Process and Thermodynamic Modeling for Separation of Aromatics from Aliphatics with Ionic Liquids

The separation of aromatics from aliphatics is an important step in oil refinery processes. Current methods for this separation include extraction, extractive distil- lation, and azeotropic distillation. The main problems with these methods are: 1) suitability only for aromatic concentration of the feed mixture higher than 20%, 2) low aromatic/aliphatic selectivities, and 3) low capacities. All of these problems call for the development of new technology for aromatic/aliphatic separation, with the design of an ionic liquid (IL) solvent for the extraction process being a promising research direction.

Well-known for their favorable properties including negligible vapor pressure, good thermal stability, large liquid range, and tunability by molecular design, ILs are highly attractive as potential substitutes for current commercial solvents [1–3]. Process and thermodynamic modeling can guide the design of ILs. For example, through fun- damental studies of the thermodynamic models for liquid-liquid equilibrium (LLE) and vapor-liquid-liquid equilibrium (VLLE), we can better understand the thermo- dynamic limits of the product purities one can achieve with various ILs, and we can evaluate energy and economic costs by building process models to compare the use of IL extraction solvents to traditional solvents for aromatic/aliphatic separation like sulfolane.

To study the aromatic/aliphatic extraction process, toluene-heptane mixtures are used as a model, with a current commercial solvent (sulfolane) used as the bench- mark for comparison. For the solvent recovery method, taking advantage of the nonvolatility of ILs, we choose to use a simple flash drum to evaporate the aro- matics out of the aromatic-IL mixture, thus recovering the IL solvent. To reduce the loss of solvent, ILs with good thermal stabilities are preferred. Therefore, we focus on ILs with the bis(trifluoromethylsulfonyl)imide ([Tf2N]−) anion paired with 1-hexyl-3-methylpyridinium ([hmpy]+), 1-hexyl-3- methylimidazolium ([hmim]+), 1- n-butylthiolanium ([bthiol]+), and triethyloctyl phosphonium ([P2228]+) cations.

For the thermodynamic modeling of heptane-toluene-sulfolane ternary LLE, tra- ditional NRTL1 method is used. For the heptane-toluene-IL ternary LLE, COSMO- SAC2 and COSMO-RS 3are investigated for all four ILs. The ability of these models to fit the experimental ternary LLE data are compared. On this basis, COSMO-SAC is chosen to be the thermodynamic model for the IL process. And among four ILs, [bthoil][Tf2N] is chosen as the solvent for the IL process for its better selectivity.

Process models are built for the separation of toluene from heptane using the com- mercial solvent sulfolane, as well as one of the four ILs [bthoil][Tf2N] as the extraction solvent. Sensitivity analysis is implemented for the process. For the sulfolane process, the product purity could reach 0.991 for heptane, and 0.999 for toluene, but further research is needed for reducing the energy consumption, especially for the solvent recovery column. For the IL process, the energy consumption is much lower than that of sulfolane process, but more ILs need to be explored to increase the product purity.

1. Wen-Churng Lin, Teh-Hua Tsai T.-Y. L. and Yang C.-H. Influence of the tem- perature on the liquid?liquid equilibria of heptane + toluene + sulfolane and heptane + m-xylene + sulfolane. Journal of Chemical & Engineering Data, 53 (3):760–764, 2008.
2. Amparo Chafer, Javier de la Torre A. F. and Lladosa E. Liquid- liquid equilibria of water + ethanol + 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ternary system: Measurements and cor- relation at different temperatures. Journal of Chemical and Engineering Data, 60(8):2426–2433, 2015.
3. Brennecke J. F. and Maginn E. J. Ionic liquids: Innovative fluids for chemical processing. AIChE Journal, 47(11):2384–2389, 2001.