Modeling of gas-phase heterogeneous photocatalytic oxidation reactor in the presence of mass transfer limitation and axial dispersion

Abstract

Mass transfer plays a critical role in the efficiency of photocatalytic oxidation (PCO) technology for air purification applications. There has been limited work on the exploration of mass transfer in the PCO reactor with the non-ideal flow. In this work, the performance of a continuous heterogeneous ultraviolet photocatalytic oxidation (UV-PCO) reactor was investigated and addressed under mass transfer limitation and axial dispersion. First, CFD modeling was used to determine the flow distribution in the reactor at various airflow rates. The residence time distribution (RTD) analysis with a tracer gas (CO2) was carried out for the experimental validation of the simulation model and good agreement between experimental and simulation data was achieved. Further, a quick and straightforward methodology employing an axial dispersion plug flow model was used to study the RTD. The proposed model could predict the residence time distribution of CO2 with high accuracy. Results of the RTD for CO2 in the presence and absence of a PCO filter (silica fiber felts (SFF) modified with TiO2) were almost identical due to the high porosity of the filter. The model was then used for the evaluation of the axial dispersion of methyl ethyl ketone (MEK) at different flow rates. Owing to a low value of the Peclet number (Pe < 100), the flow in the reactor deviated from an ideal plug flow and dispersion could not be ignored. The photocatalysis reaction of MEK by the SFF filter under the mass-transfer-controlled regime was further addressed. A steady-state mass balance equation was implemented for the assessment of the performance of the UV-PCO reactor in the presence of axial dispersion and mass transfer limitation. The mass transfer coefficient was calculated with the use of the developed model and a correlation for the Sherwood number was proposed to relate the mass transfer coefficient to the flow rate and fiber structure. The proposed correlation was assessed and compared with the other empirical formulas available in the literature.