Supercritical Fluid Fractionation (SFF)
The separation process in which one or more components of a mixture (gas, solid, liquid, enzymes, suspension, or isotope) is divided during a phase transition, into a number of smaller quantities (fractions) with the employment of a miscible or immiscible solvent.
Fractionation occurs due to a favorable repartition coefficient of the solute (supercritical fluid).
The ratio of the concentrations of a solute in two immiscible or slightly miscible liquids, or in two solids, when it is in equilibrium across the interface between them.
Fractionation is only a convenient process if the component to extract shows simultaneously:
- A favorable repartition coefficient (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, SCF plant schematic):
The column is divided between 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 was (choice of methods is generally conditioned by the plant design):
- By temperature gradient
- By extract reflux
If the column is equipped with a heating system 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.
Temperature: Generally the different compounds’ solubility in SC-CO2 decreases, as temperature increases. In the exhaustion section, where the temperature is lower, there will be a rough but efficient solubilization of the compounds to fractionate. While in the rectification section, the temperature is properly adjusted to provoke a drastic decrease in the solubility of one of the two compounds. This is 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 modulation of the selectivity factor (within certain limits), modifying the temperature in relation to the exhaustion section.
It is necessary to predetermine the solubility of the compounds being separated in SC-CO2, in relation to the temperature.
Reflux: The reflux fractionation of the extracts occurs at constant temperature, but it requires a second pump for the liquids before the separator, in order to recirculate part of the extracts toward the column head. With this method, even if the separation factor is not very favorable, it is possible to obtain the fractionation, since the mixture fed is more-rich of the desired component.
By regulating (1) the extracts’ fraction to recycle and (2) the number of recycles to apply, it is possible to obtain a total extract of the desired composition.
When the separation factor forbids separation with extraction mode, or extract richer than the possible limit reachable are requested, it is necessary to apply reflux fractionation. In the case of Supercritical Fluid Fractionation, the extracting phase is constituted by SC-CO2, and possibly an added co-solvent.
Fractionation Principles of Ethyl Esters with Column (Tower)
Let’s suppose, for simplicity reasons, that the ethyl esters mixtures are composed of two pseudo-components, meaning groups of molecules with similar behavior:
- Light ethyl esters, being 14-18 C fatty acids;
- Heavy ethyl esters, being 20-22 C fatty acids.
Since the first group of compounds is more soluble in SC-CO2, it creates an increase in the concentration of heavy ethyl esters – removing the light ethyl esters in the most efficient way.
Such removal can be conveniently performed utilizing Filling Towers, in which a solvent phase and a liquid phase (ethyl esters mixture) come in touch counter-current. The filling tower is the equipment usually utilized for the fractionation of liquid mixtures with liquid or gaseous solvents. The name tower (or column) comes from the fact that it appears to be like a recipient having a high height-diameter ratio.
In general, the fractionation is performed feeding continuously and counter-current the mixture to separate and the extraction solvent. In order to maximize the contact between the phases and the mass exchange inside the tower, there is the filling, made of elements having proper size and/or shape (nonstructured filling).
To fractionate the liquid mixture, it is fed into the top of the fully operating filling tower and moves down along the filling delivering the most soluble compounds to the extraction solvent, which is lighter and moves along the tower in the opposite direction.
The exhausted fraction (refined) is collected from the bottom of the tower, and the extract (composed of the solvent and the solubilized products) is collected from the top. In other words, in the case of ethyl esters mixtures, the lighter fraction of the mixture will be collected as extract at the top of the column (being more soluble in the supercritical phase), while the heavier fraction (less soluble) will be collected from the bottom as refined, since it will concentrate in the residual liquid phase as an effect of the light fraction removal.
Therefore, it is to be considered that the more the similarity between the compounds to separate, (1) the lower the productive capacity of the plant (expressed as Kg fractionate/column section), and (2) the higher the height of the tower needed for the requested separation.