4/28/01
Chem. 405
UWEC
Supercritical Fluids
By Cory Brosnan
A supercritical fluid is any substance above its critical
temperature and critical pressure. In the supercritical area there is only one
state-of-the-fluid and it possesses both gas- and liquid-like properties. A
substance becomes supercritical above
1
It’s critical
point of temperature and pressure.
Supercritical fluids have the following characteristics, they are a
dense gas, they have solubilities approaching liquid phase and diffusivities
approaching gas phase.
Supercritical fluids have rapid mass transfer and their
viscosities are nearer to those of gases. . In the vicinity of the critical
point, the diffusion coefficient is more than ten times that of a liquid. The viscosity and diffusivity, like density
are dependent on temperature and pressure. Viscosity and diffusivity are on order-of-magnitude
less than a liquid even at high pressures (300-400 atm.). Supercritical fluid technology has been
applied to various problems because of its properties of gas-like diffusivity,
gas-like viscosity, and liquid-like density and it’s pressure-dependent
solvating power.
Supercritical fluids are not a new technology, the phenomenon of
enhanced solubility’s in supercritical fluids has been known since the late
1800s. Supercritical fluids have been
used in the food processing industry, for decades, to extract compounds such as
caffeine and hop oil from food. In
recent years supercritical fluids have also been used in parts cleaning
applications.
Supercritical fluids were first discovered a hundred years ago but only about twenty years a go did they start being used by industry for extracting caffeine from tea and coffee, hops flavors for brewing and other flavors and spices. Supercritical fluids exhibit a pressure dependent dissolving power, the higher the pressure, the higher the dissolving power, and this property can be applied to purification, extraction, fractionation, and recrystallization of a wide host ofmaterials.
2
Solubility of Naphthalene in Supercritical Carbon Dioxide
(45 C)
The above diagram which shows the solubility of a much-studied model compound, naphthalene, in supercritical carbon dioxide. The critical point is 73 atm. and 31 °C, at pressures below the critical point the solubility of Naphthalene is virtually zero and at higher pressures the solubility increases drastically. The solubility of Naphthalene in Carbon Dioxide is quite general for compounds that dissolve in supercritical fluids.
One of the current
industrial applications of supercritical fluids is extracting flavor from Hops
in Brewing, which is illustrated schematically in the following figure.
2
The diagram is a
simplified version of the process for extracting flavor from Hops. The extractor contains the Hops from which
the flavor is to be extracted, the compressor recycles the carbon dioxide the
separator collects the flavor extract and recycles the carbon dioxide. The process works as follows the carbon
dioxide passes through the hops and the flavors and oils are extracted to an
equilibrium solubility level. The
mixture of gases is then passed through the pressure reduction valve and the
pressure of the carbon dioxide is reduced causing the flavor to precipitate out
into the separator. The Carbon dioxide
is then recompressed and the process starts over. The process for the extraction of Caffeine and the flavor of
spices is similar to the one for Hops.
1 Supercritical Fluid
Research at PNNL
http://www.pnl.gov/supercriticalfluid/index.stm
2 Supercritical fluid
technology at Phasex Corporation
http://www.phasex4scf.com/
3 Durability company
http://www.durability.com/chemical/sfe/intro.htm