Supercritical Fluid

Extraction

Ali Ahmadpour

Chemical Eng. Dept.

Ferdowsi University of Mashhad

2

Contents

Introduction

SCF state (physical & chemical properties)

Properties of NCF solutions (solubilities, EOSs, diffusivities)

NCF efficiencies (extraction & separation stages)

Equipment & techniques

Applications

3

References

Supercritical Fluid Processing of Food & Biomaterials, S.S.H. Rizvi, 1994.

Supercritical Fluid Extraction, M. McHugh, 1995.

Supercritical Fluid Extraction, L.T. Taylor, 1996.

Natural extract using supercritical carbon dioxide, M. Mukhopadhyay, 1996.

4

Introduction

First experimental work: 1879 by Hannay &

Hogarth

Commercial application: 1960

Advantage: High selectivity & solubility by

changing P & T

5

What is Supercritical Fluid Extraction?

Supercritical Fluid Extraction is the process of extracting

components such as oils and resins from solids utilizing the

special properties of CO2, or other solvents.

When these solvents are cooled and compressed, they have

the density of a liquid and the dispersion properties of a gas.

This is then able to penetrate the product and dissolve the

oils or other components wanted.

When the pressure is reduced, the solvent will then either

evaporate harmlessly into the atmosphere, or be recovered

and recompressed.

The end results leaves only the concentrated extract.

6

SCF process

7

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.

8

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

9

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 (Tc and Pc, respectively).

At critical conditions for P and T, there is no sudden

change of component properties.

10

The phase diagram of a single substance

11

P-T phase diagram of CO2

12

Phase diagram of SCF CO2

13

Phase diagram of a pure material and thermodynamic

state of various separation processes

14

Disappearance of meniscus at

the critical point

15

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.

16

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.

17

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.

18

Physico-chemical properties of SCFs

Above the critical temp., the pure gaseous component

cannot 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 best

described as intermediate to the two extremes.

This phase retains solvent power approximating liquids

as well as the transport properties common to gases.

A comparison of typical values for density, viscosity and

diffusivity of gases, liquids, and SCFs is presented in

Table 1.

19

Table 1. 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

20

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 extraction

Disadvantages• Elevated pressure required

• Compression of solvent requires elaborate recycling measures to reduce

energy costs

• High capital investment for equipment

21

Molecular Basis of SFE

22

SCF Process )fractionation system(

23

NCF CO2 plant

24

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

25

Comparison of physical properties of air, water,

mercury (at 298K, 1bar) and SCF CO2

Density Viscosity Kinematics viscosity

26

Useful SCFs with critical parameters

Fluid TC (K) PC (bar)

Carbon dioxide 304.1 73.8

Ethane 305.4 48.8

Ethylene 282.4 50.4

Propane 369.8 42.5

Propylene 364.9 46.0

Trifluoromethane 299.3 48.6

Chlorotrifluoromethane 302.0 38.7

Trichlorofluoromethane 471.2 44.1

Ammonia 405.5 113.5

Water 647.3 221.2

Cyclo-hexane 553.5 40.7

n-Pentane 469.7 33.7

Toluene 591.8 41.0

27

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 petrolchemistry.

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

28

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

29

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 low 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.

30

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.

31

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.

32

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

33

Properties of NCF solutions

Solubilities:

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.

34

Cont.

General principles about solubilities:

Effect of molecular structure

Effect of temp. and pressure

Effect of entrainers

35

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.

36

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.

37

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.

38

Properties of NCF solutions

Diffusion 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.

39

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.

40

Relationship between (a) sublimation of

a pure solid & (b) dissolution in an NCF

Solvent: 1 Solute: 2

Pure solid: ' NCF: ''

x ' = mole fraction of

solid in solid phase

y '' = mole fraction of

solid in NCF

41

Cont.

At equilibrium: T'=T''=T

P'=P''=P

fi'=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 (S) at T & P: f2' (T,P)= Ф2S P2

S (T)

At high pressure (poynting correction):

V2: molar volume of pure solid 2

P

P

2S

2

S

22 S2

dpRT

Vexp)T(P)P,T(f

42

Cont.

Assume incompressible solid:

Therefore, the fugacity of the pure solid phase at the system T and P can

be obtained from sublimation pressure and molar volume data.

For phase equilibrium f2'=f2''

PyRT

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

S

22S

2

S

2

RT

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

S

22S

2

S

22

(1)

RT

)]T(PP[Vexp

P

)T(Py

S

22

2

S

2

S

22

43

Cont.

Since: Ф2S =1 (ideal)

Solution of eqn. (2) requires an EOS:

Van der Waals:

P

0n,P,T2

2 dpP

RT

n

V

RT

1ln

2

(2)

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

2V

a

bV

RTP

44

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 solvent

circulation rates and results in high recompression

costs which may make the process unattractive.

45

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)

46

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.

47

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.

48

Extraction of solute from a slab

Shrinking core model Free diffusion model

49

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: solubility

C: concentration of solute in the matrix (solute/pore volume)

φ: fraction of pore space (porosity)

Mt & M: mass of solute extracted at time

: volume ratio of solvent to slab

qn: positive non-zero roots of tanqn= - qn

2

2

neff

1n2

n

2

t

L

tqDexp

q1

121

M

M

b

eff

DD(Carmen-Haul)

Db: bulk diffusivity

τ: tortuosity

50

Shrinking core model

It happens when: S<< C/φ

The fractional extraction of solute from a infinite slab of half-

thickness (L) is:

CL

DSt2

M

M2

t

51

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.

52

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;

1K

DD ads

ads

eff

53

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 minimum 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.

1K

SC

ads

max

54

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.

55

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

56

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.

57

Equipments and experimental

techniques in NCF Extraction

and Fractionation

58

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.

59

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 which 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.

60

Pilot plants with recirculation

Designed for

testing application

on a relatively small

scale (<10kg).

61

With 10-100 ml

scale.

It is not necessary

to recover and

recirculate CO2

Small pilot plant with total loss of CO2

62

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.

63

Cascades of separation vessels

P1>P2>P3

Or

T1<T2<T3

64

Zosel’s hot finger fractionation column

Increasing temp.

results in a drop in

solubility.

65

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.

66

SCF applications

Extraction was the first commercial use, in the extraction of hops and the decaffeination of coffee. Several research papers have been produced on a wide range of natural products, including high value pharmaceutical precursors. Advantages: speed due to rapid diffusion, less pollution in the working and general environment, less solvent residues in products, less solvent disposal costs.

Fractionation of liquid mixtures can be achieved by countercurrent extraction and this can be improved by imposing a temperature gradient on the column which causes refluxing to occur. It is largely applied to natural products such as essential oils and lipid products and can be used to concentrate substances prior to chromatography. Advantages: countercurrent extraction with reflux can be carried out in one unit, less pollution, no solvent residues in products, no solvent disposal costs.

Chromatography can be applied to high value products and chiral separations. Efficient simulated bed units are available. Advantages: narrower peaks and more efficient separations due to rapid diffusion, no solvent residues in products, less solvent disposal costs.

67

Cont.

Chemical Reactions are being researched with some in production. Advantages: product control and ease of product separation, more rapid reaction in diffusion-controlled, heterogeneous and enzyme reactions due to rapid diffusion, less pollution in the working and general environment, less solvent disposal costs.

Metals Processing, including extraction and separation and clean-up, using complexing agents in the fluid. Advantages: speed due to rapid diffusion, efficient separation processes, less pollution in the working and general environment, less solvent disposal costs.

Impregnation and Dyeing of polymers and synthetic fibres is established and the dyeing of cotton is being researched. Advantages: considerable reduction in water pollution from dyeing.

Particle Formation in the micron range with a narrow size distribution can be carried out, with the option of coating particles. Advantages: less solvent residues in products, degradation by heating during milling avoided.

68

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

69

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.

3) The high vapour pressure of CO2 enables it to be removed without losses.

4) Undesirable component are not extracted

70

Large Scale Systems

Natural Products

Rose oil residue

Essential oil extraction

Flavors and fragrances

Nicotine extraction

Natural pigment extraction

Pharmaceuticals

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

71

Pictures of SCF extractors

72

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.