Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including solid – extended surface – fluid contact reaction...
Reexamination Certificate
1999-05-07
2001-03-13
Beck, Shrive (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including solid, extended surface, fluid contact reaction...
C422S239000, C422S240000, C048S198100, C502S004000
Reexamination Certificate
active
06200541
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to composite materials for membrane reactors which include a gas-tight ceramic, a porous support, and an interfacial zone therebetween. More particularly, this invention relates to composite materials using oxygen ion-conducting dense ceramic membranes formed on a porous support comprising a metallic alloy to provide an interfacial zone of chemical interaction between the dense ceramic membrane and the porous support. Typically, chemical interactions are identifiable by a gradient of composition in at least one metallic element across the interfacial zone between the dense ceramic membrane and the porous support. Chemical interactions preferably match thermal expansion coefficients and other physical properties between the two different materials.
Processes using composite materials in accordance with the invention include converting methane gas into value-added-products, for example, production of synthesis gas comprising carbon monoxide and molecular hydrogen in which the synthesis gas advantageously is free of deleterious and/or inert gaseous diluents such as nitrogen.
BACKGROUND OF THE INVENTION
Conversion of low molecular weight alkanes, such as methane, to synthetic fuels or chemicals has received increasing attention as low molecular weight alkanes are generally available from secure and reliable sources. For example, natural gas wells and oil wells currently produce vast quantities of methane. In addition, low molecular weight alkanes are generally present in coal deposits and may be formed during mining operations, in petroleum processes, and in the gasification or liquefaction of coal, tar sands, oil shale, and biomass.
Many of these alkane sources are located in relatively remote areas, far from potential users. Accessibility is a major obstacle to effective and extensive use of remotely situated methane, ethane and natural gas. Costs associated with liquefying natural gas by compression or, alternatively, constructing and maintaining pipelines to transport natural gas to users are often prohibitive. Consequently, methods for converting low molecular weight alkanes to more easily transportable liquid fuels and chemical feedstocks are desired and a number of such methods have been reported.
Reported methods can be conveniently categorized as direct oxidation routes and/or as indirect syngas routes. Direct oxidative routes convert lower alkanes to products such as methanol, gasoline, and relatively higher molecular weight alkanes. In contrast, indirect syngas routes involve, typically, production of synthesis gas as an intermediate product.
As is well known in the art, synthesis gas (“syngas”) is a mixture of carbon monoxide and molecular hydrogen, generally having a dihydrogen to carbon monoxide molar ratio in the range of 1:5 to 5:1, and which may contain other gases such as carbon dioxide. Synthesis gas has utility as a feedstock for conversion to alcohols, olefins, or saturated hydrocarbons (paraffins) according to the well known Fischer-Tropsch process, and by other means. Synthesis gas is not a commodity; rather, it is typically generated on-site for further processing. At a few sites, synthesis gas is generated by a supplier and sold “over the fence” for further processing to value added products. One potential use for synthesis gas is as a feedstock for conversion to high molecular weight (e.g. C
50+
) paraffins which provide an ideal feedstock for hydrocracking for conversion to high quality jet fuel and superior high cetane value diesel fuel blending components. Another potential application of synthesis gas is for large scale conversion to methanol.
In order to produce high molecular weight paraffins in preference to lower molecular weight (e.g. C
8
to C
12
) linear paraffins, or to synthesize methanol it is desirable to utilize a synthesis gas feedstock having an H
2
:CO molar ratio of about 2.1:1, 1.9:1, or less. As is well known in the art, Fischer-Tropsch syngas conversion reactions using syngas having relatively high H
2
:CO ratios produce hydrocarbon products with relatively large amounts of methane and relatively low carbon numbers. For example, with an H
2
:CO ratio of about 3, relatively large amounts of C1-C8 linear paraffins are typically produced. These materials arc characterized by very low octane value and high Reid vapor pressure, and are highly undesirable for use as gasoline.
Lowering the H
2
:CO molar ratio alters product selectivity by increasing the average number of carbon atoms per molecule of product, and decreasing the amount of methane and light paraffins produced. Thus, it is desirable for a number of reasons to generate syngas feedstocks having molar ratios of hydrogen to carbon monoxide of about 2:1 or less.
Prior methods for producing synthesis gas from natural gas (typically referred to as “natural gas reforming”) can be categorized as (a) those relying on steam reforming where natural gas is reacted at high temperature with steam, (b) those relying on partial oxidation in which methane is partially oxidized with pure oxygen by catalytic or non-catalytic means, and (c) combined cycle reforming consisting of both steam reforming and partial oxidation steps.
Steam reforming involves the high temperature reaction of methane and steam over a catalyst to produce carbon monoxide and hydrogen. This process, however, results in production of syngas having a high ratio of hydrogen to carbon monoxide, usually in excess of 3:1.
Partial oxidation of methane with pure oxygen provides a product which has an H
2
:CO ratio close to 2:1, but large amounts of carbon dioxide and carbon are co-produced, and pure oxygen is an expensive oxidant. An expensive air separation step is required in combined cycle reforming systems, although such processes do result in some capital savings since the size of the steam reforming reactor is reduced in comparison to a straightforward steam reforming process.
Although direct partial oxidation of methane using air as a source of oxygen is a potential alternative to today's commercial steam-reforming processes, downstream processing requirements cannot tolerate nitrogen (recycling with cryogenic separations is required), and pure oxygen must be used. The most significant cost associated with partial oxidation is that of the oxygen plant. Any new process that could use air as the feed oxidant and thus avoid the problems of recycling and cryogenic separation of nitrogen from the product stream will have a dominant economical impact on the cost of a syngas plant, which will be reflected in savings of capital and separation costs.
Thus, it is desirable to lower the cost of syngas production as by, for example, reducing the cost of the oxygen plant, including eliminating the cryogenic air separation plant, while improving the yield as by minimizing the co-production of carbon, carbon dioxide and water, in order to best utilize the product for a variety of downstream applications.
Dense ceramic membranes represent a class of materials that offer potential solutions to the above-mentioned problems associated with natural gas conversion. Certain ceramic materials exhibit both electronic and ionic conductivities (of particular interest is oxygen ion conductivity). These materials not only transport oxygen (functioning as selective oxygen separators), but also transport electrons back from the catalytic side of the reactor to the oxygen-reduction interface. As such, no external electrodes are required, and if the driving potential of transport is sufficient, the partial oxidation reactions should be spontaneous. Such a system will operate without the need of an externally applied electrical potential. Although there are recent reports of various ceramic materials that could be used as partial oxidation ceramic membrane, little work appears to have been focused on the problems associated with the stability of the material under methane conversion reaction conditions.
European Patent Application 90305684.4, published on Nov. 28, 1990, under Publication No. EP0
Kleefisch Mark S.
Kobylinski Thaddeus P.
Masin Joseph G.
Udovich Carl A.
Beck Shrive
BP Amoco Corporation
Ohorodnik Susan
Yassen Thomas A.
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