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Ethylbenzene and styrene are currently separated by ordinary fractional distillation, which is challenging due the low relative volatility of this mixture of 1.3 to 1.4. Extractive distillation is a promising alternative to save capital and operational expenditures. Recently, ionic liquids (ILs) have been reported as a promising option to replace commonly used organic solvents like sulfolane. The IL 4-methyl-N-butylpyridinium tetrafluoroborate ([4-mebupy]-[BF 4 ]) is an IL with a strong effect on the relative volatility of the ethylbenzene/styrene mixture. In this work, binary VLE data in the range of (3 to 30) kPa, ternary VLE data at (5, 10, and 15) kPa, and binary LLE data at (313.2, 333.2, and 353.2) K have been determined for the system (ethylbenzene + styrene + [4-mebupy][BF 4 ]). The ternary VLE experiments show that [4-mebupy][BF 4 ] can enhance the relative volatility up to 2.7 to 2.8, and thereby [4-mebupy][BF 4] has a stronger effect on the relative volatility than the benchmark solvent sulfolane, which can increase the relative volatility up to 2.3. Therefore, [4-mebupy][BF 4 ] is a promising solvent for use in extractive distillation to separate ethylbenzene from styrene. The binary and ternary VLE data were correlated separately with the NRTL model and were regressed both together with the binary LLE data. The NRTL model could describe both data sets properly. The ternary VLE data could not be properly calculated by the binary NRTL parameters determined solely from the binary systems. ■ INTRODUCTIONStyrene is most commonly produced from ethylbenzene by a dehydrogenation process. 1 Since the conversion of ethylbenzene in this process is typically (50 to 70) %, 2 styrene has to be separated from ethylbenzene after the dehydrogenation reactor. This separation is currently performed by distillation and requires about 85 % of the total energy demand in the distillation section, 2 due to the low relative volatility of 1.3 to 1.4. 3 Extractive distillation can decrease the energy requirements of close boiling mixtures by the addition of a solvent to the distillation column. 4 The solvent enhances the relative volatility of the mixture, and thereby, it simplifies the distillative separation. As a consequence, the operation and capital expenditures will decrease. Sulfolane is a commonly applied organic solvent, which can increase the relative volatility of the (ethylbenzene + styrene) mixture up to 2.3. 5 Lately, ionic liquids (ILs) have been often reported as promising candidates to replace common organic solvents in extraction and extractive distillation processes. 6,7 An IL is a salt, that is liquid at room temperature (< 100°C), and commonly consists of a bulky asymmetric organic cation and an organic/ inorganic anion. 8 An IL combines the advantages of a liquid solvent and a solid salt. 9 ILs have a negligible vapor pressure 10,11 and are therefore often reported as green solvents. Many ILs also possess high chemical and thermal stability, and they exhibit a broad liquid range. 12,13 Moreover, because o...