Process Intensification: An Overview of Principles and Practice - Process Intensification for Green Chemistry - page 15

target material and the rapidly oscillating electrical field of the microwaves. Heat is
generated by molecular collision and friction.
Microwave-assisted processing is associated with many features that have positive
implications for intensification of processes. In particular, the more rapid, controlled and
uniform heating rates afforded by microwave exposure result not only in higher rates of
reaction than conventional heating methods but also in better product quality, through
improved selectivity. Recent publications by Toukoniitty et al. [27] and Leonelli and
Mason [29] provide good reviews of these aspects, with the former focusing on heteroge-
neous catalytic systems.
There are a number of examples of the industrial use of microwave heating in the food,
rubber and wood industries, as highlighted by Leonelli and Mason [29]. Although small-
scale batch and continuous reactor systems with microwave irradiation capabilities are
commercially available [47], there is still much scope for further development of
industrial-scale microwave reactors, with continuous flow systems that have microwave
irradiation or indeed coupled microwave and ultrasound irradiation capabilities being
ideally suited for this purpose [25,29].
Light Energy. The observation that certain compounds could be affected by sunlight to
give materials with the same chemical composition but very different physical properties
was first made in 1845, when Blyth and Hofmann [49] noted that styrene was converted
from a liquid to a glassy solid when exposed to sunlight. Since then, the industrial potential
of photochemistry has been widely demonstrated in a number of reactions, such as the
production of caprolactam, used in the manufacture of nylon 6, and the formation of
vitamins D 2 and D 3 [50].
Photoinitiation is an attractive alternative to thermal activation of reactions for a number
of reasons. It is an inherently clean process, requiring only the reacting molecules in order
for the reaction to be activated. Additional and often expensive and environmentally
unfriendly reagents and catalysts can be minimized. The irradiation by a specific wave-
length, and therefore a well-defined energy input in the form of photons, allows only
certain reactions to be targeted, thereby reducing byproduct formation and costly down-
stream separation processes. Since activation is by light, ambient operating temperatures
can be utilized, which not only reduce thermal energy consumption but can also result in
better control of the process, especially if side reactions can be minimized at lower
temperatures. This is particularly relevant for polymer processing, where low temperatures
generally result in (1) better tacticity control of the polymer, (2) reduced transfer effects,
which can be a cause of excessively branched macromolecules, and (3) minimal thermal
degradation risks for the polymer formed.
In spite of the clear advantages offered by photochemistry as a reaction-initiation
technique, its use in industry is surprisingly rather limited. This is mostly because of the
technical problems associated with the uniform irradiation of large reaction volumes, such
as those encountered in conventional batch set-ups. Batch reactors, especially those at
commercial scale, possess particularly low surface area to volume ratios. This poses
enormous processing challenges in that the photons emanating from the light source have
extremely limited penetration depths into the fluid (a few centimetres at most), resulting in
quite ineffective and non-uniform initiation of reactions in conventional stirred tank reactor
configurations. These issues have been addressed to a certain extent by the development of