Chemical Engineering June 2018 - 46

The progress in the domains of physical and theoretical
chemistry also fostered the understanding of the
fundamentals of photophysics and photochemistry
and opened a wide field of applications.
The present status of photochemical engineering
provides technical and ecological answers for practically
all up-coming projects and also allows a better
economic analysis and control of costs of production
and maintenance. Such viable answers need the
cooperation of dedicated specialists in the domains
of photochemical, materials, chemical and mechanical
engineering.
A new concept of a stirred-tank photochemical reactor
(Figure 3) has been developed mainly for multiphase
reactions (Table 1). It can be operated batchwise,
as fed-batch or continuously as a continuous
stirred tank reactor (CSTR). The aspect ratio H/TD of
the tanks is 1 to 1.5, reactor volumes range from 1 to
50 m3. Mixing provides the circulation of the substrate
to the radiation source, the turbulence to disperse and
dissolve a reactive gas and prevents solids from sedimentation.
The glass or quartz wells housing the radiation
sources are inserted through nozzles on the tank
head and fixed to the wall or to the tank bottom. The
upper end of the well on top of the tank nozzle has
connecting devices for the electric power and the cooling
medium of the radiation sources. The number of
radiation sources is restricted by space constraints on
the tank head; large tanks may contain up to 20 lamps.
The heat of the reaction is removed by cooling through
the tank wall with jacket or half pipes. If that is not sufficient,
additional coils can be installed within the space
between the wells and the wall. Cooling by evaporating
solvents could be another alternative.
Photochemical process
development
usually
starts with laboratory-size equipment, including
commercially available reactors with which homogeneous
liquid reaction systems, as well as heterogeneous
gas-liquid, liquid-solid and gas-liquid-solid
systems may be investigated (Figure 4). The objective
of laboratory experiments is twofold: 1) choosing
and optimizing the unit equipment and, in particular,
the geometry of the photochemical reactor;
and 2) investigating optimal conditions in terms of
rate and specificity of the process.
Absorption spectra of the photochemical reactant
and the substrate mixture allow one to determine the
wavelength(s) of excitation and to choose the corresponding
radiation source. The selection of the latter
and its power depends on the required range of excitation,
the specificity of the reaction and the projected
mass of production per unit of time.
The quantum yield is an additional design parameter
to be considered when there are restrictions in the
scaleup, for instance the geometrical parameters or the
electrical power. The quantum yield represents the efficiency
of the irradiation, and can be defined as the
number of moles of reactant converted per number of
moles of photons absorbed over the same time period
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Chemical Engineering June 2018

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