Kinetics of Pressure Cycling Extraction of Solute from Leaves of Mate (Ilex paraguariensis) Dispersed in Water

A set of seven extraction experiments was performed to investigate the influence of pressure cycles on the kinetics of solute removal from leaves of mate dispersed in water. The mass ratio of liquid to dry solid (40), the temperature (32°C), and time of extraction (3600 s) were not varied. Five extraction runs were under cyclic pressurization (1 cycle = 300 s at 91.4 kPa + 300 s at 182.8 kPa) and stirring speeds (S) of 0, 150, 500, 1500, and 2000 rpm, while the two other ones were at constant pressure (182.8 kPa) and S close to 1500 and 2000 rpm. Based on seven pairs of parameters of a reliable second-order kinetic model (R2 ≥ 0.967), cyclic pressurization had no effect on equilibrium and kinetics of extraction (p > 0.05) when the role of convection on solute transfer was negligible (S ≥ 500 rpm). In the stirring speed range from 500 to 2000 rpm, the operation was controlled by diffusion (Bi  > 1.7 × 103), so a transient two-dimensional diffusion model was able to describe correctly the changes of solute concentration with time. Below 500 rpm, solute transfer was governed by a combination of diffusion and convection with the external resistance to mass transfer as a function of S (16 ≤ Bi ≤ 28).