Oxygen scavenger reaction

Most oxygen scavengers perform best within an alkaline pH range, usually around 9.5, as a result of the catalyst or scavenger deprotonation step before the actual oxygen scavenging reaction. Typically, the minimum reaction pH is 8.5 (hence, the general minimum recommendation for FW pH). 

Where larger, semibulk quantities of sulfite-based oxygen scavengers are required, a 20 to 25% bisulfite solution is recommended. It is sufficiently concentrated to provide reasonable freeze protection. The overall oxygen scavenging reaction is ... 

Where dry, catalyzed sodium sulfite is used as the scavenger source, the provision of 2 to 3% metabisulfite into the day-tank batch provides sufficient pH level reduction to ensure the cobalt catalyst does not precipitate. The overall oxygen scavenging reaction is as follows ... 

The hydrazine-oxygen scavenging reaction is pH-dependent (like sulfite), and an increase in pH from 8 to 9 produces a threefold increase in reaction rate and a further three fold increase from pH 9 to 10. 

Homogeneous oxygen scavenging reaction, occurring in FW solution ... 

Following the formation of dehydroascorbic acid, the reaction proceeds irreversibly the degradation process produces oxalates, formates, and carbon dioxide, depending on temperature and pH. As usual, the oxygen-scavenging reaction rate is increased by raising the pH and the temperature. 

Because HQ has limited volatility at lower pressures, when acting as a catalyst with other more volatile oxygen scavengers, it will not always follow the primary scavenger around the boiler cycle system. This creates operational difficulties because scavenging reaction rates are slowed and, in lower pH condensate systems, the post-boiler oxygen scavenging reaction may cease entirely. 

Packer, J.E., Mahood, J.S., Mora-Arellano, V.O., Slater, T.F., Willson, R.L., and Wolfenden, B.S. 1981. Free radicals and singlet oxygen scavengers Reaction of a peroxy-radical with P-carotene, diphenyl furan and 1,4-diazobicyclo (2,2,2)-octane. Biochem. Biophys. Res. Commun. 98 901-906. 

In thermal oxidation, initiation (1) results Irom the thermal dissociation of chemical bonds that may arise Irom intrinsically weak links formed as by-products of the polymerization reaction (e.g. head-to-head links) or impurities formed in the polymerization reactor such as hydroperoxides, POOH, or in-chain peroxides as occur in polystyrene from oxygen scavenging. Reaction (1 ) shows that POOH may produce peroxy and alkoxy radicals that may subsequently form alkyl radicals via reaction (3). 

PACKER J E, MAHOOD J S, MORA-ARELLANO V O, SLATER T E, WILLSON R L and WOLFENDEN B s (1981), Free radicals and singlet oxygen scavengers reaction of a peroxy-radical with /3-carotene, diphei rl furane and 1,4-diazobicyclo(2,2,2)-octane , Biochem BiophysRes Commun, 98, 901-906. 

Oxygen scavenging reaction (Cobalt catalyzed oxidation) ... 

Oxygen scavengers other than hydrazine are also used, especially catalyzed sodium sulfite, which reacts rapidly with oxygen even at room temperatures to form sodium sulfate. Catalyzed hydrazine formulations are now commercially available that react with oxygen at ambient temperatures at rates comparable to catalyzed sulfite (189). At elevated temperatures, the reaction rates are all similar. Table 14 Hsts the standard hydrazine solution products offered by Olin Corp. for sale to the water-treatment market. Other concentrations are available and other companies offer similar products. 

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. 

NOTE Probably the most important junction of oxygen scavengers is, in reality, the ability to passivate boiler steel. In recognition of this, today most novel oxygen scavenger trials try to identify, not merely comparative oxygen reaction rates, but more importantly, the reduction in iron and copper transport rates through the boiler system. In other words, they seek to optimize the passivation of boiler surfaces and other system components. 

Similarly, when catalyzed the reaction rate decreases significantly as a function of pH level. The optimum reaction pH level is approximately 9.5 to 10.5. Iron, and especially copper, in the boiler may act as adventitious catalysts. However, as metal transport polymers are frequently employed, iron, copper, or cobalt may be transported away from contact with sulfite, and thus are not available for catalysis. (This may be a serious problem in high-pressure units employing combinations of organic oxygen scavengers and metal ion catalysts.)... 

In its catalyzed form [catalyzed with hydroquinone (HQ), benzo-quinone, or copper], DEHA has a very fast reaction rate, almost as fast as catalyzed sulfite. Hydroquinone is the most popular catalyst for DEHA, and it is likely that the rapid reaction rate is, in part at least, due to the catalyst simply acting as an oxygen scavenger in its own right. 

Pyrogallol, another oxygen scavenger (and a reaction product of tannins), often is used as a catalyst for HQ. 

MEKO is a very volatile scavenger with a distribution ratio between that of DEAE and cyclohexylamine, which enables it to provide postboiler oxygen scavenging protection in larger steam-condensate lines. It is fast-reacting, although in the boiler this reaction does not always proceed to completion, even in the presence of copper (perhaps only 60-70% complete). 

Several dihydrotetrazine materials have been promoted as oxygen scavengers, including 3,6-dimethyl,1-2-dihydro,1,2,4,5-tetrazine, and the diethyl- and diphenyl-derivatives. 3,6-Dimethyl,1-2-dihydro,1,2,4,5-tetrazine is a six-sided ring with a formula H3C-CN2C-CH3NHNH its MW equals 102. Its basic reaction with oxygen is shown here ... 

Dihydroxyacetone (CH20HC 0) has a MW of 90. Another novel organic oxygen scavenger introduced 20 years ago with little or no commercial success, DHA is catalyzed by HQ and Mn. The reaction requires at least 2 ppm DHA per 1 ppm oxygen. 

The results showed that the presence of oxygen caused a decrease in the yield of As 03 and — As0j> and an increase (not equal to the sum of the other decreases) in >As0. The decrease in clearly due to thermal scavenging effects, while the increase in As0 was ascribed to scavenging reactions such as... 

Ozone also reacts with ethane in the gas phase at room temperature. Rather than a direct molecular reaction, however, evidence points to the initiation of radical-chain reactions by the very small O-atom concentrations present in ozone at room temperature. Added oxygen scavenges the radicals and slows the build-up, leading to induction periods which may be in excess of 3 h. Recent advances in mechanistic investigations of gas-phase ozonolysis of alkanes have been reviewed. Oligomeric peroxides dominate the products of oxidation of nitrotoluenes with ozone in acetic acid. °... 

Kohno, Y., Egawa, Y., Itoh, S., Nagaoka, S., Takahashi, M., and Mukai, K. (1995). Kinetic study of quenching reaction of singlet oxygen and scavenging reaction of free radical by squalene in n-butanol. Biochim. Biophys. Acta 1256,52-56. 

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