INTRODUCTION
Research rationale
Organic compounds, primarily composed of carbon, are essential components of all living organisms Among these, volatile organic compounds (VOCs) are characterized by their ability to easily vaporize due to their high vapor pressure and typically low boiling points, often below 15 °C VOCs can also include various substitutes like hydrogen, oxygen, and halogens, which may pose additional health risks Common sources of VOC emissions include the combustion of fuels such as gasoline, wood, and natural gas, as well as activities involving solvents, paints, and adhesives found in both residential and workplace environments.
The petrochemical and various other industries produce and utilize numerous types of volatile organic compounds (VOCs), with investigations revealing that over 500 tons of these chemicals are released into the air annually in Thailand Exposure to multiple VOCs can lead to a range of health issues, including fatigue, nausea, impaired vigilance, confusion, drowsiness, and respiratory problems like irritant-induced asthma Short-term high exposure to VOCs can result in significant health effects, and certain VOCs, such as formaldehyde and benzene, are recognized by health authorities as probable human carcinogens.
There are five conventional techniques for controlling VOC emissions in various industries: absorption, adsorption, incineration/oxidation, bio-filtration, and condensation Each method has distinct advantages and limitations Absorption is limited by the solubility of VOCs in the chosen liquid, while smaller droplets can enhance gas solubility Adsorption offers high removal efficiency but comes with high operational and disposal costs Incineration/oxidation is effective for high-concentration VOC emissions but may incur additional costs for lower concentrations due to the need for co-fuels Bio-filtration is the most cost-effective option, utilizing microorganisms like bacteria and fungi to degrade pollutants, though it may experience bio-clogging issues Lastly, condensation efficiently recycles gaseous VOCs into liquids under high pressure or low temperature, but is only cost-effective at high pollutant concentrations.
Vacuum Ultraviolet (VUV) light, with wavelengths shorter than 200 nm, is highly absorbed by molecular oxygen in the air, while wavelengths between 150–200 nm can penetrate nitrogen, which is effective for oxidizing volatile organic compounds (VOCs) A VUV lamp operates at a wavelength of 185 nm, generating energetic photons that activate oxygen and water vapor, leading to the production of reactive species such as O(D), O(P), hydroxyl radicals (OH), and ozone Although VUV technology has been utilized to decompose various VOCs, including benzene and toluene, its application is limited due to the generation of ozone byproducts and its low capacity for degradation and mineralization of VOCs.
In this study, VUV was applied to remove toluene vapor from synthesis gas, as a case study Oxidation of VOCs was performed under VUV radiation in a continuous flow reactor.
Reasearch’s Objectives
To assess the efficiency of toluene removal using VUV radiation in a continuous flow reactor
Research questions and hypothesis
1 A breach-scale experiment was set up at the Department of Environmental Engineering, King Mongkut’s University of Technology Thonburi (KMUTT) A 3-L stainless reactor was selected in the study
2 Toluene vapor was simulated using a toluene generator developed under this study
4 The removal efficiency was assessed using measurement at inlet and outlet of the reactor
5 A multi-gas detector (MultiRae) was used for measurement of toluene concentration based on photoionization detection (PID)
6 Optimum inlet concentration was also studied by vary an inlet gas flow rate The target inlet toluene concentrations under this study were 100, 150, and 200 ppm.
Limitations
The outdated VUV lamp used in this experiment may produce weak radiation, potentially impacting the results Additionally, the reactor design is a crucial factor to consider, but time constraints prevent us from developing a new reactor.
The limited duration of the internship, lasting only three and a half months, posed a challenge for conducting the experiment This timeframe was necessary to explore the new field of air pollution, specifically focusing on understanding VUV and gaining knowledge about VOC.
Definitions
Toluene, also known as methylbenzene or phenylmethane, is a colorless, low-viscosity aromatic hydrocarbon with the chemical formula C7H8 It is slightly soluble in water, with solubility levels of 0.047 g/100ml at 160°C and 0.04 g/100ml at 150°C Toluene is highly compatible with various substances, dissolving well in lipids, oils, resins, phosphorus, sulfur, and iodine, and it readily mixes with organic solvents such as alcohol, ether, and ketones Additionally, toluene is recognized as a flammable solvent.
Figure 1.1 Chemical properties of toluene
Table 1.1 Physical properties of toluene
Physical state and appearance Clear liquid
Odor threshold 2.14 ppm (8 mg/cu.m.)
Toluene, a natural component of gasoline and crude oil, is widely utilized as an additive to enhance octane ratings in gasoline, and as a solvent in various products such as paints, coatings, inks, adhesives, and cleaners It plays a crucial role in the production of benzene, nylon, plastics, and polyurethanes Historically, toluene was also employed as a medicinal anthelmintic agent against roundworms and hookworms However, it is important to note that toluene is a highly lipophilic toxin that can lead to significant neurological damage, including loss of myelin and atrophy in cerebral and cerebellar white matter.
Toluene is a hazardous substance that irritates the skin, eyes, and respiratory system, with inhalation being the primary exposure route It poses a risk of systemic toxicity through ingestion or inhalation, and can be absorbed slowly through the skin Symptoms of toluene poisoning include central nervous system effects such as headache, dizziness, and drowsiness, as well as more severe reactions like seizures, respiratory depression, and electrolyte imbalances.
The American Conference of Governmental Industrial Hygienists (ACGIH)
In 1997, a threshold limit value of 188 mg/m³ for an 8-hour time-weighted average exposure to toluene in workplace air was recommended, accompanied by a skin notation Other countries have set standards ranging from 100 to 380 mg/m³ (International Labor Office, 1991) Additionally, the World Health Organization established a provisional guideline for toluene in drinking water at 700 μg/L (WHO, 1993).
Table 1.2 Occupational Exposure Limits of toluene from USA
OSHA PEL ( Occupational Safety and
Health Administration permissible exposure limit)
200 ppm (averaged over an 8-hour work-shift)
OSHA STEL (short-term exposure limit) 500 ppm (10-minute exposure)
NIOSH IDLH (immediately dangerous to life or health)
The ACGIH TLV (Threshold Limit Value) for exposure is set at 50 ppm, averaged over an 8-hour work shift Additionally, the AIHA ERPG-2 (Emergency Response Planning Guideline) indicates a maximum airborne concentration that allows nearly all individuals to be exposed for up to 1 hour without experiencing irreversible health effects or symptoms that could hinder their ability to take protective action.
Table 1.3 Maximum allowable concentration of some hazardous substances in ambient air in Viet Nam (Legal 2006)
Inorganic substances Chemical formula The average time Allowable concentration Toluene
Unit: Microgram per cubic meter (μg/m3)
1.5.3 Vacuum ultraviolet Vacuum Ultraviolet, or VUV, wavelengths (10 -
VUV lamps emit UV light at wavelengths around 150–200 nm, which can effectively propagate through nitrogen These wavelengths are highly active in the oxidation of volatile organic compounds (VOCs), while shorter wavelengths (below 200 nm) are strongly absorbed by molecular oxygen in the air.
The 185 nm wavelength generates energetic photons capable of activating oxygen and water vapor, resulting in the production of reactive species such as O(D), O(P), hydroxyl radicals (OH), and ozone While vacuum ultraviolet (VUV) technology has been effective in degrading various volatile organic compounds (VOCs) like benzene and toluene, its practical application is hindered by the formation of ozone as a byproduct and its limited efficiency in degrading and mineralizing VOCs.
LITERATURE REVIEW
Air pollution is a critical global issue, characterized by the contamination of the atmosphere with harmful chemicals and biological materials, particularly volatile organic compounds (VOCs), sulfur dioxide (SO2), and nitrogen dioxide (NO2) In response to this challenge, Chinese researchers have discovered a method to reduce VOC gaseous pollutants through the application of vacuum ultraviolet (VUV) radiation combined with catalytic processes.
Huang and Leung (2014) investigated the enhanced degradation of gaseous benzene using vacuum ultraviolet (VUV) radiation over transition metal-modified TiO2 Their findings revealed that the Mn/TiO2 photocatalyst achieved the highest benzene removal efficiency of 58%, while Co/TiO2, Ni/TiO2, and P25 exhibited a consistent efficiency of 50% In contrast, Fe/TiO2 and undoped TiO2 showed a decline to 45%, indicating that benzene degradation was ineffective The absence of ozone production under a 254 nm UV lamp suggested that both direct photo-oxidation and catalytic ozonation processes were not present Additionally, the study highlighted that water vapor played a dual role in benzene oxidation during the VUV-PCO process, with catalytic reactions dominating at low humidity and 185 nm photo-oxidation becoming the primary pathway at high humidity.
Huang (2016) investigated the photocatalytic oxidation of gaseous benzene using TiO2/zeolite catalysts under VUV radiation, achieving 100% benzene removal efficiency primarily due to initial absorption The absorption capacity for benzene is influenced not only by the BET (Brunauer-Emmett-Teller) surface area but also by the pore diameter of the zeolite The sole product of the benzene photocatalytic oxidation process was carbon dioxide (CO2).
In a study by Zhao (2013), the health risks associated with the vacuum ultraviolet (VUV) photolysis of naphthalene (NP) in indoor air were assessed using gas chromatograph–mass spectrometry and proton transfer reaction-mass spectrometry to detect intermediates The findings revealed that the accumulation of volatile organic compounds (VOCs), particularly harmful aldehydes, increased the health risk influence index (ᶯ) to 150 after just 2.81 minutes of VUV irradiation However, with a longer exposure time of 7.01 minutes, the mineralization of VOCs significantly reduced the health risk influence index (ᶯ) to 28 This indicates that the mineralization of VOCs plays a crucial role in mitigating the health risks associated with photolysis.
The results will give a safe and economical application of VUV photolysis technology in indoor air purification
In a study conducted by Chaolin Li on February 26, 2014, the photolysis of low concentrations of H2S malodorous gas was investigated using UV/VUV irradiation from high-frequency discharge electrodeless lamps The research revealed that over 90% of H2S removal efficiency was achieved at low concentrations (3.1–29.6 mg m^-3) across various gas residence times (2.9–23.2 s) The study highlighted the significant impact of relative humidity and oxygen concentration on H2S removal, emphasizing the role of these media in the photolysis process The possible mechanisms identified for photolysis include direct photolysis through UV/VUV light and indirect photolysis facilitated by ozone and hydroxyl radicals.
Huiling Huang and Haibao Huang (6 January 2016) investigated the efficient degradation of gaseous benzene using VUV photolysis in combination with ozone-assisted catalytic oxidation (OZCO) This study uniquely integrates a highly effective Mn/ZSM-5 catalyst with VUV photolysis to enhance the elimination of ozone and improve the degradation efficiency of volatile organic compounds (VOCs) Their findings reveal that the benzene removal efficiency achieved was only 48%.
The study demonstrated that a Mn/ZSM-5 catalyst effectively eliminated both benzene and residual O3 (83 ppm) during the VUV photolysis process By analyzing the identified products, potential degradation pathways and mechanisms were proposed for the innovative VUV-OZCO process This research offers valuable insights into an efficient method for the degradation of volatile organic compounds (VOCs).
In our study, we selected toluene as a representative volatile organic compound (VOC) due to its significant toxicity and photochemical activity Toluene was tested in a closed system under vacuum ultraviolet (VUV) radiation, with concentrations measured at both the inlet and outlet It was observed that the outlet concentration began to be measured immediately after the VUV lamp was turned on.
Toluene, a clear to amber liquid from the benzene series, has a strong, benzene-like odor and a low vapor pressure that leads to significant volatilization at room temperature It is highly flammable, with a flash point of 4.4 °C, and reacts vigorously with various chemicals, especially nitrogen-containing compounds, as well as some plastics To ensure safety, the ACGIH recommends an 8-hour time-weighted average (TWA) exposure limit of 50 ppm (189 mg/m³) to mitigate central nervous system effects, while OSHA has set a permissible exposure limit (PEL) of 200 ppm (754 mg/m³).
Toluene exposure for the general population primarily arises from gasoline, which contains 5% to 7% toluene by weight It is released into the atmosphere during gasoline production, transport, and combustion, leading to higher concentrations in areas with heavy traffic, near gas stations, and refineries Due to its reactivity with other air pollutants, toluene has a short lifespan in ambient air Additionally, toluene is utilized in aviation gasoline, high-octane blending stocks, and as a solvent for paints and coatings Other sources of toluene include tobacco smoke, petroleum and coal production, and its use as a chemical intermediate for styrene production.
Toluene is predominantly found in indoor air due to the use of everyday household items such as paints, paint thinners, adhesives, synthetic fragrances, and nail polish, as well as from cigarette smoke Intentional inhalation of substances like paint or glue can lead to significantly elevated toluene exposure, particularly among solvent abusers.
Toluene exposure may also occur in the workplace, especially in occupations such as printing or painting, where toluene is frequently used as a solvent
Levels of toluene was measured in rural, urban, and indoor air averaged 1.3, 10.8, and 31.5 micrograms per cubic meter (àg/m3), respectively.
METHOD
Materials
The list of materials used in this experiment is shown below
Table 1.4 List of materials used in the experiment
Valve 3 Use to controll flow rate of gas
Pipe 3 meters Lead the gas and toluene
Rotameter 1 Measure the flow rate of gas
Box 2 1 mix gas and toluene
Toluene bottle 1 lit 1 contain toluene
VOCs meter 1 Generrate electromagnetic radiation
Method
The experimental setup, illustrated in Figure 1.2, involved a pump that draws fresh air through a toluene bottle, causing vaporization of the liquid due to increased pressure The resulting high-concentration toluene vapor is then mixed with fresh air in a mixing box to achieve the desired dilution This mixed vapor is transported to a continuous flow VUV reactor, where the inlet concentration of toluene is measured using VOCs meter#1, while VOCs meter#2 measures the outlet concentration, allowing for the estimation of toluene removal efficiency To regulate the gas flow into the pipe, valves #1, #2, and #3 were installed, and a 3-L stainless reactor was chosen to minimize toluene adsorption on its surface.
Figure 1.2 Schematic diagram of experiment setup
3.2.2 Calculate flow rate of gas based on a retention time
To analyze the impact of retention time on efficiency, target flow rates were calculated for retention times of 0.5, 1, 2, 3, and 4 minutes in the reactor The detailed calculations are provided below, with a reactor diameter of 10 cm (0.1 m).
Table 1.5 Calculate the flow rate based on retention time
Retention time (min) Volume (m³) Flow rate(m³/min) Flow rate(L/min)
Based on the volume of reactor, the target flow rate was calculated as shown in Table 1.5 A rotameter was installed to control a flow rate of gas passing through the reactor
Step 1: Set the inlet toluene concentration at 100 ppm and controll the concentration to be stable within 30 minutes Check the toluene concentration for every 1 minute during the control period (30 minutes)
Step 2: Check the leakage of the system (with VUV lamp but not turn on) by measuring the outlet concentration for every 1 minutes during 30 minutes The outlet concentrations are expected to be equal with the inlet one if the outlet concentrations stable at 100 ppm, the experiment can be started by turn on the VUV lamp if the concentration is not stable 100 ppm, the inlet concentrations should be recheck again by going back to Step 1
Step 3: Activated carbon was installed to clean the remaining toluene at the outlet
Step 4: Start the experiment by turn on the VUV lamp and measure the outlet concentrations for every 1 minute during 30 minutes
Step 5: Adjust the inlet toluene concentrtions at 150 and 200 ppm, respectively and start step 1, 2, 3 and 4 again After the experiment, the system efficiencies for toluene removal were calculated
The experiment was conducted in the laboratory of the Department of Environmental Engineering at the Faculty of Engineering, where we analyzed toluene concentrations at 100, 150, and 200 ppm Each concentration was measured 30 times prior to activating the VUV lamp and 30 times after the lamp was turned on.
RESULT AND DISSCUSION
Result
4.1.1 Reduction of outlet concentration of 200 ppm and C/C ₀ (%)
We conducted the sample of toluene concentration at 200 ppm:
The outlet concentration of toluene was generated at 200 ppm and maintained consistently for 30 minutes Subsequently, the system's leakage was assessed using a VUV lamp (turned off) by measuring the outlet concentration every minute for 30 minutes The experiment commenced with the VUV lamp activated, followed by one-minute interval measurements of outlet concentrations for another 30 minutes Finally, the removal efficiency was calculated, with results displayed in Table 1.6, highlighting the concentrations before and after activating the VUV lamp along with the corresponding removal efficiency.
Table 1.6 Degradation and removal efficiency of outlet concentration of 200 ppm
Table 1.6 indicates that the toluene concentration decreased by approximately 9 ppm after 30 minutes of VUV lamp activation, resulting in a removal efficiency of 5% While this outcome is reasonable, it is notably lower than findings reported in other literature.
4.1.2 Reduction of outlet concentration of 150 ppm and C/C ₀ (%)
We conducted the sample of toluene concentration at 150 ppm:
The outlet concentration of toluene was generated at 150 ppm and maintained constant for 30 minutes A leakage check was conducted using a VUV lamp (turned off) by measuring the outlet concentration every minute during this period The experiment commenced with the VUV lamp turned on, and outlet concentrations were recorded every minute for another 30 minutes The removal efficiency was then calculated, with results displayed in Table 1.7, showing the concentrations before and after the VUV lamp activation along with the corresponding removal efficiency.
Table 1.7 Degradation and removal efficiency of outlet concentration of 150 ppm
Table 1.7 indicates that the concentration of toluene decreased by approximately 13 ppm after 30 minutes of VUV lamp activation, resulting in a removal efficiency of around 9% While this outcome is reasonable, it is notably lower than the efficiencies reported in other literature.
4.1.3 Reduction of outlet concentration of 100 ppm and C/C ₀ (%)
We conducted the sample of toluene concentration at 100 ppm:
The experiment involved generating a toluene outlet concentration of 100 ppm and maintaining it consistently for 30 minutes System leakage was assessed using a VUV lamp (not activated) by measuring outlet concentrations every minute for 30 minutes Following this, the VUV lamp was turned on, and outlet concentrations were recorded at one-minute intervals for another 30 minutes The removal efficiency was then calculated, with results displayed in Table 1.8, illustrating the concentrations before and after the VUV lamp activation along with the corresponding removal efficiency.
Table 1.8 Degradation and removal efficiency of outlet concentration of 100 ppm
Table 1.8 indicates that the toluene concentration decreased by approximately 11 ppm after 30 minutes of VUV lamp operation This corresponds to a removal efficiency of about 11%, which, while reasonable, is lower than findings reported in other literature.
Disscusion
The reduction of toluene concentration, initially set at 200 ppm, is illustrated in Figure 1.4 following the activation of the VUV lamp The concentration decreased gradually from 200 ppm to 197 ppm between the 30th and 34th minutes, shortly after the introduction of VUV radiation Subsequently, the outlet concentrations stabilized around 197 ppm until the 40th minute, after which a significant drop occurred, bringing the concentration down to 191.2 ppm by the 50th minute The concentration continued to decline slowly, reaching 191.1 ppm by the 60th minute.
Figure 1.4 Changes of outlet toluene concentration (inlet concentration at 200 ppm) after apply VUV radiation
After turn on the VUV lamp, the inlet concentration of toluene reduced to
147 ppm and started to fluctuated at the 35 th minute at an average concentration of 147.7 ppm The outlet concentrations were dramatically decresed to 137.2 at the
53 rd minute Then, it slowly changed to 137 ppm at the 60 th minute
Figure 1.5 Changes of outlet toluene concentration (inlet concentration at 150 ppm) after apply VUV radiation
The application of VUV lamp radiation resulted in a decrease in outlet concentrations from 100 ppm to 97.8 ppm within the first 4 minutes Throughout the 34th to 37th minutes, the concentration stabilized around 97.6 ppm Subsequently, by the 52nd minute, the toluene concentration dropped significantly to 90.5 ppm, with a gradual decline to 89 ppm observed by the 60th minute.
Figure 1.6 Changes of outlet toluene concentration (inlet concentration at 100
Removal efficiecy
4.3.1 C/C ₀ efficiency at 200 ppm inlet concentration
Figure 1.7 shows the changes of removal efficiency of toluene over the time at the inlet concentration of 100 ppm The final efficiency was (1-0.95)*100% = 5% after 30 minutes of VUV radiation
Figure 1.7 Changes of removal efficiency at the inlet concentration of 200 ppm
4.3.2 C/C ₀ efficiency at 150 ppm inlet concentration
Concentration of toluene tended to reduce after 30 minutes, the maximum efficiency for the inlet concentration of 150 ppm was (1-0.91)*100%= 9%
Figure 1.8 Changes of removal efficiency at the inlet concentration of 150 ppm
4.3.3 C/C ₀ efficiency at 100 ppm inlet concentration
The highest toluene removal efficiency was (1-0.89)*100%= 11% for the inlet concentration of 100 ppm Figure 1.9 showed the changes of removal efficiency at the inlet concentration of 100 ppm
Figure 1.9 Changes of removal efficiency at the inlet concentration of 100 ppm
4.3.4 Comparison of removal efficiency of toluene
The overall toluene removal efficiency in the experiments was low, with reductions of 11% at 100 ppm, 9% at 150 ppm, and 5% at 200 ppm, which is significantly less than reported in other studies This reduced efficiency may be attributed to the reactor design used in the experiments.
VUV radiation exhibits peak power at approximately 1-2 mm from the lamp, while gas turbulence within the reactor plays a crucial role in the process The study revealed that removal efficiencies fluctuate based on inlet concentrations, with other ambient factors like gas moisture and temperature also significantly impacting results Increased moisture levels enhance the photolysis rate of removal Figure 1.10 illustrates the comparison of toluene removal efficiency across inlet concentrations of 100, 150, and 200 ppm.
Figure 1.10 Comparison of toluene removal efficiency for the inlet concentrations of 100, 150, and 200 ppm
CONCLUSION
Recent findings indicate that volatile organic compounds (VOCs) can be effectively removed under vacuum ultraviolet (VUV) irradiation in a continuous flow reactor, achieving a maximum toluene vapor removal efficiency of 11% The flow rate plays a crucial role in influencing the removal efficiency of toluene This innovative approach presents a promising solution for reducing VOCs in industrial settings where they are typically emitted, highlighting its potential application in future technologies.
In this experiment, key factors to consider include humidity levels, reactor design, and lamp quality The reactor design must be closely aligned with the experimental system to ensure optimal performance Additionally, using a high-quality lamp will produce strong irradiation, significantly enhancing the rate of toluene removal.
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