115] 25 cooling water oxygen cooler (out)

[0116] 26 cooling water anolyte cooler (in)

[0117] 27 cooling water anolyte cooler (out)

[0118] 31a, 31b conduits of first piping system

[0119] 32a, 32b, 32c conduits of second piping system

[0120] 33 conduit of third piping system

[0121] 34 electrolysis cell stack

[0122] 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.