Dyed microspheres for quantification of UV dose distributions: Photochemical reactor characterization by Lagrangian actinometry
Résumé
Lagrangian actinometry represents a new method of photochemical reactor characterization. The method is based on an application of dyed microspheres, which were developed by attachment of (E)-5-[2-(methoxycarbonyl)ethenyl]cytidine (hereafter referred to as S) to polystyrene microspheres. S is a nonfluorescent molecule that when subjected to ultraviolet (UV) irradiation yields a single product, 3-beta-D-ribofuranosyl-2,7-dioxopyrido[2,3-d]pyrimidine (hereafter referred to as P), which displays a strong fluorescence signal. Dyed microspheres were subjected to UV irradiation under a collimated beam and using a single-lamp, monochromatic (low pressure Hg), continuous-flow reactor. In parallel with these experiments, a biodosimetry experiment was conducted using Bacillus subtilis spores as the challenge organism. Particle-specific fluorescence intensity measurements were conducted on samples from the collimated-beam experiments and the flow-through reactor experiments by flow cytometry. Estimates of the dose distribution delivered by the flow-through reactor for each operating condition were developed by deconvolution of data resulting from flow cytometry analysis of these samples. In conjunction with these experiments, a numerical model was developed to simulate the behavior of the reactor system. A commercially available computational fluid dynamics package was used to simulate the flow field, while line-source integration was used to simulate the irradiance field. A particle-tracking algorithm was employed to interrogate the flow and irradiance field simulations for purposes of developing particle-specific (Lagrangian) estimates of dose delivery. Dose distribution estimates from the microspheres assays and the numerical simulations were combined with the measured dose-response behavior of B. subtilis spores to yield estimates of spore inactivation in the flow-through experiments. For the range of operating conditions used in these experiments, predictions of spore inactivation based on dose distribution estimates from both methods were in good agreement with each other, and with the measured spore inactivation behavior. Lagrangian actinometry is capable of yielding accurate, detailed measurements of dose delivery by continuous-flow UV systems. This method represents a substantial improvement over existing experiment-based methods of UV reactor characterization (e.g., biodosimetry) in that it yields a measurement of the dose distribution for a given operating condition. This method also represents an improvement over existing methods for validation of numerical simulations. Specifically, because this method yields a measurement of the dose distribution, it is possible to compare these measurements with predicted dose distributions from the numerical simulation. The combined application of biodosimetry, numerical modeling, and Lagrangian actinometry represents an extremely robust approach to reactor characterization and validation.