Selective toluene disproportionation STDP

Intermediate pore zeolites typified by ZSM-5 (1) show unique shape-selectivities. This has led to the development and commercial use of several novel processes in the petroleum and petrochemical industry (2-4). This paper describes the selectivity characteristics of two different aromatics conversion processes Xylene Isomerization and Selective Toluene Disproportionation (STDP). In these two reactions, two different principles (5,j6) are responsible for their high selectivity a restricted transition state in the first, and mass transfer limitation in the second. 

Description Dry toluene feed and hydrogen-rich recycle gas are pumped through feed/effluent exchangers and charge heater and into the PxMax 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 efflu-... 

After the cmde BTX is formed, by reforming in this case, a heart cut is sent to extraction. Actually, the xylenes and heavier components are often sent to downstream processes without extraction. The toluene produced is converted to ben2ene, a more valuable petrochemical, by mnning it through a hydrodealkylation unit. This catalytic unit operates at 540—810°C with an excess of hydrogen. Another option is to disproportionate toluene or toluene plus aromatics to a mixture of ben2ene and xylenes using a process such as UOP s Tatoray or Mobil s Selective Toluene Disproportionation Process (STDP) (36). 

STDP [Selective toluene disproportionation process] A process for converting touene to mixed xylenes, predominately />-xylene. It takes place in the presence of hydrogen over a ZSM-5-type catalyst. Developed by Mobil in the 1980s and first operated by Enichem. 

As is apparent from the previous discussion on toluene disproportionation,the observation of high p-selectivity in STDP requires a dramatic change in selectivity. First, the primary product must be directed to be highly para-selective. Secondly, the subsequent isomerization of the primary p-xylene product must be selectively inhibited ... 

An interesting way to increase pX production of an aromatics complex is to transform less valuable aromatics such as toluene and C9+ aromatics into xylenes. This is achieved either by toluene disproportionation (TDP) or by toluene C9+ aromatics transalkylation. There are two types of disproportionation processes Normal TDP produces a xylene cut where pX content is at thermodynamic equilibrium, while with STDP pX content is much higher (para-selectivity). This is possible when a selectivated zeolite (generally MFI) is used. For normal TDP as well as for transalkylation processes, mordenites doped with a hydrogenating metal to limit coking are used as catalysts. 

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