Oxygen scavenger blends

In boilers in which tannin blends are employed both as oxygen scavengers and as sludge dispersants, the magnetite film may also incorporate iron tannates. 

Here, the use of inhibitor formulations having a less dramatic effect on TDS (such as certain tannins) may be extremely beneficial. Formulations are available that are based on tannin chemistry and contain blends that act as oxygen scavengers and metal passivators, with additional sludge dispersant and antifoam properties. 

Where water softening is provided and there is no reduction in system water TDS, treatments are primarily based on inorganic corrosion inhibitor blends (nitrite, molybdate, etc.). Under these circumstances, there is no benefit in using an expensive organic oxygen scavenger to keep the TDS level low, and a common chemical such as catalyzed sodium sulfite may be used. 

As a consequence, it is not possible to devise a single class of anodic inhibitor formulation that satisfies all water treatment requirements. Rather, there several types of formulations, and some formulators may offer each of these products in different strengths, with or without oxygen scavengers and indicator dyes in their blends. As examples, a nitrite/sUicate formulation and a molybdate/nitrite formulation are provided here ... 

Tannin blend concentrates primarily intended for oxygen-scavenging duty in steam boilers. They generally are offered as dark-brown to black, blended products of 25 to 50% active strength. Here, tannins provide an additional benefit of sludge conditioning at no extra cost. 

The hydrolysis products of the hydrolyzable tannins are not dissimilar to some of the novel oxygen scavengers developed to replace hydrazine. Oxygen reaction rates and application rates are also similar. The oxygen reaction time for tannin blends varies with pH levels and temperature but at 45 °C (113 °F) is on the order of 75% complete within 5 seconds and 90% complete within 10 seconds. 

Tannin chemistry is complex and tannin blends (composed of singletree species tannins mixed together) offer a number of useful water treatment functions. Apart from oxygen-scavenging abilities, they act as passivating agents, film-formers, sequestrants, and polymeric sludge dispersants (for more details, see Section 10.3). 

Amines can be blended into multifunctional product formulations containing alkali, polymer, and phosphate and where sulfite- or hydrazine-based oxygen scavengers are part of the formula, but not where tannins are employed. Amines must be fed separately from tannins. 

Alkaline oxygen scavengers may be used as alternative treatments to some corrosion inhibitors where toxicity restrictions prohibit proprietary blends. 

Some formulators additionally use oxygen scavengers or indicator dyes in their blends. An example of a formulation suitable for a closed-loop circuit that is primarily of carbon steel construction is as follows ... 

Fig. 19.18 Morphology of PA6-nanocomposite-PA6I/6T blend with nanoscale oxygen scavenger dispersions (Akkapeddi et al. 2004)...

Fig. 19. 23 Schematics of typical O2 permeability rates top) and the net oxygen ingress in bottles made from PET-O2 scavenger blends...

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