Extraction, partially miscible examples

Separation of two liquid phases, immiscible or partially miscible liquids, is a common requirement in the process industries. For example, in the unit operation of liquid-liquid extraction the liquid contacting step must be followed by a separation stage (Chapter 11, Section 11.16). It is also frequently necessary to separate small quantities of entrained water from process streams. The simplest form of equipment used to separate liquid phases is the gravity settling tank, the decanter. Various proprietary equipment is also used to promote coalescence and improve separation in difficult systems, or where emulsions are likely to form. Centrifugal separators are also used. 

As an alternative to distillation, extraetion with a eo-solvent that is poorly mis-eible with the ionie liquid has often been used. There are many solvents that can be used to extract product from the ionic liquid phase, whether from a monophase reaction or from a partially miscible system. Typical solvents are alkanes and ethers (15). Supercritical CO2 (SCCO2) was recently shown to be a potential alternative solvent for extraction of organics from ionic liquids (22). CO2 has a remarkably high solubility in ionic liquids. The SCCO2 dissolves quite well in ionic liquids to facilitate extraction, but there is no appreciable ionic liquid solubilization in the CO2 phase in the supercritical state. As a result, pure products can be recovered. For example, about 0.5 mol fraction of CO2 was dissolved at 40°C and 50 bar pressure in [BMIMJPFe, but the total volume was only swelled by 10%. Therefore, supercritical CO2 may be applied to extract a wide variety of solutes from ionic liquids, without product contamination by the ionic liquid (29). 

EQUILIBRIA AND PHASE COMPOSITIONS. The equilibrium relationships in liquid extraction are generally more complicated than for other separations, because there are three or more components present, and some of each component is present in each phase. The equilibrium data are often presented on a triangular diagram, such as those shown in Figs. 20.10 and 20.11. The system acetone-water-methyl isobutyl ketone (MIK), Fig. 20.10, is an example of a type I system, which shows partial miscibility of the solvent (MIK) and the diluent (water) but complete miscibility of the solvent and the component to be extracted (acetone). Aniline-7i-heptane-methylcyclohexane (MCH) form a type II system (Fig, 20.11), where the solvent (aniline) is only partially miscible with both the other components. 

---