|  | PxMax (sm) (Selective Toluene Disproportionation)
ExxonMobil now offers its PxMax technology for commercial licensing. PxMax is the new process name for our state-of-the-art Selective Toluene DisProportionation (STDP) technology, previously know as MTPX, and first put into commercial operation in 1996. PxMax services provide full support for licensees from initial consultation through technology transfer and on-going improvements.
Compared to its predecessor MSTDP, the PxMax process offers:
- Higher purity paraxylene-rich xylenes (90%+)
- Higher total xylenes yield
- Superior xylenes/benzene ratio
These advantages, combined with lower operating temperatures and reduced H2/hydrocarbon requirements, can result in increased profits and significant debottleneck opportunities.
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- Cost Advantage versus Conventional STDP
- Unique Ex-Situ Catalyst Selectivation
- A Commercial Success
- Simple Fixed Bed Process
| Cost Advantage versus Conventional STDP
Compared to conventional STDP technologies, such as MSTDP, the PxMax process can significantly lower your costs. Key advantages include:
- No on-line selectivation, resulting in:
+ simplified operation
+ easy retrofit into existing equipment
+ lower capital costs
- Lower operating costs
- Long operating cycles before regeneration is required
In addition, the high paraxylene purity leads to a reduction in the cost associated with final purification, either by downsizing of the adsorptive separation unit or a significant reduction in the required number of crystallizer stages.
| Unique Ex-situ Catalyst Selectivation
The high paraxylene selectivity of PxMax is due to the proprietary catalyst utilized in the process. This catalyst is selectivated by a novel pretreatment procedure during catalyst manufacture. This ex-situ selectivation increases the relative rate of paraxylene diffusivity over the larger ortho- and meta-isomers resulting in a xylene product with a greater than 90% paraxylene concentration. The catalyst has excellent stability, can be regenerated and never requires re-selectivation.
| A Commercial Success
The predecessor to the PxMax process is known as MSTDP. It has been one of the most successful new petrochemical technologies introduced to the market place, with many licensees in all regions of the world. These units have over 67 years of cumulative operating experience.
The PxMax process was put into commercial operation in 1996 by Mobil Oil at Chalmette Refining L.L.C.'s, Louisiana refinery and subsequently applied in 1997 at Mobil Chemical's grass-roots unit in Beaumont, Texas. Both units are still on their first cycle and have the potential to run in excess of ten years before requiring regeneration.
| Six additional units have been licensed since the PxMax technology was made available for licensing. Nippon Petroleum retrofitted the PxMax Technology into their existing MSTDP unit and started it up in late 2001. LG-Caltex built a world-scale grass roots unit in Korea that was started up in early 2003. The third licensee converted a unit from a different service to PxMax and start-up the unit in late 2002. A fourth and fifth licensee converted their existing MSTDP units to PxMax Technology in late 2003 and mid 2004. S-Oil Corporation, the sixth licensee, converted an equilibrium TDP unit to PxMax in 2006.
| Simple Fixed Bed Process
The PxMax process flow 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 catalyst bed. Here, disproportionation occurs, at moderate temperature and pressure, to produce a paraxylene-rich xylene product along with co-product benzene. The by-product yields are small.
The 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.
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