Process Intensification: An Overview of Principles and Practice - Process Intensification for Green Chemistry - page 14

separations. The essential features of the mechanism involved in such electrostatic
separations are [35]:
Charging of a liquid droplet - this can be by either (a) induced charging of polar
molecules via polarization and reorientation of molecular-dipoles charging by the
applied electric field or (b) direct or contact charging of nonpolar molecules by contact
with a DC-charged electrode
Aggregation, coalescence and settling under gravity of electrically charged droplets, in
order to achieve complete phase separation
Intensification of electrical field separations has focused on improved designs for
the coalescing vessel and the electrodes, enhanced hydrodynamics to promote electri-
cally charged droplet interactions through turbulence and higher electric field strengths
[35].
The large interfacial areas formed due to small droplet formation in electric fields can
also be beneficially applied to enhance overall rates of reaction in immiscible liquid
systems, whereby a higher degree of stable emulsification is achieved [36,37]. In one such
process involving the enzymatic hydrolysis of triglyceride esters to yield free fatty acids
and glycerol, studied by Weatherley and Rooney [38], electrostatic fields were used to
intensify the dispersion of the aqueous phase into the oil substrate by creating large
interfacial areas between the reacting species and thus enhancing the overall rate of
reaction at relatively modest temperatures and pressures.
Electric field effects on intensification of heat-transfer processes in general and of
boiling in particular are also well documented [39-42], and the mechanisms are generally
well understood [43-45]. In nucleate boiling, for instance, not only are more bubbles
released from the surface when an electric field is applied but also they are smaller than in
its absence [44].
The combined effects of electric and centrifugal fields on hydrodynamics in thin-film
flow in an SDR have been analysed via numerical simulations by Matar and Lawrence [46].
The applied electric field was shown to induce turbulence on the film surface through the
formation of an increased intensity of large-amplitude waves. These simulation results
suggest that electric fields have the potential to further enhance heat and mass transfer and
reaction rates in thin-film processing.
Electromagnetic Fields. The electromagnetic spectrum covers a wide range of energy
fields, such as microwaves, light, X-rays and g -rays. Here only microwaves and light will
be considered with respect to the intensification of chemical processes.
Microwaves. Microwaves are a form of electromagnetic energy, with frequencies in the
range of 300MHz to 300 GHz. The commonly used frequency for microwave heating and
chemical processing is 2.45 GHz. Microwave heating of materials is quite distinct from
conventional heating, where conduction and convection are the main mechanisms for the
transfer of heat. Under microwave exposure, on the other hand, either dipole interactions or
ionic conduction come into play, depending on the chemical species involved [47,48].
Dipole interactions occur with polar molecules that have high dielectric constants, such as
water and alcohols, while migration of dissolved ions in the electric field takes place in
ionic conduction. Both mechanisms require effective coupling between components of the