Article pubs.acs.org/est
Decomposition Treatment of SO2F2 Using Packed Bed DBD Plasma Followed by Chemical Absorption Yong Nie,*,† Qifeng Zheng,† Xiaojiang Liang,† Dayong Gu,‡ Meizhen Lu,† Min Min,§ and Jianbing Ji† †
College of Chemical Engineering and Material, Zhejiang University of Technology, Hangzhou 310014, China Shenzhen International Travel Health Care Center, Shenzhen 518033, China § Center for Biorefining and Department of Bioproducts and Biosystems Engineering, University of Minnesota, Saint Paul, Minnesota 55108, United States ‡
ABSTRACT: The technology of packed bed dielectric barrier discharge (DBD) plasma followed by a chemical absorption has been developed and was found to be an efficient way for decomposition treatment of sulfuryl fluoride (SO2F2) in simulated residual fumigant. The effects of energy density, initial SO2F2 concentration, and residence time on the removal efficiency of SO2F2 for the DBD plasma treatment alone were investigated. It was found that the SO2F2 could be removed completely when initial volume concentration, energy density, and residence time were 0.5%, 33.9 kJ/L, and 5.1 s, respectively. The removal mechanism of SO2F2 in the packed bed DBD reactor was discussed. Based on the detailed analysis of SO2F2 molecular stability and its exhaust products in the DBD plasma reactor, it was concluded that the energetic electrons generated in the packed bed DBD reactor played a key role on the removal of SO2F2, and the major decomposition products of SO2F2 detected were SO2, SiF4, and S (Sulfur). Among these products, SiF4 was formed by the F atom reacted with the filler-quartz glass beads (SiO2) in the packed bed DBD reactor. Aqueous NaOH solution was used as the chemical absorbent for the gaseous products of SO2F2 after plasma pretreatment. It was found that the gaseous products in the plasma exhaust could be absorbed and fixed by the subsequent aqueous NaOH solution.
1. INTRODUCTION Sulfuryl fluoride (SO2F2) is used increasingly around the world as a fumigant to replace methyl bromide, owing to its zero ozone depletion potential.1 However, as a fumigant, the toxicity of SO2F2 must be taken into account. Sulfuryl fluoride is a strong atmospheric greenhouse gas of which the global warming potential (GWP) is 4780 times higher than that of carbon dioxide, and the globally averaged atmospheric lifetime is 36 years. Moreover, SO2F2 has been accumulated in the global atmosphere with a growth rate of 5 ± 1% per year since 1978.2,3 Furthermore, long-term exposure to SO2F2 gas leads to a serious impact on human health.4 In order to protect the environment and human health, SO2F2 must be removed without delay. However, little attention has been given for the hazards of SO2F2 used as a fumigant. There is no feasible way to remove SO2F2. The general practice is to release the residual SO2F2 directly into the atmosphere after fumigation. According to a regular SO2F2 application rate of 20 g/m3 used in container fumigation, the equivalent discharging volume concentration of SO2F2 in the air is about 0.5%. In order to protect the environment and reduce global warming, it is urgent to develop a new technology for the decomposition treatment of SO2F2. Nonthermal plasma (NTP) technology with the strong ability of bond breaking and molecule reorganization has been widely used in NOx, SO2, and VOCs control.5−9 The NTP could be conventionally generated by a gas discharge, such as © 2013 American Chemical Society
glow discharge, corona discharge, dielectric barrier discharge (DBD), and microwave discharge. Among these discharge modes, DBD is an excellent source of ideal energetic electrons with 1−10 eV.10 Due to its attractive characteristic, DBD is recently widely studied for potential industrial applications, especially in the exhaust control, such as the removal of acid gases (NOX, SO2, etc.), VOCs, and PFCs (NF3, C2F6, CF4, SF6, etc.).11 However, the pure nonthermal plasma process is characterized by secondary pollution and low selectivity and could not result in harmless end products directly. In this paper, the technology of packed bed DBD plasma12 followed by a chemical absorption was first used for the treatment of SO2F2 in the simulated residual fumigant. The effects of energy density, defined as the ratio of discharge power to the fed gas-flow rate, initial SO2F2 concentration, and residence time on the removal efficiency of SO2F2 for the DBD plasma treatment alone were investigated. Meantime, the removal mechanism of SO2F2 in the packed bed DBD reactor was also investigated. The efficiency of using aqueous NaOH solution as absorbent for the removal of gaseous products of SO2F2 after plasma pretreatment was studied. According to the Received: Revised: Accepted: Published: 7934
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Figure 1. The schematic layout of the experimental setup.
2.3. Packed Bed DBD Reactor. The packed bed DBD reactor consists of a quartz tube with an inner diameter of 35 mm and an outer diameter of 40 mm, a stainless-steel rod placed along the axis of the quartz tube, quartz glass beads (diameter 3−4 mm) filled between the quartz tube and the stainless-steel rod, and a stainless-steel mesh enfolded around the quartz tube. A high voltage was applied to the stainless steel mesh acting as a discharging electrode. The quartz tube functioned as a dielectric material. The stainless-steel rod was connected with a ground wire acting as a ground electrode. The discharge distance between the stainless-steel mesh and the stainless-steel rod was set to 10 mm. The effective discharge length of the packed bed DBD reactor was 130 mm. Charge-voltage (Q-U) Lissajous figure technology13 was adopted to measure the discharge power for the packed bed DBD reactor. The voltage (U) applied to the DBD reactor was measured by a 1000:1 high voltage probe (P6015A, made by Tektronix) and a digital oscilloscope (TDS 1012B, made by Tektronix). A 0.47 μF capacitor was connected in series with the DBD reactor to determine the charge stored in the DBD reactor (Q = C × UC), and the corresponding voltage of capacitor (UC) was measured with a 1:1 voltage probe (P6139A, made by Tektronix). The area of the Q-U Lissajous figure conforms to the discharge energy per one cycle (W), and the average discharge power (P) can be calculated by multiplying the discharge energy per one cycle by AC frequency ( f). Therefore, the average discharge power (P) can be calculated by the following equation:
experimental results, an efficient and feasible method has been developed for the decomposition treatment of SO2F2 after fumigation.
2. EXPERIMENTAL SECTION 2.1. Experimental System. The schematic layout of the experimental setup is shown in Figure 1. The reactor system consists of a packed bed DBD reactor followed by a chemical absorber. The packed bed DBD reactor was energized by AC high-voltage power (made by Nanjing Suman Electronics Co. Ltd.). For the purpose of adjusting the discharge intensity, the discharge power was varied from 87 to 236 W. In the chemical absorber, the aqueous NaOH solution was used as an absorbent. As shown in Figure 1, the flow rates of mixed air and SO2F2 in a simulated fumigant were controlled by rotameters. The simulated gas was first mixed in a buffer tank and then introduced into the packed bed DBD reactor. After the DBD plasma pretreatment, the gaseous products in the plasma exhaust entered the chemical absorber and was absorbed and fixed by the aqueous NaOH solution. During the experiments, the volume concentration of SO2F2 fed to the buffer bank was controlled in the range of 0.4%∼0.9%, in which the volume concentration of 0.5% was equivalent to a regular SO2F2 application rate of 20 g/m3 used in container fumigation; the total flow rate of the simulated gas was controlled in the range of 300−800 mL/min, corresponding to residence times of 2.5− 6.8 s in the packed bed DBD plasma reactor. 2.2. Chemical Analysis. The concentration of SO2F2 was analyzed by gas chromatography (GC) equipped with a Gaspro plot column and a flame photometric detector (7890A, made by Agilent). The sensitivity of the GC for SO2F2 analysis was 0.5 ppm. The gas and solid products were determined by a FTIR (4700, made by Nicolet) and an Elemental Analyzer (Vario Macro Cube, made by Elementar), respectively. The contents in the aqueous NaOH solution after chemical absorption were analyzed by ion chromatography equipped with a Supp4-250 anion exchanged column (883 Basic IC plus, made by Metrohm). Thus, the removal efficiency of SO2F2 for the packed bed DBD reactor could be evaluated using the following equation C − Cout η = in × 100 Cin (1)
P=f×W=f×
∫0
T
U (t )d(Uc(t ) × C)
(2)
Figure 2(a) shows the voltage waveforms measured at the DBD reactor and the capacitor, respectively, and Figure 2(b) shows the Q-U plot at the corresponding voltage. For example, in Figure 2(b), the discharge energy per one cycle at 11.6 kV was 0.0262 J, hence, the average discharge power was 0.0262 J × 8000 Hz = 210 W; if the gas flow rate was 400 mL/min, the corresponding energy density was 210 W/400 mL/min = 31.4 kJ/L. In addition, a 50Ω resistance (R) substituted in the above measuring capacitor was connected in series with the DBD reactor to determine the discharge current (I) of the DBD reactor (I = UR/R), and the corresponding voltage of resistance (UR) was measured with a 1:1 voltage probe (P6139A, made by Tektronix). 2.4. Chemical Absorber. A random packed column was used as the chemical absorber, packed with ceramic rasching
where η (%) is the removal efficiency of SO2F2, and Cin and Cout are the inlet and outlet volume concentrations of SO2F2 of the packed bed DBD reactor, respectively. 7935
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Figure 3. Effect of energy density on the removal efficiency of SO2F2 (gas flow rate: 400 mL/min; initial SO2F2 concentration: 0.5%; input power varied from 87 W to 236 W).
the number and energy magnitude of electrons increased, which makes SO2F2 degrade more easily. 3.2. Effect of Initial SO2F2 Concentration on the Removal Efficiency of SO2F2 for the DBD Plasma Treatment. The fumigant application rate of SO2F2 could be varied for different fumigation objects in the container, correspondingly, the venting SO2F2 concentration varied. By varying initial SO2F2 concentration, fixing input power and gas flow rate the effect of initial SO2F2 concentration on the removal efficiency of SO2F2 was investigated. As can be seen from Figure 4, when energy density was fixed at 33.9 kJ/L and Figure 2. (a) Voltage waveforms measured at the DBD reactor and at the 0.47 μF capacitor; (b) charge−voltage Lissajous figure of the DBD reactor.
rings (Φ2 × 2 mm, specific surface area of 2500 m2/m3, porosity of 0.87). The height and inner diameter of the packed column were 780 mm and 20 mm, respectively. The column was operated under the condition of room temperature (25 °C) and atmospheric pressure, in which an aqueous NaOH solution was used as the chemical absorbent. During the experiments, the concentration of the aqueous NaOH solution was controlled in the range of 0.1−0.2 mol/L, and the corresponding gas−liquid ratio was set to 1:60.
3. RESULTS AND DISCUSSION 3.1. Effect of Energy Density on the Removal Efficiency of SO2F2 for the DBD Plasma Treatment. The energy density is often used to indicate the DBD plasma intensity and the energy consumption for per liter gas treatment, in other words, the magnitude of energy density affects the removal efficiency of SO2F2. The effect of energy density on the removal efficiency of SO2F2 was investigated with varying input power, fixing gas flow rate, and initial SO2F2 concentration. As can be seen from Figure 3, there was a positive relationship between the SO2F2 removal efficiency and the input energy density, and SO2F2 could be removed completely when initial volume concentration, gas flow rate, and energy density were 0.5%, 400 mL/min (the corresponding residence time was 5.1 s), and 33.9 kJ/L, respectively. This phenomenon is attributed to the role of energetic electrons (referred to section 3.5), with the increase of energy density,
Figure 4. Effect of initial SO2F2 concentration on the removal efficiency of SO2F2 and energy yield (gas flow rate: 400 mL/min; input power: 236 W; energy density: 33.9 J/L; initial SO2F2 concentration varied from 0.4% to 0.9%).
gas flow rate was 400 mL/min, SO2F2 in the simulated fumigant could be removed completely when the initial SO2F2 concentration was less than 0.5%; when initial SO 2 F 2 concentration was more than 0.5%, the removal efficiency of SO2F2 decreased with the increase of the initial SO2F2 concentration. The reason for this phenomenon is because the removal of SO2F2 depended on the ratio of energetic electrons to SO2F2 molecules. When operating the DBD plasma reactor at the same energy density, the number and energy magnitude of electron would not vary significantly. 7936
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Consequently, when initial SO2F2 concentration was more than 0.5%, the number and energy magnitude of electrons would not be sufficient for degrading the increased SO2F2 molecules. In this case, the energy yield, defined as the ratio of the absolute removal amounts of SO2F2 to the energy injecting into the DBD plasma reactor (g/kWh), was calculated and plotted with the change of initial SO2F2 concentration. As can be seen from Figure 4, the energy yield of SO2F2 increased rapidly from 1.95 to 3.38 g/kWh when the initial SO2F2 concentration increased from 0.4% to 0.8%. When the initial SO2 F 2 concentration was more than 0.8%, the energy yield of SO2F2 increased only slightly. The maximum energy yield was 3.4 g/ kWh in our experiments. The above results confirmed that the number and energy magnitude of the electrons at a given energy density determined the absolute removal amounts of SO2F2. According to the above results, calculating the energy consumption for the treatment of a container after fumigation with SO2F2 (2.4 g/kWh for SO2F2 concentration of 0.5%) requires 8.1 kWh per cubic meter of container with a regular application rate of 20 g/m3. 3.3. Effect of Residence Time on the Removal Efficiency of SO2F2 for the DBD Plasma Treatment. The length of residence time directly determined the processing load of the DBD plasma reactor. The longer residence time, the smaller the processing load. In this section, the effect of residence time (defined as the ratio of effective volume of the packed bed DBD plasma reactor to the gas flow rate) on the removal efficiency of SO2F2 at the same input power of 236 W was investigated. As can be seen from Figure 5, the removal
introduced into the packed bed DBD reactor under the same initial SO2F2 concentration, which results in more SO2F2 molecules colliding with the energetic electrons under the same input power; more SO2F2 molecules decomposed, consequently, the energy yield of SO2F2 improved when the residence time shortened. 3.4. Products Analysis for the DBD Plasma Treatment. In order to explore the removal mechanism of SO2F2 in the DBD plasma reactor, a detailed products analysis was necessary. In this section, FT-IR spectrum analysis was conducted for the feed gas and discharge products at different energy density. As can be seen from Figure 6, the characteristic peaks of SO2F2 in
Figure 6. FT-IR spectra at three cases: (a) the feed gas; (b) the discharging products at 26.5 kJ/L; and (c) the discharging products at 33.9 kJ/L (gas flow rate: 400 mL/min; initial SO2F2 concentration: 0.5%).
the frequency region 1535−1450 cm−1 (in Figure 6a) disappeared gradually with the increase of energy density. Meanwhile, the characteristic peaks of SO2 (1357−1330 cm−1) and SiF4 (1010−1100 cm−1) appeared gradually, which proved that SO2F2 could be removed efficiently in the packed bed DBD reactor. Integrating with the analysis of solid products collected in the packed bed DBD by the Elemental Analyzer, it was found that the main products determined in our experiments were SO2, SiF4, and S (Sulfur). Among these products, SiF4 was formed by the F atom reacted with the fillerquartz glass beads (SiO2) in the packed bed DBD reactor. 3.5. SO2F2 Removal Mechanism for the DBD Plasma Treatment. The conventional removal mechanism of gas pollutants in air by DBD plasma technology can be described as follows: many activated radicals like O3, O, and etc., were generated during the DBD process by the collision of highenergy electrons with oxygen, and these active radicals then reacted with pollutants, which resulted in the removal of the pollutants. However, it is reported that SO2F2 is very inert against attack by most atmospheric activated oxidants like O, O3, and also against photolysis,14,15 thus, we could presume that the removal of SO2F2 was primarily attributed to the direct collision of energetic electrons with SO2F2 in the packed bed DBD reactor. Integrating with the above products analysis, the hypothesis removal mechanism of SO2F2 in the case of DBD plasma treatment alone could be written as follows:
Figure 5. Effect of residence time on the removal efficiency of SO2F2 and energy yield (initial SO2F2 concentration: 0.5%; input power: 236 W; gas flow rate varied from 300 mL/min to 800 mL/min).
efficiency of SO2F2 increased with residence time, until SO2F2 in the simulated fumigant was completely decomposed. When the residence time was more than 5.1 s, no SO2F2 remained. The reason for this phenomenon is because residence time determined the collision probability of the SO2F2 molecules with the energetic electrons in the DBD plasma reactor. The longer the residence time, the more opportunities electrons collided with SO2F2, and the higher the removal efficiency of SO2F2. In this paper, the residence time of 5.1 s was preferable when input power was 236 W and the corresponding energy density was 33.9 kJ/L. In addition, energy yields of SO2F2 at different residence time were also shown in Figure 5. As can be seen from Figure 5, the energy yield of SO2F2 decreased with residence time, the reason for this phenomenon is due to the following: the shorter the residence time, the higher the gas flow rate; more SO2F2 7937
O2 + e → O + O + e
(3)
O2 + O → O3
(4)
SO2 F2 + e → SO2 F + F + e
(5)
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SO2 F + e → SO2 + F + e
(6)
SO2 + e → SO + O + e
(7)
SO + e → S + O + e
(8)
4F + SiO2 → SiF4 + O2
(9)
SO + O3 → SO2 + O2
(10)
SO + O → SO2
(11)
As verified by the mass spectrum of SO2F2, the mechanism of molecular decomposition into O, F, S, SO, SO2, and SO2F in MS detector was similar to the results of energetic electrons worked in the DBD plasma reactor. Based on the above analyses, the number and energy magnitude of electrons directly determined the removal efficiency of SO2F2. Therefore, studying the influence of energy density on the total number of electrons (N) and the energy of an electron obtained in single cycle (ES) were necessitated. The calculated equation of N and ES in DBD reactor could be expressed as follows16 N=
∫0
T
I(t )dt /(1.6 × 10−19)
Figure 7. FT-IR spectrum (a) the feed gas; (b) the feed gas after plasma treatment alone (energy density: 33.9 kJ/L); and (c) the feed gas after DBD plasma followed by a chemical absorption (energy density: 33.9 kJ/L, NaOH solution as absorbent).
after chemical absorption, which means the harmful gaseous products in the plasma exhaust could react with NaOH and be removed completely; in other words, the toxicity of the residual SO2F2 after fumigation could be eliminated using DBD plasma followed by a chemical absorption. By means of chemical absorption, the harmful gaseous products of SO2F2 in the plasma exhaust could react with NaOH and be fixed in the solution; the overall process for this chemical absorption can be described as follows:
(12)
ES = P /(f × N )
(13)
where I(t) is the discharge current of DBD reactor, P is the average discharge power of DBD reactor, T is the time of a single cycle, and f (f = 1/T) is the AC voltage frequency. Table 1 shows the effect of energy density on the N and ES. As can be seen from Table 1, when the energy density increased Table 1. Effect of Energy Density on the N and ES parameter energy density (kJ/L) N (1013) ES (10−16 J)
value 12.9 1.78 6.12
17.6 2.02 7.56
26.5 2.65 8.63
33.9 3.06 9.63
2NaOH + SO2 = Na 2SO3 + H 2O
(14)
Na 2SO3 + O3 = Na 2SO4 + O2
(15)
SiF4 + 6NaOH = Na 2SiO3 + 4NaF + 3H 2O
(16)
2NO2 + 2NaOH = NaNO2 + NaNO3 + H 2O
(17)
In eqs 15 and 17, gas species like O3 and NO2 were generated in the DBD plasma reactor because of air in the simulated fumigant. By chemical absorption, SO2 was fixed via eq 14 and eq 15 and finally converted to Na2SO4; SiF4 was fixed via eq 16 and finally converted to Na2SiO3 and NaF; a byproduct like NO2 was fixed via eq 17 and finally converted to NaNO2 and NaNO3. In this sense, the elements S and F in the plasma exhaust were fixed in the salt form confirmed by ion chromatography spectrum, as shown in Figure 8. The salt in the solution could be further recycled by the crystallization method. Thus, the technology of packed bed DBD plasma followed by a chemical absorption has been developed and was found to be an efficient way for the decomposition treatment of SO2F2 in
from 12.9 kJ/L to 17.6 kJ/L, N was increased from 1.78 × 1013 to 2.02 × 1013, meantime, ES also increased dramatically from 6.12 × 10−16 J to 7.56 × 10−16 J, which confirmed the removal efficiency of SO2F2 increasing remarkably from 56.3 to 82.1% with the energy density increased from 12.9 kJ/L to 17.6 kJ/L in Figure 3. It is reported that the bond dissociation energies of SO and S−F are only 7.70 × 10−19 J and 7.67 × 10−19 J, respectively;17,18 in other words, the energy of electron generated in the packed bed DBD reactor was powerful enough to destroy SO and S−F bond. 3.6. Decomposition Treatment of SO2F2 in Case of DBD Plasma Followed by Chemical Absorption. In order to avoid the secondary pollution of plasma exhaust, the technology of DBD plasma followed by a chemical absorption was used. In this section, for the purpose of confirming the decomposition treatment of the residual SO2F2 after fumigation using this technology, the gaseous contents before and after chemical absorber were analyzed by FT-IR. As can be seen from Figure 7(b), the characteristic peaks of SO2F2 in the frequency region of 500−1535 cm −1 in Figure 7(a) disappeared, meantime, the characteristic peaks of SO2 (1330−1357 cm−1) and SiF4 (1010−1100 cm−1) appeared after plasma treatment. As can be seen from Figure 7(c), the characteristic peaks of SO2 and SiF4 in Figure 7(b) disappeared
Figure 8. Ion chromatogram spectrum of absorption solution. 7938
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(14) Sulbaek Andersen, M. P.; Blake, D. R.; Rowland, F. S.; Hurley, M. D.; Wallington, T. J. Atmospheric chemistry of sulfuryl fluoride: Reaction with OH radicals, Cl atoms and O3, atmospheric lifetime, IR spectrum, and global warming potential. Environ. Sci. Technol. 2009, 43, 1067−1070. (15) Zhao, Z.; Laine, P. L.; Nicovich, J. M.; Wine, P. H. Reactive and nonreactive quenching of O (1D) by the potent greenhouse gases SO2F2, NF3, and SF5CF3. Proc. Natl. Acad. Sci. 2010, 107, 6610−6615. (16) Nifuku, M.; Horváth, M.; Bodnár, J.; Zhang, G.; Tanaka, T.; Kiss, E.; Woynárovich, G.; Katoh, H. A study on the decomposition of volatile organic compounds by pulse corona. J. Electrost. 1997, 40, 687−692. (17) Linstrom, P. J.; Mallard, W. G. NIST Chemistry webbook; NIST standard reference database No. 69. 2001. (18) Chase, M. W., Jr. Tables, N. J. T. Monograph No. 9. J. Phys. Chem. Ref. Data, NIST-JANAF Thermochemical Tables; NIST: Gaithersburg, 1998.
simulated residual fumigant. Improving the energy yield and reducing the energy consumption for this technology will be further studied by quantitative analysis of the products of plasma treatment by changing the configuration of the DBD plasma reactor and packing dielectric.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86 13675897635. Fax: +86 571 88320053. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
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The financial support from Natural Science Foundation of China (NSFC) (Grant No. 51107118) and Science and Technology Plan of General Administration of Quality Supervision of the P.R.C. (Grant No. 201010256651.9) are gratefully acknowledged. Furthermore, we are very grateful to Dr. Richard Griffith for his insightful advice.
NOTE ADDED AFTER ASAP PUBLICATION There were errors in Figure 1 in the version of this paper published July 5, 2013. The correct version published July 8, 2013.
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