| Supercritical fluid fractionation (SCF) has been proposed to purify several liquid mixtures. Particularly, a feed of fried peanut oil has been recently treated in order to re-use the purified oil. Although experiments have been successfully carried out obtaining a product composition similar to the original fresh oil, the understanding of the process still needs to be improved. The oil composition, obtained using High Performance Liquid Chromatography (HPLC), is usually lumped in three components groups (pseudocomponents): fatty acids (low molecular weight compounds, LMWC), triglycerides (TG) and dimeric and polymeric triglycerides (DPTG), that have been assumed to be typically triacilglycerol dimers. To describe overall mass transfer in any continuous device operated with supercritical carbon dioxide (SC-CO2), local phase equilibria, transfer rate and material balances must be simultaneously solved and integrated along the unit. Unfortunately the fried oil consists of many components and equilibrium data for a part of them are not available.
A large part of fats and oils in the world is used for the preparation of fried foods. During deep fat frying a significant quantity of oil is heated for a long period and a complex series of chemical changes during frying occur. Fried oil, as a consequence of oxidation, polymerisation and hydrolysis reactions, contains a variety of undesired substances such as oligopolymers, oxidized triglycerides, sterols and fatty acids. The discharged fried oil still has a large portion of triglycerides, but there are also volatile compounds, that include hydrocarbons, aldehydes and ketones, and polymers, that include mostly triglycerides dimers (DPTG), which were not even detectable in fresh oil. Oil composition is usually considered as a mixture of three class of compounds: LMWC, TG and DPTG. Moreover, Acidity and Peroxide Value (PV) parameters are often used to characterize the oil, these are determined according to standard methods specified in European Commission Regulation 2568/91.
Table 16. Fresh oil and fried oil properties.
Table 16 shows a fresh and a fried oil. Fried oil exhibits a strong increase in the DPTG, corresponding to a sharp decrease in TG content; moreover, DPTG increases both values, Acidity and PV. The recent availability of recovered fried oil, now possible in many countries because of environmental protection law, encourages to find new techniques able to valorize the collected material. Within this frame, the fried oil can be treated with SC-CO2 to be fractionated in a lighter part that can be reused and a heavier fraction that should be differently used. Fractionation by SC-CO2 might be an effective way for the purification of fried oils since a selective separation of the oil components can be attained. When appropriate operating conditions are set, a recovery of about 97% of TG fed into the column can be obtained, with a composition very similar to the fresh frying oil. The design of supercritical CO2 fractionation column requires the knowledge of phase equilibrium data, to estimate the driving forced for the material transfer between the phases. Local phase equilibria, mass transfer rate and material balances must be simultaneously solved and integrated along the column. Unfortunately, the fried oil consists of many components and equilibrium data for a part of them are not available. In fact, whereas equilibrium data between CO2 and the classical components of vegetable oils (LMWC; TG) are available in literature, the data between CO2 and heavier (DPTG) components has not yet been investigated. For this reason this study approaches the high pressure CO2 - DPTG equilibrium.
At the equilibrium can be observed two phases: the lighter is a supercritical fluid substantially transparent, coloured light yellow; the heavier is a liquid phase coloured from dark yellow to brown, with increase of pressure.
Table 17. Effect on volume of SC-CO2
VL,IN is the liquid mixture fed
Vtot, cell is the actual total volume of the cell
VL, EQUIL is the volume of the liquid observable at equilibrium
DVoil is the liquid volume difference between fed and equilibrium
Fig 54. Equilibrium volume ratio of the feed mixture.
Two opposite mass transfer phenomena can be observed: from liquid to supercritical phase and vice versa. The first one is shown when the oil is feed in the cell: the observable liquid volume is considerably reduced because of solubilisation in SC-CO2 of part of liquid components. The second one is related to the pressure: the liquid volume slightly increases with pressure, probably due to solubilisation of SC-CO2 in the liquid phase. A temperature increase from 40 to 55 °C slightly increases the solubilised CO2 into the liquid phase.
Particularly, the solubility of liquid phase components seems to change with pressure. In fact the colour observed for each phase changes with operating condition. When the pressure increases, supercritical phase colour becomes lighter and liquid phase becomes browner. This behaviour is suspected to be due to an improvement on TG selectivity. Experiments with static method have been planned for a near future to better define characterization of examined products. |