Essential Oil Extraction using Liquid CO2
Contributors
Summary
Many fruits and vegetables contain essential oils, which are hydrophobic liquids responsible for their distinctive fragrance. These oils are commonly extracted for use in various industries such as perfume, cosmetics, food, medicine, and cleaning products. The traditional extraction involves steam distillation followed by liquid-liquid (solvent) extraction, which is energy-intensive and often employs hazardous solvents like methylene chloride. However, a greener alternative has been discovered using carbon dioxide (CO2) at elevated pressure. In this activity, students will simulate industrial processes by using liquid CO2 to extract essential oil, specifically D-limonene from lemon peels. This method showcases the unique properties of supercritical CO2, a phase where CO2 exhibits both gas and liquid characteristics, enabling efficient separations. Importantly, the use of CO2 as an extraction solvent does not contribute to climate change and reduces energy input and the need for dangerous solvents. Currently, supercritical CO2 is used to remove caffeine from coffee beans to produce decaffeinated coffee and as a replacement for perchloroethylene in dry cleaning applications. Through this experiment, students will analyze and compare the two extraction methods, bridging the gap between classroom activities and industrial chemical processes, and exploring the citrus-like scent of D-limonene found in lemons, oranges, and limes.
Industrial methods for obtaining D-limonene.
Traditionally, essential oils have been extracted by steam distillation and/or organic solvent extraction. In recent decades, great strides have been made in technology that uses supercritical or liquid carbon dioxide in place of organic solvents. Carbon dioxide (CO2) is useful as a green alternative solvent because it provides environmental and safety advantages; it is non-flammable, relatively nontoxic, readily available, and environmentally benign. Processing with CO2 also results in minimal liability in the event of unintentional release or residual solvent in the product. Although CO2 is a greenhouse gas, using it as a solvent does not impact the environment because it is captured from the atmosphere prior to use. Large-scale CO2 processing has had commercial success in many separation and extraction processes. The tunable solubility properties, low toxicity, and ease of removal of CO2 have led to well-established CO2 technology for the extraction of various food products, including essential oils and hops, and for the decaffeination of coffee and tea.
Another major benefit of using carbon dioxide as a solvent is its accessible phase changes. Unlike other gases, relatively low temperatures and pressures can be used to form supercritical and liquid CO2. CO2 sublimes (goes directly from a solid to a gas) at normal atmospheric pressure of 1.01 bar. Looking at a phase diagram of CO2, you can see the temperature and pressure at which the CO2 will transition from solid (dry ice) to liquid. In this experiment, the supercritical point of CO2 will not be reached; this is not feasible with the equipment used in this simple experiment. In industry, pressure vessels are used to achieve the triple point, above which the supercritical point is reached.
*The steam distillation can be a demo lab and the liquid CO2 can be the hands-on student lab if you feel that you only have one class period to cover this material.
Industrial methods for obtaining D-limonene.
Traditionally, essential oils have been extracted by steam distillation and/or organic solvent extraction. In recent decades, great strides have been made in technology that uses supercritical or liquid carbon dioxide in place of organic solvents. Carbon dioxide (CO2) is useful as a green alternative solvent because it provides environmental and safety advantages; it is non-flammable, relatively nontoxic, readily available, and environmentally benign. Processing with CO2 also results in minimal liability in the event of unintentional release or residual solvent in the product. Although CO2 is a greenhouse gas, using it as a solvent does not impact the environment because it is captured from the atmosphere prior to use. Large-scale CO2 processing has had commercial success in many separation and extraction processes. The tunable solubility properties, low toxicity, and ease of removal of CO2 have led to well-established CO2 technology for the extraction of various food products, including essential oils and hops, and for the decaffeination of coffee and tea.
Another major benefit of using carbon dioxide as a solvent is its accessible phase changes. Unlike other gases, relatively low temperatures and pressures can be used to form supercritical and liquid CO2. CO2 sublimes (goes directly from a solid to a gas) at normal atmospheric pressure of 1.01 bar. Looking at a phase diagram of CO2, you can see the temperature and pressure at which the CO2 will transition from solid (dry ice) to liquid. In this experiment, the supercritical point of CO2 will not be reached; this is not feasible with the equipment used in this simple experiment. In industry, pressure vessels are used to achieve the triple point, above which the supercritical point is reached.
*The steam distillation can be a demo lab and the liquid CO2 can be the hands-on student lab if you feel that you only have one class period to cover this material.
Safety Precautions, Hazards, and Risk Assessment
Safety Information:
The most serious safety concerns in this experiment involve the possibilities of cap discharge (most common occurrence) or vessel rupture (rarely observed). During the testing phase of this experiment, caps blew off during approximately 4% of the extractions. All caps were directed upward by the plastic containment cylinders. Caps remained on during all extractions when students 1) tightened the cap as tightly as possible and 2) did not use caps that were stripped. If the cap cannot be completely tightened and continues to turn, the stripped cap should be discarded and replaced. Due to variations in the centrifuge tubes and caps, a tight seal is not always formed at their junction. In this case, the CO2 does not liquefy, and retightening of the cap or replacement of the cap or tube may be required. Although many modifications of the sealing process have been proposed, such as the use of Teflon tape or parafilm on the threads, it is important that the cap seal well enough to induce liquefaction but not so tightly that the gas cannot escape. The cap must allow the gaseous CO2 to escape slowly during the extraction and also must function as a safety valve. During experiment development, attempts were made to observe the transition from the liquid to the solid phase by opening the cap while the CO2 was liquid. In two of these cases, the centrifuge tube ruptured, and plastic shards were propelled several feet from the demonstrator. Although no injuries occurred, it is recommended that the tubes always remain in containment cylinders during liquefaction. It is important to note that, in our experience, vessel rupture only occurred while attempting to release the pressure from the vessel when liquid CO2 was present.
Additional Safety Information:
• Safety goggles and gloves should be worn at all times.
• Do not use glass containers for warm water baths/containment cylinders in the liquid CO2 experiment. If a centrifuge tube ruptures, the plastic cylinder protects students from any plastic shards propelled sideways.
• Never allow students to hold their faces above the containment cylinder, as a ruptured centrifuge tube would be propelled upwards.
• Always handle dry ice with proper PPE (oven mitts, cryogen gloves, etc.) to prevent burns.
• The mortar and pestle can get very cold while crushing dry ice. Wear proper PPE.
• Use dry ice only in well-ventilated spaces. CO2 can displace oxygen in enclosed spaces and pose risk of suffocation.
• In the steam distillation experiment, hot water can cause burns.
• If using graters or scissors for zesting the lemon, be mindful of sharp edges.
Disposal Information:
• All materials used in this lab are safe to be disposed of in the trash.
• Always follow state and district guidelines for laboratory waste disposal.
The most serious safety concerns in this experiment involve the possibilities of cap discharge (most common occurrence) or vessel rupture (rarely observed). During the testing phase of this experiment, caps blew off during approximately 4% of the extractions. All caps were directed upward by the plastic containment cylinders. Caps remained on during all extractions when students 1) tightened the cap as tightly as possible and 2) did not use caps that were stripped. If the cap cannot be completely tightened and continues to turn, the stripped cap should be discarded and replaced. Due to variations in the centrifuge tubes and caps, a tight seal is not always formed at their junction. In this case, the CO2 does not liquefy, and retightening of the cap or replacement of the cap or tube may be required. Although many modifications of the sealing process have been proposed, such as the use of Teflon tape or parafilm on the threads, it is important that the cap seal well enough to induce liquefaction but not so tightly that the gas cannot escape. The cap must allow the gaseous CO2 to escape slowly during the extraction and also must function as a safety valve. During experiment development, attempts were made to observe the transition from the liquid to the solid phase by opening the cap while the CO2 was liquid. In two of these cases, the centrifuge tube ruptured, and plastic shards were propelled several feet from the demonstrator. Although no injuries occurred, it is recommended that the tubes always remain in containment cylinders during liquefaction. It is important to note that, in our experience, vessel rupture only occurred while attempting to release the pressure from the vessel when liquid CO2 was present.
Additional Safety Information:
• Safety goggles and gloves should be worn at all times.
• Do not use glass containers for warm water baths/containment cylinders in the liquid CO2 experiment. If a centrifuge tube ruptures, the plastic cylinder protects students from any plastic shards propelled sideways.
• Never allow students to hold their faces above the containment cylinder, as a ruptured centrifuge tube would be propelled upwards.
• Always handle dry ice with proper PPE (oven mitts, cryogen gloves, etc.) to prevent burns.
• The mortar and pestle can get very cold while crushing dry ice. Wear proper PPE.
• Use dry ice only in well-ventilated spaces. CO2 can displace oxygen in enclosed spaces and pose risk of suffocation.
• In the steam distillation experiment, hot water can cause burns.
• If using graters or scissors for zesting the lemon, be mindful of sharp edges.
Disposal Information:
• All materials used in this lab are safe to be disposed of in the trash.
• Always follow state and district guidelines for laboratory waste disposal.
Digital Object Identifier (DOI)
https://doi.org/10.59877/TTWL6400
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