Patent Application
20080019903
The present invention relates generally to a process and an apparatus for producing concurrently hydrogen and metal oxide nanoparticles and to a process and an apparatus for producing ammonia.
It is known to produce hydrogen by reacting metals with steam. U.S. Pat. No. 2,635,947 discloses a process in which metal particles are reacted in a fluidized bed with steam to generate hydrogen. However, the efficiency of such a method is limited by the available specific surface area, since the oxidation of the metal to the metal oxide occurs principally on the surface of the metal particles.
It has also been proposed to generate hydrogen by bubbling steam through a bath of molten zinc (Berman A., Epstein M., The Kinetics of Hydrogen Production in the Oxidation of Liquid Zinc with Water Vapor. Int J. Hydrogen Energy 2000; 25:957-67). However, the feeding of reactants and removal of products—especially the feeding of molten Zn and the removal of solid ZnO formed inside the Zn (liquid) bath—poses a technological difficulty for a continuous mode of operation. Furthermore, the reaction rate is inhibited by the formation of a ZnO (solid) layer around the steam bubbles. Further complications arise during start-ups and shutdowns because of the volume expansion and compression during zinc phase change.
A process circumventing some of these problems has been proposed in U.S. Pat. No. 3,928,550. Metallic zinc is combined with molten lead. The mixture is then sprayed under pressure into an oxidizer. There, the zinc is reacted with steam to yield hydrogen and zinc oxide. Again, this method may suffer from reduced efficiency due to the low specific surface area of the lead/zinc droplets and from the formation of a ZnO (solid) layer on the surface of the droplets.
In U.S. Pat. No. 4,496,370 a process has been proposed in which zinc vapor is directly reacted with steam to yield zinc oxide and hydrogen. Such a reaction, however, is heterogeneous in nature and generally requires the presence of some surface, and may lead to undesired deposits of solid zinc and zinc oxide on the reactor walls, as the walls provide such a surface.
Conventional production of ammonia employs the Haber-Bosch process. An alternative is a two-step process, in which first a metal nitride is produced, which then is reacted with water to form ammonia. For example, it is known from WO 02/090255 to produce ammonia by reacting aluminum nitride with water. This process may suffer from low efficiency and yield due to passivation of the aluminum nitride particles.
It is an object of the present invention to provide a process for producing hydrogen which avoids the above-mentioned problems and enables high yields. This object is achieved by a process with the features of claim 1.
It is a further object of the present invention to provide a process for simultaneous production of metal oxide nanoparticles along with hydrogen. This object is achieved by a process with the features of claim 10.
It is a further object of the present invention to provide a process for producing ammonia which avoids the above-mentioned problems and enables high yields. This object is achieved by a process with the features of claim 12.
It is a further object of the present invention to provide apparatus for the production of hydrogen or ammonia and metal oxide nanoparticles which is suitable for carrying out the processes of the present invention. This object is achieved by an apparatus with the features of claims 15 or 24.
Both the processes for producing hydrogen and ammonia employ nanoparticles and/or nanodroplets comprising a metal or metal-containing compound, which are reacted with steam (i.e., water in the gas phase, water vapor). Specifically, for the production of ammonia, the metal-containing compound is a metal nitride. Nanoparticles and/or nanodroplets are advantageous because of their large specific surface area, which leads to an efficient reaction and high yields. The specific surface area of the nanoparticles and/or nanodroplets of the metal or metal-containing compound is significantly larger than for particles usually employed, e.g., in fluidized-bed reactors. Preferably, the nanoparticles and/or nanodroplets entirely consist of the metal or metal-containing compound, without admixtures or a carrier substance. However, the use of nanoparticles or nanodroplets comprising a carrier or admixtures is also within the scope of the present invention.
For the purposes of this document, “nanoparticles” are to be understood as solid particles having a BET surface area larger than 1 square meter per gram (m2/g). The term “nanoparticles” is thus defined by reference to a bulk property of a large collection (ensemble) of individual particles. Preferably, the BET surface area is larger than approximately 5 m2/g. More preferably, it is larger than 10 m2/g, and even more preferably larger than 20 m2/g.
For the purposes of this document, the term “BET surface area” is to be understood as the specific surface area as determined by gas adsorption and as defined in ISO 9277 (“Determination of the specific surface area of solids by gas adsorption using the BET method”). More specifically, the BET surface area is the specific surface area as determined by nitrogen adsorption at 77 K employing the BET isotherm. For droplets (including nanodroplets), the BET surface area is to be understood as the BET surface area of the particles which are obtained after solidification of the droplets by rapid cooling without fusion of droplets. Thus, the term “nanodroplets” is to be understood as droplets of liquid whose solidification by rapid cooling without fusion yields nanoparticles in the sense of the above definition.
Assuming monodispersed spherical particles, the BET surface area, hereinafter referred to with the symbol SSA, is related to the average particle size d by the formula: d[μm]=6/(ρ[g/cm3]×SSA[m2/g]). Here, ρ is the density of the solid material. Hence, the definition of nanoparticles and nanodroplets by their BET surface area can directly be translated into a definition by average particle size, provided the density of the material is known.
For producing hydrogen, suitable metals or metal-containing compounds are metals or compounds which have a negative Gibbs free energy for the hydrolysis reaction. In particular, the metal is preferably selected from the group consisting of Zn (zinc), Fe (iron), Mg (magnesium), Mn (manganese), Al (aluminum), Ti (titanium), Si (silicon), and Ca (calcium). For the purposes of this document, the semi-metal silicon (Si) is to be understood as a metal. Preferred is Zn, Fe or Mg, most preferred is Zn. The metal-containing compound is preferably a pure or mixed low-valence metal oxide. The term “low-valence metal oxide” is to be understood as meaning a metal oxide in which at least one metal component is in a oxidation state lower than its maximum oxidation state. Suitable pure metal oxides include FeO, SnO, TiO and TiOx where x represents a real number with 1<x<2, e.g. Ti2O3. Suitable mixed metal oxides include CoFexOy, MgFexOy, ZnFexOy, CdFexOy, MnFexOy, NiFexOy, InFexOy, SnFexOy, NiMnzFexOy, where x, y, and z represent real numbers and where at least one of the metal components is in an oxidation state below its maximum oxidation state. For producing ammonia, the metal nitride is preferably selected from the group consisting of aluminum nitride (AlN), silicon nitride (Si3N4), iron nitride (Fe2N), and titanium nitride (TiN). Preferred is aluminum nitride (AlN). The nanoparticles and/or nanodroplets may be provided in an immobilized form or in form of an aerosol. The aerosol form is preferred.
The steam may be provided to the reaction zone by generating steam remote from the reaction zone and injecting steam into the reaction zone or its vicinity, or by feeding liquid water into the reaction zone or its vicinity. The liquid water is then evaporated within the reaction zone or its vicinity to form steam because of the elevated temperatures generally present in the reaction zone and its vicinity. Of course, a combination of ex-situ and in-situ generation of steam is also possible.
Preferably, the nanoparticles and/or nanodroplets and the steam are continuously fed into the reaction zone as streams, and any products of the reaction as well as any unreacted reactants are continuously removed from the reaction zone as a stream. In this way, a continuous process can be accomplished.
An advantage of the proposed processes for either hydrogen or ammonia production is that they can be carried out at moderate pressures. Advantageously, the process is carried out at a pressure below 10 bar, preferably below 2 bar, most preferred at ambient pressure or at less than 100 mbar above ambient pressure.
A very simple and highly efficient process results by generating the nanoparticles and/or nanodroplets in situ, i.e. in the reaction zone or in the vicinity of the reaction zone. Thus, the nanoparticles and/or nanodroplets may be generated in a formation zone upstream from or overlapping with said reaction zone by feeding a stream of a vapor of the metal or metal-containing compound into the formation zone, and cooling the vapor of the metal or metal-containing compound in the formation zone under conditions adapted for obtaining nanoparticles and/or nanodroplets. The nanoparticles/nanodroplets may be formed homogeneously or heterogeneously on seeds provided to the reaction chamber from the outside. In particular, if the metal is Zn, seeds can be created be suspending a few tiny ZnO nanoparticles prior to the stream which contains the Zn vapor entering the reaction zone and contacting the steam. Alternatively, ZnO seed nanoparticles can be created in situ by sending a small oxygen flow to the Zn vapor before the latter enters the reaction zone and contacts the steam.
If nanoparticles are formed in situ, it may be advantageous to feed additional steam to the formation zone which is cooler than the vapor of the metal or metal-containing compound when being fed to the formation zone, in order to aid in rapid cooling (“quenching”) of the vapor of the metal or metal-containing compound. It may be particularly advantageous to feed liquid water into the formation zone or into its vicinity. The water will then be evaporated in the formation zone or its vicinity to form steam. The heat of evaporation required for this will additionally aid in the cooling of the vapor of the metal or metal-containing compound.
Preferably, the stream of the vapor of the metal or metal-containing compound, before feeding the vapor into the formation zone, is at a temperature and pressure such that the temperature is above the saturation temperature of the vapor. In addition, it is preferred that, in the formation zone, the vapor is cooled to a temperature below the saturation temperature. Preferably, the cooling rate exceeds approximately 5000 K/s.
Alternatively, the nanoparticles and/or nanodroplets may be generated in a formation zone upstream from or overlapping with said reaction zone by feeding a stream of a precursor of the metal or metal-containing compound into the formation zone, feeding a stream of a reactant gas suitable for reacting with the precursor to yield the metal or metal-containing compound into the formation zone, and reacting the precursor and the reactant gas under conditions suitable to yield nanoparticles and/or nanodroplets comprising the metal or metal-containing compound. The precursor may itself be a metal or a metal-containing compound, preferably it is a metal. Advantageously, the precursor is provided as a vapor. The term “reactant gas” is to be understood as a gas or gas mixture suitable to react with the precursor. Specifically, the reactant gas may be a nitriding gas. The term “nitriding gas” is to be understood as a gas or a gas mixture containing nitrogen and/or a nitrogen-containing compound suitable to react with the precursor to yield a metal nitride. Preferably, the nitriding gas comprises nitrogen (N2).
Specifically, in the case of ammonia production, the precursor may be a metal, e.g., aluminum, preferably in the vapor state. Alternatively, the precursor may be a metal oxide, e.g. aluminum oxide (Al2O3), and the nitriding gas may additionally comprise a reducing agent, e.g. carbon, methane, other hydrocarbons or biomass. The conditions in the formation zone are then chosen such that a reaction of the metal oxide with the nitriding gas mixture occurs, which yields the metal nitride. The metal oxide may be provided in the form of nanoparticles or nanodroplets, or nanoparticles and/or nanodroplets of the metal nitride may be formed in situ.
A side product of the proposed processes for producing hydrogen are metal oxide nanoparticles. Of course, these may as well be considered the main product rather than a side product. There is, e.g., a large market for such nanoparticles in the rubber industries, where they may be used, e.g., for improving the properties of car tires. In this sense, the proposed processes for producing hydrogen may as well be considered as processes for producing metal oxide nanoparticles. In particular, the present invention provides a process for producing such nanoparticles (along with hydrogen) from a metal vapor and water/steam, the process comprising the steps of
Of course, any considerations made in connection with the process for producing hydrogen equally well apply also to the process for producing metal oxide nanoparticles.
This process is particularly advantageous if the metal is Zn and the product of the process are ZnO nanoparticles.
The stream of the vapor of the metal, metal-containing substance or precursor may comprise a carrier gas. Likewise, any stream of steam or of a reactant gas may comprise a carrier gas. Advantageously, the carrier gas is substantially inert under the conditions of the process. The carrier gas may be a noble gas, specifically, argon.
Generally, the reaction of the steam with the metal or metal-containing compound is exothermic. Thermal energy released by this reaction may advantageously be employed for heating any of the streams fed to the formation or reaction zone (i.e., for heating any of the streams of steam, of the vapor of the metal, metal-containing compound or precursor, or of any other reactant gas, with or without carrier gases). Any thermal energy supplied to the process, e.g. for heating of the streams, may advantageously be provided by concentrated solar energy, nuclear process heat or by combustion of a portion of the hydrogen generated.
An inventive apparatus comprises a reaction chamber with at least one first inlet for nanoparticles and/or nanodroplets comprising a metal or of a metal-containing compound or for a metal vapor, at least one second inlet for steam or liquid water, and at least one outlet for products of the reaction. It further comprises a mixing zone adapted for mixing the streams entering through the first and second inlets, and a reaction zone downstream from and/or overlapping with the mixing zone adapted for reacting nanoparticles and/or nanodroplets with steam.
If nanoparticles are generated in situ in the reaction chamber, the reaction chamber further comprises a particle/droplet formation zone downstream from and/or overlapping with the mixing zone and upstream from and/or overlapping with the reaction zone. The particle/droplet formation zone is adapted for forming nanoparticles and/or nanodroplets from the metal vapor by rapid cooling.
Advantageously, the apparatus further comprises a metal evaporator for evaporating a metal vapor from a solid or liquid metal. The evaporator is connected to the first inlet of the reaction chamber for feeding the vapor to the first inlet and from there into the reaction chamber. A variety of embodiments for the evaporator may be envisaged. In the simplest case, the evaporator comprises a chamber in which a bath of liquid (molten) metal is provided, and means for passing a stream of a carrier gas such as argon over the surface of the liquid metal and towards the inlet of the reaction chamber. In a more sophisticated embodiment, the liquid metal might be sprayed through a carrier gas atmosphere, and the carrier gas together with the resulting metal vapor would then be fed to the reaction chamber. The metal vapor might also be generated by sublimation from solid metal at a sufficient temperature, where the solid metal is preferably finely dispersed to achieve a high specific surface.
Preferably, the apparatus further comprises a steam source, e.g., in the form of a steam generator, connected directly or indirectly to the second inlet of the reaction chamber.
In an advantageous embodiment, the steam is brought into thermal contact with the reaction chamber, in particular with the reaction zone, before being fed to the reaction chamber. This enables heat exchange between the reaction chamber (in which a generally exothermic reaction takes place) and the steam, which leads to heat removal from the chamber at the same time as heating of the steam to a temperature suitable for the subsequent reaction. To this end, the apparatus preferably comprises a steam conduit (steam feeding channel) in thermal contact with the reaction zone, whose inlet is connected to the steam source and whose outlet is connected to the second inlet of the reaction chamber.
The steam is advantageously fed into the reaction chamber in a manner that leads to rapid mixing with the nanoparticles or the metal vapor and to rapid cooling of same (“quenching”) in order to obtain the nanoparticles and/or nanodroplets. This minimizes agglomeration of the nanoparticles to larger particles and deposition of the metal vapor or of nanoparticles on the walls of the reaction chamber. To this end, the apparatus preferably comprises a plurality of steam nozzles for injecting the steam in a manner that creates turbulent flow in the reaction chamber, in particular in the mixing and/or particle/droplet formation zones. Preferably the nozzles have a direction of ejection of the steam which is different from the direction of flow of the metal vapor or nanoparticles/nanodroplets, and which is preferably approximately at a right angle with the latter direction.
Preferably, the reaction chamber is elongated and has a plurality of inlets for said steam or liquid water and/or a plurality of inlets for said vapor of said metal or metal-containing compound distributed along a direction of elongation of the reaction chamber. In particular, preferably the steam is fed to the reaction chamber in such a distributed manner. Some or all of the inlets may again be designed as nozzles for creating turbulent flow.
Preferably, at least a portion of the reaction chamber has porous walls and means for feeding at least a portion of the steam or liquid water and/or at least a portion of the vapor of the metal, metal-containing compound or precursor to said reaction chamber through the porous walls. By this, deposits of the metal, the metal-containing compound, or of any reaction products on the walls of the reaction chamber can be reduced or completely avoided. Preferably, the steam is fed through the porous walls.
Alternatively, an inventive apparatus may comprise a reaction chamber with at least one first inlet for a precursor, at least one second inlet for a reactant gas (which can be a nitriding gas), at least one third inlet for steam or liquid water, and at least one outlet for products of the reaction. The reaction chamber further comprises a mixing zone adapted for mixing streams entering through at least said first inlet and said second inlet, a particle/droplet formation zone downstream from and/or overlapping with said mixing zone, adapted for forming nanoparticles and/or nanodroplets from said precursor and said reactant gas or said nitriding gas, and a reaction zone downstream from and/or overlapping with said mixing zone adapted for reacting nanoparticles and/or nanodroplets with steam.
The apparatus may further comprise a source of the precursor (such as an evaporator in the case of a gaseous precursor) connected to the first inlet, a source of the reactant gas (such as a pressure tank or compressor) connected to the second inlet, and/or a steam source (such as a steam generator) connected to the third inlet. Any other considerations made for the apparatus further above equally well apply, mutatis mutandis, to the present apparatus, in particular the considerations in relation to bringing the steam into thermal contact with the reaction chamber before feeding, the provision of steam nozzles for creating turbulent flow, the provision of inlets distributed along the length of the reactor, in particular, for the steam, or the provision of porous walls, in particular for the steam.
The invention will be described in more detail in connection with exemplary embodiments illustrated in the drawings, in which
Anthropogenic emissions of greenhouse gases and other pollutants can be significantly reduced or even completely eliminated by substituting fossil fuels by cleaner fuels, e.g. hydrogen. A review of the process technology for thermochemically producing hydrogen from water using solar energy is found in (Steinfeld A., Palumbo R., Solar Thermochemical Process Technology. In: Meyers R A, editor. Encyclopedia of Physical Science and Technology, 3rd edition, Volume 15. San Diego, USA: Academic Press, 2002. p. 237-256).
A first, endothermic step 102 comprises thermal reduction of metal oxide to the metal or the lower-valence metal oxide by providing solar energy 101. The second, exothermic step 103 comprises the hydrolysis of the metal to form hydrogen and the original metal oxide; the latter is recycled to the first step. This solar thermochemical cycle is represented by:
1st step (solar): MxOy→xM+0.5yO2
2nd step (non-solar): xM+yH2O→MxOy+yH2
where M and MxOy denote the metal and the corresponding metal oxide. The net reaction is H2O→H2+0.5 O2. Hydrogen and oxygen are formed in different steps, thereby eliminating the need for high-temperature gas separation.
In
1st step (solar): ZnO→Zn+0.502
2nd step (non-solar): Zn+H2O→ZnO+H2.
The chemical aspects of the 1st step and the present status of the pertinent chemical reactor technology are summarized in (Steinfeld A., Solar Hydrogen Production via a 2-step Water-Splitting Thermochemical Cycle based on Zn/ZnO Redox Reactions, Int J. Hydrogen Energy 2002; 27:611-19).
Other metals may also be employed in this redox (reduction-oxidation) cycle, such as Fe, Mg, Al, Ti, Si, and Ca. The temperature requirement for the production of metals from the metal oxides may be reduced by conducting the chemical reduction electrolytically, or by making use of a reducing agent such as carbon or methane (see e.g. Steinfeld A., Kuhn P., Reller A., Palumbo R., Murray J., Tamaura Y., Solar-processed Metals as Clean Energy Carriers and Water-Splitters. Int. J. Hydrogen Energy 1998; 23:767-74). Mixed metal oxides may be also employed in this redox cycle. The process may also be applied by reducing a metal oxide to a lower-valence metal oxide, instead of performing the complete reduction to the metal. For example, in the first step, Fe2O3 or Fe3O4 may be reduced to wustite (FeO), which has the capability of splitting water in the second step.
Ammonia can also be produced via a 2-step process. The first, high-temperature, endothermic step is the production of metallic nitrides by, e.g. carbothermic reduction of their metal oxides in a N2 atmosphere. The second step is the hydrolysis of the metal nitride for the formation of ammonia and the original metal oxide; the latter is recycled to the first step. The reactions can be represented by the following general equations:
1st step (reduction): MxOy+C(gr)+N2→MxNy′+CO
2nd step (hydrolysis): MxNy′+H2O→MxOy+NH3
where M denotes the metal, and MxNy and MxOy the corresponding metal nitride and oxide, respectively. The stoichiometry of theses reactions will depend on the metal. For example, for aluminum:
1st step (reduction): Al2O3+3C(gr)+N2→2AlN+3CO
2nd step (hydrolysis): 2AlN+3H2O→Al2O3+2NH3
The net reaction is: 3C(gr)+N2+3H2O→3CO+2NH3
Other metals may also be employed in this 2-step process, e.g. Si, Fe, Ti, etc. Other reducing agents than C(gr) (graphite) may also be employed for the reduction step, e.g. methane, biomass, and other hydrocarbons.
Alternatively, ammonia can be produced via a 3-step process. The first step is the endothermic reduction of the metal oxide to the metal. The second step is the nitridation of the metal with a nitriding gas producing metal nitride. The third step is the hydrolysis of the metal nitride for the formation of ammonia and the original metal oxide.
In the present invention, it is suggested to employ nanoparticles and/or nanodroplets in a process for producing hydrogen or in a process for producing ammonia. Metal nanoparticles can be obtained advantageously by evaporation-condensation processes. In these processes, the metal is first evaporated from a melt. Metal nanoparticle formation then takes place by vapor condensation due to oversaturation during rapid cooling (quenching). The quenching can be achieved, e.g., by mixing of the metal vapor with a gas of lower temperature or by expansion (cf. Kodas T. T., Hampden-Smith M., Aerosol Processing of Materials, Wiley-VCH, New York, 1999; Wegner K., Walker B., Tsantilis S., Pratsinis S. E., Design of Metal Nanoparticle Synthesis by Vapor Flow Condensation. Chem. Eng. Sci. 2002; 57:1753-62). An inert gas is employed frequently to carry the metal vapor from an evaporation zone to a particle formation zone and the product particles to a collector. This process also has been employed successfully for the production of zinc nanoparticles (Daub O., Langel W., Reiner C., Kienle L., QMS-controlled Production of Nanocrystalline Metals by Inert Gas Condensation in a Flow System. Ber. Bunsenges. Phys. Chem. 1997; 101:1753-56).
The evaporation-condensation process can be coupled with a chemical reaction by introduction of a reactant gas to produce metal oxides, metal nitrides, metal carbides, or other ceramic materials (see, e.g., Pratsinis S. E., Wang G., Panda S., Guiton T., Weimer A., Aerosol Synthesis of AlN by Nitridation of Aluminum Vapor and Clusters. J. Mater. Res. 1995; 10: 512-20). Generally speaking, nanoparticles of a metal or metal-containing compound can be produced in aerosol flow reactors by evaporation-condensation processes involving chemical reactions. In these processes, a precursor is evaporated, and the vapor of the precursor is mixed with a reactant gas. Subsequently, particle formation and reaction from the precursor to the metal or metal-containing compound take place by vapor condensation during rapid cooling (quenching). In U.S. Pat. No. 5,525,320, such a process (with aluminum as precursor and nitrogen or ammonia as reactant gas) has been employed successfully for the production of aluminum nitride nanoparticles.
An overview of the general process technology is given in (Kodas T. T., Hampden-Smith M., Aerosol Processing of Materials, Wiley-VCH, New York, 1999), and reference is made to this document for details of such processes.
Advantageously, the process for producing the nanoparticles/nanodroplets and the process for producing hydrogen employing these nanoparticles/nanodroplets are combined into a single process. A combined process for producing hydrogen then may comprise the following stages: Continuous production of steam and metal vapor in separate generators; continuous feeding of separate streams of steam and metal vapor into a mixing zone; mixing of steam and metal vapor inside the mixing zone to create a single stream of metal and steam; rapid cooling (quenching) of metal vapor by steam and simultaneous formation of nanoparticles and/or nanodroplets of metal; progression of the flow of steam laden with nanoparticles and/or nanodroplets of metal to a chemical reaction zone; chemical reaction of steam and metal, generating hydrogen and metal oxide particles along the chemical reaction zone; and continuous exit of products.
Not all of the above-mentioned stages need to be carried out as described; stages can overlap and/or can be skipped. Top candidate metals for the hydrogen-production process are zinc, iron, and magnesium, but other metals can also be employed. The process may also be applied using a metal-containing compound instead of the metal, and a hydrogen-containing substance instead of water. In addition, the particles and/or droplets can form an aerosol or can be immobilized at a surface. In any of these possibilities, a key aspect is the combined formation of nanoparticles and/or nanodroplets with a BET-surface area above 1 m2/g, and their in-situ reaction to generate hydrogen. Alternatively, the nanoparticles and /or nanodroplets may be produced in separate upstream processes and fed as solids or liquids into the mixing zone.
Advantages of the proposed process for the production of hydrogen are at least three-fold:
Likewise, a combined process for the production of nanoparticles/nanodroplets of metal nitride and ammonia may comprise the following stages: Continuous production of steam, metal vapor, and a nitriding gas; Continuous feeding of separate streams of metal vapor and a nitriding gas into a mixing zone; Mixing of the metal vapor and the nitriding gas inside the mixing zone to create a single gas stream; Quenching of the vapor and simultaneous formation of nanoparticles and/or nanodroplets of metal nitride; Progression of the flow of nanoparticles and/or nanodroplets of metal nitride to a chemical reaction zone; Continuous feeding of steam into the chemical reaction zone; Chemical reaction of steam and metal nitride, generating ammonia and metal oxide particles along the chemical reaction zone; Continuous exit of products.
Again, not all of the above-mentioned stages must be carried out as described; stages can overlap and/or can be skipped.
Specifically, metal nanoparticles may be produced in a separate upstream process, then contacted with a nitriding gas to form nanoparticles comprising the metal nitride, and these nanoparticles may then be fed into the mixing zone.
Alternatively, the metal vapor, the nitriding gas and the steam may first be mixed to form nanoparticles comprising metal nitride and steam, which then react in the chemical reaction zone downstream of the mixing zone to form metal oxide particles and ammonia. Top candidate metals for the ammonia-production process are aluminum, titanium, and silicon, but other metals can also be employed.
Advantages of the proposed process for the production of ammonia are:
For producing hydrogen, metal vapor and steam are fed into the mixing zone 211. There, intimate mixing between the metal vapor and the steam occurs. The temperature, mixing ratio and total pressure in the mixing zone are chosen such that the temperature is above the saturation temperature of the metal vapor at the partial pressure of the metal vapor in the mixing zone. The mixture of metal vapor and steam is then transferred to the particle/droplet formation zone 212. In this zone, the mixture is subjected to rapid cooling (quenching) to a temperature below the saturation temperature of the metal vapor. By this, nanoparticles and/or nanodroplets of the metal are formed. The exact conditions under which such nanoparticle and/or nanodroplets formation occurs may depend on the type of metal, on the reactor design, size etc. (cf. Kodas T. T., Hampden-Smith M., Aerosol Processing of Materials, Wiley-VCH, New York, 1999; Wegner K., Walker B., Tsantilis S., Pratsinis S. E., Design of Metal Nanoparticle Synthesis by Vapor Flow Condensation. Chem. Eng. Sci. 2002; 57:1753-62). Examples for suitable conditions for a specific reactor design will be given below. Generally, a cooling rate exceeding 5000 Kelvin per second (K/s or, equivalently, ° C./s) is advantageously chosen. A higher cooling rate may be advantageous, as it may lead to smaller particle sizes. Therefore, more rapid cooling, e.g. at a rate exceeding 10000 K/s, is preferred, more preferred at a rate above 20000 K/s, even more preferred above 50000 K/s.
Additional steam may be fed to the particle/droplet formation zone 212 through valve 205. The steam may be at a lower temperature than at valve 204 in order to aid in rapid cooling of the metal vapor/steam mixture entering from the mixing zone.
The nanoparticles and/or nanodroplets generated in the particle/droplet formation zone 212 are then transferred to the reaction zone 213. Here, conditions are chosen such that a reaction of the metal nanoparticles/nanodroplets with the steam can occur. Generally, pressure and temperature conditions are such that the temperature is below the saturation temperature of the metal vapor, such that no evaporation from the nanoparticles and/or nanodroplets can occur. At the same time, the temperature should be high enough that the reaction proceeds rapidly enough. Additional steam can be fed to the reaction zone 213 through valve 206 for providing enough steam for a stoichiometric reaction and/or for regulating the temperature and pressure conditions in the reaction zone.
Finally, the reaction products (metal oxide-comprising particles and gaseous, elemental hydrogen) as well as any unreacted reactants (steam and/or metal nanoparticles/nanodroplets) are removed to form a product stream 208.
Specifically, if the metal is zinc (Zn) and the process is carried out at or near atmospheric pressure, the preferred conditions are as follows: The temperature in the mixing zone 211 should preferably exceed the saturation temperature of the zinc vapor. The saturation temperature depends on the partial pressure of Zn, which in turn depends on how much steam and Zn vapor is fed into the mixing zone. If the partial pressure approaches atmospheric pressure, the saturation temperature approaches 1180 K, and the temperature in the mixing zone should preferably exceed 1180 K. The temperature in the particle/droplet formation zone and in the reaction zone should preferably be in the range between 800 K and 1180 K, when working at atmospheric pressure. In general, for any operating pressure, the temperature in the particle/droplet formation zone and in the reaction zone should be below the saturation temperature of zinc vapor. The cooling rate in the particle/droplet formation zone should preferably exceed 80000 K/s.
Various additions and modifications to the described process and apparatus are possible. Instead of single valves 204, 205, 206, at least a portion of the reaction chamber may be equipped with a porous wall, through which the steam is fed into the chamber. A porous wall with steam entering continuously has the advantage that reactions involving the chamber walls will be minimized, and solid material is prevented from precipitating on the walls.
Instead of feeding only steam to the reaction chamber in various places along the reaction chamber, also metal vapor can be fed to the reaction chamber in more than one place for optimizing the nanoparticle/nanodroplet formation.
Instead of or in addition to feeding steam to the reaction chamber, liquid water may be fed to the chamber. Due to the elevated temperatures in the reaction chamber, the water will readily evaporate in the chamber to form steam. This may be particularly advantageous if a separate stream of liquid water is fed to the particle/droplet formation zone 212 or its vicinity, e.g., through valve 205. While evaporating, the water will remove heat from the formation zone 212. This will aid in the rapid cooling (quenching) of the metal vapor and thus improve the formation of nanoparticles or nanodroplets.
In addition to steam, some other gas—which is preferably substantially inert under the conditions present within the reaction chamber—can be fed to the reaction chamber, either through additional inlets or together with the steam and/or the metal vapor. Feeding such an additional gas into the reaction chamber can improve the mixing and/or cooling process. Feeding of the metal vapor to the reaction chamber can be facilitated by employing the additional gas as a carrier gas for the metal vapor, i.e., by feeding such an additional gas to the reaction chamber together with the metal vapor. The additional gas can be, e.g., a noble gas, specifically argon.
It may be advantageous to feed a metal as precursor, preferably in vapor form, and a reactant gas to the reaction chamber. The reactant gas reacts with the precursor to form a metal-containing compound. Alternatively, it may be advantageous to feed a metal-containing compound as precursor, preferably in vapor form, and a reactant gas to the reaction chamber. The reactant gas reacts with the precursor to form a metal or a different metal-containing compound. The reaction conditions are chosen such that the metal or metal-containing compound is obtained in the form of nanoparticles and/or nanodroplets. The precursor may be fed to the reaction chamber by means of a carrier gas.
An apparatus for carrying out a process in which a precursor is fed to a reaction chamber is shown in
The precursor and the reactant gas are fed into the mixing zone 311. There, intimate mixing between the precursor and the reactant gas occurs. The mixture of the precursor and the reactant gas is then transferred to the particle/droplet formation zone 312. In this zone, the mixture is subjected to rapid cooling (quenching) leading first to the formation of nanoparticles and/or nanodroplets of the precursor. Steam or liquid water may enter through valve 305 and may aid in the quenching process. The reactant gas then reacts with the precursor nanoparticles and/or nanodroplets to form nanoparticles and/or nanodroplets comprising the desired metal or metal compound. The exact conditions under which such nanoparticle and/or nanodroplet formation occurs will depend on the precursor, the reactant gas, on the reactor design, size etc.
The nanoparticles and/or nanodroplets generated in the particle/droplet formation zone 312 are then transferred to the reaction zone 313. Here, conditions are chosen such that a reaction of the nanoparticles/nanodroplets with the steam can occur. Additional steam can be fed to the reaction zone 313 through valve 306 for providing enough steam for a stoichiometric reaction and/or for regulating the temperature and pressure conditions in the reaction zone. Finally, the reaction products as well as any unreacted reactants are removed to form a product stream 308. Again, at least a portion of the reaction chamber may be equipped with a porous wall, through which the steam is fed into the chamber. Utilization of a precursor is especially advantageous in a process for producing ammonia according to the present invention. An example is the generation of aluminum nitride nanoparticles by feeding aluminum (as a precursor of aluminum nitride) and a nitriding gas (as a reactant gas) to the reaction chamber, under conditions as e.g. described in U.S. Pat. No. 5,525,320 or as in the references cited therein. Advantageously, the precursor is provided in the form of a vapor, and the nitriding gas is nitrogen.
An apparatus which has been employed successfully in laboratory-scale experiments for producing hydrogen according to the principles of the present invention is shown in
Gases (steam and argon) are supplied to the reactor by the following means: An argon reservoir 420 is provided. Argon is fed to the inlet tube 403 to generate a carrier gas stream, via a first mass flow controller 421 (Bronkhorst® EL-Flow 201 A). Steam is generated in a steam generator 424 with the aid of a water pump 423 by known means. Argon gas is fed to the steam generator from the reservoir 420 through a second mass flow controller 422. An argon/steam mixture (hereinafter referred to as the mixing gas stream) is fed to the reactor 401 through a superheater 425.
Reaction products are treated and analyzed as follows. Particles produced in the reactor 401 are collected on a glass fiber filter 407 (Whatman® GF/A, diameter 150 mm). Gases exiting filter 407 are cooled in a condenser 412 by cooling water (cooling water inlet 410, cooling water outlet 411), the condenser additionally comprises a water trap (not shown in
FIGS. 5 to 7 show some details of the reactor 401. As best seen in
Experiments were carried out utilizing the apparatus of
As soon as steady state at the desired temperature was reached, the carrier gas was turned on followed by the water for steam generation and the mixing gas. The carrier gas was switched on first to prevent a steam backflow into the evaporation zone, which would have led to a fast oxidation of the metal melt surface. Samples were taken by the GC every 30 seconds for the duration of an experiment. The sampling phase was 10 seconds, the analyzing phase lasted 20 seconds. The concentration of H2 was displayed in % and logged in a file. The accuracy of the GC used to determine the H2 concentration of the product gas was approved by analyzing a gas mixture (Scott Specialty Gases) containing 1% of each O2, H2, CO, CO2, and CH4 with the balance of N2 previous to the first run.
In one experiment, zinc (Zn) was placed into the crucible 404. Argon was fed to the inlet tube 403 at a carrier gas flow rate of 0.5 liters per minute (I/min) and passed over the crucible with the molten zinc. The temperature in the inside of the furnace 402 was set to 1023 K. Steam was generated at a rate of 10.2 grams per hour (g/h) and was mixed with argon at a rate of 1.5 I/min and fed to the reaction chamber as mixing gas stream 505. The pressure in the reactor 401 was only slightly above atmospheric pressure (1050 mbar absolute pressure). The particles collected on the filter were analyzed by X-ray diffraction (XRD). XRD patterns were analyzed using the software Topas® (Version 2.0) to obtain the exact particle composition (the Zn to ZnO ratio). The same software was also used to determine the crystal size of Zn and ZnO. A hydrogen yield (moles of hydrogen produced per moles of zinc evaporated) of up to 68% was achieved with these or similar parameters. With these or similar conditions, the size of the collected Zn and ZnO crystals, as determined by X-ray diffraction, varied between approximately 10 nanometers (nm) and 130 nm.
The apparatus shown in
For designing a continuous reactor for carrying out the process of the present invention commercially, the following design guidelines should be contemplated:
A clear separation of the evaporation zone and the reaction chamber avoids steam backflow, which is the main reason for a fast oxidation of the metal melt surface. The separation should be done in a manner allowing for independent temperatures of the evaporation zone and of the reaction zone. Of course, condensation of metal inside the interconnection of the two zones should be avoided. With a controllable and continuous supply of metal, it is possible to control the evaporation rate of metal such that it does not decrease over time. If oxidation of the metal melt surface which is responsible for reduction of the evaporation rate cannot be avoided, despite the separation of evaporation and reaction zone, the forming oxide layer on the melt surface should be removed. This could be done mechanically, for instance by using a stirrer. Continuous supply of steam can be achieved by a steam generator as is well known in the art.
In the embodiment of
With this setup, Zn nanoparticles were produced. The flow rate of the argon carrier gas fed to tube 403 varied between 0.5 and 3 I/min. The flow rate of the argon mixing gas stream 505 varied between 10 and 30 I/min. The furnace temperature varied between 923 and 1023 K. Other parameters (number of crucibles 404, mixing mode, mixing zone length, diameter of the hole in cap 405) were also varied. BET surface areas between 1.9 m2/g and 11.2 m2/g, corresponding to BET-equivalent particle diameters between 425 nm and 72 nm, were obtained. In the investigated range, the conditions leading to the largest BET surface area of 11.2 m2/g (i.e., to the smallest particle size) were a furnace temperature of 1023 K, a carrier gas flow rate of 1.5 I/min, a mixing gas flow rate of 30 I/min, one single crucible, T-mode mixing with one distance ring 601 of 6 mm thickness to define the mixing zone, and 6 mm hole diameter in cap 405. The Zn nanoparticles thus obtained may be collected and used in a fluidized-bed reactor to produce hydrogen in a process according to the present invention or could be directly fed to the mixing zone 503 of a modified reactor similar to the reactor 401 without melt evaporation 404.
The evaporation stage 214 (which may be considered a special embodiment of an evaporator) comprises a first furnace 402a in which a first reactor tube is disposed. A holder with an attached crucible 404 for receiving molten metal extends into the reaction tube. The holder is attached to a balance 901 for measuring the evaporation rate, enclosed in a balance box 900. Argon gas serving as a carrier gas is fed to the balance box by means of a mass flow controller (MFC) 421. The carrier gas streams into the balance box and from there into the reactor tube, being heated by the furnace 402a and carrying with it metal vapor evaporating from the crucible 404. The vapor-laden carrier gas passes through the cooling stage 215 and is cooled to a temperature just above the saturation temperature of the metal vapor in the carrier gas. From there, it passes into the reaction stage 216.
The reaction stage comprises a second furnace 402b. Disposed in the second furnace is a second reactor tube connected to the first reactor tube. One or more steam conduits 911 extend through the second reactor tube in its longitudinal direction. An argon/steam mixture is fed from a steam generator 424, which in turn is fed by argon gas through an MFC 422 and by water from a water pump 430, to these conduits from the downstream end of the reactor to steam nozzles 910. There, the steam is injected into the stream of the carrier gas carrying the metal vapor, the injection direction being at approximately right angles to the longitudinal direction of the reactor. The steam creates a turbulent flow in the region around the steam nozzles, thoroughly mixing the steam and the vapor-laden carrier gas and, at the same time, quenching the metal vapor (i.e., rapidly cooing it to below the saturation temperature of the metal), such that metal nanoparticles are formed. The stream of carrier gas, metal nanoparticles and steam is then carried down an inner tube 912 disposed within the second reactor tube, where the hydrolysis reaction takes place. A mixture containing metal oxide nanoparticles and hydrogen exits the inner tube through filter 407.
By sharing a wall with the inner tube 912, the steam conduits 911 are in thermal contact with the inner tube. Heat exchange takes place between the steam being passed to the steam nozzles 910 and the gas stream in inner tube 912. Thereby, the steam is preheated, and heat is removed from the (exothermic) reaction taking place in the inner tube.
Instead of a plurality of steam conduits, an arrangement of concentric tubes as in
It should be emphasized that, while the invention has been described in conjunctions with specific embodiments, many alternatives, modifications and variations are possible, and the invention is not limited to the embodiments described above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04 013 334.0 | Jun 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CH05/00315 | 6/6/2005 | WO | 12/4/2006 |
| Number | Date | Country | |
|---|---|---|---|
| 60576612 | Jun 2004 | US |
