Chemistry Reference
In-Depth Information
2 General Problems with
Photochemical Processes
The benefits that can be derived from the exploita-
tion of photochemistry in cleaner chemicals manu-
facturing can be realised only when they are not
outweighed by serious disadvantages. There are
several generic problems that frequently are cited as
the underlying reasons why photochemistry is not
utilised more widely. These are
• The need for specialised processing plant
• Unfamiliar process technology
• Window fouling
• The high cost of photons
Clearly, where photochemical processes already
have been commercialised successfully, these prob-
lems have been overcome. Whether they can be
overcome more generally is discussed in the next few
sections.
Fig. 1
8
.9
A stirred batch reactor adapted for photochemical
reactions by incorporation of two lamps in immersion wells.
made of much cheaper glass for reactions initiated
by longer wavelength UV or visible light.
Other ways of incorporating the necessary lamps
into a set-up based on a standard batch reactor also
can be envisaged. For example, the liquid reaction
mixture contained in the main vessel can be circu-
lated through a second chamber containing the
lamp.
Reaction conditions in the various types of adapted
batch reactor can differ markedly. This can lead to
surprising results when seemingly the same reaction
is carried out in reactors of different designs. In the
author's laboratory, for example [10], the chlorina-
tion of propionyl chloride by light-induced reaction
with sulfuryl chloride (SO
2
Cl
2
) has been investigated
in various glass reactors based on the industrial
stirred batch reactor. In an immersion-well reactor
similar to that shown in Fig. 18.9, the results were
comparable to those derived from parallel studies
with propionic acid (Fig. 18.8): mixtures of 2- and
3-chloro and 3-chlorosulfonylpropionyl chloride
were obtained, the proportions varying with tem-
perature. In contrast, photolysis of propionyl chlo-
ride and SO
2
Cl
2
in a reactor involving circulation
to a second chamber gave almost exclusively 2-
chloropropionyl chloride. The greatly different
product distributions obtained with the two reactor
types suggest that a different reaction mechanism
may be operating in each case.
The main point about Fig. 18.9 is that standard
batch reactors can be adapted relatively easily for
2.1 Specialised photochemical reactors
and process technology
It is all very well to regard the photon as an ideal,
non-material reagent, but the differences between
photons and conventional reagents become apparent
when one considers how they are introduced into a
reaction mixture. Many organisations have general-
purpose batch reactors in which it is quite feasible to
carry out almost any chemical reaction that requires
only the straightforward addition of solid or liquid
reagents and the heating and mechanical stirring of
the reaction mixture. Photons, on the other hand,
require one or more lamps to be added to the
configuration, preferably in a manner to allow
maximum utilisation of the light output. In this
respect, specialised plant is needed for photochem-
istry, but there are ways in which standard batch
reactors can be adapted fairly easily.
The most common adaptation of a batch reactor
for photochemical reactions is to enclose the lamps
in immersion wells (Fig. 18.9). The resulting facility
is a scaled-up version of the familiar immersion-well
reactor—probably the most widely used type of
reactor for laboratory photochemical syntheses—in
which the light source is surrounded completely by
the reaction mixture. Immersion wells have to be
made of quartz if UV light with wavelengths of less
than about 300 nm is to be utilised, but they can be
