Chemical Engineering June 2018 - 48

chemical processes, where high reaction specificity is
required. The substitution of Hg-arcs exhibiting a radiant
efficiency of approximately 30% by LED-arrays for
large-scale applications needs further development,
mainly for reasons of their implementation in wells, their
electronic controls and the cooling of multiple and large
arrays. Their unilateral emission and arrangement on a
support does not allow for a quasi-homogeneous emission
over the entire circumference of the well.
Commercially available medium-pressure Hg arcs
are standardized in size, and have electric power in a
range of 5 to 60 kW (Figure 6). Specific know-how is
needed for the design of the electric power connection
and supply, as well as for the cooling medium passing
through the head of the well, which is bolted onto the
tank nozzle.
Even if the outer protection tube (made of glass or
quartz) with its connection to the tank is a reliable and
proven component, its breakage should be considered
in the risk analysis. As the components of the reaction
system are usually hazardous or corrosive (or both),
such a rare event has to be detected by monitoring,
and the leakage must automatically be collected in a
containment. Those emergency procedures are fixed in
detail through the HAZOP (hazard and operability study)
of the reactor.
Relevant for the design of a photochemical reactor
is the fact that the incident radial power, I0, is not constant
throughout the reactor volume containing radiation-absorbing
components. Its attenuation is due to
chromophores, that is, functional groups of molecules
absorbing radiation of given wavelengths [1, 2] and resulting
in its exponential decrease with distance x from
the source within the absorbing medium, as described
by the Lambert-Beer law, Equation (3):
(3)
The drop of the relative radiant power, I/I0, with disPI
8
105B
AI
ORP
tance
x from the entrance of the radiation is shown in
Figure 6. The molecule-specific molar absorption coefficient,
κ, depends on wavelength, λ. The concentration,
c, of the light-absorbing substance is simplisticly
assumed to be constant over the whole distance of
the optical path. The spectral absorbance B(λ) is an
additive property, which means for mixtures containing
more than one absorbing compound, B(λ) is expressed
as:
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PACKING
(4)
Usually, decadic molar absorption coefficients ε(λ)
PI
are determined in UV/VIS spectrophotometers and can
be transformed into Naperian molar absorption coefficients
by κ = 2.303 . ε.
Figure 7 demonstrates, in principle, one of the most
important obstacles to be surmounted while up-scaling
photochemical reactions: depending on B(λ) of the substrates
and products, either their concentration or the
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Chemical Engineering June 2018

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