After the extraction process, the mixture composed by CO2 and solutes leaves the extraction vessel(s) and it is directed to the separation vessels.  By varying the pressure, flow and temperature of these vessels, it is possible to induce the selective precipitation of different chemical compounds as a function of their different saturation conditions in the supercritical fluid.

First of all, the mixture is directed in first separator and here, with the use of a particular valve, which is called Lamination Valve, the pressure goes down and (for the effect of Joule-Thomson Effect the rapidly-expanded gas during depressurization process cools because the molecules get the energy using their specific heats), also the temperature of gaseous CO2 drops dramatically, and two important effects are observed:

• Solvent properties of CO2 change istantly: Density of the fluid is reduced 10 fold and the expansion of the CO2 changes the speed of the CO2 from centimeters per second to meters per second.
• All compounds dissolved in the CO2 immediately fall out of solution because the fluid changed its status from supercritical to gaseous and it has become a weak solvent.

First separator is a heated gravimetric separator. This design is particular effective for the heaviest compounds of the extract. The gravimetric separator works by gravitational force. But this operation isn’t enough to completely clean up the CO2, in fact other lighter solutes are not separated from the solvent in this separator. For this reason there are other 2 separator in series.

Second separator is a heated cyclonic separator. While the first works by gravity force, the second works by centrifugal force. The fluid moves very quickly creating a vortex inside the vessel. The fluid continues to spin and the particles of extract begin to separate moving toward the wall of the separator, sliding to bottom. The temperature of the wall determines which compound will be condensed. Generally, essential oils, like hydrocarbon terpenes, can be found in this separator, as they are lipid-soluble compounds, while oxygenated terpenoids and sterols travel to the third separator dissolved in water micro drops as they are polar compounds and the condensation point (because of heating) is too high to condense them in the second separator.

The third is a cooled cyclonic separator. Micro droplets of water will condense and collect in this separator along with hydrophilic/oxygenated terpenes and other volatile substances due to their low condensation point.

A liquid co-solvent can be added to CO2 to increase its solvent power on polar molecules.

Indeed, supercritical CO2 is a good solvent for lipophilic non-polar compounds like oils, whereas, it has a low affinity with hydrophilic polar compounds like sugars.

Process engineers often add small quantities of liquid solvents (for example, ethyl alcohol) that are readily solubilized by supercritical CO2 to extract poliphenols or other compound with intermediate polarity.

When in solution, they modify the solvent power of supercritical CO2. This strategy has the drawback that, a larger solvent power could also mean lower process selectivity and since the co-solvent is liquid at atmospheric pressure, it will be collected in the separators together with the extracted compounds.

All oils are very non-polar compounds. CO2 is an non-polar solvent, so there is a good affinity between them. 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,….

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,… : therefore 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.

By adding a polar or medium polar co-solvent (ethanol or light alcohols, esters or ketones) in the right percentage and for a correct and pre-determinated time during extraction, it is possible to fine tune total polarity of the supercritical solvent.

Entraining agents have different properties than supercritical CO2. The critical point (CP) of CO2 is 73.8 bar and 31.5° C; ethanol’s CP is 63 bar and 241° C. A mathematical description requires computational chemistry to show how the entraining agents interact with the CO2. Suffice it to say, CO2 will be in the supercritical phase and the entraining agents will be in the liquid phase. This changes the solvent characteristics of the CO2 and improves extraction yields.

The cleaning procedure is an important practice: it has the same importance of the extraction process.

Any extraction system needs to be cleaned periodically: this period depends on many factors, and All these topics have an impact on the cleaning period:

• Type of the raw material
• Quality of the raw material
• Raw material pre-treatment
• Number of extraction before cleaning
• Use of co-solvent in the extraction process

All along the process the bacteria contamination is very low: during the extraction we have a hostile environment inside the system for bacteria: no oxygen, high pressure and saturation of CO2.  If you use a small amount of ethanol or oil as a co-solvent the viscosity of the extract is decreased and the deposit of solid parts in the system reduced dramatically. The condition will change immediately when you stop the system and open the lid of the extractor. Ambient air goes inside the extractor/circuit along with any bacteria in the air. The cleanliness of the ambient air is therefore an important factor.

Therefore, it is important to reduce the time the extractor is open and reduce the time the CO2 flow is stopped. In this way there is much less possibility of precipitation of solid parts in the pipes which generate encrustations that are difficult to remove.

Terpenes, main constituents of essential oils, commonly called “essences”, are widely spread both in the animal and plant kingdoms in the form of apparently different substances but all united by the same basic structure, all recognized as multiples of the compound 2 -methyl-1,3-butadiene or isoprene.

Supercritical CO2 has an excellent yield and produces high quality terpene mixes as the temperatures involved are very low and therefore do not undergo the typical deterioration associated with high temperatures. The extraction of terpenes into supercritical CO2 is very fast and takes place at low pressure (about 80 bar) and low temperature (about 45 ° C). Terpenes dissolve under relatively low conditions, just above the supercritical values for CO2. The extraction of CO2 with supercritical CO2 produces a clear extract, rich in essential oils in which the terpenes are dissolved.

The high selectivity of the supercritical CO2 allows to obtain a high purity.

The soaking extraction or Static-Dynamic Process (SDP) allows the supercritical CO2 to remain in contact with the raw material for the time necessary to dissolve the target compound. The raw material is brought to the set point pressure and, when reached, the system stops the pump and isolates the extractor. The system then passes to another extractor leaving the matter in the first extractor to absorb the CO2. Soaking extraction resolves problems of slow extraction kinetics and high mass transfer resistance with great effectiveness, increasing efficiency by soaking material before extraction CO2 restarts again.

The soaking cycle works in this way: when the extraction phase for extractor 1 is completed, the CO2 is transferred from extractor 1 to extractor 3. The pressure in extractor 1 is decreasing very fast and the pressure in extractor 3 is increasing very fast. Immediately after the cross point (blue-red), the pump starts to run therefore the pressure in extractor 3 increases and the extractor 1 is under venting phase. When the pressure in the extractor 3 is reached the pump stops and the soaking process takes places. When the pump starts to run again the extraction process the pressure in extractor 2 increases and the extractor process begins.

Soaking / static-dynamic extraction in the scientific literature.

The transport mechanism that occurs in supercritical fluid extraction is considered a leaching process. In leaching, the solvent must first travel to the surface of the material and diffuse through the pores. The solute then dissolves in the solvent, and is transported to the surface of the particle. Finally, the solute is transferred into the bulk fluid. This process will proceed until an equilibrium concentration of the solute is reached in the bulk fluid.

The use of static-dynamic cycling in supercritical fluid extraction is similar to an equilibrium-staged separation. In essence, each static-dynamic cycle simulates a stage of the separation. During the static soak time, the system is allowed ample time to reach equilibrium, and then released during the dynamic phase. By performing this extraction process in cycles, equilibrium stages are reached allowing for an efficient extraction that uses half the amount of CO2 that would be used in a continuous system.

Ref: Megan Matricardi, Robert Hesketh, Stephanie Farrel, Rowan University, NJ

While in the Semi Continuous Process (SCP) the system is equipped with 2 extraction vessels, in the Continuous Overlapping Process (COP) the system is equipped with 3 extraction vessels.

Taking advantage on the typical botanical extraction kinetic, the intelligent automation of the system  will apply a different extraction strategy. 

The COP automation program puts two extractors in series, doubling the volume of the extractor.

The advantage is obvious: in the same process time we will have results almost comparable to a double-sized system. All these procedures are automatically managed by the on-board automation without any intervention by the operator, who will only have to take care of the unloading/reloading of the basket and the withdrawal of the extract.

The Semi Continuous Process (SCP) reduces the down time and bacteria pollution in the system. The CO2 flows continuously in the circuit, preventing the accumulation of dirt and bacteria. In the semi-continuous extraction process, the extraction begins with the extractor 1 and proceeds with the extractor 2. Then from the extractor 2 it proceeds to the extractor 1 and so on. It newer stop until the production manager decides to clean the system.

Two extraction curves are visible.

Once the extraction step for extractor 1 is completed, CO2 is transferred from extractor 1 to extractor 2. The pressure in extractor 1 is decreasing very fast and the pressure in extractor 2 is increasing very fast.
Immediately after the crossing point, the CO2 returns to the reservoir. You can see the level of liquid CO2 rising in the reservoir. Then the pump starts, the pressure in extractor 2 increases and extractor 1 is venting at the same time. The double extraction system gives you a big advantage: semi-continuous extraction. This type of extraction ensures that the process never stops.

The semi continuous process gives many Advantages:

• The CO2 is always circulating: bacteria and dirt have no time to  deposit in the circuit
• The fast closing system reduce to 5 minutes unloading/reloading the canisters
• The production per day is increased by 25% if compared to a single extractor system
• With 25% higher output per day, the return on the investment is greatly advantaged.
• Dead times are dramatically reduced

Supercritical carbon dioxide isn’t only a efficient solvent for apolar compounds, in fact, if it’s combined with fluid modifiers like water, it becomes a very efficient solvent also for medium polar and polar substances (caffeine for coffee decaffeination).The extraction efficiency than conventional technologies based on chemical solvents is achieved by the following features offered by supercritical CO2:

Increase Of Mass’ Transport for the Zero Superficial Tension Effect, with correlated advantages in the extractive efficiency and in the duration of the extractions (minutes vs. hours);

With a minimum modulation of temperature and pressure in the process, the Solvent Properties are modified in an important way;

Totally Miscible like gasses;

• Low Viscosity, Eco-Friendly and green solvent, Not Inflammable, Inert and Not Toxic;
• High Diffusivity and so it can increase the extractive kinetics;
• The Co-Solvents (water, ethanol) can further modify the solvent proprieties;
• The relatively low temperatures of extraction help the Conservation of volatile compounds;

This process is not influenced by the Oxidation: the extraction vessel, full of supercritical CO2, is an inert space for the oxidation process also at high temperature (50-70 °C);

Antibacterial and so the final extracts are high quality products, for a microbiology point of view;

Cheap: it’s common in the atmosphere (0,04% and this value is still raising in this years) and it’s concentrated in a pure solution (99,9% of CO2, gas state,20-25 bar), simply available in safety cylinders bottles.

During the process the CO2 is continually Recycled and this reduce the primary costs of extraction. At the same time, the small quantity of solvent lost during the process, and so released in the atmosphere, doesn’t increase global CO2 emissions, because the same CO2 was concentrated from the atmosphere in cylinders;

If it was used a polar Co-Solvent (like water and ethanol) for the extraction of polar or medium polar chemical compounds, the extracted substances are easily isolated with the evaporation of the co-solvent.

The supercritical CO2 extraction is, beyond all doubt, the best extraction technique available today. It is the best not only for the quality of its extracts, but also for the absence of contamination. For this reason, the exhausted matrix (flour, etc. ..) can be used without any fear, which is impossible for solvent extraction. It is the best for its rapid process (hours instead of days) and the presence of a reducing environment in which the oxidation can not occur. Is the best because it is the only true green technology: it does not pollute, there are no solvents or even exhausted to dispose of. It is the best because it is the only one that can transform waste products (which represent only a cost) in high value-added active ingredients that would otherwise be acquired by large pharmaceutical companies at a high price. For a long time, the poorest technologies (processes with chemical solvents) were more spread. But today we all realize the damage caused by chemical solvents to human health and very soon (in Europe since 2012) they will be banned worldwide. It is clear that today, from an economic standpoint, investment in traditional technologies that use chemical solvents has no future. The reasons for economic, technical and policy underlying the choice of technology proposal are the following:

 Supercritical CO2 extraction benfits and versatility.

1) The characteristics of CO2 extraction technology are versatility combined with eco-friendly, acting in transformation processes on two different levels:

• in the initial processing phase of vegetable raw materials and in the entire food chain. SCF extraction works directly on raw materials, eg in the processing of the must SCF occurs just after the crushing of grapes. In addition, waste materials are used as primary products, (those parts that are currently disposed of) because with this technology can be found re-employment as a source of antioxidants. Following the same chain processes, SCF may also be applied in subsequent stages. In fact, SCF interventions can be included in the processing of waste (seeds and grapes), pasteurization of grape juice, into wine and brandy dealcoholization obtaining new products. In the olive sector intervention may be even more “vertical” and significant because in the extraction stage you can get a very high quality primary pharmaceutical product: olive oil with a polyphenol content of 10/20 times the average, water vegetation concentrated in polyphenols, waxes for cosmetics. In the next step is possible, in addition to ultra filtration to concentrate the waste water in polyphenols, extracted from leaves collected from the annual pruning of olive other polyphenols (eg oleuropein antioxidant) and work the exhausted matrix to produce health food (the latter processing is facilitated by the absence of water vegetation on the previous stage supercritical extraction). Another advantage given by the supercritical phase process in the oil production chain is the complete absence of vegetation waste water that, in traditional processes, pollute heavily the soil (in Italy the wastewater requires special disposal and is a considerable monetary cost).
• Only in final processes for specialized applications (eg pesticide: pyrethrum and neem). This is the case of dealcoholization of alcoholic beverages, pasteurization of fruit juices and drinks in general, powder coating of the drug, oil and liquids fractionation, concentration of active ingredients or the recovery of waste materials from previous processes.

2) Flexibility offered by SCF technology that can act on a wide range of process conditions and performs various processing stages (also in sequence). This quality can be used both in the extraction and fractionation process, getting new productions and products. Examples include: de-oiled meal, numerous extracts in co-extraction, extractions made in sequence (in a first phase apolar soluble compounds are extracted and sequentially in a second phase polar soluble compounds are extracted).

3) Cost: in the beginning this technological approach could not offer many economies as the technology did not confer special benefits to the process. With the technological growth you can see the benefits of multi-purpose approach and the significantly improved production capacity. Furthermore, with the restriction in the use of organic solvents results from the EC, this technology shown its ability to bind to the quality of production even more economic efficiency. Technical factors that determine the SCF system cost are:

• extractions in sequence and in two phases with advantages in terms of time (significant is the saving of labour time and equipment given by the process of extraction of polar and non-polar)
• full use of raw materials because the supercritical plants produce more product at the same time
• reduced process time and high productivity achieved in extraction, separation and pasteurization, associated with the ability to completely remove the substance in question (oil or other compound)
• low cost of CO2
• savings in disposal costs necessary with traditional technologies
• low numbers of workers

4) High production quality recognized by the whole market.

Consequences of the characteristics of SCF technology and its market position:

• recognized by the market as production of high quality and in some cases without competition (es. coffee decaffeinizzation, lycopene, olive oil with high content in polyphenols, extracted pesticides)
• inclusion in food platforms in both structural processes, conditioning entire supply chains, and processes to reach niche or unique products available only thanks to this technology (eg extraction of antibacterials for high efficiency demonstrated, removal of pesticides, dealcoholization with retention of original flavours, pasteurization of liquid at low temperature, extraction of compounds that are particularly active in anti-cancer therapies such as lycopene and CBD cannabis)
• insertion, at the present time, in higher-end market for the following reasons: a) barriers (now removed in EC) against and prevention of environmentally friendly technologies and that are in opposition to the use of organic solvents (hexane) b) non-recognition of costs which are the hidden environmental costs for disposal of toxic substances and lack of attention or lack of prevention in productive activities in respect of use of toxic substances (eg, hexane, etc…)
• economy (already defined above) through:
• multipurpose system and technological developments that on one hand have significantly improved the efficiency and productivity (continuous separation systems that leverage new enthalpy of the process and structured packing, can be removed in a series of water-soluble and fat-soluble compounds, techniques of co extraction possible due to the wide range of process conditions offered by the technology, continuous pasteurization at low temperatures) on the other hand have reduced the environmental costs (low cost of the “solvent” supercritical CO 2),
• low workers number related with the investment,
• encouraging the use of clean technologies and the significant limitations to the use of organic solvents (hexane).