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A packed column has been used to study the absorption of nitrogen oxide in an alkaline solution of sodium chlorite. The reactions taking place during the absorption have been examined and a lumped reaction model has been used to estimate rate constants from experimental data. Several parallel and consecutive reactions were found to take place during the absorption. NO was found to be oxidized to NO2 and/or to NO2-, and ClO2- was reduced to Cl- and/or to ClO-. The pH value of the absorbing liquid was found to have a great impact both on the absorption rate and on the extent of the different redox reactions within the liquid. Experimental results indicate that sodium chlorite mainly works as an agent to oxidize NO to NO: and that the major part of the nitrogen oxides are absorbed via the hydrolysis of N2O3 and N2O4. (Less)

A model has been developed to predict the dissolution rate of a limestone slurry as a function of particle size distribution and limestone conversion. The model is based on basic mass-transfer theory and includes a factor allowing the flux of calcium ions from the limestone surface to vary with the fraction dissolved. Changes in the flux with the fraction dissolved have been reported to be caused by the presence of sulfite but can also be caused by accumulation of inerts at the liquid-solid interface and/or by changes in the effective mass-transfer area. Calculations show that the decrease in flux reported for sulfites can have a significant impact on the slurry conditions within the reaction tank, i.e., impact on the limestone conversion and the relationship between liquid and solid alkalinity. In the absence of sulfites, the flux from limestone particles has been assumed to be constant with respect to the degree of dissolution. The modeling results have been found to be in good agreement with the measured values of a continuous stirred tank reactor. The model was able to accurately predict the impact of both the particle size distribution and reaction tank residence time on limestone conversion and dissolution rate.

A model based on the penetration theory has been developed to calculate the dynamic absorption rate of sulfur dioxide into a droplet of limestone slurry. The model includes both instantaneous equilibrium reactions and reactions with finite rates; limestone dissolution, sulfite oxidation, gypsum crystallization and the hydrolysis reaction of CO2. The model has been used to quantify the mass transfer within a spray scrubber and to estimate the impact of the reactions with finite rate of the SO2 mass transfer. The variations within the physical mass transfer conditions of a spray scrubber have been simulated by assuming high mass transfer coefficients close to the nozzles and low coefficients below the spray region. The developed concentration profiles of the diffusing species and the depth of penetration have been determined for different penetration times. The calculations show that the absorption of SO2 into a limestone spray scrubber to a large extent is liquid-side controlled. Only at the very top of the absorber, where the partial pressure of SO2 is low, is the gas film resistance above 50%. Limestone dissolution close to the gas-liquid interface has been shown to be of significance at low pH and in the parts of the absorber where the internal circulation inside the droplets is low. The impact of the hydrolysis reaction of CO2 on the absorption rate of SO2 has been studied by varying the reaction rate constant. Simulations show that the rate constant has a large impact on local absorption rates. Depending on the length of the contact time between the gas and the liquid, the assumption of an instantaneous hydrolysis reaction has an impact on the overall SO2 absorption rate. (C) 1997 Elsevier Science Ltd. (Less)

The flux of an ion in an ionic solution is due to a concentration gradient and an electrical potential gradient. The latter is a result of differences in the diffusion constants of the diffusing ions. Even though it is possible to express the electrical potential gradient in terms of concentrations and concentration gradients, it results in a complicated system to solve if more than two species diffuse. In this study the dissolution rate of limestone has been modeled with the flux equations, including the electrical potential gradient and assuming that the electrical potential gradient is zero. Under wet flue gas desulfurization (WFGD) conditions which correspond to a rather high ionic strength, > 100 mM, it was found that the impact of the potential gradient is insignificant when the pH of the slurry is above 3. Furthermore, it has been shown that when neglecting the potential gradient, the mass balances must be combined with a flux of charge condition and not a condition of electroneutrality. (C) 1997 Elsevier Science Ltd. (Less)

A packed column has been used to study the absorption of nitrogen oxide in an alkaline solution of potassium permanganate. The reactions taking; place during the absorption have been examined and the rate constants have been estimated from experimental data. The experiments show that potassium permanganate is an excellent absorbent for nitrogen oxide. However, to avoid formation of MnO2, the hydroxide concentration has to be very high, i.e. >3 mol/l. It was found that the reaction could be expressed as first-order with respect to NO and with respect to KMnO4. The rate constant may be expressed in terms of the hydroxide concentration as follows: k(mn) = 6114.9 10(1.9208 10-4 CNaOH) m(3) mol(-1) s(-1).