Supercritical fluids in separation and purification: A review

The properties of the supercritical fluid near the critical point can be changed by varying the pressure and temperature, allowing selective and faster extraction. Supercritical fluids exhibit liquid-like density and gas – like viscosity & diffusion coefficients and can penetrate more into the solid matrix inaccessible to liquids due to negligible surface tension and viscosity. They are suitable as a substitute for organic solvents in a range of industrial and laboratory processes. These properties of supercritical fluids are well suited for the extraction of solutes, from various plants, fruits, flowers, seeds, leaves, which cannot be easily extracted using conventional extraction processes. In view of above mentioned advantages, supercritical fluid extraction is becoming increasingly popular in many industries such as petroleum, chemical, food, and perfumery. Supercritical fluids are also used for the extraction of metal ions from aqueous solutions and solid and liquid matrices. This review paper provides a concise review of the applications of supercritical fluids from the perspective of feed materials, type of supercritical fluids, co solvents and modifiers used, nature of work, operating conditions, findings, limitations of the work done and scope for further research. The application of supercritical fluids in extraction and purification of various natural extracts and others, using CO2 accounts for more than 90% of the published research in the field of supercritical fluid technology. Other supercritical fluids being used are propane, hexane and butane. Carbon dioxide is a relatively non-polar solvent but has some limited affinity with polar molecules due to its large molecular quadrupole. Co-solvents and modifiers are often be added to improve the solubility of polar molecules. On addition of various co solvents and modifier such as isopropyl alcohol and ethanol, n-hexane, heptane, pentane, toluene, methanol, acetone, formic acid with ammonium formate in methanol etc enhances the performance of supercritical extraction process. Despite this, the status of supercritical fluid technology commercialization is less than satisfactory. A review studies revealed that most of the published research is at laboratory scale with little or no information on scale up and design strategies.