Liquid-liquid Separation of Aromatics from Ionic Liquids Using CO₂

Ionic liquids are solvents successfully tested for the separation of aliphatic/aromatic mixtures obtaining, in some cases, better distribution ratios and selectivities for the aromatic than common organic solvents such as sulfolane. Since ionic liquids present a wide thermal stability, several authors propose the recovery of the solvent from the extract with simple distillation or stripping, but binary mixtures of aromatic + ionic liquid are barely studied in order to determine an optimal separation procedure.

The liquid-liquid equilibrium of binary systems of toluene + ionic liquids are investigated, obtaining a solubility range from 0.75 to 0.87 mole fraction of toluene as follows: [bthiol][Tf₂N] < [hmim][Tf₂N] ≈ [hmpy][Tf₂N] < [P₂₂₂₈][Tf₂N]. [P₆₆₆₁₄][Tf₂N] is completely miscible with toluene at room temperature. Also, [hmim][Tf₂N], [hmpy][Tf₂N] and [P₂₂₂₈][Tf₂N] were investigated for the liquid-liquid separation of heptane + toluene, obtaining lower selectivities compared with sulfolane. Then, the excess volume and excess enthalpy were determined for toluene + ionic liquid mixtures, obtaining negative values.

Solubility of CO₂ is measured in binary systems with toluene, [hmim][Tf₂N], [hmpy][Tf₂N], [P₂₂₂₈][Tf₂N], [P₆₆₆₁₄][Tf₂N] and [bthiol][Tf₂N], where the lowest solubility is observed in [bthiol][Tf₂N] and the highest in [P₆₆₆₁₄][Tf₂N]. Then, solubility of CO₂ in the homogeneous phase of toluene + ionic liquid is determined until a phase split is observed or the highest pressure allowed by the experimental apparatus is reached, i.e. about 8 MPa. The lowest phase split is observed using [bthiol][Tf₂N] at 3.282 MPa and the highest with [P₆₆₆₁₄][Tf₂N] at 5.732 MPa, when temperature is 298.15 K and initial composition of ionic liquid is 0.3 mole fraction. There is a small volume expansion difference when using ionic liquid in binary and ternary mixtures. For solubility measurements including [bthiol][Tf₂N], a modified phase equilibrium apparatus was used, eliminating the mercury of all the lines and replacing a manual pump from the old apparatus with a digital syringe pump.

The measurements performed in this work give the starting phase equilibrium measurements for simulating a liquid-liquid extraction (or extractive distillation) with a regeneration of the ionic liquid using CO₂ at moderate pressures.