p-Diethylbenzene (p-DEB) is used as the desorbent in the Parex process (Derosset, et al., Sep. Sci. Technol. 15, 637 (1980)) for the separation of p-Xylene for xylene isomers and as the cross-linking agent in the synthesis of polymeric resins. One of the ways to produce p-DEB is to separate mixed DEB supplied from ethylbenzene alkylation process (Wang, et al., U.S. Pat. No. 4,950,835). Because of the very close boiling points possessed by DEB isomers: for example, ortho-diethylbenzene (o-DEB) is 183.degree. C.; meta-diethylbenzene (m-DEB) is 180.degree.-181.degree. C.; and para-diethylbenzene (p-DEB) is 184.degree. C., it is difficult to have a complete separation by distillation. The cryogenic separation is an alternative, however, it is in general costly.
At present, while several adsorption processes have been developed for separation of diethylbenzene isomers, the selective adsorption process using zeolite as a adsorbent in liquid phase is most commonly used and it is recognized as a most economic one. For example, disclosed in the U.S. Pat. No. 4,051,192 (1977), Neuzil, et al. used about 70 cm.sup.3 of the zeolite X ion-exchanged with barium (Ba) and potassium (K) to recover 90.7% of p-DEB at the temperature 450 K. and the pressure 136 atm, wherein toulene was used as the desorbent; Lu and. Lee, in their article, Ind. Eng. Chem., Res. 26, 2024 (1987), tested several adsorbents and desorbents for separation p-DEB from mixed DEB and found that the CDZ zeolite was the most appropriate one to obtain p-DEB with a purity over 95% in the temperature range from 393-463 K. and in the pressure range from 1.5 to 3.0 atm. wherein o-xylene was used as the desorbent.
Santacesaria, et al. in their article, entitled "Separation of Xylenes on Y Zeolites in the Vapor Phase. 1. Determination of the Adsorption Equilibrium Parameters and of the Kinetic Regime", Ind. Eng. Chem. Processes Des. Dev. 24, 78-83 (1985) reported the separation of xylene isomers under gas phase operation was found to be superior to that under liquid phase operation. The improvement was attributed to higher mass transfer rate in vapor. But for both operations a desorbent is required. Some of C.sub.8 aromatics were the most commonly employed desorbent.
Several methods of separation using desorbent as mentioned above require using aromatic compounds as a desorbent, therefore, a distillation operation is thereby needed in order to separate desorbent from product, which is energy intensive and increased separation cost.
TAN, Chung-Sung, one of the inventors of the present invention, and Tsay, Jeng-Leei, in their article, entitled "Separation of Xylene Isomers on Silicalite in Supercritical and Gaseous Carbon Dioxide" Ind. Eng. Chem. Res., Vol. 29, 502-504 (1990) reported an experimental study of the separation of an equal amount of p- and m-xylenes on silicalite using carbon dioxide as the carrier and the desorbent. The results showed that the operations in the gaseous phase carbon dioxide offered a better separation efficiency over those at supercritical conditions. The effects of temperature, pressure and flow rate on the effectiveness of separation were also examined. It was found that, for a pulse of 1.0 cm.sup.3 of xylene isomers and 39.5 g of silicalite, the most appropriate operating conditions were a temperature of around 358 K., a pressure of 476 atm and a flow rate of about 15.0 cm.sup.3 /min.
TAN, Chun-Sung, in his U.S. Pat. application Ser. No. 07/801,531 filed on Dec. 2, 1991 (which was allowed on Dec. 24, 1992), disclosed a process for separating ethylbenzene and p-xylene from xylene isomers mixture, wherein a compressed high pressure vapor phase CO.sub.2 was used as the carrier. A fixed amount of xylene isomers was fed into a silicalite adsorbent bed for adsorption operation, and a first stage effluent stream containing a rich m-xylene and/or o-xylene, but having substantially no ethylbenzene and p-xylene was obtained from the adsorbent bed, wherein when ethylbenzene or p-xylene contained in the mixture started to appear in the first stage effluent stream, a supercritical CO.sub.2 used as a desorbent was introduced into the adsorbent bed and a second stage effluent stream containing a rich ethylbenzene and p-xylene, but having substantially no other xylene isomers is thereby obtained. Preferably, the first and the second effluent streams could be separately sent into different activated carbon adsorbent beds to adsorb isothermally and isobarically the said xylene isomers products, and the resulting substantially pure CO.sub.2 exiting from said activated carbon adsorbent bed could be recovered and repeatedly used. Alternatively, the introduction of said high pressure gaseous CO.sub.2 could continue until m-xylene, o-xylene and p-xylene were completely separated from the mixture, and then a supercritical CO.sub.2 was introduced as a desorbent such that a second stage effluent stream containing a rich ethylbenzene (EB) was obtained.
The object of the present invention is to provide a process for separation of diethylbenzene isomers on silicalite in high pressure gaseous CO.sub.2 or propane to produce a high purity of p-diethylbenzene.
Another object of the present invention is to provide a process for separation of diethylbenzene isomers on silicalite in high pressure gaseous CO.sub.2 or propane to to produce a high purity of p-diethylbenzene with a high productivity, that is, the amount of the adsorbent used is relatively lower.
The other object of the present invention is to provide a process for separation of diethylbenzene isomers on silicalite in a high pressure gaseous CO.sub.2 or propane to produce a high purity of p-diethylbenzene, wherein CO.sub.2 or propane is used as a carrier in the first stage of the process and as a desorbent in the second stage of the process, and by reducing the pressure of the effluent stream from the adsorbent bed the diethylbenzene isomers products could be separated from the carrier or desorbent.