Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By dealkylation
Reissue Patent
1999-12-06
2004-06-08
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By dealkylation
C585S483000, C585S485000, C585S488000, C585S950000, C518S700000, C518S728000, C423S650000, C423S651000
Reissue Patent
active
RE038532
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention rotates to an improved technique for the hydrodealkylation of a hydrodealkylatable hydrocarbon, particularly under conditions of low sulfur, which minimizes carburization thus preventing premature plant shut-downs.
The hydrodealkylation of hydrodealkylatable hydrocarbons such as alkyl aromatics has been practiced for many years. The principal processes involve the conversion of toluene and like alkyl-substituted benzenes to benzene and various byproducts. Such processes are either catalytic or non-catalytic in nature. The catalytic processes employ one or more catalysis that promote the conversion of the alkyl aromatic compounds to benzene and the remaining alkyl. The non-catalytic processes typically employ heat and pressure to promote the conversion of the alkyl aromatic compounds to benzene and the remaining alkyl.
Some conventional catalytic hydrodealkylation processes employ Group VIII metals such as Rh and Pt supported on an alumina support. For example, Kovach et al., in U.S. Pat. No. 3,700,745, describes a hydrodealkylation process which includes contacting an alkyl aromatic hydrocarbon with a catalyst including an active Group VIII metal, such as, platinum, rhodium, palladium, ruthenium and nickel. Other catalytic hydrodealkylation processes employ chromia type catalysts deposited on an alumina support. For example, Daly et al., in U.S. Pat. No. 4,451,687, discloses a catalyst for the hydrodealkylation of alkylaromatic compounds containing chromia on an alumina support. Other catalytic hydrodealkylation processes employ variations of the above catalysts or even completely different catalysts. See, for example, U.S. Pat. Nos. 3,686,340, 3,966,833, 4,189,613, 4,191,632, 4,463,206 and 5,053,574.
The catalytic processes, however, are not always suitable for the commercial conversion of alkyl aromatic compounds to benzene and the remaining alkyl. In particular, the activity, selectivity and conversion rate of such catalysts are not always suitable for large scale hydrodealkylation at the temperatures and pressures suitably employed. If the reaction temperature or pressure is increased aide reactions such as hydrocracking of the aromatic ring is promoted.
Furthermore, some catalysts tend to deactivate with use, presumably due to coke formation on the catalyst surface. In this regard, it is believed that active sites promote polymerization of either hydrogenolysis products or aromatic hydrocarbons resulting in hydrocarbon condensation on the catalyst surface. Under the conditions of the process, these condensed species are dehydrogenated forming coke. The result of these reactions is a reduction in activity of the catalyst since the coke is strongly adsorbed onto the sites which promote dealkylation. In other words, this coke or carbon build-up either blocks or poisons the active catalyst sites casing deactivation. See U.S. Pat. No. 4,451,687.
Additionally, some catalysts tend to deactivate with use, due to the presence of sulfur and in particular thiophene sulfur in the process feed. Thus, catalysts such as certain noble metal catalysts deactivate over time due to the presence of sulfur in the feed. These catalysts must be replaced or regenerated when sulfur reduces the activation to an extent low enough to prevent suitable conversion of the feed.
In view of the disadvantages associated with utilizing catalytic hydrodealkylation processes, non-catalytic hydrodealkylation processes have been developed. Mainly, such processes employ the use of heat and pressure to convert alkylaromatic compounds to benzene and the disassociated alkyl compounds.
Button et al., in U.S. Pat. No. 3,607,960, and Loboda, in U.S. Pat. No. 4,058,452, disclose processes for the thermal hydrodealkylation of an alkyl aromatic, such as toluene, to produce benzene. Both processes include subjecting a gaseous mixture of at least one alkyl aromatic compound and hydrogen in a reaction zone to a reaction temperature in the range of about 1000° to 1800° F. and removing benzene from the effluent. Other patents which disclose the thermal dealkylation of hydrodealkylatable hydrocarbons include U.S. Pat. Nos. 2,929,775, 3,160,671, and 3,284,526.
Thermal hydrodealkylation processes ameliorate the disadvantages associated with the above-mentioned catalytic hydrodealkylation processes in that they do not employ the use of catalysts which are susceptible to deactivation. However, due to the use of high temperatures end pressures which are required for the conversion of alkyl aromatic compounds in the absence of a suitable catalyst, such processes have their own inherent problems.
SUMMARY OF THE INVENTION
With conventional hydrodealkylation techniques, added sulfur effectively inhibits carburization. Somehow, the sulfur interferes with the carburization reaction. But with low sulfur systems or with sulfur outages, this inherent protection no longer exists especially when the system is exposed to high temperatures such as in thermal hydrodealkylation.
The problems associated with carburization include coking, carburization of system metallurgy, and metal dusting. The embrittlement of the steel walls by carburization leads to “metal-dusting”, i.e., a release of catalytically active particles and metal droplets of metal due to an erosion of the metal. The excessive “metal-dusting” adds active metal particulates to the system, which particulates provide additional sites for coke formation in the system.
Coking is generally not a problem which must be addressed in hydrodealkylation processes, but this significant source of calm formation due to the absence of sulfur in reactor feedstreams excessively aggravates the problem. In fact, active metal particulates in coke particles metastasize more generally throughout the system. That is, the active metal particulates actually induce coke formation on themselves and anywhere that the particles accumulate in the system resulting in coke plugs and hot regions of exothermic reactions. As a result, a premature coke-plugging of the reactor system occurs which can lead to a premature shut-down of the system.
One solution to the problem associated with carburization, embrittlement, and metal-dusting is to add sulfur to the feed to thereby effectively inhibit carburization. However, the addition of sulfur increases production cost and process complexity. Moreover, sulfur usage has inherent environmental and safety hazards which are preferably avoided. Moreover, at high temperatures coking and carburization will still occur even when sulfur is added to the feed.
Consequently, there remains a need in the art for improved processes for the hydrodealkylation of hydrodealkylatable compounds with reduced carburization, especially in the absence of, or at low levels of, sulfur. Such a method would include means for inhibiting the undesirable catalytic activity which causes carburization of system metal.
Accordingly, one object of the invention is to provide the technical background necessary for solutions to the problems associated with coking, carburization, and metal-dusting. In particular, the discovery of the mechanisms involved with carburization and metal-dusting which lead to premature coke-plugging allows those skilled in the art to formulate solutions to the problems.
Another object of this invention is to provide a method for inhibiting carburization and metal-dusting in a process for thermally dealkylating a hydrodealkylatable hydrocarbon. The process includes pretreating surfaces exposed to the dealkylation atmosphere to form a protective layer which is more resistant to embrittlement, carburization and metal-dusting than the materials conventionally used to manufacture the reactor system.
The use of the process of the present invention allows for the dealkylation of hydrodealkylatable hydrocarbons especially in the absence of sulfur. Thus, an advantage of the method for inhibiting carburization includes the lack of a need for the addition of sulfur to hydrocarbon feeds and any recycle streams. In addition, hydrocarbon foods having
Cannella William J.
Hagewiesche Daniel P.
Heyse John V.
Innes Robert A.
Kramer David C.
Chevron Phillips Chemical Company LP
Griffin Walter D.
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