2007; Coyle and Oelgemöller 2008; Su et al. 2014; Heggo and Ookawara 2017). The
operating principle of such reactors is schematized in the Fig. 2.7.
The implementation of photochemical micro-reactors allows overcoming some
drawbacks of conventional batch reactors, more especially the attenuation effect of
photon transport, which makes the scale-up difficult, by ensuring a uniform irradiation of the reaction mixture and/or of the catalyst (Colina-Márquez et al. 2010;
Wang et al. 2014a; Cambié et al. 2016). Indeed, their high surface-to-volume ratio
results in a high accessible photocatalytic surface area. It was evidenced that mass
transfer from the reaction medium to the catalyst is improved (Gorges et al. 2004;
Wang et al. 2014a).
The separation of the catalyst from the reaction medium can be overcome by
coating a thin photocatalyst layer on the reactor’s walls of a continuous-flow
photochemical micro-reactor (Heggo and Ookawara 2017). Thus, the catalyst can
be fully irradiated. However, such a coating of the catalyst results in a decrease of the
accessible surface area (Mozia 2010; Cambié et al. 2016). Fortunately, this phenomenon is compensated, thanks to the high surface-to-volume ratio (up to 11,667 m
2 .
m
À3 ) of micro-photoreactors compared to conventional slurry photoreactors
(2631 m
2 .m
À3 ) (Gorges et al. 2004).
Micro-reactors can be implemented according to different configurations among
which slurry-bubble reactor, monolith reactor, membrane reactor, fluidized bed
reactor, falling film reactor, planar reactor, microchannel and capillary reactors,
silicon reactors, and multiphase micro-droplet reactors.
Flow photoreactors are mostly implemented in organic photochemical reactions
and, in a minor extent, TiO 2 -based photocatalytic degradation of dyes degradation
(Gutmann et al. 2015; Mizuno et al. 2016).
Dye Degradation Using Photochemical Flow Reactors
Several examples of photocatalytic dye degradation in micro-reactors are reported in
the literature. Most of them use methylene blue as a model dye. Some studies focus
on immobilization of the catalyst on the inner walls of the reactor. For example, Li
et al. reported that by using TiO 2 -SiO 2 coated walls of a micro-reactor, the degradation rate of methylene blue was increased by more than 150 times as compared to
conventional system (Li et al. 2003). In another work, methylene blue was
photodegraded both on TiO 2 -P25 and TiO 2 -graphène photocatalysts deposited on
the walls of a glass chip micro-reactor (Padoin et al. 2016). The degradation rate of
methylene blue was found to be one order of magnitude higher than with equivalent
macroscopic reactor. In order to increase the photocatalytic efficiency for the
methylene blue degradation, Lindstrom et al. coated a mesoporous anatase film
with a high surface area on the inner walls of the reactor (Lindstrom et al. 2007).
As a general feature of micro-reactors, they found that the rate-limiting parameter for
the photodegradation of methylene blue is not the catalyst surface area, neither the
adsorption properties but the oxygen concentration in the water phase, meaning that
the reaction is not limited by heat and mass transfer in such conditions. Lin et al.
74
B. Lebeau et al.
operating principle of such reactors is schematized in the Fig. 2.7.
The implementation of photochemical micro-reactors allows overcoming some
drawbacks of conventional batch reactors, more especially the attenuation effect of
photon transport, which makes the scale-up difficult, by ensuring a uniform irradiation of the reaction mixture and/or of the catalyst (Colina-Márquez et al. 2010;
Wang et al. 2014a; Cambié et al. 2016). Indeed, their high surface-to-volume ratio
results in a high accessible photocatalytic surface area. It was evidenced that mass
transfer from the reaction medium to the catalyst is improved (Gorges et al. 2004;
Wang et al. 2014a).
The separation of the catalyst from the reaction medium can be overcome by
coating a thin photocatalyst layer on the reactor’s walls of a continuous-flow
photochemical micro-reactor (Heggo and Ookawara 2017). Thus, the catalyst can
be fully irradiated. However, such a coating of the catalyst results in a decrease of the
accessible surface area (Mozia 2010; Cambié et al. 2016). Fortunately, this phenomenon is compensated, thanks to the high surface-to-volume ratio (up to 11,667 m
2 .
m
À3 ) of micro-photoreactors compared to conventional slurry photoreactors
(2631 m
2 .m
À3 ) (Gorges et al. 2004).
Micro-reactors can be implemented according to different configurations among
which slurry-bubble reactor, monolith reactor, membrane reactor, fluidized bed
reactor, falling film reactor, planar reactor, microchannel and capillary reactors,
silicon reactors, and multiphase micro-droplet reactors.
Flow photoreactors are mostly implemented in organic photochemical reactions
and, in a minor extent, TiO 2 -based photocatalytic degradation of dyes degradation
(Gutmann et al. 2015; Mizuno et al. 2016).
Dye Degradation Using Photochemical Flow Reactors
Several examples of photocatalytic dye degradation in micro-reactors are reported in
the literature. Most of them use methylene blue as a model dye. Some studies focus
on immobilization of the catalyst on the inner walls of the reactor. For example, Li
et al. reported that by using TiO 2 -SiO 2 coated walls of a micro-reactor, the degradation rate of methylene blue was increased by more than 150 times as compared to
conventional system (Li et al. 2003). In another work, methylene blue was
photodegraded both on TiO 2 -P25 and TiO 2 -graphène photocatalysts deposited on
the walls of a glass chip micro-reactor (Padoin et al. 2016). The degradation rate of
methylene blue was found to be one order of magnitude higher than with equivalent
macroscopic reactor. In order to increase the photocatalytic efficiency for the
methylene blue degradation, Lindstrom et al. coated a mesoporous anatase film
with a high surface area on the inner walls of the reactor (Lindstrom et al. 2007).
As a general feature of micro-reactors, they found that the rate-limiting parameter for
the photodegradation of methylene blue is not the catalyst surface area, neither the
adsorption properties but the oxygen concentration in the water phase, meaning that
the reaction is not limited by heat and mass transfer in such conditions. Lin et al.
74
B. Lebeau et al.