Phase equilibrium diagrams for the system water, inert and solutes from mate (Ilex paraguariensis) leaves at constant temperature and cyclic pressurization
Introduction
Leaching with hot water as solvent is applied in large-size plants in South America to produce a non-alcoholic beverage from dry leaves of mate (Ilex paraguariensis). As many times reported in the literature (Kawakami and Kobayashi, 1991, Zanoelo et al., 2008, Rodriguesa et al., 2010, Tussolini et al., 2014), it tastes like the worldwide consumed aqueous extracts analogously produced with dry shoots of tea (Camellia sinensis). The intake of such a kind of beverage in a commercial ready-to-drink way has increased substantially in Brazil (Kotovicz and Zanoelo, 2013), the world-leading exporter and second producer of dehydrated leaves of mate (Kotovicz and Zanoelo, 2013, Cavalcanti, 2013).
The benefits of leaching assisted by hydrostatic pressure cycles when compared to classical and innovative extraction techniques have been previously evidenced in the literature (Naviglio, 2003, Naviglio et al., 2007, Naviglio and Ferrara, 2008, Naviglio et al., 2008, Kotovicz and Zanoelo, 2013, Kotovicz et al., 2014, Ortiz et al., 2015). For the particular case of solute removal from mate leaves, it has been already demonstrated that the efficiency of extraction is increased from approximately 30% at constant atmospheric pressure to about 80% by applying hydrostatic pressure cycles at only 338.2 kPa (Kotovicz and Zanoelo, 2013). The rates of mate solubles leached under cyclic hydrostatic pressure were also revealed to be higher than those at constant pressurization, and at atmospheric pressure at the same extraction temperature (Kotovicz and Zanoelo, 2013). There are also strong evidences that extraction of mate solubles assisted by ultrasound at pressurized conditions, and assisted by pulsed hydrostatic pressure, are procedures analogously effective to enhance the extraction yield at equilibrium and to reduce the time of extraction, but the later is much more economic in terms of capital cost (Kotovicz and Zanoelo, 2013, Kotovicz et al., 2014).
Because of the above summarized advantages of cyclic pressurization for extracting water-soluble compounds from leaves of mate, it has been taken into consideration as a reliable alternative procedure to produce ready-to-drink beverages of I. paraguariensis in a commercial scale. However, it is well-known that the success of the industrial operation of leaching depends on calculations based on the knowledge of the equilibrium relationships between the slurry (raffinate or underflow) and the liquid phase (extract or overflow) (Treybal, 1980, Penney and Fair, 2012). The methods of calculation to estimate the number of theoretical stages of leaching required to increase the solute content of the effluent solution to some specific desired value by involving equilibrium diagrams are extensively reported in the academic literature on mass transfer operations of chemical and food engineering (Treybal, 1980, Blackadder and Nedderman, 1982, McCabe et al., 1985, Rao, 2010, Berk, 2013).
In this framework, the main aim of the current investigation is to obtain equilibrium data for the system water-mate inert-mate solubles in the solid-to-solvent ratio range from 1.5:30 to 6:30 at three different leaching temperatures (17 °C, 32 °C and 47 °C). The results at equilibrium were basically emerged from kinetic experiments of solute removal, which were used in a tuning procedure involving a first-order extraction model whose one of the two parameters is the solute mass fraction at equilibrium on an inert-free basis. Graphical representation of equilibrium on rectangular coordinates (McCabe–Thiele and Ponchon-Savarit constructions) were preferred to a right triangular diagram.
Section snippets
Kinetic experiments
Commercial samples of comminuted dry leaves of mate retained on a 20 mesh screen and passed by a 12 mesh screen of the Tyler standard scale were used in the kinetic leaching experiments (McCabe et al., 1985). The moisture content of the leaves determined in triplicate by oven drying them at 105 °C for 24 h was close to 8.04 ± 0.07% (AOAC, 1990).
Leaching was carried out in a batch cylindrical polycarbonate vessel where a constant mass of distilled water (30 × 10−3 kg) and a sachet containing a
Results and discussion
Fig. 1 presents plots of solute mass fraction in the liquid phase as a function of leaching time for some particular examined conditions. The discussion is not for now focused on the effect of the changed factors on the examined curves, but the point here is that the curves always have the typical general form of an asymptote approaching the equilibrium when the leaching time infinitely increases. A simplified model that properly describes such a kind of kinetics, involving a tuned parameter yAe
Conclusions
Equilibrium data for the system water−inert−mate solubles for leaching assisted by cyclic pressurization in the R range from 0.05 to 0.2at 182.8 kPa and three different temperatures were presented. They reveal that the assumption that yAe is equal to xAe, many times adopted for the calculation of the number of theoretical contact stages for a continuous operation of leaching, is not currently valid. In the examined case, it happens because the insoluble fraction B is not truly inert, but it
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