A Review of Essential Oil Extr Action Technologies

7/30/2019 A Review of Essential Oil Extr Action Technologies

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Prepared by: Scott SanfordAugust 12, 2011 (revised)

University of Wisconsin-MadisonBiological Systems Engineering460 Henry Mall

Madison, WI 53706

Phone: 608-262-5062E-mail: [email protected]

M i n t O i l E n e r g y C o n s u m p t i o n , E n e r g y U s e

E f f i c i e n c y a n d D i s t i l l a t i o n P r o c e s s e sA Rev iew o f Essen t ia l O i l Ex t rac t ion Techno log ies


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Project funded by a grant from the Mint Industry Research Council.


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Review of Essential Oil Extraction Technologies

The common method for extraction of mint oil used by mint growers in the U.S. is steam distil-lation. The mint hay is chopped directly into a movable tub (wagon) and then moved to the dis-tillation area where it is connected to the steam source and condensate recovery system. A studyby Hackleman et al. in 2006 found many things that could be done to improve energy efficiency

of current distillation systems. However the technology being used today is about 50 years oldand newer more energy efficient methods are available for the extraction of mint oil. Steam dis-tillation or water distillation have been used since antiquity. Energy for distillation ranges from2.5 kWh/kg to 4 kWh/kg (38686190 Btu/lb) of raw product (plant material) (Berka-Zougali2010). This paper will look at the different process that can be used to extract essential oilsfrom plant matter.

Water distillationWater distillation is generally used for extraction oils from dried or powdered plant ma-

terials such as spice powders, ground woody plants such as cinnamon bark, some flowers likerose or tough materials such as roots, wood or nuts. Plant materials are immersed in water, al-

lowed to soak and then boiled with direct heat. The pressure on the still can be reduced or in-creased to change the distillation products. The volatile components are mostly extracted at atemperature just below 100C by diffusion mechanism. The volatiles are transported with thesteam to a condenser and oil is separated using a Florentine Vessel. The remaining water in thevessel after distillation can be transferred to another vessel hot and redistilled reducing energyand water use. An advantage of water distillation is the plant material is always in contact withboiling water. The vessel should be stirred to prevent material from clumping or settling to thebottom of the vessel. Some disadvantages included low water levels causing overheating orcharring resulting in off-notes (off-flavors), lower quality oil, slower process resulting in higherenergy use, incomplete release of essential oil, and requires more stills because of lower densityin the still.

Water-Steam DistillationThis process uses similar equipment to water distillation but is a combination of water

and steam distillation processes. It can be used for mint or other leafy plants. Plant material isloaded in a vessel on a grate with water below. The water is boiled creating a low-pressure wetsteam. The plant material must be uniformly loaded into the vessel otherwise steam will bypasssome of the material. This method can cause overheating of the material on the vessel wallsthrough conduction if the still is direct fired resulting in off-notes. The wet steam can saturatethe plant material on the grates and slow down the distillation process. The method does de-crease processing time and improves energy use compared to water distillation along with highyields and better quality oil.

Steam DistillationThis is the most commonly used method for commercial scale extraction of essential

oils including all types of mint. This method is similar to the other methods except steam issupplied by an external source. Dry steam is injected under the plant material that is resting on aperforated floor. The material needs to be distributed evenly on the floor so the steam will flowevenly through the material. The oil is extracted by diffusion as the steam passes through thematerial. The steam-oil mixture then passes through the condenser and the oil is decanted in aFlorentine vessel. This process is more energy efficient, cost effective, better process control,produces more consistent oil and less likely to damage oils. A disadvantage of steam distillation


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is channeling of the steam through the plant material resulting in lower extraction rates. Thiscan be reduced by chopping the material which is a common practice but this can result in los-ing considerable amounts of oil (ztekin 2007) during wilting and chopping. Mixing the mate-rial in the still (such as an auger) during processing can also reduce channeling of the steam.Even though steam distillation is the most energy efficient of the distillation processes it stillrequires a large amount of energy per pound of oil and large amounts of water for cooling. Baseon supplied by MIRC, it requires about 79,500 Btu per pound of oil recovered. Distillation haslong processing times, about 2+ hours, and is energy intensive compared to other methods of oilextraction. Lawrence (1995) estimated the cost for a 4 tub distillation system in the U.S. wouldcost $218,000 ($1992) ($329,000$2009) and have the capacity to support 400 acres of mint.

Hydro DistillationThis method uses steam at atmospheric pressure passed into the plant material from

overhead. The advantage to this version is the steam saturates the plant materials more evenlyin less time than with steam distillation. The condenser is located under a basket or perforatefloor that the material is placed on. This method results in higher quality oil that smells morelike the original plant.

Continuous Steam DistillationA continuous steam distillation process has been used in Russia since the late 1970s. In

the mid 1980s, Bouchard et.al. (1986) and Bouchard and Serth (1991) describe a continuousprocess for extracting oil from cedarwood by Texarome (http://www.texarome.com/) in Texas.The logs were pulverized and pneumatically conveyed with steam as the carrier into a distilla-tion chamber which is approximately a mile of pipe. The conditions in the chamber werecontrolled to vaporize the volatile with the highest boiling point that was desired. The residencetime in the distillation chamber was only 25-30 seconds. This commercial system is capable ofprocessing 13 tons per day. The spent wood could be used for boiler fuel after extraction. Basedon the process conditions, the energy per pound of material processed would be approximately1154 Btu per pound of input plant material which would translate to approximately 23,000 Btuper pound of mint oil based on the same processing parameters as used for cedarwood. If mintcould be processed in this type of system it would reduce energy costs by approximately 60 to70%. The capacity of this plant would be too low for mint. Assuming the same capacity perday, the 13 ton plant could only process 3.25 acres per day based on 4 tons per acre. No patentwas found. Texarome does offer consulting services for distillation construction.

Carle and Fiedler describe the use of a cylindrical continuous distillation system under thebrand name of Padovan (De Silva 1995pg 96). It appears that Padovan no longer makes dis-tillation units (www.padovan.com). Arnaudo describes the plant material being fed through the

distillation chamber using an Archimedian screw (auger) and steam passage in countercurrentflow. This system works best with powdered materials and is being used in France for fennel oilproduction. Arnaudo also describes a continuous distillation system, named DCF, that cascadesmaterial through a series of auger modules. The augers configuration creates plugs of materialto prevent the escape of steam. This type of system uses less steam and is ideal for low boilingpoint oils such as peppermint.

Another continuous distillation system is described by Arnaudo (1991) and Vacchiano (1992)is known as the Biolandes process. Biolandes is a company based in France that specializes in


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essential oils and natural extracts. They developed a continuous distillation process and holdseveral patents on the equipment developed for the process. A unit comprised of two 265-cubicfoot (7.5 cu. meters) vessels can process 3 tons of pine needles per hour. Figures 1 show illus-trations from the patent for the distillation vessel. Figure 2 is an illustration of the overall sys-tem. The steam and oil vapor would traditionally be condensed using lots of cold water butBiolandes route the steam mixture through an aerothemic radiator (air cooled), item 42 in Fig-

Figure 1: Biolanders Patent illustrations (Source: US Patent 5,024,820)

Figure 2: Biolanders Continuous Steam Distillation System Schematic (Source: US Patent 4,935,104)


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Figure 3: Patent Illustration for Rathbun and Thalheimer

ure 2, with a high thermal transfer capacity to transform the 100C steam mixture to 100C water.The hot air is used to dry the distillation residues that are immediately feed into the boiler to createthe steam. This reduces water demand and energy expenditures. U.S. Patent 4,935,104 covers theprocess described above and expired June of 2010. A second U.S. Patent 5,024,820 covers the load-ing and unloading system used for the continuous still which uses the plant material to form a plug toprevent the steam from escaping, Figure 1. This patent will expire in June 2011.


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The plant material plug to prevent steam from escaping was first used in U.S. Patent 4,495,033, Figure3, which uses a series of augers in channels with steam emitted into the plant material as it is moveddown the channel. The system can be setup with a series of augers that cascades the plant material fromone auger to the next to increase distillation time and mixing of material. This was developed by AlbertThalheimer, a mint grower, and Robert Rathbun, a machine shop owner, both from Toppenish, WA. This

still was never commercialized. The patent expired in 2005. The patent application claims the unit wasmore energy efficient, a 65% energy savings, but no data was published or is available to substantiate theclaim. The process time is claimed to be 5 to 8 minutes and could be setup to operate automatically 24hours per day. No processing capacity was stated.

U.S. Patent 5,891,501 (1999) describes the use of a surfactant to improve oil yields from steam distilla-tion. The surfactant can be applied at the time of mowing, to the windrow as the mint hay is being feedinto a chopper or as it is transferred to the distillation tub. Based on data provided in the patent from 11tubs with surfactant and 11 tubs without, the surfactant resulted in a 7.6% increase in oil recovered. Theinventor was contacted several times to find out if this was commercially available but never returnedcalls. There is concern if the surfactant were to end up in the oil or affect the separation of the oil from

the water.

Cold Pressing / ExpressionCold Pressing is a method used for citrus oil extraction and oilseeds but is not practical for mint

oil.

Solvent ExtractionSolvent extraction is the most widely used extraction process for extracting oils from plants. The

plant material is immersed into the solvent and the oils diffuse into the solvent. The solvent are removedfrom the oils by distillation or evaporation. There are various materials that can be used for solvents de-pending on the material being extracted. There are four classes of solvents. Low boiling point organic

solvents which would include hexane, propane, butane, methanol, ethanol and others. Water can be usedas a solvent along with fats or waxes. There are also liquefied gaseous solvents such as carbon dioxideand freons. Each solvent has drawbacks, there isnt a perfect solvent. The properties of a perfect solventwould be as follows: doesnt dissolve water, has low viscosity, high solution capacity, low latent heat ofvaporization, low boiling point, stable and inert, non-toxic food grade, readily available, recoverable,doesnt leave a residue, nonflammable, inexpensive and environmentally friendly. Hexane appears to bethe most widely used, predominantly for oil seeds.

Types of solvent extractors include Static extractors and Rotational extractors. A static extractor is typi-cally a cylindrical vessel in which the plant material is placed and the solvent is successively circulated.The solvent is removed by draining, followed by a steam cycle to strip the solvent from the plant mate-

rial. The essential oils are extracted from the solvent by evaporation of the solvent. The solvent is gener-ally condensed and reused for environmental and cost reasons. A rotational extractor is similar to a drumwashing machine. The process is similar to the static extractor but agitation aids in increasing contactbetween the plant material and the solvent. Biolandes (Vacchiano - 1992) has a continuous solvent ex-traction process that works on the same principle as the continuous steam extraction system but the steamis replaced with a solvent. The plant material is introduced at the top and distributed with a spreadingmechanism and the solvent is introduced at the bottom of the vessel and flows up through the plant mate-rial. The exhausted material is de-solventized by compression and then run through a vessel that stripsthe remaining solvent with steam.


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Figure 5: Sliding Cell Solvent Extractor

Figure 6: Sliding Cell Solvent Extractor

Cut-away view Courtesy of Lurgi AG

Figure 4: Carrousel Extractor cut-awayview (source: Harburg-Freudenberger)

A Rotocel or Carrousel is a continuous extractor used in the oil seed industry which consists of18 pie shaped cells located in a circle, Figure 4. The cells are loaded and then rotate around thevessel while solvent is sprayed on the plant matter and allowed to percolate through. The cellbottoms are perforated to allow the solvent to drain through and be re-circulated. One rotation isabout an hour but varies with the material being extracted. At the end of one rotation the plant

matter exits the cell as the cell rotates over an open bottom cell. This type of equipment has alow energy requirement. There are several other types of extractors including the loop extractor(Crown Iron Works Company Minneapolis, MN) and sliding bed extractor (Lurgi AG -

Frankfort, Germany), see Figures 5 & 6. These ex-tractors work similar to the carrousel extractor ex-cept the material path is linear.The mixture of solvent and essential oil is calledmiscella. The solvent is removed from the miscellain a two-step process: evaporation followed bysteam stripping. The vapor from evaporation is con-

densed and the solvent separated from the water.The solvent is then recycled.Dai et.al. (2010) reported that solvent extractionwith hexane was 180 times faster than using steamdistillation.Microwave Assisted Solvent extractionMicrowave-assisted process (MAP) applies micro-wave energy to selectively heat components in a sol-vent solution. Materials have different dielectric

constants and in general the higher the absolute value, the higher the level of absorption of mi-crowave energy. The absorption level does vary with temperature of the substrate and the mi-crowave frequency used. Because of the variation in the absorption of microwave energy, it ispossible to selectively heat substances. Free water molecules have a high dielectric constant(~80) and hence absorb energy and heat quickly. Some solvents such as hexane have a low di-electric constant (1.9) and essentially allow the microwave energy to pass through without ab-sorbing energy. For the extraction of mint oil, the microwaves will interact with the free waterin the plant structure causing localized heating. More heating will occur in areas with higher


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Solvent

Extraction

Unit

Filtration

Drying

Solvent

Distillation

Oil

Dry Exhausted

Plant Material

Plant

Material

Solvent

Transducers

Solvent Recovery

Figure 7: Ultrasonic-Assited Solvent extractionprocess illustration (Vinatoru1997)

water contents such as glandular and vascular systems of the plant material. The water will in-crease in temperature to the boiling point or higher and cause rapid expansion and rupture cells.This releases the oil and other components into the solvent and produces pathways for the sol-vent into the plant materials. The soluble components are dissolved by the solvent and then re-covered by evaporating the solvent. This process requires smaller volumes of solvent, can usedless toxic solvents and reduces energy consumption over more commonly used extraction meth-

ods. Par et.al. (1994) reported exposure to 20-30 seconds of microwave (at 625 watt) showedmore glandular disruption than six hours of soxhlet extraction or 2 hours of steam distillation.They also found that oil yield on a per weigh basis was greater for the soxhlet extractor (labscale solvent extraction method) but the oil quality was superior for the MAP extracted oil be-cause in contained little chlorophyll and less pulegone (a monoterpene which is clear colorlessoily liquid and has a pleasant odor). In their U.S. Patent, 5,002,784, Par et.al., indicated thatMAP would reduce processing costs by 45% and labor costs by 50% compared to steam distil-lation which could increase net revenue from 18% to 35%. Extraction lab tests of peppermintproduced 25% more oil using MAP than traditional steam distillation. This patented processwas developed by J.R. Jocelyn Par et.al. at the Minister of the Environment in Canada and iscovered by U.S. Patents: 5,002,784 (1991), 5,338,557 (1994), 5,377,426 (1995), 5,458,897

(1995), 5,519,947 (1996), 5,675,909 (1997), 5,884,417 (1999) and 6,061,926 (2000). The appli-cation of microwave to flammable solvents can be hazardous if not carefully controlled.

A study by Dai, et.al. (2010) found the highest yield of menthone, menthofuran, and mentholfrom peppermint was achieved in lab testing using microwave assisted solvent extraction, witha 30:70 mixture of ethanol and hexane, an extraction time of about 30 minutes and a sample-to-solvent ratio of 2g to 80 mL. Using 100% ethanol or a 70:30 mixture of ethanol and hexane re-sulted in about a 10% reduction in oil yields. Oil yield for sample-to-solvent ratio down to 2g to20 mL also gave an acceptable yield. The extraction method had the largest affect on yield.

Ultrasonic Assisted Extraction

Sonication or ultrasonically assisted extraction involves immersing the plant material in a sol-vent in a vessel and then subjecting it to ultrasonic sound waves. Figure 7 is an illustration ofthe process. Solvent and plant material are placed in a temperature controlled vessel and sub-jected to ultrasonic vibrations to free the oil. After 10 to 15 minutes the solution is filtered toseparate the plant solids from the solvent and extracted oil. The liquid fraction is distilled toseparate the solvent from the oil and the solid fraction is dried to recover the solvent. The sol-vent is condensed and reused. If a low boiling point solvent is used and the temperature is kept

below its boiling point, ultrasound can increase oilyield. Solvent extraction is often done with cold sol-vent. Using sonication resulted in similar yieldscompared with conventional extraction methods but

in less time. Ultrasonic works by breaking the thincell walls of the oil glands and rinsing out the oilonce the cell walls are broken. If the plant materialhas been dried, the solvent diffuses into the cellscausing swelling and hydration. Sonication increasesthe rate of swelling and hydration. Lower frequen-cies (20 kHz) result in more cell damage and fasterrelease of oil than high frequency (500 kHz) whichleft the leaf undamaged (Vinatoru 2001). Stirring ofthe solvent also aided in decreasing the processing


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time. This can be a low temperature process when used with vacuum distillation to preserve thethermal sensitive components of the oil. Da Porto (2009) reported increases of oxygenatedmonoterpenes by up to 8 times more using ultrasound-assisted solvent extraction with 70%ethanol as a solvent compared to hydrodistillation. Carvone was the component that increasedthe most. Ultrasound does not increase oil yield with water distillation, it only produces morerapid boiling.

Solvent-Free Microwave Extraction (SFME)Microwave for cooking was discovered by Percy Spencer at Raytheon Corporation while devel-oping radar systems in 1945. In 1947 the first microwave ovens for use in restaurants and com-mercial kitchens were introduced. The first units weighed 750 pounds and stood five foot, sixinches and require water for cooling the magnetron. It took decades before it was refinedenough for the average consumer. Today, industrial applications for microwaves included foodprocessing, laboratory analysis, preheating and vulcanization of rubber, drying, baking sandcore molds for metal casting, drying resins in paper production, curing fiberboard and chip-board, and many other applications.

Microwave essential oil extraction can be done in a batch or continuous flow method. Thegreen plant material is exposed to microwave energy which causes the in-situ water of the plantmaterial to heat and boil causing the plant glandular membranes to rupture and release the oil.The oil and water is condensed to recover the oil and the water can be return to the microwaveoven to facilitate additional vaporization of the essential oil or disposed. Testing by Lucchesi etal. (2004) compared solvent-free microwave extraction with hydro-distillation and found 30minutes of processing time recover the same amount of oil as after 4.5 hours of hydro-distillation (HD). The first droplets of oil were recovered in 5 minutes compared to 90 minuteswith distillation. The oil composition for the SFME had higher amounts of oxygenated com-pounds and lower amounts of monoterpenes than the HD extracted oil. The oxygenated com-pounds are highly odoriferous and more valuable than the monoterpenes according to Lucchesi.

The mint oil contained 65% and 52% carvone, an oxygenated compound, for SFME and HD,respectively, while limonene content, a monoterpene, was 9.7% and 20% for SFME and HD,respectively. The difference in composition is not likely that they werent extracted but the re-duction in extraction times and reduced water volumes with SFME results is less degradation ofthe oils by hydrolysis, trans-esterification or oxidation. The energy requirement was alsogreatly reduced from 4.5 kWh for HD to 0.50 kWh for SFME using a 500g sample (Lucchesi2004). The oil yields of 0.095% for crispate mint which is substantially lower that is reported inmost articles on mint oil yields. The equivalent energy consumption is 14,664 Btu/lb of oil forhydro-distillation and 1,629 Btu/lb for microwave extraction. SFME has the advantage over sol-vent extraction methods of not having to be concerned with toxic solvent residue remaining inthe oil or secondary operations to remove it.

Vian (2008) published information on a new method that microwaves the plant matter in an up-side down flask, Figure 8. The concept is that the vapors will fall by gravity through a condens-ing unit into a Florentine flask. The oil yield when compared to hydro-distillation was compara-ble, 0.6% versus 0.59% but the extraction time was reduced from 90 minutes to 20 minutes.This concept would not be practical for commercial production.Microwave steam distillation (MSD) passes steam through the plant material while at the sametime irradiating the plant material. The steam is not heated by the microwaves because of a lowdielectric constant (1.0) but the water in the plant material is heated by the microwaves result-


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ing in rupturing of the cell walls and vaporization of the water and essential oils. The steampassing through the plant material acts as a carrier to move the essential oils to the condenserand Florentine flask. Figure 9 shows an illustration of the process. The microwave power set-ting is important for fast extraction but excessive power can cause the loss of volatile com-pounds. Lavender flowers were used in the study. The sample was processed until no more es-sential oils were obtained. It took less than 10 minutes to completely extract the oil. ComparingMSD to steam distillation, the oil yield was the same but it only took 6 minutes to obtain thesame oil yield as 30 minutes of

steam distillation. Using thesame process, Sahraoui (2008)found almost 100% of the ex-tractable oil was recovered in 5minutes compared to about 30minutes using steam distillationalone for lavender flowers.

Mengal and Mompon (2006)were issued a patent (U.S. Pat-ent 7,001,629 B1 (2006)) for a

microwave extraction systemthat cycles the extraction vesselfrom atmospheric pressure 100kPa to -25 kPa (14.5 to -3.6 psi) vacuum three times in a 15 minute cycle while keeping thetemperature below 75C . The lower pressure would cause lower boiling points. Water vaporfrom the extraction vessel is condensed, the oil is separated and the water is routed back to thechamber. A stirrer in the vessel increases the contact with the irradiation and reduces hot spots.The oil yield reported appears to be nearly equal to steam distillation. This method would likelybe batch process.

Figure 8: Microwave hydro-diffusion and gravity apparatus (Vian2008)

Figure 9: Microwave steam distillation (Sahraoui2008)


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MIRC has supported work on microwave extraction at Oregon State University. Velasco(2007) studied different power settings and combinations of power settings and time exposures.Her study found that the most effective power settings was a combination of 1120 W for 2minutes and 518 W for 1.25 minutes extracted the greatest oil yield. The energy consumptionwas calculated at 0.22 kWh per pound of mint hay. The energy cost was $1.22 per pound of oil

extracted. The process time was 3.25 minutes compared with 120 minutes for steam distillation.The peppermint oil composition is less than the ideal for Menthol and Menthone, about thesame for Cineole, and higher for Limonene, Pulegone and Furon. The mint oil composition wasacceptable based on the acceptable industry ranges.

Further work was done in 2009 by Hackleman (2010) setting up and running a pilot scale studyusing an industrial planar microwave system. Two trials were done, one at a growers facilityand one using frozen mint at the microwave suppliers facility. They had trouble getting the oilto be transported to the condenser unit and oil passing through the condenser unit. A blowerwas used to draw the steam and oil vapors from the microwave chamber to the condenser sincethere wasnt steam pressure to move the vapor. They reported problems getting the oil to not

condense in the pipe leading to the condenser unit. This may explain the reason why Mengaland Mompon (2006) in their patent are routing the condensed water back to the microwavechamber. An increase in water would provide more vapor to help carry the oil to the condenserunit. No oil was collected in the Florentine vessel. The 100 kW microwave unit tested should becapable of processing 8 cubic feet per minute and Hackleman concluded that if it was operated24 hours per day using an automated feeding system and the harvest season was extended, itwould replace 2 or 3 still tubs. Based on those assumptions, a complete system was estimated tocost $400,000 in capital and have maintenance of $2000 per year for cathode tube replacement(approximately every 8 years) plus maintenance for belts, motors, etc. This cost includes aclosed loop condensation cooling system to reduce water use and discharge. Based on assump-tions, Hackleman estimated the energy cost would be $0.40 per pound of oil or approximately a

93% energy cost savings over petroleum fired steam distillation which cost $5-7 per pound ofoil. However, in this trial only 11% of the oil was extracted from that mint hay based on sam-ples taken before and after so much work needs to be done if this is to be a viable extractionmethod.

The lack of success of Hackleman to extract oil can likely be explained by the difference in di-electric constants between the water and the oil components. The dielectric constant of a mate-rial is related to the ability of the material to be heated by microwave energy. A high dielectricconstant such as water results in high absorption of microwaves and heating . Water has a di-electric constant of between 55 and 80 depending on the temperature although once it turns tosteam the dielectric constant drops to 1 (no absorption of heat). The volatile components of

mint oil have dielectric constants ranging from 2.3 to 11, see Table 1. Hexane which is used inmicrowave assisted solvent extraction has a dielectric constant of 1.9 at 20C and is consideredto be transparent to microwave (very little absorption). Low dielectric constants means it willtake a long time to heat, therefore without high water content in the mint plant material thevolatilization of mint oil will take a long time. A grower at the Midwest Mint Growers confer-ence in Wisconsin in Feb 2011 told me hed tried using microwave on green-chopped mint (nowilting) and got 100 percent extraction. This would have resulted from the high water contentin the mint turning to steam and volatilizing the oil components. If the mint can be successfullygreen-chopped, the setup that Hackleman used might be successful.


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Supercritical Fluid Extraction

Supercritical fluid extraction (SFE) is a form of solvent extraction using a supercritical fluid asa solvent. The most commonly used fluids are carbon dioxide, ammonia, ethane, ethylene, pro-pane, pentane and water but they are sometimes mixed with ethanol or methanol. A substance

becomes a supercritical fluid at a temperature and pressure above its critical point, the point atwhich it changes from a liquid to a vapor, Figure 10. In the supercritical state a substance hasproperties of both a gas and liquid state. The densities are close to that of the liquid form whileviscosity is near that of the gaseous form. Supercritical fluids can flow through a solid like a gasand dissolves materials like a liquid. The solvent properties can be adjusted by changing thetemperature and pressures. Carbon dioxide is most commonly used for food applications suchas decaffeination of coffee beans or the extraction of hops for beer production. It is also usedfor the extraction of essential oils and pharmaceutical products from plants. The critical tem-perature for CO2 extraction is 31.1C (88F) while the critical pressure is 7.3 MPa (1060 psi).For traditional steam distillation, the temperatures will be greater than 100C (212F) with pres-

sures a little above atmospheric. The higher temperatures of steam distillation can cause degra-dation of thermally sensitive oils during extraction.

Supercritical fluid extraction using CO2 has many advantages. CO2 is relatively inexpensive,readily available and can be easily recycledand recovered from the extraction leaving noharmful solvent residues. It is non-flammable,

non-explosive, non-toxic, colorless and odor-less. SFE reduces processing times, results in abetter quality product with longer self life andprocess conditions are easily achievable.

The process involves loading plant materialinto a pressure vessel and pressurizing it withcarbon dioxide until the pressure and tempera-ture is above 7.4 MPa and 90F, respectively.Above critical pressure and temperatures, theCO2 will become supercritical and act like asolvent, dissolving oils, pigments and resinsfrom the plant material. The CO2 is usuallycirculated through the vessel to extract the sub-

Table 1: Dielectric constants and Boiling oils of mint oil components

Compound Dielectric Constant Boiling Pt. C

d-Limonene 2.3 175-178Cineole 4.57 176-177Menthone 8.8 210Menthol 4.0 217Pulegone 9.5 224Menthyl acetate 7.07 227-229Dihydrocarvone 8.5 221.5Carvone 11 227-231Water 80 @ 68F, 55.3 @ 212F; 1.00 steam 100

Source: Langes Handbook of Chemistry 15 th ed, McGraw Hill 1999

Figure 10: Definition of Supercritical State(Brunner2005)


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strates. To remove the oil, the CO2 is de-pressurized and allowed to evaporate leaving the oiland other soluble components behind. Ideally the CO2 can be evaporated leaving oil withoutany residues but many of the highly volatile components will pass though a dry separator there-fore the CO2 is typically passed through a solvent such as ethanol to trap the volatiles. This willrequire secondary processes but results in higher yields. Supercritical CO2 solvent propertiescan be modified by changing the temperature and pressure to be selective for the components

that are desired to be extracted. It is possible to extract more constituents than steam distillationand may require additional processing if using a single stage system. Extraction process can bebatch or continuous flow with extraction times of 15 to 120 minutes depending on conditions.Figure 11 is an illustration of the flow schematic of a single stage processing with a supercriti-cal fluid.Continuous Flow SFEGeneral Foods developed a continuous process for SCF extraction of caffeine from coffee andreceived a patent for the process in 1989. Figure 12 is a flow schematic for the process. The de-sired amount of product flows through a valve and enters a small pressure vessel that is used to

charge the extraction vessel. The charging vessel has a valve on both the inlet and outlet so itcan be isolated. During filling the outlet valve is closed and the inlet is open. Once the desiredquantity of product is in charge vessel the inlet valve is closed and the pressure is increased us-ing the extraction solvent to the same pressure as the extraction vessel. The charging vessel out-let valve is then opened allowing the product charge to flow into the extraction vessel. At thesame time as the charging outlet valve is open, the valve at the bottom of the extraction vessel isopened to the discharge vessel, allowing an equal amount of spent material to exit the extraction

vessel. The discharge vessel is also pressurized before the valve is opened to the extraction ves-sel. The valve at the top and bottom of the extraction vessel are closed. The supercritical fluidsin the charging and discharge vessel are vented and the discharge vessel is emptied. The super-critical fluids are typically vented to a holding vessel and recycled. The discharge vessel can bevented to the charging vessel to conserve supercritical fluids. The valves and charging / dis-charge vessel act as air locks so product can be loaded and unloaded from the extraction vesselwithout disrupting the process. Rotary locks can be used in place of the two valves and charg-ing / discharge vessels but are more complex, cost more and generally require more mainte-nance. The residence time for extraction can be adjusted by the charging frequency and quantity

Separator

ChillerPumpHeater

EvaporatorThrottling

Valve

Extractor

Oil

Product

Feed

Figure 11: Flow Scheme of single stage processing with super-

critical fluids (Egger & Jaeger, 2003)Product in

Valve In

Valve In

Valve Out

Valve Out

Charging vessel

Discharge vessel

Extractor

CO2 Out to

separator

CO2 In

Spent product

Out

Figure 12: Continuous Supercritical extraction

process flow (adapted from Katz, 1989)


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and the vessel size. The flow of supercritical fluids is continuous during charging and discharg-ing and flows counter-current. This aids in maximizing extraction because supercritical fluidsfree of dissolved oils are passing through product that is at the bottom of the vessel that havethe lowest amounts of oils left to extract. The supercritical fluids are routed to a separator afterexiting the extraction vessel to separate the essential oil from the solvent. The CO2 is re-pressurized and recycled back to the extractor. Hughes Aircraft Company also has a similar pat-

ent issued in May 1994 (US Patent No. 5,313,965).

The temperature and pressure of extraction will have an effect on the oil yield from spearmint.Al-Marzouqi (2007) found in lab scale experiments that increasing the pressure from 15 MPa(2175 psi) to 35 MPa (5076 psi) increased the yield by 48.5% at 30C, 18.6% at 40C and17.6% at 50C. Increasing the temperature at a constant pressure also provided similar results;44.4% increase in yield raising the temperature from 30C to 40C and 13.1% increase in yieldraising the temperature from 40C to 50C. The project sourced mint from 5 different countriesand found a difference of up to 27% between the lowest and highest oil yields based on thesource. The quality of the extract was considered better with respect to oil composition ex-tracted with supercritical CO2 extraction at operating conditions of 30C and 15 MPa. Using

the major flavor components as quality markers, carvone and limonene, the oil produced withsupercritical CO2 extraction contained 11443 and 2453 g/g of dry material, respectively, com-pared to 10780 and 510 g/g of dry material, respectively, using steam distillation. Table 1shows the principal components for steam and supercritical CO2 extraction at 30C and 15MPa.zer, et.al. (1996) reported oil extraction yields from 23 to 80% from spearmint using super-critical fluid extraction (SFE) with CO2. The highest yields were at 40C at 11 MPa (1595 psi)with an extraction time of 4 hours. The lowest yields were at 60C at 8 MPa (1160 psi) with a 1hour extraction time. In this study lower yields were extracted as temperatures increase or proc-

ess time decreased. This report also compared the composition of the extracted oil compared tosteam distillation. Limonene percentages were 31% less with SFE (5.26%) than using steamdistillation (7.63%) but carvone components were 5.2% higher at 81.15% with SFE comparedto steam distillation at 77.13%. This contradicts more recent work by Al-Marzouqi (2007) whoreported a 79% increase in limonene extraction but similar increases in carvone at 6.1%. Al-

Marzouqi study used much higher pressures which might be the reason for higher recovery per-centages.

Barton (1992) compared peppermint and spearmint oil extraction from green plants and field-dried hay with supercritical fluid extraction with CO2 at various rates and time and steam distil-lation. Bartons results indicated that the CO2 extracted oil are strongly colored, dark yellowto greenish yellow but differed only slightly in specific gravity and refractive index. The yieldsof spearmint oil from field dried hay using SFE-CO2 match the yield from steam distillationwith process temperatures of about 34C and pressures of 10 MPa (1450 psi). An extractiontime of 4 hours was used for the experiment. The minimum extraction time was not determined.

Table 2: Mint oil composition (g/g on dry basis) (from Al-Marzouqi (2007))

* Supercritical Fluid Extraction

a-pinene Limonene Cineole Linalool Menthol Dihydro-carvone Carvone

Steam 0.0 510 54.1 35.0 28.2 21.4 10780

SCFE* 48.2 2453 95.6 33.4 22.5 27.8 11443


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Barton found that the CO2 flow needed to be 7.5 g CO2 per g dry hay using a single passdown-flow extraction.

Economics of Supercritical Fluid Extractionztekin and Martinov (2007) reported that a three 500 liter (132 gallon) vessels supercriticalfluid extraction system using CO2 for processing cloves, cumin, cardamom and ginger was esti-

mated to cost $2.2 million for capital and $4 to $6 million/year for operating costs. A compari-son by Pereira and Meireles (2007) for rosemary, fennel and anise estimated the manufacturingcost in Brazil using SFE compared to steam distillation to be 44%, 55% and 58% less, respec-tively. A student project at Rowan University in New Jersey, looked at the extraction of peanutoil using supercritical CO2 extraction and compared it to a hexane solvent process. They madeenergy and cost estimates for a 10 million pound per year processing facility with a 30% oilyield (3 million pound per year). Referring to Table 3, extracting peanut oil using supercriticalCO2 extraction reduced energy use and costs by 60% and 55%, respectively.Brunner (2005) reported the cost for a 1000 ton per year batch system is in the range of $1.36per pound of feed stock but economies of scale could reduce cost to about $0.25 per pound.Continuous flow systems could reduce costs further although continuous SFE extraction has

only been carried out at the lab scale so far. Lack and Seiditz (2001) reported cost estimates for

the decaffeination of coffee beans from $0.50 to $0.65 per pound for a 3500 ton per year capac-ity system to $0.33 to $0.45 per pound for a 7000 ton per year capacity system but warns thatcost estimates can vary by 30%. The cost breakdown excluding raw material costs are as fol-lows: Interest and depreciation = 36.1%; labor = 24.5%; utilities = 17.2%; taxes = 20.5%; ad-ministration = 1%. Capital costs are about one third of the total processing cost.

Pressurized Fluid ExtractionMany terms are used to describe this technique: pressurized fluid extraction (PFE), acceleratedsolvent extraction (ASE trademarked by Dionex), pressurized liquid extraction (PLE), pres-surized solvent extraction (PSE) or enhanced solvent extraction (ESE) (Camel 2001). Thismethod is similar to supercritical fluid extraction in that it maintains a solvent in a liquid state at

an elevated temperature with pressure. Using this method, temperatures between 200 300Cmay be used with common organic solvents. The decrease in viscosity at elevated temperaturesaids in the solvent diffusing into materials and increases the solubility of compounds into thesolvent. The high pressure also aids in the solvent penetration in to compounds. Extraction timeis 5 10 minutes and has been used as a lab bench test for recovery of environmental contami-nates. Higher levels of moisture in soil samples inhibited the extraction of pesticides whichwould indicate the need for dry hay if used for mint oil extraction but high temperature may de-grade oil.

Table 3: Costs for peanut oil processing with supercritical CO2 extraction (Gifford, 2001)

Solvent type CO2 HexaneSolvent cost $0.07 / lb. $0.07 / lb.Solvent use 87 million lbs per year 38 million lbs per yearEnergy input 1.8 GWh/yr 4.6 GWh/yrOperating cost $ 6,200,000 14,000,000Energy per lb of oil 0.6 kWh/lb 1.53 kWh/lbCost per lb of oil $ 2.07 / lb $ 4.67 / lb


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Subcritical Water ExtractionSubcritical Water Extraction uses superheated water at elevated temperatures (100C to 374C)and pressures high enough to maintain liquid state. Under these conditions water is a low polar-ity extraction solvent. Advantages reported by research:

- Higher extraction of polar compounds- avoids extraction of waxes and lipids

- faster, cheaper, cleaner (solvent free)- environmentally friendly- nontoxic- lower pressures than supercritical fluid extraction- cheaper equipment than supercritical fluid extraction- more efficient extraction qualitative compounds contain higher oxygenated com-

pounds and fewer terpene fractions- better representation of natural aroma- eliminates drying stage- selective extraction based on temperature selection

Subcritical water extraction uses hot water under pressure to maintain the water in a liquid stateto act as a solvent. It may not be suitable for thermally sensitive oils. Kubtov et.al. (2001)reported nearly complete decomposition of linalool and -terpinene at temperature of 175Cduring oil extraction from peppermint. Water is corrosive under supercritical conditions and candamage equipment but can be prevented by using ultra pure and degasified water. This requiresadditional equipment and cost.

Kubtov did extraction rates at different temperatures and showed 90% or higher extractionrates for all major components except menthol acetate at 125C. At 150C all components werenearly at 100% extraction in less than 25 minutes with the exception of menthol acetate whichwas only 30% extraction. Higher temperatures and longer times would be needed to obtain

higher yields of menthol acetate. For peppermint, 30 minutes at 150C or 12 minutes at 175Cresulted in similar extraction quantities of carvone, pulegone, piperitone, eucalyptol,menthone, neomenthol and menthol compared to 1 hour of supercritical fluid extraction and 4hours of hydro-distillation.

Instant controlled pressure drop technologyThis method is based on a patent by a French group. A U.S. Patent of the process, No.5,855,941, was granted in 1999. The process involves heating dry plant material (11% mois-ture) with steam for a short period, followed by an abrupt pressure drop to a slight vacuum (5kPa). This sudden pressure drop causes vaporization of fluids and the breaking of cell wallswhich aids in releasing essential oils. This process is repeated 2 to 6 times at saturated steam

pressures of up to 0.6 MPa (85 psi) with heating time of 0.5 to 20 minutes. Higher pressuresand the number of cycles resulted in increased oil recovered from Cananga flowers. A 4 minutecycle resulted in 2.74% oil yield versus 2.60% for a 24 hour steam distillation process, a 5%yield increase. This work has also been done for other crops and typically results in higher oilyields with shorter processing times. The processing of Myrtle leaves resulted in 10% higher oilyields with a 2 minute process time compared to 180 minutes with hydrodistillation (Berka-Zougali 2010), processing Lavandin resulted in an 84% increased oil yield in 8 minutes(Besombes 2010) and Rosemary oil was processed in 10 minutes, recovering 91 to 97% of theoil compared to more than an hour with hydro-distillation (Rezzoug 1998). This process has


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the advantage of fast processing compared to hydro-distillation, selective extraction compared tosupercritical extraction, no solvent residue, low energy and water consumption (662 kWh and 42kg per ton of raw product, respectively) and produces high quality oil. The disadvantage is that ithas not yet been commercialized and is a batch process. Using multiple vessels could simulate acontinuous process. Another concern is fatigue stress on vessels from the constant cycling. Nopublished data was found on the extraction of mint oil.

Operation The plant matter is placed in vessel 1 and heated with direct saturated steam (F1)and steam through a heating jacket (F2) on pressure vessel to increase the temperature and pres-sure. This step can last from several seconds to minutes but preferable not more than one minute.Valve V2 is closed during loading and heating. Vessel 2 is evacuated to a vacuum of 0.05 MPa(-7.5 psi) with vacuum pump (3). Valve 7 is closed, valve V2 is opened allowing the pressure todrop by 0.6 to 0.9 MPa (90 to 135 psi) in approximately 0.5 seconds. This causes the water andoil in the plant matter to vaporize. Valve V2 is closed and the heating and pressurization step isrepeated followed by valve V2 opening to depressurize the system. This is repeated the numberof cycles specified for the product, typically two to six cycles. Cooling water flows (F3) into awater jacket on vessel 2 to condense the water and oil vapor, Valve V4 is opened to allow the

water and oil to flow into a Florentine type vessel to separate the water and oil. The water is re-moved through V6 and the oil recovered through valve V5.

Moderate Electric Field Extraction (MEF)MEF processing applies a voltage across a food material to rupture cell membranes and increasepermeability. Sensoy and Sastry (2004) experimented with black tea and dry and fresh mint.Samples were placed in breaker with a salt solution and electrodes on opposite sides. The saltsolution was used to improve the electrical conductivity and was varied depending on the volt-age used. Voltages ranged from 200 to 1000 volts. Three different frequencies were used: 50,500, 5000 Hz. The method was compared with hot water extraction and found that there wasnt a

Figure 12: Schematic diagram of the in-stant controlled pressure drop DIC appa-ratus: (1) autoclave with heating jacket;(V2) rapid valve; (2) vacuum tank withcooling water jacket; (3) vacuum pump;(4) extract container; F1 & F2 steamflow; F3 cooling water flow.(From Besombes 2010)


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difference when using dried tissue but the MEF process on fresh mint resulted in an 80% in-crease in total solids extracted. The extracted solids would include oils along with other sub-stances. There was no significant difference in solids extracted as voltage was increased and adecrease in solid extract at frequencies of 500 and 5000 Hz compared to 50 Hz. Heating occurswhile the samples are subjected to MEF. The process temperatures started at room temperature(25C) and the test was concluded when the solution temperature reached 80C. The process

could be done in a continuous process by pumping a slurry of the plant tissue and brine solutionthrough a pipe past a power source. The technique needs further study to determine its effi-ciency at extracting mint oil compared with current methods.

High-Voltage Electrical Discharge (HVED)High-Voltage Electrical Discharge (HVED) as described by Grmy-Gros, et.al. (2008) passesan electrical charge through plant materials that is submerged in an aqueous solution. The elec-trical charge travels through the plant material causing cells to rupture, pressure pulses and pro-duce oxidative chemistry. The electrical pulse is applied for only a few microseconds whichavoids overheating. HVED has been used in experiments to enhance extraction of mucilage ex-traction from whole linseed, solutes from tea leaves and moonflower roots and oil from linseed

cake. Extraction times for fennel gratings with HVED were about 20 minutes compared with 40minutes with MEF and 200 minutes for ultrasonic assisted extraction. All methods resulted in aextraction rate of about 97% compared to extraction without treatment resulted in 60% extrac-tion in 20 minutes. Similar work was done by Dobreva (2010) on rose pedals. Dobreva calledthe method Pulsed Electric Fields (PEF) which subjected rose pedals to electric fields of 1-5kV/cm at a specific energy of 5-20 kJ/kg. The results increased essential oil yield by 13-33%over distillation alone and reduced distillation times from 2.5 to 1.5 hours. No undesirablechanges in the properties of the oil were observed.

Oil CompositionOne of the criteria for using a different extraction method is that the oil quantity as well as qual-

ity of oil must meet the requirements of producers and consumers. The tables 4 and 5 comparethe MIRC criteria for the mint oil components reported in different studies sited in this paper.The orange highlighted cells in the tables indicate study values that are below or above theMIRC criteria (first row of the table). If a study published the oil composition for a control dis-tillation method (steam or hydro-distillation), it is listed above the alternative extractionmethod.

In Table 4 Peppermint oil composition, all of the values for the three studies sited are withinthe MIRC criteria except for Menthol which is 1-2% points less than the minimum for all stud-ies except for the microwave extraction study by Hackleman (2009). Hacklemans Microwavestudy was also below the MIRC criteria levels for Limonene and Cineole but this was likely

because the study resulted in incomplete oil extraction (11%). Menthyl acetate was low by lessthan 1% points for the study by Barton for both the control using steam and the supercriticalfluid extraction method which may reflect the plant material used.

In Table 5 Spearmint oil composition, the ultrasonic study by Da Port (2009) reported signifi-cantly low values for Limonene but higher values for Cineole than the control or the MIRC cri-teria. The hydro-distillation control in Da Ports study was 8% points lower than the MIRC cri-teria for Carvone but the ultrasonic extraction method was 21% points higher and within theMIRC criteria. The Carvone content for Lucchesi (2004) control (hydro-distillation) was 6%


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SpearmintMentha spi-

cataLimonene Cineole Menthone Dihydrocarvone Carvone

MIRC Critera 7 - 22 % 0.5 3 %


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points lower than the MIRC criteria but the results for the solvent-free microwave extractionwere within the MIRC criteria.

Mint Oil Distillation Energy Survey Summary

MRIC collected energy data from a small sample of growers on two occasions and supplied thedata pertaining to the mint oil distillation. The first set contained 6 surveys of which 4 were us-

able and the second set contained 17 surveys of which 15 were usable. Based on surveys pro-vided, the oil yield in pounds per acre ranged from 45 to 180 with an average of 82.4 poundsper acre while the energy use for steam distillation ranged from 41,325 Btu per pound of oil to265,574 Btu per pound of oil, see Appendix A. Three farms reported value for spearmint andthe rest distilled peppermint. The high energy data point was an outlier in the data set being al-most twice the next highest value and was omitted from the data summary. This grower wasburning used motor oil, the only farm that reported the use of used motor oil. The grower dis-tilled spearmint and reported lower than average yields. Excluding the one farm from the sur-vey, the range for distillation energy use would be 41,325 Btu per pound of oil to 145,635 Btuper pound of oil. The low distillation energy use system was from a farm with average yield peracre using natural gas. Three farms that reported multiple fuel sources. Two of the growers

switch between diesel fuel and natural gas during the season. The other farm has two distillationsystems. The high value of 145,635 Btu per pound of oil was from a farm in the first survey us-ing diesel fuel. The grower reported distillation energy use for peppermint and spearmint at143,556 and 145,635 Btu per pound of oil, respectively. The average distillation energy use was79,509 Btu per pound of oil. One might expect that as oil yield per acre increased the distilla-tion energy use might decline but based on Figure 13 there doesnt appear to be any relationship(trend line is nearly flat). There was a difference in efficiency depending on the fuel type used.The average of the two growers who use Propane was 68,079 Btu per pound of oil, the lowestof the four fuels. Eight systems used natural gas fired distillation systems with and average effi-ciency of 73,295 Btu per pound of oil, 7.6% higher and eleven systems used diesel fuel with an

Figure 13 - Oil Yield per Acre versus Distillation Energy Use

0

20

40

60

80

100

120

140

160

180

200

20,000 40,000 60,000 80,000 100,000 120,000 140,000

Distillation Energy Use (Btu/ ac)

OilYield

perAcre(lbs)


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average efficiency of 98,809 Btu per pound of oil or 45% more energy. The decrease in effi-ciency might be due in some part to boiler maintenance and the tendency for fuel oil to foulheat exchanger surfaces. Natural gas and propane generally burn very clean.

If a distillation system is operating at 100 to 120 psi steam pressure and the boiler efficiency is

80% (typical value for well maintained boiler) then it will require approximately 1500 Btu toproduce a pound of wet steam. If the heat exchanger is fouled with soot or the boiler is poorlyadjusted, the efficiency may drop to 60% and require 2000 Btu to produce the same pound ofsteam. The difference in energy use could also be the result of running the distillation systemlonger than necessary to harvest the mint oil from a loaded tub. How the tub is loaded and pos-sibly the length of cut could possible effect the distillation time as well.

When comparing different fuel types of energy sources it is necessary to convert all to the sameunit of measure. For energy in the English measurement system, British Thermal Units (Btu)are use. The definition for a Btu is the amount of energy required to increase the temperature ofa pound of water by one degree Fahrenheit. Different fuel types contain different quantities of

energy. A gallon of propane or liquid petroleum gas contains about 91,500 Btu per gallon whilediesel fuel contains 138,700 Btu per gallon. Therefore it takes 1.5 gallons of propane to equalone gallon of diesel fuel on an energy basis. Natural gas is sold in units of Therms or cubic feet.A Therm is defined as 100,000 Btu and a cubic foot of natural gas generally contains about1030 Btu but can vary from about 950 to 1100 Btu per cubic foot. Thus it takes 1.39 Therms ofnatural to provide the same energy as 1 gallon of diesel fuel.

For a future survey it would be advantageous to find out the number of cutting per year andthe oil yield and energy use per cutting. From the wide range of efficiency reported, there islikely substantial energy savings from doing routine boiler maintenance / tuning and using bestmanagement practices for majority of growers.


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Summary:Energy data for the different method of oil extraction are sketchy in the literature so side-by-side energy comparison could not be made for most methods. Most of the methods discusswould likely reduce the energy input but may not necessary be economical because of highercapital costs, larger use of electricity and limited number of hours of operation per year. Someof the extraction methods have been develop mainly for lab analysis and are not practical for

large production volumes. Some of the methods would not be conducive for on-farm processingbecause of the specialized equipment, flammable solvents, or high pressure vessels. A coopera-tive could be formed to share the capital costs and employ the technical talent to operate theequipment but capacity would be an issue since all of the mint would be ready to harvest at ap-proximately the same time. The two methods that appear to have the best chance of success atthe farm level would be a continuous steam distillation system or the solvent-free microwaveextraction. Both are continuous processing methods that would lend themselves to better proc-ess control, heat recovery, un-attended/automatic operation and lower capital costs. A continu-ous steam distillation system would need to be developed based on the existing patents and ad-ditional test runs would be needed to develop and refine the process for a microwave system.Based on processing parameters published by Bouchard and Serth (1991) for cedarwood andassuming mint could be processed in this system with the same parameters, the energy to ex-tract a pound of mint oil could be reduced by 60 to 70% with a 25 second process time. Thedrawback is the capacity would need to be increased to meet the needs of an average farm.Based on a 10 day, 10 hours per day harvest window and 4 tons per acre, the processing capac-ity would need to be 20 tons per hour versus the current capacity of 13 tons per day.

The microwave system would have a longer process time than the continuous steam system de-scribe by Bouchard and Serth (1991) ranging in the 1 to 3 minute processing time based on thedemonstration work by Hackleman (2009). The energy cost between batch steam distillationand microwave may not be any less expensive base on Velasco (2007) who reported steam dis-tillation energy costs of $1.26/lb of oil compare to $1.22/lb of oil for microwave at optimal con-ditions based on bench scale testing. If settings werent optimal, energy costs for microwavecould be much higher. The oil composition does not seem to be adversely affected by themethod of extraction. More work will need to be done to work out the process parameters formicrowave.

Table 6 lists the different extraction methods review and lists the process times and relativecapital cost. The methods with the shortest process times are the continuous steam, instantane-ous pressure drop technology and solvent-free microwave. There is little information on thecapital costs for the different methods in the literature. Capital costs for commercial scale sys-tems are few. Hackleman (2009) estimated the cost of a microwave system equal to a two orthree station conventional steam system at $400,000 while a supercritical CO2 extraction sys-tem is estimated at $2 million for a system with two 132 gallon extractors, likely not largeenough to process an acre per day.


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Table 6: Process type, extraction method, process times

Process type Method type Process time

Steam / Hydro distillation Batch 2+ hours

Continuous steam Continuous 25 sec 8 min.

Solvent Extraction Batch or continuous

Microwave Assisted SolventExtraction

Batch or continuous 20 minutes

Ultrasonic Assisted SolventExtraction

Batch (possible continu-ous)

10-15 minutes

Solvent-Free Microwave Continuous 1 to 3 minutes

Microwave Steam Distilla-tion

Batch 6 minutes

Supercritical Fluid Extraction Batch orContinuous Batch

60 minutes +

Subcritical Water Extraction Batch 12 30 minutes

Pressurized Fluid Extraction Batch 5-10 minutes

Instant controlled pressuredrop technology

Batch 0.5 to 20 minutes

Moderate Electric Field Ex-traction (MEF)

Batch or continuous 2 to 10 minutes

High-Voltage Electrical Dis-charge

Batch or continuous 20 minutes


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Vinatoru, M., M. Toma, O. Radu, P.I. Filip, D. Lazurca, T.J. Mason (1997), The use of ultra-sound for the extraction of bioactive principles from plant materials, Ultrasonics Sonochemis-try, Vol. 4, pg 135-139.


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Appendix AMint distillation energy estimates 8/12/2011

S.Sanford

September 2010 PDF document

Surveys # of mint oilFueluse Type fuel

Minttype Fuel/# Btu/# Comments

1 27536 28500 D P 1.0350 143,556

1 40000 42000 D S 1.0500 145,635

2 60392 NG P 0.5670 56,700

3 NG 0.4738 47,380 ^ Cu.Ft to Thermcorrection

4 32630 19960 NG P 0.6117 61,171

2 surveys could not be used

May 2011 PDF documentSurvey

# #/acre oil Fuel/acre Fuel/# Btu/#

1* 80 NG P 0.8989 89,888

20 D P 0.2247 31,169

89 P 121,056

2 56.3 35 D P 0.6217 86,226

3* 80 NG P 0.7273 72,72720 D P 0.1818 25,218

110 97,945

4 85 Custom Distilled off-site

5** 45 43.26 NG P 0.9613 96,133^ Cu.Ft to Thermcorrection

45 25 D P 0.5556 77,056

6 67.9 50 p P 0.7364 67,378

7 90 54 D P 0.6000 83,220

8 75 57.5 D P 0.7667 106,337

9 100 95 D P 0.9500 131,765

10 85 63.894 p P 0.7517 68,78011 86 40 D P 0.4651 64,512

12 83 34.3 NG P 0.4133 41,325

13 51.3 33.166 NG P 0.6465 64,651^ Cu.Ft to Thermcorrection

15 73 40 D P 0.5479 76,000

16 100 Can't use gave $ not amount 104 #/acre

17 180 68 D S 0.3778 52,398

79,509 Average

Outlier

14 61 120 UMO S 1.9672 265,574

* Growers switched fuels during season. ** Grower has two distillation systems^ Give as cu-ft/ac used 1.03 correction to estimate Therms

Mint Types Assumed Energy Values

P - Peppermint AbbreviationFuelType Btu/unit

Fuelunits

S - Spearmint D Diesel 138700 gallons

NG Natural Gas 100000 Therms

p Propane 91500 gallons

UMO Used Motor Oil 135000 gallons


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Documents

A Review of Essential Oil Extr Action Technologies