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Abstract
In this work, the CO
2
-H
2
O system species and the preferential precipitation of either CaCO
3
or Mg(OH)
2
have been studied by the MINDO-Forces method. The calculations were carried out using the standard MINDO-Forces procedure with standard parameters for the included atoms. The potential energies of the LUMO of CO
2
, , and
are positive, and this clearly indicates that there is a kinetic barrier to overcome. The activation energy (Ea) of the reaction of H
2
O with CO
2
is about 14.3 eV and this indicates that the reaction is not likely to occur. On the other hand, the reaction of hydroxide ion with CO
2
has a much smaller activation energy (3.4 eV). When the two reactions run simultaneously, the one with lower activation energy occurs preferentially. The pH value plays a decisive role in Mg(OH)
2
scale precipitation. At higher pH, the alkaline mechanism is predominant, and the net reaction (
) is preferred. Increasing the temperature has similar effect since the pH value increases due to the CO
2
release. Values of pH in the evaporator stages were 0.2 to 0.7 unit higher than seawater feed. At high pH values, Mg(OH)
2
precipitates preferentially.
Acknowledgment
This work has been partially supported by the Faculty of Scientific Research and Graduate Studies at Tafila Technical University, Jordan.
Notes
∗From Sawyer et al. (Citation1994).
a Al-Rawajfeh (Citation2007).
b Kern (Citation1960), Astarita (Citation1967), Danckwerts (Citation1970), Stumm and Morgan (Citation1981), and Pinsent et al. (Citation1956).
c Values are from Luther (Citation2004).
d Values in parentheses are from Shams El Din et al. (Citation2005).