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Environ. Sci. Technol. 1999, 33, 1931-1935 results. It has been shown that liquid-phase composition Calcium Bisulfite Oxidation Rate in (sulfite concentration, dissolved oxygen, pH), temperature, - and the presence, even in traces, of catalysts (Co2+,Cu2+, the Wet Limestone Gypsum Flue Mn2+) and inhibitors (alcohols, phenols, hydroquinone) Gas Desulfurization Process strongly affect the reaction rate. Sulfite oxidation was studied both in homogeneous ,² conditions, obtained by contacting a sulfite solution with an AMEDEO LANCIA* AND oxygen saturated solution, and in heterogeneous conditions, DINO MUSMARRA³ obtained by contacting a sulfurous solution with an oxygen Dipartimento di Ingegneria Chimica, containing gas phase. Results for homogeneous conditions UniversitaÁ di Napoli ªFederico IIº, p.le Tecchio 80, 80125 are relatively consistent. Working with high sulfite concen- Napoli, Italy, and Istituto di Ricerche sulla Combustione, trations Yagi and Inoue (2) and Srivastva et al. (3) found a CNR, p.le Tecchio 80, 80125 Napoli, Italy first-order kinetic equation with respect to both oxygen and sulfite. On the other hand, results obtained with lower 3 concentrations, indicate a /2-order dependence from sulfite In this paper oxidation of calcium bisulfite in aqueous and a zero-order dependence from oxygen, both in the absence and in the presence of such catalysts as copper, solutions was studied, in connection with the limestone- manganese, and cobalt (4-9). The dependence of the reaction gypsum flue gas desulfurization process. Experimental rate on the catalyst concentration is more uncertain; while measurements of the oxidation rate were carried out in a the reaction rate is proportional to the square root of the laboratory scale stirred reactor with continuous feeding concentration of Co2+ or Mn2+, the description of the catalytic of both gas and liquid phase. A calcium bisulfite clear solution activity of copper appears more difficult (10). was used as liquid phase, and pure oxygen or mixtures On the basis of the results reported in the literature (4-9, of oxygen and nitrogen were used as gas phase. Experiments 11-12), for a pH range of 7.5-9, low sulfite concentration + + were carried out at T ) 45 °C varying the composition and Co2 or Mn2 as catalyst, the following equation appears of the liquid phase and the oxygen partial pressure. the most appropriate to describe the kinetics of the oxidation Manganous sulfate was used as catalyst. The analysis of reaction the experimental results showed that the kinetics of ) 1/2 3/2 r kcM cS(IV) (3) bisulfite oxidation in the presence of MnSO4 follow a parallel reaction mechanism, in which the overall reaction rate 2- can be calculated as the sum between the uncatalyzed rate where r is the reaction rate expressed as moles of SO4 3 produced per unit time and volume, k is the kinetic constant, ( /2 order in bisulfite ion) and the catalyzed reaction rate cM the catalyst concentration, and cS(IV) the total sulfite (first order in manganous ion). concentration. However, the values found for the kinetic constant k are not at all consistent: in particular the reported values of k at 25 °C range between 2 and 35 × 106 m3/mol Introduction s, while the activation energy ranges from 50 to 150 kJ/mol. The wet limestone-gypsum process is the most common Equation 3 can be interpreted by making the hypothesis flue gas desulfurization treatment. The byproduct of this that the reaction takes place via a free radical mechanism, + 1 z process is a solid mixture mostly made of CaSO3‚ /2H2O. with a chain initiated by the catalyst autoxidation from M + + However, if sulfite and bisulfite ions are oxidized prior solid to M(z 1) or by the action of UV light (13). Such hypothesis formation, a precipitate mostly made of CaSO4‚2H2O (gyp- is also strengthened by the extreme sensitivity of the reaction sum) is obtained. Production of gypsum is useful not only rate to such free radicals scavengers as alcohols, phenols, 1 - because it is inert (while CaSO3‚ /2H2O has a considerable and hydroquinone (14 17). The mechanism best docu- chemical oxygen demand) but also because it can be sold as mented for the reaction is the one proposed by BaÈckstrom - - a building material it the market specifications are met. The (11, 18, 19), which considers the free radicals SO3 and SO5 overall reactions for oxidation of sulfite and bisulfite ions in as chain carriers. aqueous solutions are More recently the study of sulfite oxidation in hetero- geneous conditions received great attention, since these - - SO 2 + 1/ O ) SO 2 (1) conditions are closer to those encountered in FGD processes, 3 2 2 4 where the reaction is carried out by bubbling air into a - - + - HSO + 1/ O ) SO 2 + H (2) solution saturated with respect to calcium sulfite at pH 3.5 5 3 2 2 4 (20). In FGD plants, due to the lower pH, the prevailing sulfurous species is bisulfite ion HSO - instead of sulfite ion The kinetics of such reactions, and particularly of the 3 SO 2-; besides, since calcium sulfite solubility is rather low, absorption of oxygen by basic solutions of sodium sulfite in 3 the concentrations involved in the reaction are lower than the presence of catalysts, received great attention during the those taken into account by the researchers whose work is past decades; Linek and Vacek (1) presented a detailed review described above. However, the interpretation of the results of the literature for the period 1960-1980. The researchers relative to heterogeneous conditions is more complex, due who studied the reaction of sulfite oxidation pointed out the to the interactions between chemical reactions and diffusive extreme sensitivity of its kinetics to experimental conditions, transport of reactants, products, and catalysts (21). Barron which often prevented the achievement of reproducible and co-workers (6, 22) proposed an interpretation according to which, when the absorption rate is high, the overall reaction * Corresponding author phone: [39] (81)768-2243; fax: [39] (81)- is controlled by the rate of production of the activated form 593-6939; e-mail: [email protected]. + + ² UniversitaÁ di Napoli ªFederico IIº. of the catalyst, M(z 1) . More recently Lancia et al. (12) studied ³ Istituto di Ricerche sulla Combustione. the heterogeneous oxidation of calcium bisulfite in conditions 10.1021/es9805425 CCC: $18.00 1999 American Chemical Society VOL. 33, NO. 11, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1931 Published on Web 04/24/1999 of 8.5 × 10-7 m3/s in each experiment, corresponding to a mean residence time into the reactor t of about 460 s. At the beginning of each experiment, as soon as the liquid in the reactor reached the overflow, agitation was started and the gas stream was introduced. It was assumed that steady state was reached after a time longer than 5t had elapsed. The oxidation rate at steady state was evaluated by measuring the total sulfate concentration both in the inlet and in the outlet liquid streams. Furthermore, in both streams the concentrations of total sulfite and Ca2+ ion were measured. Total sulfite concentration was measured by iodometric titration using starch as an indicator, Ca2+ ion concentration was measured by EDTA titration using mures- side as an indicator, while the total sulfate concentration was measured by means of a turbidimeter (Hach DR/3) at 450 nm wavelength. Experimental Results Three different series of experiments were carried out to FIGURE 1. Sketch of the experimental apparatus. measure the reaction rate of calcium bisulfite oxidation in the presence of manganous sulfate as catalyst. In the first of pH and temperature comparable to those encountered in series the total sulfite concentration was varied within quite the wet limestone FGD process. They showed that in the a broad range while keeping oxygen partial pressure (pO2) 3 absence of catalyst the oxidation rate is /2-order in bisulfite fixed at 0.21 atm and using two different concentrations of ion and zero-order in oxygen. Mn2+ ion. In the second series the manganous concentration 2+ - 3 With the aim of gathering a better understanding of the cMn was varied in the range of 0.009 2 mol/m while keeping fundamental phenomena involved in forced oxidation, in pO2 fixed at 0.21 atm and the total sulfite concentration below the present paper the attention is focused on catalytic 7 mol/m3. Eventually, in the third series, different oxygen oxidation of calcium bisulfite. An experimental work is concentrations in the gas phase were considered for two + presented in which heterogeneous conditions are considered, values of Mn2 concentration, with the total sulfite concen- and pure oxygen or mixtures of oxygen and nitrogen are tration below 7 mol/m3. The experimental results for the used as gas phase. Experiments were performed in the slow three series of experiments are reported in Tables 1-3 where, reaction regime (23) using manganous sulfate as catalyst. together with the reaction rate, there are reported the total The experimental findings are compared with those previ- concentrations of sulfite, sulfate calcium, and manganese. ously obtained operating in absence of catalyst (12), with the To find a kinetic equation from the experimental results, aim of obtaining a kinetic equation for the catalyzed rate of it has been necessary to speciate the solution composition. reaction. With this aim the equilibrium equations for the following reactions were used (see Appendix) Experimental Apparatus and Procedure + - SO + H O ) H + HSO (4) The rate of calcium bisulfite oxidation was measured using 2(aq) 2 3 the laboratory scale apparatus sketched in Figure 1. Such an - + - HSO ) H + SO 2 (5) apparatus consists of a thermostated stirred reactor with 3 3 - + - lines for continuous feeding and discharging of both gas and HSO ) H + SO 2 (6) liquid phase.

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