Chemistry Reference
In-Depth Information
Figure 4.16 Overview of several high-p,T SCF-compatible microsystems from Trachsel
et al.
[212], Murphy
et al.
[195], Tiggelaar
et al.
[213], Marre
et al.
[192] and Goodwin et al.
[219] and of the operating regions corresponding to sustainable, engineering and hydro-
thermal fluids [218].
#
2011, with permission from Elsevier.
supercritically. The typical range of pressure and temperature conditions for the use
of three kinds of fluids (sustainable, engineering and hydrothermal) is also given in
the figure.
A positive effect of this is the transition from laminar to turbulent flow, achieved simply
through changes in fluid density and viscosity, with a significant corresponding increase in
Reynolds number. This is much in contrast to the liquid microfluidic case [220]. Thus, a
tight control over temperature and pressure can be used to tune the hydrodynamics within
the channels of microdevices. Furthermore, since the diffusion coefficient is much higher
for gas-like fluids, rapid mixing can be achieved without the need for specifically designed
mixers. Low viscosities and high diffusivities reduce the dispersion within the micro-
channels, facilitating narrow RTD. The two main solvents studied under supercritical
conditions are water and carbon dioxide, due to their high abundance, low cost and easy
recovery. Besides these, alcohols often find use.
Supercritical CO
2
.
Supercritical CO
2
(scCO
2
) is inexpensive, recyclable, easily obtain-
able as a byproduct of fermentation and combustion, nontoxic, nonflammable, relatively
inert, non-oxidizable and provides simple separation via evaporation. Despite being
quite inert, even to free radicals, scCO
2
can react with nucleophiles. The list of reactions
includes aldol reactions, carbonylations, cyclizations, epoxidations, esterifications,
carbon-carbon cross-coupling reactions, hydrogenations, hydroformylations and
polymerizations. Continuous flow processes on the microscale provide unique opportu-
nities for working with green scCO
2
due to the small scale and improved safety at the
required operating pressures.
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