Sulfur dioxide oxidation in slurries of activated carbon. Part I. Kinetics

Abstract

Part 1. KineticsDynamic adsorption and reaction studies were made for the catalytic oxidation of sulfur dioxide in aqueous slurries of activated carbon at room temperature and atmospheric pressure. For the small particles (d, = 0.03 mm) of carbon used, mass transfer resistances were of minor importance SO that the data could be analyzed in terms of the kinetics of the chemical reaction. Quantitative evaluation of mass transfer effects, including intraparticle diffusion, is presented in Part 11.The rate was first order in oxygen concentration in the liquid and zero order with respect to SOz. The rate decreased with increasing HzS04 concentration in the liquid. This was explained by the decrease in solubility of oxygen in the solution rather than by a change in reaction rate constant. The adsorption and rate data together suggest that the controlling step in the mechanism is the adsorption of oxygen on active sites of the carbon surface. The low concentration of oxygen in water (due to its low solubility) reduces its rate of adsorption. SCOPEActivated carbon is known to catalyze oxidation of SOz at room temperature. This provides a potential process for continuous removal of sulfur dioxide from gas streams. The comprehensive review of Hartman et al. (1971) notes that chemisorbed oxygen has a role in the oxidation process and that the presence of water removes the adsorbed SO3 molecules, thus renewing the active sites. Also these authors found that H2S04 slows the rate of oxidation. Unless water is continuously added the buildup of HzS04 in the solution (containing the activated carbon) will ultimately cause the reaction to stop.The kinetics of the reaction, with water present, are not well understood, particularly the role of oxygen and HzSO4. Hartman and Coughlin (1972) interpreted tricklebed reactor data in terms of kinetics which supposed that the surface rate between adsorbed atomic oxygen and adsorbed sulfur dioxide to be rate determining. Since the inlet concentration to the reactor was constant, the results were not a stringent test of the kinetics model. Also intraparticle diffusion was not separated from the intrinsic rate constant. These authors also pointed out the effect of H2S04 in decreasing the solubility of sulfur dioxide in aqueous solutions. Other studies of this system have not analyzed the kinetics of the oxidation reaction. Joyce et a].(1970) reported trickle-bed studies of the recovery of SOz from gas streams. Sulfuric acid of 12y0 concentration was obtained with an excess of oxygen and at 100OC. Seaburn and Engel (1973) compared several activated carbons of different particle sizes in terms of capacity for adsorbing SO, and catalytic activity for the oxidation reaction. Intraparticle diffusion resistances for the various carbons were not considered.The present work was undertaken to clarify the adsorption and reaction characteristics of the oxygen-sulfur dioxide-activated carbon-water system in a three-phase slurry reactor. Part I presents an analysis of data showing predominantly the chemica...

Performance of slurry and trickle‐bed reactors: Application to sulfur dioxide removal

“…The experimental data of Komiyama and Smith (1975b) for oxidation of sulfur dioxide in a slurry reactor provide a means for testing some of the results of the preceding section. These authors measured rates of reaction RA as a function of catalyst loading M , and particle diameter for semibatch operation, that is, continuous gas flow but no flow of liquid.…”

“…The boundary conditions, Equation ( 7) , become Equations (23) to (26) are applicable to any reaction of the form of Equation (1) for which the kinetics are first oruer in A and zero oruer in B, Por s u h r dioxide oxidation with an activated carbon catalyst in an aqueous slurry (Komiyama, 1975a), the reaction was zero order because Cbsoz was in excess. In this case, y s B > 0.…”

Adsorption rates of oxygen in aqueous slurries of activated carbon

“…From the values of K and H at 5"C, kads is 0.32 cm3/ Komiyama and Smith (1975) in their study of the oxidation of sulfur dioxide, using a slurry of the same activated carbon particles as catalyst, concluded that the rate of adsorption of oxygen controlled the oxidation rate. If we assume that their rate of reaction is equal to the rate of oxygen adsorption, a value of kads = 2.25 cm3/(g) (s) is obtained at 25°C.…”

“…Sci., A method has recentIy been deveIoped (Niiyama and Smith, 1976) for evaluating mass transfer and adsorption rates in three-phase slurry reactors by analyzing breakthrough curves in the effluent gas stream. Since slurry systems have been proposed for removing pollutants by oxidation with gaseous oxygen, and since the rate of oxygen adsorption can significantly influence the rate of oxidation (Komiyama and Smith, 1975), we have used the new Hiroo Niiyama is on leave from Tokyo Institute of Technology, Japan. method to determine mass transport and intrinsic adsorption rate parameters for oxygen in aqueous slurries of carbon particles.…”

Study of gas‐to‐liquid mass transfer by dynamic methods in trickle beds

“…Henry's law constant, H i s small (H = 0.01 21 at 298OC according to Komiyama and Smith, 1975) for the very soluble SO2 gas in water. Because the diffusivity in the gas phase is about lo4 times that in the liquid phase, k, is expected to be about lo2 times k,.…”