Lecture note : SFC/SFE ????? ???????

1
Dong-Sun Lee/ CAT-Lab / SWU
Chapter 33 Supercritical Fluid Chromatography
2
Supercritical Fluid Chromatography (SFC) In
supercritical fluid chromatography (SCF) the
mobile phase is a supercritical gas or a near
critical liquid. Compared to gas chromatography,
where gas is under ambient pressure, and liquid
chromatography, where liquid is used as mobile
phase, the solvent power of the fluid mobile
phase in SFC can be varied by density, e.g., by
pressure changes at constant temperature.
Solubility generally increases with pressure
under supercritical conditions of the mobile
phase. Since temperature is near the critical
temperature of the mobile phase, temperature
sensitive compounds can be processed.
Chromatographic separation can be carried out at
constant pressure (isobaric operation) or with
increasing pressure (pressure programmed). In
addition, temperature can be varied. Temperature
directly determines the vapour pressure of the
feed components and the density of the mobile
phase and, indirectly, adsorption equilibrium.
With higher temperature, vapour pressures of the
feed components increase exponentially. Density
decreases proportionally to temperature if
conditions are far from critical, but in the
region of the critical point of the mobile phase,
which is the main area of application of
supercritical fluid chromatography, density
varies dramatically with temperature. In this
region, the solvent power of the mobile phase,
which increases with density, is substantially
changed.
3
Introduction of the physico-chemical properties
of the supercritical fluids A pure supercritical
fluid (SCF) is any compound at a temperature and
pressure above the critical values (above
critical point). Above the critical temperature
of a compound the pure, gaseous component cannot
be liquefied regardless of the pressure applied.
The critical pressure is the vapor pressure of
the gas at the critical temperature. In the
supercritical environment only one phase exists.
The fluid, as it is termed, 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.
Supercritical fluid, as its called, is heavy
like liquid but with penetration power of gas.
These qualities make supercritical fluids
effective and selective solvents.
4
A comparison of typical values for density,
viscosity and diffusivity of gases, liquids, and
SCFs
5
(No Transcript)
6

• Some of the advantages and disadvantages of SCFs
  compared to conventional liquid solvents for
  separations
• 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

7

• 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
• Carbon dioxide is the most commonly used SCF, due
  primarily to its low critical parameters (31.1C,
  73.8 bar), low cost and non-toxicity. However,
  several other SCFs have been used in both
  commercial and development processes.

8
(No Transcript)
9
Typical Supercritical Solvents
Compound Tcº C Pc atm d
CO2 31.3 72.9 0.96
C2H4 9.9 50.5 ---
N2O 36.5 72.5 0.94
NH3 132.5 112.5 0.40
n-C5 196.6 33.3 0.51
n-C4 152.0 37.5 0.50
CCl2F2 111.8 40.7 1.12
CHF3 25.9 46.9 ---
H2O 374.1 218.3 ---
Density in g/ml at 400 atm.
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
chromatography-liquid (LC) flow programming
isothermal supercritical fluid chromatography
programmed density/pressure/temperature
supercritical fluid chromatography The coal tar
separation shown here using three methods
indicates the type of differences one encounters
using the three methods. If we select gas
chromatography, with a temperature gradient from
80C to 250C over the 50-minute retention time, we
end up with a reasonable separation. Cooling the
column would have to be added to the time of
separation. Using a Polarity gradient in HPLC
going from 50 to 100 acrylonitrile, an
acceptable separation can be effected in forty
minutes. Turning finally to SFC, a pressure
ramp is used in the solvent, such that solvent
density rose from 0.225g/L to 0.7 g/ml over the
retention period of 120 minutes. Certainly the
last is not the best analytical solution,
however, it does eliminate the need for
acrylonitrile recover and recycle. The
recompression of the supercritical solvent would
seem to be simpler. However to the man in the
lab, liquid chromatography is the clear winner.
16
The art and scientific of adding cosolvents or
modifiers in SFC is one that has had a great deal
of academic and industrial attention. Generally
the cosolvent or modifier is a material added to
the supercritical solvent at low percentage,
which makes a significant change to the solvent
properties. For example the selectivity of
2-naphthol with respect to anthracene can be
enhanced greatly by adding methanol to
supercritical CO2. Unmodified CO2 will give a
selectivity of about 7. It rises to 30 it 3.5
methanol is added as a cosolvent. Two good
solvents for non-polar mixtures are CO2 and
ethylene. Predicting the effect of cosolvents is
a tricky business, since one is dealing with
exchange between solid surfaces and a mixed
solvent. However it is important that we make
progress in understanding this frontier of phase
equilibria. One major area where limited progress
has been achieves is in ionic materials. A great
deal of work has focused on proton acceptors or
donors. There has been interest in dipolar
molecules, surfactants and other more complex
molecules. Progress in the last groups has been
the result of clever detective work, Edison
experimentation, and in a few cases dumb luck.
Hydrogen bonding is an area where significant
work is still required. The concept of producing
gradients in cosolvents promises to be a powerful
weapon.
17
From a prediction standpoint the prediction of
the behavior of cosolvents in chromatography is
far more complex than the equivalent extraction
case. In the latter there is a physical
equilibrium of the solutes between a liquid or
solid phase and the solute-cosolvent mixture. The
foundations for such predictions are firmly laid.
Dealing with unknown solids, such as soil can
greatly increase the complexity and quickly lead
to intractable situations. The case of
chromatography involves exchange of multiple
solutes between a supercritical mixture and
solutes adsorbed in often-unresolved way on the
surfaces and pores of an active solid. This
latter situation is currently theoretically
intractable. The result is that one must rely
almost totally upon the experimentation to
develop chromatographic cosolvent system.
18
(No Transcript)
19
(No Transcript)
20
The figure to the left shows the result on
solubility of modifying cosolvents in the in a
chromatographic separation or ortho benzoic acid.
Note that both cosolvents have a dramatic effect
upon solubility - almost a factor of a 50 at the
lower temperatures. As the temperature rises, the
methanol proves to be the better cosolvent. It is
an interesting exercise to theorize why this
trend occurs. The use of gradients in cosolvents
are not a viable proposition in industrial
applications, because of the additional
separation required at the end of the process to
separate the mixed solvent.
http//www.isopro.net/web8.htm
21
Supercritical Extraction For over a quarter of a
century supercritical fluids, primarily carbon
dioxide, have been used as a solvent in
extraction processes performed under
supercritical conditions. Carbon Dioxide has
several properties that recommend it for this
duty. Its critical temperature is 31.3C, making
near room temperature operations possible. It is
non-toxic, non-flammable and approved by FDA for
use in food and pharmaceutical plants. Its
critical pressure is 72.9 atm., which is
considered moderate. Its properties have been
exhaustively studied, so it continues to be the
extraction solvent of choice. Caffeine removal is
an excellent example of the application of this
technology. Most supercritical Fluid Extraction
processes are quite simple and follow the scheme
shown in the following Fig. A sample is placed
in the Sample thimble, and supercritical fluid is
pumped through the thimble. The extraction of the
soluble compounds is allowed to take place as the
supercritical fluid passes into a collection trap
through a restricting nozzle. The fluid is vented
in the collection trap, allowing the solvent to
either escape or be recompressed for future use.
The material left behind in the collection trap
is the product of the extraction. Obviously this
is a batch process. This is acceptable for the
analytical purpose to which the method is
applied, but could not be considered
commercially, unless there was some extreme
purpose to which the process was being applied.
22
(No Transcript)
23
Applications of SFC Chiral separations
Fatty acid separations Vitamin separations
Purification of pharmaceuticals Nutraceutical
products extractions Essential oils from
plant Photoresist cleaning Fractionation of
polymers Impregnations and reactions
24
Industrial applications The special properties
of supercritical fluids bring certain advantages
to chemical separation processes. Several
applications have been fully developed and
commercialized. Food and flavouring SFE is
applied in food and flavouring industry as the
residual solvent could be easily removed from the
product no matter whether it is the extract or
the extracted matrix. The biggest application is
the decaffeinication of tea and coffee. Other
important areas are the extraction of essential
oils and aroma materials from spices. Brewery
industry uses SFE for the extraction of hop. The
method is used in extracting some edible oils and
producing cholesterine-free egg powder.
Petrolchemistry The destillation residue of the
crude oil is handeled with SFE as a custom
large-scale procedure (ROSE Residum Oil
Supercritical Extraction). The method is applied
in regeneration procedures of used oils and
lubricants. Pharmaceutical industy Producing of
active ingradients from herbal plants for
avoiding thermo or chemical degradation.
Elimination of residual solvents from the
products. Other plant extractions Production of
denicotined tobacco. Environmental protection
Elimination of residual solvents from wastes.
Purification of contaminated soil.