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Compared to conventional Selective Toluene Disproportionation (STDP) technologies, the PxMaxsm process using the EM-2300 catalyst can significantly lower costs. Key advantages include:
In addition, high paraxylene purity leads to a reduction in costs associated with final purification, either by downsizing of the adsorption separation unit or a reduction in the required number of crystallizer stages.
Unique ex-situ catalyst selectivation
The high paraxylene selectivity of PxMax is achieved using the proprietary EM-2300 catalyst. This catalyst is selectivated by a novel pretreatment procedure during catalyst manufacture. Ex-situ selectivation increases the relative rate of paraxylene diffusivity over the larger ortho- and meta-isomers, resulting in a xylene product with greater than 90% paraxylene concentration. EM-2300 has excellent stability and can be regenerated.
Simple fixed-bed process
The PxMax process flow (shown) is typical for a vapor-phase reaction in a fixed-bed reactor. Toluene feed, combined with hydrogen-rich recycle gas, is preheated and passed through the EM-2300 catalyst bed. Disproportionation occurs here, at moderate temperature and pressure, to produce a paraxylene-rich xylene product along with co-product benzene. Byproduct yields are small.
Reactor effluent is cooled by heat exchange and the liquid products are separated from the recycle gas. The separated liquid is stripped to remove the light ends and then fractionated to recover a very high-purity benzene product and a highly enriched paraxylene stream for recovery of paraxylene. Unreacted toluene is recycled to extinction.