WO2022073976A1 - Membrane process for the production of hydrogen and oxygen by hydrolysis of water and related apparatus - Google Patents

Membrane process for the production of hydrogen and oxygen by hydrolysis of water and related apparatus Download PDF

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Publication number
WO2022073976A1
WO2022073976A1 PCT/EP2021/077393 EP2021077393W WO2022073976A1 WO 2022073976 A1 WO2022073976 A1 WO 2022073976A1 EP 2021077393 W EP2021077393 W EP 2021077393W WO 2022073976 A1 WO2022073976 A1 WO 2022073976A1
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hydrogen
oxygen
membrane
water
selectively permeable
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PCT/EP2021/077393
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French (fr)
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Silvano Tosti
Alfonso Pozio
Luca FARINA
Alessia Santucci
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Enea - Agenzia Nazionale Per Le Nuove Tecnologie, L’Energia E Lo Sviluppo Economico Sostenibile
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Priority to EP21783021.5A priority Critical patent/EP4225695A1/en
Publication of WO2022073976A1 publication Critical patent/WO2022073976A1/en

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/02—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen
    • C01B3/04—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by decomposition of inorganic compounds, e.g. ammonia
    • C01B3/042—Decomposition of water
    • C01B3/045—Decomposition of water in gaseous phase
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J12/00—Chemical processes in general for reacting gaseous media with gaseous media; Apparatus specially adapted therefor
    • B01J12/005—Chemical processes in general for reacting gaseous media with gaseous media; Apparatus specially adapted therefor carried out at high temperatures, e.g. by pyrolysis
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
    • B01J19/08—Processes employing the direct application of electric or wave energy, or particle radiation; Apparatus therefor
    • B01J19/12—Processes employing the direct application of electric or wave energy, or particle radiation; Apparatus therefor employing electromagnetic waves
    • B01J19/122—Incoherent waves
    • B01J19/127—Sunlight; Visible light
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
    • B01J19/18—Stationary reactors having moving elements inside
    • B01J19/1893—Membrane reactors
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J3/00—Processes of utilising sub-atmospheric or super-atmospheric pressure to effect chemical or physical change of matter; Apparatus therefor
    • B01J3/006—Processes utilising sub-atmospheric pressure; Apparatus therefor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J4/00—Feed or outlet devices; Feed or outlet control devices
    • B01J4/04—Feed or outlet devices; Feed or outlet control devices using osmotic pressure using membranes, porous plates
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J8/00—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes
    • B01J8/008—Details of the reactor or of the particulate material; Processes to increase or to retard the rate of reaction
    • B01J8/009—Membranes, e.g. feeding or removing reactants or products to or from the catalyst bed through a membrane
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B13/00—Oxygen; Ozone; Oxides or hydroxides in general
    • C01B13/02—Preparation of oxygen
    • C01B13/0203—Preparation of oxygen from inorganic compounds
    • C01B13/0207—Water
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B13/00—Oxygen; Ozone; Oxides or hydroxides in general
    • C01B13/02—Preparation of oxygen
    • C01B13/0229—Purification or separation processes
    • C01B13/0248—Physical processing only
    • C01B13/0251—Physical processing only by making use of membranes
    • C01B13/0255—Physical processing only by making use of membranes characterised by the type of membrane
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/50—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification
    • C01B3/501—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification by diffusion
    • C01B3/503—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification by diffusion characterised by the membrane
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/50—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification
    • C01B3/501—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification by diffusion
    • C01B3/503—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification by diffusion characterised by the membrane
    • C01B3/505—Membranes containing palladium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2208/00—Processes carried out in the presence of solid particles; Reactors therefor
    • B01J2208/00008—Controlling the process
    • B01J2208/00017—Controlling the temperature
    • B01J2208/00433—Controlling the temperature using electromagnetic heating
    • B01J2208/00451—Sunlight; Visible light
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
    • Y02P20/10—Process efficiency
    • Y02P20/133—Renewable energy sources, e.g. sunlight

Definitions

  • the present invention relates to the field of chemistry and more specifically to a process and related plant for the production of hydrogen and oxygen by thermal decomposition reaction of water at high temperature, preferably using solar energy.
  • Electrolysis is an industrial process that uses electrical energy to produce hydrogen from water through reaction (1) with efficiencies ranging from 50% for conventional alkaline cells to 80% for high temperature cells.
  • a number of thermochemical processes have been studied, for example the sulphur- iodine process, which achieve the thermal decomposition of water without using electricity.
  • the reaction (1) does not take place directly, which only occurs at very high temperatures, but a series of reactions involving other compounds are carried out (in the case of the sulphur-iodine cycle: HI, H 2 SO 4 , SO 2 ) and proceeding at lower temperatures (400-800 °C) so that heat from solar plants can be used as a heat source.
  • Italian Patent No. 102018000003185 describes an application using a membrane consisting of liquid Pd on a porous ceramic support that can selectively separate the hydrogen produced.
  • Chinese patent no. CN105417494 describes a K 2 NiF 4 membrane for separating oxygen from water. Oxygen is then reacted with methane to produce syngas (hydrogen and CO). This is a membrane reactor that uses oxygen separated from water to carry out the methane reforming reaction.
  • Chinese patent no. CN110844905 describes a reactor with a membrane that separates oxygen deriving from decomposing water and reacts it with syngas (H 2 and CO) to produce pure CO 2 .
  • Chinese patent CN109836153 describes ceramic materials that are selectively permeable to oxygen.
  • European patent no. EP1775014 and US patent no. US7815890 describe the use of metallic membranes selectively permeable to hydrogen and isotopes and used for the extraction of tritium from tritiated water.
  • Chinese Patent nos. CN105692548 and CN105692549 describe a process for the production of ammonia and a device for purifying a stream of hydrogen using water steam.
  • Chinese Patent no. CN108117044 A describes materials used to make membrane selectively permeable to oxygen based on Ce.
  • Chinese patent no. CN109734438 describes materials (perovskite) for making membranes selectively permeable to oxygen.
  • Chinese Patent No. CN108117388 describes a titanium-based material for making membranes selectively permeable to oxygen.
  • US Patent No. US7087211 describes a device using two membranes for separating hydrogen and oxygen, in which oxygen separated from steam water reacts with methane to produce syngas (H 2 + CO).
  • the syngas formation reaction is carried out in a reaction chamber, i.e. a membrane reactor, which houses two membranes and the methane feed.
  • US Patent Application No. US 2004/098914 describes a device for producing hydrogen and oxygen that uses membranes to separate hydrogen and oxygen that is not coupled to solar radiation. Furthermore, the permeation mechanism indicated is not of the pressure-driven type.
  • the present invention makes it possible to solve several technical problems linked to the thermal decomposition of water.
  • the addition of a membrane for the simultaneous separation of also the oxygen allows to increase the reaction conversion compared to the use of the membrane for hydrogen alone, making the thermal decomposition reaction of water thermodynamically favourable.
  • a further object of the present invention is to enable the process and the related device to be fed by solar energy, allowing operation at certain temperatures, to achieve desired yields and make the process technologically viable.
  • the risk of explosion is reduced because the concentration of oxygen in the retentate stream, which contains unreacted water, oxygen and unpermeated hydrogen, is significantly lowered.
  • the object of the present invention differs from that described in US7087211 in that it operates at much higher temperatures of interest, suitable for the applications of thermal decomposition of water, and uses membranes for the separation of hydrogen that can also operate at the temperatures of interest.
  • the object of the present invention differs from what is described in US 2004/098914 in that it is capable of using solar energy as an energy source for water splitting, since the proposed device enables higher conversion yields to be obtained than using a conventional reactor and thus enables operation at temperatures compatible with solar devices.
  • the gas separation mechanism (hydrogen and oxygen) used is based on the difference in partial pressures of the permeating gases, hydrogen and oxygen, respectively, for the two types of membrane considered, and specifically, it is a transport of pressure-driven permeation matter.
  • the membrane reactor simultaneously carries out a reaction and the separation of one or more reaction products.
  • a membrane reactor in accordance with Le Chatelier's principle of mobile equilibrium, the system in the presence of product removal causes a greater quantity of reactants to react, thereby increasing their conversion. This effect, known as the "shift effect", results in the increased reaction conversion that a membrane reactor can boast compared to a conventional reactor.
  • a membrane reactor is characterised by higher reaction conversions than the same process carried out in a conventional reactor.
  • the conventional reactor can in turn be followed by devices for separating the reaction products, but which cannot increase the reaction conversion obtained in the upstream conventional reactor.
  • the object of the present invention differs from US 2015/251905 and US 2007/269687 because the separation of hydrogen and oxygen produced by the water splitting reaction takes place simultaneously and at the same temperature as the water splitting reaction in a single device that operates as a "membrane reactor" consisting of two membranes. Due to the continuous removal of both reaction products (H2 and 02), it is possible to achieve reaction conversions that are higher than those achievable by a membrane reactor with only one membrane. Therefore, the proposed device can operate at temperatures approaching those compatible with those of a concentrating solar system. Consequently, the H2 and 02 yield obtained from the process of thermal decomposition of water is higher for all temperatures of practical interest, and this is due to the combined effect of the two membranes operating in the single reaction stage.
  • the object of the present invention makes it possible to obtain conversion values of the water decomposition reaction that are higher than those obtainable in a conventional reactor and in any case higher than those obtainable in a membrane reactor that uses a single membrane for hydrogen separation. Furthermore, the object of the present invention makes it possible to use solar energy directly as an energy source, thus reducing the environmental impact.
  • a device 1 for carrying out the reaction of thermal decomposition of water to hydrogen and oxygen comprising a main vessel 2 substantially impermeable to gases that it houses in its inside a membrane selectively permeable to oxygen 3 and a membrane selectively permeable to hydrogen 4 at least one inlet for water 5 at least one outlet for the so-called retentate stream 6, i.e.
  • a further object of the present invention is a reaction of thermal decomposition of water to hydrogen and oxygen which takes place in device 1 under conditions of temperature ranging between 1000 and 3000 °C, pressure ranging between 0.5 and 10 bar.
  • Figure 1 shows a schematic representation of a membrane reactor for water decomposition.
  • Figure 2 shows a further schematic representation of a membrane reactor for water decomposition.
  • Figure 3 graphically shows the flow rate in Nm 3 /h of hydrogen and oxygen extracted from the Ta (capable of selectively separating hydrogen) and hafnia (capable of selectively separating oxygen) membranes respectively as a function of the process temperature (feed water flow rate 1000 kg/h).
  • Figure 4 graphically shows the surface area of the Ta and hafnia membranes as a function of the process temperature (feed water flow rate 1000 kg/h).
  • water molecule splitting means the process for producing hydrogen and oxygen represented by the reaction:
  • ceramic material means a non-metallic inorganic material, consisting of metallic and non-metallic elements linked together mainly by ionic and/or covalent bonds, of formula A m X n , wherein A is a metallic element and X is a non-metallic element m and n are integers, in particular in the case in which X is oxygen and A is a monovalent, bivalent, trivalent, quadrivalent, pentavalent metal.
  • a composite material means a heterogeneous material, i.e., consisting of two or more phases with different physical properties.
  • an alloy means a combination in solution or in mixture of two or more elements of which at least one is a metal, and the resulting material of which has metallic properties different from those of its components.
  • the present invention describes a device 1 for carrying out the reaction of thermal decomposition of water to hydrogen and oxygen
  • a main vessel 2 substantially impermeable to gases that it houses in its inside a membrane selectively permeable to oxygen 3 and a membrane selectively permeable to hydrogen 4 at least one inlet for water 5 at least one outlet for the retentate stream 6 the retentate stream consists of the reaction products hydrogen and oxygen that have not been separated from the membranes and the water that has not reacted at least one outlet for hydrogen separated from the membrane 6' at least one outlet for oxygen separated from the membrane 6'' wherein the main vessel 2 is made of a material that can be exposed to solar radiation and simultaneously capable of operating at a temperature of at least 1300°C.
  • Figure 2 schematically illustrates an embodiment of the device 1 in which the main container substantially impermeable to gases 2 houses a membrane selectively permeable to oxygen 3 having the shape of a chamber with an inlet for water on one side 5 and an outlet for the retentate stream 6 (consisting of the reaction products hydrogen and oxygen not separated from the membranes and the non-reacted water) being the outlet 6'' for the oxygen produced by the reaction and permeated through the membrane 3 which in turn houses a membrane selectively permeable to hydrogen 4 having the shape of at least one tube with one end closed and the other end open being the outlet 6' for the hydrogen produced by the reaction and permeated through the membrane 4.
  • a membrane selectively permeable to oxygen 3 having the shape of a chamber with an inlet for water on one side 5 and an outlet for the retentate stream 6 (consisting of the reaction products hydrogen and oxygen not separated from the membranes and the non-reacted water) being the outlet 6'' for the oxygen produced by the reaction and permeated through the membrane 3
  • the material which can be exposed to solar radiation and at the same time is capable of operating at a temperature of at least 1300°C operates at a temperature ranging between 1300 and 3000°C, more preferably 1500 and 2500°C, even more preferably equal to
  • the material which can be exposed to solar radiation and at the same time is capable of operating at a temperature of at least 1300°C is of at least one metal, optionally in the form of composite material or alloy, selected from the group consisting of tungsten, vanadium, niobium, molybdenum, zirconium, chromium; more preferably it is tungsten.
  • the external container which is hit by the solar radiation, even concentrated, provides the heat necessary for water decomposition.
  • the hydrogen permeable membrane is made of a metal selected from the group consisting of: tantalum, niobium, molybdenum, vanadium, tungsten and alloys thereof and/or their alloys with titanium, zirconium, aluminium, palladium.
  • the membrane selectively permeable to hydrogen is tantalum.
  • the membrane selectively permeable to hydrogen having the shape of at least one metal tube is of the self-supported type, or it is of the composite type, i.e. consisting of a thin metal layer deposited on a porous support.
  • the membranes of the self-supported type consist of tubes with thicknesses ranging from a few tens of pm up to tenths of a millimetre, depending on the operating pressure values.
  • Composite membranes are typically made up of metallic layers with thicknesses of the order of pm or a fraction of pm depending on the characteristics of the porous support to which they are anchored.
  • the porous support can be made of metallic material or ceramic material.
  • the membrane selectively permeable to oxygen is made of ceramic material.
  • the ceramic material is a metal oxide.
  • the metal oxide is selected from the group consisting of: hafnia, alumina, zirconia, thoria, lithium oxide and mixtures thereof.
  • the device may further comprise a vacuum pumping system or a washing system with inert gas for extracting the hydrogen separated from the membrane selectively permeable to hydrogen and a vacuum pumping system or a washing system with inert gas for extracting the oxygen separated from the membrane selectively permeable to oxygen.
  • the device can further comprise a system for heating and steaming the incoming water.
  • the device may further comprise a system for recovering heat from the hydrogen and oxygen streams separated from the outgoing membranes (and from the retentate stream) possibly connected to a system for heating and steaming the incoming water by means of recirculation and/or heat exchange and recovery means.
  • a further object of the present invention is a reaction of thermal decomposition of water to hydrogen and oxygen which takes place in the device 1 under conditions of temperature ranging between 1300 and 3000 °C, more preferably 1500 and 2500 °C, even more preferably 1900 °C, and a pressure ranging between 0.5 and 10 bar, preferably between 0.5 and 1 bar, more preferably 1 bar.
  • the oxygen permeable chamber is hafnia (HfO 2 )
  • the hydrogen permeable tube is tantalum (Ta)
  • the external container is made of tungsten.
  • the operating temperature is from 1900 to 2500 °C and the reaction pressure is 1 bar.
  • the tube is of a composite type, with a 10 pm thick tantalum layer, the hafnia chamber has a permeation thickness of 0.1 mm, the feed water flow rate 1000 kg/h, the hydrogen and oxygen permeate streams are extracted with vacuum pumps at a pressure of 100 Pa.
  • Kp with reference to reaction (1) is in turn given by: where the partial pressures of the reaction products (numerator) and the reactant (denominator) are indicated.
  • the equilibrium constant can also be expressed in terms of molar fractions:
  • Table 1 below shows the Kx values for reaction (1) at the pressures of 0.5, 1 and 10 bar in the temperature range 1000-3000 °C. These values were calculated using the Asther software, taking into account the formation of only H2 and 02 as reaction products in the reaction (1).
  • Reaction (1) is a reaction that proceeds with an increase in the number of moles and is therefore favoured by a reduction in pressure. This behaviour is evident from the results shown in Table 1: the values of the equilibrium constant at 1 bar are greater than those of the constant at 10 bar by a factor equal to 10 A 0.5 and those at 0.5 bar are greater than those at 1 bar by a factor equal to in accordance with the relation (5).
  • the conversion of water i.e. fraction of fed H 2 O reacting to produce hydrogen and oxygen
  • Table 2 shows the mole fractions of the reactant and of the reaction products in a conventional reactor through the scheme in Table 2 below.
  • Expression (9) summarises the previous cases and, in particular, coincides with (6) in the case of g and ⁇ equal to zero (conventional reactor), with expression (7) in the case of (membrane reactor separating oxygen) and finally with (8) in the case of ⁇ (membrane reactor separating hydrogen).
  • Tables 6 and 7 show the calculated values for the conversion of water a for the conventional reactor (TR) and for the different membrane reactor configurations (MR) in the case of separation of only hydrogen, only oxygen and hence hydrogen and oxygen simultaneously.
  • Tables 6 and 7 refer to the cases of membrane efficiency (both e and q) of 0.8 and 0.9 and reaction pressure 1 bar, respectively.
  • the membrane reactor with hydrogen separation enables the conversion to be increased by a factor of 3, the one with oxygen separation by a factor of 2, and the membrane reactor that simultaneously separates hydrogen and oxygen enables the conversion to be increased by a factor of 4.5-5.
  • the membrane for separating hydrogen is more effective than that for separating oxygen to increase conversion according to the stoichiometry of reaction (1) which from one mole of water leads to the formation of one mole of hydrogen and half a mole of oxygen.
  • the materials considered are: hafnia (HfO2) for the construction of the oxygen permeable chamber (melting temperature 2900 °C), tantalum (Ta) for the construction of the hydrogen permeable tube (melting temperature 3017 °C), tungsten for the construction of the external container exposed to solar radiation (melting temperature 3422 °C).
  • the input data for the solar membrane reactor are: feed water flow rate 1000 kg/h, operating temperature from 1900 to 2500 °C, use of permeator tubes of the composite type with a 10 pm thick Ta layer, hafnia chamber with an effective permeation thickness of 0.1 mm, reaction pressure 1 bar, hydrogen and oxygen permeate streams extracted with vacuum pumps at a pressure of 100 Pa.
  • the reaction (1) proceeds with an increase in the number of moles, so it is favoured by low pressures in a conventional reactor.
  • the quantity of reaction products (hydrogen and/or oxygen) removed from the membrane increases with pressure, so that the behaviour of the reaction conversion as the pressure changes is a combination of these two effects: as the pressure increases, the conversion decreases due to the thermodynamic effect, while it tends to increase due to the permeation effect.
  • the process pressure of 1 bar allows for high yields while adequate permeation flows can be obtained by adopting a permeate side pressure of 100 Pa by vacuum pumping. While the reduction in operating pressure leads to an increase in the reaction conversion, it also leads to a reduction in the driving force of the permeation and thus to an increase in the required membrane surface.
  • the flow rate of hydrogen and oxygen recovered through the Ta and hafnia membrane varies from 120 to 540 Nm3/h for H2 and from 60 to 270 Nm3/h for 02, respectively.
  • the necessary Ta membrane surface passes from 35 to 60 m2 (it increases since the permeability of Ta in accordance with (10) reduces with temperature) while the hafnia surface is reduced from 300 to 35 m2 (in fact the permeability of hafnia in accordance with (11) increases with temperature). Still in the same temperature range and by not carrying out thermal recoveries on the unreacted water, the necessary thermal power passes from about 2 to 4 MW.

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  • Chemical Kinetics & Catalysis (AREA)
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Abstract

A device and the relative process for producing hydrogen and oxygen by means of a thermal decomposition reaction of water is described, which allows solar energy to be used as an energy source thanks to the reduction in the operating temperature achieved by the advantages introduced by the simultaneous use of a membrane to selectively separate the hydrogen and a membrane to selectively separate the oxygen produced.

Description

MEMBRANE PROCESS FOR THE PRODUCTION OF HYDROGEN AND
OXYGEN BY HYDROLYSIS OF WATER AND RELATED APPARATUS
Background of the invention
The present invention relates to the field of chemistry and more specifically to a process and related plant for the production of hydrogen and oxygen by thermal decomposition reaction of water at high temperature, preferably using solar energy.
Background
The production of hydrogen by water molecule splitting is represented by the reaction:
Figure imgf000003_0001
This reaction at room temperature and pressure is thermodynamically unfavourable. Only at very high temperatures it is possible to obtain the formation of hydrogen and oxygen and other compounds (O, H, OH) (A. Tsutsumi, ENERGY CARRIERS AND CONVERSION SYSTEMS - Vol. I Thermodynamics of water splitting, https://www.eolss.net/sample-chapters/CO8/E3-13-03-01.pdf (accessed 03.06.2020).
Electrolysis is an industrial process that uses electrical energy to produce hydrogen from water through reaction (1) with efficiencies ranging from 50% for conventional alkaline cells to 80% for high temperature cells. Apart from other minor processes, such as photocatalytic conversion, a number of thermochemical processes have been studied, for example the sulphur- iodine process, which achieve the thermal decomposition of water without using electricity. In these processes, the reaction (1) does not take place directly, which only occurs at very high temperatures, but a series of reactions involving other compounds are carried out (in the case of the sulphur-iodine cycle: HI, H2SO4, SO2) and proceeding at lower temperatures (400-800 °C) so that heat from solar plants can be used as a heat source.
The direct use of solar energy for the thermal decomposition of water is of little benefit because even at very high temperatures, e.g. 2000 °C, the quantities of hydrogen and oxygen produced by the reaction (1) are practically negligible. Temperatures of 3000 °C must be reached to obtain hydrogen and oxygen mole fractions of around 10-20 %, which is technologically impractical. CSP (concentrating solar power) technologies based on the use of mirrors and lenses to concentrate the sun's rays are estimated to be capable of reaching temperatures of 1000 °C in solar towers and in other advanced systems (Stirling dish) (Renewable Energy Cost Analysis Concentrating Solar Power, volume 1: Power Sector, Issue 2/5, June 2010, https://irena.org/-
/media/Files/IRENA/Agency/Publication/2012/RE Technologie s Cost Analysis-CSP.pdf (accessed 03.06.2020). The use of tower reflector systems has led to reaching temperatures of 1300 °C with heat flows of 10 MW/m2 (Yogev, A Kribus, M. Epstein, A. Kogan, Solar "Tower Reflector" systems: a new approach for high-temperature solar plants, Int. J. Hydrogen Energy vol. 23 no. 4 (1998) 239-245). Higher temperatures are achievable with systems using a ceramic plate device (1500 °C) and in future applications for which the use of liquid metals is envisaged (1700 °C with lead and 2600 °C with tin) (Bernhard Hoffschmidt,
Receivers for Solar Tower Systems, June 2014, https://sfera2.sollab.eu/uploads/images/network!ng/SFERAi 20SUMMER%20SCHCOL%202014%20- %20PRESENTATIONS/SolarTowerReceivers%20- %20Bernhard%2OHoffschmidt.pdf (accessed 03.06.2020)).
The use of a membrane selectively permeable to one of the reaction products continuously deprives this product of the reaction environment and thus, according to Le Chatellier's principle, the system reacts by reacting a greater quantity of reactant. Devices consisting of a reactor coupled to a selective membrane are called membrane reactors and, under the same operating conditions such as pressure and temperature, allow achieving higher reaction yields than a conventional reactor. In dehydrogenation reactions, the use of selectively hydrogen-permeable membranes has been investigated, for which there are also industrial applications.
In particular, the production of hydrogen by thermal decomposition of water is reported in the literature (R.B. Diver, E.A. Fletcher, Hydrogen and oxygen from water - II, Energy vol. 4 (1979) 1139-1150).
Italian Patent No. 102018000003185 describes an application using a membrane consisting of liquid Pd on a porous ceramic support that can selectively separate the hydrogen produced.
Chinese patent no. CN105417494 describes a K2NiF4 membrane for separating oxygen from water. Oxygen is then reacted with methane to produce syngas (hydrogen and CO). This is a membrane reactor that uses oxygen separated from water to carry out the methane reforming reaction.
Chinese patent no. CN110844905 describes a reactor with a membrane that separates oxygen deriving from decomposing water and reacts it with syngas (H2 and CO) to produce pure CO2 .
Chinese patent CN109836153 describes ceramic materials that are selectively permeable to oxygen.
European patent no. EP1775014 and US patent no. US7815890 describe the use of metallic membranes selectively permeable to hydrogen and isotopes and used for the extraction of tritium from tritiated water. Chinese Patent nos. CN105692548 and CN105692549 describe a process for the production of ammonia and a device for purifying a stream of hydrogen using water steam.
Chinese Patent no. CN108117044 A describes materials used to make membrane selectively permeable to oxygen based on Ce.
Chinese patent no. CN109734438 describes materials (perovskite) for making membranes selectively permeable to oxygen.
Chinese Patent No. CN108117389 describes an iron-based material for making membranes selectively permeable to oxygen.
Chinese Patent No. CN108117388 describes a titanium-based material for making membranes selectively permeable to oxygen.
US Patent No. US7087211 describes a device using two membranes for separating hydrogen and oxygen, in which oxygen separated from steam water reacts with methane to produce syngas (H2 + CO). The syngas formation reaction is carried out in a reaction chamber, i.e. a membrane reactor, which houses two membranes and the methane feed.
US Patent Application No. US 2004/098914 describes a device for producing hydrogen and oxygen that uses membranes to separate hydrogen and oxygen that is not coupled to solar radiation. Furthermore, the permeation mechanism indicated is not of the pressure-driven type.
US patent applications no. US 2015/251905 and US 2007/269687 describe devices and processes for the production of hydrogen and oxygen which provide for three stages in which the first stage at high temperature in which the main device behaves like a membrane reactor consisting of a single membrane for the separation of oxygen in which a water splitting reaction takes place and at the same time the separation of the oxygen produced, followed by a cooling stage and a further stage in which the process for separating the hydrogen coming from stage 1 takes place by using a membrane for the selective separation of hydrogen.
Technical problem
The present invention makes it possible to solve several technical problems linked to the thermal decomposition of water.
Most of the devices and processes known in the art are based on reactors for the decomposition of water which use a membrane for the separation of hydrogen.
In the present invention, the addition of a membrane for the simultaneous separation of also the oxygen allows to increase the reaction conversion compared to the use of the membrane for hydrogen alone, making the thermal decomposition reaction of water thermodynamically favourable.
A further object of the present invention is to enable the process and the related device to be fed by solar energy, allowing operation at certain temperatures, to achieve desired yields and make the process technologically viable.
The same inventors of the present invention, starting from what is described in Italian patent no. 102018000003185 concerning the use of a porous membrane used as a support for a liquid Pd film used to separate hydrogen, were designing a device for carrying out experimental tests at high temperature. While designing a containment system made of ceramic material, they surprisingly found that certain ceramic materials that were capable of operating at the temperatures of interest exhibited the ability of selectively separating oxygen.
This made it possible to develop an innovative membrane reactor in which a membrane for oxygen separation is used in addition to that for separating hydrogen. Certain materials are identified for this device which allow it to operate at the desired operating temperatures for extended periods.
Furthermore, in the device subject-matter of the present invention, the risk of explosion is reduced because the concentration of oxygen in the retentate stream, which contains unreacted water, oxygen and unpermeated hydrogen, is significantly lowered.
The object of the present invention differs from that described in US7087211 in that it operates at much higher temperatures of interest, suitable for the applications of thermal decomposition of water, and uses membranes for the separation of hydrogen that can also operate at the temperatures of interest.
The object of the present invention differs from what is described in US 2004/098914 in that it is capable of using solar energy as an energy source for water splitting, since the proposed device enables higher conversion yields to be obtained than using a conventional reactor and thus enables operation at temperatures compatible with solar devices. Furthermore, the gas separation mechanism (hydrogen and oxygen) used is based on the difference in partial pressures of the permeating gases, hydrogen and oxygen, respectively, for the two types of membrane considered, and specifically, it is a transport of pressure-driven permeation matter.
The membrane reactor simultaneously carries out a reaction and the separation of one or more reaction products. In a membrane reactor, in accordance with Le Chatelier's principle of mobile equilibrium, the system in the presence of product removal causes a greater quantity of reactants to react, thereby increasing their conversion. This effect, known as the "shift effect", results in the increased reaction conversion that a membrane reactor can boast compared to a conventional reactor. In other words, a membrane reactor is characterised by higher reaction conversions than the same process carried out in a conventional reactor. The conventional reactor can in turn be followed by devices for separating the reaction products, but which cannot increase the reaction conversion obtained in the upstream conventional reactor.
The object of the present invention differs from US 2015/251905 and US 2007/269687 because the separation of hydrogen and oxygen produced by the water splitting reaction takes place simultaneously and at the same temperature as the water splitting reaction in a single device that operates as a "membrane reactor" consisting of two membranes. Due to the continuous removal of both reaction products (H2 and 02), it is possible to achieve reaction conversions that are higher than those achievable by a membrane reactor with only one membrane. Therefore, the proposed device can operate at temperatures approaching those compatible with those of a concentrating solar system. Consequently, the H2 and 02 yield obtained from the process of thermal decomposition of water is higher for all temperatures of practical interest, and this is due to the combined effect of the two membranes operating in the single reaction stage.
In summary, the object of the present invention makes it possible to obtain conversion values of the water decomposition reaction that are higher than those obtainable in a conventional reactor and in any case higher than those obtainable in a membrane reactor that uses a single membrane for hydrogen separation. Furthermore, the object of the present invention makes it possible to use solar energy directly as an energy source, thus reducing the environmental impact.
Object of the invention With reference to the attachedclaims, the technical problem is therefore solved by providing a device 1 for carrying out the reaction of thermal decomposition of water to hydrogen and oxygen comprising a main vessel 2 substantially impermeable to gases that it houses in its inside a membrane selectively permeable to oxygen 3 and a membrane selectively permeable to hydrogen 4 at least one inlet for water 5 at least one outlet for the so-called retentate stream 6, i.e. the reaction products hydrogen and oxygen that have not been separated from the membranes and the water that has not reacted at least one outlet for hydrogen separated from the membrane 6' at least one outlet for oxygen separated from the membrane 6'' wherein the main reactor is made of a material that can be exposed to solar radiation and simultaneously capable of operating at a temperature of at least 1300°C.
A further object of the present invention is a reaction of thermal decomposition of water to hydrogen and oxygen which takes place in device 1 under conditions of temperature ranging between 1000 and 3000 °C, pressure ranging between 0.5 and 10 bar.
Further features of the present invention will be clear from the following detailed description with reference to the accompanying figure and to the experimental tests provided.
Brief description of the figures Figure 1 shows a schematic representation of a membrane reactor for water decomposition.
Figure 2 shows a further schematic representation of a membrane reactor for water decomposition.
Figure 3 graphically shows the flow rate in Nm3/h of hydrogen and oxygen extracted from the Ta (capable of selectively separating hydrogen) and hafnia (capable of selectively separating oxygen) membranes respectively as a function of the process temperature (feed water flow rate 1000 kg/h).
Figure 4 graphically shows the surface area of the Ta and hafnia membranes as a function of the process temperature (feed water flow rate 1000 kg/h).
Detailed description of the invention
Within the meaning of the present invention, water molecule splitting means the process for producing hydrogen and oxygen represented by the reaction:
H2O(g)<h>H2(g)+0.5O2(g) AG298K =228.6kJ/mol AH298K=241.8kJ/mol (1)
Within the meaning of the present invention, ceramic material means a non-metallic inorganic material, consisting of metallic and non-metallic elements linked together mainly by ionic and/or covalent bonds, of formula Am Xn, wherein A is a metallic element and X is a non-metallic element m and n are integers, in particular in the case in which X is oxygen and A is a monovalent, bivalent, trivalent, quadrivalent, pentavalent metal.
Within the meaning of the present invention, a composite material means a heterogeneous material, i.e., consisting of two or more phases with different physical properties.
Within the meaning of the present invention, an alloy means a combination in solution or in mixture of two or more elements of which at least one is a metal, and the resulting material of which has metallic properties different from those of its components.
The present invention describes a device 1 for carrying out the reaction of thermal decomposition of water to hydrogen and oxygen comprising a main vessel 2 substantially impermeable to gases that it houses in its inside a membrane selectively permeable to oxygen 3 and a membrane selectively permeable to hydrogen 4 at least one inlet for water 5 at least one outlet for the retentate stream 6 the retentate stream consists of the reaction products hydrogen and oxygen that have not been separated from the membranes and the water that has not reacted at least one outlet for hydrogen separated from the membrane 6' at least one outlet for oxygen separated from the membrane 6'' wherein the main vessel 2 is made of a material that can be exposed to solar radiation and simultaneously capable of operating at a temperature of at least 1300°C.
The above device is schematically shown in Figure 1.
Figure 2, on the other hand, schematically illustrates an embodiment of the device 1 in which the main container substantially impermeable to gases 2 houses a membrane selectively permeable to oxygen 3 having the shape of a chamber with an inlet for water on one side 5 and an outlet for the retentate stream 6 (consisting of the reaction products hydrogen and oxygen not separated from the membranes and the non-reacted water) being the outlet 6'' for the oxygen produced by the reaction and permeated through the membrane 3 which in turn houses a membrane selectively permeable to hydrogen 4 having the shape of at least one tube with one end closed and the other end open being the outlet 6' for the hydrogen produced by the reaction and permeated through the membrane 4.
Preferably the material which can be exposed to solar radiation and at the same time is capable of operating at a temperature of at least 1300°C operates at a temperature ranging between 1300 and 3000°C, more preferably 1500 and 2500°C, even more preferably equal to
1900 °C.
Preferably the material which can be exposed to solar radiation and at the same time is capable of operating at a temperature of at least 1300°C is of at least one metal, optionally in the form of composite material or alloy, selected from the group consisting of tungsten, vanadium, niobium, molybdenum, zirconium, chromium; more preferably it is tungsten.
The external container which is hit by the solar radiation, even concentrated, provides the heat necessary for water decomposition.
Preferably the hydrogen permeable membrane is made of a metal selected from the group consisting of: tantalum, niobium, molybdenum, vanadium, tungsten and alloys thereof and/or their alloys with titanium, zirconium, aluminium, palladium.
More preferably the membrane selectively permeable to hydrogen is tantalum.
Preferably, the membrane selectively permeable to hydrogen having the shape of at least one metal tube is of the self-supported type, or it is of the composite type, i.e. consisting of a thin metal layer deposited on a porous support.
The membranes of the self-supported type consist of tubes with thicknesses ranging from a few tens of pm up to tenths of a millimetre, depending on the operating pressure values.
Composite membranes are typically made up of metallic layers with thicknesses of the order of pm or a fraction of pm depending on the characteristics of the porous support to which they are anchored.
Preferably the porous support can be made of metallic material or ceramic material.
Preferably the membrane selectively permeable to oxygen is made of ceramic material.
Preferably the ceramic material is a metal oxide.
More preferably, the metal oxide is selected from the group consisting of: hafnia, alumina, zirconia, thoria, lithium oxide and mixtures thereof.
The device may further comprise a vacuum pumping system or a washing system with inert gas for extracting the hydrogen separated from the membrane selectively permeable to hydrogen and a vacuum pumping system or a washing system with inert gas for extracting the oxygen separated from the membrane selectively permeable to oxygen.
The device can further comprise a system for heating and steaming the incoming water.
The device may further comprise a system for recovering heat from the hydrogen and oxygen streams separated from the outgoing membranes (and from the retentate stream) possibly connected to a system for heating and steaming the incoming water by means of recirculation and/or heat exchange and recovery means.
A further object of the present invention is a reaction of thermal decomposition of water to hydrogen and oxygen which takes place in the device 1 under conditions of temperature ranging between 1300 and 3000 °C, more preferably 1500 and 2500 °C, even more preferably 1900 °C, and a pressure ranging between 0.5 and 10 bar, preferably between 0.5 and 1 bar, more preferably 1 bar.
In a preferred embodiment, the oxygen permeable chamber is hafnia (HfO2), the hydrogen permeable tube is tantalum (Ta), the external container is made of tungsten.
In a preferred embodiment in the device consisting of an oxygen permeable chamber made of hafnia, hydrogen permeable tube made of tantalum (Ta) and external container made of tungsten, the operating temperature is from 1900 to 2500 °C and the reaction pressure is 1 bar. The tube is of a composite type, with a 10 pm thick tantalum layer, the hafnia chamber has a permeation thickness of 0.1 mm, the feed water flow rate 1000 kg/h, the hydrogen and oxygen permeate streams are extracted with vacuum pumps at a pressure of 100 Pa.
Examples
With reference to reaction (1), the equilibrium constant Kp depends on the reaction AG (J/mol) according to the expression:
ΔG°= -RTlnKp (2) where R is the gas constant (8.31 J mol-1 K-l) and T (K) is the temperature.
Kp with reference to reaction (1) is in turn given by:
Figure imgf000015_0001
where the partial pressures of the reaction products (numerator) and the reactant (denominator) are indicated.
The equilibrium constant can also be expressed in terms of molar fractions:
Figure imgf000016_0001
The relation between the two equilibrium constants for the reaction (1) is:
Kx= p- 05 Kp (5) where P (bar) is the reaction pressure.
Table 1 below shows the Kx values for reaction (1) at the pressures of 0.5, 1 and 10 bar in the temperature range 1000-3000 °C. These values were calculated using the Asther software, taking into account the formation of only H2 and 02 as reaction products in the reaction (1). Reaction (1) is a reaction that proceeds with an increase in the number of moles and is therefore favoured by a reduction in pressure. This behaviour is evident from the results shown in Table 1: the values of the equilibrium constant at 1 bar are greater than those of the constant at 10 bar by a factor equal to 10A0.5 and those at 0.5 bar are greater than those at 1 bar by a factor equal to in accordance with the relation (5).
Figure imgf000016_0002
Figure imgf000016_0003
Figure imgf000017_0001
The conversion of water (i.e. fraction of fed H2O reacting to produce hydrogen and oxygen) is related to the mole fractions of the reactant and of the reaction products in a conventional reactor through the scheme in Table 2 below.
Table 2
Figure imgf000018_0002
Using expression (4) and the relations between reacted fraction a and the mole fractions shown in Table 2, the relation between Kx and in a conventional reactor is
Figure imgf000018_0001
obtained:
(Kx2 - l)a3 - 3Kx2 a+ 2Kx2 = 0 (6)
In the case of a membrane reactor capable of selectively separating hydrogen, we define g as the fraction of hydrogen produced that is recovered through the membrane in the permeate flow rate. In this case, with regards to the molar fractions of the reactant and of the reaction products reference is made to the scheme in Table 3 below. Table 3
Figure imgf000019_0001
In this case, the relation between a and Kx becomes: [KX2(1— 2η|)— (1—η))2α3 + 4Kx2i]α2 -(3+2η)kx 2 α+ 2Kx2 = 0 (7)
In the case of a membrane reactor capable of selectively separating oxygen and by defining e as the fraction of produced oxygen that is recovered through the membrane in the permeate flow rate, the scheme of Table 4 below is obtained.
Table 4
Figure imgf000019_0002
Figure imgf000020_0001
In this case, the relation between a and Kx becomes:
(Kx2 — 1)(1— e)a3 + 2Kx2ca2 -(3+ E)KX2 a+ 2Kx2 = 0
Let us now consider the case of a membrane reactor capable of selectively separating both hydrogen and 5 oxygen simultaneously. On the basis of the definitions given above, the scheme of Table 5 is obtained.
Table 5
Figure imgf000020_0002
In this case, the relation between a and Kx is governed 0 by:
[KX2(1— ε— 2η|)— (1— ε)(1—η))2]α3 + 2Kx2 (ε+2η))α2 -(3+ε+2η)kx 2 α+ 2Kx2 = 0 (9)
Expression (9) summarises the previous cases and, in particular, coincides with (6) in the case of g and ε equal to zero (conventional reactor), with expression (7) in the case of
Figure imgf000021_0001
(membrane reactor separating oxygen) and finally with (8) in the case of ε (membrane reactor separating hydrogen).
Figure imgf000021_0002
Tables 6 and 7 show the calculated values for the conversion of water a for the conventional reactor (TR) and for the different membrane reactor configurations (MR) in the case of separation of only hydrogen, only oxygen and hence hydrogen and oxygen simultaneously. Tables 6 and 7 refer to the cases of membrane efficiency (both e and q) of 0.8 and 0.9 and reaction pressure 1 bar, respectively.
In the temperature range between 1500 and 2000 °C of interest for practical applications, it can be seen in Table 6 (membrane separation efficiency of 0.8) that, compared to conventional reactors, the membrane reactor with hydrogen separation enables the conversion to be increased by a factor of 3, the one with oxygen separation by a factor of 2, and the membrane reactor that simultaneously separates hydrogen and oxygen enables the conversion to be increased by a factor of 4.5-5. The membrane for separating hydrogen is more effective than that for separating oxygen to increase conversion according to the stoichiometry of reaction (1) which from one mole of water leads to the formation of one mole of hydrogen and half a mole of oxygen.
Figure imgf000021_0003
Figure imgf000022_0001
Figure imgf000023_0001
If membranes with a separation efficiency of 0.9 are used (Table 7), the conversion values compared with the conventional reactor in the range 1500-2000 °C increase by a factor of 4.5 in the case of a membrane reactor separating hydrogen, by a factor of 2 in the case of oxygen separation and by a factor of 8.5-10 in the case of the simultaneous presence of membranes for the separation of hydrogen and oxygen. From a quantitative point of view, it can be observed that with a membrane reactor that simultaneously separates hydrogen and oxygen at 1500 °C it can be obtained a conversion value (approx. 1.5%) that can only be reached at 2000 °C in a conventional reactor. Again, the membrane reactor that simultaneously separates hydrogen and oxygen exhibits a reaction conversion of about 8% at 1800 °C, 10% at 1900
C and 15% at 2000 °C.
Table 7
Figure imgf000023_0002
Figure imgf000024_0001
The construction of a reactor for the thermal decomposition of water took into consideration the choice of materials capable of operating at high temperatures. Taking into account the reaction conversion values calculated in the previous paragraph, in the example shown below, operating temperatures at least higher than 1900°C are considered, to which a fraction of converted water of more than 10%, as reported in table 7, corresponds.
In particular, the materials considered are: hafnia (HfO2) for the construction of the oxygen permeable chamber (melting temperature 2900 °C), tantalum (Ta) for the construction of the hydrogen permeable tube (melting temperature 3017 °C), tungsten for the construction of the external container exposed to solar radiation (melting temperature 3422 °C).
The input data for the solar membrane reactor are: feed water flow rate 1000 kg/h, operating temperature from 1900 to 2500 °C, use of permeator tubes of the composite type with a 10 pm thick Ta layer, hafnia chamber with an effective permeation thickness of 0.1 mm, reaction pressure 1 bar, hydrogen and oxygen permeate streams extracted with vacuum pumps at a pressure of 100 Pa.
The following expression of permeability was considered for Ta [7]:
Figure imgf000025_0001
and for hafnia the following expression [8]:
Figure imgf000025_0002
The reaction (1) proceeds with an increase in the number of moles, so it is favoured by low pressures in a conventional reactor. In a membrane reactor, the quantity of reaction products (hydrogen and/or oxygen) removed from the membrane increases with pressure, so that the behaviour of the reaction conversion as the pressure changes is a combination of these two effects: as the pressure increases, the conversion decreases due to the thermodynamic effect, while it tends to increase due to the permeation effect. The process pressure of 1 bar allows for high yields while adequate permeation flows can be obtained by adopting a permeate side pressure of 100 Pa by vacuum pumping. While the reduction in operating pressure leads to an increase in the reaction conversion, it also leads to a reduction in the driving force of the permeation and thus to an increase in the required membrane surface.
The results of the sizing of the membrane reactor are shown in Figures 3 and 4, which refer to the study relating to the temperature range 1900-2500 °C in which the water conversion passes from about 10 to 50% (see
Table 7). Operating between 1900 and 2500 °C, the flow rate of hydrogen and oxygen recovered through the Ta and hafnia membrane varies from 120 to 540 Nm3/h for H2 and from 60 to 270 Nm3/h for 02, respectively. In the same temperature range the necessary Ta membrane surface passes from 35 to 60 m2 (it increases since the permeability of Ta in accordance with (10) reduces with temperature) while the hafnia surface is reduced from 300 to 35 m2 (in fact the permeability of hafnia in accordance with (11) increases with temperature). Still in the same temperature range and by not carrying out thermal recoveries on the unreacted water, the necessary thermal power passes from about 2 to 4 MW.
This preliminary analysis shows that, by operating between 2200 and 2400 °C, a high water conversion (between 27 and 41%) is obtained by adopting devices with a membrane surface (both Ta and hafnia) of a few tens of m2 to which a thermal flow of the order of 0.1 Mw/m2 corresponds.
Further, in a membrane plant the production of hydrogen and oxygen by hydrolysis of water is estimated using the reaction conversion values shown in Table 7. Operating at 1900 °C with a membrane efficiency of 90% and a fed water flow rate of 1000 kg/h, the results are as follows:
- reaction conversion about 10%, quantity of pure hydrogen produced 123 Nm3/h (10.8 kg/h), quantity of pure oxygen produced 61.5 Nm3/h (86.6 kg/h).
By bringing the temperature to 2400 °C the results are:
- reaction conversion about 41%,
- quantity of pure hydrogen produced 469 Nm3/h (41 kg/h), quantity of pure oxygen produced 234.4 Nm3/h (330 kg/h).

Claims

1. Device (1) comprising a main vessel substantially impermeable to gases (2) housing in its inside a membrane selectively permeable to oxygen (3) and a membrane selectively permeable to hydrogen (4) at least one inlet for water (5), at least one outlet for the retentate stream (6), at least one outlet for hydrogen (6') separated from the membrane (4) at least one outlet for oxygen (6'') separated from the membrane (3), wherein the main vessel (2) is made of a material which can be exposed to solar radiation and at the same time is capable of operating at a temperature of at least 1300°C.
2. Device according to claim 1 wherein the membrane selectively permeable to oxygen (3) has the shape of a chamber, having an inlet for water (5) on one side and an outlet for the retentate stream (6) on the opposite side, and which in turn houses a membrane selectively permeable to hydrogen (4) having the shape of at least one tube with one end closed and the other end open being the outlet (6') for hydrogen.
3. Device according to claim 1 wherein the main vessel is made of a material capable of operating at a temperature ranging between 1300 and 3000 °C.
4. Device according to claim 3 wherein the main vessel is made of a material capable of operating at a temperature ranging between 1500 and 2500 °C.
5. Device according to claim 4 wherein the main vessel is made of a material capable of operating at a temperature equal to 1900 °C.
6. Device according to claim 1 wherein the material that can be exposed to solar radiation and simultaneously is capable of operating at a temperature of at least 1300°C is at least one metal, optionally in the form of a composite material or alloy, selected from the group consisting of tungsten, vanadium, niobium, molybdenum, zirconium, chromium.
7. Device according to claim 6 wherein the metal is tungsten.
8. Device according to claim 1 wherein the membrane selectively permeable to hydrogen is made of a metal selected from the group consisting of: tantalum, niobium, molybdenum, vanadium, tungsten and alloys thereof and/or with titanium, aluminium, palladium.
9. Device according to claim 1 wherein the membrane selectively permeable to hydrogen is made of tantalum.
10. Device according to claim 2 wherein the membrane selectively permeable to hydrogen having the shape of at least one tube is of the selfsupported type or is of the composite type.
11. Device according to claim 1 wherein the selectively oxygen permeable membrane is made of ceramic material.
12. Device according to claim 11 wherein the ceramic material is an oxide of a metal selected from the group consisting of: hafnia, alumina, zirconia, thoria, lithium oxide and mixtures thereof.
13. Device according to claim 1 further comprising a vacuum pumping system or a washing system with inert gas for extracting hydrogen separated from the membrane selectively permeable to hydrogen and a vacuum pumping system or a washing system with inert gas for extracting oxygen separated from the membrane selectively permeable to oxygen.
14. Device according to claim 1 further comprising a system for heating and steaming incoming water.
15. Device according to claim 1 further comprising a system for recovering heat from the hydrogen and oxygen streams separated from the outgoing membranes and from the retentate stream possibly connected with a system for heating and steaming the incoming water by means of recirculation, exchange and heat recovery means.
16. Process of thermal decomposition of water to hydrogen and oxygen which takes place in the device (1) under conditions of temperature ranging between 1300 and 3000 °C and pressure ranging between 0.5 and 10 bar.
17. Process according to claim 16 wherein the temperature ranges between 1500 and 2500 °C.
18. Process according to claim 17 wherein the temperature is equal to 1900 °C.
19. Process according to claim 16 wherein the pressure ranges between 0.5 and 1 bar.
20. Process according to claim 19 wherein the pressure is equal to 1 bar.
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