Chemistry Reference
In-Depth Information
laboration between the author's group at Strathclyde
University and Skelton's group at Cambridge. In this
project it is hoped to develop the photochemical
OFM reactor for use with a wide range of reagent
and solvent systems and to increase the scope of
immobilised photosensitisers.
Continuous flow reactors and process intensification
There is much current interest in process intensifica-
tion through the development of small continuous-
flow reactors. Major advantages are the lower capital
costs and space requirements of the smaller reaction
vessels needed for any given throughput. Photo-
chemical reactions should, in the main, be quite
amenable to this approach.
In an immersion-well batch reactor like that
shown in Fig. 18.9, the light output from each lamp
usually will be absorbed completely within a short
distance of the lamp enclosure, unless the reaction
solution is very dilute. Thus, most of the volume of
the reaction vessel effectively is shielded from the
light. The reaction can proceed to completion
because, through stirring of the solution, all the re-
actant molecules at some stage will come close
enough to a lamp to be irradiated. This, however,
may be achieved just as easily in a flow system.
Indeed, the flow system has the advantage that, once
the reactant molecules have been irradiated, the
resulting product molecules are removed from the
reaction chamber and therefore are not exposed to
further prolonged irradiation and the possibility of
secondary photolysis. All the designs mentioned in
this chapter—immersion-well, falling-film, jet-
injection, bell and OFM reactors—are capable of
adaptation to continuous-flow processes.
Fig. 1
8
.12
Schematic diagram of a photochemical reactor with
oscillatory flow mixing.
choice of construction materials and the avoidance
of window fouling. Research on UV-assisted wet
oxidation and parallel research on oscillatory flow
mixing (OFM) [19] has led to the development, by
Skelton's group at Cambridge University, of a photo-
chemical reactor with oscillatory flow mixing of
the reactants [20,21], which seems highly promising
for heterogeneous photocatalytic synthesis (Fig.
18.12).
In OFM reactors, the mixing of phases is accom-
plished by the application of a vertical oscillation to
the reaction liquid, by means of a pulsing unit, which
causes vortexing around and between the baffles
(see Fig. 18.12). The oscillations can be varied in
amplitude and frequency to optimise mixing. The
OFM reactor therefore is specially suitable for two-
and three-phase reaction systems, e.g. the combina-
tion of heterogeneous photocatalysts and gaseous
reagents. The reactor can be operated in either batch
or continuous modes, but in the latter it is important
that a sufficient settling volume is provided, above
the top baffle, to ensure that the solid photocatalyst
is not carried over.
The OFM reactors originally were designed for
the photocatalytic destruction of pollutants, e.g.
using supported TiO
2
, but their potential for photo-
chemical synthesis now is being investigated in a col-
3.2 Light sources
In view of the need for highly efficient mono-
chromatic light sources, the development of a range
of so-called excimer lamps has been a welcome
advance [22,23]. These silent discharge sources are
driven by high-voltage power supplies operating at
frequencies from 50 Hz to several megahertz and
have narrow spectral bandwidths, readily variable
geometry—allowing flexibility in reactor design—
and relatively low operating temperatures. Table
18.1 lists some of the available systems, together
with their peak wavelengths. The two varieties of
