Experiments, Modelling and Scale-up for Supercritical Extraction from Plants

摘要

The extraction of natural products with supercritical carbon dioxide has been introduced on industrial scale in the last decades and the number of industrial plants is increasing. It is therefore important to develop and apply procedures for optimisation of the extraction process, using preferably phenomenological models. The models should describe three aspects of the extraction: phase equilibrium, mass transfer, and flow pattern of the solvent in the extractor. Technical design of industrial plant for supercritical extraction based on pilot plant experiments in the Separex company is described in congress contribution [1]. The pilot plant is used first to determine the optimal extraction conditions scanning different pressures, temperatures and solvents, and then to obtain thermodynamic and kinetic data. The appropriate extrapolation method for scale-up depends on the mechanism controlling the extraction, which could be the solubility of the extract in the fluid, internal diffusion in the extracted material, or their combination. If the extraction rate is limited by the solubility or thermodynamic equilibrium between the solid and fluid phases, the solvent mass required for the extraction from unit mass of the feed (extracted plant) should be kept constant in small and large equipment. When the extraction is controlled by internal diffusion, the mass ratio of flow rate to feed, which is proportional to solvent residence time in the extraction bed, should be kept constant, claim the authors [1]. When both diffusion and solubility are important, a versatile mathematical model for extraction is solved numerically. Thermodynamic equilibrium is described in the model either by the solubility of the extract or by a linear or Langmuir type equilibrium that is known from desorption/adsorption. Linear driving force approximation is used for the external and internal diffusion, and the axial dispersion is simulated dividing the extractor axially into a series of ideal mixers. The design procedure is illustrated on vegetable oil extraction from ground seeds [1]. Two extraction periods are distinguished in the model. First, the easily extractable fraction of total oil is extracted and the solvent flowing out of the extractor is completely saturated with oil, which solubility in CO_2 under given pressure and temperature was measured separately. The second period is controlled by internal diffusion and the internal diffusion coefficient is the same for any size of extractor. Thus, only two model parameters have to be determined from the pilot plant extraction, the fraction of easily extractable solute and the internal diffusion coefficient. In this contribution, the basic ideas are identical with the cited work but the model is more detailed and extended by the findings published in the last decade. A procedure for laboratory/pilot plant extraction data collection and scale-up is proposed.