Paraxylene isomerization catalysts

It should, however, be emphasized that new catalysts with zeolites other than MOR or MFI which give higher paraxylene yields were recently developed for the isomerization of the C8 aromatic cut. Moreover, adsorption on FAU zeolites is now the main technique used for paraxylene separation (Chapter 10). 

Description Aromatics are produced from naphtha in the Aromizing section (1), and separated by conventional distillation. The xylene fraction is sent to the Eluxyl unit (2), which produces 99.9% paraxylene via simulated countercurrent adsorption. The PX-depleted raffinate is isomerized back to equilibrium in the isomerization section (3) with either EB dealkylation-type (XyMax) processes or EB isomerization-type (Oparis) catalysts. High-purity benzene and toluene are separated from non-aromatic compounds with extractive distillation (Morphylane ) processes (4). Toluene and C9 to Cn aromatics are converted to more valued benzene and mixed xylenes in the TransPlus process (5), leading to incremental paraxylene production. 

Commercial plants Eight Eluxyl units have been licensed, accounting for 3 million tpy of paraxylene and three units that are in operation. Six isomerization units use the Oparis catalyst and 19 ExxonMobil EB deal-kylating units have been put into operation. Three TransPlus units are currently in operation. 

Application The Isomar process isomerizes C8 aromatics to mixed xylenes, to maximize the recovery of paraxylene in a UOP aromatics complex. Depending on the type of catalyst used, ethylbenzene (EB) is also converted into xylenes or benzene. 

P4-14c A total of 2500 gal/h of metaxylene is being isomerized to a mixture of orthoxylene, metaxylene, and paraxylene in a reactor containing 1000 ft of catalyst. The reaction is being carried out at 750°F and 300 psig. Under these conditions, 37% of the metaxylene fed to the reactor is isomerized. At a flow rate of 1667 gal/li, 50% of the metaxylene is isomerized at the Same temperature and pressure. Energy changes are negligible. 

Mordenite has been used since the second half of the 1970s in industrial xylene and ethylbenzene isomerization proeesses, chiefly for the production of paraxylene. the isomer for whieh there is the highest demand (Table 2). First, the isomerization of ethylbenzene into xylenes implies a bifunetional aeid catalyst. Second, it requires a temperature of around 400°C and hydrogen pressure in the region of 1-1.5 MPa. The catalyst is. therefore, composed of acid mordenite associated with a strong hydrogenation function, supplied by Pt. 

Xylene isomerization takes depieted paraxyiene and orthoxylene streams from the paraxylene unit and passes them over a duai fixed-bed cataiyst. As the process flow moves through the reactor and over the catalyst, it is combined with hydrogen-rich recycie gas. The upper section of the reactor is utiiized for EB deaikyiation, and the iower section is optimized for xylene isomerization. EB conversion rates are typicaiiy in excess of 65%, whereas paraxyiene concentrations are typically 102% greater than equilibrium. 

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