Natural convection retards supercritical CO2 extraction of essential oils and lipids from vegetable substrates

External mass-transfer coefficients (k(f)) during supercritical fluid extraction (SCFE) of high-solubility solutes, under solvent upflow conditions and low superficial velocities, can be small because of the negative influence of natural convection phenomena. A shrinking-core model for mass transfer was used to estimate best-fit values of k(f) for data on SCFE of lipids from prepressed rapeseeds. Values of k(f) at a high Reynolds number (Re = 14.1) were similar when using solvent upflow or downflow, but k(f) at lower Re (1.57) was 3.6 times smaller when using solvent upflow than that predicted from a literature correlation for downflow conditions. These k(f)'s are consistent with values estimated by fitting literature data, or gathered from various sources under similar, nonadequate conditions (solvent upflow under low Re) for the extraction of both fatty and essential oils. Care is advisable when employing best-fit values of k(f) from laboratory data for process design purposes, especially when sizing of the solvent pumps for the experiments is questionable.