Ind. Eng. Chem. Res. 2009, 48, 4307–4311
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Mechanism and Kinetics of Sulfite Oxidation in the Presence of Ethanol Li-Dong Wang,*,† Yong-Liang Ma,*,† Ji-Ming Hao,† and Yi Zhao‡ Department of EnVironmental Science and Engineering, Tsinghua UniVersity, 100084 Beijing, China, and School of EnVironmental Science and Engineering, North China Electric Power UniVersity, Baoding, 071003 Hebei, China
Oxidation of sulfite is an important process in flue gas desulfurization. The inhibitory effects of four inhibitors on the intrinsic oxidation of sulfite were compared, and ethanol was found to be an excellent inhibitor. The intrinsic oxidation kinetics of sulfite in the presence of ethanol used as an inhibitor was investigated using a batch apparatus. The reaction orders of the reagents and the activation energy were obtained. The results indicate that the intrinsic reaction proceeds in two steps: rapid reaction in an oxygen-rich state and slow reaction in an oxygen-depleted state. Integrated with the macroscopic oxidation kinetics of calcium sulfite in the presence of an ethanol inhibitor, it was concluded that the macroscopic oxidation process is controlled by the intrinsic reaction rate and the intrinsic oxidation reaction proceeds in the rapid reaction state. Furthermore, a mechanism for the intrinsic reaction is proposed based on a steady-state assumption. The results derived with this mechanism are in good agreement with the experimental results. 1. Introduction The oxidation of sulfite, a residual product in wet desulfurization, has attracted much attention in recent years. Because of the presence of oxygen in flue gas, some sulfite will inevitably be oxidized into sulfate; this is called unforced oxidation. However, the oxidation is frequently accelerated by adding catalysts and bubbling air (forced oxidation). Inhibited oxidation occurs when the oxidation is restrained through the addition of inhibitors, and it has the advantage of avoiding clogging and fouling in practical applications. Furthermore, in some desulfurization techniques, such as magnesia desulfurization,1 sulfite oxidation is prohibited in order to regenerate the absorbent and produce sulfuric acid. Consequently, the study of sulfite oxidation kinetics in the presence of inhibitors is of great importance for flue gas desulfurization. There are two kinds of kinetics, intrinsic and macroscopic. The former is used to study the mechanisms of chemical reactions; it has more theoretical significance and is usually carried out under homogeneous conditions. The latter considers not only the chemical reaction, but also mass transfer of reagents between different phases. Thus, macroscopic kinetics has more practical significance and is often studied under heterogeneous conditions. In wet desulfurization, the oxidation of sulfite accompanies mass transfer, which, as mentioned above, is a macroscopic process. Thus, it is necessary to integrate the two kinds of kinetics in order to clearly reveal the reaction characteristics. The published literature indicates that the oxidation of sulfite is a free-radical reaction and is influenced by many materials, including catalysts and inhibitors. Compared with the uncatalyzed oxidation kinetics,2-6 the catalytic kinetics of sulfite oxidation in the presence of transition metal ions such as manganese, copper, and cobalt7-15 and organic acids such as peracetic acid16 have been thoroughly investigated. In contrast, some materials have been found to have an inhibitory effect on the oxidation of sulfite. Linek and Vacek17 suggested that * To whom correspondence should be addressed. Tel.: (86)1062782030. Fax: (86) 10-62771101. E-mail:
[email protected] (L.D.W.),
[email protected] (Y.L.M.). † Tsinghua University. ‡ North China Electric Power University.
carboxymethyl, cellulose, glycerol, and ethanol inhibit the oxidation of sulfite. Sipos18 also found that phenol has an inhibitory effect during measurements of dissolved oxygen. Similarly to Miller et al.’s research,19 Mo et al.20 suggested that thiosulfate decreases the rate of sulfite oxidation. Zuo and Chen21 suggested that formaldehyde is an inhibitor for sulfite oxidation during simultaneous measurement of sulfate and sulfite. This indicates that the oxidation rate of sulfite would be inhibited by complex organic ligands, such as oxalate22,23 and hydroxyalkanesulfonates.24 Wei et al.25 applied vitamin C to prevent sulfite failure in storage. Our previous work26 also showed that phenol, hydroquinone, and ethanol are all effective inhibitors in the macroscopic oxidation of sulfite; however, this behavior is yet to be confirmed by a study of the intrinsic kinetics. In brief, previous studies mainly focused on identifying inhibitors of sulfite oxidation, and data on the inhibited kinetics are still scarce. Thus, additional studies on the inhibited oxidation kinetics are necessary, especially for the intrinsic kinetics. This article reports on the intrinsic oxidation kinetics of sulfite in the presence of inhibitors, especially ethanol, which is an effective and nontoxic inhibitor and will cause little further pollution. Using a batch apparatus, the inhibited intrinsic oxidation of sulfite was carried out and used to explain the macroscopic oxidation kinetics of calcium sulfite, reported in our previous work. In this way, a mechanism for inhibited oxidation is proposed and suggests a benefit to exploring sulfite oxidation techniques in wet flue gas desulfurization. 2. Experimental Section A batch apparatus was used to analyze the intrinsic reaction kinetics. A dissolved oxygen probe was connected to a dissolved oxygen meter. The zero dissolved oxygen value was adjusted when the probe was shaken in a sodium sulfite solution at a concentration of 5%, and the saturated dissolved oxygen value was adjusted in the air. After this calibration process had been performed for 30 min, the dissolved oxygen meter was ready for automatic measurements. Briefly, known amounts of distilled water and inhibitors were added to an opaque glass reactor. After the dissolved oxygen probe had been fixed in the solution, the reactor was sealed
10.1021/ie801731h CCC: $40.75 2009 American Chemical Society Published on Web 03/26/2009
4308 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009
Figure 1. Effect of inhibitors on the intrinsic oxidation rate of sulfite.
and placed in a water bath. The rotation speed was adjusted to 50 rpm, and the reaction was started at the same time as the sodium sulfite solution was added to the reactor to give a total volume of 1.0 L. The concentration of dissolved oxygen was recorded during the reaction process. Sodium sulfite reacted with dissolved oxygen according to the reaction
Figure 2. Effect of initial concentration of ethanol on the intrinsic oxidation rate.
SO32- + 1 / 2O2fSO42As the initial concentrations of all of the reagents were known, the concentration of sodium sulfite at any time could be calculated from the concentration of dissolved oxygen. It should be noted that the inhibitor of ethanol or hydroquinone was present in trace amounts; hence, their initial concentration (c10) could be neglected compared to the initial concentrations of sulfite (c20) and dissolved oxygen (c30). The statistical software SPSS was used for regression analysis of the experimental data, and the relationship between the dissolved oxygen concentration and the reaction time was determined. Thus, the intrinsic reaction rate at any time was given by 2(dc3/dt). 3. Results and Discussion 3.1. Inhibitor Screening. The effects of four types of additives on the reaction rate were investigated and compared to the results of uninhibited oxidation at 24.0 °C under the following conditions: 2.30 × 10-4 mol · L-1 dissolved oxygen, 6 mmol · L-1 sodium sulfite, and specified concentration of one of the inhibitors (5.42 × 10-6 mol · L-1 hydroquinone, 2.18 × 10-5 mol · L-1 ethanol, 5.35 × 10-5 mol · L-1 phenol, or 1.95 × 10-3 mol · L-1 acetonitrile). As shown in Figure 1, the results indicate that the four additives all have marked inhibitory effects and prolong the process of sulfite oxidation. Furthermore, the order of the inhibitors’ concentrations was acetonitrile > phenol > ethanol > hydroquinone and that of the residence times was hydroquinone > ethanol > phenol > acetonitrile. Thus, it is concluded that hydroquinone and ethanol are the most effective inhibitors. However, ethanol is much less poisonous than the other inhibitors and is considered to be an excellent inhibitor. 3.2. Reaction Order of Ethanol. It is noted that, in the reaction solution, the overwhelming majority of initial dissolved oxygen came from the distilled water, whose volume was about 990 mL. Thus, the initial concentration of dissolved oxygen was approximately equal to the initial concentration of the distilled water. As explained in section 2, the effect of the initial ethanol concentration on the intrinsic reaction rate was measured; the results obtained for an initial concentration of
Figure 3. Reaction order of ethanol in slow and rapid reaction states.
dissolved oxygen of 2.30 × 10-4 mol · L-1 and a sodium sulfite concentration of 6.00 × 10-3 mol · L-1 at 24.0 °C are shown in Figure 2. The results indicate that the intrinsic oxidation of sulfite inhibited by ethanol is a complicated process that is separated into a rapid reaction in an oxygen-rich state and a slow reaction in an oxygen-depleted state. Under otherwise the same conditions, the reaction rate increased with decreasing ethanol concentration, and the rapid reaction state was extended accordingly. At different ethanol concentrations, the initial reaction rate was obtained as described in section 2. The concentration of ethanol and the reaction rate were made dimensionless with respect to their initial values, and the results in Figure 3 indicate that the intrinsic oxidation is of order -0.5 in the concentration of ethanol in the rapid reaction state. In the slow reaction state, the concentration of ethanol and the reaction rate were made dimensionless for the same dissolved oxygen concentration, for example, 1.25 × 10-4 mol · L-1. The results in Figure 3 indicate that the intrinsic oxidation is also of order -0.5 in the concentration of ethanol. 3.3. Reaction Order of Sulfite. The effect of the initial sulfite concentration on the intrinsic reaction rate when the initial concentration of dissolved oxygen was 2.30 × 10-4 mol · L-1 and the ethanol concentration was 2.18 × 10-5 mol · L-1 at 24.0 °C is shown in Figure 4.
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Figure 6. Reaction order of oxygen in the rapid reaction state.
Figure 4. Effect of initial concentration of sulfite on the intrinsic oxidation rate.
Figure 7. Effect of temperature on the intrinsic reaction rate. Table 1. Intrinsic Oxidation Kinetics of Sulfite Inhibited by Ethanol reaction order Figure 5. Reaction order of sulfite in slow and rapid reaction states.
The initial reaction rate was obtained as described in section 2. As in the calculation of the ethanol reaction order, the concentration of sulfite and the reaction rate were made dimensionless with respect to the initial values, and the results shown in Figure 5 indicate that the intrinsic oxidation is of order 1.0 in the concentration of sulfite in the rapid reaction state. In the slow reaction state, the sulfite concentration and reaction rate were made dimensionless at the same dissolved oxygen concentration, for example, 1.41 × 10-4 mol · L-1. The results in Figure 5 show that the intrinsic oxidation is of order 1.5 in the concentration of sulfite. 3.4. Reaction Order of Dissolved Oxygen. When the initial concentration of sulfite was 12 mmol · L-1 (Figure 4), sulfite was greatly in excess compared to dissolved oxygen (0.23 mmol · L-1) and ethanol (21.8 µ mol · L-1) in both the rapid and slow reaction states. Therefore, the isolation method was used to calculate the reaction order with respect to dissolved oxygen. In the rapid reaction state, the dissolved oxygen concentration and the reaction rate were made dimensionless, and the reaction order of oxygen was found to be 2.0 (Figure 6). In the slow reaction state, the dissolved oxygen concentration decreased linearly with increasing reaction time, which is in agreement with the characteristics of a zeroth-order reaction. (See Figure 4.) Thus, the results imply that the intrinsic reaction is of order 0 with respect to the oxygen concentration.
status
ethanol
sulfite
dissolved oxygen
activation energy (kJ · mol-1)
oxygen-rich state oxygen-depleted state
-0.5 -0.5
1.0 1.5
2.0 0
54.5 149.6
3.5. Activation Energy. For initial concentrations of 2.30 × 10-4 mol · L-1 dissolved oxygen, 2.18 × 10-5 mol · L-1 ethanol, and 6.0 × 10-3 mol · L-1 sulfite, the effect of temperature on the intrinsic reaction rate can be seen in Figure 7. According to the Arrhenius equation, k ) k0e-Ea/RT, the activation energy is given by log
(
Ea km 1 1 )kn 2.303R Tm Tn
)
where km and kn are the reaction rate coefficients at temperatures Tm and Tn, respectively. The results indicate that the activation energy is 54.5 kJ · mol-1 in an oxygen-rich state and 149.6 kJ · mol-1 in an oxygen-depleted state. The intrinsic oxidation kinetics data in the presence of ethanol are summarized in Table 1. 4. Mechanism The published literature27 suggests that dissolved oxygen from the air might have a great effect on the free-radical reaction rate. Thus, we tentatively assumed that free radicals were readily produced in the oxygen-rich state but infrequently produced in the oxygen-depleted state. Integrated with the macroscopic oxidation kinetics of calcium sulfite from our previous work26 (see the
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Appendix), we aimed to propose a mechanism for the intrinsic oxidation kinetics of sulfite inhibited by ethanol in wet limestone scrubbing, which is the most popularly applied desulfurization technology. The macroscopic reaction rate is determined by the slowest of the three steps, including the intrinsic reaction in the liquid phase, the dissolution of calcium sulfite from the surface of the solid particles into the liquid, and the mass transfer of oxygen into the liquid. In experiment no. 4, from Table 2, the macroscopic oxidation rate was about 3.3 × 10-7 mol · L-1 · s-1. Because calcium sulfite scarcely dissolved in the reagent, the maximum sulfite concentration was about 1.8 × 10-3 mol · L-1 at pH 6.0. Assuming that the intrinsic oxidation of calcium sulfite proceeded in the slow reaction state and using the concentrations of sulfite (
