Supercritical Fluid Extraction (S.F.E.)

1. DefinitionSFE : Supercritical Fluid Extraction of solids, generally operated in batch mode on

milled materials (or pellets, granulates,…), with carbon dioxide as solvent, possibly added with a co-solvent (ethanol most often) to increase its polarity. The equipment is composed of one or several extractors consisting in high pressure autoclave(s) with fast-opening systems in which the material to be treated is set inside a “ basket ” easy to put in and to remove, and a separation section where extract and fluid are separated prior to fluid recycle through condensation and liquid pumping. As it is operated in batch and requires manpower to change the solid material at each cycle of extraction, this process generally leads to significant costs, often higher than those obtained in classical organic solvent extractions. However, it is to be noticed that no organic residue is found both in extract and solid residue and no thermal degradation appears, what results in very high quality products.A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and 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.

Extraction can be defined as the removal of soluble material from an insolubleresidue, either liquid or solid, by treatment with a liquid solvent. It is therefore, a solutionprocess and depends on the mass transfer phenomena. The controlling factor in the rate ofextraction is normally the rate of diffusion of the solute through the liquid boundary layer atthe interface.Extraction involves the separation of medicinally active portions of plant or animal tissue from the inactive or inert components using selective solvents in standard extraction procedures. The products so obtained from plants are relatively impure liquids, semisolids or powders intended only for oral or external use. So extraction continues to be of considerable interest in order to obtain improved yields of drug derived from plant and animal sources.

Solvent extractionSolvent extraction also known as liquid-liquid extraction and partitioning, is amethod to separate compounds based on their relative solubilities in two differentimmiscible liquids, usually water and an organic solvent. It is an extraction of a substancefrom one liquid phase into another liquid phase. It is a basic technique in chemicallaboratories, where it is performed using a separatory funnel. In other words, this is theseparation of a substance from a mixture by preferentially dissolving that substance in asuitable solvent. Solvent extraction may be made use analytically for concentrating orrejecting a particular substance, or for the separation of mixtures. This process usuallyseparates a soluble compound from an insoluble compound. Solvent extraction is used innuclear processing, ore processing, production of fine organic compounds, processing ofperfumes and other industries. Thus, although the method can be used for extraction andpreconcentration of a wide range of non volatile or semi volatile species from water usingonly routine laboratory equipments, its use is decreasing in most situations because solventsof the required purity tend to be expensive, and can also cause problems with proper

disposal after use.

2. Process description and variants

Except for decaffeination of green coffee beans operated at very large scale according to asemi-continuous process, all SFE processes are operated on batch mode as the raw material,generally in form of a powder obtained by milling or pellets by granulation, has to be loadedin the extractors and the spent material unloaded at atmospheric pressure. This is a very stronglimitation as it induces both investment costs (autoclave closing/opening systems) andoperating expenses (manpower, fluid losses, …). But until now, no satisfactory solution hasbeen proposed to fill and remove the material from a high-pressure autoclave. In mostequipment, the raw material is not loaded directly in the extractor, but in a “basket” consistingin a cylinder closed by filters through which the fluid will flow in and out. Few very largeextractors, like for hop processing, are directly loaded with the material, the spent materialbeing removed by aspiration with a vacuum cleaner.On the other hand, it is important to note that, at the difference with lab or pilot-scaleequipments, industrial-scale SFE units always comprise several extractors for several reasons:

Increase of productivity as the plant can work during the time needed forunloading/loading one extractor ;

Decrease of the fluid losses as it is possible to depressurize one extractor (at the end ofthe extraction) into another one (just loaded with raw material) ;

Increase productivity by reducing the total extraction time when operating a simulatedcounter-current between the fluid and the material : In many cases where extraction islimited by extract solubility at first and later by extract diffusion (see below at § 4.6),it is preferable to use the fluid until “saturation”, meaning that the fluid leaving the lastextractor in the sequence shall be saturated in extract ; it is easy to understand that thequantity of fluid required to perform a given production of extract is minimal when thefluid exits the extraction zone saturated in extract.

This requires several extractors in series that, for ease of comprehension, we will number at 4and called A, B, C, and D.Suppose D is at the end of the extraction cycle and in the phase of unloading the spentmaterial and loading the raw material ; during this period of time, extraction is performed onextractors A, B, C, C being in last third period of extraction, B in the second one and A in thefirst one ; fluid saturation can be easily obtained if the “fresh” fluid is firstly sent to autoclaveC (lean), and then to extractor B (medium) and finally to autoclave A (rich). After anextraction time t equal to one third of the total extraction time, the material in extractor C is tobe unloaded meanwhile extractor D is ready for extraction. Then, the fresh fluid is sent toextractor B (lean), then to extractor A (medium) and finally to extractor D (rich). And againafter the same period of time t, extractor B is unloaded meanwhile the fluid is sent to extractorA (lean), then to extractor D (medium), and finally to extractor C (rich). And so on….Optimization of the extraction cycle shall also deal with fluid management to avoid costlyventing to atmosphere. In most industrial SFE units, the fluid (generally carbon dioxide) ispressurized in liquid phase by a pump and recycled by gas condensation (see flow sheetbelow). As the performance of cooling machines is very dependent on the temperature

required for the cooling fluid, the condensation temperature is chosen around 8 to 10°Ccorresponding to a vapor pressure about 45 bar for CO2. This means that, when depressurizingthe extractor for unloading/loading operation, the fluid can be recycled only when its pressureremains over 45 bar ; then, it is generally vented to atmosphere as, for economic reasons, onlyvery large-scale plants can be designed with a fluid recompression system permitting torecycle the fluid at a much lower pressure. According to our experience, the best fluidmanagement consists in the following sequence of operations when extractor C is to beunloaded and extractor D is ready for extraction : The fluid pump is stopped, extractor D isslowly pressurized up with recycle gas at 45 bar and then equilibrated with extractor C. Afterequilibration, extractor C is depressurized to 45 bar, the fluid pump is re-started andextraction performed. Then, extractor C is depressurized to atmosphere by venting the fluid,prior to unloading/loading operation. This leads to minimal losses as only compressed gas (45bar, ~25°C) is vented.Optimization of the extraction cycle requires to manage the time duration of each operation.It is easy to understand that the loading/unloading operation should be shorter than the time ofextraction between commuting the circuit (one third of the total extraction time in theprecedent example). In fact, depressurizing and pressurizing extractors cannot be done“brutally” as this may cause damages to the equipment and/or the processed material :deformation of the baskets containing the material, agglomeration of the material andconsequently bad contact with the fluid (channeling), dry ice formation duringdepressurization with total plugging of lines and or basket. SEPAREX and HITEX developeda proprietary process for reducing the final depressurization time without formation of dry ice: Carbon dioxide is vented to atmosphere until its pressure reaches 10 bar or so ; at thatmoment, compressed nitrogen (or air) is pumped and the gaseous mixture is vented with anincreasing concentration in nitrogen (or air) without temperature decrease due to the Joule-Thomson depressurization of CO2.

It is very important that all these operations are automated through a reliable control system,both for safety and economic reasons.

AdvantagesEnvironmental improvement and reduced product contaminationSFE is an alternative to liquid extraction using solvents such as hexane ordichloromethane. There will always be some residual solvent left in the extract and matrix,and there is always some level of environmental contamination from their use. In contrast,carbon dioxide is easy to remove simply by reducing the pressure, leaving almost no trace,and it is also environmentally benign. The use of SFE with CO2 is approved by the SoilAssociation for organic products. The CO2 used is largely a by-product of industrialprocesses or brewing, and its use in SFE does not cause any extra emissions.

SelectivityThe solvent strength of a supercritical fluid can be varied by change in the pressureand a lesser extent in the temperature.For example, volatile oils can be extracted from a plant with low pressures (100 bar),whereas liquid extraction would also remove lipids. Lipids can be removed using pure CO2

at higher pressures, and then phospholipids can be removed by adding ethanol to the solvent. 742 G. N. Sapkale et al.: Supercritical Fluid….

SpeedIt is a fast process and completed in 10 to 60 minutes. It is a diffusion-based process,with the solvent required to diffuse into the matrix, and the extracted material to diffuse outof the matrix into the solvent. Diffusivities are much faster in supercritical fluids than inliquids, and therefore extraction can occur faster. Also, there is no surface tension andviscosities are much lower than in liquids, so the solvent can penetrate into small poreswithin the matrix inaccessible to liquids.

PurityA supercritical fluid can be separated from analyte by simply releasing pressure.Supercritical fluids are cheap, inert and nontoxic. Thus, they are readily disposed offafter an extraction is completed by allowing them to evaporate into the atmosphere.

RecoveryRecovery of analytes becomes simple.