Chemical Engineering Science 58 (2003) 3589–3600 www.elsevier.com/locate/ces Rate-based modelling of SO 2 absorption into aqueous NaHCO 3 = Na 2 CO 3 solutions accompanied by the desorption of CO 2 S. Ebrahimi a; b; * , C. Picioreanu a , R. Kleerebezem a , J. J. Heijnen a , M. C. M. van Loosdrecht a a Kluyver Laboratory for Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands b Chemical Engineering Department, Sahand University of Technology, Tabriz, Iran Received 21 February 2003; received in revised form 9 May 2003; accepted 16 May 2003 Abstract A rate-based model of a counter-current reactive absorption/desorption process has been developed for the absorption of SO2 into NaHCO3= Na2CO3 in a packed column. The model adopts the lm theory, includes diusion and reaction processes, and assumes that thermodynamic equilibrium among the reacting species exists in the bulk liquid. Model predictions were compared to experimental data from literature. For the calculation of the absorption rate of SO2 into NaHCO3= Na2CO3 solutions and concomitant CO2-desorption, it is important to take into account all reversible reactions simultaneously. It is clear that the approximate analytical based model cannot be expected to predict the absorption rates under practical conditions because of the complicated nature of the reactive absorption processes. The rigorous numerical approach described here only requires denition of the individual reactions in the system, and subsequent solution is independent of specic assumptions made, or operational variables like pH or compound concentrations. As an example of the exibility of this approach, additional calculations were conducted for SO2 absorption in a phosphate-based buer system. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Absorption; Flue gas; Sulfur dioxide; Numerical analysis; Multiphase reactions; Modelling 1. Introduction Sulfur dioxide in ue gas generated as a result of com- bustion of fossil fuel in, e.g., thermal power plants, etc., is the main cause of global environmental problems such as air pollution and acid rain. Sulfur dioxide has also been reported to support the reactions that create ozone deple- tion in the stratosphere (Karlsson, 1997). Many countries have therefore adopted strict regulations regarding SO 2 emissions from coal- and oil-red boilers in power plants, which are one of the primary sources of SO 2 emissions. The sulfur dioxide content of the ue gas generated is usually quite small and below about 0.1– 0.4% by volume (Astarita, Savage, & Bisio, 1983). However, the volume of the gas produced globally is so large that considerable amount of sulfur dioxide is introduced into the atmosphere. In view of the large number of processes which introduce sulfur dioxide ∗ Corresponding author. Kluyver Laboratory for Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Nether- lands. Tel.: +31-15-278-1551; fax: +31-15-278-2355. E-mail address: s.ebrahimi@tnw.tudelft.nl (S. Ebrahimi). into the atmosphere, it is apparent that studies on ue gas desulfurization methods and development of ue gas desul- furization plants have become numerous. Although various processes have been proposed for ue gas desulfurization, the wet-type scrubbing is still the dom- inant process. The wet-type processes include methods us- ing alkaline solutions of sodium, calcium and magnesium compounds as absorbent. The sodium method above all is excellent in reactivity between the absorbent and SO 2 , but the sodium compounds used are relatively expensive for this purpose. For this reason, the calcium method us- ing relatively cheap calcium compounds such as calcium carbonate is most widely employed as a ue gas desulfur- ization system for large boilers in power plants. However, when sodium compounds are used in a closed-loop system (with regeneration of solutions), it is estimated that the costs will be comparable or lower than those of calcium-based processes. In this study, we have developed a combined chem- ical/biological process for SO 2 removal, NaHCO 3 re- covery and elemental sulfur production. An aqueous NaHCO 3 = Na 2 CO 3 solution is used as absorbent in a closed-loop process, schematically depicted in Fig. 1. 0009-2509/03/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0009-2509(03)00231-8