A supercritical fluid is any substance at temperature and pressure above its critical point. It can diffuse through solids like a gas, and it can dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in

Large Changes in Density, allowing many properties of a supercritical fluid to be “fine-tuned”. Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes. Carbon dioxide and water are the most commonly used supercritical fluids, being used for decaffeination and power generation, respectively.

Supercritical Fluid Extraction (SFE) is the process of separating one component (the extractant) from another (the matrix) using supercritical fluids as the extracting solvent. If you need additional information, click here. Extraction is usually from a solid matrix, but can also be from liquids. SFE can be used as a sample preparation step for analytical purposes, or on a larger scale to either strip unwanted material from a product (e.g. decaffeination) or collect a desired product (e.g. essential oils). Carbon dioxide (CO2) is the Most Used supercritical fluid, sometimes modified by co-solvents such as ethanol or methanol. Extraction conditions for supercritical CO2 are above the critical temperature of 31°C and critical pressure of 74 bar. Addition of modifiers may slightly alter this. The discussion below will mainly refer to extraction with CO2, except where specified.

The Critical Point (C) is marked at the end of the gas-liquid equilibrium curve, and the shaded area indicates the supercritical fluid region. It can be shown that by using a combination of isobaric changes in temperature with isothermal changes in pressure, it is possible to convert a pure component from a liquid to a gas (and vice versa) via the supercritical region without incurring a phase transition.

The behavior of a fluid in the supercritical state can be described as that of a very mobile liquid. The solubility behavior approaches that of the liquid phase while penetration into a solid matrix is facilitated by the gas-like transport properties. As a consequence, the rates of extraction and phase separation can be significantly faster than for conventional extraction processes. Furthermore, the extraction conditions can be controlled to effect a selected separation. Supercritical fluid extraction is known to be dependent on the density of the fluid that in turn can be manipulated through control of the system pressure and temperature. The dissolving power of a SCF increases with isothermal increase in density or an Isopycnic (constant density) increase in temperature. In practical terms this means that a SCF can be used to extract a solute from a feed matrix as in conventional liquid extraction. However, unlike conventional extraction, once the conditions are returned to ambient the quantity of residual solvent in the extracted material is negligible.

The basic principle of SCF extraction is that the solubility of a given compound (solute) in a solvent varies with both temperature and pressure. At ambient conditions (25°C and 1 bar) the solubility of a solute in a gas is usually related directly to the vapor pressure of the solute and is generally negligible. In a SCF, however, solute solubilities of up to 10 orders of magnitude greater than those predicted by ideal gas law behavior have been reported.

The dissolution of solutes in supercritical fluids results from a combination of vapor pressure and solute-solvent interaction effects. The impact of this is that the solubility of a solid solute in a supercritical fluid is not a simple function of pressure.

Although the solubility of volatile solids in SCF is higher than in an ideal gas, it is often desirable to increase the solubility further in order to reduce the solvent requirement for processing. The solubility of components in SCFs can be enhanced by the addition of a substance referred to as an entrainer, or co-solvent. The Volatility of this additional component is usually intermediate to that of the SCF and the solute. The addition of a co-solvent provides a further dimension to the range of solvent properties in a given system by influencing the chemical nature of the fluid.

Co-solvents also provide a mechanism by which the extraction selectivity can be manipulated. The commercial potential of a particular application of SCF technology can be significantly improved through the use of co-solvents. A factor that must be taken into consideration when using co-solvents, however, is that even the presence of small amounts of an additional component to a primary SCF can change the critical properties of the resulting mixture considerably.

CO2 isn’t a liquid at room pressure: it’s a gas.

In the picture here on the right, taken from The Engineering ToolBox, the equilibrium curve is the colored one between the triple point and the critical point.

The Triple Point is the only plot’s point in which there are at the same time solid, liquid and gasseous phase of the material. It’s characterized by particular condition of temperature and pression. The Critical Point is characterized by the disappearance of the difference between gaseous phase and liquid phase.

In condiction of temperature and pressure over the critical point, we talk about Supercritical Fluid. The CO2 is at the same time a liquid a gas during all of the plot’s points that shape the Equilibrium Curve.

Going over other thermodynamic and technical discussions, maintaining the CO2 always along this curve will save a lot of energy. Our system is designed to maintain this equilibrium in the Reservoir, one of the most important vessel in the system.

Without a good control of temperature and pressure at the reservoir’s level, you’ll never be able to have a stable flow in the system.

In our system it’s easy to set the pressure in the reservoir at the desired value. We suggest 48 bar. The system, thanks to the full automation control, will stabilize the pressure automatically.

If essential oils are wanted, the presence of water in the matrix interferes negatively on the process, because it is extracted together with the oil, hence it is necessary to remove it in a second moment. In order to avoid this problem, vegetable matrices are usually dried before extraction, unless extracts containing also polar substances are wanted. In this case it is necessary to add other solvents (entrainers or co-solvents) directly to the matrix or to the CO2, like ethanol or water, able to extract those compounds.

CO2 is chemically inert, so isomerization, oxidation or components hydrolysis are avoided. The advantage of this technique is that at the end of the extraction, the solvent can be removed as a gas, offering the possibility to recover the extracted concentrated compounds. In the industrial processes, CO2 can be recycled minimizing its consume. This technique finds several applications such as oil extraction from seeds, caffeine extraction from coffee, nicotine extraction from tobacco, etc.; it is also very convenient at industrial level. The advantages in using supercritical CO2 are largely of a “health and safety” and environmental nature and relate to increased unease about the presence of organic solvent residues in material for human consumption. It has good solvent characteristics for non-polar and slightly polar solutes.

It has a convenient critical temperature (31ºC). This enables extractions to be carried out at comparatively low temperature (often as low as 40 or 50ºC), decreasing the risk of damage of thermolabile compounds.

Most of the volatile components, which tend to be lost in hydro-distillation, are present in the supercritical extracts. Partly because of this, extracts obtained in this way tend to have flavor and taste, which are well liked by tasty panels. Extraction of natural raw material with supercritical CO2, allows the obtaining of extracts which flavor and taste are perfectly respected and reproducible. The supercritical fluid ability to vaporize non-volatile components (at moderate temperatures) reduces the energy spent, when comparing to distillation. Once the pressure excess in the equipment prevents oxygen entry while extraction occurs, oxidation reactions don’t happen.

In chemistry, we often talk about chemicals and solvents in terms of their polarity. Some chemicals are highly polar (i.e. water) and some chemicals are highly non-polar (i.e. hexane). When describing how a particular solvent will dissolve a chemical – there is a rule of thumb that ‘like dissolves like’. Meaning, a non-polar solvent will dissolve a non-polar chemical. All fats and oils are non-polar, thus using a non-polar solvent is most appropriate.