Heavies hydrodealkylation

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeoHte catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg aHphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equiHbrium, ie, 102—104% of equiHbrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions ate temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydtocatbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

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. 

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. 

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