Towards a unified theory of alkaline scale formation from seawater

ABSTRACT

Alkaline scale formation inside the condenser tubes is one of the main operation problems encountered in seawater desalination plants. The scales are composed of CaCO3 and/or Mg[OH]2 and impair heat transfer and reduce flow velocities. Scale formation is inherent to the special composition of seawater, which contains HCO3, Ca[2+] and Mg[2+] ions. The two theories accounting for scale formation are reviewed in short. The one, Langelier's, suggests the primary precipitation of CaCO3 at intermediate temperatures followed, at higher temperatures, by the separation of Mg[OH]2. Dooly and Glater's theory, on the other hand, is based on the separation of Mg[OH]2 at all temperatures. Hitherto, no satisfactory explanation has been presented to explain the reasons behind the variation in scale composition. Like other chemical reactions, scaling can be studied either thermodynamically or kinetically. In the present paper both approaches are attempted. The free energy changes accompanying scale formation reactions at 25 and 100°C are computed and their implications discussed. It is concluded that thermodynamics alone is not adequate to explain scaling. The kinetic aspects of the reactions are then considered. The OH and CO3[2-] ions necessary for the precipitation of Mg[OH]2 and CaCO3 are considered to form as HCO3[-] = OH[-] + CO2[i] and 2 HCO3[-] = CO3[2-] + H2O + CO2 [ii] with reaction [i] being rate determining. The two reactions are assumed to have different, temperature-dependent activation energies. Because it involves bond cleavage, reaction [i] is regarded to have, at low temperatures, a high activation energy. This decreases, however, appreciably as the temperature increases. On the other hand, reaction [ii] is an acid-base neutralization reaction. It is likely to have low- activation energy with smaller dependence on temperature. A schematic presentation of the variation of the activation energy of the two reactions as function of temperature will exhibit two straight lines of different slopes, crossing at an intermediate temperature. As a result of curve crossing, the energy-restricted reaction at low temperatures becomes kinetically favoured at higher temperature. This concept adequately explains the change in scale composition from one of CaCO3-rich to that of Mg[OH]2 -rich as the temperature is raised