To selectively isomerize a paraxylene-depleted C8 aromatics mixture to greater than equilibrium paraxylene (PX) concentration using ExxonMobil Chemical’s XyMax and Advanced MHAI processes. Simultaneously, ethylbenzene (EB) and non-aromatics in the feed are converted to benzene and light paraffins, respectively. Conversion of EB is typically 60%–80%.
The para-depleted liquid C8 aromatics raffinate stream from the PX separation unit, along with hydrogen-rich recycle gas are pumped through feed/effluent exchangers and the charge heater (1) and into the reactor (2). Vapor then flows down through the fixed, dualbed catalyst system.
Dealkylation of EB and cracking of non-aromatics preferentially occurs in the top bed. The bottom bed promotes isomerization of xylenes, while minimizing loss of xylenes from side reactions. The reactor effluent is cooled by heat exchange, and the resulting liquid and vapor phases are separated in the product separator (3). The liquid is then sent to a fractionator (4) for recovery of benzene and toluene from the isomerate.
Two enhanced isomerization catalyst technologies have been developed by ExxonMobil Chemical. The first technology, referred to as Advanced Mobil High Activity Isomerization (AMHAI), provides higher selectivity and lower operating costs compared to isomerization processes used in the past. The AMHAI technology offers increased operating flexibility in terms of a greater range of EB conversion and a
lower temperature requirement. The second technology, referred to as XyMax, further increases yield performance and debottleneck potential. This technology can operate at even higher EB conversion, with higher selectivity and lower xylene loss.
Operating conditions: XyMax and AMHAI units operate with a high space velocity and a low hydrogen-to-hydrocarbon ratio, which results in increased debottlenecking potential and decreased utilities costs. By converting a high portion of EB in the feed, these technologies can provide significant savings in associated paraxylene recovery facilities. Both technologies offer long operating cycles.
Licensor: ExxonMobil Chemical Technology Licensing LLC (retrofit applications) Axens (grassroots applications)
