Method and apparatus for producing hydrogen gas using hydrogen bromide electrolysis

Abstract

A process for the production of hydrogen comprising the steps of: a) providing a starting mixture comprising bromine, water and a sulfur-containing compound, b) reacting the starting mixture provided in step a) to produce a reaction mixture effluent comprising sulfuric acid and hydrogen bromide, c) separating the reaction mixture effluent obtained in step b) into one or more hydrogen bromide-enriched compositions and one or more sulfuric acid-enriched compositions, where at least one hydrogen bromide-enriched composition contains up to 1,000 ppm sulfuric acid, where step c) comprises at least two distillation steps, d) separating the reaction mixture effluent obtained in step c) containing up to 1,000 ppm sulfuric acid, at least a portion of the at least one hydrogen bromide-enriched composition comprising at least one hydrogen bromide-enriched composition comprising at least 50,000 ppm sulfuric acid and at least 50,000 ppm sulfuric acid is subjected to electrolysis to obtain hydrogen and a bromine-containing composition wherein the electrolysis cell is operated at an operating temperature of at least 70 DEG C, and e) recycling at least a portion of the bromine-containing composition obtained in step d) back to step a).

Description

[0001] The present invention relates to a method and a device for preparing hydrogen by electrolysis of hydrogen bromide.

[0002] Hydrogen is an ecologically sustainable energy source, since it produces only water when used as an energy source for fuel cells or if burned with oxygen. Furthermore, hydrogen can be produced from non-fossil raw materials using renewable energy sources, such as by electrolyzing water using electricity that has been generated by solar energy. Hydrogen produced by such or similar methods is known as green hydrogen due to its environmental friendliness. For the reasons mentioned above, the global demand for hydrogen and hydrogen-derived fuels is expected to increase significantly in the coming years, especially since hydrogen is expected to play a key role in mitigating global warming.

[0003] It is known that there are multiple technologies for preparing hydrogen by electrolysis of water, such as alkaline electrolysis, proton exchange membrane electrolysis and solid oxide electrolysis. Different electrolysis technologies have different operating ranges and different efficiencies. Although solid oxide electrolysis is, for example, a high temperature solution, which generally provides relatively high efficiency, this technology has serious disadvantages, such as having a limited lifespan, and requiring relatively high capital expenditure, combined with a generally low level of technical readiness. Alkaline electrolysis does not allow power to be adjusted over a relatively wide range, and in addition suffers from disadvantageously low current density, which in turn increases capital expenditure. In contrast, proton exchange membrane electrolysis can generally be operated at a higher current density, which reduces capital expenditure. In addition, proton exchange membrane electrolysis is more flexible than other water electrolysis technologies (such as particularly alkaline electrolysis). In view of this, proton exchange membrane electrolysis is generally favored over water electrolysis to prepare green hydrogen.

[0004] However, water electrolysis requires a significant amount of energy. Therefore, it would be desirable to prepare an alternative electrolytic route for hydrogen, such as the electrolysis of hydrogen halides, such as the electrolysis of hydrogen fluoride, hydrogen chloride or hydrogen bromide. At the industrial level, the only relevant practical application of hydrogen bromide electrolysis is its use in flow batteries, the purpose of which is to store energy. However, such flow batteries neither produce hydrogen nor bromine for their subsequent use, but these compounds are used as energy carriers and are even usually preserved in the form of impurities, which are sufficient for storage, but are usually not pure enough to be used for subsequent use of hydrogen. In addition, in flow batteries, electrolysis is usually carried out in the liquid phase under mild conditions to avoid pressure increase and / or membrane damage, and is usually operated at a limited conversion level to avoid forming multiple phases and any inhomogeneities in the system. Hydrogen bromide electrolysis is also described for other purposes, such as for converting biowaste into usable energy components, for flue gas desulfurization, for removing hydrogen sulfide from exhaust gases and other methods. Even in these processes, where hydrogen is produced as a by-product, these processes are not suitable for producing pure hydrogen on an industrial scale at a commercially acceptable price using equipment with reasonably low capital expenditure.

[0005] In view of this, it is an object underlying the present invention to provide a process for producing hydrogen from water on an industrial scale, wherein the energy requirement for pure hydrogen is lower than in the water electrolysis processes described in the prior art and which is characterized by low production costs.

[0006] According to the present invention, this object is solved by providing a method for producing hydrogen, which comprises the following steps:

[0007] a) providing a starting mixture containing bromine, water and a sulfur-containing compound,

[0008] b) reacting the starting mixture provided in step a) to produce a reaction mixture effluent comprising sulfuric acid and hydrogen bromide,

[0009] c) separating the reaction mixture effluent obtained in step b) into one or more compositions enriched in hydrogen bromide and one or more compositions enriched in sulfuric acid, wherein at least one composition enriched in hydrogen bromide contains at most 1,000 ppm of sulfuric acid, wherein step c) comprises at least two distillation steps,

[0010] d) subjecting at least a portion of the at least one composition enriched in hydrogen bromide containing up to 1,000 ppm of sulfuric acid obtained in step c) to electrolysis to obtain hydrogen and a bromine-containing composition, wherein the electrolysis cell is operated at an operating temperature of at least 70° C., and

[0011] e) recycling at least a portion of the bromine-containing composition obtained in step d) back to step a).

[0012] The process allows not only the preparation of pure hydrogen on an industrial scale, but also the preparation of pure hydrogen with reasonably low operating costs and with reasonably low capital expenditures. The reaction of bromine, water and sulfur-containing compounds (such as elemental sulfur) to sulfuric acid and hydrogen bromide in step b) is exothermic, and the energy obtained thereby can be used to significantly reduce the amount of external energy required for the water decomposition involving the electrolysis of hydrogen bromide carried out in step d). In addition, the separation step c) separates the reaction mixture effluent containing hydrogen bromide and sulfuric acid and usually together with unreacted water and bromine and other impurities produced by the source of sulfur-containing compounds into one or more compositions enriched in hydrogen bromide and one or more compositions enriched in sulfuric acid, wherein at least one composition enriched in hydrogen bromide contains up to 1,000 ppm and most preferably up to 10 ppm of sulfuric acid, allowing such hydrogen bromide compositions depleted in sulfuric acid to be subjected to electrolysis in step d), which reliably protects the electrolyzer (especially the membrane and electrodes) from degradation, thereby resulting in a long life of the electrolyzer. Such degradation of the electrolyzer may be caused by the presence of sulfuric acid and products of the conversion of sulfuric acid in the electrolyzer (e.g., to elemental sulfur). Those products, such as, for example, elemental sulfur, may also be present due to incomplete oxidation of S species in the reaction mixture effluent in step b). Typically, the reaction of S-containing compounds may result in the presence of some individual sulfur impurities in step b), which is highly undesirable. Therefore, separation in step c) to significantly reduce the concentration of sulfuric acid in the HBr stream is important for protecting the performance of the electrolyzer.

[0013] In addition, in step c), one or more compositions enriched with sulfuric acid are generated, allowing very pure sulfuric acid with a sulfuric acid concentration of up to 95% by weight to be prepared in an easy and cost-effective manner, which can be commercially sold, and thus further reducing the overall cost of the method according to the invention. In addition, the reaction mixture effluent containing hydrogen bromide and sulfuric acid and usually accompanied by unreacted water and bromine and other impurities produced by the source of sulfur-containing compounds is separated into one or more compositions enriched with hydrogen bromide and one or more compositions enriched with sulfuric acid, concentrating water in the one or more compositions enriched with sulfuric acid, and thus allowing to easily remove pure and particularly water free of hydrogen bromide from one or more compositions enriched with sulfuric acid. Due to its purity, this pure and particularly water free of hydrogen bromide can be easily removed from the method, thus reliably and effectively allowing to avoid water accumulation in a continuous method. In addition, separation of the reaction mixture effluent into one or more compositions enriched in hydrogen bromide and one or more compositions enriched in sulfuric acid allows a portion of the one or more compositions enriched in hydrogen bromide to be recycled back to the starting mixture, thereby ensuring in a simple and effective manner that the starting mixture is a single-phase liquid mixture containing enough hydrogen bromide, so that bromide and water are reliably avoided to separate into two phases in the reaction mixture. In addition, the generation of one or more compositions enriched in sulfuric acid allows a sufficiently high amount of sulfuric acid in the mixture to be adjusted by simply recycling a portion of the one or more compositions enriched in sulfuric acid to the liquid portion of the reaction mixture obtained in step b), which destroys the hydrogen bromide-water azeotrope in the mixture. This allows the hydrogen bromide-enriched gas composition with a relatively low water content to be easily and effectively separated from the liquid composition enriched in sulfuric acid and water by subsequent distillation steps, so that the hydrogen bromide-enriched gas composition can be directly used for the electrolysis of step d), which is more energy-efficient than the electrolysis of an aqueous solution in which hydrogen bromide is dissolved in water. In summary, the process according to the invention allows the production of hydrogen on an industrial scale with a lower energy demand for pure hydrogen compared to the water electrolysis processes for pure hydrogen described in the prior art.

[0014] In principle, the present invention is not particularly limited to the type of sulfur-containing compound used in step a), as long as it can react into sulfuric acid with bromine and optional water. Therefore, it is not important whether the sulfur-containing compound is reacted into sulfuric acid in step b) by oxidizing sulfur atom, by reducing sulfur atom or by not changing the oxidation state of sulfur atom. According to an alternative, the sulfur-containing compound used in step a) can be oxidized into sulfuric acid. When the sulfur-containing compound is selected from any combination of sulfur, sulfide, disulphide, thiophene, mercaptan, sulfur dioxide, carbon disulfide, carbonyl sulfide and two or more of the aforementioned compounds, good results are particularly obtained. More preferably, the sulfur-containing compound is selected from any combination of hydrogen sulfide, sulfur, sulfur dioxide, carbon disulfide, carbonyl sulfide, methyl mercaptan and two or more of the aforementioned compounds. The particularly preferred example of sulfur-containing compound is hydrogen sulfide and / or elemental sulfur.

[0015] By way of example, the reactions of some of the above sulfur-containing compounds with bromine and water are shown:

[0016] H2S+4Br2+4H2O→8HBr+H2SO4

[0017] S+3Br2+4H2O→6HBr+H2SO4

[0018] SO2+Br2+2H2O→2HBr+H2SO4

[0019] CS2+4Br2+10H2O→8HBr+2H2SO4+CO2

[0020] 2COS+4Br2+10H2O→8HBr+2H2SO4+2CO2

[0021] For example, the raw material composition (such as unstripped gas) containing one or more sulfur-containing compounds can be used and bromine and water and optional hydrogen bromide are mixed to form a starting mixture. When the content of sulfur-containing compounds in the raw material composition (it is preferably unstripped gas) is at least 5 weight % based on the starting mixture of 100 weight %, and preferably 6 to 99.9 weight %, good results are obtained especially.

[0022] According to the present invention, the starting mixture provided in step a) contains bromine, water and sulfur-containing compounds. Of course, the presence of other compounds is not excluded. On the contrary, it is preferred that the starting mixture provided in step a) additionally contains hydrogen bromide. This is preferred to achieve a single-phase liquid reaction mixture in step b). The binary system of bromine and water (except very low bromine content) is a two-phase system, while the ternary mixture of bromine, water and hydrogen bromide is a single-phase liquid mixture at a hydrogen bromide concentration higher than a certain (e.g., 5 wt %), which depends on the relative amount of bromine and water. At 11 wt % and higher than 11 wt % hydrogen bromide concentration, all ternary mixtures of bromine, hydrogen bromide and water are single-phase liquid mixtures. Therefore, it is preferred that the starting mixture a) contains a correspondingly high amount of hydrogen bromide.

[0023] More specifically, if the starting mixture is a gas-liquid mixture at the reaction temperature and reaction pressure of step b), it is preferred that the starting mixture contains at least 5% by weight and more preferably 11 to 46% by weight of hydrogen bromide based on the total weight of bromine, hydrogen bromide and water in the liquid phase of the starting mixture. If the starting mixture is a liquid at the reaction temperature and reaction pressure of step b), it is preferred that the starting mixture contains at least 5% by weight and more preferably 11 to 46% by weight of hydrogen bromide based on the total weight of bromine, hydrogen bromide and water in the starting mixture. A 5% by weight concentration of hydrogen bromide produces a single-phase liquid mixture at least for low and high amounts of bromine relative to water. However, as described above, a hydrogen bromide concentration of at least 11% by weight produces a single-phase liquid mixture for each amount of bromine relative to water.

[0024] Preferably, if the starting mixture is a gas-liquid mixture at the reaction temperature and reaction pressure, the starting mixture comprises at least 5% by weight and preferably 11 to 46% by weight of hydrogen bromide, at least 1% by weight and preferably 2 to 35% by weight of bromine and 40 to 75% by weight of water, based on the total weight of water, bromine and hydrogen bromide in the liquid phase of the starting mixture. If the starting mixture is a liquid at the reaction temperature and reaction pressure, the starting mixture preferably comprises at least 5% by weight and preferably 11 to 46% by weight of hydrogen bromide, at least 1% by weight and preferably 2 to 35% by weight of bromine and 30 to 75% by weight of water, based on the total weight of bromine, hydrogen bromide and water in the starting mixture.

[0025] In order to achieve the addition of hydrogen bromide to the starting mixture in an easy and cost-effective manner, it is proposed in a further development of the inventive concept that the method according to the invention further comprises as step f) recycling at least a portion of the hydrogen bromide contained in one of the one or more compositions enriched in hydrogen bromide prepared in step c) to step a), wherein the recycled hydrogen bromide is not subjected to electrolysis. Thus, it is not necessary to supply the starting mixture in step a) with external hydrogen bromide, thereby further reducing the overall costs of the method and achieving a particularly efficient utilization of the hydrogen bromide formed in the reaction of step b). A further advantage of using recycled hydrogen bromide in the starting mixture that has not been subjected to the electrolysis in step d), i.e. by using recycled hydrogen bromide in the starting mixture that bypasses the electrolysis of step d), is that only a portion of the hydrogen bromide obtained in step b) needs to be sent to the electrolysis of step d) to prepare bromine, and in the case of electrolysis of aqueous hydrogen bromide solution, it is avoided to add more water to the electrolysis of step d), both of which lead to significant energy savings in the method.

[0026] Under standard pressure and temperature conditions, hydrogen bromide and water form a mixture of an azeotropic composition. Under these conditions, the relative amount of hydrogen bromide relative to the hydrogen bromide-water mixture is about 47% by weight. An important aspect of the process according to the invention is the ability to adjust the hydrogen bromide-water azeotrope composition or to completely destroy the azeotrope, if desired and depending on the embodiment. To this end, increasing amounts of sulfuric acid can be added to the hydrogen bromide-water mixture, which has the progressive effect of first reducing the hydrogen bromide fraction in the azeotropic composition and ultimately completely destroying the azeotrope, which occurs when the amount of sulfuric acid is about 55% by weight relative to the sum of hydrogen bromide and water.

[0027] Another important aspect of the present invention is that the optimum operating point of any embodiment can be that hydrogen bromide is present at the outlet of the reaction section relative to the hydrogen bromide and water in the liquid portion at a near azeotropic composition. The reason for this is twofold: on the one hand, if hydrogen bromide is present at a concentration higher than the azeotropic composition in the reaction mixture effluent, then excess hydrogen bromide may also be contained in the gaseous portion of the reactor mixture effluent, and potentially, it may not only impair the operation of the subsequent gas scrubber described further below, but may also result in insufficient hydrogen bromide remaining in the liquid portion of the reaction mixture effluent, in which case, in the treatment of step d), such as in electrolysis (or other hydrogen bromide conversion methods), hydrogen bromide is insufficiently converted to bromine. On the other hand, if hydrogen bromide is present at a concentration lower than the azeotropic composition in the reactor mixture effluent, then this means that excess water is present, which means that additional energy is consumed in the subsequent hydrogen bromide recovery and sulfuric acid purification steps to evaporate this water. In addition, this may also result in the sub-azeotropic hydrogen bromide solution entering the treatment of step d) (such as electrolysis), which is inefficient. Thus, as described further below, the optimum point can be directed by controlling the amount of additional water entering the gas scrubber.

[0028] As described above, in the present invention, particularly preferably in step a), use elemental sulfur and / or hydrogen sulfide as sulfur-containing compound. Therefore, preferably in step a), elemental sulfur (or any sulfur-containing compound) is mixed with bromine, water and optional hydrogen bromide. In this embodiment, if the starting mixture is a gas-liquid mixture at reaction temperature and reaction pressure, the concentration of sulfur in the starting mixture provided in step a) is preferably higher than 0.05 weight %, more preferably 0.05 to 40 weight % and most preferably 0.1 to 2 weight %, and if the starting mixture is a liquid at reaction temperature and reaction pressure, then based on the starting mixture total weight.

[0029] However, according to an alternative preferred embodiment of the present invention, preferably in step a), use the gas containing sulfur-containing compounds as impurities as the source of sulfur-containing compounds, so that gas is desulfurized while producing hydrogen. Therefore, it is suggested that in this alternative preferred embodiment of the present invention, in step a), the gas containing sulfur-containing compounds is mixed with bromine, water and optionally and also preferably hydrogen bromide, wherein the gas containing sulfur-containing compounds is selected from biogas, natural gas, flue gas, combustion gas from sulfur compounds, tail gas and any combination of two or more aforementioned gases. Based on the gross weight of gas, the concentration of sulfur-containing compounds in gas can be 10ppm to 99% by weight and more preferably 1 to 40% by weight. Further preferably in this embodiment, the gas containing sulfur-containing compounds contains unsaturated compounds less than 500ppm by volume, more preferably less than 200ppm by volume, even more preferably less than 100ppm by volume, and most preferably does not contain unsaturated compounds. This avoids the unnecessary consumption of bromine in the reaction of step b) due to side reactions.

[0030] Both of the aforementioned embodiments can be combined, ie in step a) a combination of elemental sulphur and any of the aforementioned gases containing sulphur-containing compounds can be added to the starting mixture in step a).

[0031] With regard to the reaction conditions of step b), i.e. with regard to the reaction temperature and the reaction pressure, the present invention is not particularly restricted. Good results are particularly obtained when the reaction in step b) is carried out at a temperature of at least 55° C. and preferably at a temperature of 60 to 200° C. Good results are also particularly obtained when the reaction in step b) is carried out at a pressure of at least 5 barg and preferably at a pressure of 6 to 15 barg.

[0032] This reaction does not require a catalyst.

[0033] The reaction mixture effluent generated during the reaction in step b) of the process according to the invention may (depending on the composition of the starting mixture and on the reaction conditions) be a single-phase liquid.

[0034] However, in most cases, the reaction mixture effluent generated during the reaction of step b) will be a gas / liquid two-phase mixture. In this case, it is suggested according to a further development of the inventive concept that the gas / liquid two-phase mixture is separated into a gas phase and a liquid phase as a first sub-step of step c), thereby allowing a certain amount of hydrogen bromide to be separated as a gas phase from the sulfuric acid generated in the reaction of step b), as well as any non-reactive gases that may be present in the reactor feed mixture in an easy manner. Good results are particularly obtained when the gas-liquid mixture of the reaction mixture effluent is fed into a container in step c) and allowed to phase separate therein to separate the reaction mixture effluent into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent. Alternatively, the phase separation can be achieved by subjecting the gas-liquid mixture of the reaction mixture effluent to at least one separation step selected from a stripping step, an absorption step, an extraction step and any combination of two or more of the aforementioned separation steps, so as to separate the reaction mixture effluent into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent. Preferably, the aforementioned separation is carried out at the temperature and pressure of the reaction mixture effluent when it leaves the reactor, ie there is no active cooling, active heating, active pressurization or active pressure reduction between reaction and separation.

[0035] Preferably, a portion of the liquid portion of the reaction mixture effluent is recycled to the reactor. If at least one sulfur-containing compound is hydrogen sulfide, this is particularly preferred. The hydrogen sulfide conversion reaction occurs in the liquid phase, and the reaction is irreversible and mass transfer limited. Therefore, it is advantageous to supply the reaction mixture with a liquid recirculation device, which helps to ensure that hydrogen sulfide and bromine are preferably maintained in the liquid phase. This also minimizes the amount of unreacted bromine. If at least one sulfur-containing compound is elemental sulfur, unreacted sulfur in the reaction mixture effluent should be filtered, and unreacted sulfur should be recycled to the reactor.

[0036] In a further development of the inventive concept, the oxidation of a portion of the S-containing compounds may stop at the elemental sulfur and not proceed to sulfuric acid. Such failures may be caused by different process upsets or occur during the start-up or shutdown of the operation (different transition stages). In this case, the reaction mixture effluent will also contain some elemental sulfur. The elemental sulfur impurities can be removed from the effluent by filtering, decantation or any other method known in the prior art and recycled back to the reactor.

[0037] In a further development of the inventive concept, it is proposed that the liquid portion of the reaction mixture effluent contains at least 20% by weight and preferably about 50% by weight of hydrogen bromide relative to the water content of the liquid portion of the reaction mixture effluent. Likewise, if the reaction mixture effluent generated in step b) is a single-phase liquid, the liquid portion of the reaction mixture effluent corresponds to the total reaction mixture effluent. However, if the reaction mixture effluent generated in step b) is a gas-liquid two-phase mixture, the liquid portion of the reaction mixture effluent corresponds to the liquid phase obtained after the aforementioned separation step. It is particularly preferred that the liquid portion of the reaction mixture effluent contains hydrogen bromide and water in a concentration close to or in the hydrogen bromide-water azeotrope. For a binary mixture of water and hydrogen bromide at standard temperature and pressure, the azeotropic composition contains about 47% by weight of hydrogen bromide and about 53% by weight of water. However, the exact content of water and hydrogen bromide of the azeotropic composition also depends on the concentration of sulfuric acid in the case of the existing pressure conditions of the liquid portion of the reaction mixture effluent and the reactor outlet conditions. It is particularly advantageous to form such a hydrogen bromide-water azeotrope in the liquid portion of the reaction mixture effluent, especially since a super-azeotropic composition may cause too much hydrogen bromide to be carried away by the gas leaving the gas-liquid reaction mixture effluent during the aforementioned separation into the gaseous portion of the reaction mixture effluent and the liquid portion of the reaction mixture effluent. In addition, a sub-azeotropic composition means in the best case scenario that too much water is circulated in the process, which needs to be removed by evaporation, which reduces the efficiency of the process, and in the worst case scenario that if sulfuric acid is not added to the liquid portion of the reaction mixture effluent in the preferred subsequent distillation step further described below, the composition of the overhead stream will depend on whether the hydrogen bromide-water azeotrope is broken inside the distillation column. If the hydrogen bromide-water azeotrope is broken in the distillation step, hydrogen bromide with a relatively low water content can be removed in the overhead composition. However, if the hydrogen bromide-water azeotrope is not broken in the distillation step, a large amount of water will be contained in the overhead composition. On the other hand, assuming that the downstream electrolysis unit can process up to 60% aqueous hydrogen bromide on a mass basis, the target concentration in the liquid phase at the reactor outlet can be adjusted accordingly, but the same general principles as laid out above remain valid. The same principles apply whenever the azeotropic hydrogen bromide composition relative to water is changed due to the operating pressure in the reactor.

[0038] In addition to the aforementioned hydrogen bromide content, it is preferred that the liquid portion of the reaction mixture effluent comprises at least 0.1 wt % sulfuric acid, 20 to 70 wt % water, and up to 2 wt % bromine, based on the total weight of the liquid portion of the reaction mixture effluent. More preferably, the liquid portion of the reaction mixture effluent comprises 0.3 to 15 wt % sulfuric acid, 25 to 65 wt % water, and up to 0.5 wt % bromine, based on the total weight of the liquid portion of the reaction mixture effluent.

[0039] The gaseous portion of the reaction mixture effluent preferably comprises up to 50% by weight of hydrogen bromide, based on the total weight of the gaseous portion of the reaction mixture effluent.

[0040] The liquid portion of the reaction mixture effluent, i.e. the reaction mixture effluent in case the reaction mixture effluent is a single-phase liquid composition and the liquid portion of the reaction mixture effluent in case the reaction mixture effluent is a gas / liquid two-phase composition, after separation from the gaseous portion of the reaction mixture effluent, is preferably separated into one or more additional compositions enriched in hydrogen bromide and one or more compositions enriched in sulfuric acid by a further separation step. According to the present invention, preferably, step c) comprises at least two distillation steps and preferably three distillation steps in addition to the aforementioned preferred gas-liquid separation step.

[0041] Before being sent to the first distillation step, the pressure of the reaction mixture effluent or the liquid part of the reaction mixture effluent, respectively, is preferably reduced, for example by sending it through a throttling control valve.

[0042] In the case where the reaction mixture effluent is a single-phase liquid composition, the reaction mixture effluent or a mixture of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) is subjected to a first distillation step as feed. As further elaborated below, if the electrolysis of step d) is carried out with a gaseous composition enriched in hydrogen bromide, it is preferred to subject a mixture of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) as feed to a first distillation step. This allows to increase the sulfuric acid concentration in the mixture of the reaction mixture effluent and the composition enriched in sulfuric acid, so that the hydrogen bromide-water azeotrope is destroyed in the mixture, or at least sufficiently modified, so that sufficient hydrogen bromide is ultimately available to undergo electrolysis and thus regenerate the required amount of bromine at the reactor inlet conditions. As elaborated above, thus during the distillation step, a gaseous hydrogen bromide composition having a relatively low water content can be separated as a tower top composition from a liquid tower bottom composition comprising sulfuric acid and water. This is advantageous because electrolysis using a gaseous hydrogen bromide-rich composition cannot tolerate excess water and thus must only dry or partially condense the overhead composition having a relatively low water content to reduce the water content in the gaseous hydrogen bromide-rich composition to an acceptable amount for the electrolysis of step d). However, if the electrolysis of step d) is carried out with a liquid (i.e., aqueous) hydrogen bromide composition, the reaction mixture effluent can be used without admixture with the sulfuric acid-enriched composition obtained in step c), since the higher water content in the overhead composition is not detrimental to the electrolysis due to the fact that the hydrogen bromide-water azeotrope is not broken during the distillation process. However, in this case, the reaction mixture effluent can also be mixed with the sulfuric acid-enriched composition obtained in step c) before using this mixture as a feed for the first distillation step. For example, this can be done to provide the electrolyzer with a liquid hydrogen bromide solution above the azeotropic composition.

[0043] In the aforementioned embodiment, the distillation step may also be settling the reaction mixture effluent or the mixture of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) in a container to separate the reaction mixture effluent or the mixture of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) into a gas composition enriched in hydrogen bromide and a liquid composition enriched in sulfuric acid. Settling in a container is considered to be a distillation step with one theoretical stage.

[0044] In the case where the reaction mixture effluent is a two-phase gas / liquid composition, the two-phase gas / liquid composition is first separated into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent as further described above, for example by feeding the reaction mixture effluent into a container and allowing it to phase separate therein or by subjecting it to at least one separation step selected from a stripping step, an absorption step, an extraction step and any combination of two or more of the aforementioned separation steps. Subsequently, the liquid portion of the reaction mixture effluent or a mixture of the liquid portion of the reaction mixture effluent and the sulfuric acid-enriched composition obtained in step c) is subjected as feed to a first distillation step. As described above, when the electrolysis in step d) is carried out with a gaseous hydrogen bromide-rich composition, it is most preferred to mix the liquid portion of the reaction mixture effluent with the sulfuric acid-enriched composition before subjecting the mixture to the first distillation step, however such a mixture may be used, but in the case where the electrolysis in step d) is carried out with a liquid (i.e., aqueous) hydrogen bromide-rich composition, it is also suitable to use only the liquid portion of the reaction mixture effluent. Also in this embodiment, the distillation step may be settling of the reaction mixture effluent or a mixture of the reaction mixture effluent and the sulphuric acid-enriched composition obtained in step c) in a vessel.

[0045] In most of the aforementioned embodiments, in which a mixture of the reaction mixture effluent (in the case where the reaction mixture effluent is a single-phase liquid composition) or the liquid portion of the reaction mixture effluent (in the case where the reaction mixture effluent is a two-phase gas / liquid composition) with the composition enriched in sulfuric acid obtained in step c) is subjected as feed to a first distillation step, for the aforementioned reasons it is preferred that the feed contains so much sulfuric acid that the hydrogen bromide-water azeotrope is broken during the distillation, or at least the azeotropic composition is changed so that sufficient hydrogen bromide will still be available for electrolysis, as mentioned above. Likewise, this allows efficient separation of hydrogen bromide in the gas phase from water in the liquid phase, so that an overhead composition having a rather low water concentration of, for example, less than 30% by weight is obtained in the distillation, which water concentration can subsequently be further reduced by drying the overhead composition. The exact amount of sulfuric acid required to break the hydrogen bromide-water azeotrope depends on the concentration of sulfuric acid, but the hydrogen bromide-water azeotrope is reliably broken at a sulfuric acid concentration of at least 53% by weight relative to the water concentration. It is therefore particularly preferred that the feed in these embodiments comprises at least 53% by weight of sulfuric acid relative to the water content. For example, the feed comprises at least 40 wt. %, more preferably at least 45 wt. %, and even more preferably at least 48 wt. % sulfuric acid and less than 38 wt. %, more preferably at least 30 wt. %, and even more preferably at least 25 wt. % water, based on the total weight of the feed.

[0046] In the first distillation step, the feed is separated into an overhead composition of a gaseous composition enriched in hydrogen bromide containing up to 1,000 ppm of sulfuric acid and a bottom composition of a liquid composition enriched in sulfuric acid containing more than 30% by weight of sulfuric acid and less than 1,000 ppm of hydrogen bromide. Preferably, the composition enriched in hydrogen bromide obtained in the first distillation step contains up to 1,000 ppm, more preferably up to 10 ppm and most preferably up to 1 ppm of sulfuric acid, 5 to 60% by weight of water and 40 to 95% by weight of hydrogen bromide, based on the total weight of the composition enriched in hydrogen bromide. On the other hand, the composition enriched in sulfuric acid obtained in the first distillation step preferably contains more than 50% by weight of sulfuric acid and less than 50% by weight of water, based on the total weight of the composition enriched in sulfuric acid. More preferably, the composition enriched in sulfuric acid obtained in the first distillation step contains 60 to 95% and even more preferably 70 to 80% of sulfuric acid and 5 to 40% and even more preferably 20 to 30% of water, based on the total weight of the composition enriched in sulfuric acid.

[0047] In a further development of the inventive concept, it is proposed to use the composition enriched in hydrogen bromide containing up to 1,000 ppm of sulfuric acid produced as the overhead composition in the first distillation step for the electrolysis of step d). As explained above, the electrolysis can be carried out with a gaseous hydrogen bromide-rich composition or with a liquid (i.e. aqueous) hydrogen bromide-rich composition.

[0048] In the case where the electrolysis of step d) is carried out with a liquid hydrogen bromide-rich composition, it is preferred that the hydrogen bromide-rich composition containing at most 1,000 ppm of sulfuric acid produced as the overhead composition in the first distillation step is condensed in a condenser, and all or a portion thereof is fed to the electrolytic cell for electrolysis. In this case, the hydrogen bromide-rich liquid composition fed to the electrolytic cell has the same composition as the hydrogen bromide-rich composition containing at most 1,000 ppm of sulfuric acid produced as the overhead composition in the first distillation step, i.e., it preferably contains at most 1,00 ppm of sulfuric acid, more preferably at most 10 ppm and most preferably at most 1 ppm of sulfuric acid, 5 to 60 wt% of water and 40 to 95 wt% of hydrogen bromide based on the total weight of the condensed hydrogen bromide-rich composition. In this embodiment, it is preferred that only a portion of the condensed hydrogen bromide-enriched composition is fed to the electrolyzer, while the remainder thereof bypasses the electrolyzer and is recycled to the starting mixture, i.e. is not fed to the electrolyzer, but directly or indirectly by mixing it with other process streams into step a). Thus, it is not necessary to supply the starting mixture in step a) with external hydrogen bromide, thereby further reducing the overall cost of the process and achieving a particularly efficient utilization of the hydrogen bromide formed in the reaction of step b). Another advantage of using hydrogen bromide in the starting mixture that is not subjected to electrolysis in step d) (i.e. by bypassing the electrolysis of a portion of the hydrogen bromide formed in the reaction of step b)) is that only a portion of the hydrogen bromide obtained in step b) needs to be fed to the electrolysis of step d) to produce bromine, and in the case of aqueous hydrogen bromide electrolysis, the addition of more water to the electrolysis of step d) is avoided, both of which lead to significant energy savings in the process.

[0049] According to a particularly preferred embodiment of the present invention, the electrolysis of step d) is not carried out with a liquid hydrogen bromide-rich composition, but with a gaseous hydrogen bromide-rich composition. A particular advantage of carrying out the electrolysis of step d) with a gaseous hydrogen bromide-rich composition is that - compared with carrying out the electrolysis of step d) with a liquid hydrogen bromide-rich composition - the operating energy requirement in the electrolyzer is reduced by 10 to 15%. In the embodiment in which the electrolysis of step d) is carried out with a gaseous hydrogen bromide-rich composition, at least a portion of the hydrogen bromide-rich composition obtained as a top composition in the first distillation step is preferably subjected to a water removal step to separate the hydrogen bromide-rich composition into a water-rich composition and a hydrogen bromide-rich, water-depleted gaseous composition comprising 90 to 99% by weight of hydrogen bromide and less than 10% by weight, preferably less than 5% by weight, more preferably less than 2% by weight and most preferably less than 1% by weight of water. At least a portion of the hydrogen bromide-rich, water-depleted gaseous composition is subjected to the electrolysis in step d). In addition, the gas composition enriched in hydrogen bromide and depleted in water contains sulfuric acid of at most 1,000 ppm, preferably at most 100 ppm, more preferably at most 10 ppm and most preferably at most 1 ppm. The water removal step can be carried out by distillation, pervaporation or partial condensation. In the latter alternative, the gas composition enriched in hydrogen bromide obtained as the overhead composition in the first distillation step can be cooled and partially condensed, and then separated into a liquid composition enriched in water and a gas composition enriched in hydrogen bromide and depleted in water. When the gaseous portion of the composition enriched in hydrogen bromide is fed to the electrolyzer for electrolysis in step d), it is preferred to allow the liquid portion of the composition enriched in hydrogen bromide to bypass the electrolysis and be recycled to the starting mixture, i.e., not to be sent to the electrolyzer, but to enter step a) directly or indirectly by mixing it with other process streams.

[0050] In order to further purify and concentrate the composition enriched in sulfuric acid obtained as the bottom composition of the first distillation step, it is proposed in a further preferred embodiment to subject it to a second distillation step to produce a top composition and a bottom composition containing 60 to 95% by weight and preferably 70 to 80% by weight of sulfuric acid and 5 to 40% by weight and preferably 20 to 30% by weight of water, based on the total weight of the bottom composition. It is further preferred that at least a portion of the bottom composition of the second distillation step is subjected to a third distillation step to produce a top composition and a bottom composition containing at least 90% by weight and preferably at least 94% by weight of sulfuric acid and less than 10% by weight and preferably less than 6% by weight of water, based on the total weight of the bottom composition. The top composition obtained in the third distillation step preferably contains at least 99.0% by weight, more preferably at least 99.5% by weight and even more preferably at least 99.9% by weight of water. This pure water can be removed without any further required purification steps, thus reliably and effectively allowing water accumulation in a continuous process to be avoided.

[0051] As further explained above, in particular in the case of electrolysis with a gas composition enriched in hydrogen bromide in step d), it is preferred to adjust the sulfuric acid and water concentrations in the feed to the first distillation column so that the hydrogen bromide-water azeotrope is broken during the distillation. This allows efficient separation of hydrogen bromide from water in the gas phase. In order to appropriately adjust the sulfuric acid and water concentrations in the feed to the first distillation column, it is proposed in a further development of the inventive concept to recycle a portion of the bottom composition obtained in the second distillation step and / or a portion of the bottom composition obtained in the third distillation step to the reaction mixture effluent (in the case where the reaction mixture effluent is a single-phase liquid composition) or to the liquid part of the reaction mixture effluent (in the case where the reaction mixture effluent is a gas / liquid two-phase composition) and to mix it as a composition enriched in sulfuric acid before subjecting the thus obtained mixture as feed to the first distillation step.

[0052] According to another preferred embodiment of the invention, the gaseous part of the reaction mixture effluent or the mixture of the gaseous part of the reaction mixture effluent and the overhead composition of the second distillation step is subjected to a scrubbing step with water to obtain a gaseous composition containing less than 10 ppm of bromine and hydrogen bromide in total and an aqueous composition containing 5 to 40% by weight of hydrogen bromide, wherein the aqueous composition is recycled to step a). Thus, it is achieved to return the unreacted bromine that may be contained in the gaseous part of the reaction mixture effluent and subjected to the scrubbing step to the reactor, which allows a more efficient use of the bromine. The aqueous composition can be recycled directly to step a) or indirectly to step a) by mixing it with other process streams. At least part of the water required for the scrubbing step can be part or all of the overhead composition obtained in the third distillation step. For example, part of the overhead composition obtained in the third distillation step can be removed, while another part can be used in the scrubbing step. The rest of the water required for the scrubbing step can be fresh water.

[0053] Alternatively, even though less preferred, the gaseous portion of the reaction mixture effluent or a mixture of the gaseous portion of the reaction mixture effluent and the overhead composition of the second distillation step can be recycled directly to step a), i.e. without being subjected to a scrubbing step. In this embodiment, part of the overhead composition obtained in the third distillation step can also be recycled directly to step a).

[0054] According to a particularly preferred embodiment of the present invention, the electrolysis of step d) is carried out with a gaseous hydrogen bromide-rich composition and comprises the following steps:

[0055] i) feeding the reaction mixture effluent in step c) into a container and allowing it to phase separate therein to separate the reaction mixture effluent into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent,

[0056] ii) subjecting a mixture of the liquid portion of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) as feed to a first distillation step to separate the feed into an overhead composition of a composition enriched in hydrogen bromide containing at most 1,000 ppm of sulfuric acid and a bottom composition of a composition enriched in sulfuric acid containing more than 30% by weight of sulfuric acid and less than 1,000 ppm of hydrogen bromide, wherein the feed preferably contains at least 53% by weight of relative sulfuric acid relative to the water concentration,

[0057] iii) subjecting at least a portion of the composition enriched in hydrogen bromide obtained as the overhead composition in the first distillation step to a water removal step to separate the composition enriched in hydrogen bromide into a composition enriched in water and a gas composition enriched in hydrogen bromide, depleted in water, comprising 90 to 99% by weight of hydrogen bromide and less than 10% by weight and preferably less than 1% by weight of water, and subjecting at least a portion of the gas composition enriched in hydrogen bromide, depleted in water to the electrolysis in step d),

[0058] iv) subjecting the composition enriched in sulfuric acid obtained as the bottom composition of the first distillation step to a second distillation step to produce an overhead composition and a bottom composition containing 60 to 95% by weight and preferably 70 to 80% by weight of sulfuric acid and 5 to 40% by weight and preferably 20 to 30% by weight of water, based on the total weight of the bottom composition, wherein preferably at least a portion of the bottom composition of the second distillation step is subjected to a third distillation step to produce an overhead composition and a bottom composition containing at least 90% by weight and preferably at least 94% by weight of sulfuric acid and less than 10% by weight and preferably less than 6% by weight of water, based on the total weight of the bottom composition,

[0059] v) recycling a portion of the bottom composition obtained in the second distillation step and / or a portion of the bottom composition obtained in the third distillation step as a composition enriched in sulphuric acid to and mixing it with the liquid part of the reaction mixture effluent to produce a mixture of the liquid part of the reaction mixture effluent and the composition enriched in sulphuric acid used in step ii),

[0060] vi) recycling at least a portion of the bromine-containing composition obtained in the electrolysis to step a), and

[0061] vii) optionally, recycling the gaseous portion of the reaction mixture effluent or the mixture of the gaseous portion of the reaction mixture effluent and the overhead composition of the second distillation step to step a) or subjecting the gaseous portion of the reaction mixture effluent or the mixture of the gaseous portion of the reaction mixture effluent and the overhead composition of the second distillation step to a scrubbing step with water to obtain a gaseous composition containing less than 10 ppm of bromine and hydrogen bromide in total and a liquid aqueous composition containing 5 to 40% by weight of hydrogen bromide, wherein the liquid aqueous composition is recycled to step a).

[0062] As an alternative to the aforementioned step i), separation can be achieved in step i) by subjecting the reaction mixture effluent to at least one separation step selected from a stripping step, an absorption step, an extraction step and any combination of two or more of the aforementioned separation steps.

[0063] Furthermore, the aforementioned method can be modified by replacing the first distillation step ii) with settling the reaction mixture effluent or the mixture of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) in a vessel to separate the reaction mixture effluent or the mixture of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) into a gas composition enriched in hydrogen bromide and a composition enriched in sulfuric acid. Settling in a vessel is considered as a distillation step with one theoretical stage.

[0064] Furthermore, it is preferred in the aforementioned process that the overhead composition obtained in the third distillation, which consists essentially of water, is recycled to step a) or to the optional scrubber step.

[0065] According to an alternative particularly preferred embodiment of the present invention, the electrolysis of step d) is carried out with a liquid hydrogen bromide-rich composition and comprises the following steps:

[0066] i) feeding the reaction mixture effluent in step c) into a container and allowing it to phase separate therein to separate the reaction mixture effluent into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent,

[0067] ii) subjecting the liquid portion of the reaction mixture effluent or a mixture of the liquid portion of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) as feed to a first distillation step to separate the feed into an overhead composition of a composition enriched in hydrogen bromide containing up to 1,000 ppm of sulfuric acid and a bottom composition of a composition enriched in sulfuric acid containing more than 30% by weight of sulfuric acid and less than 1,000 ppm of hydrogen bromide,

[0068] iii) condensing at least a portion of the composition enriched in hydrogen bromide obtained as an overhead composition in the first distillation step and subjecting at least a portion of the condensed portion to the electrolysis in step d),

[0069] iv) subjecting the composition enriched in sulfuric acid obtained as the bottom composition of the first distillation step to a second distillation step to produce an overhead composition and a bottom composition containing 60 to 95% by weight and preferably 70 to 80% by weight of sulfuric acid and 5 to 40% by weight and preferably 20 to 30% by weight of water, based on the total weight of the bottom composition, wherein preferably at least a portion of the bottom composition of the second distillation step is subjected to a third distillation step to produce an overhead composition and a bottom composition containing at least 90% by weight and preferably at least 94% by weight of sulfuric acid and less than 10% by weight and preferably less than 6% by weight of water, based on the total weight of the bottom composition,

[0070] v) optionally, recycling a portion of the bottom composition obtained in the second distillation step and / or a portion of the bottom composition obtained in the third distillation step as a composition enriched in sulphuric acid to the liquid part of the reaction mixture effluent and mixing it therewith to produce a mixture of the liquid part of the reaction mixture effluent and the composition enriched in sulphuric acid used in step ii),

[0071] vi) recycling at least a portion of the bromine-containing composition obtained in the electrolysis to step a),

[0072] vii) optionally, recycling the gaseous portion of the reaction mixture effluent to step a) or subjecting the gaseous portion of the reaction mixture effluent to a scrubbing step with water to obtain a gaseous composition containing less than 10 ppm of bromine and hydrogen bromide in total and a liquid aqueous composition containing from 5 to 40% by weight of hydrogen bromide, wherein the liquid aqueous composition is recycled to step a), and

[0073] viii) Optionally, a portion of the condensed fraction of the composition enriched in hydrogen bromide obtained in step iii) is recycled to step a) without subjecting it to the electrolysis in step d).

[0074] As an alternative to the aforementioned step i), the separation in step i) may be achieved by subjecting the reaction mixture effluent to at least one separation step selected from a stripping step, an absorption step, an extraction step and any combination of two or more of the aforementioned separation steps.

[0075] Furthermore, the aforementioned method can be modified by replacing the first distillation step ii) with settling the reaction mixture effluent or the mixture of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) in a vessel to separate the reaction mixture effluent or the mixture of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) into a gas composition enriched in hydrogen bromide and a composition enriched in sulfuric acid. Settling in a vessel is considered as a distillation step with one theoretical stage.

[0076] Furthermore, it is preferred in the aforementioned process that the overhead composition obtained in the third distillation, which consists essentially of water, is recycled to step a) or to the optional scrubber step.

[0077] In a further development of the inventive concept, it is proposed to carry out the electrolysis in step d) by using an electrolysis cell comprising an anode, a cathode and a membrane sandwiched between the anode and the cathode, wherein an electric field can be applied between the anode and the cathode. The composition enriched in hydrogen bromide obtained in step c) and containing up to 1,000 ppm of sulfuric acid is fed to the anode, optionally a second composition comprising hydrogen bromide and water is fed to the cathode, and the electrolysis cell is operated to produce hydrogen at the cathode, wherein a bromine-containing composition is produced at the anode.

[0078] As explained in further detail below, the electrolysis in step d) is preferably carried out by using an electrolysis cell comprising a membrane made of a fluoropolymer membrane having a glass transition temperature of at least 110°C.

[0079] According to the invention, the electrolysis is carried out in step d) by operating the electrolysis cell at an operating temperature of at least 70°C, wherein the operating temperature is preferably below the boiling point of the optional second composition fed to the cathode, and preferably at an operating temperature of 70 to 122°C.

[0080] Furthermore, it is preferred that the electrolysis is carried out in step d) by operating the electrolysis cell at an operating pressure which increases from the anode to the cathode.

[0081] Good results are particularly obtained when the electrolysis is carried out, ie if the electrolysis cell is operated such that the effluent from the electrolysis cell has a molar ratio of bromine to hydrogen bromide of at least 1.0, preferably greater than 1.0, more preferably greater than 1.2, even more preferably greater than 1.5.

[0082] Anode and cathode often comprise conductive materials, especially metal, coated metal, carbon cloth, graphite felt, carbon fiber composite material, carbon-loaded polymer or graphite, and are preferably made of carbon fiber composite material or coated metal. The membrane can be a sheet or layer that can be used in a standard electrolyzer, preferably as a proton exchange membrane. Therefore, preferably protons can migrate through such membranes.

[0083] As described above, electrolysis can be carried out with a gas composition enriched in hydrogen bromide or with a liquid (i.e., aqueous) composition enriched in hydrogen bromide. In an aqueous composition containing hydrogen bromide, hydrogen bromide is dissolved in water, wherein the hydrogen bromide dissolved in water will often dissociate in water, so that the composition will include ionic species, especially as protons, hydronium ions and bromine cations. At the anode of the feed hydrogen bromide, protons are generated during the electrolysis process, and protons subsequently migrate to the cathode through the membrane. According to the present invention, the aqueous composition enriched in hydrogen bromide can also be fed to the cathode. This allows operating the electrolyzer at a lower operating voltage, which reduces operating costs. In addition, this allows operating the electrolyzer at a higher temperature, which reduces the cooling and / or heating requirements of the electrolyzer, which also reduces operating costs. In addition, this allows operating the electrolyzer at a higher current density, which reduces the capital expenditure of the electrolyzer. In addition, this can reduce the concentration gradient across the membrane, and thus reduce or even alleviate the undesirable transfer of hydrogen bromide and / or bromine and / or bromine cations through the membrane from the anode to the cathode. In addition, the presence of hydrogen bromide on the cathode side can allow hydrogen bromide to pass through the membrane from the cathode to the anode side. This causes an increase in the efficiency of the system, and allows any disturbance in the electrolysis process to be avoided. Without wishing to be bound by theory, it is assumed that in this embodiment, there is actually a passage of hydrogen bromide in the reverse direction, i.e. from the cathode to the anode. Such passage of hydrogen bromide in the reverse direction is considered to be one of the reasons that the electrolysis efficiency of this embodiment can be improved. In this embodiment, it is preferred that the hydrogen bromide concentration of the composition fed to the cathode is at least 0.5mol / kg, more preferably at least 1mol / kg, more preferably at least 2mol / kg, even more preferably at least 3mol / kg, further preferably at least 4mol / kg, even further preferably at least 5mol / kg and most preferably at least 6mol / kg, wherein mol / kg refers to the mole of hydrogen bromide per kilogram of corresponding composition.

[0084] Independently, whether or not the hydrogen bromide composition is also introduced to the cathode of the electrolysis cell, it is preferred that during the electrolysis of step d) at an operating voltage U of more than 0 mV to 1,900 mV and preferably more than 0 mV to 1,700 mV op Operating the electrolyzer at such an operating voltage improves the selectivity of the reaction to hydrogen bromide electrolysis and thereby reduces the operating costs of the electrolysis.

[0085] In a further development of the inventive concept, the electrolysis in step d) is preferably carried out by operating the electrolyser at an operating temperature of 70° C. to 122° C. Operating the electrolyser at such operating temperatures can reduce cooling and / or heating requirements. Such higher temperatures increase the proton conductivity of the membrane comprising the fluoropolymer, which allows operating the electrolyser at higher current densities and / or lower voltages. Higher current densities help to keep capital expenditures low. It is thus particularly preferred to keep the operating temperature above 70° C., more preferably above 90° C. and most preferably above 100° C.

[0086] It is further preferred that, during the electrolysis process in step d), the electrolyzer is operated at an operating pressure increased from the anode to the cathode. In other words, there is a pressure gradient increased from the anode to the cathode, wherein the corresponding pressures at the anode and the cathode have the same reference point, i.e., both are absolute pressures, or both are gauge pressures relative to the atmosphere around the electrolyzer. In such cases, electrolysis often occurs in the gas phase, i.e., by feeding the gas stream to the anode and the cathode. In such cases, the pressure at the cathode is higher than the pressure at the anode, preferably at least 0.01MPa (i.e., the difference between the pressure at the cathode and the pressure at the anode is 0.01MPa or more), more preferably at least 0.02MPa, more preferably at least 0.05MPa and most preferably at least 0.1MPa. For example, when the absolute pressure at the anode is 0.1MPa, the absolute pressure at the cathode is 0.11 to 0.3MPa, more preferably 0.12 to 0.25MPa, more preferably 0.15 to 0.2MPa and most preferably 0.2MPa. However, it is also envisioned that the optional pressure at the cathode is 1MPa, 2MPa, 3MPa, 4MPa or even 5MPa higher than the pressure at the anode. With such an increased pressure or pressure gradient, the operating voltage of the electrolyzer can be reduced, which can cause a reduction in operating costs. In addition, the pressure gradient can allow hydrogen to be obtained at a higher pressure on the cathode side, so that subsequent hydrogen may not be required or less compression is required to densify the obtained hydrogen. In addition, the operation under the pressure difference improves the operating parameters.

[0087] According to another preferred embodiment of the present invention, the membrane used for the electrolysis of step d) is a fluoropolymer membrane. When the glass transition temperature of the fluoropolymer membrane is at least 110°C, preferably at least 120°C and more preferably at least 125°C, good results are particularly obtained. Preferably, the fluoropolymer membrane (in a dry state) is composed of a fluoropolymer, and more preferably a fluoropolymer in a chemically stable form. As used herein, a fluoropolymer is a fluorine-carbon-based polymer with multiple carbon-fluorine bonds. Such fluoropolymers often do not contain silicon atoms. Such fluoropolymers may further be free of chlorine, bromine and / or iodine atoms. As used herein, the glass transition temperature of the fluoropolymer membrane is determined according to DIN EN ISO 11357-2:2020-08 (half-order height method). Therefore, as used herein, the glass transition temperature of the fluoropolymer membrane is measured by differential scanning calorimetry (DSC), more specifically by the curve obtained by such DSC measurement. The glass transition temperature of the fluoropolymer membrane is determined on a sample of the fluoropolymer membrane, more specifically on a sample of the fluoropolymer membrane in a fully hydrated state. According to the present invention, a fluoropolymer membrane in a fully hydrated state is a fluoropolymer membrane that does not absorb any additional (deionized) water (deionized H2O, as described in the ASTM D5127-13 (2018) standard) at a temperature of 80°C after 24 hours, and is therefore a fully hydrated fluoropolymer membrane or a fluoropolymer membrane saturated with water. Therefore, the water absorption of the fluoropolymer membrane has reached its maximum value. A fully hydrated fluoropolymer membrane is obtained by immersing the fluoropolymer membrane in deionized water (which will result in an increase in the weight of the fluoropolymer membrane as measurable by a balance). When the weight no longer increases over time, the fluoropolymer membrane is in a fully hydrated state. A fluoropolymer membrane in a fully hydrated state is typically obtained by immersing the fluoropolymer membrane in deionized water at a temperature of 80°C for 24 hours. As used herein, glass transition temperature refers to the lowest glass transition temperature of a fluorinated polymer. It is known that certain polymers or films made of polymers, respectively, such as The type of membrane may have multiple glass transition temperatures, in particular two glass transition temperatures. The first glass transition temperature is due to the mobility of the main chain in the polymer matrix, while the second glass transition temperature is due to side chain effects, in particular effects associated with strong interactions of functional groups such as sulfonic acid groups in such side chains. Once the first glass transition temperature has been reached, the membrane undergoes irreversible changes and can no longer be used in electrolytic processes in particular. Therefore, in the context of the present invention, if a material, in particular a membrane or a membrane sample, exhibits multiple glass transition temperatures, only the lowest glass transition temperature is relevant to the definition of glass transition temperature provided herein.

[0088] Fluoropolymer membranes having a glass transition temperature of at least 110°C allow electrolysis to be performed at higher temperatures and / or higher pressures because the fluoropolymer membranes are more resilient. Without wishing to be bound by theory, it is believed that the resilience to hydrogen bromide and hydrogen and bromine after electrolysis at higher temperatures and at higher pressures is imparted by fluoropolymer membranes having a glass transition temperature of at least 110°C. In addition, especially higher temperatures can reduce the electrical resistance of the fluoropolymer membrane, which can allow higher proton conductivity. Higher proton conductivity can allow higher current density to be obtained at lower operating voltages. Higher current density helps to keep capital expenditures low, while lower operating voltages result in lower operating costs.

[0089] Preferably, the fluoropolymer of the membrane comprises -(CF2-CF2)- repeating units. Such -(CF2-CF2)- repeating units increase the resilience of the fluoropolymer membrane. The increased resilience allows the electrolysis of hydrogen bromide to be carried out at higher temperatures and at higher pressures, which allows operation at higher current densities and / or at lower voltages, thereby making the electrolysis more economical. Furthermore, preferably, the fluoropolymer of the membrane does not contain structural entities of the formula -O-CF2-CF(CF3)-O-. Such structural entities are found in the polymers marketed under the trade name In commercial films sold, however, this may result in a film with lower rigidity. The film is thus more susceptible to degradation at higher temperatures and higher pressures.

[0090] Preferably, the fluoropolymer film, more specifically the fluoropolymer that the film comprises and preferably constitutes the film, comprises -CF3 chain ends. Such -CF3 chain ends can improve the resilience of the fluoropolymer film. The improved resilience can allow HBr electrolysis to be carried out at higher temperatures and under higher pressures, and can allow operation at higher current densities and / or at lower voltages, making the method according to the present invention more economical thus.

[0091] According to a particularly preferred embodiment of the present invention, the fluoropolymer of the membrane is a sulfonated fluoropolymer. As used herein, sulfonation means that the fluoropolymer of the fluoropolymer membrane carries -SO3H groups. Such -SO3H groups increase the proton conductivity of the fluoropolymer membrane, which allows the electrolysis to be operated at higher current densities and / or at lower voltages, thereby making the electrolysis more economical. It is particularly preferred that the sulfonated fluoropolymer comprises -O-(CF2) n -SO3H group, wherein n is an integer selected from 1, 2, 3, 4 and 5, preferably 2. Such -O-(CF2) nThe -SO3H group contributes to achieving an increased proton conductivity of the fluoropolymer membrane and at the same time increases the resilience of the fluoropolymer membrane. A favorable balance between these effects is particularly achieved when n is selected from 1 to 5, and especially when n is 2. Without wishing to be bound by theory, it is believed that -O-(CF2) n The relatively short chain length of the -S03H group increases the glass transition temperature of the fluoropolymer membrane, making it more resilient at higher temperatures. Therefore, sulfonated fluoropolymer membranes are particularly advantageous from the point of view of hydrogen bromide electrolysis at higher temperatures and / or at higher pressures and therefore at higher current densities and / or at lower voltages. Therefore, sulfonated fluoropolymer membranes make electrolysis particularly economical.

[0092] Preferably, the sulfonated fluoropolymer membrane comprises a hydrolyzed copolymer of F2C=CF2 and CF2=CF-O-(CF2)2-SO2F. Such a hydrolyzed copolymer may lead to an increase in the proton conductivity of the fluoropolymer membrane and / or an increase in the resilience. Therefore, from the point of view of hydrogen bromide electrolysis at higher temperatures and / or at higher pressures and therefore at higher current densities and / or at lower voltages, the sulfonated fluoropolymer membrane is particularly advantageous. Therefore, the sulfonated fluoropolymer membrane makes electrolysis particularly economical.

[0093] It is further preferred that the fluoropolymer membrane has an acid capacity of at least 0.9meq / g, more preferably 0.95 to 1.5meq / g and most preferably 1.0 to 1.2meq / g. As used herein, meq / g refers to milliequivalents / gram, wherein equivalents are mol-H (i.e., the moles of H acid sites), preferably mol-SO3H (i.e., the moles of SO3H groups). The acid capacity represents the sum of the active sites or functional groups responsible for the proton exchange of the fluoropolymer membrane. Such acid capacity can improve proton conductivity, and can thus cause the conversion rate of hydrogen bromide in electrolysis to increase. The improved hydrogen bromide conversion rate reduces the need for separation and possible recycling of unconverted hydrogen, which can help to maintain low capital expenditures.

[0094] It is further preferred that the fluoropolymer membrane (in dry state) has a thickness of 25 to 350 μm, more preferably 40 to 150 μm, even more preferably 45 to 130 μm and most preferably 45 to 55 μm. Such thicknesses achieve a good balance between sufficient resilience on the one hand and low capital expenditure on the other hand. At lower thicknesses, there is a risk of undesired crossover of hydrogen / bromine through the fluoropolymer membrane, and at higher thicknesses, an undesirable reduction in proton conductivity may result.

[0095] In a further development of the inventive concept, it is proposed that the fluoropolymer film (in the dry state) has a viscosity of at least 1.70 g / cm 3 , more preferably at least 1.80 g / cm3 , and more preferably 1.80 to 2.00 g / cm 3 and most preferably 1.90 to 1.95 g / cm 3 The density of the fluoropolymer film can be measured on its sample according to ASTM D792-20. It is further preferred that the fluoropolymer film (in a dry state) has a tensile modulus of 100 to 500 MPa, more preferably 150 to 400 MPa, more preferably 200 to 300 MPa and most preferably 260 to 280 MPa. The tensile modulus of the fluoropolymer film can be measured on its sample according to ASTM D638-14. In addition, it is preferred that the fluoropolymer film (in a dry state) has a tensile stress at break in the longitudinal direction (MD) according to ASTM D882-18 of 10 to 70 MPa, more preferably 20 to 60 MPa, more preferably 30 to 50 MPa and most preferably 35 to 45 MPa. In addition, it is preferred that the fluoropolymer film (in a dry state) has a tensile stress at break in the transverse direction (CD) according to ASTM D882-18 of 5 to 60 MPa, more preferably 10 to 50 MPa, even more preferably 20 to 40 MPa, and most preferably 25 to 35 MPa. In addition, it is preferred that the fluoropolymer film (in a dry state) has an elongation at break in the longitudinal direction (MD) according to ASTM D882-18 of 100 to 200%, more preferably 120 to 180%, even more preferably 140 to 160%, and most preferably 145 to 155%. Similarly, it is preferred that the fluoropolymer film (in a dry state) has an elongation at break in the transverse direction (CD) according to ASTM D882-18 of 100 to 350%, more preferably 120 to 300%, even more preferably 150 to 250%, and most preferably 190 to 210%. The aforementioned properties improve the resilience of the fluoropolymer film. The increased resilience allows the hydrogen bromide electrolysis to be carried out at higher temperatures and at higher pressures. This can allow operation at higher current densities and / or at lower voltages, thereby making the electrolysis of step d) more economical.

[0096] According to a second aspect, the present invention relates to an apparatus for preparing hydrogen according to the aforementioned method, comprising:

[0097] a) a reactor comprising one or more inlet lines for bromine, water and sulfur-containing compounds and an outlet line for the effluent of a reaction mixture comprising sulfuric acid and hydrogen bromide,

[0098] b) a vessel comprising an inlet line connected to an outlet line for a reaction mixture effluent, wherein the vessel further comprises an outlet line for a gaseous portion of the reaction mixture effluent and an outlet line for a liquid portion of the reaction mixture effluent, wherein the outlet line for the gaseous portion of the reaction mixture effluent is directly or indirectly connected to the inlet line of the reactor,

[0099] c) a first distillation column comprising an inlet line connected to an outlet line for a liquid portion of the effluent of the reaction mixture, wherein the first distillation column comprises an overhead outlet line for a gaseous composition enriched in hydrogen bromide and a bottom outlet line for a composition enriched in sulfuric acid,

[0100] d) an electrolyzer comprising an anode, a cathode and a membrane sandwiched between the anode and the cathode, and an inlet line for a composition containing hydrogen bromide containing up to 1,000 ppm of sulfuric acid, an outlet line for hydrogen and an outlet line for a bromine-containing composition, wherein the inlet line of the electrolyzer is connected to the outlet line d1) of the first distillation column for a composition enriched in hydrogen bromide directly, or d2) is connected via a condenser arranged therebetween, the condenser comprising an inlet line connected to the top outlet line of the first distillation column for a gas composition enriched in hydrogen bromide and comprising an outlet line connected to the inlet line for the hydrogen bromide-containing composition of the electrolyzer, or d3) is connected via a container arranged therebetween, the container comprising an inlet line connected to the top outlet line of the first distillation column for a gas composition enriched in hydrogen bromide, comprising a liquid outlet line and comprising a gas outlet line, wherein the gas outlet line is connected to the inlet line of the electrolyzer, and wherein the outlet line of the electrolyzer for the bromine-containing composition is directly or indirectly connected to the inlet line of the reactor,

[0101] e) a second distillation column comprising an inlet line connected to the bottom outlet line of the first distillation column for a composition enriched in sulfuric acid, wherein the second distillation column comprises a top outlet line for a composition enriched in hydrogen bromide and a bottom outlet line for a composition enriched in sulfuric acid, and

[0102] f) optionally, a third distillation column comprising an inlet line connected to the bottom outlet line of the second distillation column for a composition enriched in sulfuric acid, wherein the third distillation column comprises an overhead outlet line for a composition enriched in water and a bottom outlet line for a sulfuric acid product composition.

[0103] The vessel b) can also be a stripping column, an absorption column, an extraction column or a distillation column.

[0104] In a further development of the inventive concept, it is proposed that the apparatus further comprises a recycle line which leads from the bottom outlet line of the second distillation column and / or from the bottom outlet line of the third distillation column to the inlet of the first distillation column.

[0105] Furthermore, it is preferred that the apparatus further comprises a scrubbing column comprising an inlet line connected to the outlet line of the vessel for the gaseous portion of the reaction mixture effluent, wherein preferably, the top outlet line of the second distillation column is connected to the outlet line of the vessel for the gaseous portion of the reaction mixture effluent, wherein the scrubbing column further comprises an inlet line for water, an outlet line for a gaseous composition containing less than 10 ppm of bromine and hydrogen bromide in total, and an outlet line for an aqueous composition containing 5 to 40% by weight of hydrogen bromide. Preferably, the inlet line for water of the scrubbing column is combined upstream of the scrubbing column with a recycle line from the top outlet line of the third distillation column for water produced as a top composition in the third distillation column.

[0106] Specific embodiments according to the present invention are described below by way of examples with reference to the accompanying drawings.

[0107] Figure 1 is a schematic diagram of an apparatus for producing hydrogen using gas-phase electrolysis according to one embodiment of the present invention.

[0108] Figure 2 is a schematic diagram of an apparatus for preparing hydrogen using gas-phase electrolysis according to another embodiment of the present invention.

[0109] Figure 3 is a schematic diagram of an apparatus for producing hydrogen using liquid phase electrolysis according to one embodiment of the present invention.

[0110] Figure 4 is a schematic diagram of an apparatus for preparing hydrogen using liquid phase electrolysis according to another embodiment of the present invention.

[0111] Figure 5 is a schematic diagram of an apparatus for preparing hydrogen using liquid phase electrolysis according to another embodiment of the present invention.

[0112] Using gas phase electrolysis Figure 1The apparatus 10 shown in FIG. 1 comprises a reactor 12, a separation vessel 14, three distillation columns, namely a first distillation column 16, a second distillation column 18 and a third distillation column 20, and an electrolyzer 22. The reactor 12 comprises an inlet line 24 for natural gas, an inlet connected to a recycle line for a composition containing bromine, hydrogen bromide and water, an inlet connected to a recycle line 28 for a liquid portion of a reaction mixture effluent, and an outlet line 30 for the reaction mixture effluent. The outlet line 30 for the reaction mixture (from which the recycle line 28 branches off) leads to the vessel 14, which is further connected to an outlet line 32 for a gaseous portion of the reaction mixture effluent and an outlet line 34 for a liquid portion of the reaction mixture effluent. The outlet line 34 for the liquid portion of the reaction mixture effluent leads through a throttling valve 36, then combines with a recycle line 38 for a composition enriched in sulfuric acid and then leads to the inlet of the first distillation column 16. The first distillation column 16 comprises an overhead outlet line 40 and a bottom outlet line 42, which lead to the inlet of the second distillation column 18. Likewise, the second distillation column 18 comprises an overhead outlet line 44 and a bottom outlet line 46, which lead to the inlet of the third distillation column 20. Furthermore, the third distillation column comprises an overhead outlet line 48 and a bottom outlet line 50, which split into a recycle line 38 for the composition enriched with sulfuric acid and a sulfuric acid product outlet line 52.

[0113] The top outlet line 40 of the first distillation column 16 leads to a vessel 54, which is further connected to an outlet line 56 for liquid and to an outlet line 58 for gas. The outlet line 58 for gas leads to the electrolyzer 22, which further comprises an outlet line 60 for the bromine-containing composition and an outlet line 62 for hydrogen. The apparatus 10 further comprises a scrubbing column 64, which is connected to the outlet line 32 for the gaseous portion of the effluent of the reaction mixture, which is combined upstream of the scrubbing column 64 with the top outlet line 44 of the second distillation column 18. The scrubbing column 64 further comprises an inlet line 66 for water, which is combined upstream of the scrubbing column 64 with a recycle line 68 for the water produced as the top composition in the third distillation column 20. More specifically, the top outlet line 48 of the third distillation column 20 leads to a vessel 70, a liquid recycle line 72 leads from the vessel 70 back to the third distillation column 20, and a gas line 71 leads to a compressor 74, from the compressor 74 to a heat exchanger 76 and from the heat exchanger 76 to another vessel 78. The vessel 78 is connected to a gas outlet line 80 and a recycle line 68 for liquid water. The scrubber 64 is further connected to an outlet line 81 for a gas composition containing less than 10 ppm of bromine and hydrogen bromide in total and an outlet line 82 for an aqueous composition containing 5 to 40 wt.% hydrogen bromide. When the outlet line 81 leads to a vessel 84 containing a gas outlet line 86 and a recycle line 88 leading to the water inlet line 66 of the scrubber 64, the outlet line 82 of the scrubber 64 is combined with the outlet line 60 for the bromine-containing composition into a recycle line 26 leading to the reactor 12.

[0114] During operation of the apparatus 10, natural gas containing hydrogen sulfide as a sulfur-containing compound is fed via an inlet line 24, and a composition containing bromine, hydrogen bromide and water is fed into the reactor 12 via a recycle line 26 to form a starting mixture therein, which reacts in the reactor 12 to form a reaction mixture effluent containing sulfuric acid, hydrogen bromide, unreacted bromine and water, and methane, ethane and carbon dioxide derived from the natural gas. The reaction mixture effluent is withdrawn from the reactor 12 via an outlet line 30, a portion of which is recycled to the reactor 12 via a line 28, and the remaining portion is fed into a separation vessel 14. The reaction mixture effluent is separated in the separation vessel 14 into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent. When the gaseous portion of the reaction mixture effluent leaves the separation vessel 14 via an outlet line 32, the liquid portion of the reaction mixture effluent leaves the separation vessel 14 via an outlet line 34. Prior to entering the first distillation column 16, the liquid portion of the reaction mixture effluent is combined with the composition enriched in sulfuric acid from the bottom outlet line 50 of the third distillation column 20 and directed to the line 34 via the recycle line 38 to increase the sulfuric acid concentration in the liquid portion of the reaction mixture effluent to a level high enough to break the hydrogen bromide-water azeotrope therein, thereby allowing efficient separation of hydrogen bromide in the vapor phase and water in the liquid phase in the first distillation column 16. The bottoms composition enriched in water and sulfuric acid produced in the first distillation column 16 is withdrawn from the first distillation column 16 via the bottoms outlet line 42 and introduced into the second distillation column 18, where the bottoms composition enriched in water and sulfuric acid of the first distillation column 16 is separated into a water-enriched overhead composition and a sulfuric acid-enriched bottoms composition. The bottom composition thus produced, enriched in sulfuric acid, is withdrawn from the second distillation column 20 via the bottom outlet line 46 and subsequently fed to the third distillation column 20 and separated therein into a pure water top composition comprising more than 99.9% by weight of water and a concentrated sulfuric acid bottom composition comprising at least 95% by weight of sulfuric acid. The concentrated sulfuric acid bottom composition thus produced is withdrawn from the third distillation column 20 via the outlet line 50. While a portion of the concentrated sulfuric acid composition is recycled into the liquid portion of the reaction mixture effluent directed through the line 34 via the recycle line 38, the remaining portion of the concentrated sulfuric acid composition is withdrawn from the apparatus 12 via the sulfuric acid product outlet line 52.

[0115] The composition enriched in hydrogen bromide obtained as the overhead composition in the first distillation column 16 is fed via the overhead outlet line 40 of the first distillation column 16 to a vessel 54 where the composition enriched in hydrogen bromide is partially condensed and thereby separated into a water-enriched liquid composition withdrawn from the vessel via a liquid outlet line 56 and a hydrogen bromide-enriched, water-depleted gas composition containing less than 1% by weight of water withdrawn from the vessel via an outlet line 58. When the gaseous portion of the composition enriched in hydrogen bromide is fed via line 58 to the electrolysis cell 22 for the electrolysis of step d) therein, the liquid portion of the composition enriched in hydrogen bromide is not subjected to any electrolysis and thereby bypasses the electrolysis cell 22, but is directed to the recycle line 26. Optionally, a portion of the water-enriched liquid composition leaving the vessel 54 is fed back to the top of the distillation column 16 as a liquid reflux. The composition enriched in hydrogen bromide is electrolyzed in the electrolysis cell 22 into hydrogen gas and a bromine-containing composition. While hydrogen is withdrawn from electrolyzer 22 via outlet line 62 and removed from the plant as a product, a bromine-containing composition is withdrawn from electrolyzer 22 via outlet line 60 and combined with an aqueous composition obtained as a bottom composition in scrubber 64. The combined stream thus obtained is directed to reactor 12 via recycle line 26, to which the liquid portion of the composition enriched in hydrogen bromide obtained in vessel 54, bypassing electrolyzer 22, is directed via line 56.

[0116] The gaseous portion of the reaction mixture effluent leaving the separation vessel 14 via the outlet line 32 is subjected to a scrubbing step with water in the scrubbing tower 64 to obtain a gaseous composition containing less than 10 ppm of bromine and hydrogen bromide in total, which is withdrawn from the scrubbing tower 64 via the gas outlet line 81, and an aqueous composition containing 5 to 40% by weight of hydrogen bromide, which is withdrawn from the scrubbing tower 64 via the liquid outlet line 82 and subsequently combined with the bromine-containing composition leaving the electrolyzer via the outlet line 60.

[0117] The top composition of the third distillation column 20 withdrawn from the third distillation column 20 via the outlet line 48 is degassed in the vessel 78 , and the degassed pure water thus obtained is introduced through the line 68 and combined with the water inlet line 66 leading to the scrubbing column 64 .

[0118] Figure 2 The device 10 shown in FIG. Figure 1 , except that the liquid portion of the reaction mixture effluent withdrawn from vessel 14 via outlet line 34 is not combined with the composition enriched in sulfuric acid obtained as a bottoms composition in third distillation column 20, but is combined with the composition enriched in sulfuric acid obtained as a bottoms composition in second distillation column 18 and directed to line 34 via recycle line 38.

[0119] and Figure 1 and Figure 2The devices shown in 10 are different, Figure 3 The apparatus 10 shown in FIG. 1 uses liquid phase electrolysis. Figure 3 The device 10 shown in FIG. Figure 1 The apparatus shown in FIG. 1 differs in that the top outlet line 40 of the first distillation column 40 is not directed to the vessel 54 and is not separated therein into a liquid composition enriched in water and a gaseous composition enriched in hydrogen bromide and depleted in water. Instead, the top outlet line 40 of the first distillation column 40 is directed to a condenser 90 in which the top composition obtained in the first distillation column 16, which contains up to about 50% by weight of hydrogen bromide and up to 50% by weight of water, is condensed into a liquid. A portion of the liquid thus obtained is subsequently fed to the electrolysis cell 22 via the line 58, while the remaining portion of the liquid is directed via the bypass line 92 to the outlet line of the aqueous composition of the scrubbing column 64. Figure 1 Another difference of the device 10 shown in FIG. 1 is that Figure 3 The overhead outlet line 44 of the second distillation column 18 of the apparatus 10 shown in FIG. 1 is not combined with the line 32 for the gaseous portion of the effluent of the reaction mixture. Instead, Figure 3 The top outlet line 44 of the second distillation column 18 of the apparatus 10 shown in FIG. 1 is directed to a condenser 94, in which the water-enriched top composition obtained in the second distillation column 18 is condensed into a liquid, which is then directed via a liquid line 96 to a water inlet line 66 of a scrubbing column 64. Figure 1 Still another difference of the apparatus 10 shown in FIG. 1 is that the liquid portion obtained in the vessel 70 from the overhead composition withdrawn from the third distillation column 20 via the line 48 is not recycled to the third distillation column via the liquid recycle line 72. Instead, the liquid portion obtained in the vessel 70 from the overhead composition of the third distillation column 20 is conducted via the liquid line 98 and combined with the liquid portion of the water obtained in the vessel 78, wherein the pure water mixture thus obtained is directed via the recycle line 68 to the water inlet line 66 of the scrubbing column 64. Figure 1 A further difference of the apparatus 10 shown in FIG. 1 is that the gas composition obtained in the scrubbing tower 64 is not directed to the container 84. Instead, the gas composition obtained via line 81 and leaving the scrubbing tower 64 is withdrawn from the apparatus 10 via line 81. Figure 1 The last difference in the device shown in 10 is that Figure 3 The apparatus 10 shown in does not include any recycle line 38 for recycling the composition enriched in sulfuric acid and combining it with the liquid portion of the reaction mixture effluent withdrawn from the vessel 14 via line 34 .

[0120] Figure 4 The device 10 shown in FIG. Figure 31, except that the apparatus 12 comprises a recycle line 38 for recycling a portion of the bottom composition enriched in sulfuric acid obtained in the third distillation column 20 and combining it with the liquid portion of the reaction mixture effluent withdrawn from the vessel 14 via the line 34 before conducting the mixture thus obtained to the first distillation column 16.

[0121] Figure 5 The device 10 shown in FIG. Figure 3 14 is identical to the apparatus shown in 14, except that the apparatus 12 comprises a recycle line 38 for recycling a portion of the bottoms composition enriched in sulfuric acid obtained in the second distillation column 18 and combining it with the liquid portion of the reaction mixture effluent withdrawn from the container 14 via the line 34, before conducting the mixture thus obtained to the first distillation column 16.

[0122] Reference numerals list

[0123] 10. Equipment

[0124] 12 Reactor

[0125] 14 Separation Container

[0126] 16. First Distillation Column

[0127] 18. Second distillation column

[0128] 20 Third distillation tower

[0129] 22 Electrolyzer

[0130] 24Inlet pipelines for natural gas

[0131] 26 Recycle line for a composition containing bromine, hydrogen bromide and water

[0132] 28 Recirculation line for the liquid portion of the effluent from the reaction mixture

[0133] 30 outlet pipeline for the outflow of the reaction mixture

[0134] 32 Outlet line for the gaseous portion of the effluent from the reaction mixture

[0135] 34 Outlet line for the liquid portion of the reaction mixture effluent

[0136] 36 throttle valve

[0137] 38 Recirculation line for composition enriched in sulfuric acid

[0138] 40Top outlet pipeline of the first distillation tower

[0139] 42 Bottom outlet pipeline of the first distillation tower

[0140] 44 The top outlet pipeline of the second distillation tower

[0141] 46 Bottom outlet pipeline of the second distillation tower

[0142] 48 The top outlet pipeline of the third distillation tower

[0143] 50 The bottom outlet pipeline of the third distillation tower

[0144] 52 Export pipeline for sulfuric acid product

[0145] 54 Container

[0146] 56 Liquid outlet pipeline

[0147] 58 Export pipeline / pipeline leading to the electrolyzer

[0148] 60 Export pipeline for bromine-containing composition

[0149] 62 Export line for hydrogen

[0150] 64 Scrubber

[0151] 66 Inlet line for water

[0152] 68 Recirculation line for water

[0153] 70 Containers

[0154] 71 Gas pipeline

[0155] 72 Liquid recirculation line

[0156] 74 Compressor

[0157] 76 Heat Exchanger

[0158] 78 Container

[0159] 80 Gas outlet pipeline

[0160] 81 Export pipeline for gas composition

[0161] 82 Outlet line for aqueous composition

[0162] 84 Container

[0163] 86 Gas export pipeline

[0164] 88 Recirculation line

[0165] 90 Condenser

[0166] 92 Bypass line

[0167] 94 Condenser

[0168] 96 Liquid pipeline

[0169] 98 Liquid pipeline

Claims

1. A method for preparing hydrogen, comprising the following steps: a) providing a starting mixture containing bromine, water and a sulfur-containing compound, b) reacting the starting mixture provided in step a) to produce a reaction mixture effluent comprising sulfuric acid and hydrogen bromide, c) separating the reaction mixture effluent obtained in step b) into one or more compositions enriched in hydrogen bromide and one or more compositions enriched in sulfuric acid, wherein at least one composition enriched in hydrogen bromide contains at most 1,000 ppm of sulfuric acid, wherein step c) comprises at least two distillation steps, d) subjecting at least a portion of the at least one composition enriched in hydrogen bromide containing up to 1,000 ppm of sulfuric acid obtained in step c) to electrolysis to obtain hydrogen and a bromine-containing composition, wherein the electrolysis cell is operated at an operating temperature of at least 70° C., and e) recycling at least a portion of the bromine-containing composition obtained in step d) back to step a).

2. according to the method for claim 1, wherein the sulfur-containing compound is selected from the group consisting of sulfur, sulfide, disulfide, thiophene, mercaptan, sulfur dioxide, carbon disulfide, carbonyl sulfide and any combination of two or more of the foregoing compounds, and more preferably selected from the group consisting of hydrogen sulfide, sulfur, sulfur dioxide, carbon disulfide, carbonyl sulfide, methyl mercaptan and any combination of two or more of the foregoing compounds.

3. The process according to claim 1 or 2, wherein the starting mixture provided in step a) further contains hydrogen bromide, and wherein the process further comprises as step f) recycling at least a portion of the hydrogen bromide contained in one of the one or more compositions enriched in hydrogen bromide produced in step c) to step a), wherein the recycled hydrogen bromide is not subjected to electrolysis.

4. The process according to claim 3, wherein if the starting mixture is a gas-liquid mixture at the reaction temperature and the reaction pressure, the starting mixture comprises at least 5% by weight and preferably 11 to 46% by weight of hydrogen bromide, at least 1% by weight and preferably 2 to 35% by weight of bromine and 40 to 75% by weight of water, based on the total weight of water, bromine and hydrogen bromide in the liquid phase of the starting mixture. And wherein if the starting mixture is a liquid at the reaction temperature and the reaction pressure, the starting mixture comprises at least 5% by weight and preferably 11 to 46% by weight of hydrogen bromide, at least 1% by weight and preferably 2 to 35% by weight of bromine and 30 to 75% by weight of water, based on the total weight of the starting mixture.

5. The process according to any one of the preceding claims, wherein the reaction in step b) is carried out at a temperature of at least 55° C. and preferably at a temperature of 60 to 200° C., and wherein the reaction in step b) is carried out at a pressure of at least 5 barg and preferably at a pressure of 6 to 15 barg.

6. A process according to any one of the preceding claims, wherein the reaction mixture effluent is a gas-liquid mixture and in step c) is fed into a vessel and allowed to phase separate therein, or is subjected to at least one separation step selected from a stripping step, an absorption step, an extraction step and any combination of two or more of the preceding separation steps, to separate the reaction mixture effluent into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent, wherein the liquid portion of the reaction mixture effluent comprises at least 20% by weight and preferably from 25 to 50% by weight of hydrogen bromide relative to the water content of the liquid portion of the reaction mixture effluent, and wherein the liquid portion of the reaction mixture effluent comprises at least 0.1% by weight and preferably from 0.3 to 15% by weight of sulfuric acid, 20 to 70% by weight and preferably from 25 to 65% by weight of water and at most 2% by weight and preferably at most 0.5% by weight of bromine, based on the total weight of the liquid portion of the reaction mixture effluent.

7. The process according to any one of the preceding claims, wherein step c) comprises at least two distillation steps and preferably three distillation steps, wherein the reaction mixture effluent in step c) is fed into a vessel and allowed to phase separate therein or is subjected to at least one separation step selected from a stripping step, an absorption step, an extraction step and any combination of two or more of the preceding separation steps to separate the reaction mixture effluent into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent, and then the liquid portion of the reaction mixture effluent or a mixture of the liquid portion of the reaction mixture effluent with the composition enriched in sulfuric acid obtained in step c) is subjected as feed to a first distillation step, wherein the feed is separated, preferably in the first distillation step, into a gaseous overhead composition of a composition enriched in hydrogen bromide containing up to 1,000 ppm of sulfuric acid and a liquid bottom composition of a composition enriched in sulfuric acid containing more than 30% by weight of sulfuric acid and less than 1,000 ppm of hydrogen bromide.

8. The process according to claim 7, wherein at least a portion of the gaseous composition enriched in hydrogen bromide obtained as an overhead composition in the first distillation step is condensed and subjected to electrolysis in step d), or wherein at least a portion of the composition enriched in hydrogen bromide obtained as an overhead composition in the first distillation step is subjected to a water removal step to separate the composition enriched in hydrogen bromide into a composition enriched in water and a gaseous composition enriched in hydrogen bromide, depleted in water, comprising 90 to 99% by weight of hydrogen bromide and less than 10% by weight and preferably less than 1% by weight of water, wherein at least a portion of the gaseous composition enriched in hydrogen bromide, depleted in water is subjected to electrolysis in step d).

9. The process according to claim 7 , wherein the composition enriched in sulfuric acid obtained as the liquid bottoms composition of the first distillation step is subjected to a second distillation step to produce an overhead composition and a bottoms composition containing 60 to 95% by weight and preferably 70 to 80% by weight of sulfuric acid and 5 to 40% by weight and preferably 20 to 30% by weight of water, based on the total weight of the bottoms composition, wherein preferably at least a portion of the bottoms composition of the second distillation step is subjected to a third distillation step to produce an overhead composition and a bottoms composition containing at least 90% by weight and preferably at least 94% by weight of sulfuric acid and less than 10% by weight and preferably less than 6% by weight of water, based on the total weight of the bottoms composition.

10. The process according to claim 9, wherein a portion of the bottom composition obtained in the second distillation step and / or a portion of the bottom composition obtained in the third distillation step is recycled to the reaction mixture effluent or the liquid part of the reaction mixture effluent and mixed therewith as a composition enriched in sulfuric acid before subjecting the thus obtained mixture as feed to the first distillation step.

11. A process according to claim 9 or 10, wherein the gaseous part of the reaction mixture effluent or a mixture of the gaseous part of the reaction mixture effluent and the overhead composition of the second distillation step is subjected to a scrubbing step with water to obtain a gaseous composition containing less than 10 ppm of bromine and hydrogen bromide in total and an aqueous composition containing from 5 to 40% by weight of hydrogen bromide, wherein the aqueous composition is recycled to step a).

12. A method according to any one of the preceding claims, comprising: i) feeding the reaction mixture effluent from step c) into a container and allowing it to phase separate therein, to separate the reaction mixture effluent into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent, ii) subjecting a mixture of the liquid portion of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) as feed to a first distillation step to separate the feed into an overhead composition of a composition enriched in hydrogen bromide containing up to 1,000 ppm of sulfuric acid and a bottom composition of a composition enriched in sulfuric acid containing more than 30% by weight of sulfuric acid and less than 1,000 ppm of hydrogen bromide, wherein the feed preferably contains at least 53% by weight of relative sulfuric acid relative to the water concentration, iii) subjecting at least a portion of the composition enriched in hydrogen bromide obtained as the overhead composition in the first distillation step to a water removal step to separate the composition enriched in hydrogen bromide into a composition enriched in water and a gas composition enriched in hydrogen bromide, depleted in water, comprising 90 to 99% by weight of hydrogen bromide and less than 10% by weight and preferably less than 1% by weight of water, and subjecting at least a portion of the gas composition enriched in hydrogen bromide, depleted in water to electrolysis in step d), iv) subjecting the composition enriched in sulfuric acid obtained as the bottom composition of the first distillation step to a second distillation step to produce an overhead composition and a bottom composition containing 60 to 95% by weight and preferably 70 to 80% by weight of sulfuric acid and 5 to 40% by weight and preferably 20 to 30% by weight of water, based on the total weight of the bottom composition, wherein preferably at least a portion of the bottom composition of the second distillation step is subjected to a third distillation step to produce an overhead composition and a bottom composition containing at least 90% by weight and preferably at least 94% by weight of sulfuric acid and less than 10% by weight and preferably less than 6% by weight of water, based on the total weight of the bottom composition, v) recycling a portion of the bottom composition obtained in the second distillation step and / or a portion of the bottom composition obtained in the third distillation step as a composition enriched in sulphuric acid to and mixing it with the liquid part of the reaction mixture effluent to produce a mixture of the liquid part of the reaction mixture effluent and the composition enriched in sulphuric acid used in step ii), vi) recycling at least a portion of the bromine-containing composition obtained in the electrolysis to step a), and vii) optionally, recycling the gaseous portion of the reaction mixture effluent or the mixture of the gaseous portion of the reaction mixture effluent and the overhead composition of the second distillation step to step a) or subjecting the gaseous portion of the reaction mixture effluent or the mixture of the gaseous portion of the reaction mixture effluent and the overhead composition of the second distillation step to a scrubbing step with water to obtain a gaseous composition containing less than 10 ppm of bromine and hydrogen bromide in total and a liquid aqueous composition containing from 5 to 40% by weight of hydrogen bromide, wherein the liquid aqueous composition is recycled to step a).

13. A method according to any one of claims 1 to 12, comprising: i) feeding the reaction mixture effluent in step c) into a container and allowing it to phase separate therein to separate the reaction mixture effluent into a gaseous portion of the reaction mixture effluent and a liquid portion of the reaction mixture effluent, ii) subjecting the liquid portion of the reaction mixture effluent or a mixture of the liquid portion of the reaction mixture effluent and the composition enriched in sulfuric acid obtained in step c) as feed to a first distillation step to separate the feed into an overhead composition of a composition enriched in hydrogen bromide containing at most 1,000 ppm of sulfuric acid and a bottom composition of a composition enriched in sulfuric acid containing more than 30% by weight of sulfuric acid and less than 1,000 ppm of hydrogen bromide, iii) condensing at least a portion of the composition enriched in hydrogen bromide obtained as an overhead composition in the first distillation step and subjecting at least a portion of the condensed portion to electrolysis in step d), iv) subjecting the composition enriched in sulfuric acid obtained as the bottom composition of the first distillation step to a second distillation step to produce an overhead composition and a bottom composition containing 60 to 95% by weight and preferably 70 to 80% by weight of sulfuric acid and 5 to 40% by weight and preferably 20 to 30% by weight of water, based on the total weight of the bottom composition, wherein preferably at least a portion of the bottom composition of the second distillation step is subjected to a third distillation step to produce an overhead composition and a bottom composition containing at least 90% by weight and preferably at least 94% by weight of sulfuric acid and less than 10% by weight and preferably less than 6% by weight of water, based on the total weight of the bottom composition, v) optionally, recycling a portion of the bottom composition obtained in the second distillation step and / or a portion of the bottom composition obtained in the third distillation step as a composition enriched in sulphuric acid to the liquid part of the reaction mixture effluent and mixing it therewith to produce a mixture of the liquid part of the reaction mixture effluent and the composition enriched in sulphuric acid used in step ii), vi) recycling at least a portion of the bromine-containing composition obtained in the electrolysis to step a), vii) optionally, recycling the gaseous portion of the reaction mixture effluent to step a) or subjecting the gaseous portion of the reaction mixture effluent to a scrubbing step with water to obtain a gaseous composition containing less than 10 ppm of bromine and hydrogen bromide in total and a liquid aqueous composition containing from 5 to 40% by weight of hydrogen bromide, wherein the liquid aqueous composition is recycled to step a), and viii) Optionally, a portion of the condensed fraction of the composition enriched in hydrogen bromide obtained in step iii) is recycled to step a) without subjecting it to the electrolysis in step d).

14. The process according to claim 1 , wherein in step d) the electrolysis is carried out by using an electrolysis cell comprising an anode, a cathode and a membrane sandwiched between the anode and the cathode, wherein the composition enriched in hydrogen bromide containing up to 1,000 ppm of sulfuric acid obtained in step c) is fed to the anode, optionally a second composition comprising hydrogen bromide and water is fed to the cathode, and the electrolysis cell is operated to produce hydrogen at the cathode, wherein a bromine-containing composition is produced at the anode, wherein the membrane is made of a fluoropolymer having a glass transition temperature of at least 110° C., and wherein the electrolysis cell is operated at an operating temperature of 70° C. to 122° C.

15. An apparatus for producing hydrogen according to the method of any one of the preceding claims, comprising: a) a reactor comprising one or more inlet lines for bromine, water and sulfur-containing compounds and an outlet line for the effluent of a reaction mixture comprising sulfuric acid and hydrogen bromide, b) a vessel comprising an inlet line connected to an outlet line for an effluent of the reaction mixture, wherein the vessel further comprises an outlet line for a gaseous portion of the effluent of the reaction mixture and an outlet line for a liquid portion of the effluent of the reaction mixture, wherein the outlet line for the gaseous portion of the effluent of the reaction mixture is directly or indirectly connected to the inlet line of the reactor, c) a first distillation column comprising an inlet line connected to an outlet line for a liquid portion of the effluent of the reaction mixture, wherein the first distillation column comprises an overhead outlet line for a gaseous composition enriched in hydrogen bromide and a bottom outlet line for a composition enriched in sulfuric acid, d) an electrolytic cell comprising an anode, a cathode and a membrane sandwiched between the anode and the cathode, and an inlet line for a composition containing hydrogen bromide containing up to 1,000 ppm of sulfuric acid, an outlet line for hydrogen and an outlet line for a composition containing bromine, wherein the inlet line of the electrolytic cell is connected d1) directly to the outlet line of the first distillation column for a composition enriched in hydrogen bromide, or d2) is connected via a condenser arranged therebetween, the condenser comprising a condenser connected to the outlet line of the first distillation column for a composition enriched in hydrogen bromide. or d3) connected via a container arranged therebetween, the container comprising an inlet line connected to an outlet line at the top of the first distillation column for a gas composition enriched in hydrogen bromide, comprising a liquid outlet line and comprising a gas outlet line, wherein the gas outlet line is connected to an inlet line of the electrolyzer, and wherein the outlet line of the electrolyzer for the bromine-containing composition is directly or indirectly connected to the inlet line of the reactor, e) a second distillation column comprising an inlet line connected to the bottom outlet line of the first distillation column for a composition enriched in sulfuric acid, wherein the second distillation column comprises a top outlet line for a composition enriched in hydrogen bromide and a bottom outlet line for a composition enriched in sulfuric acid, and f) optionally, a third distillation column comprising an inlet line connected to the bottom outlet line of the second distillation column for a composition enriched in sulfuric acid, wherein the third distillation column comprises an overhead outlet line for a composition enriched in water and a bottom outlet line for a sulfuric acid product composition.

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