Compositions – Gaseous compositions – Carbon-oxide and hydrogen containing
Reexamination Certificate
2001-04-24
2004-02-24
Langel, Wayne A. (Department: 1754)
Compositions
Gaseous compositions
Carbon-oxide and hydrogen containing
Reexamination Certificate
active
06695983
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of producing syngas by converting methane and other hydrocarbons in a gas feed into hydrogen and carbon monoxide. More particularly, the present invention relates to such a method in which the methane is converted in two or more stages to allow an initial catalytic reforming stage formed by an oxygen transport membrane reactor to be operated at a lower temperature than a subsequent stage to protect the structural integrity of the membrane.
BACKGROUND OF THE INVENTION
The use of oxygen transport membranes in high temperature catalytic reactors for the generation of syngas has received significant attention in the recent past because of the economic incentives created by combining oxygen separation and the syngas generating oxidation and reforming reactions in a single process unit. The process involves bringing a mixture of hydrocarbons, steam, and optionally carbon dioxide in contact with the anode side of an electron and oxygen ion conducting ceramic membrane which at high temperature permeates oxygen from an oxygen containing gas, typically air, on the cathode side to the anode. The preferred membranes consist of mixed conducting metal oxide films supported by multi-layer porous structures which near the membrane film enhance surface exchange and are mechanically and chemically compatible. The permeated oxygen will react with the fuel gas in a partial oxidation reaction, which provides the energy for a simultaneous catalytically enhanced reforming reaction to produce a mixture of hydrogen and carbon monoxide, i.e. syngas.
The partial oxidation reaction for methane is shown in Equation 1. The steam reforming reaction for methane is shown in Equation 2. Additional conversion of carbon monoxide may occur with the exothermic water gas shift reaction, Equation 3. The scope of the present invention also includes reforming reactions between methane and CO
2
as indicated by equation (4).
CH
4
+½O
2
→CO+2 H
2
(1)
CH
4
+H
2
O→CO+3 H
2
(2)
CO+H
2
O→CO
2
+H
2
(3)
CH
4
+CO
2
→2CO+2H
2
(4)
U.S. Pat. No. 4,793,904 describes an electrocatalytic process for producing synthesis gas from light hydrocarbons. The process involves passing an oxygen containing gas over the cathode side of an oxygen ion conducting membrane and permeating oxygen from the cathode to the anode to react with light hydrocarbons and thus producing a synthesis gas. U.S. Pat. No. 5,714,091 discloses a process for generating syngas using an autothermal reactor comprising an oxygen ion conducting membrane. The process stages a partial oxidation reaction with a catalytic reforming reaction where heat is transferred from the former to the latter.
Typically the syngas is produced at high pressure (10 to 40 bar) to avoid cost associated with compressing the low density syngas. To achieve high conversion of methane at the desired hydrogen to carbon monoxide ratios requires favorable equilibrium conditions for the reactant gases which at the prevailing pressures can be obtained only at relatively high temperatures, e.g. 900 to 1100° C. Unfortunately, at these temperatures the preferred membrane materials have low creep strength which leads to limited membrane life and/or complicated structural supports. The problem is further aggravated by the fact that the actual temperatures experienced by the membrane are even higher since a temperature gradient is needed to dissipate the heat of the exothermic oxidation reaction, occurring at or near the anode surface, to the endothermic reforming reaction in the adjacent catalyst structure.
Another problem relates to solid carbon formation, which can be especially acute when the feed stock contains hydrocarbons heavier than methane and it is desired to operate at low steam to carbon ratios. Even with only moderate amounts of hydrocarbons with more than one carbon atom in the natural gas feed, and at the low steam to carbon ratios (<1) desired for processes producing syngas with hydrogen to carbon monoxide ratios of about two or less, permissible inlet temperatures for the reactor, i.e. values above which free carbon formation would occur, would be below the level where appreciable oxygen ion transport rates can be realized. U.S. Pat. No. 6,077,323 addresses syngas generation using mixed conducting ceramic membranes. It discloses operating temperatures for the membrane reactor which avoid solid carbon formation and specifies a higher total pressure on the anode than on the cathode side. Beyond the use of a prereformer the patent does not involve a staged process to optimize operation of the membrane reactor-reformer or limit operating temperatures of the membrane. U.S. Pat. No. 6,048,472 by Nataraj et al. stages a catalytic prereformer, operating at a lower temperature, ahead of the membrane reactor to eliminate carbon formation, especially with higher hydrocarbons, and raise the permissible inlet temperature for the membrane react. It is to be noted that the prior art routinely employs prereformers in conjunction with conventional autothermal or externally fired reformers to eliminate solid carbon formation with heavier feed stocks and or at low steam to carbon ratios. Representative patents illustrating the use of prereformers in conjunction with autothermal and conventional steam reformers are U.S. Pat. Nos. 5,252,609, 4,631,182 and 4,824,658.
As will be discussed, the present invention presents a multi-stage processes for syngas generation by oxygen transport membrane reactors that include prereformers or catalytic partial oxidation reactors for pretreatment of the hydrocarbon feed streams and oxygen transport membrane reactor-reformers coupled with subsequent reforming or autothermal reforming reactors to allow the oxygen transport membrane reactor-reformers to operate at sufficiently low temperatures to address structural problems with preferred oxygen transport membrane materials.
SUMMARY OF THE INVENTION
The present invention provides a method of producing a crude syngas product stream or a syngas product stream by further processing of the crude syngas product stream. Both the crude and syngas product stream comprise carbon monoxide and hydrogen. The crude syngas product stream additionally comprises carbon dioxide and moisture. In accordance with the method, methane in a feed stream comprising methane is converted into the hydrogen and carbon monoxide in at least two stages, thereby to form a crude syngas stream. The methane is converted at least in part by carbon dioxide or steam methane reforming in one of the at least two stages operated at a lower temperature than a subsequent of the at least two stages. The one of the at least two stages has at least one oxygen transport membrane to separate oxygen from an oxygen containing gas, thereby permeating oxygen from a cathode to an anode side thereof, and a reforming catalyst located adjacent said anode side of the at least one oxygen transport membrane to promote the reforming of the methane. The reforming of the methane is thermally balanced through heat generated by oxidation of fuel species supported by the oxygen permeating through said at least one oxygen transport membrane. The subsequent of the at least two stages can be a fired reformer or an autothermal reformer, sometimes also called an oxygen blown reformer. In case of the formation of a syngas product stream, water and at least part of the carbon dioxide is removed from said crude syngas stream. In certain downstream processes it is advantageous to leave some of the carbon dioxide in the syngas product stream. Part of said syngas product stream can be recycled to form part of the feed stream as can be off-gases from downstream synthesis reactions.
Preferably, the methane in the feed stream can be converted into the hydrogen and carbon monoxide by partial oxidation of the methane prior to the reforming of the methane within an entrant section of the at least one oxygen transport membrane
Gottzmann Christian Friedrich
Prasad Ravi
Robinson Earl T.
Schwartz Joseph Michael
Langel Wayne A.
Praxair Technology Inc.
Rosenblum David M.
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