Pergamon Chemical Engineerin0 Science, Vol. 49, No. 24A, pp. 4523-4532, 1994 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009-2509/94 57.00+0.00 0009-2509(94)00355-6 SO2 ABSORPTION IN A BUBBLING REACTOR USING LIMESTONE SUSPENSIONS A. LANCIA,* D. MUSMARRA, t F. PEPE* and G. VOLPICELLI *;t * Dipartimento di lngegneria Chimica, Universit;i di Napoli "Federico II", P~le Tecchio 80, 80125 Napoli, Italy * Istituto di Ricerche sulla Combustione, C.N.R., P.le Tecchio 80, 80125 Napoli, Italy (Received 24 May 1994; accepted for publication 5 October 1994) Abstract--In the present work attention was focused on a wet flue gas desulfurization process using limestone suspensions, which is the most common method used to reduce SO2 emissions from power plant exhaust gases. The SO2 absorption rate was measured varying both the SOz concentration in the gas phase and the limestone concentration in the suspension. The experiments were performed by bubbling mixtures of sulfur dioxide and nitrogen in the continuous limestone aqueous suspension. The absorption phenomenon was studied by making use of the film theory to describe the 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 which 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. Furthermore, they take into account the effect of the gradient of the electric potential of diffusion on the diffusive transport of ions and molecules in the film surrounding the gas-liquid interface. The SO2 absorption rate and the limestone dissolution rate experimentally determined were used to integrate the model equations, yielding the value of the film thickness, and allowing the determination of the concentration profiles of the different species in the liquid film and of the enhancement factor for chemical absorption. Consistency between model and experimental results, on the basis of the hypothesis of the model, was found. INTROD UCTION Treatment of flue gases from power plants is required in order to satisfy the stringent emission limits, particularly to reduce the SOd emission into the atmosphere. A desulfurization process that has reached industrial scale and world-wide diffusion is the wet limestone process. In this process the SO2 scrubbing is realized by contacting the flue gases with a limestone suspension in a spray or packed tower. High SO2 absorption removal efficiency, low cost and wide availability of the absorbing reagent are the principal features that have determined the success of the treatment_ On the other hand, the very large sizes of the absorbing towers and the difficulties related to the safe disposal of the sludge produced are the main disadvantages of such a removal process. The search for different contact conditions between the polluted gas and the absorbing suspension has been the object of a good deal of attention. In particular it has been suggested (ldemura et al., 1978) that SOd could be absorbed in bubbling reactors, in which the gas containing SO~ is bubbled through the scrubbing suspension. The use of such contacting devices would, in fact, allow reduced dimensions for the gas treating plants and reduced capital investments. • *To whom correspondence should be addressed. SO 2 absorption in water has been regarded as a process of absorption with instantaneous reaction by Danckwerts (1968), who showed that the absorption rate could be calculated by means of the penetration theory equations, considering a "total" driving force, obtained by adding the contributions of unreacted SO21~q~ and of its reacted forms (HSO3 and SOl- ions). Later Hikita et al. (1977) and Teramoto et al. (1978) modeled results relative to SO2 absorption in water or in aqueous solutions of NaOH and Na2SO 3 using the equations of the penetration theory with instantaneous irreversible reactions, assuming that one or two reaction planes exist in the mass transfer boundary layer. Chang and Rochelle (1985) showed that in the conditions of interest for FGD applications (i.e_ low SO2 partial pressure), the reversibility of the SO 2 hydration reaction has to be taken into account. Furthermore the same authors (Chang and Rochelle, 1982) showed that a very good approximation to the results of the penetration theory could be obtained by making use of the film theory and replacing the diffusivity ratios by their square roots. Sada et al. (1979, 1981a,b, 1982) showed that, in the presence of rather specific experimental conditions (solid made of very fine particles with very high concentration), solid dissolution in the gas-liquid mass transfer boundary layer has to be taken into account. Therefore, according to these authors, the 4523