Photocatalytic degradation of toluene in an annular LED-based photoreactor

Abstract

In this work, gas phase toluene was degraded as model contaminant in an annular LED-based photocatalytic reactor under various light irradiance and illumination modes as a means to enhance the efficiency in illumination. The effect of controlled periodic illumination was first studied by exploring the effect of the duty cycle and the period and compared to continuous illumination at equivalent light irradiance. Controlled periodic illumination is based on the hypothesis that introducing photons in an alternate fashion rather than continuously avoids the buildup of charges and consequently, diminishes electron-hole recombination. The results show that under a kinetic-limited regime, there is no difference between the conversion of toluene under controlled periodic illumination and continuous illumination at equivalent average irradiance. Consequently, the photonic efficiency for controlled periodic illumination and continuous illumination at equivalent irradiance is also the same. Nonetheless, the photonic efficiency diminished with increasing average light irradiance. This behavior is ascribed to increasing electron-hole recombination with increasing duty cycle for controlled periodic illumination and with increasing light irradiance for continuous illumination. The second illumination technique investigated was the following of different radiation profiles along the reactor length. The LED array of the annular photoreactor was controlled with five different power supplies, each controlling one-fifth of the reactor length; thus, allowing to test experimentally different light irradiance per section of reactor. It was hypothesized that since at the inlet of the reactor the mass transfer driving force is larger than at the outlet of the reactor, a lower irradiance at the inlet would still yield good toluene conversion while at the same time decreasing the energy input. Experimentally, both an increasing and decreasing light irradiance profiles were tested and compared to a uniform profile at equivalent light irradiance. The results show that an increasing irradiance profile yielded a slightly higher conversion than uniform illumination; however, the difference was not statistically significant. In addition, a mathematical model of five photocatalytic reactors in series was developed to further explore the effect of following different irradiation profiles. By solving an optimization problem to minimize both the irradiance and outlet concentration of toluene, it was noted that the highest toluene conversion is always achieved under a uniform illumination of the reactor. Finally, this thesis presents the experiments to design the irradiation process control. The effect of relative humidity and toluene inlet concentration perturbations on the degradation of toluene, production of carbon dioxide, and mineralization are also analyzed. The results show that increasing the relative humidity to a certain value will stop catalyst deactivation and will augment the mineralization of toluene. In contrast, an increase in toluene inlet concentration will diminish the mineralization of toluene. Furthermore, a PI feedback controller was designed and validated manually. The feedback controller was able to maintain the conversion of toluene at the set-point at experimental conditions with no perturbations and at experimental conditions with perturbations such as catalyst deactivation, and changes in relative humidity and toluene inlet concentration.