An improved model for analyzing the performance of photocatalytic oxidation reactors in removing volatile organic compounds and its application

An improved photocatalytic oxidation (PCO) reactor model was developed to analyze the removal of volatile organic compounds (VOCs) in indoor air. One new parameter, the average total removing factor Kt, together with the other two parameters, the number of mass transfer units NTUm and the fractional conversion epsilon, are found to be the main parameters influencing the photooxidation performance of PCO reactors. Three new parameters, the ideal reaction number of mass transfer units, NTUm,ir; the ideal reaction fractional conversion, epsilonir; and the reaction effectiveness, eta, also are defined. These concepts are helpful to the structural design and optimization for PCO reactors. The application of the model in designing a plate-type PCO reactor is demonstrated. This study shows that the present model is an effective tool for designing PCO reactors and for evaluating VOC removal performance of available PCO reactors.     

An Improved Model for Analyzing the Performance of Photocatalytic Oxidation Reactors in Removing Volatile Organic Compounds and Its Application

Abstract

An improved photocatalytic oxidation (PCO) reactor model was developed to analyze the removal of volatile organic compounds (VOCs) in indoor air. One new parameter, the average total removing factor K _t, together with the other two parameters, the number of mass transfer units NTU_m and the fractional conversion e, are found to be the main parameters influencing the photooxidation performance of PCO reactors. Three new parameters, the ideal reaction number of mass transfer units, NTU_m,ir; the ideal reaction fractional conversion, ε_ir; and the reaction effectiveness, η, also are defined. These concepts are helpful to the structural design and optimization for PCO reactors. The application of the model in designing a plate-type PCO reactor is demonstrated. This study shows that the present model is an effective tool for designing PCO reactors and for evaluating VOC removal performance of available PCO reactors.     

Heterogeneous photocatalysis of aromatic and chlorinated volatile organic compounds (VOCs) for non-occupational indoor air application

The current study evaluated the technical feasibility of applying TiO2 photocatalysis to the removal of low-ppb concentrations of volatile organic compounds (VOCs) commonly associated with non-occupational indoor air quality issues. A series of experiments was conducted to evaluate five parameters (relative humidity (RH), hydraulic diameter (HD), feeding type (FT) for VOCs, photocatalytic oxidation (PCO) reactor material (RM), and inlet port size (IPS) of PCO reactor) in relation to the PCO destruction efficiencies of the selected target VOCs. None of the target VOCs exhibited any significant dependence on the RH, which is inconsistent with a previous study where, under conditions of low humidity and a ppm toluene inlet level, a drop in the PCO efficiency was reported with a decreasing humidity. However, the other four parameters (HD, RM, FT, and IPS) were found to be important for better VOC removal efficiencies as regards the application of TiO2 photocatalytic technology for cleansing non-occupational indoor air. The PCO destruction of VOCs at concentrations associated with non-occupational indoor air quality issues was up to nearly 100%, and the CO generated during PCO was a negligible addition to indoor CO levels. Accordingly, a PCO reactor would appear to be an important tool in the effort to improve non-occupational indoor air quality.     

Characteristics of photocatalytic oxidation of toluene, benzene, and their mixture

The investigation of the photocatalytic oxidation (PCO) of multicomponent volatile organic compounds (VOCs) is very important to the application of PCO technology, because there is seldom a single VOC component in indoor air. In this paper, the characteristics of binary indoor VOCs, toluene and benzene, were experimentally studied using a mass transfer based method that we developed. The concentration ranges for toluene and benzene were 4.48-27.4 mg/m3 and 1.82-4.08 mg/m3, respectively. We found the following: (1) the PCO of each individual contaminant studied obeys the unimolecular form of the Langmuir-Hinshelwood (L-H) rate form; (2) the PCO of the binary contaminants follow the competitive adsorption L-H rate form; (3) the reaction-coefficient for PCO of individual contaminants differs from that in the competitive adsorption L-H rate form; and (4) the component impact factor of A to B, put forward in this paper, is a useful parameter describing the influence of A on the reaction coefficient of B, and it was found that the impact factor of toluene (a chemically active component) on benzene (a chemically stable component) is high, and the impact factor of benzene on toluene is low.     

Influence of the inlet air in efficiency of photocatalytic devices for mineralization of VOCs in air-conditioning installations

Nowadays, a large proportion of photocatalytic oxidation (PCO) devices are being implemented in heating, ventilation and air-conditioning systems. However, no systematic studies have been carried out regarding the influence of inlet air preconditioning. To analyse the impact of the inlet air-conditions into photocatalytic efficiency, a simulated air-conditioning duct with flowing gas through inside was designed. Isobutylene was chosen as the target VOCs. The concentration in the gas phase was monitored using a photoionization detector. The influence of flow rate, relative humidity and temperature on the VOC removal efficiency was analysed. Experimental results were presented in terms of gas-removal efficiency (η) and clean air delivery rate (CADR) and analysed on a kinetic basis. From them, the weight of each parameter in the global process has been determined, from bigger to smaller contribution, flow>>temperature>relative humidity. Also, the relevance of the inlet air conditions has been illustrated in a model room in order to determinate the time necessary to obtain a threshold value accomplishing with enough air quality and the energy consumption of the device. Additionally, the photocatalytic decontamination has been assimilated to the “air exchange rate”, a parameter commonly used in indoor air quality studies. The results show that preconditioning of air can improve the efficiency of photocatalytic devices and bring important energy savings.     

Method for predicting photocatalytic oxidation rates of organic compounds

In designing a photocatalytic oxidation (PCO) system for a given air pollution source, destruction rates for volatile organic compounds (VOCs) are required. The objective of this research was to develop a systematic method of predicting PCO rate constants by correlating rate constants with physical-chemical characteristics of compounds. Accordingly, reaction rate constants were determined for destruction of volatile organics over a titanium dioxide (TiO2) catalyst in a continuous mixed-batch reactor. It was found that PCO rate constants for alkanes and alkenes vary linearly with gas-phase ionization potential (IP) and with gas-phase hydroxyl radical reaction rate constant. The correlations allow rates of destruction of compounds not tested in this research to be predicted based on physical-chemical characteristics.     

Effectiveness of photocatalytic filter for removing volatile organic compounds in the heating, ventilation, and air conditioning system

Nowadays, the heating, ventilation, and air conditioning (HVAC) system has been an important facility for maintaining indoor air quality. However, the primary function of typical HVAC systems is to control the temperature and humidity of the supply air. Most indoor air pollutants, such as volatile organic compounds (VOCs), cannot be removed by typical HVAC systems. Thus, some air handling units for removing VOCs should be added in typical HVAC systems. Among all of the air cleaning techniques used to remove indoor VOCs, photocatalytic oxidation is an attractive alternative technique for indoor air purification and deodorization. The objective of this research is to investigate the VOC removal efficiency of the photocatalytic filter in a HVAC system. Toluene and formaldehyde were chosen as the target pollutants. The experiments were conducted in a stainless steel chamber equipped with a simplified HVAC system. A mechanical filter coated with Degussa P25 titania photocatalyst and two commercial photocatalytic filters were used as the photocatalytic filters in this simplified HVAC system. The total air change rates were controlled at 0.5, 0.75, 1, 1.25, and 1.5 hr(-1), and the relative humidity (RH) was controlled at 30%, 50%, and 70%. The ultraviolet lamp used was a 4-W, ultraviolet-C (central wavelength at 254 nm) strip light bulb. The first-order decay constant of toluene and formaldehyde found in this study ranged from 0.381 to 1.01 hr(-1) under different total air change rates, from 0.34 to 0.433 hr(-1) under different RH, and from 0.381 to 0.433 hr(-1) for different photocatalytic filters.     

Effectiveness of Photocatalytic Filter for Removing Volatile Organic Compounds in the Heating,Ventilation, and Air Conditioning System

Abstract

Nowadays, the heating, ventilation, and air conditioning (HVAC) system has been an important facility for maintaining indoor air quality. However, the primary function of typical HVAC systems is to control the temperature and humidity of the supply air. Most indoor air pollutants, such as volatile organic compounds (VOCs), cannot be removed by typical HVAC systems. Thus, some air handling units for removing VOCs should be added in typical HVAC systems. Among all of the air cleaning techniques used to remove indoor VOCs, photocatalytic oxidation is an attractive alternative technique for indoor air purification and deodorization. The objective of this research is to investigate the VOC removal efficiency of the photocatalytic filter in a HVAC system. Toluene and formaldehyde were chosen as the target pollutants. The experiments were conducted in a stainless steel chamber equipped with a simplified HVAC system. A mechanical filter coated with Degussa P25 titania photocatalyst and two commercial photocatalytic filters were used as the photo-catalytic filters in this simplified HVAC system. The total air change rates were controlled at 0.5, 0.75, 1, 1.25, and 1.5 hr^?1, and the relative humidity (RH) was controlled at 30%, 50%, and 70%. The ultraviolet lamp used was a 4-W, ultraviolet-C (central wavelength at 254 nm) strip light bulb. The first-order decay constant of toluene and form-aldehyde found in this study ranged from 0.381 to 1.01 hr^?1 under different total air change rates, from 0.34 to 0.433 hr^?1 under different RH, and from 0.381 to 0.433 hr^?1 for different photocatalytic filters.     

Kinetics of photocatalytic VOCs abatement in a standardized reactor

A standardized micro-pilot scale continuous flow apparatus has been built up, using two types of differential photoreactors, and equipped with a refined control of the working conditions in order to study the photocatalytic degradation as a way to abate VOCs in air. Kinetic measurements have been carried out on trichloroethylene (TCE), methanol and benzene as model pollutants. The absence of diffusional limitation and of reaction product effect on the kinetics have both been demonstrated under our working conditions. The influences of concentration, light flux and temperature on the initial degradation rate have been studied. A simple Langmuir-Hinshelwood kinetic model has been verified for TCE and methanol, whereas benzene degradation was more complex and did not follow this simple mechanism. The determined experimental data aim at being useful in the scaling-up of photocatalytic reactors.     

Investigation of the Treatability of the Primary Indoor Volatile Organic Compounds on Activated Carbon Fiber Cloths at Typical Indoor Concentrations

Abstract

Volatile organic compounds (VOCs) are a major concern for indoor air pollution because of the impacts on human health. In recent years, interest has increased in the development and design of activated carbon filters for removing VOCs from indoor air. Although extensive information is available on sources, concentrations, and types of indoor VOCs, there is little or no information on the performance of indoor air adsorption systems for removing low concentrations of primary VOCs. Filter designs need to consider various factors such as empty bed contact time, humidity effects, competitive adsorption, and feed concentration variations, whereas adsorption capacities of the indoor VOCs at the indoor concentration levels are important parameters for filter design. A preliminary assessment of the feasibility of using adsorption filters to remove low concentrations of primary VOCs can be performed. This work relates the information (including VOC classes in indoor air, the typical indoor concentrations, and the adsorption isotherms) with the design of a particular adsorbent/adsorbates system. As groundwork for filter design and development, this study selects the primary VOCs in indoor air of residences, schools, and offices in different geographical areas (North America, Europe, and Asia) on the basis of occurrence, concentrations, and health effects. Activated carbon fiber cloths (ACFCs) are chosen as the adsorbents of interest. It is demonstrated that the isotherm of a VOC (e.g., toluene on the ACFC) at typical indoor concentrations—parts per billion by volume (ppbv) level—is different than the isotherm at parts per million by volume (ppmv) levels reported in the publications. The isotherms at the typical indoor concentrations for the selected primary VOCs are estimated using the Dubinin–Radushkevitch equation. The maximum specific throughput for an indoor VOC removal system to remove benzene is calculated as a worst-case scenario. It is shown that VOC adsorption capacity is an important indicator of a filter’s lifetime and needs to be studied at the appropriate concentration range. Future work requires better understanding of the realistic VOC concentrations and isotherms in indoor environments to efficiently utilize adsorbents.     

A mass transfer based method for measuring the reaction coefficients of a photocatalyst

We have developed a new method, the mass transfer based (MTB) method, for measuring the Langmuir–Hinshelwood (L–H) rate form reaction coefficients of photocatalysts. The conventional method for determining the reaction coefficients disregards the effect of mass transfer on the reaction surface by designing and controlling a reaction process to be reaction-limited. In contrast, the new MTB method takes the mass transfer effect into account by using a computational fluid dynamics (CFD) method. The reaction coefficients can be regressed by the measured reaction rates and the calculated VOC concentrations in the air adjacent to the reaction surface. Thus, by using the new method, the reaction coefficient of a reaction process can be accurately determined even if it is not reaction-limited. This is very important in cases where it is difficult to realize reaction-limited processes, such as photocatalytic oxidation of VOCs with strong UV radiation intensity. The relative error of the regressed reaction coefficients obtained by the new method is analyzed. To illustrate, we apply this method to measuring the reaction coefficients of TiO₂ photocatalytic decomposing formaldehyde. This method is very useful in determining the reaction coefficients of the photocatalytic oxidation of various VOCs simultaneously.     

Method of removal of volatile organic compounds by using wet scrubber coupled with photo-Fenton reaction - Preventing emission of by-products

The photo-Fenton reaction was applied as a novel method for the removal of volatile organic compounds (VOCs) in the gas phase, and its effectiveness was experimentally examined. In conventional VOCs removal methods using a photocatalyst or ozone, VOCs are oxidized in the gas phase. Therefore, incompletely oxidized intermediates, which may have adverse effects on health, are likely to contaminate the treated air. On the other hand, in the VOCs removal method developed in this study, because the VOCs are oxidized in the liquid phase by the photo-Fenton reaction, any incompletely oxidized intermediates produced are confined to the liquid phase. As a result, the contamination of the treated air by these harmful intermediates can be prevented. Using a semi-batch process, it was found that the removal efficiency for toluene in a one-pass test (residence time of 17 s) was 61%, for an inlet toluene gas concentration of 930 ppbv, an initial iron ion concentration of 20 mg L⁻¹, and an initial hydrogen peroxide concentration of 630 mg L⁻¹. The removal efficiency was almost constant as long as H₂O₂ was present in the solution. Proton transfer reaction mass spectrometry analysis confirmed the absence of any incompletely oxidized intermediates in the treated air.     

Can ornamental potted plants remove volatile organic compounds from indoor air? — a review

Volatile organic compounds (VOCs) are found in indoor air, and many of these can affect human health (e.g. formaldehyde and benzene are carcinogenic). Plants affect the levels of VOCs in indoor environments, thus they represent a potential green solution for improving indoor air quality that at the same time can improve human health. This article reviews scientific studies of plants’ ability to remove VOCs from indoor air. The focus of the review is on pathways of VOC removal by the plants and factors affecting the efficiency and rate of VOC removal by plants. Laboratory based studies indicate that plant induced removal of VOCs is a combination of direct (e.g. absorption) and indirect (e.g. biotransformation by microorganisms) mechanisms. They also demonstrate that plants’ rate of reducing the level of VOCs is influenced by a number of factors such as plant species, light intensity and VOC concentration. For instance, an increase in light intensity has in some studies been shown to lead to an increase in removal of a pollutant. Studies conducted in real-life settings such as offices and homes are few and show mixed results.     

Photocatalytic Oxidation of Volatile Organic Compounds Using Fluorescent Visible Light

Abstract

Photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) is a highly attractive alternative technology for purification and deodorization of indoor air. The main objectives of this study were to demonstrate that a common fluorescent visible light (FVL) lamp can be used to effectively remove by PCO low concentrations of VOCs from slightly contaminated air and to provide some fundamental and technical details on the process. The target VOC was n-butanol, which is a standard reference odorant. Its PCO was studied under a long residence time in a 3.7-L cylindrical reactor with commercial titanium dioxide (TiO₂) as the reference photocatalyst and using mostly FVL for illumination. For comparison only, a UV (black) light lamp was used. The gas-phase products were detected and quantified online by gas chromatography (GC). The effects of reactor residence time, of inlet concentration, and of the relative light intensity on the efficiency of the process were also evaluated. At a high n-butanol concentration (0.1 vol %), butanal and propanal were identified as the intermediate products of the process; ethanal appeared when the initial concentration was <850 ppm_v. This indicates that PCO leading to CO₂ and H₂O is relatively slow and proceeds in a stepwise manner. Although the efficiency of the process with an FVL lamp was significantly lower than when using a UV black light, complete PCO of low concentrations was achieved for 100 ppm_v. In a search for a material with photoactivation extended to higher wavelengths or increased photo-activity, several samples of transition metal- or silver ion-doped (2 atomic %) TiO₂ as well as SrTi_1xFe_xO₃ (x = 0.1 and 0.15) perovskites were included in the study. None of these materials was more active than pure TiO₂. The results of this study open new horizons in the area of indoor air quality (IAQ) control.     

Treatment of atrazine by integrating photocatalytic and biological processes

This research examines the degradation of atrazine by photocatalytic oxidation (PCO) under different experimental conditions. Deisopropylatrazine, deethylatrazine and deethyldeisopropylatrazine were formed as major intermediates based on gas chromatography-mass spectrometry. The reaction mixture was found to be toxic towards two bioassays, i.e. the Microtox and amphipods survival tests even when atrazine was completely degraded by PCO within 2 h under optimized conditions. The results indicate that adding H2O2 could significantly enhance the degradation of atrazine by PCO. Ammeline, ammelide and cyanuric acid (CA) became the major intermediates/products as detected by high performance liquid chromatography from 6th to the 40th h of PCO treatment. After 72 h PCO treatment, only CA was detectable in the reaction mixture. Further degradation of CA was carried out by a newly isolated CA-degrading bacterium, Sphingomonas capsulata. The photochemical pretreatment integrated with microbial degradation lead to the complete degradation and detoxification of atrazine.     

Interaction of volatile organic compounds with indoor materials—a small-scale screening method

Indoor air pollution caused by volatile organic compounds (VOCs) may affect the health and well-being of inhabitants. Uptake and release of these compounds by and from indoor materials alter their concentrations in indoor air: uptake will lower peak concentrations, whereas subsequent (slow) release at lower concentration levels will prolong the presence of VOCs in indoor air. An experimental set-up has been implemented where indoor materials are placed as a “membrane” separating two air compartments. Both compartments – consisting of Field and Laboratory Emission Cells FLECs – are constantly flushed with air, one air stream containing a mixture of 20 VOCs, and concentrations in both compartments are measured after 1 h. Ten materials usually covering extensive surfaces indoors were consecutively exposed to the vapour mixture at concentration levels typically found in indoor environments. Under the chosen experimental conditions, five of these materials exhibited a permeability high enough that VOCs could be detected on the other side. Mass transport of VOCs into and through indoor materials has therefore been confirmed by experiment. The set-up allows for a quick screening of indoor materials with respect to their sorption capacity and permeability.     

Photocatalytic oxidation of volatile organic compounds using fluorescent visible light

Photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) is a highly attractive alternative technology for purification and deodorization of indoor air. The main objectives of this study were to demonstrate that a common fluorescent visible light (FVL) lamp can be used to effectively remove by PCO low concentrations of VOCs from slightly contaminated air and to provide some fundamental and technical details on the process. The target VOC was n-butanol, which is a standard reference odorant. Its PCO was studied under a long residence time in a 3.7-L cylindrical reactor with commercial titanium dioxide (TiO2) as the reference photocatalyst and using mostly FVL for illumination. For comparison only, a UV (black) light lamp was used. The gas-phase products were detected and quantified online by gas chromatography (GC). The effects of reactor residence time, of inlet concentration, and of the relative light intensity on the efficiency of the process were also evaluated. At a high n-butanol concentration (0.1 vol %), butanal and propanal were identified as the intermediate products of the process; ethanal appeared when the initial concentration was < or = 850 ppm(v). This indicates that PCO leading to CO2 and H2O is relatively slow and proceeds in a stepwise manner. Although the efficiency of the process with an FVL lamp was significantly lower than when using a UV black light, complete PCO of low concentrations was achieved for 100 ppm(v). In a search for a material with photoactivation extended to higher wavelengths or increased photoactivity, several samples of transition metal- or silver ion-doped (2 atomic %) TiO2 as well as SrTi(1-x-)Fe(x)O3 (x = 0.1 and 0.15) perovskites were included in the study. None of these materials was more active than pure TiO2. The results of this study open new horizons in the area of in door air quality (IAQ) control.     

The use of indoor air measurements to evaluate intrusion of subsurface VOC vapors into buildings

The implementation of a risk-based corrective action approach often requires consideration of soil vapor migration into buildings and potential inhalation exposure and risk to human health. Due to the uncertainty associated with models for this pathway, there may be a desire to analyze indoor air samples to validate model predictions, and this approach is followed on a somewhat frequent basis at sites where risks are considered potentially significant. Indoor air testing can be problematic for a number of reasons. Soil vapor intrusion into buildings is complex, highly dependent on site-specific conditions, and may vary over time, complicating the interpretation of indoor air measurements when the goal is to deduce the subsurface-derived component. An extensive survey of indoor air quality data sets highlights the variability in indoor volatile organic compound (VOC) concentrations and numerous sources that can lead to elevated VOC levels. The contribution from soil vapor is likely to be small relative to VOCs from other sources for most sites. In light of these challenges, we discuss how studies that use indoor air testing to assess subsurface risks could be improved. To provide added perspective, we conclude by comparing indoor air concentrations and risks arising from subsurface VOCs, predicted using standard model equations for soil vapor fate and intrusion into buildings, to those associated with indoor sources.     

Feasibility of a tandem photocatalytic oxidation–adsorption system for removal of monoaromatic compounds at concentrations in the sub-ppm-range

Unlike previous photocatalytic oxidation (PCO) studies incorporated with adsorption, this study investigates the feasibility of applying a tandem PCO-adsorption hybrid technique regarding low-level monoaromatic compound removal. The PCO efficiencies decreased as the hydraulic diameter (HD) increased. A PCO reactor of a medium HD size was selected for further experiments. Under conditions relevant to the use of the PCO system, the CO level measured during the PCO process was minimal in comparison to indoor CO levels. Trace level formations of formaldehyde and acetaldehyde were observed during the photocatalytic process, but these compounds were undetectable at the activated carbon unit outlet. The degradation efficiencies, obtained from the PCO unit, exhibited a dependence on both the inlet concentration (IC) and relative humidity (RH), whereas those from the PCO-adsorption hybrid system did not. Under specific conditions, the PCO unit presented a high degradation efficiency of close to, or exceeding 90%, in regards to ethyl benzene, o-xylene, and m,p-xylene. However, the benzene air concentrations, after being treated by the PCO unit, substantially exceeded the USEPA inhalation reference concentration guideline of 30 μg m⁻³ (corresponding to 0.01 ppm). In contrast, the PCO-adsorption hybrid system presented a high removal efficiency of close to 100% regarding all compounds, regardless of the IC or RH range. Consequently, it is suggested that the PCO-adsorption hybrid system has a synergistic advantage of photocatalysis and adsorption in regards to the BTEX elimination process.     

Personal exposure of graduate students attending the college of natural sciences or social sciences to volatile organic compounds on campus

The present study measured the levels of 24 selected volatile organic compounds (VOCs) in the personal air samples obtained from graduate students attending the college of natural sciences (GSNSs) or social science (GSSSs) during their daily activities on campus along with associated indoor and outdoor air samples. In addition, the sources of their personal exposure were characterized using multivariate statistical models. In the personal samples of GSNSs and GSSSs, 16 and 15 different VOCs were always detected, respectively. The personal exposure of five chlorinated hydrocarbons and six aromatics was significantly higher for GSNSs than for GSSSs. Consistently, the indoor levels of these compounds were higher for GSNSs (in research and laboratory rooms) than for GSSSs (in research rooms). However, the personal exposure of two aromatic VOCs (1,2,4- and 1,3,5-trimethylbenzene) was higher for GSSSs. Moreover, the personal exposure of the five chlorinated and six aromatic compounds was significantly correlated with VOC concentrations both in the research and laboratory rooms of GSNSs and with those in the research rooms of GSSSs. For certain VOCs, outdoor sources were also a major contributor to the personal exposure of both GSNSs and GSSSs. The multivariate models identified five factors that accounted for 81% of the total variance and four factors that explained 76% of the total variance. It was further suggested that multiple indoor sources in research rooms such as office equipment, building finishing materials, and air fresheners were the main source for the personal exposure to VOCs for GSNSs, whereas building finishing materials were the main source for GSSSs.