Conceptual Process Design for Aromatic/Aliphatic Separation with Ionic Liquids

The investment and annual costs for a separation of aromatic hydrocarbons from a naphtha feed (~10 % aromatics) can be 35 to 70 % lower for an extraction with ionic liquids compared to an extraction with sulfolane. Aromatic hydrocarbons (benzene, toluene, ethyl benzene and xylenes) and C4 C10 aliphatic hydrocarbons are normally separated by liquid extraction, extractive distillation or azeotropic distillation. The application of ionic liquids for separation processes is promising because of their nonvolatile nature. This facilitates solvent recovery using techniques as simple as flash distillation or stripping. Ionic liquids are organic salts that are liquid at low temperature (T < 100 °C) and they consist of organic cations based on methylimidazolium, N-butylpyridinium and others, and organic or inorganic anions. The extraction of toluene from a mixed toluene/heptane stream with the ionic liquid 4-methyl-N-butyl-pyridinium tetrafluoroborate ([mebupy]BF4) was evaluated in a pilot plant Rotating Disc Contactor (RDC). With the NRTL interaction parameters determined for this separation, a process model could be developed using ASPEN Plus 12.1. The data measured in the pilot plant and the data calculated with the model are in reasonable agreement with each other. With the results of process simulations, an economic evaluation of the process was made and compared to the extraction process with sulfolane. The total investment costs in the sulfolane extraction were estimated by UOP, the supplier of this process, to be about M€ 86 for a naphtha feed of 300 t/h containing about 10% aromatic hydrocarbons. The investment costs for an extraction process with the ionic liquid [mebupy]BF4 was estimated to be M€ 56, including an ionic liquid inventory of M€ 20. The lower investment in the ionic liquid process is mainly due to the fact that the regeneration of the ionic liquid is much simpler than that of sulfolane. Since also the energy costs are lower, the total annual costs with the ionic liquid process are estimated to be M€ 27.4, compared to M€ 58.4 for sulfolane. The main conclusion of the process evaluation is that ionic liquids which show a high aromatic distribution coefficient, Darom = 0.6 m/m, with a reasonable aromatic/ aliphatic selectivity, Sarom/alif = 40, could reduce the investment costs of the aromatic/ aliphatic separation to about M€ 25 to 30 and the annual costs to M€ 16 to 17.