Supported nickel-magnesium oxide catalysts and processes for...

Details Supported nickel-magnesium oxide catalysts and processes for... Supported nickel-magnesium oxide catalysts and processes for...

2001-06-13

2003-10-21

Silverman, Stanley S. (Department: 1754)

Compositions

Gaseous compositions

Carbon-oxide and hydrogen containing

C423S418200, C423S651000, C502S326000, C502S328000

Reexamination Certificate

active

06635191

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to catalysts and processes for the catalytic partial oxidation of light hydrocarbons (e.g. natural gas) to produce synthesis gas, and more particularly to such catalysts comprising a mixture of nickel oxide and magnesium oxide supported on a porous metal substrate.
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 hydrocarbon waxes and other hydrocarbon products, for example, fuels boiling in the middle distillate range such as kerosene and diesel fuel, by the Fischer-Tropsch process.
Current industrial use of methane as a chemical feedstock typically begins with the 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 use of catalysts to effect the partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas, is also a process known in the art. Although it currently has limited commercial usage, catalytic partial oxidation has recently attracted much attention due to significant inherent advantages, including 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
+½O
CO+2H
2
  (2)
This ratio is more useful than the H
2
:CO ratio from steam reforming for conversion of syngas to chemicals such as methanol and to fuels. Furthermore, oxidation reactions are typically much faster than reforming reactions. Thus, much smaller reactors can be used for the partial oxidation reaction than are needed for steam reforming. 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.
U.S. Pat. No. 5,648,582 discloses a process for the catalytic partial oxidation of a feed gas mixture consisting of essentially methane. The methane-containing gas feed mixture and an oxygen-containing gas are passed over a supported metal catalyst at space velocities of 800,000 hr−1 to 12,000,000 hr−1. The catalytic metals are selected from the group consisting of rhodium, nickel and platinum.
M. Fathi et al.,
Catal. Today,
42, 205-209 (1998) disclose the catalytic partial oxidation of methane over Pt, Pt/Rh, Pt/Ir and Pd gauze catalysts at contact times of 0.21 to 0.33 milliseconds. Pt, Pt/5%Rh and Pt/10%Rh gauzes were tested under the same conditions at 700 to 1100° C. The best results were obtained at 1100° C. using the Pt/10%Rh gauze catalyst. The CH
4
conversion was about 30%; the oxygen conversion was about 60%; the CO selectivity was about 95%; and the hydrogen selectivity was about 30%.
European Patent No. 0640559A1 discloses a process for the partial oxidation of natural gas which is carried out by means of a catalyst constituted by one or more compounds of metals from the Platinum Group, which is given the shape of wire meshes, or is deposited on a carrier made from inorganic compounds, in such a way that the level of metal or metals from Platinum Group, as percent by weight, comprise within the range of from 0.1 to 20% of the total weight of catalyst and carrier. The partial oxidation is carried out at temperatures in the range of from 300 to 950° C., at pressures in the range of from 0.5 to 50 atmospheres, and at space velocities comprised in the range of from 20,000 to 1,500,000 h−1. The metal catalysts from the Platinum Group are selected from rhodium, ruthenium and iridium.
European Pat. No. 0576096A2 discloses a process for the catalytic partial oxidation of a hydrocarbon feedstock, which process comprises contacting a feed comprising the hydrocarbon feedstock, an oxygen-containing gas and, optionally, steam at an oxygen-to-carbon molecular ratio in the range of from 0.45 to 0.75, at elevated pressure with a catalyst in a reaction zone under adiabatic conditions. The catalyst comprises a metal selected from Group VIII of the Periodic Table and supported on a carrier and is retained within the reaction zone in a fixed arrangement having a high tortuosity. The process is characterized in that the catalyst comprises a metal selected from ruthenium, rhodium, palladium, osmium, iridium and platinum, and the fixed arrangement of the catalyst is in a form selected from a fixed bed of a particulate catalyst, a metal gauze and a ceramic foam.
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. Heretofore, typical 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 generally prevented these processes from becoming economically feasible.
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 conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort has been devoted in the art to the development of catalysts allowing commercial performance without coke formation.
A number of 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 loa

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