METHOD OF PRODUCING H2 AND/OR BR2 BY ELECTROLYSING HBr USING FLUOROPOLYMER MEMBRANES

Abstract

A method of producing hydrogen and/or bromine by electrolysing hydrogen bromide using a fluoropolymer membrane having a glass transition temperature T.sub.g110 C. in an electrolysis of hydrogen bromide, wherein the hydrogen bromide stems from a sulfuric acid synthesis.

Claims

1. A method of producing hydrogen and/or bromine by electrolysing hydrogen bromide comprises the following steps: i) synthesizing sulfuric acid such that hydrogen bromide is produced, ii) providing an electrolytic cell comprising an anode, a cathode, and a fluoropolymer membrane having a glass transition temperature T.sub.g equal to or greater than 110 C. as determined according to DIN EN ISO 11357-2:2020-08, the fluoropolymer membrane being sandwiched between the anode and the cathode, iii) feeding a first composition comprising hydrogen bromide and water to the anode, iv) feeding a second composition comprising hydrogen bromide and water to the cathode, and v) operating the electrolytic cell at an operation voltage of at most 1900 mV and at an operation current density of at least 3 kA/m.sup.2 to produce hydrogen at the cathode and/or bromine at the anode, wherein the hydrogen bromide produced in step i) is used to prepare the first composition fed in step iii) or both the first composition and the second composition fed in steps iii) and iv)

2. A method of producing hydrogen and/or bromine by electrolysing hydrogen bromide comprises the following steps: i) providing a stream containing hydrogen bromide obtained from the synthesis of sulfuric acid, ii) providing an electrolytic cell comprising an anode, a cathode, and a fluoropolymer membrane having a glass transition temperature T.sub.g equal to or greater than 110 C. as determined according to DIN EN ISO 11357-2:2020-08, the fluoropolymer membrane being sandwiched between the anode and the cathode, iii) feeding a first composition comprising hydrogen bromide and water to the anode, iv) feeding a second composition comprising hydrogen bromide and water to the cathode, and v) operating the electrolytic cell at an operation voltage of at most 1900 mV and at an operation current density of at least 3 kA/m.sup.2 to produce hydrogen at the cathode and/or bromine at the anode, wherein the stream containing hydrogen bromide provided in step i) is used to prepare the first composition fed in step iii) or both the first composition and the second composition fed in steps iii) and iv)

3. The method according to claim 1 is characterized in that step v) of operating the electrolyte cell comprises an operational voltage of at most 1500 mV; or from 800 to 1500 mV.

4. The method according to claim 1 is characterized in that step v) of operating the electrolyte cell comprises an operational current density of at least 4 kA/m.sup.2 or from 4 kA/m.sup.2 to 12 kA/m.sup.2.

5. The method according to claim 1 is characterized in that the temperature of the first composition fed to the anode is at least or above 60 C. and simultaneously the temperature of the second composition fed to the cathode is at least or above 60 C.

6. The method according to claim 1 is characterized in that the fluoropolymer membrane has a glass transition temperature T.sub.g in the range of 120 to 170 C., as determined according to DIN EN ISO 11357-2:2020-08.

7. The method according to claim 1 is characterized in that the fluoropolymer membrane comprises (CF.sub.2CF.sub.2) repeat units.

8. The method according to claim 1 is characterized in that the fluoropolymer membrane does not comprise structural entities of the formula OCF.sub.2CF(CF.sub.3)O.

9. The method according to claim 1 is characterized in that the fluoropolymer membrane is a sulfonated fluoropolymer membrane.

10. The method according to claim 9 is characterized in that the sulfonated fluoropolymer membrane comprises O(CF.sub.2).sub.nSO.sub.3H groups, wherein n is an integer selected from 1, 2, 3, 4 and 5, preferably 2.

11. The method according to claim 9 is characterized in that the sulfonated fluoropolymer membrane comprises a hydrolysed copolymer of F.sub.2CCF.sub.2 and CF.sub.2CFO(CF.sub.2).sub.2SO.sub.2F.

12. The method according to claim 1 is characterized in that the fluoropolymer membrane has an acid capacity of equal to or greater than 0.9 meq/g.

13. The method according to claim 1 is characterized in that electrolysis occurs at a temperature of 70 C. or more and/or in that electrolysis occurs at a temperature of less than 100 C.

14. The method according to claim 1 is characterized in that electrolysis occurs at a temperature ranging from 75 to 95 C.

15. The method according to claim 1 is characterized in that electrolysis occurs under a pressure of more than 0.1 MPa.

16. The method according to claim 1 is characterized in that the pressure increases from an anode to a cathode of an electrolytic cell in which the electrolysis of hydrogen bromide occurs.

17. The method according to claim 1 is characterized in that the electrolysis occurs in the absence of hydrogen fluoride, hydrogen chloride, and/or hydrogen iodide.

18. The method according to claim 1 is characterized in that the second composition comprises a hydrogen bromide concentration of at least 0.5 mol/kg; preferably at least 3 mol/kg.

19. The method according to claim 1 is characterized in that the second composition comprises a hydrogen bromide concentration ranging from 0.5 to 10 mol/kg; preferably from 3 to 7 mol/kg.

20. The method according to claim 1 is characterized in that the fluoropolymer membrane is selected to have a glass transition temperature of at least 30 C. higher than the operational temperature of the electrolysis; preferably higher than 40 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0131] FIG. 1 shows the general outline of an electrolysis cell that is applicable for a use according to the present disclosure.

[0132] FIG. 2 shows the temperature dependence of the voltage of HBr electrolyses.

[0133] FIG. 3 shows the results of HBr electrolyses using different membranes.

[0134] FIG. 4 shows the results of further HBr electrolyses with different membrane types.

[0135] FIG. 5 is a scheme with different routes for producing hydrogen bromide in a sulfuric acid synthesis.

[0136] FIG. 6 is a scheme with a typical reaction sequence yielding valuable hydrogen by converting a sulfur(S)-containing and potentially nitrogen(N)-containing feedstream via an oxidation with bromine.

EXPERIMENTAL SECTION

[0137] FIG. 1 is a general outline of an exemplary electrolysis cell and an exemplary electrolysis procedure, respectively, which can be employed to put a use according to the present disclosure into practice. The proton exchange membrane depicted in FIG. 1 is a fully hydrated fluoropolymer membrane having a glass transition temperature T.sub.g110 C., for example a fully hydrated Aquivion E98-05S membrane or a fully hydrated Aquivion E87-05S membrane. The membrane is sandwiched between two catalytic layers. The catalytic layers themselves are sandwiched between two gas diffusion layers. On the left-hand side, an anode is arranged on a first gas diffusion layer, namely on the surface opposite to the surface on which a first catalytic layer is arranged. On the right-hand side, a cathode is arranged on a second gas diffusion layer, namely on the surface opposite to the surface on which a second catalytic layer is arranged. A first mixture comprising HBr and H.sub.2O is fed to the anode, while a second mixture also comprising HBr and H.sub.2O is fed to the cathode. The HBr fed to the anode as well as the HBr fed to the cathode stems from a synthesis of H.sub.2SO.sub.4 according to SO.sub.2+Br.sub.2+2H.sub.2O.fwdarw.H.sub.2SO.sub.4+2HBr. An electric field is applied to the electrolysis cell such that electrons (e.sup.) are transferred from the anode to the cathode. At the anode, HBr migrates through the anode gas diffusion layer to the anode catalytic layer where it is electrooxidized so that Br.sub.2 is formed and protons are remaining. The protons migrate from the anode catalytic layer through the membrane to the cathode catalytic layer. At the cathode catalytic layer, the protons recombine with electrons to H.sub.2. Formed H.sub.2 migrates through cathode gas diffusion layer. As electrolysis products, a stream comprising Br.sub.2 and H.sub.2O is withdrawn from the anode, and a stream comprising H.sub.2, H.sub.2O, and HBr is withdrawn from the cathode. The stream withdrawn from the anode may additionally comprise unconverted HBr.

[0138] FIG. 2 shows the results of actual electrolyses of HBr using an Aquivion E98-05S membrane at different temperatures. A successful electrolysis was performed at elevated temperatures of 75 C. up to 95 C. without any indication of a degrading of the membrane despite using a highly concentrated HBr feed having an HBr concentration of 6 mol/kg as the anode feed. The operational voltage decreased from 1301 mV at a temperature of 75 C. to 1230 mV at a temperature of 95 C., keeping a current density of 3 kA/m.sup.2. The increase to higher temperatures thus led to an advantageous reduction of the to be invested voltage at a constantly high current density of 3 kA/m.sup.2, which in turn led to advantageously lowered OPEX.

[0139] FIG. 3 shows the results of actual electrolyses of HBr using two different membranes and current densities, namely an Aquivion E98-05S membrane at a current density of 10 kA/m.sup.2 and an Aquivion E87-05S membrane at a current density of 5 kA/m.sup.2. Further, the HBr concentration of an aqueous HBr solution fed to the cathode of the electrolysis cell was increased from 0 mol/kg (deionized water) over 2 mol/kg and further 4 mol/kg up to 6 mol/kg. It is seen that successful electrolyses were performed at an elevated temperature of 95 C. and at high current densities of 5 kA/m.sup.2 and even 10 kA/m.sup.2 without any indication of a degrading of the membrane. Moreover, feeding a highly concentrated aqueous HBr solution to the cathode did not impair the membrane performance. Instead, with the exception of a minor outlier at 2 mol/kg for the Aquivion E98-05S membrane, increasing the HBr concentration surprisingly led to a significant lowering of the operational voltage which in turn led to advantageously lowered OPEX.

[0140] FIG. 4 shows the results of actual electrolyses of HBr using different current densities and different membranes. Comparing the results obtained with 2 NAFION membranes at 5kA/m2 and 95 C. with the Aquivion membrane in the same conditions, it can be seen that the operational voltage is lower for Aquivion, resulting in a lower OPEX.