Paraxylene from Toluene (STDP with Pentasil Zeolite Catalyst)

The process under analysis comprises three major sections: (1) Reaction; (2) Purification; and (3) Products Separation.

Reaction. Toluene and hydrogen feeds are combined and fed to a fired heater, where it is vaporized and raised to the reaction temperature. Here, the feed stream is fed to a radial reactor, where the disproportionation reaction takes place in vapor phase with a zeolitic catalyst (pentasil zeolitic aluminosilicate), in a hydrogen atmosphere to minimize coke formation and side reactions. This reaction proceeds via a bimolecular intermediate, which cleaves to benzene and xylene. The catalyst inhibits the diffusion of ortho-xylene and metaxylene is inhibited, promoting, ultimately, the formation of para-xylene.

Purification. The reactor effluent is cooled and routed to a gas/liquid separator, in which a hydrogen-rich gaseous stream is removed from the liquid stream, is compressed and recycled to feed. The liquid stream from the separator is fed to a column in which remaining gas and low-boiling hydrocarbons are stripped off from the stream. The aromatics concentrate is then submitted to a clay treatment, in such a way that impurities are removed by polymerization and absorption.

Products Separation. The aromatics concentrate is fractionated, so benzene, toluene and heavy aromatics are recovered. Toluene is recycled to feed stream. A concentrated xylenes stream is further divided into mixed xylenes and para-xylene by means of crystallizers and wash columns.