Extraction
Supercritical carbon dioxide
Carbon dioxide pressure-temperature phase diagram
Supercritical carbon dioxide (sCO2) is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure.
Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP), or as a solid called dry ice when frozen. If the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid. More specifically, it behaves as a supercritical fluid above its critical temperature (304.25 K, 31.10 °C, 87.98 °F) and critical pressure (72.9 atm, 7.39 MPa, 1,071 psi), expanding to fill its container like a gas but with a density like that of a liquid.
Supercritical CO2 is becoming an important commercial and industrial solvent due to its role in chemical extraction in addition to its low toxicity and environmental impact. The relatively low temperature of the process and the stability of CO2 also allows most compounds to be extracted with little damage or denaturing. In addition, the solubility of many extracted compounds in CO2 varies with pressure, permitting selective extractions.
CO2 Hypercritical Extraction
Hypercritical carbon dioxide (CO2) extraction is a relatively new process used for the extraction of aromatic products. The basic concept is that CO2 under pressure will turn from a gas into a liquid that can then be used as an inert liquid solvent. This liquid solvent is able to diffuse throughout the plant material thus extracting its aromatic constituents. CO2 extracts contain most of the same constituents as their essential oil counterparts, although they can contain some elements not found in essential oils. For instance, the essential oil of ginger (Zingiber officinale) does not contain the bitter principles, however the CO2 extract does. Also, the CO2 extract of frankincense (Boswellia carterii) has immune enhancing and anti-inflammatory activity not found in the essential oil. CO2 extracts are known for their strong similarity in aroma to the actual plant aroma. Other common CO2 extracts on the market include German chamomile (Matricaria recutita) and Calendula (Calendula officinalis).
The three main disadvantages for this process are cost, potential pesticide residue, and the lack of information regarding their safety and therapeutic benefits.7 With regard to pesticide residue, Guba comments that “carbon dioxide extraction has been demonstrated to concentrate from 7 to 53 times more pesticide residues in the final extract.” Therefore, it seems pertinent to only use organic plant material for CO2 extraction. Perhaps as more CO2 extracts become available and more practitioners use them, further details regarding their applications will become apparent. Two of the most common essential oils available via CO2 extraction include frankincense and ginger.
Applications
Supercritical fluid extraction
Carbon dioxide is gaining popularity among coffee manufacturers looking to move away from classic decaffeinating solvents, because of real or perceived dangers related to their use in food preparation. sCO2 is forced through the green coffee beans which are then sprayed with water at high pressure to remove the caffeine. The caffeine can then be isolated for resale (e.g. to the pharmaceutical or beverage manufacturers) by passing the water through activated charcoal filters or by distillation, crystallization or reverse osmosis. Supercritical carbon dioxide is used to remove organochloride pesticides and metals from agricultural crops without adulterating the desired constituents from the plant matter in the herbal supplement industry.
Supercritical carbon dioxide can be used as a more environmentally friendly solvent for dry cleaning over traditional solvents such as hydrocarbons, including perchloroethylene.
Supercritical carbon dioxide is used as the extraction solvent for creation of essential oils and other herbal distillates. Its main advantages over solvents such as hexane and acetone in this process are that it is non-toxic and non-flammable. Furthermore, separation of the reaction components from the starting material is much simpler than with traditional organic solvents. The CO2 can evaporate into the air or be recycled by condensation into a cold recovery vessel. Its advantage over steam distillation is that it operates at a lower temperature, which can separate the plant waxes from the oils.
In laboratories, sCO2 is used as an extraction solvent, for example for determining total recoverable hydrocarbons from soils, sediments, fly-ash and other media, and determination of polycyclic aromatic hydrocarbons in soil and solid wastes. Supercritical fluid extraction has been used in determining hydrocarbon components in water.
Processes that use sCO2 to produce micro and nano scale particles, often for pharmaceutical uses, are under development. The gas antisolvent process, rapid expansion of supercritical solutions and supercritical antisolvent precipitation (as well as several related methods) process a variety of substances into particles.
Due to its ability to selectively dissolve organic compounds and assist the functioning of enzymes, sCO2 has been suggested as a potential solvent to support biological activity on Venus- or super-Earth-type planets.
Manufactured products
Environmentally beneficial, low-cost substitutes for rigid thermoplastic and fired ceramic are made using s CO2 as a chemical reagent. The s CO2 in these processes is reacted with the alkaline components of fully hardened hydraulic cement or gypsum plaster to form various carbonates. The primary byproduct is water.
Supercritical carbon dioxide is used in the foaming of polymers. Supercritical carbon dioxide can saturate the polymer with solvent. Upon depressurization and heating the carbon dioxide rapidly expands, causing voids within the polymer matrix, i.e., creating a foam. Research is also ongoing at many universities in the production of microcellular foams using sCO2.
An electrochemical carboxylation of a para-isobutylbenzyl chloride to ibuprofen is promoted under sCO2.
Working fluid
Supercritical CO2 is chemically stable, reliable, low-cost, non-toxic, non-flammable and readily available, making it a desirable candidate working fluid.
Power Generation
The unique properties of s CO2 present advantages for closed-loop power generation and can be applied to various power generation applications. Power generation systems that use traditional steam Brayton and Rankine cycles can be upgraded to sCO2 to increase efficiency and power output.
It presents interesting properties that promise substantial improvements in system efficiency. Due to its high fluid density, sCO2 enables extremely compact and highly efficient turbomachinery. It can use simpler, single casing body designs while steam turbines require multiple turbine stages and associated casings, as well as additional inlet and outlet piping. The high density allows for highly compact, microchannel-based heat exchanger technology.
In 2016, General Electric announced an sCO2-based turbine that operated at 50% efficiency. In it the CO2 is heated to 700 °C.It requires less compression and allows heat transfer. It reaches full power in 2 minutes, whereas steam turbines need at least 30 minutes. The prototype generated 10 MW and is approximately 10% the size of a comparable steam turbine.
Further, due to its superior thermal stability and non-flammability, direct heat exchange from high temperature sources is possible, permitting higher working fluid temperatures and therefore higher cycle efficiency. And unlike two-phase flow, sCO2’s single-phase nature eliminates the necessity of a heat input for phase change that is required for the water to steam conversion, thereby also eliminating associated thermal fatigue and corrosion.
Despite the promise of substantially higher efficiency and lower capital costs, the use of sCO2 presents material selection and design issues. Materials in power generation components must display high-temperature resistance, oxidation resistance and creep resistance. Candidate materials that meet these property and performance goals include incumbent alloys in power generation, such as nickel-based superalloys for turbomachinery components and austenitic stainless steels for piping. Components within sCO2 Brayton loops suffer from corrosion and erosion, specifically erosion in turbomachinery and recuperative heat exchanger components and intergranular corrosion and pitting in the piping.
Testing has been conducted on candidate Ni-based alloys, austenitic steels, ferritic steels and ceramics for corrosion resistance in sCO2 cycles. The interest in these materials derive from their formation of protective surface oxide layers in the presence of carbon dioxide, however in most cases further evaluation of the reaction mechanics and corrosion/erosion kinetics and mechanisms is required, as none of the materials meet the necessary goals.
Other
Work is underway to develop a sCO2 closed-cycle gas turbine to operate at temperatures near 550 °C. This would have implications for bulk thermal and nuclear generation of electricity, because the supercritical properties of carbon dioxide at above 500 °C and 20 MPa enable thermal efficiencies approaching 45 percent. This could increase the electrical power produced per unit of fuel required by 40 percent or more. Given the volume of carbon fuels used in producing electricity, the environmental impact of cycle efficiency increases would be significant.
Supercritical CO2 is an emerging natural refrigerant, used in new, low carbon solutions for domestic heat pumps. Supercritical CO2 heat pumps are commercially marketed in Asia. EcoCute systems from Japan, developed by Mayekawa, develop high temperature domestic water with small inputs of electric power by moving heat into the system from the surroundings.
Supercritical CO2 has been used since the 1980s to enhance recovery in mature oil fields.
"Clean coal" technologies are emerging that could combine such enhanced recovery methods with carbon sequestration. Using gasifiers instead of conventional furnaces, coal and water is reduced to hydrogen gas, carbon dioxide and ash. This hydrogen gas can be used to produce electrical power In combined cycle gas turbines, CO2 is captured, compressed to the supercritical state and injected into geological storage, possibly into existing oil fields to improve yields. The unique properties of sCO2 ensure that it remains out of the atmosphere.
Supercritical CO2 could be used as a working fluid in enhanced geothermal systems. Possible advantages compared to water include higher energy yield resulting from its lower viscosity, better chemical interaction, CO2 storage through fluid loss and higher temperature limit. As of 2011, the concept had not been tested in the field.
Aerogel production
Supercritical carbon dioxide is used in the production of silica, carbon and metal based aerogels. For example, silicon dioxide gel is formed and then exposed to sCO2. When the CO2 goes supercritical, all surface tension is removed, allowing the liquid to leave the aerogel and produce nanometer sized pores.
Sterilization of biomedical materials
Supercritical CO2 is an alternative for terminal sterilization of biological materials and medical devices with combination of the additive peracetic acid (PAA). Supercritical CO2 does not sterilize the media, because it does not kill the spores of microorganisms. Moreover, this process is gentle, as the morphology, ultrastructure and protein profiles of inactivated microbes are preserved.
Cleaning
Supercritical CO2 is used in certain industrial cleaning processes.
Exploring Aromatherapy
Aromatherapy is an incredibly vast and rich field.
How Are Essential Oils Extracted?
Essential oils can be extracted via two key methods: Distillation (includes hydrodistillation) and Expression.
Absolutes, other the other hand, can be extracted via Solvent extraction or Enfleurage, although enfleurage is rarely performed in the modern day.
Another type of aromatic product available on the market are CO2 extracts, referred to simply as that, CO2 extracts. They differ in chemistry from their related distilled essential oils but are becoming increasing available on the market.
The Practice of Distillation
Distillation appears to have been practiced throughout ancient times. Based upon the current interpretation Paolo Rovesti’s discovery of an earthenware distillation apparatus, the production or extraction of aromatic oils by means of steam distillation, has been known for 5000 years.1 During the fifth century AD, the famed writer, Zosimus of Panopolis, refers to the distilling of a divine water and panacea. Throughout the early Middle Ages and beyond, a crude form of distillation was known and was used primarily to prepare floral waters or distilled aromatic waters. These appear to have been used in perfumery, as digestive tonics, in cooking, and for trading.
Although an extensive trade of odoriferous material has been shown to have occurred in the ancient Orient and ancient Greece and Rome, the oils used were not essential oils per se, “rather they were obtained by placing flowers, roots, and other plant material into a fatty oil of best quality, submitting the glass bottles containing these mixtures to the warming influence of the sun and finally separating odoriferous oil from the solid constituents”.2
In 900 AD, Avicenna, the famous child prodigy from Persia who wrote many documents on plants and their uses and also instructions for massage, was accredited with refining the process of distillation by improving the cooling system.
Today distillation is still the most common process of extracting essential oils from plants. The advantage of distillation is that the volatile components can be distilled at temperatures lower than the boiling points of their individual constituents and are easily separated from the condensed water.
The Distillation Process
During distillation the plant material is placed upon a grid inside the still. Once inside, the still is sealed, and, depending upon the above methods, steam or water/steam slowly breaks through the plant material to remove its volatile constituents. These volatile constituents rise upward through a connecting pipe that leads them into a condenser. The condenser cools the rising vapor back into liquid form. The liquid is then collected in a vehicle below the condenser. Since water and essential oil do not mix, the essential oil will be found on the surface of the water where it is siphoned off. Occasionally an essential oil is heavier than water and is found on the bottom rather than the top, such as with clove essential oil.
The three types of distillation include:
Water Distillation
The plant material comes into direct contact with the water. This method is most often employed with flowers (rose and orange blossoms), as direct steam causes these flowers to clump together making it difficult for steam to pass through.
Water and Steam
This method can be employed with herb and leaf material. During this process, the water remains below the plant material, which has been placed on a grate while the steam is introduced from outside the main still (indirect steam).
Steam Distillation
This method is the most commonly used. During this process, steam is injected into the still, usually at slightly higher pressures and temperatures than the above two methods.
Note on Boiling Point: The boiling point represents the temperature at which a liquid is converted to a gas at a specified pressure. The fundamental nature of steam distillation is that it enables a compound or mixture of compounds to be distilled (and subsequently recovered) at a temperature substantially below that of the boiling point(s) of the individual constituent(s). Essential oils contain substances with boiling points up to 200°C or higher, including some that are solids at normal temperatures. In the presence of steam or boiling water, however, these substances are volatilized at a temperature close to 100°C at atmospheric pressure.3
Percolation or Hydro-diffusion
This is a relatively recent method and is very similar to steam distillation except that the steam comes in through the top rather than the bottom, and there is a shorter distillation time. It is useful in extracting essential oils from woody or tough material or seeds such as fennel and dill.
Hydrosols: A By-product of Distillation
Hydrosols, also known as hydrolats, are the by-product or product (depending on the distiller purpose) of the distillation process. Hydrosols contain the water-soluble constituents of the aromatic plant and retain a small amount of essential oil. Every liter of hydrosol contains between 0.05 and 0.2 milliliter of dissolved essential oil, depending on the water solubility of the plant’s components and the distillation parameters.4
*Please Note: The addition of essential oils to water is not at all the same as true hydrosols, and it is recommended that you read the ingredients label on products to ascertain whether or not you are getting a true hydrosol. When water and essential oils are mixed together with or without a dispersant, this is called a “spritzer” or “aromatic spritzer,” and this product should not be confused with a true hydrosol.
Expression
Expression, also referred to as cold pressing, is a method of extraction specific to citrus essential oils, such as tangerine, lemon, bergamot, sweet orange, and lime. In older times, expression was done in the form of sponge pressing, which was literally accomplished by hand. The zest or rind of the citrus would first be soaked in warm water to make the rind more receptive to the pressing process. A sponge would then be used to press the rind, thus breaking the essential oil cavities, and absorb the essential oil. Once the sponge was filled with the extraction, it would then be pressed over a collecting container, and there it would stand to allow for the separation of the essential oil and water/juice. The essential oil would finally be siphoned off.
A more modern method of extraction, and less labor-intensive, has been termed the ecuelle a piquer process that involves a prodding, pricking, sticking action to release the essential oil. During this process, the rind of the fruit is placed in a container having spikes that will puncture the peel while the device is rotated. The puncturing of the rind will release the essential oil that is then collected in a small area below the container. The end process is the same as above. The majority of modern expression techniques are accomplished by using machines using centrifugal force. The spinning in a centrifuge separates the majority of essential oil from the fruit juice.
Click to view a video of a modern extraction of citrus essential oil.
What is the difference between expressed and distilled citrus oils?
Expressed citrus oils are produced by mechanical separation (cold pressing) of the oil from the peels of various citrus fruits such as orange, grapefruit, tangerine, lemon, lime, petitgrain and bergamot. Expressed citrus oils contain small amounts of naturally occurring nonvolatile residues such as waxes. Expressed citrus oils offer the advantage of cold process which results in an aroma which is identical to fresh citrus peels.
Citrus oils can also be distilled from either the peels or whole fruits. While the aroma differs from expressed citrus oils, the distilled versions offer certain advantages:
1) Distilled citrus oils do not contain nonvolatile residues which can clog diffusers, stain fabric, and shorten the shelf life.
2) Distilled citrus oils don’t contain nonvolatile furocoumarins and are generally less photosensitizing.5,6
Extraction Techniques for Absolutes & CO2 ExtractsThe following methods of extraction, enfleurage, solvent extraction, and CO2 extraction, will only be briefly discussed. This textbook does not cover the therapeutic use of CO2 extracts or absolutes.
Enfleurage
Flowers were being processed via enfleurage in the Grasse region of Southern France long before the modern method of solvent extraction. In the early days of perfumery, many flower scents were extracted via enfleurage, now considered an ancient art that is passed down from father to son or from generation to generation.
Enfleurage is a cold-fat extraction process that is based upon the principles that fat possesses a high power of absorption, particularly animal fat. The fat used must be relatively stable against rancidity. It is a method used for flowers that continue developing and giving off their aroma even after harvesting (e.g., jasmine and tuberose).
Today, Grasse continues to be one of the few areas in the world that continues to employ enfleurage as a method of extraction, although it is rare in the aromatherapy market due to the expense. If one finds a jasmine enfleurage on the market, this would typically be considered an absolute.
Solvent Extraction
Some plant material is too fragile to be distilled and an alternative method must be employed. Solvent extraction is the use of solvents, such as petroleum ether, methanol, ethanol, or hexane, to extract the odoriferous lipophilic material from the plant. The solvent will also pull out the chlorophyll and other plant tissue, resulting in a highly colored or thick/viscous extract. The first product made via solvent extraction is known as a concrete. A concrete is the concentrated extract that contains the waxes and/or fats as well as the odoriferous material from the plant. The concrete is then mixed with alcohol, which serves to extract the aromatic principle of the material. The final product is known as an absolute.
Solvent extraction is used for jasmine, tuberose, carnation, gardenia, jonquil, violet leaf, narcissus, mimosa, and other delicate flowers. Neroli and rose can be distilled or solvent-extracted. The name neroli typically implies the essential oil, whereas the name orange blossom is commonly used for the absolute or hydrosol of neroli. The name rose is used to describe either the essential oil or the absolute. Companies selling essential oils should clarify whether the product you are purchasing is an essential oil or absolute. This information should be on the label and in the product catalog.
After the solvent extraction process has been completed, the resulting absolute will have an extremely low concentration of solvent residue, approximately 5 to 10ppm (parts per million). The current European Union standards are for less than 10 parts per million solvent residues in a finished absolute.7 However, even with such a potentially small residue (less than .0001%), many aromatherapists disagree with the use of absolutes for individuals with a compromised immune system due to the potential effect of the residual pesticide.
However, absolutes do have therapeutic value and are often used for psychological purposes and for animals, particularly horses. Many therapists incorporate absolutes, such as rose absolute, jasmine, and tuberose, as a valuable part of their therapeutic applications of aromatherapy. Ultimately the decision to use absolutes is up to the practitioner and his/her own personal preferences.
Absolutes are highly concentrated aromatic substances and are obtained from delicate flowers by either enfleurage or solvent extraction. Absolutes will most often resemble the natural aroma of the plant and are normally more colored and viscous than essential oils. Absolutes are used extensively in the cosmetic and perfume industries due to their strong aromas. There are also different grades of absolutes. The top grade is the uncut, which can be a thick or semisolid substance, making them difficult to work with. Less expensive grades are diluted with alcohol to make them more user friendly, although often the strength of aroma is slightly diminished.
CO2 Hypercritical Extraction
Hypercritical carbon dioxide (CO2) extraction is a relatively new process used for the extraction of aromatic products. The basic concept is that CO2 under pressure will turn from a gas into a liquid that can then be used as an inert liquid solvent. This liquid solvent is able to diffuse throughout the plant material thus extracting its aromatic constituents. CO2 extracts contain most of the same constituents as their essential oil counterparts, although they can contain some elements not found in essential oils. For instance, the essential oil of ginger (Zingiber officinale) does not contain the bitter principles, however the CO2 extract does. Also, the CO2 extract of frankincense (Boswellia carterii) has immune enhancing and anti-inflammatory activity not found in the essential oil. CO2 extracts are known for their strong similarity in aroma to the actual plant aroma. Other common CO2 extracts on the market include German chamomile (Matricaria recutita) and Calendula (Calendula officinalis).
The three main disadvantages for this process are cost, potential pesticide residue, and the lack of information regarding their safety and therapeutic benefits.7 With regard to pesticide residue, Guba comments that “carbon dioxide extraction has been demonstrated to concentrate from 7 to 53 times more pesticide residues in the final extract.” Therefore, it seems pertinent to only use organic plant material for CO2 extraction. Perhaps as more CO2 extracts become available and more practitioners use them, further details regarding their applications will become apparent. Two of the most common essential oils available via CO2 extraction include frankincense and ginger.
Supercritical carbon dioxide
Carbon dioxide pressure-temperature phase diagram
Supercritical carbon dioxide (sCO2) is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure.
Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP), or as a solid called dry ice when frozen. If the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid. More specifically, it behaves as a supercritical fluid above its critical temperature (304.25 K, 31.10 °C, 87.98 °F) and critical pressure (72.9 atm, 7.39 MPa, 1,071 psi), expanding to fill its container like a gas but with a density like that of a liquid.
Supercritical CO2 is becoming an important commercial and industrial solvent due to its role in chemical extraction in addition to its low toxicity and environmental impact. The relatively low temperature of the process and the stability of CO2 also allows most compounds to be extracted with little damage or denaturing. In addition, the solubility of many extracted compounds in CO2 varies with pressure, permitting selective extractions.
CO2 Hypercritical Extraction
Hypercritical carbon dioxide (CO2) extraction is a relatively new process used for the extraction of aromatic products. The basic concept is that CO2 under pressure will turn from a gas into a liquid that can then be used as an inert liquid solvent. This liquid solvent is able to diffuse throughout the plant material thus extracting its aromatic constituents. CO2 extracts contain most of the same constituents as their essential oil counterparts, although they can contain some elements not found in essential oils. For instance, the essential oil of ginger (Zingiber officinale) does not contain the bitter principles, however the CO2 extract does. Also, the CO2 extract of frankincense (Boswellia carterii) has immune enhancing and anti-inflammatory activity not found in the essential oil. CO2 extracts are known for their strong similarity in aroma to the actual plant aroma. Other common CO2 extracts on the market include German chamomile (Matricaria recutita) and Calendula (Calendula officinalis).
The three main disadvantages for this process are cost, potential pesticide residue, and the lack of information regarding their safety and therapeutic benefits.7 With regard to pesticide residue, Guba comments that “carbon dioxide extraction has been demonstrated to concentrate from 7 to 53 times more pesticide residues in the final extract.” Therefore, it seems pertinent to only use organic plant material for CO2 extraction. Perhaps as more CO2 extracts become available and more practitioners use them, further details regarding their applications will become apparent. Two of the most common essential oils available via CO2 extraction include frankincense and ginger.
Applications
Supercritical fluid extraction
Carbon dioxide is gaining popularity among coffee manufacturers looking to move away from classic decaffeinating solvents, because of real or perceived dangers related to their use in food preparation. sCO2 is forced through the green coffee beans which are then sprayed with water at high pressure to remove the caffeine. The caffeine can then be isolated for resale (e.g. to the pharmaceutical or beverage manufacturers) by passing the water through activated charcoal filters or by distillation, crystallization or reverse osmosis. Supercritical carbon dioxide is used to remove organochloride pesticides and metals from agricultural crops without adulterating the desired constituents from the plant matter in the herbal supplement industry.
Supercritical carbon dioxide can be used as a more environmentally friendly solvent for dry cleaning over traditional solvents such as hydrocarbons, including perchloroethylene.
Supercritical carbon dioxide is used as the extraction solvent for creation of essential oils and other herbal distillates. Its main advantages over solvents such as hexane and acetone in this process are that it is non-toxic and non-flammable. Furthermore, separation of the reaction components from the starting material is much simpler than with traditional organic solvents. The CO2 can evaporate into the air or be recycled by condensation into a cold recovery vessel. Its advantage over steam distillation is that it operates at a lower temperature, which can separate the plant waxes from the oils.
In laboratories, sCO2 is used as an extraction solvent, for example for determining total recoverable hydrocarbons from soils, sediments, fly-ash and other media, and determination of polycyclic aromatic hydrocarbons in soil and solid wastes. Supercritical fluid extraction has been used in determining hydrocarbon components in water.
Processes that use sCO2 to produce micro and nano scale particles, often for pharmaceutical uses, are under development. The gas antisolvent process, rapid expansion of supercritical solutions and supercritical antisolvent precipitation (as well as several related methods) process a variety of substances into particles.
Due to its ability to selectively dissolve organic compounds and assist the functioning of enzymes, sCO2 has been suggested as a potential solvent to support biological activity on Venus- or super-Earth-type planets.
Manufactured products
Environmentally beneficial, low-cost substitutes for rigid thermoplastic and fired ceramic are made using s CO2 as a chemical reagent. The s CO2 in these processes is reacted with the alkaline components of fully hardened hydraulic cement or gypsum plaster to form various carbonates. The primary byproduct is water.
Supercritical carbon dioxide is used in the foaming of polymers. Supercritical carbon dioxide can saturate the polymer with solvent. Upon depressurization and heating the carbon dioxide rapidly expands, causing voids within the polymer matrix, i.e., creating a foam. Research is also ongoing at many universities in the production of microcellular foams using sCO2.
An electrochemical carboxylation of a para-isobutylbenzyl chloride to ibuprofen is promoted under sCO2.
Working fluid
Supercritical CO2 is chemically stable, reliable, low-cost, non-toxic, non-flammable and readily available, making it a desirable candidate working fluid.
Power Generation
The unique properties of s CO2 present advantages for closed-loop power generation and can be applied to various power generation applications. Power generation systems that use traditional steam Brayton and Rankine cycles can be upgraded to sCO2 to increase efficiency and power output.
It presents interesting properties that promise substantial improvements in system efficiency. Due to its high fluid density, sCO2 enables extremely compact and highly efficient turbomachinery. It can use simpler, single casing body designs while steam turbines require multiple turbine stages and associated casings, as well as additional inlet and outlet piping. The high density allows for highly compact, microchannel-based heat exchanger technology.
In 2016, General Electric announced an sCO2-based turbine that operated at 50% efficiency. In it the CO2 is heated to 700 °C.It requires less compression and allows heat transfer. It reaches full power in 2 minutes, whereas steam turbines need at least 30 minutes. The prototype generated 10 MW and is approximately 10% the size of a comparable steam turbine.
Further, due to its superior thermal stability and non-flammability, direct heat exchange from high temperature sources is possible, permitting higher working fluid temperatures and therefore higher cycle efficiency. And unlike two-phase flow, sCO2’s single-phase nature eliminates the necessity of a heat input for phase change that is required for the water to steam conversion, thereby also eliminating associated thermal fatigue and corrosion.
Despite the promise of substantially higher efficiency and lower capital costs, the use of sCO2 presents material selection and design issues. Materials in power generation components must display high-temperature resistance, oxidation resistance and creep resistance. Candidate materials that meet these property and performance goals include incumbent alloys in power generation, such as nickel-based superalloys for turbomachinery components and austenitic stainless steels for piping. Components within sCO2 Brayton loops suffer from corrosion and erosion, specifically erosion in turbomachinery and recuperative heat exchanger components and intergranular corrosion and pitting in the piping.
Testing has been conducted on candidate Ni-based alloys, austenitic steels, ferritic steels and ceramics for corrosion resistance in sCO2 cycles. The interest in these materials derive from their formation of protective surface oxide layers in the presence of carbon dioxide, however in most cases further evaluation of the reaction mechanics and corrosion/erosion kinetics and mechanisms is required, as none of the materials meet the necessary goals.
Other
Work is underway to develop a sCO2 closed-cycle gas turbine to operate at temperatures near 550 °C. This would have implications for bulk thermal and nuclear generation of electricity, because the supercritical properties of carbon dioxide at above 500 °C and 20 MPa enable thermal efficiencies approaching 45 percent. This could increase the electrical power produced per unit of fuel required by 40 percent or more. Given the volume of carbon fuels used in producing electricity, the environmental impact of cycle efficiency increases would be significant.
Supercritical CO2 is an emerging natural refrigerant, used in new, low carbon solutions for domestic heat pumps. Supercritical CO2 heat pumps are commercially marketed in Asia. EcoCute systems from Japan, developed by Mayekawa, develop high temperature domestic water with small inputs of electric power by moving heat into the system from the surroundings.
Supercritical CO2 has been used since the 1980s to enhance recovery in mature oil fields.
"Clean coal" technologies are emerging that could combine such enhanced recovery methods with carbon sequestration. Using gasifiers instead of conventional furnaces, coal and water is reduced to hydrogen gas, carbon dioxide and ash. This hydrogen gas can be used to produce electrical power In combined cycle gas turbines, CO2 is captured, compressed to the supercritical state and injected into geological storage, possibly into existing oil fields to improve yields. The unique properties of sCO2 ensure that it remains out of the atmosphere.
Supercritical CO2 could be used as a working fluid in enhanced geothermal systems. Possible advantages compared to water include higher energy yield resulting from its lower viscosity, better chemical interaction, CO2 storage through fluid loss and higher temperature limit. As of 2011, the concept had not been tested in the field.
Aerogel production
Supercritical carbon dioxide is used in the production of silica, carbon and metal based aerogels. For example, silicon dioxide gel is formed and then exposed to sCO2. When the CO2 goes supercritical, all surface tension is removed, allowing the liquid to leave the aerogel and produce nanometer sized pores.
Sterilization of biomedical materials
Supercritical CO2 is an alternative for terminal sterilization of biological materials and medical devices with combination of the additive peracetic acid (PAA). Supercritical CO2 does not sterilize the media, because it does not kill the spores of microorganisms. Moreover, this process is gentle, as the morphology, ultrastructure and protein profiles of inactivated microbes are preserved.
Cleaning
Supercritical CO2 is used in certain industrial cleaning processes.
Exploring Aromatherapy
Aromatherapy is an incredibly vast and rich field.
How Are Essential Oils Extracted?
Essential oils can be extracted via two key methods: Distillation (includes hydrodistillation) and Expression.
Absolutes, other the other hand, can be extracted via Solvent extraction or Enfleurage, although enfleurage is rarely performed in the modern day.
Another type of aromatic product available on the market are CO2 extracts, referred to simply as that, CO2 extracts. They differ in chemistry from their related distilled essential oils but are becoming increasing available on the market.
The Practice of Distillation
Distillation appears to have been practiced throughout ancient times. Based upon the current interpretation Paolo Rovesti’s discovery of an earthenware distillation apparatus, the production or extraction of aromatic oils by means of steam distillation, has been known for 5000 years.1 During the fifth century AD, the famed writer, Zosimus of Panopolis, refers to the distilling of a divine water and panacea. Throughout the early Middle Ages and beyond, a crude form of distillation was known and was used primarily to prepare floral waters or distilled aromatic waters. These appear to have been used in perfumery, as digestive tonics, in cooking, and for trading.
Although an extensive trade of odoriferous material has been shown to have occurred in the ancient Orient and ancient Greece and Rome, the oils used were not essential oils per se, “rather they were obtained by placing flowers, roots, and other plant material into a fatty oil of best quality, submitting the glass bottles containing these mixtures to the warming influence of the sun and finally separating odoriferous oil from the solid constituents”.2
In 900 AD, Avicenna, the famous child prodigy from Persia who wrote many documents on plants and their uses and also instructions for massage, was accredited with refining the process of distillation by improving the cooling system.
Today distillation is still the most common process of extracting essential oils from plants. The advantage of distillation is that the volatile components can be distilled at temperatures lower than the boiling points of their individual constituents and are easily separated from the condensed water.
The Distillation Process
During distillation the plant material is placed upon a grid inside the still. Once inside, the still is sealed, and, depending upon the above methods, steam or water/steam slowly breaks through the plant material to remove its volatile constituents. These volatile constituents rise upward through a connecting pipe that leads them into a condenser. The condenser cools the rising vapor back into liquid form. The liquid is then collected in a vehicle below the condenser. Since water and essential oil do not mix, the essential oil will be found on the surface of the water where it is siphoned off. Occasionally an essential oil is heavier than water and is found on the bottom rather than the top, such as with clove essential oil.
The three types of distillation include:
Water Distillation
The plant material comes into direct contact with the water. This method is most often employed with flowers (rose and orange blossoms), as direct steam causes these flowers to clump together making it difficult for steam to pass through.
Water and Steam
This method can be employed with herb and leaf material. During this process, the water remains below the plant material, which has been placed on a grate while the steam is introduced from outside the main still (indirect steam).
Steam Distillation
This method is the most commonly used. During this process, steam is injected into the still, usually at slightly higher pressures and temperatures than the above two methods.
Note on Boiling Point: The boiling point represents the temperature at which a liquid is converted to a gas at a specified pressure. The fundamental nature of steam distillation is that it enables a compound or mixture of compounds to be distilled (and subsequently recovered) at a temperature substantially below that of the boiling point(s) of the individual constituent(s). Essential oils contain substances with boiling points up to 200°C or higher, including some that are solids at normal temperatures. In the presence of steam or boiling water, however, these substances are volatilized at a temperature close to 100°C at atmospheric pressure.3
Percolation or Hydro-diffusion
This is a relatively recent method and is very similar to steam distillation except that the steam comes in through the top rather than the bottom, and there is a shorter distillation time. It is useful in extracting essential oils from woody or tough material or seeds such as fennel and dill.
Hydrosols: A By-product of Distillation
Hydrosols, also known as hydrolats, are the by-product or product (depending on the distiller purpose) of the distillation process. Hydrosols contain the water-soluble constituents of the aromatic plant and retain a small amount of essential oil. Every liter of hydrosol contains between 0.05 and 0.2 milliliter of dissolved essential oil, depending on the water solubility of the plant’s components and the distillation parameters.4
*Please Note: The addition of essential oils to water is not at all the same as true hydrosols, and it is recommended that you read the ingredients label on products to ascertain whether or not you are getting a true hydrosol. When water and essential oils are mixed together with or without a dispersant, this is called a “spritzer” or “aromatic spritzer,” and this product should not be confused with a true hydrosol.
Expression
Expression, also referred to as cold pressing, is a method of extraction specific to citrus essential oils, such as tangerine, lemon, bergamot, sweet orange, and lime. In older times, expression was done in the form of sponge pressing, which was literally accomplished by hand. The zest or rind of the citrus would first be soaked in warm water to make the rind more receptive to the pressing process. A sponge would then be used to press the rind, thus breaking the essential oil cavities, and absorb the essential oil. Once the sponge was filled with the extraction, it would then be pressed over a collecting container, and there it would stand to allow for the separation of the essential oil and water/juice. The essential oil would finally be siphoned off.
A more modern method of extraction, and less labor-intensive, has been termed the ecuelle a piquer process that involves a prodding, pricking, sticking action to release the essential oil. During this process, the rind of the fruit is placed in a container having spikes that will puncture the peel while the device is rotated. The puncturing of the rind will release the essential oil that is then collected in a small area below the container. The end process is the same as above. The majority of modern expression techniques are accomplished by using machines using centrifugal force. The spinning in a centrifuge separates the majority of essential oil from the fruit juice.
Click to view a video of a modern extraction of citrus essential oil.
What is the difference between expressed and distilled citrus oils?
Expressed citrus oils are produced by mechanical separation (cold pressing) of the oil from the peels of various citrus fruits such as orange, grapefruit, tangerine, lemon, lime, petitgrain and bergamot. Expressed citrus oils contain small amounts of naturally occurring nonvolatile residues such as waxes. Expressed citrus oils offer the advantage of cold process which results in an aroma which is identical to fresh citrus peels.
Citrus oils can also be distilled from either the peels or whole fruits. While the aroma differs from expressed citrus oils, the distilled versions offer certain advantages:
1) Distilled citrus oils do not contain nonvolatile residues which can clog diffusers, stain fabric, and shorten the shelf life.
2) Distilled citrus oils don’t contain nonvolatile furocoumarins and are generally less photosensitizing.5,6
Extraction Techniques for Absolutes & CO2 ExtractsThe following methods of extraction, enfleurage, solvent extraction, and CO2 extraction, will only be briefly discussed. This textbook does not cover the therapeutic use of CO2 extracts or absolutes.
Enfleurage
Flowers were being processed via enfleurage in the Grasse region of Southern France long before the modern method of solvent extraction. In the early days of perfumery, many flower scents were extracted via enfleurage, now considered an ancient art that is passed down from father to son or from generation to generation.
Enfleurage is a cold-fat extraction process that is based upon the principles that fat possesses a high power of absorption, particularly animal fat. The fat used must be relatively stable against rancidity. It is a method used for flowers that continue developing and giving off their aroma even after harvesting (e.g., jasmine and tuberose).
Today, Grasse continues to be one of the few areas in the world that continues to employ enfleurage as a method of extraction, although it is rare in the aromatherapy market due to the expense. If one finds a jasmine enfleurage on the market, this would typically be considered an absolute.
Solvent Extraction
Some plant material is too fragile to be distilled and an alternative method must be employed. Solvent extraction is the use of solvents, such as petroleum ether, methanol, ethanol, or hexane, to extract the odoriferous lipophilic material from the plant. The solvent will also pull out the chlorophyll and other plant tissue, resulting in a highly colored or thick/viscous extract. The first product made via solvent extraction is known as a concrete. A concrete is the concentrated extract that contains the waxes and/or fats as well as the odoriferous material from the plant. The concrete is then mixed with alcohol, which serves to extract the aromatic principle of the material. The final product is known as an absolute.
Solvent extraction is used for jasmine, tuberose, carnation, gardenia, jonquil, violet leaf, narcissus, mimosa, and other delicate flowers. Neroli and rose can be distilled or solvent-extracted. The name neroli typically implies the essential oil, whereas the name orange blossom is commonly used for the absolute or hydrosol of neroli. The name rose is used to describe either the essential oil or the absolute. Companies selling essential oils should clarify whether the product you are purchasing is an essential oil or absolute. This information should be on the label and in the product catalog.
After the solvent extraction process has been completed, the resulting absolute will have an extremely low concentration of solvent residue, approximately 5 to 10ppm (parts per million). The current European Union standards are for less than 10 parts per million solvent residues in a finished absolute.7 However, even with such a potentially small residue (less than .0001%), many aromatherapists disagree with the use of absolutes for individuals with a compromised immune system due to the potential effect of the residual pesticide.
However, absolutes do have therapeutic value and are often used for psychological purposes and for animals, particularly horses. Many therapists incorporate absolutes, such as rose absolute, jasmine, and tuberose, as a valuable part of their therapeutic applications of aromatherapy. Ultimately the decision to use absolutes is up to the practitioner and his/her own personal preferences.
Absolutes are highly concentrated aromatic substances and are obtained from delicate flowers by either enfleurage or solvent extraction. Absolutes will most often resemble the natural aroma of the plant and are normally more colored and viscous than essential oils. Absolutes are used extensively in the cosmetic and perfume industries due to their strong aromas. There are also different grades of absolutes. The top grade is the uncut, which can be a thick or semisolid substance, making them difficult to work with. Less expensive grades are diluted with alcohol to make them more user friendly, although often the strength of aroma is slightly diminished.
CO2 Hypercritical Extraction
Hypercritical carbon dioxide (CO2) extraction is a relatively new process used for the extraction of aromatic products. The basic concept is that CO2 under pressure will turn from a gas into a liquid that can then be used as an inert liquid solvent. This liquid solvent is able to diffuse throughout the plant material thus extracting its aromatic constituents. CO2 extracts contain most of the same constituents as their essential oil counterparts, although they can contain some elements not found in essential oils. For instance, the essential oil of ginger (Zingiber officinale) does not contain the bitter principles, however the CO2 extract does. Also, the CO2 extract of frankincense (Boswellia carterii) has immune enhancing and anti-inflammatory activity not found in the essential oil. CO2 extracts are known for their strong similarity in aroma to the actual plant aroma. Other common CO2 extracts on the market include German chamomile (Matricaria recutita) and Calendula (Calendula officinalis).
The three main disadvantages for this process are cost, potential pesticide residue, and the lack of information regarding their safety and therapeutic benefits.7 With regard to pesticide residue, Guba comments that “carbon dioxide extraction has been demonstrated to concentrate from 7 to 53 times more pesticide residues in the final extract.” Therefore, it seems pertinent to only use organic plant material for CO2 extraction. Perhaps as more CO2 extracts become available and more practitioners use them, further details regarding their applications will become apparent. Two of the most common essential oils available via CO2 extraction include frankincense and ginger.