Design and Modelling of Photoreactors for Waste Treatment and Purification

Photochemistry is a research field with high potential, promising important future industrial and environmental applications such as pollutant removal, CO2 reduction back to fuel, alternative solar energy storages and novel materials. On the other hand, making these promises feasible and therefore applied, falls within the purpose of photoreactor design branch of chemical engineering science. However, photoreactor design could not yet live up to its whole potential as many promising photochemical reactions have not yet been integrated to industrial usage.

The issue with photoreactor design is that it adds a new type of transport phenomenon to the pre-existing mass and heat transfer, which is the photon transfer. The intuitive solution to this new problem has been to apply microflow reactors which had already achieved drastic increases in apparent reaction rates for reactions without light. When light was added to the intensified mass and heat transfer medium of microflow reactors, up to five orders of magnitude increase in apparent photochemical reaction rate was observed when compared to stirred tank photoreactors. This enabled the photo-microflow chemistry branch to blossom.

Photo-microflow strategy aims to improve the practical usage of photochemistry by increasing the reaction rate. However, it also inherits the biggest drawback from the microflow reactors, which is the lack of throughput. Increase of the reaction rate may mean higher production rate per volume, but if it comes at the expense of reactor volume itself, at a certain point, further decrease in the volume may be counter-productive and may decrease the product output. This suggests an optimization case.

This work is a compilation of case studies on two applications namely photocatalytic waste degradation in wastewater and photochemical waste purification from lamp phosphor waste.  

After a brief state-of-the-art on various photoreactor designs, the thesis focuses on the development and validation of a computational model for immobilized catalyst reactors for micropollutant removal from wastewater. The model does not only predict the apparent reaction rate, but is also able to calculate the concentration gradient within the catalyst coating and the free-flowing medium. 

In the following chapter, the thesis focuses on the waste purification application and proposes a new mixer - phase separator for solvent extraction of Eu. The phase separation does not rely on gravity, but is based on differences in wettability. This chapter, focusing on a non-photochemical system, shows that as the reactor channel dimension increases, the space-time yield decreases, but throughput increases, which requires less need for numbering up to meet the industrial throughput. This work has also shown that the 30 min long mixing-settling process can be intensified such that it is completed in 10 s in industrial concentrations.

The next chapter focuses on the scale up of intensified systems and iterates on the optimization problem stated above. It proposes a new benchmark, namely the photochemical space-time yield (PSTY), and proves that it can compare waste degradation reactors of very different scales and sizes. Furthermore, PSTY is proven to predict the industrial applicability of a reactor as the pilot-scale reactors scored higher than the laboratory scale ones.

In the following chapter, using PSTY, the said optimization case was proven and a method was proposed to calculate the optimum reactor characteristic length for a case study of photochemical Eu precipitation. The results prove that PSTY is not only a benchmark, but also a design parameter. The irradiance in the reactor was also modelled with a newly developed model, which uses P1 approximation of radiative transfer equation in participating media and a simple empirical equation for the air medium.

In summary, this thesis studies photoreactor design and iterates on the strategies of scale-up of these promising platforms.