Kinetics of the reaction between gaseous sulfur trioxide and solid calcium oxide

Abstract

Tiny particles (diameter < 100 µm) of porous CaO have been reacted with SO₃ in inert N₂ at atmospheric pressure and temperatures between 600 and 900 °C. These particles of CaO were well characterized by BET analysis and mercury porosimetry. Conditions were such that the kinetics of the reaction: CaO + SO₃→ CaSO₄, whereby one solid gives another, controlled the rate of production of CaSO₄. Thus it was established that, for the initial stages of reaction, the diffusion of SO₃ to a particle of CaO in the thermogravimetric balance used was relatively rapid, as was the diffusion of SO₃ both inside the pores of a particle of CaO and through the layer of product, CaSO₄. The reaction is first order in SO₃, the rate constant (defined per unit area of solid CaO) being 3.2 ± 0.3 × 10^–6 exp(–746 ± 108/T) m s^–1. Thus the apparent activation energy is only 6.2 ± 0.9 kJ mol^–1. The rate constant is almost identical to that for SO₂ reacting with CaO to give CaSO₃; in each case the steric factor is extremely low, being ca. 2 × 10^–8. The diffusivity for SO₃ through the product, CaSO₄, is ca. 2 × 10^–13 m² s^–1.