Hydrodealkylation Dealkylation

The feedstock is usually extracted toluene, but some reformers are operated under sufftciendy severe conditions or with selected feedstocks to provide toluene pure enough to be fed directiy to the dealkylation unit without extraction. In addition to toluene, xylenes can also be fed to a dealkylation unit to produce benzene. Table 20 Hsts the producers and their capacities for manufacture of benzene by hydrodealkylation of toluene. Additional information on hydrodealkylation is available in References 50 and 52. 

A typical catalytic hydrodealkylation scheme is shown ia Figure 3 (49). The most common feedstock is toluene, but xylenes can also be used. Recent studies have demonstrated that and heavier monoaromatics produce benzene ia a conventional hydrodealkylation unit ia yields comparable to that of toluene (51). The use of feeds containing up to 100% of C —aromatics iacreases the flexibiUty of the hydrodealkylation procedure which is sensitive to the price differential of benzene and toluene. When toluene is ia demand, benzene suppHes can be maintained from dealkylation of heavy feedstocks. 

Biphenyl has been produced commercially in the United States since 1926, mainly by The Dow Chemical Co., Monsanto Co., and Sun Oil Co. Currently, Dow, Monsanto, and Koch Chemical Co. are the principal biphenyl producers, with lesser amounts coming from Sybron Corp. and Chemol, Inc. With the exception of Monsanto, the above suppHers recover biphenyl from high boiler fractions that accompany the hydrodealkylation of toluene [108-88-3] to benzene (6). Hydrodealkylation of alkylbenzenes, usually toluene, C Hg, is an important source of benzene, C H, in the United States. Numerous hydrodealkylation (HDA) processes have been developed. Most have the common feature that toluene or other alkylbenzene plus hydrogen is passed under pressure through a tubular reactor at high temperature (34). Methane and benzene are the principal products formed. Dealkylation conditions are sufficiently severe to cause some dehydrocondensation of benzene and toluene molecules. 

Toluene is dealkylated to benzene over a hydrogenation-dehydrogenation catalyst such as nickel. The hydrodealkylation is essentially a hydrocracking reaction favored at higher temperatures and pressures. The reaction occurs at approximately 700°C and 40 atmospheres. A high benzene yield of about 96% or more can be achieved ... 

HDA [Hydrodealkylation] A proprietary dealkylation process for making benzene from toluene, xylenes, pyrolysis naphtha, and other petroleum refinery intermediates. The catalyst,... 

Hydeal [Hydrodealkylation] A process for making benzene by de-alkylating other aromatic hydrocarbons. Generally similar to the Litol process. Developed in the 1950s by UOP and Ashland Oil Company, but abandoned in favor of UOP s THDA process. See dealkylation. 

THDA [Thermal hydrodealkylation] A process for dealkylating alkyl benzenes to produce benzene. The by-product is mainly methane. Developed by UOP and licensed by that company. 

Dealkylation. The removal of an alkyl group (a straight chain) from a molecule. Sometimes the alkyl group is replaced by hydrogen, in which case the process is called hydrodealkylation. 

A cracking process, the dealkylation of alkylbenzenes, became an established industrial synthesis for aromatics production. Alkylbenzenes (toluene, xylenes, tri-methylbenzenes) and alkylnaphthalenes are converted to benzene and naphthalene, respectively, in this way. The hydrodealkylation of toluene to benzene is the most important reaction, but it is the most expensive of all benzene manufacturing processes. This is due to the use of expensive hydrogen rendering hydrodealkylation too highly dependent on economic conditions. 

The converse reactions dealkylation and hydrodealkylation are practiced extensively to convert available feedstocks into other more desirable (marketable), products. Two such processes are (1) the conversion of toluene or xylene, or the higher-molecular-weight alkyl aromatic compounds, to benzene in the presence of hydrogen and a suitable presence of a dealkylation catalyst and (2) the conversion of toluene in the presence of hydrogen and a fixed bed catalyst to benzene plus mixed xylenes. 

MFI zeolites seem to be the most efficient for EB dealkylation, in terms of activity, selectivity and stability. In the 70s, on metal-free MFI catalysts, EB was disproportionated into benzene and diethylbenzenes. As indicated above, with MFI catalysts, ethylbenzene disproportionation occurs through a deethylation-ethylation mechanism, with ethylene as desorbed intermediate. The addition of a metal (carried out early 80s) allows a rapid and irreversible conversion of ethylene into ethane with a consequent shift of ethylbenzene transformation from disproportionation to hydrodealkylation. The selectivity is highly sensitive to temperature that must be in the range 380°C-460°C to limit both alkylation and naphthene cracking. 

HDA [HydroDeAlkylation] A proprietary dealkylation process for making benzene from toluene, xylenes, pyrolysis naphtha, and other petroleum refinery intermediates. The catalyst, typically chromium oxide or molybdenum oxide, together with hydrogen gas, removes the methyl groups from the aromatic hydrocarbons, converting them to methane. The process also converts cresols to phenol. Developed by Hydrocarbon Research with Atlantic Richfield Corporation and widely licensed worldwide. 

Toluene is the most abundant and lowest cost aromatic material. This is a reason to be the valuable raw material for the production of various chemical products. The main chemical use for toluene Is, in fact, production of benzene by dealkylation. The reaction can be carried out either thermally or catalytically using variety of catalysts including supported metals or metal oxides [1-6], amorphous silica-alumina [7] and zeolites [8-19]. The simplest toluene hydrodealkylation reaction is the reaction in which the methyl group is removed in the presence of the forming mainly methane CeHsCHs + H2 —> CeH0 + CH4... 

This study was undertaken with the objective of closely following the relationships of silica/alumina ratio with catalytic properties of ZSM-5 zeolite in toluene transformation under hydrodealkylation process conditions. It was hoped that the data would reveal the reaction pathway of the dealkylation and subsequently shed light on the mechanism of H-ZSM-5 catalyzed transformation of toluene. 

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