Chemical Engineering June 2018 - 55

Cover Story
tank pressure, the reactive gas may
be injected and spread underneath
the primary disperser. The fraction
of the gas that is not immediately
dissolved rises into the head-space
of the reactor above the liquid level
and is drawn back into the liquid
by way of the upper self-inducing
turbine. For slow photochemical
reactions, the self-inducing turbine
would suffice on its own to achieve
sufficient transfer rates if the gas
would be fed into the reactor from
the top of the tank (Figure 3). Such
an arrangement would reduce expensive
piping of corrosion-resistant
material within the reactor.
Heat transfer. Heat from the exothermic
processes, from the stirring
and emitted by the radiation sources
(despite cooling by transparent liquids
circulating within the wells),
must be removed from the reactor
through the tank wall with a jacket
or half-pipe coils. However, with
low reaction temperature, the driving
temperature gradient with the
coolant is rather low. Installation of
heat-exchangers within the reactor
is possible, but limited by the space
required for the lamps and by the additional
costs for corrosion-resistant
materials. Chilled water or cooling
brine may be used to collect the residual
heat of the process. The heattransfer
coefficient for the stirred side
is calculated based on the known
dimensionless numbers contained in
Equation (15):
(15)
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Where:
Nu = Nusselt number
Re = Reynolds Number
Pr = Prandtl number
Geo = factor considering the impeller
type and the geometry of the heatexchanging
surface in case of cooling
coils, as laid out in detail in Ref. 13.
Mechanical design and safety
Many substances involved in photochemical
reactions (Table 1)
are
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31701
highly corrosive, which precludes the
use of conventional stainless steels.
The material of construction of the
product-wetted parts of the agitator
(impeller, shaft, mounting flange and
mechanical seal) and lamp holders
must be, for example, nickel-based
alloys, such as Hastelloy, or titanium.
Tanks may be made of, or cladded
with, these materials, or made of
glass-lined carbon steel.
The measures for a safe operation
of the glass or quartz wells with the
lamps have been described previously.
Given the toxic and corrosive
properties of some of these reaction
systems and the emanating liquids,
gases or aerosols, the sealing of
the rotating agitator shaft is a fundamental
safety element [18]. Even
if pressure and temperature levels
are moderate during most photochemical
processes, only doubleacting
mechanical seals or hermetically
sealed magnetic couplings are
suitable options to ensure complete
separation of the reactor content
54
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Chemical Engineering June 2018

Table of Contents for the Digital Edition of Chemical Engineering June 2018
