Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2000-08-21
2002-12-24
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C252S373000, C422S198000, C422S198000, C422S198000, C422S201000, C422S211000, C423S655000
Reexamination Certificate
active
06497856
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an integrated chemical reactor for the production of hydrogen from hydrocarbon fuels such as natural gas, propane, liquefied petroleum gas, alcohols, naphtha and other hydrocarbon fuels and having a unique unitized, multifunctional structure. The integrated reactor offers significant advantages such as lower heat loss, lower parts count, lower thermal mass, and greater safety than the many separate components employed in conventional systems to achieve the same end. The integrated reactor is especially well-suited to applications where less than 15,000 standard cubic feet per hour of hydrogen are required.
The present invention also relates to the generation of hydrogen for use in industrial applications, as a chemical feedstock, or as a fuel for stationary or mobile power plants.
2. Discussion of the Background
Hydrogen production from natural gas, propane, liquefied petroleum gas (LPG), alcohols, naphtha and other hydrocarbon fuels is an important industrial activity. Typical industrial applications include feedstock for ammonia synthesis and other chemical processes, in the metals processing industry, for semiconductor manufacture and in other industrial applications, petroleum desulfurization, and hydrogen production for the merchant gas market. The demand for low-cost hydrogen at a smaller scale than produced by traditional industrial hydrogen generators has created a market for small-scale hydrogen production apparatus (<15,000 standard cubic feet per hour (scfh)). This demand has been augmented by the growing enthusiasm for hydrogen as a fuel for stationary and mobile powerplants, especially those employing electrochemical fuel cells, which require hydrogen as a fuel.
Hydrogen is typically produced from hydrocarbon fuels industrially via chemical reforming using combinations of steam reforming and partial oxidation. This is typically achieved at scales larger than one ton per day using well-known process and catalyst designs. For several reasons, it is difficult to adapt these large-scale technologies to economically produce hydrogen at small scales. Typical industrial applications produce far more than 15,000 standard cubic feet per hour (~1 ton per day), and often employ catalytic steam reforming of light hydrocarbons in radiantly-fired furnaces. Steam reforming of hydrocarbons is illustrated for the simple case of methane below.
CH
4
+H
2
O→CO+3H
2
The above reaction is highly endothermic, and the reacting fluid must have energy transferred to it for the reaction to proceed. Further, the extent of the reaction is low at low temperatures, such that greatly elevated temperatures, often as high as 800° C., are required by conventional systems to convert an acceptable amount of hydrocarbon to hydrogen and carbon monoxide. The catalyst employed in industrial reactors is typically composed of an active nickel metal component supported on a ceramic support.
The radiantly-fired furnaces employed in large-scale industrial reactors have many disadvantages that make them unsuitable for small-scale systems. The most important disadvantage is the very high temperature of the radiant burners and the gas contacting the reactor surfaces, which are usually tubular in form. The temperature of the radiant burners often approaches or exceeds the melting temperature of the alloy from which the tubes are fabricated. Melting of the tubes is prevented by the rapid endothermic catalytic reaction inside the tubes. If, however, the catalyst fails due to carbon formation, sulfur poisoning or other causes, then the tubes form what is referred to in the literature as a “hot spot,” which greatly accelerates the failure of the reactor tube in question. In large-scale systems, careful monitoring and control of the furnace and tube temperatures as well as exceptionally rugged construction of the tubes makes the risks of hot spots acceptable. For systems producing below 1 ton per day, however, the complexity and cost of such safety measures can become prohibitive. Nonetheless, small-scale steam reformers utilizing radiant heat transfer are known and described, for example, in U.S. Pat. No. 5,484,577 to Buswell, et al. The extreme measures necessary to control the temperature in arrays of reformer tubes are likewise documented in U.S. Pat. No. 5,470,360 to Sederquist.
A means of transferring the necessary heat to the reacting gases without radiant heat transfer and its attendant risks, which is especially well-suited to small-scale steam reforming, is the use of compact heat exchange surfaces, such as arrays of tubes or finned-plates. The heat transfer mechanism in such devices is dominated by convection and conduction with minimal radiant transfer. An example of this approach is described in U.S. Pat. No. 5,733,347 to Lesuir, wherein finned plates are employed to increase heat transfer. Tubular compact heat exchangers for steam reforming are sold by Haldor Topsoe, Inc. of Houston, Tex.
Conventional hydrogen generation systems employing steam reforming of hydrocarbon fuels typically include three main reaction steps for producing hydrogen; steam reforming, high-temperature water gas shift, and low temperature water gas shift. The important reactions for methane are as follows:
CH
4
+H
2
O→CO+3H
2
steam reforming
CO+H
2
O→CO
2
+H
2
water gas shift
It is evident from the equation for steam reforming of hydrocarbon fuel that the principal products are hydrogen and carbon monoxide. The carbon monoxide may be converted into additional hydrogen via a catalytic reaction with steam (water gas shift reaction).
The water gas shift reaction is mildly exothermic and thus is thermodynamically favored at lower temperatures. However, the kinetics of the reaction are superior at higher temperatures. Thus, it is common practice to first cool the reformate product from the steam reformer in a heat exchanger to a temperature between 350° C. and 500° C. and conduct the reaction over a catalyst composed of finely divided oxides of iron and chromium formed into tablets. The resulting reformate gas is then cooled once again to a temperature between 200° C. and 250° C. and reacted over a catalyst based upon mixed oxides of copper and zinc. An example of this approach is given in U.S. Pat. No. 5,360,679 to Buswell, et al. In cases where an exceptionally pure hydrogen product is required, the temperature of the low-temperature shift converter is controlled by including a heat exchanger in the reactor itself, and an example of this approach is given in U.S. Pat. No. 5,464,606 to Buswell, et al. In all cases, the low temperature shift converter is quite large because of the poor catalyst activity at low temperatures.
In conventional systems, subsets of the process components are connected to one another via external plumbing; each component of the process being typically referred to as a “unit process,” in the chemical engineering literature. This approach is preferred in large, industrial units because standard hardware may be used. Owing to the large size of industrial units, the unit process approach also makes shipping of the components to the site of the installation feasible, as combinations of the components are sometimes too large to be transported by road or rail.
For systems producing less than 1 ton per day, however, the unit process approach has many disadvantages. The first disadvantage is the high proportion of the total system mass dedicated to the hardware and plumbing of the separate components. This high mass increases startup time, material cost, and system total mass, which is undesirable for mobile applications such as powerplants for vehicles.
Another disadvantage of the unit process approach in small systems is the complexity of the plumbing system to connect the components. The complexity increases the likelihood of leaks in the final system, which presents a safety hazard, and also significantly increases the cost of the assembly process itself. Moreover, the requiremen
Barbour Jason P.
Lomax, Jr. Franklin Delano
Reardon John P.
H2GEN Innovations, Inc.
Medina Maribel
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Silverman Stanley S.
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