Sulphur Dioxide Solubility in Aqueous Solutions of Calcium or Magnesium Sulphites

Abstract

Based on the physical and chemical equilibria conditions, a model for gaseous sulphur dioxide solubility in a saturated solution of calcium or magnesium sulphites was developed. The dissociation constant of the reaction Ca2++HSO–3CaHSO+3 as a function of temperature is presented. The value of a similar parameter for the system with magnesium sulphite is evaluated as less than 10–2.

Equilibria in a Limestone Based FGD Process: A Pure System and with Chloride Addition

“…According to assumptions 2 and 3, the concentration of dissolved SO 2 or CO 2 in the solution is assumed to be the sum of the gas solubility in pure water and the amount converted into an ion pair involving CaHSO 3 or CaHCO 3 . It has to be stressed that the model formulated according to assumption 2 [15] reproduces experimental data very well [14] for systems consisting of SO 2 (with inert gas)-water with CaSO 3 crystals. Considering experimental data [16] for the system consisting of CO 2 (with inert gas)-water with CaCO 3 particles, assumption 3 is also adequate.…”

“…However, the content of dissolved limestone in the solution has to be equal to zero (assumption 1) in view of its complete reaction with ªsulfurousº acid. The presence of dissolved CaSO 3 in the solution causes conversion of calcium sulfite to calcium hydrogen sulfite at the beginning of sulfur dioxide absorption [15,18]. Before the transformation of calcium sulfite to hydrogen sulfite is completed whole absorbed SO 2 are consumed in the conversion.…”

Equilibria in Limestone-Based FGD Process: Magnesium Addition

“…Additionally, CaHSO 3 + ions formation and/or the neutralization of SO 2 by sorbent result in a decrease of the concentration of HSO 3 - ions in the solution. In such a case, formation of pyrosulfite ions can be neglected. , …”

“…Points 2 and 3 allow one to assume that the amounts of dissolved SO 2 or CO 2 in the solution are sums of their content in pure water and the amount converted into MeHSO 3 + or MeHCO 3 + forms, respectively. Considering experimentally verified results of modeling of sulfur dioxide solubility in calcium sulfite water solutions and moderately concentrated solutions of magnesium sulfite, 9 the influence of calcium and magnesium salts content on Henry's constant can be neglected. A similar assumption can be introduced for Henry's constant describing carbon dioxide solubility.…”

“…In such a case, formation of pyrosulfite ions can be neglected. 5,9 Considering salts and gases dissolved in the FGD solution, the equilibria of the following liquid-phase reactions have to be taken into account: However, because of the precipitation conditions, the following reactions have to be completed:…”