Empirical kinetic models for the resinoid extraction from aerial parts of St. John's wort ( Hypericum perforatum L.)

Abstract

The empirical kinetic models for the resinoid extraction from St. John's worth ( Hypericum perforatum L.) aerial parts were analyzed to choose the optimum one regarding their relative simplicity and accuracy of fitting the experimental data obtained at different operating conditions. The following two-parametric models were analyzed: a parabolic diffusion model, a power law equation, a hyperbola equation, an exponential equation of the Weibull type and a logarithmic relation of the Elovich type. The aqueous solutions of ethanol (70 and 95% by volume) were used to isolate the resinoid from a charge of the ground plant material (the plant material-to-solvent ratio: 1:5 and 1:10 g/mL; the extraction temperature: 25, 50 and about 80 °C; and the mean plant particle size: 0.23, 0.57 and 1.05 mm). All empirical models gave a good fit to the experimental data (root mean square, RMS < ±5%), but the best one was Elovich's equation having the smallest RMS (±2.5%) and the highest linear correlation coefficient (0.975). The best empirical model was somewhat better than the physical model based on the film theory (RMS = ±2.8%). The effects of the process factors on the kinetic model parameters were assessed using the full factorial test plan 2 4.