CFD modeling of pharmaceuticals and CECs removal by UV/H2O2 process in helical microcapillary photoreactors and evaluation of OH radical rate constants

Abstract

Process intensification by tailored secondary flow in helical microcapillary film (MCF) photoreactors was unveiled by computational fluid dynamics, and it was revealed for the removal of six common contaminants of emerging concern CECs (the antiviral Acyclovir, the antiretrovirals Stavudine and Zidovudine, and the biocidal antifungal agents Methylisothiazolinone, Benzisothiazolinone and Isoxazole) in water by UV hydrogen peroxide. The MCF photoreactors consisted of fluoropolymer films containing 10 microchannels with diameter varying from 100 to 1000 µm coiled around a UVC lamp. In contrast to a MCF with straight channels, mixing intensification by secondary flow (Dean vortices) caused by the helical shape of the microcapillary strongly enhanced the radial fluid mixing, further supplementing the transport of the reacting species by Taylor-Aris dispersion. The intensity of the Dean vortices formed was correlated to the Dean (De) and Schmidt (Sc) numbers through a new correlation for the radial Peclet, which established that these become significant when De1.94Sc>67. Thus, the second-order reaction rate constant of the six CECs with OH• radicals (kOH) determined in a helical MCF photoreactor increased (4.4% up to 37.9%) in comparison to those determined assuming a MCF photoreactor with plug flow. In addition, the helical shape of the MCF significantly diminished mass transfer limitations and decreased the CECs Electrical Energy per Order Reduction (EEO), paving the way for scaling-up of helical microcapillary photoreactor technology. This study shows how micromixing can be successfully exploited to design more efficient microcapillary photoreactors.