August 26, 2013
Boost p-xylene production with a shape-selective catalyst. Over the past 20 years, the demand for poly(ethylene terephthalate) polyester has made it the fastest-growing polymer in the petrochemical industry. Consequently, the two intermediates used to produce polyester, purified terephthalic acid (PTA) and ethylene glycol, are in high demand. The production of p-xylene, the feedstock for making PTA, is also growing. Boosting p-xylene yields from catalytic reformers is important for integrated refining–petrochemical companies.
The major source of xylenes is refineries’ reforming processes, which are primarily intended to boost gasoline octane. In reformers, naphtha is catalytically converted to a mixture of aromatics, including the three xylenes. The ratio of the xylene isomers produced in reformers (as determined by thermodynamic equilibria), however, is the direct opposite of what the petrochemical marketplace needs. m-Xylene is made in the largest amount and is in the least demand; whereas p-xylene, which has the largest demand, is made in the smallest amount. The isomerization technology needed to fix this imbalance increases costs.
C.-Y. Chen and co-inventors disclose a reforming process that uses a medium-pore zeolite catalyst to increase p-xylene concentrations in reformate to higher than thermodynamic equilibrium levels. In a control example, a naphtha feedstock with >10% C8 paraffinic hydrocarbons is introduced into a fixed-bed reactor that contains a commercial reforming catalyst consisting of platinum and a rhenium promoter on alumina. At a temperature of 479 ºC, a liquid hourly space velocity of 1.5 h–1, a pressure of 350 psig, and a H2/hydrocarbon mol ratio of 5:1, the p-/m-xylene ratio in the product is 0.41:1.
When the same experiment is run with a catalyst containing platinum and rhenium supported on ZSM-5 zeolite, the p/m ratio jumps to 1.24:1. The inventors say that this type of reforming catalyst is shape-selective and produces more p-xylene than is normally allowed by thermodynamic equilibrium at these temperatures. (Chevron USA [San Ramon, CA]. US Patent 8,471,083, June 25, 2013; Jeffrey S. Plotkin)