Incorrect Ternary Liquid-Liquid Equilibria Description versus Extractor Design Simulation

The presented study focuses on the separation of aromatics from aliphatic hydrocarbons by liquid-phase extraction using ionic liquids. The aim of this study was to show the impact of incorrect description of liquid-liquid equilibria of the separated system on the extractor design parameters. Extraction is usually applied for the separation or concentration of dilute solutions with the aim to achieve minimum concentration of the separated component in the final raffinate. Therefore, correct description of the liquid-liquid (L-L) equilibrium in this concentration region is very important for the industrial practice. For thermodynamic description of a ternary L-L equilibrium, excess Gibbs energy dependencies on the mixture composition are applied. Model parameters of the mentioned equations can be evaluated from different experimental equilibrium data. The most commonly used experimental data are ternary tie lines. Parameters of the G(E) equations evaluated from experimental tie lines usually provide good or even excellent description of the given ternary L-L area. Extrapolation of the L-L data beyond this region, however, can sometimes lead to incorrect L-L equilibrium description. This problem is shown on the real ternary system heptane - toluene - 3-methyl-N-butylpyridinium tetracyanoborate ([3-mebupy][B(CN)(4))]). This study proves serious discrepancies in the extractor design parameters using the NRTL equation parameters evaluated from experimental ternary equilibrium data. Depending on the initial guess, the mathematical model of a continuous counter-current extraction column offered at least three different values of the extraction solvent consumption for the preset purity of the final raffinate and of the number of theoretical stages.