588
The operation mode of the photoreactor can be either batch or continuous. CO 2
bubbling takes place initially, and then the reactor is sealed for carrying out the reaction by irradiating the lamp. On the other hand, continued injection of CO 2 is
required into the irradiated reactor for the continuous photoreactor’s configuration.
The liquid products (e.g., CH 3 OH, HCOOH) and gaseous products (e.g., CH 4 , CO,
H 2 ) are collected continuously in specific time intervals for analyzing quantitatively
and qualitatively by chromatographic or titration techniques. The limitations of the
slurry photoreactors in either configurations are:
• After reaction, it is challenging to separate the catalyst from the reaction mixture.
• As the reaction proceeds in the liquid medium suspended with catalyst, due to
the light scattering and absorption, there is a decrement in light utilization
efficiency.
4.5.2 Fixed Bed Photoreactors
Fixed bed reactor configuration came into the picture to overcome product separation limitations in the slurry reactor system. In this system, the catalysts are usually
set up over fixed support, say beads, monoliths, etc. The photocatalyst can be coated
on support or kept directly over the photoreactor wall. The advantages of fixed over
slurry reactors are [127]:
• As the catalysts are immobilized on a fixed support, the process can be operated
continuously avoiding the cost of catalyst separation.
• The reaction completes faster in the fixed bed photoreactor system. Larger grain
size catalysts can be utilized, providing the advantage of better external mass
transfer and lower pressure drop.
• Higher surface volume ratio and lesser pressure drop enable achieving a higher
flow rate, which leads to more straightforward modification.
• Flow regime relatively closer to plug flow can be obtained leading to higher conversion rate per unit mass of catalyst.
• Photons in this system configuration directly reach the exposed catalyst surface
avoiding any light scattering or absorption.
The downsides of fixed bed reactors are:
• The problem of the temperature gradient between gas and solid surface arises.
• Maximum usage of optical fixed bed reactors is difficult to achieve.
• Due to the heat buildup, it can lead to deactivating the catalyst rapidly.
However, both the reactor systems have their advantage, and the choice of the
system configuration depends on the product requirement and application.
K. Tripathi et al.
The operation mode of the photoreactor can be either batch or continuous. CO 2
bubbling takes place initially, and then the reactor is sealed for carrying out the reaction by irradiating the lamp. On the other hand, continued injection of CO 2 is
required into the irradiated reactor for the continuous photoreactor’s configuration.
The liquid products (e.g., CH 3 OH, HCOOH) and gaseous products (e.g., CH 4 , CO,
H 2 ) are collected continuously in specific time intervals for analyzing quantitatively
and qualitatively by chromatographic or titration techniques. The limitations of the
slurry photoreactors in either configurations are:
• After reaction, it is challenging to separate the catalyst from the reaction mixture.
• As the reaction proceeds in the liquid medium suspended with catalyst, due to
the light scattering and absorption, there is a decrement in light utilization
efficiency.
4.5.2 Fixed Bed Photoreactors
Fixed bed reactor configuration came into the picture to overcome product separation limitations in the slurry reactor system. In this system, the catalysts are usually
set up over fixed support, say beads, monoliths, etc. The photocatalyst can be coated
on support or kept directly over the photoreactor wall. The advantages of fixed over
slurry reactors are [127]:
• As the catalysts are immobilized on a fixed support, the process can be operated
continuously avoiding the cost of catalyst separation.
• The reaction completes faster in the fixed bed photoreactor system. Larger grain
size catalysts can be utilized, providing the advantage of better external mass
transfer and lower pressure drop.
• Higher surface volume ratio and lesser pressure drop enable achieving a higher
flow rate, which leads to more straightforward modification.
• Flow regime relatively closer to plug flow can be obtained leading to higher conversion rate per unit mass of catalyst.
• Photons in this system configuration directly reach the exposed catalyst surface
avoiding any light scattering or absorption.
The downsides of fixed bed reactors are:
• The problem of the temperature gradient between gas and solid surface arises.
• Maximum usage of optical fixed bed reactors is difficult to achieve.
• Due to the heat buildup, it can lead to deactivating the catalyst rapidly.
However, both the reactor systems have their advantage, and the choice of the
system configuration depends on the product requirement and application.
K. Tripathi et al.