Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including solid – extended surface – fluid contact reaction...
Reexamination Certificate
2001-03-15
2004-09-28
Elve, M. Alexandra (Department: 1725)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including solid, extended surface, fluid contact reaction...
C422S224000
Reexamination Certificate
active
06797243
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to Fischer-Tropsch catalysts systems and processes for the production of heavier hydrocarbons from lighter hydrocarbons.
BACKGROUND OF THE INVENTION
A. Introduction to the Fischer Tropsch Process
The synthetic production of hydrocarbons by the catalytic reaction of carbon monoxide and hydrogen is well known and is generally referred to as the Fischer-Tropsch reaction. The Fischer-Tropsch process was developed in early part of the 20
th
century in Germany. It has been practiced commercially in Germany during World War II and later in South Africa.
The Fischer-Tropsch reaction for converting synthesis gas (primarily CO and H
2
) has been characterized by the following general reaction:
2
H
2
+
CO
à
Catalyst
—CH
2
—
+
H
2
O
The hydrocarbon products derived from the Fischer-Tropsch reaction range from methane to high molecular weight paraffinic waxes containing more than 100 carbon atoms.
Numerous catalysts have been used in carrying out the reaction, and both saturated and unsaturated hydrocarbons can be produced. The synthesis reaction is very exothermic and temperature sensitive whereby temperature control is required to maintain a desired hydrocarbon product selectivity.
B. Introduction to Synthesis Gas Production
Synthesis gas may be made from natural gas, gasified coal, and other sources. Three basic methods have been employed for producing the synthesis gas (“syngas”), which is substantially carbon monoxide and molecular hydrogen, utilized as feedstock in the Fischer-Tropsch reaction. The two traditional methods are steam reforming, wherein one or more light hydrocarbons such as methane are reacted with steam over a catalyst to form carbon monoxide and hydrogen, and partial oxidation, wherein one or more light hydrocarbons are combusted sub-stoichiometrically to produce synthesis gas. The steam reforming reaction is endothermic, and a catalyst containing nickel is often utilized. Partial oxidation is the catalytic or non-catalytic, sub-stoichiometric combustion of light hydrocarbons such as methane to produce the synthesis gas. The partial oxidation reaction is typically carried out using high purity oxygen. High purity oxygen, however, can be quite expensive.
In some situations these synthesis gas production methods may be combined to form the third method. A combination of partial oxidation and steam reforming, known as autothermal reforming, wherein air (or O
2
) is used as a source of oxygen for the partial oxidation reaction has also been used for producing synthesis gas heretofore. Autothermal reforming is a combination of partial oxidation and steam reforming where the exothermic heat of the partial oxidation supplies the necessary heat for the endothermic steam reforming reaction. The autothermal reforming process can be carried out in a relatively inexpensive refractory lined carbon steel vessel whereby low cost is typically involved.
The autothermal process results in lower hydrogen to carbon monoxide ratio in the synthesis gas than does steam reforming alone. That is, the steam reforming reaction with methane results in a ratio of about 3:1 or higher while the partial oxidation of methane results in a ratio of less than about 2:1. A good ratio for the hydrocarbon synthesis reaction carried out at low or medium pressure (i.e., in the range of about atmospheric to 500 psig) over a cobalt catalyst is about 2:1. When the feed to the autothermal reforming process is a mixture of light shorter-chain hydrocarbons, such as a natural gas stream, some form of additional control is required to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas at the optimum ratio (for cobalt based FT catalysts) of about 2:1. For this reason steam and/or CO
2
may be added to the synthesis gas reactor to adjust the H
2
/CO ratio to the desired value with the goal of optimizing process economics.
C. Introduction to F-T Reactors and Techniques
Numerous types of reactor systems have been used for carrying out the Fischer-Tropsch reaction. The developed Fischer-Tropsch reactor systems have included conventional fixed-bed, three-phase slurry bubble column designs, fluidized and/or moving bed, and ebullating beds to name a few. Due to the complicated interplay between heat and mass transfer and the relatively high cost of Fischer-Tropsch catalysts, no single reactor design has dominated the recent commercial development efforts.
Fixed-bed reactors have individual catalyst particles (typically less than 15 mm in the characteristic diameter) packed into tubes in cylindrical vessels. The individual particles of various shapes such as trilobes, spheres or cylinders, typically contain voidages on the order of about 0.3 to 0.5 depending upon the specific particle shape. These reactors offer simplicity and conversion kinetics that are easy to scale up to commercial sized units. Due to the high heat release and relatively low mass velocity associated with commercial operations, however, the reactor tube sizes are kept relatively small (typically less than two or three inches) when operating with a gas continuous system.
Fixed-bed Fischer-Tropsch reaction systems are primarily constrained by pressure drop and heat transport limitations. High productivity and good methane selectivity generally can be achieved with small particle sizes, typically on the order of less than 200 microns. In this context, “selectivity” refers to the following ratio: (moles of referenced product formed)/(mole of CO converted). The pressure drop, however, limits the practical application to much larger particle sizes for use in fixed-bed reactor systems. Shaped extrudates (trilobes, quadralobes, etc.) in the range of about {fraction (1/64)} to ⅛ inch in diameter are frequently used. Smaller size extrudates are infrequently used because they are difficult to manufacture in commercial quantities and create high pressure drops across the bed.
The heat transfer characteristics of such fixed-bed reactors are generally poor because of the relatively low mass velocity. If one attempts, however, to improve the heat transfer by increasing the velocity, higher CO conversion, which is the commercial goal, can be obtained but there is an excessive pressure drop across the reactor, which limits commercial viability. In order to obtain the CO conversions desired and gas throughputs of commercial interest, the needed conditions result in a high radial temperature profile. Due to the high heat of reaction, Fischer-Tropsch fixed-bed reactor diameters are generally less than three inches to avoid excessive radial temperature profiles. The use of high-activity catalysts in Fischer-Tropsch fixed-bed reactors, which must handle a large exothermic heat of reaction and which have poor effective thermal conductivity in the packed bed, may cause large radial temperature profiles to exist.
Further, the use of catalyst particle sizes greater than {fraction (1/64)} of an inch to avoid excessive pressure drop through the reactor results in high methane selectivity and low selectivities toward the high molecular weight paraffins, which generally have more economic value. This selectivity is due to a disproportional catalyst pore diffusion limitation on the rate of transport of reactants—CO and H
2
—into the interior of the catalyst pellets. To address the situation, the use of catalysts having the active metal component restricted to a thin layer about the outer edge of the particle has been suggested. These catalysts appear costly to prepare and do not appear to make good use of the available reactor volume. Other fixed-bed reactor system alterations have also been proposed.
U.S. Pat. No. 5,786,393 presents the use of liquid recycles as a means of improving the overall performance in a fixed-bed design. This art has been referred to by some as a “trickle bed” reactor (as part of a subset of fixed-bed reactor systems) in which both reactant gas and an inert liquid are introduced (preferably in an upflow or down flow orientation wit
Agee Kenneth L.
Agee Mark A.
Arcuri Kym B.
Baker & Botts L.L.P.
Elve M. Alexandra
Syntroleum Corporation
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