以紫外光/臭氧程序增進光觸媒對室內揮發性有機物去除效率之研究

Abstract

揮發性有機物 (Volatile organic compounds, VOC) 在室內環境中幾乎是無所不在，且與室內空氣品質不良有關。揮發性有機物的暴露對人體造成許多不良的健康影響以及引起所謂的病態建築症候群。 市面上用於控制室內揮發性有機物的方法很多，如活性碳吸附、臭氧等。其中，光觸媒 (Photocatalytic oxidation, PCO) 對於大部份之室內揮發性有機物有效，且能在室溫下將揮發性有機物完全分解成水及二氧化碳，因此成為近年來發展最快且應用最廣之室內空氣清淨技術。然而，光觸媒反應產生若干中間產物會造成觸媒活性之下降。過去文獻報告指出臭氧在光觸媒反應中具正面之效應。本研究之目的在於探討臭氧對於光觸媒對室內揮發性有機物去除效率之增進效應。 回顧文獻發現，常見之揮發性有機物物種如芳香族碳氫化物一直都是榜上有名，且未有改善之跡象。因此，本研究選擇甲苯 (toluene)、對-二甲苯 (p-xylene)、間-二甲苯 (m-xylene)、1,3,5-三甲基苯 (1,3,5-trimethyl- benzene, or mesitylene) 等四種芳香族碳氫化合物及正己烷 (n-hexane) 及異丁醇 (iso-butanol) 作為實驗之揮發性有機物物種。實驗使用之光反應器是以一45 cm石英管構成，以Degussa P25 TiO2 作為光觸媒，紫外光光源為15瓦，波長254 nm之長型燈管。實驗探討揮發性有機物濃度、氣體流率、濕度及臭氧濃度等因子對揮發性有機物去除之影響。 實驗結果顯示，在氣體流率大於1000 L/min時，氣相質傳效應可被忽略，此時光觸媒反應之動力模式符合雙分子之Langmuir-Hinshelwood (L-H) 模式。光觸媒反應之揮發性有機物轉化率及二氧化碳產出率隨揮發性有機物之濃度增加而減少，但其氧化速率之趨勢則為相反。光觸媒反應之中間產物的殘餘量隨著進流之揮發性有機物濃度減少而降低。甲苯、對-二甲苯、間-二甲苯、1,3,5-三甲基苯之光觸媒反應常數介於1.03 ~ 4.00 μ-mole m-2s-1之間且與氫氧自由基反應常數 (kOH) 成正比關係。VOC與水分子之 Langmuir 吸附常數分別介於 0.95 ~ 1.35 ppm-1及5.61×10-3 ~ 1.44×10-3 ppm-1之間。VOC之 Langmuir吸附常數的倒數與亨利定律常數 (Henry’s Law constant) 呈現線性正相關；相反地，水分子之 Langmuir吸附常數的倒數與亨利定律常數呈現線性負相關。此相關性在本研究中之適用於芳香族碳氫化合物。 濕度對於光觸媒具有增進氫氧自由基生成之正效應及競爭吸附之負效應。不同揮發性有機物物種與水分子之競爭吸附之作用強弱不同，競爭吸附之負效應之強弱也不同，這與各物種之親水性有關，而親水性可以亨利定律常數或是辛醇/水分配係數 (octanol/water partition coefficient, KOW)，實驗物種之芳香族揮發性有機物之競爭吸附之作用之強弱順序為：甲苯 > 對-二甲苯 ≈ 間-二甲苯 > 1,3,5-三甲基苯。 揮發性有機物之氧化速率與臭氧濃度成正比。以揮發性有機物之氧化速率與臭氧濃度作圖所得之斜率定義為臭氧對光觸媒反應增進效應指標。臭氧對於甲苯、對-二甲苯、間-二甲苯及1,3,5-三甲基苯之光觸媒反應增進效應指標 (enhancement index) 介於 1.41×10-6至1.80×10-6 (μ-mole-m-2-s-1/ppb-O3) 之間，且與氫氧自由基反應常數成正比。 在有揮發性有機物及無揮發性有機物狀態下，TiO2/UV/O3反應之臭氧去除效率分別為61.1 ~ 99.9% 及38.1 ~ 95.1%。臭氧去除效率隨揮發性有機物增加或停留時間增加而上升，但隨濕度增加或臭氧濃度增加而下降。UV/O3之臭氧去除效率隨停留時間及濕度增加而上升，且為臭氧濃度之一級反應。 另一部份實驗在一裝有光觸媒濾網之小型通風空調系統 (heating ventilation and air-conditioning system, HVAC system) 中進行，以探討光觸媒在通風空調系統之應用。此一實驗揮發性有機物物種為甲苯及甲醛，實驗分別在相對濕度30%、50% 及70% 下進行，分別代表乾、中、濕三種條件，系統之換氣率為0.5 ~ 1.5 hr-1。光觸媒濾網對甲苯之去除效率介於0.264 ~ 0.532，對甲醛之去除效率介於0.348 ~ 0.736。光觸媒濾網對甲苯及甲醛之去除效率隨濕度增加遞增，與過濾面速增加呈線性地遞減。實驗系統之清潔空氣釋放率 (clean air delivery rate, CADR) 隨過濾面速增加，且在過濾面速444 m/hr 時達到最大值。每單位面積光觸媒濾網之CADR 值 (CADR per unit area, CADR/A) 可作為將實驗數據運用至實際環境時 (up-scale system) 之參考依據，而CADR/A可代表實際之光觸媒氧化速率 (與濾材面積無關)。

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.