Chemistry of hydrocarbon compounds – Adding hydrogen to unsaturated bond of hydrocarbon – i.e.,... – Hydrocarbon is aromatic
Reexamination Certificate
2002-08-20
2004-06-15
Dang, Thuan D (Department: 1764)
Chemistry of hydrocarbon compounds
Adding hydrogen to unsaturated bond of hydrocarbon, i.e.,...
Hydrocarbon is aromatic
C585S270000
Reexamination Certificate
active
06750374
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for producing cyclohexane by benzene hydrogenation, and, more particularly, to a process for producing cyclohexane by benzene hydrogenation using a hydrogen source that contains impurities.
BACKGROUND OF THE INVENTION
Over the years, researchers have developed numerous processes for manufacturing cyclohexane from the hydrogenation of benzene. For the most part, the majority of these various processes differ from each other in the techniques used to compensate for impurities, found in either the reaction components themselves or that are generated during the hydrogenation process.
For example, U.S. Pat. No. 3,711,566 (Estes et al.) describes a process in which aromatic hydrocarbon feedstocks containing sulfur are hydrogenated using a fluorided-platinum catalyst. Sulfur, a known poison to platinum catalysts, causes rapid deactivation of the catalyst as the hydrogenation process proceeds. Adding fluorine to the catalyst reduces sulfur poisoning; however, this undesirably increases hydrocracking activity that also deactivates the catalyst. Estes et al. inhibited hydrocracking activity by adding extremely small amounts, of carbon monoxide (a poison of metal catalysts itself) to the pure-hydrogen feed stream. This allowed the carbon monoxide to interact with the acidity of the fluorided-catalyst surface and prevent reactions, like hydrocracking, from taking place. Because carbon monoxide can also poison and deactivate the catalyst, care must be exercised in both purifying the hydrogen feed stream and in adding the carbon monoxide to the pure-hydrogen feed stream in order to achieve proper hydrogenation. This type of hydrogenation process therefore appears most useful when the hydrocarbon feedstock contains substantial amounts of sulfur requiring the catalyst to contain fluorine to prevent the sulfur from poisoning the catalyst.
U.S. Pat. No. 4,626,604 (Hiles et al.) describes a process in which hydrogenation occurs in a series of catalytic stages using at least three adiabatic reaction vessels. Because hydrogenation occurs in stages, lower operating temperatures can be used, which in turn reduces the formation of byproducts such as esters that can poison the catalysts and decrease, catalytic activity. However, Hiles et al. requires that the liquid unsaturated aromatic hydrocarbon be vaporized prior to mixing with the hydrogen gas. Portions of the vaporized unsaturated aromatic hydrocarbon are then hydrogenated in each catalytic stage before the saturated hydrocarbon is cooled and condensed back to liquid-form.
Of particular concern in a conventional hydrogenation of benzene process are impurities found in the hydrogen source, because such impurities often deactivate the catalyst used to promote the hydrogenation reaction. Carbon monoxide is one such impurity that can reversibly poison catalysts, like nickel, used in benzene hydrogenation processes. In the poisoning process, carbon monoxide is adsorbed onto the active sites of the nickel catalyst surface, thereby reducing the activity of the catalyst. Depending on the concentration of carbon monoxide in the hydrogen source, the nickel catalyst can rapidly deactivate.
Once the nickel catalyst has deactivated, the catalyst may be regenerated by heating the catalyst at a temperature from about 220° C. to about 260° C. Because this regeneration process may not be completed in the presence of benzene or cyclohexane (the temperatures required for regeneration tend to promote the formation of large quantities of undesirable cracking products), the reactor must be taken off-line before regeneration of the catalyst. Due to the obvious inconvenience associated with taking the reactor off-line, most conventional benzene hydrogenation processes are designed to limit or prevent deactivation of the catalysts.
In order to prevent or limit deactivation of the nickel catalysts commonly used in benzene hydrogenation processes, most conventional processes require that a highly pure hydrogen source be used. Relatively pure hydrogen sources may be obtained from a steam reformer, and such hydrogen streams typically contain about 96 mole % hydrogen, about 4 mole % methane, and less than about 10 ppm of carbon monoxide and other impurities. Even with such low carbon monoxide levels, these hydrogen streams must still often be further purified to reduce the carbon monoxide levels to less than about 1 ppm before use. As such, these hydrogen streams tend to be expensive, yet they are frequently used because no other alternatives have been available.
Less pure sources of hydrogen are available from steam cracking, catalytic reforming, and hydroalkylation. Hydrogen streams obtained from these sources typically contain from about 10 mole % to about 80 mole % hydrogen, with the remainder comprising impurities such as methane, other light hydrocarbons, and/or carbon monoxide. The level of carbon monoxide in hydrogen streams from these sources is often as great as about 5000 ppm, which often prevents the use of these hydrogen sources in conventional benzene hydrogenation processes.
Therefore, what is needed is a process that: (i) promotes the hydrogenation of benzene to cyclohexane that operates using a lower purity, and thereby, a less expensive source of hydrogen; (ii) proceeds without deactivation of the catalyst due to the presence of carbon monoxide or other impurities in the hydrogen source; and (iii) promotes the hydrogenation of benzene without contributing to the formation of a significant amount of cracking products, such as methylcyclopentane.
SUMMARY OF THE INVENTION
The present invention, accordingly, provides for a process of producing cyclohexane by benzene hydrogenation using a hydrogen source that contains impurities. The supported catalysts used in the present invention reduce benzene to cyclohexane, and reduce carbon monoxide to methane and water. Alkenes, such as ethylene, are also reduced to their alkane counterparts. An advantage of the present invention is that the catalysts used in the disclosed process, if used under the reaction conditions disclosed, do not deactivate in the presence of carbon monoxide or other impurities typically found in hydrogen sources. Another advantage is that the disclosed process proceeds without the formation of a significant amount of cracking products, such as methylcyclopentane.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides for a process for causing the simultaneous production of cyclohexane from the hydrogenation of benzene and chemical reduction of certain impurities that may be present in the reactants. The process involves providing a first stream comprising benzene; providing a second stream comprising hydrogen and impurities; mixing the first and the second streams to form a reactive mixture; and contacting the reactive mixture with a catalyst to effectuate the reduction of the benzene and impurities. Under the preferred reaction conditions of the present invention, the catalyst will not deactivate rapidly, high benzene and hydrogen conversions will be obtained, and cracking product formation will be held within acceptable limits.
The hydrogen stream used in the process of the present invention may be obtained from a variety of sources, including, but not limited to, steam cracking, catalytic reforming, and/or hydroalkylation. Preferably, the hydrogen source should contain no more than about 15 mole % of impurities, such as, but not limited to, carbon monoxide or light hydrocarbons. More preferably, the hydrogen source should contain no more than about 5 mole % of carbon monoxide and about 10 mole % of light hydrocarbons. The light hydrocarbons may comprise alkanes and/or alkenes with from about one to about three carbon atoms, including, but not limited to, methane and/or ethylene.
The benzene stream may be obtained from any number of sources, including, but not limited to hydrodealkylation, pyrolysis, catalytic reforming or fractional distillation.
The catalysts used in the process of the present invention may be prepa
McKinney Michael Wayne
Renken Terry Lee
Sanderson John Ronald
Dang Thuan D
Gardere Wynne & Sewell LLP
Huntsman Petrochemical Corporation
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