Ionic Liquids for Separation of Aromatics and Aliphatics: Extraction and Solvent Regeneration Using CO2

In the production of aromatic compounds, it is necessary to separate aromatic products from aliphatic compounds. This separation is achieved via liquid-liquid extraction. Due to several drawbacks from the current industrial processes for this separation, ionic liquids (ILs) have drawn interest as potential solvents for this separation.

In this work, four ionic liquids consisting of the bis(trifluoromethylsulfonyl)imide anion paired with a phosphonium, pyridinium, imidazolium, and thiolanium cation were investigated as potential solvents to separate aromatic and aliphatic compounds in a liquid-liquid extraction process. Equilibrium data of n-heptane + aromatic + IL was measured at 298.15 K. The nonrandom two-liquid model was fit to the systems, and collaborators applied the perturbed chain statistical association fluid theory, COSMO-RS, and COSMO-SAC models, which are compared, each showing different strengths. Experimental results surprisingly show that the thiolanium cation achieves separation factors similar to the imidazolium cation, despite being more aliphatic than aromatic in structure.

Solvent regeneration is also an important part of a liquid-liquid extraction process. While regeneration by methods such as stripping can be very effective, they can also be energy intensive. The gas antisolvent process is a method where dissolving a gas such as CO₂ in an IL/solvent mixture induces a vapor+liquid to vapor+liquid+liquid phase transition as a means of separating the IL and solvent. Here, the feasibility of using a gas anti-solvent to separate aromatics from ILs is studied by measuring CO₂ solubility in aromatics, [bthiol][Tf₂N] and mixtures. These systems exhibited VLàVLL transitions, and at lower pressure and CO₂ composition than required for the other ILs in this study.

The results given in this thesis show that ionic liquids are promising solvents for the separation of aromatics from aliphatics, which can be regenerated using CO₂ as a gas antisolvent. Furthermore, understanding the underlying thermodynamics can aid in optimizing this process.