(299b) A Study of Photon Transport in Gas-Liquid Flow: Scalability of Photooxidations from the Micro- to the Milli-Scale

The use of light energy constitutes a green
activation mode of organic molecules. A photoreaction of increasing interest is
the photosensitized addition of singlet oxygen, as it is an atom-economic
method of functionalization and is applied in the synthesis of
commercial chemical products as fragrances and pharmaceuticals [1]. Many singlet oxygen oxidations were
applied to organic substrates in flow photo microreactors due to their small
penetration depth and the presence of segmented flow which leads to enhanced interfacial
mass transfer [2,3]. However, as microreactors lack the throughput required by
industry, scale-up could be realized using milli-scale reactors which combine
the advantages of microreactors with the throughput of conventional batch
reactor systems [4]. However, scale-up is usually preceded by an extensive
parameter study (e.g. variation of the light source intensity, concentration of
the sensitizer, transport gas fraction) realized for each reaction system on
the small and large scale [5,6]. If the light absorption and
scattering phenomena in the employed gas-liquid photoreactors are understood at
a fundamental level, they can be predicted and the scale-up procedure becomes faster and cost efficient.

This study aims to investigate the photon
transport in gas-liquid flows on the micro- and milli-scale. The microreactor
consists of a glass plate with a serpentine channel characterized by a volume
of 0.6 mL and a diameter of 1 mm.
The milli-scale reactor is a Corning® Advanced-Flow^TM
G1 Photo Reactor which is
composed of heart-shaped elements and is characterized by
a volume of 8 mL and a
channel height of 1.1 mm. Both reactors are irradiated by green
Light-Emitting Diodes (LEDs). Firstly,
the photon flux and the optical pathlength are experimentally determined by visible-light
actinometry following a methodology previously reported [7]. The actinometric
measurements are performed in single liquid phase and in gas-liquid two-phase
flows. Nitrogen is used as inert gas with the volumetric gas transport
fractions comprised between 0.2 and 0.5. An example of the different gas-liquid
flow patterns obtained in the microreactor and G1
Photo Reactor is shown in
Figure 1. Secondly, a modelling tool based on the ray tracing technique implemented
in Matlab is used to support the experimental
quantification of the optical pathlength in single and two-phase flow.

As application, the gas-liquid photoxidation
of 2-methoxyfuran in the presence of molecular oxygen and Rose Bengal in
ethanol is scaled-up from the microreactor to G1
Photo Reactor. The
optimal concentration of the photosensitizer on micro- and milli-scale is
correlated to the optical pathlength determined by actinometry. It is found
that the photoreaction is not limited by the gas-liquid interfacial mass
transfer on both micro- and milli-scale. Therefore, the maximum conversion of
2-methoxyfuran occurs for a gas fraction where the liquid receives the highest
photon flux.

This study will contribute to the understanding of the light
absorption and scattering phenomena in different gas-liquid flow patterns at the
micro- and milli-scale. This knowledge is crucial for making the scale-up
process more efficient, but also for designing improved multiphase flow
photochemical systems.  

Figure 1. Example of gas-liquid flow
patterns in the studied microreactor (left) and Corning® Advanced-Flow^TM G1 Photo Reactor (right).

References

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