Environ. Sci. Technol. 2001, 35, 2792-2796
A Kinetic Study of the Oxidation of S(IV) in Seawater F. VIDAL B.* AND P. OLLERO Department of Chemical and Environmental Engineering, University of Seville, Camino de los Descubrimientos s/n, 41092, Sevilla, Spain
Flue gas desulfurization by means of SO2 absorption in seawater is a well-known process. However, it can be optimized if the oxidation kinetics of dissolved S(IV) is known. Laboratory reactor experiments in which the pH and temperature were controlled, determined the oxidation kinetics of S(IV) in seawater. This study conclusively shows that the order with respect to S(IV) is one and that the order with respect to oxygen is zero. The kinetic constant depends greatly on the temperature and on the pH; consequently, the activation energy and a relationship between the kinetic constant and the hydrogen proton concentration (2 < pH < 6) were also obtained.
Introduction The desulfurization of flue gas from coal or fuel-oil boilers using seawater is a simple, well-known process with clear advantages over other desulfurization technologies. Bromley (1), using a glass absorber, studied the absorption of SO2 into seawater in the laboratory and reported technical data supporting the process. Tokerud (2) presented a detailed description of the commercial process and compared the effluent from the aeration basin to be disposed of into the sea with normal seawater. Radojevic (3) reviewed industrial applications of seawater scrubbing and emphasized the technical and economic advantages of the process. Tilly et al. (4) made a comparative study of four desulfurization processes (Spray-Dryer, Wet-Limestone, Wellman Lord, and Seawater Scrubbing) for a specific Scottish coastal power station and concluded that, with one possible area of concern, seawater scrubbing was BPEO, i.e., the “best practicable environmental option”. Compared with the most widely used technology, i.e., wet desulfurization using limestone, the main advantages are (1) an alkaline absorbent is not necessary since the seawater itself is alkaline (pH = 8.2) and (2) the water treatment plant needed is much simpler. In the most widely employed industrial wet desulfurization process using seawater, the flue gas comes into contact with the seawater in a spray tower or in a packed tower. The effluent liquid from the tower (with a pH between 2 and 3) is carried to an aerated oxidation basin, where more seawater is added to raise the pH to a level at which the S(IV) absorbed into the tower can be oxidized to sulfate (S(VI)). Then, the seawater is carried to a neutralization basin, where it is mixed with more water from the power plant condensers. This dilution, along with the desorption of the CO2 produced by the aeration, causes a pH > 7 in the liquid waste discharged into the sea. * Corresponding author phone: (+34)954487222; fax: (+34)954461775; e-mail:
[email protected]. 2792
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 13, 2001
There are not many desulfurization units based on this technology, in part, obviously, because its use is limited to plants located on the coast, and, in part, because of other drawbacks associated with it. Two of the main problems are the amount of seawater required and the amount of space that the treatment plants take up. These factors are related to each other as well as to the oxidation rate from S(IV) to S(VI). As is well-known, the oxidation reaction rate increases with the pH when the pH is between 2 and 6, so seawater has to be added to the oxidation basin to raise the pH. However, the beneficial effect of increasing the reaction rate is partially counteracted by the need to process a greater amount of seawater, which means a larger oxidation basin. Thus, there must be an optimal operating pH, whose determination requires detailed knowledge of the oxidation kinetics. An analysis of the existing bibliography regarding the oxidation reaction of S(IV) reveals a lack of kinetics studies, specifically in seawater. The objective of the study presented here is to determine an empirical kinetic equation that describes the oxidation rate for S(IV) in seawater. Of the kinetic studies available in the bibliography, the one that comes closest to the oxidation conditions for S(IV) in seawater is the one by Clarke and Radojevic (5, 6). These authors studied the catalytic effect of the chlorine ion in the oxidation reaction of S(IV) from atmospheric SO2 in aqueous brines. As a result, they proposed the following kinetic equation:
(-rS(IV)) ∝ [S(IV)]2[Cl-]1.3[H+]-0.5 pH 3÷7
(1)
In regard to the reaction mechanism, there is no general consensus. While Beike et al. (7) assumed that the only reactant species is SO32- and that direct oxidation is not produced by HSO3-, Kumar et al. (8) showed that at low pH levels, in which the only reactive species present is HSO3-, oxidation is also produced. Based on experiments carried out in distilled water, Ermakov et al. (9) argued that oxidation takes place by means of a complex mechanism, which involves both SO32- and HSO3-. Only a complex mechanism, such as the one proposed, could explain the dependence of the oxidation rate on the pH observed in some of the studies cited (7, 9). Given that the oxidation mechanism in seawater could be even more complex than in distilled water and considering that the ultimate goal is to find the optimum processing conditions for the global reaction S(IV) + 1/2O2 w S(VI), the following empirical kinetic equation was selected
(-rS(IV)) ) kS(IV)[S(IV)]R[O2]β
(2)
where R and β are the partial order in S(IV) and O2, respectively, and kS(IV) is the kinetic constant, which is a function of the temperature and the proton concentration:
kS(IV) ) k(T)f([H+])
(3)
Experimental Section Apparatus. For these experiments, seawater from Algeciras Bay (Cadiz, Spain) was used. Table 1 shows the seawater composition. The oxidizing gas (air or oxygen) was forced through a bubbler with deionized distilled water in order to saturate it with water. Next it passed through a glass wool filter to eliminate the particles or drops that might have been pulled 10.1021/es000229e CCC: $20.00
2001 American Chemical Society Published on Web 05/26/2001
TABLE 1. Characterization of Seawater Used in the Experiments analyzed parameter
result, mg/L
analyzed parameter
result, mg/L
pH dissolved oxygen (O2) dissolved total solids ammonia (NH3) carbonates (CaCO3) bicarbonates (CaCO3) sulfates (SO42-) chlorides (Cl-) silicates (SiO2) oil and fats calcium (Ca) magnesium (Mg) sodium (Na) potassium (K)
8.2 7.5 38050
