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RESEARCH NOTES Effects of Pulsed Corona Discharge on SO2 Absorption into Water from an SO2/Air Mixture Joo-Youp Lee,† Soon-Jai Khang,*,† and Tim C. Keener‡ Departments of Chemical Engineering and Civil and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221
The absorption of SO2 from an SO2/air mixture into tap water was measured under pulsed corona discharge at 22 °C and 1 atm as a continuation of the previous study3 in which an SO2/N2 mixture and deionized distilled water were used. The mass-transfer enhancement in the gas phase showed almost the same trend as that obtained from the previous study. However, compared to the previous study, the effect of mass-transfer enhancement due to the higher alkalinity in the liquid phase was attenuated as the corona power increased. Introduction SO2 absorption from flue gas with aqueous solutions or slurries is an important process in the area of air pollution control, and extensive studies have been done in order to improve the performance of the widely used wet flue-gas desulfurization (FGD) systems. The most commonly used and studied wet FGD system is the wet limestone-gypsum process that has received considerable attention. A large volume of experimental data are available, and limestone dissolution and gypsum crystallization as well as chemical absorption of SO2 gas into the liquid have been taken into account in various models. This process is frequently limited by liquidphase mass-transfer resistance and consumes substantial amounts of energy generated from power plants. Meanwhile, another approach using nonthermal plasma processes has been directed to remove various toxic gas compounds. Corona discharge utilizing low-energy electrons is one of the most promising nonthermal plasma processes, and many studies on SO2 removal have been done in recent years. A few experimental results showed that a better performance in SO2 removal was obtained in the presence of water as a result of either enhanced absorption rate of SO2 by corona discharge1-3 or the formation of SO2 clusters4 or sulfuric acid vapor by means of radical (OH, O, HO2, etc.) generation.5 Additionally, it was shown that the use of fast-rising narrow pulses superimposed on dc bias gives a high-power efficiency because it enables one to drive electrons rather than ions, hence generating radicals by electron collision without raising the temperature. In the present work, the effect of corona discharge on an SO2/air absorption into tap water is studied in a batch system as a continuation of the previous study.3 The same batch experimental system as that used in the previous work was employed to study SO2 absorp* To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Civil and Environmental Engineering.
tion into the liquid phase under corona and noncorona conditions. Positive pulsed streamer corona generated from a ring-shaped discharge wire was used for all of the experiments as in the previous work. The experimental results with corona discharge are compared to those without corona discharge. A diffusion model based on film theory is used to determine the mass-transfer enhancement due to the corona discharge as well as the mass-transfer enhancement due to SO2 hydrolysis reactions. Gas- and Liquid-Phase Oxidations Although the possibility of conversion of SO2 to H2SO4 by corona discharge in the gas phase was reported in the literature, it was practically impossible to completely analyze and predict the formation of H2SO4 vapor because of the following problems: (1) a difficulty in corroborating the formation of the metastable and excited states of species and measuring the concentrations of those species generated in the gas phase; (2) the lack of information on the detailed temporal and spatial variations of the electric field for a positive corona. The main objective of corona discharge was to provide active radicals such as OH, O, N, HO2, etc., starting with the generation of electrons. However, it was also difficult to estimate the distribution of electrons as a function of energy (i.e., the local electric field) and calculate the subsequent radical and ionic reactions involving the innumerable metastable and excited states of species with respect to space and time. In addition, the predicted removal efficiency of SO2 heavily depends on the combination of possible reaction pathways and the initial electron density (or the initial concentrations of radicals) and was known to be low according to an experience-based data.8 Therefore, sulfuric acid vapor formation in the gas phase was not taken into account in this work. The kinetics of sulfite oxidation has received great attention to dispose of the solid byproduct, gypsum, in all limestone scrubbing methods of the FGD process. It has been reported that the reaction rate depends on the
10.1021/ie020795d CCC: $25.00 © 2003 American Chemical Society Published on Web 03/26/2003
Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003 2031 Table 1. Experimental Results for SO2/Air Absorption into Tap Water run
power (W)
KOG × 106 a
1 2 3
0 7.4 15.3
11.60 64.31 65.37
4 5 6 7 8 9 10
0 4.7 6.4 9.6 11.5 15.5 16.6
3.01 3.74 4.02 3.82 4.17 4.58 4.82
pH
φl,avg
kl × 103 (cm/s)
φE,l measured
kg × 105 a
φg
1/KOGb
1.160 6.431 6.537
1.00 5.54 5.64
86 243 15 550 15 298
1.160 4.508 5.719 6.460 6.486 6.540 6.554
1.00 3.89 4.93 5.57 5.59 5.64 5.65
332 371 267 239 248 970 261 839 239 632 218 264 207 683
H/klb
1/kgb
246 164 245 055 231 481 246 361 224 214 202 973 192 425
86 207 22 184 17 489 15 478 15 418 15 291 15 258
NaOH Solution
a
13.43 13.41 13.16 8.58-6.47 8.62-6.10 8.80-6.06 8.68-5.95 8.62-6.50 8.73-6.10 8.68-5.95
5.89 5.53 5.43 5.48 5.46 5.41 5.54
Tap Water 3.03 3.04 3.22 3.03 3.33 3.67 3.88
Units of mol/(atm cm2 s). b Units of (atm cm2 s)/mol.
Table 2. Tap Water Quality species Na+ Ca2+ Clsulfite a
1.00 1.00 1.06 1.00 1.10 1.17 1.24
content (mg/L) 18.11 28 171-192 NDa
species
content (mg/L)
sulfate nitrite nitrate
123-141 NDa 5.75-7.34
ND ) not detectable.
Table 3. Parameters Used in a Modified Debye-Hu 1 ckel Equation at 22 °C C3j H+ OHHSO3SO32-
6.0 3.0 4.5 4.5
C4j 0.4 0.3 0.0 0.0
-
HSO4 SO42A(T) B(T)
C3j
C4j
3.0 3.0
0.3 0.0 0.508 0.328
liquid-phase composition, temperature, and presence of catalysts (Co2+, Cu2+, and Mn2+). Previous results for homogeneous oxidation (sulfite oxidation with dissolved oxygen in liquid) in the literature presented different rate equations in the presence of catalysts. For heterogeneous oxidation (contact of a sulfite solution with gaseous oxygen), it was reported that the kinetics was so sensitive to experimental conditions that it was extremely difficult to obtain consistent results. Furthermore, most of studies were achieved in the presence of catalysts that tap water does not contain. A recent study9 showed that the heterogeneous oxidation reaction was 3/2-order in sulfite and zero-order in dissolved oxygen in the absence of the catalysts. In this work, calculations were made by incorporating the kinetic data reported in the study into the model to examine the effect of sulfite oxidation in the bulk liquid phase on the SO2 absorption rate. However, because the effect of sulfite oxidation on mass-transfer enhancement turned out to be so small, sulfite oxidation was not taken into account in the model. Results and Discussion Experimental results for the absorption of SO2/air into an NaOH solution and tap water are listed in Table 1. All of the mass-transfer coefficients and parameters listed in Table 1 were obtained by following the same procedure reported in the previous work.3 The tap water was analyzed using an ion chromatograph to estimate the ionic strength. The presence of sulfates was detected as shown in Table 2. The concentrations of bisulfate and sulfate ions in tap water were estimated by assuming that those two ionic species are in equilibrium and are considered as background concentrations in water. The parameters used in a modified Debye-Hu¨ckel equation to estimate the activity coefficients for ionic species including two sulfate ions are also listed in Table 3 for reference. The values of the gas-phase mass-transfer coefficient, kg, listed in Table 1 were similar to those measured in the previous work. The liquid-phase mass-transfer coefficients, kl, were evaluated for the absorption of SO2/ air into tap water and are plotted as a function of corona
Figure 1. kl with respect to input corona power for SO2 absorption into water.
power along with the results of the previous study in Figure 1. When the corona power is not applied to the system, the liquid-phase mass-transfer coefficient, kl, for the absorption of SO2/air into tap water shows a higher value than those for the absorption of SO2/N2 into deionized distilled water because of the higher initial alkalinity of tap water. However, as the power increases, it seems to be difficult to distinguish the effect of alkalinity on the absorption of SO2 into water. When the mass-transfer enhancements in the liquid phase are considered, two effects are involved: (1) mass-transfer enhancement in the liquid phase due to the effect of boundary-layer thinning on the liquid film and (2) an absorption capacity increase due to chemical reactions in the liquid phase. From the values of the liquid-phase mass-transfer coefficients, kl, in Figure 1, it appears that the effect of boundary-layer thinning dominates the effect of chemical absorption within the liquid film as
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Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003
the corona power increases. It is unclear at the current stage what attenuates the effects of alkalinity in a highcorona-power region, and further investigation is to be performed to elucidate the observed phenomena. Nomenclature A(T), B(T) ) parameters in a modified Debye-Hu¨ckel equation C3j, C4j ) specific parameters for each ionic component in a modified Debye-Hu¨ckel equation H ) Henry’s law constant, (atm cm3)/mol KOG ) overall mass-transfer coefficient, mol/(atm cm2 s) kg ) gas-phase mass-transfer coefficient, mol/(atm cm2 s) kl ) liquid-phase mass-transfer coefficient, cm/s Greek Letters φg ) mass-transfer enhancement factor due to electrostatics in the gas phase φl ) mass-transfer enhancement factor due to chemical reactions and electrostatics (or just chemical reactions) in the liquid phase φl,avg ) average mass-transfer enhancement factor over time due to chemical reactions and electrostatics (or just chemical reactions) in the liquid phase φE,l ) mass-transfer enhancement factor due to electrostatics in the liquid phase Subscripts g ) gas j ) concentration of species j, mol/cm3 l ) liquid
Literature Cited (1) Masuda, S.; Nakao, H. Control of NOx by Positive and Negative Pulsed Corona Discharges. IEEE Trans. Ind. Appl. 1990, 26, 374. (2) Tseng, C.-H.; Keener, T. C.; Khang, S.-J.; Lee, J.-Y. Sulfur Dioxide Removal by Pulsed Corona Enhanced Wet Electrostatic Precipitation. Adv. Environ. Res. 1999, 3, 309. (3) Lee, J.-Y.; Khang, S.-J.; Tseng, C.-H.; Keener, T. C. The Effects of Pulsed Corona Discharge on SO2 Absorption into Water. Ind. Eng. Chem. Res. 2001, 40, 5822. (4) Tamon, H.; Sano, N.; Okazaki, M. Influence of Oxygen and Water Vapor on Removal of Sulfur Compounds by Electron Attachment. AIChE J. 1996, 42, 1481. (5) Mizuno, A.; Clements, J. S.; Davis, R. H. A Method for the Removal of Sulfur Dioxide from Exhaust Gas Utilizing Pulsed Streamer Corona for Electron Energization. IEEE Trans. Ind. Appl. 1986, IA-22, 516. (6) Onda, K.; Kasuga, Y.; Kato, K.; Fujiwara, M.; Tanimoto, M. Electric Discharge Removal of SO2 and NOx from Combustion Flue Gas by Pulsed Corona Discharge. Energy Convers. Manage. 1997, 38, 1377. (7) Vasishtha, N.; Someshwar, A. V. Absorption Characteristics of Sulfur Dioxide in Water in the Presence of a Corona Discharge. Ind. Eng. Chem. Res. 1988, 27, 1235. (8) Civitano, L. Industrial Application of Pulsed Corona Processing to Flue Gas. In Non-Thermal Plasma Techniques for Pollution Control; Penetrante, B. M., Schultheis, S. E., Eds.; NATO Advanced Study Institute Series B34; Springer-Verlag: Berlin, Germany, 1993; pp 103-130. (9) Lancia, A.; Musmarra, D.; Pepe, F. Uncatalyzed Heterogeneous Oxidation of Calcium Bisulfite. Chem. Eng. Sci. 1996, 51, 3889.
Received for review October 8, 2002 Revised manuscript received February 11, 2003 Accepted February 15, 2003 IE020795D
