Rapid Expansion of Supercritical Solution (RESS)

Rapid Expansion of Supercritical Solution (RESS).

RESS (Rapid Expansion of Supercritical Solution) is used generally to produce fine particles for the food, cosmetic and pharmaceutical industry.

Organic material is dissolved in supercritical carbon dioxide and is used to rapidly expand the supercritical solution through an expansion nozzle. Due to an abrupt decrease of pressure to atmospheric, very high Supersaturation values is achieved which led to small particle sizes. Furthermore, as the pressure change traveled at the speed of sound, a rapid propagating mechanical perturbation is obtained and led to an uniform condition within the nucleating medium, and hence in principle to narrow particle size distributions. Previous RESS reports indicate that the RESS process is an extremely attractive technology to form small particles with a narrow particle size distribution.

RESS process is consisting of two steps:

Dissolving the solid substance in a SCF,
Formation of particles due to supersaturation.

In the RESS process, at first SCF-CO2 is pumped at desired pressure and temperature to extraction chamber containing solid substance(s) through heat exchanger. The supercritical solution expands rapidly in the precipitator causing a collapse of pressure and temperature, which favors the formation of drops and particles.

The SCF percolates and dissolves the solid substance(s) in the extractor and then the resulted solution is depressurized through a heated nozzle or capillary at supersonic speed into a low pressure chamber. The supercritical solution is expanded adiabatically in the chamber, which leads to a rapid drop in temperature and pressure and spontaneous formation of droplets/particles. During the rapid expansion of the supercritical solution, the density and solvent power decrease significantly, resulting in super saturation of the solution and consequently precipitation of desire particles free of a residual solvent.The RESS process advantages are its simplicity, effectiveness and the absence of organic solvents.

There is also a variation of the traditional RESS that uses a solid cosolvent (RESS-SC). This concept is widely used in the field of extraction with SCF and is called co-extraction. Generally, the RESS process is batch type, but there is also a continuous RESS process, which allows to obtain micro-particles in a constant and continuous way.

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Supercritical Controlled Espansion Atomization (SCEA)

Supercritical Controlled Espansion Atomization (SCEA).

SCEA (Supercritical Controlled Espansion Atomization) is a technique for the production of microparticles of different materials of relatively low melting temperatures, controlling expansion parameters for a better particle dispersion. The process is based on the capacity of those materials to dissolve large amounts of CO2 at moderate pressures. Upon depressurization down to ambient conditions, the dissolved CO2 is rapidly released and expanded, producing an intense cooling effect that promotes the formation of microparticles.

The SCEA process is quite similar to the PGSS process with one important difference: in SCEA process, the material is solubilized in supercritical CO2 and than depressurized through a nozzle with the formation of droplets or solid particles.

Unlike to RESS technique, the principle governing SCEA process involves both the pressure and temperature and solvent-induced phase separation.

Advantages of PGSS process are:

simplicity of this process, leading to low processing cost and wide range of application
can be applied to process inorganic powders to pharmaceutical compounds
low solvent gas usage and pressure than RESS process as operational condition.

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Near-critical Expansion Atomization (NEA)

Near-critical Expansion Atomization (NEA).

In NEA (Near-critical Expansion Atomization) process, the carbon dioxide is used in this process for the atomization and the crystallization of the product. The product subjected to the process is maintained in the liquid phase in a feed tank at a controlled temperature and subsequently conveyed, at the desired pressure, to the “atomization tower” where there is contact with the carbon dioxide released to atmospheric pressure.

The result is the formation of microscopic droplets (atomization) immediately cooled to temperatures much lower than 0 ° C.

The lipid is crystallized in a stable form, and particulate matter obtained has a narrow size distribution in the range 8-100 microns. The process parameters (carbon dioxide pressure and temperature and product) are regulated in function of the desired particle size. The possibility of working small quantities of product even with very low melting point is an indisputable advantage compared to conventional processes. The direct contact between the product and the carbon dioxide also ensures the optimal exploitation of the properties of cryogenic gas.

The product obtained can be immediately inserted into the stages of production, mixed or packaged in a modified atmosphere using the carbon dioxide derived from the process. With NEA process special applications are possible, for example in the pharmaceutical field, where fine particulates are required (5 – 30 micron) for the insertion of active ingredients in a matrix.

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Supercritical Fluid Chromatography (SFC)

Supercritical Fluid Chromatography (SFC).

La Chromatographie en Fluide Supercritique (SFC) est une technique de fractionnement largement utilisée dans l'industrie pharmaceutique, où les productions sont de grande valeur, car c'est une technique très efficace, et elle permet de séparer des composés qui ne seraient pas isolés avec des techniques telles que la distillation ou l’extraction par solvant. Il existe différents types de chromatographie, mais elles utilisent essentiellement la capacité d'un solide poreux (phase stationnaire) à lier sélectivement les composés à isoler. , cette dernière portée par un Fluide (phase mobile). Un tel phénomène est appelé Adsorption. La chromatographie d'élution supercritique est particulièrement utilisée pour le fractionnement de mélanges d'esters ayant un poids moléculaire similaire.

La différence substantielle entre la chromatographie traditionnelle et la chromatographie supercritique est que, alors que dans le premier type l'éluant est un gaz ou un solvant liquide, dans le second type la phase mobile est un fluide supercritique. Cette figure de droite montre avec un niveau de détail croissant, un système chromatographique. Le mélange à séparer est injecté dans la colonne et est absorbé dès le premier trait de la phase stationnaire.

Ensuite, l'éluant s'écoule et, dans la chromatographie en fluide supercritique (SFC), cela contribue à la séparation des substances. Dans le cas du système SC CO2-huile, par exemple, à la température et à la pression de fonctionnement, les solvants sont dissous dans la masse d'huile, la gonflant et créant un Système biphasique, dans lequel un La phase riche en CO2 est à l'état supercritique, tandis que l'autre, riche en huile, est liquide. Le flux d'éluant perturbe l'état d'équilibre et génère des gradients de concentration entre la phase liquide, la phase supercritique et la phase solide, il y a donc des échanges matériels qui tendent à amener le système à l'équilibre.

En particulier, il existe un flux entre les phases supercritiques et liquides qui, en contact strict les unes avec les autres, se déplacent à contre-courant le long des canaux.

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Supercritical Fluid Fractionation (SFF)

Fractionnement en fluide supercritique (FFS).

For Fractionation 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 Fractionation Tower (or 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 tower 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.

Fractionation principles of ethylesters in tower.

Let’s suppose, for simplicity reasons, that the ethylesters mixtures are composed by two pseudocomponents, meaning groups of molecules with similar behavior:
Light ethylesters, being 14-18 C fatty acids;
Heavy ethylesters, being 20-22 C fatty acids.

Since the first group of compounds is more soluble in SC-CO2, it derives that an increase in the concentration of a heavy ethylesters mixture, it means to remove in the most efficient way the light ethylesters. Such removal can be conveniently performed utilizing Filling Towers, in which a solvent phase and a liquid phase (ethylesters mixture) come in touch countercurrent. The filling tower is the equipment usually utilized for liquid mixtures fractionation 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 countercurrent 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 (non structured filling ).

When the filling tower is at regime, the liquid mixture to fractionate, fed at the top of the tower, 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 by the solvent and the solubilized products) is collected from the top. In other words, in the case of ethylesters mixtures the lighter fraction of the mixture will be collected as extract at the top of the tower (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, the lower will be the productive capacity of the plant (expressed as Kg fractionate/column section), and the higher will be the height of the tower needed for the requested separation.

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Static-Dynamic Process (SDP)

Processus statique-dynamique (PSD).

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

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Continuous Overlapping Process (COP)

Processus continu de overlapping (PCO).

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.

As we see, the most of the extract comes out in the first half of the extraction. The new process program will apply a new approach: after the first half of the extraction, the system puts in series the extractors nr. 1 and nr.2

As the most of the compounds has been extracted from extractor nr. 1, after the first half of the extraction the CO2 is no more saturated by the compounds like before.

This means the CO2 has room to solubilize more compounds than what available in extractor nr. 1.

Connecting the extractor nr. 2 in series to extractor nr. 1 after the first half of the process will give to the production manager the advantage to recover the last 15% to 30% left in extractor nr. 1 while extracting the most of the compounds from extractor nr. 2, and so on with extractor nr. 2 and extractor nr. 3 and then again with extractor nr. 3 and extractor nr. 1 in an endless circle.

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Semi Continuous Process (SCP)

Processus semi-continu (PSC).

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

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Extraction efficiency of CO2

Efficacité de l'extraction du CO2.

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.

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Supercritical CO2 extraction benefits

Avantages de l'extraction par CO2 supercritique.

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).

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