| dc.description.abstract | Air pollution by sulfur dioxide is of great concern due to its harmful effects on environment, human beings, fauna and flora. Thermal power plants and industries employing combustion of carbonaceous fuels are the major source of SO? emissions. Typically the concentration of SO? in flue gases of these plants is in the range of 2000 to 20000 ppm. Flue gas desulfurisation (FGD) is one of the widely practiced strategies to control SO? emissions. Aqueous?phase oxidation of SO? in the presence of a catalyst is an attractive alternative to the conventional processes for FGD because, amongst other reasons, sulfuric acid, the product of aqueous?phase oxidation, finds extensive application in industry. Salts of metals such as copper(II) are reported to catalyze the oxidation of SO? in aqueous phase effectively. Copper(II) is easily anchored on poly?4?vinylpyridine resin particles to form a complex PVP?Cu which is a potential heterogeneous catalyst for the oxidation of SO? in aqueous solution. The PVP?Cu complex is always in equilibrium with copper(II) in the liquid phase. The objective of the present work is to evaluate systematically oxidation of SO? in aqueous phase by oxygen catalyzed by dissolved cupric chloride in the homogeneous phase and by PVP?Cu complex in the heterogeneous phase.
The first part of the work deals with homogeneous?phase oxidation. Cupric chloride forms the copper(II) catalyst. It is found that concentration of CuCl? has no effect on the solubility of SO? in dilute solutions of CuCl? and H?SO? while concentration of H?SO? has a significant effect. In solution SO? exists as SO?(aq) and HSO??. Experiments with high concentrations of H?SO? in the solution indicated that SO?(aq) does not react and HSO?? is the only reactive species in the aqueous?phase oxidation. External mass?transfer effects are found to be not significant. Based on these observations a rate model is developed which takes into consideration the adverse effect of concentration of sulfuric acid (produced during oxidation) on the solubility of SO? in aqueous solutions. The rate model includes a power?law type term for the rate of homogeneous?phase reaction obtained from a proposed free?radical chain mechanism for the oxidation. The mechanism involves chain initiation by reaction of HSO?? with Cu(II) to produce HSO?• radicals and propagation by formation of SO?•? radical by the reaction of HSO?• with dissolved oxygen which is then followed by reaction of HSO?• with HSO?? to regenerate HSO?• radicals. Consideration of different first? and second?order termination reactions leads to different expressions for the kinetic rate, with a unique combination of orders in CuCl?, O? and HSO??. Experiments are conducted at various levels of concentrations of SO? and O? in the gas phase and CuCl? in the liquid phase. The observed orders are one in each of O?, CuCl? and HSO??. This suggests a first?order termination of the free radicals of bisulfite ion. The predictions of the model match very well with the experimentally observed concentration profiles of SO? in the gas phase and HSO?? and S(VI) in the liquid phase.
The second part of the work relates to the combined homogeneous and heterogeneous?phase oxidation where SO? is bubbled through a dilute solution of CuCl? in which PVP?Cu particles are kept immersed in a rotating basket. Adsorption isotherms of Cu(II) and S(VI) on PVP are obtained; these are of Langmuir type. Effects of external mass transfer processes and of internal mass transfer of S(IV) species are not significant whereas effects of internal mass transfer of S(VI) have marked effect on the system. Intra?particle diffusion coefficient of S(VI) is determined separately from transient experiments to be 1.12 × 10?? m² s?¹.
Preliminary experiments have clearly indicated that SO?(aq) does not react and HSO?? is the only reactant for the heterogeneous?phase oxidation reaction also. The non?reactant SO?(aq) deactivates the oxidation reaction by competing with HSO?? for adsorption on the active PVP?Cu sites on the catalyst particles. However, the particles become saturated with SO?(aq) beyond a certain value of its concentration (saturation limit). A mechanism is proposed for the heterogeneous?phase catalysis based on these observations to develop a rate model for combined homogeneous and heterogeneous?phase oxidation of SO? in a rotating catalyst basket reactor. The rate model also takes into account the effects of the concentration of H?SO? on the solubility of SO?, and of the intraparticle diffusion of S(VI). The model predicts first order in HSO??, half order in dissolved oxygen, first order in concentration of Cu(II) on the solid, and deactivation effect of SO?(aq).
The oxidation reaction is evaluated experimentally at various levels of the operating variables such as concentration of SO? and O? in the gas phase and CuCl? on the solid phase in equilibrium with the corresponding concentration of CuCl? in the liquid phase. In all experiments a pseudo?steady?state region is observed where the gas?phase concentration of SO? reaches a steady value but the concentration of HSO?? and H?SO? in the liquid phase continue to change. Pseudo?steady?state considerations lead to the determination of the initial estimates for the parameters of the rate model—namely, the rate constant and the equilibrium adsorption constant of SO?(aq) on PVP?Cu. The parameters are estimated from the data on the entire transient profile of the product (H?SO?) by solving the model equations by Runge–Kutta method and finite?difference technique using Thomas algorithm along with Marquardt’s non?linear parameter?estimation technique. The procedure also leads to independent predictions of the transient profiles in concentration of SO? in the gas phase and HSO?? in the liquid phase. The predictions of the model with the estimated parameters match very well with the experimentally observed concentration profiles of S(VI) and HSO?? in the liquid phase and SO? in the gas phase. The deactivation constant in the saturation range is found to be 0.26, which indicates that the intrinsic rate constant is about four times greater than the observed rate constant.
The studies indicate that PVP?Cu is an effective catalyst for aqueous?phase oxidation of sulfur dioxide. HSO?? is the only reactive species in both the homogeneous and heterogeneous?phase reactions. The Cu(II)?catalyzed homogeneous?phase oxidation follows a free?radical chain mechanism with the overall rate showing first order in each of oxygen, cupric chloride and HSO??. The PVP?Cu?catalyzed heterogeneous reaction leads to a rate expression which is half order in CO2C_{O_2}CO2??, first order in CHSO3?C_{HSO_3^-}CHSO3??? and first order in catalyst concentration on the solid. | |
