Paraxylene from Mixed Xylenes (Ethylbenzene Dealkylation)

The process under analysis comprises three major sections: (1) Adsorption; (2) Isomerization/Dealkylation; and (3) Fractionation.

Adsorption. The top product of the xylene splitter is fed to an adsorption unit, which generates an extract containing paraxylene and a raffinate with the ortho- and metaxylene, and ethylbenzene. Both streams are distilled (separately) to recover the desorption fluid. The raffinate then proceeds to the isomerization/dealkylation stage and the extract is sent to further fractionation.

Isomerization/Dealkylation. The Paraxylene-lean raffinate separated during adsorption is fed, along with hydrogen, to a reactor where ortho- and metaxylene are isomerized, and ethylbenzene is dealkylated to benzene and ethane. The product from the reaction comprises a near-equilibrium mixture of all xylene isomers, unreacted ethylbenzene and hydrogen, benzene, and minor amounts of by-products generated. This stream is cooled and sent to a gas-liquid separation step to recover the unreacted hydrogen.

Fractionation. The liquid product from reaction is sent to a stabilizer column, to be first separated in three streams: light hydrocarbons, a benzene/toluene mixture, and mixed xylenes. The lights are burned for fuel. The benzene-toluene mixture is sent to a benzene column for separation of benzene co-product from toluene (also sold as co-product to gasoline blending facilities). The xylenes obtained from the bottom of the stabilizer column are fed to the xylene column, which also receives the process feed of fresh mixed xylenes. This column removes heavy aromatics as its bottoms (to be sold as co-product), producing a mixed xylenes distillate that proceeds to adsorption. The extract stream produced during adsorption also goes through a final distillation stage to purify the paraxylene, generating a second stream of the toluene co-product.