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A mathematical model for modeling solid-liquid extraction from plants was developed. Two extraction systems, System INicotiana tabacumL.-waterand System IIGeranium sanguineumL.-water, were investigated. The effect of agitation rate on the mode of the extraction process was studied, as the limit values in the case of internal diffusion for System I and System II were determined. The kinetics of extraction for both systems at determined technological parameters (temperature and solid/liquid ratio) was experimentally investigated. The values of the partial mass transfer coefficient,k, were determined by the Regular regime method. A four-parameter empirical model for prediction of the effective diffusivity,Deff, was also presented. The model adequacy is verified with the experimental data and shows very good coincidence.

Experimental data and mathematical models are presented for extraction from plants in a continuous countercurrent screw extractor operating with solvent recycling. The working process of the device was analyzed for two kinetically different solid-liquid systems: Geranium macrorhizum L.-water and Nicotiana tabacum L.-water. A dimensionless convection-diffusion model, adapted for the relevant flow configuration, was solved numerically under dynamic conditions. From independent experiments in a periodically stirred vessel and in a continuous screw extractor, the model parameters (effective diffusivity, mass-transfer coefficient, and axial dispersion) were obtained by comparing the model solutions to the experimental data. It was found that, for systems containing dilute solutions at high solvent velocity with an internaldiffusion-controlled process (Bi . 40), a simplified perfect-mixing approximation successfully fits the experimental data for the larger particle sizes studied. IntroductionScrew contactors are extensively used in solvent extraction for chemical, biological, and wood species valorization because of their simple mechanical configuration and reliable exploitation mode. The major difficulty in the scale-up of this design can be attributed to complex solid-liquid transport phenomena. As a whole, convection-dispersion models coupled with experimental measurements of the residence time distributions are used to assess the level of axial mixing. 1-3 To predict extraction yields, mass-transfer models are added to the transport models, under general, simplified, steady-state operating conditions. 4,5The present study aims to describe, experimentally and theoretically, the behavior of a new type of countercurrent screw extractor. The apparatus has the advantage of providing reliable conveyance outside the screw through the ascending hydrotransport of a dispersed solid material. Because the multistage process equipment presents some differences with respect to conventional continuous processing, a mathematical model that can adequately describe the processing is required. The proposed model is based on integration of the differential mass balances that takes into account the coupled processes of internal diffusion, external convection, and axial dispersion.All of the results are compared with the predictions obtained using the standard function derived from the experimental kinetics of the process, 6,7 which gives an approximation of the extraction kinetics under the assumption of perfect mixing for both phases. The use of the standard function allows for an integral description of the internal diffusion resistance without knowledge of the corresponding kinetic parameters. The ability of this standard function to predict the variation in the kinetics for a change in the phase ratio is commonly employed in the case of periodic processes, but no consideration has been given to the accuracy of the approximation for dynamic conditions.

Extraction kinetics of polyphenols and flavonoids from plant material and their separation and concentration by nanofiltration were investigated. The kinetics experiments were carried out with Cotinus coggygria. The influence of the extraction solvent on the extraction rate was defined. Four different extraction solvents or mixtures were applied -methanol, azeotropic ethanol, 50/50 ethanol/water mixture, and pure water. The optimum extraction rate of flavonoids and polyphenols was achieved by using 50 % ethanol as a solvent for extraction. This solvent mixture was used for generating extracts for the nanofiltration experiments. Organic solvent nanofiltration membranes from DuraMem TM series with different pore sizes (200, 300 500 and 900 Da) were tested in dead-end and cross-flow filtration systems. Flux and rejection data were obtained for every membrane type. Very good separation and respectively concentration of the extracted useful compounds was achieved (rejection for polyphenols and flavonoids is 91 and 93 %, respectively). During a feed/extract concentration experiment, the constant polyphenols and flavonoids rejection was measured. The results suggest that the nanofiltration technology could be combined successfully with solid-liquid extraction for natural sourced valuable compounds enrichment.