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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.