WO2018033850A1

WO2018033850A1 - Process for separation of hydrogen and oxygen from a photocatalytic water-splitting process

Info

Publication number: WO2018033850A1
Application number: PCT/IB2017/054944
Authority: WO
Inventors: Sari AL-SAYEGH; Justin R. Johnson; Xiaotong WEI
Original assignee: Sabic Global Technologies B.V.
Priority date: 2016-08-15
Filing date: 2017-08-14
Publication date: 2018-02-22

Abstract

Embodiments of the invention are directed to methods, processes, and systems for safely and reliably purifying hydrogen from a gas mixture containing hydrogen and oxygen. A process comprises injecting a flammability suppressor into a photocatalytic water-splitting reactor to form a suppressed feed source gas, introducing the suppressed feed source gas to a first membrane separation unit, and collecting a hydrogen product gas stream.

Description

PROCESS FOR SEPARATION OF HYDROGEN AND OXYGEN FRO A PHOTOCATALYTIC WATER- SPLITTING PROCESS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Patent Application No. 62/375,024 filed August 15, 2016, and U.S. Provisional Patent Application No. 62/459, 189 filed February 15, 2017. The entire contents of each of the above-referenced disclosures are specifically incorporated herein by reference without disclaimer.

BACKGROUND OF THE INVENTION

A. Field of the Invention [0002] The invention generally concerns processes to separating an explosive mixture of hydrogen (H₂) and oxygen (0₂) produced in the same environment from a photocatalytic water-splitting process. Specifically, the process includes utilization of a flammability suppressor to form a suppressed feed gas source that is outside the flammability limit of a produced gaseous hydrogen containing composition. B. Description of Related Art

[0003] Hydrogen fuel production has gained increased attention as oil and other nonrenewable fuels become increasingly depleted and expensive. Methods such as photocatalytic water-splitting are being investigated to produce hydrogen fuel, which burns cleanly and can be used in a hydrogen fuel cell. Water-splitting holds particular interest since it utilizes water, an inexpensive renewable resource.

[0004] Technologies are currently under development for producing energy from renewable and sustainable resources such as water. Water-splitting has been investigated as a source of hydrogen. However, there is currently a lack of commercial methods or technologies for purifying hydrogen gas produced via the water-splitting process. The process produces a highly explosive gas mixture, which requires using as yet undefined techniques and/or systems to separate and purify hydrogen from oxygen.

[0005] Various attempts to use and/or separate the hydrogen from the water-splitting reaction have been described. By way of example, International Patent Application No. WO 2008054230 to Fareid et al, describes producing hydrogen and oxygen from water-splitting and then subsequent use of the hydrogen in methanation reactions. In another example, U.S. Patent No. 5,397,559 to Kogan describes thermal water-splitting in one chamber, which is connected to a second chamber by a porous membrane permeable to hydrogen in a Knudsen flow regime, which is then passed to a third chamber and reacting the hydrogen with oxygen in the presence of a catalyst to produce water. The currently known methods for separating the gas mixture produced by water lack reliability and safety, and are considered unrealistic from an engineering point of view.

[0006] While there have been various attempts to use hydrogen from water-splitting reactions, there remains a need for additional methods, processes, and systems for purifying hydrogen from water-splitting reactions for commercial uses.

SUMMARY OF THE INVENTION [0007] Flammable gases and vapors constitute a wide spread explosion hazard in many industrial or domestic situations. Each such gas has critical values of concentration in air, namely a lower explosive limit, LEL, and an upper explosive limit, UEL (flammability limits). When a gas mixture is between these limits any ignition source such as a spark can trigger an explosion. [0008] The lower and upper flammability limits (LFL and UFL, respectively) are the limiting mixture concentrations under particular conditions that can support flame propagation and lead to an explosion. Mixture concentrations outside those limits are nonflammable. The progressive addition of a flammability suppressor (e.g., an inert gas) to the mixture causes the narrowing of the flammability range. Outside the flammability limit is defined as a mixture whose components are at a concentration and condition (e.g., pressure and temperature) that will not support flame propagation.

[0009] Embodiments of the current disclosure relate to methods, processes, and systems for safely and reliably purifying hydrogen from a gas mixture containing hydrogen and oxygen, such as the gas produced by photocatalytic water-splitting. The process of the present invention separates a highly flammable and explosive gas mixture containing hydrogen and oxygen using a flammability suppressant to produce a mixture that is outside the flammability limits under the conditions used to separate hydrogen from oxygen obtained through a water-splitting reaction.

[0010] Certain embodiments are directed to a process for separating hydrogen from a photocatalytic water-splitting product gas can include: (i) injecting a flammability suppressor into a photocatalytic water-splitting reactor that is producing a hydrogen containing feed source gas to form a suppressed feed source gas that is outside of flammability limits; (ii) introducing the suppressed feed source gas to a first membrane separation unit and performing a first hydrogen separation process producing a permeate stream that is enriched in hydrogen and outside of fiammability limits; and (iii) collecting a hydrogen product stream that includes hydrogen from the suppressed feed source gas. In the water-splitting reaction, the fiammability suppressor can be used (1) as a sweep gas in the water-splitting reaction to remove produced 0₂(g) and H₂(g) from a reaction zone of the water-splitting reaction, and (2) as a diluent such that the mixture in step (i) is outside its fiammability zone. In step (ii), the 0₂(g) can be separated from the H₂(g) and the fiammability suppressor by passing the H₂(g), C0₂(g), and 0₂(g) mixture through one or more membranes obtain the H₂(g) stream and the fiammability suppressor stream. In some embodiments, the suppressed feed source gas can be compressed prior to introduction to the first membrane separation unit. One or more permeate and/or retentate streams produced from one or more separation units can be recycled to one or more membrane separation units or combined with one or more process streams (e.g., a compressed suppressed feed gas stream). A fiammability suppressor refers to a compound or element that when mixed with a fuel source in sufficient amounts maintains the concentration of a fuel mixture outside the fiammability limits of the fuel mixture, suppressing the fiammability of the mixture under the particular conditions. In certain aspects, the fuel mixture is hydrogen and oxygen produced in the same environment by the splitting of water. Non-limiting examples of a fiammability suppressor includes nitrogen gas, carbon dioxide gas, carbon monoxide gas, water vapor, methane, ethane, ethylene, propane, propylene, hydrocarbon mixture, or a combination thereof.

[0011] In certain aspects, the fiammability suppressor is injected in an amount so that at least 50, 60, 70, 80, 90 or 95 mol% of the suppressed feed source gas is the fiammability suppressor. In a further aspect, the process can include capturing and recycling the fiammability suppressor and/or hydrogen at the appropriate points in the process or system. In one embodiment, the fiammability suppressor includes at least 70 mol.% carbon dioxide with the balance being nitrogen, argon, water vapor, or mixtures thereof. In certain aspects, the product gas of the photocatalytic reactor is approximately 50 to 80 mol % hydrogen, greater than 0 to 40 mol % oxygen, and 0 to 20 mol % carbon dioxide before injection of the fiammability suppressor. In a further aspect, the suppressed product gas of the photocatalytic reactor has a hydrogen content of approximately 25 mol. % or less.

[0012] In certain aspects, the separation process can be performed at about 20-200 °C. In a further aspect, the first separation process can be performed at a permeate side pressure of about 0.01 to 1 MPa or all values and ranges there between.

[0013] In certain aspects, the suppressed feed source gas can be compressed to about 0.01 to 1 MPa including all values and ranges there between prior to being introduced to the membrane separation unit of step (ii). In a further aspect, the suppressed feed gas can be sent to a compressor at a temperature between 15 up to 200 °C, preferably about 150 °C.

[0014] In a further aspect, the first permeate stream can include at least 30, 40, 50, 60, 70, or 80 mol.% hydrogen. The first permeate stream is outside flammability limits. Notably, all streams produced via the method of the present invention are outside flammability limits.

[0015] Certain embodiments are directed to a purified hydrogen stream produced by the process described herein. In certain aspects, the hydrogen stream can include at least 50, 60, 70, or 80 mol.% hydrogen. The purified hydrogen stream can be maintained outside flammability limits {e.g., 0₂ concentrations are less than 5 mol.%).

[0016] Further embodiments are directed to a gas purification system that can include: (a) a flammability suppressor source configured to provide a flammability suppressor; (b) a photocatalytic water-splitting reactor configured to receive the flammability suppressor and mix a feed source gas produced by the reactor with a flammability suppressor forming a suppressed feed gas under conditions that are beyond the flammability limits for the given conditions; (c) a first separation unit fluidly coupled to the reactor, the first separation unit being configured to separate hydrogen from a suppressed feed source and produces a first product gas that includes hydrogen under conditions that are outside flammability limits.

[0017] In the context of the present invention 20 embodiments are described. Embodiment 1 is a process for separating hydrogen from a photocatalytic water-splitting product gas that comprises: injecting a flammability suppressor into a photocatalytic water- splitting reactor that is producing a hydrogen containing feed source gas to form a suppressed feed source gas that is outside of flammability limits; introducing the suppressed feed source gas to a first membrane separation unit and performing a first hydrogen separation process producing a first permeate stream that is enriched in hydrogen and outside of flammability limits; and collecting a hydrogen product gas stream. Embodiment 2 is the process of embodiment 1, wherein the flammability suppressor is at least 50, 60, 70, 80, 90 or 95 mol.% of the suppressed feed source gas. Embodiment 3 is the process of any one of embodiments 1 to 2, wherein the flammability suppressor comprises gaseous nitrogen gas, carbon dioxide gas, carbon monoxide gas, water vapor, methane, ethane, ethylene, propane, propylene, hydrocarbon mixture, or a combination thereof. Embodiment 4 is the process of any one of embodiments 1 to 3, further comprising producing a first retentate stream enriched in the flammability suppressor and recycling the flammability suppressor to the photocatalytic water-splitting reactor, a second membrane separation unit, or both. Embodiment 5 is the process of any one of embodiments 1 to 4, wherein the product gas of the photocatalytic reactor is approximately 50 to 80 mol % hydrogen, greater than 0 mol.% to 40 mol % oxygen, and 0 to 20 mol % carbon dioxide before injection of the flammability suppressor. Embodiment 6 is the process of any one of embodiments 1 to 5, wherein the suppressed feed source gas is compressed to about 0.1 MPa to 1.5 MPa, or 0.1, 0.2, 0.3, 0.4, 0.5, 1.0 or 2.5 MPa prior to introducing said gas to the first membrane separation unit. Embodiment 7 is the process of claim 6, wherein suppressed feed source gas is sent to a compressor at a temperature between 15 and 150 °C. Embodiment 8 is the process of any one of embodiments 1 to 7, wherein the first separation process is performed at about 20 to 200 °C, a pressure of about 0.1 MPa to 2.5 MPa, or both. Embodiment 9 is the process of any one of embodiments 1 to 8, wherein the permeate side of the first separation process is maintained at a pressure from 0.01 MPa to 1 MPa. Embodiment 10 is the process of any one of embodiments 1 to 9, wherein the first permeate comprises at least 30 mol.% hydrogen. Embodiment 11 is the process of any one of embodiments 1 to 10, further comprising introducing the first permeate to a second membrane separation unit and performing a second hydrogen purification process. Embodiment 12 is the process of embodiment 11, further comprising recycling a portion of a permeate stream, a retentate stream, or both streams of the second hydrogen purification process to the first hydrogen purification process. Embodiment 13 is the process of embodiment 11, further comprising introducing the retentate of the first hydrogen purification process or the retentate of the second hydrogen purification process to a third membrane separation unit and performing a third hydrogen purification process. Embodiment 14 is the process of embodiment 13, further comprising recycling a permeate of the third hydrogen purification process to the first hydrogen purification process feed source or the second hydrogen purification process feed source. Embodiment 15 is the process of embodiment 13, wherein the retentate of the second hydrogen purification process is introduced to the third membrane separation unit and the method further comprises: recycling a permeate of the third hydrogen purification process to the second hydrogen purification process feed source; and collecting the H₂ product gas stream from the second hydrogen purification process, selective. Embodiment 16 is the process of embodiment 13, wherein the retentate of the first hydrogen purification process is introduced to the third membrane separation unit, and the method further comprises: recycling a permeate of the third hydrogen purification process to the first hydrogen purification process; recycling a retentate of the second hydrogen purification process to the first hydrogen purification process; and collecting the H₂ product gas stream from the second hydrogen purification process. Embodiment 17 is The process of embodiment 13, wherein the retentate of the first hydrogen purification process is introduced to the third membrane separation unit, and the method further comprises: providing a permeate of the first hydrogen purification process to the second membrane separation unit, wherein the second hydrogen purification process comprises a C0₂ selective membrane; and providing a permeate of the third hydrogen to the first hydrogen purification process; and collecting the H₂ product gas stream from the second hydrogen purification process. Embodiment 18 is the process of embodiment 13, wherein the retentate of the second hydrogen purification process is introduced to the third membrane separation unit and the method further comprises: providing a permeate of the first hydrogen purification process to the second membrane separation unit, wherein the second hydrogen purification process comprises a C0₂ selective membrane; and providing a permeate of the second hydrogen purification process to the first hydrogenation purification process; providing a portion of the retentate of the third hydrogen separation process to the second hydrogen purification process; and collecting the H₂ product gas stream from the third hydrogen purification process. Embodiment 19 is a gas purification system comprising: (a) a flammability suppressor source configured to provide a flammability suppressor; (b) a photocatalytic water-splitting reactor configured to receive the flammability suppressor and mix a product gas produced by the reactor with a flammability suppressor forming a suppressed feed gas under conditions that are beyond the flammability limits for the given conditions; and (c) a first separation unit fluidly coupled to the reactor, the first separation unit being configured to separate hydrogen from a suppressed feed source. Embodiment 20 is the gas purification system of embodiment 19, further comprising a second or a third separation unit fluidly coupled to the first separation unit, the second separation unit being configured to further separate hydrogen or carbon dioxide from a permeate from the first separation unit, and the third separation unit being configured to further separate hydrogen from the flammability suppressor.

[0018] Other embodiments of the invention are discussed throughout this application. Any embodiment discussed with respect to one aspect of the invention applies to other aspects of the invention as well and vice versa. Each embodiment described herein is understood to be embodiments of the invention that are applicable to all aspects of the invention. It is contemplated that any embodiment discussed herein can be implemented with respect to any method or composition of the invention, and vice versa. The following includes definitions of various terms and phrases used throughout this specification. [0019] The term "raffinate" or "retentate" refers to a product of a separation process which has had a component or components removed.

[0020] The term "permeate" refers to a product of a separation process product that includes a component that has passed through a membrane.

[0021] The term "enriched" means that the stream produced from a separation process has a greater concentration of compounds(s) (i.e., hydrogen) than the originating stream that enters the separation unit(s) used in the separation process.

[0022] The terms "about" or "approximately" are defined as being close to as understood by one of ordinary skill in the art. In one non-limiting embodiment, the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%.

[0023] The terms "wt.%", "vol.%", or "mol.%" refers to a weight, volume, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of component in 100 grams of the material is 10 wt.% of component. [0024] The term "substantially" and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0025] The terms "inhibiting" or "reducing" or "preventing" or "avoiding" or any variation of these terms, when used in the claims and/or the specification includes any measurable decrease or complete inhibition to achieve a desired result. [0026] The term "effective," as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result.

[0027] The use of the words "a" or "an" when used in conjunction with any of the terms "comprising," "including," "containing," or "having" in the claims, or the specification, may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one."

[0028] The words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. [0029] The process of the present invention can "comprise," "consist essentially of," or "consist of particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phase "consisting essentially of," in one non- limiting aspect, a basic and novel characteristic of the processes of the present invention are their abilities to separate hydrogen from oxygen of a gaseous mixture at conditions above the flammability limits of the mixture.

[0030] Other objects, features and advantages of the present invention will become apparent from the following figures, detailed description, and examples. It should be understood, however, that the figures, detailed description, and examples, while indicating specific embodiments of the invention, are given by way of illustration only and are not meant to be limiting. Additionally, it is contemplated that changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. In further embodiments, additional features may be added to the specific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings. [0032] FIGS. 1A and IB are flammability diagrams for H₂/0₂/N₂ and H₂/0₂/C0₂ mixtures, respectively. All stream compositions of the current invention are outside the explosion zone, i.e., outside flammability limits.

[0033] FIG. 2 is a schematic of an overview of the process of the present invention using a flammability suppressor. The membrane cascade separation unit can include more than one membrane unit.

[0034] FIG. 3A is a schematic of an embodiment of a process of the present invention for separation of hydrogen from a photocatalytic water-splitting product using two separation units with recycle of the permeate from the second separation unit to the first separation unit.

[0035] FIG. 3B is a schematic of an embodiment of a process of the present invention for separation of hydrogen from a photocatalytic water-splitting product using two separation units with recycle of the retentate and permeate from the second separation unit to the first separation unit and the second separation unit, respectively.

[0036] FIG. 4 is a schematic of an embodiment of a process of the present invention for separation of hydrogen from a photocatalytic water-splitting product using a hydrogen selective membrane separation unit and a carbon dioxide selective membrane separation unit.

[0037] FIG. 5 is a schematic of an embodiment of a process of the present invention for separation of hydrogen from a photocatalytic water-splitting product using three hydrogen selective separation units with recycle of the permeate from the third separation unit to the second separation unit.

[0038] FIG. 6 is a schematic of an embodiment of a process of the present invention with three hydrogen separation units with recycle of the retentate from the second membrane separation unit and the permeate from third membrane separation unit to the first separation unit.

[0039] FIG. 7 is a schematic of an embodiment of a process of the present invention with a carbon dioxide separation unit and two hydrogen separation units.

[0040] FIG. 8 is a schematic of an embodiment of a process of the present invention with a carbon dioxide separation unit between two hydrogen separation units.

[0041] FIG. 9A shows the influence of temperature on the explosion limit of hydrogen.

[0042] FIG. 9B shows the influence of pressure on the explosion limit of hydrogen.

[0043] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings. The drawings may not be to scale.

DESCRIPTION

[0044] Embodiments of the current disclosure relate to methods, processes, and systems that provide solutions to the purification of gaseous mixtures containing hydrogen and oxygen in concentrations within gaseous mixtures flammability limit. The discovery provides for safely and reliably purifying hydrogen from a gas mixture containing hydrogen and oxygen produced in the same environment, such as that produced by the photocatalytic water-splitting process. The process separates hydrogen from a feed source gas using a flammability suppressing process or system. Methods, apparatus and systems described herein provide for separating hydrogen from a suppressed feed source by using one or more membrane separation processes. These and other non-limiting aspects of the present invention are discussed in further detail with reference to the figures.

[0045] The methods, processes, and systems suppress the flammability of a mixture during the processing of the feed source. The flammability of the mixtures produced during the process can be modulated by injecting another compound or element into the mixture to maintain the mixture outside flammability limits that is under conditions that do not sustain combustion of the mixture. The flammability limit may be expressed as a function of pressure, temperature, and the composition of the mixture {See, for example, FIGS. 1A and IB). FIG. 1A depicts flammability diagram 100 for an oxygen (0₂)/nitrogen (N₂)/air mixture. Data point 102 is the upper flammability limit for 94% H₂ in 0₂ (about 6% 0₂). Data point 104 is the upper flammability limit for 75% H₂ in air (about 25% air). Data point 106 is the lower flammability limit for 4% H₂ in air (about 96% air). Data point 108 is the lower flammability limit for 4% H₂ in 0₂ (about 96% 0₂). FIG. IB depicts flammability diagram 110 for an H₂/0₂/C0₂ mixture. Data point 112 is the extinction effect (about 73% C0₂). As a feed source is processed its composition can change as well as the temperature and pressure, these differences in composition and conditions can cause the flammability limits of the stream during processing to vary at different points along the process. To avoid the risk of fire/explosion, the stream can be maintained above the upper explosive limit or below the lower explosion limit (both instances are conditions are outside of the flammability zone for the mixture) at all points along the process or system. [0046] Hydrogen has a wide flammability range in comparison with other fuels. As a result, hydrogen can be combusted over a wide range of gas mixtures. Thus, hydrogen can combust in a mixture in which the gas content is less than the theoretical, stoichiometric or chemically ideal amount needed for combustion. Hydrogen has very low ignition energy. The amount of energy needed to ignite hydrogen is about one order of magnitude less than that required for gasoline.

A. Overall Process

[0047] Referring to FIG. 2, an overall system and process for the purification of hydrogen from a flammable mixture produced from a photocatalytic water-splitting process is depicted. Photocatalytic water-splitting system 200 can include photocatalytic water-splitting unit 202 and separation unit 204. Separation unit 204 can be a cascade of membranes capable of separating oxygen, flammability suppressor, hydrogen and mixtures thereof. In the photocatalytic water-splitting process, water feed stream 206 and flammability suppressor 208 can enter photocatalytic water-splitting unit 202. In some embodiments, water feed stream 206 and flammability suppressor 208 can be mixed together and enter photocatalytic water-splitting unit 202 as one stream. In some embodiments, flammability suppressor 208 can be injected into the product stream as it exits water-splitting unit 202. The flammability suppressor can be nitrogen, carbon dioxide, water vapor, or any mixture thereof. In one embodiment, the flammability suppressor is at least 70% gaseous C0₂, at least 75 mol.% C0₂(g), at least 80 mol.% C0₂(g), at least 85 mol.% C0₂(g), at least 90 mol.% C0₂(g), at least 95 mol.% C0₂(g), 99 mol.% C0₂(g), 100 mol.% C0₂(g) or any value or range there between. In some instances, the flammability suppressor is at least 80% gaseous N₂, at least 85 mol.% N₂(g), at least 90 mol.% N₂(g), at least 95 mol.% N₂(g), at least 100 mol.% N₂(g) or any value or range there between.

1. Photocatalytic Water-splitting

[0048] Photocatalytic water-splitting is the light-induced conversion reaction of water to hydrogen and oxygen. Photocatalytic water-splitting is an artificial process for the dissociation of water into its constituent parts, hydrogen (H₂) and oxygen (0₂), using either artificial or natural light. In some embodiments, the H₂ and 0₂ are produced without producing greenhouse gases or having many adverse effects on the atmosphere.

[0049] There are several requirements for a photocatalyst to be useful for water-splitting. The minimum potential difference (voltage) needed to split water is 1.23 eV at 0 pH. Since the minimum band gap for successful water-splitting at pH=0 is 1.23 eV the electrochemical requirements can theoretically reach down into infrared light, albeit with negligible catalytic activity. Theoretically, infrared light has enough energy to split water into hydrogen and oxygen; however, this reaction is kinetically very slow because the wavelength is greater than 380 nm. The potential must be less than 3.0 V to make efficient use of the energy present across the full spectrum of sunlight.

[0050] In photocatalytic water-splitting unit 202, water 206 can be contacted with a photocatalyst in the presence of light source 218 to induce splitting of the water into hydrogen (H₂) and oxygen (0₂) in the same environment In some embodiments, sacrificial agents such as methanol, ethanol, propanol, zso-propanol, «-butanol, /so-butanol, ethylene glycol, propylene glycol, glyceral, oxalic acid, malonic acid, succinic acid, maleic acid, tartaric acid, citric acid and their water soluble salts can be used to increase the production of H₂. Ethanol and/or ethylene glycol are used in some instances.

[0051] Materials used in photocatalytic water-splitting fulfill the band requirements and typically have dopants and/or co-catalysts added to optimize their performance. A sample semiconductor with the proper band structure is titanium dioxide (Ti0₂). However, due to the relatively positive conduction band of Ti0₂, there is little driving force for H₂ production, so Ti0₂ is typically used with a co-catalyst such as platinum (Pt) to increase the rate of H₂ production. It is routine to add co-catalysts to spur H₂ evolution in most photocatalysts due to the conduction band placement. Most semiconductors with suitable band structures to split water absorb mostly UV light; in order to absorb visible light, it is necessary to narrow the band gap. Photocatalysts

[0052] Any photocatalyst that can generate H₂ and 0₂ from water upon irradiation can be used. Photocatalysts are described by Liao et al. {Catalysis, 2012, 2:490), which is incorporated herein by reference. Non-limiting examples of photocatalysts include Pt/Ti0₂ based compounds, Pd/Ti0₂ based compounds, Au/Ti0₂ based compounds, Pd/Zr0₂, Pt/Zr0₂, Ru₂0/Zr0₂, Cu/Zr0₂, cobalt-based compositions, cadmium-based compositions, or combinations thereof.

[0053] Ti0₂ based compounds can be used a photocatalyst as they yield both a high quantum number and a high rate of H₂ gas evolution. For example, Pt/Ti0₂ (anatase phase) is a catalyst used in water-splitting. These photocatalysts can combine with a thin NaOH aqueous layer to make a solution that can split water into H₂ and 0₂. Ti0₂ absorbs only ultraviolet light due to its large band gap (greater than 3.0 eV), but outperforms most visible light photocatalysts because it does not photocorrode as easily. Most ceramic materials have large band gaps and thus have stronger covalent bonds than other semiconductors with lower band gaps. Co-catalysts and dopants can be added to Pt loaded Ti0₂. Non-limiting examples of co-catalysts and dopants include Pd, Rh, Au, Ni, Cu, Ag, Fe, Mo, Ru, Os, Re, V, Sr, metals or combinations thereof. Addition of NiO particles as co-catalysts assisted in H₂ production; this step can be done by using an impregnation method with an aqueous solution of Νί(Νθ3)·6Η₂0 and evaporating the solution in the presence of the photocatalyst. The TiO-based photocatalysts can include dye sensitizers that help absorb light. Non-limiting example of organic dyses include erosin Y, riboflavin, cyanine, cresyl violet, hemicyanine, and merocynaine. [0054] Cobalt based compositions can include nanocrystalline cobalt (II) oxide, tris(bipyridine) cobalt(II), compounds of cobalt ligated to certain cyclic polyamines, and certain cobaloximes. Chromophores can be connected to organic rings that complex to a cobalt atom. Processes using cobalt-based compositions can be less efficient than a platinum catalyst, however, cobalt is less expensive, potentially reducing total costs. The process can use one of two supramolecular assemblies based on Co(II)-templated coordination as photosensitizers and electron donors to a cobaloxime macrocycle.

2. Hydrogen Purification

[0055] Referring back to FIG. 2, the product gas produced in photocatalytic water- splitting unit 202 can include product gas of the photocatalytic reactor containing 50 to 80 mol.% hydrogen, greater than 0 mol.% (e.g., 1 mol.%) to 40 mol.% oxygen, and 0 to 20 mol.%) carbon dioxide before injection of the flammability suppressor. Flammability suppressor 208 can be injected into photocatalytic water-splitting unit through a flow path to form the suppressed feed source gas 210 that is outside of flammability limits of the gaseous mixture. [0056] Suppressed feed source gas 210 can exit photocatalytic water-splitting unit 202 and enter separation unit 204. Suppressed feed source gas 210 can include 50 to 95 mol.%>, 60 to 90 mol.%, 70 to 80 mol.% or 50 mol.%, 55 mol.%, 60 mol.%, 65 mol.%, 70 mol.%, 75 mol.%, 80 mol.%, 85 mol.%, 90 mol.%, 95 mol.%, 99 mol.% or any value or range there between of the flammability suppressor. In a further aspect, the suppressed gas can have a hydrogen content of approximately 25 mol.% or less, or 5 to 25 mol.%, or 5 mol.%, 10 mol.%), 15 mol.%), 20 mol.% or 25 mol.% or any value or range there between. It should be understood that suppressed feed source gas 210 can pass through multiple heat exchangers, compressors, or other equipment to change the pressure and/or temperature of the stream to conditions suitable for separation in separation unit 204. In separation unit 204, hydrogen product gas 212, flammability suppressor stream 214, and gaseous oxygen-containing stream 216 can be produced. Certain aspects (e.g., membrane separations) can be performed at a pressure below 0.101 MPa (1 atm), at which point a vacuum pump can be employed to attain the appropriate pressure.

[0057] For the process described herein, feed source gas 210 produced in the photocatalytic reactor is suppressed. In some embodiments, feed source gas 210 can be compressed to approximately 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, 2.0, or 2.5 MPa (including all values and ranges there between) before entering a first membrane separation unit. The separation unit can include a gas selective separation membrane, in particular a hydrogen selective membrane, a carbon dioxide selective membrane or both.

[0058] FIGS. 3-8 are schematics of membrane separation units that can be used for separation of H₂ product gas streams, 0₂-containing gas streams, and flammability suppressor gas streams (e.g., N₂ or C0₂ gas streams) from the suppressed feed source gas 210. Referring to FIGS. 3A and 3B, system 300 is depicted, which includes two hydrogen selective membrane separation units. In system 300, suppressed feed source gas 210 can be introduced into a pre-separation unit 302, where suppressed feed source gas 210 can be passed through a series of compressors, optional heat exchangers, and the like to form compressed feed source 304 that is outside flammability limits. Pre-processing system 302 can also be referred to as inter-stage systems. By way of example, suppressed feed source 210 can exit photocatalytic water-splitting unit 202 and pass through one or more heat exchangers to cool the suppressed gas feed source to a temperature of less than 60 °C or between 15 and 150 °C, or 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150 °C and then pass through a compressor to be compressed to about 0.1 MPa to 2.5 MPa, or 0.5 to 1.5 MPa, or 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, 2.0, or 2.5 MPa or any range or value there between. In some embodiments, suppressed feed source gas 210 is only cooled, or not compressed or cooled.

[0059] Compressed suppressed feed 304 source can be introduced to first membrane separation unit 306 and subjected to conditions suitable to separate oxygen from the hydrogen to produce first permeate stream 308 (e.g., H₂/N₂ or a H₂/C0₂ containing stream) and first retentate stream 310 (e.g., oxygen-containing stream). In a preferred embodiment, first membrane separation unit 306 is a hydrogen selective membrane unit. First permeate stream 308 can include H₂, N₂, C0₂, and 1 mol.% to less than 5 mol.% 0₂. Separation conditions in first membrane separation unit 306 can include temperatures of 20 to 200 °C and/or a pressure of the permeate side of the membrane can be maintained at 0.01 MPa to 1.5 MPa, 0.05 to 1 MPa, or 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, MPa any value or range there between. First permeate stream 308 can be hydrogen enriched (i.e., have a higher concentration of H₂ than feed source gas stream 210 or compressed gas stream 304). First retentate stream 310 can include oxygen and be enriched in flammability suppressor (e.g., enriched in N₂, and/or C0₂). Permeate stream 308 can exit first membrane separation unit 306 and enter second membrane separation unit 312. In some embodiments, second membrane separation unit 312 is a H₂ selective membrane unit. In some embodiments, first permeate stream 308 be further processed through, for example, one or more compressors and heat exchangers to produce a second permeate stream that is at the temperature and pressure suitable for separation in second membrane unit 312. In second membrane unit 312, first permeate stream 308 (or processed first permeate) can be subjected to conditions suitable to produce a second retentate stream 314 (e.g., C0₂ or N₂ stream) and a second permeate stream 316 (e.g., H₂ containing stream, or H₂ product gas stream). Second permeate stream 316 can be collected, further processed, or transported (e.g., stream 320). In certain aspects, second permeate stream 316 can be a hydrogen enriched product gas. A portion of permeate stream 316, recycle stream 318, can be recycled to preprocessing unit 302, mixed with suppressed feed source 210, mixed with compressed suppressed feed source 304, or combinations thereof. In some embodiments, permeate stream 316 is not recycled. Permeate stream 316 (H₂ product stream, (e.g., stream 320)) can include at least 30 mol.% H₂, at least 50 mol.% H₂, at least 80 mol.% H₂, at least 90 mol.% H₂ or about 99 mol.% and less than 5 mol.% 0₂ or 0 to 5 mol.% 0₂. Referring to FIG. 3B, all or a portion of retentate stream 314 can be recycled to preprocessing unit 302, mixed with suppressed feed source 210, mixed with compressed suppressed feed source 304, or combinations thereof. As shown, a portion of second permeate stream 316, stream 318, can be combined with first permeate stream 308, and the other portion, stream 320, can be collected, transported, or stored. It should be understood that a portion of second permeate stream can be directly fed to membrane separation unit 312 or recycled to the first membrane separation process as described in FIG. 3 A. In one aspect, the systems of FIGS. 3 A and 3B are used to separate hydrogen from oxygen when nitrogen is used the flammability suppressor.

[0060] FIG. 4 depicts a schematic for a process of the present invention to separate H₂ from compressed suppressed feed source gas 304 that includes H₂, 0₂, and C0₂. Compressed suppressed feed source gas 304 can enter membrane separation unit 306 and be subjected to conditions suitable to separate 0₂ from H₂ to form first retentate stream 310 and first permeate stream 308. Membrane separation unit 306 can be a hydrogen selective membrane. First permeate stream 308 can include H₂, C0₂, and 0₂ at concentrations that are outside the flammability limits. First permeate stream 308 can enter carbon dioxide selective membrane 402. In carbon dioxide selective membrane 402, first permeate stream 308 is subjected to conditions suitable to separate H₂ from the C0₂ to produce hydrogen enriched product stream 316 (second retentate stream) and carbon dioxide permeate stream 404. In some embodiments, a portion, or all, of H₂ enriched product stream 316 can be recycled to processing unit 302, mixed with compressed suppressed feed gas 304, mixed with suppressed feed gas 210, or combinations thereof. H₂ product stream 316 can include at least 30 mol.% H₂, at least 50 mol.% H₂, at least 80 mol.% H₂, at least 90 mol% H₂ or about 99.9 mol.% and less than 5 mol.% 0₂ or 0 to 5 mol.% 0₂.

[0061] FIG. 5 is a schematic of system and process of the present invention that includes three hydrogen membrane separation units. In FIG. 5, suppressed feed 210 can be introduced into pre-separation process 302, which includes compression, heat exchange etc. as described above to produce pre-processed feed source 304. In some embodiments, suppressed feed 210 include a mixture of H₂, 0₂, and C0₂. Pre-processed feed source 304 {e.g., a compressed suppressed feed source) can be introduced to first membrane separation 306 and subjected to conditions to produce first permeate stream 308 {e.g., H₂/C0₂ gas stream) and first retentate stream 310 {e.g., oxygen-containing stream) as previously described. First permeate stream 308 can exit first membrane separation unit 306, enter second membrane separation unit 312, and be subjected to conditions to produce H₂ product stream 316 (second permeate) and second retentate stream 314. H₂ product stream 316 can be collected, and, in certain aspects, can be a hydrogen enriched product gas. Retentate stream 314 can be introduced to third H₂ selective membrane separation unit 502. Retentate stream 314 can include H₂ and flammability suppressor {e.g., C0₂, N₂, CH₄, flue gas or mixtures thereof) and optionally 0₂. In third membrane separation unit 502, retentate stream 314 can be subjected to conditions suitable to separate H₂ from the C0₂ to produce third permeate stream 504 and third retentate stream 506. As shown, all of permeate stream 504 can be recycled to first permeate stream 308. It should be understood that a portion of permeate stream 504 can be recycled to a processing unit prior to membrane unit 312 or directly to membrane separation unit 312. Permeate stream 510 can include H₂ and, optionally 0₂ and/or C0₂. Retentate stream 506 can be a C0₂ gas stream, which can be collected, processed, or transported, or combinations thereof. In a preferred embodiment, membrane separation units 306, 312, and 502 are all hydrogen selective membrane units.

[0062] FIG. 6 depicts another embodiment to separate hydrogen from oxygen above to produce a H₂ stream outside the flammability limits using three membrane separation units. In some embodiments, the feed suppressed feed 210 (e.g., a mixture of H₂, flammability suppressor, and 0₂) can be introduced into pre-separation processing unit 302, which includes compression, heat exchange etc. as described above. The pre-processed feed source 304 {e.g., a compressed suppressed feed source) can be introduced to a first membrane separation 306 and be subjected to conditions to produce first permeate stream 308 {e.g., H₂/C0₂ gas stream) and first retentate steam 310 {e.g., an oxygen-containing stream). First permeate stream 308 can be introduced to second membrane separation unit 312 and be subjected to conditions to produce H₂ product stream 316 (second permeate) and retentate stream 314 (second retentate stream). In some embodiments, permeate stream 308 can be preprocessed prior to entering membrane separation unit 312. H₂ product stream 316 can be collected, and, in certain aspects, can be suitable for sale. Retentate stream 314 can include H₂ and C0₂, and can be recycled to preprocessing unit 302. As shown, all of retentate stream 314 can be recycled to pre-processing unit 302. It should be understood that a portion of retentate stream 314 can be recycled to preprocessing unit 302, processed first permeate stream 304, feed stream 210, directly into membrane separation unit 306, or combinations thereof.

[0063] First retentate stream 310 can enter third membrane separation unit 502, and be subjected to conditions suitable to separate H₂ from the C0₂ to produce permeate stream 504 (third permeate stream) and retentate stream 506 (third retentate stream). Permeate stream 504 can include H₂, C0₂, and 0 mol.% to 5 mol.% 0₂. Permeate stream 504 can be recycled to preprocessing unit 302. As shown, all of permeate stream 504 can be recycled to preprocessing unit 302. It should be understood that a portion of permeate stream 504 can be recycled to preprocessing unit 302, processed first permeate stream 304, feed gas source stream 210, directly into membrane separation unit 306, or combinations thereof. Retentate steam 506 can include flammability suppressor {e.g., C0₂, N₂, methane, flue gas, or combinations thereof), 0₂, and less than 4 mol.% H₂. In a preferred embodiment, retentate stream 506 includes C0₂ and 0₂. Retentate stream 506 and be collected, stored, or further processed. In a preferred embodiment, membrane separation units 306, 312, and 502 are all hydrogen selective membrane units.

[0064] Referring to FIG. 7, compressed suppressed feed gas 304 {e.g., H₂/0₂/C0₂ gas stream) can enter hydrogen selective membrane unit 306 to produce first permeate stream 308 {e.g., H₂/C0₂ gas stream) and first retentate stream 310 {e.g., C0₂/0₂ stream). First permeate stream 308 can be preprocessed prior to entering carbon dioxide selective membrane unit 702 {e.g., compressed, passed through heat exchangers, subjected to vacuum, or the like). In carbon dioxide selective membrane unit 702, permeate stream 308 can be subjected to conditions suitable to separate the permeate stream into permeate C0₂ stream 704 and H₂ product stream 316 (retentate product stream). As shown, all of C0₂ stream 704 is recycled to preprocessing unit 302. It should be understood that all, or a portion, of C0₂ stream 704 can be recycled to compressed suppressed feed gas stream 304, mixed with suppressed feed gas stream 210, directly to hydrogen membrane separation unit 306, or combinations thereof.

[0065] First retentate stream 310 can enter third H₂ membrane separation unit 502, and be subjected to conditions suitable to separate H₂ from the flammability suppressor (e.g., C0₂, N₂, methane, flue gas, or combinations thereof) to produce permeate stream 504 and retentate stream 506. Permeate stream 504 can include H₂, flame suppressor and/or less than 5 mol.% 0₂. Permeate stream 504 can be recycled to preprocessing unit 302 or other streams/units as previously described. As shown, all of permeate stream 504 can be recycled to preprocessing unit 302. It should be understood that a portion of permeate stream 504 can be recycled to preprocessing unit 302, processed first permeate stream 304, feed stream 210, directly into membrane separation unit 306, or combinations thereof. Retentate steam 506 can include flammability suppressor (e.g., C0₂, N₂, methane, flue gas, or combinations thereof), 0₂, and less than 4 mol.% H₂. In a preferred embodiment, retentate stream 506 includes C0₂ and 0₂. Retentate stream 506 and be collected, stored, or further process. [0066] Referring to FIG. 8, compressed suppressed feed gas 304 (e.g., H₂/0₂/C0₂ gas stream) can enter hydrogen selective membrane unit 306 to produce first permeate stream 308 (e.g., H₂/C0₂ gas stream) and first retentate stream 310 (e.g., C0₂/0₂ stream). First permeate stream 308 can be preprocessed prior to entering carbon dioxide selective membrane unit 702. In carbon dioxide selective membrane unit 702, first permeate stream 308 can be subjected to conditions suitable to separate the permeate stream into permeate C0₂ stream 704 and retentate stream 802 that includes H₂, flammability suppressor, and 0 mol.% to 5 mol.%) 0₂. As shown, all of C0₂ permeate stream 704 is recycled to preprocessing unit 302 (inter-stage systems). It should be understood that all, or a portion, of C0₂ permeate stream 704 can be recycled to compressed suppressed feed gas stream 304, mixed with suppressed feed gas stream 210, directly to hydrogen membrane separation unit 306, or combinations thereof.

[0067] Retentate H₂ stream 802 can enter third H₂ membrane separation unit 502, and be subjected to conditions suitable to separate H₂ from the flammability suppressor (e.g., C0₂, N₂, methane, flue gas, or combinations thereof) to produce permeate stream 504 as a H₂ product stream and retentate stream 506. Retentate steam 506 can include H₂, flammability suppressor (e.g., C0₂, N₂, methane, flue gas, or combinations thereof) and less than 5 mol.% 0₂. A portion of retentate stream 506 can be recycled to first permeate stream 308. The amount of retentate stream 506 added to first permeate stream 308 can be adjusted to maintain the concentration of components of the first permeate stream outside flammability limits (e.g., less than 5 mol.% 0₂). A portion of retentate stream 506 can be burned, transported, stored, or provided to other processing units.

[0068] In certain embodiments, a plurality of the hydrogen and/or carbon dioxide separation membranes or can be used in the process. In certain aspects, other purification procedures can be included in the process or systems described herein, such as differential solvent adsorption, pressure swing adsorption, or the like can be used in combination with the membrane separation unit. Various inter-stage units between the membranes are not described, however, it should be understood that the various gas processing units can be included such as, but not limited to, compressors, vacuum pumps, blowers, heat exchangers, condensers, filters, dryers, or combinations thereof.

[0069] The feed source gas produced from the photocatalytic water-splitting process is at near atmospheric pressure, for example, the feed source can contain about 70 mol.% H₂, 25 mol.%) 0₂ and 5 mol.%> C0_2. This feed source gas is suppressed by injecting a flammability suppressor to an appropriate mole percentage to bring the suppressed feed source gas outside flammability limits. The suppressed feed source gas can then be compressed to increase the pressure of the suppressed feed source gas to the desired delivery pressure. The compressor, for example, can be a piston compressor, a diaphragm compressor, a scroll compressor, or other type of compressor, or combinations of compressors. In certain aspects the gas is compressed using a piston compressor. In certain aspects the suppressed feed source gas can be compressed to approximately 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, 2.0, 2.5 MPa, including all values and ranges there between, and sent to a separation unit for hydrogen separation. For safety, the compressor can be a spark-free or spark-suppressed compressor.

[0070] The hydrogen separation membrane is characterized by allowing hydrogen to preferably permeate through the membrane under a pressure differential between the feed and permeate side of the membrane.

[0071] Although varying according to the type and performance of the hydrogen separation membrane, the temperature of the hydrogen separation membrane during use is preferably 20 to 200 °C and more preferably 50 to 150 °C. The feed gas is preferably 0.1 MPa to 2.5 MPa and more preferably 0.4 MPa to 1.0 MPa before entering the separation unit. Permeate pressure can be operated between 0.01 MPa to 1 MPa. [0072] In certain aspects, the hydrogen product gas isolated and can be further purified, if needed. The hydrogen product gas (product gas) can be compressed and/or further suppressed, and transmitted to a second hydrogen purification unit.

[0073] FIGS. 3-8 are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples or figures represent techniques discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. 3. Gas selective membranes

[0074] Hydrogen selective membrane and carbon dioxide selective membranes can be manufactured or be obtained from commercial sources. Non-limiting examples of commercial membrane sources are Air Products (U.S.A.), Air Liquide (U.S.A.) UBE Industries, LTD. (JAPAN), or the like. [0075] Non-limiting examples of materials that compose the hydrogen separation membrane include polymeric and carbon membranes. Polymeric membranes typically achieve hydrogen selective molecular separation via control of polymer free volume. Polymeric membranes may be comprised, for example, of glassy polymers, epoxies, polysulfones, polyimides (e.g., polyimide membrane from UBE, or Proteus™ membranes from Membrane Technology and Research, Inc., and other materials, and may include crosslinks and matrix fillers of non-permeable (e.g., dense clay) and permeable (e.g., zeolites) varieties to modify polymer properties. Carbon membranes are generally microporous and substantially graphitic layers of carbon prepared by pyrolysis of polymer membranes or hydrocarbon layers. Carbon membranes may include carbonaceous or inorganic fillers, and are generally applicable at both low and high temperature. The hydrogen separation membrane may be a dense membrane composed only of the above-mentioned materials, or may be a dense thin membrane composed of the above-mentioned materials supported on a porous body. In the case of the former, the thickness of the hydrogen separation membrane is preferably 0.1 μιη or more and more preferably 0.5 μιη to 5 μιη from the viewpoints of mechanical strength and hydrogen permeability. In the case of the latter, the thickness of the thin membrane is 0.1 to 25 μιη or more and more preferably 0.1 μιη to 2 μιη from the viewpoint of processability.

[0076] In cases where the hydrogen separation membrane comprises the dense thin membrane composed of the above-described materials and the porous body supporting the membrane thereon, the replacement of gaseous species tends to be inhibited on the side of the porous body and, thus, it is preferable for a dense thin membrane to be the side contacted with a mixed gas, and a porous body to be the side contacted with permeated hydrogen.

[0077] Non-limiting examples of materials that C0₂ selective membranes can include at least one of poly(ethylene oxide) containing polymer membranes, silicon-containing polymers, and amine functionalized polymers. A commercial C0₂ selective membrane is a Polaris™ membrane (Membrane Technology and Research, Inc (MTR), U.S. A). EXAMPLES

[0078] The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

[0079] Purification of stream that includes a flammability suppressant, hydrogen, and oxygen from a water-splitting process was simulated using ASPEN PLUS with a SRK thermodynamic package and including custom membrane models developed using Aspen Custom Modeler (Aspen Tech, U.S.A.). The following assumptions were used: 1) the system was in steady state; 2) cross-flow with unhindered permeate withdrawal; 3) no temperature or pressure drop along or across the membrane; 4) no interaction between the components were considered; and 5) constant permeability along the membranes. Table 1 lists the main process parameters used in the simulations. The molar composition used in both systems was based on the flammability discussed throughout the specification. Referring to FIG. 2, it was determined that as operating temperature increases, the explosive range widened. Assuming that all the mixtures behave as a hydrogen and air mixture, the influence of temperature and pressure on the explosion limits of hydrogen were determined. FIG. 9A shows the influence of temperature on the explosion limit of hydrogen. FIG. 9B shows the influence of pressure on the explosion limit of hydrogen. From the data, it was determined that as pressure increases, the explosive range decreases lower pressures (up to 2.0 MPa (20 bar) then expands again. For pressures above 5 MPa (50 bar), the explosive range remains relatively constant for the mixture. From the data in FIGS. 9 A and 9B it was determined that the first 0₂ limits did not change with the influence of temperature and pressure. Thus, for the water- splitting system any stream received from the the water-splitting system can have a mixture composition can be outside the explosion regime in the flammability diagram.

Table 1

Examples 1A and IB

(N₂ Diluent System)

[0080] Simulations for the N₂ diluent system were performed for two membrane stages using hydrogen selective material using the schematic shown in FIGS. 3A and 3B. Table 2 lists the types of membranes used and Table 3 lists the results for Example 1 A (FIG. 3B) and Table 4 lists the results for Example IB (FIG. 3 A). For the processes of Example 1A and IB feed stream 210 was heated and compressed by passing the feed stream through one or more inter-stage systems 302 to form heated and compressed feed stream 304. Permeate stream 308 was passed through one or more inter-stage systems (not shown) to adjust the pressure and maintain the temperature at 60 °C prior to entering membrane unit 312. Other inter-stage units were used to maintain the desired pressures and temperatures of the streams as they entered the membranes or combined with other streams. For the process of Example IB (FIG. 3 A) Example 1A had a H₂ recovery of 92.1% and a product purity of 99.4%. Example had a H₂ recovery of 92.1% and a product purity of 95.4%.

Table 2

(Ube Industries, Ltd., Japan.)

TABLE 3

TABLE 4

Example 2A

(C0₂ Diluent System)

[0081] Simulations for the C0₂ diluent system were performed for three membrane stages using hydrogen selective material using the schematic shown in FIG. 5. Referring to FIG. 5, feed stream 210 was heated and compressed by passing the feed stream through an interstage system to form heated and compressed feed stream 304. Permeate stream 308 was passed through one or more inter-stage units 302 to adjust the pressure to 1.3 and maintain the temperature at 150 °C prior to entering membrane system 312. Table 5 lists the types of membranes used in FIG. 5 and Table 6 lists the results for Example 2A. Example 2A had a H₂ recovery of 83.6% and a product purity of 99.0%.

Table 5

* (Membrane Technology and Research, Inc., U.S.A.)

TABLE 6

Example 2B

(C0₂ Diluent System)

[0082] Simulations for the C0₂ diluent system were performed for three membrane stages using hydrogen selective material using the schematic shown in FIG. 6. Referring to FIG. 6, feed stream 210 was heated and compressed by passing the feed stream through one or more interstage systems 302 to form heated and compressed feed stream 304. Table 7 lists the types of membranes used in FIG. 6 and Table 8 lists the results for Example 2B. Example 2B had a H₂ recovery of 87.0% and a product purity of 99.1%.

Table 7

* (Membrane Technology and Research, Inc., U.S.A.)

TABLE 8

Example 2C

(C0₂ Diluent System)

[0083] Simulations for the C0₂ diluent system were performed for three membrane stages using hydrogen selective material using the schematic shown in FIG. 8. Referring to FIG. 8, feed stream 210 was heated and compressed by passing the feed stream through an interstage system to form heated and compressed feed stream 304. Permeate stream 308 was passed through one or more inter-stage units 302 to adjust the pressure to 1.3 and maintain the temperature at 150 °C prior to entering membrane system 702. Retentate stream 802 was passed through a heat exchanger (not shown) to heat the stream to 150 °C prior to entering membrane unit 502. Table 9 lists the types of membranes used in FIG. 8 and Table 10 lists the results for Example 2C. Example 2C had a H₂ recovery of 92.1% and a product purity of 99.0%.

Table 9

*Membrane Technology and Research, Inc., U.S.A. TABLE 10

Claims

1. A process for separating hydrogen from a photocatalytic water-splitting product gas, the method comprising:

injecting a flammability suppressor into a photocatalytic water-splitting reactor that is producing a hydrogen containing feed source gas to form a suppressed feed source gas that is outside of flammability limits;

introducing the suppressed feed source gas to a first membrane separation unit and performing a first hydrogen separation process producing a first permeate stream that is enriched in hydrogen and outside of flammability limits; and collecting a hydrogen product gas stream.

2. The process of claim 1, wherein the flammability suppressor is at least 50, 60, 70, 80, 90 or 95 mol.% of the suppressed feed source gas.

3. The process of claim 1, wherein the flammability suppressor comprises gaseous nitrogen gas, carbon dioxide gas, carbon monoxide gas, water vapor, methane, ethane, ethylene, propane, propylene, hydrocarbon mixture, or a combination thereof.

4. The process of claim 1, further comprising producing a first retentate stream enriched in the flammability suppressor and recycling the flammability suppressor to the photocatalytic water-splitting reactor, a second membrane separation unit, or both.

5. The process of claim 1, wherein the produced a hydrogen containing feed source gas comprises hydrogen and oxygen produced in the same environment, and the produced feed source gas is approximately 50 to 80 mol % hydrogen, greater than 0 mol.% to 40 mol % oxygen, and 0 to 20 mol % carbon dioxide.

6. The process of claim 1, wherein the suppressed feed source gas is compressed to about 0.1 MPa to 1.5 MPa, or 0.1, 0.2, 0.3, 0.4, 0.5, 1.0 or 2.5 MPa prior to introducing said gas to the first membrane separation unit.

7. The process of claim 6, wherein suppressed feed source gas is sent to a compressor at a temperature between 15 and 150 °C.

8. The process of claim 1, wherein the first separation process is performed at about 20 to 200 °C, a pressure of about 0.1 MPa to 2.5 MPa, or both.

9. The process of claim 1, wherein the permeate side of the first separation process is maintained at a pressure from 0.01 MPa to 1 MPa.

10. The process of claim 1, wherein the first permeate comprises at least 30 mol.% hydrogen.

11. The process of claim 1, further comprising introducing the first permeate to a second membrane separation unit and performing a second hydrogen purification process.

12. The process of claim 11, further comprising recycling a portion of a permeate stream, a retentate stream, or both streams of the second hydrogen purification process to the first hydrogen purification process.

13. The process of claim 11, further comprising introducing the retentate of the first hydrogen purification process or the retentate of the second hydrogen purification process to a third membrane separation unit and performing a third hydrogen purification process.

14. The process of claim 13, further comprising recycling a permeate of the third hydrogen purification process to the first hydrogen purification process feed source or the second hydrogen purification process feed source.

15. The process of claim 13, wherein the retentate of the second hydrogen purification process is introduced to the third membrane separation unit and the method further comprises:

recycling a permeate of the third hydrogen purification process to the second hydrogen purification process feed source; and

collecting the H₂ product gas stream from the second hydrogen purification process, selective.

16. The process of claim 13, wherein the retentate of the first hydrogen purification process is introduced to the third membrane separation unit, and the method further comprises:

recycling a permeate of the third hydrogen purification process to the first hydrogen purification process;

recycling a retentate of the second hydrogen purification process to the first hydrogen purification process; and

collecting the H₂ product gas stream from the second hydrogen purification process. The process of claim 13, wherein the retentate of the first hydrogen purification process is introduced to the third membrane separation unit, and the method further comprises:

providing a permeate of the first hydrogen purification process to the second membrane separation unit, wherein the second hydrogen purification process comprises a C0₂ selective membrane; and

providing a permeate of the third hydrogen to the first hydrogen purification process; and

collecting the H₂ product gas stream from the second hydrogen purification process.

The process of claim 13, wherein the retentate of the second hydrogen purification process is introduced to the third membrane separation unit and the method further comprises:

providing a permeate of the second hydrogen purification process to the first hydrogenation purification process;

providing a portion of the retentate of the third hydrogen separation process to the second hydrogen purification process; and

collecting the H₂ product gas stream from the third hydrogen purification process.

A gas purification system comprising:

(a) a flammability suppressor source configured to provide a flammability suppressor;

(b) a photocatalytic water-splitting reactor configured to receive the flammability suppressor and mix a product gas produced by the reactor with a flammability suppressor forming a suppressed feed gas under conditions that are beyond the flammability limits for the given conditions; and

(c) a first separation unit fluidly coupled to the reactor, the first separation unit being configured to separate hydrogen from a suppressed feed source.

The gas purification system of claim 19, further comprising a second or a third separation unit fluidly coupled to the first separation unit, the second separation unit being configured to further separate hydrogen or carbon dioxide from a permeate from the first separation unit, and the third separation unit being configured to further separate hydrogen from the flammability suppressor.