All pure compounds can be found in three states : Solid, Liquid or Vapor (or Gas). On the (pressure, temperature) diagram, the three regions corresponding to these three states are separated by curves that meet at the triple point. Surprisingly, thevaporization/liquefaction curve presents an end point called critical point (Pc - Tc ) : Beyond this point, (P> Pc et T> Tc ), only one phase exists, called Supercritical Fluid (SCF) . At the critical point itself, the fluid compressibility becomes infinite :
meaning that the fluid specific gravity rapidly varies with a slight change in pressure at constant temperature. Moreover, even out of the critical region itself, SCF exhibit large changes in specific gravity – and consequently its solvent power - , as shown on figure 1, and other physico-chemical properties with pressure or temperature.
Figure : Diagram (P,T,r) of carbon dioxide
Most compounds exhibit a critical pressure in the range of 35 to 60 bar, except water (221 bar) and ammonia (113.5 bar). Critical temperature increases with the complexity of the molecule and very few compounds exhibit a critical temperature between 0 and 50°C (ethane, ethylene, CO2, N2O). Mixtures behave in a more complex way depending on their composition, but “ critical ” phenomena are also observed.
The most interesting characteristics of SCF, on which are based all the SCF processes, is related to their “tunability” with pressure and temperature, especially of the tunability of their solvent power.
Solvents are classified according to a scale of polarity depending on their ability to dissolve “ polar ” or “ non-polar ” molecules. Water is the most polar solvent, dissolving all kinds of compounds that can be ionized or that contains hydrophilic moieties like sugars, proteins, amino-acids,….On the contrary, organic solvents like light alkanes (hexane, heptane,…) or chlorinated hydrocarbons do not dissolve these compounds, but only hydrophobic molecules that are not at all soluble in water like fats, oils, hydrocarbons, essential oils,… : they are called non-polar solvents. Other solvents like alcohols, amines, ketones,…exhibit intermediate behavior. Most supercritical fluids behave like non-polar solvents exhibiting a strong affinity with lipids and hydrocarbons, but a weak affinity with oxygenated or hydroxylated molecules ; it is possible to tune their polarity by adding a polar co-solvent (ethanol or light alcohols, esters or ketones).
Solvent power :
Fluid phase equilibria of mixtures are very complex, and many types of phase diagrams can be found. Recent progresses in thermodynamc modeling permit to predict the phase equilibria of many mixtures ; however, some measurements – that are difficult to perform! - continue to be required to set some interaction parameters that cannot be calculated yet, especially for polar liquids (for example in the case of strong hydrogen-bonding). Nevertheless, for “simple” systems and relatively low solubility, the empirical correlation proposed by Chrastil can be used to interpret experimental results with a good reliability without any complicated calculations :
where C is the solute concentration, a, b and k empirical constants ; this correlation shows the extreme dependence of the solubility to the fluid specific gravity r. It also shows that :
Ø Solubility increases with density (or pressure) at constant temperature ;
Ø Solubility may increase or decrease when temperature is raised at constant pressure.
In all cases, the solubility dramatically decreases when the fluid is depressurized at constant temperature below its critical pressure, with solubility variation of several orders of magnitude. This is the basis of most SCF processes : SCF are used as solvents in the supercritical fluid region to selectively extract some compound(s) before being depressurized to cause the solute(s) precipitation permitting the fluid-solute separation.
It is to be noticed that although water is only slightly soluble in SCF carbon dioxide (1-2 g/kg), it plays a very important role as “co-solvent” for many polar molecules ; in fact, water is present in most applications, especially when natural products are processed.
Finally, it is also important to say that, due to their non-polar character, SCF are also used as “anti-solvent” in polar organic solvents where they readily dissolve, leading to a significant decrease of their polar character, and causing precipitation of compounds previously dissolved in these solvents.
Thermodynamic cycle :
The basic diagram when designing and operating a SCF plant (whatever its size !) is the Pressure-Enthalpy diagram (so-called Mollier diagram), or possibly the Temperature-Entropy diagram for those who prefer. On these diagrams, it is easy to represent the behavior of the fluid that, in most cases, can be assimilated as a pure fluid, except when a significant content of co-solvent is added to the pure fluid. It becomes easy to read the phase state (liquid, gas, supercritical fluid), the enthalpy to supply at the heat exchangers or to remove at the condenser.
In literature, thousands of articles can be found on high-pressure fluid phase equilibria covering a very wide range of compounds and operating conditions. But happily, it is not always necessary to handle detailed thermodynamic data on the processed mixtures to design the SCF processes, especially when complex mixtures are treated.