Adsorption of sulphur dioxide from multicomponent mixtures on hydrophobic zeolites.

摘要

Several flue gas desulphurisation processes have been developed over the last decades. The main technologies are capital intensive and usually involve equipment that is space demanding because flue gas flowrates are high and SO₂ concentrations are very low. Adsorption on zeolites has been proposed as an alternative method to treat SO₂-laden streams by taking advantage of the roll-up effect. This phenomenon is based on the competition between SO₂ and H₂O for adsorption sites. The more strongly adsorbed water displaces SO₂ generating a more concentrated stream with respect to SO₂, which can be fed to a conventional process, resulting in better overall economics.; In this study, adsorption equilibrium and kinetic parameters were measured for SO₂, H₂O and CO₂ on hydrophobic zeolites. Breakthrough curves for these species were obtained from their ternary mixtures containing different concentrations of H₂O (1.5–8.0 vol%), two levels of SO₂ concentration (950 and 1800 ppmv) and an almost constant concentration of CO₂ (about 9 vol%). Single component and binary data for a few systems were also investigated. MOR and MFI type zeolites with different SiO₂/Al₂O₃ ratios and a Cs+ exchanged sample were used. The competition with water led to the displacement of SO₂, resulting in very small capacities for this species in the mixtures. The affinity of the zeolites for CO ₂ was very low and the presence of this species in the mixtures did not significantly affect the adsorption capacities for SO₂ and H₂O.; Roll-up peaks of as much as 8 times the SO₂ concentration in the mixture were observed. The magnitude of this effect was found to increase with decreasing SiO₂/Al₂O₃ ratios. At high ratios the phenomenon practically did not exist. On the other hand, the SO ₂ capacities from the mixtures increased with increasing SiO₂/Al ₂O₃ ratio.; A semi-predictive mathematical model was used to predict the breakthrough curves for SO₂ and H₂O. The model was based on axially dispersed plug flow, linear driving force and an adequate equilibrium relationship assuming isothermal behaviour. A biporous diffusion model was used successfully to predict the overall mass transfer coefficients for both SO₂ and H₂O using experimentally derived tortuosity factors. The equilibrium parameters for the multicomponent models were derived from adsorption isotherms. Langmuir 1, Langmuir-Freundlich and Langmuir 2 extended models were used to characterise the multicomponent equilibrium data. The Extended Langmuir 2 based model gave the best agreement with experimental breakthrough curves.