(567e) Light Absorption Efficiency of Gas-Liquid Continuous Flow Microreactors Illuminated By Visible LED Light Sources

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].

Continuous photo microreactors are a promising tool for e.g. photooxidation transformations due to their small penetration depth and the presence of segmented flow which leads to enhanced mass transfer of the gas in the liquid [2]. In addition, the large photon flux through the thin liquid film located between the bubble and the reactor wall and light scattering at the gas-liquid interface were also reported as possible factors to enhance the photoreaction yield in comparison to single phase flow [3, 4]. However, designing an efficient photoreactor does not only involve optimizing its geometry, but also the light source needs to be adapted to the chosen chemistry, reactor channel layout and the phase of the reaction medium [5, 6]. In recent years, the use of LEDs has become popular in flow photochemistry due to their energy efficiency and the possibility to match their emitted wavelength to the photosensitizer absorption and their size to the reactor microchannels. However, the influence of the LED light distribution and potential scattering of light at the gas-liquid interface on the yield of the photochemical reactions performed in segmented flow is so far not reported.

This study aims to investigate the influence of the LED board configuration on the photon absorption efficiency in gas-liquid continuous flow microreactors. Firstly, the distribution and number of photons emitted by LED boards with different configurations is experimentally determined by near field goniophotometric measurements (NFGM). Secondly, the number of photons absorbed in the microreactor channels is quantified using an actinometer system. The balance of the photon fluxes for various LED light sources and for operating in single phase liquid or two-phase gas-liquid flow are compared and discussed. Furthermore, we developed a modelling tool based on the ray tracing technique to simulate the distribution of the scattered light rays from the gas-liquid interface, which supports the discussion of the experimental observations.

This study will contribute to the understanding of the incident radiation and dispersed phase influence on the photon absorption in gas-liquid photochemical transformations, knowledge which will be crucial for the efficient design of multiphase flow photochemical systems.

References

1. J. P. Knowles, L. D. Elliott, and K. I. Booker-Milburn, Flow photochemistry: Old light through new windows. Beilstein Journal of Organic Chemistry, 2012. 8: p. 2025-2052.

2. A. Gavriilidis, A. Constantinou, K. Hellgardt, K. K. (Mimi) Hii, G. J. Hutchings, G. L. Brett, S. Kuhn, and S. P. Marsden, Aerobic oxidations in flow: opportunities for the fine chemicals and pharmaceuticals industries, Reaction Chemistry & Engineering, 2016. 1: p. 595-612.

3. A. Yavorskyy, O. Shvydkiv, C. Limburg, K. Nolan, Y. M. C. Delauré, and M. Oelgemöller, Photooxygenations in a bubble column reactor, Green Chemistry, 2012. 14: p. 888-892.

4. M. Nakano, Y. Nishiyama, H. Tanimoto, T. Morimoto, and K. Kakiuchi, Remarkable Improvement of Organic Photoreaction Efficiency in the Flow Microreactor by the Slug Flow Condition Using Water, Organic Process Research and Development, 2016. 20: p. 1626−1632.

5. M. E. Leblebici, G. D. Stefanidis, T. Van Gerven, Comparison of photocatalytic space-time yields of 12 reactor designs for wastewater treatment, Chemical Engineering and Processing, 2015. 97: p. 106-111.

6. D. Ziegenbalg, G. Kreisel, D. Weiß, and D. Kralisch, OLEDs as prospective light sources for microstructured photoreactors, Photochemical & Photobiological Sciences, 2014. 13: p. 1005-1015.