Benzene by hydrodealkylation

The benzene—toluene fraction is further fractionated in a small column, not shown in Figure 5, to recover benzene for recycle to the alkylation unit and toluene for sale. This toluene can be converted to benzene by hydrodealkylation but the high selectivity catalyst has reduced the formation of toluene in the dehydrogenation reactor to the point where the cost of installing a hydrodealkylation unit is difficult to justify even in a large styrene plant. 

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 ...

The principal chemical uses of BTX are illustrated in Figure 1 and Hsted in Table 1 (2). A very wide range of consumer products from solvents to fibers, films, and plastics are based on BTX. The consumption of BTX is approximately in the proportions of 67 5 28, respectively. However, no BTX process gives BTX in these proportions. The economic value of benzene and xylenes (especially -xylene) is normally higher than that of toluene. Because of this, processes that convert toluene to benzene by hydrodealkylation (3) and disproportionate toluene to benzene and xylenes (4) have been commercialized. In addition, reforming processes that emphasize production of either benzene or -xylene [106 2-3] have been described (5). Since these are not classified as BTX processes they are not discussed in detail here. 

Table 8.9 shows the non-fuel uses of toluene. Some of the toluene goes into gasoline depending on its supply and price compared to other octane enhancers. Of the other uses of toluene about half is converted into benzene by hydrodealkylation, though this amount varies with the price difference between benzene and toluene. 2,4-Toluene diisocyanate (TDI) is a monomer for polyurethanes. Included in miscellaneous uses is 2,4,6-trinitrotoluene (TNT) as an explosive. 

Chronologically, the effon was im tially dirreted towards a better petrochemical upgrading of toluene and m-xylene, which continue to be used as solvents and in the gasoline pool. Toluene is thus converted to benzene by hydrodealkylation. Through i omerizatioo,. m-.xyiene yields the onho and para isomers in proportions corresponding to thermodynamic equilibrium in the reaction conditions, namely a mixture of Cg from which the... 

Toluene is converted to benzene by hydrodealkylation. Typically, a 75% conversion is used in the reactor, which necessitates the recovery and recycle of unreacted toluene. In addition, a side reaction occurs that produces a small amount of a biphenyl byproduct, which is separated from the toluene. A hydrodealkylation process is being designed that includes a distillation column for separating toluene from biphenyl. The feed to the column is 3.4 Ibmoiyhr of benzene, 84.6 IbmoVhr of toluene, and 5.1 Ibmol/hr of biphenyl at 264°F and 37.1 psia. The distillate is to contain 99.5% of the toluene and 2% of the biphenyl. If the column operates at a bottoms pressure of 38.2 psia, determine the bottoms temperature and select a suitable heat source for the reboiler. Steam is available at pressures of 60, 160, and 445 psig. The barometer reads 14 psia. 

Craig, R.G., Doelp, L.C., Logwinuk, A.K. Benzene by Hydrodealkylation Using the Detol Process Erdol, Kohle, Erdgas, Petrochem., 18, 527 (1965)... 

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