Ind. Eng. Chem. Res. 1988,27, 1235-1241 Friday, D. K.; LeVan, M. D. AZChE J. 1982,28,86. Friday, D. K.; LeVan, M. D. AZChE J. 1984, 30,679. Gelbin, D.; Bunke, G.; Wolff, H. J.; Neimass, J. Chem. Eng. Sci. 1983,38, 1993. Grevillot, G.; Tondeur, D.; Dodds, J. A. J. Chromatogr. 1974, 102, 421. Helfferich, F.; Klein, G. Multicomponent Chromatography;Marcel Dekker: New York, 1970; pp 105-272. Hindmarsh, A. C. ACM-SIGNUM Newsletter 1980, 15(4), 10. James, D. H.; Phillips, C. S. G. J. Chem. SOC.1954, 1954, 1066. Kumar, R.; Dissinger, G. R. Znd. Eng. Chem. Process Des. Dev. 1986, 25, 456. LeVan, M. D.; McAvoy, R. L., Jr.; Davis, M. M.; Dolan, W. B. In Fundamentals of Adsorption; Liapis, A. I., Ed.; Engineering Foundation: New York, 1987, pp 349-358. Pan, C. Y.; Basmadjian, D. Chem. Eng. Sci. 1971, 26, 45.
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Rhee, H. K.; Amundson, N. R. Chem. Eng. J. 1970, 1,241. Rhee, H. K.; Amundson, N. R. AZChE J. 1982, 28, 423. Rhee, H. K.; Aris, R.; Aumndson, N. R. Phil. Trans. R. SOC.1970, A267, 419. Rhee, H.-K.; Aris, R.; Amundson, N. R. First-Order Partial Differential Equations with Applications. Vol. I. Single Equations; Prentice-Hall: Englewood Cliffs, NJ, 1986. Rhee, H. K.; Heerdt, E. D.; Amundson, N. R. Chem. Eng. J . 1972, 3, 22. Richardson, L. F. Phil. Trans. R. SOC.1927, 226, 199. Ruthven, D. M. AZChE J. 1978,24, 540. Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley-Interscience: New York, 1984.
Received for review August 12, 1987 Accepted February 22, 1988
Absorption Characteristics of Sulfur Dioxide in Water in the Presence of a Corona Discharge Niraj Vasishtha and Arun V. Someshwar* Department of Chemical Engineering, University of New Hampshire, Durham, New Hampshire 03824
The absorption characteristics of sulfur dioxide into a stagnant body of distilled water are investigated in the presence of point-plane ionic discharges in a closed chamber a t 30 f 0.1 "C. SO2 removal is seen to be enhanced when the liquid surface is subjected to the discharge, with the bulk of the enhancement resulting in acidic deposition within the chamber. Gas-phase oxidation of SO2 in the absence of liquid water is seen to be minimal. The effects of varying the discharge intensity and polarity and gas oxygen content on the observed absorption rate enhancements are reported. The removal of sulfur dioxide from effluent gases emanating from coal-fired electric-generating plants in ways that are efficient and inexpensive continues to be a problem of paramount concern. Existing flue gas desulfurization (FGD) technologies make use of expensive scrubbers with alkaline reagents as the principal scrubbing medium. Extreme levels of corrosion caused by these reagents have necessitated the use of expensive corrosion-resistant scrubber materials, resulting in high capital costs. The problem of waste disposal of the alkaline sludge is an additional unwelcome factor. In recent years, several researchers (Koppang, 1977; Pilat and Raemhild, 1977; Marks, 1970) have observed that when liquid water is sprayed from electrified nozzles its SO2absorption capacity increases dramatically. While the experimental evidence presented by these authors points clearly in the direction of an advanced technique for SO2 removal, the precise nature of these charge-induced effects is as yet unclear. Considering that spray dryers using liquid reagents are still the dominant FGD method of operation, a further elucidation of the fundamental underlying parameters is imperative.
Background Previous work related to enhanced, charge-induced, SOz absorption can be grouped into three categories. The first category comprises mainly pilot-plant studies in which a liquid absorbent was sprayed from nozzles maintained at a high electric potential into a flowing gas stream containing SOz. Koppang (1977) and Pilat and Raemhild (1977),while working primarily with particulate emission control, found that charged liquid water droplets issuing from electrified nozzles were capable of removing substantial amounts of SO2, far exceeding even that attributable to saturation. Marks (1970) provides further experimental evidence of the superior absorption capabilities of charged aerosols for certain noxious gaseous species. 0888-5885/88/2627-1235$01.50/0
The second category of researchers was primarily interested in accounting for possible enhancements in mass-transfer rates of SO2 absorption into liquid absorbents. Uchigasaki et al. (1967) conducted a detailed laboratory study looking into the absorption of SOz (among other gases) into water and 1 N NaOH in a wetted wall column and in the presence of a wire-cylinder electric discharge. The SOz concentrations studied in this work were typically over 2 mol % or 20000 ppm, which far exceeds the levels commonly found in effluent gases from coal-fired power plants (typically 0 to 3000 ppm). This study was conducted under various conditions of discharge density, polarity, and SO2 and Oz content in the gas stream. On the basis of their results, the authors have suggested that the enhanced removal of SOz in a short-contact-time apparatus was due primarily to a vast improvement in the overall mass-transfer coefficient in the presence of the discharge. However, they also seem to indicate that the enhancement of the transfer coefficient is a strong function of the gas Oz content (being negligible in the absence of 0,) and that the discharge leads to partial oxidation of the SOz with the resulting HzS04being detected in both the exiting gas and liquid phases. Uchigasaki and Endo (1969) and Asano and Uchigasaki (1970) have also reported on significantly enhanced absorption rates of SOz into other alkaline solutions such as N,N-dimethylamine and aqueous Ca(OH), solution. Sheldstad et al. (1974) have also corroborated the observations of increased absorption rates of SO2 into water and several other alkaline reagents in the presence of a wire-cylinder discharge identical with the one used by Uchigasaki et al. (1967). They concluded that the largest effect of the corona occurs in those liquids which give the highest absorption rate without corona (primarily the most alkaline liquids). Carleson and Berg (1983) investigated the absorption of pure SO2 into water droplets formed by electrified nozzles in an attempt to observe any enhanced mass0 1988 American Chemical Society
1236 Ind. Eng. Chem. Res., Vol. 27, No. 7,1988
transfer rates resulting from surface-charge-induced internal Circulation. Working in an oxygen-free atmosphere, they failed to detect any improvements in the coefficients due to the surface charge densities alone. What improvements they did observe they attributed to the reduced drop diameter (and hence increased surface area per unit mass) of the charged droplets issuing from electrified nozzles and increased velocities of these droplets while traversing in an externally applied electric field. Matteson and Giardina (1974) also looked into the absorption rates of SOz (750-4000ppm) into growing droplets of electrically charged water. They concluded that the presence of the electrical charges provided for additional sites for SOz adsorption, which then increased the overall rate of absorption during the initial stages of drop formation. A third category of researchers have focused on the gas-phase oxidation of SOz in the presence of electric discharges. Matteson et al. (1972) have conducted a detailed study of the gas-phase oxidation of SOz (500-3000 ppm) in a humid, air mixture subjected to an electrical discharge. They were able to demonstrate that when the flowing, humid, air stream was subjected to a wire-cylinder discharge significant fractions of the SOz content were converted to an acid mist over a residence time of the order of minutes. They have also reported on various other observations such as the disappearance of ozone normally produced in the discharge in direct proportion to the SO2 removed, the role of gas oxygen content, and the apparent order of the SO2 oxidation reaction. Several other researchers have looked into the feasibility of using electrical discharges for the oxidation and subsequent removal of SOz (Palumbo and Fraas, 1971; Moyes and Smith, 1965; Glockler and Lind, 1939) from air. The general consensus is that discharge-induced gas-phase SOz oxidation is feasible, but the practicality of utilizing such a technique for the treatment of waste gases is in serious doubt. The need for an absorbing media such as water or other dilute alkaline solutions therefore becomes imperative for desulfurization on a practical scale.
Present Work In the present work, the results obtained from the absorption of SOzinto a stagnant body of distilled water, with the latter subjected to point corona discharges, are presented. The corona discharge was produced at the tips of four needle points placed directly above the water surface. The SOz absorption was conducted in a closed chamber, with the initial gas-phase SOzcontent usually around 1600 ppm. Both negative and positive corona discharges were tested and the effects of varying the corona current density observed. The role of Oz content in the gas stream was also briefly studied. In the bulk of the experiments, gasphase SOz concentration profiles (with time) during absorption into a body of 30 cm3 of distilled water were generated, both with and without the imposed discharge. Several runs were also conducted specifically to account for the material balance of the initial input of SOz into the system at the end of a run. In order to estimate the role played solely by discharge-induced, gas-phase SO2 oxidation, tests were also conducted in the absence of a body of liquid water and for various levels of humidity in the closed chamber. Experimental Section Figure 1 gives a schematic representation of the experimental setup. The absorption chamber consisted of a Pyrex dessicator (with an internal gas volume of about 6.24 L) which was fitted with a PVC top and maintained
Figure 1. Schematic representation of the experimental setup: (1) glass absorption chamber, (2) high-voltage electrode, (3) ground electrode, (4) Teflon tray with liquid sample, (5) corona needle electrode, (6) hypodermic needle, (7) circulation pump, (8) syringe pump, (9) pressure gauge, (10)electrometer, (11)high-voltage supply, (12) nitrogen, (13)temperature-controlled air bath, (14)septum, (15) temperature controller, (16) to liquid analysis, (17) to vacuum pump, and (18) to atmosphere.
in a temperature-controlled air furnace at 30 f 0.1 "C.A high-voltage supply (Spellman RHSR 25PN60) provided between 6 and 7 kV of potential to four stainless steel needle points in order to initiate the corona discharge. The discharge current was measured by a Keithley electrometer (Model 617). A syringe pump was used to deliver 30 cm3 of distilled water through a blunted hypodermic needle (22 gauge, stainless steel) into a Teflon tray placed directly beneath the needle points. A circulation pump was used to keep the gas mixture in the chamber fairly well mixed. A septum fitted to the chamber top allowed for the injection of pure SOz samples into the chamber and the extraction of well-mixed samples of the chamber gas at selected time intervals. The experiments were conducted in three distinct configurations. These are depicted in Figure 2. In the type I configuration,no liquid body of water was present within the chamber. The discharge was created between the needle points and a grounded metal plate, and the effects of the discharge on the SOzconcentration profile with time were recorded for several chamber humidity conditions. In the type I1 configuration, 30 cm3of distilled water was placed in the Teflon tray. However, the tray was placed below the grounded metal plate, with the result that although liquid water was present in the chamber the ionic discharge traversed from the points to the plate without impinging on the water surface. In the type I11 configuration, the water was placed directly in the path of the discharge in between the points and the grounded metal plate, with the water itself being grounded. In this latter configuration, the water may be expected to have been significantly charged, although the measurement of this charge with the discharge being active has not been feasible. Also, a significant water mist is created by the bombardment of the ions on the water surface, generally leading to a displacement of about 3% of the water in the tray. On the basis of the power dissipated by the discharge and the circulation of the gases in the chamber, temperature rises in both the liquid water and the gases are expected to be minimal. Procedure. Prior to the start of a run, the chamber was evacuated over extended periods of time (several hours) after which room air was allowed to fill the chamber to a pressure slightly above atmospheric. Once temperature
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1237
\2
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Figure 2. Electrode configurationtypes: (a, top) type I, (b, middle) type 11, (c, bottom) type 111; (1) high-voltage electrode, (2) ground electrode, (3) corona needle, (4) Teflon tray with liquid sample.
equilibrium had been achieved, a sample of pure SOz (generated separately) was injected into the chamber, the circulation pump turned on, and the experiment begun. At intervals of nearly every 20 min samples of the gas were extracted from the chamber and analyzed for their SO2 content. For type I1 and type I11 experiments, after about 50-min duration, distilled water was fed through the hypodermic needle into the Teflon tray. For the “with discharge” runs, at about t = 90 min the corona discharge was initiated and maintained at the required discharge intensity for the remainder of the experiment. At the end of a run (typically about 3-h duration), the liquid sample was analyzed for both dissolved SO2 and SO2 oxidized to HzSOk To conduct the liquid-side analysis, the water sample was siphoned out due to the slight positive pressure maintained in the system. This water was then split into two streams, one of which was fed directly into an analysis flask containing an iodine solution for iodometric analysis of the absorbed SO2 content. The other stream was fed to a similar flask which was subjected to vacuum (to extract any dissolved SOz) and then titrated with a 0.01 N NaOH solution to determine the acid content of the water. The gas-side measurement consisted of a dilution system capable of accurately diluting concentrated 5-mL gas samples (0-2000 ppm) extracted from the chamber with nitrogen down to levels measurable by a Thermoelectron (Model 43A) ambient, pulsed, flouroscent, SO2analyzer. The purity of the SO2 samples injected at the beginning of each experiment was periodically checked on a gas chromatograph fitted with a Poropak QS column. Typically, the above analyses provided the profile of gas-phase SO2 concentration with time and the sulfur content in the liquid phase at the end of a particular run. These profiles provided a qualitative picture of the roles of discharge current density, polarity, and gas Oz content
on the SOz absorption. They also indicated the extents to which gas-phase oxidation of the SO2was taking place. To obtain a more quantitative picture of how the SOz distributed itself at the end of a discharge application, a set of consecutive runs was conducted as above, each for a period of about 3 h, with the discharge being active for half this duration. From the material balance calculations, which included the categories of dissolved and oxidized SO2 in the liquid phase and SO2 remaining in the gas phase, it became apparent that significant portions of the SOz were unaccounted for during the discharge runs. Owing to repeated observations of significant mist formation in the chamber when the discharge was active and acidic corrosion of the chamber metallic parts, a second set of runs was then conducted with an additional analysis performed for possible acidic deposition on the glass chamber walls. Distilled water was used to rinse the glass walls of the chamber at the end of a run and the washings tested for acidity. Acidic depositions on the metallic electrode components and the Teflon circulation tubing were not tested for. Results Type I Experiments. As previously mentioned, in the absence of any liquid water within the chamber, these experiments were conducted to verify whether the point corona discharges utilized here were by themselves capable of oxidizing the SO2. Parts A, B, and C of Figure 3 represent the SO2concentration profiles with time for three levels of relative humidity, viz., (a) about 30%, (b) 4570, and (c) 63%, with and without a negative corona discharge (i = -50 PA). To obtain the desired humidity, air, humidified by a water bubbler, was fed into the chamber for over an hour and vented out through a humidity sensor (EG&G,Model 911). The feed was then discontinued and the humid air within the chamber circulated with the pump. The results for the type I runs seem to indicate that in the present case, wherein only small fractions of chamber gas (estimated to be less than 1%)are subjected to point discharges, the oxidation of SO2 to SO3 is indeed rather negligible. The gradual dip in the profiles with time, especially at the higher humidities, may be attributed to the corrosion reactions on the various metallic components within the chamber. In spite of using type 316 stainless steel components as far as possible, the severe corrosive effects of moist, SOz-laden air could not be avoided. Type I1 Experiments. In these runs, 30 cm3of liquid water was placed below the grounded metal plate, with the result that the discharge did not impinge upon the water surface. Tests for the level of charging of the water at the end of the experiment confiimed the lack of any significant charging in this configuration. Although not measured (as humidity sensor was sensitive to acidic gaseous streams), the chamber humidity in these runs is expected to have been close to saturation for the bulk of the discharge application. Figure 4 represents the SO2profiles generated with and without the ionic discharge for a current intensity of -50 FA. It is evident from this figure that even at extremely high levels of humidity SO2oxidation reactions are negligible in the present configuration, in apparent contradiction to the findings of several investigators. Figure 5 shows typical SO2absorption profiles obtained for a 30-cm3body of water placed within the chamber, in the absence of any discharge. These profiles, which were found to be quite reproducible, will be depicted as a “nofield” solid curve in later figures. The continued decline of the SO2 profile even after over 2 h of gas-liquid contact time may be attributed to primarily two factors, viz.; (a) the ongoing SO2liquid-phase oxidation in the 02-rich en-
1238 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 2000.0
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vironment, and (b) the corrosion reactions on the various metallic parts. Type I11 Experiments. As shown in Figure 2, in the type I11 runs, the water was placed directly in the path of the discharge, with the water itself being grounded. Figures 6 , 7, and 8 show the SO2 profiles for negative discharges of i = -25, -50, and -75 MA,respectively. These runs were conducted with the gas phase comprised of air (0, content of 21%). The significant increase in the SO2 removal rate once the corona is initiated definitely points to some additional removal mechanisms being brought into play. Figure 9 provides a summary of the negative discharge effects at various current densities.
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