1,721,052 research outputs found
Calcium bisulfite oxidation rate in the wet limestone-gypsum flue gas desulfurization process
No full textIn this paper oxidation of calcium bisulfite in aqueous solutions was studied, in connection with the limestone-gypsum flue gas desulfurization process. Experimental measurements of the oxidation rate were carried out in a laboratory scale stirred reactor with continuous feeding of both gas and liquid phase. A calcium bisulfite clear solution was used as liquid phase, and pure oxygen or mixtures of oxygen and nitrogen were used as gas phase. Experiments were carried out at T = 45 degrees C varying the composition of the liquid phase and the oxygen partial pressure. Manganous sulfate was used as catalyst. The analysis of the experimental results showed that the kinetics of bisulfite oxidation in the presence of MnSO4 follow a parallel reaction mechanism, in which the overall reaction rate can be calculated as the sum between the uncatalyzed rate (3/2 order in bisulfite ion) and the catalyzed reaction rate (first order in manganous ion)
Mass transfer between a fixed bed of limestone particles and acid solutions
No full textLimestone dissolution in acid solutions was experimentally studied by means of a fixed-bed reactor. A diffusive model was proposed to describe experimental results. The model, based on the film theory, takes into account the effect of the electric potential of diffusion on the dissolution rate. In order to validate the model, experiments were carried out to study the effect of both particle size and liquid-solid relative velocity on the dissolution rate. Furthermore, the limestone dissolution rate in aqueous solutions of SO2(aq), HCl, and H2SO4 was measured at different acid concentrations. The results showed that the diffusive model, associated with the correlation proposed by Chu et al. [Chu, J. C.; Kalil, J.; Wetteroth, W. A. Chem. Eng. Prog. 1953, 49 (3), 141] for the evaluation of the liquid-solid mass-transfer coefficient, is capable of describing the dependence of the dissolution rate on both the size of the limestone particles and the liquid-solid relative velocity. In addition, it was found that, in agreement with the physical meaning of the film theory, the thickness of the liquid film is not dependent on the kind of acid present in the solution nor on its concentration but only on the fluid dynamic conditions of the liquid-solid system
Uncatalyzed heterogeneous oxidation of calcium bisulfite
No full textWet limestone scrubbing is the most common flue gas desulfurization process for control of sulfur dioxide emissions from combustion of fossil fuels. Forced oxidation in the scrubber loop improves the dewatering properties of the sludge, leading to the formation of gypsum (Ca50 . 2H(2)O). A literature analysis revealed that uncertainties on the mechanisms of the oxidation reaction and on the values of the kinetic parameters still remain. In the present work the oxidation rate was experimentally studied by contacting pure oxygen or mixtures of oxygen and nitrogen with a calcium bisulfite solution. The experiments were carried out in a well-mixed bubbling reactor varying temperature, oxygen partial pressure and sulfite concentration, in the absence of solid calcium sulfite and of catalytic species. It was shown that the rate of the process is controlled by reaction kinetics, and that the reaction rate is zero order in dissolved oxygen and 3/2-order in bisulfite ion. Copyright (C) 1996 Elsevier Science Lt
Calcium sulphate dihydrate nucleation in the presence of calcium and sodium chloride salts
No full textCalcium sulfate dihydrate nucleation in the presence of calcium and sodium chloride salts
No full textCalcium sulfate dihydrate primary nucleation was experimentally studied in a batch crystallizer with a related optical device at a supersaturation ranging from 2 to 4 and a fixed temperature T = 25 degreesC. The mother liquor was supersaturated with calcium sulfate in which calcium or sodium chloride salts were added to approach flue gas desulfurization stack-gas liquor. The experimental results were compared with those obtained in the absence of foreign substances in solutions, showing that chloride salts influence gypsum nucleation by increasing the induction period, thus retarding nucleation kinetics
Measuring induction period for calcium sulfate dihydrate precipitation
No full textHomogeneous nucleation of CaSO4. 2H(2)O (gypsum) based on an optical diagnostic technique was studied within a supersaturation range of 1-4 at 25-90 degrees C. The experiments were carried out using an experimental apparatus consisting of a batch crystallizer with the related measurement devices. Signals of scattered and transmitted light coming from a He Ne laser source were analyzed to measure the induction period (t(ind)), that is, the time delay necessary for homogenous nucleation to take place. As expected from theory, it was found that t(ind) decreases when either temperature or supersaturation increase; from the dependence of t(ind) on supersaturation, it was possible to distinguish between the mechanism of homogenous and heterogenous nucleation. From the experimental data relative to homogeneous nucleation, the interfacial tension (gamma(s)) between CaSO4. 2H(2)O and the surrounding aqueous solution and the activation energy (E-att) for CaSO4. 2H(2)O crystallization were evaluated. In particular, the dependence of t(ind) on temperature made it possible to evaluate E-att at 30 kJ/mol and the dependence of t(ind) on supersaturation offered the value of gamma(s) to be about 37 mJ/m(2), which does snot vary with temperature in the interval explored
Modeling of SO2 absorption into limestone suspensions
No full textThe wet limestone flue gas desulfurization process, and more specifically absorption of SO2 limestone suspensions, was studied. Experiments of SO2 absorption were carried out using a bubbling reactor with a mixture of sulfur dioxide and nitrogen in the gas phase and an aqueous limestone suspension in the liquid phase. The SO2 absorption rate was measured at different compositions of both gas and liquid phases and at different gas flow rates and agitator speeds. A model based on the film theory was proposed to describe liquid-side mass transfer. It was assumed that the liquid-phase diffusional resistance is concentrated in a layer, the thickness of which depends on fluid dynamics, but is independent of the nature of the reactions taking place. The equations considered by the model describe conditions of thermodynamic equilibrium as well as material and electrical balances and use the experimentally determined gas- and liquid-side mass-transfer coefficients, rather than empirical parameters. Model calculations and experimental results were compared, and a good consistency was found. Eventually the model was used to evaluate the absorption enhancement factor as a function of gas- and liquid-phase composition
Fly ash capture of mercuric chloride vapors from exhaust combustion gas
No full textEmissions of mercuric chloride from incineration of Municipal Solid Waste (MSW) constitute a severe environmental problem. Recent studies of the emissions from MSW incinerators indicate that, when fabric filters are used, fly ash may promote Hg emission control. In this investigation, a laboratory-scale apparatus was used to evaluate HgCl2 adsorption on MSW fly ash. A mixture of NS and HgCl2 was used as gas phase, with HgCl2 concentration varying between 0.5 and 10 mg/m(3). Fly ash samples collected, on the fabric filter of an industrial size MSW incinerator, were tested to understand the mechanisms involved in mercury fly ash sorption. The experiments led to the determination of the breakthrough curves for the fixed bed and of the adsorption isotherms at different temperatures in the range of 150-250 degrees C. The adsorption isotherms, which have a characteristic "Langmuir" shape, were used to evaluate the Langmuir parameters at the temperatures investigated and the heat of adsorption for the phenomenon
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