Reaction Product Composition and Characteristics

The ideal product from the C8A isomerization reaction is pure xylenes and depleted EB. However, as discussed in Section 14.4.1.3, there are numerous side reactions that deviate from this desired goal. The amount of deviation is typically defined as xylene loss, C8 aromatic loss, or C8 ring loss, each of which represents particular yield losses across the reactor. They are defined by the general equation  

These losses require additional feed to make a given amount of product Feed costs are 80-95% of the final product cost, so yield losses significantly reduce profitability [59]. 

The most common by-product losses are due to transalkylation, dealkylation, saturation and cracking. Transalkylation results in toluene, trimethylbenzenes, methylethyl benzenes, benzene and ClOAs. These are the best by-products to have, because they are the easiest to react back into C8A in a transalkylation unit (if the aromatics complex is so equipped) without any loss of carbon atoms [59-61]. Dealkylation results in benzene, toluene, methane and ethane. The benzene and toluene are aromatics and represent valuable by-products, but the C1-C6 nonaromatics represent carbons that are lost from the complex as less valuable LPG and fuel gas. 

Cracked by-products are C7-paraffins. Due to the difficulty in converting such paraffins back into aromatics, cracked by-products are the least desirable byproducts, and catalyst and process design go to great length to avoid their formation. Since it is easier to crack C8N than C8A, the level of C8N must be carefuUy balanced between their benefit to assist EB isomerization versus their contribution to cracking. In EB dealkylation systems, C8N are rapidly cracked by the stronger acid sites on such catalysts. 

Historically, the earliest C8 aromatic isomerization catalysts tended to use amorphous supports with a halogen such as chloride or fluoride. Due to water sensitivity and corrosion issues, these were replaced by large-pore zeolites such as mordenite. The larger pore size was more favorable toward bimolecular transalkylation, whereas the chlorided alumina support tended to promote cracking. In both 

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