Process for separation of hydrogen and oxygen

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.

Claims

1. A process for separating hydrogen and oxygen from a gas mixture comprising: separating the hydrogen and the oxygen at about atmospheric pressure by passing the feed source through an adsorption medium comprising a carbon molecular sieve that selectively adsorbs hydrogen from the feed source producing an enriched oxygen product gas and an adsorption medium enriched in hydrogen; and desorbing the adsorbed hydrogen by exposing the hydrogen enriched adsorption medium to a vacuum forming a hydrogen enriched purge gas and a regenerated adsorption medium.

2. A process for separating hydrogen and oxygen comprising: providing a gas mixture feed comprising oxygen and hydrogen; passing the gas mixture feed through an adsorption medium that selectively adsorbs hydrogen from the gas mixture feed producing a hydrogen enriched adsorption medium; desorbing the adsorbed hydrogen by exposing the hydrogen enriched adsorption medium to a vacuum forming a hydrogen enriched purge gas and a regenerated adsorption medium, and injecting a hydrogen product gas to the gas mixture feed.

3. The process of claim 1, wherein the adsorption medium further comprises a zeolite.

4. The process of claim 1, wherein the adsorption process is performed at 15 to 60 C.

5. The process of claim 1, wherein the desorption process is performed at about 15 to 60 C.

6. The process of claim 1, wherein the adsorption process is performed at a pressure of about 0.8 to 1.2 bar.

7. The process of claim 1, wherein the adsorption selectively adsorbs hydrogen at about atmospheric pressure and desorbs hydrogen at a pressure below 0.5 bar.

8. The process of claim 1, wherein the adsorption medium further comprises a zeolite.

9. The process of claim 8, wherein the zeolite is a 5 angstrom zeolite.

10. The process of claim 1, wherein the adsorption medium further comprises a binder.

11. The process of claim 8, wherein the adsorption medium further comprises a binder.

12. The process of claim 2, wherein the adsorption medium comprises a zeolite.

13. The process of claim 2, wherein the adsorption process is performed at 15 to 60 C.

14. The process of claim 2, wherein the desorption process is performed at about 15 to 60 C.

15. The process of claim 2, wherein the adsorption process is performed at a pressure of about 0.8 to 1.2 bar.

16. The process of claim 2, wherein the adsorption selectively adsorbs hydrogen at about atmospheric pressure and desorbs hydrogen at a pressure below 0.5 bar.

17. The process of claim 14, wherein the adsorption medium comprises a zeolite.

18. The process of claim 17, wherein the zeolite is a 5 angstrom zeolite.

19. The process of claim 2, wherein the adsorption medium further comprises a binder.

20. The process of claim 2, wherein the desorption process is performed at about 60 C.

Description

DESCRIPTION OF THE DRAWINGS

(1) 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.

(2) FIG. 1. A block diagram of one embodiment of a hydrogen purification system disclosed herein.

DESCRIPTION

(3) 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 that produced by the photocatalytic water splitting process. The solution separates a gas containing hydrogen and oxygen produced by a photocatalytic water-splitting reaction using a vacuum swing adsorption (VSA) process.

(4) I. Photocatalytic Water Splitting

(5) Photocatalytic water splitting is the light-induced conversion reaction of water to hydrogen and oxygen. This reaction has attracted attention as one of the most promising hydrogen production processes. Photocatalytic water splitting is an artificial process for the dissociation of water into its constituent parts, hydrogen (H.sub.2) and oxygen (O.sub.2), using either artificial or natural light without producing greenhouse gases or having many adverse effects on the atmosphere. When H.sub.2O is split into O.sub.2 and H.sub.2, the stoichiometric ratio of its products is 2:1.

(6) There are several requirements for a photocatalyst to be useful for water splitting. The minimum potential difference (voltage) needed to split water is 1.23V 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. These values are true only for a completely reversible reaction at standard temperature and pressure (1 bar and 25 C.). 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 3.0V to make efficient use of the energy present across the full spectrum of sunlight. Water splitting can transfer charges, but not be able to avoid corrosion for long term stability. Defects within crystalline photocatalysts can act as recombination sites, ultimately lowering efficiency.

(7) 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 (TiO.sub.2). However, due to the relatively positive conduction band of TiO.sub.2, there is little driving force for H.sub.2 production, so TiO.sub.2 is typically used with a co-catalyst such as platinum (Pt) to increase the rate of H.sub.2 production. It is routine to add co-catalysts to spur H.sub.2 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.

(8) Photocatalysts can suffer from catalyst decay and recombination under operating conditions. In certain aspects catalyst decay becomes a problem when using a sulfide-based photocatalyst such as cadmium sulfide (CdS), as the sulfide in the catalyst is oxidized to elemental sulfur at the same potentials used to split water. Thus, sulfide-based photocatalysts are not viable without sacrificial reagents such as sodium sulfide to replenish any sulfur lost, which effectively changes the main reaction to one of hydrogen evolution as opposed to water splitting. Recombination of the electron-hole pairs needed for photocatalysis can occur with any catalyst and is dependent on the defects and surface area of the catalyst; thus, a high degree of crystallinity is required to avoid recombination at the defects.

(9) Examples of photocatalyst include, but are not limited to NaTaO.sub.3:La, K.sub.3Ta.sub.3B.sub.2O.sub.12, (Ga.sub..82Zn.sub..18)(N.sub..82O.sub..18), Pt/TiO.sub.2, and Cobalt based systems.

(10) NaTaO.sub.3:LaNaTaO.sub.3:La yields a high water splitting rate of photocatalysts without using sacrificial reagents. The nanostep structure of the material promotes water splitting as edges functioned as H.sub.2 production sites and the grooves functioned as O.sub.2 production sites. Addition of NiO particles as co-catalysts assisted in H.sub.2 production; this step can be done by using an impregnation method with an aqueous solution of Ni(NO.sub.3).Math.6H.sub.2O and evaporating the solution in the presence of the photocatalyst.

(11) K.sub.3Ta.sub.3B.sub.2O.sub.12K.sub.3Ta.sub.3B.sub.2O.sub.12 is activated by UV light and above, does not have the performance or quantum yield of NaTaO.sub.3:La. However, it does have the ability to split water without the assistance of co-catalysts. This ability is due to the pillared structure of the photocatalyst, which involves TaO.sub.6 pillars connected by BO.sub.3 triangle units.

(12) (Ga.sub..82Zn.sub..18)(N.sub..82O.sub..18)(Ga.sub..82Zn.sub..18)(N.sub..82O.sub..18) has a high quantum yield in visible light for visible light-based photocatalysts that do not utilize sacrificial reagents. Tuning the catalyst is done by increasing calcination temperatures for the final step in synthesizing the catalyst. Temperatures up to 600 C. helped to reduce the number of defects, though temperatures above 700 C. can destroy the local structure around zinc atoms.

(13) Pt/TiO.sub.2TiO.sub.2 is a very efficient photocatalyst, as it yields both a high quantum number and a high rate of H.sub.2 gas evolution, e.g., Pt/TiO.sub.2 (anatase phase). These photocatalysts combine with a thin NaOH aqueous layer to make a solution that can split water into H.sub.2 and O.sub.2. TiO.sub.2 absorbs only ultraviolet light due to its large band gap (>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.

(14) Cobalt based systemsPhotocatalysts based on cobalt have been reported. Members are tris(bipyridine) cobalt(II), compounds of cobalt ligated to certain cyclic polyamines, and certain cobaloximes. Chromophores have reportedly been connected to part of a larger organic ring that surrounded a cobalt atom. The process is less efficient than using a platinum catalyst, but cobalt is less expensive, potentially reducing total costs. The process uses one of two supramolecular assemblies based on Co(II)-templated coordination as photosensitizers and electron donors to a cobaloxime macrocycle.

(15) II. Hydrogen Purification

(16) The gas produced from the photocatalytic water-splitting process is at or near atmospheric pressure (about 1 bar) and can contain about 70% mol H.sub.2, 25% mol 02 and 5% mol CO.sub.2. This gas can be used as a feed source for processing by VSA.

(17) A feed source comprising H.sub.2 and O.sub.2 is sent to a vacuum swing adsorption (VSA) separation unit. VSA is a non-cryogenic gas separation technology. Using special solids, or adsorbents, VSA segregates certain gases from a gaseous mixture under minimal pressure according to the species' molecular characteristics and affinity for the adsorbents. These adsorbents (e.g., CMS or zeolites) form a molecular sieve and preferentially adsorb the target gas species at near ambient pressureproducing a product gas. Once a certain volume or mass of feed source has been processed, a vacuum is applied to the adsorbent to regenerate the adsorbent material and alternatively capture the adsorbed molecules.

(18) VSA differs from cryogenic distillation techniques of gas separation as well as pressure swing adsorption (PSA) techniques because it operates at near-ambient temperatures and pressures. VSA differs primarily from PSA in that PSA typically vents to atmospheric pressures, and uses a pressurized gas feed in the separation process. VSA typically draws the gas through the separation process with a vacuum or blower. Hybrid VPSA systems also exist. VPSA systems apply pressurized gas to the separation process and also apply a vacuum for desorption.

(19) Certain embodiments of a VSA process can include passing a feed source through an adsorption vessel and collecting an enriched product gas, and regenerating the adsorbing material or adsorption medium by exposing the adsorption medium to a vacuum. A feed source can be passed through an adsorption vessel and adsorption zone at about a pressure of approximately 0.8 to 1.2 bar, the adsorption zone can contain an adsorbent selective to retain oxygen, wherein the oxygen is adsorbed and hydrogen gas passes through the zone essentially unadsorbed producing a hydrogen enriched product gas, or vice versa. When needed, the adsorption material can be regenerated by evacuating the adsorption vessel to a predetermined reduced pressure at which point the oxygen is desorbed and collected as a purge gas from the adsorption vessel. After application of a vacuum for a sufficient time the vacuum is released and the adsorption vessel is brought back to an appropriate feed pressure and the process of adsorption is repeated.

(20) The adsorbent of the present invention can be incorporated into solid particles in which the adsorbent is present in an amount effective to promote the desired separation. Solid particles comprising the molecular sieve and a binder may be formed into shapes such as pills, pellets, granules, rings, spheres, etc. Generally, a slurry mixture comprising the molecular sieve, binder, and water is prepared and the slurry mixture is blended by sonification, milling, etc. prior to formation of the solid particles by extrusion, spray-drying, etc. In one aspect, the solid particles comprise an adsorptively effective amount of the adsorbent and at least one matrix material, preferably selected from the group consisting of binder materials, filler materials, and mixtures thereof to provide a desired property or properties, e.g., desired adsorbent dilution, mechanical strength, and the like to the solid particles. Filler and binder materials include, for example, synthetic and naturally occurring substances such as metal oxides, clays, silicas, aluminas, silica-aluminas, silica-magnesias, silica-zirconias, silica-thorias, silica-berylias, silica-titanias, silica-alumina-thorias, silica-alumina-zirconias, aluminophosphates, mixtures of these, and the like. The preparation of solid particles comprising adsorbent and matrix materials is conventional and well known in the art and, therefore, need not be discussed in detail.

(21) FIG. 1 illustrates a flow diagram for one embodiment of the system. FIG. 1 illustrates a scheme where reactants (e.g., water and sacrificial agent) and catalyst are provided in reactor 100 where photocatalytic water splitting is performed. Once the water is split the resulting gas is transferred to absorber (VSA) vessel 102 via blower 104. In absorber vessel 102 the feed source is exposed to a adsorption medium that selectively adsorbs oxygen or hydrogen. The product gas (hydrogen or oxygen) can be recycled via blower 106 or further processed. The absorbate is collected as a purge gas by applying a vacuum to the absorber vessel to desorb the adsorbed gas. In one embodiment blower 104 or 106 can be used to move a source feed gas through the absorber vessel or apply a vacuum to the absorber vessel to desorb and regenerate the adsorption medium. Product gas or purge gas can be transferred to one or more buffer tank for storage or further processing needs. In one embodiment a hydrogen product gas can be injected into the feed stream to modulate the explosivity/flammability of the feed source.

(22) One embodiment of the vacuum swing adsorption process can comprise one or more vacuum swing adsorption cycles being performed concurrently in 2 or more adsorption vessels, and in certain aspects asynchronously (i.e., each concurrent process being at different stagesadsorption, purge, etc.) that can be employed to purify the feed source to produce a high purity hydrogen product. Included in certain aspects of a vacuum swing adsorption cycle is an adsorption step, a vacuum purge step, and a repressurization step. An adsorption bed receives a feed source and undergoes an adsorption step to adsorb a feed source component on the adsorbent producing a product gas. At the conclusion of the adsorption step, prior to breakthrough, the adsorption bed can purged with a co-purge stream such as an adsorbent gas or inert gas purge to force non-adsorbed gas from the void spaces of the adsorption bed. The adsorption bed undergoes a depressurization step to a purge pressure or vacuum and a purge stream is withdrawn. The adsorption bed can be evacuated to a pressure of about 0.5 psi. The adsorption bed be repressurized or allowed to return to ambient pressure. The adsorption and desorption process can be repeated.

(23) The examples as well as the figures 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.