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Fractionation process

or extraction it is meant the separation of one or more components (from a mixture) with the employment of a miscible or immiscible solvent, thanks to a favorable repartition coefficient of the solute. The repartition coefficient is given by the ratio between the solute concentration in the SCF at the equilibrium and the concentration of the solute in the starting matrix at the equilibrium.The extraction is a convenient process only if the component to extract shows simultaneously:·         

  • A favorable repartition coefficient (meaning a high solubility in the SCF);·         
  • A favorable separation factor (if evaluated in relation to possible compounds being extracted from the mixture together with the compound of interest).

In the other cases, in order to obtain a satisfactory separation level, it is necessary to recur to the fractionation process, modifying the chromatographic column configuration (see figure): the column is divided in a rectification section (the portion of the column above the feeding section) and an exhaustion section (the portion under the feeding section). The fractionation can occur in two different ways: by temperature gradient and by extract reflux. The choice of one of the two methods is generally conditioned by the plant design. If the column is equipped with a heating system differentiated along the column height, it is convenient to realize the temperature gradient fractionation with the tower heated at different temperatures, generally increasing along the height.  The different compounds' solubility in SC-CO2 decreases, most of the times, with the temperature increase. In the exhaustion section, where the temperature is lower, there will be a rough but efficient solubilization of the compounds to fractionate. In the rectification section instead, temperature is set properly in order to provoke a drastic decrease in the solubility of one of the two compounds. This will be released by the SC-CO2 and it will undergo an internal reflux, while the most soluble compound will be concentrating in the extract. This kind of fractionation allows to modulate the selectivity factor (within certain limits) modifying the temperature in relation to the exhaustion section. Anyway it is necessary to know in advance the solubility of the compounds to separate in SC-CO2 in relation to the temperature. The reflux fractionation of the extracts occurs at constant temperature, but it needs the insertion of a second pump for the liquids before the separator, in order to recirculate part of the extracts toward the column head. In this way, therefore, even if the separation factor is not very favorable, it is possible to obtain the fractionation, since the mixture fed is more and more rich in the desired component. Regulating the extracts fraction to recycle and the number of recycles to apply, it is possible to obtain a total extract having the desired composition.In the case in which the separation factor is such to forbid separation with extraction mode or, instead, extract richer than the possible limit reachable are requested, it is necessary to apply reflux fractionation. In the case of SFF the extracting phase is constituted by SC-CO2, which might be added of a co-solvent.

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