No abstract

No abstract

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.

A scale-up methodology based on the characteristic function approximation for the mass-transfer rate is proposed that provides a reliable and inexpensive method for designing and controlling an extraction apparatus. The characteristic function is derived in terms of the usual kinetic and equilibrium parameters. An analytical expression for the overall resistance to mass transfer is also obtained, providing a linearly variable mass-transfer resistance during the extraction. Experimental data for the kinetics of extraction in a laboratory-scale batch extractor from four solid-liquid systems are presented and discussed in relation to the theory. The calculation procedure is applied to estimate the extraction times in a continuous-action apparatus corresponding to an assigned extraction yield. The calculation results are also supported by experiments in a pilot screw extractor.

No abstract