Chemical Engineering June 2018 - 45

Cover Story
Process
TABLE 1. INDUSTRIAL PHOTOCHEMICAL PROCESSES, REACTIONS, PRODUCTS
Mechanism
Product
Chlorination
Bromination
Sulfoxidation
Sulfochlorination
Nitrosylation
Hydrodimerization
Oxidation
Intermediates,
solvents,
C-PVC, rubbers
Intermediates
Intermediates, sulfonic
acids, surface-active
materials
Intermediates, sulfonic
acids, surface-active
materials
Intermediates, Nylon 6
and 12
Intermediates
Intermediates, surfaceactive
materials
Isomerization
Intermediates
Industry
Specialties,
polymers
Specialties
Specialties, consumer
products,
polymers
Specialties, consumer
products,
polymers
Specialties,
polymers
Specialties,
agrochemicals
Specialties,
perfumes,
pharmaceuticals
Specialties,
pharmaceuticals
Cycloaddition,
electrocyclic
reactions
Polymerization
Intermediates
Specialties,
pharmaceuticals
Polymers
Specialties,
polymers
Note:
L = liquid phase, G = gas phase, S = solid phase
(2)
At ambient temperature, k is too small for
FIGURE 2. In the reaction
mechanism of chlorination,
light is first absorbed by Cl2
to form Cl. radicals, which
subsequently substitute a
hydrogen in a hydrocarbon
compound
most reactions to get them started. To raise
it to an economic rate, the temperature T
must be increased. Therefore, most industrial
reactions run at higher temperatures.
High temperatures not only enhance the desired
reaction, but also side reactions, leading
to undesirable byproducts or to thermal
decomposition of reactants, intermediates
and products. Both entail costs for starting
materials, waste treatment and an increased
effort to purify the product.
Catalysts allow high conversion rates
at lower temperatures as they reduce the
Stirred-tank photochemical reactors
Large-scale industrial applications of photochemical
reactions have been known
for more than 100 years, when first attempts
to up-scale photochemical chlorinations
were rendered possible by the
development of mercury vapor lamps [10].
Immersion-type photochemical reactors
[1] were conceived to profit from the radiation
emitted 360 deg radially from cylindrically
shaped light sources. Such processes
required modest knowledge in the
domain of photochemistry, but proved to
be a real challenge for engineers. Challenges
include concerns primarily over the
corrosion-resistant materials (for example,
in the case of chlorinations), the new design
of mechanically resistant quartz-metal
interfaces and the design of new concepts
ensuring high-turbulent conditions in the
presence of fragile lamp installations. Particular
attention had to be devoted to handling
the exothermicity of chain reactions.
44
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JUNE 2018
Radical
Radical
Radical
Radical
Radical
Radical
Radical,
sensitized,
catalytic
Electron
density
distribution,
sensitized
Electron
density
distribution,
sensitized
Radical, ionic,
catalytic
L
L-G
L-S
L-G-S
L
L-G
L-G-S
L
L-G
L-G
L-G-S
L
L
L-G
G-S
L-G-S
L
Multiphase
System
activation energy, EA. But catalysts involve
costs for their procurement or make up,
handling, feeding, separation and recovery
or disposal.
Process initiation by radiant energy allows
for running reactions independently of
temperature and in most cases, well below
100°C and without a catalyst. Photocatalysts
for specific reactions are not used to
decrease EA, but to impose a specific mechanism
of chemical transformation.
The absorption of electromagnetic radiation
of an appropriate wavelength promotes
the reactant molecule from its ground state
to an electronically
excited
state,
from
which it subsequently undergoes a mostly
specific chemical transformation to a stable
product or to a reactive intermediate capable
of initiating a second thermal reaction.
The most relevant industrial photochemical
processes and their primary mechanisms
are listed in Table 1 [1].
A description of the immense diversity of
the reaction mechanisms would be beyond
the scope of this article. For detailed information,
the interested reader is referred to
the literature [1-9]. Representative for the
predominant radical mechanisms is the
photochemical halogenation: electronic
excitation of ground-state molecules Cl2
or Br2 leads to a dissociative state generating
the respective radicals Cl· or Br· that
may add to C=C double bonds or abstract
a hydrogen atom [1]. The latter is shown in
Figure 2.
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