Chemistry Reference
In-Depth Information
(4)
It is suitable for concentrated solutions because
a relatively thin liquid film is irradiated.
(5)
It is relatively simple to construct and operate.
To date, bell reactors have been used almost exclu-
sively in the photosterilisation of aqueous media,
and there has been little endeavour to assess their
suitability for chemicals manufacturing. There is
clearly a potential for this design, however.
The photochemical oscillatory flow reactor
An attractive way of utilising photochemistry in
manufacturing processes would be in heterocatalytic
systems. In this type of process, light is absorbed by
an insoluble photosensitiser and the excitation
energy subsequently is transferred to the reactant
molecules. The photosensitiser returns to its original
ground state and then is available for another light-
absorption and energy-transfer cycle. At the end of
the reaction, the insoluble photosensitiser can be
removed readily by filtration and ideally used again.
Photosensitisers can be immobilised by adsorption
on silica or alumina surfaces, by complexation with
ion-exchange resins, by incorporation into zeolite
cages or by covalent bonding to organic polymers.
The processing advantages of reactions activated
by reagents or catalysts immobilised on polymer
supports, for example, have long been recognised
[15]. Within this field, however, the immobilisation
of photosensitisers has received relatively little atten-
tion, although there have been some studies of im-
mobilised dyes, such as Rose Bengal and Methylene
Blue [16,17] for singlet oxygen generation.
The practical implementation of heterogeneous
photocatalytic reactions for chemical synthesis on a
large scale presents significant difficulties. Firstly,
there is the problem of achieving efficient mixing
and mass transfer in a heterogeneous reaction
mixture, with the additional requirement of getting
light into the system to initiate the reaction. Sec-
ondly, there is the question of optimising the design
of the immobilised photosensitisers.
The general problems encountered in the design
of suitable reactors for heterogeneous photocatalysis
have been discussed by Augugliaro
et al.
[18] and
include both modelling difficulties (non-ideal fluido-
dynamics and complex kinetics) and practical con-
siderations such as the opacity and light-scattering
properties of suspended photosensitiser particles,
Fig. 1
8
.11
Schematic diagram of a photochemical forced-jet
bell reactor.
outside of the bell. The irradiated reaction mixture is
collected at the bottom of the bell, from where it can
be recirculated or transferred to the next step of a
process.
Although the bell reactor greatly reduces window
fouling, experience gained with the disinfection of
cutting fluids (G. Shama, personal communication)
has shown that some fouling still occurs, apparently
caused by droplets of liquid created by the falling
liquid sheet as it impinges on a liquid or solid surface,
becoming deposited on the lamp enclosures. It is
hoped that in further refinements of the design this
fouling can be substantially reduced or even elimi-
nated, e.g. by incorporating splash shields and by
carefully selecting the location of the light sources.
In addition to a reduction in window fouling com-
pared with conventional immersion-well reactors,
the bell reactor has the following advantages:
(1)
It has a high ratio of surface area to reacting
volume, favouring gas-liquid reactions and
reactions producing off-gases.
(2)
It works well with mixtures containing sus-
pended solids, e.g. in heterogeneous photocat-
alytic systems.
(3)
It could be incorporated readily into either batch
or continuous processes.
