| Phase equilibrium controls supercritical fluid extraction from plants in the first extraction period when mass transfer resistance is low; this period is decisive for the economics of the process. Phase equilibrium during the CO2 extraction of different compounds from several plants is being studied and in this work we present the results for the extraction from stinging nettle (Urtica dioica) leaves and roots. The solutes occurr in the solid phase in two forms, as free solute and as solute interacting with matrix. According to their polarity they form two groups, the polar compounds and the low-polar compounds that prevail in the extract due to the non-polar character of the solvent. Phase behaviour of major components practically is not affected by the presence of minor components. Equilibrium fluid-phase concentration of the examined minor low-polar component was, surprisingly, dependent on the equilibrium of major component rather than on the solubility of the pure component in CO2. On the other hand, the equilibrium fluid-phase concentration of minor polar components was most dependent on the solvent composition (we used ethanol in different concentrations as carbon dioxide modifier) and on the extraction temperature, and it was independent of the phase equilibrium of major, low-polar components. The consequence for the supercritical fluid extraction was that the extraction of minor low-polar components was synchronised with the extraction of major components, it was relatively fast, and the percentage of minor low-polar components in extract could not be significantly varied by changes in extraction pressure and temperature. The extraction of polar components was slower than that of low-polar compounds. It could be, however, as well as their percentage in the extract, controlled in a wide range primarily by changes in the ethanol concentration in the solvent and in the extraction temperature.
In supercritical fluid extraction from plants the driving force is the difference between the concentration in the bulk of supercritical solvent and the equilibrium concentration at the solvent-solid interface. Phase equilibrium controls the extraction process particularly in the first extraction period when mass transfer resistance is low, and this period is decisive for the process economics. As the extracted plants consist of many components, usually hundreds of them, which are present in the plant in a wide range of concentrations from tens percent to trace amounts, phase equilibrium is being established in a complex multi-component system.
The equilibrium of a compound extracted from plant may be affected by matrix, it is by the insoluble part of the plant, and/or by co-extracted compounds. Its equilibrium fluid-phase concentration may therefore substantially differ from its solubility measured in a twocomponent system compound + solvent, though the same pressure and temperature are applied.
The extraction operating conditions were adjusted to measure fluid-phase concentrations at equilibrium or very close to equilibrium. In interpretation of experimental concentrations and their changes in the course of extraction we distinguish between major and minor components and between low-polar and polar compounds. Effect of ethanol added to the solvent as a modifier is studied, too.
When plotted in one graph, the extraction curves measured at the same operating conditions but with different charges of extracted material in the extractor overlapped. It is a proof that equilibrium was established at the extractor outlet and the fluid-phase equilibrium concentration can be read from the slope of extraction curves. The quick achievement of equilibrium is a result of fast mass transfer from the particle core to its surface, which is enabled by breaking cell walls during the vegetable material grinding to very small particles.
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