Hydrodealkylation, of toluene

Benzene, toluene, and a mixed xylene stream are subsequently recovered by extractive distillation using a solvent. Recovery ofA-xylene from a mixed xylene stream requires a further process step of either crystallization and filtration or adsorption on molecular sieves. o-Xylene can be recovered from the raffinate by fractionation. In A" xylene production it is common to isomerize the / -xylene in order to maximize the production of A xylene and o-xylene. Additional benzene is commonly produced by the hydrodealkylation of toluene to benzene to balance supply and demand. Less common is the hydrodealkylation of xylenes to produce benzene and the disproportionation of toluene to produce xylenes and benzene. 

Hydrogenolysis is analogous to hydrolysis and ammonolysis, which involve the cleavage of a bond induced by the action of water and ammonia, respectively. Chemical bonds that are broken by hydrogenolysis reactions include carbon—carbon, carbon—oxygen, carbon—sulfur, and carbon—nitrogen bonds. An example of hydrogenolysis is the hydrodealkylation of toluene to form benzene and methane ... 

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. 

Table 20. U.S. Producers of Benzene by Hydrodealkylation of Toluene and Their Annual Capacities ...

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. 

Hydrodealkylation of toluene and xylenes with hydrogen is noted in Chapter 3. 

Raw materials for obtaining benzene, which is needed for the production of alkylbenzenes, are pyrolysis gasoline, a byproduct of the ethylene production in the steam cracking process, and coke oven gas. Reforming gasoline contains only small amounts of benzene. Large amounts of benzene are further produced by hydrodealkylation of toluene, a surplus product in industry. 

An important process for the manufacturing of benzene is the hydrodealkylation of toluene [66] ... 

Manufacture Benzene used to be made from coal, but it is now made from petroleum by two different routes catalytic reforming of naptha and hydrodealkylation of toluene. The first route provides toluene as a byproduct for the second route. 

Benzene is to be produced by the hydrodealkylation of toluene according to the reaction ... 

A preliminary assessment of a process for the hydrodealkylation of toluene is to be made. The reaction involved is ... 

These same notions can be extended to an entire plant in which several unit operations are connected together. The HDA process for hydrodealkylation of toluene to form benzene is a good example of where an eigenstructure can be found that provides a more easily and simply controlled plant. See Fig. 8.15. Assuming that the toluene feed rate to the unit is fixed, this plant has 22 valves that must be set. There are 11 inventory loops (levels and pressures), so they require 11 valves. One possible conventional control structure is shown in Fig. 8.15. 

For many years benzene was made from coal tar even as late as 1949, when all of it was made by this old process. New processes began to take over in the 1950s, which were used for 50% of the benzene in 1959, 94% in 1972, 96% in 1980, and near 100% in the 1990s. These new processes consist of catalytic reforming of naphtha and hydrodealkylation of toluene in a 70 30 capacity ratio. 

Fig. 8.9 shows U.S. prices for the aromatics in dollars per gallon. As in the olefins we see very steady pricing to the mid 70s, then heavy increases through the late 70s because of the oil embargo and early 80s because of higher inflation rates, followed by ups and downs, especially for benzene. For the hydrodealkylation of toluene to be profitable as a production method for benzene the price of toluene must be 50C/gal lower than that for benzene. Sometimes this happens, sometimes not. / -Xylene is the more expensive isomer of the two commercial xylenes because of the crystallization process required. The commercial value of benzene is approximately 2 billion. 

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. 

Dow, Monsanto, and Koch Chemical Company are the principal biphenyl producers, with lesser amounts coming from Sybron Corporation and Chemol, Inc. With the exception of Monsanto, the above suppliers recover biphenyl from high boiler fractions that accompany the hydrodealkylation of toluene to benzene. 

Hydrodealkylation of toluene, produce naphthalene from alkyl... 

Reactors can operate at low temperature (e.g., C4 sulfuric acid alkylation reactors run at 10°C) and at high temperatures (hydrodealkylation of toluene reactors run at 600°C). Some reactors operate in a batch or fed-batch mode, others in a continuous mode, and still others in a periodic mode. Beer fermentation is conducted in batch reactors. Ammonia is produced in a continuous vapor-phase reactor with a solid promoted iron catalyst. 

For many years benzene (benzol) was made from coal tar, but new processes that consist of catalytic reforming of naphtha and hydrodealkylation of toluene are more appropriate. Benzene is a natural component of petroleum, but it cannot be separated from crude oil by simple distillation because of azeotrope formation with various other hydrocarbons. Recovery is more economical if the petroleum fraction is subjected to a thermal or catalytic process that increases the concentration of benzene. 

HDA1—hydrodealkylation of toluene to produce benzene CYHEX1—cyclohexane production by hydrogenation of benzene STYR1—styrene production from ethylbenzene XYL1—production of m-xylene from toluene... 

Still larger quantities of aromatic hydrocarbons are needed, and these are synthesized from alkanes through the process of catalytic reforming (Sec. 9.3). This can bring about not only dehydrogenation as in the formation of toluene from methylcyclohexane, but also cyclization and isomerization as in the formation of toluene from //-heptane or 1,2-dimethylcyclopentane. In an analogous way, benzene is obtained from cyclohexane and methylcyclopentanc, as well as from the hydrodealkylation of toluene. 

This is an abstraction for the work space. A process flowsheet is developed and presented at the LayoutWindow. All design tasks are applied to an instance of LayoutWindow to transform a current process flowsheet. A snapshot of the transition in the process flowsheet is a design state at the moment. Figures 22 and 23 show the snapshots of a LayoutWindow during the design of hydrodealkylation-of-toluene process. 

Derivation (1) Hydrodealkylation of toluene or pyrolysis of gasoline (2) transalkylation of toluene by disproportionation reaction (3) catalytic reforming of petroleum (4) fractional distillation of coal tar. 

The third group of processes to benzene, toluene, and xylenes, or p-xylene is discretionary and depends on the proportion of these products that are available from the first two classes of processes in comparison to the current demand. Benzene consumption is more than twice that of toluene. It may be conveniently produced in almost 90% yields by the catalytic hydrodealkylation of toluene (Eq. 19.14) [14]. Or if both benzene and xylenes are in short supply, toluene may be catalytically transalkylated to mainly benzene and xylenes [15]. 

Earlier hydrodealkylation of toluene into benzene, disproportionation of toluene into benzene and xylenes and transalkylation of toluene and C9 aromatics into xylenes were developed and commercialized to strike a right balance in desired aromatic... 

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