Catalytic oxidation process

Details Catalytic oxidation process Catalytic oxidation process

2000-08-01

2002-09-10

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Hydrogen or compound thereof

Elemental hydrogen

C423S437100, C423S651000, C423S418200, C252S373000

Reexamination Certificate

active

06447745

ABSTRACT:

FIELD OF THE INVENTION
Process for the catalytic oxidation of hydrocarbons and other compounds utilizing ceramic foam supported catalysts. Included are processes for the preparation of carbon monoxide and hydrogen (certain mixtures of which are also known as synthesis gas or syngas) by catalytic partial oxidation (CPO) of low carbon number hydrocarbon feed streams, such as methane; catalytic oxidation of ethylene; catalytic oxidation of ammonia, etc. Syngas is useful for the preparation of a variety of other valuable chemical compounds, such as by application of the Fischer-Tropsch process; ethylene oxide and nitric acid also have significant commercially utility. Additionally, catalytic oxidation can be applied to processes for the reaction of hydrocarbon fuel with air and steam under conditions suitable for use in fuel cell reformer applications.
1. Background of the Invention
The combustion of methane gas at elevated temperature, e.g., 1000° F. (538° C.) is highly exothermic and produces CO
2
and H
2
O according to the following stoichiometry:
CH
4
+2O
2
→CO
2
+2H
2
O  (−190.3 kcal/g mol CH
4
)
The gases formed in such a reaction are not directly useful for the production of valuable chemical compounds. Furthermore, if efforts were made to produce valuable products from water and carbon dioxide, the high temperatures generated would present problems with respect to reactors, catalysts and other process equipment.
Conversely, it is known to produce a chemically useful mixture of CO and H
2
gases, also known as synthesis gas or syngas, from methane and other light hydrocarbon gases, by various reactions, including partial oxidation, steam- or CO
2
-reforming, or a combination of these chemistries. The partial oxidation reaction of methane is a less highly exothermic reaction which, depending upon the relative proportions of the methane and oxygen and the reaction conditions, can proceed according to the following reaction paths:
It is most desirable to enable the partial oxidation reaction to proceed according to the last reaction scheme. This results in certain advantages, including: (1) producing the most valuable syngas mixture; (2) minimizing the amount of heat produced (thereby protecting the apparatus and the catalyst bed); and (3) reducing the formation of steam (thereby increasing the yield of hydrogen and carbon monoxide). Any incidentally produced, or added, steam can be further converted by the steam-reforming reaction into additional useful syngas components.
Fuel cells produce electricity by converting reactants such as hydrogen and oxygen into products such as water. One method of providing hydrogen for use in a fuel cell is to oxidize a liquid hydrocarbon fuel in a fuel cell reformer that is a component of the fuel cell system. Improving the conversion of liquid hydrocarbons to hydrogen for use as a fuel in fuel cell processes is part of the continuing effort to commercialize fuel cell technology.
Ammonia oxidation is used to produce nitric acid by oxidizing ammonia in the presence of the catalyst, typically a pad of platinum-containing gauzes, to produce hot nitric oxide. The nitric oxide is then quenched and oxidized, e.g., in the presence of air, to form nitrogen (IV) oxides, which then react with water to form nitric acid. Uneven flow distribution of the ammonia through the gauze pad (i.e., the catalyst bed) can result in an undesirable reaction that produces nitrogen and water; improvements in catalyst technology could improve such a commercially important process.
The oxidation of ethylene to ethylene oxide is typically carried out in the presence of a silver-containing catalyst supported on a low surface area (<1 m
2
/g) alpha-alumina carrier or substrate. It has been suggested that such a substrate provides large pores that are effective in avoiding a diffusion limited reaction regime having reduced selectivity (C. N. Satterfield, “Heterogeneous Catalysis in Industrial Practice” , 282, 2d Ed.,1991, McGraw-Hill). Improved catalyst technology, including improvements to the carrier or substrate per se, may thus lead to improvements in this commercially valuable process.
Copending patent application, U.S. Ser. No. 08/484378, filed Jan. 14, 2000, describes a multistage CPO process wherein control of specific process conditions, including preheat of the gaseous feedstreams and control of interstage temperature and the number of stages, leads to improved methane conversion and syngas product (hydrogen and CO) selectivity (the text of this application is incorporated herein for all permitted purposes).
A catalytic partial oxidation process utilizing a ceramic foam catalyst support is described by Vonkeman et al. in EP 576 096 B1 (Shell). The support is disclosed as having a high “tortuosity”, which is defined as the ratio of the length of the path followed by a gas flowing through a bed of the catalyst to the length of the shortest straight line path through the catalyst bed. The general range of conditions disclosed for conducting a CPO process with such a catalyst includes an oxygen to carbon molar ratio in the range of from 0.45 to 0.75, elevated pressure (up to 100 bar), space velocity of from 20,000 to 25,000,000 Nl/l/hr, hydrocarbon and oxygen-containing gas, feed preheat, and a reaction zone necessarily under adiabatic conditions. The catalyst was prepared by impregnating a commercially available, particulate alpha-alumina carrier with an aqueous solution of chloroplatinic acid, followed by drying and calcining in order to deposit platinum.
Kumar et al., WO 96/16737 (Shell) also describe a specific process for the preparation of a high tortuosity ceramic foam support impregnated with high loadings of an inorganic oxide. A catalyst containing a catalytically active component is prepared with the support for use in a hydrocarbon feedstock oxidation process.
Van Grinsven, et al., EP 537 862 A1 (Shell), describe a CPO process using a noble metal catalyst supported on an alpha-alumina support having “very large pores”, defined as requiring that at least 90% of the pore volume are pores greater than 2,000 nm (equivalent to 20,000 angstroms or 2 microns). Optionally the supported catalyst can be prepared using a co-impregnated salt of at least one metal which forms, upon calcination, an oxide that cannot easily be reduced (Al
2
O
3
being most preferred).
Jacobs et al., U.S. Pat. No. 5,510,056 (Shell) disclose that successful operation of a CPO process on a commercial scale requires high conversion of the hydrocarbon feedstock at high space velocities, using mixtures of an oxygen-containing gas and methane in a preferred O
2
to carbon atom ratio (in the region of the stoichiometric ratio of about 1:2, or 0.5), which mixtures are preferably preheated and are at elevated pressures. The advance described in Jacobs also requires the use of a high tortuosity, high porosity catalyst carrier.
It is disclosed in EP 303 438 (assigned to Davy McKee Corp.), to conduct a CPO process using previously formed mixtures of high temperature, high pressure methane and oxygen gases and, optionally, steam at space velocities up to 500,000 hr
−1
, using a mixing and distributing means in order to thoroughly premix the gases prior to introduction to the catalyst. It is the objective in this disclosure to operate the CPO process in a mass-transfer-controlled regime and to introduce the gas mixture at or above its autoignition temperature.
EP 842 894 A1 discloses a process and apparatus for catalytic partial oxidation of a hydrocarbon wherein the use of several stages is proposed. The reference states that in each stage there is used “a small fraction of the stoichiometric amount of oxygen required for the reaction” to prevent the generation of high temperatures in the reactor as a consequence of “excessive” concentrations of oxygen. Furthermore, it is disclosed that the hydrocarbon feed is mixed with oxygen and preheated to a temperature in the range of 300-400° C. and the reaction is performed at substantially the same temperature in all s

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