Toluene dealkylation process

Cyclic Hydrocarbons. The cyclic hydrocarbon intermediates are derived principally from petroleum and natural gas, though small amounts are derived from coal. Most cycHc intermediates are used in the manufacture of more advanced synthetic organic chemicals and finished products such as dyes, medicinal chemicals, elastomers, pesticides, and plastics and resins. Table 6 details the production and sales of cycHc intermediates in 1991. Benzene (qv) is the largest volume aromatic compound used in the chemical industry. It is extracted from catalytic reformates in refineries, and is produced by the dealkylation of toluene (qv) (see also BTX Processing). 

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

A process flowsheet of the dealkylation of toluene to benzene is in Figure 2.4 the material and enthalpy flows and temperature and pressures are tabulated conveniently, and basic instrumentation is represented. 

Figure 2.4. Process flowsheet of the manufacture of benzene by dealkylation of toluene (Wells, Safety in Process Design, George Godwin, London, I960). 

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. 

UOP Benzene Toluene Thermal dealkylation process produces high-purity benzene from toluene 41 1992... 

In the above process, usually 2 mol of isobutylene react with each mole of cresol in the presence of acidic catalyst. Dilute H2SO4 is the most popular catalyst for both alkylation and dealkylation process. Some of the plants use p-toluene sulfonic acid or even a mixture of sulfuric acid and p-toluene sulfonic acid. It is reliably learnt that at least one plant has been using some quantities of a very strong Friedel Crafts alkylation catalyst—Triflic acid or trifluoromethane... 

The catalytic dealkylation of toluene to benzene involves recycling of unreacted toluene after removal of by-product phenylbenzene. Using the information shown on the process flowsheet (Figure 7.11) determine... 

The dealkylation of toluene and xylenes to benzene is carried out not only catalyt-ically but also as a purely thermal reaction in the presence of hydrogen. In the catalytic process, pressures ranges from 35 to 70 bar and temperatures from 550 to 650 °C chromium oxides on alumina are used as catalysts. Thermal dealkylation takes place at temperatures up to 750 °C and pressures of around 45 bar. 

The Hydeal process developed by UOP, the Houdry Detol process and the BASF process for the dealkylation of toluene operate in a similar way to the... 

DETOL [DEalkylation of TOLuene] A process for making benzene by dealkylating toluene and other aromatic hydrocarbons. Developed by the Houdry Process and Chemical Company and generally similar to its Litol process for the same purpose. The catalyst is chromia on alumina. Licensed by ABB Lummus Global. Twelve plants had been licensed in 1987. 

More than 90% of today s petrochemicals are produced from refineiy products. Most are based on the use of C2-C4 olefins and aromatics finm hydrocarbon steam cracking units, which are even more closely linked to refineries. In North America, the feedstock for steam cracker units have generally been ethane, propane, or LPG. As a result, most of the propylene and aromatics have been provided by FCC units and catalytic reformers. In maity other parts of the world where naphtha feed has been more readily available, suppUes of propylene and aromatics have been produced directly by steam cracking. When necessary, the catalytic dehydrogenation of paraffins or dealkylation of toluene can balance the supply of olefins or benzene. In Table 7.2 some of the catalytic processes that convert olefins and benzene from a steam cracker into basic petrochemicals for the modem chemical industry are shown. 

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. 

Extractive distillation, using similar solvents to those used in extraction, may be employed to recover aromatics from reformates which have been prefractionated to a narrow boiling range. Extractive distillation is also used to recover a mixed ben2ene—toluene stream from which high quaUty benzene can be produced by postfractionation in this case, the toluene product is less pure, but is stiU acceptable as a feedstock for dealkylation or gasoline blending. Extractive distillation processes for aromatics recovery include those Hsted in Table 4. 

There are many variations of the basic processing loop shown in Figure 8. Processing to produce only BT is common, often in conjunction with a toluene-to-benzene dealkylation unit. If benzene and toluene ate not to be recovered. Column B may be used to remove toluene and lighter components. 

Toluene is used more commonly than the other BTXs as a commercial solvent. There are scores of solvent applications, though environmental constraints and health concerns diminish the enthusiasm for these uses. Toluene also is used to make toluene diisocyanate, the precursor to polyurethane foams. Other derivatives include phenol, benzyl alcohol, and benzoic acid. Research continues on ways to use toluene in applications that now require benzene. The hope is that the dealkylation-to-benzene or disproportionation steps can be eliminated. Processes for manufacturing styrene and terephthalic acid—the precursor to polyester fiber—are good, commercial prospects. 

The presence of metal may catalyze demethylation and can occur to some extent in catalysts where the metal function is under-passivated, as by incomplete sulfiding. This would convert valuable xylenes to toluene. The demethylation reaction is usually a small contributor to xylene loss. Metal also catalyzes aromatics saturation reactions. While this is a major and necessary function to facilitate EB isomerization, any aromatics saturation is undesirable for the process in which xylene isomerization and EB dealkylation are combined. Naphthenes can also be ring-opened and cracked, leading to light gas by-products. The zeolitic portion of the catalyst participates in the naphthene cracking reactions. Cracked by-products can be more prevalent over smaller pore zeolite catalysts. 

After the separator, the liquid product is sent to a deheptanizer to remove toluene, benzene and other lighter products. If this is an EB isomerization-style process, the deheptanizer operation may be constrained by the need to send the C8N to the bottoms, which also results in more toluene in the bottoms than would be present in an EB dealkylation system (which does not require C8N recirculation). The elevated toluene is not generally detrimental to catalyst performance, primarily acting as a diluent, although in some cases it may actually be beneficial, by pushing the toluene -i- C9A transalkylation equilibrium back toward C8A. 

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. 

---