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Xilenes isomers separation by absorption in K-Y zeolites

LITERATURE

Summary of the work made

A process has been developed for separating pure m-xylene from p-xylene isomer by selective absorption of p-xylene in a K-Y zeolite. The separation occurs with the isomers in vapour phase. Toluene is used as desorbing agent. Four tubular columns containing a fixed bed of zeolite of opportune length are used in the plant, one operating in absorption and three in desorption. A serie of on-off valves allow s to change automatically an absorption column in a desorption one.

 

Work

The separation of aromatic C8 isomers is a relevant industrial problem, because, p-xylene is employed to produce terephtalic acid, o-xylene to produce phtalic annhydride and m-xylene is used to produce isophtalic acid. Therefore, it is of fundamental importance to separate each isomer at high level of purity. Distillation cannot be used because m- and p-xylene have a very low value of relative volatility. One of the most efficient method of separation is based on the use of zeolites through the adsorption. We have intensively studied this type of separation [1-7] and developed a separation process consisting on making the separation of a m- p-xylenes mixture, diluted with toluene, by this vaporized mixture into a tubular column filled with Y-zeolite, exchanged as much as possible with a potassium salt. K-Y zeolite adsorbes selectively p-xylene and if the length of the column is enough and the residence time appropriate, pure m-xylene mixed with tolune is collected at the column outlet. When a trace of p-xylene appears the feeding of the mixture is stopped and the column is washed with toluene for desorbing p-xylene. We have also automated the operation by working with 4 columns of the same length, one working in adsorption and the other three in desorption, being the desorption operation much slower than the adsorption one. The same principles can be also applied to other compounds that are difficult to separate and to adsorption with reaction in chromatographic reactors.

 

Literature on Xilenes isomers separation by absorption in K-Y zeolites

  1. S. Carrà, E. Santacesaria, M. Morbidelli, F. Codignola, L. Di Fiore;"Process for the separation of meta-xylene from mixtures of aromatic hydrocabrons" (SISAS SpA) B.I. 24044A/80; Eur. Pat. 811106176.1; US Pat. 4,368,347

  2. E. Santacesaria, M. Morbidelli, P. Danise, M. Mercenari, S. Carrà;"Separation of xylenes of Y zeolites. Note 1: Determination of the adsorption equilibrium parameters, selectivities and mass transfer coefficients through finite bath experiments". Ind. Eng. Chem. Proc. Des. Dev., 21, 440 (1982).

  3. E. Santacesaria, M.Morbidelli, A. Servida, G. Storti, S. Carrà; Separation of xylenes on Y zeolites. Note 2: Breakthrough curves and their interpretation". Ind. Eng. Chem. Proc. Des. Dev., 21, 446 (1982).

  4. S. Carrà, E. Santacesaria, M. Morbidelli, G. Storti, D. Gelosa; "Separation of xylenes on Y zeolites. Note 3: Pulse curves and their interpretation". Ind. Eng. Chem. Proc. Des. Dev., 21, 451 (1982).

  5. E. Santacesaria, D. Gelosa, P. Danise, S. Carrà; "Separation of xylenes on Y zeolites in vapor phase. Part 1: Determination of the adsorption equilibrium parameters and of kinetic regime". Ind. Eng. Chem. Proc. Des. Dev., 24, 78-83 (1985).

  6. M. Morbidelli, E. Santacesaria, G. Storti, S. Carrà "Separation of xylenes on Y zeolites in vapor phase. Part 2: Breakthrough and pulse curves and their interpretation". Ind. Eng. Chem. Proc. Des. Dev., 24, 83-88 (1985).

  7. G. Storti, E. Santacesaria, M. Morbidelli, S. Carrà; "Separation of xylenes on Y zeolites in vapor phase. Part 3: Choice of the suitable desorbent". Ind. Eng. Chem. Proc. Des. Dev. 24, 89-92 (1985).