Compositions – Gaseous compositions – Carbon-oxide and hydrogen containing
Reexamination Certificate
2000-01-14
2004-04-27
Griffin, Steven P. (Department: 2754)
Compositions
Gaseous compositions
Carbon-oxide and hydrogen containing
C252S376000, C423S418200, C423S651000, C585S250000
Reexamination Certificate
active
06726850
ABSTRACT:
FIELD OF THE INVENTION
Catalytic partial oxidation (CPO) of low carbon number hydrocarbon feed streams, such as methane, to produce useful products, such as mixtures of hydrogen and carbon monoxide, the latter mixture also known as synthesis gas or syngas. Syngas is useful for the preparation of a variety of other valuable chemical compounds, such as by application of the Fischer-Tropsch process.
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, and the high temperatures generated present problems with respect to reactors, catalysts and other process equipment if efforts were made to produce valuable products from water and carbon dioxide.
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:
CH
4
+ O
2
→ CO + H
2
+ H
2
O
(−64 kcal/g mol CH
4
)
2CH
4
+ 1.5O
2
→ 2CO + 3H
2
+ H
2
O
(−34.9 kcal/g mol CH
4
)
CH
4
+ ½ O
2
→ CO + 2H
2
(−5.7 kcal/g mol CH
4
)
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.
Jacobs et al., U.S. Pat. No. 5,510,056 (Shell) discloses 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 at elevated pressures. The advance described in Jacobs is directed to the specifics of the catalyst support.
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 stage by cooling the reaction mixture in each stage.
GB Patent Application 2311790 discloses a two stage process and the use of a specifically defined catalyst whereby in a second stage a second synthesis gas is produced utilizing a first synthesis gas, as feed gas, with oxygen to cause partial oxidation of unreacted methane.
Staged oxygen addition has been disclosed as providing possible improvements solution to some of the difficulties encountered in CPO processes. “CO
2
Reforming and Partial Oxidation of Methane,” Topics in Catalysis 3 (1996) 299-311, recommends a staged addition of O
2
to the reactor during methane oxidation in a two stage process, including total methane oxidation followed by reforming in the presence of the formed CO
2
and H
2
O. Oxygen staging is said to lead to a flattening of the temperature profile along the reactor. However, it is also stated that “lowering the O
2
/CH
4
ratio will make carbon deposition thermodynamically more favorable and thus lead to deactivation of the catalyst.” (Id., p.308). The experimental results reported in this reference are expressed as a function of the catalyst-bed exit temperature rather than feed temperature.
“Partial Oxidation of Methane to Synthesis Gas via the Direct Reaction Scheme Over Ru/TiO
2
Catalyst,” Catalysis Letters 40 (1996) 189-195, discusses achieving an increase in selectivity to syngas in the presence of oxygen over a Ru/TiO
2
catalyst by multi-feeding oxygen. The increase is attributed to suppression of deep oxidation of H
2
and CO. Based on observations therein, the authors imply that “if oxygen is fed along the length of the catalyst bed instead of only at the entrance . . . and its local partial pressure is maintained at low levels, CO selectivity via the direct reaction scheme may be increased by attenuating oxidation of methane to CO
2
.” (Id., p.194).
“Methane Conversion to Ethylene and Acetylene: Optimal Control with Chlorine, Oxygen and Heat Flux,” Ind. Eng. Chem. Res. 35 (1996) 683-696, discloses conversion of methane to ethylene with controlled oxygen and heat flux. The process was conducted in the gas phase, i.e., the reference is directed to a non-catalytic gas phase methane reaction. Furthermore, oxygen is not present in the reactor initially; rather it was used as a manipulative variable.
“Kinetic-Transport Models of Bimodal Reaction Sequences-1. Homogeneous and Heterogeneous Pathways in Oxidative Coupling of Methane,” Chemical Engineering Science, 48 (1993) 2643-2661, discloses a reaction transport model. Staging oxygen introduction along the reactor length is found to minimize secondary oxidation reactions by lowering the local O
2
pressures, which leads to a slight increase in maximum yield, but also requires much larger reactor volumes. However, oxidative coupling differs from the technology disclosed by the present inventors in that it has as its objective the production of ethane and ethylene, rather than CO and H
2
(i.e., syngas).
“Partial Oxidation of Methane to Syngas Using Fast Flow Membrane Reactors,” ACS Div. Fuel Chem. Reprints 42(2) Apr. 13, 1997, discloses the use of a double bed reactor wherein both beds can be operated autothermally if oxygen is used as the downstream feed instead of carbon dioxide. The temperature of the second bed rapidly increases upon introduction of the downstream oxygen feed.
While the above references disclosed certain aspects of CPO and staged oxygen addition, there still exists a need for a CPO process that avoids undesirable gas phase reactions and maximizes yield and selectivity.
SUMMARY OF THE INVENTION
A multistage catalytic partial oxidation (CPO) process for oxidizing a hydrocarbon feedstream comprising C
1
-C
4
hydrocarbons, with an oxygen-containing feedstream to produce a product comprising CO and H
2
, said process conducted under CPO conditions in the presence of a CPO catalyst, wherein:
(A) the total oxygen requirement for said process is introduced incrementally from an oxygen-containing feedstream, the first of said incremental additions taking place in a first reaction stage, and subsequent said incremental addi
Androulakis Ioannis P.
Deckman Harry W.
Feeley Jennifer S.
Hershkowitz Frank
Reyes Sebastian C.
Griffin Steven P.
Medina Maribel
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