| Summary: | 碩士 === 國立臺灣大學 === 環境工程學研究所 === 96 === Volatile organic compounds(VOCs) is one of the major indoor air pollutants, and is considered as an cause for sick building syndrome. Using wood furniture , paint, cleanser and perfume even for freshening and improving indoor air quality by Aromatherapy will produce biogenic volatile organic compounds like limonene. Because of long time exposure and possible production of secondary organic aerosols from reaction between limonene and oxidant so in this essay I choose limonene as a target pollutant in this study.
Among the indoor air-cleaning technology for controlling VOCs, photocatalytic oxidation (PCO) reaction is widely used in recent years because of the advantages such as higher cleaning efficiency, the lower energy consumption, and the fully oxidation and lower operation cost. Some researches have demonstrated that ozone has positive effects on effectiveness of PCO, and also ozone has high oxidation with limonene. The objective of this research was to investigate the enhancement effect of ozone on VOCs removal efficiency and the ozone removal efficiency.
There are many factors influencing PCO efficiency. In this study, the effects of limonene concentration, reactive humidity, gas flow rate and the ozone concentration on the removal of limonene were investigated. Other factors were fixed like controlling temperature on 25±1℃, using UV light source (wavelength is 254 nm), constant UV intensity, and choosing Degussa P25 TiO2 for the catalyst in this study.
The effect of gas-phase mass transfer was negligible when gas flow rate was higher than 1600 ml/min. And the PCO kinetics fitted Langmuir-Hinshelwood model for bimolecular competitive adsorption form. The VOCs conversion and CO2 evolution decreased with increasing limonene concentrations. However, the VOCs oxidation rate has opposite effect. In this experiment, removal efficiency of ozone ranged from 45.01% to 53.77%. After the reaction, concentration of ozone are close or below to 50 ppb.
The VOCs-hydroxyl radical rate constants (k_OH) of limonene is 15.6, 19.7, 16.0 μ-mole/m2-s respectively for humidity 20%, 50%, 80%. PCO rate constants of toluene, p-xylene, m-xylene, mesitylene, and limonene were proportional to k_OH no matter what humidity is. Getting together with Langmuir adsorption constants of the above-mentioned VOCs and water. A linear positive relationship was found between reciprocal of Langmuir adsorption constants and Henry’s Law constants no matter what reactive humidity is.
Increasing reactive humidity showed a dual effect on VOCs conversion, CO2 evolution, and the residual intermediate. The reactive humidity turning point is 40％. The experiment showed that adding ozone can reduce it significantly when the humidity move up to 30％. And it showed that in high humidity, adding ozone can control the residual intermediate on 10~20%, and extending the catalyst effectiveness.
In PCO reaction, the VOCs conversion and CO2 evolution increased with increasing concentration of ozone. But it will enhance the residual intermediate relatively. The slopes of plot of VOCs oxidation rates & ozone concentration were defined as enhancement indices of ozone. The experiment result showed enhancement indices of ozone on limonene is 4.15×〖10〗^(-4) μ-mole-m-2-s-1/ppb-O3. Getting together with above-mentioned species, enhancement indices of ozone were proportional to k_OH.
In PCO reaction, removing efficiency of ozone and VOCs oxidation increased with increasing the ozone removal efficiency. The ozone removal efficiency in the presence and absence of limonene ranged from 50.13% to 89.93% and from 40.23% to 77.29%. So this study found that VOCs oxidation is related to removing efficiency of ozone.
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