Enhancement Effect of UV/O3 on the Effectiveness of Photocatalytic Oxidation for Removing Indoor Volatile Organic Compounds

• Main Authors: Kuo-Pin Yu, 余國賓
• Other Authors: Grace Whei-May Lee
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/84711287968886366461

博士 === 國立臺灣大學 === 環境工程學研究所 === 94 === Volatile organic compounds (VOCs) are omnipresent indoors and relevant to the aggravation of indoor air quality (IAQ). Long-term exposure to VOCs may cause some harmful health effects and the sick building syndrome (SBS). Among those available air-cleaning techniques for controlling indoor VOCs (e.g. active carbon, photocatalytic, and ozone air cleaners), photocatalytic oxidation (PCO) is one of the fastest developed and most widely used in recent years because PCO can oxidize a verity of VOCs to CO2 and H2O under room temperature. However, some intermediates and by-products generated during the PCO reactions might result in the deactivation of photocatalyst. Some researches demonstrated that ozone has positive effects on the effectiveness of PCO. The objective of this research was to investigate the enhancement effect of ozone on indoor VOCs removal efficiency of PCO. Previous literatures showed that aromatic hydrocarbons were one of the most encountered indoor VOCs. Thus, four aromatic hydrocarbons—toluene, p-xylene, m-xylene, and mesitylene—were chosen as the target pollutants in this study. n-Hexane and iso-butanol were used as the target compounds for alkane and alcohol, respectively. A 45-cm-quartz tube was used as the photoreactor to conduct the experiments. Degussa P25 TiO2 was used as photocatalyst. A 15-Watt, UV-C (ultraviolet; the central wavelength is 254 nm) long-life strip light bulb was used as the UV light source. In this study, the effects of VOCs concentration, gas flow rate, humidity, and ozone concentration on the removal of VOCs were investigated. The effect of gas-phase mass transfer was negligible when gas flow rate was higher than 1000 mL/min. And the PCO kinetics fitted a Langmuir-Hinshelwood (L-H) model for bimolecular competitive adsorption form. The VOCs oxidation rate and CO2 yield rate increased with the increase of VOCs concentrations. However, the VOCs conversions and CO2 evolution decreased with the increase of VOCs concentrations. The PCO rate constants of toluene, p-xylene, m-xylene, and mesitylene ranged from 1.03 to 4.00 μ-mole m-2s-1, and were proportional to the VOCs-hydroxyl radical rate constants (kOH). The Langmuir adsorption constants of VOCs and water ranged from 0.95 to 1.35 ppm-1 and from 1.44×10-3 to 5.61×10-3 ppm-1, respectively. A linear positive relationship was found between the reciprocal of Langmuir adsorption constants and Henry’s Law constants of aromatic VOCs. Oppositely, the reciprocal of Langmuir adsorption constants of water showed a linear negative relationship with Henry’s Law constants of aromatic VOCs. The increase of humidity could enhance the formation of hydroxyl radicals. However, the VOCs confronted the competition from the water molecules for the OH adsorption sites on the surface of photocatalyst. Therefore, humidity showed a dual effect on the PCO reaction. The degrees of competitive adsorption were relevant to the hydrophilicities, Henry’s Law constants, and octanol/water partition coefficients (KOW) of VOCs. In this study, the degrees of competitive adsorption between the four aromatic hydrocarbons and water molecules were in the following order: toluene > p-xylene ≈ m-xylene > 1,3,5-trimethylbenzene. The VOCs oxidation rates were proportional to the ozone concentration. The slopes of the plot of VOCs oxidation rates & ozone concentration were defined as the enhancement indices of ozone. The enhancement indices of ozone on toluene, p-xylene, m-xylene, and mesitylene oxidation rates ranged from 1.41×10-6 to 1.80×10-6 μ-mole-m-2-s-1/ppb-O3, and were proportional to kOH. The ozone removal efficiency (ORE) of TiO2/UV/O3 reaction in the presence and absence of VOCs ranged from 61.1% to 99.9% and from 38.1% to 95.1%, respectively. The ORE of TiO2/UV/O3 reaction increased with VOCs concentration and retention time, and decreased with humidity and O3 concentration increasing. The ORE of UV/O3 reaction increased with retention time and humidity increasing. The O3 removals of UV/O3 reaction were first-order rate form regarding O3. VOCs removal efficiency in the heating ventilation and air-conditioning (HVAC) system by the PCO filter was also investigated. The target compounds were toluene and formaldehyde. The experiments were conducted under relative humidity of 30%, 50%, and 70%, which represented for the dry, mediate, and humid condition. The air change rates of the HVAC system were set between 0.5 and 1.5 hr-1. The toluene removal efficiency of the PCO filter ranged from 0.264 to 0.532, and the formaldehyde removal efficiency ranged from 0.348 to 0.736. The toluene and formaldehyde removal efficiency increased with the increase of relative humidity and decreased with the increase of face velocity. The clean air delivery rate, CADR, increased with face velocity increasing and reached a maximum when the face velocity was 444 m/hr. CADR per unit area (CADR/A) could be applied for “real-world” implications. And CADR/A represented for the VOCs oxidation rate independent of filtration area.