WO2018037315A1

WO2018037315A1 - Integrated method and system for hydrogen and ethylene production

Info

Publication number: WO2018037315A1
Application number: PCT/IB2017/054985
Authority: WO
Inventors: Abdullah BAUBAID; Justin R. Johnson; Farhan A. PASHA
Original assignee: Sabic Global Technologies B.V.
Priority date: 2016-08-22
Filing date: 2017-08-16
Publication date: 2018-03-01

Abstract

Integrated Systems and methods for producing ethylene and hydrogen are described. The systems and methods integrate a photocatalytic water splitting (PCWS) process with the oxidative dehydrogenation of ethane (ODhE) process to produce hydrogen and ethylene production.

Description

INTEGRATED METHOD AND SYSTEM FOR HYDROGEN AND ETHYLENE

PRODUCTION

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Patent Application No. 62/377,886 filed August 22, 2016, which is hereby incorporated by reference in its entirety.

BACKGROUN D

A. Field of the Invention

[0002] The invention generally concerns in integrated process of a photocatalytic water splitting process and oxidative dehydrogenation of ethane process to produce hydrogen and ethylene.

B. Description of Related Art

[0003] There is increasing global demand for hydrogen gas and ethylene. Conventional technology produces hydrogen from steam reforming of methane, while ethylene is produced from steam cracking of ethane, propane, butane, liquid petroleum gas (LPG), or heavier hydrocarbons. Due to the depletion of fossil fuels, there is a necessity to find an alternative feedstock to meet the growing demand for hydrogen production globally.

[0004] One alternative to steam reforming to produce hydrogen is photocatalytic water splitting (PCWS). Hydrogen production from photocatalytic water splitting can provide potential benefits for the energy section and chemical sectors. A limitation to water splitting reactions is that high concentrations of flammable hydrogen (H₂) and oxygen (0₂) concentrations greater than 4.2% (LFL, lower flammable limit) and 0₂ greater than 4% (MOC, minimum oxygen concentration) can be produced. When coproduced oxygen species are not removed, elevated oxygen partial pressure and oxygen species concentrations can hinder the water splitting reaction and decrease hydrogen generation. To control the amount of oxygen released, sacrificing reagents such as glycerin or glycols, have been used to consume oxygen generated in the system. Hence, the cost of hydrogen production is tied to the cost of sacrificing reagent. The oxidation of sacrificing reagent can also produce environmentally unfriendly volatile organic compounds (VOCs) and contaminate the water pool used for electrolysis. Various attempts to avoid the flammable region of the gas mixture and reduce oxygen partial pressure include: (1) use of a oxygen sacrificing reagent to reduce free oxygen concentration in hydrogen, (2) addition of sweeping inert gas, such as steam, carbon dioxide, or nitrogen to reduce hydrogen concentration in the reactor exit gas, (3) use of air sweeping to maintain H₂ below the LFL, e.g., less than 4%, and/or (4) use of a membrane reactor to produce and separate hydrogen and oxygen simultaneously. However, the above methods have significant drawbacks and challenges in both economic terms and technical feasibility. Thus, the separation of oxygen and hydrogen at photocatalytic site remains costly and difficult.

[0005] An alternative to steam cracking of C₂+ hydrocarbons to produce ethylene is the oxidative dehydrogenation of ethane reaction. In an oxidative dehydrogenation of ethane (ODhE) reaction, C₂ to C₆ hydrocarbons can be contacted with an oxygen containing gas in a fluidized catalyst bed containing an ODE catalyst (See, for example U.S. Patent No. 5,639,929 to Bharadway, et al., and Bodke et al., Journal of Catalysis, 2000, 191 :62-74) to produce alkenes, hydrogen and water. The ODE reaction, with no external heating, requires an ethane to oxygen to hydrogen (C₂H₆:0₂:H₂) molar ratio of 2: 1 :2 to achieve an ethylene yield competitive to steam cracking. Reaction mixtures having this molar ratio can be dangerous because of the highly exothermic and spontaneous reaction of the hydrogen/oxygen components. Furthermore, these reactions suffer in that the addition of raw oxygen source to ODhE reactor is not economically feasible compared to the existing ethylene technology at the same product yield. [0006] While various methods to produce hydrogen and ethylene from sources other than steam reforming and steam cracking have been attempted, more efficient and economically feasible methods are needed.

SUMMARY

[0007] An alternative process and system for hydrogen and ethylene production that does not require fossil fuel {e.g., methane) has been discovered. The process and system integrates photocatalytic water splitting (PCWS) process with the oxidative dehydrogenation of ethane (ODhE) process to produce hydrogen and ethylene. Notably, the integration process provides ethane as a sweep gas to a PCWS reactor in amounts sufficient to reduce flammability of the H₂ and 0₂ and consume all oxygen present in an ODhE reactor, thereby avoiding the required separation of H₂ and 0₂ from the PCWS product stream. Such an integrated process provides a solution to many of the issues related to separation of H₂ and 0₂ produced by PCWS, the required feed composition for ethylene production, and the energy source for the dehydrogenation reaction.

[0008] In a particular aspect of the invention, an integrated process for the production of hydrogen and an alkene is described. The integrated process can include mixing an incombustible gaseous C₂ to C₅ alkane into a product gas derived from photocatalytic water splitting to form an oxidative dehydrogenation source gas, and introducing the oxidative dehydrogenation source gas into an oxidative dehydrogenation reactor where the oxidative dehydrogenation source gas is converted to an oxidative dehydrogenation product gas that includes the C₂ to C₅ alkane, a C₂ to C₅ alkene, hydrogen, water, and optional carbon oxides (e.g., carbon monoxide and/or carbon dioxide). In some embodiments, a supplemental gas source that includes an oxygen source and/or H₂ can be combined with the oxidative dehydrogenation source gas. The oxidative dehydrogenation product gas can be processed in a recovery unit to produce ethylene and hydrogen for use in other chemical reactions. The oxidative dehydrogenation product gas can be processed to remove water from the oxidative dehydrogenation product gas to produce a dried dehydrogenation product gas. The dried dehydrogenation product gas can be introduced into a gas separation unit that separates hydrocarbons from the dried dehydrogenation product gas producing a gaseous nonhydrocarbon stream that includes hydrogen and optional carbon oxides, and a gaseous hydrocarbon product that includes alkanes/alkene products. Introduction of the gaseous nonhydrocarbon stream to a hydrogen separation unit can produce a hydrogen product gas. Introduction of the gaseous hydrocarbon product stream into a hydrocarbon separation unit can produce an alkane product gas and an alkene product gas. The process can further include quenching the oxidative dehydrogenation product gas prior to drying and/or recycling unreacted alkane to be mixed with the product gas derived from photocatalytic water splitting to form an oxidative dehydrogenation source gas.

[0009] Systems for integrated photocatalytic water splitting and ethylene production are described. A system can include a photocatalytic water splitting unit configured for producing an oxidative dehydrogenation source gas, an ODhE unit configured for receiving the oxidative dehydrogenation source gas, and a recovery system configured to produce hydrogen product streams, alkene product streams and alkane product streams. In some embodiments, the system can include: a photocatalytic water splitting unit configured for mixing an alkane with the product gas from the photocatalytic water splitting reaction to form an oxidative dehydrogenation source gas; an oxidative dehydrogenation reactor configured to receive the oxidative dehydrogenation source gas from the photocatalytic water splitting unit and to perform an oxidative dehydrogenation reaction that converts (i) an alkane to an alkene and (ii) hydrogen and oxygen to water, forming oxidative dehydrogenation reaction products; and a recovery unit configured to receive the dehydrogenation reaction products and separate alkenes and H₂ from the oxidative dehydrogenation reaction products. The recovery unit can include a drying unit configured to receive the oxidative dehydrogenation reaction product and to dry the oxidative dehydrogenation reaction product, producing a dry dehydrogenation product gas; a compressor configured to receive the dry dehydrogenation product gas and to compress the dry dehydrogenation product gas; a gas separation unit configured to receive the compressed dry dehydrogenation product gas and to separate nonhydrocarbon products from hydrocarbon products and produce a gaseous nonhydrocarbon stream that can includes hydrogen and carbon oxides, and a gaseous hydrocarbon stream that can include an alkane/alkene mixture; a hydrogen separation unit configured to receive the gaseous nonhydrocarbon stream to produce a hydrogen product gas by isolating hydrogen from the gaseous nonhydrocarbon stream; a separation unit configured to receive the gaseous hydrocarbon stream and to separate the alkane from the alkene producing an alkane product stream and an alkene product stream or both. The system can further include a quenching unit configured to receive and cool the oxidative dehydrogenation product gas from the oxidative dehydrogenation reactor and/or a recycling pathway for unreacted alkane. The pathway being configured to include alkane produced from the separation unit into the oxidative dehydrogenation source gas.

[0010] In the integrated methods and/or systems of the present invention, the incombustible C₂ to C₅ alkane can include ethane, propane, butane and pentane, and the resulting alkene can be ethylene, propylene, butenes, butadiene, or pentene. A molar ratio of alkane to oxygen in the oxidative dehydrogenation source gas can be at least 2: 1, or at least 4: 1. In some instances, the oxidative dehydrogenation source gas can be heated prior to introduction into the ODhE reactor. In the ODhE reactor, the alkane can be converted to an alkene. In certain instances, the oxidative dehydrogenation source gas can be contacted with catalyst that includes a metal or compound thereof from Columns 5-1 1 of the Periodic Table, (e-g-, a Pt-Sn/ A1₂0₃ catalyst) to produce the alkene product. In certain instances, an oxygen source gas is provided to the ODhE reactor to facilitate propagation of heat for the oxidative hydrogenation of alkane reaction. Oxidative hydrogenation reaction conditions can include a temperature of about 800 °C to 1200 °C, preferably 960 °C and a pressure of about 0.01 MPa to 0.5 MPa, preferably 0.1 MPa. In some instances, the gas separation unit can be pressurized to a pressure (e.g., 1 MPa to 5 MPa) sufficient to separate the gases in the gas separation step.

[0011] The methods and systems of the present invention can produce a hydrogen product gas that includes at least 95% hydrogen and/or an alkene product that includes at least 85% alkene product.

[0012] 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. Furthermore, compositions or product streams of the invention can be used to achieve methods of the invention.

[0013] The following includes definitions of various terms and phrases used throughout this specification.

[0014] The phrase "carbon oxides" includes carbon monoxide, carbon dioxide, or a mixture thereof.

[0015] 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."

[0016] 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%.

[0017] The use of the term "or" in the claims is used to mean "and/or" unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and "and/or."

[0018] The terms "wt.%", "vol.%", or "mol.%" refers to a weight percentage of a component, a volume percentage of a component, 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.

[0019] The term "substantially" and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%. [0020] 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.

[0021] The term "effective," as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result. [0022] As used in this specification and claim(s), 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. [0023] The processes and systems 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 and systems of the present invention is their ability to integrate a photocatalytic water- splitting process with the oxidative dehydrogenation of alkanes to produce hydrogen and alkenes.

[0024] Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

DESCRIPTION OF TH E DRAWINGS

[0025] The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of the specification embodiments presented herein.

[0026] FIG. 1 is a schematic of the integrated system used to perform the integrated process of producing hydrogen and ethylene from a photocatalytic water-splitting process and an oxidative dehydrogenation of alkane process.

[0027] FIG. 2 is a schematic of the integrated system used to perform the integrated process of producing hydrogen and ethylene from a photocatalytic water-splitting process and an oxidative dehydrogenation of alkane process with an additional gas source stream.

[0028] FIG. 3 is a schematic an integrated photocatalytic water-splitting unit, an oxidative dehydrogenation of alkane unit, and a product recovery unit.

DESCRIPTION

[0029] The invention provides solutions to the problems associated with generating hydrogen and ethylene. The solution is premised on an integrated PCWS/ODhE process, which resolves the flammability risks associated with the H₂/0₂ effluent stream obtained from a PCWS reactor by sweeping ethane into the PCWS reactor. The PCWS reactor effluent from the PCWS contains the requisite amount of H₂, 0_2; and alkane to achieve a desired alkane conversion and alkene selectivity (e.g., 70% conversion of ethane and ethylene selectivity >80%) can be used as the feed source for the ODhE reaction. Furthermore, the exothermic H₂ and 0₂ reaction in the feed source can provide sufficient heat to enable the oxidation of the alkane to the alkene as shown below in reactions (1) and (2) for the oxidative dehydrogenation of ethane.

2H₂ + 0₂→ 2H₂0 (1)

C₂H₆→C₂H₄ + H₂ (2)

[0030] The integrated PCWS/ODhE process solves the problems associated with the flammability of the product stream from photocatalytic water splitting (PCWS) reactions and the explosive nature of an alkane/hydrogen/oxygen mixture for oxidative hydrogenation of alkane reactions. Notably, equivalent amounts of hydrogen and ethylene are produced as compared to PCWS and ODhE reactors operated independently, thereby, providing a viable alternative technology to methane steam reforming for hydrogen production and steam cracking for alkene production. [0031] FIG. 1 is a schematic of the integrated PCWS and ODhE processes. System 10 can include PCWS reactor 12 and ODhE reactor 14. In PCWS reactor 12, water can be split into H₂ and 0₂ through a photocatalytic reaction at ambient conditions using known PCWS methods. For example, International Application Publication No. WO 2016/110773 to Alghamdi et al. describes photocatalytic systems and methods. PCWS reactor 12 can have at least one side able to receive transmitted light from a light source 18 {e.g., sunlight). The interior of reactor 12 can be isolated from atmosphere. Liquid medium 16 {e.g., water/sacrificial agent mixture) can enter PCWS reactor 12 at a rate sufficient to sustain the water splitting reaction and produce the desired amount of H₂. For example, 2600 MT/day of the liquid medium {e.g., water/sacrificial agent mixture) can be feed to PCWS reactor 12. The liquid medium can be agitated continuously to promote escape of gases therefrom. Agitation can be accomplished by any suitable means, as by a magnetically driven armature, a mixer in the liquid, or fluid flow. Liquid medium 16 can include water and/or a sacrificial agent {e.g., alcohols, diols, amines or the like) and/or a water-dispersion of any suitable catalyst-containing photoactive material 20. The photocatalyst can include Column 8-11 metals on a semiconductor support. Non-limiting examples of Column 8-11 metals include ruthenium (Ru), rhodium (Rh), cobalt (Co), nickel (Ni), palladium (Pd), platinum (Pt), silver (Ag), copper (Cu) gold (Au), alloys thereof, or any combination thereof. Semiconductor supports can include titanium dioxide, zirconium dioxide. Non-limiting examples of sacrificial agents include ethanolamines, alcohols, methanol, ethanol, diols, polyols, dioic acids, or any combination thereof. Irradiation of the water in the presence of the photocatalyst can generate hydrogen and oxygen. Irradiation of water-sacrificial agent mixture in the presence of the photocatalyst can generate hydrogen oxygen an impurities.

[0032] An incombustible C₂ to C₅ alkane (C_2-5 alkane) stream 24 can enter the PCWS reactor 12 prior to, during, or after the water splitting reaction has been initiated as a sweep gas to carry the products {e.g., H₂ and 0₂) out of the PCWS reactor and into the ODhE reactor 14. The amount of alkane provided to PCWS reactor 12 can be sufficient to achieve at least 70%, at least 80%, at least 90% or 100% conversion of alkane in ODhE reactor 14 with a alkene selectivity of at least 80%, at least 90% or 100%. By way of example, 3600 MT/day of ethane can be provided to PCWS reactor 12 and 3600 MT/day of the ethane can be consumed in the ODhE reactor 14 to produce ethylene.

[0033] Sweep gas 26 exiting PCWS reactor 12 can include H₂, 0₂, C_2-5 alkane, water, and optionally, carbon oxides if a carbon based sacrificial agent is used, and can be used as the oxidative dehydrogenation source gas for the ODhE reactor. Oxidative dehydrogenation gas source 26 can, in some instances, be preheated prior to entering ODhE reactor 14. By way of example, oxidative dehydrogenation gas source 26 can flow through a heating unit (e.g., a heat exchanger) and be heated to a temperature approximate the temperature required to initiate the H₂ and 0₂ reaction. The heat generated from the H₂ and 0₂ reaction can be sufficient to drive the dehydrogenation of the alkane, and thus, no external heat is required. In some embodiments, preheating is not necessary. In certain embodiments, liquid hydrocarbons (e.g., naphtha) can be combined with the oxidative dehydrogenation gas source 26 at an ODhE reactor inlet or directly provided to the ODhE reactor 14. When liquid hydrocarbons are added to the ODhE reactor, the amount of 0₂ and H₂ provided to the ODhE reactor can be adjusted to provide adequate heat to promote the dehydrogenation reaction.

[0034] In ODhE reactor 14, oxidative dehydrogenation gas source 26 can contact a dehydrogenation catalyst to produce oxidative dehydrogenation product gas stream 28. Catalysts for oxidative dehydrogenation include metals that can form a metal-oxo intermediate, which includes metals or compounds thereof from Columns 5, 6, 7, 8, 9, 10, and/or 11 of the Periodic Table of Elements and/or any combination thereof. Moreover, these metals may be combined with any other element or elements that form an alloy that can form the metals-oxide (M=0) intermediate via oxidation under suitable conditions. The catalyst can be supported or unsupported. Supported catalysts can be supported on alumina, silica, titania or zirconia based supports, or combination thereof. Non-limiting examples of oxidative dehydrogenation catalysts include a Pt-Sn on alumina catalyst, Cr₂0₃/Al₂0₃-Zr0₂, V₂O₅/AI₂O₃, Fe-Cr₂/Zr0₂, or any mixture thereof. Oxidative dehydrogenation product gas stream 28 includes hydrocarbon products (e.g., C₂-5 alkane and/or C₂-5 alkenes) and nonhydrocarbon products (e.g., H₂, water, and optionally carbon oxides). Notably, ODhE reactor 14 produces the same, or substantially the same, amount of H₂ that is consumed by the H₂ and 0₂ combustion reaction, while the 0₂ is fully, or substantially, consumed. Since a molar excess of C₂-5 alkane is used, only sufficient heat from the exothermic combustion reaction is produced to convert a portion of the C₂-5 alkane to the corresponding C₂-5 alkene (e.g., about 1.4 mole of ethane can be converted to ethylene) based on the composition of the oxidative hydrogenation gas source 26 as it enters the ODhE reactor 14 (e.g., inlet composition). Oxidative dehydrogenation conditions can include temperature and pressure. The operating average temperature can range from 800 to 1200 °C, 850 to 1150 °C, 900 to 1100 °C, 950 to 1000 °C any range or value there between (e.g., 800 °C, 810 °C, 820 °C, 830 °C, 840 °C, 850 °C, 860 °C, 870 °C, 880 °C, 890 °C, 900 °C, 910 °C, 920 °C, 930 °C, 940 °C, 950 °C, 960 °C, 970 °C, 980 °C, 990 °C, 1000 °C, 1010 °C, 1020 °C, 1030 °C, 1040 °C, 1050 °C, 1060 °C, 1070 °C, 1080 °C, 1090 °C, 1 100 °C, 1 1 10 °C, 1 120 °C, 1 130 °C, 1 140 °C, 1 150 °C, 1 160 °C, 1 170 °C, 1 180 °C, 1 190 °C, and 1200 °C). Average operating pressures can include 0.05 to 0.5 MPa, 0.1 to 0.4 MPa, and 0.2 to 0.3 MPa.

[0035] Oxidative dehydrogenation product gas stream 28 can exit ODhE reactor 14 and enter quenching unit 30. In quenching unit 30, oxidative dehydrogenation product gas stream 28 can be cooled to a temperature below the boiling point of water to condense any liquid (e.g., water) from the gas stream to produce cooled oxidative dehydrogenation product gas stream 32. Quenching unit 30 can be any known unit capable of liquid/gas separation (e.g., a knock-out pot). Cooled oxidative dehydrogenation product gas stream 32 can exit quenching unit 30 and enter drying unit 34. In drying unit 34, cooled oxidative dehydrogenation product gas stream 32 can be subjected to drying conditions that remove all, or substantially all, of the water from the gas stream 32 to produce dried oxidative dehydrogenation product gas stream 36 and liquid (e.g., water) stream 38. Water obtained from quenching unit 30 and/or drying unit 34 can be recycled or reused after any necessary treatment.

[0036] Dried oxidative dehydrogenation product gas stream 36 can be pressurized in compressor unit 40 to produce compressed gas stream 42, which can enter separation unit 44. Separation unit 44 can be any known separation unit capable of separating hydrocarbons from non-hydrocarbons (e.g., membrane system, vacuum and/or pressure swing adsorption systems). In separation unit 44, the dried oxidative dehydrogenation product gas stream 36 can be separated into hydrocarbon stream 46 and gaseous nonhydrocarbon stream 48.

[0037] Hydrocarbon stream 46 can include C2-5 alkane and C2-5 alkenes, and can enter hydrocarbon separation unit 50. Hydrocarbon separation unit 50 can be any known separation unit capable of separating alkanes from alkenes (e.g., membrane systems and/or distillation systems). In hydrocarbon separation unit 50, the hydrocarbon stream 46 can be separated into a C2-5 alkane stream 52 and a C2-5 alkene product stream 54. Portions of C2-5 alkane stream 52 can be combined with incombustible C2-5 alkane stream 24, provided directly to PCWS reactor 12 (not shown) or provided to ODhE reactor 14 as C2-5 alkane stream 62. C2-5 alkene product stream 54 can be collected, stored, sold, transported, or provided directly to other processing units. [0038] Nonhydrocarbon stream 48 can include H₂ and optional carbon oxides, and can enter H₂ purification unit 56. H₂ purification unit 56 can be any unit (e.g., membrane unit, pressure swing adsorption unit, and the like) capable of separating H₂ from other gaseous components (e.g., CO and C0₂). In H₂ purification unit 56, H₂ can be purified to produce H₂ product stream 58 and nonhydrocarbon gaseous stream 59. H₂ product stream 58 can be at least 80 mol% H₂, at least 99 mol% H₂, at least 99 mol% H₂, or 100 mol% H₂, based on the total moles of product in the stream. The H₂ can be transported, collected, sold, or used directly in other processing units. In some embodiments, excess H₂ stream 64 can be produced from PCWS reactor 12 and combined with H₂ product stream 58. In the context of this invention, excess H₂ produced from the PCWS reactor is more hydrogen than the amount required to sustain the reaction performed in ODhE reactor 14. Nonhydrocarbon gaseous stream 60 can include carbon oxides and H₂, and can be recycled to H₂ purification unit 56 to recover additional H₂, collected, or provided to other processing units.

[0039] In some embodiments, oxidative dehydrogenation gas source 26 is oxygen lean and/or hydrogen lean (i.e., does not include a sufficient amount of oxygen and/or hydrogen to initiate the H₂ and 0₂ reaction in ODhE reactor 14). FIG. 2 is a schematic of the integrated process of system 10 that include additional gaseous oxygen and/or hydrogen feed sources for the ODhE reactor. As shown in system 10 of FIG. 2, supplemental gas source 66 that includes an oxygen source and/or H₂ can be provided to the ODhE reactor 14. Gaseous oxygen source can be air, oxygen enriched air, or gaseous 0_2.

[0040] FIG. 3 depicts a schematic of system 10 that includes an integrated process of the PCWS process, the ODhE process, and a recovery process. As shown in FIG. 3, PCWS recovery unit 66 is combined with the integrated PCWS and ODhE process. In system 10, gaseous oxidative dehydrogenation product stream 28 can enter product recovery unit 66. Product recovery unit 68 can include the necessary units to produce pure hydrogen stream 70, alkene product stream 72, alkane recycle stream 52, and water stream 74. By way of example, recovery unit 68 can include drying unit 34, separation units 44 and 50, and hydrogen purification unit 56. In embodiments when carbon oxides are produced, a carbon oxides stream can be separated from the gaseous nonhydrocarbon stream in recovery unit 68. Examples

[0041] The present invention will be described 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.

[0042] Table 1 presents compositions of the streams of FIG. 1 for the entire process based on a hypothetical production of one million metric tons of ethylene and 76 thousands metric ton of hydrogen annually.

Tab e 1

Claims

An integrated process for the production of hydrogen and an alkene, the integrated process comprising

mixing an incombustible gaseous C₂ to C₅ alkane into a product gas derived from photocatalytic water splitting to form an oxidative dehydrogenation source gas;

introducing the oxidative dehydrogenation source gas into an oxidative dehydrogenation reactor where the oxidative dehydrogenation source gas is converted to an oxidative dehydrogenation product gas that comprises the C₂ to C₅ alkane, a C₂ to C₅ alkene, water, carbon oxides, and hydrogen;

processing the oxidative dehydrogenation product gas to remove water from the oxidative dehydrogenation product gas to produce a dried dehydrogenation product gas;

introducing the dried dehydrogenation product gas into a gas separation unit that separates hydrocarbons from the dried dehydrogenation product gas producing a gaseous nonhydrocarbon stream comprising hydrogen and carbon oxides, and a gaseous hydrocarbon product comprising alkanes/alkene products;

introducing the gaseous nonhydrocarbon stream to a hydrogen separation unit that produces a hydrogen product gas; and

introducing the gaseous hydrocarbon product stream into a separation unit that produces an alkane product gas and an alkene product gas.

The process of claim 1, wherein the alkane is ethane.

The process of claim 1, wherein the alkene is ethylene.

The process of claim 1, wherein the molar ratio of alkane to oxygen in the oxidative dehydrogenation source gas is at least 2: 1.

The process of claim 1, wherein the molar ratio of alkane to oxygen in the oxidative dehydrogenation source gas is at least 4: 1.

The process of claim 1, wherein the alkane is a liquid alkane.

The process of claim 1, wherein the oxidative dehydrogenation reactor converts the alkane to alkene at conversion rate of about 70% and an alkene selectivity of greater than 80%.

8. The process of claim 1, wherein the oxidative dehydrogenation reactor comprises a catalyst comprising a metal or compound thereof from Columns 5-11 of the Periodic Table.

9. The process of claim 1, further comprising heating oxidative dehydrogenation source gas prior to introduction into the oxidative dehydrogenation reactor.

10. The process of claim 1, wherein conversion of the oxidative dehydrogenation source gas to an oxidative dehydrogenation product gas is performed at about 800 to 1200 °C, preferably 960 °C at a pressure of about 0.01 MPa to 0.5 MPa, preferably 0.1 MPa.

11. The process of claim 1, wherein the processing the oxidative dehydrogenation product gas includes quenching the oxidative dehydrogenation product gas prior to drying.

12. The process of claim 1, wherein the gas separation step is performed at about 1 MPa to 5 MPa.

13. The process of claim 1, further comprising recycling unreacted alkane to be mixed with the product gas derived from photocatalytic water splitting to form an oxidative dehydrogenation source gas.

14. The process of claim 1, further comprising providing an oxygen source to the

oxidative dehydrogenation reactor.

15. A hydrogen product gas produced by the method of claim 1 comprising at least 95% hydrogen.

16. An alkene product produced by the method of claim 1 comprising at least 85% alkene product.

17. A system for integrated photocatalytic water splitting and ethylene production, the system comprising:

a photocatalytic water splitting unit configured for mixing an alkane with a

product gas from a photocatalytic water splitting reaction to form an oxidative dehydrogenation source gas;

an oxidative dehydrogenation reactor configured to receive the oxidative

dehydrogenation source gas from the photocatalytic water splitting unit and to perform an oxidative dehydrogenation reaction that converts (i) an alkane to an alkene and (ii) hydrogen and oxygen to water, forming oxidative dehydrogenation reaction products; and

a recovery unit configured to receive the dehydrogenation reaction products and separate alkenes and H₂ from the oxidative dehydrogenation reaction products.

The system of claim 17, wherein the alkane is ethane and the alkene is ethylene.

The system of claim 17, further comprising a quenching unit configured to receive and cool the oxidative dehydrogenation product gas from the oxidative

dehydrogenation reactor.

The system of claim 17, further comprising a recycling pathway for unreacted alkane, the pathway configured to include alkane produced from the separation unit into the oxidative dehydrogenation source gas.