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Supercritical Fluid

Extraction

Ali AhmadpourChemical Eng. Dept.Ferdowsi University of Mashhad

Contents Introduction SCF state (physical & chemical properties) Properties of NCF solutions (solubilities,

EOSs, diffusivities) NCF efficiencies (extraction & separation

stages) Equipment & techniques Applications

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References Supercritical Fluid Processing of Food &

Biomaterials, S.S.H. Rizvi, 1994 Supercritical Fluid Extraction, M. McHugh,

1994 Fundamentals of Food Process Engineering,

2nd edition, R.T. Toledo, Natural Extracts Using Supercritical CO2,

M. Mukhopadhyay, 2000

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Introduction

First experimental work: 1879 by Hannay & Hogarth

Commercial application: 1960

Advantage: High selectivity & solubility by changing P & T

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What is Supercritical Fluid Extraction?

Supercritical Fluid Extraction is the process of extracting oilsand resins from plants and herbs utilizing the specialproperties of CO2, or other solvents. When these solvents arecooled and compressed, they have the density of a liquid andthe dispersion properties of a gas. This is then able topenetrate the product and dissolve the oils or othercomponents wanted. When the pressure is reduced, thesolvent will then either evaporate harmlessly into theatmosphere, or be recovered and recompressed. The endresults leaves only the concentrated extract.

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SCF process

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SCF extraction characteristics SCF extraction may be done on solids or liquids. The solvent is a gas at conditions of T and P at which

the gas will not condense into a liquid phase. The gas has a density almost that of a liquid, but it is

not a liquid. The solubility of solutes in a SCF approaches the solubility in a liquid.

The low viscosity and near zero surface tension of the gas at the supercritical conditions are unique properties, which are superior to liquid solvents as an extracting agents.

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Cont. Among different gases, CO2 is selected as

a safe SCF solvent for food industry because of: Non-toxic Non-flammable Low critical temp. Low critical pressure Low cost Good solubility

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The Supercritical State A pure component is considered to be in a

supercritical state if its temperature and its pressure are higher than the critical values (Tcand Pc, respectively).

At critical conditions for P and T, there is no sudden change of component properties.

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The phase diagram of a single substance

١٠

P-T phase diagram of CO2

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Phase diagram of SCF CO2

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Phase diagram of a pure material and thermodynamic state of various separation processes

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Disappearance of meniscus at the critical point

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Phase transition of CO2 to SCF

Separate phases of carbon dioxide. The meniscus is easily observed.

With an increase in temperature, the meniscus begins to diminish.

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Increasing the temperature causes the gas and liquid densities to become more similar. The meniscus is less easily observed but still evident.

Once the critical temperature and pressure have been reached the two distinct phases of liquid and gas are no longer visible. One homogenous phase called the "supercritical fluid" phase occurs which shows properties of both liquids and gases.

Cont.

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Near critical fluid (NCF) The term “near -critical liquid (NCL)” used to

distinguish the state of a compressed gas just blow Tc from a “normal liquid” at NTP, for which T<Tc.

The term “near critical fluid” (NCF) will be used to represent both SCF and NCL state of compressed-gas solvents.

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Physico-chemical properties of SCFs

١٨

Above the critical temp., the pure gaseous componentcannot be liquefied regardless of the pressure applied.

In the supercritical environment only one phase exists.The fluid is neither a gas nor a liquid and is bestdescribed as intermediate to the two extremes.

This phase retains solvent power approximating liquidsas well as the transport properties common to gases.

A comparison of typical values for density, viscosityand diffusivity of gases, liquids, and SCFs is presentedin the next Table.

Comparison of physical and transport properties of gases, liquids, and SCFs.

Property Density (kg/m3)

Viscosity (cP)

Diffusivity (mm2/s)

Gas 1 0.01 1-10

SCF 100-800 0.05-0.1 0.01-0.1

Liquid 1000 0.5-1.0 0.001

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Advantages & Disadvantages of SFE

Advantages• Dissolving power of the SCF is controlled by pressure and/or temperature • SCF is easily recoverable from the extract due to its volatility • Non-toxic solvents leave no harmful residue • High boiling components are extracted at relatively low temperatures • Separations not possible by more traditional processes can sometimes be

effected • Thermally labile compounds can be extracted with minimal damage as

low temperatures can be employed by the extractionDisadvantages• Elevated pressure required • Compression of solvent requires elaborate recycling measures to reduce

energy costs • High capital investment for equipment

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Molecular Basis of SFE

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SCF Process (fractionation system)

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NCF CO2 plant

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Solvents of supercritical fluid extraction

The choice of the SFE solvent is similar to the regular extraction. Principle considerations are the followings: Good solving property Inert to the product Easy separation from the product Cheap Low PC because of economic reasons

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Comparison of physical properties of air, water, mercury (at 298K, 1bar) and SCF CO2

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Useful SCFs with critical parameters

Fluid TC (K) PC (bar)Carbon dioxide 304.1 73.8Ethane 305.4 48.8Ethylene 282.4 50.4Propane 369.8 42.5Propylene 364.9 46.0Trifluoromethane 299.3 48.6Chlorotrifluoromethane 302.0 38.7Trichlorofluoromethane 471.2 44.1Ammonia 405.5 113.5Water 647.3 221.2Cyclo-hexane 553.5 40.7n-Pentane 469.7 33.7Toluene 591.8 41.0

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Organic solvents are usually explosive so a SFE unit working with them should be explosion proof and this fact makes the investment more expensive. The organic solvents are mainly used in petrochemicals.

CFC-s are very good solvents in SFE due to their high density, but the industrial use of chloro-fluoro hydrocarbons are restricted because of their effect on the ozonosphere.

CO2 is the most widely used fluid in SFE. Beside CO2, water is the other increasingly applied solvent.

One of the unique properties of water is that, above its critical point (374°C, 218 atm), it becomes an excellent solvent for organic compounds and a very poor solvent for inorganic salts. This property gives the chance for using the same solvent to extract the inorganic and the organic component, respectively.

SCF solvents

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Density: In NCL phase, CO2 densities are typical of normal liquid solvent

(900-1000kg/m3). The SCF state of CO2 includes a wide range of densities from “gas-

like” values at low P (<100kg/m3) to “liquid-like” values at elevated pressure.

The region near the critical point has the highest compressibility. The solubility is directly related to the number of solvent molecules

per unit volume. Therefore, density is the key parameter in determining the effect of T & P on solubilities.

Above the critical point, solubilities have steep rise with P at constant T. Therefore, ability of controlling solubilities with P is one of the main features that distinguish NCFs from liquid solvents.

Physical properties of NCF CO2

٢٨

Viscosity: NCFs have high degree of molecular mobility (low viscosity) and

higher diffusivity than liquid solvents. To reach reasonable solubility, the density of NCF must be modest

(>400kg/m3). NCFs-CO2 usually have viscosities (600μP) when the density is around 770kg/m3.

Low viscosity of NCF provide several benefits in extraction processes: In leaching, it enables effective percolation of solvent through packed

bed and rapid penetration to the internal pore structure of particles. In extracting liquid, the NCF solvent dissolve in the liquid phase and

lower its viscosity. With high viscous liquid, part of mass transfer problem could be solved.

It facilitates solvent transfer and reduces pipeline dimensions in extraction plants.

Cont.

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Diffusion: In low viscosity media, diffusion is enhanced and

diffusion coefficients in NCFs are significantly higher than in liquid solvents (about 10 times).

At constant density, the diffusion coefficient is not greatly affected by T or P.

Cont.

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Volatility (vapor pressure): NCFs are highly volatile and can be completely

removed and recycled at low T. This has important implications for improving the quality of extracts, since: Highly volatile components in the extract are retained

(flavors and fragrances) The extract is not subjected to thermal or chemical

degradation at high T. High volatility ensures complete removal of solvent

residues.

Cont.

٣١

Although, CO2 is the safest medium in extraction, it undergo chemical reactions with water and need to be considered when extracting food materials.

One reaction is the dissolution of CO2 that gives carbonic acid. The acid is then dissociates and lowers the pH of the aqueous phase.

If acidity is problematic, it is possible to add bicarbonate anion. Also, the pressure of CO2 can be used to control the pH of water.

Another reaction of CO2 with water is the formation of solid hydrate below about 10˚C. This restricts the use of NCF CO2 in the extraction of aqueous systems to temps. Up to 10˚C higher than the freezing point of water.

Chemical properties of NCF CO2

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Properties of NCF solutionsSolubilities: Intermolecular attractive interactions must be

weak. Therefore, all NCFs are essentially non-polar solvents.

NCFs offer greater selectivity than liquid solvents. Any attempt at increasing solubilities by changing conditions or adding entrainers usually reduce selectivity. Therefore, opposing effects of selectivity and solubility should be optimized.

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Cont.General principles about solubilities: Effect of molecular structure Effect of temp. and pressure Effect of entrainers

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Cont.. Effect of molecular structure

In NCFs, the molecular structure of the solute is very important as small changes in MW and functional groups can affect solubility to a greater extent than with liquid solvents. Solubility is reduced by increasing polarity. Branching increases solubility. Unsaturation increases solubility. Aromaticity decreases solubility.

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Cont.. Effect of temp. and pressure

For liquid solvents, the pressure has very little effect on solubility. In NCFs, the effect of pressure is related to the solute-solvent interactions which depends on solvent density.

At very high pressures, the solubilities decrease with increasing pressure.

The solubility increases with increasing temp. at constant density.

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Cont.. Effect of entrainers

A liquid cosolvent (or entrainer) is sometimes added to NCFs to improve solubility level of polar or high MW substances.

Entrainers are liquid solvents (e.g. ethanol, acetone, ethyl acetate, …) that completely miscible with the NCF and added at low levels (<10%).

Although, they improve solubility, they reduce selectivity and introduce further operations for their removal.

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Properties of NCF solutionsDiffusion coefficients: Diffusion coefficients for solutes in NCFs are

significantly higher than in liquid solvents. Above the density of about 500 kg/m3, the solute

diffusion coefficients are of order 10-8 m2/s which is about an order of magnitude greater than that of liquid solvents.

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Cont.EOSs for solubility prediction: Among all theoretical methods, the solution phase

equilibrium using EOSs are most widely applied to predict the solubilities in NCFs.

The most familiar EOS is van der Waals. One limiting factor that restricts application of EOS

models for food materials is the lack of available data for the fundamental properties of pure components.

Another problem is the ambiguity of mixing rules and adjusted parameters in equations.

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Relationship between (a) sublimation of a pure solid & (b) dissolution in an NCF

Solvent: 1 Solute: 2Pure solid: ' NCF: '' x ' = mole fraction of solid in solid phasey '' = mole fraction of solid in NCF

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Cont. At equilibrium: T'=T''=T

P'=P''=Pfi'=fi'' (for all i)

Assume that NCF doesn’t dissolve in the solid: f2'=f2'' For ideal gas: fi= yi P In general: fi= Фi yi P For pure solid phase at T & P: f2' (T,P)= Ф2

S P2S (T)

At high pressure (poynting correction):

V2: molar volume of pure solid

Φ=′ ∫

P

P2S

2S22 S

2

dpRTVexp)T(P)P,T(f

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Cont. Assume incompressible solid:

f2'=f2'' ⇒

⇒

PyRT

)]T(PP[Vexp)T(P 22

S22S

2S2 ′′Φ ′′=

−

Φ

−

Φ=′RT

)]T(PP[Vexp)T(P)P,T(fS22S

2S22

(1)

−

Φ ′′Φ

=′′

RT)]T(PP[Vexp

P)T(Py

S22

2

S2

S2

2

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Cont. Since: Ф2

S =1 (ideal)

Solution of eqn. (2) requires an EOS:

Van der Waals:

∫

−

∂∂

=Φ ′′P

0n,P,T2

2 dpP

RTnV

RT1ln

2

(2)

)x,V,T(fP i= (3)

( ) 2Va

bVRTP −−

=

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Efficiency of NCF extraction The efficiency of solvent extraction process is

judged by: Rate of process operation Energy consumption

The rate-limiting step is most frequently the rate of mass transfer at the extraction stage.

This mass transfer resistance requires high solventcirculation rates and results in high recompressioncosts which may make the process unattractive.

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Cont.

The energy input is determined by: The conc. of solute in the NCF solvent leaving

the extraction vessel (determines the No. of cycles to complete the extraction)

The differential conditions of P & T between the extraction and separation vessels (determine the energy consumption per cycle)

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Extraction stage In leaching, the rate of solute removal for any solvent

depends on: The amount of solute in the particle Its distribution within the matrix The particle size and shape The geometry of the porous network

The solubility is a prime factor for determination of solvent effectiveness in an extraction process.

When solubility is low, it can also determine the extraction mechanism.

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Extraction mechanism

There are two different mechanisms in extraction: The free diffusion model The shrinking core model

In SCF processes, the solubilities are often low and shrinking core model exist.

In liquid solvent extraction, the common free diffusion happens.

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Extraction of solute from a slab

Shrinking core model Free diffusion model

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Free diffusion model It happens when: S>>C It is usually expressed as: S>>C/φ The fractional extraction of solute from a infinite slab of half-

thickness (L) is :

S: solubilityC: concentration of solute in the matrix (solute/pore volume)φ: fraction of pore space (porosity)Mt & M∞: mass of solute extracted at timeα: volume ratio of solvent to slabqn: positive non-zero roots of tanqn= -α qn

( )

−

α+α+α+α

−= ∑∞

=∞2

2neff

1n2n

2t

LtqDexp

q1121

MM

τϕ

= beff

DD(Carmen-Haul)

Db: bulk diffusivityτ: tortuosity

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Shrinking core model It happens when: S<< C/φ The fractional extraction of solute from a infinite slab of half-

thickness (L) is:

=

∞ CLDSt2

MM

2t

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Parameters in the shrinking core model

Solubility: The most important feature of the model is that the rate of extraction is determined not only by diffusion coefficient but also by the solubility of the solute. When the solubility is low, rate of extraction can be very low. Therefore, it is good for the controlled drug delivery.

Diffusivity: Usually higher diffusivity in SCF gives rapid extraction. This is true when the free diffusion model applies. Low solubility in SCFs often switch the mechanism to shrinking core in which the enhanced diffusion is offset by the solubility.

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Cont. Adsorption: The extent of extraction is determined by

adsorption energy and solvation energy. In leaching. The adsorption/desorption of solute can be accounted by a linear isotherm. Here, the diffusion coefficient is replaced by the modified one inversely related to the adsorption coefficient.

When Kads>1 Low extraction rate

solutediffusingfreelyofConc.soluteadsorbedofConc.K;

1KDD ads

adseff =

+=

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Cont. In fixed-bed extraction, in the absence of adsorption, maximum

conc. of solute in the solvent (Cmax) is given by the solubility. This determines the min. amount of solvent required.

The implications for extraction efficiency are twofold: More solvent is required for complete extraction (more operation cycles). A greater pressure is required as the level of solute in the SCF is low.

Both factors increase the energy consumption of the process.

1KSC

adsmax +

=

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Cont. Role of water: In the extraction of plant materials it is

found that the addition of water is essential to achieve a good extraction rate.

Water does not usually increase solubilities in SCF (different from entrainers).

Water affects the rate through rehydrating and swelling the internal cellular structure of died plant. It has two opposing effects: Increasing the particle size will increase the diffusion distance. Expansion of the internal structure will shorten the diffusion path

by opening channels. ٥٥

Cont. In some cases, water play a crucial role in determining not

only the rate but also the mechanism of the extraction. For example, dry tea contains 3% w/w caffeine. Its solubility

in SCF CO2 is low (S<<c/φ) ⇒ Shrinking core model The solubility of caffeine in water is significantly higher than

in CO2 ⇒ Free diffusion model

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Separation stage

Since the solubility of solutes in SCFs decline with decreasing pressure, the separation stage operates at low pressure ⇒ High energy consumption

One solution is to partition the solute to a coexisting solid (AC) or liquid phase in the separation vessel (at the same pressure as extractor). This is useful when the extract presents at low level (contaminants).

Another new idea for separation stage is crystallization. Pressure variation has rapid response and can be used to control the crystal size.

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Equipments and experimental techniques in NCF Extraction and Fractionation

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Major problems in SCF extraction

Two of the major problems of SCF extraction are: Channeling of fluid flow through the bed of solids Entrainment of the non-extractable component by the SCF.

The contact time is related to the solubility of the solute in the SCF and the rate of flow of the fluid through the bed of solids. A large quantity of solute is extracted within a reasonable length of time.

SCF penetration into the interior of a solid is rapid, but solute diffusion from the solid into the SCF may be slow and may contribute to the prolonged contact time needed for extraction.

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Extraction SCF extraction is done in a single-stage contractor

with or without recycling of the fluid. When recycling is used, the process involves

pressure reduction in a separator in witch the solid is separate by gravity and then the gas compress back to the supercritical conditions and recycled.

Temperature reduction may also be used to drop the solute and the solvent is reheated for recycling without the need for recompression.

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Pilot plants with recirculation

Designed for testing application on a relatively small scale (<10kg).

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With 10-100 ml scale.It is not necessary to recover and recirculate CO2

Small pilot plant with total loss of CO2

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Fractionation In order to fractionate mixtures it is more efficient (in

terms of time) to employ equipment specifically designed for this purpose.

In cascade configuration, the CO2 stream passes to the first separation vessel where the condition P1,T1 are set to precipitate the first fraction of least soluble components. The output stream from the first separator is then passed to a second vessel at lower pressure P2 where the second fraction precipitates.

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Cascades of separation vessels

P1>P2>P3OrT1<T2<T3 ٦٤

Zosel’s hot finger fractionation column

Increasing temp. results in a drop in solubility.

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Industrial applications

Supercritical extraction offers many advantages such as high purity, low residual solvent content, and environment protection.

It has also some disadvantages such as high capital cost, high pressure and low solubilities.

Several applications have been fully developed and commercialized.

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Applications in the food industries Decaffeination of coffee and tea Extraction of essential oils (vegetable and fish oils) Extraction of flavors and fragrances from natural

resources Extraction of aroma and ingredients from spices and

red peppers Extraction of fat from food products Extraction of vitamin additives Production of cholesterol-free egg powder De-fat potato chips

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Examples

Almonds, Anise, Basil, Bergamot, Black pepper, camomile, Caraway, Cardamon, Cinnamon, Clove, Coriander, Cumin, Fennel, Ginger, Hop, Horseradish, Juniper, Lemon, Lime, Lovage, Mace, Marjoram, Nutmeg, Orange, Paprika, Parsley, Patchouli, Peppermint, Pimento, Red pepper, Rosemary, Sage, Sandalwood, Thyme,Vanila.

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EXTRACTION OF FLAVOURS AND FRAGRANCES

Flavours and fragrances are conventionally isolated from botanical sources either as an absolute using solvent extraction or by steam distillation.

The main drawbacks to these methods are thermal degradation, loss of volatile and indiscriminate separation of high molecular weight components.

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The main advantages

1) The extraction and separation can be carried out at low temperature in an inert environment thereby avoiding thermal damage and chemical degradation.

2) The extract has improved solubility in formulations since less terpenes (C10H16) are extracted.

3) The high vapour pressure of CO2 enables it to be removed without losses in the highly “top notes”.

4) Undesirable component such as proteins, waxes, sugars, chlorophyll are not extracted.

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Large Scale SystemsNatural Products Rose oil residue Essential oil extraction Flavors and fragrances Nicotine extraction Natural pigment extractionPharmaceuticals Synthetic drug production Separation of isomers Ethical drug purification Enzyme catalyzed reactions Residual solvent removal Drug micro particle

crystallization

Foods Hops extraction Decaffeination Cholesterol from butter Fatty acids from barley

Seed oil extraction

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Pictures of SCF extractors

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SFE applications list Extraction of Fat from Liquid Milk Products and Infant

Formula Extraction of PCBs from River Sediment Fat Extraction from Chocolate Products, Cocoa Powders,

and Chocolate Liquors Extraction of Oil from Oilseeds Extraction of Fat from Animal Feed Extraction of Binders from Ceramics and Powder Injection

Moldings Extraction of Fat from Meat Extraction of Rosehip Seed Oil

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Extraction of Total Petroleum Hydrocarbons (TPH) from Soil Extraction of Lycopenes from Tomato By-products Extraction of Drugs and other Chemical Residues from Tissues

using SPE Trapping Techniques Extraction of β-agonists from Bovine Liver Tissue Extraction of Irganox 1076 from Polystyrene Extraction of Antioxidant from High Density Polyethylene Recovery of alpha tocopherol Removal of water from fatty acid/trigliceride mixtures Separation of oil from fried chips

Cont.

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Extraction of Sulfonamides from Chicken Liver using Supercritical Fluoroform

The Micronization of Drug Particles by the Rapid Expansion of Supercritical Solution

Precipitation of Protein Powders into a Compressed Anti-Solvent Precipitation of Protein Powders by Gas Anti-Solvent (GAS) Drying of Aerogels Impregnation of Organometallic Silver into Polyimide Film Oil Reduction of Whole Pecans Selective Extraction of Fatty Acids and Carotenoids from

Microalgae Recovery of Nitrosamines from Frankfurters

Cont.

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Supercritical water Water has obvious attractions as a solvent for clean chemistry. Both near-

critical and supercritical water (SCH2O) have increased acidity, reduced density and lower polarity, greatly extending the possible range of chemistry which could be carried out in water.

SCH2O can be applied most effectively for organic synthesis leading to useful products.

As water is heated towards its critical point (Tc=374°C, Pc=218 atm.), it undergoes a transformation considerably more dramatic than that of most other substances.

It changes from the polar liquid to an almost non-polar fluid. The change occurs over a relatively wide temperature range; even at 200°C, the density drops to 0.8 g/ml and, at Tc, the fluid becomes miscible both with organics and with gases. Diffusivity increases and the acidity is enhanced more than would be expected purely on the basis of higher temperatures.

A major research effort has been focused on the total oxidation of toxic organics and hazardous wastes in SCH2O, "incineration without a smokestack". The process is highly effective but there can be serious problems of corrosion associated with large scale waste destruction.

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