Catalytic partial oxidation processes and catalysts with...

Details Catalytic partial oxidation processes and catalysts with... Catalytic partial oxidation processes and catalysts with...

2000-07-27

2002-12-03

Langel, Wayne A. (Department: 1754)

Chemistry of inorganic compounds

Carbon or compound thereof

Oxygen containing

C252S373000, C423S651000

Reexamination Certificate

active

06488907

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to catalysts and processes for the catalytic partial oxidation of hydrocarbons (e.g., natural gas), for the preparation of a mixture of carbon monoxide and hydrogen using a supported metal catalyst. More particularly, the invention relates to syngas production processes employing catalysts having a diffusion barrier layer between a metal support and a catalytically active species.
2. Description of Related Art
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.
CH
4
+H
2
O
CO+3H
2
  (1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H
2
:CO ratio of 2:1, as shown in Equation 2.
CH
4
+1/2O
2
CO+2H
2
  (2)
This ratio is more useful than the H
2
:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by prior art catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.
For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high gas hourly space velocities are difficult to achieve at reasonable gas pressure drops, particularly with fixed beds of catalyst particles. Accordingly, substantial effort has been devoted in the art to the development of catalyst support structures and the design of the catalytic reaction zone.
Fixed reaction zone processes, wherein the reaction zone comprises a fixed bed of solid catalyst particles, have been known for some time and are described in the patent literature. For example, U.S. Pat. No. 5,149,464 describes such a process and catalyst. A number of other process regimes have been proposed in the art for the production of syngas via partial oxidation reactions. For example, the process described in U.S. Pat. No. 4,877,550 employs a syngas generation process using a fluidized reaction zone. Such a process however, requires downstream separation equipment to recover entrained supported-nickel catalyst particles.
To overcome the relatively high pressure drop associated with gas flow through a fixed bed of catalyst particles, which can prevent operation at the high gas space velocities required, various structures for supporting the active catalyst in the reaction zone have been proposed. U.S. Pat. No. 5,510,056 discloses a monolithic support such as a ceramic foam or fixed catalyst bed having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. The preferred catalysts for use in the process comprise ruthenium, rhodium, palladium, osmium, iridium, and platinum. Data are presented for a ceramic foam supported rhodium catalyst at a rhodium loading of from 0.5-5.0 wt %.
U.S. Pat No. 5,648,582 also discloses a process for the catalytic partial oxidation of a feed gas mixture consisting essentially of methane. The methane-containing feed gas mixture and an oxygen-containing gas are passed over an alumina foam supported metal catalyst at space velocities of 120,000 hr.
−1
to 12,000,000 hr.
−1
The catalytic metals exemplified are rhodium and platinum, at a loading of about 10 wt %.
U.S. Pat. No. 5,744,419 (Choudhary et al.) describes certain Ni and Co catalysts on an inert support, the surface of which is precoated with an oxide of Be, Mg or Ca. These catalysts are employed for converting methane to syngas.
U.S. Pat. No. 5,338,488 (Choudhary et al) describes certain composite catalysts having the general formula T
m
AO
n
. T is a transition metal (including Ni, Co, Pd, Ru, Rh and Ir), A is an alkaline earth metal (including Mg, Ca, Ba and Sr), O is oxygen, m is the T/A mole ratio from 0.01-100 and n is the number of oxygen atoms needed to form a catalyst composite wherein each element has a complete set of valence electrons. These catalysts are said to have activity for catalyzing the production of synthesis gas by oxidative conversion of methane.
Hofstad et al. (Catalysis Today 40:157-170 (1998)) describe certain alumina supported rhodium catalysts with activity for catalyzing the partial oxidation of methane to synthesis gas.
As mentioned above, the partial oxidation of methane is a very exothermic reaction, and temperatures at typical reaction conditions in excess of 1,000° C. may be required for successful operation. It is known that ceramic monolith catalyst supports are susceptible to thermal shock; that is, either rapid changes in temperature with time or substantial thermal gradients across the catalyst structure. Catalysts and catalyst supports for use in such a process must therefore be very robust, and avoid structural and chemical breakdown under the relatively extreme conditions prevailing in the reaction zone.
U.S. Pat. No. 5,639,401 discloses a porous monolithic foam catalyst support of relatively high tortuosity and porosity, preferably comprising at least 90 wt % zirconia for thermal shock resistance. The catalyti

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