ISSN 10637850, Technical Physics Letters, 2009, Vol. 35, No. 7, pp. 657–660. © Pleiades Publishing, Ltd., 2009. Original Russian Text © N.N. Aruev, A.A. Bogdanov, M.I. Petrov, A.M. Polyanskii, V.A. Polyanskii, R.V. Tyukal’tsev, I.L. Fedichkin, 2009, published in Pis’ma v Zhurnal Tekhnicheskoі Fiziki, 2009, Vol. 35, No. 14, pp. 40–47.
MassSpectrometric Study of the ElectronBeamStimulated Conversion of Sulfur Dioxide N. N. Aruev*, A. A. Bogdanov, M. I. Petrov, A. M. Polyanskii, V. A. Polyanskii, R. V. Tyukal’tsev, and I. L. Fedichkin Ioffe PhysicalTechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia Electron and Beam Technologies, Research and Production Corporation, St. Petersburg, Russia LUMASS Company, St. Petersburg, Russia *email:
[email protected] Received March 16, 2009
Abstract—The dynamics of electronbeamstimulated conversion of sulfur dioxide into ammonium sulfate has been studied using a highly sensitive, fastresponse timeofflight mass spectrometer of the reflectron type. The conversion of various gas mixtures containing SO2, CO2, O2, N2, and Ar in the presence of an ammonia and water vapor was carried out in a reactor with a volume of 0.04 m3. Optimum conditions for the electronbeamstimulated conversion of SO2 are determined. The reaction product ((NH4)2SO4) is a valu able nitrogenous fertilizer. PACS numbers: 82.80.Rt DOI: 10.1134/S1063785009070207
Nitrogen oxides and sulfur oxides are formed in the course of metallurgical processes, cement production, and organic fuel combustion. As a rule, their concen trations in industrial waste gases are rather small (below 2%), but the total volume of wastes in the Rus sian industry alone amounts to several hundred thou sand tons for nitrogen oxides and several million tons for sulfur oxides. These oxides are toxic for humans and animals. Moreover, upon being entrained by warm waste gases and carried to superstandard atmospheric layers, these oxides form stable nitric and sulfuric acids under the action of cosmic rays and eventually fall to the ground as acid rain, bringing heavy damage to agriculture and ecology. International agreements pose stringent limitations on sulfur oxide wastes, the emission of which is several times greater than that of nitrogen oxides. Proceeding from the maximum permissible SO2 yield, it is possible to estimate the minimum necessary degree of purifica tion of waste gases from sulfur oxides. For example, this degree amounts to 90–96% for the wastes of coal and oilsupplied boiler stations in which the SO2 con centration typically does not exceed 2% (here and below, concentrations are measured in vol %). At the same time, for the waste gases of metallurgical plants where the SO2 concentration can vary within 1–55%, the minimum degree of purification must be 96– 99.7%. The only method capable of ensuring these high degrees of purification is the electronbeamstimu lated conversion [1, 2]. According to this, waste gases are processed by electron beams with energies of sev
eral hundred kiloelectronvolts, which initiate the rapid oxidation and binding of SO2 to NH3. This purifica tion process is not accompanied by an additional emission of any gases leading to the greenhouse effect, does not yield acids in the liquid phase, and does not contaminate the atmosphere, while the products (ammonium salts) are valuable mineral fertilizers. Experiments on the electronbeam (EB) purifica tion of waste gases with high SO2 concentrations (5– 20%) typical of metallurgical plants have been reported in the available literature. The degree of puri fication achieved in these experiments did not exceed 90%. Therefore, the task of increasing the degree of EB purification processes to 96–99% is topical. Mass spectrometry is a universal method capable of simultaneously monitoring the dynamics of concen tration variation for many gases and following several chains of radical reactions involved in the EB conver sion of SO2. However, the massspectrometric deter mination of the concentration of SO2 during this pro cess is complicated by the simultaneous presence of ammonia, water vapor, and sulfur oxides in the ana lyzed gaseous medium, which can react with structural elements of the instrument. Sulfur dioxide exhibits high adsorption ability and decomposes under the electron bombardment, which leads to the deposition of sulfur on all surfaces in the ionization region. Reactions involved in the EB conversion of waste gases were studied in an experimental prototype of a commercial purification system based on a hermetic reaction chamber with a volume of ~0.04 m3. The flow rate and composition of a model waste gas mixture
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Example of tuning of the detection system of a Lumas50 reflectron in a synchronous regime +
+
+
N2
O2
Ar+
CO 2
81.31877 80.95591 80.87045 81.42074 81.17167 81.64762
17.44877 17.44528 17.48186 17.54036 17.45842 17.47253
0.811175 0.906182 0.838008 0.878528 0.885228 0.856333
0.027254 0.030155 0.027376 0.033272 0.029176 0.032157
Relative intensity
could be varied within broad limits using valves of the gas supply system. The flow rate of each component was automatically controlled with an error not exceed ing 1%. The interaction of a beam of accelerated electrons with molecules of a gas mixture leads to the formation of active H, OH, N, O, and HO2 radical species. These radicals oxidize SO2 to sulfuric acid, which interacts with ammonia with the formation of solid ammonium salts. The characteristic time of radiolysis processes is on the order of 10–8 s, while the times of radical chain reactions range from 10–5–10–1 s. Therefore, the investigation of the process of EBstimulated conver sion of sulfur dioxide into ammonium sulfate requires a highly sensitive fastresponse instrument. Taking into account the above considerations, we performed a gas analysis using a Lumas50 timeof flight mass spectrometer of the reflectron type with a linear trajectory of ion motion [3], which was devel oped at the LUMASS Company (St. Petersburg). This instrument comprises the ion source, reflector, and detector arranged on the same line. At an analyzer length of ~300 mm, the resolution power at half max imum of the mass peak is R0.5 = 150–200 in a mass range of 20–50 amu. Ions are detected by a combina tion of two microchannel plates with a diameter of 25 mm. With the detector operating in the current mea surement mode, the instrument sensitivity with respect to argon is ≤10–10 Torr. In the ion count mode, the instrument is capable of detecting single ions in a bunch at a repetition frequency of 10 kHz. The
2
14
28
40
Mass of ion, amu
Fig. 1. Mass spectrum of residual gases in the analyzer chamber of the reflectron.
dynamic range amounts to 104 in the current mode and 104–105 in the ion count mode. With a proper adjustment of the detection regimes and sufficiently large time (several minutes) of signal accumulation, the total dynamic range of the given reflectron can reach up to 107–108. The instrument has rather small dimensions and weight (~25 kg) and is simply con nected to the reaction chamber, which makes possible the monitoring of chemical processes in various sec tions of the chamber. The mass spectrometer is automatically controlled by a computer so as to simultaneously measure the temporal variations in the concentrations Ci(t) of sep arate components of the gas mixture. In a synchronous regime, the instrument detects only ion bunches with definite masses and flight times. The results of Ci(t) determination are processed and displayed by the computer as graphs and tables. Lumas50 is capable of simultaneously measuring and processing data for twelve components of a gas mixture. The concentra tion of each component present in the mass spectrum measured in the current mode is calculated using the total area under the corresponding peak, which pro vides a greater accuracy of determination as compared to that of the method that uses peak amplitudes. Indeed, the peak amplitude strongly depends on the line shape, which is influenced by numerous factors such as the imperfect ion beam focusing, ion scatter ing on molecules and atoms of residual gases, angular and energy aberrations of the beam propagating from the ion source to detector [4]. The error of determination of the concentrations of gas components by the Lumas50 reflectron operating in the current mode does not exceed 0.02%. This value + is comparable with the background CO 2 peak inten sity, since the average content of carbon dioxide in the atmosphere is ~0.03%. Figure 1 shows the mass spectrum of residual gases present in the analyzer chamber of the reflectron at a pressure of ~5 × 10–8 Torr (without baking), which was recorded using an oscilloscope. The most intense sig + nals in this spectrum are the peak due to H 2 and the 14
+
unresolved peak of N2 –12C16O+. A rather large amplitude is observed for the peak of 40Ar+ (M = 40 anu), the content of which in the atmosphere is + 0.9%, while the peak of CO 2 (M = 44 amu) is only manifested on the background level. Data in the table present a fragment of the procedure of instrument tun ing for residual gases in a synchronous regime, show + + + ing the intensities of N 2 , O 2 , Ar+, and CO 2 peaks in the order of increasing ion masses. Prior to measurements, the mass spectrometer was calibrated using a reference Ar + SO2 mixture pre pared with an uncertainty of component concentra tions not exceeding 1%. Analogous calibrations were
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MASSSPECTROMETRIC STUDY Content, %
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Content, % 50
SO2
NH3 40
O2
20
30
SO2
O2
20 NH3 10
10 0 N2 0
40
80
+
H2S+, 40Ar+, CO 2 , SO 2 , and Ar 2 (cluster ion). For investigations of the process of radiationstim ulated oxidation of sulfur dioxide and its binding to ammonia in the experimental setup, we used an Ar + SO2 mixture modeling the waste gases with SO2 con centration varied in the 5–30% range. The reactor chamber was preliminarily evacuated so as to reduce a contribution due to the residual gas components. The experiments were performed both in static and flow regimes. In a static regime, the chamber was initially filled with an Ar + SO2 mixture, after which NH3, H2O, and O2 components were added and the ionizing electron beam was switched on. The concentrations of NH3, H2O, and O2 components were estimated pro ceeding from the initial concentration of SO2 in accor dance with the following reaction equation: SO 2 + 2NH 3 + H 2 O + 1/2O 2 = ( NH 4 ) 2 SO 4 ↓.
(1)
Figures 2 and 3 show the typical temporal variation of the concentrations of components of reaction (1) in different initial mixtures. Figure 2 illustrates the dynamics of SO2 conversion into ammonium sulfate for an initial mixture containing 26% SO2 and 74% Ar (data for Ar are omitted in order to simplify the pre sentation). The other reactants (N2, O2, NH3, and water vapor) are introduced and the reaction initiated at tin ~ 25 s, after which the content of SO2 in the gas mixture almost immediately begins to decrease. The slope of the descending curve of the SO2 concentration TECHNICAL PHYSICS LETTERS
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800 Time, s
Fig. 3. Dynamics of the EBstimulated conversion of SO2 added (to 30%) to an initial mixture containing 1% Ar, 4.5% CO2, 30% O2, and N2 (to balance). The ammonia and water vapor are introduced (under pressure) and the reaction initiated at tin = 600 s.
also performed for NH3 and O2. As a result of the pre liminary tuning and calibration, the instrument was capable of simultaneously analyzing the dynamics of variation for twelve components of a gas mixture 1 + + + + including N2 , 14N+, NH 3 , 16O+, H2O+, N 2 , O 2 , +
200
Time, s
Fig. 2. Dynamics of EBstimulated SO2 conversion into ammonium sulfate in an initial mixture containing 26% SO2 and 74% Ar. The reactants (N2, O2, NH3, and water vapor) are introduced and the reaction initiated at tin ~ 25 s.
+
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CO2
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is apparently determined by the rate of accumulation of the reactants and the rates of radical chain reac tions. These reactions yield solid ammonium sulfate in the form of a white crystalline powder, which can be observed through glass windows of the reaction cham ber. At tf = 70 s, the SO2 concentration attains a small constant level (~0.03%) and the NH3 content begins to grow at a rate determined by the flow rate of ammo nia supplied to the reactor. Figure 3 shows the dynamics of the same process observed under quite different conditions. In this case, the initial mixture contained 1% Ar, 4.5% CO2, 30% O2, and N2 (to balance). Then, at the moment taken as the onset of the time count, sulfur dioxide was introduced so that its concentration reached 30% within 8–10 min. The reactants (NH3 and water vapor) were introduced and the reaction initiated at tin = 600 s. As can be seen from Fig. 3, the SO2 concen tration in the reactor smoothly decreases for about two minutes and then drops sharply to a level of ~0.03%. From this moment on, the NH3 content exhibits rapid growth. The process is accompanied by the intense formation of ammonium sulfate crystals. The analysis of a large number of curves analogous to those presented in Figs. 2 and 3 led to the following conclusions: (i) A high absolute sensitivity of the Lumas50 reflectron makes it possible to measure the concentra tions of all gas components, even at an extremely low level (~10–8 Torr), which is very important in studying aggressive substances such as ammonia and sulfur dioxide. A high response speed of the reflectron allows the dynamics of processes to be studied in sufficient detail. The automated control and special vacuum sys
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tem design make possible the instrument operation in a continuous roundtheclock duty regime. (ii) The efficiency of EBstimulated sulfur dioxide conversion into ammonia sulfate reaches 97–99%, depending on the initial SO2 concentration. (iii) The relative error of determining the SO2 con version efficiency depends on the initial SO2 concen tration and does exceed 5% in cases under consider ation. (iv) The meansquare deviation of the random component of the relative error in all series of experi ments does not exceed 0.5%. Acknowledgments. The authors are grateful to V.G. Smorodin for his active participation in all exper iments.
REFERENCES 1. V. M. Belogrivtsev, A. S. Koroteev, R. N. Rizakhanov, I. I. SHishkanov, and A. M. Yartsev, Izv. Akad. Nauk SSSR, Énerg. Transport 33, 26 (1991). 2. A. G. Chmielewski, E. Iller, J. Licki, and Z. Zimek, Radiat. Phys. Chem. 40, 321 (1992). 3. B. A. Mamyrin and D. V. Shmikk, Zh. Éksp. Teor. Fiz. 76, 1500 (1979) [Sov. Phys. JETP 49, 762 (1979)]. 4. N. N. Aruev, V. T. Zhdan, A. V. Kozlovskii, S. N. Mar kovskii, and I. I. Pilyugin, Mass Spektrom. 5, 289 (2008).
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