Hydrodealkylation processes

Abstract

Carburization and metal-dusting while hydrodealkylating a hydrodealkylatable hydrocarbon are reduced even in the substantial absence of added sulfur.

Description

BACKGROUND OF THE INVENTIONThe 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 alumi...

AI Technical Summary

Benefits of technology

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.

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.

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 a low sulfur content may be used.

Another aspect of the invention includes providing a reactor system which prevents embrittlement. Preventing embrittlement significantly reduces metal-dusting and coking in the reactor system, and permits operation for longer periods of time. Furthermore, higher temperatures may be used during hydrodealkylation, especially in thermal hydrodealkylation processes, allowing for increased production.

In the case of a stannide outer protective layer and a stainless steel substrate, the stannide layer is nickel-enriched and comprises carbide inclusions, while the intermediate carbide-rich, nickel-depleted bonding layer comprises stannide inclusions. Preferably the carbide inclusions are continuous extensions or projections of the bonding layer as they extend, substantially without interruption, from the intermediate carbide-rich, nickel-depleted bonding layer into the stannide phase, and the stannide inclusions are likewise continuous extending from the stannide layer into the intermediate carbide-rich, nickel-depleted bonding layer. The aforementioned presence of carbide inclusions in the stannide layer, and stannide inclusions in the intermediate carbide-rich, nickel-depleted bonding layer, provide improved anchoring of the protective layer thereby increasing abrasion resistance. The interface between the intermediate carbide-rich, bonding layer and the nickel-enriched stannide layer is irregular, but is otherwise substantially without interruption.

Additionally, it has been found that certain preferred coatings are sulfur-tolerant, for example, the tin-based protective layers can tolerate up to 200 ppm sulfur. The protective layers eliminate the need to presulfide the metallurgy, reduce sulfide corrosion and improve product values and waste disposal due to induced levels of sulfur. Chromium-, Sb- and Ge-based protective layers can tolerate even higher sulfur levels, up to 5 or more wt. %. Most preferably, the layers can tolerate the respective amounts of sulfur for a period of at least 200, preferably at least 400, and most preferably 600 hours without degrading to an extent that carburization will occur, resulting in shut-down of the system due to excessive coking.

Problems solved by technology

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 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.

As a result, a premature coke-plugging of the reactor system occurs which can lead to a premature shut-down of the system.

However, the addition of sulfur increases production cost and process complexity.

Moreover, at high temperatures coking and carburization will still occur even when sulfur is added to the feed.

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