Liquid–Liquid Equilibria and Process Simulation of Thiophene and n-Paraffins with Imidazolium-Based [NTf₂] Ionic Liquids

New liquid–liquid equilibrium (LLE) data are presented for ternary systems relevant to extractive desulfurization, comprising thiophene, long-chain n -alkanes ( n -dodecane, n -tetradecane, and n -hexadecane), and a homologous series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, [Cₙmim][NTf₂]. Phase equilibria were measured at 313.15 K and atmospheric pressure to systematically assess the coupled influence of ionic-liquid cation alkyl-chain length and paraffinic hydrocarbon chain length on phase behavior and sulfur partitioning. Extraction performance was quantified in terms of distribution coefficients and selectivity, revealing a pronounced and tunable affinity of thiophene for the ionic-liquid-rich phase. Distribution coefficients ranging from 1.88 to 2.92 and selectivity values between 49 and 177 were obtained, depending on cation structure, indicating clear structure–property relationships within the [Cₙmim][NTf₂] family. The complete LLE datasets were successfully correlated using the Non-Random Two-Liquid (NRTL) activity-coefficient model, yielding thermodynamically consistent binary interaction parameters with low root-mean-square deviations. The resulting ternary phase diagrams provide quantitative insight into the non-ideal interactions governing the selective solvation of aromatic sulfur compounds in [NTf₂]⁻-based ionic liquids. Overall, this work establishes benchmark thermodynamic data and molecular-level structure–property relationships linking ionic-liquid structure to extraction behavior. Process simulations indicate that a three-stage extractor employing [C 8 mim][NTf₂] can achieve ultra-low-sulfur fuel (sulfur <10 ppm) at solvent-to-feed ratios above 1.3, while maintaining moderate and industrially feasible regeneration energy requirements.