Handbook of Solvents - George Wypych - ChemTech - Ventech!

1438 Aydin K. Sunol and Sermin G. Sunol
Swelling can be advantageous in that it enables permeation and diffusion of the supercritical
fluid into the polymer network. Fragrances, dyes, or medicinal substances loaded in
the supercritical fluid can readily impregnate into the polymer and load the polymer with
the aforementioned additives. Upon release of the pressure, only the supercritical fluid (i.e.,
carbon dioxide) flashes off. This avenue has led to a plethora of controlled (timed) release
products.
Naturally, swelling may be undesirable in many instances as well. For example, swelling
of organic polymer based membranes decreased selectivity. Other possible deleterious
effects could include malfunctions due to solubilization and swelling of sealants such as
gaskets or o-rings.
21.1.2.7 Surfactants and micro-emulsions
Most highly polar and ionic species are not amenable to processing with desirable solvents
such as carbon dioxide or any other solvent such as water that has a higher critical temperature
well above the decomposition temperature of many solutes. In such instances, the combination
of the unique properties of supercritical fluids with those of micro-emulsions can
be used to increase the range of applications of supercritical fluids. 57 The resulting thermodynamically
stable systems generally contain water, a surfactant and a supercritical fluid (as
opposed to a non-polar liquid in liquid micro-emulsions). The possible supercritical fluids
that could be used in these systems include carbon dioxide, ethylene, ethane, propane, propylene,
n-butane, and n-pentane while many ionic and non-ionic surfactants can be used.
The major difference between the liquid based emulsions and the supercritical ones is the
effect of pressure. The pressure affects the miscibility gaps as well as the microstructure of
the micro-emulsion phase.
The incorporation of the micro-emulsion phase creates interesting potential advantages
for reactions as well as separations. Isolation of components from fermentation broths
and garment cleaning appear to be two of the more competitive applications of these systems.
21.1.3 SEPARATION WITH SUPERCRITICAL SOLVENTS
Supercritical fluids are effective at much lower temperatures than distillation, and their application
in separation avoids degradation and decomposition of heat-labile compounds.
Attractiveness of supercritical extraction processes are due to the sensitivity of responses to
process variables, promise of complete and versatile regeneration of solvents, energy savings,
enhanced solute volatilities, solvent selectivities, favorable transport properties for
solvents, and state governed effectiveness of solvents which enables the use of low cost,
non-toxic, environmentally acceptable solvents. The impact of inherent characteristics of
supercritical fluids on separations is summarized in Table 21.1.5.
Table 21.1.5 The characteristics and challenges with supercritical separations
Inherent characteristics of systems
@ supercritical conditions
Enhanced solubility Effective at lower temperatures
Favorable transport properties
Resulting promise Challenges to be met
High mass transfer rates especially
in porous media
Recovery of substances
inadvertently extracted