Chemical Engineering June 2018 - 47

Cover Story
FIGURE 3. This stirred-tank
photochemical reactor incorporates
eight light sources
Ekato/Peschl
power calculated from the exitance necessary
to produce the projected mass of
product per unit of time
* Agitation
❍
Optimized residence time in the irradiated
reactor volume for homogeneous
reaction systems and high circulation
rates of particles and slurries to thelight
source to compensate for the
short penetration depth of radiation
❍
Optimized gas dispersion and gasliquid
mass transfer
❍ Maintenance of suspension of solids
* Heat transfer (reaction enthalpy and radiation
sources) to maintain the process
temperature
* Mechanical integrity of the wells housing
the radiation sources
* Corrosion-resistant materials of construction
* Safety aspects concerning radiation, radiation
sources, corrosion and toxicity of
chemicals
and in the reference volume.
the
Additional parameters to consider are
residence time
within
the
irradiated
volume, the spectral range of excitation,
temperature, substrate concentrations, solvent
effects and reaction kinetics - also of
subsequent thermal reactions and safety
considerations.
The scaleup to and the design of the
production plant on the basis of such
laboratory testing involves, in most cases,
further investigations in pilot reactors with
typical volumes of 20 to 200 L. The engineer
must consider and tune the following
parameters, which are described in the following
sections:
* Adaptation of the photochemical reactor
to the type and number of radiation source
originally chosen and its/their electrical
46
Irradiation and absorption
A new and reliable design of an agitated
photochemical reactor with an elaborated
concept of agitation and fixing of the wells
with corrosion-resistant materials is, at present,
the answer to an increasing demand for
the processes summarized in Table 1. Figure
3 depicts the arrangement of a given
number of light sources inserted into the required
cooling and protection tubes (wells)
that are located and secured among each
other as well as connected to the reactor
wall. Given the fact of a relatively limited
chain reaction, as in the case of the chlorination
of polymer granulates (for example,
chlorinated polyvinyl chloride; C-PVC), and
the rather low quantum efficiencies of most
of the investigated photochemical reactions
in heterogeneous media, a rather high incident
radiant power density, I0, is required for
a large production scale.
Up to the present, mercury (Hg) mediumpressure
arcs are mostly used for the excitation
within the ultraviolet and visible (UV/VIS)
spectral domain of wavelengths >250 nm
[1]. Because of their operating temperature
of approximately 900°C, water-cooling and
in some cases the implementation of insulating
vacuum spaces are necessary.
Hg-arcs are contained in cylinders of borosilicate
glass or quartz and emit over their
entire circumference. They provide a broad
line-spectrum with peaks of various intensities
(Figure 5). Some light-emitting diodes
(LEDs)
emit
quasi-monochromatic
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radiation
(Figure 5) and may be used for photoJUNE
2018
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Chemical Engineering June 2018

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