Chang-Seo Lee

Performance of ultraviolet photocatalytic oxidation for indoor air applications: systematic experimental evaluation.

Photocatalytic oxidation (PCO) is a promising technology that has potential to be applied in mechanically ventilated buildings to improve indoor air quality (IAQ). However, the major research studies were done in bench-top scale reactors under ideal reaction conditions. In addition, no study has been carried out on the investigation of the ozonation and photolysis effect using a pilot duct system. The objective of this study is the development of methodologies to evaluate the performance of PCO systems. A systematic parametric evaluation of the effects of various kinetic parameters, such as compounds type, inlet concentration, airflow rate, light intensity, and relative humidity, was conducted, and new interpretations were provided from a fundamental analysis. In addition, the photolysis effect under vacuum ultraviolet (VUV) irradiation for a variety of volatile organic compounds (VOCs) was examined for the first time in a pilot duct system. The performance comparison of ultraviolet C (UVC)-PCO and VUV-PCO was also discussed due to the presence of ozone. Moreover, the formation of by-products generated with or without ozone generation was fully compared to evaluate the PCO technology.

Adsorption performance of titanium dioxide (TiO2) coated air filters for volatile organic compounds

The photocatalytic oxidation (PCO) technology as an alternative method for air purification has been studied for decades and a variety of PCO models indicate that the adsorption of reactants on the catalyst surface is one of the major physical and chemical processes occurring at a heterogeneous photocatalytic reaction. However, limited study explored the adsorption effect of a photocatalyst. This study carried out a systematic evaluation of adsorption performance of titanium dioxide (TiO(2)) coated fiberglass fibers (FGFs), TiO(2) coated carbon cloth fibers (CCFs), and original CCFs air filters at various relative humidity conditions for nine volatile organic compounds. TiO(2)/FGFs, TiO(2)/CCFs, and CCFs were characterized by SEM for morphology and N(2) adsorption isotherm for BET surface area and pore structure. A bench-scale adsorption test setup was constructed and adsorption tests were performed at various relative humidity conditions and four different injected concentrations for each compound. The isothermal adsorption curves at low concentration levels were obtained and they were well described by Langmuir isotherm model. It was noticed that there were significant differences between the adsorption behaviors and photocatalytic activities of TiO(2)/FGFs and TiO(2)/CCFs. It was concluded that adsorption performance is closely related to the characteristics of substrates and therefore, the development of a substrate with high adsorption ability is a promising trend for improving the performance of the UV-PCO technology.

Conjugate mass transfer modeling for VOC source and sink behavior of porous building materials : When to apply it?

Volatile organic compounds (VOC) are major indoor air pollutants. Physical models have been developed to predict VOC source (emission) and sink behavior (sorption) of building materials. They frequently adopt the conventional convection approach using a third-kind boundary condition. This conventional convection approach in conjunction with the commonly used Sherwood number correlation is based on the assumptions of constant wall concentration at the material-air interface and quasi-steady convective mass transfer in the fluid (air). In this study, the validity of these assumptions is theoretically investigated. An analytical model using the conventional convection approach and a numerical conjugate mass transfer model are developed. The conjugate mass transfer models consider unsteady two-dimensional laminar forced convection over a flat plate coupled with unsteady one-dimensional diffusion and sorption within the porous solid through the concentration and the flux continuities at the material-air interface. The simulation results indicate that the assumptions can lead to a significant overestimation of the wall concentration especially in the early transfer phase. When the effect on the VOC source/sink behavior is quantified by the total transfer time, which is the time required to emit/absorb 99% of the maximum transferable VOC mass, the analytical model results in less than 5% error in the predicted value when VOC transfer is controlled by internal diffusion, i.e., Biot number larger than 9 for (ε + K) 100.

Evaluation of ultraviolet-photocatalytic oxidation of light alcohols at sub-parts per million concentrations

Ultraviolet photocatalytic oxidation can be a promising technology as it has shown great potential in improving indoor air quality while concurrently providing an energy-efficient solution for application in the heating, ventilating, and air-conditioning systems. At this time, there are some concerns regarding possible formation of toxic by-products from photocatalytic oxidation reactions that undermine the application of this technology in non-industrial buildings. This article reports the outcome of an experimental study on the evaluation of photocatalytic oxidation reactions of light alcoholic volatile organic compounds with nano TiO2 catalysts at different indoor air conditions. The removal efficiencies of tested individual volatile organic compounds and their by-products were compared at three different parts per billion-level challenge concentrations. Acetaldehyde and formaldehyde were identified as primary by-products, and no significant catalyst deactivation was found during the experiment. The reaction pathways and the selectivity of the reactions were investigated at different relative humidity levels. It is concluded that the humidity had dual effects on the oxidation rate in the case where the degradation rate of the compound under study was relatively higher under low relative humidity levels.

A systematic approach for evaluation of gas-phase filter model

This article reports the development of a systematic methodology for the evaluation of gas-phase filtration models. In this approach, two sets of experiments are performed. For the first, all of the required model input parameters are quantified either experimentally or empirically. The second set of experiments is needed for the overall model validation process. The proposed methodology was applied to an existing gas-phase filter model that was developed for application to a single or a mixture of contaminants. The model was evaluated for two gases, namely n-hexane and methyl ethyl ketone, and four scenarios: (1) single methyl ethyl ketone at a dry air condition, (2) single n-hexane at a dry air condition, (3) a mixture of methyl ethyl ketone and n-hexane at a dry air condition, and (4) a mixture of methyl ethyl ketone and n-hexane at a humid air condition. The model was able to predict the lifetime of the filter for a single contaminant with less than 10% relative error. For the binary mixture, the model could not predict the lifetime of the heavier compound; however, it was able to predict the lifetime of the filter for the lighter compound with about 25% relative error. For the case of a mixture, the model underestimates the displacement phenomenon of a lighter compound. It was also noted that in the case of a heavier compound, there is good agreement between the models prediction, when it was applied to a single gas, and the experimental data for the single and mixture gas. It was also concluded that humidity has little effect on the breakthrough profile.

Improving HVAC System Performance: Towards the Design of Sustainable and Immune Buildings

This paper describes how superstructures, shopping centers and/or high-rise buildings are becoming part of today’s city landscapes. On the one hand, concerns about occupants’ health, comfort, energy consumption and environment are becoming part of the main design consideration for these buildings’ ventilation systems, and, on the other hand, threats to public safety and security such as the intentional and/or non-intentional release of chemical and other agents into indoor environments have become imminent in everyday life. Therefore, designers now need to integrate in their design not only consideration for the occupants’ comfort and building energy consumption but also occupants’ health and safety. Activated carbon filters have been used for purification of air and water in industrial applications. However these technologies have not been applied to the non-industrial built environment in general and there is no standard to quantify or to classify the performance of these systems for in-duct mechanical system application. The development of a standard testing procedure and design tool are a very timely effort, since it would create a benchmark for evaluating the contaminant reduction and energy savings of these systems. This paper describes the experimental set-up for testing activated carbon filters for in-duct mechanical system application first and then it presents the experimental results of twelve different activated carbon filters and then discusses the recommendations for the future works.

Deactivation and ultraviolet C-induced regeneration of photocatalytic oxidation air filters

Ultra-violet photocatalytic oxidation has been regarded as one of the promising air purification technologies for improving indoor air quality. However, limited availability of experimental data in terms of photocatalyst deactivation and regeneration has hindered successful implementation of ultra-violet photocatalytic oxidation air cleaners in mechanical ventilation systems. The objective of this study is to obtain knowledge of the ultraviolet C-induced regeneration method, the simplest on-site approach, for the recovery of photocatalytic activity of photocatalytic oxidation filters after challenging ultra-violet photocatalytic oxidation systems with approximately 100 ppb of acetone or methyl ethyl ketone. Experimental observations of photocatalyst deactivation, and characterization of fresh and deactivated photocatalyst with the scanning electron microscope technique were presented. During the regeneration process, the production rates of formaldehyde, acetaldehyde, and acetone were hourly quantified under a short-term and a long-term ultraviolet C illumination. The regeneration performance was also examined and compared by testing the single-pass removal efficiency of regenerated photocatalytic oxidation filters by two methods: ultraviolet C illumination and O3-included ultraviolet C illumination. The results indicate that the ultraviolet C-induced regeneration method, superior to O3-assisted ultraviolet C method, plays a certain role in partial recovery of the photocatalytic activity. The degree of recovery would depend on the nature of contaminant gases previously processed in ultra-violet photocatalytic oxidation since different VOCs generate various types and amounts of surface adsorbed by-products, which resist regeneration at a different level. Graphical Abstract. UV-PCO represents a new generation technology for improving indoor air quality. However, little is known about photocatalyst deactivation and regeneration which is a major concern for the purpose of commercialization. The current objective is to explore the UVC-induced regeneration method, the simplest on-site approach, employed in an HVAC system. This work demonstrates a systematic evaluation of deactivation and UV-induced regeneration performance under the conditions relevant to the actual applications for two VOCs. In addition, the gaseous by-product generation rates and O3-assited UVC-induced recovery method were examined for the first time. GRAPHICAL ABSTRACT