A client ran a carbohydrazide analysis on the boiler water, and the result came back close to zero. The team's immediate conclusion: the scavenger is not working and the product needs to be replaced.
Wrong. At the temperature the sample was collected, the carbohydrazide had already decomposed into hydrazine, and it was the hydrazine doing the scavenging work — not detected because no one requested the right analysis.
Wrong conclusions lead to wrong actions, and this is especially true for scavenger selection. It depends on boiler operating pressure, required reaction speed, whether the product generates dissolved solids, compatibility with attemperation water, regulatory restrictions when steam contacts food, handling toxicity, and total cost, including the side effects of a wrong choice.
Sodium Sulfite
Sodium sulfite (Na₂SO₃) is an inorganic powder compound, widely used in low-pressure boilers. It is normally used with catalysts, such as cobalt, which can increase the reaction rate by 10 to 100 times. However, cobalt becomes inactive above pH 9.3, precipitating and forming deposits at the bottom of the tank, which requires strict monitoring.
Because it generates solids, sulfite should not be used in attemperation water, and it is also a limiting factor for concentration cycles. Decomposition products are sodium sulfate (Na₂SO₄), sodium sulfide (Na₂S) and sulfur dioxide (SO₂), which generate acids and are corrosive. It can be used in the food industry under FDA 21 CFR 173.310(c), as neutral or alkaline sodium sulfite. It has a relatively low cost.
Hydrazine
Hydrazine (N₂H₄) is a traditional oxygen scavenger, in declining use due to its high toxicity. It is classified by IARC as Group 2A (probable human carcinogen), requiring rigorous occupational health and safety evaluation.
It is a liquid compound. Reaction speed is slow at low temperature and nearly instantaneous above 150°C. Catalysts such as cobalt significantly accelerate the reaction, by 25 to 100 times. It can be used in attemperation.
It cannot be used in systems in contact with food or food-contact packaging. FDA 21 CFR 173.310(d) requires zero tolerance for hydrazine in steam that contacts food. It can be used at any pressure range. Decomposition products, occurring above 200°C, are nitrogen (N₂) and water, and under thermal decomposition, ammonia (NH₃). Cost is medium to high, depending on the formulation (catalyzed or not) and the handling costs associated with its toxicity.
Carbohydrazide
Carbohydrazide (CH₆N₄O) was specifically developed to replace hydrazine because of the latter's toxicity. It is used in medium- and high-pressure boilers, and decomposes thermally starting at 80°C. Decomposition products are hydrazine (generated in situ), ammonia and CO₂.
This brings up an important observation: if the carbohydrazide analysis is performed at temperatures within or above the decomposition range, the result will be low or zero, leading to possibly false concerns. The decomposition product is hydrazine, which is also a scavenger. In other words, at low temperatures carbohydrazide is analyzed; at higher temperatures, hydrazine is analyzed — and only then should conclusions be drawn.
Comparative tests by Nalco showed that carbohydrazide outperformed hydrazine in oxygen-scavenging efficiency. It does not generate dissolved solids and can therefore be used in attemperation. Because it generates hydrazine in situ, its use in food-contact steam is not recommended. It has a high cost relative to traditional inorganic scavengers.
DEHA
DEHA, diethylhydroxylamine ((C₂H₅)₂NOH), gained ground as hydrazine use declined. Its reaction speed is lower, which is why it is used in higher-pressure boilers, where water temperature is higher. DEHA has relatively high vapor pressure for an organic scavenger, meaning that, when dosed into the feedwater, a fraction of the compound travels with the steam through the system, protecting steam lines and, in some arrangements, even the condensate return.
In the reaction with dissolved oxygen, DEHA forms acetate, nitrogen and water, and does not generate dissolved solids, so it can be used in attemperation. Thermal decomposition tends to generate acetaldehyde, acetic acid, dimethylamine and ethylamine. Its use is not recommended for food applications, since it is not on the FDA's 21 CFR 173.310 positive list. It has moderate toxicity (GHS warning classification), being irritating to the eyes, skin and respiratory tract on direct exposure. Cost is high relative to traditional inorganic scavengers.
Sodium Erythorbate
Sodium erythorbate (C₆H₇NaO₆) is a diastereoisomer of vitamin C, L-ascorbic acid. It is a solid oxygen scavenger, used in low- and medium-pressure systems, reacting quickly with oxygen even at moderate temperatures.
In its conventional form, it is not recommended for attemperation water. However, Nalco Chemical Company developed and patented, in the 1980s, erythorbate neutralized with ammonia or with amines such as morpholine, cyclohexylamine, diethanolamine or triethanolamine, which can be used in attemperation water. Erythorbic acid/sodium erythorbate is GRAS (Generally Recognized as Safe) as a direct food additive under FDA rules, and is cited by industry manufacturers as a non-toxic alternative for steam-generation systems with food contact. Cost is medium to high.
Conclusion
Six products, six distinct characteristics. Correct scavenger selection is decisive for the success of steam-generation water treatment, and in field practice most problems don't come from choosing the wrong product: they come from the right choice applied without understanding the system's real operating conditions.
References
- Chardon Labs. Sulfite for Steam Boilers. Available at: chardonlabs.com.
- Sipilä, K.; Ferreirós, P.; Ikäläinen, T.; Mikkelson, A.; Betova, I.; Bojinov, M. Decomposition products of oxygen scavengers and their effect on corrosion of steam generator materials, I: Diethyl-hydroxylamine and carbohydrazide.
- US Patent 4,269,717. Boiler additives for oxygen scavenging. Inventor: Manuel Slovinsky. Assignee: Nalco Chemical Company.
- US Patent 4,419,327 (also filed in Canada as CA 1,188,594). Inventors: John A. Kelly and Cynthia A. Soderquist. Assignee: Nalco Chemical Company.