COOLING WATER CORROSION MONITORING PHOSPHONATE SCALING WATER TREATMENT

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Abstract

Previous papers by the authors have documented the impact of pH on inhibitor form, and efficacy. It was shown that the dissociated inhibitor specie is the active form for the polymers and phosphonates scale inhibitors studied. This article expands the inhibitors evaluated, and adds the impact of ionic strength and temperature on the inhibitor speciation via the dissociation constants, and pK_a’s. Scale inhibitor dosage inhibition studies are included that quantify the relative efficacy of inhibitor species. The test methods and their development are described along with the data evaluation methods. The article concludes with the application of the active inhibitor specie concept to practical water treatment situations, including high versus low-pH barium sulfate, and calcium sulfate scale control, and calcium phosphate inhibition.

Some of several important points to understand from this article are the following:

• Scale inhibitors are weak acids and chemically behave like any other weak acid. They are constrained by equilibrium between “protonated” and “dissociated” species.

• The “dissociated” species tends to be the active inhibitor form, while the “protonated” form tends to be inactive.

• A dissociation constant defines the equilibrium. By definition, the pK_a is the pH where half of the total inhibitor is in the active, dissociated form, and half in the inactive, protonated form.

• As pH increases above the pK_a, the equilibrium moves to favor the active, dissociated form. As pH drops below the pK_a, the equilibrium moves towards the inactive protonated form.

The impact of pH and protonation state on treatment efficacy is observed in many areas of water treatment. The protonated form of hypochlorous acid has much more biocidal activity than the dissociated hypochlorite form. The dissociated inhibitor form is more effective in adsorbing on the formations during oil field squeeze treatments. Similar observations have been made concerning the impact of pH and protonation state on efficacy of scale inhibitors by phosphonates and polymers.

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