Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2000-05-02
2003-02-25
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C423S651000, C423S652000, C423S655000, C423S656000
Reexamination Certificate
active
06524550
ABSTRACT:
TECHNICAL FIELD
The present invention relates to methods for producing hydrogen from hydrocarbon fuels and reactors for carrying out the methods; and more particularly to methods, apparatus, and catalysts for conducting water gas shift reactions on a reactant stream of hydrocarbon fuels having been previously reformed by partial oxidation, steam reforming, or both.
BACKGROUND OF THE INVENTION
Reforming of hydrocarbon fuels to make hydrogen is well known in the art. Conventionally, hydrocarbons are reformed predominately in large-scale industrial facilities providing hydrogen for bulk storage and redistribution, or producing hydrogen as an on-line, upstream reagent for another large-scale chemical process. For the most part, these prior processes operate continuously and at steady-state conditions.
More recently, however, a strong interest has developed in providing hydrocarbon-reforming reactors integrated with an end use of the hydrogen. Also, there is a strong interest to develop a low-cost, small-scale source for hydrogen that can replace the need for storing hydrogen gas on site or on board. More particularly, a great interest has developed in providing reactors for producing hydrogen, which can be integrated with a fuel cell which uses hydrogen as a fuel source to generate electricity. Such hydrogen generator/fuel cell systems are being pursued for stationary uses such as providing electrical power to a stationary facility (home or business), for portable electric power uses, and for transportation.
There are many technical requirements for reactors used in such applications, which are not required of traditional large or small-scale hydrogen generating reactors. For example, it is of particular interest to have such a system where the fuel cell can provide “power on demand.” Hence, hydrogen must be produced at required variable levels on demand. In other words, the hydrogen producing reactors must be sufficiently dynamic to follow the load. It is also of interest that such systems perform well upon start up and shutdown cycling. In particular, it is desirable to have these integrated systems be stable through repeated on-off cycling, including being ready to come back on-line in a relatively short time after periods of non-use.
Another marked difference between proposed integrated systems and traditional reactors is that there must be sufficient processing in the integrated system itself, and of the hydrocarbon feed stock so as to not only give a yield of hydrogen sufficient to meet the demand, but also to minimize byproducts of reaction including contaminants. In large-scale reactor systems, which produce enormous volumes and run continuously, space, weight, and cost of auxiliary systems is not so critical as in the integrated, smaller-scale reformers, especially those proposed for portable power or transportation applications. For example, carbon monoxide may be considered an undesirable reaction product on board a fuel cell powered automobile. However, in a steady state conventional process, the carbon monoxide can easily be handled by auxiliary separation systems, and may in fact be welcomed for its use in a synthesis gas to make acetic acid, dimethyl ether and alcohols.
In short, the challenge for the smaller-scale, dynamic, integrated processors is the idea that what goes in the reformer must come out at the same end as the desired hydrogen gas. Accordingly, processing has to be more complete and more efficient, while cost effective, lightweight, and durable. The processing must be sufficient to reduce or eliminate species in the product gas which are harmful to the end use (for example, fuel cells) or other down stream components.
Another challenge exists for the proposed integrated systems with respect to the hydrocarbon feed stock. To be of maximum benefit, the proposed integrated systems should be able to use existing infrastructure fuels such as gasoline or diesel fuels. These fuels were not designed as a feed stock for generating hydrogen. Because of this, integrated systems are challenged to be able to handle the wide variety of hydrocarbons in the feed stock. For example, certain reforming byproducts such as olefins, benzene, methyl amide, and higher molecular weight aromatics can cause harm to catalysts used in reforming or purifying steps and may harm the fuel cell itself. Impurities in these fuels such as sulfur and chlorine can also be harmful to reactor catalysts and to the fuel cell.
It is also important to note that a natural byproduct of hydrocarbon reforming is carbon monoxide. Carbon monoxide can poison proton exchange membrane (“PEM”) fuel cells, even at very low concentrations, e.g., less than 100 PPM. This poses a problem for an integrated reactor system that is not faced by traditional reforming processes where significant carbon monoxide concentrations are either a useful co-product, or can be separated from the product gas without undue burden on the system economics as a whole.
Also, as noted above, integrated systems proposed to date are expected to transfer the total of the reformate to a fuel cell. Accordingly, techniques which separate carbon monoxide from hydrogen, such as pressure swing adsorption (“PSA”) or hydrogen permeable membrane separation, have the deficit of having to provide an alternate means for disposal or storage of the carbon monoxide. Both of the aforementioned techniques also suffer in efficiency as neither converts the carbon monoxide (in the presence of water) to maximize hydrogen production. PSA also suffers from high cost and space requirements. Most notably, PSA presents a likely unacceptable parasitic power burden for portable power or transportation applications. At the same time, hydrogen permeable membranes are expensive, are sensitive to fouling from impurities in the reformate, and reduce the total volume of hydrogen provided to the fuel cell.
At the levels of carbon monoxide present in the reformate stream after partial oxidation, steam reforming or ATR (for example, less than or equal to about 20% carbon monoxide), catalytic techniques such as preferential oxidation (“PROX”) or selective methanation are not efficient options. Although it should be noted that PROX and selective methanation may both be appropriate as a secondary, or clean up, process at suitably low carbon monoxide levels. For example, PROX appears to be suitable for oxidizing carbon monoxide at residuals of 20,000 PPM or less.
On the other hand, implementing and using water gas shift reactions does not present the impairments of the above-discussed techniques. Hence use of a water gas shift reactor is highly preferred.
Reformation of hydrocarbons (for example, alcohols, methane, propane, butane, pentane, hexane, and various other gaseous and liquid petroleum fractions saturated and unsaturated, cyclic compounds, aromatic compounds, etc.) may be subjected to some form of partial oxidation to create a reformate enriched in hydrogen. This partial oxidation can be accomplished by a flame-type gas-phase reaction or can be catalytically promoted, for example by a nickel-containing catalyst. Water in the form of steam may be added to prevent coking of the hydrocarbons during oxidation. Reformate composition varies widely with the type of hydrocarbon fuel or feed stock and with the efficacy of the particular partial oxidation process employed. However, reformate generated in this way generally includes varying amounts of carbon monoxide, carbon dioxide, water, nitrogen, trace amounts of hydrogen sulfide, and in the case of partial oxidation, ammonia. Beyond these chemicals, the remainder of the reformate being methane, ethane and depending on the fuel, other higher molecular weight hydrocarbons including: unsaturated and aromatic species; oxygenated species such as ethers, esthers, alcohols, aldehydes, etc.
Steam reforming may also be used to produce hydrogen by promoting the following reaction Equation 1, with a catalyst such as a nickel supported on a refractory material:
C
n
H
m
+nH
2
O→nCO+(m/2+n)H
2
where n=an inte
Chintawar Prashant S.
Hagan Mark R.
Thompson Craig
Langel Wayne A.
Wallenstein and Wagner, Ltd.
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