Application: To selectively convert toluene to high-purity (90%+) paraxylene-rich xylenes and benzene using ExxonMobil Chemical’s technologies PxMax and ASTDP.
Description: Dry toluene feed and hydrogen-rich recycle gas are pumped through feed/effluent exchangers and charge heater and into the reactor (1). Selective toluene disproportionation (STDP) occurs in the vapor phase to produce the paraxylene-rich xylene and benzene co-product. Byproduct yields are small.
Reactor effluent is cooled by heat exchange, and liquid products are separated from the recycle gas. Hydrogen-rich gas from the separator (2) is recycled back to the reactor together with makeup hydrogen. Liquid product is stripped of remaining light gas in the stabilizer (3) and sent to product fractionation. Unconverted toluene is recycled to extinction.
The PxMax technology uses catalyst, which is ex-situ selectivated by pretreatment during catalyst manufacture. The ASTDP technology uses catalyst, which is in-situ coke selectivated. Both technologies provide significantly higher selectivity and longer operating cycles than other STDP technologies. Operating costs associated with downstream recovery are also reduced by the high-purity paraxylene from PxMax and ASTDP.
Operating conditions: PxMax operates at lower start-of-cycle temperatures and lower hydrogen to hydrocarbon recycle ratios than other STDP technologies, resulting in longer cycles and lower utilities. By eliminating the in-situ selectivation step, the PxMax version of this technology results in simplified operation and lower capital costs. Both catalysts offer long operating cycles and are regenerable.
Licensor: ExxonMobil Chemical Technology Licensing LLC (retrofit applications) Axens (grassroots applications)