Abstract
The modified database in the pH, redox equilibrium calculations code (PHREEQC) with a Tipping and Hurley database (T_H.DAT) coupled with the Windermere’s humic acid model (WHAM) was used to simulate scale formation potential in cooling water circuitry, at Eskom power generating stations in South Africa. This study reports a semi-empirical simulative approach in which organic matter fractions, metals and anions in raw and cooling water were used as modelling experimental inputs. By using the saturation index profiles of Ca2+/ Mg2+ with fulvic acid in a modified Tipping and Hurley (T_H.DAT) database, fulvate complex species such as CaFulvate, MgFulvate, and geochemical modelling predictions, mineral phases that potentially precipitate are discussed. Speciation calculations showed that the increase in fulvic acid levels decreased saturation indices of scaling metal phases due to reduced levels of Ca2+ and Mg2+ in the water. Furthermore, if the concentrations of fulvic acid are known, semi-empirical calculations using the geochemical PHREEQC code with a modified T_H.DAT arepossible. Consequently, mineral phase equilibria outputs may give an indication of how the pH and temperature is to be manipulated to optimally predict and control the incidence of scaling.
Highlights
Eskom, a power-generating company in South Africa, draws water for cooling turbines mainly from surface water systems.is type of water has high levels of dissolved solids, mainly organic and inorganic compounds (Pather, 2004). e interaction of the various chemical components may produce ions, compounds in solid or liquid phases and complexes
PHREEQC is able to carry out the following simulations: ion speciation, batch-reaction modelling, surface complexation and ion exchange, reaction, mixing waters and changing temperature, transport modelling, advection with reactions with and without dispersion, and inverse modelling, and can calculate reactions that account for changes in composition of water along a ow path (Parkhurst and Appelo, 2013)
Using pH values (Table 2), alkalinity, alkaline earth metal cations (Ca2+, Mg2+) and anions (SO42, Cl, NO3, CO32, PO43-), various temperature and natural organic matter (NOM) dependent simulations were carried out. e relatively similar raw water characteristics obtained in this study con rmed water use from the same source, the Vaal River
Summary
A power-generating company in South Africa, draws water for cooling turbines mainly from surface water systems. Current research has shown that natural organic matter (NOM) in uences chemical interactions of certain species in natural water. E current study describes (i) the modi cation of the Tipping and Hurley database with the Windermere Humic Acid Model (WHAM) database (T_H.DAT) to incorporate humate and fulvate equilibrium de nitions with Ca and Mg ions, and (ii) speciation calculations generating saturation indices of scaling mineral phases. Successful modi cation of the T_H.DAT was achieved by coding log k values for Mg and Ca equilibrium reactions with predominant NOM fractions (fulvates and humates) into it. Fulvate-2 and Humate-2, previously de ned as SOLUTION_MASTER_ SPECIES in the Tipping and Hurley’s WHAM model (2003), generated MgFulvate, CaHumate and MgHumate in addition to other mineral phases.
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