The present invention provides for methods for the partial oxidation of hydrocarbons to produce hydrogen and carbon monoxide or syngas from a mixture of a hydrocarbon-containing feed gas stream and an oxygen-containing feed gas stream using a two stage reactor system.
The first stage reactor is a catalyst composition which has two layers. The first layer is a catalytically active metal dispersed on an inert carrier support. The second layer is a reduced metal catalyst which consists essentially of a transition metal supported on or in a stable inorganic metal oxide washcoated on a support. The second stage reactor comprises a shift reactor. For purposes of the present invention, the shift reactor can be catalytic monolith or a shift reactor such as a low temperature shift catalyst or water gas shift catalyst. The hydrocarbon and oxygen containing gas streams are fed to this two-stage reactor and will react to form the hydrogen carbon monoxide or syngas.
The present invention also provides for a reactor system apparatus which is used in the partial oxidation of hydrocarbons to produce hydrogen and carbon monoxide or syngas which comprises a two stage reactor system.
The first stage comprises a catalyst composition of two layers. The first layer being a catalytically active metal dispersed on an inert carrier support and the second layer is a reduced metal catalyst consisting essentially of a transition metal supported on or in a stable inorganic metal oxide washcoated on a support.
The second stage reactor in the reactor system comprises a shift reactor. The shift reactor may be selected from a catalytic monolith or a low temperature shift catalyst or water gas shift catalyst. The catalytic monolith which may be used in the second stage reactor, is a metal catalyst which consists essentially of a metal supported by a ceria coating disposed on a ceramic monolith. The metal is selected from the group consisting of nickel, cobalt, iron, platinum, palladium, iridium, rhenium, ruthenium, rhodium, and osmium. The ceramic material is selected from the group consisting of zirconia, alumina, yttria, titania, magnesia, ceria, and cordierite. The ceria coating has a weight percentage between about 5% and about 30% with respect to the ceramic monolith. In one embodiment of the monolith catalyst, the ceramic is selected from the group consisting of zirconia, yttria, titania, magnesia, ceria, and cordierite.
A further description of this metal catalyst monolith ceramic may be found in co-pending application Ser. No. 10/143,705 published Jan. 9, 2003 as US 2003/0007926 A1 to Jiang et al.
In a preferred embodiment of the catalytic monolith, a zirconium monolith is coated by about 1 to 20 weight percent of ceria which supports 0.5 to 5 weight percent rhodium on the ceria-coated monolith.
The apparatus for the partial oxidation of hydrocarbons to produce hydrogen and carbon monoxide comprising a first stage reactor which comprises a catalyst composition of a first layer of a catalytically active metal dispersed on an inert carrier support and a second layer of a reduced metal catalyst consisting essentially of a transition metal supported on or in a stable inorganic metal oxide washcoated on a support, and a second stage reactor comprising a shift reactor.
The preferred catalyst substrates to use in the first layer at the feed side are particulates including spheres, pellets and rings; wagonwheels, Saint-Gobain Norpro's Ty-Pak®, HexPak™, Snowflake® media, and Norton® saddles, gauze wire, honeycomb monoliths or any other suitable supports in any acceptable manufactured shapes such as those listed herein, although reticulated monoliths with improved mechanical strength can also be used. This mechanical strength can be illustrated for purposes of the present invention as “crush strength” which is the maximum stress that a material can sustain under crush loading. The crush loading is that weight and/or pressure that will fracture or compact the catalyst substrate. The catalyst substrates of the present invention will have a crush strength that can withstand the typical operating conditions of a partial oxidation process of hydrocarbons. Accordingly the catalyst substrates of the present invention will not lose crush strength during the normal operation of a partial oxidation process for hydrocarbons. The layered catalysts at the product side may catalyze the partial oxidation, but mainly promote reforming and water-gas shift reactions to achieve high level of carbon conversion with high syngas selectivity. The reticulated monoliths are more suitable for these subsequent layers, particularly as the last layer.
When more than two layers are used, they can be layered in any order provided the first layer is more stable to thermal shock and higher temperatures in excess of 1100° C. and the subsequent layers can further catalyze the subsequent carbon conversion to syngas. The pore structure of monoliths and the particle size of particulates are such that there is no significant pressure drop through the bed. US 2003/0083198 A1 teaches a fixed bed catalyst system comprising first and second zones disposed in stacked serial flow arrangement in order to achieve better flow distribution, where first zone (upstream section) has a greater resistance to flow of the feed stream than second zone (down stream section). However, in the current invention, the first layer (upstream) can be less resistant than the second layer by using a larger pore size honeycomb rather than a reticulate monolith. A catalytically inactive substrate may be placed between each of the catalyst layers to improve the gas distribution.
Protection of the reticulated monolith catalyst either from active metal sintering or mechanical strength weakening at high temperatures can be achieved by shielding it from the high temperature using the first layer as the combustion catalyst. This concept of multi-layer catalysts is shown in the following drawing, where NG stands for natural gas.
The first layer catalyst desirably should be able to operate in an oxidizing and reducing environment effectively without significant deactivation over a temperature range from about 1000° to about 1800° C. The first layer catalyst preferably comprises catalytically active metals dispersed on an inert carrier support. The metals used in the present catalysts are selected from certain transition and noble metals of the Periodic Table of Elements. Active metals can be selected from the group consisting of transition or noble metal selected from the group of nickel, cobalt, iron, platinum, palladium, iridium, rhenium, ruthenium, rhodium and osmium and combinations thereof. Preferred metals are nickel and rhodium.
The most preferred metal of this group is rhodium. The metals may be present on the support in the form of metals, metal oxides, metal halides or other metal salts prior to being reduced. Upon reduction, as described below, the metals are substantially in the metallic form. Generally, from about 0.1% to 8% by weight of the metal will be deposited on the support. Optionally Rh/Pt gauze catalyst can be used as the exothermic reaction catalyst.
US 2003/0198592 A1 discloses multiple serially aligned reaction zones and multiple hydrocarbon feeds for converting a hydrocarbon and oxygen feed stream to syngas. The first reaction zone catalyzes the net partial oxidation of the feed hydrocarbon and the subsequent zone catalyzes either a water-gas shift reaction, net partial oxidation, steam or dry reforming. Some of the most selective catalysts, such as Rh, are not able to withstand the heat and retain their activity due to problems such as sintering and a limited tolerance to carbon formation. The present invention provides an advantage inasmuch as it places more of a thermally stable catalyst, such as a mixed metal oxide or a stable crystalline structure such as a spinel or perovskite, more preferably CoCrOxide, at the front of the reactor and most preferably 9.9 wt % Co0.2Cr0.8Oxide/1 wt % Rh/6.1 wt % Yb supported on a refractory support such as alumina or zirconia, preferably partially stabilized zirconia (PSZ).
In the process of the present invention, the exothermic reaction is almost complete instantaneously after contacting the first catalyst layer. The primary function of subsequent layer is to effect the endothermic reforming reaction. The “net partial oxidation” catalyst zone is considered as the two layers comprising a combustion catalyst and an endothermic catalyst. It is important to point out that PSZ reticulate material is not preferred due to the inherent phase transition at high temperature and clearly Rh coating on Al2O3 is stable even after more than 7 months of continuous operation as the first layer. It retains its catalytic activity as well as its mechanical strength.
A preferred form of inert carrier is made of a refractory, substantially inert, rigid material which is capable of maintaining its shape with sufficient degree of mechanical strength at high temperatures. Typically, a material is selected for the support, which exhibits a low thermal coefficient of expansion, good thermal shock resistance and high melting point. For example, cordierite material is not suitable as the front layer support for pure oxygen-based partial oxidation due to its lower melting point around 1450° C.
Several general types of material for construction of such carriers are known. Honeycomb monolithic supports are commercially available in various sizes and configuration. A suitable shape and size of pellet or granule type catalyst can also be purchased commercially. Both types of support structures are made from conventional refractory or ceramic materials selected from the group consisting of alumina, mullite, zirconia, partially stabilized zirconia, ceria, silica, silicon carbide, silicon nitride, Sialon™ (a silicon nitride ceramic with a small percentage of aluminum oxide added), aluminum nitride, boron nitride, aluminosilicate, magnesium aluminosilicates, and mixtures of these. Carriers can be formed by any suitable process, including molding, pressing, extruding, spray drying or die stamping, and so on. Inert support also includes metal monolith, prepared as metal foam or sintered particles of metal at high temperature. The most preferred support is a stabilized alumina, silicon carbide or partially stabilized zirconia in the form of honeycomb, sphere, pellet, ring, wagonwheel, Saint-Gobain Norpro's Ty-Pak®, HexPak™, Snowflake® media and Norton® saddles or granule. Preferably the surface area of support is relatively low ranging from between about 0.1 and about 50 m2/g.
Subsequent Catalyst Layer
The subsequent catalyst layer beneath the first layer desirably should be able to operate in oxidizing and reducing environments effectively without significant deactivation over a temperature range below about 1100° C. The endothermic reaction catalyst comprises any of the catalytic metal catalysts known for such purpose. The metal catalysts employed in the present invention are monolith support structures composed primarily of partially stabilized zirconia washcoated with cerium oxide and impregnated with a noble or transition metal or combinations thereof. As used herein, “metal catalyst” refers to the entire catalyst structure including the metal, inorganic metal oxide washcoat and the monolith support or monolith substrate. The active metals used in the present catalysts are selected from the group consisting of a transition or noble metal selected from the group of nickel, cobalt, iron, platinum, palladium, iridium, rhenium, ruthenium, rhodium and osmium and combinations thereof. Preferably the active metals are rhodium and/or nickel.
A monolith support is generally a ceramic foam-like or porous structure formed from a single structural unit having passages disposed in either an irregular or regular pattern with spacing between adjacent passages. Preferred are the ceramic foam structures having irregular passages. The support structures are made from conventional refractory or ceramic materials selected from the group consisting of alumina, mullite, zirconia, partially stabilized zirconia, ceria, silica, silicon carbide, silicon nitride, Sialon™, a silicon nitride ceramic with a small percentage of aluminum oxide added, aluminum nitride or boron nitride, aluminosilicate, magnesium aluminosilicates, and mixtures of these. Metal monoliths prepared as metal foams can also be used. The most preferred supports are partially stabilized zirconia or silicon carbide.
It is preferred to incorporate the active metals on a high surface area for improved overall catalyst effectiveness and activity. The surface area of the support can be increased by depositing a high-surface area inorganic metal oxide such as gamma alumina, ceria, etc. by impregnation, washcoating, adsorption, ion-exchange, precipitation, co-precipitation, deposition precipitation, sol-gel method, slurry dip-coating, microwave heating, etc. Processes for coating substrates with inorganic metal oxides comprise contacting a substrate with a metal oxide precursor, preferably maintaining the precursor coated substrate at conditions to equilibrate the coating, and then oxidizing the precursor to form a substrate containing metal oxide. It is preferred to have macropores by adding decomposable organic matter to the washcoat and then decomposing it at high temperatures in an oxidizing atmosphere.
The stable high surface area inorganic oxides that can be washcoated are selected from but are not limited to oxides comprising one or more cations selected from groups IA, IIA, IIIA and IVA of the Periodic Table of the Elements and the transition metals, preferably from groups IA, IIA, IIIA, IIIB, IVA, IVB and the lanthanides, more preferably from ceria, aluminum, lanthanum, zirconium and barium and mixtures of these. The foam may be impregnated with more than one inorganic oxide simultaneously or sequentially.
Suitably the impregnating medium is in the form of an aqueous or organic solution, slurry, sol, gel, suspension or dispersion of inorganic oxide(s) particles, preferably of a sol of colloidal inorganic oxide(s) particles. The precursors of these washcoats are in any form that can be decomposed to an oxide by thermal decomposition. A stabilizing agent such as an oxide of alkaline or rare earth oxide may be added to the washcoat to prevent the sintering of washcoat during the reaction or pretreatment.
The most preferred support is a partially stabilized zirconia foam monolith washcoated with about 5% to about 30% inorganic metal oxide, most preferably ceria. Optionally ceramic honeycomb or metal monolith coated with noble metal can also be used as endothermic reaction catalyst.
In one aspect, the invention provides a process for the partial oxidation of hydrocarbons to produce hydrogen and carbon monoxide comprising contacting a mixture of a hydrocarbon-containing feed gas and an oxygen-containing feed gas with a catalytically effective multi-layer configuration, at a pressure between 1 and 20 atmospheres, a feed gas standard gas hourly space velocity of about 5,000 to about 500,000 per hour, and a linear velocity of about 0.2 to about 50.0 meter per second (mls). In addition, the feed gas mixture may also comprise steam and/or CO2. The two stages of the reactor apparatus will operate at different temperatures with the first stage operating at about 800° to about 100° C. and the second stage operating at about 250° to about 600° C. The reactant gas mixture can also be preheated to a temperature of about 10 to about 450° C.
Turning now to the figures,
In
While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.