Modelling and simulation of countercurrent fractional extraction with supercritical solvents

Fractionating fluid mixtures by supercritical fluid extraction has received increasing attention within the last few years. In this paper, a simulation model of a countercurrent fractional extraction column operated with supercritical fluids is presented. The model is based on ideal separation stages and incorporates the equation of state due to Redlich-Kwong and Soave for describing phase equilibria. Simulation studies were performed on the separation of a binary feed using supercritical carbon dioxide as extraction solvent. The system is also used when treating a reaction mixture from an enzymatic conversion effected in supercritical carbon dioxide. To achieve a symmetric fractionation with equal purities in both top and bottom product flow of the column, the reflux ratio has to be set to a distinct 'optimal' value. All other column parameters remaining unchanged, the optimal reflux ratio increases as the solvent flow is increased. As a consequence, product purities also increase.