Environ. Sci. Technol. 2010, 44, 2585–2591
VOC Removal and Deodorization of Effluent Gases from an Industrial Plant by Photo-Oxidation, Chemical Oxidation, and Ozonization CELIA DOMEN ˜ O,† A ´ N G E L R O D R ´I G U E Z - L A F U E N T E , † JM MARTOS,‡ RAFAEL BILBAO,‡ AND C R I S T I N A N E R ´I N * , † Department of Analytical Chemistry and Department of Chemical and Environmental Engineering, I3A, CPS-University of Zaragoza, Marı´a de Luna st. 3, Torres Quevedo Building, E-50018 Zaragoza, Spain
Received September 10, 2009. Revised manuscript received January 22, 2010. Accepted February 11, 2010.
The efficiency of photo-oxidation, chemical oxidation by sodium hypochlorite, and ozonization for the industrial-scale removal of volatile organic compounds (VOCs) and odors from gaseous emissions was studied by applying these treatments (in an experimental system) to substances passing through an emission stack of a factory producing maize derivatives. Absorption and ozonization were the most efficient treatment, removing 75% and 98% of VOCs, respectively, while photooxidation only removed about 59%. The emitted chemical compounds and odors were identified and quantified by gas chromatography-mass spectrometry (in full-scan mode). In addition to presenting the results, their implications for selecting optimal processes for treating volatile emissions are discussed
Introduction Volatile organic compounds (VOCs) are among the most common pollutants emitted by process industries. They often include organic compounds that have powerful and unpleasant odors, even at low concentrations. VOCs should therefore be considered to include compounds that contribute to unpleasant odor profiles of emissions, as well as recognized pollutants. Recent environmental legislation has obliged industries to search for effective abatement technologies to reduce the emissions of pollutants from their plants. However, there is currently no standardized limit or threshold value for odors that, if exceeded, would be a breach of law. Most attempts to reduce or eliminate VOCs, therefore, pay little or no attention to the emission of malodorous VOCs. While pollutant emissions are controlled by measurement prior to discharge, the control of odor is based on its impact on the surrounding environment. However, it is very difficult to evaluate odors emanating from VOC emissions, since they are strongly affected by diverse variables, such as concentration, temperature, moisture, and chemical reactions that might occur among the mix of compounds present in the * Corresponding author phone: (+34)-976761873; fax: (+34)976762388; e-mail:
[email protected]. † Department of Analytical Chemistry. ‡ Department of Chemical and Environmental Engineering. 10.1021/es902735g
2010 American Chemical Society
Published on Web 03/01/2010
emissions. Consequently, any study of odorous compounds emitted by an industrial stack should be specifically designed to trap and quantify as many, and as much, of the emitted organic compounds as possible. There are a large number of potential options for reducing VOCs and treating odorous exhaust emissions, which can be broadly categorized into two groups: (i) absorption (1-3), including water and chemical scrubbing, and adsorption by activated carbon or other porous substrates (4-6) and (ii) incineration, including thermal, catalytic, and biological oxidation (7-11). However, there are potentially large differences between the various options in terms of their efficacy and their capital and operating costs. Conventional methods for treating VOCs from gas streams have inherent limitations, and none are definitely costeffective (12). The optimal treatment, or combination of treatments, for a particular situation depends on factors such as site characteristics (including operational parameters and maintenance schedules), treatment objectives, contaminant loading patterns, and the characteristics of the odorous air. Devices that are extensively used in odor control applications include wet scrubbers, which absorb odorous components in an appropriate liquid scrubbing medium. Such scrubbers are commercially available in a number of designs and variations, all of which are claimed to provide high levels of gas-liquid contact and highly efficient odor removal (due to the high solubility of the odorous compounds in the scrubbing medium). The rate at which a given odorous substance is removed from an air stream by a wet scrubber depends on its degree of saturation at the interface of the gaseous phase and the solvent within the absorber. This rate mechanism determines the removal efficiency for a particular size of absorption plant and a particular air flow rate. Thus, the removal efficiency is a function of the reaction time, the degree of saturation at the surface of the liquor, and the reactivity of the gas components within the absorbing solvent. Since it is generally impractical to remove odorants effectively using water alone, other absorbents are also often employed (13), generally after water systems, which are usually applied in the first stage of emission treatment. Wet scrubbers with oxidizing chemicals are currently used to treat VOCs, but little information is available on scrubber efficiency for many of the VOCs generated, for instance in the rendering process (14, 15). The efficiency of absorption can be increased if the absorbing liquid contains a reagent that reacts with the odorants present in the airstream. There are generally oxidizing solutions to achieve this and the most widely applied include sodium hypochlorite, hydrogen peroxide, ozone, and potassium permanganate (14-16). Due to its high reactivity, sodium hypochlorite is probably the most widely used oxidizing agent. It is generally used at an alkaline pH in order to prevent its dissociation and the release of free chlorine. Scrubbers using chlorine dioxide have low removal efficiencies for aldehydes but can be combined with biofilters to improve the overall removal efficiency, as reported by Kastner et al. (17). Ozone is also a powerful oxidizing agent, although its oxidative power is more pronounced in the liquid phase as opposed to the gas phase (18-20). Nevertheless, kinetic data suggest that typical oxidizing agents used in wet scrubbers do no react, or react only slowly, with many of the VOCs in waste gases from industrial plants (21-23). However, most relevant data have come from laboratory-scale experiments (24), and there have been few industrial-scale studies in commercially operating plants. Photooxidation to decompose VOCs in air has also been explored, initially by Bhowmick and Semmens, who invesVOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Experimental plant for the absorption experiments with ozone. tigated the ability of UV light to oxidize VOCs present in off-gas from air-stripping (25). Ultraviolet oxidation techniques, including photocatalytic oxidation and photochemical oxidation, have been extensively studied since then as promising methods for removing and destroying various VOCs in polluted air (26-28). However, a limiting factor for these methods is the generation of undesirable intermediates, some of which are more toxic than the parent compounds. Hence, a range of techniques have been proposed and developed for removing both VOCs and odor from emissions produced by industrial plants, but to our knowledge, their efficiency and utility have not been previously compared on an industrial scale. This may be partly due to the limited availability of industrial facilities with managers that are both able and willing to permit experimental studies to be conducted in situ and partly to the high cost of industrialscale tests. To address this lack, this paper describes an indepth comparison of the removal efficiencies of VOCs and odors by photo-oxidation using short wavelength UV (254 and 185 nm) and two oxidizing agents in wet scrubbers (sodium hypochlorite and ozone). Following detailed analysis of the resulting emission inventories, a strategy for the control of VOCs and odors is recommended. It should be emphasized that the treatments described in the present paper were applied in a commercially operating emissions stack of an industrial plant that continuously produces maize derivatives. Thus, the in situ evaluation under normal operating conditions allowed the company to choose the most appropriate technique for removing VOCs and odor from their emissions.
Experimental Section Experimental Plant for Absorption. Venturi and packedbed wet scrubbers are sometimes coupled, since the Venturi in a single-stage scrubber reduces the temperature and particulate levels, as well as trapping water-soluble polar pollutants. In the present study on absorption, therefore, an 2586
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experimental scrubber consisting of two 1 m3 tanks, coupled in series, was connected to the emissions stack at a industrial plant, located in Zaragoza, Spain. The second tank comprised the scrubber where the oxidation process occurred, in which either the water or the oxidant mixture were recirculated. The first tank was always filled with osmotized water, the aim of this step being to trap water-soluble polar pollutants. The first set of experiments was carried out with 150-180 g/L of NaClO in a 50% (w/w) of NaOH in water as the oxidant in the scrubber, following the experimental conditions (with redox potentials of 350-400 mV and 650-700 mV) previously described in ref 13. The contact time was 4.8 s and the space velocity was 0.21 s-1. Redox potentials were controlled through the concentration of NaClO and continuously measured with a redox monitor. A second set of experiments was carried out in which the first tank was filled with water and the second scrubber tank contained ozone generated from a WEDECO ozonizer (see Figure 1), which provided the required O3 stream in the system. Three experimental runs were carried out, each with a low gas flow (482 m3 N/h) from the stack and a constant flow (1.25 m3 N/h) of added gas containing either 103, 70, or 25 g/h of O3. The chemical reactions between the compounds and ozone were taken into account and the consumption of ozone was calculated in each case. A great excess of ozone (>80% over the stoichiometric requirements) was present in each assay. The contact time was 7.5 s and the space velocity 0.13 s-1 In order to analyze the concentrations of pollutants throughout the process and evaluate the efficiency of the tested oxidation processes, three points were selected for sampling: before and after the first tank and at the outlet of the scrubber, as shown in Figure 1. Samples were taken on alternate days in duplicate, each sample comprising one liquid fraction (condensates) and one gas fraction trapped on the carbon filter as described below.
FIGURE 2. Experimental plant for the photo-oxidation experiments. Experimental Plant for Photo-Oxidation. The experimental plant for the photo-oxidation experiment (Figure 2), commercially available and supplied by Solairpur and Bioclimatic, has two sections. The gas from the stack first enters a cyclonic heat exchanger, where some of the vapor from the gas condenses, small particulates are removed, and the temperature of the gas is decreased. The hot water flowing out of the heat exchanger can be used elsewhere in the plant in other thermal applications. The gas from this stage then passes to the second section, where organic compounds contained in the gas are oxidized in a cylindrical chamber into which oxygen radicals and ozone, produced from the ambient air in the photo-oxidation module (at 254 and 185 nm), are continuously injected. The ratio of emission/fresh air flow is around 4/1. Since the fan from the cyclone is giving a flow of maximum 1000 m3/h, a maximum of 200 m3/h fresh air enriched with radicals has to be mixed with the off-gas. Dimensions of the oxidation chamber are 3 × 0.4 × 0.4 m, and dimensions of the photooxidation module are 2 × 0.7 × 0.7 m. The photooxidation module consists of 64 UV-lamps with a maximum power input of 1.4 kW. Contact time was 8.4 s and the space velocity 0.12 s-1. The main mechanistic steps to generate the free radicals in the photooxidation module, which are responsible for the oxidation, are described in Figure 2. Three points were selected for sampling: before and after the cyclonic heat exchanger and at the outlet of the second section, as shown in Figure 2. In order to generate sufficient data to obtain indications of both mean levels and variations in levels of VOCs before and after the treatment, three separate experiments were carried out under the following identical experimental conditions: 428 m3 N/h of gas entering the cyclonic exchanger; 402 m3 N/h of gas treated in the oxidation chamber; and 122 m3 N/h of oxidizing air current used in the oxidation chamber, which corresponds to about 30% of the gas flow from the stack. Chemicals and Analytical Methods. All chemicals used in the procedures described below, including the calibrations and the analytical chromatography, were analytical grade reagents. The analytical procedure for sampling and analysis here applied was previously developed by Domen ˜ o et al. (13).
TABLE 1. Standards (Aldrich, Madrid, Spain) Used for Generating Calibration Curves and Their Analytical Parametersa compound
DL (µg)
Gas chlorocyclohexane hexadecane benzophenone 1-eicosene dichloromethane tiophene benzene dimethyl sulfide m-xylene toluene ethylbenzene guaiacol furfural cis-jasmone nonanal benzophenone coumarin guaiacol 1-hexene furfural
QL (µg)
RSD (%)
Fraction Standards 8 220 12 22 80 11 20 75 5 78 275 8 75 250 10 72 225 12 61 21 8 41 125 15 68 250 8 51 185 7 98 275 5 0.7 1 21 0.5 1 18
Liquid Fraction 0.9 0.4 0.8 0.2 0.1 0.8 1.2
Standards 2.2 14 0.9 9 1.2 7 0.8 9 0.4 18 1.1 7 2.3 5
linear range (µg) 404-1963 110-917 95-415 378-3220 363-7834 359-2401 311-6717 205-4096 396-1725 253-6114 543-1631 2.0-5.7 2.0-5.7 5-95 1-4.8 3-102 1-3.8 0.6-3.2 3-421 2-5.7
a DL, detection limit (3σ); QL, quantitation limit (10σ); RSD, relative standard deviation (midrange, n ) 5).
Two different calibration plots were generated for quantitative purposes. For the gas fraction, a solution of a selected series of standards (Table 1) in ethanol (Scharlab, Barcelona, Spain) was added to 4.5 g of active carbon (12-40 mesh of high purity with a specific surface of 600 m2/g and pore volume of 0.95 mL/g dry matter) (Aldrich, Madrid, Spain) in a closed vial. The compounds in the headspace were then sampled using a polyacrylate (PA) solid-phase microextraction (SPME) fiber, as described below. For the calibration of the liquid fraction, 10 µL of ethanol solutions containing the selected standards was added to 3 mL of distilled water, and VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the PA-SPME fiber was used in total immersion mode. Calibration plots and detection and quantification limits derived under these conditions are shown in Table 1. For quantitative purposes, a series of 20 standards were selected according to the chemical structure, retention time in the chromatogram, and the presence of the compounds in each fraction. The response factor of these individual standards was used to quantify all the compounds emitted by the stack. This is a common practice in real life when many compounds are involved in complex matrices (13, 29, 30). Statistical treatment was done with the standards, and the RSD values are given in Table 1. Gas Sampling System. To sample the gas, three impingers were coupled in series between the stack and the pump, each of which was installed in a cold box filled with ice. The first impinger in the series was empty, while the other two contained silica gel, which was activated for 2 h in an oven before use. The outlet of the third impinger was coupled to a filter of active carbon consisting of 4.5-5 g of carbon sandwiched between two glass fiber filters. The resulting compound filter was contained within a stainless steel holder and used as the adsorbing filter for the VOCs. The carbon was activated at 270 °C for 1 h before use. The series of impingers and the filter were connected to a pump working at a flow rate of 25 L/min while sampling a total volume of 900 L of gas for each experimental run (13). During each sampling run, water vapor from the stack condensed in the first impinger, together with the most polar compounds, while the most volatile fractions were collected on the carbon filter. The liquid fraction from the first impinger and the carbon filter were analyzed separately. Analysis of the Liquid Fraction. The organic compounds captured in the liquid phase (condensates) obtained in each sampling test were extracted by adding 5 g of analytical grade NaCl (Merck, Darmstadt, Germany) to a 3 mL subsample taken from the liquid fraction, to facilitate SPME by an 85 µm thick polyacrylate SPME fiber, in total immersion mode (with stirring at 600 rpm), which was subsequently desorbed in the injection port of the GC-MS system described below under the following optimized conditions: adsorption temperature, 80 °C; desorption temperature, 250 °C in the injection port of the GC; adsorption time, 5 min; desorption time, 2 min. Analysis of the Gas Fraction. To analyze compounds from the gas fraction adsorbed on the carbon filter, they were first extracted by SPME using the polyacrylate fiber in headspace mode, as follows. The active carbon was placed in a 20 mL vial, crimped with a high-temperature PTFE-lined septum. A small quantity of anhydrous sodium sulfate was added to the vial to prevent humidity from causing adsorption problems on the fiber. After the fiber had been introduced into the vial through the septum, the temperature was progressively increased from room temperature to 175 °C and held there for 1 h to adsorb the analytes, which were subsequently desorbed in the injection port of the GC-MS at 250 °C, for 10 min (13). Gas Chromatography-Mass Spectrometry. All GC-MS analyses were performed using a Hewlett-Packard 6890 gas chromatograph (Wilmington, DE) equipped with a 5973 mass selective detector and a SGE-5 (60 m × 0.25 mm, 0.25 µm film thickness) capillary column (Sugelabor, Madrid, Spain). A cryogenic oven was used to start the chromatography. The temperature program was as follows. The initial temperature of 0 °C was held for 3 min, then raised to 250 °C at a rate of 3 °C/min. The injector temperature was set at 250 °C and the transfer line at 280 °C. Due to the likelihood of a very diverse range of compounds being present in these samples from a normally operating commercial plant, SCAN mode (50-500 amu) was used in the MS detector. The ionization system was electronic impact, and C-50 quality 2588
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helium (Carburos, Meta´licos, Zaragoza, Spain) was used as the carrier gas at constant flow mode set at 1.1 mL/min. Olfactometric Analysis. Olfactometry is the most commonly used method worldwide for odor measurement. Samples were taken from each of the sampling points of the plant, and olfactometrically analyzed following the European Committee of Normalization (CEN) norm EN13725 (31). The results were expressed as odor units defined in the norm. Three replicate samples were taken for analysis from each sample point and stored in “Tedlar” bags before the olfactometric analysis.
Results and Discussion Characterization of Exhaust Gas. Since the emissions investigated are generated by an industrial process in which maize derivatives are produced, compounds related to this type of product were expected. However, although the plant operates continuously, the material being processed varies over the course of any 24 h period. Consequently, a range of compounds at various concentrations were obtained in samples obtained at different sampling periods, dates, and times. The composition of the mixture of VOCs that requires treatment by the control equipment at a given factory clearly influences the applicability of systems that could be employed, since the destruction and removal efficiency of the system will depend on its overall ability to remove or destroy all emitted compounds that need to be removed or destroyed. Hence, increases in the diversity of substances that need to be controlled will inevitably increase the demands placed on the system and (hence) reduce the range of appropriate choices. Thus, it is essential in analyses such as this to obtain detailed information regarding the compounds present in the gas emitted via the stack, since the methods that can be used to treat them depend on their chemical nature. Figure 3 displays the compounds identified in the present study, grouped according to their chemical properties. The wide variation of the exhaust components detected in samples collected on different sampling dates illustrate the normal variation due to variations in the industrial processes and raw materials (vegetable residues) processed by the plant. The mean concentrations of the various chemical groups identified were as follows: alkanes, 14.788 mg/m3 N; alkenes, 0.14 mg/m3 N; alcohols, 1.316 mg/m3 N; aldehydes, 3.622 mg/m3 N; ketones, 0.0184 mg/m3 N; aromatic compounds (benzene derivatives), 0.734 mg/m3 N; and other miscellaneous compounds, 0.172 mg/m3 N. In total, more than 100 different compounds were identified, which highlights the importance of identifying and quantifying specific compounds in emissions by comprehensive GC-MS analysis. Due to the daily variations in the gas composition, each series of experiments included the analysis of the gas at the inlet of the plant, as well as the gases from the sampling points specified above. Efficiency of the Absorption with Water-NaClO/NaOH. The removal efficiencies of each compound group, by each stage of the process, are given in Table 3. In the first stage, in which only water was used to trap the compounds, the total removal efficiency ranged between 42% and 61%, while a 20% reduction of VOCs was observed in the second step when NaClO and a redox potential of 350-400 mV was used. This increased to up to 70% reduction of VOCs when the potential applied was raised to 650-700 mV. Hence, the efficiency of VOC absorption was enhanced by using NaClO/ NaOH rather than by using water alone. The applied redox potential proved to be a highly important factor in this case, since almost 100% of alcohols were eliminated at the high redox potential, whereas only a 50% reduction was obtained at the low potential. Similar results were observed for aldehydes, ketones, and other compounds, levels of which
FIGURE 3. Chemical composition of the exhaust gases emitted by the stack. Compounds are grouped according their chemical nature. were generally reduced by approximately 60% at the low redox potential and by between 80% and 90% at the higher potential. These results are similar to those reported by Kastner et al. using ClO2 in the wet scrubber (14). Individual compounds indentified in each fraction were reported in ref (13). No different pathway was observed when changing the redox potential and only the intensity of treatment influenced the results. As was previously reported, some organochlorine compounds were detected, demonstrating that the use of NaClO is not the best idea to remove VOCs, even being efficient (13). Absorption Efficiency with Water-Ozone. The total removal efficiency achieved by the water-ozone oxidant mixture ranged between 53.3 and 92% in the first step, in which only water was trapping the compounds. When VOCs entered the system at a higher concentration, e.g. 113.7 mg/N m3, the elimination rate was correspondingly higher (92.6%), demonstrating that absorption efficiency depends to some extent on the quantity of compounds in the emissions being processed. Significant differences in the removal efficiencies of different types of compound were also observed (Table 2). This is an important observation, since it strongly affects the amounts of different compounds that need to be eliminated in the second step of the oxidation process. The results obtained using three different concentrations of O3 show that the best reductions were obtained with the highest (103 g/h) concentrations of ozone, and the lowest reductions when using the lowest (25 g/h) concentration of ozone. This finding demonstrates a direct relationship between ozone concentration supplied and the reduction capacity of VOCs. The reductions in VOCs achieved varied from 87 to 93%, 77 to 83%, and
