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PxMax (selective toluene disproportionation)

PxMaxsm is ExxonMobil’s newest Selective Toluene Disproportionation (STDP) technology. The key ingredient is the novel EM-2300 catalyst that offers improved yields, excellent stability and ease of operation. As part of a PxMax license, ExxonMobil provides full support for licensees from initial consultation through technology transfer and ongoing improvements.

Compared to its predecessor, the PxMax process using EM-2300 catalyst offers:

  • higher purity paraxylene-rich xylenes (90%+)
  • higher total xylenes yield
  • superior xylenes/benzene ratio

These performance benefits, combined with lower operating temperatures and reduced H2/hydrocarbon requirements, can result in increased profits and significant debottlenecking opportunities.

PxMax advantages

Lower costs

Compared to conventional Selective Toluene Disproportionation (STDP) technologies, the PxMaxsm process using the EM-2300 catalyst can significantly lower costs. Key advantages include:

no on-line selectivation, resulting in:

  • lower capital costs
  • easy retrofits
  • simplified operation
  • lower operating costs
  • long operating cycles before regeneration is required

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.

Photo — As part of a PxMax license, ExxonMobil Chemical provides full support for licensees from initial consultations through technology transfer and ongoing improvements.

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.